MANF Promotes Diabetic Corneal Epithelial Wound Healing and Nerve Regeneration by Attenuating Hyperglycaemia-Induced Endoplasmic Reticulum Stress

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a neurotrophic factor widely expressed in mammalian tissues and exerts critical protective effects on neurons and other cell types in various disease models, such as those for diabetes. However, to date, the expression and roles of MANF in the cornea, with or without diabetic keratopathy (DK), remain unclear. Here, we demonstrated that MANF is abundantly expressed in normal corneal epithelial cells, however, MANF expression was significantly reduced in both unwounded and wounded corneal epithelium in STZ-induced type 1 diabetic C57BL/6 mice. Recombinant MANF significantly promoted normal and diabetic corneal epithelial wound healing and nerve regeneration. Furthermore, MANF inhibited hyperglycaemia-induced endoplasmic reticulum (ER) stress and ER stress-mediated apoptosis. Attenuation of ER stress with 4-phenylbutyric acid (4-PBA) also ameliorated corneal epithelial closure and nerve regeneration. However, the beneficial effects of MANF and 4-PBA were abolished by an Akt inhibitor and Akt-specific siRNA. Finally, we revealed that the subconjunctival injection of MANF-specific siRNA prevented corneal epithelial wound healing and nerve regeneration. Our results provide important evidence that hyperglycaemia-suppressed MANF expression may contribute to delayed corneal epithelial wound healing and impaired nerve regeneration by increasing ER stress, and MANF may be a useful therapeutic modality for treating DK.