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 1

Cyclin I binds and activates constitutive Cdk5 in postmitotic cells.

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Cyclin I binds and activates constitutive Cdk5 in postmitotic cells. (A,...
(A, B) To determine whether cyclin I and Cdk5 coimmunoprecipitate in glomerular podocytes, cyclin I–null (–/–) podocytes were infected with either myc-tagged cyclin I or GFP. Reciprocal coimmunoprecipitation studies with either α-myc (A, lanes 1, 2) or α-Cdk5 (B, lanes 3, 4) antibodies revealed that cyclin I bound to endogenous Cdk5 in podocytes. (C) Histone H1 kinase activity was abundant in cyclin I–null podocytes infected with cyclin I–myc (lane 1). In contrast, kinase activity was not detected in cultured cyclin I–null podocytes infected with GFP (lane 2). (D) To prove that the cyclin I–associated histone H1 kinase activity shown in C was specifically due to the activation of Cdk5, WT (+/+) podocytes were transfected with siRNA. Cyclin I–associated histone H1 kinase activity was present in podocytes transfected with control siRNA (lane 1). In contrast, cyclin I–associated kinase activity was not detected in cells transfected with siRNA targeting Cdk5 (lane 2). As expected, an IP with a control preimmune IgY antibody showed no kinase activity (lane 3). Ctrl, control. (E and F) To determine whether cyclin I–Cdk5 was also active in tissues in vivo, kinase activity was measured in protein extracts from kidney glomeruli and brain of WT and cyclin I–null mice. Cdk5 was active in WT cultured podocytes, glomeruli, and brain tissue. In contrast, histone H1 kinase activity was not detected in corresponding tissues from cyclin I–null mice. Densitometric analysis (n = 3). Data shown represent mean + SD.