Cav3.2 overexpression in L4 dorsal root ganglion neurons after L5 spinal nerve cutting involves Egr-1, USP5 and HMGB1 in rats: An emerging signaling pathway for neuropathic pain

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

T-type calcium channel modulation by hydrogen sulfide in neuropathic pain conditions

Neuropathic pain can appear as a direct or indirect nerve damage lesion or disease that affects the somatosensory nervous system. If the neurons are damaged or indirectly stimulated, immune cells contribute significantly to inflammatory and neuropathic pain. After nerve injury, peripheral macrophages/spinal microglia accumulate around damaged neurons, producing endogenous hydrogen sulfide (H₂S) through the cystathionine-γ-lyase (CSE) enzyme. H₂S has a pronociceptive modulation on the Ca_v3.2 subtype, the predominant Ca_v3 isoform involved in pain processes. The present review provides relevant information about H₂S modulation on the Ca_v3.2 T-type channels in neuropathic pain conditions. We have discussed that the dual effect of H₂S on T-type channels is concentration-dependent, that is, an inhibitory effect is seen at low concentrations of 10 µM and an augmentation effect on T-current at 100 µM. The modulation mechanism of the Ca_v3.2 channel by H₂S involves the direct participation of the redox/Zn²⁺ affinity site located in the His191 in the extracellular loop of domain I of the channel, involving a group of extracellular cysteines, comprising C114, C123, C128, and C1333, that can modify the local redox environment. The indirect interaction pathways involve the regulation of the Ca_v3.2 channel through cytokines, kinases, and post-translational regulators of channel expression. The findings conclude that the CSE/H₂S/Ca_v3.2 pathway could be a promising therapeutic target for neuropathic pain disorders.

The Yin/Yang Balance of Communication between Sensory Neurons and Macrophages in Traumatic Peripheral Neuropathic Pain

Traumatic peripheral neuropathic pain is a complex syndrome caused by a primary lesion or dysfunction of the peripheral nervous system. Secondary to the lesion, resident or infiltrating macrophages proliferate and initiate a cross-talk with the sensory neurons, at the level of peripheral nerves and sensory ganglia. The neuron–macrophage interaction, which starts very early after the lesion, is very important for promoting pain development and for initiating changes that will facilitate the chronicization of pain, but it also has the potential to facilitate the resolution of injury-induced changes and, consequently, promote the reduction of pain. This review is an overview of the unique characteristics of nerve-associated macrophages in the peripheral nerves and sensory ganglia and of the molecules and signaling pathways involved in the neuro-immune cross-talk after a traumatic lesion, with the final aim of better understanding how the balance between pro- and anti-nociceptive dialogue between neurons and macrophages may be modulated for new therapeutic approaches.

T-Type Calcium Channels: A Mixed Blessing

The role of T-type calcium channels is well established in excitable cells, where they preside over action potential generation, automaticity, and firing. They also contribute to intracellular calcium signaling, cell cycle progression, and cell fate; and, in this sense, they emerge as key regulators also in non-excitable cells. In particular, their expression may be considered a prognostic factor in cancer. Almost all cancer cells express T-type calcium channels to the point that it has been considered a pharmacological target; but, as the drugs used to reduce their expression are not completely selective, several complications develop, especially within the heart. T-type calcium channels are also involved in a specific side effect of several anticancer agents, that act on microtubule transport, increase the expression of the channel, and, thus, the excitability of sensory neurons, and make the patient more sensitive to pain. This review puts into context the relevance of T-type calcium channels in cancer and in chemotherapy side effects, considering also the cardiotoxicity induced by new classes of antineoplastic molecules.

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.

A Synthetically Accessible Small-Molecule Inhibitor of USP5-Cav3.2 Calcium Channel Interactions with Analgesic Properties

Cav3.2 calcium channels are important mediators of nociceptive signaling in the primary afferent pain pathway, and their expression is increased in various rodent models of chronic pain. Previous work from our laboratory has shown that this is in part mediated by an aberrant expression of deubiquitinase USP5, which associates with these channels and increases their stability. Here, we report on a novel bioactive rhodanine compound (II-1), which was identified in compound library screens. II-1 inhibits biochemical interactions between USP5 and the Cav3.2 domain III-IV linker in a dose-dependent manner, without affecting the enzymatic activity of USP5. Molecular docking analysis reveals two potential binding pockets at the USP5-Cav3.2 interface that are distinct from the binding site of the deubiquitinase inhibitor WP1130 (a.k.a. degrasyn). With an understanding of the ability of some rhodanines to produce false positives in high-throughput screening, we have conducted several orthogonal assays to confirm the validity of this hit, including in vivo experiments. Intrathecal delivery of II-1 inhibited both phases of formalin-induced nocifensive behaviors in mice, as well as abolished thermal hyperalgesia induced by the delivery of complete Freund's adjuvant (CFA) to the hind paw. The latter effects were abolished in Cav3.2 null mice, thus confirming that Cav3.2 is required for the action of II-1. II-1 also mediated a robust inhibition of mechanical allodynia induced by injury to the sciatic nerve. Altogether, our data uncover a novel class of analgesics─well suited to rapid structure-activity relationship studies─that target the Cav3.2/USP5 interface.

The terpenes camphene and alpha-bisabolol inhibit inflammatory and neuropathic pain via Cav3.2 T-type calcium channels

T-type calcium channels are known molecular targets of certain phytocannabinoids and endocannabinoids. Here we explored the modulation of Cav3.2 T-type calcium channels by terpenes derived from cannabis plants. A screen of eight commercially available terpenes revealed that camphene and alpha-bisabolol mediated partial, but significant inhibition of Cav3.2 channels expressed in tsA-201 cells, as well as native T-type channels in mouse dorsal root ganglion neurons. Both compounds inhibited peak current amplitude with IC50s in the low micromolar range, and mediated an additional small hyperpolarizing shift in half-inactivation voltage. When delivered intrathecally, both terpenes inhibited nocifensive responses in mice that had received an intraplantar injection of formalin, with alpha-bisabolol showing greater efficacy. Both terpenes reduced thermal hyperalgesia in mice injected with Complete Freund's adjuvant. This effect was independent of sex, and absent in Cav3.2 null mice, indicating that these compounds mediate their analgesic properties by acting on Cav3.2 channels. Both compounds also inhibited mechanical hypersensitivity in a mouse model of neuropathic pain. Hence, camphene and alpha-bisabolol have a wide spectrum of analgesic action by virtue of inhibiting Cav3.2 T-type calcium channels.

Structure-Activity Relationship of USP5 Inhibitors

USP5 is a deubiquitinase that has been implicated in a range of diseases, including cancer, but no USP5-targeting chemical probe has been reported to date. Here, we present the progression of a chemical series that occupies the C-terminal ubiquitin-binding site of a poorly characterized zinc-finger ubiquitin binding domain (ZnF-UBD) of USP5 and competitively inhibits the catalytic activity of the enzyme. Exploration of the structure-activity relationship, complemented with crystallographic characterization of the ZnF-UBD bound to multiple ligands, led to the identification of 64, which binds to the USP5 ZnF-UBD with a K_D of 2.8 μM and is selective over nine proteins containing structurally similar ZnF-UBD domains. 64 inhibits the USP5 catalytic cleavage of a di-ubiquitin substrate in an in vitro assay. This study provides a chemical and structural framework for the discovery of a chemical probe to delineate USP5 function in cells.

L5-6 Spinal Nerve Ligation-induced Neuropathy Changes the Location and Function of Ca2+ Channels and Cdk5 and Affects the Compound Action Potential in Adjacent Intact L4 Afferent Fibers

Voltage-gated Ca2+ (CaV) channels regulate multiple cell processes, including neurotransmitter release, and have been associated with several pathological conditions, such as neuropathic pain. Cdk5, a neuron-specific kinase, may phosphorylate CaV channels, altering their functional expression. During peripheral nerve injury, upregulation of CaV channels and Cdk5 in the dorsal root ganglia (DRG) and the spinal cord, has been correlated with allodynia. We recently reported an increase in the amplitude of the C component of the compound action potential (cAP) of afferent fibers in animals with allodynia induced by L5-6 spinal nerve ligation (SNL), recorded in the corresponding dorsal roots. This was related to an increase in T-type (CaV3.2) channels generated by Cdk5-mediated phosphorylation. Here, we show that CaV channel functional expression is also altered in the L4 adjacent intact afferent fibers in rats with allodynia induced by L5-6 SNL. Western blot analysis showed that both Cdk5 and CaV3.2 total levels are not increased in the DRG L3-4, but their subcellular distribution changes by concentrating on the neuronal soma. Likewise, the Cdk5 inhibitor olomoucine affected the rapid and the slow C components of the cAP recorded in the dorsal roots. Patch-clamp recordings revealed an increase in T- and N-type currents recorded in the soma of acute isolated L3-4 sensory neurons after L5-6 SNL, which was prevented by olomoucine. These findings suggest changes in CaV channels location and function in L3-4 afferent fibers associated with Cdk5-mediated phosphorylation after L5-6 SNL, which may contribute to nerve injury-induced allodynia.

Targeting T-type/CaV3.2 channels for chronic pain

T-type calcium channels regulate neuronal excitability and are important contributors of pain processing. CaV3.2 channels are the major isoform expressed in nonpeptidergic and peptidergic nociceptive neurons and are emerging as promising targets for pain treatment. Numerous studies have shown that CaV3.2 expression and/or activity are significantly increased in spinal dorsal horn and in dorsal root ganglia neurons in different inflammatory and neuropathic pain models. Pharmacological campaigns to inhibit the functional expression of CaV3.2 for treatment of pain have focused on the development of direct channel blockers, but none have produced lead candidates. Targeting the proteins that regulate the trafficking or transcription, and the ones that modify the channels via post-translational modifications are alternative means to regulate expression and function of CaV3.2 channels and hence to develop new drugs to control pain. Here we synthesize data supporting a role for CaV3.2 in numerous pain modalities and then discuss emerging opportunities for the indirect targeting of CaV3.2 channels.

Macrophage as a Peripheral Pain Regulator

A neuroimmune crosstalk is involved in somatic and visceral pathological pain including inflammatory and neuropathic components. Apart from microglia essential for spinal and supraspinal pain processing, the interaction of bone marrow-derived infiltrating macrophages and/or tissue-resident macrophages with the primary afferent neurons regulates pain signals in the peripheral tissue. Recent studies have uncovered previously unknown characteristics of tissue-resident macrophages, such as their origins and association with regulation of pain signals. Peripheral nerve macrophages and intestinal resident macrophages, in addition to adult monocyte-derived infiltrating macrophages, secrete a variety of mediators, such as tumor necrosis factor-α, interleukin (IL)-1β, IL-6, high mobility group box 1 and bone morphogenic protein 2 (BMP2), that regulate the excitability of the primary afferents. Neuron-derived mediators including neuropeptides, ATP and macrophage-colony stimulating factor regulate the activity or polarization of diverse macrophages. Thus, macrophages have multitasks in homeostatic conditions and participate in somatic and visceral pathological pain by interacting with neurons.

Role of HMGB1 in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN), one of major dose-limiting side effects of first-line chemotherapeutic agents such as paclitaxel, oxaliplatin, vincristine, and bortezomib is resistant to most of existing medicines. The molecular mechanisms of CIPN have not been fully understood. High mobility group box 1 (HMGB1), a nuclear protein, is a damage-associated molecular pattern protein now considered to function as a pro-nociceptive mediator once released to the extracellular space. Most interestingly, HMGB1 plays a key role in the development of CIPN. Soluble thrombomodulin (TMα), known to degrade HMGB1 in a thrombin-dependent manner, prevents CIPN in rodents treated with paclitaxel, oxaliplatin, or vincristine and in patients with colorectal cancer undergoing oxaliplatin-based chemotherapy. In this review, we describe the role of HMGB1 and its upstream/downstream mechanisms in the development of CIPN and show drug candidates that inhibit the HMGB1 pathway, possibly useful for prevention of CIPN.