The Impact of the Maternal X Chromosome on Cognition and Brain Aging in Female Mice

Background Introduction

In mammals, female cells possess two X chromosomes, one from the mother (maternal X chromosome, Xm) and one from the father (paternal X chromosome, Xp). During embryonic development, one of the X chromosomes is randomly inactivated, a process known as X chromosome inactivation (X inactivation). This inactivation mechanism results in cellular mosaicism in females, where some cells express the maternal X chromosome while others express the paternal X chromosome. This mosaicism varies among individuals, with some showing significant skewing of X chromosome inactivation. The parental origin of the X chromosome may influence gene expression through epigenetic mechanisms such as DNA methylation, potentially buffering against aging and disease processes. However, whether skewing or mosaicism of X chromosome inactivation affects female functions, particularly cognition and brain aging, remains an unresolved question.

This study aimed to investigate whether skewing towards the maternal X chromosome affects brain and body functions in female mice and to further reveal the unique characteristics of Xm and Xp neurons in cognition and brain aging.

Source of the Paper

This paper was co-authored by Samira Abdulai-Saiku, Shweta Gupta, Dan Wang, and others from the Weill Institute for Neurosciences at the University of California San Francisco, the Gladstone Institute of Cardiovascular Disease, and the Bakar Aging Research Institute. The paper was published in Nature in 2024 under the title “The maternal X chromosome affects cognition and brain ageing in female mice.”

Research Process and Results

1. Study Design and Mouse Model Construction

To investigate the impact of maternal X chromosome skewing on female mice, the research team constructed two groups of mouse models: one with Xm+Xp mosaicism (normal random X chromosome inactivation) and the other with Xm skewing (using gene editing to ensure the maternal X chromosome remained the only active X chromosome). The specific steps included: - Gene Editing: By deleting the Xist gene (a key regulator of X chromosome inactivation) and utilizing the Cre-loxP system, the maternal X chromosome was maintained as the only active X chromosome in all cells. - Mouse Model Validation: Immunofluorescence was used to verify mosaicism in Xm+Xp mice and the inactivation of the paternal X chromosome in Xm mice.

2. Organ Function Assessment

The research team conducted a comprehensive evaluation of multiple organ functions in Xm and Xm+Xp mice, including cardiac function, bone density, body composition, and energy metabolism. The results showed: - Cardiac Function: Measurements of left ventricular volume, ejection fraction, and other parameters via echocardiography revealed no significant differences between the two groups during middle age (16-19 months). - Bone Density and Body Composition: Dual-energy X-ray absorptiometry (DEXA) measurements of bone density, lean body mass, and fat percentage showed no significant differences between the two groups. - Energy Metabolism: Measurements of oxygen consumption (VO2), carbon dioxide production (VCO2), and respiratory exchange ratio (RER) using metabolic cages (CLAMS) revealed no significant differences in metabolic parameters between the two groups.

3. Cognitive Function Testing

The research team conducted cognitive function tests across the lifespan of Xm and Xm+Xp mice, focusing on spatial learning and memory. Key experiments included: - Morris Water Maze Test: Used to assess spatial learning and memory. The results showed that Xm mice exhibited no significant differences in spatial learning ability compared to Xm+Xp mice during youth (4-8 months), but showed significant memory deficits in memory tests. - Open Field Test: Used to assess repeated spatial memory testing. The results showed that Xm mice exhibited significant forgetfulness during middle age (9-11 months) and old age (20-24 months). - Y-Maze Test: Used to assess working memory and spatial memory. The results showed that Xm mice exhibited significant working memory deficits in old age.

4. Brain Aging and Epigenetic Analysis

To explore whether the Xm chromosome accelerates brain aging, the research team conducted epigenetic clock analysis on the hippocampus of the mice. The results showed: - Epigenetic Age: The hippocampus of Xm mice exhibited significant epigenetic age acceleration in old age, while no significant differences were observed in the epigenetic age of blood. - Neuron-Specific Analysis: Fluorescence-activated cell sorting (FACS) was used to isolate Xm and Xp neurons, revealing significant epigenetic age acceleration in Xm neurons during old age.

5. Gene Imprinting and CRISPR Activation Experiments

The research team further investigated whether the Xm chromosome affects cognitive function through gene imprinting. RNA sequencing (RNA-seq) and real-time quantitative PCR (RT-qPCR) validation revealed that nine genes (including SASH3, TLR7, and CYSLTR1) were imprinted in Xm neurons. To validate the function of these genes, the research team used CRISPR activation (CRISPRa) to simultaneously upregulate the expression of SASH3, TLR7, and CYSLTR1 in the hippocampus of old mice. The results showed: - CRISPRa Validation: In vitro experiments successfully increased the mRNA expression of SASH3, TLR7, and CYSLTR1 by approximately twofold. - Behavioral Testing: In old mice, CRISPRa significantly improved spatial learning and memory.

Conclusions and Significance

This study found that skewing towards the maternal X chromosome significantly affects cognitive function in female mice and accelerates epigenetic aging in the hippocampus. Xm neurons silence some cognition-related genes through imprinting mechanisms, and activating these genes using CRISPRa technology can improve cognitive function in old mice. This research reveals the impact of parental origin of the X chromosome on cognition and brain aging, providing new insights into understanding heterogeneity in cognitive health among females.

Research Highlights

1. Key Finding: First to reveal that the maternal X chromosome affects cognitive function and brain aging through gene imprinting mechanisms.
2. Methodological Innovation: Successfully activated multiple imprinted genes in old mice using CRISPRa, significantly improving cognitive function.
3. Application Value: Provides a theoretical foundation for developing epigenetic intervention strategies targeting cognitive decline and brain aging.

Additional Valuable Information

This study also found that the impact of the Xm chromosome on brain function may be more significant than on other organs, consistent with the high expression of the X chromosome in the brain. Additionally, the findings align with cognitive deficits observed in humans with Turner syndrome, further supporting the importance of parental origin of the X chromosome in cognitive function.