Role of tRNA fragments in islet macrophage activation and in the crosstalk with β-cells during obesity

Islet-resident macrophages (Mφs) play a key role in pancreatic islet homeostasis but their activation in obesity results in β-cell dysfunction. This activation is likely to be elicited by modulatory molecules highly sensitive to environmental changes. The pool of transfer RNAs (tRNAs) is strictly regulated by nutrient availability and under stress conditions tRNAs can be cleaved, generating fragments (tRFs) with regulatory functions. Moreover, tRFs play a crucial role in extracellular vesicles (EVs)-mediated cell-to-cell crosstalk. The aim of the present work was to determine the impact of obesity on the tRF profile of islet Mφs and to investigate the role of these RNAs in Mφ activation and in the crosstalk with β-cells. Islet Mφs and β-cells were isolated from obese db/db and lean wild type mice. Small RNA sequencing identified 15 modulated tRFs (≥1.5-fold change, p≤0.05, n=4) in db/db islet Mφs. To assess their role in Mφ activation, we compared the tRF profile of db/db islet Mφs to that of bone marrow derived Mφs (BMDMs), polarized into M1 pro- and M2 anti-inflammatory Mφs. Interestingly, 7 of the tRFs upregulated in db/db islet Mφs were enriched in M2 BMDMs (≥1.5-fold change, p≤0.05, n=3). Inhibition of one of these fragments, 5’tRFGlu(CTC), led to profound transcriptomic changes in both M1 and M2 BMDMs (n=3), with a preferential impact on genes involved in T-cell stimulation and in acute immune responses. Among the tRFs upregulated in obese islet Mφs, 5’tRFGlu(CTC), 5’tRFGly(GCC) and 5’tRFAsp(GTC) were also increased in β-cells of db/db mice and were abundant in EVs released by M2 BMDMs, suggesting a possible role in the crosstalk between islet cells. Our data suggest that tRFs may act as key modulators of Mφs activation and that obesity induces an upregulation of anti-inflammatory tRFs in islet Mφs. RNA labelling and EVs transfer techniques are currently being used to investigate the contribution of tRFs in Mφ-β-cell crosstalk.