The present study demonstrates that persistent neuroinflammation bridges α-syn pathologic alterations and progressive neurodegeneration in mediating chronic PD progression. The two-hit (neuroinflammation and mutant α-syn overexpression) progressive animal model created for this study mimics PD multifactorial etiology and reproduces the following key features of PD: a) chronic, progressive, and relatively selective degeneration of dopaminergic neurons and fibers in the nigrostriatal pathway; b) Lewy body-like neuronal inclusions containing aggregated α-syn; c) persistent neuroinflammation, a common feature shared by all neurodegenerative diseases including PD; and d) a chronic progressive disease course that is absent in most available PD models. These key features of PD were replicated in LPS-injected α-syn Tg mice, but not in NS-injected Tg mice or in LPS-injected WT mice, indicating synergistic effects of environmental exposures and genetic predisposition in the pathogenesis of PD.
α-Syn plays a prominent role in PD onset, ongoing disease progression, and host-to-graft disease propagation. In general, Tg overexpression of human α-syn results in widespread α-syn pathologies and variable neurodegeneration in different animal species (Farrer 2006
; Lee et al. 2002
). Among different α-syn Tg mouse models, only a few exhibit overt degeneration of nigrostriatal DA fibers and even fewer show loss of nigral DA neurons (Dawson et al. 2010
; Lee et al. 2002
; Masliah et al. 2000
). The lack of DA neuron loss in most gene-based PD animal models and the dearth of α-syn pathology in most toxin-based PD animal models underscore a crucial role of gene–environment interactions in PD. The low rate of concordance for PD in monozygotic and dizygotic twins (Marttila et al. 1988
; Tanner 2003
) strongly suggests the involvement of environmental factors in PD. Epidemiologic and case–control studies have implicated rural living, well water consumption, and pesticide exposure as potential environmental risk factors for PD (Elbaz et al. 2009
). Interestingly, it has been reported that overexpression of mutant α-syn in mice protects against (Manning-Bog et al. 2003
) or fails to affect paraquat neurotoxicity (Norris et al. 2007
), augments neurodegeneration elicited by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and LPS (Gao et al. 2008
; Nieto et al. 2005
), or does not enhance rotenone-induced neurotoxicity (Nieto et al. 2005
). These findings indicate that interactions between α-syn and environmental toxins on PD neurodegeneration require further investigation. Another newly developed two-hit mouse model combining a genetic lesion (Parkin−/−
) and neuroinflammation has provided additional experimental evidence supporting a crucial role of gene–environment interactions in PD (Frank-Cannon et al. 2008
Compared with a previous mouse model, generated by an intraperitoneal injection of a higher dose of LPS (5 mg/kg, equal to ~ 15 × 106
EU/kg) into WT mice (Qin et al. 2007
), our two-hit model created by a moderately toxic dose of LPS (3 × 106
EU/kg) revealed accelerated DA neurodegeneration and provided additional insight into α-syn pathologic alterations under inflammatory states. Specifically, significant loss of DA neurons started to manifest at 7 months after LPS injection in the prior model (Qin et al. 2007
) and at 2.5 months in our two-hit model; at these two time points, a 23% and 37% loss of DA neurons was observed, respectively. A single nigral injection of LPS resulted in acute, dramatic nigral neuroinflammation and DA neuron loss in mutant α-syn Tg mice (Gao et al. 2008
), but that acute two-hit model did not replicate the chronic, progressive course of PD, and acute, robust, and localized nigral inflammation may not be highly relevant to potential etiologies or pathogenesis of PD. The present study therefore advances research to elucidate a mechanistic link among persistent neuroinflammation, chronic α-syn pathology, and progressive neurodegeneration in chronic PD progression.
Persistent activation of iNOS and NADPH oxidase in LPS-treated Tg mice () and blockage of nitrated α-syn formation by continuous inhibition of these enzymes () indicated that increased nitric oxide, superoxide, and peroxynitrite may nitrosylate and/or oxidize α-syn and lower its solubility to form aggregates ( and ). Aggregated α-syn may overwhelm the ubiquitin–proteasome system and accumulate over time, leading to endoplasmic reticulum stress and mitochondrial impairment (Cooper et al. 2006
). Loss of physiologic function of normal α-syn and oxidative insults from microglia-derived free radicals further exacerbate neuronal damage, eventually causing neuronal death. DA neurons in the SN are known to be uniquely vulnerable to oxidative insults (Jenner and Olanow 1998
), and they encounter an excessively high level of oxidative stress during brain inflammation because of the high density of microglia in this region (Kim et al. 2000
). The combination of these factors may be partially responsible for the relatively selective degeneration of DA neurons in the SN in PD.
We have proposed that dying/dead neurons further activate microglia by releasing noxious compounds (e.g., α-syn) into the extracellular milieu (Gao and Hong 2008
). Indeed, monomeric and aggregated α-syn can be released from cultured cells and is recovered from the cerebrospinal fluid of patients with PD (Liu et al. 2009
). Exogenous application of aggregated α-syn activates cultured microglia (Zhang et al. 2005
). Therefore, ongoing neurodegeneration and chronic neuroinflammation cause a vicious self-propelling cycle that makes it possible for neuroinflammation to persist long after peripheral inflammation has abated, and for neurodegeneration to become a chronic progressive process. Without positive feedback from damaged neurons, the low-grade acute neuroinflammation induced by LPS was not sustained in WT mice, and consequent effects on α-syn pathology and neurodegeneration were negligible.