DNA-fragmentation and expression of apoptosis-related proteins in experimentally denervated and reinnervated rat facial muscle

Abstract

Muscle fibres may undergo apoptotic cell death in several neuromuscular disorders such as denervated muscle fibres in spinal muscular atrophies. We investigated DNA-fragmentation (in situ by the TUNEL-method) and expression of apoptosis-associated proteins in experimentally denervated and reinnervated rat facial muscle up to 24 weeks after surgery to evaluate the rate and time lapse of apoptotic muscle fibre loss. While denervated muscle displayed constantly high rates of DNA-fragmentation, denervated and immediately reinnervated muscle showed a distinct decrease of primarily elevated DNA-cleavage, finally resembling rates of normal controls. Denervated muscle fibres revealed strong immunoreactivity of the anti-apoptotic proteins bcl-2 and bcl-xL, and the pro-apoptotic factor bax. In reinnervated muscle fibres, only bcl-2 was constantly upregulated while bcl-xL and bax diminished after the 7th week. The present findings indicate that denervation may prompt muscle fibres to activate an intrinsic 'suicide' programme to undergo apoptosis. High levels of bcl-2 after denervation may sustain cell survival until reinnervation, e.g. after accidental nerve damage or in neurodegenerative disorders. Furthermore, increasing levels of bcl-2 are able to neutralize high apoptosis-promoting bax levels. Interventions modifying DNA-fragmentation and the expression of apoptosis-related proteins may lead to new therapeutic concepts in denervating disorders of muscle in the absence of other primary therapies.