The possible role of Myosin light chain in myoblast proliferation

Skeletal muscles have the potential to regenerate by activation of quiescent satellite cells, however, the molecular signature that governs satellite cells during muscle regeneration is not well defined. Myosin light chains (Myls) are sarcomere-related proteins as traditional regulator of muscle contraction. In this report, we studied the possible role of Myl in the proliferation of skeletal muscle-derived myoblasts. Compared to diaphragm-derived myoblasts, the extraocular muscle-derived myoblasts with lower levels of Myl proliferated faster, maintained a longer proliferation phase, and formed more final myotubes. It was found that blockading Myl with anti-Myl antibody or knockdown of Myll by siRNA targeted against Myll could enhance the myoblast proliferation and delay the differentiation of myoblasts. Our results suggested that Myl, likely Myll, can negatively affect myoblast proliferation by facilitating myoblast withdrawal from cell cycle and differentiation.

Ventricular myosin light chain-2 gene expression in developing heart of chicken embryos

Recent gene knock-out studies in mice have suggested that ventricular myosin light chain-2 (vMLC2) has a role in the regulation of cardiogenic development and that perturbation in expression of vMLC2 is linked to the onset of dilated cardiomyopathy. In an attempt to develop an avian model for such studies, we examined the expression pattern of vMLC2 in chicken embryos at various stages and analyzed the effect of antisense oligonucleotide-mediated interference of vMLC2 function in cultures of whole embryos. Our results showed vMLC2 to be a specific marker for ventricular chamber throughout chicken embryonic development and antisense vMLC2 treatment of primitive streak stage (stage 4) embryos to produce pronounced dilation of heart tube with severe deficiency in formation of striated myofibrils. Further studies with antisense mRNA techniques of whole embryo cultures should, therefore, be useful to evaluate the role of vMLC2 and other putative regulatory factors in cardiac myofibrillogenesis.

Coexistence of potentiation and fatigue in skeletal muscle

Twitch potentiation and fatigue in skeletal muscle are two conditions in which force production is affected by the stimulation history. Twitch potentiation is the increase in the twitch active force observed after a tetanic contraction or during and following low-frequency stimulation. There is evidence that the mechanism responsible for potentiation is phosphorylation of the regulatory light chains of myosin, a Ca2+ -dependent process. Fatigue is the force decrease observed after a period of repeated muscle stimulation. Fatigue has also been associated with a Ca2+ -related mechanism: decreased peak Ca2+ concentration in the myoplasm is observed during fatigue. This decrease is probably due to an inhibition of Ca2+ release from the sarcoplasmic reticulum. Although potentiation and fatigue have opposing effects on force production in skeletal muscle, these two presumed mechanisms can coexist. When peak myoplasmic Ca2+ concentration is depressed, but myosin light chains are relatively phosphorylated, the force response can be attenuated, not different, or enhanced, relative to previous values. In circumstances where there is interaction between potentiation and fatigue, care must be taken in interpreting the contractile responses.