Biological sex, as well as gender in humans, affect the physiology of health and disease in major ways. Neurodevelopmental disorders, autoimmune disease, stroke – all of these affect men and women at different frequencies, and in different ways.
Biomedical research, however, has been slow to notice this. Especially in preclinical studies.
While the National Institutes of Health have been requiring researchers to account for sex as a variable since 2016, "in Europe, there is no such policy," Susanne Wolf told BioWorld.
The predictable result is that "a lot of researchers use male mice for all of their research," she said.
In the Sept. 4, 2018, issue of Cell Reports, Wolf, who is a researcher in the Cellular Neurosciences Lab at the Max Delbrück Center for Molecular Medicine (MDC) in Berlin, and her team published a comprehensive resource detailing differences in microglia, the immune cells of the brain, between male and female mice.
Wolf's pithy summary of the study's big picture implication was to "use female mice, especially if you want to find a cure for something."
The team decided to look at microglia because "a lot of work has been done on the immune system already because it is easily accessible," she said. "Microglia are the immune cells [of] the brain – we wanted to know whether they mirrored situation in the periphery."
In addition, microglia in a sense span the nervous system and the immune system – both systems that are sexually dimorphic.
Sex matters, from biophysics to behavior
In their studies, the team found myriad differences between microglia isolated from the brains of male and female mice.
Those differences began at the physical level, with male mice having a different cell membrane potential than females.
They continued through gene expression, where the scientists found almost a thousand genes that were differentially expressed between males and females, and protein expression, with differences in more than 350 proteins. Those proteins included multiple myosin-related proteins, which affect cell stability, trafficking, shape and size; and innate immune proteins related to Toll-like receptor and interferon signaling.
Another group of proteins whose expression differed between male and female cells was the purinoreceptors, which accounted for a functional difference that particularly surprised Wolf and her colleagues.
Cells from male and female mice "reacted to ATP differently," she said. "ATP is released when other cells are damaged, [and] male cells reacted much more strongly than females" to such damage.
Wolf and her team plan to look at mouse models of sexually dimorphic disease to see whether microglia function accounts for observed differences.
Clinical studies, too, could be mined for such differences, she said. "People, companies, clinicians could go back and check their huge databases to see whether there is difference in treatment response."
Looking beyond the binary
Ironically, researchers are realizing that it is important to study both males and females just as society at large is coming to the realization that neither sex nor gender are binary categories, and that they can align in more ways than are dreamt of in some philosophies.
Whether a transgender person's physiology reflects their genetic sex or their gender is largely unexplored territory, but will depend on many factors, including both current hormone therapy and whether they went through the puberty of their natal sex.
There is some work suggesting that microglia retain characteristics of their chromosomal sex when transplanted. Female mice, like female humans, are less likely to suffer a stroke than males. In a study published in Cell Reports in June by researchers from the University of Milan, transplanting microglia from female mice into males protected the males from ischemic stroke.
Wolf said the findings likely reflected the longevity of the cells themselves. "These are the cells that they are born with, and they stay with you," she said. Unlike organs such as the skin, which renews itself every few years, for brain cells and brain immune cells "there is... turnover, but it is very slow."
As a result, "everything you do in life, they kind of memorize," she said. "If you are female microglia, you are educated by [female] hormones, and you have certain properties that you keep, even if you are then in a different environment."
She cautioned, though, that studies done to date have been short-term, and no one has attempted to specifically model the state of transgender individuals who are being treated with cross-sex hormones.
Over time, she said, being in the physiological milieu characteristic of the opposite sex could well be a continuing education of sorts for such cells, leading to functional changes.
"Long-term," she said, "who knows?"