Loss of SARM1 Improves Phenotypes in a Mouse Model of Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay.

BACKGROUND AND OBJECTIVES: Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS) is a neurodegenerative disease caused by pathogenic variants in SACS. ARSACS is characterized by mitochondrial abnormalities and disruptions of the neurofilament cytoskeleton. In other conditions, these features have been linked to activation of Sterile Alpha and TIR Motif Containing 1 (SARM1), an enzyme that can trigger axon degeneration and neuronal death. Inhibition of SARM1 is an attractive therapeutic strategy because SARM1 is inactive in healthy cells and knockout of SARM1 has little or no deleterious effects. We therefore asked whether SARM1 activity contributed to Purkinje cell degeneration and motor defects present in a Sacs -/- mouse model of ARSACS.

METHODS: We studied 4 cohorts of mice: (1) Sacs -/- ; Sarm1 +/+ ; (2) Sacs +/+ ; Sarm1 +/- ; (3) Sacs -/- ; Sarm1 +/- ; and (4) Sacs -/- ; Sarm1 -/- . In 9-month-old mice, we analyzed protein markers of Purkinje cells (n = 3-4 mice per genotype) and counted surviving Purkinje cells in folia III, IV, and VIII of cerebellar sections (n = 3 mice per genotype, 6 sections per mouse), and tested motor function at 3, 6, and 9 months by quantifying parameters of gait (Digigait) and coordination and balance (Rotarod) (8 male, 8 female mice of each genotype).

RESULTS: Probing of cerebellar extracts showed that the Purkinje cell protein markers Calbindin-1, RGS8, and PCP2 were decreased in Sacs -/- mice but restored to normal levels in Sacs -/- ; Sarm1 -/- mice. Purkinje cell loss in Sacs -/- mice was most prominent in anterior folia, as previously noted. Sarm1 loss partially mitigated the Purkinje cell death in folium III of 9-month-old Sacs -/- mice. Similarly, longitudinal behavioral assessment of motor functions showed that disturbances in gait pattern (slower cadence, prolonged swing, and stance phases) were partially alleviated. Rotarod tests gave more ambivalent results, as the homozygous loss of Sarm1 was less effective than heterozygous loss in ameliorating the Sacs -/- phenotype.

DISCUSSION: We conclude that SARM1 contributes to neurodegeneration in ARSACS, and its downregulation or inhibition could constitute a significant therapeutical strategy in the treatment of the disease.