Elevated level of protein phosphatase 2A activity in retinas of rd mice

Abstract

Phosphorylation of rhodopsin is not detectable in vitro in the retina of the rd mouse. We investigated the enzymatic system responsible for this abnormality by measuring the levels of rhodopsin kinase and protein phosphatase 2A in normal ( rd/+) and diseased ( rd rd ) mouse retinas of several ages. For each enzyme, we developed micro assays that were suitable for measuring enzyme activity in one-half mouse retina. Our results indicate that rhodopsin kinase activity is identical in rd/+ and rd rd retinas until post-natal day 11, and it decreases thereafter in the rd rd retina, correlating with the loss of rod photoreceptors that occurs in this tissue. Protein phosphatase 2A has a constant level of activity in rd/+ retinas from ages 5 to 32 days but it is higher than normal in rd rd retinas from post-natal days 5 to 10. It then decreases to levels that are comparable to those in rd/+ retina. Although the rd rd extract contains the elevated protein phosphatase 2A activity, when rd rd and rd/+ retinal extracts are each subjected to gel filtration, the elution profiles of protein phosphatase 2A activity appear to be quantitatively identical. This apparent loss of rd rd phosphatase activity suggests a difference in the regulatory behavior of the enzyme in the normal and degenerative retinas. Thus, the failure to detect in vitro phosphorylation of rhodopsin in the rd rd retina seems to result from the elevated level of protein phosphatase 2A activity which could more rapidly remove the phosphate from phosphorylated rhodopsin. Since protein phosphatase 2A is a ubiquitous enzyme with broad specificity, an elevation in its activity also could affect other protein phosphorylations. Disruption of control functions of photoreceptor cell phosphoproteins may be an additional feature of the rd disease, which appears to be caused by a defect in the β-subunit of cGMP-phosphodiesterase.