shi/mld MBP1/- mice shiver less than shi/shi MBP1/- animals, do not have convulsions, and live longer, although the abundances of. MBP mRNA in these ...
MBP mRNA, more axons become myelinated, the number of turns increases and the myelin appears compacted, with prominent major dense lines. Axons with larger diameters appear to be more readily myelinated than smaller axons and to contain more myelin turns. Even small differences in MBP gene expression resulted in distinct phenotypes: for example, shi/mld MBP1/- mice shiver less than shi/shi MBP1/- animals, do not have convulsions, and live longer, although the abundances of MBP mRNA in these animals are estimated to be 13.5% and 12.5% of normal, respectively, and there are no apparent differences in the myelin ultrastructure. These elegant studies provide formal genetic proof that the phenotype of the shiverer mouse is the consequence of a deletion in the gene for MBP. In addition, this work indicates that myelination is not an all-or-none event but that the degree of myelination is dependent on the levels of expression of at least one of its protein components. The production of lines of mice with varying degrees of myelination will be an important resource for the study of myelination and the physiological consequences of dysmyelination. One also assumes that similar studies will be carried out t !H , i
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for other dysmyelinating mutants as their molecular defects are defined. The mutant mouse jimpy, for example, displays severe CNS hypomyelination as a result of aberrant splicing of proteolipid protein (PLP) gene transcripts 19'2°. KlausArmin Nave in this correspondent's laboratory has recently demonstrated that the defect results from a single base change in a consensus splice site of the PLP gene 21. We eagerly await a similar molecular genetic analysis of other dysmyelinating mutants to increase our understanding of the vital process of myelination.
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Selected references
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1 Redhead, C., Popko, B., Takahashi, N., Shine, H. D., Saavedra, R. A., Sidman, R. L. and Hood, L. (1987) Ce1148,703712 2 Popko, B., Puckett, C., Lai, E., Shine, H.D., Redhead, C., Takahashi, N., Hunt, S. W., Sidman, R. L. and Hood, L. (1987) Cell 48, 713-721 3 Biddle, F., March, E. and Miller, J. R. (1973) Mouse News Lett. 48, 24 4 Chernoff, G. F. (1981) J. Hered. 72, 128 5 Bird, T. D., Farrell, D. F. and Sumo, S.M. (1978) J. Neurochem. 31, 387-391 6 Kirschner, D.A. and Ganser, A.L. (1980) Nature 283,207-210 7 Roach, A., Boylan, K., Horvath, S., Prusiner, S. B. and Hood, L. E. (1983) Cell 34, 799-806 8 Zeller, N. K., Hunkeler, M. J., CampagJ
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Whathappenswhengrowthconesmeetneurites: attractionor repulsion? Paul C. Letourneau
he routes taken by elongating axons to their synaptic targets Departmentof Cell are controlled by the growth cone, Biologyand a sensory-effector system that Neuroanatomy, senses local cues and responds with Universityof five activities that determine neurMinnesota, onal form: elongation, branching, Minneapolis, MN, USA. turning, retraction and synaptogenesis. Recent in-vitro studies show that movements of vertebrate growth cones are both promoted and, surprisingly, inhibited by contacts with axons. Thus, the classical terms, contact guidance and contact inhibition1, have renewed significance in understanding the navigation of growth cones.
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Axons are pathways for growth cone migration Growth cone navigation involves sensory functions provided by surface receptors for adhesive ligands, 390
growth factors, neurotransmitters, and ions, while the effector system includes transmembrane signals and second messengers that control secretion and extension, adhesion and contraction of motile processes 2. The paths taken by different growth cones result from differences in surface sensitivities that modulate effector functions. The importance of axon bundles as pathways for neurite elongation is clearly illustrated in simple embryos like Daphnia and grasshoppers. Corey Goodman and colleagues at Stanford University have shown that growth cones can recognize a single axon or small group of axons3'4. Do vertebrate growth cones also follow labelled axonal pathways? If so, how many labels exist, and bow do growth cones respond to axonal labels?
© 1987.ElsevierPublications.Cambridge 0378- 5912187150200
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noni, A. T., Sprague, J. and Lazzarini, R. A. (1984) Proc. Natl Acad. Sci. USA 81, 18-22 Kimura, M., Inoko, H., Katsuki, M., Ando, A., Sato, T., Hirose, T., Takashima, H., Inayama, S., Okano, H., Takamutsu, K., Mikoshiba, F., Tsukada, Y. and Watanabe, I. (1985) J. Neurochem. 42, 692-696 Roach, A., Takahashi, N., Pravtcheva, D., Ruddle, F. and Hood, L. (1985) Cell 42, 149-155 Molineaux, S. M., Engh, H., de Ferra, F., Hudson, L. and Lazzarini, R.A. (1986) Proc. Natl Acad. Sci. USA 83, 7542-7546 de Ferra, F., Engh, H., Hudson, L., Kamholz, J., Puckett, C., Molineaux, S. and Lazzarini, R.A. (1985) Cell 43, 721-727 Takahashi, N., Roach, A., Teplow, D.B., Prusiner, S.B. and Hood, L. (1985) Cell 42, 139-148 Newman, S., Kitamura, K. and Campagnoni, A. T. (1987) Proc. NatlAcad. Sci. USA 84, 886-890 Sidman, R. L., Conover, C. S. and Carson, J.H (1985) Cytogenet. Cell Genet. 39, 241-245 Charlton, H.M. (1987) Trends Neurosci. 10, 229-231 Doolittle, D. P. and Schweikart, K. M. (1977) J. Hered. 68, 331-332 Matthieu, J-M. Ginalski, H., Friede, R. L., Cohen, S. R. and Doolittle, D. P. (1980) Brain Res. 191,278-283 Nave, K-A., Lai, C., Bloom, F. E. and Milner, R.J. (1986) Proc. Natl Acad. Sci. USA 83, 9264-9268 Hudson, L. D., Berndt, J. R., Puckett, C., Kosak, C.A. and Lazzarini, R.A. (1987) Proc. Natl Acad. Sci. USA 84, 1454-1458 Nave, K. A., Bloom, F. E. and Milner, R. J. J. Neurochem. (in press) m,,,,i
F r i e d r i c h B o n h o e f f e r and his colleagues at the M a x - P l a n c k d n s t i tfit ~ E n t w i c k l u n g s b i o l o g i e in T f i b i n g e n have d e v e l o p e d elegant
tissue culture paradigms for analysing growth cone navigation5'6. Jonathan Raper, who studied labelled pathways with Goodman, has joined Bonhoeffer and co-authored some interesting papers on interactions of growth cones with neurites 7-1°. In the February, 1987 issue of the Journal of Cell Biology, Chang, Rathjen and Raper described an ingenious method for assessing neurite elongation on axon bundles. Reproducible arrays of fascicles extending from chick sympathetic ganglia were prepared and seeded with fluorescently labelled sympathetic neurons, which rapidly attached to the cables and extended neurites (Figs 1, 2). After one day, the cultures were fixed and labelled neurites were measured. Raper and his colleagues used this system to test the effects of antibodies against three axonal surface glycoTINS, VOI. 10, No. 10, 1987
proteins: NCAM, G4, and F l l . Neurite elongation on axons was 40% slower in the presence of Fab fragments to G4 and F l l , but antiNCAM had no effect. These effects were specific to growth on axons, since neither antibody reduced neurite elongation on laminin. The authors conclude that the antibodies interfere with specific components involved in neurite elongation on axons. The G4 and F l l glycoproteins may be generally involved in fasciculation, since anti-G4 and anti-F11 stain many regions of chick brain and peripheral neurons. This is not the vertebrate equivalent of 'labelled pathways'. However, this approach can be developed to probe growth cone-axon interactions involving these glycoproteins, other glycoproteins, and their asyet unidentified growth cone receptors.
Improper contacts may signal growth cone retraction Using co-cultures of sympathetic and retinal neurons, Bray, Wood and Bungen reported that growth cones of one neuronal type fasciculate with like axons, but not with unlike axons. Kapfhammar and Raper re-examined this phenomenon and found that not only did growth cones not fasciculate with the other type, but they actually retracted from unlike neurites 8. Expanding their observations they concluded that central growth cones retract when they contact peripheral, but not central neurites, and peripheral growth cones retract from central, but not peripheral neurites 9. Detailed observations of growth cones revealed a sequence in which fingerlike projections, filopodia, contacted an unlike neurite, the growth cone collapsed and retracted several microns, and eventually a new growth cone was organized and extension resumed 1° (Fig. 3). This behavior seemed to require contact with an unlike neurite and, thus, strongly resembles contact inhibition of fibroblast locomotion 12. The authors suggest that the central and peripheral neurites they studied differ by at least one label, and that this difference evokes opposite responses from central and peripheral growth
Fig. 1. High magnification view of axonal cables with an attached test neuron elongating on the axons. (B) Phase contrast, (C) fluorescence. (Taken, with permission, from Ref. 7.)
and peripheral neurites share G4 and F l l glycoproteins, should not growth cones of either type recognize these markers and elongate on any neurite? Yet, unknown markers on central and peripheral neurites may inhibit stable interactions involving G4 and F l l and cones. induce unlike growth cones to reThese experiments suggest tract. This response may arise from complexity in the sensory-effector a distinct signal to retract, like the activities of growth cones. If central reflexive withdrawal of a finger
TINS, Vol. 10, No. 10, 1987
from a hot stove. However, there may be another explanation for growth cone retraction from unlike neurites (see figures in Ref. 10). It appears that retraction does not occur until the main body of a growth cone advances and touches an unlike neurite after an initial filopodial contact. Perhaps, retraction is prompted by non-adhesion between a growth cone and an unlike neurite, and tensile forces 391
Fig. 2. The axonal substrate assay. Shadedarea is a ring of fixed cell; laminin is represented as +, anti-laminin as -; an explant in the center has fascicles radiating out to the ring of fixed cells. Test neurons are round dots, which elongate axons afterO.5 d. The graph displays percent of neurites longer than a given length (horizontal axis). (Taken, with permission, from Ref. 7.)
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Fig. 3. Retina/growth cone meeting a sympathetic neurite. Filopodia touch the neutite first in (B), and cytoplasmic strands remain attached to the neurite after the growth cone re~dts in (D). (Taken, with permission, from
Ref. 10.)
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always present in neurites spontaneously retract a detached growth cone until it regains anchorage 1:~. Kapfhammer and Raper discount the possibility of growth cone non-adhesion to unlike neurites, because strands of cytoplasm from the growth cone remained adherent to an unlike neurite as the growth cone retracted (Fig. 3). Yet, filopodia, especially the tips, are specialized for adhering to other surfaces, because of their small radii of curvature and other properties 14-17. Strands pulled out from a retreating growth cone may be remnants of adhesive contacts that were first established by filopodia, while the main body of the growth cone could not subsequently adhere to an unlike neurite. The significant point here is that growth cones may not adhere to all surfaces to which their filopodia do. Must we distinguish a distinct signal to retract from simple loss of growth cone anchorage? The answer is yes, since these possibilities involve different sensoryeffector strategies for growth cone navigation. The hypothesis of nonadhesion demands an explanation for non-adhesion between growth cones and axons that express ligands, like NCAM, NgCAM, G4 or F l l . This explanation is needed to support any hypothesis of labelled pathways in vertebrates, unless additional adhesive mechanisms or growth cone responses are invoked. A distinct signal for retraction allows adhesive interactions to be overridden by involving other growth cone sensitivities, perhaps like the effects of serotonin, which induce retraction of growth cones extended from neuron 19, but not neuron 5 of the snail buccal ganglion18. It may help to resolve this issue by examining whether contact by filopodia alone will produce growth cone retraction from unlike neurites. These studies are a fascinating example of a good marriage of biochemistry and cell biology. Elucidating growth cone navigation is clearly a demanding task, and simple, but relevant approaches such as those devised by the researchers in Tfibingen are key elements in unravelling this mystery. Selected references 1 Exp. Cell Res. (Suppl. 8) (1961) 2 Kater, S. B. and Letourneau, P. C., eds (1985) Biology of the Nerve Growth
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perspectives Cone, Alan R. Liss 3 Bastiani,M. J., du Lac, S. and Goodman, C. S.(1986)J. Neurosci. 6, 3518-3531 4 du Lac, S., Bastiani, M. J. and Goodman, C. S.(1986)J. Neurosci. 6, 35323541 5 Bonhoeffer, F. and Huf, J. (1980) Nature 288, 162-164 6 Bonhoeffer, F. and Huf, J. (1985) Nature 315, 409-410 7 Chang, S., Rathjen, F. G. and Raper, J. A. (1987) J. Cell Biol. 104, 355-362
on disease 8 Kapfhammer, J.P., Grunwald, B.E. 14 Howard, P., Wetzel, B., Walther, B., Chipowsky,S. and Roseman,S.(1975) and Raper,J. A. (1986) J. Neurosci. 6, J. Cell Biol. 67, 179a 2527-2534 9 Kapfhammer, J.P. and Raper, J.A. 15 Pethica, B.A. (1961) Exp. Cell Res. (Suppl. 8), 123-140 (1987) J. Neurosci. 7, 1595-1600 10 Kapfhammer, J.P. and Raper, J.A. 16 Albrecht-Buehler, G. (1976) J. Cell Biol. 69, 275-286 (1987) J. Neurosci. 7, 201-212 11 Bray, D., Wood, P. and Bunge, R. P. 17 Tsui, H.C.T., Lankford, K.L. and Klein, W. L. (1985) Proc. Natl Acad. (1980) Exp. Cell Res. 130, 241-250 Sci. USA 82, 8256-8260 12 Trinkaus, J.P. (1984) Cells into OrEans: The Forces That Shape the 18 Haydon, P.G., McCobb, D.B. and Kater, S. B. (1984) Science 226, 561Embryo, Prentice-Hall 564 13 Bray,D. (1979)J. CellSci. 37,391-410 [
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Motor neuron diseasesand amyotrophiclateral sclerosis: research progress Lewis P. Rowland In the past five years, a &reat diversity of motor neuron diseases has emerged from new evidence of different etiologies. The theory of persistent viral infection has been revived by a caseattributed to injections of human &rowth hormone, by cases of myelopathy attributed to HlV infections, and by the role of HTL V-1 in tropical spastic paraparesis. Dietary factors- calcium deprivation, ingestion of cycad, and lathyrism - seem to be important in different parts of the world. An immunological disorder is suggested in some cases by inordinate frequency of paraproteinemia. Finally, genetic forms are reco&nized in hexosaminidase deficiency, and the chromosomal locus of X-linked recessive motor neuron diseasehas been identified. The tasksnow are to understand the pathogenesis of these (and other) different forms of motor neuron diseases, and to devise rational therapy. Three years ago, in these pages, I reviewed theories of the etiology and pathogenesis of motor neuron diseases and amyotrophic lateral sclerosis (ALS)I; the terms are used interchangeably. These diseases are not common, accounting for about 1 of 1000 adults deaths, and with a prevalence of 2-7 per 1000002. However, these diseases are linked to the more common Alzheimer's disease and Parkinson's disease for reasons summarized in the earlier report 1. (1) All three categories affect the aging nervous system. (2) Pairs of the syndromes, or all three, may occur together in the same individual. (3) In all three conditions, most cases are sporadic and, in the clearly familial cases, an autosomal dominant pattern is the usual form of inheritance. (4) All are 'degenerative diseases', i.e. histopathology shows neuronal changes and loss of neurons without indication of immunologic, infectious, or vascular disorder. (5) In all three, there are abnormal intraneuronal inclusions and collections of neurofilaments (although the specific lesions differ) 2'3, (6) Each affects a different but specific set of neurons; they are different examples of 'selective vulnerability'. Since the last TINS review ~, there has been some progress in relation to ALS. Tandan and Bradley4, in a admirably comprehensive review of ALS, concluded that there is no 'unifying hypothesis' to explain how 'viruses, metals, TINS, Vol. 10, No. 10, 1987
endogenous toxins, immune dysfunction, endocrine abnormalities, impaired DNA repair, altered axonal transport, and trauma' have all been 'etiologically linked to ALS'. That view is a reasonable response to the apparently conflicting evidence provided that there is a single motor neuron disease and a single ALS. However, there is another view: that there are many motor neuron diseases. This view is bolstered by evidence cited elsewhere 1'5, and is summarized below. In some, persistent viral infection is causal. In others, the neurological complications of occupational intoxication by lead may cause the syndrome. In others, there is a known inherited biochemical abnormality. In some, hyperparathyroidism or hyperthyroidism is responsible. There is a difference between believing that the neurological disorders of human lead intoxication or hexosaminidase deficiency are truly forms of ALS, or that they merely 'mimic' motor neuron disease. If we could understand how these diseases arise, we might find clues to the nature of the more numerous cases of idiopathic ALS and other motor neuron diseases. It may therefore be useful to review the new evidence related to viruses, the environment, immune disorder, genetics, and therapy.
LewisP.Rowlandisat theNeurological Institute,ColumblaPresbyterianMedical Center,New York,NY 10032-3784, USA.
Viruses For more than a decade, starting in the mid-1960s, ALS research was dominated by the search for persistent viral infection. There were several reasons to believe that this might be fruitful. Most important was the demonstration by Gajdusek and his associates that Creutzfeldt-Jakob disease (CJD) is transmissible; although dementia and myoclonus are the dominant features of that disorder, some cases of CJD show evidence of upper and lower motor neuron pathology (the diagnostic hallmarks of ALS). Poliomyelitis provided another reason to suspect persistent viral infection, because that neurotropic virus also affects motor neurons, and because it was thought that survivors of paralytic poliomyelitis
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