The glial scar is more than just astrocytes

Abstract

Cellular responses in glia play a key role in regulating brain remodeling post-stroke. However, excessive glial reactivity impedes post-ischemic neuroplasticity and hampers neurological recovery. While damage-associated molecular patterns and activated microglia were shown to induce astrogliosis, the molecules that restrain astrogliosis are largely unknown. We explored the role of tenascin-C (TnC), an extracellular matrix component involved in wound healing and remodeling of injured tissues, in mice exposed to ischemic stroke induced by transient intraluminal middle cerebral artery occlusion. In the healthy adult brain, TnC expression is restricted to neurogenic stem cell niches. We previously reported that TnC is upregulated in ischemic brain lesions. We herein show that the de novo expression of TnC post-stroke is closely associated with reactive astrocytes, and that astrocyte reactivity at 14 days post-ischemia is increased in TnC-deficient mice (TnC−/−). By analyzing the three-dimensional morphology of astrocytes in previously ischemic brain tissue, we revealed that TnC−/− reduces astrocytic territorial volume, branching point number, and branch length, which are presumably hallmarks of the homeostatic regulatory astrocyte state, in the post-acute stroke phase after 42 days. Interestingly, TnC−/− moderately increased aggrecan, a neuroplasticity-inhibiting proteoglycan, in the ischemic brain tissue at 42 days post-ischemia. In vitro in astrocyte-microglia cocultures, we showed that TnC−/− reduces the microglial migration speed on astrocytes and elevates intercellular adhesion molecule 1 (ICAM1) expression. Post-stroke, TnC−/− did not alter the ischemic lesion size or neurological recovery, however microglia-associated ICAM1 was upregulated in TnC−/− mice during the first week post stroke. Our data suggest that TnC plays a central role in restraining post-ischemic astrogliosis and regulating astrocyte-microglial interactions.