Distinct Signaling Pathways Regulate TREM2 Phagocytic and NFκB Antagonistic Activities.

Hailan Yao,Michael Mullan,Kyle Coppola,Daniel Paris,Jonas Elias Schweig,Fiona Crawford

doi:10.3389/fncel.2019.00457

Abstract

Several genetic variants of the Triggering Receptor Expressed on Myeloid Cells-2 (TREM2) have been shown to increase the risk of developing Alzheimer’s disease (AD) supporting a role of microglia and immune cells in the pathobiology of AD. We have employed an ectopic model of TREM2 and DAP12 expression in HEK293 cells to study selectively TREM2 dependent signaling and phagocytic functions and evaluated the effects of some of the TREM2 mutations associated with AD. We show that shedding of the TREM2 N-terminal domain does not affect the inhibition of NFκB activation induced by TREM2 while it completely blocks phagocytosis suggesting that TREM2 anti-inflammatory properties can be mediated by the TREM2 C-terminal fragment while the phagocytic activity requires the full-length receptor. In addition, we confirm in that model that apolipoprotein E (APOE) is a ligand for TREM2 and triggers TREM2 signaling. In particular, we show that APOE4 stimulates spleen tyrosine kinase (SYK) activation more potently than APOE2 in a TREM2 dependent manner. Interestingly, TREM2 appears to antagonize NFκB activation induced by phorbol ester but is unable to prevent TNFα induction of NFκB activation suggesting that TREM2 antagonizes inflammatory events triggered downstream of PKC. TREM2 mutations drastically impact TREM2 phagocytosis as well as its ability to antagonize NFκB activation and notably prevent the activation of the PI3K/AKT pathway observed with wild-type TREM2. Overall our data suggest that TREM2 dependent phagocytosis requires an activation of the SYK/PI3K/AKT/PLCγ pathways while the suppression of NFκB activation by TREM2 is independent of SYK, PI3K, and PLCγ activities. This model of ectopic TREM2-DAP12 co-expression appears suitable to study TREM2 signaling as several biological functions of TREM2 and TREM2 mutations that have been previously described in myeloid and microglial cells were also replicated in this model.

Highlights

The brain of Alzheimer’s disease (AD) patients is characterized by the presence of three pathologies that are central to the AD process: extracellular accumulation of cerebral β-amyloid (Aβ); intraneuronal accumulation of hyperphosphorylated and aggregated tau, and; inflammation (Hardy and Selkoe, 2002; De Strooper and Karran, 2016; Hammond et al, 2019)
Triggering Receptor Expressed on Myeloid Cells-2 (TREM2) is a phagocytic receptor exclusively expressed in myeloid cells which signals trough the adaptor protein DNAX-activating protein of 12 kDa (DAP12) following the binding of ligands to TREM2
It has been shown that some TREM2 ligands induce the phosphorylation of two tyrosine residues within the immunoreceptor tyrosine-based activation motifs (ITAMs) domain of DAP12 resulting in the recruitment and activation of spleen tyrosine kinase (SYK) and downstream signaling molecules including phosphatidyl inositol 3-kinase (PI3K) and phospholipase Cγ (PLCγ) (Takahashi et al, 2005; Otero et al, 2009; Peng et al, 2010; Wang et al, 2015; Colonna and Wang, 2016)

Summary

Introduction

The brain of Alzheimer’s disease (AD) patients is characterized by the presence of three pathologies that are central to the AD process: extracellular accumulation of cerebral β-amyloid (Aβ); intraneuronal accumulation of hyperphosphorylated and aggregated tau, and; inflammation (Hardy and Selkoe, 2002; De Strooper and Karran, 2016; Hammond et al, 2019). TREM2 mutations are rare, their observed effect size is comparable to the APOEε4 allele, which represents the strongest genetic risk factor for late onset AD (Ulrich et al, 2017; Yeh et al, 2017; Carmona et al, 2018). Aβ oligomers have been shown to interact with TREM2 with high affinity and to induce NFAT (nuclear factor of activated T cell) signaling while in TREM2 AD variants, the Aβ affinity for TREM2 remains unchanged, NFAT signaling induced by Aβ oligomers is reduced suggesting a partial loss of TREM2 function (Lessard et al, 2018). BV2 microglial cells expressing AD TREM2 variants show impaired phagocytic activity suggesting TREM2 loss of function for these variants (Kleinberger et al, 2014)

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Conclusion