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

Abstract

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.

Keywords: CaV3.2; Ubiquitination; glycosylation; inflammatory pain; neuropathic pain; phosphorylation.

Figure 1.

Schematic representation of CaV3.2 topology and regulation. A) Location of histidine (His) 191 (cian), asparagines (Asn) 192, 345, 1466 and 1780 (orange), serines (Ser) 532, 561, 1144, 1987 and 2188 (green) and the USP5 interaction site in the CaV3.2α1 subunit. B) (i) High mobility group box 1 (HMGB1) binds to the receptor for advanced glycation end-products (RAGE) and triggers upregulation of transcriptional factor early growth response 1 (Egr-1) and CaV3.2 overexpression. This can be prevented by low molecular weight heparin (LMWH), a RAGE antagonist. (ii) Phosphorylation (P) of CaV3.2 α1 subunit by protein kinase C (PKC), cyclin-dependent kinase 5 (Cdk5) and its activator p25, enhances surface expression of the channel. Homocysteine potentiates T-type currents; Phorbol 12-myristate 13-acetate (PMA), a specific PKC activator, and the Cdk5 inhibitor, olomoucine prevent this effect. (iii) Repressor element 1-silencing transcription factor (REST) represses the activation of the promoter of CaV3.2. (iv) Insulin-like growth factor 1 (IGF-1) increase CaV3.2 currents acting through the IGF-1 receptor (IGF-1R), which activates Gαo proteins and subsequentially activates PKCα by Gβɣ subunits. This effect can be blocked by 2,2′,3,3′,4,4′-Hexahydroxy-1,1′-biphenyl-6,6′-dimethanol Dimethyl Ether (HBDDE), a PKCα inhibitor. (v) Ubiquitin-specific proteinase 5 (USP5) binds to CaV3.2 and decreases channel ubiquitination (Ub). (vi) Asparagine (N)-linked glycosylation promotes the functional expression of the channel (purple) and this can be prevented by neuraminidase. (vii) L-cysteine and H₂S increase T-type currents, while Zn²⁺ and ascorbic acid can inhibit this effect.

Figure 2.

Effects of chronic neuropathic pain models in CaV3.2 expression and function in the sensory system. Chronic compression of DRG, chronic constriction injury, L5/L6 SNL, partial sciatic nerve ligation, L5 spinal nerve cutting, spared nerve injury, paclitaxel-induced peripheral neuropathy and diabetic neuropathy rodent pain models increase CaV3.2 expression and/or activity in the dorsal horn of the spinal cord and in DRG neurons.