ALS is characterized by progressive inability to execute movements. Motor neurons innervating fast-twitch muscle-fibers preferentially degenerate. The reason for this differential vulnerability and its consequences on motor output is not known. Here, we uncover that fast motor neurons receive stronger inhibitory synaptic inputs than slow motor neurons, and disease progression in the SOD1G93A mouse model leads to specific loss of inhibitory synapses onto fast motor neurons. Inhibitory V1 interneurons show similar innervation pattern and loss of synapses. Moreover, from postnatal day 63, there is a loss of V1 interneurons in the SOD1G93A mouse. The V1 interneuron degeneration appears before motor neuron death and is paralleled by the development of a specific locomotor deficit affecting speed and limb coordination. This distinct ALS-induced locomotor deficit is phenocopied in wild-type mice but not in SOD1G93A mice after appearing of the locomotor phenotype when V1 spinal interneurons are silenced. Our study identifies a potential source of non-autonomous motor neuronal vulnerability in ALS and links ALS-induced changes in locomotor phenotype to inhibitory V1-interneurons.

ALS is characterized by progressive inability to execute movements. Motor neurons innervating fast-twitch muscle fibers exhibit preferential degeneration. The reason for differential vulnerability of fast motor neurons, and its consequence on motor output is not known. Here, we show that fast motor neurons receive more inhibitory synaptic inputs than slow motor neurons, and loss of inhibitory synapses onto fast motor neurons precedes disease progression in the SOD1 G93A mouse model of ALS. Loss of inhibitory synapses on fast motor neurons is accounted for by a loss of synapses from inhibitory V1 spinal interneurons. Deficits in V1-motor neuron connectivity appear prior to motor neuron death and are paralleled by development of specific SOD1 G93A locomotor deficits. These distinct SOD1 G93A locomotor deficits are phenocopied by silencing of inhibitory V1 spinal interneurons in wild-type mice. Silencing inhibitory V1 spinal interneurons does not exacerbate SOD1 G93A locomotor deficits, suggesting phenotypic pathway interaction. Our study identifies a potential cell non-autonomous source of motor neuronal vulnerability in ALS, and links ALS-induced changes in locomotor phenotypes to inhibitory V1 interneurons.

Loss of connectivity between spinal V1 inhibitory interneurons and motor neurons is found early in disease in the SOD1G93A ALS mice. Such changes in premotor inputs can contribute to homeostatic imbalance of vulnerable motor neurons. Here, we show, for the first time, that stabilization of V1 synapses by overexpression of the Extended Synaptotagmin 1 presynaptic organizer increases motor neuron survival and ameliorates motor phenotypes, demonstrating that interneurons can be a potential target to attenuate ALS symptoms.

Loss of connectivity between spinal V1 inhibitory interneurons and motor neurons is found early in disease in the SOD1G93A mice. Such changes in premotor inputs can contribute to homeostatic imbalance of vulnerable motor neurons. Here, we show, for the first time, that stabilization of V1 synapses by overexpression of a presynaptic organizer, increases motor neuron survival and ameliorates motor phenotype, demonstrating that interneurons can be a potential target to attenuate ALS symptoms.