Rationale: The elderly experience profound systemic responses after stroke, which contribute to higher mortality and more severe long-term disability. Recent studies have revealed that stroke outcomes can be influenced by the composition of gut microbiome. However, the potential benefits of manipulating the gut microbiome after injury is unknown. Objective: To determine if restoring youthful gut microbiota after stroke aids in recovery in aged subjects, we altered the gut microbiome through young fecal transplant gavage in aged mice after experimental stroke. Further, the effect of direct enrichment of selective bacteria producing short-chain fatty acids (SCFAs) was tested as a more targeted and refined microbiome therapy. Methods and Results: Aged male mice (18–20 months) were subjected to ischemic stroke by middle cerebral artery occlusion. We performed fecal transplant gavage 3 days after middle cerebral artery occlusion using young donor biome (2–3 months) or aged biome (18–20 months). At day 14 after stroke, aged stroke mice receiving young fecal transplant gavage had less behavioral impairment, and reduced brain and gut inflammation. Based on data from microbial sequencing and metabolomics analysis demonstrating that young fecal transplants contained much higher SCFA levels and related bacterial strains, we selected 4 SCFA-producers ( Bifidobacterium longum , Clostridium symbiosum , Faecalibacterium prausnitzii , and Lactobacillus fermentum ) for transplantation. These SCFA-producers alleviated poststroke neurological deficits and inflammation, and elevated gut, brain and plasma SCFA concentrations in aged stroke mice. Conclusions: This is the first study suggesting that the poor stroke recovery in aged mice can be reversed via poststroke bacteriotherapy following the replenishment of youthful gut microbiome via modulation of immunologic, microbial, and metabolomic profiles in the host.

Objective: Chronic systemic inflammation contributes to the pathogenesis of many age‐related diseases. Although not well understood, alterations in the gut microbiota, or dysbiosis, may be responsible for age‐related inflammation.Methods: Using stroke as a disease model, we tested the hypothesis that a youthful microbiota, when established in aged mice, produces positive outcomes following ischemic stroke. Conversely, an aged microbiota, when established in young mice, produces negative outcomes after stroke. Young and aged male mice had either a young or an aged microbiota established by fecal transplant gavage (FTG). Mice were subjected to ischemic stroke (middle cerebral artery occlusion; MCAO) or sham surgery. During the subsequent weeks, mice underwent behavioral testing and fecal samples were collected for 16S ribosomal RNA analysis of bacterial content.Results: We found that the microbiota is altered after experimental stroke in young mice and resembles the biome of uninjured aged mice. In aged mice, the ratio of Firmicutes to Bacteroidetes (F:B), two main bacterial phyla in gut microbiota, increased ∼9‐fold (p < 0.001) compared to young. This increased F:B ratio in aged mice is indicative of dysbiosis. Altering the microbiota in young by fecal gavage to resemble that of aged mice (∼6‐fold increase in F:B ratio, p < 0.001) increased mortality following MCAO, decreased performance in behavioral testing, and increased cytokine levels. Conversely, altering the microbiota in aged to resemble that of young (∼9‐fold decrease in F:B ratio, p < 0.001) increased survival and improved recovery following MCAO.Interpretation: Aged biome increased the levels of systemic proinflammatory cytokines. We conclude that the gut microbiota can be modified to positively impact outcomes from age‐related diseases. Ann Neurol 2018;83:23–36

Ischemic stroke is a devastating brain injury, resulting in high mortality and significant loss-of-function. Understanding the pathophysiology of ischemic stroke risk, mortality and functional loss is critical to novel therapeutic development. Age and sex have a complex and interactive effect on ischemic stroke risk and pathophysiology. Aging is the strongest non-modifiable risk factor for ischemic stroke, and aged stroke patients have higher mortality, morbidity and poorer functional recovery than their young counterparts. Importantly, patient age modifies the influence of patient sex in ischemic stroke. Early in life, the burden of ischemic stroke is higher in males, but becomes more common and debilitating for women in elderly populations. The profound effects of sex and age on clinical ischemic stroke are mirrored in the results of experimental in vivo and in vitro studies. Here, we will review current knowledge on the influence of age and sex in ischemic stroke incidence, mortality and functional outcome in clinical populations. We will also discuss the experimental evidence for sex- and age- differences in stroke pathophysiology, and how a better understanding of these biological variables can improve clinical care and enhance novel therapeutic development.

The peripheral immune system plays a critical role in aging and in the response to brain injury. Emerging data suggest inflammatory responses are exacerbated in older animals following ischemic stroke; however, our understanding of these age-related changes is poor. In this work, we demonstrate marked differences in the composition of circulating and infiltrating leukocytes recruited to the ischemic brain of old male mice after stroke compared to young male mice. Blood neutrophilia and neutrophil invasion into the brain were increased in aged animals. Relative to infiltrating monocyte populations, brain-invading neutrophils had reduced phagocytic potential, and produced higher levels of reactive oxygen species and extracellular matrix-degrading enzymes (i.e., MMP-9), which were further exacerbated with age. Hemorrhagic transformation was more pronounced in aged versus young mice relative to infarct size. High numbers of myeloperoxidase-positive neutrophils were found in postmortem human brain samples of old (> 71 years) acute ischemic stroke subjects compared to non-ischemic controls. Many of these neutrophils were found in the brain parenchyma. A large proportion of these neutrophils expressed MMP-9 and positively correlated with hemorrhage and hyperemia. MMP-9 expression and hemorrhagic transformation after stroke increased with age. These changes in the myeloid response to stroke with age led us to hypothesize that the bone marrow response to stroke is altered with age, which could be important for the development of effective therapies targeting the immune response. We generated heterochronic bone marrow chimeras as a tool to determine the contribution of peripheral immune senescence to age- and stroke-induced inflammation. Old hosts that received young bone marrow (i.e., Young → Old) had attenuation of age-related reductions in bFGF and VEGF and showed improved locomotor activity and gait dynamics compared to isochronic (Old → Old) controls. Microglia in young heterochronic mice (Old → Young) developed a senescent-like phenotype. After stroke, aged animals reconstituted with young marrow had reduced behavioral deficits compared to isochronic controls, and had significantly fewer brain-infiltrating neutrophils. Increased rates of hemorrhagic transformation were seen in young mice reconstituted with aged bone marrow. This work suggests that age alters the immunological response to stroke, and that this can be reversed by manipulation of the peripheral immune cells in the bone marrow.

Microglia are highly active and vigilant housekeepers of the central nervous system that function to promote neuronal growth and activity. With advanced age, however, dysregulated inflammatory signaling and defects in phagocytosis impede their ability to perform the most essential of homeostatic functions, including immune surveillance and debris clearance. Microglial activation is one of the hallmarks of the aging brain and coincides with age-related neurodegeneration and cognitive decline. Age-associated microglial dysfunction leads to cellular senescence and can profoundly alter the response to sterile injuries and immune diseases, often resulting in maladaptive responses, chronic inflammation, and worsened outcomes after injury. Our knowledge of microglia aging and the factors that regulate age-related microglial dysfunction remain limited, as the majority of pre-clinical studies are performed in young animals, and human brain samples are difficult to obtain quickly post-mortem or in large numbers. This review outlines the impact of normal aging on microglial function, highlights the potential mechanisms underlying age-related changes in microglia, and discusses how aging can shape the recovery process following injury.