Motoneurons inhibitory synapses homeostatically respond to neuronal activity and modulate Amyotrophic Lateral Sclerosis pathogenesis

Alterations in Excitation/inhibition (E/I) balance and changes in motor neurons (MN) activity may contribute to MN vulnerability in ALS. The balance of pathogenic vs adaptive changes occurring in inhibitory synapses and affecting E/I balance remain unclear. Confocal microscopy of MN from P45 male SOD1G93A mice reveal downregulated GlyR but upregulated GABAR clusters at inhibitory synapses. GlyR and GABAR respond to PSAM and DREADD chemogenetic alterations of MN excitability, with increased activity driving increase in inhibitory clusters. An E3 ligase-conjugated intrabody (GFE3) degrades Gephyrin, decreases GABAR and GlyR clusters, increases net activity and downregulates disease markers. However, simultaneous decrease of inhibition and increased activity by actPSAM and GFE3 shows no net beneficial effects on disease markers. Thus inhibitory synapses are involved in the early phases of ALS pathogenesis and respond to persistent homeostatic loops and their suppression delivers a net activity increase, offering potential benefits on disease pathways.<b>Significance Statement</b> This study reveals significant changes in inhibitory connections onto motor neurons (MN) during the early stages of ALS, highlighting their involvement in the excitation/inhibition (E/I) imbalance seen at this disease stage. Using advanced chemogenetic tools, we show that inhibitory synapses respond homeostatically to modulations in MN activity. Moreover, decreasing inhibitory receptor components using a functionalized nanobody increases net MN excitability and reduces disease markers. These findings show the dynamic nature of inhibitory synapses in ALS MN, emphasizing their abilities to adapt to activity changes which can in turn influence disease progression.