| Though mutant proteins are broadly expressed in neurodegenerative diseases, only some neuronal subsets are vulnerable. In patients with amyotrophic lateral sclerosis (ALS), most motor neurons degenerate but those innervating extraocular muscles, pelvic sphincters and slow limb muscles exhibit selective resistance. Previous work in our lab used laser capture microdissection followed by microarray analysis of vulnerable lumbar level (L5) motor neurons and the resistant motor neuron populations that innervate the extraocular muscles (III) and the pelvic sphincter muscles (DL) to identify hundreds of molecular markers of each subset. By looking at the genes that had >10 fold differential expression and that were selectively and strongly expressed in motor neurons into adulthood, we identified matrix metalloproteinase-9 (MMP-9) as a potential susceptibility gene. We first examined the expression of MMP-9 in wild-type mice and found that it is expressed only after postnatal day 5 in fast motor neurons, which are selectively vulnerable in ALS. Further, MMP-9 was the only secreted member of the MMP family constitutively expressed in this population. In mutant SOD1 mice, MMP-9 expression was strongly positively correlated with cell loss at end-stage of the disease and with early induction of endoplasmic reticulum stress, as measured by p-EIF2alpha.;To test whether MMP-9 is a driver of disease, we crossed Mmp9 null mice with mSOD1 mice and found that both partial reduction and complete ablation of MMP-9 levels delayed muscle denervation, prolonged survival, and improved motor function, measured both behaviorally and electrophysiologically. Importantly, even acute knock-down in motor neurons with AAV6 gene therapy or central inhibition of MMP-9 after symptom-onset were able to protect muscle innervation of the fast hindlimb muscle, TA. Further, MMP-9 expressed by motor neurons is required for full ER stress activation, suggesting it may be a very early intermediate in the disease pathway triggered by mutant SOD1. However, virally introducing MMP-9 into resistant pools does not confer susceptibility, implying that there is an additional factor (or factors) also present in fast motor pools that is necessary to induce axonal die-back. These studies suggest that MMP-9 is a promising candidate therapeutic target for ALS. Importantly, these data support the study of neuronal diversity as a potential way to define novel therapeutic strategies for the treatment of neurodegenerative diseases. |
