Cyclin I activates Cdk5 and regulates expression of Bcl-2 and Bcl-XL in postmitotic mouse cells
Paul T. Brinkkoetter, … , James M. Roberts, Stuart J. Shankland
Paul T. Brinkkoetter, … , James M. Roberts, Stuart J. Shankland
Published September 1, 2009
Citation Information: J Clin Invest. 2009;119(10):3089-3101. https://doi.org/10.1172/JCI37978.
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Research Article Nephrology

Cyclin I activates Cdk5 and regulates expression of Bcl-2 and Bcl-XL in postmitotic mouse cells

Abstract

Cyclin I is an atypical cyclin because it is most abundant in postmitotic cells. We previously showed that cyclin I does not regulate proliferation, but rather controls survival of podocytes, terminally differentiated epithelial cells that are essential for the structural and functional integrity of kidney glomeruli. Here, we investigated the mechanism by which cyclin I safeguards against apoptosis and found that cyclin I bound and activated cyclin-dependent kinase 5 (Cdk5) in isolated mouse podocytes and neurons. Cdk5 activity was reduced in glomeruli and brain lysates from cyclin I–deficient mice, and inhibition of Cdk5 increased in vitro the susceptibility to apoptosis in response to cellular damage. In addition, levels of the prosurvival proteins Bcl-2 and Bcl-XL were reduced in podocytes and neurons from cyclin I–deficient mice, and restoration of Bcl-2 or Bcl-XL expression prevented injury-induced apoptosis. Furthermore, we found that levels of phosphorylated MEK1/2 and ERK1/2 were decreased in cyclin I–deficient podocytes and that inhibition of MEK1/2 restored Bcl2 and Bcl-XL protein levels. Of interest, this pathway was also defective in mice with experimental glomerulonephritis. Taken together, these data suggest that a cyclin I–Cdk5 complex forms a critical antiapoptotic factor in terminally differentiated cells that functions via MAPK signaling to modulate levels of the prosurvival proteins Bcl-2 and Bcl-XL.

Authors

Paul T. Brinkkoetter, Paul Olivier, Jimmy S. Wu, Scott Henderson, Ronald D. Krofft, Jeffrey W. Pippin, David Hockenbery, James M. Roberts, Stuart J. Shankland

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Figure 5

Reducing p35 or Cdk5 activity augments caspase-3 cleavage in cyclin I–null podocytes.

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Reducing p35 or Cdk5 activity augments caspase-3 cleavage in cyclin I–nu...
To determine the relative roles of cyclin I, p35, and Cdk5 on apoptosis measured by caspase-3 cleavage, cyclin I–null podocytes were transfected (tx) with siRNA targeting either Cdk5 or p35, and in separate studies, Cdk5 activity was reduced in cyclin I–null cells with roscovitine. Reducing Cdk5 protein levels augmented caspase-3 cleavage in cyclin I–null cells following UV-C irradiation compared with control siRNA–transfected cells (lanes 1, 2). Lowering p35 protein levels increased caspase-3 cleavage following UV-C irradiation in cyclin I–null cells compared with nontransfected and control siRNA–transfected cells (lanes 3–5). Roscovitine increased caspase-3 cleavage (lane 6) following exposure to UV-C to the levels seen in cyclin I–null cells transfected with siRNA that targets p35.