Lamins are crucial proteins for nuclear functionality. Here, we provide new evidence showing that increased lamin B1 levels contribute to the pathophysiology of Huntington’s disease (HD), a CAG repeat‐associated neurodegenerative disorder. Through fluorescence‐activated nuclear suspension imaging, we show that nucleus from striatal medium‐sized spiny and CA1 hippocampal neurons display increased lamin B1 levels, in correlation with altered nuclear morphology and nucleocytoplasmic transport disruption. Moreover, ChIP‐sequencing analysis shows an alteration of lamin‐associated chromatin domains in hippocampal nuclei, accompanied by changes in chromatin accessibility and transcriptional dysregulation. Supporting lamin B1 alterations as a causal role in mutant huntingtin‐mediated neurodegeneration, pharmacological normalization of lamin B1 levels in the hippocampus of the R6/1 mouse model of HD by betulinic acid administration restored nuclear homeostasis and prevented motor and cognitive dysfunction. Collectively, our work points increased lamin B1 levels as a new pathogenic mechanism in HD and provides a novel target for its intervention.

Huntington’s disease (HD) is an inherited neurodegenerative disorder with onset of characteristic motor symptoms at midlife, preceded by subtle cognitive and behavioral disturbances. Transcriptional dysregulation emerges early in the disease course and is considered central to HD pathogenesis. Using wild-type (wt) and HD knock-in mouse striatal cell lines we observed a HD genotype-dependent reduction in the protein levels of transcription factor 4 (TCF4), a member of the basic helix-loop-helix (bHLH) family with critical roles in brain development and function. We characterized mouse Tcf4 gene structure and expression of alternative mRNAs and protein isoforms in cell-based models of HD, and in four different brain regions of male transgenic HD mice (R6/1) from young to mature adulthood. The largest decrease in the levels of TCF4 at mRNA and specific protein isoforms were detected in the R6/1 mouse hippocampus. Translating this finding to human disease, we found reduced expression of long TCF4 isoforms in the postmortem hippocampal CA1 area and in the cerebral cortex of HD patients. Additionally, TCF4 protein isoforms showed differential synergism with the proneural transcription factor ASCL1 in activating reporter gene transcription in hippocampal and cortical cultured neurons. Induction of neuronal activity increased these synergistic effects in hippocampal but not in cortical neurons, suggesting brain region-dependent differences in TCF4 functions. Collectively, this study demonstrates isoform-specific changes in TCF4 expression in HD that could contribute to the progressive impairment of transcriptional regulation and neuronal function in this disease.

Lamins are crucial proteins for nuclear functionality. Here, we provide new evidence showing an involvement of increased lamin B1 levels in the pathophysiology of Huntington's disease (HD), a CAG repeat-associated neurodegenerative disorder. Through fluorescence-activated nuclear suspension imaging we demonstrate that nucleus from striatal medium-sized spiny and CA1 hippocampal neurons display increased lamin B1 levels, in correlation with altered nuclear morphology and nucleocytoplasmic transport disruption. Moreover, ChIP-sequencing analysis shows an alteration of lamin-associated chromatin domains in hippocampal nuclei, which could contribute to transcriptional alterations we determined by RNA sequencing. Supporting lamin B1 alterations as a causal role in mutant-huntingtin mediated neurodegeneration, pharmacological normalization of lamin B1 levels by betulinic acid administration in the R6/1 mouse model of HD restored nuclear homeostasis and prevented motor and cognitive dysfunction. Collectively, our work point out increased lamin B1 levels as a new pathogenic mechanism in HD and provides a novel target for its intervention.Keywords: chromatin accessibility, LAD, R6/1 mouse, nuclear morphology, nuclear permeability of the polyglutamine chain at the amino terminus of the huntingtin (Htt) protein inducing selfassociation and aggregation. Consequently, mutant Htt (mHtt) loses its biological functions and becomes toxic 13 . In HD, medium-sized spiny neurons (MSNs), the GABAergic output projection neurons that account for the vast majority (90-95%) of all striatal neurons, are mainly affected. Although motor symptoms are the most prominent, psychiatric alterations and cognitive decline appear first in HD patients, which become more evident as the disease progresses. Cognitive deficits are related to the dysfunction of the corticostriatal pathway and the hippocampus and, together with motor deficits, have been replicated in most HD mouse models 14 .Molecular mechanisms leading to nuclear lamina alterations in neurons expressing mHtt remain to be elucidated. Previous results from our lab suggested that decreased levels of the pro-apoptotic kinase PKC would lead to an aberrant accumulation of lamin B 15 which, in turn, could have a significant influence in the nuclear lamina structure 16,17 . Therefore, here we sought to deeply characterize the impact of lamin alterations in HD brain at physiological (studying nuclear lamina morphology and nucleo-cytoplasmic transport), transcriptomic (by generating RNA-sequencing (RNA-seq) data) and epigenetic (analyzing lamin chromatin binding and chromatin accessibility) levels by using the R6/1 transgenic mouse model of HD and human post-mortem brain samples. ResultsLamin B levels are increased in a region-specific manner in HD brain. Lamin B1, B2, and A/C protein levels were analyzed in the striatum, cortex and hippocampus of wild-type and R6/1 mice, a transgenic mouse model of HD over-expressing the exon 1 of the human mutant huntingtin 18 , at different ages. Western blot a...