TREM2 splice isoforms generate soluble TREM2 species that disrupt long-term potentiation

Abstract

Background TREM2 is a transmembrane receptor expressed by myeloid cells and acts to regulate their immune response. TREM2 governs the response of microglia to amyloid and tau pathologies in the Alzheimer’s disease (AD) brain. TREM2 is also present in a soluble form (sTREM2), and its CSF levels fluctuate as a function of AD progression. Analysis of stroke and AD mouse models revealed that sTREM2 proteins bind to neurons, which suggests sTREM2 may act in a non-cell autonomous manner to influence neuronal function. sTREM2 arises from the proteolytic cleavage of the membrane-associated receptor. However, alternatively spliced TREM2 species lacking a transmembrane domain have been postulated to contribute to the pool of sTREM2. Thus, both the source of sTREM2 species and its actions in the brain remain unclear. Methods The expression of TREM2 isoforms in the AD brain was assessed through the analysis of the Accelerating Medicines Partnership for Alzheimer’s Disease Consortium transcriptomics data, as well as qPCR analysis using post-mortem samples of AD patients and of the AD mouse model 5xFAD. TREM2 cleavage and secretion were studied in vitro using HEK-293T and HMC3 cell lines. Synaptic plasticity, as evaluated by induction of LTP in hippocampal brain slices, was employed as a measure of sTREM2 actions. Results Three distinct TREM2 transcripts, namely ENST00000373113 (TREM2230), which encodes the full-length transmembrane receptor, and the alternatively spliced isoforms ENST00000373122 (TREM2222) and ENST00000338469 (TREM2219), are moderately increased in specific brain regions of patients with AD. We provide experimental evidence that TREM2 alternatively spliced isoforms are translated and secreted as sTREM2. Furthermore, our functional analysis reveals that all sTREM2 species inhibit LTP induction, and this effect is abolished by the GABAA receptor antagonist picrotoxin. Conclusions TREM2 transcripts can give rise to a heterogeneous pool of sTREM2 which acts to inhibit LTP. These results provide novel insight into the generation, regulation, and function of sTREM2 which fits into the complex biology of TREM2 and its role in human health and disease. Given that sTREM2 levels are linked to AD pathogenesis and progression, our finding that sTREM2 species interfere with LTP furthers our understanding about the role of TREM2 in AD.

Long-read transcript sequencing identifies differential isoform expression in the entorhinal cortex in a transgenic model of tau pathology

“…This association was confirmed in our PacBio Iso-Seq targeted dataset with both transcripts associated with genotype (LR.Trem2.62: logFC = 1.73, FDR = 5.2 × 10 −4 ; LR.Trem2.54: log2FC = 1.55, FDR = 3.3 × 10 −6 ) and pathology in TG mice (LR.Trem2.62: logFC = 2.43, FDR = 9.96 × 10 −1 ; LR.Trem2:54: log2FC = 2.20, FDR = 3.85 × 10 −2 ). A significant increase of these two known Trem2 isoforms was also reported in a recent study using qPCR in two transgenic amyloid models of AD 16 ,suggesting that these transcripts are upregulated in response to both tau and amyloid pathology. Notably, the up-regulated mouse LR.Trem2.54 isoform shows high homology with the human TREM2 isoform (ENST00000373113.8, 75.3% homology, 81% query cover) that is most abundantly expressed in the human cortex 17 .…”

“…We leveraged our full-length Iso-Seq reads to identify a specific differentially expressed transcript (DET) of Gfap (LR.Gfap. 16, Figure 2C) -the most abundantly expressed Gfap isoform (Gfap-201, ENSMUST00000067444.9) in the mouse EC -that was the primary driver of overall differential Gfap expression in TG mice (log2FC = 3.57, FDR = 1.16 x 10 -3 , Figure 2D, Supplementary Table 4). The upregulation of LR.Gfap.16 was confirmed using the hybrid method incorporating normalized RNA-Seq read counts (log2FC = 1.06, P = 2.1 x 10 -2 ), although this approach also identified upregulation of several novel Gfap-associated isoforms (Figure 2E).…”

Different Splice Isoforms of Peripheral Triggering Receptor Expressed on Myeloid Cells 2 mRNA Expressions are Associated With Cognitive Decline in Mild Dementia Due to Alzheimer’s Disease and Reflect Central Neuroinflammation

“… 26 Similarly, a recent research also demonstrated elevated levels of TREM2 alt mRNA and other isoforms in several AD brain regions. 25 One of the few studies addressing the implication of peripheral sTREM2 in AD demonstrated that plasma sTREM2 levels correlate with inflammatory activity in the peripheral circulation and vary across different stages of the disease. 42 Therefore, the findings of the present study suggest that the measurement of TREM2 alt expression in the peripheral blood may reflect the activation of TREM2-related signaling and an altered peripheral immune activity in response to early-stage AD pathology.…”

“…A recent study conducted by Moutinho and colleagues have presented experimental findings demonstrating that in the human brain, TREM2 alternatively spliced isoforms (TREM2 alt ) are translated and secreted as sTREM2. 25 In accordance with these findings, we compared the peripheral expression of all TREM2 mRNA isoforms and TREM2 alt , which encodes sTREM2, 26 at the same time in patients with mild AD to healthy controls to examine the clinical significance of different mRNA isoforms in this study. MRS was used to investigate the association between peripheral TREM2 expression and central neuroinflammation.…”

Proteo-genomics of soluble TREM2 in cerebrospinal fluid provides novel insights and identifies novel modulators for Alzheimer’s disease

“…In addition to PBMC-derived macrophages, we have performed experiments using the immortalized human microglial clone 3 cell line, HMC3 (ATCC®CRL-3304). This cell line has been used in multiple studies to investigate intracellular TREM2 biology but not sTREM2 derived from HMC3 cells [ 84 – 86 ]. Although we were able to successfully knockdown our genes of interest in HMC3 cell line by transient transfection with siRNAs, we were unable to detect any sTREM2 in the conditioned medium even after the medium was concentrated prior to ELISA experiments (Data not show).…”