An emergent disease-associated motor neuron state precedes cell death in ALS.

To define molecular determinants of motor neuron degeneration in amyotrophic lateral sclerosis (ALS), we generated longitudinal single-nucleus transcriptomes and chromatin accessibility profiles of spinal motor neurons together with spatial transcriptomics from the SOD1-G93A mouse model. Vulnerable alpha motor neurons showed thousands of molecular changes, marking a transition into a distinct cell state we named "disease-associated motor neurons" (DMs). We identified transcription factor networks that govern how healthy cells transition into DMs and those associated with motor neuron subtype-selective vulnerability. Upregulation of DM-associated transcription factors in human motor neurons induced key features of DMs, demonstrating an active regulatory component. Human ALS spinal cord single-nucleus RNA sequencing data demonstrated conservation of the DM signature in alpha motor neurons, and human orthologs of regions differentially accessible in SOD1-G93A mouse motor neurons were enriched for ALS genetic risk variants. Together, these findings establish a conserved, genetically linked motor neuron signature in ALS.