Post-stroke Delivery of Valproic acid Promotes Functional Recovery and Differentially Modifies Responses of Peri-infarct Microglia

Abstract Background: Recently, microglia, being the determinant of environment in peri-infarct tissue and strongly influence the potential for neuronal plasticity, has been implicated in post-ischemic secondary injury and functional recovery. However, the specific role of peri-infarct microglia and the timing of its morphological changes following ischemic stroke are not well understood. Valproic acid (VPA) can protect against ischemic damage and promote recovery. These effects are usually attributed, at least partially, to the anti-inflammatory ability of this drug to suppress microglial activation. In this study, we explored whether a low dose of VPA after stroke could modify reactive responses in microglia/macrophages and optimize peri-infarct microenvironments to improve functional recovery.Methods: Male Sprague-Dawley rats were subjected to distal middle cerebral artery occlusion (dMCAo) for 90 minutes, followed by reperfusion. Some received a single injection of VPA (200 mg/kg) 90 minutes after the induction of ischemia, while vehicle-treated animals underwent the same procedure with physiological saline. Infarction volume was calculated at 48 hours after reperfusion, and neurological symptoms were evaluated through 48 hours thereafter. The production of cytokines and biomarkers after insult was determined using enzyme-linked immunosorbent assays (ELISAs) and western blot. The effects of VPA on the activation of peri-infarct CD11b-positive cells were assessed based on cellular density and quality observation of morphology with fractal analysis and circularity index. The expressions of genes within peri-infarct zones at 3 days were determined by RNA-sequencing analysis. To determine whether VPA modulates microglial polarization, gentleMACS Dissociator was used to isolate CD11b-positive cells from the peri-infarct cortex of rats treated with or without VPA 3 days after dMCAo, after which the cells were subjected to qPCR analysis.Results: 200 mg/kg of VPA injected 90 minutes after ischemia induction did not significantly reduce infarct volume but did improve neurological deficit at least partially compared with vehicle. Meanwhile, VPA significantly reduced dMCAo-induced elevation of IL-6 at 24 hours post-stroke and significantly decreased the number of CD11b-positive microglia/macrophages within peri-infarct cortex at 7 but not at 2 days. Morphological analysis revealed that VPA therapy leads to higher fractal dimensions and lower circularity index of CD11b-positive cells within peri-infarct cortex at both 2 days and 7 days, suggesting that VPA has core effects on microglial activation. The attenuation of microglia/macrophage activation caused by VPA might involve HDAC inhibition-mediated suppression of galectin-3 production. Analysis of VPA-induced changes to gene expressions in the peri-infarct cortex at 3 days post-stroke indicated the upregulation of wound healing, collagen trimmer, and extracellular matrix genes. Furthermore, qPCR analysis of CD11b-positive cells suggested that VPA could partially enhance M2 subset polarization of microglia/macrophages in peri-infarct cortex.Conclusions: Our results are the first to show that a low dose of VPA promotes short-term functional recovery but does not alter infarct volume. The decreases in the expression of both IL-6 and galectin-3 might influence the extracellular matrix remodeling and morphological characteristics and transcriptional profiles of microglia/macrophages, which could contribute to the improved recovery.