The role of alpha2delta subunits in calcium channel function under physiological and pathological conditions

We are trying to understand the mechanism of action of a class of drugs that are used in the treatment of chronic pain.
The drugs, gabapentin, and its newer relative pregabalin, are used to alleviate various forms of chronic pain, including post-herpetic neuralgia and diabetic neuropathy, although they were originally developed for the treatment of certain types of epilepsy, for which they are also effective.
Recent research has shown that these drugs have their effect by binding to a protein that makes up part of a calcium channel called the alpha2delta subunit. These calcium channels are present in all brain cells, and are essential for communication between them. In several animal models of chronic pain, the levels of alpha2delta subunits are found to increase in the damaged nerve cells.
In this study we now hope to understand, at the molecular level, how these alpha2delta subunits work, and how the binding of gabapentin to these proteins interferes with their function. To do this we will mainly use cells in tissue culture, but also where necessary, rodent models of disease.

Many anti-epileptic drugs inhibit voltage-gated cation channels, including calcium channels. However, gabapentin and its higher affinity analogue pregabalin are unique in this regard, because they bind to certain accessory alpha2delta subunits of calcium channels, rather than directly blocking the channel. These drugs are also effective in the treatment of neuropathic pain and allodynia following nerve damage. The alpha2delta subunits are involved in enhancing the number of calcium channels at the plasma membrane, and the main effect appears to be on trafficking. However, the mechanism of action of the gabapentinoid drugs is unknown at the molecular level. If this were better understood, it is possible that improved drugs would be forthcoming. Since these drugs also have other potential indications for the treatment of certain psychiatric disorders, such as anxiety, it is now very important to understand their mechanism of action.
Now that we have available mutant alpha2delta-1 and alpha2delta-2 subunits that do not bind gabapentinoids, but remain partially functional, the potential to understand how these drugs produce their effect is much enhanced. These mutant alpha2deltas will be extensively utilised in this proposal.
We will study the mechanism of trafficking of calcium channel alpha1 subunits by alpha2delta subunits, and the altered trafficking abilities of mutant alpha2delta-1 and -2 subunits that do not bind gabapentin. We will investigate the effect of gabapentinoid drugs on trafficking.
We will also investigate the site and mechanisms of proteolytic processing of the alpha2delta subunits and the effect on function of blocking proteolytic processing by mutations and drugs.
It is known that alpha2delta-1 is up-regulated in DRGs in rat models of neuropathic pain. We will now investigate whether there is altered proteolytic processing of up-regulated alpha2delta subunits.
We will examine the effect of gabapentinoid drugs on calcium currents in cultured DRG neurons following over-expression of wild-type alpha2delta subunits or their gabapentin-resistant counterparts.
One section is directly relevant to human epilepsy as it involves the use of a rodent model of epilepsy in which rats have intermittent seizures. We will investigate whether there is up- or down- regulation of either alpha2delta-1 or -2 in epileptic foci.
Finally we will investigate the subcellular distribution of alpha2delta-2 in Purkinje cells, to gain further insight into the severe phenotype associated with its loss.
This research programme has the potential to elucidate further the mechanism of action both of alpha2delta subunits and of the gabapentinoid drugs which bind to them.