Project Summary: The long-term objective of this work is to define the molecular mechanisms underlying proprioceptive signaling that are required for sensory-driven motor behaviors. Proprioception encodes body and limb position, which is essential for purposeful movement. Yet, the lack of tools to selectively target genes in proprioceptors, the sensory neurons that carry proprioceptive signals, has impeded our understanding of the molecular mechanisms underlying this vital "sixth sense." Voltage-gated sodium channels (NaVs) are critical for neuronal signaling in the nervous system, and proprioceptors express three isoforms: NaV1.1, NaV1.6, and NaV1.7. We recently discovered that loss of NaV1.1 in sensory neurons causes ataxic-like behaviors and abnormal limb positioning and impaired proprioceptor transmission during static muscle stretch. While loss of NaV1.7 has no impact on motor function in mice or humans, NaV1.6 is known to contribute to the function of brain neurons involved in motor control. Thus, our lab created a sensory neuron-wide knock out of NaV1.6 and found that these animals have extremely severe motor coordination deficits distinct from those observed in NaV1.1 conditional knockout animals. This proposal will uncover the mechanistic role of NaV1.6 in proprioceptive signaling by developing the first CRISPR/Cas9 intersectional genetic and viral strategy to manipulate genes in selectively proprioceptors. The work outlined herein will test the central hypothesis that the unique cellular localization patterns and intrinsic properties of NaV1.6 underly the transmission of proprioceptive signals to spinal motor circuits for motor behaviors. I test this hypothesis using the following mechanistic aims: Aim 1 will analyze the cellular distribution patterns of NaV1.6 channels in proprioceptive sensory neurons. Aim 2 will investigate how loss of NaV1.6 changes proprioceptor biophysical properties and excitability. Aim 3 will leverage an intersectional gene knock out model to investigate how acute deletion of NaV1.6 proprioceptors affects sensory-motor circuit function and motor behaviors, overcoming a long-standing challenge in the field. NaV1.6 is associated with brain-related diseases like epilepsy and autism spectrum disorder. Symptoms of these diseases include motor-related deficits; however, our understanding of the mechanisms that give rise to these comorbidities outside the brain is understudied. The proposed herein are directly in line with the goals of NINDS as it will 1) advance our knowledge about mammalian proprioception and 2) shed new light on critical roles of peripheral sensory neuron signaling in neurological disorders. Furthermore, this proposal will not only provide critical me new training in molecular biology and biochemistry techniques, but also offer the scientific community a new and exciting approach to gene manipulation in challenging- to-target neuronal populations.