Microbiome studies explore why more women develop Alzheimer's disease

According to the Alzheimer's Association, almost two-thirds of Americans with Alzheimer's dementia are women. While some of this discrepancy can be attributed to women living longer than men on average, researchers believe biological factors play a role as well.

A pair of new studies from the University of Chicago explores sex-specific differences in the development of Alzheimer's-like symptoms in mice, including the impact of estrogen, the primary female reproductive hormone, on the formation of amyloid plaques and inflammation in the brain—two hallmark symptoms of the disease. The research also strengthens evidence of the gut microbiome's role in mediating these symptoms, providing clues that could someday help develop treatments.

Clues pointing to the microbiome

Alzheimer's disease is characterized by the formation of amyloid plaques, or clumps of the protein amyloid beta (Aβ) that accumulate in the brain. The disease also activates immune cells present in the brain known as microglia, which can help remove amyloid plaques but may also exacerbate the disease by causing inflammation.

In 2019, a research team led by Sangram Sisodia, Ph.D., the Thomas A. Reynolds Sr. Family Professor of Neurobiology at UChicago, treated mouse models of Alzheimer's disease with a cocktail of antibiotics during the second week of life.

The antibiotics lessened the formation of amyloid plaques and microglia activation in males—but surprisingly, not female mice—by 3 months of age. Although this antibiotic regimen initially wipes out all gut bacteria, the gut becomes repopulated with numerous additional bacterial species over the next three months.

Sisodia reasoned that the microbiome must play a major role in these changes in amyloid deposition and neuroinflammation. To prove that the improvements in Alzheimer's symptoms were caused by alterations in the gut microbiome, they also transplanted fecal matter from untreated mice into antibiotic-treated animals.

This procedure restored the gut microbiome and caused an increase in amyloid plaque formation and microglial activation. These findings have since been confirmed and reported in several labs around the country.

Sex-specific microbiome changes

In the first of new papers, published in Molecular Neurodegeneration, Sisodia and his colleagues tested the effects of a new drug compound called sodium oligomannate, or GV-971, on the formation of amyloid deposits and neuroinflammation.

The compound was originally derived from brown seaweed by the Chinese pharmaceutical company Shanghai Green Valley Pharmaceuticals. In the company's testing, GV-971 reduced amyloid deposits and neuroinflammation in Alzheimer's mouse models. The compound has also undergone Phase III clinical trial testing in China and is now clinically approved for patients with Alzheimer's.

When Sisodia and his team tested GV-971 on a mouse model of Alzheimer's, they saw a significant drop in amyloid deposits, even at the lowest doses, and a reduction in inflammatory markers in the microglia—but again, these changes were only observed in male animals. They also noted significant changes in the composition and abundance of several types of gut bacteria in male mice but fewer changes in the microbiome of females.

Independently, and unbeknownst to Sisodia, David Holtzman, MD, the Barbara Burton and Reuben M. Morriss III Distinguished Professor of Neurology at Washington University in St. Louis and co-author of the paper, conducted a similar set of experiments with GV-971 in a different line of mice and saw similar results: The levels of amyloid deposition and neuroinflammation were significantly decreased, but only in male mice.

Moreover, a host of bacterial species that were changed by GV-971 in the studies from the Sisodia lab also appeared to be changed in Holtzman's experiments.

"It was kind of crazy because the microbiomes of these mice differ between UChicago and WashU. But at the end of the day, after doing the treatments, we found out what's in the microbial composition, and there are two or three bacteria that stand out," Sisodia said. "It's hard to believe it's a coincidence, but there must be something there."

Sisodia said further studies are needed to understand the links between GV-971, the microbiome, amyloid deposition, and inflammation, either by introducing or removing these key bacteria and analyzing the effects of the metabolites they produce.

"How do those pathways interact? And how does that lead to changes in brain function? That's all yet to be determined," he said.

Impacts of estrogen

The second study, published in Scientific Reports, looked at sex-specific differences in Alzheimer's more directly. Working with the UChicago Microbiome Center, postdoctoral scholar and first author of the study, Piyali Saha, Ph.D., investigated whether levels of circulating estrogen might be the reason female mice do not exhibit reductions in amyloid deposition and neuroinflammation after antibiotic treatment.

Saha treated mice that exhibited amyloid deposition with antibiotics and measured levels of estrogen circulating in their blood plasma and saw that estrogen levels increased threefold over mice treated with just saline.

Saha reasoned that increased estrogen might have something to do with the differences in amyloid deposits seen between male and female mice treated with antibiotics in previous studies. To test this, she conducted a second set of experiments in which she removed the ovaries (termed ovariectomy, or OVX) of female mice when they were just a few weeks old, in effect stopping estrogen production.

This procedure reduced both amyloid deposition and levels of inflammatory microglia. When another cohort of OVX-treated mice were later given estradiol in their drinking water to restore estrogen levels, amyloid deposits increased again, as did inflammatory microglia. The composition of the gut microbiome also varied significantly among the mice undergoing OVX, those later receiving estradiol and controls.

"That came out of the blue; I had no idea that manipulating estrogen levels was going to change things that dramatically," Sisodia said. "Estrogen seems to be the driver of the changes we see in Alzheimer's pathology, but we also know the microbiome is changing. So, there's this crosstalk between the two."

This counters longstanding practices of using hormone replacement therapy to restore estrogen levels in postmenopausal women to help prevent cognitive decline, a strategy brought into question by recent epidemiological studies. For example, a large-scale study of more than 20,000 women in Denmark from 2000 to 2018 showed that women who took estrogen replacement therapy had a higher risk of Alzheimer's and other dementias than those who did not receive this treatment.

"This evidence would suggest that estrogen replacement therapy is not the right thing to do," Sisodia said. "We see in the current study that estrogen levels always have an impact on amyloid deposition. If you take away the source of estrogen in mice at a very early stage, amyloid deposition goes away. It's pretty remarkable."

Sisodia stresses that there is still a lot to learn about the chain of events that leads from estrogen levels to changes in the gut microbiome and changes in amyloid deposition. It could be that estrogen affects the composition and abundance of certain types of bacteria, which in turn changes the metabolites and enzymes they produce that further impact brain function.

Timing also matters because once symptoms of Alzheimer's become apparent, it is far too late to reverse the damage. Shutting down estrogen production completely in women isn't a solution, but the clues from these studies hint at possible intermediate steps.

"If we can identify some target molecules that are involved in this biological cascade of estrogen metabolism, maybe we can develop some sort of medicine to mitigate the effects," Sisodia said. "I think that's potentially a great therapeutic avenue, at least for 50% of the population."