Aging activates, represses, and remodels the silent X chromosome and its escape across cell types of the female mouse hippocampus

Females show an advantage in lifespan and cognition in aging human populations. The X chromosome is a major source of sex difference, as female mammals harbor an inactive (Xi) and active (Xa) following X chromosome inactivation (XCI). Whether Xi or the silent X can activate in the aging brain and thus increase X dose is unknown. Here, we performed single nuclei RNA-sequencing in the young and aging hippocampus of female mice and applied allele-specific computational analysis. We show that aging remodels transcription of Xi and Xa across hippocampal cell types. Aging preferentially induced gene expression changes on the X chromosomes compared to the autosomes. Xi underwent activation and repression of select genes, particularly in dentate gyrus neurons, critical to learning and memory. Female M. musculus XistloxP +/-,Zp3-cre mice with a congenic C57BL/6J background, were crossed with male M. castaneous mice. XX progeny carrying the XistloxP +/-, Zp3-cre allele were used for single nuclei RNA sequencing. The fresh hippocampi of four young (3-4 month) and four old (23-24 month) mice were dissected and frozen for downstream snRNAseq studies.在衰老的人类群体中，女性在寿命与认知能力方面表现出优势。X染色体是造成性别差异的主要因素，因为雌性哺乳动物在经历X染色体失活（X chromosome inactivation, XCI）后，会分别拥有一条失活X染色体（inactive X chromosome, Xi）与一条激活X染色体（active X chromosome, Xa）。目前尚不清楚失活X染色体（Xi）或是沉默的X染色体能否在衰老的大脑中被激活，进而提升X染色体的剂量。本研究对雌性小鼠年轻与衰老状态下的海马体开展单细胞核RNA测序（single nuclei RNA-sequencing），并运用等位基因特异性的计算分析方法。研究结果显示，衰老会重塑海马体各类细胞中Xi与Xa的转录模式。相较于常染色体，衰老更倾向于诱导X染色体上的基因表达发生改变。失活X染色体（Xi）会对特定基因进行激活与沉默调控，这一现象在齿状回神经元中尤为显著，而齿状回神经元对学习与记忆至关重要。本研究将带有同基因C57BL/6J背景的XistloxP +/-、Zp3-cre雌性小家鼠（M. musculus）与雄性卡氏小家鼠（M. castaneus）进行杂交，选取携带XistloxP +/-、Zp3-cre等位基因的雌性（XX型）后代开展单细胞核RNA测序实验。研究人员分别解剖4只年轻（3-4月龄）与4只老年（23-24月龄）小鼠的新鲜海马体并进行冷冻保存，用于后续的单细胞核RNA测序研究。