Much of the latest pain research has focused on calcium channels, chemical channels that transmit pain signals. Researchers have been finding various techniques to affect their chemistry and make them less excitable, which slows down the transmission of pain signals and thus dulls pain.
Calcium Channel Modification Offers Novel Target for Pain Relief | Pain Research Forum by Michele Solis on 11 Sep 2014
Ubiquitin-removing enzyme USP5 regulates Cav3.2 levels, pain in mice
Researchers have identified a potent regulator of pain signaling, called ubiquitin-specific protease 5 (USP5), in sensory neurons. Published in Neuron September 3, the study found that USP5 cleaves ubiquitin groups off Cav3.2 calcium channels, thus preventing the channels’ degradation and expanding the pool of channels available for passing along pain signals to the spinal cord.
the researchers also found that USP5 is upregulated after injury in mice, and that disrupting USP5’s interaction with Cav3.2 channels provides relief in models of inflammatory or neuropathic pain
The findings suggest for the first time that ubiquitination—the cell’s system of flagging proteins for removal by adding ubiquitin molecules—can be harnessed to regulate pain. USP5 essentially cancels the order for removal and, in mammals, is one of about 90 de-ubiquitinases, about which not much is known.
Currently, T-type calcium channel blockers are used as analgesics, but these come with side effects because they prevent the baseline function of the channels. To get around this, researchers have been looking for subtler ways to control the alterations in T-type channel number or function that occur in pain states.
The new study finds that cells also use ubiquitination and de-ubiquitination to control Cav3.2 channel activity. In particular, blocking USP5’s action on Cav3.2 channels seemed to remove the pain-promoting channels added after inflammatory insult or nerve injury.
Khanna previously found an endogenous modulator for N-type calcium channels (see PRF related news story) and suggests that the mechanism discovered in the new study may also act on other channels or receptors involved in pain.
USP5 proved to be a potent regulator of Cav3.2 expression. Knocking it down with shRNA resulted in decreased levels of Cav3.2 protein and halved the electrical current carried by Cav3.2 channels in cultured neurons.
In vivo, USP5 levels rose after injuries resulting in chronic pain, either with injection of inflammation-inducing complete Freund’s adjuvant (CFA) to a paw or with a chronic constriction injury (CCI) to the sciatic nerve, which models neuropathic pain. Each condition resulted in hypersensitivity to touch, along with enhanced USP5 in DRGs and in the dorsal horn of the spinal cord.
Uncoupling USP5 from Cav3.2 channels with a peptide also diminished pain. The peptide contained the Cav3.2 loop targeted by USP5 and sopped up endogenous USP5, thus preventing its action on Cav3.2 channels.
Because peptides degrade quickly and are expensive to produce, however, Zamponi foresees only limited therapeutic use of the peptides used in these experiments. Instead, he advocates finding small molecules to interfere with USP5’s interaction with Cav3.2’s loop.
Curbing a Calcium Channel | Pain Research Forum by Megan Talkington on 13 Jun 2011
A peptide inhibitor of CaV2.2–CRMP-2 interaction shows analgesic activity
The N-type voltage-gated calcium channel (CaV2.2) plays crucial roles in pain transmission, making it a tantalizing therapeutic target. Blocking the channel directly, however, produces profound side effects. Last week, researchers reported a subtler strategy: They inhibited channel activity by thwarting the interaction of CaV2.2 with collapsin response mediator protein 2 (CRMP-2), a protein that facilitates proper trafficking of the channel to the cell surface in neurons.
CaV2.2 is involved in neurotransmitter release from presynaptic terminals at multiple points along pain pathways. One CaV2.2 blocker, ziconotide (a synthetic version of ω-conotoxin), is used to treat pain, but that drug causes severe side effects, including hypotension, memory loss, and ataxia
they discovered that CRMP-2 regulated CaV2.2 activity. Overexpression of CRMP-2 increased expression of CaV2.2 at the cell surface of neurons, enhanced calcium currents, and augmented the stimulus-dependent release of the neuropeptide calcitonin gene-related peptide (CGRP) from dorsal root ganglion (DRG) sensory neurons
The peptide also showed efficacy against a model of long-lasting, painful neuropathy, caused by the AIDS drug 2’,3’ dideoxycytidine (ddC): Intraperitoneal injection of the peptide seven days after an injection of ddC reversed tactile hypersensitivity.
Pharmacokinetic experiments revealed that the injected peptide made it into the DRG, spinal cord, and even the brain. Nonetheless, at doses that reduced pain, TAT-CBD3 produced no ill effects in tests of motor function, spatial memory, and anxiety-associated behaviors.
Bruce Bean, Harvard Medical School
This is an intriguing paper. The data suggest that pain sensation can be decreased by indirectly modifying the function of voltage-dependent calcium channels by altering the interaction of the main pore-forming subunit with a newly-recognized accessory protein, CRMP-2.
A remarkable and very surprising aspect of the results in the paper is the speed with which the effects occur.
A remarkable and very surprising aspect of the results in the paper is the speed with which the effects occur. In principle, one might expect that an intervention that works by altering cellular trafficking or plasma membrane expression of ion channels might take many hours to have its effects.
Remarkably, peptide application was found to completely abolish plasma membrane expression of Cav2.2 in DRG cell bodies studied in tissue culture, as 15 minutes of peptide treatment did not just reduce the Cav2.2-mediated component of calcium current but completely eliminated it. Whether such complete inhibition also occurs in synaptic terminals remains to be seen.
It is easy to imagine that adaptive or compensatory mechanisms might up-regulate calcium channel expression at longer times. Indeed, the effect of the peptide in a neuropathic pain model was smaller after 4 hours than after one hour. While this might be accounted for degradation or redistribution of the peptide, as suggested by the authors, it could also reflect compensatory mechanisms controlling calcium channel expression on a longer time scale.
the very poor record of treatments in rat models translating to chronic pain in human patients,
Nevertheless, the paper is remarkable for introducing a completely new molecular target for possible pain therapies, and the data in rat models is truly spectacular for the size of the effects in pain models and for lack of serious side effects.