Shh regulates M2 microglial polarization and fibrotic scar formation after ischemic stroke

Fibrotic scar formation is a critical pathological change impacting tissue reconstruction and functional recovery after ischemic stroke. The regulatory mechanisms behind fibrotic scarring in the central nervous system (CNS) remain largely unknown. While macrophages are known to play a role in fibrotic scar formation in peripheral tissues, the involvement of microglia, the resident immune cells of the CNS, in CNS fibrosis requires further exploration. The Sonic Hedgehog (Shh) signaling pathway, pivotal in embryonic development and tissue regeneration, is also crucial in modulating fibrosis in peripheral tissues. However, the impact and regulatory mechanisms of Shh on fibrotic scar formation post-ischemic stroke have not been thoroughly investigated. This study explores whether Shh can regulate fibrotic scar formation post-ischemic stroke and its underlying mechanisms through in vivo and in vitro manipulation of Shh expression. Our results showed that Shh expression was upregulated in the serum of acute ischemic stroke patients, as well as in the serum, CSF, and ischemic regions of MCAO/R mice. Moreover, the upregulation of Shh expression was positively correlated with fibrotic scar formation and M2 microglial polarization. Shh knockdown inhibited fibrotic scar formation and M2 microglial polarization while aggravating neurological deficits in MCAO/R mice. In vitro, adenoviral knockdown or Smoothened Agonist (SAG) activation of Shh expression in BV2 cells following OGD/R regulated their polarization and influenced the expression of TGFβ1 and PDGFA, subsequently affecting fibroblast activation. These results suggest that Shh regulates M2 microglial polarization and fibrotic scar formation after cerebral ischemia. Schematic illustration of Shh regulating M2 microglial polarization and fibrotic scar formation after ischemic stroke. After ischemic stroke, microglia highly express Shh. Elevated Shh further promotes M2 polarization of microglia. M2-polarized microglia, through the upregulation of TGFβ1 and PDGFA, work in concert with Shh to promote the formation of fibrotic scars following ischemic stroke. [Display omitted] •Shh is highly expressed in both ischemic stroke patients and MCAO/R mice.•Shh regulates fibrotic scar formation after ischemic stroke.•Shh regulates microglial polarization after ischemic stroke.•Shh knockdown exacerbates neurological deficits in mice following MCAO/R.