DAP12 Stabilizes the C-terminal Fragment of the Triggering Receptor Expressed on Myeloid Cells-2 (TREM2) and Protects against LPS-induced Pro-inflammatory Response

Li Zhong,Xiao-Fen Chen,Zhen-Lian Zhang,Zhe Wang,Xin-Zhen Shi,Kai Xu,Yun-Wu Zhang,Huaxi Xu,Guojun Bu

doi:10.1074/jbc.m115.645986

Abstract

Triggering receptor expressed on myeloid cells 2 (TREM2) is a DAP12-associated receptor expressed in microglia, macrophages, and other myeloid-derived cells. Previous studies have suggested that TREM2/DAP12 signaling pathway reduces inflammatory responses and promotes phagocytosis of apoptotic neurons. Recently, TREM2 has been identified as a risk gene for Alzheimer disease (AD). Here, we show that DAP12 stabilizes the C-terminal fragment of TREM2 (TREM2-CTF), a substrate for γ-secretase. Co-expression of DAP12 with TREM2 selectively increased the level of TREM2-CTF with little effects on that of full-length TREM2. The interaction between DAP12 and TREM2 is essential for TREM2-CTF stabilization as a mutant form of DAP12 with disrupted interaction with TREM2 failed to exhibit such an effect. Silencing of either Trem2 or Dap12 gene significantly exacerbated pro-inflammatory responses induced by lipopolysaccharides (LPS). Importantly, overexpression of either full-length TREM2 or TREM2-CTF reduced LPS-induced inflammatory responses. Taken together, our results support a role of DAP12 in stabilizing TREM2-CTF, thereby protecting against excessive pro-inflammatory responses.

Highlights

Triggering receptor expressed on myeloid cells 2 (TREM2) is a DAP12-coupled receptor associated with neurodegenerative diseases
Triggering receptor expressed on myeloid cells 2 (TREM2) is a DAP12-associated receptor expressed in microglia, macrophages, and other myeloid-derived cells
We detected a band for the full-length TREM2 (TREM2-FL) protein at about 38 kDa and a lower molecular mass species at about 15 kDa, which is consistent with the size of the C-terminal fragment of TREM2 (TREM2-CTF) (Fig. 1A)

Summary

Background

TREM2 is a DAP12-coupled receptor associated with neurodegenerative diseases. Results: Co-expression of DAP12 increased the level of TREM2 C-terminal fragment (TREM2-CTF) which suppressed the release of pro-inflammatory cytokines. Conclusion: A major function of DAP12 is to stabilize TREM2-CTF, which regulates inflammatory responses in microglia. Silencing of either Trem or Dap gene significantly exacerbated pro-inflammatory responses induced by lipopolysaccharides (LPS). TREM2 is a type I transmembrane protein mainly expressed in microglia cells within the brain In association with another transmembrane protein DAP12, TREM2 regulates critical functions of microglia including inhibition of pro-inflammatory responses and stimulation of phagocytosis of apoptotic neurons (3, 10 –12). Mounting evidence implicates both TREM2 and DAP12 as key regulators of immune-responses within microglia It remains largely unknown how DAP12 and TREM2 coordinate to execute their important physiological and pathophysiological functions. Our studies illuminate a novel function of DAP12 in stabilizing TREM2-CTF, which protects against the deleterious pro-inflammatory responses induced by LPS, and provide a functional link between TREM2/DAP12 signaling and neuroinflammation

Experimental Procedures

Results

Discussion