Role of cyclin-dependent kinases in stroke models of neuronal injury
Creators: name: Rashidian, Juliet
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2009
Subjects: subject: Biology, Neuroscience.

description: Ischemic stroke results from a transient or permanent reduction in cerebral blood flow. In spite of much research in trying to develop intervention strategies, most clinical trials have reported disappointing outcomes. These failures demonstrate that developing effective therapeutic treatments requires a more complete understanding of intracellular signals that lead to neuronal death. The understanding of stroke is made more difficult by the fact that neurons in different regions of brain may die through distinct mechanisms including rapid and delayed cell death pathways. The signaling pathways regulating these mechanisms, however, are not fully defined. Previous studies had suggested that inappropriate activation of cyclin-dependent kinases (CDKs) might be potential regulators of ischemic damage. Although correlative evidence had increasingly implicated upregulation of multiple CDKs signais in neuronal death models, the relevance of these signais was not known. Accordingly, this thesis research explored the functional role of G1-phase CDKs (Cdk2, Cdk4) and non-cell cycle Cdk5 in different models of neuronal death evoked by ischemia. The data indicates that these CDKs act preferentially to regulate damage in distinct models. While Cdk4 is essential in apoptotic and delayed type of death, Cdk5 is critical for excitotoxic death, in vitro. In adult and mature neuronal models (in vivo), these two CDKs also contributed differently. In global ischemia induced by 10min global ischemia, Cdk4 is a mediator of death signaling. Cdk5, however, plays the main role in 5min global ischemia and in a model of focal ischemia. To further decipher the significance of these signals, we investigated downstream effectors of Cdk4 and Cdk5. Our data shows that ischemia induces phosphorylation of Rb by Cdk4. This study suggests that Rb likely plays a functional role in this pathway. Consistent with this notion, expression of a constitutively active Rb is protective. Studying Cdk5 targets, however, is more complicated. Cdk5 is localized in different compartments of the cell and is suggested to have both pro-death and pro-survival functions. Accordingly, we initially localized its activity in stroke models. This research shows that Cdk5 function is context-dependent. While cytoplasmic Cdk5 signal increases in both models of global and focal ischemia, nuclear Cdk5 is essential only in the focal model. We further demonstrate that peroxidase Prx2 is a cytoplasmic target for Cdk5. Phosphorylation and inactivation of this enzyme by Cdk5 is a critical signal leading to stroke damage. Moreover, our data implicates the survival factor MEF2D, as a nuclear target for Cdk5 in focal ischemia. Nevertheless, the significance of this signal in ischemic death pathway is not clear at present, as attempts to increase MEF2 activity did not appear to affect stroke damage. Taken together, our results both indicate the functional significance of the cell cycle Cdk4 and neuronal Cdk5 signals as well as identify the pathways and circumstances by which they regulate ischemic damage.