Project Summary (2006-2009)

Paraoxonases (PON) belong to a family of enzymes coded by genes (PON1, PON2 and PON3) located on human chromosome 7. PON1 and PON2 are synthesized in the liver and a portion of each is secreted into the serum in association with high density lipoprotein particles. PON1 hydrolyzes several organophosphorus (OP) compounds in addition to oxidized lipids and several drugs. PON1 displays several coding and regulatory region polymorphisms, which influence its catalytic efficiency and its level of expression. PON2 is a less studied PON that has two coding region polymorphisms, whose significance has not been yet established, and is widely distributed in tissues, including brain. PON2 does not hydrolyze OPs but has strong antioxidant properties. The overall goal of this project is to carry out a series of experiments that would add important knowledge on PON1 and PON2 functions and roles in determining susceptibility to environmentally-induced neurotoxicity and neurodegenerative diseases. Specific goals are: 1) To investigate the roles of PON1 in modulating the toxicity resulting from exposure to mixtures of OP compounds. While previous studies have established and characterized the role of PON1 in determining susceptibility to individual OPs and their active metabolites (e.g. chlorpyrifos oxon, diazoxon, paraoxon), in a real-life situation exposure to multiple OPs usually occurs. These studies, which are being carried out in PON1 knockout mice and in PON1 transgenic mice expressing one or the other of the two human PON192 alleles, focus in particular on the contribution of PON1 and carboxylesterase (CarE) in modulating toxicity due to combined OP exposures. 2) To investigate the role of PON1 in modulating the developmental neurotoxicity of a specific OP (chlorpyrifos oxon) following prenatal exposure. Evidence is emerging that gestational exposure to OPs may have even more deleterious effects on the developing nervous system than postnatal exposure, underlying the importance of maternal PON1 status for protection of the fetus. Such studies are being carried out in PON1 transgenic mice and include a series of biochemical, behavioral, histopathological and molecular end-points. 3) To investigate the role of PON1 and PON2 in Parkinson's disease (PD). As oxidative stress is believed to be a major contributor in the etiopathogenetic series of events leading to degeneration of dopaminergic neurons, and PON2 has strong antioxidant properties and is expressed in brain tissue, project researchers are investigating whether PON2 knockout mice have increased susceptibility to a dopaminergic neurotoxin, MPTP. Studies aimed at localizing and examining PON2 levels in brain and at developing substrates for enzymatic assays are also being carried out. Finally, as a follow-up to the previous studies that examined only PON1 genotypes, PON1 status in PD patients are being determined, to understand whether PON1 may also play a role in PD. Previous project studies on PON1 genetic variability and risk for carotid artery disease have shown that it is PON1 levels that determine risk and not any of the PON1 SNPs characterized to date.