Optic neuritis, inflammation of the optic nerve (ON), is one of the main symptoms in multiple sclerosis (MS) and leads to visual disability. Astrocytes are pivotal regulators of neuroinflammation in MS. However, it remains unclear that the detailed roles and mechanisms of astrocytes in the neuroinflammation and demyelination in optic neuritis of MS.
To assess the role of YAP in ON and retina in response to experimental autoimmune encephalomyelitis (EAE), mice that conditionally knockout (CKO) YAP in astrocytes, namely YAP GFAP -CKO mice, were successfully generated. Immunostaining, Nissl staining, Hematoxylin-Eosin (HE) staining, TUNEL staining, luxol fast blue (LFB) staining, electron microscopy (EM), qRT-PCR and gene set enrichment analysis (GSEA) and gene set variation analysis (GSVA) by RNA sequencing were used to examine the roles of YAP pathway in EAE based on these conditional knockout mice. Inhibitors including SRI-011381 [an agonist of transforming growth factor-β (TGF-β) pathway] and XMU-MP-1 (an inhibitor of Hippo kinase MST1/2 to activate YAP) were used to further explore the molecular mechanism of YAP in ON and retina of EAE mice. Additionally, microglia and retinal ganglion cells (RGCs) counts, and demyelination and astrocytes were assessed by immunohistological staining.
We found that yes-associated protein (YAP) was significantly upregulated and activated in the astrocytes of ON in EAE. Conditional knockout YAP in astrocytes caused more severe inflammatory infiltration and demyelination in ON, and damage of the RGCs in EAE mice. Moreover, YAP deletion in astrocytes promoted the activation of astrocytes and microglia, but inhibited the proliferation of astrocytes of ON in EAE mice. Mechanically, TGF-β signaling pathway was significantly down-regulated after YAP deletion in astrocytes . Additionally, both qPCR and immunofluorescence assays confirmed the reduction of TGF-β 1 in YAP knockout ON astrocytes of EAE mice. Interestingly, SRI-011381 partially rescued the deficits in ON and retina of YAP knockout EAE mice. Finally, activation of YAP pathway relieved the neuroinflammation and demyelination in optic neuritis of EAE mice.
These results suggest astrocytic YAP may prevent the neuroinflammatory infiltration and demyelination through upregulation of TGF-β signaling and provide targets for the development of therapeutic strategies tailored for optic neuritis in MS.