Proteoglycans are present in the transverse tubule system of skeletal muscle

Two-Pore Channel 2 activity is required for slow muscle cell-generated Ca(2+) signaling during myogenesis in intact zebrafish

We have recently characterized essential inositol 1,4,5-trisphosphate receptor (IP 3R) and ryanodine receptor (RyR)-mediated Ca(2+) signals generated during the differentiation of slow muscle cells (SMCs) in intact zebrafish embryos. Here, we show that the lysosomal two-pore channel 2 (TPC2) also plays a crucial role in generating, and perhaps triggering, these essential Ca(2+) signals, and thus contributes to the regulation of skeletal muscle myogenesis. We used a transgenic line of zebrafish that expresses the bioluminescent Ca(2+) reporter, aequorin, specifically in skeletal muscle, in conjunction with morpholino (MO)-based and pharmacological inhibition of TPC2, in both intact embryos and isolated SMCs. MO-based knock-down of TPC2 resulted in a dramatic attenuation of the Ca(2+) signals, whereas the introduction of TPCN2-MO and TPCN2 mRNA together partially rescued the Ca(2+) signaling signature. Embryos treated with trans-ned-19 or bafilomycin A1, a specific NAADP receptor inhibitor and vacuolar-type H(+)ATPase inhibitor, respectively, also displayed a similar disruption of SMC Ca(2+) signaling. TPC2 and lysosomes were shown via immunohistochemistry and confocal laser scanning microscopy to be localized in perinuclear and striated cytoplasmic domains of SMCs, coincident with patterns of IP 3R and RyR expression. These data together imply that TPC2-mediated Ca(2+) release from lysosomes acts upstream from RyR- and IP 3R-mediated Ca(2+) release, suggesting that the former might act as a sensitive trigger to initiate the SR-mediated Ca(2+)-induced-Ca(2+)-release essential for SMC myogenesis and function.

The determinants of transverse tubular volume in resting skeletal muscle

The transverse tubular (t)-system of skeletal muscle couples sarcolemmal electrical excitation with contraction deep within the fibre. Exercise, pathology and the composition of the extracellular fluid (ECF) can alter t-system volume (t-volume). T-volume changes are thought to contribute to fatigue, rhabdomyolysis and disruption of excitation–contraction coupling. However, mechanisms that underlie t-volume changes are poorly understood. A multicompartment, history-independent computer model of rat skeletal muscle was developed to define the minimum conditions for t-volume stability. It was found that the t-system tends to swell due to net ionic fluxes from the ECF across the access resistance. However, a stable t-volume is possible when this is offset by a net efflux from the t-system to the cell and thence to the ECF, forming a net ion cycle ECF→t-system→sarcoplasm→ECF that ultimately depends on Na⁺/K⁺-ATPase activity. Membrane properties that maximize this circuit flux decrease t-volume, including P_Na(t) > P_Na(s), P_K(t) < P_K(s) and N_(t) < N_(s) [P, permeability; N, Na⁺/K⁺-ATPase density; (t), t-system membrane; (s), sarcolemma]. Hydrostatic pressures, fixed charges and/or osmoles in the t-system can influence the magnitude of t-volume changes that result from alterations in this circuit flux. Using a parameter set derived from literature values where possible, this novel theory of t-volume was tested against data from previous experiments where t-volume was measured during manipulations of ECF composition. Predicted t-volume changes correlated satisfactorily. The present work provides a robust, unifying theoretical framework for understanding the determinants of t-volume.

Sulfated glycosaminoglycans in the extracellular matrix of muscle tissue in Atlantic cod (Gadus morhua) and Spotted wolffish (Anarhichas minor)

Two species of commercially important cold water fish were investigated for content of sulfated glycosaminoglycans (GAGs) in muscle tissue by use of in vivo 35S-sulfate labeling combined with different digestions (papain, chondroitinase ABC, keratanase and nitrous acid treatment), DEAE chromatography, SDS-PAGE and histology techniques. The species investigated in this study have different gaping properties. The non-gaping species, spotted wolffish (Anarhichas minor), contained 3-4 times more 35S-sulfated anionic components than the gaping species, Atlantic cod (Gadus morhua). The higher level of sulfation in wolffish was supported by light microscopy studies using Alcian blue staining with different concentrations of MgCl2 as critical electrolyte. Furthermore, the muscular connective tissue in the non-gaping species was dominated by chondroitin sulfate (CS)/dermatan sulfate (DS), whereas the gaping species was more dominated by heparan sulfate (HS). Moreover, structural differences were observed in the junctions between the myofibers, which were more pronounced in the wolffish. The histological studies revealed that the basement membrane area was rich in acidic mucopolysaccharides in both species.

Heparan sulfates in skeletal muscle development and physiology

Recent years have seen an emerging interest in the composition of the skeletal muscle extracellular matrix (ECM) and in the developmental and physiological roles of its constituents. Many cell surface-associated and ECM-embedded molecules occur in highly organized spatiotemporal patterns, suggesting important roles in the development and functioning of skeletal muscle. Glycans are historically underrepresented in the study of skeletal muscle ECM, even though studies from up to 30 years ago have demonstrated specific carbohydrates and glycoproteins to be concentrated in neuromuscular junctions (NMJs). Changes in glycan profile and distribution during myogenesis and synaptogenesis hint at an active involvement of glycoconjugates in muscle development. A modest amount of literature involves glycoconjugates in muscle ion housekeeping, but a recent surge of evidence indicates that glycosylation defects are causal for many congenital (neuro)muscular disorders, rendering glycosylation essential for skeletal muscle integrity. In this review, we focus on a single class of ECM-resident glycans and their emerging roles in muscle development, physiology, and pathology: heparan sulfate proteoglycans (HSPGs), notably their heparan sulfate (HS) moiety.

Osmotic properties of the sealed tubular system of toad and rat skeletal muscle

A method was developed that allows conversion of changes in maximum Ca(2+)-dependent fluorescence of a fixed amount of fluo-3 into volume changes of the fluo-3-containing solution. This method was then applied to investigate by confocal microscopy the osmotic properties of the sealed tubular (t-) system of toad and rat mechanically skinned fibers in which a certain amount of fluo-3 was trapped. When the osmolality of the myoplasmic environment was altered by simple dilution or addition of sucrose within the range 190-638 mosmol kg(-1), the sealed t-system of toad fibers behaved almost like an ideal osmometer, changing its volume inverse proportionally to osmolality. However, increasing the osmolality above 638 to 2,550 mosmol kg(-1) caused hardly any change in t-system volume. In myoplasmic solutions made hypotonic to 128 mosmol kg(-1), a loss of Ca(2+) from the sealed t-system of toad fibers occurred, presumably through either stretch-activated cationic channels or store-operated Ca(2+) channels. In contrast to the behavior of the t-system in toad fibers, the volume of the sealed t-system of rat fibers changed little (by <20%) when the osmolality of the myoplasmic environment changed between 210 and 2,800 mosmol kg(-1). Results were also validated with calcein. Clear differences between rat and toad fibers were also found with respect to the t-system permeability for glycerol. Thus, glycerol equilibrated across the rat t-system within seconds to minutes, but was not equilibrated across the t-system of toad fibers even after 20 min. These results have broad implications for understanding osmotic properties of the t-system and reversible vacuolation in muscle fibers. Furthermore, we observed for the first time in mammalian fibers an orderly lateral shift of the t-system networks whereby t-tubule networks to the left of the Z-line crossover to become t-tubule networks to the right of the Z-line in the adjacent sarcomere (and vice versa). This orderly rearrangement can provide a pathway for longitudinal continuity of the t-system along the fiber axis.

Reversible vacuolation of T-tubules in skeletal muscle: mechanisms and implications for cell biology

The majority of investigations of the transverse tubules (T-system) of skeletal muscle have been devoted to their role in excitation-contraction coupling, with particular reference to contact with the sarcoplasmic reticulum and the mechanism of Ca2- release. By contrast, this review is concerned with structural and functional aspects of the vacuolation of T-tubules. It covers experimental procedures used in reversible vacuolation induced by the efflux-influx of glycerol and other small nonelectrolytes, sugars, and ions. The characteristics of the phenomenon, associated alterations in muscle function, and the swelling of analogous structures in nonmuscle cells are considered. Possible functions of reversible vacuolation in water balance, transport, membrane repair, muscle pathology, and fatigue are considered, and the potential application of reversible vacuolation in the transfection of skeletal muscle is discussed. In relation to the possible mechanisms involved in reversible vacuolation, particular attention is given to the dynamic and structural aspects of the opening and closing of T-tubules, the origin of vacuolar membranes, and the localized character of tubular swelling.

The T-tubular network and its triads in the sole plate sarcoplasm of the motor end-plate of mammals

The transition between the subsynaptic folds and the T-tubules demonstrated in a former paper was further investigated in the sole plate area by using the extracellular marker Lanthanum. A tubular network of the T-system of the sole plate area which is connected to the subsynaptic folds and to the t-tubular elements between the myofibrils is described for the first time. T-tubules of this network criss-cross through the sarcoplasm of the sole plate and lie in close proximity to sole plate nuclei and mitochondria. Cisterns of sarcoplasmic reticulum of the sole plate area make contact with these t-tubules forming triads. The possible physiological role of this sole plate network and its triads will be discussed with regard to a transport of substances in tubules with the dimension of nanotubes and Ca2+ activated processes in the sole plate area.