Both cyclin I and p35 are required for maximal survival benefit of Cyclin-dependent kinase 5 in kidney podocytes

Cyclin-dependent kinase (Cdk)-5 is activated by both cyclin I and the noncyclin activator p35 in terminally differentiated cells such as kidney podocytes and neurons. Cyclin I and p35 are restricted to podocytes in the kidney, and each limit podocyte apoptosis by activating Cdk5. To determine whether both activators are necessary, or whether they serve backup roles, a double cyclin I-p35 null mouse was generated. Experimental glomerular disease characterized by podocyte apoptosis was then induced by administering an anti-podocyte antibody. The results showed that under nonstressed conditions double mutants had normal kidney structure and function and were indistinguishable from wild-type, cyclin I−/−, or p35−/− mice. In contrast, when stressed with disease, podocyte apoptosis increased fourfold compared with diseased cyclin I−/− or p35−/− mice. This resulted in a more pronounced decrease in podocyte number, proteinuria, and glomerulosclerosis. Under normal states and nephritic states, levels for the prosurvival protein Bcl-2 were lower in double cyclin I−/− p35−/− mice than the other mice. Similarly, levels of Bcl-xL, another prosurvival member, were lower in normal and nephritic double cyclin I−/− p35−/− mice but similar to single-cyclin I−/− mice. Moreover, levels of ERK1/2 and MEK1/2 activation were lower in nephritic double cyclin I−/− p35−/− mice but similar to single-cyclin I−/− mice. The results demonstrate that the activators of Cdk5, p35, and cyclin I are not required for normal kidney function. However, they play pivotal coordinated roles in maintaining podocyte survival during stress states in disease.