Thrombospondin-4 divergently regulates voltage-gated Ca2+ channel subtypes in sensory neurons after nerve injury

The role of voltage-gated calcium channel α2δ-1 in the occurrence and development in myofascial orofacial pain

Patients who suffer from myofascial orofacial pain could affect their quality of life deeply. The pathogenesis of pain is still unclear. Our objective was to assess Whether Voltage-gated calcium channel α2δ-1(Cavα2δ-1) is related to myofascial orofacial pain. Rats were divided into the masseter tendon ligation group and the sham group. Compared with the sham group, the mechanical pain threshold of the masseter tendon ligation group was reduced on the 4th, 7th, 10th and 14th day after operation(P < 0.05). On the 14th day after operation, Cavα2δ-1 mRNA expression levels in trigeminal ganglion (TG) and the trigeminal spinal subnucleus caudalis and C1-C2 spinal cervical dorsal horn (Vc/C2) of the masseter tendon ligation group were increased (PTG=0.021, PVc/C2=0.012). Rats were divided into three groups. On the 4th day after ligating the superficial tendon of the left masseter muscle of the rats, 10 ul Cavα2δ-1 antisense oligonucleotide, 10 ul Cavα2δ-1 mismatched oligonucleotides and 10 ul normal saline was separately injected into the left masseter muscle of rats in Cavα2δ-1 antisense oligonucleotide group, Cavα2δ-1 mismatched oligonucleotides group and normal saline control group twice a day for 4 days. The mechanical pain threshold of the Cavα2δ-1 antisense oligonucleotides group was higher than Cavα2δ-1 mismatched oligonucleotides group on the 7th and 10th day after operation (P < 0.01). After PC12 cells were treated with lipopolysaccharide, Cavα2δ-1 mRNA expression level increased (P < 0.001). Cavα2δ-1 may be involved in the occurrence and development in myofascial orofacial pain.

Fabry disease Schwann cells release p11 to induce sensory neuron hyperactivity

Patients with Fabry disease suffer from chronic debilitating pain and peripheral sensory neuropathy with minimal treatment options, but the cellular drivers of this pain are unknown. Here, we propose a mechanism we believe to be novel in which altered signaling between Schwann cells and sensory neurons underlies the peripheral sensory nerve dysfunction we observed in a genetic rat model of Fabry disease. Using in vivo and in vitro electrophysiological recordings, we demonstrated that Fabry rat sensory neurons exhibited pronounced hyperexcitability. Schwann cells probably contributed to this finding because application of mediators released from cultured Fabry Schwann cells induced spontaneous activity and hyperexcitability in naive sensory neurons. We examined putative algogenic mediators using proteomic analysis and found that Fabry Schwann cells released elevated levels of the protein p11 (S100A10), which induced sensory neuron hyperexcitability. Removal of p11 from Fabry Schwann cell media caused hyperpolarization of neuronal resting membrane potentials, indicating that p11 may contribute to the excessive neuronal excitability caused by Fabry Schwann cells. These findings demonstrate that sensory neurons from rats with Fabry disease exhibit hyperactivity caused in part by Schwann cell release of the protein p11.

Pathophysiological roles of thrombospondin-4 in disease development

Thrombospondin-4 (TSP-4) belongs to the extracellular matrix glycoprotein family of thrombospondins (TSPs). The multidomain, pentameric structure of TSP-4 allows its interactions with numerous extracellular matrix components, proteins and signaling molecules that enable its modulation to various physiological and pathological processes. Characterization of TSP-4 expression under development and pathogenesis of disorders has yielded important insights into mechanisms underlying the unique role of TSP-4 in mediating various processes including cell-cell, cell-extracellular matrix interactions, cell migration, proliferation, tissue remodeling, angiogenesis, and synaptogenesis. Maladaptation of these processes in response to pathological insults and stress can accelerate the development of disorders including skeletal dysplasia, osteoporosis, degenerative joint disease, cardiovascular diseases, tumor progression/metastasis and neurological disorders. Overall, the diverse functions of TSP-4 suggest that it may be a potential marker or therapeutic target for prognosis, diagnosis, and treatment of various pathological conditions upon further investigations. This review article highlights recent findings on the role of TSP-4 in both physiological and pathological conditions with a focus on what sets it apart from other TSPs.

Schwann cell release of p11 induces sensory neuron hyperactivity in Fabry disease

Patients with Fabry disease suffer from chronic debilitating pain and peripheral sensory neuropathy with minimal treatment options, but the cellular drivers of this pain are unknown. Here, we propose a novel mechanism by which altered signaling between Schwann cells and sensory neurons underlies the peripheral sensory nerve dysfunction we observe in a genetic rat model of Fabry disease. Using in vivo and in vitro electrophysiological recordings, we demonstrate that Fabry rat sensory neurons exhibit pronounced hyperexcitability. Schwann cells likely contribute to this finding as application of mediators released from cultured Fabry Schwann cells induces spontaneous activity and hyperexcitability in naïve sensory neurons. We examined putative algogenic mediators using proteomic analysis and found that Fabry Schwann cells release elevated levels of the protein p11 (S100-A10) which induces sensory neuron hyperexcitability. Removal of p11 from Fabry Schwann cell media causes hyperpolarization of neuronal resting membrane potential, indicating that p11 contributes to the excessive neuronal excitability caused by Fabry Schwann cells. These findings demonstrate that rats with Fabry disease exhibit sensory neuron hyperexcitability caused in part by Schwann cell release of the protein p11.

Targeting intrinsically disordered regions facilitates discovery of calcium channels 3.2 inhibitory peptides for adeno-associated virus-mediated peripheral analgesia

ABSTRACT

Ample data support a prominent role of peripheral T-type calcium channels 3.2 (Ca V 3.2) in generating pain states. Development of primary sensory neuron-specific inhibitors of Ca V 3.2 channels is an opportunity for achieving effective analgesic therapeutics, but success has been elusive. Small peptides, especially those derived from natural proteins as inhibitory peptide aptamers (iPAs), can produce highly effective and selective blockade of specific nociceptive molecular pathways to reduce pain with minimal off-target effects. In this study, we report the engineering of the potent and selective iPAs of Ca V 3.2 from the intrinsically disordered regions (IDRs) of Ca V 3.2 intracellular segments. Using established prediction algorithms, we localized the IDRs in Ca V 3.2 protein and identified several Ca V 3.2iPA candidates that significantly reduced Ca V 3.2 current in HEK293 cells stably expressing human wide-type Ca V 3.2. Two prototype Ca V 3.2iPAs (iPA1 and iPA2) derived from the IDRs of Ca V 3.2 intracellular loops 2 and 3, respectively, were expressed selectively in the primary sensory neurons of dorsal root ganglia in vivo using recombinant adeno-associated virus (AAV), which produced sustained inhibition of calcium current conducted by Ca V 3.2/T-type channels and significantly attenuated both evoked and spontaneous pain behavior in rats with neuropathic pain after tibial nerve injury. Recordings from dissociated sensory neurons showed that AAV-mediated Ca V 3.2iPA expression suppressed neuronal excitability, suggesting that Ca V 3.2iPA treatment attenuated pain by reversal of injury-induced neuronal hypersensitivity. Collectively, our results indicate that Ca V 3.2iPAs are promising analgesic leads that, combined with AAV-mediated delivery in anatomically targeted sensory ganglia, have the potential to be a selective peripheral Ca V 3.2-targeting strategy for clinical treatment of pain.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na⁺ and Ca²⁺ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K⁺ channels. Because they display limited central side effects, peripherally restricted Na⁺ and Ca²⁺ channel blockers and K⁺ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Na_v1.3, Na_v1.7, Na_v1.8, Ca_v3.2, and HCN2 and activators of K_v7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K⁺ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na⁺ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca²⁺ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing “pain” as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Sigma-1 receptor activity in primary sensory neurons is a critical driver of neuropathic pain

The Sigma-1 receptor (σ1R) is highly expressed in the primary sensory neurons (PSNs) that are the critical site of initiation and maintenance of pain following peripheral nerve injury. By immunoblot and immunohistochemistry, we observed increased expression of both σ1R and σ1R-binding immunoglobulin protein (BiP) in the lumbar (L) dorsal root ganglia (DRG) ipsilateral to painful neuropathy induced by spared nerve injury (SNI). To evaluate the therapeutic potential of PSN-targeted σ1R inhibition at a selected segmental level, we designed a recombinant adeno-associated viral (AAV) vector expressing a small hairpin RNA (shRNA) against rat σ1R. Injection of this vector into the L4/L5 DRGs induced downregulation of σ1R in DRG neurons of all size groups, while expression of BiP was not affected. This was accompanied by attenuation of SNI-induced cutaneous mechanical and thermal hypersensitivity. Whole-cell current-clamp recordings of dissociated neurons showed that knockdown of σ1R suppressed neuronal excitability, suggesting that σ1R silencing attenuates pain by reversal of injury-induced neuronal hyperexcitability. These findings support a critical role of σ1R in modulating PSN nociceptive functions, and that the nerve injury-induced elevated σ1R activity in the PSNs can be a significant driver of neuropathic pain. Further understanding the role of PSN-σ1R in pain pathology may open routes to exploit this system for DRG-targeted pain therapy.

Neuronal contact upregulates astrocytic sphingosine-1-phosphate receptor 1 to coordinate astrocyte-neuron cross communication

Astrocytes, the most abundant glial cells in the mammalian brain, directly associate with and regulate neuronal processes and synapses and are important regulators of brain development. Yet little is known of the molecular mechanisms that control the establishment of astrocyte morphology and the bi-directional communication between astrocytes and neurons. Here we show that neuronal contact stimulates expression of S1PR1, the receptor for the bioactive sphingolipid metabolite sphingosine-1-phosphate (S1P), on perisynaptic astrocyte processes and that S1PR1 drives astrocyte morphological complexity and morphogenesis. Moreover, the S1P/S1PR1 axis increases neuronal contact-induced expression of astrocyte secreted synaptogenic factors SPARCL1 and thrombospondin 4 that are involved in neural circuit assembly. Our findings have uncovered new functions for astrocytic S1PR1 signaling in regulation of bi-directional astrocyte-neuron crosstalk at the nexus of astrocyte morphogenesis and synaptogenesis.

Enhanced T-type calcium channel 3.2 activity in sensory neurons contributes to neuropathic-like pain of monosodium iodoacetate-induced knee osteoarthritis

The monosodium iodoacetate knee osteoarthritis model has been widely used for the evaluation of osteoarthritis pain, but the pathogenesis of associated chronic pain is not fully understood. The T-type calcium channel 3.2 (Ca_V3.2) is abundantly expressed in the primary sensory neurons, in which it regulates neuronal excitability at both the somata and peripheral terminals and facilitates spontaneous neurotransmitter release at the spinal terminals. In this study, we investigated the involvement of primary sensory neuron-Ca_V3.2 activation in monosodium iodoacetate osteoarthritis pain. Knee joint osteoarthritis pain was induced by intra-articular injection of monosodium iodoacetate (2 mg) in rats, and sensory behavior was evaluated for 35 days. At that time, knee joint structural histology, primary sensory neuron injury, and inflammatory gliosis in lumbar dorsal root ganglia, and spinal dorsal horn were examined. Primary sensory neuron-T-type calcium channel current by patch-clamp recording and Ca_V3.2 expression by immunohistochemistry and immunoblots were determined. In a subset of animals, pain relief by Ca_V3.2 inhibition after delivery of Ca_V3.2 inhibitor TTA-P2 into sciatic nerve was investigated. Knee injection of monosodium iodoacetate resulted in osteoarthritis histopathology, weight-bearing asymmetry, sensory hypersensitivity of the ipsilateral hindpaw, and inflammatory gliosis in the ipsilateral dorsal root ganglia, sciatic nerve, and spinal dorsal horn. Neuronal injury marker ATF-3 was extensively upregulated in primary sensory neurons, suggesting that neuronal damage was beyond merely knee-innervating primary sensory neurons. T-type current in dissociated primary sensory neurons from lumbar dorsal root ganglia of monosodium iodoacetate rats was significantly increased, and Ca_V3.2 protein levels in the dorsal root ganglia and spinal dorsal horn ipsilateral to monosodium iodoacetate by immunoblots were significantly increased, compared to controls. Perineural application of TTA-P2 into the ipsilateral sciatic nerve alleviated mechanical hypersensitivity and weight-bearing asymmetry in monosodium iodoacetate osteoarthritis rats. Overall, our findings demonstrate an elevated Ca_V3.2 expression and enhanced function of primary sensory neuron-T channels in the monosodium iodoacetate osteoarthritis pain. Further study is needed to delineate the importance of dysfunctional primary sensory neuron-Ca_V3.2 in osteoarthritis pain.

Neuronal α2δ proteins and brain disorders

α2δ proteins are membrane-anchored extracellular glycoproteins which are abundantly expressed in the brain and the peripheral nervous system. They serve as regulatory subunits of voltage-gated calcium channels and, particularly in nerve cells, regulate presynaptic and postsynaptic functions independently from their role as channel subunits. α2δ proteins are the targets of the widely prescribed anti-epileptic and anti-allodynic drugs gabapentin and pregabalin, particularly for the treatment of neuropathic pain conditions. Recently, the human genes (CACNA2D1-4) encoding for the four known α2δ proteins (isoforms α2δ-1 to α2δ-4) have been linked to a large variety of neurological and neuropsychiatric disorders including epilepsy, autism spectrum disorders, bipolar disorders, schizophrenia, and depressive disorders. Here, we provide an overview of the hitherto identified disease associations of all known α2δ genes, hypothesize on the pathophysiological mechanisms considering their known physiological roles, and discuss the most immanent future research questions. Elucidating their specific physiological and pathophysiological mechanisms may open the way for developing entirely novel therapeutic paradigms for treating brain disorders.

Neuropathic Pain: From Mechanisms to Treatment

Neuropathic pain caused by a lesion or disease of the somatosensory nervous system is a common chronic pain condition with major impact on quality of life. Examples include trigeminal neuralgia, painful polyneuropathy, postherpetic neuralgia, and central poststroke pain. Most patients complain of an ongoing or intermittent spontaneous pain of, for example, burning, pricking, squeezing quality, which may be accompanied by evoked pain, particular to light touch and cold. Ectopic activity in, for example, nerve-end neuroma, compressed nerves or nerve roots, dorsal root ganglia, and the thalamus may in different conditions underlie the spontaneous pain. Evoked pain may spread to neighboring areas, and the underlying pathophysiology involves peripheral and central sensitization. Maladaptive structural changes and a number of cell-cell interactions and molecular signaling underlie the sensitization of nociceptive pathways. These include alteration in ion channels, activation of immune cells, glial-derived mediators, and epigenetic regulation. The major classes of therapeutics include drugs acting on α₂δ subunits of calcium channels, sodium channels, and descending modulatory inhibitory pathways.

Thrombospondin-4 induces prolongation of action potential duration in rat isolated ventricular myocytes

Expression of thrombospondin-4 (TSP-4), a matricellular protein, is increased in the heart tissue of various cardiac disease models. In dorsal root ganglion neurons, TSP-4 inhibits L-type Ca²⁺ channel (LTCC) activity. Although TSP-4 might be related to the electrophysiological properties in heart, it remains to be clarified. The present study aimed to clarify the effects of TSP-4 on action potential (AP), LTCC current (I_CaL) and voltage-dependent K⁺ (Kv) channel current (I_Kv) in rat isolated ventricular myocytes by a patch clamp technique. Ventricular myocytes were isolated from the heart of adult male Wistar rats. The ventricular myocytes were treated with TSP-4 (5 nM) or its vehicle for 4 hr. Then, whole-cell patch clamp technique was performed to measure AP (current-clamp mode) and I_CaL and I_Kv (voltage-clamp mode). The mRNA expression of Kv channels was examined by reverse transcription-polymerase chain reaction. TSP-4 had no effect on the resting membrane potential and peak amplitude of AP. On the other hand, TSP-4 significantly prolonged AP duration (APD) at 50% and 90% repolarization. TSP-4 significantly inhibited the peak amplitudes of I_CaL and I_Kv. TSP-4 had no effect on mRNA expression of Kv channels (Kcna4, Kcna5, Kcnb1, Kcnd2 and Kcnd3). The present study for the first time demonstrated that TSP-4 prolongs APD in rat ventricular myocytes, which is possibly mediated through the suppression of Kv channel activity.

Effects of thrombospondin-4 on pro-inflammatory phenotype differentiation and apoptosis in macrophages

Thrombospondin-4 (TSP-4) attracted renewed attention recently as a result of assignment of new functions to this matricellular protein in cardiovascular, muscular, and nervous systems. We have previously reported that TSP-4 promotes local vascular inflammation in a mouse atherosclerosis model. A common variant of TSP-4, P387-TSP-4, was associated with increased cardiovascular disease risk in human population studies. In a mouse atherosclerosis model, TSP-4 had profound effect on accumulation of macrophages in lesions, which prompted us to examine its effects on macrophages in more detail. We examined the effects of A387-TSP-4 and P387-TSP-4 on mouse macrophages in cell culture and in vivo in the model of LPS-induced peritonitis. In tissues and in cell culture, TSP-4 expression was associated with inflammation: TSP-4 expression was upregulated in peritoneal tissues in LPS-induced peritonitis, and pro-inflammatory signals, INFγ, GM-CSF, and LPS, induced TSP-4 expression in macrophages in vivo and in cell culture. Deficiency in TSP-4 in macrophages from Thbs4^−/− mice reduced the expression of pro-inflammatory macrophage markers, suggesting that TSP-4 facilitates macrophage differentiation into a pro-inflammatory phenotype. Expression of TSP-4, especially more active P387-TSP-4, was associated with higher cellular apoptosis. Cultured macrophages displayed increased adhesion to TSP-4 and reduced migration in presence of TSP-4, and these responses were further increased with P387 variant. We concluded that TSP-4 expression in macrophages increases their accumulation in tissues during the acute inflammatory process and supports macrophage differentiation into a pro-inflammatory phenotype. In a model of acute inflammation, TSP-4 supports pro-inflammatory macrophage apoptosis, a response that is closely related to their pro-inflammatory activity and release of pro-inflammatory signals. P387-TSP-4 was found to be the more active form of TSP-4 in all examined functions.

Direct, gabapentin-insensitive interaction of a soluble form of the calcium channel subunit α2δ-1 with thrombospondin-4

The α₂δ‐1 subunit of voltage-gated calcium channels binds to gabapentin and pregabalin, mediating the analgesic action of these drugs against neuropathic pain. Extracellular matrix proteins from the thrombospondin (TSP) family have been identified as ligands of α₂δ‐1 in the CNS. This interaction was found to be crucial for excitatory synaptogenesis and neuronal sensitisation which in turn can be inhibited by gabapentin, suggesting a potential role in the pathogenesis of neuropathic pain. Here, we provide information on the biochemical properties of the direct TSP/α₂δ-1 interaction using an ELISA-style ligand binding assay. Our data reveal that full-length pentameric TSP-4, but neither TSP-5/COMP of the pentamer-forming subgroup B nor TSP-2 of the trimer-forming subgroup A directly interact with a soluble variant of α₂δ-1 (α₂δ-1_S). Interestingly, this interaction is not inhibited by gabapentin on a molecular level and is not detectable on the surface of HEK293-EBNA cells over-expressing α₂δ‐1 protein. These results provide biochemical evidence that supports a specific role of TSP-4 among the TSPs in mediating the binding to neuronal α₂δ‐1 and suggest that gabapentin does not directly target TSP/α₂δ-1 interaction to alleviate neuropathic pain.

In Vitro Nociceptor Neuroplasticity Associated with In Vivo Opioid-Induced Hyperalgesia

Opioid-induced hyperalgesia (OIH) is a serious adverse event produced by opioid analgesics. Lack of an in vitro model has hindered study of its underlying mechanisms. Recent evidence has implicated a role of nociceptors in OIH. To investigate the cellular and molecular mechanisms of OIH in nociceptors, in vitro, subcutaneous administration of an analgesic dose of fentanyl (30 μg/kg, s.c.) was performed in vivo in male rats. Two days later, when fentanyl was administered intradermally (1 μg, i.d.), in the vicinity of peripheral nociceptor terminals, it produced mechanical hyperalgesia (OIH). Additionally, 2 d after systemic fentanyl, rats had also developed hyperalgesic priming (opioid-primed rats), long-lasting nociceptor neuroplasticity manifested as prolongation of prostaglandin E₂ (PGE₂) hyperalgesia. OIH was reversed, in vivo, by intrathecal administration of cordycepin, a protein translation inhibitor that reverses priming. When fentanyl (0.5 nm) was applied to dorsal root ganglion (DRG) neurons, cultured from opioid-primed rats, it induced a μ-opioid receptor (MOR)-dependent increase in [Ca²⁺]_i in 26% of small-diameter neurons and significantly sensitized (decreased action potential rheobase) weakly IB4⁺ and IB4^- neurons. This sensitizing effect of fentanyl was reversed in weakly IB4⁺ DRG neurons cultured from opioid-primed rats after in vivo treatment with cordycepin, to reverse of OIH. Thus, in vivo administration of fentanyl induces nociceptor neuroplasticity, which persists in culture, providing evidence for the role of nociceptor MOR-mediated calcium signaling and peripheral protein translation, in the weakly IB4-binding population of nociceptors, in OIH.SIGNIFICANCE STATEMENT Clinically used μ-opioid receptor agonists such as fentanyl can produce hyperalgesia and hyperalgesic priming. We report on an in vitro model of nociceptor neuroplasticity mediating this opioid-induced hyperalgesia (OIH) and priming induced by fentanyl. Using this model, we have found qualitative and quantitative differences between cultured nociceptors from opioid-naive and opioid-primed animals, and provide evidence for the important role of nociceptor μ-opioid receptor-mediated calcium signaling and peripheral protein translation in the weakly IB4-binding population of nociceptors in OIH. These findings provide information useful for the design of therapeutic strategies to alleviate OIH, a serious adverse event of opioid analgesics.

Gabapentin prevents synaptogenesis between sensory and spinal cord neurons induced by thrombospondin-4 acting on pre-synaptic Cav α2 δ1 subunits and involving T-type Ca2+ channels

BACKGROUND AND PURPOSE

Nerve injury induces concurrent up-regulation of the voltage-gated calcium channel subunit Cav α2 δ1 and the extracellular matrix protein thrombospondin-4 (TSP4) in dorsal root ganglia and dorsal spinal cord, leading to the development of a neuropathic pain state. Interactions of these proteins promote aberrant excitatory synaptogenesis that contributes to neuropathic pain state development through unknown mechanisms. We investigated the contributions of Cav α2 δ1 subunits and TSP4 to synaptogenesis, and the pathways involved in vitro, and whether treatment with gabapentin could block this process and pain development in vivo.

EXPERIMENTAL APPROACH

A co-culture system of sensory and spinal cord neurons was used to study the contribution from each protein to synaptogenesis and the pathway(s) involved. Anti-synaptogenic actions of gabapentin were studied in TSP4-injected mice.

KEY RESULTS

Only presynaptic, but not postsynaptic, Cav α2 δ1 subunits interacted with TSP4 to initiate excitatory synaptogenesis through a pathway modulated by T-type calcium channels. Cav α2 δ1 /TSP4 interactions were not required for maintenance of already formed synapses. In vivo, early, but not delayed, treatment with low-dose gabapentin blocked this pathway and the development of the pain state.

CONCLUSIONS AND IMPLICATIONS

Cav α2 δ1 /TSP4 interactions were critical for the initiation, but not for the maintenance, of abnormal synapse formation between sensory and spinal cord neurons. This process was blocked by early, but was not reversed by delayed, treatment with gabapentin. Early intervention with gabapentin may prevent the development of injury-induced chronic pain, resulting from Cav α2 δ1 /TSP4-initiated abnormal synapse formation.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Injury-induced maladaptation and dysregulation of calcium channel α2 δ subunit proteins and its contribution to neuropathic pain development

Voltage-gated calcium channels (VGCCs) play important roles in physiological functions including the modulation of neurotransmitter release, neuronal network activities, intracellular signalling pathways and gene expression. Some pathological conditions, including nerve injuries, can cause the dysregulation of VGCCs and their subunits. This in turn can lead to a functional maladaptation of VGCCs and their subunits, which can contribute to the development of disorders such as pain sensations. This review has summarized recent findings related to maladaptive changes in the dysregulated VGCC α2 δ1 subunit (Cav α2 δ1 ) with a focus on exploring the mechanisms underlying the contribution of Cav α2 δ1 to pain signal transduction. At least under neuropathic pain conditions, the dysregulated Cav α2 δ1 can modulate VGCC functions as well as other plasticity changes. The latter includes abnormal excitatory synaptogenesis resulting from its interactions with injury-induced extracellular matrix glycoprotein molecule thrombospondins, which is independent of the VGCC functions. Blocking Cav α2 δ1 with gabapentinoids can reverse neuropathic pain significantly with relatively mild side effects, but only in a small population of neuropathic pain patients due to reasons yet to be explored. There are emerging data suggesting that early preventive treatment with gabapentinoids can prevent aberrant excitatory synapse formation and the development of chronic pain. If these findings are confirmed clinically, this could be an attractive approach for neuropathic pain management.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Orai1 and Orai3 Mediate Store-Operated Calcium Entry Contributing to Neuronal Excitability in Dorsal Root Ganglion Neurons

Store-operated calcium channels (SOCs) are highly calcium-selective channels that mediate calcium entry in various cell types. We have previously reported that intraplantar injection of YM-58483 (a SOC inhibitor) attenuates chronic pain. A previous study has reported that the function of SOCs in dorsal root ganglia (DRG) is enhanced after nerve injury, suggesting that SOCs may play a peripheral role in chronic pain. However, the expression, functional distribution and significance of the SOC family in DRG neurons remain elusive and the key components that mediate SOC entry (SOCE) are still controversial. Here, we demonstrated that the SOC family (STIM1, STIM2, Orai1, Orai2, and Orai3) was expressed in DRGs and STIM1 was mainly present in small- and medium-sized DRG neurons. Using confocal live cell imaging, Ca2+ imaging and electrophysiology techniques, we demonstrated that depletion of the endoplasmic reticulum Ca2+ stores induced STIM1 and STIM2 translocation, and that inhibition of STIM1 or blockage of Orai channels with pharmacological tools attenuated SOCE and SOC currents. Using the small inhibitory RNA knockdown approach, we identified STIM1, STIM2, Orai1, and Orai3 as the key components of SOCs mediating SOCE in DRG neurons. Importantly, activation of SOCs by thapsigargin induced plasma membrane depolarization and increased neuronal excitability, which were completely abolished by inhibition of SOCs or double knockdown of Orai1 and Orai3. Our findings suggest that SOCs exert an excitatory action in DRG neurons and provide a potential peripheral mechanism for modulation of pain hypersensitivity by SOC inhibition.

Thrombospondin-4 in tissue remodeling

Thrombospondin-4 (TSP-4) belongs to the thrombospondin protein family that consists of five highly homologous members. A number of novel functions have been recently assigned to TSP-4 in cardiovascular and nervous systems, inflammation, cancer, and the motor unit, which have attracted attention to this extracellular matrix (ECM) protein. These newly discovered functions set TSP-4 apart from other thrombospondins. For example, TSP-4 promotes angiogenesis while other TSPs either prevent it or have no effect on new blood vessel growth; TSP-4 reduces fibrosis and collagen production while TSP-1 and TSP-2 promote fibrosis in several organs; unlike other TSPs, TSP-4 appears to have some structural functions in ECM. The current information about TSP-4 functions in different organs and physiological systems suggests that this evolutionary conserved protein is a major regulator of the extracellular matrix (ECM) organization and production and tissue remodeling during the embryonic development and response to injury. In this review article, we summarize the properties and functions of TSP-4 and discuss its role in tissue remodeling.

The effect of pregabalin or duloxetine on arthritis pain: a clinical and mechanistic study in people with hand osteoarthritis

Osteoarthritis (OA) is the most prevalent arthritis worldwide and is characterized by chronic pain and impaired physical function. We hypothesized that heightened pain in hand OA could be reduced with duloxetine or pregabalin. In this prospective, randomized clinical study, we recruited 65 participants, aged 40–75 years, with a Numerical Rating Scale (NRS) for pain of at least 5. Participants were randomized to one of the following three groups: duloxetine, pregabalin, and placebo. The primary endpoint was the NRS pain score, and the secondary endpoints included the Australian and Canadian Hand Osteoarthritis Index (AUSCAN) pain, stiffness, and function scores and quantitative sensory testing by pain pressure algometry. After 13 weeks, compared to placebo, ANOVA found significant differences between the three groups (P=0.0078). In the intention-to-treat analysis, the pregabalin group showed improvement for NRS pain (P=0.023), AUSCAN pain (P=0.008), and AUSCAN function (P=0.009), but no difference between duloxetine and placebo (P>0.05) was observed. In the per protocol analysis, NRS pain was reduced for pregabalin (P<0.0001) and duloxetine (P=0.029) compared to placebo. We conclude that centrally acting analgesics improve pain outcomes in people with hand arthritis, offering new treatment paradigms for OA pain.

Microarray analysis of bone marrow lesions in osteoarthritis demonstrates upregulation of genes implicated in osteochondral turnover, neurogenesis and inflammation

Objective

Bone marrow lesions (BMLs) are well described in osteoarthritis (OA) using MRI and are associated with pain, but little is known about their pathological characteristics and gene expression. We evaluated BMLs using novel tissue analysis tools to gain a deeper understanding of their cellular and molecular expression.

Methods

We recruited 98 participants, 72 with advanced OA requiring total knee replacement (TKR), 12 with mild OA and 14 non-OA controls. Participants were assessed for pain (using Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)) and with a knee MRI (using MOAKS). Tissue was then harvested at TKR for BML analysis using histology and tissue microarray.

Results

The mean (SD) WOMAC pain scores were significantly increased in advanced OA 59.4 (21.3) and mild OA 30.9 (20.3) compared with controls 0.5 (1.28) (p<0.0001). MOAKS showed all TKR tissue analysed had BMLs, and within these lesions, bone marrow volume was starkly reduced being replaced by dense fibrous connective tissue, new blood vessels, hyaline cartilage and fibrocartilage. Microarray comparing OA BML and normal bone found a significant difference in expression of 218 genes (p<0.05). The most upregulated genes included stathmin 2, thrombospondin 4, matrix metalloproteinase 13 and Wnt/Notch/catenin/chemokine signalling molecules that are known to constitute neuronal, osteogenic and chondrogenic pathways.

Conclusion

Our study is the first to employ detailed histological analysis and microarray techniques to investigate knee OA BMLs. BMLs demonstrated areas of high metabolic activity expressing pain sensitisation, neuronal, extracellular matrix and proinflammatory signalling genes that may explain their strong association with pain.

Increased thrombospondin-4 after nerve injury mediates disruption of intracellular calcium signaling in primary sensory neurons

Painful nerve injury disrupts Ca²⁺ signaling in primary sensory neurons by elevating plasma membrane Ca²⁺-ATPase (PMCA) function and depressing sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) function, which decreases endoplasmic reticulum (ER) Ca²⁺ stores and stimulates store-operated Ca²⁺ entry (SOCE). The extracellular matrix glycoprotein thrombospondin-4 (TSP4), which is increased after painful nerve injury, decreases Ca²⁺ current (I_Ca) through high-voltage-activated Ca²⁺ channels and increases I_Ca through low-voltage-activated Ca²⁺ channels in dorsal root ganglion neurons, which are events similar to the effect of nerve injury. We therefore examined whether TSP4 plays a critical role in injury-induced disruption of intracellular Ca²⁺ signaling. We found that TSP4 increases PMCA activity, inhibits SERCA, depletes ER Ca²⁺ stores, and enhances store-operated Ca²⁺ influx. Injury-induced changes of SERCA and PMCA function are attenuated in TSP4 knock-out mice. Effects of TSP4 on intracellular Ca²⁺ signaling are attenuated in voltage-gated Ca²⁺ channel α₂δ₁ subunit (Ca_vα₂δ₁) conditional knock-out mice and are also Protein Kinase C (PKC) signaling dependent. These findings suggest that TSP4 elevation may contribute to the pathogenesis of chronic pain following nerve injury by disrupting intracellular Ca²⁺ signaling via interacting with the Ca_vα₂δ₁ and the subsequent PKC signaling pathway. Controlling TSP4 mediated intracellular Ca²⁺ signaling in peripheral sensory neurons may be a target for analgesic drug development for neuropathic pain.