Dysregulation of synaptic transcripts underlies network abnormalities in ALS patient-derived motor neurons

Amyotrophic lateral sclerosis (ALS) is characterized by dysfunction and loss of upper and lower motor neurons. Several studies have identified structural and functional alterations in the motor neurons before the manifestation of symptoms, yet the underlying cause of such alterations and how they contribute to the progressive degeneration of affected motor neuron networks remain unclear. Importantly, the short- and long-term spatiotemporal dynamics of neuronal network activity make it challenging to discern how ALS-related network reconfigurations emerge and evolve. To address this, we systematically monitored the structural and functional dynamics of motor neuron networks with a confirmed endogenous <i>C9orf72</i> mutation. We show that ALS patient-derived motor neurons display time-dependent neural network dysfunction, specifically reduced firing rate and spike amplitude, impaired bursting, but higher overall synchrony in network activity. These changes coincided with altered neurite outgrowth and branching within the networks. Moreover, transcriptional analyses revealed dysregulation of molecular pathways involved in synaptic development and maintenance, neurite outgrowth, and cell adhesion, suggesting impaired synaptic stabilization. This study identifies early synaptic dysfunction as a contributing mechanism resulting in network-wide structural and functional compensation, which may over time render the networks vulnerable to neurodegeneration.<b>NEW & NOTEWORTHY</b> RNA-sequencing of ALS patient-derived motor neurons revealed altered expression of genes involved in cell adhesion, neurite outgrowth, synaptic development and maintenance, and synaptic plasticity. These alterations were accompanied by time-dependent structural impairments and disrupted neuronal activity, suggesting that early synaptic changes and network-wide structural and functional compensations contribute to motor neuron vulnerability in ALS.