Sex-specific effects of microbiome perturbations in an Alzheimer’s mouse model: an interview with Hemraj Dodiya

Written by Hemraj Dodiya (University of Chicago, IL, USA)

Hemraj Dodiya is a third-year postdoctoral scholar working with Professor Sangram Sisodia at the University of Chicago (IL, USA). He started his PhD in the field of Parkinson’s disease, where his dissertation focused on the microbiome gut–brain axis. Hemraj is currently utilizing his PhD as a basis to implement his expertise in this area to examine the microbiota gut–brain axis in Alzheimer’s disease.

In this interview, Hemraj speaks to us about his work that was presented at SfN Neuroscience (19–23 October 2019, Chicago) on the sex-specific effects of microbiome perturbation on cerebral amyloid-β amyloidosis and microglia phenotypes in an Alzheimer’s transgenic mouse model. We also wanted to find out more about how these gut microbiome changes might be affecting Alzheimer’s pathology in a sex-specific manner and whether these changes observed were causes, rather than effects, of differences in health.

Can you tell us more about your work on the sex-specific effects of microbiome perturbations on cerebral amyloid-β amyloidosis and microglia phenotypes in an Alzheimer’s transgenic mouse model?

In 2017, I started working with Professor Sisodia at the University of Chicago and in the last 2 years of the research studies I was focusing on exploring the role of gut bacteria in Alzheimer’s pathology, specifically considering sexes because we know that women – for an unknown reason – have an increased risk of Alzheimer’s disease. For me, I was really interested in finding out what happens when we changed the gut bacteria in female Alzheimer’s mice and whether we see the same response in male mice.

When I joined this lab in 2017, we already had a great study published by a previous postdoc in the lab that demonstrated that when we changed gut bacteria using antibiotics, male Alzheimer’s mice had reduced pathology. This was explored in female mice, however, there wasn’t a deep analysis included. Thus, I wanted to carry that forward in my studies and when I started working with the APPPS1-21 transgenic mouse line of Alzheimer’s disease, I started to include both male and female mice.

When these male and female mice received antibiotic treatment to alter their gut bacteria, we saw different microbiota taxa present in both groups. These readouts correlated strongly with brain readouts (e.g., Alzheimer’s pathology and inflammatory-related biomarkers). To specify, male mice demonstrated reduced amyloidosis whilst female mice exhibited no changes in amyloidosis both at two different timepoints: early (7 weeks) and later (3 months).

At present, we are trying to connect the dots – are these bacteria in female mice treated with antibiotic, at pro-inflammatory milieu, which put the female mice for not having reduced beneficial effects as in the male mice? That’s something we are working on right now, but we clearly see a sex-specific effect of the gut microbiome on Alzheimer’s pathology in this second paper, published from the Sisodia lab.

In your opinion, how might these gut microbiome changes be affecting Alzheimer’s pathology in mice in a sex-specific manner?

Currently, the paper that we have published in the Journal of Experimental Medicine demonstrated that we were able to establish a strong causality between gut bacteria and Alzheimer’s pathology. When we treated these mice, they were in the pre-weaning phase – so male and female pups were living with their mothers in the same cage. When we administered the antibiotic to male and female mice, they went through the same changes in their gut microbiota during the pre-weaning phase. At the time of sacrifice, they started to exhibit different microbiota profiles.

For us to understand their exact role, we are transplanting these male and female antibiotic-treated microbiome into  female mice to investigate if the female mice could benefit from the antibiotic-treated male microbiome and how they are playing a role in aggregating Alzheimer’s pathology or inflammation in a sex-specific manner (supported by BrightFocus® Foundation). Right now, we are hypothesizing that the antibiotic-treated female microbiome in our model, does have few taxa (e.g., Akkermansia muciniphila, S247). We aren’t 100% sure what their exact role is but these are the putative pro-inflammatory taxa that are observed in the female gut after antibiotic treatment, which are significantly lower in antibiotic-treated male mice.

Thus, it is possible that these putative pro-inflammatory taxa in the female gut could be participating in brain pathology and that’s why we don’t see female mice going through reduced amyloidosis, unlike male mice, which presumably have an anti-inflammatory microbiome in their gut after antibiotic treatment.

One of the major considerations in microbiome research is whether the changes observed are causes, rather than effects, of differences in health. What are your thoughts on this? For example, do we know enough about the microbiome to establish one or the other?

When I joined this lab in 2017 that was my first question – I asked Professor Sisodia, how do we know it’s not a consequence? As I mentioned previously in the work that was done by a previous postdoc, antibiotic treatment resulted in changes in the gut microbiome as well as changes in the brain pathology. Thus, how do we know that it’s gut bacteria that are causing this Alzheimer’s pathology aggregation?

To answer that question, after brainstorming with our collaborator Professor Jack Gilbert (University of California San Diego, CA, USA), I initiated fecal microbiota transfer studies and that is the second set of studies that were published in the Journal of Experimental Medicine. I utilized two sets of antibiotic-treated male mice – one group received antibiotics only and the second group received antibiotic plus fecal slurry from Alzheimer’s-controlled transgenic mice. Ultimately, I restored the microbiome in one group using controlled microbiota. To establish causality, we wanted to see if we restored the microbiome back, whether there would be an increase in pathology. To answer that question – yes. When we restored the microbiome after antibiotic treatment, we observed an increase in pathology that helped us establish causality that gut bacteria were causing Alzheimer’s pathology and inflammatory-related changes.

We are following up on those questions right now to further address what component of the gut microbiome is corresponding to these brain-related changes.

What further work needs to be done in order to translate these findings into a clinical setting?

In a clinical aspect, I highly encourage researchers to start including sex-specific microbiome effects. First, we need more rigorous and longitudinal studies from clinical patients investigating microbiome differences. However, at the same time, we need to have researchers clustering male and female microbiomes separately for us to start identifying the differences between Alzheimer’s male and female patients. That is the first suggestion I have for the field.

Second, we are in a growing field where we are trying to target microbiome manipulation. There are several ways we could initiate microbiome manipulation and some of them could be dietary interventions. There are many studies around this, including one from Martha Clara Morris (Rush University Medical Center, IL, USA), who received USD $14.5 million to utilize the MIND diet, which is a combination of the Mediterranean diet and DAS diet, to see if that reduces Alzheimer’s pathology in patients. These clinical studies will help us to investigate the relationship between diet, microbiota and Alzheimer’s, however, to establish the causality and to discover personalized dietary fibers targeting an individual microbiome, we will have to incubate these human fecal samples in human-like gastrointestinal culture models or incubate them into mouse models that are raised in a germ-free bubble.

There are many great studies and clinical trials happening worldwide and we need to start exploring the gut microbiota in all of these dietary interventions. Other directions such as prebiotics and probiotics research could be another avenue for modelling gut microbiota in Alzheimer’s patients. However, how do these diet interventions including prebiotics and probiotics affect Alzheimer’s in a sex-specific manner? This is what we need to focus on in the longitudinal studies in near-future settings, specifically including male and female groups separately in preclinical as well as clinical models.

Where do you anticipate the field might be in 5–10 years’ time? What future work do you have planned?

Currently, we were able to establish a strong causality between gut bacteria and Alzheimer’s pathology, specifically microglia cell-related inflammation in the brain. To understand the exact role or mechanism, such as how these gut bacteria or their metabolites could be contributing to immune cell education or aggravating disease pathology, we are trying to tease out individual components of this gut–brain axis.

One of the models that I am really fascinated in, which we have been developing over the last 2 years, is the germ-free mice Alzheimer’s transgenic line. With the tremendous help of the University of Chicago’s germ-free facility, Professor Sisodia and I were able to successfully rederive two transgenic mouse lines of Alzheimer’s in a germ-free condition. This model will serve us as a great tool as an incubator for us to start cooking the bacteria of our interest and see what their connection is with the brain pathology, specifically microglia, and we can start identifying individual bacterial roles or bacterial metabolites using this incubator.

However, there have been several efforts in other degenerative diseases, specifically Parkinson’s disease and multiple sclerosis, where researchers have transplanted human gut microbiota into mouse models to understand the disease-related changes in gut bacteria and how they could be fueling the source of inflammation, or even initiating the onset of Parkinson’s and multiple sclerosis pathology. Those are the research avenues that we need to explore in the field of Alzheimer’s disease to start pinpointing bacterial taxa and their role in Alzheimer’s pathology.

However, since our data currently advocates that sex-specific differences are present, I would suggest the field to start incorporating these sex-specific gut microbiota from Alzheimer’s patients and also consider where are we transplanting them, either germ-free male mice or germ-free female mice, to see and understand the male- and female-related sexual dimorphism of Alzheimer’s disease.

Do you agree with the classification of ‘good’ versus ‘bad’ bacteria? Do we know enough about these bacteria to classify them into these categories?

That’s exactly what I mentioned earlier when I spoke about Akkermansia muciniphila, which is considered to be ‘good’ bacteria for metabolic disorders. However, these bacteria keep appearing in higher numbers for many neurodegenerative disease patients. So, are they playing a ‘good’ role in these neurodegenerative disease conditions or not? To understand that question, we will need to start transplanting this individual species or taxa into germ-free mice to pinpoint their exact role.

To generally answer that question, it is surprising that we are still using the terms ‘good bacteria’ and ‘bad bacteria’. However, what we have seen in my experimental design so far is that it could be the whole gut environment per se (e.g., bacteria, virus, fungi, bacteriophages and their metabolites or the byproducts that are being produced by all these microorganisms).

Personally, I think that the newer technologies such as deep sequencing, metagenomics and metabolomics, will allow us to understand the environment’s overall role in terms of the gut–brain interaction. I think that’s what we should start including instead of focusing on individual bacteria. However, if we do find that a certain bacteria plays a significant role, I think it would be a great therapeutic intervention for us to go forward with as a probiotic.

In one incidence, chronic stress is known to affect Lactobacillus. Thus, if we put Lactobacillus back and increase their amount in the gut, can we restore or benefit from the stress-induced deleterious effects from that? Similar to that we need to identify these potential microbiome avenues in regards to Alzheimer’s and start exploring them in the future. Hopefully, longitudinal research including both sexes will allow us to tease out these important questions.

The opinions expressed in this interview are those of the interviewee and do not necessarily reflect the views of Neuro Central or Future Science Group.

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