The making of a proprioceptor: a tale of two identities.

Recent single-cell transcriptome analyses have defined the molecular correlates of mouse proprioceptor subtypes innervating different muscles and end-organ receptors. Proprioceptor muscle subtypes present distinct molecular profiles according to the identity of the muscle they innervate and express gene programs involved in the control of target selectivity. Transcriptional profiling of proprioceptor receptor subtypes reveals up to seven molecularly distinct muscle spindle afferents, but only a single class of Golgi tendon organs, suggesting a higher need for precisely calibrated spindle feedback. Proprioceptor muscle-type identity becomes apparent concomitantly with muscle innervation, while receptor-type appears to be defined after end-organ innervation, indicating a differential reliance on intrinsic and extrinsic signals. The developmental dynamic of molecular identities suggests that proprioceptors undergo plastic changes in gene expression that reflect the needs of their different functional states. Proprioception, the sense of body position in space, has a critical role in the control of posture and movement. Aside from skin and joint receptors, the main sources of proprioceptive information in tetrapods are mechanoreceptive end organs in skeletal muscle: muscle spindles (MSs) and Golgi tendon organs (GTOs). The sensory neurons that innervate these receptors are divided into subtypes that detect discrete aspects of sensory information from muscles with different biomechanical functions. Despite the importance of proprioceptive neurons in motor control, the developmental mechanisms that control the acquisition of their distinct functional properties and positional identity are not yet clear. In this review, we discuss recent findings on the development of mouse proprioceptor subtypes and challenges in defining them at the molecular and functional level. [ABSTRACT FROM AUTHOR]