Dena B. Dubal

Sex differences have traditionally been attributed to gonadal hormones, but accumulating evidence shows that sex chromosomes exert distinct, cell-autonomous effects on health, aging, and disease. XX and XY complements alter gene dosage, epigenetic programming, immune signaling, metabolism, and cellular stress responses, which shape aging and disease trajectories in women and men. This Review integrates mechanistic advances in sex chromosome biology—including escape from X chromosome inactivation, X reactivation, parent-of-X-origin, X-skew, X- and Y-linked gene functions, and Y loss—with aging, neurologic disease, immunity, cancer, and cardiometabolic disease. We highlight insights from mouse models, human studies, and emerging technologies that disentangle hormone-driven effects from chromosome-mediated effects and discuss translational implications for diagnostics, biomarkers, therapeutics, and clinical trial design, emphasizing the integration of sex chromosome biology into precision medicine.

Aging is the primary risk factor for Parkinson's disease (PD), and PD-related cognitive impairment remains a major unmet biomedical challenge. Klotho, a pleiotropic protein, extends lifespan and enhances cognition, but whether it confers resilience to cognitive impairments in PD is unclear. Here, we show that in humans, the KL-VS genetic variant of KLOTHO , linked to higher circulating klotho levels, associated with better executive cognition in individuals with PD across two independent cohorts. To test causality and explore mechanisms, we turned to mouse models. Transgenic elevation of klotho in a mouse model increased lifespan, improved synaptic and cognitive, but not motor, functions in mice, and decreased steady-state α-synuclein (α-syn) levels in the brains of male mice expressing wild-type human α-syn. Complementary in vitro studies showed that klotho rescued α-syn–induced deficits in NMDAR-dependent signaling through GluN2B and augmented α-syn microglial-related uptake, suggesting a potential mechanism by which klotho counters PD-related toxicity. Together, these findings indicate that klotho can counteract cognitive deficits related to PD, possibly by modulating α-syn levels, and these findings may be relevant to new therapeutic pathways for PD.

Women live longer than men and exhibit less cognitive aging. The X chromosome contributes to sex differences, as females harbor an inactive X (Xi) and active X (Xa), in contrast to males with only an Xa. Thus, reactivation of silent Xi genes may contribute to sex differences. We use allele-specific, single-nucleus RNA sequencing to show that aging remodels transcription of the Xi and Xa across hippocampal cell types. Aging preferentially changed gene expression on the X’s relative to autosomes. Select genes on the Xi underwent activation, with new escape across cells including in the dentate gyrus, critical to learning and memory. Expression of the Xi escapee Plp1 , a myelin component, was increased in the aging hippocampus of female mice and parahippocampus of women. AAV-mediated Plp1 elevation in the dentate gyrus of aging male and female mice improved cognition. Understanding how the Xi may confer female advantage could lead to novel targets that counter brain aging and disease in both sexes.

INTRODUCTION: While hallmarked by the accumulation of β-amyloid plaques (Aβ) and neurofibrillary tangles (tau) in the brain, Alzheimer’s disease (AD) is a multifactorial disorder that involves additional pathological events, including neuroinflammation, neurodegeneration and synaptic dysfunction. AD-associated biomolecular changes seem to be attenuated in carriers of the functionally advantageous variant of the KLOTHO gene (KL-VS _HET ). Independently, better cardiorespiratory fitness (CRF) is associated with better health outcomes, both in general and specifically with regard to AD pathology. Here we investigate whether the relationships between CRF (peak oxygen consumption (VO _2peak )) and cerebrospinal fluid (CSF) core AD biomarkers and those of neuroinflammation, neurodegeneration, and synaptic dysfunction differ for KL-VS _HET compared to non-carriers (KL-VS _NC ). METHODS: The cohort, enriched for AD risk, consisted of cognitively unimpaired adults (N=136; Mean _AGE (SD)=62.5(6.7)) from the Wisconsin Registry for Alzheimer’s Prevention and the Wisconsin Alzheimer’s Disease Research Center. Covariate-adjusted (age, sex, parental AD history, APOE 4+ status, and age difference between CSF sampling and exercise test) linear models examined the interaction between VO _2peak and KLOTHO genotype on core AD biomarker levels in CSF [phosphorylated tau 181 (pTau ₁₈₁ ), Aβ ₄₂ /Aβ ₄₀ , pTau ₁₈₁ /Aβ ₄₂ ]. Analyses were repeated for CSF biomarkers of neurodegeneration [total tau (tTau), α-synuclein (α-syn), neurofilament light polypeptide (NfL)], synaptic dysfunction [neurogranin (Ng)], and neuroinflammation [glial fibrillary acidic protein (GFAP), soluble triggering receptor expressed in myeloid cells (sTREM2), chitinase-3-like protein 1 (YKL-40), interleukin 6 (IL-6), S100 calcium-binding protein B (S100B)]. RESULTS: The interaction between VO _2peak and KL-VS _HET was significant for tTau ( P =0.05), pTau ₁₈₁ ( P =0.03), Ng ( P =0.02), sTREM2 ( P =0.03), and YKL-40 ( P =0.03), such that lower levels of each biomarker were observed for KL-VS _HET who were more fit. No significant KL-VSxVO _2peak interactions were observed for Aβ ₄₂ /Aβ ₄₀ , pTau ₁₈₁ /Aβ ₄₂ , α-syn, NfL, GFAP, IL-6 or S100B (all P s>0.09). CONCLUSIONS: We report a synergistic relationship between KL-VS _HET and CRF with regard to pTau ₁₈₁ , tTau, Ng, sTREM2 and YKL-40, suggesting a protective role for both KL-VS _HET and better cardiovascular fitness against unfavorable AD-related changes. Their potentially shared biological mechanisms will require future investigations. 1 Research in Context: Systematic Review: PubMed literature review suggests that both KLOTHO KL-VS genotype and cardiorespiratory fitness (CRF) are associated with pathophysiological processes related to Alzheimer’s Disease (AD). Both KL-VS heterozygotes (KL-VS _HET ) and those with higher CRF fare better when faced with age-related biomolecular changes of relevance to AD. The present study investigates whether the relationships between CRF and cerebrospinal fluid biomarkers (CSF) of core AD neuropathology, neuroinflammation, neurodegeneration, and synaptic dysfunction differ for KL-VS _HET compared to non-carriers. Interpretation: Our findings suggest a synergistic relationship between KL-VS _HET and higher CRF against core AD pathology along a range of unfavorable biomolecular changes implicated in this multifactorial disease. This supports the idea that CRF may interact with genetic factors to confer resilience against a multitude of adverse AD-associated processes. Future Directions: Future studies should examine longitudinal changes in CSF biomarkers to determine whether maintaining or improving CRF over time enhances AD resilience in KL-VS _HET .

Female mammalian cells have two X chromosomes, one of maternal origin and one of paternal origin. During development, one X chromosome randomly becomes inactivated^1–4. This renders either the maternal X (X_m) chromosome or the paternal X (X_p) chromosome inactive, causing X mosaicism that varies between female individuals, with some showing considerable or complete skew of the X chromosome that remains active^5–7. Parent-of-X origin can modify epigenetics through DNA methylation^8,9 and possibly gene expression; thus, mosaicism could buffer dysregulated processes in ageing and disease. However, whether X skew or its mosaicism alters functions in female individuals is largely unknown. Here we tested whether skew towards an active X_m chromosome influences the brain and body—and then delineated unique features of X_m neurons and X_p neurons. An active X_m chromosome impaired cognition in female mice throughout the lifespan and led to worsened cognition with age. Cognitive deficits were accompanied by X_m-mediated acceleration of biological or epigenetic ageing of the hippocampus, a key centre for learning and memory, in female mice. Several genes were imprinted on the X_m chromosome of hippocampal neurons, suggesting silenced cognitive loci. CRISPR-mediated activation of X_m-imprinted genes improved cognition in ageing female mice. Thus, the X_m chromosome impaired cognition, accelerated brain ageing and silenced genes that contribute to cognition in ageing. Understanding how X_m impairs brain function could lead to an improved understanding of heterogeneity in cognitive health in female individuals and to X-chromosome-derived pathways that protect against cognitive deficits and brain ageing.

Family history is one of the biggest risk factors—behind advanced age—for developing late-onset Alzheimer disease (AD). This is especially true if the

Women show resilience to cognitive aging, in the absence of dementia, in many populations. To dissect sex differences, we utilized the FCG and XY ^* mouse models. Female gonads and sex chromosomes improved cognition in aging mice of both sexes. Further, presence of a second X in male and female mice conferred cognitive resilience while its absence in females blocked it. In the hippocampal proteome of aging female mice, the second X increased proteins involved in synaptogenesis signaling – a potential pathway to improved cognition.