Role of Mitochondria on Muscle Cell Death and Meat ...

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Role of Mitochondria on Muscle Cell Death and Meat Tenderization Verónica Sierra1,2 and Mamen Oliván1* 1

Servicio Regional de Investigación y Desarrollo Agroalimentario (SERIDA), Apdo. 13, 33300 Villaviciosa, Asturias, Spain; 2Departamento de Morfología y Biología Celular, Facultad de Medicina, Universidad de Oviedo, C/ Julián Clavería s/n, 33006 Oviedo, Asturias, Spain Received: November 06, 2012; Accepted: December 18, 2012; Revised: February 07, 2013

Abstract: The possibility that mitochondria are involved in cellular dysfunction is particularly high in situations associated with increases in free radical activity, like hypoxia or ischemia; therefore its potential role in the muscle post-mortem metabolism is reviewed. In the dying muscle, different routes of cell death catabolism (apoptosis, autophagy) may occur having great influence on the process of conversion of muscle into meat. Mitochondria are the first and also one of the main organelles affected by post-mortem changes; therefore they are decisive in the subsequent cellular responses influencing the pathway to cell demise and thus, the final meat quality. Depending on the cell death programme followed by muscle cells after exsanguination, diverse proteases would be activated to a different extent, which is also reviewed in order to understand how they affect meat tenderization. This review also summarizes recent patents relating cell death processes and meat tenderness. Further research is encouraged as there is still a need of knowledge on cell death post-mortem processes to increase our understanding of the conversion of muscle into meat.

Keywords: Apoptosis, autophagy, meat quality, mitochondria, muscle, tenderization. INTRODUCTION Meat is one of the most widely consumed protein sources in the world. Therefore meat quality, a generic term used to describe properties and perceptions of meat by consumers (comprising color, flavor, texture, tenderness, and juiciness) is of high economic importance [1]. The mechanisms responsible for the development of these meat quality traits are interdependent; highlighting the complexity of the conversion of muscle into meat after animal’s slaughtering [2]. This conversion process has been theoretically defined as a threestep process composed of a short pre-rigor phase, during which muscle still remains excitable, the rigor phase, during which energy rich compounds (ATP, PC, glycogen) are exhausted and tissue reaches its maximum toughness and the post-rigor tenderizing phase, variable in length depending on chilling conditions, muscle types, individual animals and animal species [3]. It is known that tenderness is the most influencing factor on consumers repurchase intention [4] but the rate and extent of meat tenderization are known to be highly variable, and are influenced by numerous factors. Different patents concerning meat tenderness have been developed in the last years. Due to meat tenderness variability, many different devices have been created to mechanically measure this parameter using different apparatus and probes [5-7]. Moreover, great efforts have been made to estimate, early in the processing channel and in a non-destructive manner, meat *Address correspondence to this author at the Área de Sistemas de Producción Animal. SERIDA, Apdo 13, 33300 - Villaviciosa, Asturias, Spain; Tel: +34-98-5890066; Fax: +34-98-5891854; E-mail: [email protected] 1872-2148/13 $100. 00+. 00

tenderness resulting in different patented methods based on fluorescent spectroscopy, near infrared spectroscopy, high resolution images of meat surfaces, X ray or ultrasounds [817]. However an in depth knowledge of cellular post-mortem metabolism will help us to find many other different assays and molecules that can be used as tenderness markers. It has been shown that weakening of muscle cells in postmortem conditions is highly dependent on the content and activity of endogenous proteases within the skeletal muscle including calpains, cathepsins, proteasomes, caspases, serinpeptidases and metalloproteases [3, 18-22]. However, meat tenderness is not just influenced by proteolysis, but other non-enzymatic aspects such as temperature, pH, calcium concentration, sarcomere length, and connective tissue/collagen content of the muscles can all affect meat quality, due to their effects on the evolution of biochemical processes that directly or indirectly impact muscles' proteolytic potential. Mechanisms involved in the post-rigor tenderizing phase have been deeply studied by meat scientists along decades. However the potential role of Programed Cell Death (PCD) processes at the first steps of the muscle conversion into meat has only been considered in the last decade, even when PCD may have a profound effect on the speed and ultimate degree of meat tenderization. The hypothesis that muscle engages in a form of cell death post-mortem was elucidated in 2006 by Ouali’s group [2, 23]. These authors hypothesised that the hypoxic/isquemic conditions that are induced through the process of animal’s slaughter and exsanguination could activate caspases and apoptosis in the skeletal muscle, in a similar way to what is observed during neuronal or cardiac ischemia. They proposed that apoptosis and its effects on cell structure and proteins could pro© 2013 Bentham Science Publishers

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vide partial answers to still unexplained variability of meat tenderness as well as explain some early changes (acidification, juice loss) occurring in the post-mortem muscle tissue. However, when taking into account the current knowledge on the biochemical processes related to different types of PCD occurring in the living tissues under similar circumstances that the ones suffered by the dying muscle cells (i. e. ischemia and oxidative stress) it becomes likely that apoptosis (PCD type I) is not the only possibility but some other processes such as autophagy (PCD type II) should be considered as could be implicated in the pre-rigor step of the muscle conversion into meat. In fact, recent studies have shown the presence of autophagy markers in post-mortem bovine Longissimus dorsi muscle [24]. Skeletal muscle is one of the major oxygen-consuming tissues. It has been reported that skeletal muscle mitochondria show a higher respiration rate than those in the liver, kidney and brain [25]. In the post-mortem muscle, the anoxia situation caused by the sudden cut of blood flow will reduce drastically energy production in the cells, most of which was provided by ATP generated in the mitochondria in an oxygen-dependent process. Mitochondria (a double-membrane organelle with their own genome) is the main source of energy for eukaryotic cells through oxidative phosphorylation, which generates ATP coupled to electron transfer from respiratory substrates to oxygen by a series of oxidation-reduction reactions that pump protons across the mitochondrial inner membrane from the matrix space [26, 27]. They also participate in a wide range of other cellular processes, including signal transduction, cell cycle regulation, oxidative stress, thermogenesis and cell death. In mitochondria, normal respiration produces reactive oxygen species (ROS) including superoxide anion (O2) and hydrogen peroxide (H2O2). In fact, mitochondria are the major source of ROS production and, at same time, an organelle especially prone to ROS damage [28]. Cells are endowed with various non-enzymatic and enzymatic antioxidants to detoxify ROS and prevent oxidative stress. These protective agents include glutathione, thioredoxin, superoxide dismutase, catalase, glutathione peroxidase and a number of low molecular weight direct free radical scavengers with beneficial effects against oxidative/nitrosative stress, including that involving mitochondrial dysfunction [29-32]. However, if respiration is inhibited or otherwise disordered, ROS production increases [33-37]. Excessive generation of ROS leads to oxidative stress, a condition in which cellular constituents, including proteins, DNA and lipids are oxidized and damaged and may lead to cellular and mitochondrial dysfunction [38-40]. The possibility that mitochondria are involved in cellular dysfunction is particularly high in situations associated with increases in free radical activity such as prolonged exercise, starvation, hypoxia or ischemia/reperfusion (I/R) injury [41-44]. In response to this oxidative challenge, cells may trigger physiological turnover of organelles via autophagy or may result in the complete destruction of the cell via one or more of the PCD pathways, so both autophagy and apoptosis could be activated in an attempt to restore homeostasis [45].

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Whether these two pathways will be activated depends on the cell context and on the availability of specific modulators of ROS activity [46]. The relative presence of different cell death subroutines may be influenced by the availability of ATP, and their occurrence may critically depend on the release of mitochondrial proteins like cytochrome c (Cyt C), apoptosis-inducing factor and possibly caspases 3 and 9. Ca2+-dependent onset of the permeability transition, may play a major role in cell death through ATP depletion, disruption of Ca2+ homeostasis, and release of specific mitochondrial proteins. Therefore mitochondria not only have a central role in energy production but in reactive oxygen species homeostasis, and cell death subroutines. These processes are interdependent and may occur under various stressing conditions among which low oxygen levels (hypoxia) are certainly prominent. Cells exposed to hypoxia respond acutely with endogenous metabolites and proteins promptly regulating metabolic pathways. The aim of this review is to describe the role of mitochondria in the different cell death processes triggered by oxidative stress conditions that could occur in the physiological conditions of the dying muscle and could affect deeply the process of muscle conversion into meat and thus, ultimate meat tenderness and quality acquisition. MITOCHONDRIA AND APOPTOSIS IN SKELETAL MUSCLE Skeletal muscle is a unique tissue in terms of apoptosis, particularly in regards to differentiated myotubes. Muscle cells are multi-nucleated, and myonuclei can undergo apoptosis individually, maintaining cell integrity and function [47, 48]. It is known that skeletal muscle can undergo apoptosis, both in response to specific physiological stimuli or via various pathological processes [49-51]. Apoptosis is a genetically regulated and finely tuned process for selective elimination of excessive damaged or potentially dangerous cells without damaging surrounded cells, occurring in all tissues as part of the normal cellular turnover. Apoptosis classified as highly organized programmed cell death is characterized by rounding-up of the cell, reduction of cellular and nuclear volume (pyknosis), peripheral chromatin condensation, phosphorylated histones, ssDNA accumulation, DNA fragmentation in 180 pb fragments, and final nuclear fragmentation (karyohexis). Also minor changes in cytoplasmic organelles have been described such as m dissipation and mitochondria membrane permeabilization. A critical stage of apoptosis involves the acquisition of surface changes by dying cells that eventually results in the recognition and the uptake of these cells by phagocytes. Cells undergoing apoptosis break up the phospholipid asymmetry of their plasma membrane and expose phosphatidylserine which is translocate to the outer layer of the membrane. Finally, as a result of the apoptotic program, an overall cell shrinkage, blebbing of the plasma membrane (but maintenance of its integrity until the final stages of the process) and final formation of small round bodies surrounded by membranes and containing intact cytoplasmic organelles or fragments of the nucleus are produced. These apoptotic bodies result from progressive cellular condensa-

Mitochodria on Meat Tenderization

Recent Patents on Endocrine, Metabolic & Immune Drug Discovery 2013, Vol. 7, No. 2

tion and budding, and eventually are engulfed by resident phagocytic cells in vivo without inflammation [52, 53]. This process of apoptosis is orchestrated by the family of cysteine aspartate-proteases: caspases. Caspases are specific proteases, requiring an aspartic acid residue at the C-terminal side of the cleavage point of the substrate [54]. Activation of apoptosis by caspases constitutes a minimal two-step signaling pathway. The apical (initiator) caspases (caspases 8, 9 and 12) are activated in response to apoptotic stimuli, once activated; these caspases directly activate the executioner (effector) caspases (caspases 3, 6 and 7) downstream by limited proteolytic cleavage [54]. In caspase mediated apoptosis there are three main pathways of activation: 1) the intrinsic pathway that originates from within the cell in response to environmental stress such as hypoxia and ischemia. This cellular stress causes physiological changes to the cell including mitochondrial outer membrane permeability, a reduction in the inner mitochondrial membrane's transmembrane potential and Cyt-C release, leading to the formation of the apoptosome, a complex essential for caspase 9 activation [55]; 2) the extrinsic pathway, sometimes referred to as the death receptor pathway, is the pathway through which caspases 8 and 10 are activated. This pathway involves the assembly of a cell membrane receptor-adaptor-caspase complex [56] and 3) the caspase 12 pathway, which has been demonstrated to be involved in cell death, initiating the apoptotic cascade via stress directly upon the endoplasmic reticulum [57]. ROS may exert direct actions in cell death such as oxidation of cellular proteins and lipids, damage of nucleic acids and functional alteration of organelles; but ROS may also modulate cell death processes indirectly affecting various signaling cascades [58]. Indeed, ROS participate in early and late steps of the regulation of apoptosis, affecting different apoptotic signaling cascades in both intrinsic and extrinsic pathways [59-61]. In addition to their established function in energy metabolism, mitochondria play a key role in the control of apoptosis. Mitochondria can induce apoptosis through either caspase-dependent or caspase-independent signaling mechanisms [62-64]. Caspase-dependent signaling is dependent on the release of Cyt-C from within the mitochondria, which triggers a cascade of actions by caspases. This release of Cyt-C may act as an intracellular intermediate that directly deregulates the sarcoplasmic reticulum Ca2+ flux and handling, which results in caspase-7 and calpain activation. Furthermore, ROS may cause mitochondrial swelling and fragmentation, and/or alter the conformation of the mitochondrial permeability transition pores (MPTPs), thus facilitating their opening and the release of proapoptotic proteins such as Cyt-C. Once released, Cyt-C joins with apoptotic protease activating factor-1 (Apaf-1) and procaspase-9 to form a complex called apoptosome. The apoptosome then cleaves and activates procaspase-9, which acts on caspase-3. Caspase-3 in turn activates a caspase activated DNAse (CAD) to degrade DNA and initiates cellular degradation [62-64]. The apoptosome appears to be under the regulation of the inhibitor of apoptosis protein, XIAP, which has been shown to bind the activated forms of caspase-3 and -9, blocking apoptosome-mediated caspase activation [65]. The

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inhibitory effects of XIAP can be overcome by the antagonizing functions of Smac/DIABLO and Omi/HtrA2, two mitochondrial intermembrane space proteins possessing tetrapeptide inhibitor apoptosis (IAP) binding domains. Released upon apoptotic stimulation, both Smac/DIABLO [66, 67] and Omi/HtrA2 [68-70] antagonize IAP inhibition of caspases, although the latter is also a serine protease that can proteolytically cleave and inactivate IAP proteins. Moreover, recent data has shown that Omi/HtrA2 can induce caspaseindependent cell death via its serine protease activity and direct association with cell surface death receptors [71, 72]. There are multiple points in these pathways that are regulated by pro- and anti-apoptotic proteins. In particular, the release of apoptotic triggers appears to be modulated through two mechanisms: (1) the balance of pro-apoptotic (e. g., Bax) and anti-apoptotic proteins (e. g., Bcl-2), particularly from the Bcl-2 family, which control outer mitochondrial membrane stability and form the mitochondrial apoptosisinduced channel (MAC) and (2) the mitochondrial permeability transition pore (MPTP) [73]. In this second mechanism a voltage-dependent anion selective channel protein 1 (VDAC1) is coupled with ROS-induced apoptosis. This transmembrane protein has been defined a ROS sensor opening of the mitochondrial permeability transition pores (MPTP) complex under conditions of oxidative stress [74]. Indeed VDAC1 is the main channel within the mitochondrial outer membrane and upon ROS accumulation exhibits an increased conductance associated with MPTP opening and dissipation of , thus favoring the efflux of apoptotic proteins located in the intermembrane space and thus, cell death [75]. Finally, it must be taken into account that ROS accumulation within the mitochondrial matrix, as well as their capacity of triggering apoptosis, will be counteracted/regulated by mitochondrial antioxidant enzymes, namely, phospholipids hydroperoxide glutathione peroxidase, GPx, and MnSOD [76, 77]. In skeletal muscle, mitochondrial ROS production is relevant to physiological (i.e., contraction, exercise) or pathological situations. Indeed, oxidative stress appears to be associated with myogenic cell death activation, promoting apoptosis which participates in the aetiology and progression of numerous pathologies including aging, muscle dystrophy and sarcopenia, and I/R [78-84]. However, the relationship between ROS production and the control of cell death in skeletal muscle is still little understood. MITOCHONDRIA AND AUTOPHAGY IN SKELETAL MUSCLE The daily contractions of the skeletal muscle can mechanically and metabolically damage/alter muscle proteins and organelles. For example, physical exercise requires energy whose production in mitochondria can generate reactive oxygen species (ROS) that can have deleterious effects on many cellular components. Therefore muscle cells require an efficient system for removing and eliminating unfolded and toxic proteins as well as abnormal and dysfunctional organelles, this mechanism is termed autophagy.


Autophagy is a process in which eukaryotic cells selfdigest part of their cytosolic components so as to degrade proteins and organelles to survive starvation and eliminate oxidatively-damaged, aberrant macromolecules and organelles for the purpose of maintaining homeostasis during stress. The morphological hallmark of autophagy is the formation of a double-membrane structure called autophagosome [85]. Upon autophagy induction, a membrane structure termed the phagophore, the precursor to the autophagosome, gradually expands and engulfs a portion of the cytosol or specific molecular cargos and delivers them to the vacuole/lysosome for degradation [86]. The mechanism of autophagosome formation, which entails the sequential expansion of the phagophore, provides autophagy with the capacity to sequester essentially any cellular component and deliver it into the vacuole for degradation [87]. Autophagy can be involved in both survival and death processes of cells [88]. It is generally accepted that ROS induce autophagy and that autophagy, in turn, serves to reduce oxidatively-damaged molecules and organelles [45, 89, 90]. If autophagy removes damaged organelles that would otherwise activate programmed cell death as apoptosis, then autophagy would be protective [91, 92]. Several lines of evidence demonstrate that constitutive basal autophagy, which preserves mitochondrial function, is crucial for both cardiac and skeletal muscle homeostasis. However depending on the degree of cell damage this defense (repair) mechanism may become exhausted and cells proceed to commit autophagic cell death [93]. The signal to proceed from the normal degradation of damaged organelles to the complete autophagic destruction of the cell remains to be determined. Guimaraes and Linden [94] propose the mitochondrial permeability transition (MPT) to be the critical determinant in the execution of cell death, as the permeabilization of mitochondria has been observed in apoptosis, autophagy, and necrosis. Limited MPT was suggested to result in autophagy, with more extensive permeabilization inducing apoptosis. This milder mitochondrial damage would provide sufficient energy for ATPdependent autophagic and apoptotic PCD. However, MPT in the vast majority of mitochondria would stimulate a necrotic response [94], produced when the extensive mitochondrial damage resulted in complete uncoupling of oxidative phosphorylation. Such uncoupling was suggested by Lemasters et al. [95] to result in ROS production, as well as uncontrolled hydrolysis of ATP by the inner membrane ATPase. Mitochondrial dysfunction involves the activation of mitochondrial fission and mitophagy (autophagyc remove of damaged mitochondria). The selectivity of mitophagy is controlled by the processes of mitochondrial fusion and fission and by the key mitophagy proteins Bnip3L/Nix; Bnip3; Parkin, PINK1 and p62/SQSTM1. Bnip3(for Bcl-2 Nineteen Kilodalton Interacting Protein) and Bnip3L(for Bcl-2 Nineteen Kilodalton Interacting like protein X) are a subclass of evolutionary conserved BH3-domain-like members of the Bcl-2 gene family that provoke mitochondrial perturbations and cell death in response to distinct biological stresses (specially hypoxia) and are implicated in either apoptosis or mitophagy. It has been shown that splice variants of these proteins alter the normal regulation of cellular apoptosis, necro-

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sis, autophagy, or the combination thereof [96]. These proteins translocate to mitochondria and disrupt mitochondrial membrane potential [97]. Recent observations showed that both mitochondrial proteins, Bnip3 and Bnip3L recruit the autophagy machinery to mitochondria [98, 99]. Indeed, autophagic removal of abnormal and dysfunctional mitochondria would enable their replacement with new, more efficient ones. However excessive deregulated autophagy may promote cell death [100, 101] since enzymes leaking from lysosomes, such as cathepsins and other hydrolases, can initiate mitochondrial permeabilization, release of proapoptotic factors and ROS production. There are many circumstances that may trigger autophagic cell death in different tissues and particularly in muscle [102, 103]. One of the strongest and better characterized stimuli of autophagy is starvation, where mitochondrial ROS production is enhanced and autophagy increased [46]. I/R injury condition also results in oxidative stress and extensive molecular damage due to changes in oxygen availability. In response to this oxidative challenge, both autophagy and apoptosis are activated in an attempt to restore homeostasis [45]. Without any concomitant treatment, I/Rinduced activation of autophagy was observed in cardiac myocytes, kidney and hepatocytes [104-106]. During ageing there is also a progressive deterioration of mitochondrial function and activation of autophagy [107]. Recent data suggest that autophagy may also contribute to sarcopenia, the excessive loss of muscle mass that occurs in elderly people [108]. Based on the similarities that can be found between some of the aforementioned disorders where autophagy is activated and changes occurring in the animal muscle at early post-mortem times, autophagy seems an obvious candidate response to be triggered in the muscle cells after animal’ slaughtering. OVERVIEW OF MITOCHONDRIA AND PCD PROCESSES EFFECTS ON MEAT TENDERIZATION: After animal bleeding, deprivation of oxygen supply will plunge muscle cells in an ischemic-like state which leads to a massive accumulation of ROS in the mitochondria, a stop in ATP production, a cytoplasmic acidification and a calcium deregulation in the cells. All these changes make cells prone to engaged in a PCD process. Ouali and coworkers [2] suggested that exsanguination process would probably activate the intrinsic apoptotic pathway. Since them, many other authors have pointed out the importance of considering apoptosis to understand beef aging and tenderization [109113]. Previous studies in our group have shown that oxidative stress is directly related to meat tenderization at early postmortem stages [114, 115]. As aforementioned, excessive oxidative stress can trigger not only apoptosis, but also autophagic cell death, therefore a deeper study into these PCD processes in the muscle tissue at early post-mortem time could report some explanations to the still unexplained meat quality variability. Understanding the intricate biochemical mechanisms governing PCD processes after slaughter may help to provide better solutions for preslaughter animal handling and post-slaughter interventions to



manage meat toughness. In this section we will review the main published findings relating mitochondria and cell death processes with meat tenderization and/or quality acquisition. However, the studies in animal cells particularly pertaining to post-mortem muscle biology are still in an incipient phase and the role of autophagy in meat tenderization has only being proposed by a recent study from our group [24]. The muscle-to-meat transition process is strongly influenced by a rapid pH drop. Ultimate pH (pHu) has been widely used as an indicator of potential meat tenderness [116]. As the physiological pH of muscle cells is around 7.0 a lower pH results in altered functionality and (partial) denaturation of cellular proteins e.g. those of the citrate cycle and respiratory chain as well as mitochondrial transport proteins. Different works [117, 118] showed that in porcine muscle fibers from the Longissimus muscle collected at 24 h post-mortem the mitochondria were partly disrupted and swollen. They also showed that in ‘‘low-quality muscles” with low pH values, measured at 45 min post-mortem, the disruption and swelling was already visible shortly after slaughter [118]. Mitochondria control the sarcoplasmic calcium homeostasis via specific in- and efflux-systems within the membrane. The mitochondrial membrane potential not only affects the ATP production, but also influences the calcium transport systems and the maintenance of the alkaline matrix pH [119, 120]. In fact, Gursahani and Schaefer [120] showed in cardiac I/R experiments that the pH reduction during ischemia results in the acidification of the mitochondrial matrix and the decrease of the membrane potential accompanied with a lower calcium influx into the matrix. Considering this theoretical background, it could be suggested that the reduced pH and accompanying lower mitochondrial respiratory activity in the porcine muscle samples also results in a reduced calcium influx into the intact muscle mitochondria. The consequence would be a higher sarcoplasmic calcium concentration enforced by the increased calcium release from degenerate mitochondria and sarcoplasmic reticulum. As the sarcoplasmic calcium concentration has an important influence on the muscle-to-meat transition [121] an impact of the altered mitochondrial function after slaughter has also to be considered. Some methods to activate or enhance post-mortem tenderization based on the positive calcium effects on calpains activation have been patented; they are based on the administration of excessive doses of vitamin D or hypercalcemic agents to animals prior slaughter [122, 123]. However, further investigations are under progress to show how the mitochondria, especially the enzymes of the citrate cycle and respiratory chain, change during the muscleto-meat transition process and their effects on meat quality. As aforementioned in the last years many works have pointed out the importance of considering apoptosis to understand beef aging and tenderization. From these, it is worth mentioning the work from Laville et al. suggesting the importance of apoptosis for beef tenderization in a paper describing proteome changes along meat aging by comparing tender and tough meat from Charolais young bulls [110]. This work results showed a significant increase of proteolytic products in the soluble fraction of the tender meat. Six of these proteins were constitutive of the mitochondrial membrane (from the inner and outer membranes). Detection of these products occurred very early during the post-mortem

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conditioning period and thus could not have been derived from meat ageing. These data indicate an increase in mitochondrial membrane disruption in the tender group, which occurs early in the intrinsic apoptotic pathway and is subsequently involved in caspase activation [110]. Later on, Becila et al. [112] examined three selected hallmarks of apoptosis (phosphatidylserine externalization (PS), cell shrinkage and actin degradation) in rat muscles during the 72h following exsanguination and compared to usual cell behaviour in apoptotic and necrotic processes. Their results clearly demonstrate a rapid PS externalization and a cell shrinkage extending during the first 24 h postexsanguination together with a progressive degradation of cytoskeletal and thin filaments of actin, concluding that in post-mortem muscle, cells commit suicide soon after animal bleeding through apoptosis. Apoptosis is also related with heat shock proteins (Hsp), that are a family of protective proteins, that show diverse anti-apoptotic actions in response to cellular stress [124], with Hsp 27 and Hsp70 identified to directly inhibit both the intrinsic and extrinsic apoptotic pathways [125, 126]. Is has been shown that Hsp expression is stimulated after slaughter in response to cell stress such as ischemia, toxins, heat and oxidation [127]. Different studies found that Hsps gene expression was negatively correlated with beef sensory quality and suggest that the down-regulation of the anti-apoptotic properties of DNAJA1, Hsp70 and Hsp27 could facilitate cell death and caspase activity during the post-mortem conditioning period and subsequently increase meat tenderisation [109, 128, 129]. A method based on these results, using the quantification of the expression level of DNAJA1 gene as an indicator of tenderness have been recently patented [130]. The process of meat tenderization is largely a consequence of enzymatic degradation of key myofibrillar and cytoskeletal proteins that under in vivo conditions maintain the structural integrity of myofibrils. The degree of alteration and weakening of myofibrillar structures directly affects ultimate meat tenderness, with modification of several key proteins including myosin, actin, desmin and troponin, during the ageing period. Herrera-Méndez et al., [23] suggested that caspases are probably the first peptidases to act after animal death at early post-mortem stage, degrading key proteins involved in the complex spatial organisation of myofibrils within muscle cells. Caspases are specific proteases, requiring an aspartic acid residue at the C-terminal side of the cleavage point of the substrate [131]. To date at least thousand cellular substrates have been identified as targets of these executioner caspases [132], including the myofibrillar proteins actin, myosin, desmin, -actinin, and troponin-T [133]. The caspase proteolytic system was first directly examined in post-mortem skeletal muscle by Kemp et al. [134], who found high levels of caspase 9 activity in different porcine muscles but very low and no expression of the active isoforms of caspases 8 and 12, respectively. These findings were further concurred by Pulford et al. [127] who identified significantly higher caspase 9 activity than caspase 8 activity in beef muscle samples. Furthermore, caspase 9 activity across the post-mortem conditioning period has been found to positively correlate to activity of effector caspases


3/7 [134, 135]. Collectively these data indicate that the predominant apoptotic pathway active in skeletal muscle postmortem is the intrinsic pathway, involving caspase 9. Moreover, caspase3/7 and caspase 9 activity has been demonstrated to decrease along the post-mortem conditioning period [127, 134, 135] showing a negative relationship with shear force in porcine LD muscle; thus, the more caspase activated, the greater the change and the lower the shear force value, and therefore the higher the level of meat tenderness [134]. In this study a negative relationship was also detected between shear force and expression of the caspase generated alpha II spectrin 120 kDa degradation product (SBDP120) a peptide considered an excellent marker for caspase mediated apoptosis [136]. The degradation of alpha II spectrin during apoptosis could significantly compromise the membrane permeability and also cytoskeletal integrity [137], a process that is associated with meat tenderisation [138]. Later works [133] have shown that human recombinant caspase-3 is active at in vitro conditions (4ºC and pH 5. 8) being capable of degradate porcine myofibrillar proteins, such as desmin and troponin I. However, in the same year, Underwood et al. [139] showed that despite caspase-3 is active immediately after slaughter; it has not increasing activity along a post-mortem period from 0 to 240 h. Moreover, they did not find caspase-3 activity correlation with meat shear force and conclude that it is unlikely that caspase3 participate significantly in the proteolysis of post-mortem beef muscle. More recently, a new study described a significant increase in caspase-3 activity at 4 h post-mortem in different skeletal muscles of crossbreed beef [113]. These contradictory results evidence that more work has to be done to elucidate the real role of caspases on meat tenderization. The involvement of additional proteolytic activities, such as lysosomal cathepsins, calpains, proteasome and serine proteases have been described not only in relation to meat tenderization but in different experimental models of apoptotic cell death [140-141]. Calpains have been considered during decades the most important proteases in early tenderization [142]. Therefore many different tenderization markers have been patented based on different assays to identify tender meat within the calpain system, such as post-mortem analysis of amount or activity of m-calpain [143], different DNA markers of calpastatin (the endogenous calpain inhibitor) [144-146], calpain3 (CAPN3) gen [147] and also analysis of single nucleotide polymorphisms (SNPs) in the -calpain gene correlated to higher tenderness [148]. Referring to PCD, calpains have a direct impact on the execution of apoptosis as they can cleave key elements in the apoptotic machinery. A cross-talk has been suggested between calpain system and caspases, as many proteins can be cleared by both proteolytic systems [140, 149]. On the other hand, calpain inhibition through over-expression of calpastatin has been shown to increase caspase 3 activity and apoptosis [140]. Additionally calpastatin is also cleaved by caspases 1, 3 and 7 [150]. Therefore, if caspases are active in the muscle post-mortem, they may influence meat quality also by proteolysis of calpastatin [142] and again this would implicate a positive role of apoptosis in meat tenderization.

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A method to increase meat tenderness by inducing postmortem breakdown in muscle tissue combining the calpain system and apoptosis have been recently published. This is achieved by the use of beta-agonist blockers in the premortem period, which results in higher calpastatin levels that increase caspase 3 and apoptosis combined with the use of agents immediately before slaughter that induce apoptosis in muscle tissue [151]. Also lysosomal enzymes, cathepsins, are known to contribute to meat quality at later stages of the maturation process, when lysosomes are disrupted and pH conditions are favorable for acidic enzymes [19, 20, 152]. However, it have been shown that lysosomal permeabilization is often an early event not only in the autophagic but also in the apoptotic cascade, therefore cathepsins have been reported participate in PCD processes [141]. Cellular viability has been related to high cathepsin B and low cathepsin D activities [153]. Therefore, alterations in the cathepsin D/B activity ratio indicate the capability of triggering cell death, as the ratio reflects peculiarities in the autophagy-lysosomal pathway [154, 155] or in the apoptotic pathway directly, as cathepsins have also been reported to cleave and activate caspases [156] or indirectly by triggering mitochondrial dysfunction and subsequent release of mitochondrial proteins [141, 157]. A recent study by our group in meat samples from two local beef breeds (Asturiana de los Valles and Asturiana de la Montaña) showing different tenderization rate (fast and slow respectively) found higher values of CathepsinD/B activity ratio in slow tenderization muscle samples [24]. In the same study the expression of two typical autophagic markers (Beclin-1 and LC3) were also measured. The results showed that autophagic markers were expressed in both breeds but the slow tenderizing meat showed higher and earlier expression. Our findings confirm the occurrence of autophagy in postmortem muscle and as more efficient this protective mechanism is, more delay in the tenderization process occur which could have implications on meat maturation. Further research in this sense has to be done as the role of autophagy in meat tenderization still remains unclear. CURRENT & FUTURE DEVELOPMENTS A revision of the current knowledge of biochemical and cellular changes occurring in PCD processes and their parallelism with changes occurring in the muscle tissue at early post-mortem, seems to indicate that muscle cells can be susceptible to suffer PCD processes (apoptosis and autophagy) after animal’ slaughtering being mitochondria one of the main contributors to the oxidative stress that muscle cells support derivate from the exsanguinations and subsequent anoxia. Mitochondria are also the first and one of the main organelles affected by this situation; therefore its responses are decisive in the subsequent cellular response influencing the pathway to cell demise and thus, the final meat quality. However, more research is needed to elucidate if mitochondrial action have a positive, negative or both effects on the meat tenderness acquisition depending on the biochemical route triggered by muscle cells. It has been postulated that the dominant cell death phenotype is determined by the relative speed of the available death programs. So, there is a need to know if both apoptosis



and autophagy responses may occur in the muscle tissue at the same time or at different moments and if they exert a synergic or antagonic action on ultimate meat quality. Many genes promoters or inhibitors of autophagy and many autophagy modulators that can be used in the treatment of autophagy related diseases have been identified in the last years [158-161]. Similarly, different methods for apoptosis detection, based on apoptosis-specific markers released into an extracellular medium or using an annexin derivative as a biosensor for real-time visualization of phosphatidylserine exposure, and different compounds for the selective inhibition of caspases have been patented [162-164]. The use of these methods, within the post-mortem muscle cells, will help us to better understand the role of mitochondria and the influence of PCD processes on meat tenderization. Moreover, if any relationship is proved, as it seems probable from the different results here reviewed, the different assays and molecules used as biomarkers of cell death subroutines could also be applied as indirect meat quality markers. Thus, this study only further highlight our need for continued research in this area to increase our understanding of the process of conversion of muscle into meat and to provide biomarkers of meat tenderization. CONFLICT OF INTEREST

[14] [15] [16] [17] [18] [19]

[20]

[21] [22] [23]

[24]

[25]

The authors declare no conflict of interest. ACKNOWLEDGEMENTS This work was supported by: RTA2007-00087-C02; AGL2010-21578-C03-02; AGL2011-30598-c03-03; BFU201020919 and FEDER funds.

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