Purpose: Midkine (MK), a 13-kDa heparin-binding growth factor, is known to exert neurotrophic activities on various nerve cells including retinal cells. To initiate studies toward determining the physiological role of endogenous MK, we investigated the spatial and temporal expression profile of MK before and after intraocular pressure-induced retinal ischemia.
Methods: Retinal ischemia was induced in Wistar strain rats by increasing the intraocular pressure to 110 mm Hg for 45 min via cannulation into the anterior chamber. The localization and abundance of the MK protein and mRNA were determined by the use of immunohistochemistry and in situ hybridization in the normal retina, as well as the retina after reperfusion. The protein expression profile was confirmed by Western blot analysis.
Results: Immunohistochemical analysis showed that MK protein was expressed in the ganglion cell layer, the inner portion of the inner nuclear layer, and in the retinal pigment epithelium of the normal rat. MK expression transiently decreased 3 h to 2 days after reperfusion, and then dramatically increased to a level higher than normal after 7 to 28 days. The temporal expression profile of the MK protein was confirmed by Western blot analysis. In situ hybridization analysis gave results comparable to those obtained with immunohistochemistry.
Conclusions: MK was expressed in the neural cells of the retina in the normal state, but became more abundant after pressure-induced retinal ischemia. Thus, endogenous MK responds to ischemic treatment by an initial decrease in expression and then a period of expression above basal levels.