Artemin Immunotherapy Is Effective in Preventing and Reversing Cystitis-Induced Bladder Hyperalgesia via TRPA1 Regulation

Artemin sensitizes nociceptors that innervate the osteoarthritic joint to produce pain

OBJECTIVE

There have been significant developments in understanding artemin/GFRα3 signaling in recent years, and there is now accumulating evidence that artemin has important roles to play in pain signaling, including that derived from joint and bone, and that associated with osteorthritis (OA).

METHODS

A total of 163 Sprague-Dawley rats were used in this study. We used an animal model of mono-iodoacetate (MIA)-induced OA, in combination with electrophysiology, behavioral testing, Western blot analysis, and retrograde tracing and immunohistochemistry, to identify roles for artemin/GFRα3 signaling in the pathogenesis of OA pain.

RESULTS

We have found that: 1) GFRα3 is expressed in a substantial proportion of knee joint afferent neurons; 2) exogenous artemin sensitizes knee joint afferent neurons in naïve rats; 3) artemin is expressed in articular tissues of the joint, but not surrounding bone, early in MIA-induced OA; 4) artemin expression increases in bone later in MIA-induced OA when pathology involves subchondral bone; and 5) sequestration of artemin reverses MIA-induced sensitization of both knee joint and bone afferent neurons late in disease when there is inflammation of knee joint tissues and damage to the subchondral bone.

CONCLUSIONS

Our findings show that artemin/GFRα3 signaling has a role to play in the pathogenesis of OA pain, through effects on both knee joint and bone afferent neurons, and suggest that targeted manipulation of artemin/GFRα3 signaling may provide therapeutic benefit for the management of OA pain.

DATA AVAILABILITY

Data are available on request of the corresponding author.

Animal models of interstitial cystitis/bladder pain syndrome

Interstitial Cystitis/Bladder Pain Syndrome (IC/BPS) is a chronic disorder characterized by pelvic and/or bladder pain, along with lower urinary tract symptoms that have a significant impact on an individual's quality of life. The diverse range of symptoms and underlying causes in IC/BPS patients pose a significant challenge for effective disease management and the development of new and effective treatments. To facilitate the development of innovative therapies for IC/BPS, numerous preclinical animal models have been developed, each focusing on distinct pathophysiological components such as localized urothelial permeability or inflammation, psychological stress, autoimmunity, and central sensitization. However, since the precise etiopathophysiology of IC/BPS remains undefined, these animal models have primarily aimed to replicate the key clinical symptoms of bladder hypersensitivity and pain to enhance the translatability of potential therapeutics. Several animal models have now been characterized to mimic the major symptoms of IC/BPS, and significant progress has been made in refining these models to induce chronic symptomatology that more closely resembles the IC/BPS phenotype. Nevertheless, it's important to note that no single model can fully replicate all aspects of the human disease. When selecting an appropriate model for preclinical therapeutic evaluation, consideration must be given to the specific pathology believed to underlie the development of IC/BPS symptoms in a particular patient group, as well as the type and severity of the model, its duration, and the proposed intervention's mechanism of action. Therefore, it is likely that different models will continue to be necessary for preclinical drug development, depending on the unique etiology of IC/BPS being investigated.

Monoclonal antibodies against GFRα3 are efficacious against evoked hyperalgesic and allodynic responses in mouse join pain models but, one of these, REGN5069, was not effective against pain in a randomized, placebo-controlled clinical trial in patients with osteoarthritis pain

The artemin-GFRα3 signaling pathway has been implicated in various painful conditions including migraine, cold allodynia, hyperalgesia, inflammatory bone pain, and mouse knees contain GFRα3-immunoreactive nerve endings. We developed high affinity mouse (REGN1967) and human (REGN5069) GFRα3-blocking monoclonal antibodies and, following in vivo evaluations in mouse models of chronic joint pain (osteoarthritic-like and inflammatory), conducted a first-in-human phase 1 pharmacokinetics (PK) and safety trial of REGN5069 (NCT03645746) in healthy volunteers, and a phase 2 randomized placebo-controlled efficacy and safety trial of REGN5069 (NCT03956550) in patients with knee osteoarthritis (OA) pain. In three commonly used mouse models of chronic joint pain (destabilization of the medial meniscus, intra-articular monoiodoacetate, or Complete Freund's Adjuvant), REGN1967 and REGN5069 attenuated evoked behaviors including tactile allodynia and thermal hyperalgesia without discernably impacting joint pathology or inflammation, prompting us to further evaluate REGN5069 in humans. In the phase 1 study in healthy subjects, the safety profiles of single doses of REGN5069 up to 3000 mg (intravenous) or 600 mg (subcutaneous) were comparable to placebo; PK were consistent with a monoclonal antibody exhibiting target-mediated disposition. In the phase 2 study in patients with OA knee pain, two doses of REGN5069 (100 mg or 1000 mg intravenous every 4 weeks) for 8 weeks failed to achieve the 12-week primary and secondary efficacy endpoints relative to placebo. In addition to possible differences in GFRα3 biology between mice and humans, we highlight here differences in experimental parameters that could have contributed to a different profile of efficacy in mouse models versus human OA pain. Additional research is required to more fully evaluate any potential role of GFRα3 in human pain.

Investigating the Role of Artemin and Its Cognate Receptor, GFRα3, in Osteoarthritis Pain

Osteoarthritis (OA) associated pain (OA-pain) is a significant global problem. OA-pain limits limb use and mobility and is associated with widespread sensitivity. Therapeutic options are limited, and the available options are often associated with adverse effects. The lack of therapeutic options is partly due to a lack of understanding of clinically relevant underlying neural mechanisms of OA-pain. In previous work in naturally occurring OA-pain in dogs, we identified potential signaling molecules (artemin/GFRα3) that were upregulated. Here, we use multiple approaches, including cellular, mouse genetic, immunological suppression in a mouse model of OA, and clinically relevant measures of sensitivity and limb use to explore the functional role of artemin/GFRα3 signaling in OA-pain. We found the monoiodoacetate (MIA)-induced OA-pain in mice is associated with decreased limb use and hypersensitivity. Exogenous artemin induces mechanical, heat, and cold hypersensitivity, and systemic intraperitoneal anti-artemin monoclonal antibody administration reverses this hypersensitivity and restores limb use in mice with MIA-induced OA-pain. An artemin receptor GFRα3 expression is increased in sensory neurons in the MIA model. Our results provide a molecular basis of arthritis pain linked with artemin/GFRα3 signaling and indicate that further work is warranted to investigate the neuronal plasticity and the pathways that drive pain in OA.

Irradiation of the Normal Murine Tongue Causes Upregulation and Activation of Transient Receptor Potential (TRP) Ion Channels

Signal transduction at sensory neurons occurs via transmembrane flux of cations, which is largely governed by the transient receptor potential (TRP) family of ion channels. It is unknown whether TRP channel activation contributes to the pain that accompanies radiation-induced oral mucositis. This study sought to characterize changes in TRP channel expression and function that occur in the locally irradiated tissues and afferent neurons of mice. Female CD-1 mice received singe high-dose (27 Gy) tongue irradiation, or sham irradiation. Animals were euthanized either before overt glossitis developed (days 1 and 5 postirradiation), when glossitis was severe (day 11), or after mice had recovered (days 21 and 45). Tongue irradiation caused upregulation of the Trpv1 gene in trigeminal ganglia (TG) neurons. Other TRP genes (Trpv2, Trpv4, Trpa1, Trpm8) and Gfrα3 (which acts upstream of several TRP channels) were also upregulated in TGs and/or tongue tissue, in response to radiation. Ex vivo calcium imaging experiments demonstrated that the proportions of TG neurons responding to histamine (an activator of TRPV1, TRPV4 and TRPA1), TNF-α (an activator of TRPV1, TRPV2 and TRPV4), and capsaicin (a TRPV1 agonist), were increased as early as one day after tongue irradiation; these changes persisted for at least 21 days. In a subsequent experiment, we found that genetic deletion of TRPV1 mitigated weight loss (a surrogate marker of pain severity) in mice with severe glossitis. The results intimate that various TRP channels, and TRPV1 in particular, should be explored as analgesic targets for patients experiencing pain after oral irradiation.

A Systematic Review of Therapeutic Approaches Used in Experimental Models of Interstitial Cystitis/Bladder Pain Syndrome

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a multifactorial, chronic bladder disorder with limited therapeutic options currently available. The present review provides an extensive overview of therapeutic approaches used in in vitro, ex vivo, and in vivo experimental models of IC/BPS. Publications were identified by electronic search of three online databases. Data were extracted for study design, type of treatment, main findings, and outcome, as well as for methodological quality and the reporting of measures to avoid bias. A total of 100 full-text articles were included. The majority of identified articles evaluated therapeutic agents currently recommended to treat IC/BPS by the American Urological Association guidelines (21%) and therapeutic agents currently approved to treat other diseases (11%). More recently published articles assessed therapeutic approaches using stem cells (11%) and plant-derived agents (10%), while novel potential drug targets identified were proteinase-activated (6%) and purinergic (4%) receptors, transient receptor potential channels (3%), microRNAs (2%), and activation of the cannabinoid system (7%). Our results show that the reported methodological quality of animal studies could be substantially improved, and measures to avoid bias should be more consistently reported in order to increase the value of preclinical research in IC/BPS for potential translation to a clinical setting.

Artemin Is Upregulated by TrkB Agonist and Protects the Immature Retina Against Hypoxic-Ischemic Injury by Suppressing Neuroinflammation and Astrogliosis

Hypoxic-ischemia (HI) is a major cause of acquired visual impairment in children from developed countries. Previous studies have shown that systemic administration of 7,8-dihydroxyavone (DHF), a selective tropomyosin receptor kinase B (TrkB) agonist, provides long-term neuroprotection against HI injury in an immature retina. However, the target genes and the mechanisms of the neuroprotective effects of TrkB signaling are not known. In the present study, we induced an HI retinal injury through unilateral common carotid artery ligation followed by 8% oxygen for 2 h in P7 rat pups. DHF was administered intraperitoneally 2 h before and 18 h after the HI injury. A polymerase chain reaction (PCR) array was used to identify the target genes upregulated after the DHF treatment, which was then confirmed with quantitative real-time reverse transcriptase PCR and a western blot. Effects of the downstream mediator of DHF were assessed using an intravitreal injection of neutralizing antibody 4 h after DHF administration (24 h after HI). Meanwhile, the target protein was injected into the vitreous 24 h after HI to validate its protective effect when exogenously supplemented. We found that systemic DHF treatment after HI significantly increased the expression of the artemin (ARTN) gene and protein at P8 and P10, respectively. The neuroprotective effects of DHF were inhibited after the ARTN protein blockade, with an increase in neuroinflammation and astrogliosis. ARTN treatment showed long-term protection against HI injury at both the histopathological and functional levels. The neuroprotective effects of ARTN were related to a decrease in microglial activation at P17 and attenuation of astrogliosis at P29. ARTN enhances phosphorylation of RET, ERK, and JNK, but not AKT or p38 in the immature retina. Altogether, these results suggest that the neuroprotective effect of a TrkB agonist is partially exerted through a mechanism that involves ARTN because the protective effect is ameliorated by ARTN sequestration. ARTN treatment after HI injury protects the immature retina by attenuating late neuroinflammation and astrogliosis in the immature retina relating to the ARTN/RET/JNK/ERK signaling pathway. ARTN may be a strategy by which to provide long-term protection in the immature retina against HI injury.

Role of TRPA1 in Tissue Damage and Kidney Disease

TRPA1, a nonselective cation channel, is expressed in sensory afferent that innervates peripheral targets. Neuronal TRPA1 can promote tissue repair, remove harmful stimuli and induce protective responses via the release of neuropeptides after the activation of the channel by chemical, exogenous, or endogenous irritants in the injured tissue. However, chronic inflammation after repeated noxious stimuli may result in the development of several diseases. In addition to sensory neurons, TRPA1, activated by inflammatory agents from some non-neuronal cells in the injured area or disease, might promote or protect disease progression. Therefore, TRPA1 works as a molecular sentinel of tissue damage or as an inflammation gatekeeper. Most kidney damage cases are associated with inflammation. In this review, we summarised the role of TRPA1 in neurogenic or non-neurogenic inflammation and in kidney disease, especially the non-neuronal TRPA1. In in vivo animal studies, TRPA1 prevented sepsis-induced or Ang-II-induced and ischemia-reperfusion renal injury by maintaining mitochondrial haemostasis or via the downregulation of macrophage-mediated inflammation, respectively. Renal tubular epithelial TRPA1 acts as an oxidative stress sensor to mediate hypoxia-reoxygenation injury in vitro and ischaemia-reperfusion-induced kidney injury in vivo through MAPKs/NF-kB signalling. Acute kidney injury (AKI) patients with high renal tubular TRPA1 expression had low complete renal function recovery. In renal disease, TPRA1 plays different roles in different cell types accordingly. These findings depict the important role of TRPA1 and warrant further investigation.

Serum artemin is not correlated with sensitivity within dogs with naturally occurring osteoarthritis pain

Osteoarthritis (OA) pain is associated with peripheral and central sensitization in humans and results in widespread increased sensitivity across the body. Sensitization contributes to the OA-associated pain (OAP) state. We recently identified increased levels of an endogenous neurotrophic factor, artemin (ARTN), in dogs with OAP compared to healthy pain-free controls. Circulating ARTN released from damaged tissues in OA, may play a central role in widespread sensitivity and pain. However, the relationship between ARTN and somatosensory sensitivity remains unknown. The study aimed to assess the relationship between serum ARTN concentrations and measures of sensitivity in dogs with OAP using quantitative sensory testing. We hypothesized that there would be a positive association between circulating ARTN and increased sensitivity to mechanical and thermal stimuli in dogs with OAP. We used linear and logistic regression models to assess the relationship between ARTN, sensitization, and pain within a cohort of 43 dogs with spontaneous OAP. Serum ARTN was not associated with the degree of sensitization within dogs with OAP. Further, across dogs with varying OAP severity, we did not find any association between ARTN, and clinical measures of joint pain and disability. Although a relationship between ARTN and joint pain was not ruled out.

A ligand-receptor interactome platform for discovery of pain mechanisms and therapeutic targets

In the peripheral nervous system, ligand-receptor interactions between cells and neurons shape sensory experience, including pain. We set out to identify the potential interactions between sensory neurons and peripheral cell types implicated in disease-associated pain. Using mouse and human RNA sequencing datasets and computational analysis, we created interactome maps between dorsal root ganglion (DRG) sensory neurons and an array of normal cell types, as well as colitis-associated glial cells, rheumatoid arthritis-associated synovial macrophages, and pancreatic tumor tissue. These maps revealed a common correlation between the abundance of heparin-binding EGF-like growth factor (HBEGF) in peripheral cells with that of its receptor EGFR (a member of the ErbB family of receptors) in DRG neurons. Subsequently, we confirmed that increased abundance of HBEGF enhanced nociception in mice, likely acting on DRG neurons through ErbB family receptors. Collectively, these interactomes highlight ligand-receptor interactions that may lead to treatments for disease-associated pain and, furthermore, reflect the complexity of cell-to-neuron signaling in chronic pain states.

Tumor Immune Evasion Induced by Dysregulation of Erythroid Progenitor Cells Development

Cancer cells harness normal cells to facilitate tumor growth and metastasis. Within this complex network of interactions, the establishment and maintenance of immune evasion mechanisms are crucial for cancer progression. The escape from the immune surveillance results from multiple independent mechanisms. Recent studies revealed that besides well-described myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs) or regulatory T-cells (Tregs), erythroid progenitor cells (EPCs) play an important role in the regulation of immune response and tumor progression. EPCs are immature erythroid cells that differentiate into oxygen-transporting red blood cells. They expand in the extramedullary sites, including the spleen, as well as infiltrate tumors. EPCs in cancer produce reactive oxygen species (ROS), transforming growth factor β (TGF-β), interleukin-10 (IL-10) and express programmed death-ligand 1 (PD-L1) and potently suppress T-cells. Thus, EPCs regulate antitumor, antiviral, and antimicrobial immunity, leading to immune suppression. Moreover, EPCs promote tumor growth by the secretion of growth factors, including artemin. The expansion of EPCs in cancer is an effect of the dysregulation of erythropoiesis, leading to the differentiation arrest and enrichment of early-stage EPCs. Therefore, anemia treatment, targeting ineffective erythropoiesis, and the promotion of EPC differentiation are promising strategies to reduce cancer-induced immunosuppression and the tumor-promoting effects of EPCs.

Interplay of BDNF and GDNF in the Mature Spinal Somatosensory System and Its Potential Therapeutic Relevance

The growth factors BDNF and GDNF are gaining more and more attention as modulators of synaptic transmission in the mature central nervous system (CNS). The two molecules undergo a regulated secretion in neurons and may be anterogradely transported to terminals where they can positively or negatively modulate fast synaptic transmission. There is today a wide consensus on the role of BDNF as a pro-nociceptive modulator, as the neurotrophin has an important part in the initiation and maintenance of inflammatory, chronic, and/or neuropathic pain at the peripheral and central level. At the spinal level, BDNF intervenes in the regulation of chloride equilibrium potential, decreases the excitatory synaptic drive to inhibitory neurons, with complex changes in GABAergic/glycinergic synaptic transmission, and increases excitatory transmission in the superficial dorsal horn. Differently from BDNF, the role of GDNF still remains to be unraveled in full. This review resumes the current literature on the interplay between BDNF and GDNF in the regulation of nociceptive neurotransmission in the superficial dorsal horn of the spinal cord. We will first discuss the circuitries involved in such a regulation, as well as the reciprocal interactions between the two factors in nociceptive pathways. The development of small molecules specifically targeting BDNF, GDNF and/or downstream effectors is opening new perspectives for investigating these neurotrophic factors as modulators of nociceptive transmission and chronic pain. Therefore, we will finally consider the molecules of (potential) pharmacological relevance for tackling normal and pathological pain.

Revisiting the Role of Neurotrophic Factors in Inflammation

The neurotrophic factors are well known for their implication in the growth and the survival of the central, sensory, enteric and parasympathetic nervous systems. Due to these properties, neurturin (NRTN) and Glial cell-derived neurotrophic factor (GDNF), which belong to the GDNF family ligands (GFLs), have been assessed in clinical trials as a treatment for neurodegenerative diseases like Parkinson’s disease. In addition, studies in favor of a functional role for GFLs outside the nervous system are accumulating. Thus, GFLs are present in several peripheral tissues, including digestive, respiratory, hematopoietic and urogenital systems, heart, blood, muscles and skin. More precisely, recent data have highlighted that different types of immune and epithelial cells (macrophages, T cells, such as, for example, mucosal-associated invariant T (MAIT) cells, innate lymphoid cells (ILC) 3, dendritic cells, mast cells, monocytes, bronchial epithelial cells, keratinocytes) have the capacity to release GFLs and express their receptors, leading to the participation in the repair of epithelial barrier damage after inflammation. Some of these mechanisms pass on to ILCs to produce cytokines (such as IL-22) that can impact gut microbiota. In addition, there are indications that NRTN could be used in the treatment of inflammatory airway diseases and it prevents the development of hyperglycemia in the diabetic rat model. On the other hand, it is suspected that the dysregulation of GFLs produces oncogenic effects. This review proposes the discussion of the biological understanding and the potential new opportunities of the GFLs, in the perspective of developing new treatments within a broad range of human diseases.

Correlation of Artemin and GFRα3 With Osteoarthritis Pain: Early Evidence From Naturally Occurring Osteoarthritis-Associated Chronic Pain in Dogs

Arthritis, including osteoarthritis (OA) and other musculoskeletal-associated pain, is a worldwide problem, however, effective drug options are limited. Several receptors, neurotransmitters, and endogenous mediators have been identified in rodent models, but the relevance of these molecules in disease-associated pain is not always clear. Artemin, a neurotrophic factor, and its receptor, glial-derived neurotrophic factor (GDNF) family receptor alpha-3 (GFRα3), have been identified as involved in pain in rodents. Their role in OA-associated pain is unknown. To explore a possible association, we analyzed tissue from naturally occurring OA in dogs to characterize the correlation with chronic pain. We used behavioral assessment, objective measures of limb use, and molecular tools to identify whether artemin and GFRα3 might be associated with OA pain. Our results using banked tissue from well-phenotyped dogs indicates that artemin/GFRα3 may play an important, and hitherto unrecognized, role in chronic OA-associated pain. Elevated serum levels of artemin from osteoarthritic humans compared to healthy individuals suggest translational relevance. Our data provide compelling evidence that the artemin/GFRα3 signaling pathway may be important in OA pain in both non-humans and humans and may ultimately lead to novel therapeutics.

Extrinsic Primary Afferent Neurons Link Visceral Pain to Colon Motility Through a Spinal Reflex in Mice

BACKGROUND & AIMS

Proper colon function requires signals from extrinsic primary afferent neurons (ExPANs) located in spinal ganglia. Most ExPANs express the vanilloid receptor TRPV1, and a dense plexus of TRPV1-positive fibers is found around myenteric neurons. Capsaicin, a TRPV1 agonist, can initiate activity in myenteric neurons and produce muscle contraction. ExPANs might therefore form motility-regulating synapses onto myenteric neurons. ExPANs mediate visceral pain, and myenteric neurons mediate colon motility, so we investigated communication between ExPANs and myenteric neurons and the circuits by which ExPANs modulate colon function.

METHODS

In live mice and colon tissues that express a transgene encoding the calcium indicator GCaMP, we visualized levels of activity in myenteric neurons during smooth muscle contractions induced by application of capsaicin, direct colon stimulation, stimulation of ExPANs, or stimulation of preganglionic parasympathetic neuron (PPN) axons. To localize central targets of ExPANs, we optogenetically activated TRPV1-expressing ExPANs in live mice and then quantified Fos immunoreactivity to identify activated spinal neurons.

RESULTS

Focal electrical stimulation of mouse colon produced phased-locked calcium signals in myenteric neurons and produced colon contractions. Stimulation of the L6 ventral root, which contains PPN axons, also produced myenteric activation and contractions that were comparable to those of direct colon stimulation. Surprisingly, capsaicin application to the isolated L6 dorsal root ganglia, which produced robust calcium signals in neurons throughout the ganglion, did not activate myenteric neurons. Electrical activation of the ganglia, which activated even more neurons than capsaicin, did not produce myenteric activation or contractions unless the spinal cord was intact, indicating that a complete afferent-to-efferent (PPN) circuit was necessary for ExPANs to regulate myenteric neurons. In TRPV1-channel rhodopsin-2 mice, light activation of ExPANs induced a pain-like visceromotor response and expression of Fos in spinal PPN neurons.

CONCLUSIONS

In mice, ExPANs regulate myenteric neuron activity and smooth muscle contraction via a parasympathetic spinal circuit, linking sensation and pain to motility.

Visceral Pain

Most of us live blissfully unaware of the orchestrated function that our internal organs conduct. When this peace is interrupted, it is often by routine sensations of hunger and urge. However, for >20% of the global population, chronic visceral pain is an unpleasant and often excruciating reminder of the existence of our internal organs. In many cases, there is no obvious underlying pathological cause of the pain. Accordingly, chronic visceral pain is debilitating, reduces the quality of life of sufferers, and has large concomitant socioeconomic costs. In this review, we highlight key mechanisms underlying chronic abdominal and pelvic pain associated with functional and inflammatory disorders of the gastrointestinal and urinary tracts. This includes how the colon and bladder are innervated by specialized subclasses of spinal afferents, how these afferents become sensitized in highly dynamic signaling environments, and the subsequent development of neuroplasticity within visceral pain pathways. We also highlight key contributing factors, including alterations in commensal bacteria, altered mucosal permeability, epithelial interactions with afferent nerves, alterations in immune or stress responses, and cross talk between these two adjacent organs.

Mechanisms Underlying Overactive Bladder and Interstitial Cystitis/Painful Bladder Syndrome

The bladder is innervated by extrinsic afferents that project into the dorsal horn of the spinal cord, providing sensory input to the micturition centers within the central nervous system. Under normal conditions, the continuous activation of these neurons during bladder distension goes mostly unnoticed. However, for patients with chronic urological disorders such as overactive bladder syndrome (OAB) and interstitial cystitis/painful bladder syndrome (IC/PBS), exaggerated bladder sensation and altered bladder function are common debilitating symptoms. Whilst considered to be separate pathological entities, there is now significant clinical and pre-clinical evidence that both OAB and IC/PBS are related to structural, synaptic, or intrinsic changes in the complex signaling pathways that mediate bladder sensation. This review discusses how urothelial dysfunction, bladder permeability, inflammation, and cross-organ sensitisation between visceral organs can regulate this neuroplasticity. Furthermore, we discuss how the emotional affective component of pain processing, involving dysregulation of the HPA axis and maladaptation to stress, anxiety and depression, can exacerbate aberrant bladder sensation and urological dysfunction. This review reveals the complex nature of urological disorders, highlighting numerous interconnected mechanisms in their pathogenesis. To find appropriate therapeutic treatments for these disorders, it is first essential to understand the mechanisms responsible, incorporating research from every level of the sensory pathway, from bladder to brain.

Attenuated lipopolysaccharide-induced inflammatory bladder hypersensitivity in mice deficient of transient receptor potential ankilin1

Transient receptor potential ankyrin 1 (TRPA1) channel expressed by urothelial cells and bladder sensory nerve fibers might act as a bladder mechanosensor and nociceptive transducer. To disclose the role of TRPA1 in bladder function and inflammation-associated hypersensitivity, we evaluated in vitro and in vivo bladder function and inflammatory mechanosensory and nociceptive responses to intravesical lipopolysaccharide (LPS)-instillation in wild type (WT) and TRPA1-knock out (KO) mice. At baseline before treatment, no significant differences were observed in frequency volume variables, in vitro detrusor contractility, and cystometric parameters between the two groups in either sex. LPS-instillation significantly increased voiding frequency and decreased mean voided volume at 24-48 hours after instillation in WT but not in TRPA1-KO mice. LPS-instillation also significantly increased the number of pain-like behavior at 24 hours after instillation in WT mice, but not in TRPA1-KO mice. Cystometry 24 hours after LPS-instillation revealed shorter inter-contraction intervals in the WT mice compared with TRPA1-KO mice. In contrast, inflammatory cell infiltration in the bladder suburothelial layer was not significantly different between the two groups. These results indicate that TRPA1 channels are involved in bladder mechanosensory and nociceptive hypersensitivity accompanied with inflammation but not in physiological bladder function or development of bladder inflammation.

Sequestration of artemin reduces inflammation-induced activation and sensitization of bone marrow nociceptors in a rodent model of carrageenan-induced inflammatory bone pain

BACKGROUND

Pathologies that affect the bone marrow have a significant inflammatory component; however, it is not clear how inflammatory mediators affect nociceptive nerve terminals within the marrow cavity.

METHODS

In this study, an in vivo bone-nerve preparation was used to directly record the physiological response properties of bone marrow nociceptors innervating the tibial marrow cavity of rats, before and after application of the inflammatory agent carrageenan. In addition, endogenous artemin was sequestered by application of an artemin neutralizing antibody to determine if this could prevent the inflammation-induced physiological changes observed.

RESULTS

A single injection of carrageenan administered into the tibial marrow cavity produced rapid changes in weight bearing (pain-like behaviour) in conscious animals. Carrageenan, but not saline, activated bone marrow nociceptors in whole-nerve recordings and sensitized a subtype of Aδ-bone marrow nociceptors to mechanical stimulation. The activation and sensitization had a rapid time course that matched that of pain-like behaviours. Sequestration of endogenous artemin significantly reduced carrageenan-induced increases in ongoing activity and completely abolished sensitization of bone marrow nociceptors to mechanical stimulation.

CONCLUSIONS

These observations indicate that inflammation affects the activity and sensitivity of bone marrow nociceptors; that artemin plays a role in these changes; and that artemin might be a promising target for pharmacological manipulations in the treatment of inflammatory bone pain.

SIGNIFICANCE

Most pathologies that affect the bone marrow have an inflammatory component. We have used a model of carrageenan-induced inflammation to show that sequestration of artemin reduces inflammation-induced activation and sensitization of bone marrow nociceptors. Our findings suggest that artemin signalling is a target for the treatment of inflammatory bone pain.

Optogenetic Activation of Colon Epithelium of the Mouse Produces High-Frequency Bursting in Extrinsic Colon Afferents and Engages Visceromotor Responses

Epithelial cells of the colon provide a vital interface between the internal environment (lumen of the colon) and colon parenchyma. To examine epithelial-neuronal signaling at this interface, we analyzed mice in which channelrhodopsin (ChR2) was targeted to either TRPV1-positive afferents or to villin-expressing colon epithelial cells. Expression of a ChR2-EYFP fusion protein was directed to either primary sensory neurons or to colon epithelial cells by crossing Ai32 mice with TRPV1-Cre or villin-Cre mice, respectively. An ex vivo preparation of the colon was used for single-fiber analysis of colon sensory afferents of the pelvic nerve. Afferents were characterized using previously described criteria as mucosal, muscular, muscular-mucosal, or serosal and then tested for blue light-induced activation. Light activation of colon epithelial cells produced robust firing of action potentials, similar to that elicited by physiologic stimulation (e.g., circumferential stretch), in 50.5% of colon afferents of mice homozygous for ChR2 expression. Light-induced activity could be reduced or abolished in most fibers using a cocktail of purinergic receptor blockers suggesting ATP release by the epithelium contributed to generation of sensory neuron action potentials. Using electromyographic recording of visceromotor responses we found that light stimulation of the colon epithelium evoked behavioral responses in Vil-ChR2 mice that was similar to that seen with balloon distension of the colon. These ex vivo and in vivo data indicate that light stimulation of colon epithelial cells alone, without added mechanical or chemical stimuli, can directly activate colon afferents and elicit behavioral responses.SIGNIFICANCE STATEMENT Abdominal pain that accompanies inflammatory diseases of the bowel is particularly vexing because it can occur without obvious changes in the structure or inflammatory condition of the colon. Pain reflects abnormal sensory neuron activity that may be controlled in part by release of substances from lining epithelial cells. In support of this mechanism we determined that blue-light stimulation of channelrhodopsin-expressing colon epithelial cells could evoke action potential firing in sensory neurons and produce changes in measures of behavioral sensitivity. Thus, activity of colon epithelial cells alone, without added mechanical or chemical stimuli, is sufficient to activate pain-sensing neurons.

Role of neurogenic inflammation in local communication in the visceral mucosa

Intense research has focused on the involvement of the nervous system in regard to cellular mechanisms underlying neurogenic inflammation in the pelvic viscera. Evidence supports the neural release of inflammatory factors, trophic factors, and neuropeptides in the initiation of inflammation. However, more recently, non-neuronal cells including epithelia, endothelial, mast cells, and paraneurons are likely important participants in nervous system functions. For example, the urinary bladder urothelial cells are emerging as key elements in the detection and transmission of both physiological and nociceptive stimuli in the lower urinary tract. There is mounting evidence that these cells are involved in sensory mechanisms and can release mediators. Further, localization of afferent nerves next to the urothelium suggests these cells may be targets for transmitters released from bladder nerves and that chemicals released by urothelial cells may alter afferent excitability. Modifications of this type of communication in a number of pathological conditions can result in altered release of epithelial-derived mediators, which can activate local sensory nerves. Taken together, these and other findings highlighted in this review suggest that neurogenic inflammation involves complex anatomical and physiological interactions among a number of cell types in the bladder wall. The specific factors and pathways that mediate inflammatory responses in both acute and chronic conditions are not well understood and need to be further examined. Elucidation of mechanisms impacting on these pathways may provide insights into the pathology of various types of disorders involving the pelvic viscera.

GDNF, Neurturin, and Artemin Activate and Sensitize Bone Afferent Neurons and Contribute to Inflammatory Bone Pain

Pain associated with skeletal pathology or disease is a significant clinical problem, but the mechanisms that generate and/or maintain it remain poorly understood. In this study, we explored roles for GDNF, neurturin, and artemin signaling in bone pain using male Sprague Dawley rats. We have shown that inflammatory bone pain involves activation and sensitization of peptidergic, NGF-sensitive neurons via artemin/GDNF family receptor α-3 (GFRα3) signaling pathways, and that sequestering artemin might be useful to prevent inflammatory bone pain derived from activation of NGF-sensitive bone afferent neurons. In addition, we have shown that inflammatory bone pain also involves activation and sensitization of nonpeptidergic neurons via GDNF/GFRα1 and neurturin/GFRα2 signaling pathways, and that sequestration of neurturin, but not GDNF, might be useful to treat inflammatory bone pain derived from activation of nonpeptidergic bone afferent neurons. Our findings suggest that GDNF family ligand signaling pathways are involved in the pathogenesis of bone pain and could be targets for pharmacological manipulations to treat it.SIGNIFICANCE STATEMENT Pain associated with skeletal pathology, including bone cancer, bone marrow edema syndromes, osteomyelitis, osteoarthritis, and fractures causes a major burden (both in terms of quality of life and cost) on individuals and health care systems worldwide. We have shown the first evidence of a role for GDNF, neurturin, and artemin in the activation and sensitization of bone afferent neurons, and that sequestering these ligands reduces pain behavior in a model of inflammatory bone pain. Thus, GDNF family ligand signaling pathways are involved in the pathogenesis of bone pain and could be targets for pharmacological manipulations to treat it.

Attenuated Activation of Homeostatic Glucocorticoid in Keratinocytes Induces Alloknesis via Aberrant Artemin Production

Intense chronic itch significantly reduces quality of life for atopic dermatitis patients, impairing daily activity. Although abnormal itch sensation can be induced by innocuous stimuli, known as alloknesis, the mechanisms driving this process remain obscure. Psychological and environmental stimuli are known to aggravate atopic dermatitis symptoms. Recently, the enzyme 11β-hydroxysteroid dehydrogenase-1 (HSD11β1), which is expressed in keratinocytes, has been implicated in maintaining homeostasis against environmental stimuli by activating endogenous glucocorticoids. To investigate the role of HSD11β1 in keratinocytes, we generated keratinocyte-specific Hsd11b1-knockout (Hsd11b1^KC-/-) mice and analyzed skin phenotype. Hsd11b1^KC-/- mice exhibited abnormal cutaneous innervation and skin sensitivity, including light mechanical stimulus-evoked itch (i.e., alloknesis). Attenuated endogenous glucocorticoid activation induced by aberrant artemin production in keratinocytes was involved in alloknesis in Hsd11b1^KC-/- mice. Finally, we observed a significant negative correlation between expression of HSD11β1 and artemin in human skin with and without AD. These results suggest that endogenous glucocorticoids that maintain skin homeostasis in the epidermis affect both skin innervation and cutaneous sensation. Modulation of HSD11β1 activation could be a therapeutic target for sensitive or itchy skin.

Differential Regulation of Bladder Pain and Voiding Function by Sensory Afferent Populations Revealed by Selective Optogenetic Activation

Bladder-innervating primary sensory neurons mediate reflex-driven bladder function under normal conditions, and contribute to debilitating bladder pain and/or overactivity in pathological states. The goal of this study was to examine the respective roles of defined subtypes of afferent neurons in bladder sensation and function in vivo via direct optogenetic activation. To accomplish this goal, we generated transgenic lines that express a Channelrhodopsin-2-eYFP fusion protein (ChR2-eYFP) in two distinct populations of sensory neurons: TRPV1-lineage neurons (Trpv1^Cre;Ai32, the majority of nociceptors) and Na_v1.8⁺ neurons (Scn10a^Cre;Ai32, nociceptors and some mechanosensitive fibers). In spinal cord, eYFP+ fibers in Trpv1^Cre;Ai32 mice were observed predominantly in dorsal horn (DH) laminae I-II, while in Scn10a^Cre;Ai32 mice they extended throughout the DH, including a dense projection to lamina X. Fiber density correlated with number of retrogradely-labeled eYFP+ dorsal root ganglion neurons (82.2% Scn10a^Cre;Ai32 vs. 62% Trpv1^Cre;Ai32) and degree of DH excitatory synaptic transmission. Photostimulation of peripheral afferent terminals significantly increased visceromotor responses to noxious bladder distension (30–50 mmHg) in both transgenic lines, and to non-noxious distension (20 mmHg) in Scn10a^Cre;Ai32 mice. Depolarization of ChR2+ afferents in Scn10a^Cre;Ai32 mice produced low- and high-amplitude bladder contractions respectively in 53% and 27% of stimulation trials, and frequency of high-amplitude contractions increased to 60% after engagement of low threshold (LT) mechanoreceptors by bladder filling. In Trpv1^Cre;Ai32 mice, low-amplitude contractions occurred in 27% of trials before bladder filling, which was pre-requisite for light-evoked high-amplitude contractions (observed in 53.3% of trials). Potential explanations for these observations include physiological differences in the thresholds of stimulated fibers and their connectivity to spinal circuits.

Serotonergic paraneurones in the female mouse urethral epithelium and their potential role in peripheral sensory information processing

AIM

The mechanisms underlying detection and transmission of sensory signals arising from visceral organs, such as the urethra, are poorly understood. Recently, specialized ACh-expressing cells embedded in the urethral epithelium have been proposed as chemosensory sentinels for detection of bacterial infection. Here, we examined the morphology and potential role in sensory signalling of a different class of specialized cells that express serotonin (5-HT), termed paraneurones.

METHODS

Urethrae, dorsal root ganglia neurones and spinal cords were isolated from adult female mice and used for immunohistochemistry and calcium imaging. Visceromotor reflexes (VMRs) were recorded in vivo.

RESULTS

We identified two morphologically distinct groups of 5-HT+ cells with distinct regional locations: bipolar-like cells predominant in the mid-urethra and multipolar-like cells predominant in the proximal and distal urethra. Sensory nerve fibres positive for calcitonin gene-related peptide, substance P, and TRPV1 were found in close proximity to 5-HT+ paraneurones. In vitro 5-HT (1 μm) stimulation of urethral primary afferent neurones, mimicking 5-HT release from paraneurones, elicited changes in the intracellular calcium concentration ([Ca2+ ]i ) mediated by 5-HT2 and 5-HT3 receptors. Approximately 50% of 5-HT responding cells also responded to capsaicin with changes in the [Ca2+ ]i . In vivo intra-urethral 5-HT application increased VMRs induced by urethral distention and activated pERK in lumbosacral spinal cord neurones.

CONCLUSION

These morphological and functional findings provide insights into a putative paraneurone-neural network within the urethra that utilizes 5-HT signalling, presumably from paraneurones, to modulate primary sensory pathways carrying nociceptive and non-nociceptive (mechano-sensitive) information to the central nervous system.

Urothelial bladder afferent neurons in the rat are anatomically and neurochemically distinct from non-urothelial afferents

There is mounting evidence underscoring a role for the urothelium in urinary bladder sensation. Previous functional studies have identified bladder primary afferents with mechanosensitive properties suggesting urothelial innervation and/or communication. The current study identifies a group of urothelium-innervating afferent neurons in rat, and characterizes and compares the properties of these and non-urothelial afferent neuron populations. Lumbosacral (LS) primary afferent neurons were retrogradely labeled using intraparenchymal (IPar) microinjection or intravesical (IVes) infusion of tracer into the bladder. Using these techniques, separate populations of neurons were differentiated by dorsal root ganglion (DRG) somata labeling and dye distribution within the bladder. IPar- and IVes-labeled neurons accounted for 85.0% and 14.4% of labeled L6-S1 neurons (P < .001), respectively, with only 0.6% of neurons labeled by both techniques. Following IVes labeling, dye was contained only within the periurothelial bladder region in contrast to non-urothelial distribution of dye after IPar labeling. Electrophysiological characterization by in situ patch-clamp recordings from whole-mount DRG preparations indicated no significant difference in passive or active membrane properties of IPar and IVes DRG neurons. However, calcium imaging of isolated neurons indicates that a greater proportion of IPar- than IVes-labeled neurons express functional TRPA1 (45.7% versus 25.6%, respectively; P < .05). This study demonstrates that two anatomically distinct groups of LS bladder afferents can be identified in rat. Further studies of urothelial afferents and the phenotypic differences between non-/urothelial afferents may have important implications for normal and pathophysiological bladder sensory processing.

Urothelial Tight Junction Barrier Dysfunction Sensitizes Bladder Afferents

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a chronic voiding disorder that presents with pain in the urinary bladder and surrounding pelvic region. A growing body of evidence suggests that an increase in the permeability of the urothelium, the epithelial barrier that lines the interior of the bladder, contributes to the symptoms of IC/BPS. To examine the consequence of increased urothelial permeability on pelvic pain and afferent excitability, we overexpressed in the urothelium claudin 2 (Cldn2), a tight junction (TJ)-associated protein whose message is significantly upregulated in biopsies of IC/BPS patients. Consistent with the presence of bladder-derived pain, rats overexpressing Cldn2 showed hypersensitivity to von Frey filaments applied to the pelvic region. Overexpression of Cldn2 increased the expression of c-Fos and promoted the activation of ERK1/2 in spinal cord segments receiving bladder input, which we conceive is the result of noxious stimulation of afferent pathways. To determine whether the mechanical allodynia observed in rats with reduced urothelial barrier function results from altered afferent activity, we examined the firing of acutely isolated bladder sensory neurons. In patch-clamp recordings, about 30% of the bladder sensory neurons from rats transduced with Cldn2, but not controls transduced with GFP, displayed spontaneous activity. Furthermore, bladder sensory neurons with tetrodotoxin-sensitive (TTX-S) action potentials from rats transduced with Cldn2 showed hyperexcitability in response to suprathreshold electrical stimulation. These findings suggest that as a result of a leaky urothelium, the diffusion of urinary solutes through the urothelial barrier sensitizes bladders afferents, promoting voiding at low filling volumes and pain.

Inflammatory and neuropathic cold allodynia are selectively mediated by the neurotrophic factor receptor GFRα3

Tissue injury prompts the release of a number of proalgesic molecules that induce acute and chronic pain by sensitizing pain-sensing neurons (nociceptors) to heat and mechanical stimuli. In contrast, many proalgesics have no effect on cold sensitivity or can inhibit cold-sensitive neurons and diminish cooling-mediated pain relief (analgesia). Nonetheless, cold pain (allodynia) is prevalent in many inflammatory and neuropathic pain settings, with little known of the mechanisms promoting pain vs. those dampening analgesia. Here, we show that cold allodynia induced by inflammation, nerve injury, and chemotherapeutics is abolished in mice lacking the neurotrophic factor receptor glial cell line-derived neurotrophic factor family of receptors-α3 (GFRα3). Furthermore, established cold allodynia is blocked in animals treated with neutralizing antibodies against the GFRα3 ligand, artemin. In contrast, heat and mechanical pain are unchanged, and results show that, in striking contrast to the redundant mechanisms sensitizing other modalities after an insult, cold allodynia is mediated exclusively by a single molecular pathway, suggesting that artemin-GFRα3 signaling can be targeted to selectively treat cold pain.