The distribution and arrangement of microtubules in mammalian skeletal muscle fibers

Filamin-A-interacting protein 1 (FILIP1) is a dual compartment protein linking myofibrils and microtubules during myogenic differentiation and upon mechanical stress

Variations in the gene encoding filamin-A-interacting protein 1 (FILIP1) were identified to be associated with a combination of neurological and muscular symptoms. While FILIP1 was shown to regulate motility of brain ventricular zone cells, a process important for corticogenesis, the function of the protein in muscle cells has been less well characterized. The expression of FILIP1 in regenerating muscle fibres predicted a role in early muscle differentiation. Here we analysed expression and localization of FILIP1 and its binding partners filamin-C (FLNc) and microtubule plus-end-binding protein EB3 in differentiating cultured myotubes and adult skeletal muscle. Prior to the development of cross-striated myofibrils, FILIP1 is associated with microtubules and colocalizes with EB3. During further myofibril maturation its localization changes, and FILIP1 localizes to myofibrillar Z-discs together with the actin-binding protein FLNc. Forced contractions of myotubes by electrical pulse stimulation (EPS) induce focal disruptions in myofibrils and translocation of both proteins from Z-discs to these lesions, suggesting a role in induction and/or repair of these structures. The immediate proximity of tyrosylated, dynamic microtubules and EB3 to lesions implies that also these play a role in these processes. This implication is supported by the fact that in nocodazole-treated myotubes that lack functional microtubules, the number of lesions induced by EPS is significantly reduced. In summary, we here show that FILIP1 is a cytolinker protein that is associated with both microtubules and actin filaments, and might play a role in the assembly of myofibrils and their stabilization upon mechanical stress to protect them from damage.

Myristic acid selectively augments β-tubulin levels in C2C12 myotubes via diacylglycerol kinase δ

Effective amelioration of type II diabetes requires therapies that increase both glucose uptake activity per cell and skeletal muscle mass. Myristic acid (14:0) increases diacylglycerol kinase (DGK) δ protein levels and enhances glucose uptake in myotubes in a DGKδ-dependent manner. However, it is still unclear whether myristic acid treatment affects skeletal muscle mass. In this study, we found that myristic acid treatment increased the protein level of β-tubulin, which constitutes microtubules and is closely related to muscle mass, in C2C12 myotubes but not in the proliferation stage in C2C12 myoblasts. However, lauric (12:0), palmitic (16:0) and oleic (18:1) acids failed to affect DGKδ and β-tubulin protein levels in C2C12 myotubes. Moreover, knockdown of DGKδ by siRNA significantly inhibited the increased protein level of β-tubulin in the presence of myristic acid, suggesting that the increase in β-tubulin protein by myristic acid depends on DGKδ. These results indicate that myristic acid selectively affects β-tubulin protein levels in C2C12 myotubes via DGKδ, suggesting that this fatty acid improves skeletal muscle mass in addition to increasing glucose uptake activity per cell.

Reverse-engineering forces responsible for dynamic clustering and spreading of multiple nuclei in developing muscle cells

How cells position their organelles is a fundamental biological question. During Drosophila embryonic muscle development, multiple nuclei transition from being clustered together to splitting into two smaller clusters to spreading along the myotube’s length. Perturbations of microtubules and motor proteins disrupt this sequence of events. These perturbations do not allow intuiting which molecular forces govern the nuclear positioning; we therefore used computational screening to reverse-engineer and identify these forces. The screen reveals three models. Two suggest that the initial clustering is due to nuclear repulsion from the cell poles, while the third, most robust, model poses that this clustering is due to a short-ranged internuclear attraction. All three models suggest that the nuclear spreading is due to long-ranged internuclear repulsion. We test the robust model quantitatively by comparing it with data from perturbed muscle cells. We also test the model using agent-based simulations with elastic dynamic microtubules and molecular motors. The model predicts that, in longer mammalian myotubes with a large number of nuclei, the spreading stage would be preceded by segregation of the nuclei into a large number of clusters, proportional to the myotube length, with a small average number of nuclei per cluster.

Nuclear positioning in muscle development and disease

Muscle disease as a group is characterized by muscle weakness, muscle loss, and impaired muscle function. Although the phenotype is the same, the underlying cellular pathologies, and the molecular causes of these pathologies, are diverse. One common feature of many muscle disorders is the mispositioning of myonuclei. In unaffected individuals, myonuclei are spaced throughout the periphery of the muscle fiber such that the distance between nuclei is maximized. However, in diseased muscles, the nuclei are often clustered within the center of the muscle cell. Although this phenotype has been acknowledged for several decades, it is often ignored as a contributor to muscle weakness. Rather, these nuclei are taken only as a sign of muscle repair. Here we review the evidence that mispositioned myonuclei are not merely a symptom of muscle disease but also a cause. Additionally, we review the working models for how myonuclei move from two different perspectives: from that of the nuclei and from that of the cytoskeleton. We further compare and contrast these mechanisms with the mechanisms of nuclear movement in other cell types both to draw general themes for nuclear movement and to identify muscle-specific considerations. Finally, we focus on factors that can be linked to muscle disease and find that genes that regulate myonuclear movement and positioning have been linked to muscular dystrophy. Although the cause-effect relationship is largely speculative, recent data indicate that the position of nuclei should no longer be considered only a means to diagnose muscle disease.

Functional properties of the titin/connectin-associated proteins, the muscle-specific RING finger proteins (MURFs), in striated muscle

The efficient functioning of striated muscle is dependent upon the proper alignment and coordinated activities of several cytoskeletal networks including myofibrils, microtubules, and intermediate filaments. However, the exact molecular mechanisms dictating their cooperation and contributions during muscle differentiation and maintenance remain unknown. Recently, the muscle specific RING finger (MURF) family members have established themselves as excellent candidates for linking myofibril components (including the giant, multi-functional protein, titin/connectin), with microtubules, intermediate filaments, and nuclear factors. MURF-1, the only family member expressed throughout development, has been implicated in several studies as an ubiquitin ligase that is upregulated in response to multiple stimuli during muscle atrophy. Cell culture studies suggest that MURF-1 specifically has a role in maintaining titin M-line integrity and yeast two-hybrid studies point toward its participation in muscle stress response pathways and gene expression. MURF-2 is developmentally down-regulated and is assembled at the M-line region of the sarcomere and with microtubules. Functionally, its expression is critical for maintenance of the sarcomeric M-line region, specific populations of stable microtubules, desmin and vimentin intermediate filaments, as well as for myoblast fusion and differentiation. A recent study also links MURF-2 to a titin kinase-based protein complex that is reportedly activated upon mechanical signaling. Finally, MURF-3 is developmentally upregulated, associates with microtubules, the sarcomeric M-line (this report) and Z-line, and is required for microtubule stability and myogenesis. Here, we focus on the biochemical and functional properties of this intriguing family of muscle proteins, and discuss how they may tie together titin-mediated myofibril signaling pathways (perhaps involving the titin kinase domain), biomechanical signaling, the muscle stress response, and gene expression.

Muscle-specific RING finger-2 (MURF-2) is important for microtubule, intermediate filament and sarcomeric M-line maintenance in striated muscle development

The efficient functioning of striated muscle is dependent upon the structure of several cytoskeletal networks including myofibrils, microtubules, and intermediate filaments. However, little is known about how these networks function together during muscle differentiation and maintenance. In vitro studies suggest that members of the muscle-specific RING finger protein family (MURF-1, 2, and 3) act as cytoskeletal adaptors and signaling molecules by associating with myofibril components (including the giant protein, titin), microtubules and/or nuclear factors. We investigated the role of MURF-2, the least-characterized family member, in primary cultures of embryonic chick skeletal and cardiac myocytes. MURF-2 is detected as two species (approximately 55 kDa and approximately 60 kDa) in embryonic muscle, which are down-regulated in adult muscle. Although predominantly located diffusely in the cytoplasm, MURF-2 also colocalizes with a sub-group of microtubules and the M-line region of titin. Reducing MURF-2 levels in cardiac myocytes using antisense oligonucleotides perturbed the structure of stable microtubule populations, the intermediate filament proteins desmin and vimentin, and the sarcomeric M-line region. In contrast, other sarcomeric regions and dynamic microtubules remained unaffected. MURF-2 knock-down studies in skeletal myoblasts also delayed myoblast fusion and myofibrillogenesis. Furthermore, contractile activity was also affected. We speculate that some of the roles of MURF-2 are modulated via titin-based mechanisms.

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

The M-cadherin catenin complex interacts with microtubules in skeletal muscle cells: implications for the fusion of myoblasts

M-cadherin, a calcium-dependent intercellular adhesion molecule, is expressed in skeletal muscle cells. Its pattern of expression, both in vivo and in cell culture as well as functional studies, have implied that M-cadherin is important for skeletal muscle development, in particular the fusion of myoblasts into myotubes. M-cadherin formed complexes with the catenins in skeletal muscle cells similar to E-cadherin in epithelial cells. This suggested that the muscle-specific function of the M-cadherin catenin complex might be mediated by additional interactions with yet unidentified cellular components, especially cytoskeletal elements. These include the microtubules which also have been implicated in the fusion process of myoblasts. Here we present evidence that the M-cadherin catenin complex interacts with microtubules in myogenic cells by using three independent experimental approaches. (1) Analysis by laser scan microscopy revealed that the destruction of microtubules by nocodazole leads to an altered cell surface distribution of M-cadherin in differentiating myogenic cells. In contrast, disruption of actin filaments had little effect on the surface distribution of M-cadherin. (2) M-cadherin antibodies coimmunoprecipitated tubulin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated epithelial cells ectopically expressing M-cadherin. Vice versa, tubulin antibodies coimmunoprecipitated M-cadherin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated M-cadherin-expressing epithelial cells. (3) M-cadherin and the catenins, but not a panel of control proteins, were copolymerized with tubulin from myogenic cell extracts even after repeated cycles of assembly and disassemly of tubulin. Moreover, neither M-cadherin nor E-cadherin could be found in a complex with microtubules in epithelial cells ectopically expressing M-cadherin. Our data are consistent with the idea that the interaction of M-cadherin with microtubules might be essential to keep the myoblasts aligned during fusion, a process in which both M-cadherin and microtubules have been implicated.

The anabolic effects of IGF-1 in skeletal muscle after burn injury are not caused by increased cell volume

BACKGROUND

In a recent report, insulin-like growth factor 1 (IGF-1) stimulated protein synthesis and inhibited protein breakdown in skeletal muscle after bum injury. The mechanism of the anabolic effects of IGF-1 in skeletal muscle is not known. We tested the hypotheses that IGF-1 stimulates protein synthesis and inhibits protein breakdown in skeletal muscle secondary to cell swelling and that cell swelling in itself induces an anabolic response in muscle tissue.

METHODS

Extensor digitorum longus muscles from control and burned rats were incubated in the absence or presence of 1 microg/mL of IGF-1. Protein synthesis and breakdown rates were determined by measuring incorporation of 14C-phenylalanine into protein and net release of tyrosine, respectively. Cell volume was measured by determining wet and dry weight and by using 3H-mannitol as an extracellular marker.

RESULTS

IGF-1 stimulated protein synthesis and inhibited protein breakdown in muscles from nonburned and burned rats without influencing cell volume. Incubating muscles in hypo-osmotic medium increased cell volume by 17% and inhibited protein breakdown by 14% but did not influence protein synthesis.

CONCLUSIONS

The anabolic effects of IGF-1 in skeletal muscle are not caused by increased cell volume. The results differ from those reported previously in liver cells in which the anabolic effects of IGF-1 were associated with cell swelling. The role of changes in cell volume in the regulation of protein metabolism may be different in skeletal muscle than in other tissues.

Effects of tissue protectants on the kinetics of lactate dehydrogenase in cells

We studied the effects of two tissue protectants, polyvinyl alcohol (PVA) and agarose gel, on a kinetic parameter of lactate dehydrogenase LDH that is assumed to be related to the extent of diffusion of the enzyme out of tissue sections during its histochemical assay. the kinetics of the enzyme in mouse gastrocnemius (skeletal) muscle fibers and periportal hepatocytes were determined in unfixed sections incubated either on substrate (L-lactate)-containing agarose gel films or in aqueous assay media in the presence or absence of 18% PVA. The absorbances of the formazan final reaction products at their isobestic point were measured continuously in the cytoplasm of individual cells as a function of incubation time, using a real-time image analysis system. Whichever incubation medium was used, the absorbances in the two cell types increased nonlinearly during the first minute of incubation but linearly for incubation times between 1 and 3 min. The nonlinearity of the LDH reaction was analyzed using the equation vi-v = a0A, where vi is the observed initial velocity determined from the absorbance changes during the first 10 sec of incubation and v and 0 A are respectively the gradient and intercept on the absorbance axis of the linear regression line of the absorbance on incubation times between 1 and 3 min. The plots of the observed (vi-v) against 0 A were linear. Their gradients a were characteristic for each cell type and tissue protectant. The a values for skeletal muscle fibers were 12-43% lower than those for hepatocytes. The a value for hepatocytes obtained with the PVA method was 32% lower than that determined with the gel film method. For skeletal muscle fibers, the a values determined by the two methods were almost the same. Addition of excess pyruvate to the aqueous assay medium had no effects on a for either muscle fibers or hepatocytes. In contrast, a was zero for sections of polyacrylamide gels containing purified enzyme, whether incubated on agarose films or in PVA media. These data confirmed that the constant a is related to the extent to which the enzyme diffuses out of sections during incubation but not to product inhibition of LDH by pyruvate. PVA was more effective for protecting diffusion of LDH from hepatocytes than from skeletal muscle fibers, possibly because hepatocytes contain a greater proportion of diffusable (unbound) LDH than skeletal muscle fibers.

Remodeling of the cytoskeletal lattice in denervated skeletal muscle

The effect of denervation-induced atrophy on the cytoskeletal lattice in rat fast- and slow-twitch skeletal muscle has been investigated. Immunochemical analyses and immunofluorescence microscopy experiments employing monospecific antibodies to dystrophin, desmin, and alpha-tubulin were carried out on intact and denervated muscles. The relative cellular content of dystrophin and desmin were reduced in the soleus muscle (slow-twitch), while significant increases were shown in the gastrocnemius muscle (fast-twitch). In both muscles, alpha-tubulin levels increased up to 12-fold as a function of time compared to control values. Immunofluorescence microscopy revealed a distinct rearrangement of the microtubule network toward a predominantly longitudinal alignment, which was accompanied by an increase in the density of the fluorescence. It is concluded that the relative increase of the three structural proteins in the fast-twitch gastrocnemius muscle may be related to the apparent resistance of this muscle type to denervation-induced atrophy. The increased alpha-tubulin content in denervated slow- and fast-twitch muscles could be indicative of an adaptive mechanism designed to maintain the integrity of the muscle fiber in view of eventual regenerative activities.

Dystrophin-dependent efficiency of metabolic pathways in mouse skeletal muscles

Muscles from the mdx mouse (X-linked genetic disorder similar to Duchenne muscular dystrophy) lack dystrophin-associated transsarcolemmal proteins and show reduced maintenance metabolic rates. Here, microcalorimetric comparisons of metabolic stimulation by exogenous substrates in isolated muscles revealed substrate-selective limitation of chemical reaction rates through both glycolytic and TCA-cycle pathways, identical in slow- and fast-twitch mdx muscles. This systemic approach, as opposed to comparisons of single-enzyme activities, sheds new light on the function of dystrophin and associated proteins. The in vivo efficiency of metabolic pathways may depend on stabilization of enzyme complexes by dystrophin-associated elements of the cytoskeleton.

Incorporation of microinjected biotin-labelled actin into nascent myofibrils of cardiac myocytes: an immunoelectron microscopic study

Incorporation of microinjected biotin-labelled actin into nascent myofibrils of cultured cardiac muscle cells was investigated by immunogold electron microscopy. At the proximal parts of myofibrils, gold labelling was first found (at about 4 min after injection) around the A-band level. This observation suggests that polymerization of actin or the addition of newly-formed actin filaments occurs preferentially in association with myosin filaments to increase the myofibrillar girth. The distal terminals of developing myofibrils were also labelled at about 4 min after injection. This rapid incorporation of actin subunits at the myofibrillar ends suggests the continued reorganization and/or de novo formation of myofibrils at these positions. Along the extending direction of the myofibrillar terminals, gold particles were arranged in rows on the inner surface of the sarcolemma. These rows of particles continued to become longer with incubation. It appears that actin subunits are added at the membrane-associated ends of pre-existing actin filaments to increase the length of myofibrils.