Transcriptional evidence for failed reinnervation in aging muscle

Abstract

Muscle displays a marked accumulation of denervated myofibers at ages associated with an acceleration of atrophy and weakness. As muscle undergoes repeating cycles of denervation and reinnervation throughout adulthood, we hypothesize that the accumulation of denervated myofibers in advanced age is due to failed reinnervation. To address this issue, we quantified neuromuscular junction (NMJ) associated genes including five AChR subunits, MuSK, Rapsyn, Agrin, Lrp4 and APC in three different model systems. Firstly, we examined vastus lateralis (VL) muscle from young adult (YA) and very old (VO) Fisher 344xBrown Norway F1‐hybrid rats. Secondly, we examined biopsies from human VL in a young physically active group (23.7±2.7), an old active group (71.2±4.9), an old inactive group (64.8±3.1) and a very old inactive group (82.5±4.8). Lastly, we examined Extensor digitorum longus(EDL) & Tibialis anterior (TA) muscle from wild type and neurotrypsin over‐expressing (Sarco) mice as a model of unstable NMJs secondary to reduced MuSK activation. Interestingly, transcripts of MuSK (21 fold), Rapsyn, Agrin and AChR subunits α (68 fold), β, and the fetal isoform γ (47 fold) were upregulated in very old muscle of rat. In contrast, only Musk, Agrin and Rapsyn were increased in aging human muscle of very old inactive group compared with a young active group, consistent with the milder denervation phenotype by morphology. In contrast, in Sarco mice these NMJ transcripts were largely preserved despite a muscle atrophy phenotype and small angular fibers characteristic of long‐term denervation, which were frequent in VO rat and very old inactive human muscle, were rare in Sarco mice suggesting a high fidelity of reinnervation. We conclude that the dramatic changes in the NMJ transcriptional profile in aging muscle is indicative of a failed reinnervation response.