Parkinson's disease-like neuromuscular defects occur in prenyl diphosphate synthase subunit 2 (Pdss2) mutant mice

Abstract

The Pdss2 gene product is needed for the isoprenylation of benzoquinone to generate coenzyme Q (CoQ). A fatal kidney disease occurs in mice that are homozygous for a missense mutation in Pdss2, which can be recapitulated in conditional Pdss2 knockouts targeted to glomerular podocytes. We now report that homozygous missense mutants also demonstrate significant neuromuscular deficits, as validated by behavioral and coordination assays, and these deficits are recapitulated in conditional Pdss2 knockouts targeted to dopaminergic neurons. Both conditional knockout and missense mutant mice demonstrate deficiencies in tyrosine hydroxylase-positive neurons in the substantia nigra, implicating a pathology similar to sporadic Parkinson's disease (PD).

Introduction

Parkinson's disease (PD) is the second most common neurodegenerative disease, affecting 1% of the population above the age of 65. The characteristic pathology involves degeneration of dopaminergic neurons in the substantia nigra pars compacta and the appearance of intracytoplasmic inclusions known as Lewy bodies (Beal, 2001). The specific etiology of PD is unknown, although both genetic and environmental factors that influence this disease have been identified. Genetic forms of PD can be caused by mutations in SCNA (PARK1), which encodes alpha-synuclein, a key component of Lewy bodies; PARK2, which encodes parkin, a ubiquitin E3 ligase; PINK1 (PARK6), which encodes a serine-threonine kinase; DJ-1 (PARK7), an oxidative stress sensor; or leucine-rich repeat kinase 2 (LRRK2, PARK8), which encodes a serine-threonine kinase (Henchcliffe and Beal, 2008).

A number of animal models of PD have been developed that either involve mutations at candidate gene loci or injection of neurotoxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 6-hydroxydopamine (6-OHDA), or rotenone (Abeliovich et al., 2000, Betarbet et al., 2000, Bloem et al., 1990, Goldberg et al., 2003, Sauer and Oertel, 1994). While each of these models has made a significant contribution to our understanding of PD, the complexity of the disease suggests multiple levels of pathogenesis. Among the many etiologies of PD that have been proposed, one common pathogenic factor is mitochondrial dysfunction (Henchcliffe and Beal, 2008). Reduced coenzyme Q (CoQ) levels have been observed in platelets of PD patients (Beal, 2001, Shults et al., 1997), and some studies have shown that patients with PD may benefit from CoQ₁₀ supplementation (Beal, 2003). CoQ₁₀ supplementation has also been shown to protect against MPTP-induced damage in aged mice (Beal, 2001). CoQ is an essential co-factor that shuttles electrons between complexes I or II and complex III in the electron transport chain, and also functions as a potent antioxidant in mitochondria. An NIH-sponsored Phase 3 clinical research trial is in progress to test the effectiveness of CoQ₁₀ in slowing the progression of Parkinson's disease (http://www.pdtrials.org/en/browse/all/1/view/238).

To further investigate how CoQ might impact a PD phenotype, we have studied two mouse models of primary CoQ deficiency. These animals have either a homozygous missense mutation in a gene encoding a CoQ biosynthetic pathway enzyme with resultant CoQ deficiency in all tissues, or a conditional knockout of the same gene that is specifically targeted to the dopaminergic neurons. This gene was initially identified as a spontaneous mutation designated kd for kidney disease (Lyon and Hulse, 1971), and was subsequently shown to encode a mitochondrial enzyme with prenyltransferase-like activity (Peng et al., 2004). It is now called prenyl diphosphate synthase subunit 2 (Pdss2), and its gene product is one of two enzymes needed for the isoprenylation of CoQ (Saiki et al., 2005). Upon targeted knockout of Pdss2 specifically in glomerular podocytes, the kidney disease phenotype of kd/kd (B6.Pdss2^kd/kd) mice was recapitulated (Peng et al., 2008). This renal phenotype could be significantly mitigated when mutant B6.Pdss2^kd/kd mice were supplemented from weaning throughout adulthood with CoQ₁₀ in their drinking water (Saiki et al., 2008). In contrast, when Pdss2 was deleted specifically in hepatocytes, myeloid cells, or renal proximal tubular epithelial cells, no disease phenotype was evident (Peng et al., 2008), suggesting that differentiated cells differ significantly with regard to their susceptibility to CoQ deficiency. We therefore examined whether behavioral deficiencies that resemble PD might appear in either the Pdss2 conditional knockouts or missense mutants.