TNF-α/TNFR1 signaling is required for the development and function of primary nociceptors

Immune drivers of physiological and pathological pain

Physiological pain serves as a warning of exposure to danger and prompts us to withdraw from noxious stimuli to prevent tissue damage. Pain can also alert us of an infection or organ dysfunction and aids in locating such malfunction. However, there are instances where pain is purely pathological, such as unresolved pain following an inflammation or injury to the nervous system, and this can be debilitating and persistent. We now appreciate that immune cells are integral to both physiological and pathological pain, and that pain, in consequence, is not strictly a neuronal phenomenon. Here, we discuss recent findings on how immune cells in the skin, nerve, dorsal root ganglia, and spinal cord interact with somatosensory neurons to mediate pain. We also discuss how both innate and adaptive immune cells, by releasing various ligands and mediators, contribute to the initiation, modulation, persistence, or resolution of various modalities of pain. Finally, we propose that the neuroimmune axis is an attractive target for pain treatment, but the challenges in objectively quantifying pain preclinically, variable sex differences in pain presentation, as well as adverse outcomes associated with immune system modulation, all need to be considered in the development of immunotherapies against pain.

Kinesin family member 2A gates nociception

Nociceptive axons undergo remodeling as they innervate their targets during development and in response to environmental insults and pathological conditions. How is nociceptive morphogenesis regulated? Here, we show that the microtubule destabilizer kinesin family member 2A (Kif2a) is a key regulator of nociceptive terminal structures and pain sensitivity. Ablation of Kif2a in sensory neurons causes hyperinnervation and hypersensitivity to noxious stimuli in young adult mice, whereas touch sensitivity and proprioception remain unaffected. Computational modeling predicts that structural remodeling is sufficient to explain the phenotypes. Furthermore, Kif2a deficiency triggers a transcriptional response comprising sustained upregulation of injury-related genes and homeostatic downregulation of highly specific channels and receptors at the late stage. The latter effect can be predicted to relieve the hyperexcitability of nociceptive neurons, despite persisting morphological aberrations, and indeed correlates with the resolution of pain hypersensitivity. Overall, we reveal a critical control node defining nociceptive terminal structure, which is regulating nociception.

The good, the bald, and the hairy: A mechanosensor meets its fate at the target

In this issue of Developmental Cell, Koutsioumpa et al. (2023) investigate the maturation of low-threshold mechanoreceptor nerve endings in both hairy and glabrous skin types and discover a critical role for target-derived BMP in the development of Meissner corpuscles in glabrous (i.e., hairless) skin.

Roles and regulation of microglia activity in multiple sclerosis: insights from animal models

As resident macrophages of the CNS, microglia are critical immune effectors of inflammatory lesions and associated neural dysfunctions. In multiple sclerosis (MS) and its animal models, chronic microglial inflammatory activity damages myelin and disrupts axonal and synaptic activity. In contrast to these detrimental effects, the potent phagocytic and tissue-remodelling capabilities of microglia support critical endogenous repair mechanisms. Although these opposing capabilities have long been appreciated, a precise understanding of their underlying molecular effectors is only beginning to emerge. Here, we review recent advances in our understanding of the roles of microglia in animal models of MS and demyelinating lesions and the mechanisms that underlie their damaging and repairing activities. We also discuss how the structured organization and regulation of the genome enables complex transcriptional heterogeneity within the microglial cell population at demyelinating lesions.

Dexmedetomidine Prolongs the Analgesic Effects of Periarticular Infiltration Analgesia following Total Knee Arthroplasty: A Prospective, Double-blind, Randomized Controlled Trial

BACKGROUND

Periarticular infiltration analgesia (PIA) is widely administered to relieve postoperative pain following total knee arthroplasty (TKA). The present study aimed to evaluate the effect of prolonging the analgesic duration by adding dexmedetomidine to PIA for pain management after TKA.

METHODS

One hundred and sixteen patients were randomly allocated into 3 groups based on PIA regimens including Group R (ropivacaine), Group E (ropivacaine plus epinephrine), and Group D (ropivacaine plus dexmedetomidine). The primary outcomes were postoperative visual analog scale (VAS) scores, time until the administration of first rescue analgesia, and opioid consumption. The secondary outcomes included postoperative inflammatory biomarkers and functional recovery. The tertiary outcomes were postoperative complications and adverse events.

RESULTS

The patients in Group D had significantly lower resting VAS scores than those in Groups R and E at 6 hours after surgery. Group R showed the higher pain scores at rest and motion than Groups D and E 12 hours postoperatively. The use of dexmedetomidine or epinephrine postponed the time until the administration of first rescue analgesia and led to lower opioid consumption in the first 24 hours after TKA. The levels of IL-8 and TNF-α in Groups D and E were significantly lower than those in Group R on postoperative Day 3. Furthermore, no significant differences were observed in functional recovery, postoperative complications, or adverse events among the three groups.

CONCLUSIONS

Adding dexmedetomidine as an adjuvant to PIA could potentiate and prolong the analgesic effect in the early stage following TKA without increasing the risk of adverse events.

A developmental atlas of somatosensory diversification and maturation in the dorsal root ganglia by single-cell mass cytometry

Precisely controlled development of the somatosensory system is essential for detecting pain, itch, temperature, mechanical touch and body position. To investigate the protein-level changes that occur during somatosensory development, we performed single-cell mass cytometry on dorsal root ganglia from C57/BL6 mice of both sexes, with litter replicates collected daily from embryonic day 11.5 to postnatal day 4. Measuring nearly 3 million cells, we quantified 30 molecularly distinct somatosensory glial and 41 distinct neuronal states across all timepoints. Analysis of differentiation trajectories revealed rare cells that co-express two or more Trk receptors and over-express stem cell markers, suggesting that these neurotrophic factor receptors play a role in cell fate specification. Comparison to previous RNA-based studies identified substantial differences between many protein-mRNA pairs, demonstrating the importance of protein-level measurements to identify functional cell states. Overall, this study demonstrates that mass cytometry is a high-throughput, scalable platform to rapidly phenotype somatosensory tissues.

Systemic pharmacological verification of Guizhi Fuling decoction in treating endometriosis-associated pain

ETHNOPHARMACOLOGICAL RELEVANCE

Guizhi Fuling decoction (GZFL decoction) is a famous formula in the Synopsis of the Golden Chamber, which has a long history in treating endometriosis. However, its exact mechanism remains unclear.

AIM OF STUDY

This study aims to explore the mechanism of GZFL decoction in treating endometriosis, especially in alleviating endometriosis-associated pain.

MATERIALS AND METHODS

A combination of system pharmacology and pharmacodynamics was used to explore the specific mechanism of GZFL decoction in the treatment of endometriosis-associated pain. First, the TCMSP database was used to search the components of the GZFL decoction; the parameter index was set as oral bioavailability (OB) ≥ 30% and drug-likeness (DL) ≥ 0.18, while the active ingredients of the drug were screened out. The disease targets of endometriosis were obtained from the TTD, OMIM, Genecards, and DisGeNET databases; the keyword was "endometriosis pain". Network construction and analysis were performed using Cytoscape 3.7.2 software; the David database was used to enrich and analyze the pathways for alleviating endometriosis pain after GZFL decoction treatment. In addition, the network results were verified using experimental animal and cell research.

RESULTS

The results showed the following targets: 76 for the effective chemical components in the prescription, 1329 for disease pain, and 278 for the intersection of drugs and endometriosis pain. The enrichment results for these targets showed that the TNF-PI3K/Akt pathway exhibited research significance. In endometriosis rat models, the GZFL decoction reduced the volume of lesions and relieved pain symptoms. It also reduced the serum levels of IL-6, IL-1β, and TNF-α as well as their expression in the lesion tissues. The GZFL decoction also suppressed the activation of PI3K/Akt downstream signaling proteins.

CONCLUSIONS

GZFL decoction could reduce the volume of lesions, suppress inflammation, and decrease the sensitivity to pain in endometriosis rat models through inhibiting PI3K/Akt pathway. This study provides a possible target for traditional Chinese medicine in treating endometriosis-associated pain.

Double-negative T cells mediate M1 polarization of microglial cells via TNF-α-NLRP3 to aggravate neuroinflammation and cognitive impairment in Alzheimer's disease mice

We mainly study the role and regulatory mechanism of double-negative T cells (DNTs) in Alzheimer's disease (AD). The mice splenic DNTs were separated and amplified by Rosettesep antibody adsorption method and Easysep magnetic activated cell sorting. DNTs were intraperitoneally injected into the APP/PS1-AD mice model, which was found to aggravate cognitive impairment in mice. DNTs secreted tumor necrosis factor α (TNF-α) to promote the activation of NLRP3 and the M1 polarization of microglial cells, and silencing NLRP3 with small interfering RNA (siRNA) suppressed the effect of DNTs. DNTs were later cocultured with mice microglial cell line BV2, then fluorescence staining was conducted to detect NLRP3 expression, and enzyme-linked immunoassay was performed to measure the expression of inflammatory factors. Moreover, the levels of NLRP3, ASC, and TNFR1 proteins were detected by western-blot assay, and the proportion of F4/80 + CD11b + M1 cells was detected by flow cytometry. DNTs promoted the M1 polarization of BV2 cells and the activation of NLRP3 inflammasome. After treatment of BV2 cells with NLRP3 inhibitor, the effect of DNTs was weakened. Later, TNF-α siRNA was transfected into DNTs, and it was found that DNTs with TNF-α silencing had markedly weakened polarization effect on BV2 cells. We discovered that the proportion of DNTs increased in AD patients. DNTs secreted TNF-α to regulate the activation of NLRP3 inflammasome and the M1 polarization of microglial cells, thus promoting the central inflammatory response and aggravating the cognitive impairment in AD mice.

Depolarizing NaV and Hyperpolarizing KV Channels Are Co-Trafficked in Sensory Neurons

Neuronal excitability relies on coordinated action of functionally distinction channels. Voltage-gated sodium (NaV) and potassium (KV) channels have distinct but complementary roles in firing action potentials: NaV channels provide depolarizing current while KV channels provide hyperpolarizing current. Mutations and dysfunction of multiple NaV and KV channels underlie disorders of excitability, including pain and epilepsy. Modulating ion channel trafficking may offer a potential therapeutic strategy for these diseases. A fundamental question, however, is whether these channels with distinct functional roles are transported independently or packaged together in the same vesicles in sensory axons. We have used Optical Pulse-Chase Axonal Long-distance imaging to investigate trafficking of NaV and KV channels and other axonal proteins from distinct functional classes in live rodent sensory neurons (from male and female rats). We show that, similar to NaV1.7 channels, NaV1.8 and KV7.2 channels are transported in Rab6a-positive vesicles, and that each of the NaV channel isoforms expressed in healthy, mature sensory neurons (NaV1.6, NaV1.7, NaV1.8, and NaV1.9) is cotransported in the same vesicles. Further, we show that multiple axonal membrane proteins with different physiological functions (NaV1.7, KV7.2, and TNFR1) are cotransported in the same vesicles. However, vesicular packaging of axonal membrane proteins is not indiscriminate, since another axonal membrane protein (NCX2) is transported in separate vesicles. These results shed new light on the development and organization of sensory neuron membranes, revealing complex sorting of axonal proteins with diverse physiological functions into specific transport vesicles.SIGNIFICANCE STATEMENT Normal neuronal excitability is dependent on precise regulation of membrane proteins, including NaV and KV channels, and imbalance in the level of these channels at the plasma membrane could lead to excitability disorders. Ion channel trafficking could potentially be targeted therapeutically, which would require better understanding of the mechanisms underlying trafficking of functionally diverse channels. Optical Pulse-chase Axonal Long-distance imaging in live neurons permitted examination of the specificity of ion channel trafficking, revealing co-packaging of axonal proteins with opposing physiological functions into the same transport vesicles. This suggests that additional trafficking mechanisms are necessary to regulate levels of surface channels, and reveals an important consideration for therapeutic strategies that target ion channel trafficking for the treatment of excitability disorders.

Chronic cerebral aspects of long COVID, post-stroke syndromes and similar states share their pathogenesis and perispinal etanercept treatment logic

The chronic neurological aspects of traumatic brain injury, post‐stroke syndromes, long COVID‐19, persistent Lyme disease, and influenza encephalopathy having close pathophysiological parallels that warrant being investigated in an integrated manner. A mechanism, common to all, for this persistence of the range of symptoms common to these conditions is described. While TNF maintains cerebral homeostasis, its excessive production through either pathogen‐associated molecular patterns or damage‐associated molecular patterns activity associates with the persistence of the symptoms common across both infectious and non‐infectious conditions. The case is made that this shared chronicity arises from a positive feedback loop causing the persistence of the activation of microglia by the TNF that these cells generate. Lowering this excess TNF is the logical way to reducing this persistent, TNF‐maintained, microglial activation. While too large to negotiate the blood‐brain barrier effectively, the specific anti‐TNF biological, etanercept, shows promise when administered by the perispinal route, which allows it to bypass this obstruction.

TNF-α impairs EP4 signaling through the association of TRAF2-GRK2 in primary fibroblast-like synoviocytes

Our previous study showed that chronic treatment with tumor necrosis factor-α (TNF-α) decreased cAMP concentration in fibroblast-like synoviocytes (FLSs) of collagen-induced arthritis (CIA) rats. In this study we investigated how TNF-α impairs cAMP homeostasis, particularly clarifying the potential downstream molecules of TNF-α and prostaglandin receptor 4 (EP4) signaling that would interact with each other. Using a cAMP FRET biosensor PM-ICUE3, we demonstrated that TNF-α (20 ng/mL) blocked ONO-4819-triggered EP4 signaling, but not Butaprost-triggered EP2 signaling in normal rat FLSs. We showed that TNF-α (0.02-20 ng/mL) dose-dependently reduced EP4 membrane distribution in normal rat FLS. TNF-α significantly increased TNF receptor 2 (TNFR2) expression and stimulated proliferation in human FLS (hFLS) via ecruiting TNF receptor-associated factor 2 (TRAF2) to cell membrane. More interestingly, we revealed that TRAF2 interacted with G protein-coupled receptor kinase (GRK2) in the cytoplasm of primary hFLS and helped to bring GRK2 to cell membrane in response of TNF-α stimulation, the complex of TRAF2 and GRK2 then separated on the membrane, and translocated GRK2 induced the desensitization and internalization of EP4, leading to reduced production of intracellular cAMP. Silencing of TRAF2 by siRNA substantially diminished TRAF2-GRK2 interaction, blocked the translocation of GRK2, and resulted in upregulated expression of membrane EP4 and intracellular cAMP. In CIA rats, administration of paroxetine to inhibit GRK2 effectively improved the symptoms and clinic parameters with significantly reduced joint synovium inflammation and bone destruction. These results elucidate a novel form of cross-talk between TNFR (a cytokine receptor) and EP4 (a typical G protein-coupled receptor) signaling pathways. The interaction between TRAF2 and GRK2 may become a potential new drug target for the treatment of inflammatory diseases.

TNFα in MS and Its Animal Models: Implications for Chronic Pain in the Disease

Multiple Sclerosis (MS) is a debilitating autoimmune disease often accompanied by severe chronic pain. The most common type of pain in MS, called neuropathic pain, arises from disease processes affecting the peripheral and central nervous systems. It is incredibly difficult to study these processes in patients, so animal models such as experimental autoimmune encephalomyelitis (EAE) mice are used to dissect the complex mechanisms of neuropathic pain in MS. The pleiotropic cytokine tumor necrosis factor α (TNFα) is a critical factor mediating neuropathic pain identified by these animal studies. The TNF signaling pathway is complex, and can lead to cell death, inflammation, or survival. In complex diseases such as MS, signaling through the TNFR1 receptor tends to be pro-inflammation and death, whereas signaling through the TNFR2 receptor is pro-homeostatic. However, most TNFα-targeted therapies indiscriminately block both arms of the pathway, and thus are not therapeutic in MS. This review explores pain in MS, inflammatory TNF signaling, the link between the two, and how it could be exploited to develop more effective TNFα-targeting pain therapies.

Heart neurons use clock genes to control myocyte proliferation

Neurons can regulate the development, pathogenesis, and regeneration of target organs. However, the role of neurons during heart development and regeneration remains unclear. We genetically inhibited sympathetic innervation in vivo, which resulted in heart enlargement with an increase in cardiomyocyte number. Transcriptomic and protein analysis showed down-regulation of the two clock gene homologs Period1/Period2 (Per1/Per2) accompanied by up-regulation of cell cycle genes. Per1/Per2 deletion increased heart size and cardiomyocyte proliferation, recapitulating sympathetic neuron–deficient hearts. Conversely, increasing sympathetic activity by norepinephrine treatment induced Per1/Per2 and suppressed cardiomyocyte proliferation. We further found that the two clock genes negatively regulate myocyte mitosis entry through the Wee1 kinase pathway. Our findings demonstrate a previously unknown link between cardiac neurons and clock genes in regulation of cardiomyocyte proliferation and heart size and provide mechanistic insights for developing neuromodulation strategies for cardiac regen5eration.

P2X4R Contributes to Central Disinhibition Via TNF-α/TNFR1/GABAaR Pathway in Post-stroke Pain Rats

Central post-stroke pain (CPSP) is a disabling condition in stroke patients. It is a type of neuropathic pain for which the mechanism and relevant drug pathways remain unknown. Inflammatory response and central disinhibition have been suggested recently. Our previous research has shown targeting P2X4 receptors (P2X4R) may be effective in the treatment of CPSP, but the downstream pathway of the P2X4R has not been studied. In this study, we found the increase in tumor necrosis factor alpha (TNF-α) level and endocytosis of surface gamma-aminobutyric acid a receptors (GABAaR) in CPSP, and these effects were inhibited by blocking P2X4R. Furthermore, antagonizing TNF-α can increase surface GABAaR expression and mechanical pain threshold. Meanwhile, knocking down TNFR1 but not TNFR2 reversed the endocytosis of surface GABAaR and alleviated mechanical allodynia. Thus, the neuropathic pain was mediated, in part, through P2X4R/TNF-α/TNFR1/GABAaR signaling, which was induced after stroke. PERSPECTIVE: P2X4R regulates the pathophysiological mechanism of CPSP through central disinhibition mediated by TNF-α/TNFR1. Our results suggest that modulation of P2X4R-TNF-α/TNFR1-GABAaR signaling could provide a new therapeutic strategy to treat CPSP.

Involvement of Mast Cells in the Pathophysiology of Pain

Mast cells (MCs) are immune cells and are widely distributed throughout the body. MCs are not only classically viewed as effector cells of some allergic diseases but also participate in host defense, innate and acquired immunity, homeostatic responses, and immunoregulation. Mounting evidence indicates that activation of MCs releasing numerous vasoactive and inflammatory mediators has effects on the nervous system and has been involved in different pain conditions. Here, we review the latest advances made about the implication of MCs in pain. Possible cellular and molecular mechanisms regarding the crosstalk between MC and the nervous system in the initiation and maintenance of pain are also discussed.

Transient receptor potential vanilloid subtype 1 depletion mediates mechanical allodynia through cellular signal alterations in small-fiber neuropathy

Transient receptor potential vanilloid subtype 1 (TRPV1) is a polymodal nociceptor that monitors noxious thermal sensations. Few studies have addressed the role of TRPV1 in mechanical allodynia in small-fiber neuropathy (SFN) caused by sensory nerve damage. Accordingly, this article reviews the putative mechanisms of TRPV1 depletion that mediates mechanical allodynia in SFN. The intraepidermal nerve fibers (IENFs) degeneration and sensory neuronal injury are the primary characteristics of SFN. Intraepidermal nerve fibers are mainly C-polymodal nociceptors and Aδ-fibers, which mediated allodynic pain after neuronal sensitization. TRPV1 depletion by highly potent neurotoxins induces the upregulation of activating transcription factor 3 and IENFs degeneration which mimics SFN. TRPV1 is predominately expressed by the peptidergic than nonpeptidergic nociceptors, and these neurochemical discrepancies provided the basis of the distinct pathways of thermal analgesia and mechanical allodynia. The depletion of peptidergic nociceptors and their IENFs cause thermal analgesia and sensitized nonpeptidergic nociceptors respond to mechanical allodynia. These distinct pathways of noxious stimuli suggested determined by the neurochemical-dependent neurotrophin cognate receptors such as TrkA and Ret receptors. The neurogenic inflammation after TRPV1 depletion also sensitized Ret receptors which results in mechanical allodynia. The activation of spinal TRPV1(+) neurons may contribute to mechanical allodynia. Also, an imbalance in adenosinergic analgesic signaling in sensory neurons such as the downregulation of prostatic acid phosphatase and adenosine A1 receptors, which colocalized with TRPV1 as a membrane microdomain also correlated with the development of mechanical allodynia. Collectively, TRPV1 depletion-induced mechanical allodynia involves a complicated cascade of cellular signaling alterations.

Tumor Necrosis Factor Alpha in Amyotrophic Lateral Sclerosis: Friend or Foe?

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by a massive neuroinflammatory reaction, which plays a key role in the progression of the disease. One of the major mediators of the inflammatory response is the pleiotropic cytokine tumor necrosis factor α (TNFα), mainly released within the central nervous system (CNS) by reactive astrocytes and microglia. Increased levels of TNFα and its receptors (TNFR1 and TNFR2) have been described in plasma, serum, cerebrospinal fluid and CNS tissue from both ALS patients and transgenic animal models of disease. However, the precise role exerted by TNFα in the context of ALS is still highly controversial, since both protective and detrimental functions have been reported. These opposing actions depend on multiple factors, among which includes the type of TNFα receptor activated. In fact, TNFR2 seems to mediate a harmful role being involved in motor neuron cell death, whereas TNFR1 signaling mediates neuroprotective effects, promoting the expression and secretion of trophic factors. This suggests that a better understanding of the cytokine impact on ALS progression may enable the development of effective therapies aimed at strengthening the protective roles of TNFα and at suppressing the detrimental ones.

Long-distance regressive signaling in neural development and disease

Nervous system development proceeds via well-orchestrated processes involving a balance between progressive and regressive events including stabilization or elimination of axons, synapses, and even entire neurons. These progressive and regressive events are driven by functionally antagonistic signaling pathways with the dominant pathway eventually determining whether a neural element is retained or removed. Many of these developmental sculpting events are triggered by final target innervation necessitating a long-distance mode of communication. While long-distance progressive signaling has been well characterized, particularly for neurotrophic factors, there remains relatively little known about how regressive events are triggered from a distance. Here we discuss the emergent phenomenon of long-distance regressive signaling pathways. In particular, we will cover (a) progressive and regressive cues known to be employed after target innervation, (b) the mechanisms of long-distance signaling from an endosomal platform, (c) recent evidence that long-distance regressive cues emanate from platforms like death receptors or repulsive axon guidance receptors, and (d) evidence that these pathways are exploited in pathological scenarios. This article is categorized under: Nervous System Development > Vertebrates: General Principles Signaling Pathways > Global Signaling Mechanisms Establishment of Spatial and Temporal Patterns > Cytoplasmic Localization.

TNFα promotes oral cancer growth, pain, and Schwann cell activation

Oral cancer is very painful and impairs a patient's ability to eat, talk, and drink. Mediators secreted from oral cancer can excite and sensitize sensory neurons inducing pain. Cancer mediators can also activate Schwann cells, the peripheral glia that regulates neuronal function and repair. The contribution of Schwann cells to oral cancer pain is unclear. We hypothesize that the oral cancer mediator TNFα activates Schwann cells, which further promotes cancer progression and pain. We demonstrate that TNFα is overexpressed in human oral cancer tissues and correlates with increased self-reported pain in patients. Antagonizing TNFα reduces oral cancer proliferation, cytokine production, and nociception in mice with oral cancer. Oral cancer or TNFα alone increases Schwann cell activation (measured by Schwann cell proliferation, migration, and activation markers), which can be inhibited by neutralizing TNFα. Cancer- or TNFα-activated Schwann cells release pro-nociceptive mediators such as TNFα and nerve growth factor (NGF). Activated Schwann cells induce nociceptive behaviors in mice, which is alleviated by blocking TNFα. Our study suggests that TNFα promotes cancer proliferation, progression, and nociception at least partially by activating Schwann cells. Inhibiting TNFα or Schwann cell activation might serve as therapeutic approaches for the treatment of oral cancer and associated pain.

Animal models for Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a neurodevelopmental disorder characterized by hyperphagia, hypotonia, learning disability, as well as a range of psychiatric conditions. The conservation of the PWS genetic interval on chromosome 15q11-q13 in human, and a cluster of genes on mouse chromosome 7, has facilitated the use of mice as animal models for PWS. Some models faithfully mimic the loss of all gene expression from the paternally inherited PWS genetic interval, whereas others target smaller regions or individual genes. Collectively, these models have provided insight into the mechanisms, many of which lead to alterations in hypothalamic function, underlying the core symptoms of PWS, including growth retardation, hyperphagia and metabolism, reproductive maturation and endophenotypes of relevance to behavioral and psychiatric problems. Here we review and summarize these studies.

Myosin light chain kinase regulates intestinal permeability of mucosal homeostasis in Crohn's disease

Introduction: Researchers have investigated the potential role of intestinal permeability in Crohn's disease pathogenesis. Intestinal permeability is usually mediated by cytoskeleton and intercellular junctions. The myosin light chain kinase (MLCK) is an enzyme that activates the myosin light chain to exert its function related to cytoskeleton contraction and tight junction regulation. The correlation between MLCK and Crohn's disease pathogenesis has been consistently proven. Areas covered: This study aims to expand the understanding of the regulation and function of MLCK in Crohn's disease. An extensive literature search in the MEDLINE database (via PubMed) has been performed up to Oct. 2020. The roles of MLCK in tight junction activation, intestinal permeability enhancement, and cell signal regulation are comprehensively discussed. Expert opinion: Targeting the MLCK-related pathways such as TNF-α in CD treatment has been put into clinical use. More accurate targeting such as MLCK and TNFR2 has been proposed to reduce side effects. MLCK may also have the potential to become biomarkers in fields like CD activity. With the application of cutting age research methods and tools, the MLCK research could be accelerated.

Vertebrate Sensory Ganglia: Common and Divergent Features of the Transcriptional Programs Generating Their Functional Specialization

Sensory fibers of the peripheral nervous system carry sensation from specific sense structures or use different tissues and organs as receptive fields, and convey this information to the central nervous system. In the head of vertebrates, each cranial sensory ganglia and associated nerves perform specific functions. Sensory ganglia are composed of different types of specialized neurons in which two broad categories can be distinguished, somatosensory neurons relaying all sensations that are felt and visceral sensory neurons sensing the internal milieu and controlling body homeostasis. While in the trunk somatosensory neurons composing the dorsal root ganglia are derived exclusively from neural crest cells, somato- and visceral sensory neurons of cranial sensory ganglia have a dual origin, with contributions from both neural crest and placodes. As most studies on sensory neurogenesis have focused on dorsal root ganglia, our understanding of the molecular mechanisms underlying the embryonic development of the different cranial sensory ganglia remains today rudimentary. However, using single-cell RNA sequencing, recent studies have made significant advances in the characterization of the neuronal diversity of most sensory ganglia. Here we summarize the general anatomy, function and neuronal diversity of cranial sensory ganglia. We then provide an overview of our current knowledge of the transcriptional networks controlling neurogenesis and neuronal diversification in the developing sensory system, focusing on cranial sensory ganglia, highlighting specific aspects of their development and comparing it to that of trunk sensory ganglia.

Neuroimmune System as a Driving Force for Plasticity Following CNS Injury

Following an injury to the central nervous system (CNS), spontaneous plasticity is observed throughout the neuraxis and affects multiple key circuits. Much of this spontaneous plasticity can elicit beneficial and deleterious functional outcomes, depending on the context of plasticity and circuit affected. Injury-induced activation of the neuroimmune system has been proposed to be a major factor in driving this plasticity, as neuroimmune and inflammatory factors have been shown to influence cellular, synaptic, structural, and anatomical plasticity. Here, we will review the mechanisms through which the neuroimmune system mediates plasticity after CNS injury. Understanding the role of specific neuroimmune factors in driving adaptive and maladaptive plasticity may offer valuable therapeutic insight into how to promote adaptive plasticity and/or diminish maladaptive plasticity, respectively.

How CD40L reverse signaling regulates axon and dendrite growth

CD40-activated CD40L reverse signaling is a major physiological regulator of axon and dendrite growth from developing hippocampal pyramidal neurons. Here we have studied how CD40L-mediated reverse signaling promotes the growth of these processes. Cultures of hippocampal pyramidal neurons were established from Cd40^-/- mouse embryos to eliminate endogenous CD40/CD40L signaling, and CD40L reverse signaling was stimulated by a CD40-Fc chimera. CD40L reverse signaling increased phosphorylation and hence activation of proteins in the PKC, ERK, and JNK signaling pathways. Pharmacological activators and inhibitors of these pathways revealed that whereas activation of JNK inhibited growth, activation of PKC and ERK1/ERK2 enhanced growth. Experiments using combinations of pharmacological reagents revealed that these signaling pathways regulate growth by functioning as an interconnected and interdependent network rather than acting in a simple linear sequence. Immunoprecipitation studies suggested that stimulation of CD40L reverse signaling generated a receptor complex comprising CD40L, PKCβ, and the Syk tyrosine kinase. Our studies have begun to elucidate the molecular network and interactions that promote axon and dendrite growth from developing hippocampal neurons following activation of CD40L reverse signaling.

Pro-inflammatory TNF-α and IFN-γ Promote Tumor Growth and Metastasis via Induction of MACC1

Colorectal cancer (CRC) is one of the most common malignancies worldwide. Early stage CRC patients have a good prognosis. If distant metastasis occurs, the 5-year survival drops below 10%. Despite treatment success over the last decades, treatment options for metastatic disease are still limited. Therefore, novel targets are needed to foster therapy of advanced stage CRC patients and hinder progression of early stage patients into metastasis. A novel target is the crucial oncogene Metastasis-Associated in Colon Cancer 1 (MACC1) involved in molecular pathogenesis of CRC metastasis. MACC1 induces cell proliferation and motility, supports cellular survival and rewires metabolism resulting in increased metastasis in vivo. MACC1 is a prognostic biomarker not only for CRC but for more than 20 solid cancer entities. Inflammation plays a pivotal role in tumorigenesis, tumor progression and metastasis. For CRC, inflammatory bowel disease and ulcerative colitis are important inflammation associated risk factors. Certain cytokines, such as TNF-α and IFN-γ, are key factors in determining the contribution of the inflammatory process to CRC. Knowledge of the connection between inflammation and MACC1 driven tumors remains unclear. Gene expression analysis of CRC cells after cytokine stimulation was analyzed by qRT-PCR and Western blot. Cellular motility was assessed by Boyden chamber assays. MACC1 promoter activity after stimulation with pro-inflammatory cytokines was measured using promoter-luciferase constructs. To investigate signal transduction from receptor to effector molecules, blocking experiments using neutralizing antibodies and knockdown experiments were performed. Following TNF-α stimulation, MACC1 and c-Jun expression were significantly increased at the mRNA and protein level. Knockdown of c-Jun reduced MACC1 inducibility following TNF-α stimulation. TNF-α promoted MACC1-induced cell migration that was reverted following MACC1 knockdown. Moreover, MACC1 and c-Jun expression were downregulated by blocking TNFR1, but not TNFR2. Knock down of the NF-κB subunit, p65, reduced basal MACC1 and c-Jun mRNA expression levels. Adalimumab, a clinically approved monoclonal anti-TNF-α antibody, hindered MACC1 induction. The present study highlights that TNF-α regulates the induction of MACC1 via the NF-κB subunit p65 and the transcription factor c-Jun in CRC cells. This finding unravels a novel signaling pathway upstream of MACC1 and provides a potential therapeutic target for the treatment of CRC patients with an associated inflammation.

CARD6 protects against collagen-induced rheumatoid arthritis in mice through attenuating the inflammatory response and joint destruction via suppression of TNFR1/TRAF2 signaling

Rheumatoid arthritis (RA) is one of the most common autoimmune diseases, characterized by chronic inflammation and bone destruction. However, the pathogenesis that contributes to RA is still unclear. Caspase recruitment domain protein 6 (CARD6) is a typical member of CARD domain-containing proteins, and shows regulatory effects on nuclear factor-κB (NF-κB) activation to meditate inflammation. In the present study, the role of CARD6 in the progression of inflammatory bone erosion in RA was investigated using the in vitro and in vivo experiments. In vitro results indicated that CARD expression was markedly down-regulated in the activated macrophages induced by lipopolysaccharide (LPS), accompanied with time-dependently increased expression of pro-inflammatory cytokines. Notably, over-expressing CARD6 in macrophages by adenoviral (Ad) vector significantly abolished the expression levels of pro-inflammatory cytokines and chemokines. We found that CARD6 over-expression-suppressed inflammatory response was associated with the blockage of tumor necrosis factor receptor-1/tumor necrosis factor receptor-associated factor-2 (TNFR1/TRAF2) signaling, inhibiting NF-κB pathway subsequently. In addition, LPS-induced apoptosis in macrophages was also blunted due to AdCARD6 infection. CARD6-alleviated inflammatory response and apoptotic cell death were further confirmed in TNF-α-stimulated macrophages. Then, the in vivo studies showed that promoting CARD6 expression using adeno-associated virus (AAV) effectively attenuated the severity of arthritis, improved histopathological damage, and hindered the bone erosion in collagen-induced arthritis (CIA) mice. Moreover, pro-inflammatory factors in the joint samples were also markedly decreased in CIA mice with CARD6 over-expression, which was related to the down-regulation of TNFR1/TRAF2/NF-κB signaling pathway. Meanwhile, apoptosis in joint of CIA mice was also ameliorated by AAV-CARD6, as evidenced by the obviously reduced expression of cleaved Caspase-3. These results clearly suggested that CARD6 might have anti-inflammatory and anti-apoptotic effects during RA progression, and thus could be defined as a novel therapeutic target for RA treatment in future.

Molecular dissection of TNFR-TNFα bidirectional signaling reveals both cooperative and antagonistic interactions with p75 neurotrophic factor receptor in axon patterning

During neural development, complex organisms rely on progressive and regressive events whereby axons, synapses, and neurons are overproduced followed by selective elimination of a portion of these components. Tumor necrosis factor α (TNFα) together with its cognate receptor (Tumor necrosis factor receptor 1; TNFR1) have been shown to play both regressive (i.e. forward signaling from the receptor) and progressive (i.e. reverse signaling from the ligand) roles in sympathetic neuron development. In contrast, a paralog of TNFR1, p75 neurotrophic factor receptor (p75NTR) promotes mainly regressive developmental events in sympathetic neurons. Here we examine the interplay between these paralogous receptors in the regulation of axon branch elimination and arborization. We confirm previous reports that these TNFR1 family members are individually capable of promoting ligand-dependent suppression of axon growth and branching. Remarkably, p75NTR and TNFR1 physically interact and p75NTR requires TNFR1 for ligand-dependent axon suppression of axon branching but not vice versa. We also find that p75NTR forward signaling and TNFα reverse signaling are functionally antagonistic. Finally, we find that TNFα reverse signaling is necessary for nerve growth factor (NGF) dependent axon growth. Taken together these findings demonstrate several levels of synergistic and antagonistic interactions using very few signaling pathways and that the balance of these synergizing and opposing signals act to ensure proper axon growth and patterning.

CD40L reverse signaling suppresses prevertebral sympathetic axon growth and tissue innervation

CD40‐activated CD40L reverse signaling is a major physiological regulator of the growth of neural processes in the developing nervous system. Previous work on superior cervical ganglion (SCG) neurons of the paravertebral sympathetic chain has shown that CD40L reverse signaling enhances NGF‐promoted axon growth and tissue innervation. Here we show that CD40L reverse signaling has the opposite function in prevertebral ganglion (PVG) sympathetic neurons. During a circumscribed perinatal window of development, PVG neurons cultured from Cd40^–/– mice had substantially larger, more exuberant axon arbors in the presence of NGF than PVG neurons cultured from wild‐type mice. Tissues that receive their sympathetic innervation from PVG neurons were markedly hyperinnervated in Cd40^–/– mice compared with wild‐type mice. The exuberant axonal growth phenotype of cultured CD40‐deficient perinatal PVG neurons was pared back to wild‐type levels by activating CD40L reverse signaling with a CD40‐Fc chimeric protein, but not by activating CD40 forward signaling with CD40L. The co‐expression of CD40 and CD40L in PVG neurons suggests that these proteins engage in an autocrine signaling loop in these neurons. Our work shows that CD40L reverse signaling is a physiological regulator of NGF‐promoted sympathetic axon growth and tissue innervation with opposite effects in paravertebral and prevertebral neurons.

Phase I/II parallel double-blind randomized controlled clinical trial of perispinal etanercept for chronic stroke: improved mobility and pain alleviation

Background: Previous open-label studies showed that chronic post-stroke pain could be abated by treatment with perispinal etanercept, although these benefits were questioned. A randomized double-blind placebo controlled clinical trial was conducted to test perispinal etanercept for chronic post-stroke pain.Research design and methods: Participants received two treatments, either perispinal etanercept (active) or saline (control). Primary outcomes were the differences in daily pain levels between groups analyzed by SPSS.Results: On the 0-100 points visual analog scale, perispinal etanercept reduced mean levels for worst and average daily pain from baseline after two treatments by 19.5 - 24 points (p < 0.05), and pain alleviation was maintained in the etanercept group, with no significant change in the control group. Thirty percent of etanercept participants had near complete pain abatement after first treatment. Goniometry of the paretic arm showed improved mean shoulder rotation by 55 degrees in active forward flexion for the etanercept group (p = 0.003) only.Conclusions: Perispinal etanercept can provide significant and ongoing benefits for the chronic post-stroke management of pain and greater shoulder flexion by the paretic arm. Effects are rapid and highly significant, supporting direct action on brain function.Trial registration: ACTRN12615001377527 and Universal Trial Number U1111-1174-3242.

p75NTR and DR6 Regulate Distinct Phases of Axon Degeneration Demarcated by Spheroid Rupture

The regressive events associated with trophic deprivation are critical for sculpting a functional nervous system. After nerve growth factor withdrawal, sympathetic axons derived from male and female neonatal mice maintain their structural integrity for ∼18 h (latent phase) followed by a rapid and near unison disassembly of axons over the next 3 h (catastrophic phase). Here we examine the molecular basis by which axons transition from latent to catastrophic phases of degeneration following trophic withdrawal. Before catastrophic degeneration, we observed an increase in intra-axonal calcium. This calcium flux is accompanied by p75 neurotrophic factor receptor-Rho-actin-dependent expansion of calcium-rich axonal spheroids that eventually rupture, releasing their contents to the extracellular space. Conditioned media derived from degenerating axons are capable of hastening transition into the catastrophic phase of degeneration. We also found that death receptor 6, but not p75 neurotrophic factor receptor, is required for transition into the catastrophic phase in response to conditioned media but not for the intra-axonal calcium flux, spheroid formation, or rupture that occur toward the end of latency. Our results support the existence of an interaxonal degenerative signal that promotes catastrophic degeneration among trophically deprived axons.SIGNIFICANCE STATEMENT Developmental pruning shares several morphological similarities to both disease- and injury-induced degeneration, including spheroid formation. The function and underlying mechanisms governing axonal spheroid formation, however, remain unclear. In this study, we report that axons coordinate each other's degeneration during development via axonal spheroid rupture. Before irreversible breakdown of the axon in response to trophic withdrawal, p75 neurotrophic factor receptor-RhoA signaling governs the formation and growth of spheroids. These spheroids then rupture, allowing exchange of contents ≤10 kDa between the intracellular and extracellular space to drive death receptor 6 and calpain-dependent catastrophic degeneration. This finding informs not only our understanding of regressive events during development but may also provide a rationale for designing new treatments toward myriad neurodegenerative disorders.

Attenuating Neurogenic Sympathetic Hyperreflexia Robustly Improves Antibacterial Immunity After Chronic Spinal Cord Injury

Spinal cord injury (SCI) disrupts critical physiological systems, including the cardiovascular and immune system. Plasticity of spinal circuits below the injury results in abnormal, heightened sympathetic responses, such as extreme, sudden hypertension that hallmarks life-threatening autonomic dysreflexia. Moreover, such sympathetic hyperreflexia detrimentally impacts other effector organs, including the spleen, resulting in spinal cord injury-induced immunodeficiency. Consequently, infection is a leading cause of mortality after SCI. Unfortunately, there are no current treatments that prophylactically limit sympathetic hyperreflexia to prevent subsequent effector organ dysfunction. The cytokine soluble tumor necrosis factor α (sTNFα) is upregulated in the CNS within minutes after SCI and remains elevated. Here, we report that commencing intrathecal administration of XPro1595, an inhibitor of sTNFα, at a clinically feasible, postinjury time point (i.e., 3 d after complete SCI) sufficiently diminishes maladaptive plasticity within the spinal sympathetic reflex circuit. This results in less severe autonomic dysreflexia, a real-time gauge of sympathetic hyperreflexia, for months postinjury. Remarkably, delayed delivery of the sTNFα inhibitor prevents sympathetic hyperreflexia-associated splenic atrophy and loss of leukocytes to dramatically improve the endogenous ability of chronic SCI rats to fight off pneumonia, a common cause of hospitalization after injury. The improved immune function with XPro1595 correlates with less noradrenergic fiber sprouting and normalized norepinephrine levels in the spleen, indicating that heightened, central sTNFα signaling drives peripheral, norepinephrine-mediated organ dysfunction, a novel mechanism of action. Thus, our preclinical study supports intrathecally targeting sTNFα as a viable strategy to broadly attenuate sympathetic dysregulation, thereby improving cardiovascular regulation and immunity long after SCI.SIGNIFICANCE STATEMENT Spinal cord injury (SCI) significantly disrupts immunity, thus increasing susceptibility to infection, a leading cause of morbidity in those living with SCI. Here, we report that commencing intrathecal administration of an inhibitor of the proinflammatory cytokine soluble tumor necrosis factor α days after an injury sufficiently diminishes autonomic dysreflexia, a real time gauge of sympathetic hyperreflexia, to prevent associated splenic atrophy. This dramatically improves the endogenous ability of chronically injured rats to fight off pneumonia, a common cause of hospitalization. This preclinical study could have a significant impact for broadly improving quality of life of SCI individuals.

CD40 forward signalling is a physiological regulator of early sensory axon growth

Multiple members of the tumour necrosis factor superfamily (TNFSF) regulate the growth and branching of neural processes late in development, when neurons are establishing and refining connections. Here, we present the first evidence that a TNFSF member acts much earlier in development, when axons are growing to their targets. CD40L transiently enhanced axon growth from embryonic mouse DRG neurons cultured at this early stage. Early spinal nerves of embryos lacking the CD40L receptor (Cd40^−/− mice) were significantly shorter in vivo than those of Cd40^+/+ littermates. CD40L was synthesized in early DRG targets and was co-expressed with CD40 in early DRG neurons. Whereas CD40L enhanced early axon growth independently of neurotrophins, disruption of a CD40L/CD40 autocrine loop impaired early neurotrophin-promoted axon growth. In marked contrast to the widespread regulation of axon and dendrite growth by CD40L reverse signalling later in development, CD40-Fc, which activates reverse signalling, had no effect on early sensory axon growth. These results suggest that CD40 forward signalling is a novel physiological regulator of early axon growth that acts by target-derived and autocrine mechanisms.

Summary: CD40L, a novel physiological regulator of early sensory axon growth at the stage when sensory axons are growing to their targets, activates CD40 forward signalling by target-derived and autocrine mechanisms.

Regional Differences in the Contributions of TNF Reverse and Forward Signaling to the Establishment of Sympathetic Innervation

Members of the TNF and TNF receptor superfamilies acting by both forward and reverse signaling are increasingly recognized as major physiological regulators of axon growth and tissue innervation in development. Studies of the experimentally tractable superior cervical ganglion (SCG) neurons and their targets have shown that only TNF reverse signaling, not forward signaling, is a physiological regulator of sympathetic innervation. Here, we compared SCG neurons and their targets with prevertebral ganglion (PVG) neurons and their targets. Whereas all SCG targets were markedly hypoinnervated in both TNF-deficient and TNFR1-deficient mice, PVG targets were not hypoinnervated in these mice and one PVG target, the spleen, was significantly hyperinnervated. These in vivo regional differences in innervation density were related to in vitro differences in the responses of SCG and PVG neurons to TNF reverse and forward signaling. Though TNF reverse signaling enhanced SCG axon growth, it did not affect PVG axon growth. Whereas activation of TNF forward signaling in PVG axons inhibited growth, TNF forward signaling could not be activated in SCG axons. These latter differences in the response of SCG and PVG axons to TNF forward signaling were related to TNFR1 expression, whereas PVG axons expressed TNFR1, SCG axons did not. These results show that both TNF reverse and forward signaling are physiological regulators of sympathetic innervation in different tissues.

Balance between BDNF and Semaphorins gates the innervation of the mammary gland

The innervation of the mammary gland is controlled by brain-derived neurotrophic factor (BDNF), and sexually dimorphic sequestering of BDNF by the truncated form of TrkB (TrkB.T1) directs male-specific axonal pruning in mice. It is unknown whether other cues modulate these processes. We detected specific, non-dimorphic, expression of Semaphorin family members in the mouse mammary gland, which signal through PlexinA4. PlexinA4 deletion in both female and male embryos caused developmental hyperinnervation of the gland, which could be reduced by genetic co-reduction of BDNF. Moreover, in males, PlexinA4 ablation delayed axonal pruning, independently of the initial levels of innervation. In support of this, in vitro reduction of BDNF induced axonal hypersensitivity to PlexinA4 signaling. Overall, our study shows that precise sensory innervation of the mammary gland is regulated by the balance between trophic and repulsive signaling. Upon inhibition of trophic signaling, these repulsive factors may promote axonal pruning.

Tumor necrosis factor-α mediated pain hypersensitivity through Ret receptor in resiniferatoxin neuropathy

Neurogenic inflammation is an onset characteristic of small fiber neuropathy (SFN), which is attributed to neuropathic manifestations. Tumor necrosis factor-α (TNFα) is a cytokine that mainly mediates neurogenic inflammation through the ligand receptor TNF receptor 1 (TNFR1), and targeting TNFα/TNFR1 signaling is a direction toward treating inflammatory diseases and injury-induced neuropathy. However, the relationships between TNFα/TNFR1 signaling and Ret signaling, which mediates pain hypersensitivity, remains elusive. This study used resiniferatoxin (RTX), an ultrapotent analog of capsaicin, to generate a mouse model of SFN, leading to marked hindpaw edema (p = 0.013) and parallel the release of TNFα (p = 0.014), which was associated with the upregulation of Ret(+) neurons (p = 0.0043) and partial depletion of TNFR1 caused by colocalization with TRPV1 depleted by RTX. Pharmacological intervention of TNFα with etanercept (Enbrel^®, Wyeth), a clinical application of TNFα blockers, relieved neurogenic inflammation and caused a reduction in hindpaw thickness (p = 0.03) and TNFα releases (p = 0.01), which were determined to be associated with the normalization of mechanical allodynia (p = 0.22). The extraction of either TNFR1(+) or Ret(+) neurons from total of TNFR1(+):Ret(+) neurons indicated that TNFR1(-)/Ret(+) neurons correlated with the mechanical threshold in an antiparallel fashion (r = -0.84, p < 0.0001) but had no relationship with thermal latencies. This study confirmed that TNFα rather than TNFα mediated neuropathic manifestation through the Ret receptor, specifically mechanical allodynia in RTX neuropathy.

TWE-PRIL reverse signalling suppresses sympathetic axon growth and tissue innervation

TWE-PRIL is a naturally occurring fusion protein of components of two TNF superfamily members: the extracellular domain of APRIL; and the intracellular and transmembrane domains of TWEAK with no known function. Here, we show that April^−/− mice (which lack APRIL and TWE-PRIL) exhibited overgrowth of sympathetic fibres in vivo, and sympathetic neurons cultured from these mice had significantly longer axons than neurons cultured from wild-type littermates. Enhanced axon growth from sympathetic neurons cultured from April^−/− mice was prevented by expressing full-length TWE-PRIL in these neurons but not by treating them with soluble APRIL. Soluble APRIL, however, enhanced axon growth from the sympathetic neurons of wild-type mice. siRNA knockdown of TWE-PRIL but not siRNA knockdown of APRIL alone also enhanced axon growth from wild-type sympathetic neurons. Our work reveals the first and physiologically relevant role for TWE-PRIL and suggests that it mediates reverse signalling.

Summary:In vivo and in vitro studies of superior cervical ganglion neurons of April^−/− mice reveal that TWE-PRIL is a physiological regulator of NGF-promoted sympathetic axon growth, acting as a reverse signalling receptor.

Emerging neurotechnology for antinoceptive mechanisms and therapeutics discovery

The tolerance, abuse, and potential exacerbation associated with classical chronic pain medications such as opioids creates a need for alternative therapeutics. Phenotypic screening provides a complementary approach to traditional target-based drug discovery. Profiling cellular phenotypes enables quantification of physiologically relevant traits central to a disease pathology without prior identification of a specific drug target. For complex disorders such as chronic pain, which likely involves many molecular targets, this approach may identify novel treatments. Sensory neurons, termed nociceptors, are derived from dorsal root ganglia (DRG) and can undergo changes in membrane excitability during chronic pain. In this review, we describe phenotypic screening paradigms that make use of nociceptor electrophysiology. The purpose of this paper is to review the bioelectrical behavior of DRG neurons, signaling complexity in sensory neurons, various sensory neuron models, assays for bioelectrical behavior, and emerging efforts to leverage microfabrication and microfluidics for assay development. We discuss limitations and advantages of these various approaches and offer perspectives on opportunities for future development.

Temporally restricted death and the role of p75NTR as a survival receptor in the developing sensory nervous system

The peripheral somatosensory system overproduces neurons early in development followed by a period of cell death during final target innervation. The decision to survive or die in somatosensory neurons of the dorsal root ganglion (DRG) is mediated by target-derived neurotrophic factors and their cognate receptors. Subsets of peripheral somatosensory neurons can be crudely defined by the neurotrophic receptors that they express: peptidergic nociceptors (TrkA+), nonpeptidergic nociceptors (Ret+), mechanoreceptors (Ret+ or TrkB+), and proprioceptors (TrkC+). A direct comparison of early developmental timing between these subsets has not been performed. Here we characterized the accumulation and death of TrkA, B, C, and Ret+ neurons in the DRG as a function of developmental time. We find that TrkB, TrkC, and Ret-expressing neurons in the DRG complete developmental cell death prior to TrkA-expressing neurons. Given the broadly defined roles of the neurotrophin receptor p75NTR in augmenting neurotrophic signaling in sensory neurons, we investigated its role in supporting the survival of these distinct subpopulations. We find that TrkA+, TrkB+, and TrkC+ sensory neuron subpopulations require p75NTR for survival, but proliferating progenitors do not. These data demonstrate how diverging sensory neurons undergo successive waves of cell death and how p75NTR represses the magnitude, but not developmental window of this culling. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 701-717, 2018.

Control of neurite growth and guidance by an inhibitory cell-body signal

The development of a functional nervous system requires tight control of neurite growth and guidance by extracellular chemical cues. Neurite growth is astonishingly sensitive to shallow concentration gradients, but a widely observed feature of both growth and guidance regulation, with important consequences for development and regeneration, is that both are only elicited over the same relatively narrow range of concentrations. Here we show that all these phenomena can be explained within one theoretical framework. We first test long-standing explanations for the suppression of the trophic effects of nerve growth factor at high concentrations, and find they are contradicted by experiment. Instead we propose a new hypothesis involving inhibitory signalling among the cell bodies, and then extend this hypothesis to show how both growth and guidance can be understood in terms of a common underlying signalling mechanism. This new model for the first time unifies several key features of neurite growth regulation, quantitatively explains many aspects of experimental data, and makes new predictions about unknown details of developmental signalling.

T-type Ca2+ channels are required for enhanced sympathetic axon growth by TNFα reverse signalling

Tumour necrosis factor receptor 1 (TNFR1)-activated TNFα reverse signalling, in which membrane-integrated TNFα functions as a receptor for TNFR1, enhances axon growth from developing sympathetic neurons and plays a crucial role in establishing sympathetic innervation. Here, we have investigated the link between TNFα reverse signalling and axon growth in cultured sympathetic neurons. TNFR1-activated TNFα reverse signalling promotes Ca²⁺ influx, and highly selective T-type Ca²⁺ channel inhibitors, but not pharmacological inhibitors of L-type, N-type and P/Q-type Ca²⁺ channels, prevented enhanced axon growth. T-type Ca²⁺ channel-specific inhibitors eliminated Ca²⁺ spikes promoted by TNFα reverse signalling in axons and prevented enhanced axon growth when applied locally to axons, but not when applied to cell somata. Blocking action potential generation did not affect the effect of TNFα reverse signalling on axon growth, suggesting that propagated action potentials are not required for enhanced axon growth. TNFα reverse signalling enhanced protein kinase C (PKC) activation, and pharmacological inhibition of PKC prevented the axon growth response. These results suggest that TNFα reverse signalling promotes opening of T-type Ca²⁺ channels along sympathetic axons, which is required for enhanced axon growth.

CD40 is a major regulator of dendrite growth from developing excitatory and inhibitory neurons

Dendrite size and morphology are key determinants of the functional properties of neurons and neural circuits. Here we show that CD40, a member of the TNF receptor superfamily, is a major regulator of dendrite growth and elaboration in the developing brain. The dendrites of hippocampal excitatory neurons were markedly stunted in Cd40^-/- mice, whereas those of striatal inhibitory neurons were much more exuberant. These striking and opposite phenotypic changes were also observed in excitatory and inhibitory neurons cultured from Cd40^-/- mice and were rescued by soluble CD40. The changes in excitatory and inhibitory neurons cultured from Cd40^-/- mice were mimicked in neurons of Cd40^+/+ mice by treatment with soluble CD40L and were dependent on PKC-β and PKC-γ, respectively. These results suggest that CD40-activated CD40L reverse signalling has striking and opposite effects on the growth and elaboration of dendrites among major classes of brain neurons by PKC-dependent mechanisms.

Mechanism and therapeutic effectiveness of nerve growth factor in osteoarthritis pain

Osteoarthritis (OA) is the most common form of articular joint arthritis and a cause of significant morbidity. In this review, we present the role of nerve growth factor (NGF) in pain generation, relationship between NGF and OA pain, and pathogenic factors (interleukin-1β, transforming growth factor-β1, mechanical loading, and adipokines) involved in OA development. Since NGF blocking is an efficient way to inhibit OA-associated pain, we summarize four categories of drugs that target NGF/tropomyosin receptor kinase A (TrkA) signaling. In addition, we discuss the future of NGF/TrkA antagonists and underline their potential for use in OA pain relief. A better understanding of the causes and treatment of OA will facilitate the development of more effective methods of OA pain management.

TNFa/TNFR2 signaling is required for glial ensheathment at the dorsal root entry zone

Somatosensory information from the periphery is routed to the spinal cord through centrally-projecting sensory axons that cross into the central nervous system (CNS) via the dorsal root entry zone (DREZ). The glial cells that ensheath these axons ensure rapid propagation of this information. Despite the importance of this glial-axon arrangement, how this afferent nerve is assembled during development is unknown. Using in vivo, time-lapse imaging we show that as centrally-projecting pioneer axons from dorsal root ganglia (DRG) enter the spinal cord, they initiate expression of the cytokine TNFalpha. This induction coincides with ensheathment of these axons by associated glia via a TNF receptor 2 (TNFR2)-mediated process. This work identifies a signaling cascade that mediates peripheral glial-axon interactions and it functions to ensure that DRG afferent projections are ensheathed after pioneer axons complete their navigation, which promotes efficient somatosensory neural function.

TNF-α/TNFR1 Signaling is Required for the Full Expression of Acute and Chronic Itch in Mice via Peripheral and Central Mechanisms

Increasing evidence suggests that cytokines and chemokines play crucial roles in chronic itch. In the present study, we evaluated the roles of tumor necrosis factor-alpha (TNF-α) and its receptors TNF receptor subtype-1 (TNFR1) and TNFR2 in acute and chronic itch in mice. Compared to wild-type (WT) mice, TNFR1-knockout (TNFR1-KO) and TNFR1/R2 double-KO (DKO), but not TNFR2-KO mice, exhibited reduced acute itch induced by compound 48/80 and chloroquine (CQ). Application of the TNF-synthesis inhibitor thalidomide and the TNF-α antagonist etanercept dose-dependently suppressed acute itch. Intradermal injection of TNF-α was not sufficient to evoke scratching, but potentiated itch induced by compound 48/80, but not CQ. In addition, compound 48/80 induced TNF-α mRNA expression in the skin, while CQ induced its expression in the dorsal root ganglia (DRG) and spinal cord. Furthermore, chronic itch induced by dry skin was reduced by administration of thalidomide and etanercept and in TNFR1/R2 DKO mice. Dry skin induced TNF-α expression in the skin, DRG, and spinal cord and TNFR1 expression only in the spinal cord. Thus, our findings suggest that TNF-α/TNFR1 signaling is required for the full expression of acute and chronic itch via peripheral and central mechanisms, and targeting TNFR1 may be beneficial for chronic itch treatment.

Death Receptor 6 Promotes Wallerian Degeneration in Peripheral Axons

Axon degeneration during development is required to sculpt a functional nervous system and is also a hallmark of pathological insult, such as injury [1, 2]. Despite similar morphological characteristics, very little overlap in molecular mechanisms has been reported between pathological and developmental degeneration [3-5]. In the peripheral nervous system (PNS), developmental axon pruning relies on receptor-mediated extrinsic degeneration mechanisms to determine which axons are maintained or degenerated [5-7]. Receptors have not been implicated in Wallerian axon degeneration; instead, axon autonomous, intrinsic mechanisms are thought to be the primary driver for this type of axon disintegration [8-10]. Here we survey the role of neuronally expressed, paralogous tumor necrosis factor receptor super family (TNFRSF) members in Wallerian degeneration. We find that an orphan receptor, death receptor 6 (DR6), is required to drive axon degeneration after axotomy in sympathetic and sensory neurons cultured in microfluidic devices. We sought to validate these in vitro findings in vivo using a transected sciatic nerve model. Consistent with the in vitro findings, DR6^-/- animals displayed preserved axons up to 4 weeks after injury. In contrast to phenotypes observed in Wld^s and Sarm1^-/- mice, preserved axons in DR6^-/- animals display profound myelin remodeling. This indicates that deterioration of axons and myelin after axotomy are mechanistically distinct processes. Finally, we find that JNK signaling after injury requires DR6, suggesting a link between this novel extrinsic pathway and the axon autonomous, intrinsic pathways that have become established for Wallerian degeneration.