A Functional Characterization of the Mouse Huntingtin N-terminus in the Adult Brain

The N-terminus of the Huntingtin (HTT) protein is comprised of three functional domains encoded by exon 1 of the Huntingtin (HTT) gene. The polyglutamine stretch (polyQ), which is expanded in Huntington’s disease (HD), is flanked by the first 17 amino acids (N17) and the proline rich region (PRR). In vitro, the N17 domain can regulate HTT’s subcellular localization in response to increased oxidative stress [1], and the PRR is a protein-protein interaction domain [2, 3]. Mice lacking regions of the Htt N-terminus are born at the expected Mendelian frequency, suggesting that the Htt N-terminus is not required for Htt’s critical role during embryonic development [4-7]. In aged mice, deletion of the polyQ domain results in an increase in autophagy markers [8], while deletion of the N17 domain results in a reduction in thalamostriatal glutamatergic synapses [4]. The behavioral consequences of single- or double-domain deletions within the Htt N-terminus, however, are not consistent [4-6], suggesting that these domains do not function in isolation. Characterizing a model with a deletion in all three Htt N-terminal domains should contribute to our understanding the function of the Htt N-terminus in vivo.

I have characterized a mouse model with a deletion of the N17, polyQ, and PRR domains of the Htt N-terminus (HttΔE1), and found that while the progeny from HttΔE1/+ intercrosses were born at the expected Mendelian frequency, there was a sex-specific distortion in the number of HttΔE1/ΔE1 and Htt+/+ progeny. I performed longitudinal examinations of motor and spatial learning and memory performance in HttΔE1/ΔE1, HttΔE1/+, and HttΔE1/-, Htt+/+, and Htt+/- mice, and found that HttΔE1/ΔE1 mice exhibit a non-progressive rotarod deficit compared to Htt+/+ mice. Additionally, I investigated Htt subcellular localization, DNA damage repair, oxidative stress markers, synapse numbers, and autophagosome marker levels in HttΔE1/ΔE1 and Htt+/+ mice, and found no differences between genotypes. I did, however, observe an increase in DNA double strand breaks (DSBs) in the cortex and striatum of 3-month-old mice along with elevated pan-nuclear levels of the DNA DSB repair protein 53bp1.

I have also characterized the HttΔE1 allele in trans to an HD model allele (Htt140Q/ΔE1), and found that the HttΔE1 allele modifies HD phenotype progression. Htt140Q/ΔE1 mice exhibit delayed onset of HD mouse model motor phenotypes including reduced grip strength and hypoactivity. The HttΔE1 allele did not modify spatial learning and memory performance. Additionally, Htt subcellular localization and autophagy levels were not significantly altered and levels of synaptic markers were not consistently changed in Htt140Q/ΔE1 mice compared to Htt140Q/+ mice, or in single- or double-domain deletion mice (Htt140Q/ΔQ, Htt140Q/ΔQP, or Htt140Q/ΔN17) compared to Htt140Q/+ mice.

Based on these results, I conclude that the Htt N-terminus is not required for Htt’s critical functions in development and in the brain, but instead may modulate the efficiency of some Htt functions both under basal conditions and in the presence of an HD model allele.