Inhibition of hyperactive cyclin dependent kinase 5/p25 is protective in the 6-hydroxydopamine model of Parkinson’s disease

Background Cyclin-dependent kinase 5 (CDK5) is a multifunctional enzyme involved in neuronal development, maturation and survival. CDK5 activity is tightly regulated by association with regulatory proteins p35 and p39. Upon neuronal insults, increased intracellular calcium activates calpain, cleaving p35 into p25, which has a higher affinity for CDK5. p25 hyperactivates CDK5, initiating apoptotic cascades that lead to significant dopaminergic (DAergic) loss that can leads to neurodegenerative disorders, such as Parkinson’s disease (PD). Objective This study investigates hyperactivation of CDK5/p25 in the 6-hydroxydopamine (6-OHDA) rat model of PD and specific inhibition of CDK5/p25 by truncated peptide 5 (TP5). TP5 was investigated for amelioration of 6-OHDA induced behaviour impairments and significant protection of dopamine neurons through tyrosine hydroxylase (TH). Methods 6-OHDA induced motor impairments and reduced TH. Motor assessments included locomotor activity, beam transversal, fixed speed rotarod and amphetamine-induced rotations. Immunohistochemistry investigated DAergic neurodegeneration using TH levels and immunoprecipitation and assay investigated CDK5 activity. Results Pre-administration of TP5 maintained locomotor activity, preserved beam transversal scores, protected motor coordination and attenuated amphetamine induced rotations in 6-OHDA lesioned rats, all indicative of neuroprotection by TP5. 6-OHDA without pretreatment of TP increased CDK5 activation. CDK5 activity in TP5+6-OHDA animals was not significantly different from artificial cerebrospinal fluid (aCSF) treated sham surgery controls. Immunohistochemistry revealed significant TH protection within the substantia nigra (SN) of TP5 pretreated animals. Conclusions 6-OHDA increases CDK5 activity. Hyperactive CDK5/p25 inhibition in the 6-OHDA model has neuroprotective capability, protecting against the development of a toxin-based induction of PD-like motor phenotypes and pathology. This supports CDK5/p25 specific inhibition as a target for further neuroprotective therapeutic development. ### Competing Interest Statement The authors have declared no competing interest.