Experimental glaucoma and optic nerve transection induce simultaneous upregulation of proapoptotic and prosurvival genes

Abstract

Purpose: To investigate changes in gene expression induced by elevated intraocular pressure (IOP) and complete optic nerve transsection (ONT) over time.

Methods: A gene array of 18 signal transduction pathways was used to examine the changes in RNA profiles of retinas post-ONT in rats. Among the seven genes that were determined to be upregulated, four were confirmed to have higher expression by semiquantitative RT-PCR analysis: Ei24 and Gadd45a (both associated with apoptosis induced via the p53 pathway), IAP-1 (inhibitor of apoptosis protein 1), and Cdk2 (cell cycle regulation and apoptosis). Their mRNA levels were then studied by quantitative RT-PCR in experimental glaucoma and ONT over time. Levels of the corresponding proteins were evaluated by Western blot analysis and immunohistochemistry.

Results: Proapoptotic genes from the p-53 pathway (Ei24 and Gadd45a), Cdk2 and the prosurvival gene IAP-1 (a caspase inhibitor) were simultaneously and significantly upregulated early after ONT, returning to baseline at 2 weeks. In experimental glaucoma, Gadd45a was significantly upregulated 1 week after induction of increased IOP and stayed upregulated for 2 months and long after IOP returned to baseline. The prosurvival gene IAP-1 was simultaneously upregulated but returned to baseline earlier than the proapoptotic gene. Ei24 and Cdk2 were only slightly upregulated in glaucoma. Western blot analysis demonstrated upregulation of Gadd45a and IAP-1 proteins. Immunohistochemistry localized these changes to the retinal ganglion cell layer.

Conclusions: Members of the p-53 signal transduction pathway are significantly involved in glaucoma and ONT. The endogenous caspase inhibitor IAP-1 is upregulated simultaneously, possibly as part of an intrinsic neuroprotective mechanism. Changes in glaucoma are gradual and last long after IOP returns to normal.