Final answer:
The toxic effects of the mutant huntingtin protein in Huntington's disease include dysregulation of NMDA receptor expression, calcium homeostasis imbalance, and disruptions to mitochondrial membrane potential, leading to neuronal death.
Step-by-step explanation:
The mutant huntingtin protein in Huntington's disease (HD) induces several toxic effects leading to neuronal cell death. This mutated protein contains expanded polyglutamine tracts which make it toxic to neural tissue. Some of the key mechanisms through which the mutated huntingtin protein causes toxicity include dysregulation of NMDA receptor expression, especially at extrasynaptic sites, and disruptions to mitochondrial membrane potential. The interaction with extrasynaptic NMDA receptors increases intracellular calcium levels leading to cell death pathways rather than the beneficial pathways induced by synaptic NMDA receptors. This imbalance in calcium homeostasis can result in neuronal dysfunction and ultimately, neurodegeneration and the manifestation of HD symptoms such as spastic movements, cognitive decline, and personality changes.