Proximal Giant Neurofilamentous Axonopathy in Mice Genetically Engineered to Resist Calpain and Caspase Cleavage of α-II Spectrin

We use 1,2-diacetylbenzene (1,2-DAB) to probe molecular mechanisms of proximal giant neurofilamentous axonopathy (PGNA), a pathological hallmark of amyotrophic lateral sclerosis. The spinal cord proteome of rodents displaying 1,2-DAB PGNA suggests a reduction in the abundance of alpha-II spectrin (Spna2), a key protein in the maintenance of axonal integrity. Protein immunoblotting indicates that this reduction is due to Spna2 degradation. We investigated the importance of such degradation in 1,2-DAB PGNA. Spna2 mutant mice lacking a calpain- and/or caspase-sensitive domain (CSD), thus hypothetically resistant to 1,2-DAB, and wild-type littermates, were treated with 1,2-DAB, 35 mg/kg/day, or saline control, for 3 weeks. 1,2-DAB induced motor weakness and PGNA, irrespective of the genotype. Spna2-calpain breakdown products were not detected in mutant mice, which displayed a normal structure of the nervous system under saline treatment. Intriguingly, treatment with 1,2-DAB reduced the abundance of the caspase-specific 120-kDa Spna2 breakdown products. Our findings indicate that degradation of Spna2 by calpain- and/or caspase is not central to the pathogenesis of 1,2-DAB axonopathy. In addition, the Spna2-CSD seems to be not required for the maintenance of the cytoskeleton integrity. Our conceptual framework offers opportunities to study the role of calpain-caspase cross talk, including that of the protease degradomics, in models of axonal degeneration.