Activation of cerebral Ras-related C3 botulinum toxin substrate (Rac) 1 promotes post-ischemic stroke functional recovery in aged mice

Brain functional impairment after stroke is common; however, the molecular mechanisms of post-stroke recovery remain unclear. It is well-recognized that age is the most important independent predictor of poor outcomes after stroke as older patients show poorer functional outcomes following stroke. Mounting evidence suggests that axonal regeneration and angiogenesis, the major forms of brain plasticity responsible for post-stroke recovery, diminished with advanced age. Previous studies suggest that Ras-related C3 botulinum toxin substrate (Rac) 1 enhances stroke recovery as activation of Rac1 improved behavior recovery in a young mice stroke model. Here, we investigated the role of Rac1 signaling in long-term functional recovery and brain plasticity in an aged (male, 18 to 22 months old C57BL/6J) brain after ischemic stroke. We found that as mice aged, Rac1 expression declined in the brain. Delayed overexpression of Rac1, using lentivirus encoding Rac1 injected day 1 after ischemic stroke, promoted cognitive (assessed using novel object recognition test) and sensorimotor (assessed using adhesive removal tests) recovery on days 14-28. This was accompanied by the increase of neurite and proliferative endothelial cells in the peri-infarct zone assessed by immunostaining. In a reverse approach, pharmacological inhibition of Rac1 by intraperitoneal injection of Rac1 inhibitor NSC23766 for 14 successive days after ischemic stroke worsened the outcome with the reduction of neurite and proliferative endothelial cells. Furthermore, Rac1 inhibition reduced the activation of p21-activated kinase 1, the protein level of brain-derived neurotrophic factor, and increased the protein level of glial fibrillary acidic protein in the ischemic brain on day 28 after stroke. Our work provided insight into the mechanisms behind the diminished plasticity after cerebral ischemia in aged brains and identified Rac1 as a potential therapeutic target for improving functional recovery in the older adults after stroke.

Restoring a balanced pool of host-derived and microbiota-derived ligands of the aryl hydrocarbon receptor is beneficial after stroke

Background

Stroke is a major cause of morbidity and mortality, and its incidence increases with age. While acute therapies for stroke are currently limited to intravenous thrombolytics and endovascular thrombectomy, recent studies have implicated an important role for the gut microbiome in post-stroke neuroinflammation. After stroke, several immuno-regulatory pathways, including the aryl hydrocarbon receptor (AHR) pathway, become activated. AHR is a master regulatory pathway that mediates neuroinflammation. Among various cell types, microglia (MG), as the resident immune cells of the brain, play a vital role in regulating post-stroke neuroinflammation and antigen presentation. Activation of AHR is dependent on a dynamic balance between host-derived and microbiota-derived ligands. While previous studies have shown that activation of MG AHR by host-derived ligands, such as kynurenine, is detrimental after stroke, the effects of post-stroke changes in microbiota-derived ligands of AHR, such as indoles, is unknown. Our study builds on the concept that differential activation of MG AHR by host-derived versus microbiome-derived metabolites affects outcomes after ischemic stroke. We examined the link between stroke-induced dysbiosis and loss of essential microbiota-derived AHR ligands. We hypothesize that restoring the balance between host-derived (kynurenine) and microbiota-derived (indoles) ligands of AHR is beneficial after stroke, offering a new potential avenue for therapeutic intervention in post-stroke neuroinflammation.

Method

We performed immunohistochemical analysis of brain samples from stroke patients to assess MG AHR expression after stroke. We used metabolomics analysis of plasma samples from stroke and non-stroke control patients with matched comorbidities to determine the levels of indole-based AHR ligands after stroke. We performed transient middle cerebral artery occlusion (MCAO) in aged (18 months) wild-type (WT) and germ-free (GF) mice to investigate the effects of post-stroke treatment with microbiota-derived indoles on outcome. To generate our results, we employed a range of methodologies, including flow cytometry, metabolomics, and 16S microbiome sequencing.

Results

We found that MG AHR expression is increased in human brain after stroke and after ex vivo oxygen-glucose deprivation and reperfusion (OGD/R). Microbiota-derived ligands of AHR are decreased in the human plasma at 24 hours after ischemic stroke. Kynurenine and indoles exhibited differential effects on aged WT MG survival after ex vivoOGD/R. We found that specific indole-based ligands of AHR (indole-3-propionic acid and indole-3-aldehyde) were absent in GF mice, thus their production depends on the presence of a functional gut microbiota. Additionally, a time-dependent decrease in the concentration of these indole-based AHR ligands occurred in the brain within the first 24 hours after stroke in aged WT mice. Post-stroke treatment of GF mice with a cocktail of microbiota-derived indole-based ligands of AHR regulated MG-mediated neuroinflammation and molecules involved in antigen presentation (increased CD80, MHC-II, and CD11b). Post-stroke treatment of aged WT mice with microbiota-derived indole-based ligands of AHR reduced both infarct volume and neurological deficits at 24 hours.

Conclusion

Our novel findings provide compelling evidence that the restoration of a well-balanced pool of host-derived kynurenine-based and microbiota-derived indole-based ligands of AHR holds considerable therapeutic potential for the treatment of ischemic stroke.

Stroke-Induced Respiratory Dysfunction Is Associated With Cognitive Decline

BACKGROUND

Respiratory dysfunction is a common complication of stroke, with an incidence of over 60%. Despite the high prevalence of stroke-induced respiratory dysfunction, how disordered breathing influences recovery and cognitive outcomes after ischemic stroke is unknown. We hypothesized that stroke induces chronic respiratory dysfunction, breathing instability, and apnea in mice, which would contribute to higher mortality and greater poststroke cognitive deficits.

METHODS

Mice were subjected to a 60-minute transient middle cerebral artery occlusion or permanent distal middle cerebral artery occlusion. Whole body plethysmography was performed on C57BL/6 young (2-3 months) and aged (20 months) male and female mice. Animals were exposed to a variety of gas conditions to assess the contribution of peripheral and central chemoreceptors. A battery of cognitive tests was performed to examine behavioral function.

RESULTS

Middle cerebral artery occlusion led to disordered breathing characterized by hypoventilation and apneas. Cognitive decline correlated with the severity of disordered breathing. Distal permanent middle cerebral artery occlusion, which produces a smaller cortical infarct, also produced breathing disorders and cognitive impairment but only in aged mice.

CONCLUSIONS

Our data suggest that poststroke apnea is associated with cognitive decline and highlights the influence of aging on breathing disorders after stroke. Therefore, the treatment of respiratory instability may be a viable approach to improving cognitive outcomes after stroke.

Abstract 16: Inhibiting Ezh2 Is Neuroprotective After Stroke In Aged Mice

Introduction: Ischemic stroke results in activation of microglia, which may polarize towards a pro-inflammatory (M1) phenotype and/or an anti-inflammatory (M2) phenotype. Enhancer of zeste homolog (EZH) 2 is a histone-lysine N-methyltransferase enzyme, a key modulator of microglia polarization. We here investigated whether microglial-specific deletion of EZH2 leads to a beneficial protective effect in stroke and microglial over-expressing microglial EZH2 exacerbates stroke outcome in vivo .

Methods: Aged male mice were subjected to 60-minutes middle cerebral artery stroke. Tamoxifen administration was started 30 days prior to stroke to induce genetic deletion of microglial EZH2 in CX3CR1-creER/EZH2-floxed mice. EZH2 floxed mice were used as controls. Microglial EZH2 over-expressing was performed using lentiviral vectors. Mice were sacrificed for immunohistochemistry and crystal violet staining (brain infarct assay) after behavior tests at 3 days after stroke.

Results: The expression of microglial EZH2 was significantly abrogated with tamoxifen injection in KO mice compared to the control floxed mice (144±15.43 vs. 50.65±4.99 cells/mm 2 , P<0.01, N=5/each group). EZH2 deletion reduced brain infarct volume (29.27±2.23% vs. 6.07±0.88%, P<0.001, N=7/each group) and improved functional outcome assayed by adhesive removal test (59±13.1 seconds, N=7 in control vs. 26.28±4.1 seconds, N=12 in KO, P<0.01). Mechanistically, microglial EZH2 deletion led to a decrease in expression of M1 marker iNOS (170±14.78 vs. 76.65±11.38 cells/mm 2 , P<0.05, N=4/each group), an increase in M2 marker Arg1 (96.64±11.48 vs. 203.3±22.02 cells/mm 2 , P<0.05, N=4/each group) co-stained in microglia (Iba1). We additionally found over-expressing EZH2 in microglial cells exacerbated stroke outcome including performance in adhesive tape removal and hang-wire tests 3 days after stroke. We observed decreased Arg1 and increased iNOS in stroke mice with over-expression of EZH2 (n= 5/6, p=<0.05).

Conclusions: Genetic deletion of EZH2 in microglia improved stroke outcome in aged while overexpression of EZH2 exacerbated stroke outcome. The effect of protective effect of EZH2 inhibition may be due to limiting microglial M1 polarization and enhancing M2 polarization.