Sensory neurons in the dorsal spinal cord (dINs) represent the first site in the CNS to receive somatosensory information from the periphery and are a crucial component in sensorimotor circuits that control posture and behavior. Defining the developmental principles of somatosensory circuit formation and the molecular identity of neurons that serve specific sensory functions is key to our understanding of the logic of sensorimotor integration and for strategic interventions following injury or disease. All dINs arise from a small array of molecularly defined embryonic cell populations, but markers are expressed only transiently and how these early neurons diversify and integrate into sensory circuits serving distinct sensory modalities in the mature animal remains largely unknown. There is an urgent need for genetic access to express tracers and to manipulate individual classes of developing dINs as they emerge during the embryonic period but establish circuits postnatally. Here, we will generate for the first time, mice that bridge the gap between early identity and mature function by providing access selectively to one population of dINs, called dI1s, throughout development. We will create mouse lines to introduce anterograde and retrograde viral tracers to reveal and map the synaptic targets of dI1s in brain and spinal cord, and to identify input neurons to dI1s, in DRG, brain and spinal cord. These lines will extend temporal genetic access selectively to developing dI1s into postnatal stages, by, first, a Cre-dependent Cre approach, permitting access by Cre-dependent axonal and synaptic tracers, and second, mice that express the proteins required for transsynaptic tracing using rabies virus. We will also develop platforms to analyze efficiently the anatomy and connectivity of the circuitry. Furthermore, we will create a widely needed 3D spinal cord atlas for assessment of neuronal identity and mapping circuitry. To understand the programs that regulate dI1 development, we will also identify and follow dynamic changes of transcriptional signatures in dI1s throughout embryonic and postnatal ontogeny by single cell RNA sequencing. The tools established in the proposed experiments and analyses will be applicable to all dIN subsets and will contribute both resources and information to the field, and provide a template for manipulation and functional analysis of somatosensory pathways in health and in disease.
The circuitry for somatosensation to the spinal cord and from the spinal cord to the brain is surprisingly poorly understood. To interrogate this circuitry, novel mouse lines will be created to prolong transient genetic access in spinal somatosensory neuron classes, so that viral tracing can be used to illuminate the development of their circuitry. New histological and analysis methods and molecular profiling will permit mapping of emergent circuits with high spatial and functional resolution.