Estudio de la fosforilación de parkina y sus implicaciones en la enfermedad de parkinson

Resumen

Parkinson disease (PD) is a progressive and substantially disabling neurodegenerative disorder [1]. Its clinical symptoms primarily result from the progressive and selective death of dopaminergic neurons of the substantia nigra pars compacta. Besides cell death, a pathological hallmark of PD in surviving neurons comprises Lewy bodies, ubiquitylated intraneuronal inclusions rich in ¿-synuclein [2]. Even though largely a sporadic disorder, there are several genes associated with inherited forms of PD. One commonly implicated is PARK2, the gene encoding for parkin [3]. Mutations in the parkin gene are responsible for a large percentage of autosomal recessive juvenile parkinsonism [4, 5]. Parkin functions as an E3 ubiquitin ligase [6], and inactivation of its catalytic activity may lead to dopaminergic cell death due to accumulation of its toxic substrates. Studies suggest that changes in parkin solubility comprise the major mechanism of parkin inactivation, both in familial and sporadic PD [7-9]. In addition, some post-translational modifications lead to dramatic changes in parkin solubility, highlighting a mechanism for parkin inactivativation [7, 8]. This thesis work demostrates that parkin is subject to compound phosphorylation by Casein Kinase 1 and Cdk5 in vitro and in cultured cells. Such compound phosphorylation enhances its insolubility, leading to aggregation and concomitant inactivation. Although phosphorylation does not change parkin E3 ligase activity, increased aggregation effectively decreases the amount of soluble parkin protein able to exert a neuroprotective role. An increase in parkin phosphorylation was observed in brain samples from PD brains as compared to controls patients in caudate, with no changes in cortex and no detection of phosphorylated parkin in cerebellum. The neuroanatomical differences in parkin phosphorylation between control and PD correlate with the relative extent to which these distinct brain areas are affected by disease pathology. Further, an increase in p25, the activator of Cdk5, was observed in the caudate of PD when compared to control samples, indicating that such increase may lead to parkin phosphorylation. Importantly, compound inhibition of kinase activity was found to display beneficial effects in decreasing the aggregative properties of pathogenic parkin mutants. In conclusion, the results presented in this thesis indicate that regulating the phosphorylation status of parkin has beneficial effects in reducing parkin aggregation and concomitant inactivation. These findings may help in the design of novel therapeutic strategies against PD.