Identification of p10 as a neurotoxic product generated from the proteolytic cleavage of the neuronal Cdk5 activator

Post-translational modifications of CDK5 and their biological roles in cancer

Post-translational modifications (PTMs) of Cyclin-dependent kinase 5 (CDK5) have emerged as important regulatory mechanisms that modulate cancer development in patients. Though CDK5 is an atypical member of the cyclin-dependent kinase family, its aberrant expression links to cell proliferation, DNA damage response, apoptosis, migration and angiogenesis in cancer. Current studies suggested that, new PTMs on CDK5, including S-nitrosylation, sumoylation, and acetylation, serve as molecular switches to control the kinase activity of CDK5 in the cell. However, a majority of these modifications and their biological significance in cancer remain uncharacterized. In this review, we discussed the role of PTMs on CDK5-mediated signaling cascade, and their possible mechanisms of action in malignant tumors, as well as the challenges and future perspectives in this field. On the basis of the newly identified regulatory signaling pathways of CDK5 related to PTMs, researchers have investigated the cancer therapeutic potential of chemical compounds, small-molecule inhibitors, and competitive peptides by targeting CDK5 and its PTMs. Results of these preclinical studies demonstrated that targeting PTMs of CDK5 yields promising antitumor effects and that clinical translation of these therapeutic strategies is warranted.

Induction of CRMP-2 phosphorylation by CDK5 restricts the repair of damaged optic nerve

OBJECTIVE

To study the mechanism of collapsin response mediator protein-2 (CRMP-2) phosphorylation changes and cyclin-dependent kinase 5 (CDK5) expression after optic nerve injury.

METHODS

Optic nerve injury rat models were constructed, the messenger RNA (mRNA) level of CRMP-2 in optic nerve tissues was determined by quantitative reverse transcription polymerase chain reaction (qRT-PCR) after building models 0, 3, 7, and 14 days. The protein expression of CRMP-2, phospho-CRMP-2 (p-CRMP-2), and CDK5 were also determined by western blot analysis. Lentivirus overexpressing CRMP-2 and CRMP-2 small interfering RNA (siRNA) plasmid were designed and transfected to retina ganglion cells (RGCs), and then the neurites outgrowth of RGCs were cultured with CDK5 inhibitor or CDK5 activator was determined by tubulin staining. Inhibition on CDK5 promotes injured optic nerve by using carrying CDK5 siRNA inject into vitreous chamber.

RESULTS

There was no significant change in CRMP-2 expression in optic nerve injury rat, while p-CRMP-2 expression was evidently increased compared with sham operation group. The expression level of CDK5 in optic nerve tissue was upregulated after optic nerve injury in rat, and the upward trend of p-CRMP-2 and CDK5 was consistent with the time after the injury was prolonged. Inhibition on CDK5 evidently decreased the expression of p-CRMP-2. CDK5 siRNA had an obvious repair effect on the injured optic nerve.

CONCLUSION

The increase of CDK5 activity can lead to CRMP-2 hyperphosphorylation, which results in the difficult repair of damaged optic nerve. Therefore, inhibition on CDK5 could promote the repair of damaged optic nerve.

Cdk5 Inhibitory Peptide Prevents Loss of Dopaminergic Neurons and Alleviates Behavioral Changes in an MPTP Induced Parkinson's Disease Mouse Model

Parkinson’s disease (PD) is one of the most affected neurodegenerative diseases in the world. Deregulation of cyclin-dependent kinase 5 (Cdk5) is believed to play an important role in neurodegenerative diseases including PD. p25 is a cleavage peptide of p35, a physiologic activator of Cdk5. p25 combines to Cdk5 and leads to the hyperactivity of Cdk5, which in turn hyperphosphorylates downstream substrates and leads to neuroinflammation and apoptosis of neurons. Previously, we have demonstrated that adeno-associated virus serotype-9 (AAV9) mediated Cdk5 inhibitory peptide (CIP) inhibits the activity of Cdk5/p25 complex and alleviates pathologic and behavioral changes in Alzheimer’s disease mouse model. In this study, we evaluated whether AAV9-CIP protected dopaminergic (DA) neurons in 1-methyl-4-phe-nyl-1,2,3,6-tetrahydropyridine-probenecid (MPTP/p) induced PD mouse model. The data showed that administration of AAV9-CIP by intracerebroventricular injection 1 week before MPTP/p exposure protected loss of DA neurons in substantia nigra compact of the model mice. Importantly, AAV9-CIP also alleviated the motor and anxiety-like symptoms of the disease animals. In summary, AAV9 mediated CIP might be a potential intervention for PD.

The interaction of Munc 18 (p67) with the p10 domain of p35 protects in vivo Cdk5/p35 activity from inhibition by TFP5, a peptide derived from p35

In a series of studies, we have identified TFP5, a truncated fragment of p35, the Cdk5 kinase regulatory protein, which inhibits Cdk5/p35 and the hyperactive Cdk5/p25 activities in test tube experiments. In cortical neurons, however, and in vivo in Alzheimer's disease (AD) model mice, the peptide specifically inhibits the Cdk5/p25 complex and not the endogenous Cdk5/p35. To account for the selective inhibition of Cdk5/p25 activity, we propose that the "p10" N-terminal domain of p35, absent in p25, spares Cdk5/p35 because p10 binds to macromolecules (e.g., tubulin and actin) as a membrane-bound multimeric complex that favors p35 binding to Cdk5 and catalysis. To test this hypothesis, we focused on Munc 18, a key synapse-associated neuronal protein, one of many proteins copurifying with Cdk5/p35 in membrane-bound multimeric complexes. Here we show that, in vitro, the addition of p67 protects Cdk5/p35 and has no effect on Cdk5/p25 activity in the presence of TFP5. In cortical neurons transfected with p67siRNA, we also show that TFP5 inhibits Cdk5/p35 activity, whereas in the presence of p67 the activity is protected. It does so without affecting any other kinases of the Cdk family of cyclin kinases. This difference may be of significant therapeutic value because the accumulation of the deregulated, hyperactive Cdk5/p25 complex in human brains has been implicated in pathology of AD and other neurodegenerative disorders.

Extracellular alpha-synuclein induces calpain-dependent overactivation of cyclin-dependent kinase 5 in vitro

Extracellular alpha-synuclein (ASN) could be involved in the pathomechanism of Parkinson's disease (PD) via disturbances of calcium homeostasis, activation of nitric oxide synthase and oxidative/nitrosative stress. In this study we analyzed the role of cyclin-dependent kinase 5 (Cdk5) in the molecular mechanism(s) of ASN toxicity. We found that exposure of PC12 cells to ASN increases Cdk5 activity via calpain-dependent p25 formation and by enhancement of Cdk5 phosphorylation at Tyr15. Cdk5 and calpain inhibitors prevented ASN-evoked cell death. Our findings, indicating the participation of Cdk5 in ASN toxicity, provide new insight into how extracellular ASN may trigger dopaminergic cell dysfunction in PD.

p10, the N-terminal domain of p35, protects against CDK5/p25-induced neurotoxicity

Cyclin-dependent kinase 5(CDK5) in complex with its activator, p35 (protein of 35 kDa), is essential for early neurodevelopment in mammals. However, endogenous cleavage of p35 to p25 is associated with neuron death and neurodegenerative disease. Here we show that a peptide (p10') encoding the N-terminal domain of p35 protects against CDK5/p25-induced toxicity in neurons. p10' also prevented the death of neurons treated with the neurotoxin, 1-methyl-4-phenylpyridinium (MPP(+)), which induces conversion of endogenous p35 to p25, and Parkinson disease (PD)-like symptoms in animals. MPP(+) induces CDK5/p25-dependent phosphorylation of peroxiredoxin 2 (Prx2), resulting in inhibition of its peroxireductase activity and accumulation of reactive oxygen species (ROS). We found that p10' expression inhibited both Prx2 phosphorylation and ROS accumulation in neurons. In addition, p10' inhibited the p25-induced appearance of antigen of the Ki67 antibody (Ki67) and phosphohistone H2AX (γH2AX), classic markers of cell cycle activity and DNA double-strand breakage, respectively, associated with neuron death. Our results suggest that p10 (protein of 10 kDa) is a unique prosurvival domain in p35, essential for normal CDK5/p35 function in neurons. Loss of the p10 domain results in CDK5/p25 toxicity and neurodegeneration in vivo.