Immunohistochemistry of Synaptic and Receptor Proteins in the Hippocampus of YAC128 Huntington's Disease Mice and Wildtype Littermates

Abstract

Huntington’s disease (HD) is a fatal inherited neurogenerative disorder which results from an unstable polyglutamine repeat expansion in the huntingtin (HTT). HD is associated with permanent loss of neurons in the cortex and striatum leading to profound neurological deficits and functional impairment. While the precise mechanisms underlying the disease pathology remain unclear, clinical studies and HD animal model studies have suggested that cellular dysfunction may occur in the early stages of the disease, even in the absence of cell loss. The YAC128 line is a transgenic mouse model that carries a full-length human HTT gene. The behavioural phenotype of these mice is well characterised however cellular dysfunction has not been studied in the YAC128 line. This study aimed to investigate whether the level and distribution of key proteins involved in neuronal signalling and neurotransmitter release were altered in the hippocampus of pre-symptomatic YAC128 mice. This was addressed by performing comparative immunohistochemistry of synaptic markers (synapsin 1, PSD95, NR1 and GluR1) in young YAC128 and age-matched wildtype littermates. Functional testing in these animals showed that our colony of YAC128 mice exhibits motor deficit at 6 months of age which correlates with previous reports within the literature. We also report for the first time that YAC128 mice show impairment in spontaneous alterations in the T maze from 1 month of age suggesting early deficits in working memory. Immunohistochemical analysis revealed two key findings: (1) increased of synapsin 1 immunoreactivity in the hilus of YAC128 at 1 month; and (2) a decrease in the relative area of GluR1 immunoreactivity within the entire hippocampus at 1 month and 6 months when compared to wildtype animals. No significant differences in the level or distribution of PSD95 and NR1 were observed in hippocampus between YAC128 and wildtype littermates. Our study provides the first report of an early cognitive deficit in YAC128 HD mice. This study also provides further evidence that key proteins involved in neuronal signalling and neurotransmitter release are altered in the hippocampus of pre-symptomatic YAC128 mice. While a direct link between altered synapsin 1 and GluR1 immunoreactivity and cognitive impairment cannot be made, these data support the central hypothesis of this thesis that dysfunction of the synaptic system and ultimately the impairment of neurotransmission may contribute to the onset of HD.