CTSS and amyotrophic lateral sclerosis: The notion that unbalanced lysosomal proteolytic activity is not only a hallmark of ALS, but may also represent a first insult contributing to MN death is further supported by previous reports of altered expression or activity of the cathepsin protein family in the glial cells [48,57], skeletal muscle, and MNs of ALS mouse models [58,59,60] and in the MNs and glial cells of ALS patients [61].