Brain Sex Research Articles

Temporal sex specific brain gene expression pattern during early rat embryonic development.

Background: The classical concept of brain sex differentiation suggests that steroid hormones released from the gonads program male and female brains differently. However, several studies indicate that steroid hormones are not the only determinant of brain sex differentiation and that genetic differences could also be involved. Methods: In this study, we have performed RNA sequencing of rat brains at embryonic days 12 (E12), E13, and E14. The aim was to identify differentially expressed genes between male and female rat brains during early development. Results: Analysis of genes expressed with the highest sex differences showed that Xist was highly expressed in females having XX genotype with an increasing expression over time. Analysis of genes expressed with the highest male expression identified three early genes, Sry2, Eif2s3y, and Ddx3y. Discussion: The observed sex-specific expression of genes at early development confirms that the rat brain is sexually dimorphic prior to gonadal action on the brain and identifies Sry2 and Eif2s3y as early genes contributing to male brain development.

Modeling brain sex in the limbic system as phenotype for female-prevalent mental disorders

BackgroundSex differences exist in the prevalence and clinical manifestation of several mental disorders, suggesting that sex-specific brain phenotypes may play key roles. Previous research used machine learning models to classify sex from imaging data of the whole brain and studied the association of class probabilities with mental health, potentially overlooking regional specific characteristics.MethodsWe here investigated if a regionally constrained model of brain volumetric imaging data may provide estimates that are more sensitive to mental health than whole brain-based estimates. Given its known role in emotional processing and mood disorders, we focused on the limbic system. Using two different cohorts of healthy subjects, the Human Connectome Project and the Queensland Twin IMaging, we investigated sex differences and heritability of brain volumes of limbic structures compared to non-limbic structures, and subsequently applied regionally constrained machine learning models trained solely on limbic or non-limbic features. To investigate the biological underpinnings of such models, we assessed the heritability of the obtained sex class probability estimates, and we investigated the association with major depression diagnosis in an independent clinical sample. All analyses were performed both with and without controlling for estimated total intracranial volume (eTIV).ResultsLimbic structures show greater sex differences and are more heritable compared to non-limbic structures in both analyses, with and without eTIV control. Consequently, machine learning models performed well at classifying sex based solely on limbic structures and achieved performance as high as those on non-limbic or whole brain data, despite the much smaller number of features in the limbic system. The resulting class probabilities were heritable, suggesting potentially meaningful underlying biological information. Applied to an independent population with major depressive disorder, we found that depression is associated with male–female class probabilities, with largest effects obtained using the limbic model. This association was significant for models not controlling for eTIV whereas in those controlling for eTIV the associations did not pass significance correction.ConclusionsOverall, our results highlight the potential utility of regionally constrained models of brain sex to better understand the link between sex differences in the brain and mental disorders.

Insight into brain sex differences of typically developed infants and brain pathologies: A systematic review.

The continually advancing landscape of neuroscientific and imaging research has broadened our comprehension of sexdifferences encoded in the human brain, expanding from the hypothalamus and sexual behaviour to encompass the entire brain, including its diverse lobes, structures, and functions. However, less is known about sex differences in the brains of neonates and infants, despite their relevance to various sex-linked diseases that develop early in life. In this review, we provide a synopsis of the literature evidence on sex differences in the brains of neonates and infants at the morphological, structural and network levels. We also briefly overview the present evidence on the sex bias in some brain disorders affecting infants and neonates.

Prediction of brain sex from EEG: using large-scale heterogeneous dataset for developing a highly accurate and interpretable ML model

This study presents a comprehensive examination of sex-related differences in resting-state electroencephalogram (EEG) data, leveraging two different types of machine learning models to predict an individual's sex. We utilized data from the Two Decades-Brainclinics Research Archive for Insights in Neurophysiology (TDBRAIN) EEG study, affirming that gender prediction can be attained with noteworthy accuracy. The best performing model achieved an accuracy of 85% and an ROC AUC of 89%, surpassing all prior benchmarks set using EEG data and rivaling the top-tier results derived from fMRI studies. A comparative analysis of LightGBM and Deep Convolutional Neural Network (DCNN) models revealed DCNN's superior performance, attributed to its ability to learn complex spatial-temporal patterns in the EEG data and handle large volumes of data effectively. Despite this, interpretability remained a challenge for the DCNN model. The LightGBM interpretability analysis revealed that the most important EEG features for accurate sex prediction were related to left fronto-central and parietal EEG connectivity. We also showed the role of both low (delta and theta) and high (beta and gamma) activity in the accurate sex prediction. These results, however, have to be approached with caution, because it was obtained from a dataset comprised largely of participants with various mental health conditions, which limits the generalizability of the results and necessitates further validation in future studies. . Overall, the study illuminates the potential of interpretable machine learning for sex prediction, alongside highlighting the importance of considering individual differences in prediction sex from brain activity.

Transition in the cognitive continuum from normal cognition to dementia: the interplay of lifelong cognitive reserve with brain aging and genetic predisposition

AbstractBackgroundHigher cognitive reserve (CR) has been associated with lower risks of cognitive decline and dementia. In this study, we sought to investigate associations of lifelong CR capacity with transitions of cognitive phenotypes in older adults, while accounting for age, sex, APOE genotype, and pathological load of brain aging.MethodThis population‐based Swedish National study on Aging and Care‐Kungsholmen included 2631 participants (age≥60 years, 61.4% female) free of dementia at baseline (2001‐2004); among them 517 had brain MRI. At baseline, lifelong CR capacity was assessed by integrating early‐life education, midlife occupational complexity, later‐life social network and leisure activity. The load of pathological brain aging was assessed using volumes of grey matter (GM), white matter, hippocampus, and white matter hyperintensity, and count of perivascular space and lacunes. Cognitive impairment no dementia (CIND) and dementia were defined at baseline and across up to 15 years until 2016‐2019. Data were analyzed using the Markov multi‐state model.ResultDuring the mean 10.7 years of follow‐up, 9.0% had incident dementia (95%CI: 7.8%–10.3%). Among people free of CIND at baseline (n = 2023), 27.4% had incident CIND during follow‐up (25.3%–29.6%). Higher lifelong CR capacity (mean value of composite CR score = 0.2; range: ‐4.2‐3.6) was associated with a lower risk of direct transitions from non‐dementia (normal or CIND) to dementia (multivariable‐adjusted HR = 0.84; 95% CI 0.73–0.95) and from normal cognition to CIND (0.79; 0.73–0.85), but not with the direct transitions from normal cognition to dementia (0.86; 0.72–1.03), from CIND to dementia (0.93; 0.76–1.14), or from CIND to normal cognition (1.15; 0.98–1.35). Higher lifelong CR capacity was associated with a lower risk of direct transition from normal cognition to dementia in people with APOE‐ε4 and those with high GM volume (2nd‐4th quartiles), but not in those without APOE‐ε4 or with low GM volume (1st quartile) (P‐for‐interaction<0.05). There was no statistical interaction of CR with age, sex, or other brain MRI markers on cognitive transitions.ConclusionHigher lifelong CR capacity may benefit late‐life cognitive status by reducing the risk of cognitive deterioration to dementia, especially in people with genetic predisposition or with low neurodegenerative load.

Brain sex classification in the limbic system as female-specific phenotype for depression

Brain sex classification in the limbic system as female-specific phenotype for depression

Sex differences in gene regulatory networks during mid-gestational brain development.

Neurodevelopmental disorders differ considerably between males and females, and fetal brain development is one of the most critical periods to determine risk for these disorders. Transcriptomic studies comparing male and female fetal brain have demonstrated that the highest difference in gene expression occurs in sex chromosomes, but several autossomal genes also demonstrate a slight difference that has not been yet explored. In order to investigate biological pathways underlying fetal brain sex differences, we applied medicine network principles using integrative methods such as co-expression networks (CEMiTool) and regulatory networks (netZoo). The pattern of gene expression from genes in the same pathway tend to reflect biologically relevant phenomena. In this study, network analysis of fetal brain expression reveals regulatory differences between males and females. Integrating two different bioinformatics tools, our results suggest that biological processes such as cell cycle, cell differentiation, energy metabolism and extracellular matrix organization are consistently sex-biased. MSET analysis demonstrates that these differences are relevant to neurodevelopmental disorders, including autism.

Effects of developmental exposure to FireMaster® 550 (FM 550) on microglia density, reactivity and morphology in a prosocial animal model

Microglia are known to shape brain sex differences critical for social and reproductive behaviors. Chemical exposures can disrupt brain sexual differentiation but there is limited data regarding how they may impact microglia distribution and function. We focused on the prevalent flame retardant mixture Firemaster 550 (FM 550) which is used in foam-based furniture and infant products including strollers and nursing pillows because it disrupts sexually dimorphic behaviors. We hypothesized early life FM 550 exposure would disrupt microglial distribution and reactivity in brain regions known to be highly sexually dimorphic or associated with social disorders in humans. We used prairie voles (Microtus ochrogaster) because they display spontaneous prosocial behaviors not seen in rats or mice and are thus a powerful model for studying chemical exposure-related impacts on social behaviors and their underlying neural systems. We have previously demonstrated that perinatal FM 550 exposure sex-specifically impacts socioemotional behaviors in prairie voles. We first established that, unlike in rats, the postnatal colonization of the prairie vole brain is not sexually dimorphic. Vole dams were then exposed to FM 550 (0, 500, 1000, 2000 µg/day) via subcutaneous injections through gestation, and pups were directly exposed beginning the day after birth until weaning. Adult offspring’s brains were assessed for number and type (ramified, intermediate, ameboid) of microglia in the medial prefrontal cortex (mPFC), cerebellum (lobules VI-VII) and amygdala. Effects were sex- and dose-specific in the regions of interests. Overall, FM 550 exposure resulted in reduced numbers of microglia in most regions examined, with the 1000 µg FM 550 exposed males particularly affected. To further quantify differences in microglia morphology in the 1000 µg FM 550 group, Sholl and skeleton analysis were carried out on individual microglia. Microglia from control females had a more ramified phenotype compared to control males while 1000 µg FM 550-exposed males had decreased branching and ramification compared to same-sex controls. Future studies will examine the impact on the exposure to FM 550 on microglia during development given the critical role of these cells in shaping neural circuits.

Gene regulation by gonadal hormone receptors underlies brain sex differences

Oestradiol establishes neural sex differences in many vertebrates1–3 and modulates mood, behaviour and energy balance in adulthood4–8. In the canonical pathway, oestradiol exerts its effects through the transcription factor oestrogen receptor-α (ERα)9. Although ERα has been extensively characterized in breast cancer, the neuronal targets of ERα, and their involvement in brain sex differences, remain largely unknown. Here we generate a comprehensive map of genomic ERα-binding sites in a sexually dimorphic neural circuit that mediates social behaviours. We conclude that ERα orchestrates sexual differentiation of the mouse brain through two mechanisms: establishing two male-biased neuron types and activating a sustained male-biased gene expression program. Collectively, our findings reveal that sex differences in gene expression are defined by hormonal activation of neuronal steroid receptors. The molecular targets we identify may underlie the effects of oestradiol on brain development, behaviour and disease.

Philip William Davidson, Ph.D. obituary

Microglia are known to shape brain sex differences critical for social and reproductive behaviors. Chemical exposures can disrupt brain sexual differentiation but there is limited data regarding how they may impact microglia distribution and function. We focused on the prevalent flame retardant mixture Firemaster 550 (FM 550) which is used in foam-based furniture and infant products including strollers and nursing pillows because it disrupts sexually dimorphic behaviors. We hypothesized early life FM 550 exposure would disrupt microglial distribution and reactivity in brain regions known to be highly sexually dimorphic or associated with social disorders in humans. We used prairie voles (Microtus ochrogaster) because they display spontaneous prosocial behaviors not seen in rats or mice and are thus a powerful model for studying chemical exposure-related impacts on social behaviors and their underlying neural systems. We have previously demonstrated that perinatal FM 550 exposure sex-specifically impacts socioemotional behaviors in prairie voles. We first established that, unlike in rats, the postnatal colonization of the prairie vole brain is not sexually dimorphic. Vole dams were then exposed to FM 550 (0, 500, 1000, 2000 µg/day) via subcutaneous injections through gestation, and pups were directly exposed beginning the day after birth until weaning. Adult offspring’s brains were assessed for number and type (ramified, intermediate, ameboid) of microglia in the medial prefrontal cortex (mPFC), cerebellum (lobules VI-VII) and amygdala. Effects were sex- and dose-specific in the regions of interests. Overall, FM 550 exposure resulted in reduced numbers of microglia in most regions examined, with the 1000 µg FM 550 exposed males particularly affected. To further quantify differences in microglia morphology in the 1000 µg FM 550 group, Sholl and skeleton analysis were carried out on individual microglia. Microglia from control females had a more ramified phenotype compared to control males while 1000 µg FM 550-exposed males had decreased branching and ramification compared to same-sex controls. Future studies will examine the impact on the exposure to FM 550 on microglia during development given the critical role of these cells in shaping neural circuits.

The sexual brain, genes, and cognition: A machine‐predicted brain sex score explains individual differences in cognitive intelligence and genetic influence in young children

Sex impacts the development of the brain and cognition differently across individuals. However, the literature on brain sex dimorphism in humans is mixed. We aim to investigate the biological underpinnings of the individual variability of sexual dimorphism in the brain and its impact on cognitive performance. To this end, we tested whether the individual difference in brain sex would be linked to that in cognitive performance that is influenced by genetic factors in prepubertal children (N = 9,658, ages 9–10 years old; the Adolescent Brain Cognitive Development study). To capture the interindividual variability of the brain, we estimated the probability of being male or female based on the brain morphometry and connectivity features using machine learning (herein called a brain sex score). The models accurately classified the biological sex with a test ROC–AUC of 93.32%. As a result, a greater brain sex score correlated significantly with greater intelligence (p fdr < .001, ηp2 = .011–.034; adjusted for covariates) and higher cognitive genome‐wide polygenic scores (GPSs) (p fdr < .001, ηp2 < .005). Structural equation models revealed that the GPS‐intelligence association was significantly modulated by the brain sex score, such that a brain with a higher maleness score (or a lower femaleness score) mediated a positive GPS effect on intelligence (indirect effects = .006–.009; p = .002–.022; sex‐stratified analysis). The finding of the sex modulatory effect on the gene–brain–cognition relationship presents a likely biological pathway to the individual and sex differences in the brain and cognitive performance in preadolescence.

Brain Sex in Transgender Women Is Shifted towards Gender Identity.

Transgender people report discomfort with their birth sex and a strong identification with the opposite sex. The current study was designed to shed further light on the question of whether the brains of transgender people resemble their birth sex or their gender identity. For this purpose, we analyzed a sample of 24 cisgender men, 24 cisgender women, and 24 transgender women before gender-affirming hormone therapy. We employed a recently developed multivariate classifier that yields a continuous probabilistic (rather than a binary) estimate for brains to be male or female. The brains of transgender women ranged between cisgender men and cisgender women (albeit still closer to cisgender men), and the differences to both cisgender men and to cisgender women were significant (p = 0.016 and p < 0.001, respectively). These findings add support to the notion that the underlying brain anatomy in transgender people is shifted away from their biological sex towards their gender identity.

Metabolic Sex Dimorphism of the Brain at the Gene, Cell, and Tissue Level.

The palpable observation in the sex bias of disease prevalence in the CNS has fascinated scientists for several generations. Brain sex dimorphism has been visualized by imaging and analytical tools at the tissue, cellular, and molecular levels. Recent work highlighted the specificity of such sex bias in the brain and its subregions, offering a unique lens through which disease pathogenesis can be investigated. The brain is the largest consumer of energy in the body and provides a unique metabolic environment for diverse lineages of cells. Immune cells are increasingly recognized as an integral part of brain physiology, and their function depends on metabolic homeostasis. This review focuses on metabolic sex dimorphism in brain tissue, resident, and infiltrating immune cells. In this context, we highlight the relevance of recent advances in metabolomics and RNA sequencing technologies at the single cell resolution and the development of novel computational approaches.

Comprehensive Sex Steroid Profiling in Multiple Tissues Reveals Novel Insights in Sex Steroid Distribution in Male Mice.

A comprehensive atlas of sex steroid distribution in multiple tissues is currently lacking, and how circulating and tissue sex steroid levels correlate remains unknown. Here, we adapted and validated a gas chromatography tandem mass spectrometry method for simultaneous measurement of testosterone (T), dihydrotestosterone (DHT), androstenedione, progesterone (Prog), estradiol, and estrone in mouse tissues. We then mapped the sex steroid pattern in 10 different endocrine, reproductive, and major body compartment tissues and serum of gonadal intact and orchiectomized (ORX) male mice. In gonadal intact males, high levels of DHT were observed in reproductive tissues, but also in white adipose tissue (WAT). A major part of the total body reservoir of androgens (T and DHT) and Prog was found in WAT. Serum levels of androgens and Prog were strongly correlated with corresponding levels in the brain while only modestly correlated with corresponding levels in WAT. After orchiectomy, the levels of the active androgens T and DHT decreased markedly while Prog levels in male reproductive tissues increased slightly. In ORX mice, Prog was by far the most abundant sex steroid, and, again, WAT constituted the major reservoir of Prog in the body. In conclusion, we present a comprehensive atlas of tissue and serum concentrations of sex hormones in male mice, revealing novel insights in sex steroid distribution. Brain sex steroid levels are well reflected by serum levels and WAT constitutes a large reservoir of sex steroids in male mice. In addition, Prog is the most abundant sex hormone in ORX mice.

Can Open Science be a Tool to Dismantle Claims of Hardwired Brain Sex Differences? Opportunities and Challenges for Feminist Researchers

Feminist scholars have long been concerned with claims of hardwired brain sex differences emanating from neuroscience and evolutionary psychology. Past criticisms of these claims have rightfully questioned the impact of this research on gender equality, pointing out how findings can be used to vindicate gender stereotypes. In this article, we appraise the brain sex differences literature through the lens of open science, a movement aimed at improving the robustness and reliability of science. In this discussion, we offer a feminist evaluation of the strategies (e.g., pre-registration, data sharing, and accountability) provided by open science, and we question whether these may be the novel and disruptive tools needed to dismantle claims about hardwired brain sex differences. We suggest that open science strategies can be useful in challenging some of these claims, and we note that promising initiatives are already being developed in neuroscience and allied fields. We end by acknowledging the distinct challenges that feminist researchers wishing to engage in open science face, particularly in the context of limited diversity. We conclude that open science presents considerable opportunity for feminist researchers, and that it will be crucial for feminists to be involved in shaping the future of this movement.

Sex, Brains, and Individuals: Machine--Learned Brain Sex Explains Individual-Specific Cognitive Development and its Genetic Factors in Prepuberty Children

Is biological sex binary or continuous? Individual variability within each sex provides key information understanding human individuality. However, little is known about this within-sex individual variability of the brain. Here, we investigated the brain and genetic underpinnings of the individual difference within and between sex.

Identification of plasma proteins relating to brain neurodegeneration and vascular pathology in cognitively normal individuals.

IntroductionThis study aims to first discover plasma proteomic biomarkers relating to neurodegeneration (N) and vascular (V) damage in cognitively normal individuals and second to discover proteins mediating sex‐related difference in N and V pathology.MethodsFive thousand and thirty‐two plasma proteins were measured in 1061 cognitively normal individuals (628 females and 433 males), nearly 90% of whom had magnetic resonance imaging measures of hippocampal volume (as N) and white matter hyperintensities (as V).ResultsDifferential protein expression analysis and co‐expression network analysis revealed different proteins and modules associated with N and V, respectively. Furthermore, causal mediation analysis revealed four proteins mediated sex‐related difference in N and one protein mediated such difference in V damage.DiscussionOnce validated, the identified proteins could help to select cognitively normal individuals with N and V pathology for Alzheimer's disease clinical trials and provide targets for further mechanistic studies on brain sex differences, leading to sex‐specific therapeutic strategies.

Microglia Regulate Cell Genesis in a Sex-dependent Manner in the Neonatal Hippocampus

Microglia, the innate immune cells of the brain, regulate brain development through many processes such as synaptic pruning, supporting cell genesis and phagocytosing living and dying cells. There are sex differences in these same developmental processes throughout the brain, thus microglia may contribute to brain sex differences. We examined whether microglia support a known sex difference in neonatal hippocampal neurogenesis and whether juvenile hippocampal neurogenesis was impacted by the loss of neonatal microglia. We used central infusion of liposomal clodronate to selectively deplete microglia and found decreased cell genesis in the male, but not female, dentate gyrus and hippocampus. We found that loss of microglia decreased cell genesis in the cortex and amygdala of both males and females. We assessed the expression of several cytokines and growth factors that have previously been shown to support cell genesis. We found that expression of Il1b and Tnf were decreased in the hippocampus due to microglia depletion however, there were no sex differences in the expression of any immune genes. In adolescence, there was an increase in the number of mitotic cells in the subgranular zone of the dentate gyrus of previously microglia depleted rats however, the number of newly-born neurons was unchanged in the adolescent animals. We also sought to determine whether there was a sex difference in the number of progenitor cells in the dentate gyrus in the neonatal period. We found no sex differences in the number of progenitor cells. Overall, these studies show that microglia are important for regulating region-specific sex differences in cell genesis in the developing brain.

Development of sex differences in the human brain

ABSTRACT Sex differences in brain anatomy have been described from early childhood through late adulthood, but without any clear consensus among studies. Here, we applied a machine learning approach to estimate ‘Brain Sex’ using a continuous (rather than binary) classifier in 162 boys and 185 girls aged between 5 and 18 years. Changes in the estimated sex differences over time at different age groups were subsequently calculated using a sliding window approach. We hypothesized that males and females would differ in brain structure already during childhood, but that these differences will become even more pronounced with increasing age, particularly during adolescence. Overall, the classifier achieved a good performance, with an accuracy of 80.4% and an AUC of 0.897 across all age groups. Assessing changes in the estimated sex with age revealed a growing difference between the sexes with increasing age. That is, the very large effect size of d = 1.2 which was already evident during childhood increased even further from age 11 onward, and eventually reached an effect size of d = 1.6 at age 17. Altogether these findings suggest a systematic sex difference in brain structure already during childhood, and a subsequent increase of this difference during adolescence.

Sex-related patterns of intrinsic functional connectivity in children and adolescents with autism spectrum disorders.

We investigated whether children and adolescents with autism spectrum disorders show sex-specific patterns of brain function (using functional magnetic resonance imaging) that are well documented in typically developing males and females. We found, unexpectedly, that boys and girls with autism do not differ in their brain functional connectivity, whereas typically developing boys and girls showed differences in a brain network involved in thinking about self and others (the default mode network). Results suggest that autism may be characterized by a lack of brain sex differentiation.