The Role And Mechanism Of TREM2 In MPTP-induced Parkinson’s Disease

BackgroundParkinson’s disease (PD) is one of the most common neurodegenerative diseases, which is characterized by the loss of dopamine (DA) neurons within the substantia nigra pars compacta (SNpc) and axon terminals in the striatum. Although the pathogenesis of PD remains elusive, there is growing evidence supporting the role of microglia and inflammation in neurodegenerationMicroglia are the inherent immune cells in the central nervous system (CNS). In the normal mature brain, microglia are heterogeneously distributed in the resting state throughout the CNS for immune surveillance. The change of the internal environment can cause microglia activation, which can remove potentially deleterious debris, release neurotrophic factor, oxygen free radicals and promote tissue repair by secreting growth factors. Microglia can be long-termly activated in some disease process, which may cause massive amplification of the inflammatory process.Triggering receptors expressed by myeloid cells (TREMs) belong to a new type of rapidly expanding family of receptors. TREM2 is mainly expressed on microglia in CNS. Recent evidences indicate that TREM2 plays an important role in neurodegenerative disease, including multiple sclerosis (MS), stroke and Alzheimer’s disease (AD). Genetic study also showed that TREM2 mutation significantly increase the risk of AD and PD. Taken together, these studies highlight a critical role for TREM2 during inflammatory responses in the pathogenesis of PD. Here, we aimed to explore whether TREM2 play a role in MPTP mouse model of PD, and attempt to demonstrate the underlying signaling pathways that mediate TREM2 function.ObjectiveThe objective of the study is to investigate the role of TREM2 in MPTP-induced neurotoxicity in animal model of PD and to explore the underlying mechanism involved.Materials and MethodsIn vitro studies1. Adenovirus was used to upreguate TREM2 expression in BV2 microglia cells in vitro. The mRNA levels of TNF-α, IL-1β, iNOS and COX-2 were detected by realtime RT-PCR after exposed to LPS for 24 h.2. Protein levels of iNOS and COX-2 were detected by Western blot after exposed to LPS for 24 h.In vivo studies1. Protein levels of TH and TREM2 in the striatum at different time points are detected after MPTP injection by Western blot analysis. The mRNA levels of TREM2 were detected by realtime RT-PCR.2. The cellular localization of TREM2 in the SN and striatum of MPTP mouse was investigated by double immunofluorescence staining with CD11b (a microglia marker) and TREM2.3. Adenovirus was stereologically injected into the right striatum. Two weeks later, MPTP was intraperitoneally injected to produce the acute MPTP PD model. Mice were killed 7 days later. Protein levels of TH were detected by immunohistochemistry, immunofluorescence and Western blot. Striatal levels of DA, HVA and DOPAC were detected by HPLC.4. Activation of microglia and astrocytes in the SN and striatum were visualized using immunofluorescence staining with anti-CD11b antibody and anti-GFAP antibody.5. The mRNA levels of TNF-α、IL-1β、iNOS、COX-2 were measured by realtime RT-PCR in the SN 7 days after MPTP injection. Protein levels of iNOS and COX-2 were detected by Western blot analysis.6. Adenovirus were stereologically injected into the right striatum.Two weeks later, MPTP was intraperitoneally injected to produce the acute MPTP PD model. Mice were killed 3 days later. Protein levels of TLR4, TRAF6, p-p65, p-JNK and p-p38 were detected by Western blot analysis.Results1. The protein levels of TH were significantly decreased in MPTP-induced animal model of PD. The protein and mRNA levels of TREM2 were upreguated in the MPTP-induced PD models.2. TREM2 was colocalized with CD11b (a microglia marker) and was upregulated in MPTP-induced PD models.3. TREM2 suppressed LPS-induced pro-inflammatory responses in BV2 microglial cells.4. TREM2 attenuated MPTP-induced decrease of TH expression in the SN and striatum, and improved striatal levels of DA, HVA and DOPAC.5. TREM2 attenuated MPTP-induced microglia and astrocytes activation in the SN and striatum.6. TREM2 suppressed pro-inflammatory responses in MPTP-induced PD models.7. TREM2 attenuated MPTP-induced increases of protein levels of TLR4, TRAF6, p-p65, p-JNK and p-p38.ConclusionThe results of our study revealed that overexpression of TREM2 has a protect role in MPTP-induced PD models by inhibiting the inflammatory response.