SummaryHuman monogenic pain syndromes have provided important insights into the molecular mechanisms that underlie normal and pathological pain states. We describe an autosomal-dominant familial episodic pain syndrome characterized by episodes of debilitating upper body pain, triggered by fasting and physical stress. Linkage and haplotype analysis mapped this phenotype to a 25 cM region on chromosome 8q12–8q13. Candidate gene sequencing identified a point mutation (N855S) in the S4 transmembrane segment of TRPA1, a key sensor for environmental irritants. The mutant channel showed a normal pharmacological profile but altered biophysical properties, with a 5-fold increase in inward current on activation at normal resting potentials. Quantitative sensory testing demonstrated normal baseline sensory thresholds but an enhanced secondary hyperalgesia to punctate stimuli on treatment with mustard oil. TRPA1 antagonists inhibit the mutant channel, promising a useful therapy for this disorder. Our findings provide evidence that variation in the TRPA1 gene can alter pain perception in humans.Video Abstract

The protease β-secretase 1 (Bace1) was identified through its critical role in production of amyloid-β peptides (Aβ), the major component of amyloid plaques in Alzheimer's disease. Bace1 is considered a promising target for the treatment of this pathology, but processes additional substrates, among them Neuregulin-1 (Nrg1). Our biochemical analysis indicates that Bace1 processes the Ig-containing β1 Nrg1 (IgNrg1β1) isoform. We find that a graded reduction in IgNrg1 signal strength in vivo results in increasingly severe deficits in formation and maturation of muscle spindles, a proprioceptive organ critical for muscle coordination. Further, we show that Bace1 is required for formation and maturation of the muscle spindle. Finally, pharmacological inhibition and conditional mutagenesis in adult animals demonstrate that Bace1 and Nrg1 are essential to sustain muscle spindles and to maintain motor coordination. Our results assign to Bace1 a role in the control of coordinated movement through its regulation of muscle spindle physiology, and implicate IgNrg1-dependent processing as a molecular mechanism.

Neuregulin-1 (NRG1) plays a crucial role in axoglial signaling during the development of the peripheral nervous system, but its importance in adulthood after peripheral nerve injury remains unclear. We used single-neuron labeling with inducible Cre-mediated knock-out animals, which enabled visualization of a subset of adult myelinated sensory and motoneurons neurons in which Nrg1 was inducibly mutated by tamoxifen treatment. In uninjured mice, NRG1-deficient axons and the associated myelin sheath were normal, and the neuromuscular junction demonstrated normal apposition of presynaptic and postsynaptic components. After sciatic nerve crush, NRG1 ablation resulted in severe defects in remyelination: axons were either hypomyelinated or had no myelin sheath. NRG1-deficient axons were also found to regenerate at a slower rate. After nerve injury, the neuromuscular junction was reinnervated, but excess terminal sprouting was observed. Juxtacrine Neuregulin-1 signaling is therefore dispensable for maintenance of the myelin sheath in adult animals but has a key role in reparative processes after nerve injury.

Axons and Schwann cells exist in a highly interdependent relationship: damage to one cell type invariably leads to pathophysiological changes in the other. Greater understanding of communication between these cell types will not only give insight into peripheral nerve development, but also the reaction to and recovery from peripheral nerve injury. The type III isoform of neuregulin-1 (NRG1) has emerged as a key signaling factor that is expressed on axons and, through binding to erbB2/3 receptors on Schwann cells, regulates multiple phases of their development. In adulthood, NRG1 is dispensable for the maintenance of the myelin sheath; however, this factor is required for both axon regeneration and remyelination following nerve injury. The outcome of NRG1 signaling depends on interactions with other pathways within Schwann cells such as Notch, integrin and cAMP signaling. In certain circumstances, this signaling pathway may be maladaptive; for instance, direct binding of Mycobacterium leprae onto erbB2 receptors produces excessive activation and can actually promote demyelination. Attempts to modulate this pathway in order to promote nerve repair will therefore need to give consideration to the exact isoform used, as well as how it is processed and the context in which it is presented to the Schwann cell. Keywordsneuregulin-1; neuropathy; peripheral nerve injury; regeneration; remyelination; repair; Schwann cell Peripheral nerve injury causes significant morbidity and reduced quality of life as a consequence of weakness, sensory loss and neuropathic pain. In total, 6% of the elderly population suffer from peripheral neuropathy [1], which can be caused by metabolic diseases such as diabetes, inherited genetic disorders such as Charcot-Marie-Tooth disease, infectious and inflammatory disorders including Guillan-Barré syndrome and traumatic injury to the nerve (which alone effects up to 300,000 people in Europe per year [1,2]). In contrast to the CNS, peripheral nerves have a significant regenerative capacity [3]; however, in patients, regeneration and the clinical outcome are often poor. Axons regenerate at a rate of approximately 1 mm per day, depending on the site of the lesion. There may be a delay of months if not years before the target zone is re-innervated, and by this time, the target organ and supporting cells may have irreversibly changed, becoming much less receptive to reinnervation. The current approaches to peripheral nerve repair following nerve transection are either to surgically suture the nerve or to bridge the gap between the two cut ends. Financial & competing interests disclosureThe authors have no other rel evant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript. Europe PMC Funders Group NRGs & the PNSNRGs Neuregulins are a family of growth factors en...

Neuregulin 1 acts as an axonal signal that regulates multiple aspects of Schwann cell development including the survival and migration of Schwann cell precursors, the ensheathment of axons and subsequent elaboration of the myelin sheath. To examine the role of this factor in remyelination and repair following nerve injury, we ablated neuregulin 1 in the adult nervous system using a tamoxifen inducible Cre recombinase transgenic mouse system. The loss of neuregulin 1 impaired remyelination after nerve crush, but did not affect Schwann cell proliferation associated with Wallerian degeneration or axon regeneration or the clearance of myelin debris by macrophages. Myelination changes were most marked at 10 days after injury but still apparent at 2 months post-crush. Transcriptional analysis demonstrated reduced expression of myelin-related genes during nerve repair in animals lacking neuregulin 1. We also studied repair over a prolonged time course in a more severe injury model, sciatic nerve transection and reanastamosis. In the neuregulin 1 mutant mice, remyelination was again impaired 2 months after nerve transection and reanastamosis. However, by 3 months post-injury axons lacking neuregulin 1 were effectively remyelinated and virtually indistinguishable from control. Neuregulin 1 signalling is therefore an important factor in nerve repair regulating the rate of remyelination and functional recovery at early phases following injury. In contrast to development, however, the determination of myelination fate following nerve injury is not dependent on axonal neuregulin 1 expression. In the early phase following injury, axonal neuregulin 1 therefore promotes nerve repair, but at late stages other signalling pathways appear to compensate.

Following traumatic spinal cord injury, acute demyelination of spinal axons is followed by a period of spontaneous remyelination. However, this endogenous repair response is suboptimal and may account for the persistently compromised function of surviving axons. Spontaneous remyelination is largely mediated by Schwann cells, where demyelinated central axons, particularly in the dorsal columns, become associated with peripheral myelin. The molecular control, functional role and origin of these central remyelinating Schwann cells is currently unknown. The growth factor neuregulin-1 (Nrg1, encoded by NRG1) is a key signalling factor controlling myelination in the peripheral nervous system, via signalling through ErbB tyrosine kinase receptors. Here we examined whether Nrg1 is required for Schwann cell-mediated remyelination of central dorsal column axons and whether Nrg1 ablation influences the degree of spontaneous remyelination and functional recovery following spinal cord injury. In contused adult mice with conditional ablation of Nrg1, we found an absence of Schwann cells within the spinal cord and profound demyelination of dorsal column axons. There was no compensatory increase in oligodendrocyte remyelination. Removal of peripheral input to the spinal cord and proliferation studies demonstrated that the majority of remyelinating Schwann cells originated within the injured spinal cord. We also examined the role of specific Nrg1 isoforms, using mutant mice in which only the immunoglobulin-containing isoforms of Nrg1 (types I and II) were conditionally ablated, leaving the type III Nrg1 intact. We found that the immunoglobulin Nrg1 isoforms were dispensable for Schwann cell-mediated remyelination of central axons after spinal cord injury. When functional effects were examined, both global Nrg1 and immunoglobulin-specific Nrg1 mutants demonstrated reduced spontaneous locomotor recovery compared to injured controls, although global Nrg1 mutants were more impaired in tests requiring co-ordination, balance and proprioception. Furthermore, electrophysiological assessments revealed severely impaired axonal conduction in the dorsal columns of global Nrg1 mutants (where Schwann cell-mediated remyelination is prevented), but not immunoglobulin-specific mutants (where Schwann cell-mediated remyelination remains intact), providing robust evidence that the profound demyelinating phenotype observed in the dorsal columns of Nrg1 mutant mice is related to conduction failure. Our data provide novel mechanistic insight into endogenous regenerative processes after spinal cord injury, demonstrating that Nrg1 signalling regulates central axon remyelination and functional repair and drives the trans-differentiation of central precursor cells into peripheral nervous system-like Schwann cells that remyelinate spinal axons after injury. Manipulation of the Nrg1 system could therefore be exploited to enhance spontaneous repair after spinal cord injury and other central nervous system disorders with a demyelinating pathology.media-1vid110...

Neuregulin-1 has a key role in mediating signaling between axons and Schwann cells during development. A limitation to studying its role in adulthood is the embryonic lethality of global Nrg1 gene deletion. We used the Cre-loxP system to generate transgenic mice in which neuregulin-1 is conditionally ablated in the majority of small-diameter and a proportion of large-diameter sensory neurons that have axons conducting in the C-and A␦-fiber range, respectively. Sensory neuron-specific neuregulin-1 ablation resulted in abnormally large Remak bundles with axons clustered in "polyaxonal" pockets. The total number of axons in the sural nerve was unchanged, but a greater proportion was unmyelinated. In addition, we observed large-diameter axons that were in a 1:1 relationship with Schwann cells, surrounded by a basal lamina but not myelinated. There was no evidence of DRG or Schwann cell death; the markers of different DRG cell populations and cutaneous innervation were unchanged. These anatomical changes were reflected in a slowing of conduction velocity at the lower end of the A-fiber conduction velocity range and a new population of more rapidly conducting C-fibers that are likely to represent large-diameter axons that have failed to myelinate. Conditional neuregulin-1 ablation resulted in a reduced sensitivity to noxious mechanical stimuli. These findings emphasize the importance of neuregulin-1 in mediating the signaling between axons and both myelinating and nonmyelinating Schwann cells required for normal sensory function. Sensory neuronal survival and axonal maintenance, however, are not dependent on axon-derived neuregulin-1 signaling in adulthood.

RalA and RalB are small GTPases that are involved in cell migration and membrane dynamics. We used transgenic mice in which one or both GTPases were genetically ablated to investigate the role of RalGTPases in the Schwann cell (SC) response to nerve injury and repair. RalGTPases were dispensable for SC function in the naive uninjured state. Ablation of both RalA and RalB (but not individually) in SCs resulted in impaired axon remyelination and target reinnervation following nerve injury, which resulted in slowed recovery of motor function. Ral GTPases were localized to the leading lamellipodia in SCs and were required for the formation and extension of both axial and radial processes of SCs. These effects were dependent on interaction with the exocyst complex and impacted on the rate of SC migration and myelination. Our results show that RalGTPases are required for efficient nerve repair by regulating SC process formation, migration, and myelination, therefore uncovering a novel role for these GTPases.