Proprioception, the sense of body position in space, is critical for generating coordinated movements and reflexive actions. Proprioceptive sensory neurons (pSN) reside in the dorsal root ganglia and constantly monitor muscle stretch and tension with their mechanoreceptive organs (muscle spindles and Golgi-tendon organs) and relay this information to central circuits that generate coordinated motor actions. In particular, group Ia pSN afferents (muscle spindle) provide direct sensory feedback to motor neurons controlling the activity of the same muscle while avoiding motor neurons of antagonistic muscle groups. This precise connectivity pattern represents the basis of the stretch reflex arc and suggests the existence of proprioceptor subtypes defined by the muscle they innervate. However, molecular programs controlling critical aspects of pSN subtype identities, such as the central and peripheral connectivity, are mainly unknown. In this study, we devised a single-cell transcriptomic approach that takes advantage of the topographic organization of the pSN system to reveal molecular features of cardinal proprioceptor subtypes defined by their connectivity to limb, back, and abdominal muscles. First, we identified and validated molecular signatures for each pSN muscle-type population. Second, we found that molecular programs defining these identities are acquired early in development and maintained until early postnatal stages. Last, we discovered distinct expression patterns of axon guidance molecules of the ephrin-A/EphA family that distingush axial- and limb-pSN. In particular, we found that the absence of ephrin-A5 affects the peripheral connectivity of limb-pSN with specific hindlimb muscles, thus implying an important role for ephrin-A signaling in controlling the assembly of sensorimotor circuits. Altogether, this work reveals the molecular foundation of pSN muscle-type identity and paves the way for studying the development and function of muscle-specific sensory feedback circuits.
