Abstract
The presence of macrophages surrounding lipotoxic tubular epithelial cells (TECs) is a hallmark of diabetic nephropathy (DN). Nevertheless, the mechanisms of communication between these cell types are not well understood. Previous studies have revealed a unique subset of macrophages that express triggering receptor expressed on myeloid cells 2 (Trem2) in kidneys of human patients and mice with DN. Here, we explored the characteristics and the function of Trem2+ macrophages in the progress of DN. RNA-sequencing of macrophages in kidneys of Trem2 knockout (KO) mice fed a high-fat diet plus streptozotocin (HFD/STZ) revealed functional enrichment of metabolic processes, cytokine production, positive regulation of extracellular signal-regulated kinase (ERK) cascades, and the regulation of phagocytosis. In vivo studies demonstrated that Trem2+ macrophages reduced lipid accumulation and mitigated ferroptosis of TECs in diabetic mice. Mechanistically, Trem2-deficient macrophages amplified the production of interleukin-1β (IL-1β) through activating the ERK signaling pathway. Furthermore, IL-1β triggered CD36 expression via the transcription factor NF-κB. Bioinformatics and functional assays showed NF-κB binds the CD36 promoter, which directly bound to the promoters of CD36 to facilitate its transcription. Inhibition of NF-κB blocked IL-1β-induced CD36 production. This mechanism is exacerbated in Trem2-deficient macrophages, which release excess IL-1β to activate NF-κB in tubular cells, promoting CD36-dependent lipid uptake and ferroptosis. Additionally, we found Trem2 plays a role in enhancing the phagocytosis and clearance of ferroptotic cells by bone marrow-derived macrophages. Altogether, our results suggest Trem2+ macrophages maintain homeostasis of the renal microenvironment and exert a protective function in DN.
Article highlights:
Levels of triggering receptor expressed on myeloid cells 2 (Trem2) in macrophages are increased in human patients and in mice with diabetic nephropathy. Trem2 suppresses the extracellular signal-regulated kinase signaling pathways, thereby inhibiting IL-1β production in macrophages. Macrophage Trem2 deficiency exacerbates tubular cell lipid deposition and ferroptosis by increasing CD36 expression in an IL-1β-dependent manner.