Nerve growth factor R221W responsible for insensitivity to pain is defectively processed and accumulates as proNGF

Sex-dependent effects of the targeted NGF mutation (R100E) on pain behavior, joint inflammation, and bone erosion in mice

ABSTRACT

Nerve growth factor (NGF)-R100E is a mutated form of human recombinant NGF that reduces the binding of NGF to its p75NTR receptor while retaining its affinity toward the TrkA receptor. Here, we used human wild type NGF and NGF-R100E knock-in mice to investigate the effects of this NGF mutation on inflammation-induced pain-related behaviors and bone loss. The hNGF-R100E mutation did not alter the nerve fiber density in the sciatic nerve, ankle joint synovium, and skin of naïve mice. Withdrawal responses to mechanical, thermal, and cold stimuli before and after joint inflammation induced by intra-articular injection of complete Freund adjuvant (CFA) were similar between human recombinant nerve growth factor-wild type and hNGF-R100E male and female mice while weight bearing and gait analysis revealed significant differences. Intriguingly, hNGF-R100E male and female mice showed only mild changes, indicating lower degrees of deep joint-related pain compared to their wild type counterparts. Furthermore, micro-CT analysis demonstrated that hNGF-R100E female mice, but not males, were protected from CFA-induced bone loss, and mRNA analysis showed a different gene regulation indicating a sex-dependent relationship between NGF, inflammation, and bone loss. In conclusion, our study reveals that the hNGF-R100E mutation renders mice insensitive to inflammation-induced impact on joint loading and gait while preserving the development of the peripheral nociceptive neurons and sensitivity to punctate stimulation of the skin. Notably, the mutation uncovers a sex-dependent relationship between NGF and inflammation-induced bone loss. These findings offer valuable insights into NGF as a target for pain management and the interplay between NGF and bone architecture.

The NGF R100W Mutation, Associated with Hereditary Sensory Autonomic Neuropathy Type V, Specifically Affects the Binding Energetic Landscapes of NGF and of Its Precursor proNGF and p75NTR

Nerve Growth Factor (NGF), the prototype of the neurotrophin family, stimulates morphological differentiation and regulates neuronal gene expression by binding to TrkA and p75NTR receptors. It plays a critical role in maintaining the function and phenotype of peripheral sensory and sympathetic neurons and in mediating pain transmission and perception during adulthood. A point mutation in the NGFB gene (leading to the amino acid substitution R100W) is responsible for Hereditary Sensory and Autonomic Neuropathy type V (HSAN V), leading to a congenital pain insensitivity with no clear cognitive impairments, but with alterations in the NGF/proNGF balance. The available crystal structures of the p75NTR/NGF and 2p75NTR/proNGF complexes offer a starting point for Molecular Dynamics (MD) simulations in order to capture the impact of the R100W mutation on their binding energetic landscapes and to unveil the molecular determinants that trigger their different physiological and pathological outcomes. The present in silico studies highlight that the stability and the binding energetic fingerprints in the 2p75NTR/proNGF complex is not affected by R100W mutation, which on the contrary, deeply affects the energetic landscape, and thus the stability in the p75NTR/NGF complex. Overall, these findings present insights into the structural basis of the molecular mechanisms beyond the clinical manifestations of HSAN V patients.

Lipid Nanoparticle Delivery of Chemically Modified NGFR100W mRNA Alleviates Peripheral Neuropathy

Messenger RNA (mRNA) carries genetic instructions to the cell machinery for the transient production of antigens or therapeutic proteins and shows enormous potential in vaccine development, cancer immunotherapy, protein replacement therapy, and genome engineering. Here, the synthesis of chemically modified nerve growth factor mutant (NGF^R100W ) mRNA through in vitro transcription is described. After the replacement of the original signal peptide sequence with the Ig Kappa leader sequence, codon-optimized NGF^R100W mRNA yielded high secretion of mature NGF^R100W , which promotes axon growth in PC12 cells. Using lipid nanoparticle (LNP)-delivery of N1-methylpseudouridine-modified mRNA in mice, NGF^R100W -mRNA-LNPs result in the successful expression of NGF^R100W protein, which significantly reduces nociceptive activity compared to that of NGF^WT . This indicates that NGF^R100W derived from exogenous mRNA elicited "painless" neuroprotective activity. Additionally, the therapeutic value of NGF^R100W mRNA is established in a paclitaxel-induced peripheral neuropathy model by demonstrating the rapid recovery of intraepidermal nerve fibers. The results show that in vitro-transcribed mRNA has significant flexibility in sequence design and fast in vivo functional validation of target proteins. Furthermore, the results highlight the therapeutic potential of mRNA as a supplement to beneficial proteins for preventing or reversing some chronic medical conditions, such as peripheral neuropathy.

Understanding pain perception through genetic painlessness diseases: The role of NGF and proNGF

Nerve growth factor (NGF), by binding to TrkA and p75^NTR receptors, regulates the survival and differentiation of sensory neurons during development and mediates pain transmission and perception during adulthood, by acting at different levels of the nervous system. Key to understanding the role of NGF as a pain mediator is the finding that mutations (namely, R121W, V232fs and R221W) in the NGF gene cause painlessness disease Hereditary Sensory and Autonomic Neuropathy type V (HSAN V). Here we shall review the consequences of these NGF mutations, each of which results in specific clinical signs: R221W determines congenital pain insensitivity with no overt cognitive disabilities, whereas V232fs and R121W also result in intellectual disability, thus showing similarities to HSAN IV, which is caused by mutations in TrkA, rather than to HSAN V. Comparing the cellular, biochemical and clinical findings of these mutations could help in better understanding not only the possible mechanisms underlying HSAN V, but also mechanisms of NGF signalling and roles. These mutations alter the balance between NGF and proNGF in favour of an accumulation of the latter, suggesting a possible role of proNGF as a molecule with an analgesic role. Furthermore, the neurotrophic and pronociceptive functions of NGF are split by the R221W mutation, making NGF variants based on this mutation interesting for designing therapeutic applications for many diseases. This review emphasizes the possibility of using the mutations involved in "painlessness" clinical disorders as an innovative approach to identify new proteins and pathways involved in pain transmission and perception. OUTSTANDING QUESTIONS: Why do homozygous HSAN V die postnatally? What is the cause of this early postnatal lethality? Is the development of a mouse or a human feeling less pain affecting higher cognitive and perceptual functions? What is the consequence of the HSAN V mutation on the development of joints and bones? Are the multiple fractures observed in HSAN V patients due exclusively to the carelessness consequent to not feeling pain, or also to an intrinsic frailty of their bones? Are heterodimers of NGF^WT and NGF^R221W in the heterozygote state formed? And if so, what are the properties of these heterodimeric proteins? How is the processing of proNGF^R221W to NGF^R221W affected by the mutation?

The development of somatosensory neurons: Insights into pain and itch

Primary nociceptors are a heterogeneous class of peripheral somatosensory neurons, responsible for detecting noxious, pruriceptive, and thermal stimuli. These neurons are further divided into several molecularly defined subtypes that correlate with their functional sensory modalities and morphological features. During development, all nociceptors arise from a common pool of embryonic precursors, and then segregate progressively into their mature specialized phenotypes. In this review, we summarize the intrinsic transcriptional programs and extrinsic trophic factor signaling mechanisms that interact to control nociceptor diversification. We also discuss how recent transcriptome profiling studies have significantly advanced the field of sensory neuron development.

A missense point mutation in nerve growth factor (NGFR100W) results in selective peripheral sensory neuropathy

The R100W mutation in nerve growth factor is associated with hereditary sensory autonomic neuropathy V in a Swedish family. These patients develop severe loss of perception to deep pain but with apparently normal cognitive functions. To better understand the disease mechanism, we examined a knockin mouse model of HSAN V. The homozygous mice showed significant structural deficits in intra-epidermal nerve fibers (IENFs) at birth. These mice had a total loss of pain perception at ∼2 months of age and often failed to survive to adulthood. Heterozygous mutant mice developed a progressive degeneration of small sensory fibers both behaviorally and functionally: they showed a progressive loss of IENFs starting at the age of 9 months accompanied with progressive loss of perception to painful stimuli such as noxious temperature. Quantitative analysis of lumbar 4/5 dorsal root ganglia revealed a significant reduction in small size neurons, while analysis of sciatic nerve fibers revealed the heterozygous mutant mice had no reduction in myelinated nerve fibers. Significantly, the amount of NGF secreted from mouse embryonic fibroblasts were reduced from both heterozygous and homozygous mice compared to their wild-type littermates. Interestingly, the heterozygous mice showed no apparent structural alteration in the brain: neither the anterior cingulate cortex nor the medial septum including NGF-dependent basal forebrain cholinergic neurons. Accordingly, these animals did not develop appreciable deficits in tests for brain function. Our study has thus demonstrated that the NGFR100W mutation likely affects the structure and function of peripheral sensory neurons.

Nerve growth factor-mediated photoablation of nociceptors reduces pain behavior in mice

Supplemental Digital Content is Available in the Text.

A ligand-guided, light-activated photosensitizer tool targets TrkA-expressing nociceptors, reversing acute and chronic pain in mice.

Nerve growth factor (NGF) and its receptors TrkA and p75 play a key role in the development and function of peripheral nociceptive neurons. Here, we describe novel technology to selectively photoablate TrkA-positive nociceptors through delivery of a phototoxic agent coupled to an engineered NGF ligand and subsequent near-infrared illumination. We demonstrate that this approach allows for on demand and localized reversal of pain behaviors in mouse models of acute, inflammatory, neuropathic, and joint pain. To target peripheral nociceptors, we generated a SNAP-tagged NGF derivative NGF^R121W that binds to TrkA/p75 receptors but does not provoke signaling in TrkA-positive cells or elicit pain behaviors in mice. NGF^R121W-SNAP was coupled to the photosensitizer IRDye700DX phthalocyanine (IR700) and injected subcutaneously. After near-infrared illumination of the injected area, behavioral responses to nociceptive mechanical and sustained thermal stimuli, but not innocuous stimuli, were substantially reduced. Similarly, in models of inflammatory, osteoarthritic, and neuropathic pain, mechanical hypersensitivity was abolished for 3 weeks after a single treatment regime. We demonstrate that this loss of pain behavior coincides with the retraction of neurons from the skin which then reinnervate the epidermis after 3 weeks corresponding with the return of mechanical hypersensitivity. Thus NGF^R121W-SNAP-mediated photoablation is a minimally invasive approach to reversibly silence nociceptor input from the periphery, and control pain and hypersensitivity to mechanical stimuli.

Unveiling functional motions based on point mutations in biased signaling systems: A normal mode study on nerve growth factor bound to TrkA

Many receptors elicit signal transduction by activating multiple intracellular pathways. This transduction can be triggered by a non-specific ligand, which simultaneously activates all the signaling pathways of the receptors. However, the binding of one biased ligand preferentially trigger one pathway over another, in a process called biased signaling. The identification the functional motions related to each of these distinct pathways has a direct impact on the development of new effective and specific drugs. We show here how to detect specific functional motions by considering the case of the NGF/TrkA-Ig2 complex. NGF-mediated TrkA receptor activation is dependent on specific structural motions that trigger the neuronal growth, development, and survival of neurons in nervous system. The R221W mutation in the ngf gene impairs nociceptive signaling. We discuss how the large-scale structural effects of this mutation lead to the suppression of collective motions necessary to induce TrkA activation of nociceptive signaling. Our results suggest that subtle changes in the NGF interaction network due to the point mutation are sufficient to inhibit the motions of TrkA receptors putatively linked to nociception. The methodological approach presented in this article, based jointly on the normal mode analysis and the experimentally observed functional alterations due to point mutations provides an essential tool to reveal the structural changes and motions linked to the disease, which in turn could be necessary for a drug design study.

Nerve Growth Factor Signaling and Its Contribution to Pain

Nerve growth factor (NGF) is a neurotrophic protein essential for the growth, differentiation, and survival of sympathetic and sensory afferent neurons during development. A substantial body of evidence, based on both animal and human studies, demonstrates that NGF plays a pivotal role in modulation of nociception in adulthood. This has spurred development of a variety of novel analgesics that target the NGF signaling pathway. Here, we present a narrative review designed to summarize how NGF receptor activation and downstream signaling alters nociception through direct sensitization of nociceptors at the site of injury and changes in gene expression in the dorsal root ganglion that collectively increase nociceptive signaling from the periphery to the central nervous system. This review illustrates that NGF has a well-known and multifunctional role in nociceptive processing, although the precise signaling pathways downstream of NGF receptor activation that mediate nociception are complex and not completely understood. Additionally, much of the existing knowledge derives from studies performed in animal models and may not accurately represent the human condition. However, available data establish a role for NGF in the modulation of nociception through effects on the release of inflammatory mediators, nociceptive ion channel/receptor activity, nociceptive gene expression, and local neuronal sprouting. The role of NGF in nociception and the generation and/or maintenance of chronic pain has led to it becoming a novel and attractive target of pain therapeutics for the treatment of chronic pain conditions.

Mutation Carriers with Reduced C-Afferent Density Reveal Cortical Dynamics of Pain-Action Relationship during Acute Pain

The evidence that action shapes perception has become widely accepted, for example, in the domain of vision. However, the manner in which action-relevant factors might influence the neural dynamics of acute pain processing has remained underexplored, particularly the functional roles of anterior insula (AI) and midanterior cingulate cortex (mid-ACC), which are frequently implicated in acute pain. To address this, we examined a unique group of heterozygous carriers of the rare R221W mutation on the nerve growth factor (NGF) gene. R221W carriers show a congenitally reduced density of C-nociceptor afferent nerves in the periphery, but can nonetheless distinguish between painful and nonpainful stimulations. Despite this, carriers display a tendency to underreact to acute pain behaviorally, thus exposing a potential functional gap in the pain–action relationship and allowing closer investigation of how the brain integrates pain and action information. Heterozygous R221W carriers and matched controls performed a functional magnetic resonance imaging (fMRI) task designed to dissociate stimulus type (painful or innocuous) from current behavioral relevance (relevant or irrelevant), by instructing participants to either press or refrain from pressing a button during thermal stimulation. Carriers’ subjective pain thresholds did not differ from controls’, but the carrier group showed decreased task accuracy. Hemodynamic activation in AI covaried with task performance, revealing a functional role in pain–action integration with increased responses for task-relevant painful stimulation (“signal,” requiring button-press execution) over task-irrelevant stimulation (“noise,” requiring button-press suppression). As predicted, mid-ACC activation was associated with action execution regardless of pain. Functional connectivity between AI and mid-ACC increased as a function of reported urge to withdraw from the stimulus, suggesting a joint role for these regions in motivated action during pain. The carrier group showed greater activation of primary sensorimotor cortices—but not the AI and mid-ACC regions—during pain and action, suggesting compensatory processing. These findings indicate a critical role for the AI–mid-ACC axis in supporting a flexible, adaptive action selection during pain, alongside the accompanying subjective experience of an urge to escape the pain.

Fracture repair requires TrkA signaling by skeletal sensory nerves

Bone is richly innervated by nerve growth factor-responsive (NGF-responsive) tropomyosin receptor kinase A-expressing (TrKa-expressing) sensory nerve fibers, which are required for osteochondral progenitor expansion during mammalian skeletal development. Aside from pain sensation, little is known regarding the role of sensory innervation in bone repair. Here, we characterized the reinnervation of tissue following experimental ulnar stress fracture and assessed the impact of loss of TrkA signaling in this process. Sequential histological data obtained in reporter mice subjected to fracture demonstrated a marked upregulation of NGF expression in periosteal stromal progenitors and fracture-associated macrophages. Sprouting and arborization of CGRP+TrkA+ sensory nerve fibers within the reactive periosteum in NGF-enriched cellular domains were evident at time points preceding periosteal vascularization, ossification, and mineralization. Temporal inhibition of TrkA catalytic activity by administration of 1NMPP1 to TrkAF592A mice significantly reduced the numbers of sensory fibers, blunted revascularization, and delayed ossification of the fracture callus. We observed similar deficiencies in nerve regrowth and fracture healing in a mouse model of peripheral neuropathy induced by paclitaxel treatment. Together, our studies demonstrate an essential role of TrkA signaling for stress fracture repair and implicate skeletal sensory nerves as an important upstream mediator of this repair process.

The NGFR100W Mutation Specifically Impairs Nociception without Affecting Cognitive Performance in a Mouse Model of Hereditary Sensory and Autonomic Neuropathy Type V

Nerve growth factor (NGF) is a key mediator of nociception, acting during the development and differentiation of dorsal root ganglion (DRG) neurons, and on adult DRG neuron sensitization to painful stimuli. NGF also has central actions in the brain, where it regulates the phenotypic maintenance of cholinergic neurons. The physiological function of NGF as a pain mediator is altered in patients with Hereditary Sensory and Autonomic Neuropathy type V (HSAN V), caused by the 661C>T transition in the Ngf gene, resulting in the R100W missense mutation in mature NGF. Homozygous HSAN V patients present with congenital pain insensitivity, but are cognitively normal. This led us to hypothesize that the R100W mutation may differentially affect the central and peripheral actions of NGF. To test this hypothesis and provide a mechanistic basis to the HSAN V phenotype, we generated transgenic mice harboring the human 661C>T mutation in the Ngf gene and studied both males and females. We demonstrate that heterozygous NGF^R100W/wt mice display impaired nociception. DRG neurons of NGF^R100W/wt mice are morphologically normal, with no alteration in the different DRG subpopulations, whereas skin innervation is reduced. The NGF^R100W protein has reduced capability to activate pain-specific signaling, paralleling its reduced ability to induce mechanical allodynia. Surprisingly, however, NGF^R100W/wt mice, unlike heterozygous mNGF^+/- mice, show no learning or memory deficits, despite a reduction in secretion and brain levels of NGF. The results exclude haploinsufficiency of NGF as a mechanistic cause for heterozygous HSAN V mice and demonstrate a specific effect of the R100W mutation on nociception.SIGNIFICANCE STATEMENT The R100W mutation in nerve growth factor (NGF) causes Hereditary Sensory and Autonomic Neuropathy type V, a rare disease characterized by impaired nociception, even in apparently clinically silent heterozygotes. For the first time, we generated and characterized heterozygous knock-in mice carrying the human R100W-mutated allele (NGF^R100W/wt). Mutant mice have normal nociceptor populations, which, however, display decreased activation of pain transduction pathways. NGF^R100W interferes with peripheral and central NGF bioavailability, but this does not impact on CNS function, as demonstrated by normal learning and memory, in contrast with heterozygous NGF knock-out mice. Thus, a point mutation allows neurotrophic and pronociceptive functions of NGF to be split, with interesting implications for the treatment of chronic pain.

Identification of amino acid residues of nerve growth factor important for neurite outgrowth in human dorsal root ganglion neurons

Nerve growth factor (NGF) is an essential neurotrophic factor for the development and maintenance of the central and the peripheral nervous system. NGF deficiency in the basal forebrain precedes degeneration of basal forebrain cholinergic neurons in Alzheimer's disease, contributing to memory decline. NGF mediates neurotrophic support via its high‐affinity receptor, the tropomyosin‐related kinase A (TrkA) receptor, and mediates mitogenic and differentiation signals via the extracellular signal‐regulated protein kinases 1 and 2 (ERK1/2). However, the molecular mechanisms underlying the different NGF/TrkA/ERK signalling pathways are far from clear. In this study, we have investigated the role of human NGF and three NGF mutants, R100E, W99A and K95A/Q96A, their ability to activate TrkA or ERK1/2, and their ability to induce proliferation or differentiation in human foetal dorsal root ganglion (DRG) neurons or in PC12 cells. We show that the R100E mutant was significantly more potent than NGF itself to induce proliferation and differentiation, and significantly more potent in activation of ERK1/2 in DRG neurons. The W99A and K95A/Q96A mutants, on the other hand, were less effective than the wild‐type protein. An unexpected finding was the high efficacy of the K95A/Q96A mutant to activate TrkA and to induce differentiation of DRG neurons at elevated concentrations. These data demonstrate an NGF mutant with improved neurotrophic properties in primary human neuronal cells. The R100E mutant represents an interesting candidate for further drug development in Alzheimer's disease and other neurodegenerative disorders.

Targeted Mutation (R100W) of the Gene Encoding NGF Leads to Deficits in the Peripheral Sensory Nervous System

Nerve growth factor (NGF) exerts multifaceted functions through different stages of life. A missense mutation (R100W) in the beta-NGF gene was found in hereditary sensory autonomic neuropathy V (HSAN V) patients with severe loss of pain perception but without overt cognitive impairment. To better understand the pathogenesis of HSAN V, we generated the first NGFR100W knock in mouse model for HSAN V. We found that the homozygotes exhibited a postnatal lethal phenotype. A majority of homozygous pups died within the first week. Some homozygous pups could ingest more milk and survived up to 2 months by reducing litter size. Whole mount in situ hybridization using E10.5 embryos revealed that, compared to wild type, R100W mutation did not alter the gene expression patterns of TrkA and P75NTR in the homozygotes. We also found that the homozygotes displayed normal embryonic development of major organs (heart, lung, liver, kidney, and spleen). Furthermore, the homozygotes exhibited severe loss of PGP9.5-positive intra-epidermal sensory fibers. Taken together, our results suggest that, as with HSAN V patients, the R100W mutation primarily affects the peripheral sensory nervous system in the mouse model. This novel mouse model makes it possible to further study in vivo how NGFR100W uncouple trophic function from nociception of NGF.

Swedish Nerve Growth Factor Mutation (NGFR100W) Defines a Role for TrkA and p75NTR in Nociception

Nerve growth factor (NGF) exerts multiple functions on target neurons throughout development. The recent discovery of a point mutation leading to a change from arginine to tryptophan at residue 100 in the mature NGFβ sequence (NGFR100W) in patients with hereditary sensory and autonomic neuropathy type V (HSAN V) made it possible to distinguish the signaling mechanisms that lead to two functionally different outcomes of NGF: trophic versus nociceptive. We performed extensive biochemical, cellular, and live-imaging experiments to examine the binding and signaling properties of NGFR100W Our results show that, similar to the wild-type NGF (wtNGF), the naturally occurring NGFR100W mutant was capable of binding to and activating the TrkA receptor and its downstream signaling pathways to support neuronal survival and differentiation. However, NGFR100W failed to bind and stimulate the 75 kDa neurotrophic factor receptor (p75NTR)-mediated signaling cascades (i.e., the RhoA-Cofilin pathway). Intraplantar injection of NGFR100W into adult rats induced neither TrkA-mediated thermal nor mechanical acute hyperalgesia, but retained the ability to induce chronic hyperalgesia based on agonism for TrkA signaling. Together, our studies provide evidence that NGFR100W retains trophic support capability through TrkA and one aspect of its nociceptive signaling, but fails to engage p75NTR signaling pathways. Our findings suggest that wtNGF acts via TrkA to regulate the delayed priming of nociceptive responses. The integration of both TrkA and p75NTR signaling thus appears to regulate neuroplastic effects of NGF in peripheral nociception.SIGNIFICANCE STATEMENT In the present study, we characterized the naturally occurring nerve growth factor NGFR100W mutant that is associated with hereditary sensory and autonomic neuropathy type V. We have demonstrated for the first time that NGFR100W retains trophic support capability through TrkA, but fails to engage p75NTR signaling pathways. Furthermore, after intraplantar injection into adult rats, NGFR100W induced neither thermal nor mechanical acute hyperalgesia, but retained the ability to induce chronic hyperalgesia. We have also provided evidence that the integration of both TrkA- and p75NTR-mediated signaling appears to regulate neuroplastic effects of NGF in peripheral nociception. Our study with NGFR100W suggests that it is possible to uncouple trophic effect from nociceptive function, both induced by wild-type NGF.

Rare human nerve growth factor-β mutation reveals relationship between C-afferent density and acute pain evaluation

The rare nerve growth factor-β (NGFB) mutation R221W causes a selective loss of thinly myelinated fibers and especially unmyelinated C-fibers. Carriers of this mutation show altered pain sensation. A subset presents with arthropathic symptoms, with the homozygous most severely affected. The aim of the present study was to investigate the relationship between peripheral afferent loss and pain evaluation by performing a quantification of small-fiber density in the cornea of the carriers, relating density to pain evaluation measures. In vivo corneal confocal microscopy (CCM) was used to quantify C-fiber loss in the cornea of 19 R221W mutation carriers (3 homozygous) and 19 age-matched healthy control subjects. Pain evaluation data via the Situational Pain Questionnaire (SPQ) and the severity of neuropathy based on the Neuropathy Disability Score (NDS) were assessed. Homozygotes, heterozygotes, and control groups differed significantly in corneal C-nerve fiber density, with the homozygotes showing a significant afferent reduction. Importantly, peripheral C-fiber loss correlated negatively with pain evaluation, as revealed by SPQ scores. This study is the first to investigate the contribution of small-fiber density to the perceptual evaluation of pain. It demonstrates that the lower the peripheral small-fiber density, the lower the degree of reported pain intensity, indicating a functional relationship between small-fiber density and higher level pain experience.

Pro-neurotrophins, sortilin, and nociception

Nerve growth factor (NGF) signaling is important in the development and functional maintenance of nociceptors, but it also plays a central role in initiating and sustaining heat and mechanical hyperalgesia following inflammation. NGF signaling in pain has traditionally been thought of as primarily engaging the classic high-affinity receptor tyrosine kinase receptor TrkA to initiate sensitization events. However, the discovery that secreted proforms of nerve NGF have biological functions distinct from the processed mature factors raised the possibility that these proneurotrophins (proNTs) may have distinct function in painful conditions. ProNTs engage a novel receptor system that is distinct from that of mature neurotrophins, consisting of sortilin, a type I membrane protein belonging to the VPS10p family, and its co-receptor, the classic low-affinity neurotrophin receptor p75NTR. Here, we review how this new receptor system may itself function with or independently of the classic TrkA system in regulating inflammatory or neuropathic pain.

Nerve growth factor-mediated regulation of pain signalling and proposed new intervention strategies in clinical pain management

Nerve growth factor (NGF) is the founding member of the neurotrophins family of proteins. It was discovered more than half a century ago through its ability to promote sensory and sympathetic neuronal survival and axonal growth during the development of the peripheral nervous system, and is the paradigmatic target-derived neurotrophic factor on which the neurotrophic hypothesis is based. Since that time, NGF has also been shown to play a key role in the generation of acute and chronic pain and in hyperalgesia in diverse pain states. NGF is expressed at high levels in damaged or inflamed tissues and facilitates pain transmission by nociceptive neurons through a variety of mechanisms. Genetic mutations in NGF or its tyrosine kinase receptor TrkA, lead to a congenital insensitivity or a decreased ability of humans to perceive pain. The hereditary sensory autonomic neuropathies (HSANs) encompass a spectrum of neuropathies that affect one's ability to perceive sensation. HSAN type IV and HSAN type V are caused by mutations in TrkA and NGF respectively. This review will focus firstly on the biology of NGF and its role in pain modulation. We will review neuropathies and clinical presentations that result from the disruption of NGF signalling in HSAN type IV and HSAN type V and review current advances in developing anti-NGF therapy for the clinical management of pain.

Mechanisms of disease in hereditary sensory and autonomic neuropathies

Hereditary sensory and autonomic neuropathies (HSANs) are a clinically and genetically heterogeneous group of disorders of the PNS. Progressive degeneration, predominantly of sensory and autonomic neurons, is the main pathological feature in patients with HSAN, and causes prominent sensory loss and ulcerative mutilations in combination with variable autonomic and motor disturbances. Advances in molecular genetics have enabled identification of disease-causing mutations in 12 genes, and studies on the functional effects of these mutations are underway. Although some of the affected proteins--such as nerve growth factor and its receptor--have obvious nerve-specific roles, others are ubiquitously expressed proteins that are involved in sphingolipid metabolism, vesicular transport, transcription regulation and structural integrity. An important challenge in the future will be to understand the common molecular pathways that result in HSANs. Unraveling the mechanisms that underlie sensory and autonomic neurodegeneration could assist in identifying targets for future therapeutic strategies in patients with HSAN. This Review highlights key advances in the understanding of HSANs, including insights into the molecular mechanisms of disease, derived from genetic studies of patients with these disorders.

Nerve growth factor β polypeptide (NGFB) genetic variability: association with the methadone dose required for effective maintenance treatment

Opioid addiction is a chronic disease with high genetic contribution and a large inter-individual variability in therapeutic response. The goal of this study was to identify pharmacodynamic factors that modulate methadone dose requirement. The neurotrophin family is involved in neural plasticity, learning, memory and behavior and deregulated neural plasticity may underlie the pathophysiology of drug addiction. Brain-derived neurotrophic factor (BDNF) was shown to affect the response to methadone maintenance treatment. This study explores the effects of polymorphisms in the nerve growth factor (β polypeptide) gene, NGFB, on the methadone doses required for successful maintenance treatment for heroin addiction. Genotypes of 14 NGFB polymorphisms were analyzed for association with the stabilizing methadone dose in 72 former severe heroin addicts with no major co-medications. There was significant difference in methadone doses required by subjects with different genotypes of the NGFB intronic single-nucleotide polymorphism rs2239622 (P=0.0002). These results may have clinical importance.

Taking pain out of NGF: a "painless" NGF mutant, linked to hereditary sensory autonomic neuropathy type V, with full neurotrophic activity

During adulthood, the neurotrophin Nerve Growth Factor (NGF) sensitizes nociceptors, thereby increasing the response to noxious stimuli. The relationship between NGF and pain is supported by genetic evidence: mutations in the NGF TrkA receptor in patients affected by an hereditary rare disease (Hereditary Sensory and Autonomic Neuropathy type IV, HSAN IV) determine a congenital form of severe pain insensitivity, with mental retardation, while a mutation in NGFB gene, leading to the aminoacid substitution R100W in mature NGF, determines a similar loss of pain perception, without overt cognitive neurological defects (HSAN V). The R100W mutation provokes a reduced processing of proNGF to mature NGF in cultured cells and a higher percentage of neurotrophin secreted is in the proNGF form. Moreover, using Surface Plasmon Resonance we showed that the R100W mutation does not affect NGF binding to TrkA, while it abolishes NGF binding to p75NTR receptors. However, it remains to be clarified whether the major impact of the mutation is on the biological function of proNGF or of mature NGF and to what extent the effects of the R100W mutation on the HSAN V clinical phenotype are developmental, or whether they reflect an impaired effectiveness of NGF to regulate and mediate nociceptive transmission in adult sensory neurons. Here we show that the R100 mutation selectively alters some of the signaling pathways activated downstream of TrkA NGF receptors. NGFR100 mutants maintain identical neurotrophic and neuroprotective properties in a variety of cell assays, while displaying a significantly reduced pain-inducing activity in vivo (n = 8–10 mice/group). We also show that proNGF has a significantly reduced nociceptive activity, with respect to NGF. Both sets of results jointly contribute to elucidating the mechanisms underlying the clinical HSAN V manifestations, and to clarifying which receptors and intracellular signaling cascades participate in the pain sensitizing action of NGF.

A novel NGF mutation clarifies the molecular mechanism and extends the phenotypic spectrum of the HSAN5 neuropathy

Background

Nerve growth factor β (NGFβ) and tyrosine kinase receptor type A (TRKA) are a well studied neurotrophin/receptor duo involved in neuronal survival and differentiation. The only previously reported hereditary sensory neuropathy caused by an NGF mutation, c.661C>T (HSAN5), and the pathology caused by biallelic mutations in the TRKA gene (NTRK1) (HSAN4), share only some clinical features. A consanguineous Arab family, where five of the six children were completely unable to perceive pain, were mentally retarded, did not sweat, could not discriminate temperature, and had a chronic immunodeficiency, is reported here. The condition is linked to a new homozygous mutation in the NGF gene, c.[680C>A]+[681_682delGG].

Methods

Genetic linkage and standard sequencing techniques were used to identify the causative gene. Using wild-type or mutant over-expression constructs transfected into PC12 and COS-7 cells, the cellular and molecular consequences of the mutations were investigated.

Results

The mutant gene produced a precursor protein V232fs that was unable to differentiate PC12 cells. V232fs was not secreted from cells as mature NGFβ.

Conclusions

Both the clinical and cellular data suggest that the c.[680C>A]+[681_682delGG] NGF mutation is a functional null. The HSAN5 phenotype is extended to encompass HSAN4-like characteristics. It is concluded that the HSAN4 and HSAN5 phenotypes are parts of a phenotypic spectrum caused by changes in the NGF/TRKA signalling pathway.

Transient receptor potential vanilloid 1, vanilloid 2 and melastatin 8 immunoreactive nerve fibers in human skin from individuals with and without Norrbottnian congenital insensitivity to pain

Transient receptor potential vanilloid 1 (TRPV1), vanilloid 2 (TRPV2) and melastatin 8 (TRPM8) are thermosensitive cation channels expressed on primary sensory neurons. In contrast to TRPV1, which is present on nociceptive primary afferents and keratinocytes in human skin, less is known about the distribution of TRPV2 and TRPM8 in this tissue. Immunohistochemistry of human forearm skin identified TRPV2 and TRPM8 immunoreactive nerve fibers in epidermis-papillary dermis and around blood vessels and hair follicles in dermis, although these nerve fibers were less abundant than TRPV1 immunoreactive nerve fibers throughout the skin. The TRPV2 and TRPM8 immunoreactive nerve fibers also showed immunoreactivity for calcitonin gene-related peptide (CGRP) and to a lesser extent substance P (SP). Neither of the TRP ion channels co-localized with neurofilament 200 kDa (NF200), vasoactive intestinal peptide (VIP) or tyrosine hydroxylase (TH). Nerve fibers immunoreactive for TRPV1, TRPV2, TRPM8, CGRP and SP were absent or substantially reduced in number in individuals with Norrbottnian congenital insensitivity to pain, an autosomal disease selectively affecting the development of C-fiber and Adelta-fiber primary afferents. Quantitative real time PCR detected mRNA transcripts encoding TRPV1 and TRPV2, but not TRPM8, in skin from healthy volunteers, suggesting that these ion channels are also expressed extraneuronally. In conclusion, nerve fibers in human skin express TRPV1, TRPV2 and TRPM8 that co-localize with the sensory neuropeptides CGRP and SP, but not with NF200, VIP or TH. A dramatic loss of such nerve fibers was seen in skin from individuals with Norrbottnian congenital insensitivity to pain, further suggesting that these ion channels are expressed primarily on nociceptive primary sensory neurons in human skin.