Photo of Cindi Morshead and two trainees

Cindi Morshead (centre) and graduate students Rebecca Ruddy (left) and Kelsey Adams. The study by Morshead’s team highlights how sex bias in preclinical work can lead to failed clinical trials and inappropriate treatments (photo by Jovanna Drinjakovic)

A research team funded in part by Medicine by Design has found that a female sex hormone plays a key role in promoting brain repair, highlighting the importance of studying male and female differences when developing new therapies.

Led by Cindi Morshead, chair of the Division of Anatomy in the Department of Surgery and a professor at the Donnelly Centre for Cellular and Biomolecular Research, the researchers found that metformin, a widely prescribed drug to treat diabetes, promotes repair in adult female brains and is dependent on the sex hormone estradiol.

The findings are described in a study published in 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 have had strokes. 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.

The diabetes drug that shows promise for treating childhood brain injury

Because brain injury in early life can lead to lifelong problems with learning and memory, the researchers now 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, also a professor in U of T’s institutes of Medical Science and Biomaterials & 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.

To the researchers’ delight, metformin was able to activate neural stem cells in the brain and promote cognitive recovery, as shown by mice who were able to learn new tasks and form short and long-term memories. But the data also revealed something unexpected. Metformin did not affect all the animals equally. 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 scientists are reckoning with the data bias that is skewed against women due to the predominant use of male animals in 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 article in Science.

Human trials of repurposing metformin as a brain repair drug are ongoing, with a small study at SickKids testing it in children with brain injury, led by Morshead’s collaborators Donald Mabbott and Dr. Eric Bouffet, directors of SickKids’ Neuroscience and Mental Health and Brain Tumour programs, respectively. 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, said Morshead.

The research was supported by funding from the Canadian Institutes of Health Research, which in 2016 mandated its grant holders to account for sex dependent differences in animal 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.