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Males are straightforward while females are complicated. This misguided view prompted a decades-long exclusion of female animals from research out of fear that their fluctuating hormone levels will muddle the data. 

But now a new study by researchers at the University of Toronto shows that a female sex hormone plays a key role in promoting brain repair and opens the door to the development of more effective treatments.

The team of researchers led by Cindi Morshead, a professor at the Donnelly Centre for Cellular and Biomolecular Research and the department of surgery, found that metformin, a widely prescribed drug to treat diabetes, promotes repair in adult female brains and is dependent on the sex hormone estradiol.

Their findings published in the Sept. 11, 2019 issue of the journal Science Advances.

The research builds on a previous study, in collaboration with Freda Miller’s group at the Hospital for Sick Children, that sought to find treatment for childhood brain injury. They found that the widely prescribed drug metformin can induce brain repair and improve motor function in newborn mice that had a stroke injury. Metformin works by activating stem cells in the brain, which can self-renew and give rise to different types of brain cells to replace those killed by injury.

Because brain injury in early life can lead to lifelong cognitive problems, the researchers wanted to find out if metformin also promoted cognitive recovery.

“You can fix a hole in someone’s brain, but if they don’t function better it’s irrelevant to them,” says Morshead, who is also a professor at U of T’s Institute of Medical Science and the Institute of Biomaterials and Biomedical Engineering.

Graduate student Rebecca Ruddy induced stroke in newborn mice, followed by daily metformin treatment before the animals were tested in a puzzle box test that measured learning and memory.

The metformin was able to activate neural stem cells in the brain and promote cognitive recovery. But the data also revealed something unexpected: Metformin did not affect all the animals in equal manner. It only worked in adult females.

“When we first looked at the data, we did not see the benefit of the metformin treatment,” says Morshead. “Then we noticed that adult females tended to do better than the males.”

A closer look revealed that metformin selectively activated the adult female neural stem cells while having no effect on the males. This turned out to be due to the female sex hormone estradiol which somehow enhances the stem cells’ ability to respond to metformin. Conversely, the male hormone testosterone appears to inhibit this process. When female mice had their ovaries removed and lacked the female sex hormone, the stem cells did not respond to metformin treatment.  

“To know that there are both age and sex dependent effects – it has such implications for treatment and therapeutics,” says Morshead.

The findings come at a time when the research community is reckoning with the data sex bias stemming from an overwhelming exclusion of female mice from research. 

“The thinking was that we’re going to study males because everything you need to know is found in the male brain, and then the female brain just complicates things with hormones,” says Morshead. “It’s very misguided and troublesome for advancing neurological health.”

As preclinical research informs human studies, the sex bias is believed to have led to failed clinical trials, misdiagnosis and inappropriate therapies for women, as highlighted in a recent in Science. 

Human trials focused on repurposing metformin as a brain repair drug are ongoing. Morsehead’s collaborators – Dr. Donald Mabbott, the head of neurosciences and mental health at SickKids, and Dr. Eric Bouffet, the director of the brain tumour program at SickKids – are leading a pilot study testing the drug in children who suffered brain injury. Although the current patient cohort is too small to detect any sex effects, the plan is to increase the number of patients to see if sex also affects treatment outcome in people, Morshead says.

The research was supported by the funding from the Canadian Institutes of Health Research, which in 2016 mandated its grant holders to consider animals of both sexes in their research, as well as from the Ontario Institute for Regenerative Medicine, Medicine by Design, Brain Canada, the Stem Cell Network and the Ontario Brain Institute.