Purpose: Mitogen-activated protein kinases (MAPKs), consisting of three major enzymes-extracellular signal-regulated kinase (ERK), p38, and c-jun N-terminal kinase (JNK)-couple cell-surface receptors to critical regulatory targets and gene transcription. We hypothesized that MAPKs are differentially expressed and have distinct functions after retinal ischemia.
Methods: Rats were subjected to retinal ischemia by elevation of intraocular pressure. Changes in MAPK expression were examined by Western blot of whole retinal homogenates and by immunohistochemistry of retinal cryosections. Phosphorylated (activated) ERK, p38, and JNK proteins were localized by fluorescent double labeling. The functional significance of activated MAPKs was assessed using pharmacological antagonists. Specific MAPK blockade was documented by kinase assay and immunohistochemistry for phosphorylated target proteins. The outcome after ischemia was examined with electroretinography (ERG), by measuring retinal cell layer thickness in paraffin-embedded sections, and by TUNEL staining on retinal cryosections. Data were analyzed using ANOVA and post hoc t-test, with P < 0.05 considered significant.
Results: Expression of phosphorylated JNK and p38 increased significantly after ischemia and followed a specific time course, beginning at 1 hour, and persisting up to 1 week later. JNK and p38 were expressed in the nuclei of ganglion and amacrine cells, the outer plexiform layer, the nerve fiber layer, and the axonal terminals of bipolar cells. Phosphorylated ERK was expressed in Müller cells, peaking at 1 to 6 hours after ischemia. Blocking activation of p38 or ERK significantly improved recovery of the ERG b-wave after ischemia, dramatically decreased thinning of the inner nuclear layers, and decreased the percentage of TUNEL-positive cells.
Conclusions: The MAPKs each demonstrate a specific cellular distribution after ischemia, and ERK and p38 are linked to apoptosis. Blockade of p38 or ERK provides significant protection from ischemic damage, suggesting a novel therapeutic role for MAPK inhibition in neuroprotection.