REVIEW Institute of Neurosurgery, The PLA Navy General Hospital, Beijing, China Extracellular ␣-synuclein - a possible initiator of inflammation in Parkinson’s disease Wen-Qing Ren, Zeng-Min Tian, Feng Yin, Jun-Zhao Sun, Jian-Ning Zhang Received April 18, 2015, accepted July 17, 2015 Jian-Ning Zhang, Institute of Neurosurgery, General Hospital of Navy, 6 Fucheng Rd., Beijing 100048, China [email protected] Pharmazie 71: 51–55 (2016) doi: 10.1691/ph.2016.5070 Parkinson’s disease (PD) is a progressive neurodegenerative disease involving the loss of dopamine- producing neurons of the substantia nigra and the presence of Lewy bodies which contain high levels of ␣-synuclein. Although the causative factors of PD remain unclear, the progression of PD is accompanied by a highly localized inflammatory response mediated by reactive microglia. Recently, attention has focused on the relationship between ␣-synuclein and microglial activation. This review examines the role of ␣-synuclein on microglia in PD pathogenesis and progression, we also discuss the way of ␣-synuclein induced microglial activation. 1. Introduction 2. Structure and function of ␣-synuclein in PD Parkinson’s disease (PD) is the second most prevalent age- ␣-Synuclein is an abundant and intrinsically disordered pro- related neurodegenerative disease, after Alzheimer’s disease tein that is predominantly localized around synaptic vesicles in (AD). PD affects 3% of people over 60 years (Chauhan and presynatic terminals. ␣-Synuclein is a small protein (140 amino Jeans 2015). The pathological hallmarks of PD are the loss of acid) firstly described in Torpedo californica (Maroteaux et al. dopaminergic neurons in the substantia nigra of the brain and the 1988). The protein is composed of three distinct regions (Fig. 1): presence of intraneuronal cytoplasmic inclusions, termed “Lewy (1) an amino terminus, residues1–60, contains four 11-amino Bodies” (LBs) (Schulz and Falkenburger 2004). Although the acid imperfect repeats. It is coding for amphipathic helices, etiology of PD is incompletely understood, dysfunction of the with a conserved motif (KTKEGV); (2) a central hydropho- ubiquitin proteasome pathway, protein aggregation, mitochon- bic, residues 61–95, is called non-A␤ component (NAC). This drial dysfunction, oxidative stress and inflammation have all domain was first discovered as the non-amyloid component of been identified as important pathways leading to excitotoxic amyloid-␤ plaques in Alzheimer disease, and allows for fibril- and apoptotic death of dopaminergic neurons (Barzilai and lization of ␣-synuclein to undergo a conformational change from Melamed 2003; Moore et al. 2005; Ryan et al. 2015). Among random coil to ␤-sheet structure (el-Agnaf and Irvine 2002); these possible causes of PD, microglia and neuroinflammation (3) a carboxyl terminus, residues 96-140, a highly negatively have gained great attention during the last years. Further epi- charged, is thought to contain protein–protein and protein–small demiological studies have shown that NSAIDs (Non-Steroidal molecule interaction sites. The carboxy-terminus of ␣-synuclein Anti-inflammatory Drugs) could attenuate the development of may regulate aggregation of full-length ␣-synuclein and deter- PD (Bornebroek et al. 2007; Gagne and Power 2010; Gao et al. mine the diameter of ␣-synuclein filaments (Murray et al. 2003). 2011; Rees et al. 2011). In recent years, it has been hypothesized that the extent of ␣- ␣-Synuclein, a main component of LBs, can be secreted from synuclein may be a key factor in mediating transmission of neuronal cells. It is confirmed that extracellular ␣-synuclein disease pathology from one brain area to another. Braak and col- stimulated the abnormal microglial activation in many mod- leagues described their widely accepted staging system of PD els of Parkinson’s disease, accompanied by a large number of progression. They identified six stages in PD. Starting first with inflammatory factor secretion in vitro (Klegeris et al. 2008; Su the appearance of ␣-synuclein aggregates in the olfactory bulb et al. 2008; Codolo et al. 2013; Lei et al. 2014). Therefore, and anterior olfactory nucleus. Thereafter, the pathology spreads the interaction between microglial activation and extracellular rostral, indicated by LBs. During the second stage, the pathology ␣-synuclein seems to have become paramount in the pathogen- manifests itself in the caudal raphe nucleus, in the giganto- esis of PD. A clear understanding about possible mechanisms cellular reticular nucleus, and in the coeruleus–subcoeruleus of microglial activation by extracellular ␣-synuclein may be a complex. It is not until stage 3 where ␣-synuclein-positive valuable reference to potential strategy of PD treatment. Thus, deposits appear in the substantia nigra. At stage 4, lesions are we do a literature research and make a review about the structure detected in proencephalic regions and finally reach areas of and function of ␣-synuclein, the effect of microglial activation neocortex and prefrontal cortex at the last stages. in PD and the possible pathway of microglial activation induced In its native monomeric state, ␣-synuclein behaves as an intrin- by extracellular ␣-synuclein. sically unstructured protein, characterized by the lack of a stable Pharmazie 71 (2016) 51 REVIEW Fig. 1: Schematic structure of ␣-synuclein. tertiary structure. However, it may adopt a secondary structure of this protein may also be a crucial factor in the patho- upon binding to protein interactors or lipid membranes. The genesis of PD (Pacheco et al. 2012; Gassowska et al. 2014; assembly of ␣-synuclein is accompanied by transition from ran- Lee et al. 2014). dom coil to a-pleated sheet conformation (Roberts and Brown 2015). X-ray fiber and electron diffraction studies show ␣- synuclein filaments to exhibit a conformational conversion and 3. Microglial activation in PD fibril formation (Bousset et al. 2013). The pathological form of ␣-synuclein consists of oligomers and fibrils being rich in ␤- Microglia, the brain-resident macrophages, are the major sheets. The conversion of its ␣-helical structure to the ␤-sheet immune cells in PD (Chung et al. 2010; Fellner and Stefanova rich fibril is a defining pathologic feature of ␣-synuclein. 2013). Under physiological conditions, microglia continually ␣-Synuclein is a member of the family of synuclein proteins, monitor brain microenvironment in a resting state characterized which also include ␤- and ␥-synuclein. Although the physiolog- by ramified morphology. In response to certain cues such as brain ical function of ␣-synuclein has not been completely defined, injury or immunological stimuli, microglia are readily activated studies have shown that it may be closely related to synaptic (Fetler and Amigorena 2005). Key features of microglial acti- vesicle (Nakamura et al. 2008) transport function and synap- vation are morphological changes, increased expression of cell tic plasticity, signal transmission, and the dopamine synthesis surface receptors, the production and release of proinflamma- tory cytokines as well as neurotoxic factors (e.g: tumor necrosis and so on. Several models have been made to investigate the ␣ ␣ ␤ ␤ pathological features of ␣-synuclein in PD. Early studies sing factor- (TNF- ) and interleukin-1 (IL-1 ), nitric oxide (NO) the zebra finch indicated that the expression of ␣-synuclein in and reactive oxygen species (Block et al. 2007). Furthermore neuron terminals is related to synaptic plasticity. Abeliovich microglia-mediated neuroinflammation may contribute signif- and collaborators first developed feasible ␣-synuclein-knockout icantly to neurodegeneration in parkinsonian animals or even mice models, having normal synaptic architecture and brain human Parkinson’s disease (Kohutnicka et al. 1998; Ouchi et al. morphology, showing that ␣-synuclein deletion only led to slight 2005; Qian and Flood 2008). Subsequently, microglia evoke a changes in synaptic transmission (Abeliovich et al. 2000). The cascade of cell signaling, resulting in the activation of classi- second type of model are rodent and primate models where cell cal pathways (Block et al. 2007). The generation of reactive death is induced using neurotoxins such as MPTP and rotenone, oxygen species proinflammatory cytokines are believed to be 6-OHDA. These models have studied how oxidative stress and largely responsible for dopaminergic cell death in PD (Tansey metabolic challenges alter the structure, expression and aggrega- and Goldberg 2010; Li et al. 2013; Frakes et al. 2014). tion of ␣-synuclein. Otherwise, there are genetic models, where mutations (A53T, A30P, and E46K as well as G51D and H50Q) in proteins are associated with the development of PD (Appel- 4. Mechanisms of microglial activation by extracellular ␣ Cresswell et al. 2013; Proukakis et al. 2013). They have been -synuclein expressed in transgenic mice or delivered using viral vectors in In recent years, extracellular ␣-synuclein has received a lot of Drosophila. In all cases, ␣-synuclein accumulates in neurons, scientific attention due to its potential role in the initiation and leading to the formation of protofibrils, fibrils and LBs. progression of PD. Extracellular ␣-synuclein exerts extracellular ␣-Synuclein is known as an intraneuronal protein. However, pathogenic actions as well. Recent studies suggest that neuronal a series of studies demonstrated the presence of ␣-synuclein cells release small amounts of ␣-synuclein by unconventional in extracellular fluids. A nanomolar concentration of extracel-