Neurotrophin-3 ameliorates sensory-motor deficits in Er81-deficient mice

Molecular mechanisms underlying monosynaptic sensory-motor circuit development in the spinal cord

Motor behaviors are precisely controlled by the integration of sensory and motor systems in the central nervous system (CNS). Proprioceptive sensory neurons, key components of the sensory system, are located in the dorsal root ganglia and project axons both centrally to the spinal cord and peripherally to muscles and tendons, communicating peripheral information about the body to the CNS. Changes in muscle length detected by muscle spindles, and tension variations in tendons conveyed by Golgi tendon organs, are communicated to the CNS through group Ia /II, and Ib proprioceptive sensory afferents, respectively. Group Ib proprioceptive sensory neurons connect with motor neurons indirectly through spinal interneurons, whereas group Ia/II axons form both direct (monosynaptic) and indirect connections with motor neurons. Although monosynaptic sensory-motor circuits between spindle proprioceptive sensory neurons and motor neurons have been extensively studied since 1950s, the molecular mechanisms underlying their formation and upkeep have only recently begun to be understood. We will discuss our current understanding of the molecular foundation of monosynaptic circuit development and maintenance involving proprioceptive sensory neurons and motor neurons in the mammalian spinal cord. Developmental Dynamics 247:581-587, 2018. © 2017 Wiley Periodicals, Inc.

Assembly and function of spinal circuits for motor control

Control of movement is a fundamental and complex task of the vertebrate nervous system, which relies on communication between circuits distributed throughout the brain and spinal cord. Many of the networks essential for the execution of basic locomotor behaviors are composed of discrete neuronal populations residing within the spinal cord. The organization and connectivity of these circuits is established through programs that generate functionally diverse neuronal subtypes, each contributing to a specific facet of motor output. Significant progress has been made in deciphering how neuronal subtypes are specified and in delineating the guidance and synaptic specificity determinants at the core of motor circuit assembly. Recent studies have shed light on the basic principles linking locomotor circuit connectivity with function, and they are beginning to reveal how more sophisticated motor behaviors are encoded. In this review, we discuss the impact of developmental programs in specifying motor behaviors governed by spinal circuits.

Etv1 inactivation reveals proprioceptor subclasses that reflect the level of NT3 expression in muscle targets

The organization of spinal reflex circuits relies on the specification of distinct classes of proprioceptive sensory neurons (pSN), but the factors that drive such diversity remain unclear. We report here that pSNs supplying distinct skeletal muscles differ in their dependence on the ETS transcription factor Etv1 for their survival and differentiation. The status of Etv1-dependence is linked to the location of proprioceptor muscle targets: pSNs innervating hypaxial and axial muscles depend critically on Etv1 for survival, whereas those innervating certain limb muscles are resistant to Etv1 inactivation. The level of NT3 expression in individual muscles correlates with Etv1-dependence and the loss of pSNs triggered by Etv1 inactivation can be prevented by elevating the level of muscle-derived NT3-revealing a TrkC-activated Etv1-bypass pathway. Our findings support a model in which the specification of aspects of pSN subtype character is controlled by variation in the level of muscle NT3 expression and signaling.

Mechanisms regulating the specificity and strength of muscle afferent inputs in the spinal cord

We investigated factors controlling the development of connections between muscle spindle afferents, spinal motor neurons, and inhibitory Renshaw cells. Several mutants were examined to establish the role of muscle spindles, muscle spindle-derived NT3, and excess NT3 in determining the specificity and strength of these connections. The findings suggest that although spindle-derived factors are not necessary for the initial formation and specificity of the synapses, spindle-derived NT3 seems necessary for strengthening homonymous connections between Ia afferents and motor neurons during the second postnatal week. We also found evidence for functional monosynaptic connections between sensory afferents and neonatal Renshaw cells although the density of these synapses decreases at P15. We conclude that muscle spindle synapses are weakened on Renshaw cells while they are strengthened on motor neurons. Interestingly, the loss of sensory synapses on Renshaw cells was reversed in mice overexpressing NT3 in the periphery, suggesting that different levels of NT3 are required for functional maintenance and strengthening of spindle afferent inputs on motor neurons and Renshaw cells.

Permanent reorganization of Ia afferent synapses on motoneurons after peripheral nerve injuries

After peripheral nerve injuries to a motor nerve, the axons of motoneurons and proprioceptors are disconnected from the periphery and monosynaptic connections from group I afferents and motoneurons become diminished in the spinal cord. Following successful reinnervation in the periphery, motor strength, proprioceptive sensory encoding, and Ia afferent synaptic transmission on motoneurons partially recover. Muscle stretch reflexes, however, never recover and motor behaviors remain uncoordinated. In this review, we summarize recent findings that suggest that lingering motor dysfunction might be in part related to decreased connectivity of Ia afferents centrally. First, sensory afferent synapses retract from lamina IX, causing a permanent relocation of the inputs to more distal locations and significant disconnection from motoneurons. Second, peripheral reconnection between proprioceptive afferents and muscle spindles is imperfect. As a result, a proportion of sensory afferents that retain central connections with motoneurons might not reconnect appropriately in the periphery. A hypothetical model is proposed in which the combined effect of peripheral and central reconnection deficits might explain the failure of muscle stretch to initiate or modulate firing of many homonymous motoneurons.

Role of neurotrophin signalling in the differentiation of neurons from dorsal root ganglia and sympathetic ganglia

Manipulation of neurotrophin (NT) signalling by administration or depletion of NTs, by transgenic overexpression or by deletion of genes coding for NTs and their receptors has demonstrated the importance of NT signalling for the survival and differentiation of neurons in sympathetic and dorsal root ganglia (DRG). Combination with mutation of the proapoptotic Bax gene allows the separation of survival and differentiation effects. These studies together with cell culture analysis suggest that NT signalling directly regulates the differentiation of neuron subpopulations and their integration into neural networks. The high-affinity NT receptors trkA, trkB and trkC are restricted to subpopulations of mature neurons, whereas their expression at early developmental stages largely overlaps. trkC is expressed throughout sympathetic ganglia and DRG early after ganglion formation but becomes restricted to small neuron subpopulations during embryogenesis when trkA is turned on. The temporal relationship between trkA and trkC expression is conserved between sympathetic ganglia and DRG. In DRG, NGF signalling is required not only for survival, but also for the differentiation of nociceptors. Expression of neuropeptides calcitonin gene-related peptide and substance P, which specify peptidergic nociceptors, depends on nerve growth factor (NGF) signalling. ret expression indicative of non-peptidergic nociceptors is also promoted by the NGF-signalling pathway. Regulation of TRP channels by NGF signalling might specify the temperature sensitivity of afferent neurons embryonically. The manipulation of NGF levels "tunes" heat sensitivity in nociceptors at postnatal and adult stages. Brain-derived neurotrophic factor signalling is required for subpopulations of DRG neurons that are not fully characterized; it affects mechanical sensitivity in slowly adapting, low-threshold mechanoreceptors and might involve the regulation of DEG/ENaC ion channels. NT3 signalling is required for the generation and survival of various DRG neuron classes, in particular proprioceptors. Its importance for peripheral projections and central connectivity of proprioceptors demonstrates the significance of NT signalling for integrating responsive neurons in neural networks. The molecular targets of NT3 signalling in proprioceptor differentiation remain to be characterized. In sympathetic ganglia, NGF signalling regulates dendritic development and axonal projections. Its role in the specification of other neuronal properties is less well analysed. In vitro analysis suggests the involvement of NT signalling in the choice between the noradrenergic and cholinergic transmitter phenotype, in the expression of various classes of ion channels and for target connectivity. In vivo analysis is required to show the degree to which NT signalling regulates these sympathetic neuron properties in developing embryos and postnatally.

Transcriptional networks in the early development of sensory-motor circuits

The emergence of coordinated locomotor behaviors in vertebrates relies on the establishment of selective connections between discrete populations of neurons present in the spinal cord and peripheral nervous system. The assembly of the circuits necessary for movement presumably requires the generation of many unique cell types to accommodate the intricate connections between motor neurons, sensory neurons, interneurons, and muscle. The specification of diverse neuronal subtypes is mediated largely through networks of transcription factors that operate within progenitor and postmitotic cells. Selective patterns of transcription factor expression appear to define the cell-type-specific cellular programs that govern the axonal guidance decisions and synaptic specificities of neurons, and may lay the foundation through which innate motor behaviors are genetically predetermined. Recent studies on the developmental programs that specify two highly diverse neuronal classes-spinal motor neurons and proprioceptive sensory neurons-have provided important insights into the molecular strategies used in the earliest phases of locomotor circuit assembly. This chapter reviews progress toward elucidating the early transcriptional networks that define neuronal identity in the locomotor system, focusing on the pathways controlling the specific connections of motor neurons and sensory neurons in the formation of simple reflex circuits.

Onset of ETS expression is not accelerated by premature exposure to signals from limb mesenchyme

The ETS transcription factors ER81 and PEA3 are expressed in discrete populations of sensory and motor neurons and regulate late events in neuronal development and limb innervation. Although initiation of ETS expression requires limb-derived signals, we show here that precocious axon growth into transplanted older donor limbs, which prematurely exposes neurons to limb-derived signals, does not accelerate the onset of expression of Er81 or Pea3. Similarly, neither MN-cadherin, which is reportedly regulated by ER81, nor T-cadherin is expressed precociously in neurons innervating older donor limbs. Thus, neurons must attain a particular level of differentiation to respond to inducing signals from limb. We also show that signals emanating from limb mesenchyme are sufficient to initiate Er81 and Pea3 expression in sensory and motor neurons in the absence of myogenic cells in Sp(d) mutant mice and that induction of ETS expression is unlikely to directly involve retinoid signaling from limb mesenchyme.

Prenatal exposure to elevated NT3 disrupts synaptic selectivity in the spinal cord

Monosynaptic connections between muscle spindle (Ia) afferents and motoneurons (MNs), the central portion of the stretch reflex circuit, are highly specific, but the mechanisms underlying this specificity are primarily unknown. In this study, we report that embryonic overexpression of neurotrophin-3 (NT3) in muscles disrupts the development of these specific Ia-MN connections, using transgenic (mlc/NT3) mice that express elevated levels of NT3 in muscles during development. In mlc/NT3 mice, there is a substantial increase in the amplitudes of monosynaptic EPSPs evoked by Ia afferents in MNs as measured with extracellular recordings from ventral roots. Despite this increased functional projection of Ia afferents, there is no obvious change in the anatomical density of Ia projections into the ventral horn of the spinal cord. Intracellular recordings from MNs revealed a major disruption in the pattern of Ia-MN connections. In addition to the normal connections between Ia afferents and MNs supplying the same muscle, there were also strong monosynaptic inputs from Ia afferents supplying unrelated muscles, which explains the increase seen in extracellular recordings. There was also a large variability in the strength of Ia input to individual MNs, both from correct and incorrect Ia afferents. Postnatal muscular administration of NT3 did not cause these changes in connectivity. These results indicate that prenatal exposure to elevated levels of NT3 disrupts the normal mechanisms responsible for synaptic selectivity in the stretch reflex circuit.