Biochemical and immunological adaptation in ... - Semantic Scholar

Comparative Biochemistry and Physiology Part B 136 (2003) 227–243

Review

Biochemical and immunological adaptation in schistosome parasitism Afaf El-Ansary* Biochemistry Department, King Saud University, P.O. Box 22452, Riyadah, Saudi Arabia Received 2 February 2002; received in revised form 20 November 2002; accepted 28 April 2003

Abstract The objective of this review is to clarify aspects of immunological and biochemical adaptations of schistosomes to their intermediate and final mammalian hosts. Adaptations to the mammalian hosts are traced in relation to cercarial penetration of mammalian skin, glucose transport and metabolism. The unusual ability of schistosome surface membrane to escape immune recognition and damage are reviewed. Moreover, the behavioural changes induced in the intermediate hosts by schistosomes are considered. The evolutionary adaptation to molluscan hosts aims to increase the probability of transmission of the parasite into its mammalian host. This review inspires more hope for further design of antischistosome drugs through disturbing aspects of biochemical and immunological adaptations in schistosome parasitism. 䊚 2003 Elsevier Science Inc. All rights reserved. Keywords: Parasitism; Schistosomes; Adaptation; Proteinases; Cytokines; Biochemical; Immunological; Drug design

1. Biochemicalyimmunological adaptations in schistosomes Parasites have evolved behavioural adaptations to find hosts, to locate sites of infection within final and intermediate hosts, to escape immune responses, to entice another host to consume the host in which they are residing, etc. Many of these mechanisms are highly complex, but they are sometimes also quite subtle. While the mechanisms or processes involved in a few parasites are understood, most are not. Schistosomes are digenetic trematodes with a complex life cycle involving a mammalian host in which adults of the two sexes mate and deposit *Tel.: q9661-291-8919; fax: q9661-476-9137. E-mail address: [email protected] (A. El-Ansary).

eggs, a free aquatic stage (miracidium) derived from eggs excreted into the environment, a molluscan stage (sporocysts) with active asexual multiplication, and another free aquatic stage (cercariae) that is capable of infecting mammalian host by rapidly penetrating through intact skin. The newly penetrated larvae (schistosomulum) migrate to their final location in the venous system of the mammalian host and differentiate into adult male and female worms (Fig. 1). This review is an attempt to understand aspects of biochemical and immunological adaptations in schistosomes as a digenean parasite characterised by dramatic morphological transformations and by rapid physiological adaptations (Minchella et al., 1995). Biochemicalyimmunological adaptations in schistosomes will be elucidated either to mammalian hosts or molluscan intermediate hosts.

1096-4959/03/$ - see front matter 䊚 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S1096-4959(03)00124-6

228

A. El-Ansary / Comparative Biochemistry and Physiology Part B 136 (2003) 227–243

Fig. 1. Schistosome life cycle. Hatched in waters carrying faecal waste, the schistosome parasite goes through several stages during its life cycle, passing from human hosts to snails attached to mud and back again. Reprinted with permission from the American Chemical Society, Modern Drug Discovery, June 2001, 4(6), pp. 18–19.

1.1. Adaptation to definitive hosts 1.1.1. Cercarial penetration of mammalian skin Schistosome cercariae infect the mammalian host by penetration of the skin, which involves exploration of the outer squamous layers and then entry and migration through the epidermal and dermal layers. Concurrently, the cercariae lose their glycocalyx and transform into schistosomules. Although the glycocalyx plays a vital role in protecting the cercariae against the hypo-osmotic environment of freshwater, it activates immune response of the host making cercariae and newly transformed schistosomula susceptible to lysis and hence schistosomula be shed rapidly after skin penetration (Dalton et al., 1997). Cercariae released from snails vary greatly in a number of physiological and biochemical characteristics for example glutathione and Ca2q concentrations (Modha et al., 1998). This variability has been interpreted as being the result of the different microenvironments in which the cercariae develop within snail tissues. During swimming, cercariae can lose Ca2q if Ca2q levels are low in the surrounding water (Fusco et al., 1991). This loss might be a significant factor in the successful penetration and infection of the host. Modha et al. (1998), who studied the role of Ca2q in Schistosoma mansoni infection, speculated that cercarial Ca2q might have other effects during skin penetration, for example, it might: (1) bind phospholipids

and prevent damage to the membrane; (2) enhance tail loss during penetration and stimulate linoleateinduced proteinase release; (3) prevent dispersal of the glycocalyx antigen and delay presentation of antigen presenting cells; (4) stimulate blood clotting, which might seal ruptured capillaries during parasite migration. Hara et al. (1993) proved that linoleate (18:2) induced tail removal in S. mansoni cercariae through enhancing calcium influx. Biomphalaria glabrata, B. alexandrina and Bulinus truncatus as molluscan intermediate hosts to schistosomes have considerably higher calcium and linoleate levels compared to other non-schistosome target snails (Lymnaea truncatula and Physa acuta) (Abulaban et al., 1996; Nabih et al., 1998). Being in an environment rich in Ca2q and linoleate within the snail tissue could help the schistosome parasites to be transformed into cercariae that are more efficient for penetration and development in the final host and thus complete their life cycle. Stagespecific expression of the mRNA encoding a cercarial-specific 8 kDa calcium binding protein (CaBP) has been described (Ram et al., 1994). Western blots showed high levels of CaBP in cercariae and 3 h schistosomula, trace amounts in 24 h schistosomula, and none in miracidia, sporocysts or adult worms. Expression of CaBP in these two stages could be easily correlated with calciummediated cercarial skin penetration and early trans-


formation within the mammalian tissues. CaBP was not found in muscles and mitochondria, suggesting that it is not involved in the rapid motility and aerobic metabolism characteristic of cercariae. Moreover, the time-course of CaBP detection suggest that the CaBP synthesised in the head gland and subtegumental cells is translocated to the tegument where it plays a role in tegument modifications required for adaptation to parasite life in the mammalian host. Hamdan and Ribeiro (1999) were able to demonstrate and characterise for the first time, a stable form of S. mansoni tryptophan hydroxylase (SmTPH), an enzyme catalysing the rate-limiting step in the biosynthesis of serotonin. They showed that the level of SmTPH in the cercariae is 2.5 times higher than in adult S. mansoni suggesting differences in the level of enzyme expression between these developmental stages. Previous studies proved the importance of serotonin in the increase of glucose uptake, glycogen utilisation and lactate excretion by S. mansoni cercariae (Mettrick, 1989). Expression of the genes encoding tryptophan hydroxylase at higher rate in cercariae could be considered as a prerequisite for the subsequent transformation of S. mansoni larvae. Serotonin as a product of tryptophan hydroxylase could accelerate glycolytic flux as an emergency pathway for generating ATP during the aerobic– anaerobic transition induced within host tissues. Of all the schistosome proteinases reported to date, the 28y30 kDa serine proteinase from cercariae is perhaps the best characterised, and has become known as cercarial elastase. The enzyme represents 90% of the total activity in the cercarial secretions and shows distinct biochemical properties, including susceptibility to inhibitors and activation by calcium. Both skin penetration and glycocalyx shedding are believed to be facilitated by cercarial elastase (Price et al., 1997). Cercariae release most of their serine proteinase activity when induced to secrete the contents of their acetabular glands. In contrast, newly transformed 3 h and 24 h schistosomula do not express this activity (McKerrow et al., 1991). In addition, cercarial serine proteinase activities seem to facilitate skin penetration and degrade human skin dermal elastin (Salter et al., 2000). Recently, Valle et al. (1999) reported the expression of a stathmin-like protein (SmSLP) in S. mansoni. The timing of SmSLP appearance is striking, because the protein is absent during the

229

first 27 days of intramolluscan development-comprising sporocyst formation and active asexual multiplication of cercarial precursors—and it appears to be first synthesised when it is time for the sporocysts to break open and release mature cercariae. Even though correlation does not prove causation, it seems logical to assume that SmSLP has a function in the emergence of the larva from the snail into the water, the penetration of cercariae from the water into the mammalian host, or both. Because no SmSLP is detectable at the eggy miracidium stage, it appears that the passage from the egg to the free-swimming miracidium is not regulated by similar mechanisms. It is clear that stage-specific expression of proteins is a common feature in Schistosoma mansoni parasitism. As an adaptation strategy, this helps the parasite to complete its complex life cycle. 1.1.2. Glucose transport and metabolism in mammalian stage of schistosomes Despite possessing a mouth and functional gut, glucose is taken up by the parasites across their outer body surface or tegument. All cells appear to move glucose molecules across specific glucose transporter proteins. Because the mammalian stage of schistosomes lives in a high-glucose medium, it seems likely that they use the facilitated diffusion glucose transport proteins to take up sugar from the bloodstream (Camacho and Agnew, 1995). Recently, three different S. mansoni facilitated diffusion glucose transport proteins were identified on the basis of cloned cDNAs, designated, SGTP1, SGTP2 and SGTP4 Western blot analysis shows that SGTP1 is present in all schistosome life stages, while SGTP4 is detected only in mammalian-stage parasites, where it appears upon cercarial infection (Skelly and Shoemaker, 1996). SGTP4 is undetected in cercariae, but noticeable 15–30 min after the initiation of cercarial transformation into the schistosomulum. Presumably, the rapid appearance of glucose transport proteins at the surface of the invading schistosomulum reflects the crucial importance of swift sugar uptake to replenish depleted reserves and ensure parasite maturation (Skelly et al., 1998). This could be explained on the basis that the Km for glucose transport by the apical tegumental membrane transporter SGTP4 is greater than that of the basal transporter SGTP1 which has a lower Km for glucose. This favours the basal membrane

230


Fig. 2. Diagramatic representation of glucose movement through the tegument.

transporter to associate with free glucose not utilised in the tegument, so that it can be moved into the body of the worm (Fig. 2). Skelly and Shoemaker (2000) were able to track the process of new tegumental membrane formation, finding that introduction of glucose together with an increase in the incubation temperature synergistically promotes cercarial transformation rates. 1.1.2.1. Glucose metabolism in cercariae and adult. During cercarial transformation, a switch occurs from aerobic to predominantly anaerobic glucose metabolism (Tielens, 1994). This switch from Krebs cycle to the production of lactate is induced by the mere presence of external glucose, which results in an increased glycolytic flux (Horemans et al., 1992). This increase is probably induced by the rapid uptake of glucose that occurs upon expression of SGTP4 at the schistosomula surface, and is maintained as a result of the specific kinetic properties of schistosome hexokinase (Tielens et al., 1994). The rapid switch towards lactate production occurs only in cercarial heads (Horemans et al., 1992). The tails have little or no SGTP4 or hexokinase and degenerate following separation from the penetrating schistosomula (Skelly and Shoemaker, 1996). Skelly and Shoemaker (1995) cloned a set of representative cDNAs encoding proteins involved in glucose uptake, glycolysis, Krebs cycle and oxidative phosphorylation. The different cDNAs were used as probes to examine the expression of

glucose metabolism genes during the schistosome life cycle. These studies revealed that: (1) Transcripts encoding glycogen metabolic enzymes are expressed at much higher levels in cercarial tails than in whole cercariae or schistosomula, while the opposite pattern is found for glucose transporters and hexokinase transcripts; (2) Schistosomula contain low levels of transcripts encoding respiratory enzymes, but regain the capacity for aerobic glucose metabolism as they mature; (3) Male and female adults contain similar levels of the different transcripts involved in glucose metabolism. 1.1.2.2. The movement of glucose to internal tissues. To fulfil the energy requirement of internal tissues, glucose must be transported inwards and all tissues must possess GTPs to facilitate sugar movement across membranes. While schistosomes lack a circulatory system, most schistosome tissues–and not just the tegument—are syncytial in nature (Spence and Silk, 1971). Syncytial organisation might be essential for the movement of small molecules in organisms that depend on diffusion as a mechanism for substrate distribution. To allow free sugar to diffuse to deeper tissues, it is important that not all the imported glucose is metabolised rapidly. The first step in glucose catabolism is its phosphorylation by hexokinase, the rate limiting step for schistosome glycolysis (Skelly et al., 1998). Maintaining low hexokinase levels might be one way the adult parasites ensure that


231

sufficient free glucose exists to feed the entire organism by diffusion. Glycogen degradation and replenishment occur through the body of the parasite (Tielens et al., 1989). This metabolism of glucose ‘through glycogen’ should help maintain a low internal free glucose concentration and so promote sufficient glucose diffusion to deeper tissues. 1.1.3. Host immunity and parasitic interaction Concomitant immunity is a special kind of immune response generally associated with Schistosoma. Concomitant immunity refers to the condition where adult parasites in an initial infection are unaffected immunologically by a host, but in which challenge infections are. The effect is to keep the host from becoming overwhelmed by the parasite in subsequent invasions by water-borne cercariae. It is a subtle adaptation toward the establishment of a stable host-parasite relationship (Butterworth, 1990). The mechanisms by which adult schistosomes escape the host’s immune response are unknown despite intensive study over the years. However, some investigators are convinced that the parasite incorporates host proteins in a process known as antigen sharing, and thus masks the parasite from the host. Others suggest that the parasite becomes immunologically ‘inert’ with the loss of capacity by the parasite to express antigens in its tegumental surface that would be perceived as foreign by the final host (Butterworth, 1990). Since antigens provoking immunity are produced by all stages of development within the host (schistosomula through to adult) two major questions arise in immune animals (1) How are schistosomula killed? and (2) How do adult worms survive? This could be answered through studies of Mei et al. (1996) who measured the antioxidant enzyme glutathione peroxidase (GP) activity in different fractions of worm extracts from several developmental stages of S. mansoni. Fig. 3 demonstrates that enzyme activity is developmentally regulated, with higher specific activities in the extract of adult worms—the stage least susceptible to immune killing—than in the larval stages, which are most susceptible to immune elimination. This suggests that antioxidants may play a role in immune evasion by schistosome parasites. Maturating and adult schistosomes obtain amino acids by catabolizing haemoglobin from ingested host erythrocytes. A variety of proteinases, such

Fig. 3. Developmental expression of glutathione peroxidase in adult worms and other larval stages of schistosomes. The adult worms are least susceptible to immune killing while the other larval stages are most susceptible to immune elimination.

as cathepsin B (Sm31), cathepsin L1, cathepsin L2, cathepsin D, cathepsin C and legumain (Sm32) are believed to be involved in this process. These proteinases together with cercarial elastase, appear capable of modulating the host immune response and regulating the synthesis of specific and other IgE antibodies in vitro and in vivo in rats (Brindley et al., 1997). A developmentally regulated S. mansoni serine protease (SmSP1) has been identified (Cocude et al., 1999). However, infected animals did not produce specific antibodies to recombinant SmSP1. The lack of such response could be advantageous to the parasite by avoiding host effector mechanisms. Newly transformed schistosomula rapidly turn over their membranes, shedding membrane sheets into the blood stream of the host (Caulfield et al., 1991). This is believed to be an important evasive mechanism in schistosomula, but probably less so in adult schistosomes. Adult schistosomes have a slower turnover of their outer tegumental membranes, but the phospholipids of their membranes are subjected to very rapid deacylationyreacylation (Brouwers et al., 1997). The fatty acids required for this deacylationyreacylation have to be obtained from the host, as schistosomes are unable to synthesise fatty acids de novo. Incorporated fatty acids can be modified by chain elongation, resulting in a fatty acid profile significantly different from that of the host (Brouwers et al., 1997).

232


Phosphatidylcholine (PC) species from schistosomes, and those of the outer tegumental membranes in particular, are more saturated than those from the blood of the host (Brouwers et al., 1998a). The high amount of (16:0-20:1)-PC, which comprises nearly 50% of the tegumental membrane PC fraction, can make the outer tegumental membranes more resistant to the damage caused by reactive oxygen species secreted by neutrophils and macrophages. PC composition of the outer tegumental membranes therefore seems to reflect another adaptation of the parasite to the intravascular habitat and exposure to the host’s immune system. Plasmogens (PE) are enriched in the parasites’ outer tegumental membranes, which form the site of interaction between parasite and host. These membranes mediate the resistance of the parasite against the actions of the immune system of the host. This resistance depends upon the activity of specific membrane bound enzymes that counteract or prevent activation of the complement system as a sequentially activated plasma proteins, some of which opsonize or lyse cells. The activation of some of the complement components involves their cleavage into fragments, which then bind to corresponding complement receptor on the surface of target cells and it contributes to the destruction of schistosomes (Jokiranta et al., 1995). In addition, schistosomes are capable of another fatty acid modification, resulting in the formation of 5-octadecenoic acid (D5-C18:1). This highly unusual fatty acid, that is absent in the blood of the host, was shown to be almost exclusively located in the outer membrane complex of the schistosome suggesting important tegument-mediated role for this lipid (Brouwers et al., 1998b). In conclusion, the lipid composition of the outer tegumental membranes of schistosomes is distinct from the blood on which they feed. Apparently, a correlation exists between the inhibition of the NayK-ATPase with ouabain as a specific inhibitor, and the lysis and death of the complement-resistant parasite (Hazadi et al., 1997). The most exciting categories in the immunological interactions between schistosome parasites and the invaded host are the cytokine environment and signals received from antigenpresenting cells. Naive CD4q T helper lymphocytes (Th cells) differentiate following activation to secrete a restricted set of cytokines that serve to categorise them as either Th1 or Th2 cells

(Abbas et al., 1996). Th1 cells, whose differentiation is promoted by interleukin 12 (IL-12), secrete interferon g (IFN-g), IL-2, tumour-necrosis factor (TNF)-b and lymphotoxin. In contrast, Th2 cells produce IL-4, -5, -6, -9, -10 and -13. Th1 and Th2 cells differed in the types of antibody response they stimulated, that Th1 cells mediated delayed-type hypersensitivity (DTH) reactions, and Th2 cells were more potent helpers for antibody reactions. In addition, IFN-g stimulates the production of IgG antibodies, which bind to highaffinity Fcg receptors and complement proteins and are therefore the principal antibodies involved in the opsonisation, phagocytosis and antibodydependent cell-mediated cytotoxicity (ADCC) of microbes. The signature cytokines of Th2 cells are IL-4 and IL-5. IL-4 is the major inducer of B-cell switching to IgE production and is therefore a key initiator of IgE- dependent, mast-cell-mediated reactions. IL-5 is the principal eosinophil-activating cytokine, and mice lacking IL-5 or its receptor show marked defects in eosinophil-responses to helminths. This may explain why down-regulation of Th2 cytokines correlates with low response to vaccination against S. mansoni (Sher and James, 1998). The balance between Th1 and Th2 responses to an infectious agent can influence pathogen growth and immunopathology. The granuloma surrounding S. mansoni eggs causes pathology in murine schistosomiasis, and its formation is driven by egg Ag-stimulated type 1 and type2 cytokines. Granulomas are areas of cellular infiltration consisting mainly of eosinophils, macrophages, fibroblasts and lymphocytes. In immunocompetent animals, the formation of granulomas is orchestrated by CD4 T helper (Th2) cells (Hernandez et al., 1997). Eosinophils are a numerically dominant cell population within the schistosome granuloma. In these granulomas, eosinophils can produce a variety of cytokines, including IL-2, IL-4, IL-5 and IFN-gamma. Granuloma eosinophils were highly activated at 7 and 11 week postinfection. Activation and cytokine production were greatest at the time of maximum granuloma formation, i.e. 10–12 weeks after initial cercarial exposure. Therefore, locally activated eosinophils, not Th2 lymphocytes, produce the majority of Th2 cytokines in the granuloma milieu and may be important determinators of immunopathology in schistosomiasis (Rumbley et al., 1999). While granulomas are themselves pathogenic, they serve an essential host-protective func-


233

Fig. 4. Signal transduction in cells and schistosomes. The binding of TNFa to its receptor on the cell membrane of the adult worm activates neutral spingomyelinase (N-SM ase), the ceramide-activated protein kinase cascade results in cell life. Binding of TNFa to its receptor on schistosomula cell membrane activates acidic sphingomyelinase (A-SMase) through diacylglyceral a product of phospholipase (PLC). This activates different kinase cascades resulting in cell death.

tion, by walling-off toxic egg products, such as the hepatotoxic antigen omega-1 which would otherwise kill the host and consequently, the parasite (Dunne et al., 1991). In infected T-celldepleted and in mice with severe combined immunodeficiency (SCID), granulomas fail to form and liver failure can follow (Amiri et al., 1992). Other experiments demonstrate a major advantage to the parasite as well, in facilitating the excretion of parasite eggs and thereby enabling completion of the life cycle (Doenhoff et al., 1985). Moreover, Fc-g receptors are involved in human eosinophil activation by allergen-antibody immune- complexes and in the IgG-dependent cytotoxic activity against parasites (Kaneko et al., 1995). Purified eosinophils from hepatic S. mansoni granulomas express on the surface Fc-g receptors necessary for IgG ADCC responses and this has been proposed to have a protective function. Studies attempting to identify the principal cytokine responsible for granuloma formation have implicated tumour necrosis factor alpha (TNFa). While TNFa can be toxic to schistosomula at high concentrations, it results in increased fecundity in adult parasites (Amiri et al., 1992). If this effect

is receptor mediated, it might indicate that two pathways exist in schistosomes, whose opposite effects dominate at different stages of the life cycle. It is possible that parasite populations (adult worms) already established in the host stimulate TNF-a production, resulting in the death of newly arriving schistosomula, again limiting population size. Fig. 4. demonstrates effects of TNFa in schistosomes. In adult schistosomes, binding of TNFa activates neutral sphingomyelinase (NSMase), the ceramide (Cer)-activated protein kinase cascade results in cell life (i.e. growth andy or differentiation). However, in schistosomula, phospholipase C (PLC) is activated and the diacylglycerol (DAG) released activates acidic sphingomyelinase (A-SMase) in the lysosome. The ceramide released by A-SMase action stimulates a different kinase cascade, resulting in cell death. Thus, alteration in cell membrane signalling in schistosomes could provide a way of biochemical adaptation to host immune responses and explains the concomitant immunity associated with this parasite. The host protective role of granuloma formation was ascertained by Fallon and Dunne (1999) who

234


induced tolerization of mice to S. mansoni egg antigens by combined cyclophosphamide treatment and thymectomy. In the early acute stages of schistosome infection, egg-tolerizing mice suffered high mortalities with significantly reduced granuloma size and deposition of collagen in the liver. Similarly, limited granuloma responses were detected in the intestines of these mice, and this was associated with )90% in egg excretion. Histologically egg-tolerizing mice had exacerbated hepatocyte damage and impaired proliferation responses to egg Ag but intact responses to worm Ag. Moreover, these animals failed to downregulate type1 cytokines that are normally elicited during early schistosome infection. In situ hybridisation studies of schistosome egginduced liver granuloma demonstrated that Th1 cells were present at very early granuloma development, while Th2 cells were required for full development (Hassan et al., 2000). As an immune evasion strategy, cercarial and schistosomular extracts can cleave human, mouse and rat IgE, but not human IgA1, IgA2 or Ig G1 (Pleass et al., 2000). An elastase-like serine protease in S. mansoni is able to render IgE nonfunctional. Further, the chloromethyl ketone derivatized peptide MeO–Suc–Ala–Ala–Pro– Leu–CMK, a specific inhibitor of schistosome elastase, prevented IgE cleavage. 1.2. Adaptation to molluscan intermediate hosts 1.2.1. Schistosoma mansoni miracidial host-finding Snails act as hosts to schistosomes and many studies have dealt with the question of how miracidia find and recognise their snail hosts. Several results indicate the possibility that the schistosome miracidia possess the means to achieve a high degree of host specificity in their host-finding behaviour as well as in their attachment and penetration responses (Haberl et al., 1995). 1.2.1.1. Miracidiae and snail host cues. S. mansoni miracidial host location and behaviour after contact with the host are stimulated by macromolecular glycoconjugates presumably originating from the snail’s mucous coat (Haberl et al., 1995). At least the miracidia of S. mansoni and S. haematobium respond to glycoconjugates with molecular masses exceeding 30 kDa, whose saccharide chains are O-glycosidically linked via serine and Nacetylgalactosamine.

The detection of macromolecules may require highly developed receptor processes, and this raises the question whether the complex nature of gastropod glycoconjugates is also used by the miracidia to attain a certain degree of specificity in host recognition. For a deeper understanding of the biochemical basis of host specificity, the molecular structures of the signalling carbohydrate moieties have to be characterised in more detail. Kalbe et al. (1996) investigated the effect of snail conditioned water (SCW) from different snail stocks on each of four behaviour patterns related with host-finding in three strains of S. mansoni. Dispersal, microhabitat selection, orientation to the host, and behaviour after contact with the host are the four phases of the host-finding process in schistosome miracidia. Miracidia of the Egyptian strain of S. mansoni were capable of discriminating between SCW from different snails with their patterns of host location and behaviour after contact with the host (Kalbe et al., 1996). How far these laboratory findings agree with conditions in the field remains to be investigated. Miracidia achieve wide dispersal by high swimming speed and rotation on their long axis that supports a straight path of movement minimising energy costs (Haas et al., 1995). In contrast to the Egyptian S. mansoni strain, echinostome miracidia were unable to differentiate between different snail species (Haberl et al., 2000) confirming previous reports on the efficient host-finding process in schistosome miracidia (Kalbe et al., 1996). 1.2.1.2. Ageysize- host choice by schistosome miracidia. Results obtained using the Schistosoma mansoni–Biomphalaria glabrata complex clearly demonstrate that miracidial success in terms of prevalence and abundance (number of mother sporocysts (MSp) per exposed snail) was significantly higher for subadult snails than for juvenile and adult snails when the three snail-size classes were mixed and exposed to miracidia (Theron et al., 1998). In contrast, when snails of different size classes were exposed separately to schistosome miracidia, the infection success decreased with increasing snail size as previously described (McKindsey and McLaughlin, 1995). Assuming that for laboratory-bred snails size can be regarded as a function of age and then of snail physiology, some authors have suggested that metabolic end products could act as miracidial stimulant factors (Thomas, 1973).


For an understanding of the adaptive benefits of this differential miracidial success related to snail size, we have to keep in mind that snails differing in ageysize constitute different resource environments with different energy budgets and resource allocation patterns (Gerard and Therone, 1997; Theron et al., 1998) that influence parasite development and host responses to infection (Gerard et al. 1993; Theron and Gerard, 1994). Subadult snails show (1) an intermediate level of individual unsuitabilityyresistance; (2) a limited retardation of their growth rate when infected (Raymond and Probert, 1993); and (3) a high degree of energy availability since, during this period of transition between differentiation (development) to reproduction, growth is continued and remains uninterrupted after the onset of egg-laying in healthy subadult snails (Gerard and Therone, 1997). Taking into account these arguments, one can consider subadult individuals among the snail host population as having the most favourable ‘space-time-energy budget’ for parasite development and reproduction and constituting the best compromise for parasite fitness. In addition, Magalhaes et al. (1997) reported that SCW of B. glabrata containing secondary sporocysts lost the ability to attract miracidia. This can be considered as an aspect of biochemical adaptation reducing the number of newly invading miracidia to limit population size to enable snail host survival during parasitic infection. 1.2.2. Snail–larval schistosome relationship Exploitation of the snail host is effectively accomplished by an integration of host and parasite physiologies accompanied by dramatic and dynamic changes in host survival, behaviour, defenceimmune function, nutrition, metabolism and reproduction. A reasonable inference from this integration is that the infected snail host represents an extended- phenotype of the parasite (Thompson, 1997). These ‘extended phenotypes’ symbolise the essence of parasitism (Thompson, 1990, 1993). Within the spectrum of compatibility, these host-parasite associations are generally characterised by a high rate of infection, relatively low host mortality, a short prepatent period and high cercarial output (Brown, 1994). 1.2.2.1. Infection and host intermediary metabolism. Few studies have addressed the effects of schistosome infection and development on food

235

consumption and host intermediary metabolism. S. mansoni development and cercarial maturation are significantly affected by host nutrition (De JongBrink et al., 1995). Gerard and Therone (1996) proved that S. mansoni infected B. glabrata has higher food assimilation and enhanced growth of the digestive gland during the prepatent period, i.e. the start of cercarial shedding. Moreover, infection of the posterior digestive gland effectively blocks the supply of haemolymph to the gonads, thereby contributing to the inhibition of reproductive activity that characterises snail-schistosome association. Although the results of early studies suggested that schistosome infection had little effect on host respiration (Malek and Cheng, 1974), decreased oxygen consumption has been reported in S. mansoni infected B. alexandrina during the prepatent period (Ishak et al., 1975). Subsequent studies with tissue homogenates of B. alexandrina and Bulinus truncatus demonstrated reduced oxidation of exogenously supplied tricarboxylic acid cycle intermediates and an inhibition of respiratory chain enzymes in infected snails. Lactate production was increased over 50% in tissue homogenates of infected individuals implying a shift towards anaerobic metabolism (Ishak et al., 1975). Differences exist between miracidia of S. mansoni and other parasitic helminths. Energy metabolism of S. mansoni miracidia is pre-adapted to occasional hypoxic conditions within their molluscan hosts (Boyunaga et al., 2001). Molluscan hosts of schistosomes, B. alexandrina and B. truncatus have biochemical profiles enabling them to withstand longer anaerobic periods induced by the developing parasite compared to non-target (non-schistosome) snails such as Lymnaea truncatula and Physa acuta (Abulaban et al., 1996; Nabih et al., 1998). Consistent with decreased oxygen consumption, several investigations have demonstrated elevated glycolytic rate and glycolytic enzymes in parasitized host (Marshall et al., 1974). Stimulation of glycolytic flux in S. mansoni infected B. alexandrina was recently ascertained by an elevated lactateypyruvate ratio, hexokinase (HK), glucose phosphate isomerase (GPI), pyruvate kinase (PK) and lactate dehydrogenase (LDH) in parasitized snails two weeks post exposure to schistosome larvae El-Ansary (1999) and El-Ansary et al. (2000a). Stimulation of the glycolytic pathway in schistosome-infected snails could be considered as

236


a critical aspect of biochemical adaptation during the aerobic–anaerobic transition induced by the developing schistosome larvae. Inhibition of HK, PK and GPI using sublethal concentration (LC10) of leaves of Solanum nigrum effectively induced a longer prepatent period, lower infection rate and reduced cercarial production in S. nigrum-treated B. alexandrina (El-Ansary et al., 2000b). This provides further support for the adaptive role of glycolysis in schistosome parasitism. 1.2.2.2. Carbohydrate metabolism. A striking effect of trematode infection on host intermediary metabolism is a dramatic reduction in tissue carbohydrate levels. For example, haemolymph glucose level is depleted in B. glabrata three weeks postinfection with S. mansoni Cheng and Lee (1971). At patency, infected snails had less than half the glucose level of uninfected snails (Thompson, 1997). In the digestive gland of S. mansoni-infected B. glabrata glycogen is decreased within two weeks of infection, and by four weeks postinfection little glycogen remained (Christie et al., 1974). Similar results were reported in B. alexandrina infected with S. mansoni and Bulinus truncatus infected with S. haematobium (Mohamed and Ishak, 1981). The ratio of glycogen phosphorylase a to glycogen synthase a activities indicated that at 5 weeks post infection snails favoured glycogen breakdown whereas at 6 weeks there was a moderate rate of glycogen synthesis. Rapid glucose absorption by developing sporocysts was responsible for the effects observed at 5 weeks, and that by 6 weeks sporocyst disruption was complete and glucose uptake likely was reduced (Schwartz and Carter (1982). These patterns of glycogen synthase and glycogen phosphorylase activities in infected B. glabrata could be mimicked in uninfected snails by exposure to a feeding and fasting cycle, consistent with the observations on the altered feeding habits of infected snails (Williams and Gilbertson, 1983). Tissue homogenates of schistosome infected B. alexandrina and B. truncatus synthesised glucose from pyruvate, lactate and alanine, at approximately one half the rate of that observed in uninfected snails (Ishak et al., 1975). However, the contribution of the parasite to the tissue mass, was not considered; if that contribution was similar in the preceding host species to that reported by S. mansoni-infected B. glabrata, the rate of gluconeogenesis is likely similar in infected and

uninfected B. alexandrina and B. truncatus (ElAnsary, 1999; El-Ansary et al., 2000b). Higher glucose and glycogen levels could be considered as a prerequisite for the survival and development of schistosomes as lactate fermenters (Horemans et al., 1992). 1.2.2.3. Lipid metabolism. Little is known about the potential role of lipids in trematode-mollusc associations. Molluscs actively synthesise and store lipids, including sterols, fatty acids and triglycerides (Voogt, 1984). Although the latter generally are considered of lesser importance than carbohydrates in molluscan energy metabolism, some species, including Bulinus africanus and B. glabrata (Duncan et al., 1987), utilise fat reserves during starvation. The nutritional requirements for lipids in molluscan development are poorly understood. Adult schistosomes apparently have limited capacity to synthesise and metabolise lipids (Perez et al., 1994), but require specific lipids for development and maturation. The potential importance of lipid nutrients for intramolluscan trematode development is as yet unknown. In vivo NMR investigations demonstrated that the free phosphatide pool of the digestive gland in B. glabrata is significantly reduced in infected snails (Thompson and Lee, 1987). This suggests that rapid parasite absorption of phospholipid may be responsible for the decline in host phosphatide levels, and that the intramolluscan stages also have specific lipid requirements for development. In B. glabrata maintained on various high-fat diets after infection with S. mansoni, the time to patency was significantly reduced, reflecting the importance of the intramolluscan lipid profile for the development of the schistosome (Thompson et al., 1991). Nabih et al. (1998) proved that molluscan hosts of schistosomes have higher linoleate levels compared to non-schistosome snails. As mentioned before, this unsaturated fatty acid enhances calcium influx into cercariae, resulting in triggering tail loss and helps efficient penetration of the mammalian host skin (Hara et al., 1993). 1.2.2.4. Protein metabolism. Reduction in free amino acids levels in haemolymph and other tissues of molluscs during schistosome infection have been reported (Schnell et al., 1985). Certain amino acids (Glu, Asp, Gly and Gln) have stimulatory effects on the development of parasitic helminths (Hata, 1994). Because schistosomes have the usual


nutritional requirement for essential amino acids, parasite absorption and utilisation likely explain much of the observed decrease. Amino acids may provide an alternate energy source to carbohydrate for host metabolism or contribute to glucose synthesis and carbohydrate repletion through gluconeogenesis in infected snails. This is consistent with results demonstrating significant increase in urea cycle activity and tissue levels of nitrogenous excretory products, including urea and ammonia in infected B. glabrata and B. alexandrina (Rizk and Farrag, 1999). This, in turn, indicates a high level of trans-deamination ultimately leading to an increased supply of tricarboxylic cycle intermediates for gluconeogenesis under schistosome infection (Nabih et al., 1990). Reports that S. mansoni sporocysts produce proteinaceous excretory-secretory products (Crews and Yoshino, 1995) suggest that the parasite may be partially responsible for the appearance of new proteins in the haemolymph of infected hosts. Many of the effects of infection on haemolymph protein composition appear to be intimately associated with parasite evasion of host defence and mediation of host response (De Jong-Brink et al., 1995; Crews and Yoshino, 1995). As an example, three different cDNAs with sequence similarities to peptide derived from the 65 kDa lectins were obtained and unexpectedly found to encode fibrinogen-related proteins (FREPs). These FREPs also contained regions with sequence similarity to Ig superfamily members. B. glabrata has at least five FREP genes, three of these are expressed at increased levels after infection (Adema et al., 1997). Induction of PK (56 kDa) and LDH (36 kDa) in S. mansoni infected B. alexandrina was recently reported as an adaptive physiological response to the aerobic–anaerobic transition induced by the developing schistosome larvae (ElAnsary et al., 2000b). Moreover, Schistosoma parasites just like the majority of helminths appear to be unable to synthesise purines de novo and rely on host supplies of bases or nucleosides. A network of reactions converts these into the nucleotides required for DNA and RNA synthesis and other processes for the parasite (Wang et al., 1984). As an aspect of biochemical integration, RNAases of molluscan hosts of schistosomes have lower Km and higher affinities towards RNA compared to non-schisto-

237

some snails. This may be favourable during hostparasite association to meet the requirement of the parasite (El-Ansary, 1997). 1.2.2.5. Infection and locomotory activity of molluscan hosts. If snail movement was restricted, infected individuals had a significantly higher energy consumption than uninfected snails (Becker, 1980). The author hypothesised that infected snails have a significantly higher basal metabolic rate than uninfected individuals, but to compensate restrict their locomotor activity and maintain a constant rate of energy conversion. Infected B. glabrata move at approximately one-half of the speed of uninfected snails (Gerard, 1996). This reduced locomotion coincides with significantly decreased phosphoarginineyATP ratios and reduced phosphoarginine-ATP phosphate exchange rates in the foot of infected snails. Reduction of locomotory activity together with the stimulation of the glycolytic pathway could be easily correlated to the stabilisation of the adenylate energy charge (AEC) within the non-stressed range in S. mansoni-parasitized B. alexandrina snails (El-Ansary, 1999). 1.2.2.6. Infection and fecundity of molluscan hosts. A loss of host reproductive potential, often referred to as parasitic castration is a common feature of schistosome-mollusc interactions. Pan (1965) reported that egg laying by adult B. glabrata was reduced during the early prepatent period and ceased by 5 weeks after infection with S. mansoni. In adult B. glabrata during the late prepatent period, corresponding to the period of migration of secondary sporocysts into the digestive gland, the gonad of S. mansoni infected snails was significantly smaller than that of uninfected individuals (Crews and Yoshino, 1989). In S. mansoni–infected B. glabrata growth and development of female sexual organs ceased prior to that of male organs (Theron and Gerard, 1994). Moreover, schistosomes induce production of a peptide, schistosomin that was observed in the haemolymph of parasitized snails at the time of cercarial maturation. It interferes with the host’s neuroendocrine systems including the actions of the dorsal body hormone and calfluxin on the albumin gland and egg laying (Hordijk et al., 1991; De Jong-Brink et al., 1995). Repeated treatment with 10 mM 5-HT promoted

238


both ovulation and oviposition in B. glabrata. Infected snails treated with 5-HT exhibited similar egg-laying rates as those of both serotonin-treated and untreated, uninfected snail groups, thus reversing the castrating effects of larval infection (Manger et al., 1996; Santhanagopalan and Yoshino, 2000). These results suggest that 5-HT acts as a stimulant for egg production in B. glabrata and that parasitic castration may be due, at least in part, to suppression of 5-HT in the CNS and plasma during the course of infection with S. mansoni. Based on these aspects of snail host–schistosome relationship, we can conclude that the overall physiological response is a highly integrative process, whereby all facets of the host’s biology, including reproduction, locomotor activity, and metabolism are moderated and coordinated. These act in concert and in a manner that provides an optimal environment for long-term parasite survival and reproduction, while at the same time ensuring that the host survives. 1.2.2.7. Schistosome evasion of molluscan hosts defence response. When a schistosome miracidium penetrates a potential molluscan host, its fate is dependent on both parasite infectivity and host susceptibility. Survival of the parasite can occur only in the absence of a successful immune response, and continued development of the parasite is possible only if the host is physiologically suitable. Host–parasite compatibility is a highly specific phenomenon in schistosomes. The mechanisms leading to survival of schistosomes in otherwise fully immunocompetent but compatible molluscan hosts are still poorly understood. It is generally accepted that successful establishment of the parasite could be brought about by evasion of the host response through either molecular mimicry (Bayne and Stephens, 1983), active acquisition of host components, or by interference with the host internal defence system. It is likely that larval trematodes use a combination of these mechanisms, either simultaneously or in series (Fryer and Bayne, 1990). Excretoryysecretory (ES) products from S. mansoni mother sporocysts have been implicated in the interference of host immune recognition, with the host haemocytes serving as a possible target which is helpful for the parasite to escape detection by the host’s immune system (Davies and Yoshino,

1995). ES products of schistosomes would affect the snail’s cellular defence system through (1) inhibiting haemocyte motility (Lodes and Yoshino, 1990), decreasing haemocyte phagocytic activity (Connors and Yoshino, 1990), (3) decreasing production of cytotoxic superoxide (Connors and Yoshino, 1990) and (4) inhibiting haemocyte protein syntheticysecretory activity. Recognition of specific carbohydrate structures common on the surface of invading schistosomes is thought to elicit internal defence response in their molluscan hosts through generation of reactive oxygen species (ROS). When six carbohydrate moieties conjugated to bovine serum albumin (BSA) thought to be present on the S. mansoni sporocyst surface were tested, three of these BSAcarbohydrate conjugates (BSA-galactose, BSAmannose, and BSA-fucose), stimulated ROS generation in the molluscan haemocytes (Hahn et al., 2000). In vitro studies with haemocytes derived from one susceptible and one resistant B. glabrata strains, showed that their responses were similar. These results suggest that parasite killing may involve either qualitative differences in production of ROS, or additional factors, which might be related to one of the previously mentioned mechanisms used by the parasite to escape molluscan hosts immune responses. 1.2.2.8. Potential of biochemical adaptation in schistosomes for rational drug design. Although targeted chemotherapy and other public health measures are employed to control schistosomiasis and its spread, there is a need for development of vaccines and new anti-schistosome drugs. Schistosome proteases are involved in different aspects of host-parasite interactions. They facilitate the invasion of host tissues, nutrition and the survival of the parasite in its host (Dalton et al., 1997). Proteases also participate in the parasite’s evasion from the host’s immune response (Brindley et al., 1997). In this respect, proteases are proposed as major potential targets for immunotherapy and chemotherapy against schistosomes (Trap and Boireau, 2000). Cercarial elastase has been considered a worthy target for immunological intervention, as blocking of its action may block infection by cercariae and developing schistosomules (Darani et al., 1997; Doenhoff, 1998). Specific inhibitors of protease, when applied to human skin in formulations


designed to retain the inhibitor on and in the upper stratum corneum layers, block cercarial invasion of human skin (Salter et al., 2000). Both peptidebased, irreversible inhibitors and non-peptide, reversible inhibitors block cercarial invasion when applied in a propylene glycol: isopropyl alcohol (3:1) formulation in vitro. Arrest of cercarial invasion could be achieved even after immersion of treated skin in water for 2 h and there was 97– 100% inhibition of cercarial invasion. When inhibitor was included in skin lotion 80% reduction in worm burden and a 92% in egg burden was observed (Lim et al., 1999). Since schistosome parasites utilise haemoglobin as a major energy source for their metabolism, degradation of host haemoglobin has been hypothesised to be mediated by cysteine and aspartyl proteases secreted into the lumen of the parasite intestine. Wasilewski et al. (1996) showed that two distinct types of irreversible cysteine proteasespecific inhibitors arrest schistosome haemoglobin degradation in vitro. Arrest of haemoglobin degradation is followed by death of developing schistosomula 1 week later. Schistosome-infected mice treated with protease inhibitors 1 week early in infection, and 2 weeks at the time of egg production, showed a significant reduction in worm burden, hepatomegaly, and the number of eggs produced per female worm. Indeed, the potential antischistosomal effects of drugs targeted at cysteine proteinases has been ascertained by Wasilewski et al. (1996), using morphilinourea–Phe– Ala–CHN2 and analogues. In view of sequence differences between schistosomes and human cathepsin L, including divergence in their active site residues and differential sensitivity to diazomethanes (Brady et al., 1999), it is feasible that inhibitors that selectively inhibit schistosome cathepsin L’s could be developed. Since it is now clear that schistosome cathepsins including SmCL1, can be produced in sufficient quantities in yeast, development of specific inhibitors of these proteinases can now be addressed (Brady et al., 1999). Cyclosporin A (CsA) exerts potent anti-helminth actions. In schistosomes, evidence indicates that the drug damages parasites by mechanisms independent of its immuneosuppressive properties (Serra et al., 1999). S. mansoni possess NF-ATlike transcription factors, a protein family already characterised by its sensitivity to CsA (Serra et

239

al., 1999). The observation that CsA treatment of S. mansoni larvae inhibited the expression of the antioxidant enzyme Sm28GST and the characterisation of a functional NF-AT- like site in the gene encoding this protein, provide new insights into the understanding of the antischistosomal effect of CsA. Use of sublethal concentrations of plant molluscicides such as Solanum nigrum could be effective in disturbing snail metabolism (El-Ansary et al., 2000b). Attenuated cercariae released from metabolically disturbed snails are less pathogenic to the final mammalian host resulting in significant reduction in worm burden and egg count. Acknowledgments The author wishes to express her deep thanks to Prof Dr E. Farrag, Medicinal Chemistry Department, National Research Centre for her great efforts in collecting some of the presented articles and for her valuable comments which improved the manuscript. References Abbas, A.K., Murphy, K.M., Sher, A., 1996. Functional diversity of helper T lymphocytes. Nature 383, 787–793. Abulaban, F., El-Ansary, A., Abou el-Ela, S.H., 1996. Trace elements profile and kinetic properties of pyruvate kinase in fresh water snails, target and non-target to schistosome parasite. Egypt. J. Bilh. 18, 27–38. Adema, C.M., Hertel, L.A., Miller, R.D., Loker, E.S., 1997. A family of fibrinogen-related proteins that participates parasite-derived molecules is produced by an invertebrate after infection. Proc. Natl. Acad. Sci. USA 94, 8691–8696. Amiri, P., Locksley, M.L., Parslow, T.G., Sadick, M., Mckerrow, J., 1992. Tumor necrosis factor a restores granulomas and induces parasite egg—laying in schistosome—infected SCID mice. Nature 356, 604–606. Bayne, C.J., Stephens, J.A., 1983. Schistosoma mansoni and Biomphalaria glabrata share epitopes: antibodies to sporocysts bind host-snail hemocytes. J. Invert. Pathol. 42, 221–231. Becker, W., 1980. Microcalorimetric studies in Biomphalaria glabrata: the influence of Schistosoma mansoni on the basal metabolism. J. Comp. Physiol. Part B 135, 101–108. Boyunaga, H., Schmitz, M.G., Brouwers, J.F., Van Hellemond, J.J., Tielens, A.G., 2001. Fasciola hepatica miracidia are dependent on respiration and endogenous glycogen degradation for their energy generation. Parasitology 122, 169–173. Brady, C.P., Dowd, A.J., Brindely, P.J., Ryan, T., Day, S.R., Dalton, J.P., 1999. Recombinant expression and localisation

240


of Schistosoma mansoni cathepsin L1 support its role in the degradation of host hemoglobin. Infect. Immun. 67, 368–374. Brindley, P.J., Kalinna, B.M., Dalton, J.P., Day, S.R., Wong, J.Y.M., Smythe, M.L., et al., 1997. Proteolytic degradation of host hemoglobin by schistosomes. Molec. Biochem. Parasitol. 89, 1–9. Brouwers, J.F.H.M., Smeenk, I.M.B., van Golde, L.M.G., Tielens, A.G.M., 1997. The incorporation, modification and turnover of fatty acids in adult Schistosoma mansoni. Mol. Biochem. Parasitol. 88, 175–185. Brouwers, J.F.H.M., Gadella, B.M., van Golde, L.M.G., Tielens, A.G.M., 1998a. Quantitative analysis of phosphatidylcholine molecular species using HPLC and light scattering detection. J. Lipid Res. 39, 344–353. Brouwers, J.F.H.M., Versluis, C., Van Golde, L.M., Tielens, A.G.M., 1998b. 5-Octadecenoic acid: evidence for a novel type of fatty acid modification in schistosomes. Biochem. J. 334, 315–319. Brown, D., 1994. Snails and schistosomes. Freshwater snails in Africa and Their Medical importance. Taylor and Francis, London, pp. 303. Butterworth, , 1990. Immunology of schistosomiasis. In: Wyler, D.J. (Ed.), Modern Parasite Biology: Cellular, immunological and molecular aspects. W.H. Freeman and Company, New York, pp. 262–288. Camacho, M., Agnew, A., 1995. Glucose uptake rates by Schistosoma mansoni, S. haematobium and S. bovis adults using a flow in vitro culture system. J. Parasitol. 81, 637–640. Caulfield, J.P., Chiang, C.P., Yacono, P.W., Smith, L.A., Golan, D.E., 1991. Low-density lipoproteins bound to Schistosoma mansoni do not alter the rapid lateral diffusion or shedding of lipids in the outer surface membrane. J. Cell. Sci. 99, 167–173. Cheng, T.C., Lee, R.O., 1971. Glucose levels in the mollusc Biomphalaria glabrata infected with Schistosoma mansoni. J. Invertebr. Pathol. 18, 395–399. Christie, J.D., Foster, W.B., Stauber, L.A., 1974. The effect of parasitism and starvation on carbohydrate reserves of Biomphalaria glabrata. J. Invertebr Pathol. 23, 297–301. Cocude, C., Pierrot, C., Cetre, C., Fontaine, J., Godin, C., Capron, A., et al., 1999. Identification of a developmentally regulated Schistosoma mansoni serine protease homologous to mouse plasma kallikrein and human factor1. Parasitology 118, 389–396. Connors, V.A., Yoshino, T.P., 1990. In vitro effect of larval Schistosoma mansoni excretory-secretory products of phagocytosis-stimulated superoxide production in hemocytes from Biomphalaria glabrata. J. Parasitol. 76, 895–902. Crews, A.E., Yoshino, T.P., 1989. Schistosoma mansoni: effect of infection on reproduction and gonadal growth in Biomphalaria glabrata. Exp. Parasitol. 68, 326–334. Crews, A.E., Yoshino, T.P., 1995. Schistosoma mansoni: characterisation of excretory-secretory polypeptides synthesised in vitro by daughter sporocysts. Exp. Parasitol. 80, 27–34. Dalton, J.P., Clough, K.A., Jones, M.K., Brindley, P.J., 1997. The cysteine proteinases of Schistosoma mansoni cercariae. Parasitology 114, 105–112. Darani, H.Y., Curtis, R.H.C., McNeice, C., Price, H.P., Doenhoff, M.J., 1997. Schistosoma mansoni: anomalous immu-

nogenic properties of a 27-kDa larval serine protease associated with protective immunity. Parasitology 115, 237–247. Davies, B.J., Yoshino, T.P., 1995. Schistosoma mansoni characterisation of excretory-secretory polypeptides exhibit selective binding to plasma components of the snail Biomphalaria glabrata. Exp. Parasitol. 81, 292–301. De Jong-Brink, M., Hoek, R.M., Smit, A.B., Bergamin-Sassen, M.J.M., Lageweg, W., 1995. Schistosoma parasites evoke stress responses in their snail host by a cytokine-like factor interfering with neuro-endocrine mechanisms. Neth. J. Zool. 45, 113–119. Doenhoff, M.J., 1998. A vaccine for schistosomiasis: alternative approaches. Parasitol. Today 14, 105–109. Doenhoff, M.J., Hassounah, O.A., Lucas, S.B., 1985. Does immunopathology induced by schistosome eggs potentiate parasite survival? Immunol. Today 6, 203–206. Duncan, M., Fried, B., Sherma, J., 1987. Lipids in fed and starved Biomphalaria glabrata (Gastropoda). Comp. Biochem. Physiol. B 86, 663–671. Dunne, D.W., Jones, F.M., Doenhoff, M.I., 1991. The purification, characterisation, serological activity and hepatotoxic properties of two cationic glycoproteins from Schistosoma mansoni eggs. Parasitology 103, 225–236. El-Ansary, A., 1997. Some properties and specificity of deoxyribonuclease and ribonuclease of freshwater molluscs hosts and non-hosts to Schistosoma parasite. J. Egypt. Soc. Zool. 24, 35–47. El-Ansary, A., 1999. Biochemical alterations of the molluscan hosts tissues infected with Schistosoma mansoni. Egypt. J. Bilharziasis 21, 71–87. El-Ansary, A., El-Shaikh, K., El-Sherbini, M., 2000a. SDSPAGE- separated tissue proteins of Biomphalaria alexandrina snails in the presence and absence of Schistosoma mansoni. Egypt. J. Parasitol. 30, 25–136. El-Ansary, A., Sammour, E.M., Mohamed, A.M., 2000b. Susceptibility of Biomphalaria alexandrina to infection with Schistosoma mansoni: Correlation with the activity levels of certain glycolytic enzymes. J. Egypt. Soc. Parasitol. 30, 547–560. Fallon, P.G., Dunne, D.W., 1999. Tolerization of mice to Schistosoma mansoni egg antigen causes elevated type1and diminished type 2 cytokine responses and increased mortality in acute infection. J. Immunol. 162, 4122–4132. Fryer, S.E., Bayne, C.J., 1990. Schistosoma mansoni modulation of phagocytosis in Biomphalaria glabrata. J. Parasitol. 76, 45–52. Fusco, A.C., Salafsky, B., Vanderkooi, G., Shibuya, T., 1991. Schistosoma mansoni:the role of calcium in the stimulation of cercarial proteinase release. J. Parasitol. 77, 649–657. Gerard, C., 1996. Energy content exerted by the parasite Schistosoma mansoni on the locomotion of its snail host, Biomphalaria glabrata. Can. J. Zool. 74, 594–601. Gerard, C., Therone, A., 1996. Altered nutrition and assimilation of the snail host (Biomphalaria glabrata) as a consequence of the parasitic spatial constraint (Schistosoma mansoni). Acta. Trop. 61, 51–55. Gerard, C., Therone, A., 1997. Ageysize and time specific effects of Schistosoma mansoni on energy allocation patterns of its snail host Biomphalaria glabrata. Oecologia (Berlin) 112, 447–452.

A. El-Ansary / Comparative Biochemistry and Physiology Part B 136 (2003) 227–243 Gerard, C., Mone, H., Therone, A., 1993. Schistosoma mansoni–Biomphalaria glabrata: dynamics of the sporocyst population in relation to the miracidial dose and the host size. Can. J. Zool. 71, 1880–1885. Haas, W., Haberl, B., Kalbe, M., Komer, M., 1995. Snail-hostfinding by miracidia and cercariae: Chemical host cues. Parasitol. Today 12, 468–472. Haberl, B., Kalbe, M., Fuchs, H., Strobel, M., Schmalfuss, G., Haas, W., 1995. Schistosoma mansoni and S. haematobium: miracidial-host-finding behaviour is stimulated by macromolecules. Int. J. Parasitol. 25, 551–560. Haberl, B., Korner, M., Spengler, Y., Kalbe, M., Haas, W., 2000. Host- finding in Echinostoma caproni: miracidia and cercaria use different signals to identify the same snail species. Parasitology 120, 479–486. Hahn, U.K., Bender, R.C., Bayne, C.J., 2000. Production of reactive oxygen species by hemocytes of Biomphalaria glabrata: carbohydrate-specific stimulation. Dev. Comp. Immunol. 24, 531–541. Hamdan, F.F., Ribeiro, P., 1999. Characterization of a stable form of tryptophan hydroxylase from the human parasite Schistosoma mansoni. J. Biol. Chem. 274, 21746–21757. Hara, L., Hara, S., Fusco, A.C., Salafsky, B., Shibuya, T., 1993. Role of calcium ion in Schistosoma mansoni cercarial tail loss induced by unsaturated fatty acids. J. Parasitol. 79, 504–509. Hassan, M.M., Ramadan, M.A., Mostafa, N.M., Fekry, A.A., Gaber, O.A., 2000. Different cytokines profiles in spleen cells and liver granuloma of Schistosoma mansoni experimentally infected mice during disease development. J. Egypt. Soc. Parasitol. 30, 245–256. Hata, H., 1994. Essential amino acids and other essential components for development of Angiostrongylus costaricensis from third- stage larvae of young adults. J. Parasitol. 80, 518–520. Hazadi, T.R., Camacho, M., Mendelovic, F., Schechtman, D., 1997. An association between activity of the NayK -pump and resistance of Schistosoma mansoni towards complement-mediated killing. Parasite Immunol. 19, 395–400. Hernandez, H.J., et al., 1997. Expression of class II, but not class I, major histocompatibility complex molecules is required for granuloma formation in infection with Schistosoma mansoni. Eur. J. Immunol. 27, 1170–1176. Hordijk, P.L., Schalling, H.D., Ebberink, R.H., De Jong-Brink, M., Joosse, J., 1991. Primary structure and origin of Schistosomin an anti-gonadotropic neuropeptide of the pond snail Lymnaea stagnalis. Biochem. J. 279, 837–842. Horemans, A.M.C., Tielens, A.G.M., Van Den Bergh, S.G., 1992. The reversible effect of glucose on the energy metabolism of Schistosoma mansoni cercariae and schistosomula. Mol. Biochem. Parasitol. 51, 73–80. Ishak, M.M., Mohamed, A.M., Sharaf, A.A., 1975. Carbohydrate metabolism in uninfected and trematode-infected snails Biomphalaria alexandrina and Bulinus truncatus. Comp. Biochem. Physiol. 51, 499–505. Jokiranta, T.S., jokipii, L., Meri, S., 1995. Complement resistance of parasites (editorial). Scand. J. Immunol. 42, 9–20. Kalbe, M., Haberl, B., Haas, W., 1996. Schistosoma mansoni miracidial host finding: species specificity of an Egyptian strain. Parasitol. Res. 82, 8–13. Kaneko, M., Suanson, M.C., Gleich, G.J., Kita, H., 1995. Allergen-specific IgG1 and IgG3 through Fc gamma RII

241

induce eosinophil degranulation. J. Clin. Invest. 95, 2813–2821. Lim, K.C., Sun, E., Bahgat, M., Bucks, D., Guy, R., Hinz, R.S., et al., 1999. Blockage of skin invasion by schistosome cercariae by serine protease inhibitors. Am. J. Trop. Med. Hyg. 60, 487–492. Lodes, M.J., Yoshino, T.P., 1990. The effect of Schistosoma mansoni excretory-secretory products on Biomphalaria glabrata haemocyte motility. J. Invert. Pathol. 56, 75–85. Magalhaes, L.A., Zanotti, M., Carvalho, J.F., 1997. Observation on the miraxonal attraction exercised by sexually immature or adult Biomphalaria glabrata infected with Schistosoma mansoni. Revista de Saude Publica 31, 121–124. Malek, E.A., Cheng, T.C., 1974. Parasite induced pathology, in Medical and Economic Malacology. Academic Press, New York, pp. 204–219. Manger, P., Li, J., Christensen, B.M., Yoshino, T.P., 1996. Biogenic monoamines in the freshwater snail, Biomphalaria glabrata: Influence of infection by the human blood fluke, Schistosoma mansoni. Comp. Biochem. Physiol. Part A 114, 227–234. Marshall, I., McManus, D.P., James, B.L., 1974. Glycolysis in the digestive gland of healthy and parasitized Littorina saxatilis rudis (Maton) and in the daughter sporocysts of Microphallus similis (Jag.) (Digenea: Microphallidae). Comp. Biochem. Physiol. B 49, 291–298. McKindsey, C.W., McLaughlin, J.D., 1995. Species-and sizespecific infection of snails by Cycloccoelum mutabile (Digenea: Cyclocoelidae). J. Parasitol. 81, 513–519. McKerrow, J.H., Newport, G., Fishelson, Z., 1991. Recent insights into the structure and function of the larval proteinase involved in host infection by a multicellular parasite. Proc. Soc. Exper. Biol. Med. 192, 119–124. Mei, H., Thakur, A., Schwartz, J., Lo verde, P.T., 1996. Expression and characterisation of glutathione peroxidase activity in the human blood fluke Schistosoma mansoni. Infect. Immun. 64, 4299–4306. Mettrick, D.F., 1989. The role of 5-hydroxytryptamine (5-HT; serotonin) in glucose transport, intermediary carbohydrate metabolism and helminths neurobiology. In: Bennet, M., Behm, C., Bryant, C. (Eds.), Comparative Biochemistry of Parasitic Helminths. Chapman and Hall, London, New York, pp. 13–24. Minchella, D.J., Sollenberger, K.M., Pereira De Souza, C., 1995. Distribution of schistosome genetic diversity within molluscan intermediate hosts. Parasitology 11, 217–226. Modha, J., Redman, C.A., Thornhill, J.A., Kusel, J.R., 1998. Schistosomes: Unanswered questions on the basic biology of the host-parasite relationship. Parasitol. Today 14, 396–401. Mohamed, A.M., Ishak, M.M., 1981. Growth rate and changes in tissue carbohydrates during Schistosoma infection of the snail Biomphalaria alexandrina. Hydrobiologia 70, 2658–2662. Nabih, I., El-Dardiri, Z., El-Ansary, A., Rizk, M., 1990. Measurement of some selected enzymatic activities in infected Biomphalaria alexandrina snails. Cell. Mol. Biol. 36, 372–637. Nabih, I., El-Ansary, A., Abdel Galil, F., Zayed, N., 1998. On the factors controlling metabolic integration between Schis-

242


tosoma parasites and their molluscan hosts. J. Egypt. Ger. Soc. Zool. 26, 87–102. Pan, C.T., 1965. Generalized and focal tissue responses in the snail Australorbis glabratus, infected with Schistosoma mansoni. Ann. NY Acad. Sci. 113, 475. Perez, M.K., Fried, B., Sherma, J., 1994. High performance thin-layer chromatographic analysis of sugars in Biomphalaria glabrata (Gastropoda) infected with Schistosoma mansoni (Trematoda). J. Parasitol. 80, 336. Pleass, R.J., Kusel, J.R., Woof, J.M., 2000. Cleavage of human IgE mediated by Schistosoma mansoni. Int. Arch. Allergy Immunol. 121 (3), 194–204. Price, H.P., Doenhoff, M.J., Sayers, J.R., 1997. Cloning, heterologous expression and antigenicity of a schistosome cercarial protease. Parasitology 114, 447–453. Ram, D., Romano, B., Schechter, I., 1994. Immunological studies on the cercarial-specific calcium binding protein of Schistosoma mansoni. Parasitology 108, 289–300. Raymond, K., Probert, A.J., 1993. The effect of infection with Schistosoma margrebowiei on the growth of Bulinus natalensis. J. Helminthol. 67, 10–16. Rizk, M., Farrag, K.E., 1999. Some aspects of nitrogen metabolism in Biomphalaria alexandrina snails treated with selenium. J. Egypt. Soc. Parasitol. 29, 531–546. Rumbley, C.A., Sugaya, H., Zekavat, S.A., El-Refaei, M., Perrin, P.J., Philips, S.M., 1999. Activated eosinophils are the major source of Th2-associated cytokines in the schistosome granuloma. J. Immunol. 162, 1003–1009. Salter, J.P., Chong Lim, K., Hansell, E., Hsieh, I., Mckerrow, J.H., 2000. Schistosome invasion of human skin and degradation of dermal elastin are mediated by a single serine protease. J. Biol. Chem. 275, 38667–38673. Santhanagopalan, V., Yoshino, T.P., 2000. Monoamines and their metabolites in the freshwater snail Biomphalaria glabrata. Comp. Biochem. Physiol. Part A 125, 469–478. Schnell, S., Becker, W., Winkler, A., 1985. Amino acid metabolism in the freshwater pulmonate Biomphalaria glabrata infected with the trematode Schistosoma mansoni. Comp. Biochem. Physiol. Part B 81, 1221–1229. Schwartz, C.F.W., Carter, C.E., 1982. Effect of Schistosoma mansoni on glycogen synthase and phosphorylase from Biomphalaria glabrata. J. Parasitol. 68, 236. Serra, E.C., Lardans, V., Dissous, C., 1999. Identification of NF-AT- like transcription factor in Schistosoma mansoni: its possible involvement in the antiparasitic action of cyclosporin A. Mol. Biochem. Parasitol. 101, 33–41. Sher, S.A., James, S.L., 1998. Failure of P strain mice to respond to vaccination against schistosomiasis correlates with impaired production of IL-12 and up-regulation of Th2 cytokines that inhibit macrophage activation. Eur. J. Immunol. 28, 1762–1772. Skelly, P.J., Shoemaker, C.B., 1995. A molecular genetic study of the variations in metabolic function during schistosome development. Mem. Inst. Oswaldo. Cruz. 90, 281–284. Skelly, P.J., Shoemaker, C.B., 1996. Rapid appearance and asymmetric distribution of glucose transporter SGTP4 at the apical surface of intramammalian-stage Schistosoma mansoni. Proc. Nat. Acad. Sci. USA 93, 3642–3646. Skelly, P.J., Shoemaker, C.B., 2000. Induction cues for tegument formation during the transformation of Schistosoma mansoni cercariae. Int. J. Parasitol. 30, 625–631.

Skelly, P.J., Tielens, A.G.M., Shoemaker, C.B., 1998. Glucose transport and metabolism in mammalian-stage schistosomes. Parasitol. Today 14, 402–406. Spence, I.M., Silk, M.H., 1971. Ultrastructural studies of the blood fluke- Schistosoma mansoni. S. Afr. J. Med. Sci. 36, 69–76. Theron, A., Gerard, C., 1994. Development of accessory sexual organs in Biomphalaria glabrata as related to infection timing by Schistosoma mansoni: consequences on the energy utilisation patterns by the parasite. J. Mol. Stud. 60, 23–31. Theron, A., Rognon, A., Pages, J.R., 1998. Host choice by larval parasites: a study of Biomphalaria glabrata snails and Schistosoma mansoni miracidia related to host size. Parasitol. Res. 84, 727–732. Thomas, J.D., 1973. Schistosomiasis and the control of molluscan hosts of human schistosomiasis with particular reference to possible self-regulatory mechanisms. Adv. Parasitol. 11, 307–394. Thompson, S.N., 1990. Physiological alterations during parasitism and their effects on host behaviour. In: Barnard, C.J., Behnke, J.M. (Eds.), Parasitism and host Behaviour. Taylor and Francis, London, pp. 64–73. Thompson, S.N., 1993. Redirection of host metabolism and effects on parasite nutrition. In: Beckage, N.E., Thompson, S.N., Federici, B.A. (Eds.), Parasites and Pathogens of Insects. San Diego, Academic Press, pp. 125–144. Thompson, S.N., 1997. Physiology and Biochemistry of snaillarval trematode relationships. In: Thompson, S.N. (Ed.), Advances in Trematode Biology. Academic Press, pp. 150–196. Thompson, S.N., Lee, R.W.K., 1987. Characterization of the NMR spectrum of the schistosome vector Biomphalaria glabrata and of the changes following infection by Schistosoma mansoni. J. Parasitol. 73, 64–76. Thompson, S.N., Mejia-scales, V., Borchardt, D.B., 1991. Physiologic studies of snail-schistosome interactions and potential for in vitro culture of schistosomes. In Vitro Cell. Dev. Biol. Part A 27, 497–504. Tielens, A.G.M., Celik, C., van den Heuvel, J.M., Elfring, R.H., van den Berg, S.G., 1989. Synthesis and degradation of glycogen by Schistosoma mansoni worms in vitro. Parasitology 98, 67–73. Tielens, A.G.M., 1994. Energy generation in parasitic helminths. Parasitol. Today 10, 346–352. Tielens, A.G.M., Horemans, A.M.C., Dunnewijk, R., van der Meer, P., van den Bergh, S.C., et al., 1994. The 50KD a glucose-6- phosphatase sensitive hexokinase of Schistosoma mansoni. J. Biol. Chem. 269, 24736–24741. Trap, C., Boireau, P., 2000. Proteases in helminthic parasites. Vet. Res. 31 (5), 461. Valle, C., Festucci, A., Calogero, A., Macri, P., Mecozzi, B., Liberti, P., et al., 1999. Stage-specific expression of a Schistosoma mansoni polypeptide similar to the vertebrate regulatory protein stathmin. J. Biol. Chem. 274, 33869–33874. Voogt, P.A., 1984. Lipids: their distribution and metabolism. In: Hochachka, P.W. (Ed.), The Mollusca, Vol. 1. Academic Press, New York, pp. 329–351. Wang, C.C., Verham, R., Cheng, H.W., Rice, A., Wang, A.L., 1984. Differential effects of inhibitors of purine metabolism on two trichomonad species. Biochem. Pharmacol. 33, 1323–1329.

A. El-Ansary / Comparative Biochemistry and Physiology Part B 136 (2003) 227–243 Wasilewski, M.M., Lim, K.C., Phillips, J., McKerrow, J.H., 1996. Cysteine protease inhibitors block schistosome hemoglobin degradation in vitro and decrease worm burden and egg production in vivo. Molec. Biochem. Parasitol. 81, 179–189.

243

Williams, C.L., Gilbertson, D.E., 1983. Altered feeding response as a cause for the altered heartbeat rate and locomotor activity of Schistosoma mansoni infected Biomphalaria glabrata. J. Parasitol. 69, 671–676.