Huntington's Disease and Apoptosis
Huntington's disease (HD) is an inherited neurodegenerative disease characterized by chorea, personality changes, dementia, and early death. The characteristic symptoms result from the selective death and dysfunction of specific neuronal subpopulations within the central nervous system. Significant neuronal death is observed within the striatum (the subcortical brain structure that controls body movements) and to a lesser extent within the cortex. Within the striatum, the enkephalin-containing medium spiny neurons are particularly vulnerable, whereas many of the neighbouring neurons remain unaffected. The gene responsible for HD is the huntingtin gene (350 kDa) which contains a polymorphic stretch of repeated CAG trinucleotides that encodes a polyglutamine tract within huntingtin. Healthy people possess a huntingtin gene with less than 20 repeats, while huntingtin with > 35 repeats results with high probability in the outbreak of HD. The polyGln expansion was hypothesized to create an epitope that results in a modified interaction between huntingtin and other proteins thereby leading to disease, though this mechanism would not explain the specificity with which the huntigtin mutation affects certain subsets of neuronal cells. In a transgenic mouse model of HD it was shown that mutant huntingtin can form aggregates within the nuclei, known as neuronal intranuclear inclusions (Davies et al., 1997, Lancet, 351, 131-133). The presence of intranuclear inclusions is specific to mutant huntingtin, and inclusions form before neurological symptoms and neurodegeneration occur, suggesting, that the aggregation of huntingtin into intranuclear inclusions is a required step in neurodegeneration, but is has never been shown directly that huntingtin aggregation is toxic.
The Huntingtin protein contains several cleavage sites for Caspase-3, and N-terminal cleavage fragments of Huntingtin cause disease in mouse transgenic models.
An in vitro model of HD was established (Saudou et al., 1998, Cell, 95: 55-66) by transfecting striatal neurons with a N-terminal fragment of huntingtin, including the polyGln stretch; a wildtype fragment with 17 CAG triplets and a mutant form with 68 CAGs was transfected. A subpopulation of neurons transfected with the mutant huntingtin fragment showed clear signs of neurodegeneration as indicated by neurite loss or destruction, chromatin condensation, and nuclear pyknosis and fragmentation. Neurons from the hippocampus transfected in the same way did not show increase of neuronal death what is consistent with observations in vivo where the hippocampus is not affected by HD. Interestingly, the neurodegenerative cell death induced by mutant huntingtin is due to an apoptotic mechanism, since the TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP end labeling) assay is positive and huntingtin- induced neurodegenaration is inhibited by coexpression of Bcl-XL and by the caspase inhibitor peptide Ac-DEVD-CHO. This suggests that mutant huntingtin induces degeneration of striatal neurons by an apoptotic mechanism. It was found that mutant huntingtin has apoptotic effect only when it is located in the nucleus of cells, where it forms nuclear inclusions. A typical feature of huntingtin within those inclusions is its ubiquitination. In an experiment in which the ubiquitination of the huntingtin inclusions was inhibited, a dramatic decrease in the number of intranuclear inclusions was observed. At the same time the decrease in intranuclear inclusions resulted in a significant increase in apoptosis! Thus, the formation of intranuclear inclusions is obviously no trigger of apoptosis but - on the contrary - might be a cellular strategy for degradading or inactivating toxic forms of huntingtin, and thereby, protecting the cell from the deleterious effects of mutant huntingtin which acts within the nucleus by an yet unidentified apoptotic mechanism.
Saudou et al., 1998, Cell, 95: 55-66