We have began to use TFP5 to study its therapeutic role in PD. Parkinsons disease (PD) is a chronic neurodegenerative disorder characterized by the loss of dopamine neurons in the substantia nigra, decreased striatal dopamine levels, and consequent extrapyramidal motor dysfunction. Recent evidence indicates that cyclin-dependent kinase 5 (Cdk5) is inappropriately activated in several neurodegenerative conditions including PD. To date, strategies to specifically inhibit Cdk5 hyperactivity have not been successful without affecting normal Cdk5 activity. Previously, we reported that TFP5 peptide has neuroprotective effects in animal models of Alzheimer's disease (AD). Here, we show that TFP5/TP5 selective inhibition of Cdk5/p25 hyperactivation in vivo and in vitro, rescues nigrostriatal dopaminergic neurodegeneration induced by 1-methyl- 4-phenyl-1,2,3,6-tetrahydropyridine (MPTP/MPP+),in a mouse model of PD. TP5 peptide treatment also blocked dopamine depletion in the striatum and improved gait dysfunction after MPTP administration. The neuroprotective effect of TFP5/TP5 peptide is also associated with marked reduction in neuroinflamation and apoptosis. Here, we show selective inhibition of Cdk5/p25 hyperactivation by TFP5/TP5 peptide, which identifies the kinase as a potential therapeutic target to reduce neurodegeneration in Parkinson's disease. Because of its involvement in PD, Cdk5/p25 has been identified as a prime therapeutic target for PD. Accordingly, compounds like roscovitine, an inhibitor targeting the ATP binding site in Cdk5 and other cell cycle kinases, have been studied as potential therapeutic agents, however, these drugs are toxic. Our approach to this problem, based on structure and kinetics of the Cdk5/p25complex, resulted in the production of several small truncated peptides of p35, which competed with p25 binding and inhibited Cdk5 hyperactivation in vitro. A small peptide, P5, comprising 24 aa, specifically inhibited Cdk5/p25 activity in cultured cortical neurons, reduced hyperphosphorylated tau and apoptosis, without affecting the normal endogenous Cdk5/p35 activity, nor the activity of several cell cycle kinases (Zheng et al., 2005, Zheng et al., 2010a).The P5 peptide was modified as TFP5 with a transactivator of transcription (Tat) peptide conjugated at the C terminus to facilitate passage through the blood-brain barrier, and fluoresceinisothiocyanate (FITC; a green fluorescent tag) attached at the N terminus as a marker. When injected intraperitoneally into 5XFAD AD model mice, significantly reduced Cdk5/p25 hyperactivity, hyperphosphorylated tau and rescued behavior deficits of spatial working memory and motor deficits (Shukla et al., 2013). Moreover, TFP5 also reduced toxicity in cortical neurons exposed to high glucose (Binukumar et al., 2014). We now find the intraperitoneal injection of TP5 (TFP5 without FITC) into MPTP-induced mice effectively blocks degeneration of dopamine neurons in the SNpc, and almost completely prevents the loss of striatal dopamine and its metabolites. The peptide treatment also ameliorates the MPTP-induced behavioral deficits, inhibits neuroinflamation in vivo and protects MPP+ neurotoxicity in vitro.These results suggest that TFP5/TP5 may be effective in the treatment of Parkinsons disease. We believe TFP5 acts on multiple pathways inhibiting hyper activation of Cdk5 and other kinases involved in inducing neuropathologies to reduce / prevent neurodegenerative phenotypes.
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