A YAC Mouse Model for Huntington’s Disease with Full-Length Mutant Huntingtin, Cytoplasmic Toxicity, and Selective Striatal Neurodegeneration

Abstract

We have produced yeast artificial chromosome (YAC) transgenic mice expressing normal (YAC18) and mutant (YAC46 and YAC72) huntingtin (htt) in a developmental and tissue-specific manner identical to that observed in Huntington's disease (HD). YAC46 and YAC72 mice show early electrophysiological abnormalities, indicating cytoplasmic dysfunction prior to observed nuclear inclusions or neurodegeneration. By 12 months of age, YAC72 mice have a selective degeneration of medium spiny neurons in the lateral striatum associated with the translocation of N-terminal htt fragments to the nucleus. Neurodegeneration can be present in the absence of macro- or microaggregates, clearly showing that aggregates are not essential to initiation of neuronal death. These mice demonstrate that initial neuronal cytoplasmic toxicity is followed by cleavage of htt, nuclear translocation of htt N-terminal fragments, and selective neurodegeneration.

Specific caspase interactions and amplification are involved in selective neuronal vulnerability in Huntington's disease

“…Caspase-2 was required for the death of primary striatal cells derived from YAC72 transgenic mice. Analysis of YAC72 transgenic mice 30 and post-mortem brain from HD patients confirmed the contribution of caspase-2 to the pathogenesis of HD. Furthermore, caspase-2 mRNA levels are regulated by trophic support from the growth factor, BDNF.…”

“…Evidence for region-specific cleavage of Htt in vivo comes from studies of YAC transgenic mice expressing mutant full-length huntingtin (YAC72). In this model, N-terminal fragments of Htt are clearly found in the medium spiny neurons of the striatum 30 and cortex, 31 but not in other regions of the brain unaffected in HD. These results suggest that cleavage of Htt by caspases and other proteases such as calpains 32 may produce toxic fragments.…”

“…This transgenic mouse model shows selective loss of medium spiny neurons in the striatum, translocation of N-terminal fragments to the nucleus, gliosis in the striatum, and loss of neurons in the cerebral cortex by 1 year of age. 30 As shown in Figure 7, immunostaining for caspase-2 (panels i-k) and caspase-7 (panel l) in mouse YAC72 transgenics (1 year of age) was enriched in the striatum and cortex when compared to YAC18 age-matched control (panels m,n). Similar to that found in HD post-mortem tissue, caspase-2 was found in the neuritic profiles of the striatum and cortex of the HD transgenic mouse model (Figure 7i-k).…”

“…Line 2511 contains 1-2 copies of the YAC72 and has been described previously. 30 Mouse brains were sectioned horizontally (8 mm) on the automated rotary microtome (Leica) and deparaffinized in xylene. Antigen retrieval was carried out by microwaving sections in 10 mM citrate buffer, pH 6.0, for 5 min at 40% power in an 1100 W microwave oven.…”

Mouse models of Huntington's disease

“…HD models in which the full-length HTT transgene is carried in either a yeast or a bacterial artificial chromosome (YAC and BAC, respectively) are also available (Hodgson et al, 1999;Slow et al, 2003;Gray et al, 2008); see Table 1. Unlike the shorter N-terminal fragment models, these larger transgenic constructs have a tendency to integrate into the genome at a single genomic locus and typically in low copy numbers (i.e., 1-3 transgene copies).…”