Specificity of sensory–motor connections encoded by Sema3e–Plxnd1 recognition

Abstract

Spinal reflexes are mediated by synaptic connections between sensory afferents and motor neurons [1][2][3] . The organization of these circuits exhibits several levels of specificity. Only certain classes of proprioceptive sensory neurons make direct, monosynaptic, connections with motor neurons 4 . Those that do are bound by rules of motor pool specificity -they form strong connections with motor neurons supplying the same muscle, but avoid motor pools supplying antagonistic muscles 1,[5][6][7] . This pattern of connectivity is initially accurate and is maintained in the absence of activity 8 , implying that wiring specificity relies on the matching of recognition molecules on the surface of sensory and motor neurons. But determinants of fine synaptic specificity here, as in most regions of the central nervous system (CNS), have yet to be defined. To address the origins of synaptic specificity in these reflex circuits we have used molecular genetic methods to manipulate recognition proteins expressed by subsets of sensory and motor neurons. We show here that a recognition system involving expression of Sema3e by selected motor neuron pools, and its high-affinity receptor PlexinD1 by proprioceptive sensory neurons, is a critical determinant of synaptic specificity in sensory-motor circuits. Changing the profile of Sema3e-PlexinD1 signaling in sensory or motor neurons results in functional and anatomical rewiring of monosynaptic connections, but does not alter motor pool specificity. Our findings indicate that patterns of monosynaptic connectivity in this prototypic CNS circuit are constructed through a recognition program based on repellent signaling.Several observations led us to focus on the potential contribution of the class 3 semaphorin Sema3e and its receptors as mediators of sensory-motor synaptic specificity. Sema3e is expressed by a restricted set of brachial motor neurons and can serve as a bifunctional ligand, eliciting repellent responses through engagement of PlexinD1 and attractant responses through interactions with a Neuropilin-1/PlexinD1 receptor complex 9-12 . Moreover, Sema3e expression is lost, and the pattern of monosynaptic sensory-motor connections altered, in mice mutant for Pea3, an ETS transcription factor expressed by several brachial motor neuron pools 11, 13 . We analyzed the role of Sema3e and its receptors in two sensory-motor reflex arcs. In one reflex arc that supplies the triceps (Tri) forelimb muscle, motor neurons receive monosynaptic input from Tri sensory afferents 13 . But in a second, atypical, reflex arc that controls the cutaneous maximus (Cm) muscle, motor neurons fail to receive monosynaptic input from Cm afferents [13][14][15] . Cm motor neurons also lack monosynaptic input from Tri proprioceptive neurons, or from any other proprioceptive afferents, and conversely, Tri motor neurons lack monosynaptic input from Cm proprioceptive afferents 13 . The contrasting circuitry of these two reflex arcs permitted us to examine monosynaptic connectivity between proprioce...

Genetic dissection of plexin signaling in vivo

“…We extended our analysis of plexindependent signaling to the prototypic member of another plexin subfamily, Plexin-D1, which is mainly expressed in endothelial cells and neurons during embryonic development (27). Plexin-D1-deficient mice exhibit defects in vascular patterning, angiogenesis and outflow tract septation of the heart, skeletal malformations, errors in axonal projections and synapse formation, and defects in thymocyte migration (28)(29)(30)(31)(32)(33)(34)(35)(36). Mice lacking Plexin-D1 have only one great vessel arising from the heart (persistent truncus arteriosus) instead of two (aorta and pulmonary artery) and therefore die shortly after birth.…”

“…This crucial importance of the GAP domain of plexins during mouse development is in line with findings in Drosophila and Caenorhabditis elegans, where the GAP domains of PlexA and PLX-1 have been shown to be critical for development of the nervous system (47,48). Owing to the perinatal lethality of Plexin-B2 and Plexin-D1 GAP domain mutant mice, we could not systematically address the functional significance of the GAP domain at postnatal stages of development or in physiological and pathophysiological processes in the adult [e.g., for Plexin-B2 in the postnatal migration and proliferation of neuroblasts (26) and in wound healing (49), or for Plexin-D1 in postnatal development of the nervous system (34,35) and in angiogenesis (36,50)]. Further studies using mice with floxed alleles crossed to the BAC transgenic and specific Cre mice will be required to address these open questions.…”

Gamma and alpha motor neurons distinguished by expression of transcription factor Err3

“…First, in PV Cre Isl2 DTA mice, where all proprioceptive sensory-motor connections are eliminated, we do not detect an increase in the incidence of Err3 on gamma motor neurons within specific motor pools. Second, and more telling, analysis of the gamma/alpha status of neurons in the Cm motor pool of wild-type mice reveals both Err3 on gamma and NeuN on alpha motor neurons, despite the absence of proprioceptive sensory input (26,36). Thus the program of gamma motor neuron differentiation appears sensitive to peripheral muscle-derived factors but not to the vagaries of central sensory connectivity.…”

“…To assess this, we analyzed the Err3/NeuN status of motor neurons innervating the cutaneous maximus (Cm) muscle. The Cm muscle contains muscle spindles supplied by group Ia proprioceptive afferents, yet Cm motor neurons fail to receive direct proprioceptive input (26,36). We therefore analyzed the Err3 on /NeuN off status of the ventral-most ChAT on motor neurons at cervical level c8 in wild-type mice, the location of most Cm motor neuron cell bodies (26,36).…”

Axon Guidance Molecules and Neural Circuit Remodeling After Spinal Cord Injury

“…In the developing spinal cord, various semaphorins dictate the projections of distinct neural populations. Semaphorins prevent inappropriate monosynaptic connections between primary sensory neurons of the peripheral nervous system (PNS) and motor neurons in the spinal cord [30], and control projections at the CNS-PNS interface (Fig. 1) [27].…”