Cellular co-localization of phosphorylated tau- and NACP/α-synuclein-epitopes in Lewy bodies in sporadic Parkinson's disease and in dementia with Lewy bodies

Abstract

The precursor of the non-Aβ-component of Alzheimer's disease (AD) amyloid (NACP, α-synuclein) aggregates into insoluble filaments of Lewy bodies (LBs) in Parkinson's disease (PD) and dementia with LBs (DLB). The microtubule-associated protein tau is an integral component of filaments of neurofibrillary tangles (NFTs). NFTs are occasionally found in brains of PD and DLB; however, the presence of NFTs or tau-epitopes within LB-containing neurons is rare. Double-immunofluorescence study and peroxidase-immunohistochemical study in serial sections, performed to examine the co-localization of tau- and NACP-epitopes in the brainstem of PD and DLB, demonstrated that four different epitopes of tau including phosphorylation-dependent and independent ones were present in a minority of LBs, but more often than previously considered. A tau (tau2)-epitope was localized to filaments in the outer layers of brainstem-type LBs by immunoelectron microscopy. Therefore, we conclude that tau is incorporated into filaments in certain LBs. Extensive investigation has enabled us to classify this co-localization into four types: type 1, LBs with ring-shaped tau-immunoreactivity; type 2, LBs surrounded by NFTs; type 3, NACP- and tau-immunoreactive filamentous and granular masses; and type 4, NACP- and tau-immunoreactive dystrophic neurites. This study raises a new question whether aggregation and hyperphosphorylation of tau in PD and DLB are triggered by the collapse of intraneuronal organization of microtubules due to NACP-filament aggregation in neuronal perikarya and axons.

Introduction

NACP (also known as α-synuclein [18]) is the precursor protein of non-Aβ component (NAC) of Alzheimer's disease (AD) amyloid [31], which is present physiologically in presynaptic structures [17]. Two missense mutations were identified in the NACP gene (G209A [26]and G88C [19]) from several independent families with Parkinson's disease (PD), and subsequent immunohistochemical investigations revealed that Lewy bodies (LBs), pale bodies (PBs), and Lewy-related neurites (LRNs) in PD and in dementia with LBs (DLB) were reactive for anti-NACP antibodies 3, 4, 29, 30, 32. Recent immunoelectron microscopic (immuno-EM) studies demonstrated that the full-length NACP is assembled into the filamentous elements (LB-filaments) of LBs, PBs, and LRNs in PD and DLB 3, 27. Two mutant forms of NACP (Ala53Thr and Ala30Pro, respectively) showed accelerated in vitro formation of filaments than the wild-type NACP, closely resembling LB-filaments 7, 8. Thus, NACP is considered to be involved primarily in the formation of LBs and LRNs.

The microtubule-associated protein tau is the major constituent of insoluble paired helical filaments (PHF) of neurofibrillary tangles (NFTs) in AD 13, 14. LBs and NFTs are two distinct neuronal inclusions made up of filamentous elements, which typify synucleinopathies and tauopathies, respectively [15]. The consensus has been that LBs and LRNs do not show immunoreactivity for tau protein either before 16, 21or after 4, 30the initial report of NACP in LBs [29]. In our immunohistochemical study, however, we have noticed that some brainstem-type LBs showed immunoreactivity to both anti-tau and anti-NACP antibodies. In this report, we have attempted to characterize and classify this co-localization by means of immunohistochemical, immunofluorescence, and immuno-EM studies.