Background: Ischemic stroke (IS) is a cerebrovascular disease with high incidence and mortality. White matter repair plays an important role in the long-term recovery of neurological function after cerebral ischemia. Neuroprotective microglial responses can promote white matter repair and protect ischemic brain tissue. The aim of this study was to investigate whether hypoxic post-conditioning (HPC) can promote white matter repair after IS. This study also assesses the role and mechanism of microglial polarization in white matter repair after HPC treatment.

Methods: Male C57/BL6 mice (20-22g) were housed in a 12-hour light-dark cycle. Transient focal ischemia was induced by right middle cerebral artery occlusion (MCAO). HPC was performed by placing mice in hypoxic chambers and decreasing oxygen level to 5% for 40 minutes. Rotarod and balance beam test were subsequently performed to assess motor integration and coordination. The brain tissues of the mice were harvested and used to measure inflammatory cytokines (IL-1β, IL-10, TGF-β1, and iNOS) by ELISA. Luxol fast blue staining was used to visualize myelination. Immunofluorescence staining was performed to measure anti-Iba‐1, anti‐CD16, anti‐CD206, anti-F4/80, anti-MPO, anti-MBP, anti-NF200, anti-BrdU and anti-CNPase. Western blot was performed to detect anti-MBP, anti-β-actin, anti-PDGFR-α, and anti-APC.

Results: The results showed that HPC reduced the infarct size and pro-inflammatory level of immune cells. Furthermore, HPC promoted the transformation of microglia to anti-inflammatory phenotype on the third day after the procedure. HPC promoted the proliferation of oligodendrocyte progenitors and increased the expression of myelination-related proteins. HPC increased the expression of mature oligodendrocytes, which enhanced myelination. At the same time, the motor nerve function of mice was greatly improved.

Conclusions: During the acute phase of cerebral ischemia, the function of pro-inflammatory immune cells is enhanced, long-term white matter damage is aggravated, and motor sensory function is decreased. HPC promotes protective microglial responses and white matter repair after MCAO, which may be related to the proliferation and differentiation of oligodendrocytes.

IMAGE 1: HPC promotes oligodendrocyte differentiation and maturation.