[no abstract available]
The degeneration of neurons in the striatum, cortex and other diffuse brain regions in Huntington's disease (HD) is a progressive process, leading to debilitating symptoms and ultimately death. Despite the identification of the huntingtin gene, whose genetic mutation underlies the disease, the pathophysiology of HD remains ill-defined and there are currently no drugs in clinical use which effectively delay the progression of the disease. Recently, the CB1 cannabinoid receptor (CB1R) has gained attention as a therapeutic target in neurodegenerative diseases such as Parkinson's, and investigations have also begun in animal models of HD. However, these studies have yielded mixed reports of the efficacy of cannabinoids in this context. This may relate to the finding that CB1R binding is dramatically reduced early in the human disease. This thesis undertook the investigation of the therapeutic potential of the CB1R through the development of a PC12 cell model of HD. This model was found to recapitulate expression and aggregation of mutant huntingtin, loss of CB1Rs early in the pathological process, and cell death. Using the Alamar Blue assay, it was determined that the CB1R agonists HU210 and WIN55,212-2, produced a small but significant reduction in huntingtin-associated cell death (6-8%). This rescue was dependent upon the activation of the Gαi G-protein subtype and ERK 1/2, which was necessary but not sufficient to modulate cell survival. Conversely, compounds which increased cAMP formation exacerbated cell death and this correlated with enhanced formation of mutant huntingtin aggregates. Interestingly, the protective CB1R agonist HU210 also increased cellular aggregate load (6%). While the reduction of cell death by HU210 had been mediated by Gαi-coupled CB1Rs, the enhancement of aggregate formation was likely to have occurred through CB1Rs coupling to Gαs. The enhancement of huntingtin aggregation was generally associated with toxicity. Therefore the ability of the CB1R to promiscuously couple to Gαs, and this detrimental aggregation pathway, may limit its therapeutic usefulness and account for the modest survival effects described here. However, compounds which are selective for Gαi activation by the CB1R, or compounds which target protective downstream effectors, could represent novel therapeutic strategies to alleviate neurodegeneration in HD.
