Cyclin-dependent kinase 5 in synaptic plasticity, learning and memory

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase with a multitude of functions. Although Cdk5 is widely expressed, it has been studied most extensively in neurons. Since its initial characterization, the fundamental contribution of Cdk5 to an impressive range of neuronal processes has become clear. These phenomena include neural development, dopaminergic function and neurodegeneration. Data from different fields have recently converged to provide evidence for the participation of Cdk5 in synaptic plasticity, learning and memory. In this review, we consider recent data implicating Cdk5 in molecular and cellular mechanisms underlying synaptic plasticity. We relate these findings to its emerging role in learning and memory. Particular attention is paid to the activation of Cdk5 by p25, which enhances hippocampal synaptic plasticity and memory, and suggests formation of p25 as a physiological process regulating synaptic plasticity and memory.

Neuronal nuclear organization is controlled by cyclin-dependent kinase 5 phosphorylation of Ras Guanine nucleotide releasing factor-1

RasGRF1 is a member of the Ras guanine nucleotide exchange factor (RasGEF) family of proteins which are directly responsible for the activation of Ras and Rac GTPases. Originally identified as a phosphoprotein, RasGRF1 has been shown to be phosphorylated by protein kinase A and more recently, by the non-receptor tyrosine kinases Ack1 and Src. In this report we show that RasGRF1 interacts with and is phosphorylated by Cdk5 on serine 731 to regulate its steady state levels in mammalian cells as well as in neurons. Phosphorylation on this site by Cdk5 leads to RasGRF1 degradation through a calpain-dependent mechanism. Additionally, cortical neurons from Cdk5 knockout mice have higher levels of RasGRF1 which are reduced when wild-type Cdk5 is transfected into these neurons. In mitotic cells, nuclei become disorganized when RasGRF1 is overexpressed and this is rescued when RasGRF1 is co-expressed with active Cdk5. When RasGRF1 levels are elevated in neurons through overexpression of either the wild-type RasGRF1, or the phosphorylation mutant of RasGRF1 and by the transfection of a dominant negative Cdk5 construct, nuclei appeared condensed and fragmented. On the other hand, a reduction of RasGRF1 levels through p35/Cdk5 overexpression also leads to nuclear condensation in neurons. These data show that phosphorylation of RasGRF1 by Cdk5 tightly regulates its levels, which is essential for proper cellular organization.

CDK5 regulates cell-cell and cell-matrix adhesion in human keratinocytes

BACKGROUND

CDK5 is a member of proline-directed serine/threonine kinases. Although its cDNA was originally cloned as a homologue to those for the other members of the cyclin-dependent kinase (CDK) family, CDK5 has been shown to function differently from other CDKs. CDK5 is activated by non-cyclin partners, p35 and p39, and important during brain development by influencing adhesion, migration and differentiation of neurones.

OBJECTIVES

We sought to investigate the expression and functions of CDK5 in human keratinocytes.

METHODS

Expression of CDK5/p35, interaction of CDK5/p35 with adhesion molecules, and its roles in cell-cell and cell-matrix adhesion were studied by reverse transcriptase-polymerase chain reaction, immunoblotting and aggregation/adhesion assays in primary cultured normal human keratinocytes from infant foreskins and a human keratinocyte HaCaT cell line. Localization of CDK5 and p35 in normal human epidermis and psoriatic epidermis was studied by immunohistochemistry.

RESULTS

Both CDK5 and p35 were expressed in primary cultured keratinocytes, HaCaT cells and normal human epidermis. Roscovitine, an inhibitor of CDK5, enhanced Ca2+-dependent (cadherin-dependent) aggregation of HaCaT cells whereas it inhibited adhesion of HaCaT cells to fibronectin associated with reduced active states of beta1 integrin. Interestingly, psoriatic skin showed reduced CDK5 and p35 expression in the lower half of the epidermis, which might be associated with decreased amount of activated beta1 integrin in the epidermis of psoriatic skin.

CONCLUSIONS

CDK5/p35 may be involved in cell-cell and cell-matrix adhesion in human keratinocytes by differently regulating cadherins and integrins. Furthermore, reduced expression of CDK5/p35 in the epidermis might be involved in the pathogenesis of psoriasis.

Cyclin-dependent kinase 5 (CDK5) and neuronal cell death

Many neurological disorders like Parkinson's and Alzheimer's disease, amyotrophic lateral sclerosis (ALS) or stroke have in common a definite loss of CNS neurons due to apoptotic or necrotic neuronal cell death. Previous studies suggested that proapoptotic stimuli may trigger an abortive and, therefore, eventually fatal cell cycle reentry in postmitotic neurons. Neuroprotective effects of small molecule inhibitors of cyclin-dependent kinases (CDKs), which are key regulators of cell cycle progression, support the cell cycle theory of neuronal apoptosis. However, growing evidence suggests that deregulated CDK5, which is not involved in cell cycle control, rather than cell cycle relevant members of the CDK family, promotes neuronal cell death. Here we summarize the current knowledge about the involvement of CDK5 in neuronal cell death and discuss possible up- or downstream partners of CDK5. Moreover, we discuss potential therapeutic options that might arise from the identification of CDK5 as an important upstream element of neuronal cell death cascades.