Let's talk about sex as a biological variable — KI Case Studies, Series 2
Funding agencies across the globe are pushing for sex to be considered as a biological variable in all stages of the research they fund, from study design to data analysis and reporting. Likewise, an increasing number of journals require that sex aspects be addressed in the papers they publish. Here, four KI researchers share their experience of integrating sex as a biological variable (SABV) in their own work and explain how doing so benefits their research.
Bertrand Joseph: Microglia in the development and dysfunction of the brain
Bertrand Joseph’s team investigate the role of microglia—resident immune cells of the central nervous system—in both the development of the brain and the pathogenesis of brain disorders, such as Alzheimer’s disease, glioblastoma, and autism spectrum disorders. By integrating SABV into their research, they recently uncovered microglial mechanisms responsible for shaping cognition that they would have otherwise missed:
“As the animals to be analysed were difficult to obtain, we decided to pool male and female data, and we could not see any significant effects of microglial deficiency in Arg1+. But when we thereafter analysed the data taking SABV into account, we found that female mice, but not male mice, exhibited a robust phenotype.”
Asked whether he plans to follow up on SABV in the future, Joseph replies, “Definitely, and more as a routine for all projects. We also now aim at developing new in vitro models that include SABV. As an illustration, most microglial cell lines are ‘defined’ as sex unknown… we are now trying to develop male and female cell lines from primary microglia.”
He emphasizes that insights into sex differences in microglial cellular functions could, in turn, lead to better understanding of brain dysfunction and potentially open new avenues for therapeutic approaches that include SABV.
Rochellys Diaz Heijtz: Microbiota-gut-brain axis and neurodevelopmental disorders
Rochellys Diaz Heijtz wants to understand the biological basis of neurodevelopmental disorders, such as autism spectrum disorder, which feature important sex differences in both prevalence and manifestation. She has been addressing SABV in her work for some time now.
“I became aware early in my career about the importance of addressing sex as a biological variable in my research. I was very much inspired by the work of Prof. Roger Gorski at the University of California Los Angeles, who… was a pioneer in the field of brain sex differences,” says Heijtz.
Heijtz’s lab is currently focusing on the role of the gut microbiota in typical and atypical brain development in both humans and animal models, as well as the cellular and molecular pathways involved in the crosstalk between the microbiota and the developing brain. They have learned from their own work that some effects of the gut microbiota on the brain are age- and sex-dependent:
"For example, we showed that the absence of the bacterial peptidoglycan-sensing molecule Pglyrp2 leads to major sex-dependent alterations in motor and anxiety-like behavior. Pglyrp2-deficient female mice, but not males, show better motor performance. However, they display increased levels of anxiety-like behavior."
Asked what she thinks are effective strategies for raising awareness about SABV at KI and beyond, Heijtz urges researchers to present and discuss sex differences in various venues (e.g., meetings, workshops, websites, etc) and suggests having regular workshops about SABV policy as well as support for biomedical researchers at KI who wish to address SABV in their work. She also underscores the need to bring together KI’s clinical and preclinical scientists to speed up the translation of biomedical findings into practical use.
Rikard Holmdahl: Chronic inflammatory disorders
Women are more prone to developing certain types of autoimmune disease (e.g., rheumatoid arthritis and systemic lupus erythematosus), while men are more prone to others (e.g., ankylosing spondylitis). Rikard Holmdahl’s lab strives to determine why we develop autoimmune diseases—and understanding sex differences is, he says, simply part of the solution:
“We are always aware of sex differences, and we address the influence of both hormones and genetic differences in all projects… I think it is very important to understand the biology of both sexes.”
Holmdahl has a long track record of incorporating SABV into his research. In the 80s and 90s, for example, his group showed that several female sex hormones suppress the development of arthritis but promote the development of lupus. More recently, they positionally identified a polymorphic estrogen receptor-binding site that regulates expression of the T-cell activation marker Cd2, leading to female-specific differences in T cell-dependent mouse models of autoimmunity.
However, Holmdahl also cautions against misinterpreting apparent sex differences, “For example, in mouse models of rheumatoid arthritis, males are more susceptible to experimental arthritis— but the opposite is true for human rheumatoid arthritis, to which women are more susceptible. This susceptibility of male mice could simply result from stress due to the grouping of animals after puberty in a nonphysiological way that allows social hierarchies to develop.”
As much as he advocates for the integration of SABV in research, he decries misconceived regulations that restrict animal experimentation, resulting in underpowered and unreproducible studies: “It is bad for science and will, in the end, lead to the use of more animals.”
Ivan Nalvarte: Nuclear receptor signaling in neurodevelopment and neurodegeneration
Ivan Nalvarte’s group do research on Alzheimer’s disease (AD) and envision a future in which we have a better understanding of how AD develops, can identify men and women who are at increased risk of developing AD, and have new, personalized recommendations or treatments for combatting AD in both men and women.
To help attain this vision, they are deciphering the role of sex hormones in the onset and progression of AD, given that women run a 2-3-fold higher risk of being diagnosed with AD. His group addresses SABV on multiple levels, integrating epidemiologic data on menopause, hormone therapy, and AD patient outcome with experimental studies in male and female mouse models and male and female cell models, as well as post-mortem brains of men and women.
“From my personal experience in the field of neuroendocrinology, we almost always see that male and female animal models show different neurological experimental outcomes... [And] cell models are important in discriminating between effects of genetic sex (e.g., XX vs XY chromosome) and sex-hormone signalling,” Nalvarte explains.
Nalvarte is a strong proponent of integrating SABV in biomedical research and would welcome workshops and seminars on the topic, as well as the inclusion of SABV in education at all levels.