Skeletal Muscle Denervation Increases Satellite Cell Susceptibility to Apoptosis

Abstract

Peripheral motor nerve trauma severely compromises skeletal muscle contractile function. Satellite cells respond to denervation by dividing multiple times, ultimately fusing with other satellite cells or myocytes to form new muscle fibers. After chronic denervation, satellite cell numbers decline dramatically, impairing the ability to regenerate and repair myofibers. This satellite cell depletion may contribute to the mechanical deficit observed in denervated or reinnervated muscle. Apoptosis, an evolutionarily conserved form of cell suicide, is a potential mechanism for satellite cell depletion in denervated skeletal muscle. This work tested the hypothesis that skeletal muscle denervation increases satellite cell susceptibility to apoptotic cell death. Adult rats underwent sciatic nerve transection to denervate the distal hindlimb musculature; rats of similar age without the operation served as controls. Two, 6, 10, or 20 weeks after denervation (n = 6 each group), the gastrocnemius and soleus were excised, enzymatically digested, and plated for satellite cell culture. After reaching 95 percent confluence, satellite cells were treated for 24 hours with tumor necrosis factor-alpha (20 ng/ml) and actinomycin D (250 ng/ml), known pro-apoptotic agents. Immunostaining for activated caspases, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), and hematoxylin and eosin staining were performed to identify apoptotic satellite cells. Percentages of apoptotic cells were quantified histomorphometrically. In addition, the presence or absence of bcl-2 and bax was determined by Western blot analysis of control, 6 weeks of denervation, and 10 weeks of denervation specimens. At 6 and 10 weeks after nerve transection, TUNEL and caspase activity were increased more than two-fold in satellite cells isolated from denervated muscle compared with those isolated from control muscle (p < 0.05). In all experimental groups, retention of adherence to the collagen-coated substrate was strongly associated with satellite cell survival. Western blot analysis revealed that adherent satellite cells from all groups expressed both bcl-2 and bax. These data support the authors' hypothesis that skeletal muscle denervation increases satellite cell susceptibility to apoptotic cell death. Apoptosis may play a causative role in the depletion of satellite cells in long-term denervated skeletal muscle.

Role(s) of Mechanical Load and Satellite Cells in The Regulation of The Size of Soleus Muscle Fiber in Rats

“…Therefore, it is suggested that the mechanical load applied to the central region of fibers may be very low or zero even after 16 days of unloading. This view is consistent with the report of Jejurikar et al (2002), who reported that chronic denervation of soleus and gastrocnemius muscles of rats increased the susceptibility of satellite cells to apoptosis. Combined, these data suggest that the suicide-related pathways for the removal of satellite cells may be induced in response to chronic unloading.…”

Satellite Cells and the Muscle Stem Cell Niche

“…This implies that long-term denervation may cause satellite cells to lose their capability to enter the mitotic cell cycle (282). Second, satellite cells from muscle denervated for 6 and 10 wk displayed a twofold increase of apoptosis, compared with control cells from normal innervated muscle (248). Although the underlying mechanism of a neural influence on satellite cell behavior is still elusive, neurotrophins such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are involved.…”

Are Human and Mouse Satellite Cells Really the Same?

“…However, satellite cells do not seem to be involved in muscle hypertrophy mediated by this pathway (Amthor et al 2009). In rodent models of unloading-induced muscle atrophy, satellite cells initially become activated (Ferreira et al 2006), but eventually decrease in number (Schultz et al 1994;Mozdziak et al 2000;Hawke and Garry 2001;Jejurikar et al 2002;Jejurikar and Kuzon 2003), and those that remain are dysfunctional (Mitchell and Pavlath 2004). These studies have implications not only for repair and maintenance of skeletal muscle during periods of immobilization but also for using satellite cells to repair muscle that is already affected by disuse atrophy as, for example, in muscular dystrophies.…”