The role of RER1 in the assembly and transport of TREM2-DAP12 complex and cellular lipid metabolism

Abstract

DNAX-activating protein of 12 kDa (DAP12) is a transmembrane adapter protein expressed on lymphoid and myeloid lineage cells, and interacts with several immune receptors to form functional complexes that trigger intracellular signaling pathways. One of the DAP12 associated receptors is the triggering receptor expressed on myeloid cells 2 (TREM2), and mutations in both, DAP12 and TREM2, have been linked to neurodegenerative disease.<br /> However, mechanisms involved in the assembly of TREM2-DAP12 complexes and the turnover of both proteins are not well understood. Here, we demonstrate that DAP12 is efficiently targeted to proteasomal degradation in the absence of TREM2. Interestingly, unassembled DAP12 was retained in the early secretory compartments, including the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC) while the mutation of aspartic acid residue at position 50 (D50) to alanine (A) in the transmembrane of DAP12 leads to the escape of unassembled DAP12 to the post-translational compartments as well as cytoplasm. Interaction studies showed that unassembled DAP12 interacts with the Retention in ER Sorting Receptor 1 (RER1) by electrostatic interaction, indicating that RER1 might play a role in unassembled DAP12 retention and/or retrieval. However, the RXR motif localized in the C-terminal of DAP12 has no effect on DAP12 retention. Additionally, the deletion of endogenous RER1 decreased expression of functional TREM2-DAP12 complexes and membrane proximal signaling, and resulted in almost complete inhibition of phagocytic activity in THP-1 differentiated macrophage-like cells. Furthermore, RER1 deficiency resulted in the accumulation of cellular triacylglycerols (TAGs) in both undifferentiated and differentiated THP-1 cells. The increased cholesterol precursors and the differentially expressed genes involved in sterol synthesis pathways in RER1-deficient monocytes indicated a critical involvement of RER1 in cholesterol biosynthesis of this cell type. Additionally, the altered expression of genes related to the uptake, intracellular transport and efflux of cholesterol at the mRNA and protein level as well as the altered TAGs composition in RER1 ko macrophage-like cells indicated the critical involvement of RER1 in the complex regulation of lipid metabolism also in this cell type. These results indicate that RER1 acts as an important regulator of DAP12 containing immune receptor complexes, immune cell function as well as lipid metabolism in immune cells.