Huntingtin reduction results in altered nuclear structure and heterochromatic instability

Huntington's disease (HD), a fatal neurodegenerative disease, arises due to a CAG repeat expansion in the huntingtin (HTT) gene. Non-pathogenic wild type HTT (wtHTT) is essential for neurodevelopment as well as many vital cellular functions within the adult brain; however, the consequences of wtHTT reduction in adulthood and particularly in extrastriatal regions of the brain have not been well characterized. Understanding the implications of wtHTT loss is essential as numerous genetic therapies for HD non-specifically reduce the expression levels of both mutant and wtHTT. The aim of the current study was to characterize the effect of wtHTT reduction from the whole cell to synaptic level in primary hippocampal neurons using conventional and super-resolution imaging methods. Our results identified the nucleus as an organelle that is particularly vulnerable to wtHTT reduction, with hippocampal neurons exhibiting increased nuclear size relative to the soma, DNA decompaction and a progressive loss of heterochromatin, and biphasic changes in nuclear pCREB signaling following siRNA-mediated wtHTT knockdown. Other structural assessments including dendritic complexity, spine density and synaptic morphology appeared to be largely unaffected in our wtHTT-lowered cells. These findings highlight the nucleus as an organelle that may be particularly sensitive to huntingtin-lowering in the mammalian brain.