Fish muscle cytoskeleton integrity is not dependent on intact thin filaments

Proteolysis and Its Control Using Protease Inhibitors in Fish and Fish Products: A Review

Texture is one of the food quality attributes affecting the consumer's acceptability and the market value. Fish and shellfish undergo weakening or softening of muscle, particularly during extended storage under inappropriate conditions. The phenomenon is governed by endogenous proteases, both digestive and muscle proteases. Proteases present in the gastrointestinal tract that leach out to muscle tissue can induce proteolysis of myofibrillar and collagenous proteins. Furthermore, the muscle proteins present in gels fabricated from fish or shellfish meat also encounter degradation during thermal processing. Endogenous heat-activated proteases strongly bind to muscle proteins and are activated during heating, thereby degrading myofibrillar proteins, which are abundant in muscle tissue. This deterioration of the proteins directly leads to a weakened gel with poor water-holding capacity. Both cysteine and serine proteases are responsible for the degradation of myofibrillar proteins in several aquatic animals. Effective pretreatment of fish and shellfish, as well as the use of food-grade protease inhibitors (PIs), have been implemented to inactivate endogenous muscle and digestive proteases. For this review, proteolysis of muscle proteins and its control by food-grade PIs are revisited. Improved and effective lowering of proteolysis should be gained, thereby maintaining the quality of fish and their products.

Adjustments of Protein Metabolism in Fasting Arctic Charr, Salvelinus alpinus

Protein metabolism, including the interrelated processes of synthesis and degradation, mediates the growth of an animal. In ectothermic animals, protein metabolism is responsive to changes in both biotic and abiotic conditions. This study aimed to characterise responses of protein metabolism to food deprivation that occur in the coldwater salmonid, Arctic charr, Salvelinus alpinus. We compared two groups of Arctic charr: one fed continuously and the other deprived of food for 36 days. We measured the fractional rate of protein synthesis (K_S) in individuals from the fed and fasted groups using a flooding dose technique modified for the use of deuterium-labelled phenylalanine. The enzyme activities of the three major protein degradation pathways (ubiquitin proteasome, lysosomal cathepsins and the calpain systems) were measured in the same fish. This study is the first to measure both K_S and the enzymatic activity of protein degradation in the same fish, allowing us to examine the apparent contribution of different protein degradation pathways to protein turnover in various tissues (red and white muscle, liver, heart and gills). K_S was lower in the white muscle and in liver of the fasted fish compared to the fed fish. There were no observable effects of food deprivation on the protease activities in any of the tissues with the exception of liver, where the ubiquitin proteasome pathway seemed to be activated during fasting conditions. Lysosomal proteolysis appears to be the primary degradation pathway for muscle protein, while the ubiquitin proteasome pathway seems to predominate in the liver. We speculate that Arctic charr regulate protein metabolism during food deprivation to conserve proteins.

Biomarkers: Non-destructive Method for Predicting Meat Tenderization

Meat tenderness is the primary and most important quality attribute for the consumers worldwide. Tenderness is the process of breakdown of collagen tissue in meat to make it palatable. The earlier methods of tenderness evaluation like taste panels and shear force methods are destructive, time consuming and ill suited as they requires removing a piece of steak from the carcass for performing the test. Therefore, a non-destructive method for predicting the tenderness would be more desirable. The development of a meat quality grading and guarantee system through muscle profiling research can help to meet this demand. Biomarkers have the ability to identify if an exposure has occurred. Biomarkers of the meat quality are of prime importance for meat industry, which has ability to satisfy consumers' expectations. The biomarkers so far identified have been then sorted and grouped according to their common biological functions. All of them refer to a series of biological pathways including glycolytic and oxidative energy production, cell detoxification, protease inhibition and production of Heat Shock Proteins. On this basis, a detailed analysis of these metabolic pathways helps in identifying tenderization of meat having some domains of interest. It was, therefore, stressed forward that biomarkers can be used to determine meat tenderness. This review article summarizes the uses of several biomarkers for predicting the meat tenderness.

Calpain involvement in the remodeling of cytoskeletal anchorage complexes

Cells offer different types of cytoskeletal anchorages: transitory structures such as focal contacts and perennial ones such as the sarcomeric cytoskeleton of muscle cells. The turnover of these structures is controlled with different timing by a family of cysteine proteases activated by calcium, the calpains. The large number of potential substrates present in each of these structures imposes fine tuning of the activity of the proteases to avoid excessive action. This phenomenon is thus guaranteed by various types of regulation, ranging from a relatively high calcium concentration necessary for activation, phosphorylation of substrates or the proteases themselves with either a favorable or inhibitory effect, possible intervention of phospholipids, and the presence of a specific inhibitor and its possible degradation before activation. Finally, formation of multiprotein complexes containing calpains offers a new method of regulation.

Calpain 1-titin interactions concentrate calpain 1 in the Z-band edges and in the N2-line region within the skeletal myofibril

Calpain 1, a ubiquitous calcium-dependent intracellular protease, was recently found in a tight association with myofibrils in skeletal muscle tissue [Delgado EF, Geesink GH, Marchello JA, Goll DE & Koohmaraie M (2001) J Anim Sci79, 2097-2107). Our immunofluorescence and immunoelectron microscopy investigations restrain the protease location at the periphery of the Z-band and at the midpoint of the I-band. Furthermore, calpain 1 is found to localize in myofibril fractures, described as proteolysis sites, in postmortem bovine skeletal red muscles, near the calcium deposits located at the N1 and N2 level. This in situ localization of calpain 1 is substantiated by binding assays with two titin regions covering the I-band region: a native fragment of 150 kDa (identified by mass spectrometry) that includes the N-terminal Z8-I5 region and the N1-line region of titin, and an 800 kDa fragment external to the N1 line that bears the PEVK/N2 region. These two titin fragments are shown to tightly bind calpain 1 in the presence of CaCl(2) and E64, a calpain inhibitor. In the absence of E64, they are cleaved by calpain 1. We conclude that titin affords binding sites to calpain 1, which concentrates the protease in the regions restrained by the Z-band edge and the N1-line as well as at the N2-line level, two sarcomeric regions where early postmortem proteolysis is detected.

Salmon spawning migration and muscle protein metabolism: the August Krogh principle at work

The August Krogh principle, stating that for any particular question in biology, nature holds an ideal study system, was applied by choosing the anorexic, long-distance migration of salmon as a model to analyze protein degradation and amino acid metabolism. Reexamining an original study done over 20 years ago on migrating sockeye salmon (Oncorhynchus nerka), data on fish migration and starvation are reviewed and a general model is developed on how fish deal with muscle proteolysis. It is shown that lysosomal activation and degradation of muscle protein by lysosomal cathepsins, especially cathepsin D and sometimes cathepsin L, are responsible for the degradation of muscle protein during fish migration, maturation and starvation. This strategy is quite the opposite to mammalian muscle wasting, including starvation, uremia, cancer and others, where the ATP-ubiquitin proteasome in conjunction with ancillary systems, constitutes the overwhelming pathway for protein degradation in muscle. In mammals, the lysosome plays a bit part, if any. In contrast, the proteasome plays at best a subordinate role in muscle degradation in piscine systems. This diverging strategy is put into the context of fish metabolism in general, with its high amino acid turnover, reliance on amino acids as oxidative substrates and flux of amino acids from muscle via the liver into gonads during maturation. Brief focus is placed on structure, function and evolution of the key player in fishes: cathepsin D. The gene structure of piscine cathepsin D is outlined, focusing on the existence of duplicate, paralogous, cathepsin D genes in some species and analyzing the relationship between a female and liver-specific aspartyl protease and fish cathepsin Ds. Evolutionary relationships are developed between different groups of piscine cathepsins, aspartyl proteases and other cathepsins. Finally, based on specific changes in muscle enzymes in fish, including migrating salmon, common strategies of amino acid and carbon flux in fish muscle are pointed out, predicting some metabolic concepts that would make ideal application grounds for the August Krogh principle.

The calpain 1-alpha-actinin interaction. Resting complex between the calcium-dependent protease and its target in cytoskeleton

Calpain 1 behaviour toward cytoskeletal targets was investigated using two alpha-actinin isoforms from smooth and skeletal muscles. These two isoforms which are, respectively, sensitive and resistant to calpain cleavage, interact with the protease when using in vitro binding assays. The stability of the complexes in EGTA [Kd(-Ca2+) = 0.5 +/- 0.1 microM] was improved in the presence of 1 mm calcium ions [Kd(+Ca2+) = 0.05 +/- 0.01 microM]. Location of the binding structures shows that the C-terminal domain of alpha-actinin and each calpain subunit, 28 and 80 kDa, participates in the interaction. In particular, the autolysed calpain form (76/18) affords a similar binding compared to the 80/28 intact enzyme, with an identified binding site in the catalytic subunit, located in the C-terminal region of the chain (domain III-IV). The in vivo colocalization of calpain 1 and alpha-actinin was shown to be likely in the presence of calcium, when permeabilized muscle fibres were supplemented by exogenous calpain 1 and the presence of calpain 1 in Z-line cores was shown by gold-labelled antibodies. The demonstration of such a colocalization was brought by coimmunoprecipitation experiments of calpain 1 and alpha-actinin from C2.7 myogenic cells. We propose that calpain 1 interacts in a resting state with cytoskeletal targets, and that this binding is strengthened in pathological conditions, such as ischaemia and dystrophies, associated with high calcium concentrations.

Phosphoinositide regulation of the actin cytoskeleton

Phosphoinositides [PPIs, which collectively refer to phosphorylated derivatives of phosphatidylinositol (PI)] have a pivotal role as precursors to important second messengers and as bona fide signaling and scaffold targeting molecules. This review focuses on recent advances that elucidate how PPIs, particularly PI(4,5)P2 (PIP2), directly regulate the actin cytoskeleton in vivo by modulating the activity and targeting of actin regulatory proteins. The role of PIP2 in stimulating actin polymerization and in establishing cytoskeleton-plasma membrane linkages is emphasized. In addition, the review presents tantalizing evidence that suggests how binding of selected cytoskeletal proteins to membrane PPIs may promote PPI clustering into raft lipid microdomains, alter their accessibility to other proteins, and even distort the bilayer conformation. These actions have profound implications for many other PPI-regulated membrane functions that are beginning to be uncovered, and they suggest how PPIs can mediate crosstalk between the actin cytoskeleton and an expanding spectrum of essential cellular functions.

The structure and functional significance of variations in the connective tissue within muscle

The amount of intramuscular connective tissue (IMCT) and its morphological distribution is highly variable between muscles of differing function. The functional roles of this component of muscle have been poorly understood, but a picture is gradually emerging of the central role this component has in growth, transmission of mechanical signals to muscle cells and co-ordination of forces between fibres within a muscle. The aim of this review is to highlight recent advances that begin to show the functional significance of some of the variability in IMCT. IMCT has a number of clearly defined roles. It patterns muscle development and innervation, and mechanically integrates the tissue. In developing muscles, proliferation and growth of muscle cells is stimulated and guided by cell-matrix interactions. Recent work has shown that the topography of collagen fibres is an important signal. The timing and rates of expression of connective tissue proteins also show differences between muscles. Discussion of mechanical roles for IMCT has traditionally been limited to the passive elastic response of muscle. However, it is now clear that IMCT provides a matrix to integrate the contractile function of the whole tissue. Mechanical forces are co-ordinated and passed between adjacent muscle cells via cell-matrix interactions and the endomysial connective tissue that links the cells together. An emerging concept is that division of a muscle into fascicles by the perimysial connective tissue is related to the need to accommodate shear strains as muscles change shape during contraction and extension.

Interaction of actin with the capping protein, CapZ from sea bass (Dicentrarchus labrax) white skeletal muscle

We have compared the functional properties of CapZ from fish white skeletal muscle with those of CapZ from chicken muscle. CapZ is a heterodimer, which enhances actin nucleation and inhibits the depolymerization process by binding to the barbed ends of microfilaments. Here, we report the interaction of CapZ not only with F-actin, but also with monomeric actin. The affinity of sea bass CapZ for G-actin estimated by enzyme-linked immunosorbent assay (ELISA) was in the microM range. This association was PIP2 dependent. Binding contacts with the barbed end of actin were delimited by both ELISA and fluorescence approaches. One site (actin sequence 338-348) was located in a helical region of the subdomain 1, region already implicated in the interaction with other actin binding proteins such as gelsolin. Another site implicates the C-terminal region (sequence 360-372) of actin. Finally, the partial competition of antibodies directed against CapZ alpha or beta-subunits towards CapZ interaction with actin filaments suggests both subunits participate in the complex with actin.

Location of and post-mortem changes in some cytoskeletal proteins in pork and cod muscle

The cytoskeletal proteins actin, nebulin, spectrin, desmin, vinculin and talin were labelled immunohistochemically in sections of muscle from commercially available pigs and cod (Gadus morhua) taken pre-rigor and from samples stored for several days. Actin, nebulin and spectrin gave similar labelling patterns in both pork and cod muscle which remained the same in stored samples. Desmin was intensely labelled at the cell boundaries and within the body of the cells in both pork and cod in the initial and the stored samples. Vinculin was readily labelled in pork muscle but showed only diffuse labelling in fish. Labelling for talin in pork muscle was intense at the sarcolemma but was not present in samples stored for 4 days. In contrast, the label for talin was concentrated at the myotendinous junction of the cod muscle throughout the storage period. These are the first reports of the detection and location of spectrin and vinculin in fish muscle and of the location of talin. The results are discussed in terms of muscle structure, function and post-mortem tenderisation. © 2000 Society of Chemical Industry.

Alpha actinin-CapZ, an anchoring complex for thin filaments in Z-line

CapZ is a widely distributed and highly conserved, heterodimeric protein, that nucleates actin polymerization and binds to the barbed ends of actin filaments, preventing the addition or loss of actin monomers. CapZ interaction with actin filaments was shown to be of high affinity and decreased in the presence of PIP2. CapZ was located in nascent Z-lines during skeletal muscle myofibrillogenesis before the striated appearance of thin filaments in sarcomers. In this study, the stabilization and the anchorage of thin filaments were explored through identification of CapZ partners in the Z-line. Fish (sea bass) striated white muscle and its related Z-line proteins were selected since they correspond to the simplest Z-line organization. We report here the interaction between purified CapZ and alpha-actinin, a major component of Z filaments and polar links in Z-discs. Affinity of CapZ for alpha-actinin, estimated by fluorescence and immunochemical assays, is in the microM range. This association was found to be independent of actin and shown to be weakened in the presence of phosphoinositides. Binding contacts on the alpha-actinin molecule lie in the 55 kDa repetitive domain. A model including CapZ/alpha-actinin/titin/actin interactions is proposed considering Luther's 3D Z-line reconstruction.

Defining actin filament length in striated muscle: rulers and caps or dynamic stability?

Actin filaments (thin filaments) are polymerized to strikingly uniform lengths in striated muscle sarcomeres. Yet, actin monomers can exchange dynamically into thin filaments in vivo, indicating that actin monomer association and dissociation at filament ends must be highly regulated to maintain the uniformity of filament lengths. We propose several hypothetical mechanisms that could generate uniform actin filament length distributions and discuss their application to the determination of thin filament length in vivo. At the Z line, titin may determine the minimum extent and tropomyosin the maximum extent of thin filament overlap by regulating alpha-actinin binding to actin, while a unique Z filament may bind to capZ and regulate barbed end capping. For the free portion of the thin filament, we evaluate possibilities that thin filament components (e.g. nebulin or the tropomyosin/troponin polymer) determine thin filament lengths by binding directly to tropomodulin and regulating pointed end capping, or alternatively, that myosin thick filaments, together with titin, determine filament length by indirectly regulating tropomodulin's capping activity.

Molecular structure of the sarcomeric Z-disk: two types of titin interactions lead to an asymmetrical sorting of alpha-actinin

The sarcomeric Z-disk, the anchoring plane of thin (actin) filaments, links titin (also called connectin) and actin filaments from opposing sarcomere halves in a lattice connected by alpha-actinin. We demonstrate by protein interaction analysis that two types of titin interactions are involved in the assembly of alpha-actinin into the Z-disk. Titin interacts via a single binding site with the two central spectrin-like repeats of the outermost pair of alpha-actinin molecules. In the central Z-disk, titin can interact with multiple alpha-actinin molecules via their C-terminal domains. These interactions allow the assembly of a ternary complex of titin, actin and alpha-actinin in vitro, and are expected to constrain the path of titin in the Z-disk. In thick skeletal muscle Z-disks, titin filaments cross over the Z-disk centre by approximately 30 nm, suggesting that their alpha-actinin-binding sites overlap in an antiparallel fashion. The combination of our biochemical and ultrastructural data now allows a molecular model of the sarcomeric Z-disk, where overlapping titin filaments and their interactions with the alpha-actinin rod and C-terminal domain can account for the essential ultrastructural features.