Actin in striated muscle: recent insights into assembly and maintenance

Dynamic remodeling of septin structures fine-tunes myogenic differentiation

Controlled myogenic differentiation is integral to the development, maintenance and repair of skeletal muscle, necessitating precise regulation of myogenic progenitors and resident stem cells. The transformation of proliferative muscle progenitors into multinuclear syncytia involves intricate cellular processes driven by cytoskeletal reorganization. While actin and microtubles have been extensively studied, we illuminate the role of septins, an essential yet still often overlooked cytoskeletal component, in myoblast architecture. Notably, Septin9 emerges as a critical regulator of myoblast differentiation during the initial commitment phase. Knock-down of Septin9 in C2C12 cells and primary mouse myoblasts accelerates the transition from proliferation to committed progenitor transcriptional programs. Furthermore, we unveil significant reorganization and downregulation of Septin9 during myogenic differentiation. Collectively, we propose that filmamentous septin structures and their orchestrated reorganization in myoblasts are part of a temporal regulatory mechanism governing the differentiation of myogenic progenitors. This study sheds light on the dynamic interplay between cytoskeletal components underlying controlled myogenic differentiation.

An in vitro agent-based modelling approach to optimization of culture medium for generating muscle cells

Methodologies for culturing muscle tissue are currently lacking in terms of quality and quantity of mature cells produced. We analyse images from in vitro experiments to quantify the effects of culture media composition on mouse-derived myoblast behaviour and myotube quality. Metrics of early indicators of cell quality were defined. Images of muscle cell differentiation reveal that altering culture media significantly affects quality indicators and myoblast migratory behaviours. To study the effects of early-stage cell behaviours on mature cell quality, metrics drawn from experimental images or inferred by approximate Bayesian computation (ABC) were applied as inputs to an agent-based model (ABM) of skeletal muscle cell differentiation with quality indicator metrics as outputs. Computational modelling was used to inform further in vitro experiments to predict the optimum media composition for culturing muscle cells. Our results suggest that myonuclei production in myotubes is inversely related to early-stage nuclei fusion index and that myonuclei density and spatial distribution are correlated with residence time of fusing myoblasts, the age at which myotube-myotube fusion ends and the repulsion force between myonuclei. Culture media with 5% serum was found to produce the optimum cell quality and to make muscle cells cultured in a neuron differentiation medium viable.

Alpha-cardiac Actin Serum Expression Levels Detect Acute Cellular Rejection in Heart Transplant Patients

BACKGROUND

Given the central role of sarcomeric dysfunction in cardiomyocyte biology and sarcomere alterations described in endomyocardial biopsies of transplant patients with rejection, we hypothesized that the serum expression levels of genes encoding sarcomeric proteins were altered in acute cellular rejection (ACR). The aim of this study is to identify altered sarcomere-related molecules in serum and to evaluate their diagnostic accuracy for detecting rejection episodes.

METHODS

Serum samples from transplant recipients undergoing routine endomyocardial biopsies were included in an RNA sequencing analysis (n = 40). Protein concentrations of alpha-cardiac actin were determined using a specific enzyme-linked immunoassay (n = 80).

RESULTS

We identified 17 sarcomeric genes differentially expressed in patients with clinically relevant rejection (grade ≥2R ACR). A receiver operating characteristic curve was done to assess their accuracy for ACR detection and found that 6 relevant actins, myosins, and other sarcomere-related genes showed great diagnostic capacity with an area under the curve (AUC) > 0.800. Specifically, the gene encoding alpha-cardiac actin ( ACTC1 ) showed the best results (AUC = 1.000, P < 0.0001). We determine ACTC1 protein levels in a larger patient cohort, corroborating its overexpression and obtaining a significant diagnostic capacity for clinically relevant rejection (AUC = 0.702, P < 0.05).

CONCLUSIONS

Sarcomeric alterations are reflected in peripheral blood of patients with allograft rejection. Because of their precision to detect ACR, we propose sarcomere ACTC1 serum expression levels as potential candidate for to be included in the development of molecular panel testing for noninvasive ACR detection.

Diabetes induces differences in the F-actin spatial organization of striated muscles

Studies have shown the cytoskeleton disorganization produced by diabetes and quantified F-actin fluorescence in the striated muscles of diabetic animals. However, at present, there are no studies that have quantified F-actin spatial organization (F-actin-SO). Through our research, we analyzed the effect of diabetes on F-actin-SO in the cardiac and skeletal muscles of a mouse model. The muscle samples were labeled with phalloidin-rhodamine and analyzed with confocal microscopy. The analysis was done in two dimensions using four approaches: quantitation of (a) phalloidin-occupied areas; (b) number of F-actin-unoccupied areas per muscular fiber; (c) F-actin filament discontinuity; and (d) costamere periodicity. Our results showed that both the cardiac and skeletal muscles of the control mice had more phalloidin-occupied areas than the diabetic mice. The skeletal muscles had a significantly higher number of F-actin-unoccupied areas per muscular fiber and more F-actin discontinuities. Additionally, using western blot analyses, we showed that those differences were not due to α-actin protein expression. Finally, we considered the importance of these findings in dysfunctional contraction, disassembly in cell-cell communication, conduction of muscle impulse, and changes in cell nanomechanics. Our results quantitatively demonstrated that diabetes severely affects F-actin-SO in striated muscles.

Quantitative high-precision imaging of myosin-dependent filamentous actin dynamics

Over recent decades, considerable effort has been made to understand how mechanical stress applied to the actin network alters actin assembly and disassembly dynamics. However, there are conflicting reports concerning the issue both in vitro and in cells. In this review, we discuss concerns regarding previous quantitative live-cell experiments that have attempted to evaluate myosin regulation of filamentous actin (F-actin) turnover. In particular, we highlight an error-generating mechanism in quantitative live-cell imaging, namely convection-induced misdistribution of actin-binding probes. Direct observation of actin turnover at the single-molecule level using our improved electroporation-based Single-Molecule Speckle (eSiMS) microscopy technique overcomes these concerns. We introduce our recent single-molecule analysis that unambiguously demonstrates myosin-dependent regulation of F-actin stability in live cells. We also discuss the possible application of eSiMS microscopy in the analysis of actin remodeling in striated muscle cells.

Actin-associated proteins and cardiomyopathy-the 'unknown' beyond troponin and tropomyosin

It has been known for several decades that mutations in genes that encode for proteins involved in the control of actomyosin interactions such as the troponin complex, tropomyosin and MYBP-C and thus regulate contraction can lead to hereditary hypertrophic cardiomyopathy. In recent years, it has become apparent that actin-binding proteins not directly involved in the regulation of contraction also can exhibit changed expression levels, show altered subcellular localisation or bear mutations that might lead to hereditary cardiomyopathies. The aim of this review is to look beyond the troponin/tropomyosin mechanism and to give an overview of the different types of actin-associated proteins and their potential roles in cardiomyocytes. It will then discuss recent findings relevant to their involvement in heart disease.

The formin FHOD1 in cardiomyocytes

Members of the formin family are known to be involved in the regulation of the actin cytoskeleton. We have recently identified a muscle specific splice variant of the formin FHOD3 and demonstrated its role in the maintenance of the contractile filaments of cardiomyocytes. Here, we characterize the expression and subcellular localization of FHOD3's closest relative, FHOD1, in the heart. Confocal microscopy shows that FHOD1 is mainly located at the intercalated disc, the special type of cell-cell contact between cardiomyocytes, but also partially associated with the myofibrils. Subcellular targeting of FHOD1 is probably mediated by its N-terminal domain, since expression constructs lacking this domain show aberrant localization in primary cultures of neonatal rat cardiomyocytes. Finally, we show that in contrast to FHOD3, FHOD1 shows increased expression levels in dilated cardiomyopathy, suggesting that the two formins play distinct roles and are differentially regulated in cardiomyocytes.

Two distinct phosphorylation events govern the function of muscle FHOD3

Posttranslational modifications such as phosphorylation are universally acknowledged regulators of protein function. Recently we characterised a striated muscle-specific isoform of the formin FHOD3 that displays distinct subcellular targeting and protein half-life compared to its non-muscle counterpart and which is dependent on phosphorylation by CK2 (formerly casein kinase 2). We now show that the two isoforms of FHOD3 are already expressed in the vertebrate embryonic heart. Analysis of CK2 alpha knockout mice showed that phosphorylation by CK2 is also required for proper targeting of muscle FHOD3 to the myofibrils in embryonic cardiomyocytes in situ. The localisation of muscle FHOD3 in the sarcomere varies depending on the maturation state, being either broader or restricted to the Z-disc proper in the adult heart. Following myofibril disassembly, such as that in dedifferentiating adult rat cardiomyocytes in culture, the expression of non-muscle FHOD3 is up-regulated, which is reversed once the myofibrils are reassembled. The shift in expression levels of different isoforms is accompanied by an increased co-localisation with p62, which is involved in autophagy, and affects the half-life of FHOD3. Phosphorylation of three amino acids in the C-terminus of FHOD3 by ROCK1 is sufficient for activation, which results in increased actin filament synthesis in cardiomyocytes and also a broader localisation pattern of FHOD3 in the myofibrils. ROCK1 can directly phosphorylate FHOD3, and FHOD3 seems to be the downstream mediator of the exaggerated actin filament formation phenotype that is induced in cardiomyocytes upon the overexpression of constitutively active ROCK1. We conclude that the expression of the muscle FHOD3 isoform is characteristic of the healthy mature heart and that two distinct phosphorylation events are crucial to regulate the activity of this isoform in thin filament assembly and maintenance.