Basic and Applied Myology最新文献

Accelerated proteolysis through the ubiquitin-proteasome system has been recognized as a major contributor to muscle wasting, a serious complication frequently associated with a number of inflammatory disorders. Muscle expression of atrogin-1/MAFbx, a rate-limiting ubiquitin ligase for muscle wasting, is upregulated in various inflammatory conditions, and is considered a therapeutic target for muscle wasting. As one of the free radicals whose production is elevated in inflammatory conditions, nitric oxide (NO) is implicated in the pathogenesis of muscle wasting. To understand how inflammatory mediators upregulate atrogin-1/MAFbx expression, we tested the hypothesis that NO mediates the upregulation of atrogin-1/MAFbx expression. C2C12 myotubes were incubated with a cocktail comprised of TNF-α, interferon γ and lipopolysaccharide (LPS), which stimulated NO production and atrogin-1/MAFbx expression. Pre-incubation of the myotubes with nitric oxide synthase (NOS) inhibitor L-NAME or S-ethylisothiourea (SETU) blocked the stimulation of NO production by the cocktail. However, the stimulation of atrogin-1/MAFbx expression was not disrupted. Intraperitoneal administration of LPS to mice resulted in elevated atrogin-1/MAFbx expression in gastrocnemius muscle. But, pretreatment of the mice with L-NAME did not alter LPS stimulation of atrogin-1/MAFbx expression. Therefore, NO does not mediate upregulation of atrogin-1/MAFb expression by inflammatory mediators.

Myogenic precursors in adult skeletal muscle (satellite cells) are mitotically quiescent but can proliferate in response to a variety of stresses including muscle injury. To gain further understanding of adult myoblasts, we are analyzing myogenesis of satellite cells on fibers isolated from adult rat muscle. In this culture model, satellite cells are maintained in their in situ position underneath the fiber basement membrane. Employing two different approaches to monitor proliferation of satellite cells on isolated fibers (autoradiography following ³H-thymidine incorporation and immunofluorescence of cells positive for proliferating cell nuclear antigen (PCNA)), we show in the present study that satellite cells initiate cell proliferation at 12 to 24 hours following fiber culture establishment and that cell proliferation is reduced to minimal levels by 60 to 72 hours in culture. Maximal number of proliferating cells is seen at 36 to 48 hours in culture. These PCNA+ satellite cells transit into the differentiated, myogenin+ state following about 24 hours in the proliferative state. Continuous exposure of the fiber culture to FGF2 (basic FGF; added at the time of culture establishment) leads to a 2 fold increase in the number of PCNA+ cells by 48 hours in culture but the overall schedule of proliferation and transition into the myogenin+ state is not affected. Delaying the addition of FGF2 until 15 to 18 hours following the initiation of the fiber culture does not reduce its effect. However, the addition of FGF2 at 24 hours or later results in a progressive reduction in the number of proliferating satellite cells. Exposure of fiber cultures to transforming growth factor β (TGFβ1) leads to a reduction in the number of proliferating cells in both the absence or presence of FGF2. We propose that FGF2 enhances the number of proliferating cells by facilitating the recruitment of additional satellite cells from the quiescent state. However, satellite cells on isolated fibers conform to a highly coordinated program and rapidly transit from proliferation to differentiation regardless of the presence of FGF. The identification of agents that can prolong the proliferative state of satellite cells when the cells undergo myogenesis in their native position by the intact myofiber might be useful in improving myoblast transplantation into skeletal muscle for cell-mediated gene therapy.

Myoblasts from the pectoralis muscle of developing and adult chickens were cultured in various media and the number of differentiated myoblasts was monitored via indirect immunofluorescence using an antibody against sarcomeric myosin. Myoblasts from all age groups proliferated to a lesser extent in media lacking chicken embryo extract (CEE) when compared to myoblasts in standard medium containing 5% CEE. However, CEE influenced differentiation of myoblasts from 19-day-old embryos and adults but not that of myoblasts from 10-day-old embryos. Differentiation in cultures from 10-day embryos began at the same time and reached the same levels regardless of the presence or absence of the CEE in the medium. In contrast, differentiation of myoblasts from 19-day-old embryos and adult chickens was delayed by several days in cultures maintained with CEE compared to cultures in media lacking CEE. We do not know yet what is the active factor(s) in the CEE causing this delay of myogenic differentiation in cultures from older embryos and adults. Nevertheless, the present finding is in accordance with our previously published studies where we demonstrated that adult type chicken myoblasts, which differ from those dominant during mid-fetal development, emerge at late stages of embryogenesis and are the only one present during post-hatch and adult stages.