Up-Regulated Autophagy in Skeletal Muscle of Young Amyotrophic Lateral Sclerosis Mouse Model Prior to Disease Onset

Abstract

Autophagy is a cellular process that targets damaged organelles for lysosomal degradation, which may play crucial roles in amyotrophic lateral sclerosis (ALS), a neuromuscular disease characterized by motor neuron and muscle degeneration. Studies from Morimoto (2007) and Zhang (2011) identified increased autophagy in motor neurons of ALS mice (G93A) only at the age after the disease onset (> 90 days). To test if muscle plays an active role in the disease progression, we analyzed autophagy activity in skeletal muscle at three different age groups: before axon withdrawal ( 90 days) by expressing LC3-RFP in muscle fibers or loading muscle fibers with LysoTracker. We found dramatic increases of autophagosome formation and lysosomal vesicles at all age groups of G93A mice compared to the age-matched wild type (WT). The portion of fibers with the ratio of autophagosome-area/fiber-area more than 15∗10−3 is summarized as G93A (WT): [ 90 days: 15% (2%); n=31-74 (31-66)]. The portion of fibers with more than 100 lysosome vesicles is summarized as: [ 90 days: 44% (2%); n=39-77 (38-81)]. A majority of autophagosomes are in close contact with lysosome vesicles, indicating a coordinated up-regulation of autophagy-lysosomal pathway in G93A muscle. Western blot analysis also show increased LC3-II level in G93A muscle at all age groups: [ 90 days: 7.1±1.8, n=12 (2.2±0.9, n=4)]. Our study identified up-regulated autophagy activity in muscle as an early event before disease onset, suggesting that skeletal muscle may actively contribute to ALS progression. Supported by MDA and NIAMS/NIH.