Hedgehog Signaling Mediates Astrocytic Response and Blood-Brain Barrier Repair Following Traumatic Central Nervous System Injury

INTRODUCTION: Injury to the central nervous system (CNS) provokes a multicellular response to facilitate restoration of tissue homeostasis. Astrocytes consolidate at the lesion site to form a “glial scar” which restricts leukocyte infiltration and repairs the blood-brain barrier (BBB). However, this “scar” has been considered an impediment to regeneration and functional recovery. Therefore, uncovering molecular regulators that may be therapeutically targeted to promote pro-regenerative astrocyte phenotypes is of great interest. METHODS: Genetic reporter and conditional mutant mice were used to monitor, constitutively activate, or ablate Hh signaling following stereotactically-induced penetrating cortical or spinal cord injury. Outcomes including reactive astrocyte proliferation and polarization, leukocyte restriction, and blood-brain barrier repair were assessed via immunohistochemistry. RESULTS: Astrocytic expression of the Hh target gene Gli1 was downregulated acutely following injury but restored in the subacute phase. Constitutive activation of Hh signaling reduced astrocytic proliferation (%BrdU/EdU+ = 9.6% vs. 51.0%, p<0.01) and polarization (L/W Ratio = 2.8 vs. 1.5, p < 0.01). Additionally, in the spinal cord, constitutive activation impaired migration and integration of reactive astrocytes into the white matter lesion site. In animals with intact Hh signaling, the BBB recovered by 28 days after injury as evidenced by low levels of plasma protein and leukocyte infiltration. In contrast, conditional ablation of Hh signaling from 14-16 days after injury resulted in persistent BBB defects at 28 days. CONCLUSIONS: Our results suggest that acute downregulation of Hh signaling is required to specify reactive astrocyte properties including polarization and cell cycle re-entry. However, restoration of Hh signaling, which occurs subacutely from 10-28 days after injury and coincides with glial scar maturation, is required for leukocyte restriction and BBB repair. Understanding the temporospatial dynamics of postinjury Hh signaling is essential to modeling future therapeutic targets.