3.1. Dysferlin Functions in Sarcolemma Repair
The best-studied function of dysferlin is its role in repair of lesions in the surface membrane of striated muscle fibers, the sarcolemma
[19]. Muscle fiber contraction mechanically stresses the sarcolemma resulting in micro-lesions. These need to be repaired quickly and efficiently to prevent leakage and death of damaged muscle fibers. The repair process is triggered by Ca
2+-influx into the sarcoplasm through the lesion and depends on a set of proteins including dysferlin as one of the key players
[19][20]. It is likely, that dysferlin exerts its role during membrane repair promoting membrane aggregation and fusion via its Ca
2+-triggered interactions with negatively charged phospholipids
[21][22][19]. Dysferlin trafficking and dysferlin-dependent membrane repair are supported by partnering proteins. These include:
-
Ca
2+- and PS-binding proteins annexins A1, A2 and A6
[23][24];
-
Muscle-specific proteins mitsugumin 53 (MG53) and caveolin 3, which are important for the nucleation of the sarcolemma repair machinery and for regulating the trafficking of dysferlin to and from the PM, respectively
[25][26];
-
A giant scaffolding protein AHNAK participating in the regulation of Ca
2+ homeostasis, signaling and structure of cytoskeleton
[27][28];
-
Myoferlin, another member of the ferlin protein family
[29];
-
Affixin (β-parvin), a protein linking integrins and cytoskeleton
[30]; and;
-
A focal adhesion protein vinculin, cytoplasmic dynein participating in the retrograde vesicle transport along the microtubules and tubulin A
[29][31].
These dysferlin-interacting proteins link its function as a Ca
2+-sensitive membrane-binding protein important for vesicular trafficking during sarcolemma repair to cytoskeleton remodeling. It is likely that dysferlin is participating not only in exocytosis of vesicles or organelles dedicated for sarcolemma repair
[32], but also in concomitant endocytosis
[2][33]. More than that, in vivo function of dysferlin in sarcolemma repair extends to PS sorting to the site of membrane damage leading to the recruitment of macrophages, which removes the patch or plug (see below), as it was shown in zebrafish
[34].
A number of cellular mechanisms have been shown to contribute to PM repair: contraction of membrane wounds, plugging (protein-based crosslinking of intracellular vesicles or membranous organelles without their fusion), patching (restoration of PM integrity by fusion of intracellular vesicles), endocytosis and externalization or membrane shedding
[20][35]. The mechanisms could coexist and participate in resealing of the same PM lesion depending on the cell type or the stage of myogenic differentiation and on the extent of the PM injury. Most if not all of these mechanisms could contribute to sarcolemma resealing and proceed to a certain degree dependently on dysferlin.
Relatively large (up to 4 µm) sarcolemma lesions of mature myofibers could be resealed by one of the two mechanisms called patching and plugging, while fusion of the membranous organelles within the patch or plug was not proven. Formation of a dysferlin-containing patch or plug on the sites of sarcolemma wounds in zebrafish was paralleled by an increase in a PS-sensor signal and BODIPY-cholesterol fluorescence, confirming the presence of membranous organelles in the patch or plug
[34][36]. These data are supported by earlier observations of vesicle accumulation below membrane lesions in non-necrotic muscle fibers from biopsies of dysferlinopathy patients and dysferlin knock-out mice
[19][37][38]. These findings point to a defect in vesicle aggregation or fusion in the absence of dysferlin or under the conditions of severe reduction of its level. Resealing of smaller lesions (≥120 nm) also requires dysferlin, but without the formation of a dysferlin-containing patch
[24]. It is likely that in such cases the repair of sarcolemma wounds requires formation of a proteinaceous plug or repair cap made up of several annexins. Indeed, when fluorescently labeled, dysferlin was not found in the repair cap, which was also devoid of the negatively charged lipids PS and PIP2, questioning the presence of the membranous organelles in the cap. During the repair process, dysferlin accumulates around the repair cap in a ‘shoulder’ area, possibly via lateral diffusion within the sarcolemma
[34][33][24] and its interactions with the cytoskeleton, recruiting PS and hence macrophages to the injury sites
[34]. The fusion of dysferlin-containing vesicles with the shoulder regions in this repair process was not demonstrated yet, but could not be excluded.
Nevertheless, the function of dysferlin in vesicular trafficking, which could underlie its role in PM repair, is supported by the observed defects in the injury triggered lysosome exocytosis across the surface of dysferlin-deficient myotubes and myoblasts
[23][32]. Of note, dysferlin does not localize to lysosomes in intact myotubes, but dysferlin-containing vesicles fuse with lysosomes upon sarcolemma damage
[39]. The lysosomal exocytosis could serve at least two functions: (i) acid sphingomyelinase secretion, which promotes membrane invagination and endocytosis, e.g. of caveolae
[40][41][42] and (ii) it is likely that rather uniform secretion of lysosomal enzymes along the surface of the damaged muscle fiber could digest the basal lamina surrounding it and thereby reduce the mechanical stress on the fiber. The origin of the organelles that form the repair patch or fuse next to the site of sarcolemma injury has not been identified yet. Candidate compartments are T-tubule derived vesicles and vesicles originating from the sarcolemma or its subcompartments caveolae as well as enlargosomes
[41][43][44].
Another mechanism of sarcolemma repair, which could depend on the function of dysferlin, is the contraction of the membrane wounds. This idea is supported by several observations: (i) dysferlin is accumulating on the rims of the lesions likely by lateral diffusion
[34][33][24][45], (ii) the process of lesion contraction is Ca
2+-dependent
[45], (iii) dysferlin C2-domains are Ca
2+-sensitive
[21], (iv) dysferlin directly or indirectly interacts with cytoskeleton
[27][29][30][31][46] and (vi) dysferlin recruitment to a wound site is dependent on annexin A6, a protein that previously has been shown to be involved in membrane lesion constriction in another cell type
[24][47].
In summary, dysferlin bears a potential to participate in sarcolemma repair by at least four mechanisms ():
Figure 3. Mechanisms of dysferlin function in membrane repair. The model shows four possible and nonexclusive contributions of dysferlin to plasma membrane repair: (1) local formation of membranous patch or plug, triggered by calcium entry and supported, amongst others, by MG53 and annexins; (2) biogenesis and maintenance of the T-tubule system as a possible membrane reservoir; (3) cytoskeleton-dependent sorting of phosphatidylserine (PS) for the recruitment of macrophages, simultaneous contraction and subsequent sealing of the membrane wound and (4) exocytosis of lysosomes.
-
Membranous repair patch or plug formation;
-
T-tubule stabilization (see below) with T-tubule as a possible membrane reservoir;
-
PS-sorting; recruitment of macrophages and contraction of the membrane wound, and;
-
Lysosome exocytosis.
3.2. Dysferlin Functions in Triad Biology
Besides its function in the repair of sarcolemma of striated muscle fibers, dysferlin plays a role in T-tubule system development and in triad function upon injury. It participates in sculpturing the membranes during T-tubule biogenesis, especially in regenerating muscles and possibly also during repair of the system upon injury
[1][48]. More than that, dysferlin takes part in the regulation of Ca
2+-metabolism of injured muscle fibers via mechanochemical stabilization of the triad junction and its Ca
2+-release and thus decreasing triad and, in particular, T-tubule sensitivity to stress
[49][50][51][52].
Anatomically, the triad is defined by three membrane compartments: one T-tubule in the center, surrounded by two terminal cisternae of the sarcoplasmic reticulum. The main function of the triad is excitation–contraction coupling of the striated muscle fibers, which is achieved by physical binding of voltage-gated Ca2+ channels (Cav1.1), also known as L-type Ca2+-channels (LTCC) or dihydropyridine receptors (DHPRs) localized to T-tubules, and the ryanodine receptors (RyRs), calcium channels mediating calcium-induced calcium release from terminal cisternae of sarcoplasmic reticulum.
Dysferlin was found in a complex with DHPRs and caveolin 3, and also with RyRs
[53][31]. Dysferlin possibly stabilizes the Ca
2+-metabolism of the injured muscle fibers inhibiting DHPRs and preventing injury-induced Ca
2+-leak into the sarcoplasm through RyRs, model supported by sustained Ca
2+ influxes in dysferlin-deficient muscle fibers sensitive to the DHPR inhibitor diltiazem and reduction of the extracellular Ca
2+ concentration
[50]. However, later the sarcoplasmic reticulum and RyRs were identified as the primary source of the Ca
2+ leak in injured muscle fibers in the absence of dysferlin
[52]. Thus, dysferlin localized to T-tubules forms a complex with both DHPRs and RyRs and modulates their function in the case of injury.
Dysferlin-deficient muscles also show T-tubule system abnormalities upon regeneration
[1]. Dysferlin interacts with the T-tubule proteins caveolin 3 and amphiphysin 2 as well as with negatively charged lipid PIP2 required for T-tubule biogenesis
[1][22][54][55]. It was shown that dysferlin induces the formation of tubular structures upon expression in non-muscle cells
[1]. Otoferlin and myoferlin did not induce such intracellular membranes when overexpressed under similar conditions. When truncation mutations and pathogenetic point mutations in dysferlin were analyzed, all except one (L1341P located in C2E) failed to induce membrane tubulation in non-muscle cells. In C2C12 myoblasts, dysferlin colocalizes with PIP2 at the PM and at the T-tubule system. Dysferlin also binds to PIP2-rich vacuoles that were generated by expression of phosphatidylinositol phosphate kinase or a constitutively active Arf6 mutant
[1]. Interestingly, most analyzed dysferlin deletions and truncations were not recruited to PIP2-vacuoles, indicating that PIP2 binding is a critical feature of dysferlin. Moreover, when cellular PIP2 have been degraded, dysferlin was lost from the T-tubule system, the T-tubule system was altered and, in non-muscle cells, dysferlin overexpression, could no longer induce formation of T-tubule-like intracellular membrane structures
[1]. These data suggest that dysferlin together with PIP2 is critical for the biogenesis of the T-tubule system. In addition, the repair of the T-tubule system could be mediated by sarcolemma resealing complex of dysferlin, MG53 and annexin A1, as these proteins are enriched at longitudinal tubules of the system under overstretch conditions
[48].
3.3. Dysferlin in the Differentiation, Growth and Regeneration of Skeletal Muscle
Skeletal muscles develop normally in pre-symptomatic dysferlinopathy patients and dysferlin-deficient mice. However, after reaching a certain age, several muscle groups begin to show initial signs of pathology: centrally nucleated fibers and variability in fiber size, which deteriorate with time. So, if dysferlin plays a role in muscle growth, myoblast differentiation and fusion, then it, likely, takes place after a defined developmental stage. There are contradicting reports concerning dysferlin role in skeletal muscle differentiation, grow and regeneration. Some authors present evidence for the absence of an effect of dysferlin-deficiency on myoblast differentiation in vitro
[32][56][57]. However, none of them presented fusion indices or the numbers of nuclei per myotube differentiated in culture. There are also reports about higher regeneration capacity upon injury in mice bearing a mutation in
Dysf [58][59][60]. Other authors argue for a role of dysferlin in myoblast differentiation and cytokine secretion so that myoblast fusion is affected indirectly only. In their studies, dysferlin-deficiency leads to decreased levels of myogenesis regulatory factors like MyoD and myogenin and delays myogenic differentiation in vitro
[61][62]. Accordingly, induction of dysferlin expression in myoblasts was shown to promote their myogenic differentiation
[63]. Myoblasts isolated from dysferlinopathy patients or derived from dysferlin-deficient mice proliferated with the normal rate
[61][62], but showed decreased fusion efficiency in vitro as a result of activated signaling of the pro-inflammatory network inhibiting myogenesis
[62]. In this context it is important to mention that dysferlin has been found in a protein complex with minion/myomerger, a fusogenic protein, which together with myomaker conveys the ability to form syncytia to myogenic and non-myogenic cells
[64].
Impaired adult satellite cell differentiation, myoblast-to-myotube fusion and muscle growth in the absence of dysferlin can be attributed to a defect in insulin-like growth factor-1 receptor (IGF1R) trafficking, since IGFs are known to promote muscle cell differentiation
[65][66]. Furthermore, dysferlin-deficiency attenuated muscle regeneration resulting in the presence of an increased number of immature fibers and suggesting that regenerative process is delayed or incomplete in dysferlinopathy
[67]. Normal muscle regeneration process requires temporally acute and transient immune response for well-timed removal of necrotic fibers
[68], however, the extended inflammatory response in a mouse model of dysferlinopathy is probably due to the defect in stimulated cytokine release by myoblasts
[67].
In summary, dysferlin could have additional functions in vesicular trafficking of growth factors receptors, secreted pro-inflammatory molecules and even fusogenic proteins that promote muscle growth and regeneration. We hypothesize that dysferlin-dependent trafficking of such signaling molecules can modulate gene expression and the function of the adult muscle stem (or satellite) cells responsible for the skeletal muscle growth and regeneration in mature individuals.