Regulation of TRPV2 by axotomy in sympathetic, but not sensory neurons

Effects of peripheral nerve injury on parvalbumin expression in adult rat dorsal root ganglion neurons

Background

Parvalbumin (PV) is a calcium binding protein that identifies a subpopulation of proprioceptive dorsal root ganglion (DRG) neurons. Calcitonin gene-related peptide (CGRP) is also expressed in a high proportion of muscle afferents but its relationship to PV is unclear. Little is known of the phenotypic responses of muscle afferents to nerve injury. Sciatic nerve axotomy or L5 spinal nerve ligation and section (SNL) lesions were used to explore these issues in adult rats using immunocytochemistry.

Results

In naive animals, the mean PV expression was 25 % of L4 or L5 dorsal root ganglion (DRG) neurons, and this was unchanged 2 weeks after sciatic nerve axotomy. Colocalization studies with the injury marker activating transcription factor 3 (ATF3) showed that approximately 24 % of PV neurons expressed ATF3 after sciatic nerve axotomy suggesting that PV may show a phenotypic switch from injured to uninjured neurons. This possibility was further assessed using the spinal nerve ligation (SNL) injury model where injured and uninjured neurons are located in different DRGs. Two weeks after L5 SNL there was no change in total PV staining and essentially all L5 PV neurons expressed ATF3. Additionally, there was no increase in PV-ir in the adjacent uninjured L4 DRG cells. Co-labelling of DRG neurons revealed that less than 2 % of PV neurons normally expressed CGRP and no colocalization was seen after injury.

Conclusion

These experiments clearly show that axotomy does not produce down regulation of PV protein in the DRG. Moreover, this lack of change is not due to a phenotypic switch in PV immunoreactive (ir) neurons, or de novo expression of PV-ir in uninjured neurons after nerve injury. These results further illustrate differences that occur when muscle afferents are injured as compared to cutaneous afferents.

Nerve Growth Factor Regulates Transient Receptor Potential Vanilloid 2 via Extracellular Signal-Regulated Kinase Signaling To Enhance Neurite Outgrowth in Developing Neurons

Neurite outgrowth is key to the formation of functional circuits during neuronal development. Neurotrophins, including nerve growth factor (NGF), increase neurite outgrowth in part by altering the function and expression of Ca(2+)-permeable cation channels. Here we report that transient receptor potential vanilloid 2 (TRPV2) is an intracellular Ca(2+)-permeable TRPV channel upregulated by NGF via the mitogen-activated protein kinase (MAPK) signaling pathway to augment neurite outgrowth. TRPV2 colocalized with Rab7, a late endosome protein, in addition to TrkA and activated extracellular signal-regulated kinase (ERK) in neurites, indicating that the channel is closely associated with signaling endosomes. In line with these results, we showed that TRPV2 acts as an ERK substrate and identified the motifs necessary for phosphorylation of TRPV2 by ERK. Furthermore, neurite length, TRPV2 expression, and TRPV2-mediated Ca(2+) signals were reduced by mutagenesis of these key ERK phosphorylation sites. Based on these findings, we identified a previously uncharacterized mechanism by which ERK controls TRPV2-mediated Ca(2+) signals in developing neurons and further establish TRPV2 as a critical intracellular ion channel in neuronal function.

TRP channels and analgesia

Since cloning and characterizing the first nociceptive ion channel Transient Receptor Potential (TRP) Vanilloid 1 (TRPV1), other TRP channels involved in nociception have been cloned and characterized, which include TRP Vanilloid 2 (TRPV2), TRP Vanilloid 3 (TRPV3), TRP Vanilloid 4 (TRPV4), TRP Ankyrin 1 (TRPA1) and TRP Melastatin 8 (TRPM8), more recently TRP Canonical 1, 5, 6 (TRPC1, 5, 6), TRP Melastatin 2 (TRPM2) and TRP Melastatin 3 (TRPM3). These channels are predominantly expressed in C and Aδ nociceptors and transmit noxious thermal, mechanical and chemical sensitivities. TRP channels are modulated by pro-inflammatory mediators, neuropeptides and cytokines. Significant advances have been made targeting these receptors either by antagonists or agonists to treat painful conditions. In this review, we will discuss TRP channels as targets for next generation analgesics and the side effects that may ensue as a result of blocking/activating these receptors, because they are also involved in physiological functions such as release of vasoactive neuropeptides and regulation of vascular tone, maintenance of the body temperature, gastrointestinal motility, urinary bladder control, etc.

Advances in modulating thermosensory TRP channels

INTRODUCTION

Thermosensory channels are a subfamily of the transient receptor potential (TRP) channel family that are activated by changes in the environmental temperature. These channels, known as thermoTRPs, cover the entire spectrum of temperatures, from noxious cold (< 15°C) to injurious heat (> 42°C). In addition, dysfunction of these channels contributes to the thermal hypersensitivity that accompanies painful conditions. Moreover, because of their wide tissue and cellular distribution, thermoTRPs are also involved in the pathophysiology of several diseases, from inflammation to cancer.

AREAS COVERED

Although the number of thermoTRPs is increasing with the identification of novel members such as TRPM3, we will cover the recent advances in the pharmacology of the classical thermosensory channels, namely TRPV1, TRPV2, TRPV3, TRPV4, TRPM8 and TRPA1. This review will focus on the therapeutic progress carried out for all these channels and will highlight the tenet that TRPV1, TRPM8 and TRPA1 are the most exploited channels, and that the interest on TRPV3 and TRPV4 is growing with the first TRPV3 antagonist that moves into Phase-II clinical trials. In contrast, the pharmacology of TRPV2 is yet in its infancy.

EXPERT OPINION

Despite the tremendous academic and industrial investment to develop therapeutic modulators of thermoTRPs, it apparently seems that we are still far from the first successful product, although hope is maintained high for all compounds currently in clinical trials. A major concern has been the appearance of side effects. A better knowledge of the thermosensory protein networks (signal-plexes), along with the application of system biology approaches may provide novel strategies to modulate thermoTRPs activity with improved therapeutic index. A case in point is TRPV1, where acting on interacting proteins is providing new therapeutic opportunities.

AAV8(gfp) preferentially targets large diameter dorsal root ganglion neurones after both intra-dorsal root ganglion and intrathecal injection

Adeno-associated viral vectors (AAV) are increasingly used to deliver therapeutic genes to the central nervous system (CNS) where they promote transgene expression in post mitotic neurones for long periods with little or no toxicity. In adult rat dorsal root ganglia (DRG), we investigated the cellular tropism of AAV8 containing the green fluorescent protein gene (gfp) after either intra-lumbar DRG or intrathecal injection and showed that transduced DRG neurones (DRGN) expressed GFP irrespective of the delivery route, while non-neuronal cells were GFP(-). After intra-DRG delivery of AAV8(gfp), the mean DRGN transduction rate was 11%, while intrathecal delivery transduced a mean of 1.5% DRGN. After intra-DRG injection, 2% of small DRGN (<30 μm in diameter) were GFP(+) compared with 32% of large DRGN (>60 μm in diameter). Axons of transduced DRGN were also GFP(+); no intra-spinal neurones were transduced. A small number of contralateral DRGN were transduced after intra-DRG injection, suggesting that AAV8 may diffuse from injected DRG into the spinal canal. Microglia and astrocytes were highly ramified with increased GFAP(+) immunoreactivity (i.e. activated) in the neuropil around GFP(+) DRG axon projections within the cord after intra-DRG injection. This study showed that after both intra-DRG and intrathecal delivery, strong preferential AAV8 tropism exists for large DRGN unassociated with cell death, but GFP(+) axons projecting in the spinal cord induced local glial activation. These results open up opportunities for targeted delivery of therapeutics such as neurotrophic factors to the injured spinal cord.

New strategies to develop novel pain therapies: addressing thermoreceptors from different points of view

One approach to develop successful pain therapies is the modulation of dysfunctional ion channels that contribute to the detection of thermal, mechanical and chemical painful stimuli. These ion channels, known as thermoTRPs, promote the sensitization and activation of primary sensory neurons known as nociceptors. Pharmacological blockade and genetic deletion of thermoTRP have validated these channels as therapeutic targets for pain intervention. Several thermoTRP modulators have progressed towards clinical development, although most failed because of the appearance of unpredicted side effects. Thus, there is yet a need to develop novel channel modulators with improved therapeutic index. Here, we review the current state-of-the art and illustrate new pharmacological paradigms based on TRPV1 that include: (i) the identification of activity-dependent modulators of this thermoTRP channel; (ii) the design of allosteric modulators that interfere with protein-protein interaction involved in the functional coupling of stimulus sensing and gate opening; and (iii) the development of compounds that abrogate the inflammation-mediated increase of receptor expression in the neuronal surface. These new sites of action represent novel strategies to modulate pathologically active TRPV1, while minimizing an effect on the TRPV1 subpopulation involved in physiological and protective roles, thus increasing their potential therapeutic use.

ThermoTRP channels in nociceptors: taking a lead from capsaicin receptor TRPV1

Nociceptors with peripheral and central projections express temperature sensitive transient receptor potential (TRP) ion channels, also called thermoTRP's. Chemosensitivity of thermoTRP's to certain natural compounds eliciting pain or exhibiting thermal properties has proven to be a good tool in characterizing these receptors. Capsaicin, a pungent chemical in hot peppers, has assisted in the cloning of the first thermoTRP, TRPV1. This discovery initiated the search for other receptors encoding the response to a wide range of temperatures encountered by the body. Of these, TRPV1 and TRPV2 encode unique modalities of thermal pain when exposed to noxious heat. The ability of TRPA1 to encode noxious cold is presently being debated. The role of TRPV1 in peripheral inflammatory pain and central sensitization during chronic pain is well known. In addition to endogenous agonists, a wide variety of chemical agonists and antagonists have been discovered to activate and inhibit TRPV1. Efforts are underway to determine conditions under which agonist-mediated desensitization of TRPV1 or inhibition by antagonists can produce analgesia. Also, identification of specific second messenger molecules that regulate phosphorylation of TRPV1 has been the focus of intense research, to exploit a broader approach to pain treatment. The search for a role of TRPV2 in pain remains dormant due to the lack of suitable experimental models. However, progress into TRPA1's role in pain has received much attention recently. Another thermoTRP, TRPM8, encoding for the cool sensation and also expressed in nociceptors, has recently been shown to reduce pain via a central mechanism, thus opening a novel strategy for achieving analgesia. The role of other thermoTRP's (TRPV3 and TRPV4) encoding for detection of warm temperatures and expressed in nociceptors cannot be excluded. This review will discuss current knowledge on the role of nociceptor thermoTRPs in pain and therapy and describes the activator and inhibitor molecules known to interact with them and modulate their activity.

Dense transient receptor potential cation channel, vanilloid family, type 2 (TRPV2) immunoreactivity defines a subset of motoneurons in the dorsal lateral nucleus of the spinal cord, the nucleus ambiguus and the trigeminal motor nucleus in rat

The transient receptor potential cation channel, vanilloid family, type 2 (TRPV2) is a member of the TRPV family of proteins and is a homologue of the capsaicin/vanilloid receptor (transient receptor potential cation channel, vanilloid family, type 1, TRPV1). Like TRPV1, TRPV2 is expressed in a subset of dorsal root ganglia (DRG) neurons that project to superficial laminae of the spinal cord dorsal horn. Because noxious heat (>52 degrees C) activates TRPV2 in transfected cells this channel has been implicated in the processing of high intensity thermal pain messages in vivo. In contrast to TRPV1, however, which is restricted to small diameter DRG neurons, there is significant TRPV2 immunoreactivity in a variety of CNS regions. The present report focuses on a subset of neurons in the brainstem and spinal cord of the rat including the dorsal lateral nucleus (DLN) of the spinal cord, the nucleus ambiguus, and the motor trigeminal nucleus. Double label immunocytochemistry with markers of motoneurons, combined with retrograde labeling, established that these cells are, in fact, motoneurons. With the exception of their smaller diameter, these cells did not differ from other motoneurons, which are only lightly TRPV2-immunoreactive. As for the majority of DLN neurons, the densely-labeled populations co-express androgen receptor and follow normal DLN ontogeny. The functional significance of the very intense TRPV2 expression in these three distinct spinal cord and brainstem motoneurons groups remains to be determined.

RNA interference-mediated knock-down of transient receptor potential vanilloid 1 prevents forepaw inflammatory hyperalgesia in rat

Transient receptor potential vanilloid (TRPV)1 is a ligand-gated cation channel expressed by primary sensory neurons, including those in the dorsal root ganglia (DRG). TRPV1 plays an essential role in development of inflammatory thermal hyperalgesia after tissue injury and its expression in rat lumbar DRG is increased after hindpaw inflammation. However, the identity of factors mediating forepaw inflammatory hyperalgesia has remained elusive. Here, we examined behavioral responses to noxious thermal stimuli after forepaw inflammation in rats and found that inflammation induced by intraplantar injection of complete Freund's adjuvant significantly reduced hot-plate latency (HPL) at 50 degrees C. TRPV1 expression levels in the ipsilateral cervical DRG were also elevated after forepaw inflammation. By contrast, HPL at 56 degrees C was not shortened after forepaw inflammation and expression of TRPV2, a TRPV1 homolog, in the DRG was not increased. Paratracheal injection of short interfering RNA targeting TRPV1 blocked TRPV1 up-regulation in cervical DRG and abolished inflammation-mediated HPL reductions seen at 50 degrees C. However, thermal hyperalgesia previously established by inflammation was not reversed by short interfering RNA injection. These results indicate that: (i) enhanced TRPV1 expression in cervical DRG is closely associated with development of inflammatory thermal hyperalgesia in the forepaw after tissue injury and (ii) RNA interference targeting TRPV1 prevents inflammatory thermal hyperalgesia after forepaw injuries but does not ameliorate it when already established in a rat model of nociceptive pain representing upper limb injury in humans.

P2X3 receptor mediates heat hyperalgesia in a rat model of trigeminal neuropathic pain

The present study was undertaken to determine the role of P2X3 receptor (P2X3R) on heat hyperalgesia in a newly developed rat model of trigeminal neuropathic pain. The unilateral infraorbital nerve (IoN) was partially ligated by 6-0 silk. To assess heat sensitivity, a vibrissal pad (VP) was placed on a hot plate and the latency until the rats withdrew their head was measured. Mechanical sensitivity of VP was also assessed by the use of von Frey filament. Both heat and mechanical hyperalgesia were observed at the VP ipsilateral to the IoN ligation. The latency to heat stimuli was prolonged after subcutaneous administration of pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, P2X1,2,3,5,7,1/5,2/3R antagonist) and 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP, P2X1,3,2/3,1/5R antagonist). The latency was shortened after administration of alpha,beta-methylene ATP (alpha,beta-meATP, P2X1,3,2/3R agonist), although no changes appeared after administration of beta,gamma-methylene-L-ATP (beta,gamma-me-L-ATP, P2X1R agonist). The protein gene product-9.5 and calcitonin gene-related peptide immunoreactive nerve fibers significantly decreased in the VP skin of ipsilateral to the IoN ligation. In the ipsilateral trigeminal ganglion, the number of P2X3-immunoreactive neurons significantly increased in the small cell group. In this study, we developed an experimental model of trigeminal neuropathic pain by partial ligation of IoN, which produced heat and mechanical hyperalgesia in the VP. Pharmacological and immunohistochemical studies revealed that the P2X3R plays an important role in the heat hyperalgesia observed in this model.

PERSPECTIVE

The study describes the development of a novel model of trigeminal neuropathic pain. Heat hyperalgesia in this model was inhibited by peripheral injection of P2XR antagonists. The results suggest that P2X3R is a potential target for development of a novel therapy for trigeminal neuropathic pain.

Activation properties of heterologously expressed mammalian TRPV2: evidence for species dependence

TRPV2 has been proposed as a potential pain target, in part due to its relatedness to the nociceptor TRPV1 and to its reported activation by noxious high temperatures (>52 degrees C). However, TRPV2 responses to heat as well as to the nonselective agonist 2-aminoethoxydiphenyl borate (2-APB) have not been universally reproduced in other laboratories, leading to debate about the activation properties of this channel. Here, we report the expression of rat, mouse, and human TRPV2 in HEK293 cells and the differential properties of their responses to heat and 2-APB. Expression of mouse or rat TRPV2 in HEK293 cells resulted in robust channel activation when induced by either temperature (>53 degrees C) or 2-APB. By contrast, expression of human TRPV2 did not lead to detectable activation by either of these stimuli. Human TRPV2 protein was expressed at levels comparable with those of rat TRPV2, exhibited similar surface localization and responded to a novelly identified TRPV2 agonist, Delta(9)-tetrahydrocannabinol, indicating that human TRPV2 is functionally expressed on the cell surface. Studies using deletion mutants and chimeras between rat and human TRPV2 indicated that both amino- and carboxyl-cytoplasmic termini of rat TRPV2 are important for responses to heat and 2-APB but can be supplied in trans to form an active channel. The present study not only confirms and extends previous reports demonstrating that rat and mouse TRPV2 respond to 2-APB and noxious heat but also indicates that further investigation will be required to elucidate TRPV2 activation and regulatory mechanisms.

Regulation of neuronal and glial galectin-1 expression by peripheral and central axotomy of rat primary afferent neurons

Galectin-1 (Gal1) is an endogenously-expressed protein important for the embryonic development of the full complement of primary sensory neurons and their synaptic connections in the spinal cord. Gal1 also promotes axonal regeneration following peripheral nerve injury, but the regulation of Gal1 by axotomy in primary afferent neurons has not yet been examined. Here, we show by immunohistochemistry and in situ hybridization that Gal1 expression is differentially regulated by peripheral nerve injury and by dorsal rhizotomy. Following peripheral nerve injury, the proportion of Gal1-positive DRG neurons was increased. An increase in the proportion of large-diameter DRG neurons immunopositive for Gal1 was paralleled by an increase in the depth of immunoreactivity in the dorsal horn, where Gal1-positive terminals are normally restricted to laminae I and II. Dorsal rhizotomy did not affect the proportions of neurons containing Gal1 mRNA or protein, but did deplete the ipsilateral dorsal horn of Gal1 immunoreactivity, indicating that it is transported centrally by dorsal root axons. Dorsal rhizotomy also resulted in an increase in Gal1 mRNA the nerve peripheral to the PNS-CNS interface (likely within Schwann cells and/or macrophages), and to a lesser extent within deafferented spinal cord regions undergoing Wallerian degeneration. This latter increase was notable in the dorsal columns and along the prior trajectories of myelinated afferents into the deeper dorsal horn. These results show that neuronal and glial expressions of Gal1 are tightly correlated with regenerative success. Thus, the differential expression pattern of Gal1 following peripheral axotomy and dorsal rhizotomy suggests that endogenous Gal1 may be a factor important to the regenerative response of injured axons.

New targets for neuropathic pain therapeutics

Neuropathic pain (NeP) is initiated by a lesion or dysfunction in the nervous system. Unlike physiological pain it serves no useful purpose and is usually sustained and chronic. NeP encompasses a wide range of pain syndromes of diverse aetiologies which together account for > 12 million sufferers in the US. Currently, there are a number of therapies available for NeP, including gabapentin, pregabalin, anticonvulsants (tiagabine HCl), tricyclic antidepressants (amitriptyline, nortriptyline) and acetaminophen/opioid combination products (Vicodin, Tylenol #3). However, these products do not provide sufficient pain relief and a significant proportion of sufferers are refractory (60%). Therefore, there is a need for new therapies that provide more predictable efficacy in all patients with improved tolerability. Over the last decade, understanding of the basic mechanisms contributing to the generation of NeP in preclinical animal models has greatly improved. Together with the completion of the various genome sequencing projects and significant advances in microarray and target validation strategies, new therapeutic approaches are being rigourously pursued. This article reviews the rationale behind a number of these mechanism-based approaches, briefly discusses specific challenges that they face, and finally, speculates on the potential of emerging technologies as alternative therapeutic strategies to the traditional 'small-molecule' approach.

Deafferentation and neurotrophin-mediated intraspinal sprouting: a central role for the p75 neurotrophin receptor

Axonal plasticity in the adult spinal cord is governed by intrinsic neuronal growth potential and by extracellular cues. The p75 receptor (p75(NTR)) binds growth-promoting neurotrophins (NTs) as well as the common receptor for growth-inhibiting myelin-derived proteins (the Nogo receptor) and so is well situated to gauge the balance of positive and negative influences on axonal plasticity. Using transgenic mice lacking the extracellular NT-binding domain of p75(NTR) (p75-/- mice), we have examined the influence of p75(NTR) on changes in the density of primary afferent (calcitonin gene-related peptide-expressing) and descending monoaminergic (serotonin- and tyrosine hydroxylase-expressing) projections to the dorsal horn after dorsal rhizotomy, with and without concomitant application of exogenous nerve growth factor and NT-3. We found that, in intact p75-/- mice, the axon density of all populations was equal to or less than that in wild-type mice but that rhizotomy-induced intraspinal sprouting was significantly augmented. Monoaminergic axon sprouting was enhanced in both nerve growth factor- and NT-3-treated p75-/- mice compared with similarly treated wild-type mice. Primary afferent sprouting was particularly robust in NT-3-treated p75-/- mice. These in vivo results illustrate the interactions of p75(NTR) with NTs, with their respective tropomyosin-related kinase receptors and with inhibitory myelin-derived molecules. Our findings illustrate the pivotal role of p75(NTR) in spinal axonal plasticity and identify it as a potential therapeutic target for spinal cord injury.

Altered primary afferent anatomy and reduced thermal sensitivity in mice lacking galectin-1

The transmission of nociceptive information occurs along non-myelinated, or thinly myelinated, primary afferent axons. These axons are generally classified as peptidergic (CGRP-expressing) or non-peptidergic (IB4-binding), although there is a sub-population that is both CGRP-positive and IB4-binding. During neuronal development and following injury, trophic factors and their respective receptors regulate their survival and repair. Recent reports also show that the carbohydrate-binding protein galectin-1 (Gal1), which is expressed by nociceptive primary afferent neurons during development and into adulthood, is involved in axonal pathfinding and regeneration. Here we characterize anatomical differences in dorsal root ganglia (DRG) of Gal1 homozygous null mutant mice (Gal1(-/-)), as well as behavioural differences in tests of nociception. Gal1(-/-) mice have a significantly reduced proportion of IB4-binding DRG neurons, an increased proportion of NF200-immunoreactive DRG neurons, increased depth of central terminals of IB4-binding and CGRP-immunoreactive axons in the dorsal horn, and a reduced number of Fos-positive second order neurons following thermal (cold or hot) stimulation. While there is no difference in the total number of axons in the dorsal root of Gal1(-/-) mice, there are an increased number of myelinated axons, suggesting that in the absence of Gal1, neurons that are normally destined to become IB4-binding instead become NF200-expressing. In addition, mice lacking Gal1 have a decreased sensitivity to noxious thermal stimuli. We conclude that Gal1 is involved in nociceptive neuronal development and that the lack of this protein results in anatomical and functional deficits in adulthood.