Mutant Sod1-expressing Astrocytes Release Toxic Factors That Trigger Motor Neuron Death

42 Amyotrophic lateral sclerosis (ALS) is a devastating paralytic disorder caused by dysfunction 43 and degeneration of motor neurons starting in adulthood. Recent studies using cell or animal 44 models document that astrocytes expressing disease-causing mutations of human 45 superoxide dismutase 1 (hSOD1) contribute to the pathogenesis of ALS by releasing a 46 neurotoxic factor(s). Neither the mechanism by how this neurotoxic factor induces motor 47 neuron death nor its cellular site of action has been elucidated. Here we show that acute 48 exposure of primary wild-type spinal cord cultures to conditioned medium derived from 49 astrocytes expressing mutant SOD1 (ACM-hSOD1) increases persistent sodium currents 50 (PCNa), repetitive firing and intracellular calcium transients, leading to specific motor neuron 51 death days later. In contrast to TTX, which paradoxically increased twofold the amplitude of 52 calcium transients and killed motor neurons, reduction of hyper-excitability by other specific 53 (mexiletine) and non-specific (spermidine and riluzole) blockers of Nav channels restored 54 basal calcium transients and prevented motor neuron death induced by ACM-hSOD1. 55 These findings suggest that riluzole, the only FDA-approved drug with known benefits for 56 ALS patients, acts by inhibiting hyper-excitability. Together, our data document that a critical 57 element mediating the non-cell autonomous toxicity of ACM-hSOD1 on motor neurons is 58 increased excitability, an observation with direct implications for therapy of ALS. 59 60 61