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 3

Cyclin I–Cdk5 preferentially phosphorylates tau.

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Cyclin I–Cdk5 preferentially phosphorylates tau. (A) HEK293 cells were t...
(A) HEK293 cells were transfected with HA-Cdk5, then cotransfected with either myc-p35 or myc–cyclin I. Following an IP to the myc epitope tag on either p35 or cyclin I, in vitro kinase assays were performed using either histone H1 (HH1) or tau as substrates. Histone H1 and tau (lanes 2, 6) were strongly phosphorylated by p35-Cdk5. In contrast, cyclin I–Cdk5 preferentially phosphorylated tau (lane 6) rather than histone H1 (lane 2). No kinase activity was detected in the presence of a kinase-inactive dominant negative Cdk5 mutant (HA-D145N-Cdk5, lane 3, 7) or in the absence of either Cdk5 (lanes 1, 5) or p35/cyclin I (lanes 4, 8). (B) To compare the kinase kinetics by which cyclin I–Cdk5 phosphorylates tau in comparison with p35-Cdk5, a time-course experiment was performed in HEK293 cells cotransfected with myc-p35 and HA-Cdk5 or myc–cyclin I and HA-Cdk5. Following IP for the myc tag, phosphorylation of tau was assessed by incorporation of 32P-ATP quantified on a phosphoimager system. Cyclin I–Cdk5 showed phosphorylation kinetics similar to those of p35-Cdk5. Phosphorylation of tau was already evident after 5 minutes and reached a plateau after 40–50 minutes.