The authors from Ireland, Italy, and the UK transplanted fecal microbiota from patients with Alzheimer’s disease and healthy controls into microbiota-depleted young adult rats to investigate the involvement of gut microbiota from Alzheimer’s disease patients in host physiology and behavior. They also used an in vitro neurogenesis assay to investigate whether serum from patients with Alzheimer’s disease modulates adult hippocampal neurogenesis. The results showed, for the first time, that transfer of fecal material from Alzheimer’s disease patients to healthy young adult rats resulted in cognitive symptoms and impaired hippocampal neurogenesis. This confirmed a significance of gut microbiota in Alzheimer’s disease.
Alzheimer’s disease is a complex neurodegenerative disorder that results in a decline in cognitive function and mental health. The hippocampus, which plays a pivotal role in the processes of learning and memory, is particularly susceptible to Alzheimer’s pathology, and is one of the earliest brain regions to be affected. Alteration of the process called adult hippocampal neurogenesis, in which neural stem cells in the hippocampus generate new neurons, precedes neurofibrillary tangles and amyloid-β plaque formation.
Recent investigations have shown specific alterations in the gut microbiome composition in individuals with Alzheimer’s disease and in rodent models, including changes in microbial composition and metabolites, as well as bacterial endotoxins. Also, the microbiota–gut–brain axis has been shown to impact adult hippocampal neurogenesis.
About the study
The study involved 69 patients with Alzheimer’s disease and 64 healthy control subjects. All participants underwent a cognitive function assessment and a physical examination. Stool samples were collected from 54 participants with Alzheimer’s disease and 41 controls.
The authors first assessed plasma levels of interleukin (IL)-1β, IL-6, IL-4, IL-10, interferon (IFN)-γ, tumor necrosis factor (TNF)-α and, an inflammasome marker NLRP3 (NOD, LPR- and pyrin domain-containing protein 3), and the macrophage migration inhibitory factor (MIF). Compared to healthy controls, Alzheimer’s disease patients exhibited increased levels of IL-1β, IL-10, NLRP3, and MIF, and decreased expression of IL-4. The levels of fecal calprotectin, which is associated with the presence and severity of intestinal inflammation, were higher in patients with Alzheimer’s disease than in healthy controls.
The analysis of the composition of the gut microbiota by bacterial 16S rRNA gene sequencing revealed no significant variation in alpha and beta diversities between control subjects and patients with Alzheimer’s disease. At the phylum level, Alzheimer’s disease patients exhibited a higher abundance of Bacteroidetes and a lower abundance of the phyla Firmicutes and Verruocomicrobiota. At the genus level, Alzheimer’s disease patients had a reduction in the relative abundance of Clostridium sensu stricto 1 and the genera Coprococcusm, and an increase in the relative abundance of the pathobiont genera Desulfovibrio.
To elucidate the functional contribution of human gut microbiota to the etiology of Alzheimer’s disease, researchers transplanted fecal samples from cognitively healthy subjects and patients with Alzheimer’s disease into a microbiota-depleted young adult rat. For seven consecutive days before fecal microbiota transplantation, an antibiotic cocktail of ampicillin, vancomycin, ciprofloxacin and imipenem was administered to male Sprague-Dawley rats. After 7 days of antibiotic treatment, rats were randomly assigned to one of two groups: rats receiving fecal microbiota transplantation from control subjects (n= 16) or rats receiving fecal microbiota transplantation from patients with Alzheimer’s disease (n= 16).
The results showed that transfer of the fecal microbiota from cognitively healthy subjects to rats resulted in relatively stable taxa diversity over time. In contrast, rats colonized with fecal material from Alzheimer’s disease patients showed greater changes in microbial genera between 10 and 59 days after fecal microbiota transplantation. It is noteworthy that among the genera that increased significantly on day 59 in comparison to day 10 was Desulfovibrio, a genus that was also found to be enriched in participants with Alzheimer’s disease.
To determine whether changes in the composition of the gut microbiota in patients with Alzheimer’s disease accelerated symptoms associated with the disease, researchers conducted a series of behavioral tests in young adult rats colonized with fecal material. A battery of behavioral tests were conducted in the following order: open field test, elevated plus maze test, modified spontaneous location recognition test, novel object recognition, novel location recognition, Morris water maze test and forced swimming test. Rats colonized with fecal material from Alzheimer’s disease patients exhibited reduced discrimination between familiar and novel locations in the “large separation condition” and the “small separation condition”. These rats also had impairments in their long-term spatial memory and recognition memory. There was no change in locomotor parameters, anxiety-related behaviors or antidepressant-like behaviors.
The authors then evaluated hippocampal neurogenesis in the dentate gyrus using immunohistochemistry of rat brain tissue. Reduction in the number of BrdU/NeuN positive cells was observed in rats colonized with fecal material from Alzheimer’s disease patients, indicating reduced survival of newborn neurons in the dentate gyrus. A 3D reconstruction of cells expressing neural stem cell differentiation (DCX) markers showed a reduction in total dendritic length and dendritic complexity of DCX cells in rats colonized with fecal material from Alzheimer’s disease patients. These results suggest that gut microbiota from Alzheimer’s disease patients negatively affected the survival and dendritic arborization of neurons. There were no significant differences in microglia density in the dentate gyrus, which suggests that neuroinflammatory processes in this brain region play a minimal role in the cognitive dysfunction observed in rats.
Finally, the authors used an in vitro neurogenesis assay to study how serum from patients with Alzheimer’s disease modifies hippocampal neurogenesis. Human embryonic hippocampal progenitor cells were exposed to serum derived from Alzheimer’s patients and healthy control subjects. The percentage of cells expressing markers for neural stem cell proliferation (Ki67), differentiation (MAP2, DCX), and programmed cell death (CC3) was determined.
The expression of Ki67 positive cells was decreased in the presence of serum from patients with Alzheimer’s disease, indicating a decreased proliferative capacity of differentiating embryonic hippocampal progenitor cells. The percentage of Map2-positive immature neurons and DCX-positive neuroblasts decreased in response to serum from Alzheimer’s patients after 7 days, indicating impaired neurogenesis. These results demonstrated that administration of serum from patients with Alzheimer’s disease has a direct and negative effect on neuron proliferation and differentiation. In addition, these in vitro findings were in agreement with those previously obtained in rats in vivo.
The authors concluded that this study confirmed a significance of gut microbiota in Alzheimer’s disease. The findings revealed cognitive symptoms and impaired hippocampal neurogenesis in healthy young adult rats, induced by colonization with the gut microbiota of Alzheimer’s disease patients. The researchers suggested that future studies should investigate the underlying mechanisms by which the genera affect neurogenesis and their potential impact on the pathogenesis of Alzheimer’s disease, potentially opening avenues for new therapeutic approaches.
This study was published in Brain.
Grabrucker, S, Marizzoni, M, Silajdžić E, et al. Microbiota from Alzheimer’s patients induce deficits in cognition and hippocampal neurogenesis. BRAIN 2023: 00; 1–19. https://doi.org/10.1093/brain/awad303