Biphasic pattern of gelsolin expression and variations in gelsolin-actin interactions during myogenesis

Role of Actin-Binding Proteins in Skeletal Myogenesis

Maintenance of skeletal muscle quantity and quality is essential to ensure various vital functions of the body. Muscle homeostasis is regulated by multiple cytoskeletal proteins and myogenic transcriptional programs responding to endogenous and exogenous signals influencing cell structure and function. Since actin is an essential component in cytoskeleton dynamics, actin-binding proteins (ABPs) have been recognized as crucial players in skeletal muscle health and diseases. Hence, dysregulation of ABPs leads to muscle atrophy characterized by loss of mass, strength, quality, and capacity for regeneration. This comprehensive review summarizes the recent studies that have unveiled the role of ABPs in actin cytoskeletal dynamics, with a particular focus on skeletal myogenesis and diseases. This provides insight into the molecular mechanisms that regulate skeletal myogenesis via ABPs as well as research avenues to identify potential therapeutic targets. Moreover, this review explores the implications of non-coding RNAs (ncRNAs) targeting ABPs in skeletal myogenesis and disorders based on recent achievements in ncRNA research. The studies presented here will enhance our understanding of the functional significance of ABPs and mechanotransduction-derived myogenic regulatory mechanisms. Furthermore, revealing how ncRNAs regulate ABPs will allow diverse therapeutic approaches for skeletal muscle disorders to be developed.

Proteomics-based investigation in C2C12 myoblast differentiation

Skeletal muscle cell differentiation is a multistage process extensively studied over the years. Even if great improvements have been achieved in defining biological process underlying myogenesis, many molecular mechanisms need still to be clarified.

To further highlight this process, we studied cells at undifferentiated, intermediate and highly differentiated stages, and we analyzed, for each condition, morphological and proteomic changes. We also identified the proteins that showed statistical significant changes by a ESI-Q-TOF mass spectrometer. This work provides further evidence of the involvement of particular proteins in skeletal muscle development. Furthermore, the high level of expression of many heat shock proteins, suggests a relationship between differentiation and cellular stress. Intriguingly, the discovery of myogenesis-correlated proteins, known to play a role in apoptosis, suggests a link between differentiation and this type of cell death.

Multifunctional roles of gelsolin in health and diseases

Gelsolin, a Ca(2+) -regulated actin filament severing, capping, and nucleating protein, is an ubiquitous, multifunctional regulator of cell structure and metabolism. More recent data show that gelsolin can act as a transcriptional cofactor in signal transduction and its own expression and function can be influenced by epigenetic changes. Here, we review the functions of the plasma and cytoplasmic forms of gelsolin, and their manifold impacts on cancer, apoptosis, infection and inflammation, cardiac injury, pulmonary diseases, and aging. An improved understanding of the functions and regulatory mechanisms of gelsolin may lead to new considerations of this protein as a potential biomarker and/or therapeutic target.

Creatine supplementation prevents the inhibition of myogenic differentiation in oxidatively injured C2C12 murine myoblasts

Creatine (Cr), one of the most popular nutritional supplements among athletes, has been recently shown to prevent the cytotoxicity caused by different oxidative stressors in various mammalian cell lines, including C2C12 myoblasts, via a direct antioxidant activity. Here, the effect of Cr on the differentiating capacity of C2C12 cells exposed to H(2)O(2) has been investigated. Differentiation into myotubes was monitored using morphological, ultrastructural, and molecular techniques. Treatment with H(2)O(2) (1 h) not only caused a significant (30%) loss of cell viability, but also abrogated the myogenic ability of surviving C2C12. Cr-supplementation (24 h prior to H(2)O(2) treatment) was found to prevent these effects. Interestingly, H(2)O(2)-challenged cells preconditioned with the established antioxidants trolox or N-acetyl-cysteine, although cytoprotected, did not display the same differentiating ability characterizing oxidatively-injured, Cr-supplemented cells. Besides acting as an antioxidant, Cr increased the level of muscle regulatory factors and IGF1 (an effect partly refractory to oxidative stress), the cellular availability of phosphocreatine and seemed to exert some mitochondrially-targeted protective activity. It is concluded that Cr preserves the myogenic ability of oxidatively injured C2C12 via a pleiotropic mechanism involving not only its antioxidant capacity, but also the contribution to cell energy charge and effects at the transcriptional level which common bona fide antioxidants lack.

Actin associated proteins function as androgen receptor coregulators: an implication of androgen receptor's roles in skeletal muscle

This review of androgen receptor (AR) coregulators, which also function as actin-binding proteins, intends to establish the connection between actin cytoskeletal components and androgen signaling, especially in skeletal muscle. In cellular and animal models, androgen activated AR modulates myoblasts proliferation, promotes sexual dimorphic muscle development, and alters muscle fiber type. In the clinical setting, administration of anabolic androgens can decrease cachexia and speed wound healing. During myogenesis and regeneration of skeletal muscle in embryo and adult, the membrane of myoblasts fuse and the actin cytoskeleton is rearranged to form an alignment with myosin to form myotubes then ultimately the myofibrils. Contraction of skeletal muscle promotes the growth of myocytes by coordinating signals from the neuromuscular junction to intra-myofibrils through costameres, the functional structure comprised of signal proteins closely associated with actin filaments and involved in muscular dystrophy. Therefore, the discovery of actin-binding proteins functioning as AR coregulators implies that androgen signaling is tightly regulated during the process of the development and regeneration of skeletal muscle. The search for selective androgen receptor modulators (SARM) that act precisely in skeletal muscle instead of other tissues could target the engineering of a SARM-AR complex that selectively recruits these coregulators.

Coordinate induction of the actin cytoskeletal regulatory proteins gelsolin, vasodilator-stimulated phosphoprotein, and profilin during capillary morphogenesis in vitro

The formation of capillaries during development and tissue repair is likely to involve active reorganization of the actin cytoskeleton, although few studies have addressed this issue. Here, we have utilized an in vitro model of capillary morphogenesis whereby human umbilical vein endothelial cells are suspended within three-dimensional type I collagen gels. The cells undergo dramatic morphogenic changes to develop capillary lumens, tubes, and networks over 72 h of culture. Western blots using cell extracts of these gels over this time frame were performed using antibodies directed to various proteins associated with the actin cytoskeleton. Three proteins showed altered expression during the time course, and they were gelsolin, which increased fivefold; vasodilator-stimulated phosphoprotein (VASP), which increased twofold; and profilin, which increased threefold in expression between the 24- and the 72-h time points. Reverse transcriptase-polymerase chain reaction and Northern blot analysis revealed a similar increase in mRNA expression of the three proteins. After the onset of network formation, the differentiated endothelial cells (dECs) undergoing capillary morphogenesis were removed from collagen gels at 48 h of culture to compare their properties with untreated endothelial cells (uECs). These dECs showed two- to threefold increased spontaneous migration in Boyden chamber assays compared to uECs. The dECs also displayed a prominent spindle-shaped morphology and the novel presence of intranuclear gelsolin compared to uECs when both cell types were replated on type I collagen-coated microwells and glass coverslips. These data suggest that increased gelsolin, VASP, and profilin expression may play an important role in the regulation of capillary tube and network formation in three-dimensional extracellular matrix.

Phototactic migration of Dictyostelium cells is linked to a new type of gelsolin-related protein

The molecular and functional characterization of a 125-kDa Ca2+-extractable protein of the Triton X-100-insoluble fraction of Dictyostelium cells identified a new type of a gelsolin-related molecule. In addition to its five gelsolin segments, this gelsolin-related protein of 125 kDa (GRP125) reveals a number of unique domains, two of which are predicted to form coiled-coil regions. Another distinct attribute of GRP125 concerns the lack of sequence elements known to be essential for characteristic activities of gelsolin-like proteins, i.e. the severing, capping, or nucleation of actin filaments. The subcellular distribution of GRP125 to vesicular compartments suggests an activity of GRP125 different from actin-binding, gelsolin-related proteins. GRP125 expression is tightly regulated and peaks at the transition to the multicellular pseudoplasmodial stage of Dictyostelium development. GRP125 was found indispensable for slug phototaxis, because slugs fail to correctly readjust their orientation in the absence of GRP125. Analysis of the GRP125-deficient mutant showed that GRP125 is required for coupling photodetection to the locomotory machinery of slugs. We propose that GRP125 is essential in the natural environment for the propagation of Dictyostelium spores. We also present evidence for further representatives of the GRP125 type in Dictyostelium, as well as in heterologous cells from lower to higher eukaryotes.

Actin-binding proteins in mouse C2 myoblasts and myotubes: a combination of affinity chromatography and two-dimensional gel electrophoresis

This paper analyzes proteins expressed in a mouse muscle precursor cell line (C2 myoblasts) and compares them with those observed in differentiated myotubes from the same cell line. We observed hundreds of proteins in myoblasts using IPG two-dimensional gel electrophoresis but this number is greatly reduced using Mini-Leak (divinylsulfone-activated agarose) affinity chromatography. Two kinds of affinity columns were prepared. One contained a chemically modified monomeric actin bound to the affinity matrix. The second matrix contained a high-affinity actin-binding protein (DNase I) which was bound to the actin Mini-Leak column to block specific sites on actin. Actin-binding proteins in homogenates of myoblasts or myotubes were passed through the affinity columns and eluted under high salt conditions. The Mini-Leak affinity medium itself appeared to have little ability to bind proteins. Our two-dimensional (2-D) gels identified a small number of proteins and we are currently focusing our attention on a particular protein spot which could correspond to cofilin. Comparison of myoblast and myotube proteins using affinity chromatography shows no qualitative, clearly identifiable differences but the analysis is still in progress. These findings are discussed in relation to reports in which the myoblast-myotube transformation was associated with the up-regulation or de novo synthesis of more than ten proteins.

Functional characteristics and the complete primary structure of ascidian gelsolin

The body wall of the ascidian is composed of unusual multi-nucleated smooth muscle cells enriched with thin actin filaments containing troponin-tropomyosin which run along the longitudinal cell axis without being organized into striated structures. We purified an actin-binding protein of 80 kDa, tentatively termed 80K protein, from the body wall muscle of ascidian, Halocynthia roretzi, and characterized the functional properties and molecular structures. In the presence of Ca2+, the 80K protein accelerated the initial phase of actin polymerization, namely the nucleation process, decreased the level of polymerization at the steady state, caused marked reduction in viscosity of an F-actin solution, and fragmented F-actin filaments, while in the absence of Ca2+, it remained associated with F-actin without severing the filaments. The interaction of the 80K protein with actin was inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2). When actin was polymerized in the presence of acrosome actin bundles from horseshoe crab sperm, the 80K protein inhibited the growth of actin filaments at the barbed end but not at the pointed end, indicating that the 80K protein functions as a barbed-end capping protein. In order to characterize the molecular structure of the 80K protein, cDNAs encoding this protein were isolated from the lambda gt11 cDNA library of the ascidian muscle by using a monoclonal antibody (AS23) specific for this protein and the entire sequence was determined. The deduced peptide sequence showed about 44% homology in amino acid residues with the human gelsolin sequence, and in addition, 6 repeating segments were observed in the sequence of the 80K protein as has been described in the gelsolin sequence. These results indicate strongly that the 80K protein belongs to the gelsolin family.

Tissue distribution and levels of gelsolin mRNA in normal individuals and patients with gelsolin-related amyloidosis

We measured quantitatively the mRNA levels of intracellular and secretory forms of gelsolin, an actin-modulating protein, in human tissues from subjects of different ages. The intracellular gelsolin mRNA constituted the major type of gelsolin steady-state mRNA in all tissues analyzed. Both forms of gelsolin were expressed in most adult tissues, with particularly high mRNA levels in all types of muscle and interestingly in skin. Between the adult and infantile tissues the most striking difference in expression levels was found in liver, as the adult liver contained only a subtle amount of gelsolin mRNA. Skin and muscle samples from patients with gelsolin-related amyloidosis (FAF), with significantly increased concentrations of serum gelsolin, did not reveal an increased expression of the gene, and both mutant and wild-type alleles were expressed in equal amounts. The high level of expression of the gelsolin gene in the skin in general could locally contribute to the characteristic skin amyloidosis in FAF patients.

Dependence of fibroblast migration on actin severing activity of gelsolin

Gelsolin nucleates actin filament assembly, blocks the fast-exchanging ends of actin filaments, and severs filaments, processes that may play an important role in cell motility. We studied the relationship between cell migration, gelsolin content, and actin severing activity in human gingival fibroblasts. These cells were keratin negative and desmin negative but expressed vimentin and myosin II. Cells were separated by their ability to migrate in response to a chemoattractant stimulus. Northern analysis of mRNA, [35S]methionine incorporation into immunoprecipitated gelsolin, immunoblots of cell lysates, and quantitative confocal microscopy showed 1.4-2-fold higher levels of gelsolin in nonmigrant compared with migrant cells. Because the concentration of intracellular gelsolin did not appear to be a central determinant of cell migration, we assessed its requirement for motility. Cells that were electroinjected with a gelsolin antibody that inhibits actin severing by gelsolin in vitro showed a 72% reduction of the number of migrant cells compared with controls treated with an irrelevant antibody. Cells that were electroinjected with free gelsolin exhibited a 33% increase in migration compared with cells electroinjected with bovine serum albumin. Compared with nonmigrant cells, migrant cells contained abundant free gelsolin and exhibited gelsolin-dependent F-actin severing activity, which required Ca2+. Serum stimulation of cell migration required increases in [Ca2+]i because incubation with 3 microM 1,2-bis-(o-aminophenoxy)-ethane-N,N, N',N'-tetraacetic acid tetra(acetoxymethyl)-ester blocked calcium flux and cell migration. Serum also stimulated the recruitment of gelsolin into the supernatants of lysates from migrant but not from nonmigrant cells. In fibroblasts, gelsolin concentration alone does not apparently determine migratory capacity. Instead, the Ca2+-dependent actin severing activity of free gelsolin appears to be a major determinant of cell migration.