Caspase-6 Inhibitors as Therapeutics for Huntington Disease

Huntington disease (HD) is a neurodegenerative disorder, characterized by the onset of motor abnormalities and psychiatric disturbances usually in middle age. As the disease progresses, a decline in cognition and worsening of the symptoms are observed, with death occurring typically 15-20 years after the initial symptoms. The disorder is caused by an elongation of a polyglutamine stretch in the Huntingtin (htt) protein above the pathological threshold of 36 glutamines. Neurodegeneration occurring in the cortex and striatum brain regions is characteristic for HD, and to date no effective therapy is available to stop this process. Previous research has shown that cleavage of htt is a crucial factor in the pathogenesis of HD. Fragments derived from mutant htt are toxic to living cells, and have been shown to accumulate in neurons of HD patients and animal models of the disease. Notably, htt fragments are generated prior to the clinical onset of symptoms, suggesting that htt cleavage may be a causal, early event in the pathogenesis of HD. Similarly, cleavage of the disease protein into toxic fragments has also been shown for other diseases of the central nervous system including Alzheimer`s disease and Spinocerebellar ataxia. The laboratory of Prof. Hayden has developed a transgenic mouse model for HD that expresses full-length human mutant htt. Importantly, these mice recapitulate many features of the human disease including cognitive deficits, motor abnormalities and neuronal degeneration. Recently it has been demonstrated that mice expressing mutant htt resistant to cleavage by caspase-6 maintain their normal neuronal function and do not develop HD-like symptoms. Therefore, inhibiting the generation of htt fragments represents a promising strategy for the development of novel therapeutics. The proposed project deals with the establishment and realization of a screening assay that is capable of identifying such inhibitory compounds. A peptide containing the htt caspase-6 cleavage site will be exposed to caspase-6 and the inhibitors, and cleavage will be detected through the generation of a luminescent signal. Such a system can be used to screen large compound libraries for potential inhibitors, and the impact of the compounds on the cleavage event will be monitored directly. Compounds that are able to inhibit htt cleavage will be further evaluated in mammalian cell culture and mouse model systems, providing detailed information on the cell permeability, potential toxicity, pharmacological properties and efficiency of the inhibitors in a cellular environment. Ultimately, compounds that are effective and have a well-defined mechanism of action are promising leads for the development of a novel treatment for HD and could be evaluated in clinical trials. Additionally, identified compounds can also be tested for their ability to inhibit the aberrant caspase cleavage of the Alzheimer`s disease protein, providing another potential application for the results of the proposed study.