Circuit dysfunction in SOD1-ALS model first detected in sensory feedback prior to motor neuron degeneration is alleviated by BMP signaling

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease whose origin and underlying cellular defects are still not fully understood. While motor neuron degeneration is the signature feature of ALS, it is not yet clear if motor neurons, or other cells of the motor circuit, are the site of disease initiation. To better understand the contribution of multiple cell types in ALS, we made use of a Drosophila Sod1G85R knock-in model, in which all cells harbor the disease allele. End-stage dSod1G85R animals exhibit severe motor deficits with clear degeneration of motor neurons. Interestingly, earlier in dSod1G85R larvae motor function is also compromised, but their motor neurons exhibit only subtle morphological and electrophysiological changes, that are unlikely to cause the observed decrease in locomotion. We analyzed the intact motor circuit and identified a defect in sensory feedback that likely accounts for the altered motor activity of dSod1G85R . Furthermore, we found that the cell-autonomous activation of BMP signaling in proprioceptor sensory neurons that relay the contractile status of muscles back to the central nerve cord, is able to completely rescue early stage motor defects and partially rescue late stage motor function to extend lifespan. Identifying a defect in sensory feedback as a potential initiating event in ALS motor dysfunction, coupled with the ability of modified proprioceptors to alleviate such motor deficits, underscores the critical role that non-motor neurons play in disease progression and highlights their potential as a site to identify early-stage ALS biomarkers and for therapeutic intervention. Significance Statement At diagnosis, many cellular processes are already disrupted in the ALS patient. Identifying the initiating cellular events is critical for achieving an earlier diagnosis in order to slow or prevent disease progression. Our findings indicate that neurons relaying sensory information underlie early stage motor deficits in a Drosophila knock-in model of ALS that best replicates gene dosage in familial ALS (fALS). Importantly, studies on intact motor circuits revealed defects in sensory feedback before evidence of motor neuron degeneration. These findings strengthen our understanding of how neural circuit dysfunctions lead to neurodegeneration and coupled with our demonstration that activation of BMP signaling in proprioceptors alleviates both early and late motor dysfunction, underscores the importance of considering non-motor neurons as therapeutic targets.