Localization of retinal calmodulin kinase

The localization of calmodulin kinase II (CaM kinase) was studied in the retina by light and electron microscopic immunocytochemistry, and by enzymatic and immunoblot assay of cellular and subcellular tissue fractions. By light microscopy, both mono- and polyclonal antibodies revealed CaM kinase-like immunoreactivity in the inner and outer plexiform regions (synaptic layers), retinal pigment epithelium (RPE), and ganglion cells. The inner nuclear layer and photoreceptor outer segments stained much less intensely, and the outer nuclear layer did not stain. Electron microscopy confirmed the high concentration of immunoreactive protein in RPE and minimal outer segment staining. In addition, photoreceptor inner segments also contained CaM kinase-like immunoreactivity. Calcium and calmodulin stimulated phosphate incorporation into proteins of retinal cytosol and of isolated and cultured RPE. Calcium- and calmodulin-dependent kinase activity was present to a lesser degree in crude nuclei and synaptic membranes and was absent in isolated rod outer segments. Immunoblot analyses were consistent with enzymatic assays and immunocytochemistry. These data suggest that retinal CaM kinase is ideally located to play an important role in synaptic transmission and modulation of visual processes. Furthermore, its presence in RPE implies that CaM kinase may have a more ubiquitous role in regulating cellular processes than was previously recognized.