The Role of Natural Killer Cells in Parkinson's Disease
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Earls and Lee Experimental & Molecular Medicine (2020) 52:1517–1525 https://doi.org/10.1038/s12276-020-00505-7 Experimental & Molecular Medicine REVIEW ARTICLE Open Access The role of natural killer cells in Parkinson’s disease Rachael H. Earls1 and Jae-Kyung Lee 1 Abstract Numerous lines of evidence indicate an association between sustained inflammation and Parkinson’s disease, but whether increased inflammation is a cause or consequence of Parkinson’s disease remains highly contested. Extensive efforts have been made to characterize microglial function in Parkinson’s disease, but the role of peripheral immune cells is less understood. Natural killer cells are innate effector lymphocytes that primarily target and kill malignant cells. Recent scientific discoveries have unveiled numerous novel functions of natural killer cells, such as resolving inflammation, forming immunological memory, and modulating antigen-presenting cell function. Furthermore, natural killer cells are capable of homing to the central nervous system in neurological disorders that exhibit exacerbated inflammation and inhibit hyperactivated microglia. Recently, a study demonstrated that natural killer cells scavenge alpha-synuclein aggregates, the primary component of Lewy bodies, and systemic depletion of natural killer cells results in exacerbated neuropathology in a mouse model of alpha-synucleinopathy, making them a highly relevant cell type in Parkinson’s disease. However, the exact role of natural killer cells in Parkinson’s disease remains elusive. In this review, we introduce the systemic inflammatory process seen in Parkinson’s disease, with a particular focus on the direct and indirect modulatory capacity of natural killer cells in the context of Parkinson’s disease. 6,7 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Introduction dopaminergic (DA) neurons such as astrocytes . The Parkinson’s disease (PD) is the second most common exact cause of α-syn aggregation is debated, but the neurodegenerative disease that affects over one million importance of this protein in PD is highlighted through people in the United States, with predictable healthcare- genetic studies revealing missense, duplication, or tripli- associated costs of nearly $52 billion per year1.PDis cation mutations in the SNCA gene encoding α-syn pathologically characterized by the misfolding and leading to early-onset PD with prominent dementia8. aggregation of the alpha-synuclein (α-syn) protein into Importantly, human patient samples revealed aberrant α- insoluble inclusions known as Lewy bodies (LBs) and syn distribution peripherally within blood plasma9, Lewy neurites2. Monomeric α-syn protein is highly interstitial fluid10,11, and the enteric nervous system expressed in presynaptic terminals3 and the nucleus4 of (ENS)12, implicating prion-like replication and spreading cells. However, α-syn can acquire toxic properties through of α-syn. The accumulation of α-syn aggregates can pro- the pathogenic process of misfolding into α-syn fibrils that mote sustained activation of pattern-recognition recep- comprise the major component of LBs. LBs are present in tors (PRRs), leading to the production of proinflammatory PD, dementia with Lewy bodies (DLB), and Alzheimer’s cytokines in microglia13,14. These inflammatory mediators disease (AD) brains; however, each disease shows distinct from microglia and chemoattractant molecules from regional specificity, with PD patients displaying LBs most damaged neurons possibly play a role in infiltrating per- prominently in the substantia nigra (SN)5. Furthermore, ipheral cells into the central nervous system (CNS)15. in human sporadic PD cases and murine models of PD, Peripheral inflammation has been shown to exacerbate LB formation is present in numerous cell types other than DA neurodegeneration in numerous animal models of PD16,17. Extracellular α-syn aggregates may elicit a self- amplifying cycle of immune responses in the CNS14,18 and Correspondence: Jae-Kyung Lee ([email protected]) 19 1Department of Physiology and Pharmacology, University of Georgia College periphery through an overproduction of inflammatory of Veterinary Medicine, Athens, GA 30602, USA © The Author(s) 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a linktotheCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Official journal of the Korean Society for Biochemistry and Molecular Biology Earls and Lee Experimental & Molecular Medicine (2020) 52:1517–1525 1518 mediators, thus providing the tertiary hit required for PD- transfer of α-syn is conducted via direct contact and associated dysfunction to spread to neighboring neurons tunneling nanotubes (TNTs)38. α-Syn is localized to the in the CNS20,21 and the periphery22. Therefore, lysosomal compartment of astrocytes and neurons; how- immunomodulatory-based approaches aimed at halting ever, a progressive increase in cleaved and full-length α- the propagation and burden of extracellular α-syn, and in syn was displayed in neurons over time, while astrocytes turn diminishing the inflammatory response, are currently displayed efficient degradation of α-syn37. However, fail- being tested as a therapeutic for PD and related synu- ure of the lysosomal digestion of excess α-syn oligomers cleinopathies (reviewed in23). Recently, it was discovered in astrocytes results in α-syn deposits in the trans-Golgi that natural killer (NK) cells efficiently internalize and network followed by endoplasmic reticulum swelling and degrade α-syn aggregates via the endosomal/lysosomal mitochondrial disturbances38. Moreover, exogenous α- pathway, a novel and highly relevant function of NK cells syn induced increased mRNA expression of proin- in synucleinopathies24. The number of circulating NK flammatory cytokines (IL-1β, TNF-α), iNOS, and cells in PD patients is increased compared to non-PD cyclooxygenase-2 (COX-2) from primary mouse astro- controls25. Furthermore, NK cells are present in the cytes in a TLR4-dependent manner39. human postmortem PD SN24 and PD mouse brain26, and Peripheral inflammation has been deemed a significant depletion of NK cells in a mouse model of synucleino- contributing factor in PD pathogenesis. Serum levels of pathies is associated with increased neuropathology24.In IL-1β, IL-2, IL-10, interferon-gamma (IFN-γ), and TNF-α this review, we review α-syn-associated immune respon- have been correlated with the severity of PD symp- ses, the potential role of NK cells, and the mechanisms by toms40,41 and rate of disease progression42. Serum auto- which NK cells exert their effects on PD pathogenesis. immune antibodies against α-syn are significantly elevated in PD patients43. Autoantibody titers, α-syn monomers, Implications of α-syn-associated immune and oligomers plus fibrils in 72%, 56%, and 17% of PD responses in PD patients are elevated, respectively, within a 5-year disease Although the etiology of PD is considered multifactorial duration44, which implicates systemic adaptive immune in nature, genetic studies have identified several loci responses against different α-syn species. PD patients also involved in inflammation, including the human leukocyte have a T helper cell 1 (Th1) bias in peripheral blood with antigen-DR isotype (HLA-DR), that lead to an increased naïve CD4+ T cells from patients preferentially differ- risk of PD27. It is hypothesized that misfolded α-syn may entiating to a Th1 proinflammatory lineage and showing potentiate the observed increase in inflammation seen in augmented production of IFN-γ and TNF-α45. Moreover, PD, as excessive aggregated α-syn can hyperactivate peripheral blood mononuclear cell (PBMC)-derived microglia18 and promote neurotoxic effects by enhancing CD4+ T cells from PD patients specifically react to the release of tumor necrosis factor-alpha (TNF-α), antigenic major histocompatibility class two (MHC-II) interleukin (IL) 1-beta (IL-1β), IL-6, nitric oxide (NO), epitopes derived from α-syn46, revealing T cell specificity inducible nitric oxide synthase (iNOS), and reactive oxy- to the α-syn antigen. Substantial increases in inflamma- gen species (ROS). Microglia phagocytose extracellular α- tory cytokines (IL-1β, TNF-α, IL-6)47 and intestinal per- – syn28 via various receptors, including Toll-like recep- meability48 50 are correlated with the presence of α-syn tors13,14. As microglia clear α-syn aggregates, they become aggregates within the gut51. Increased intestinal perme- activated independent of internalization and phagocy- ability is thought to result from reductions in barrier- tosis29,30. The distinct mechanisms of α-syn-induced promoting proteins and disruptions of tight-junction activation seem to be α-syn species-specific28. For exam- networks49, a phenotype consistent with low-grade ple, monomeric and mutant α-syn can bind to CD36 on inflammation52,53. microglia,