Aging Microbiota-Gut-Brain Axis in Stroke Risk and Outcome

Abstract

The microbiota-gut-brain-axis (MGBA) is a bidirectional communication network between gut microbes and their host. Many environmental and host-related factors affect the gut microbiota. Dysbiosis is defined as compositional and functional alterations of the gut microbiota that contribute to the pathogenesis, progression and treatment responses to disease. Dysbiosis occurs when perturbations of microbiota composition and function exceed the ability of microbiota and its host to restore a symbiotic state. Dysbiosis leads to dysfunctional signaling of the MGBA, which regulates the development and the function of the host's immune, metabolic, and nervous systems. Dysbiosis-induced dysfunction of the MGBA is seen with aging and stroke, and is linked to the development of common stroke risk factors such as obesity, diabetes, and atherosclerosis. Changes in the gut microbiota are also seen in response to stroke, and may impair recovery after injury. This review will begin with an overview of the tools used to study the MGBA with a discussion on limitations and potential experimental confounders. Relevant MGBA components are introduced and summarized for a better understanding of age-related changes in MGBA signaling and its dysfunction after stroke. We will then focus on the relationship between the MGBA and aging, highlighting that all components of the MGBA undergo age-related alterations that can be influenced by or even driven by the gut microbiota. In the final section, the current clinical and preclinical evidence for the role of MGBA signaling in the development of stroke risk factors such as obesity, diabetes, hypertension, and frailty are summarized, as well as microbiota changes with stroke in experimental and clinical populations. We conclude by describing the current understanding of microbiota-based therapies for stroke including the use of pre-/pro-biotics and supplementations with bacterial metabolites. Ongoing progress in this new frontier of biomedical sciences will lead to an improved understanding of the MGBA's impact on human health and disease.

Keywords: aging; brain-gut axis; dysbiosis; microbiota; risk factors; stroke.

Figure 1.

Illustration of the major participants in MGBA signaling with their anatomical position and serves as a reference throughout this review. MG (microglia), EEC (enteroendocrine cell), Neut (neutrophil), Mac (macrophage), DC (dendritic cell), LP (lamina propria), PP (Peyer’s patch). Three barriers exist between the luminal content and the blood: a mucus layer maintained by goblet cells, a selectively-permeable single layer of enterocytes interconnected with tight junctions, and GBB (composed of enteric glia, pericytes, and endothelial cells). PPs are located along the antimesenteric border of the small intestine and have lymphoid follicles surrounded by APCs and lymphocytes (predominantly B cells). Crypt stem cells at the base of the villi and replenish the epithelium. Goblet cells secrete mucins to maintain the mucus barrier. Paneth cells secrete AMPs that modulate the gut microbiota composition. EECs are differentiated neuroendocrine cells, involved in GI physiology, coordination with the ANS, and feeding behavior. M cells are specialized antigen processing cells in the intestinal PPs capable of transcytosis of luminal antigens. BBB is a highly selective semipermeable barrier comprised of endothelial cells attached by tight junctions, astrocytic end-feet, and pericytes embedded in the vascular basement membrane. CNS lymphatic vessels reside in the dura of meninges with limited access to brain parenchyma through lymphatic protrusions that allows brain-derived antigens to move from the brain to the CSF-filled spaces and cervical lymph nodes. The skull bone marrow houses significant populations of monocytes and neutrophils, with transcriptional signatures distinct from their blood-derived counterparts, that can infiltrate into the brain in response to injury. [Fig 1 artistic rendering by Nicolle R. Fuller, SayoStudio].

Figure 2.

Representative fluorescence in situ hybridization (FISH) showing close proximity of bacteria to gut epithelium in naïve age mice (red arrow), compared to young mice (green arrow) (a, top two panels, Red: bacteria (non-specific FISH probe), Blue: DAPI, 10x magnification). Representative immunohistochemistry showing a loss of mucin secretion (pink arrows) in naïve aged mice (2a, bottom two panels, brown: MUC-2 stain, blue: hematoxylin, 40X Magnification). Representative immunohistochemistry showing the expression of intestinal E-cadherin tight junction protein (orange arrow) by intestinal epithelial cells (b, Cecum, brown = E-cadherin, blue = hematoxylin, 10/40x magnification). Signaling network between afferent vagal fibers, EECs (neuropods), Paneth cells, and LP Th2 lymphocytes. Colonization of GF mice with microbiota from SPF mice or even a single bacteria (e.g., Bacteroides thetaiotaomicron) can restore the serotonin and GLP1 production in EECs in GF mice. Th2 cytokines IL-4, IL-9, IL-13, and EEC-derived GLP2 stimulate the production of AMPs by Paneth cells. EECs express TLR-2 and TLR-6 that sense bacterial lipopolysaccharide (LPS) to activate the canonical NF-kB pathway. Afferent sensory signals propagate toward the brainstem and efferent (CNS-originated) signals travel to the intestinal neuropods in the order of milliseconds to regulate the release of hormones, including GLP1, CCK, ghrelin, and serotonin (c). [created with BioRender.com]

Figure 2.

Representative fluorescence in situ hybridization (FISH) showing close proximity of bacteria to gut epithelium in naïve age mice (red arrow), compared to young mice (green arrow) (a, top two panels, Red: bacteria (non-specific FISH probe), Blue: DAPI, 10x magnification). Representative immunohistochemistry showing a loss of mucin secretion (pink arrows) in naïve aged mice (2a, bottom two panels, brown: MUC-2 stain, blue: hematoxylin, 40X Magnification). Representative immunohistochemistry showing the expression of intestinal E-cadherin tight junction protein (orange arrow) by intestinal epithelial cells (b, Cecum, brown = E-cadherin, blue = hematoxylin, 10/40x magnification). Signaling network between afferent vagal fibers, EECs (neuropods), Paneth cells, and LP Th2 lymphocytes. Colonization of GF mice with microbiota from SPF mice or even a single bacteria (e.g., Bacteroides thetaiotaomicron) can restore the serotonin and GLP1 production in EECs in GF mice. Th2 cytokines IL-4, IL-9, IL-13, and EEC-derived GLP2 stimulate the production of AMPs by Paneth cells. EECs express TLR-2 and TLR-6 that sense bacterial lipopolysaccharide (LPS) to activate the canonical NF-kB pathway. Afferent sensory signals propagate toward the brainstem and efferent (CNS-originated) signals travel to the intestinal neuropods in the order of milliseconds to regulate the release of hormones, including GLP1, CCK, ghrelin, and serotonin (c). [created with BioRender.com]

Figure 2.

Representative fluorescence in situ hybridization (FISH) showing close proximity of bacteria to gut epithelium in naïve age mice (red arrow), compared to young mice (green arrow) (a, top two panels, Red: bacteria (non-specific FISH probe), Blue: DAPI, 10x magnification). Representative immunohistochemistry showing a loss of mucin secretion (pink arrows) in naïve aged mice (2a, bottom two panels, brown: MUC-2 stain, blue: hematoxylin, 40X Magnification). Representative immunohistochemistry showing the expression of intestinal E-cadherin tight junction protein (orange arrow) by intestinal epithelial cells (b, Cecum, brown = E-cadherin, blue = hematoxylin, 10/40x magnification). Signaling network between afferent vagal fibers, EECs (neuropods), Paneth cells, and LP Th2 lymphocytes. Colonization of GF mice with microbiota from SPF mice or even a single bacteria (e.g., Bacteroides thetaiotaomicron) can restore the serotonin and GLP1 production in EECs in GF mice. Th2 cytokines IL-4, IL-9, IL-13, and EEC-derived GLP2 stimulate the production of AMPs by Paneth cells. EECs express TLR-2 and TLR-6 that sense bacterial lipopolysaccharide (LPS) to activate the canonical NF-kB pathway. Afferent sensory signals propagate toward the brainstem and efferent (CNS-originated) signals travel to the intestinal neuropods in the order of milliseconds to regulate the release of hormones, including GLP1, CCK, ghrelin, and serotonin (c). [created with BioRender.com]

Figure 3.

Microbiota-dependent activation of the MG AHR reduces VEGFB by MG which increases TGF-a secretion by astrocytes, which in turn dampens astrocyte-mediated neuroinflammation (a). Age-related decrease in protective bacteria (e.g., Akkermansia or SCFA-producers) can lead to reduced intestinal barrier function, which activates DC to stimulate gd T cells, increasing their migration to the brain and enhancing the production of IL-17, leading to the additional recruitment of neutrophils into the aged brain after stroke (b). [created with BioRender.com]

Figure 3.

Microbiota-dependent activation of the MG AHR reduces VEGFB by MG which increases TGF-a secretion by astrocytes, which in turn dampens astrocyte-mediated neuroinflammation (a). Age-related decrease in protective bacteria (e.g., Akkermansia or SCFA-producers) can lead to reduced intestinal barrier function, which activates DC to stimulate gd T cells, increasing their migration to the brain and enhancing the production of IL-17, leading to the additional recruitment of neutrophils into the aged brain after stroke (b). [created with BioRender.com]

Figure 4.

The composition of myeloid cell populations in the skull bone marrow changes with aging with significantly higher relative frequencies of neutrophils in aged skull (a). Skull bone marrow lymphocyte compartment contains significantly higher CD8+ T lymphocytes and activated CD11b^high B lymphocytes in naïve aged mice when compared to young skull (b).

Figure 5.

Key contributors to differences between aged and young stroke and the role of age-related changes gut microbiota in response after stroke. A substantial proportion of elderly stroke patients are frail, which is linked to both cerebrovascular disease incidence and predicts shorter post-stroke survival. Post-stroke neutrophil infiltration into the brain and their ROS production are increased in aged animals after stroke. The pre-stroke relative frequency of brain APCs is significantly higher in aged mice. Skull bone marrow contains significantly higher neutrophils, CD8+ T lymphocytes, and activated CD11b^high B lymphocytes in naïve aged mice compared to young skull. Major age-related changes in the immune and autonomic nervous system contribute to age-specific response after stroke. Aged microbiota is associated with reduced production of SCFAs and Trp metabolites, both of which are major regulators of immunity and physiological barriers (e.g., BBB and intestinal barrier). Post-stroke supplementation with SCFA-producers and inulin significantly reduces neuroinflammation and improves stroke outcomes. (Illustration credit: Ben Smith).