Summary: Researchers identified over 1,000 genes that are expressed at markedly different levels in male and female mouse brains. These findings offer insight into how sex influences brain function and behavior in mammals.
Source: Stanford
New research led by Stanford Medicine investigators shows that male and female mouse brains differ in many significant ways.
The team reports that these molecular and cellular differences likely have parallels in human brains, offering clues to behavioral distinctions between men and women.
The researchers examined four small brain regions known to regulate social behaviors related to assessing, courting, mating and defending — behaviors that contribute directly to reproduction and offspring survival. Examples include male mice quickly identifying the sex of an intruder, female mating receptivity that varies with cycle stage, and maternal aggression to protect young.
From tissue samples enriched for cells that respond to sex hormones, the investigators identified more than 1,000 genes whose activity levels vary substantially between males and females. Gene activity reflects how often a gene’s instructions are used to make proteins, and those activity patterns determine what a cell does. The results, reported in a paper to be published online Jan. 21 in Cell, illuminate biological mechanisms behind sex-typical behaviors in mammals.
“By using these sex-biased genes as starting points, we have pinpointed specific groups of brain cells that coordinate sex-typical behaviors,” said senior author Nirao Shah, PhD, professor of psychiatry and behavioral sciences and of neurobiology.
Joseph Koestler, PhD, a postdoctoral scholar in Shah’s lab, is the study’s lead author.
In addition to sex differences, the team found more than 600 genes whose activity varies across phases of the female estrous cycle (the rodent analogue of the human menstrual cycle). “Finding several hundred genes in these four tiny brain regions whose expression depends strictly on cycle stage was unexpected,” Shah said. His work focuses on how sex hormones shape instinctive social behaviors.
The four targeted brain regions are conserved across mammals, including humans. “Mice aren’t humans,” Shah cautioned, “but it is reasonable to expect analogous cell types in human brains to contribute to our own sex-typical social behaviors.”
Implications for neurological and psychiatric conditions
Some sex-biased genes the team cataloged are already linked to brain disorders that show sex differences in prevalence. Of 207 genes previously associated with high risk for autism spectrum disorder — a condition diagnosed more often in males — the investigators found 39 with sex-differential expression: 29 more active in males and 10 more active in females. They also observed female-biased activation for genes connected to Alzheimer’s disease and multiple sclerosis, conditions that tend to affect women more frequently.
The researchers suggest that when a gene that must be highly active for healthy function carries a mutation, the consequences may be worse than when the same mutation affects a gene with low activity. Hormone-driven shifts in gene expression may underlie documented fluctuations in migraine, seizure frequency and some psychiatric symptoms across the menstrual cycle, Shah noted.
Built-in social behaviors
Sex-typical social behaviors are deeply rooted in mammalian brains by evolutionary pressures. Male mice rapidly judge the sex of an unfamiliar mouse and respond accordingly: confrontation and territorial aggression toward other males, or courtship toward females. Female mice tend to show maternal aggression to protect pups, are more likely to retrieve wandering young, and display mating receptivity that changes markedly with cycle stage.
“These instinctive behaviors are essential for survival and reproduction,” Shah said. “They are largely innate — if animals had to learn how to fight or mate at the moment they were needed, chances of survival would be reduced. Our findings reinforce that the brain contains built-in circuits shaped by biological factors as well as the environment.”
Previous studies comparing gene expression between male and female rodent brain cells had identified roughly 100 sex-biased genes — far fewer than expected given clear behavioral differences. By focusing on rare, hormone-responsive cells, Shah’s team uncovered about ten times that number, plus the hundreds of cycle-stage–sensitive genes. Altogether, roughly 6% of mouse genes in these regions appear to be regulated by sex or by cycle stage.
Finding needles within needles
Shah compared the experimental challenge to “finding needles within needles in a haystack.” The critical cells his team isolated likely constitute less than 0.0005% of all brain cells. Identifying them required separating these scarce cells from surrounding tissue and profiling gene expression at single-cell resolution.
The investigators enriched samples for cells that express estrogen receptors, because estrogen is a major regulator of sex-typical behavior (present in both sexes but at higher levels in females). Estrogen and progesterone levels in females rise and fall across a roughly monthly cycle, and corresponding behaviors follow those hormonal rhythms. In mice, estrus (or “heat”) aligns with hormone peaks and maximum sexual receptivity; diestrus corresponds to hormone troughs.
By comparing males, females in estrus and females in diestrus, the researchers identified 1,415 genes whose expression differed among these groups. The estrogen-responsive cells themselves were diverse. In the bed nucleus of the stria terminalis (BNST), a structure also present in humans, the team classified estrogen-responsive cells into 36 distinct cell types based on their gene-activation signatures.
Of those 36 BNST cell types, the team demonstrated that a single cell type is essential for male mice to rapidly recognize the sex of another mouse and respond in a typical male manner. In the ventromedial hypothalamus (VMH), another brain region conserved in humans, researchers distinguished 27 estrogen-responsive cell types. Disrupting just one VMH cell type — while leaving the other 26 intact — changed females that would normally be receptive into females that rejected mating advances, even during estrus.
Those two cell types — one in the BNST influencing males’ sex recognition and one in the VMH affecting female sexual receptivity — exemplify the “needles within needles.” The roles of the remaining dozens of hormone-responsive cell types in these regions remain to be determined.
Beginning of a broader picture
“This work likely represents just the beginning,” Shah said. “There are probably many more sex-differentiated features to discover across these and other brain regions if we apply the right approaches.”
Stanford’s Office of Technology Licensing has filed for patent protection related to aspects of this study.
Shah is affiliated with Stanford Bio-X and the Wu Tsai Neurosciences Institute and is a faculty fellow of Stanford ChEM-H.
Other Stanford co-authors include research scientists Sayaka Inoue, PhD, and Taehong Yang, PhD; postdoctoral scholars Daniel Bayless, PhD, and Nicole Leung, PhD; graduate students Adarsh Tantry and Chung-ha Davis; undergraduate Grace Wang; life science research professional Maricruz Alvarado; laboratory manager Charu Ramakrishnan; instructor Lief Fenno, MD, PhD; and Karl Deisseroth, MD, PhD. Collaborators from Accent Therapeutics and Columbia University also contributed.
Funding: This research was supported by grants from the National Institutes of Health (R01NS049488, R01NS083872, R01NS091144, F32MH125593, U01DA052783, R01HD104565 and R01MH123047), the National Science Foundation, the Paul and Daisy Soros Fellowship for New Americans Program, the AE Foundation, the Lucile Packard Foundation for Children’s Health, the Chan-Zuckerberg Initiative, the GG Gift Fund, the Human Frontier Science Program, the Bio-X Undergraduate Summer Research Program, and the Stanford Vice Provost for Undergraduate Education.
About this genetics research news
Author: Bruce Goldman
Source: Stanford
Contact: Bruce Goldman – Stanford
Image: The image is in the public domain
Original Research: The findings will appear in Cell