July 30, 2020
Research into the microbiome is yielding some positive new potential treatment options for Alzheimer's disease, according to George T. Grossberg, MD.
"I think the growing focus on the gut-brain axis is opening doors to new Alzheimer's disease and other brain disorders, and I think the first of a possible future generation of compounds for prevention or treatment of Alzheimer's disease may indeed be emerging," Dr. Grossberg said at a virtual meeting presented by Current Psychiatry and the American Academy of Clinical Psychiatrists.
Focus on the microbiome and microbiota is "a really hot, really new, really emerging area," said Dr. Grossberg, professor in the department of psychiatry & behavioral neuroscience at Saint Louis University. But the microbiota, which is the microorganisms within a specific organ such as the colon, is sometimes confused with the microbiome – which is defined as all of the bacteria, viruses, fungi and other microorganisms within a habitat as well as their genomes and the environment around them. "These are often used interchangeably, but they're not the same," Dr. Grossberg said at the meeting, presented by Global Academy for Medical Education.
A person's microbiome is unique to them, and nearly all of the microbiome is contained in the gut. A reduction in diversity of the microbiota in the digestive system has been linked to a wide variety of diseases, Dr. Grossberg explained. Inflammatory diseases, asthma, diabetes, obesity, and allergies are all conditions that have been linked to reduced microbiota diversity. Conversely, a microbial imbalance or dysbiosis has been implicated in anxiety and/or depression, dementia, and certain cancers, he noted.
Bacteria that positively affect the microbiome come from two main genera: Lactobacillus and Bifidobacterium. Factors such as diet, medications, geography, stage of life, birthing process, infant feeding method, and stress can all affect a person's microbiome. "We're all beginning to understand that trying to manage or trying to diversify, trying to manipulate the microbiota may have a lot of remote effects – even effects on weight or diabetes, or other disorders," Dr. Grossberg said.
Fecal microbiota transplantation (FMT), or the process of administering a donor's fecal matter into a recipient's intestinal tract, has proved beneficial in improving the health of patients suffering from recurrent Clostridioides difficile infection. A recent Harvard Health Letter, written by Jessica Allegretti, MD, MPH, observed that FMT is standard of care for patients with C. diff, and the procedure has a success rate of between 80% and 90%.
"It shows us very directly, in a very practical way, how addressing the dysbiosis – the imbalance of the gut microbiome – by infusing healthy bacteria may make a potential lifesaving difference," Dr. Grossberg said.
Research is beginning to show that the link between gut microbiota and health extends to Alzheimer's disease as well. Within the last few years, "we've started to understand that the microbial diversity in Alzheimer's disease versus healthy age-matched controls is decreased," Dr. Grossberg said.
In a study published by Nicholas M. Vogt and colleagues, there was decreased fecal microbial diversity among individuals with Alzheimer's, compared with healthy individuals matched for age. Another study by Ping Liu, PhD, and colleagues found that patients with Alzheimer's disease had decreased fecal microbial diversity, compared with individuals who had pre-onset amnestic mild cognitive impairment and normal cognition.
Dr. Grossberg noted that, while these studies do not prove that less fecal microbial diversity is responsible for mild cognitive impairment or Alzheimer's disease, "it makes us think that, maybe, there's a contributing factor."
"What happens with the dysbiosis of the gut microbiome is increased permeability of the epithelial area of the gut, which can then lead to the gut-brain axis dysregulation and may in fact allow the selective entry of bacteria into the central nervous system because the blood-brain barrier comes to be dysfunctional," he said.
Early evidence suggests that the gut-brain axis can affect cognition. In an animal model study, transferring the microbiota of a mouse with Alzheimer's disease to one that had been bred to be germ-free resulted in cognitive decline – but there was no cognitive decline for germ-free mice that received a microbiota transplant from a mouse in a healthy control group. Results from another animal study showed that transferring healthy microbiota from a mouse model into a mouse with Alzheimer's disease reduces amyloid and tau pathology. "The conclusions of these studies seems to be that microbiota mediated intestinal and systemic immune changes or aberrations seem to contribute to the pathogenesis of Alzheimer's disease in these mouse models," Dr. Grossberg said. "Consequently, restoring the gut microbial homeostasis may have beneficial effects on Alzheimer's disease treatment."
Periodontal disease also might be linked to Alzheimer's disease, Dr. Grossberg said. Several studies have shown gingipains secreted from Porphyromonas gingivalis, which contribute to inflammation in the brain, have been found in cadavers of patients with Alzheimer's disease (Sci Adv. 2019 Jan 23;5:eaau3333). "There's reason to think that the same changes may be occurring in the human brain with periodontal disease," he said.
The relationship also might extend to the gut microbiota and the central nervous system. "There seems to be a direct communication, a direct relationship between normal gut physiology and healthy central nervous system functioning, and then, when you have abnormal gut function, it may result in a variety of abnormal central nervous system functions," Dr. Grossberg said.
Studies that have examined a relationship between Alzheimer's disease and gut microbiota have highlighted the potential of probiotics and prebiotics as a method of restoring the gut microbiota (Aging [Albany NY]. 2020 Mar 31; 12:5539-50). Probiotics are popularly sold in health food aisles of grocery stores, and prebiotics are available in foods such as yogurts, tempeh, sauerkraut, and kimchi, as well as in drinks such as Kombucha tea. The effectiveness of probiotics and prebiotics also are being examined in randomized, controlled trials in patients with mild cognitive decline and mild Alzheimer's disease, Dr. Grossberg said. One therapy, Sodium oligomannate, a marine algae–derived oral oligosaccharide, has shown effectiveness in remodeling gut microbiota and has been approved in China to treat patients with mild or moderate Alzheimer's disease. Currently, no approved gut microbiota therapies are approved in the United States to treat Alzheimer's disease; however, encouraging use of a prebiotic, a probiotic, or a Mediterranean diet is something clinicians might want to consider for their patients.
"The fact that we're studying these things has really led to the notion that it may not be a bad idea for people to consume these healthy bacteria in later life, either as a way to prevent or delay, or to treat Alzheimer's disease," Dr. Grossberg said. "There's really no downside."
Global Academy and this news organization are owned by the same parent company. Dr. Grossberg reported that he is a consultant for Acadia, Alkahest, Avanir, Axsome, Biogen, BioXcel, Karuna, Lundbeck, Otsuka, Roche, and Takeda; receives research support from the National Institute on Aging, Janssen, and Roche; performs safety monitoring for EryDel, Merck, and Newron; and serves on data monitoring committees for Avanex and ITI Therapeutics.