Ligand-dependent kinase activity of MERTK drives efferocytosis in human iPSC-derived macrophages.

Removal of apoptotic cells by phagocytes (also called efferocytosis) is a crucial process for tissue homeostasis. Professional phagocytes express a plethora of surface receptors enabling them to sense and engulf apoptotic cells, thus avoiding persistence of dead cells and cellular debris and their consequent effects. Dysregulation of efferocytosis is thought to lead to secondary necrosis and associated inflammation and immune activation. Efferocytosis in primarily murine macrophages and dendritic cells has been shown to require TAM RTKs, with MERTK and AXL being critical for clearance of apoptotic cells. The functional role of human orthologs, especially the exact contribution of each individual receptor is less well studied. Here we show that human macrophages differentiated in vitro from iPSC-derived precursor cells express both AXL and MERTK and engulf apoptotic cells. TAM RTK agonism by the natural ligand growth-arrest specific 6 (GAS6) significantly enhanced such efferocytosis. Using a newly-developed mouse model of kinase-dead MERTK, we demonstrate that MERTK kinase activity is essential for efferocytosis in peritoneal macrophages in vivo. Moreover, human iPSC-derived macrophages treated in vitro with blocking antibodies or small molecule inhibitors recapitulated this observation. Hence, our results highlight a conserved MERTK function between mice and humans, and the critical role of its kinase activity in homeostatic efferocytosis.

MEN1 missense mutations impair sensitization to apoptosis induced by wild-type menin in endocrine pancreatic tumor cells.

BACKGROUND & AIMS: Missense mutations account for 30% of mutations identified in patients with the multiple endocrine neoplasia type 1 (MEN1) syndrome. They raise several issues: the distinction between pathogenic mutations and polymorphisms is sometimes difficult and the functional effects of missense mutations are unclear. We aimed to evaluate the functional consequences of missense MEN1 mutations in an appropriate endocrine cellular context. METHODS: From the INS-1 insulinoma cell line, we established clones conditionally over expressing wild-type (WT) menin or its A160T, H317Y, and A541T variants. We compared the consequences of WT or variant menin over expression on apoptotic response after gamma-irradiation and analyzed the interactions of these proteins with p53. RESULTS: WT menin over expression sensitized INS-r3 cells to apoptosis through amplification of caspase-3 activation, increased p53 acetylation, and accelerated p21 activation; moreover, over expressed WT menin could be recovered in p53-containing complexes. For all 3 missense mutations tested, the functional effects observed with WT were impaired significantly and only low amounts of variant menin proteins were recovered in p53-containing complexes. CONCLUSIONS: Taking advantage of a new endocrine cellular model, we show a loss of function for 2 missense disease-related menin mutants and for a controversial variant as well. Furthermore, our results suggest the existence of functional interactions between p53 and menin for the control of apoptosis, which may cast new light on the mechanisms of endocrine tumorigenesis.