Immunohistochemical assessment of the glial mitogen-activated protein kinase activation in glaucoma

To determine whether retinal glial cells exhibit an activated phenotype in glaucomatous human eyes and whether the mitogen-activated protein kinases (MAPKs) are associated with glial activation in glaucoma.

Activated phenotypes of retinal macroglia (astrocytes and Müller cells) and microglia were identified by morphologic assessment and immunostaining for the cell markers glial fibrillary acidic protein (GFAP) and HLA-DR, respectively, in 30 eyes obtained from glaucomatous donor eyes in comparison with normal control eyes from 20 age-matched donors. Cellular localization of the activated forms of MAPKs, including extracellular signal-regulated kinases (ERK), c-Jun amino(N)-terminal kinase (JNK), and p38, were studied in the retina of these eyes by immunoperoxidase staining and double immunofluorescence labeling with phosphorylation site-specific antibodies.

Retinal astrocytes and Müller cells exhibited a hypertrophic morphology and increased immunostaining for GFAP in the glaucomatous retina. Although an increase was detectable in the number and size of cells positive for HLA-DR immunostaining in the glaucomatous retina compared with the control retina, microglial activation was not as prominent or widespread as the macroglial activation detected in the same eyes. The intensity of immunostaining and the number of immunostained cells for the activated MAPKs were greater in retina sections from glaucomatous eyes than in control eyes, being most prominent for phospho-ERK. Double immunofluorescence labeling demonstrated that the increased retinal immunostaining for phospho-ERK was predominantly, but not exclusively, localized to glial cells, whereas, the immunostaining for phospho-JNK or phospho-p38 was mainly associated with nonglial cells.

These findings provide evidence that retinal glial cells undergo activation in the glaucomatous human retina. A prominent and persistent activation of ERK in activated glial cells suggests that this signaling pathway is probably associated with the induction and/or maintenance of the activated glial phenotype in glaucoma. Because MAPKs are involved in determination of ultimate cell fate, their differential activity in neuronal and activated glial cells in the glaucomatous retina may be associated, in part, with the differential susceptibility of these cell types to glaucomatous injury.