FKBP12 inhibits hepcidin expression by modulating BMP receptors interaction and ligand responsiveness in hepatocytes.

The expression of the iron regulatory hormone hepcidin in hepatocytes is regulated by the BMP-SMAD pathway through the type I receptors ALK2 and ALK3, the type II receptors ACVR2A and BMPR2, and the ligands BMP2 and BMP6. We previously identified the immunophilin FKBP12 as a new hepcidin inhibitor that acts by blocking ALK2. Both the physiologic ALK2 ligand BMP6 and the immunosuppressive drug Tacrolimus (TAC) displace FKBP12 from ALK2 and activate the signaling. However, the molecular mechanism whereby FKBP12 regulates BMP-SMAD pathway activity and thus hepcidin expression remains unclear. Here, we show that FKBP12 acts by modulating BMP receptor interactions and ligand responsiveness. We first demonstrate that in primary murine hepatocytes TAC regulates hepcidin expression exclusively via FKBP12. Downregulation of the BMP receptors reveals that ALK2, to a lesser extent ALK3, and ACVR2A are required for hepcidin upregulation in response to both BMP6 and TAC. Mechanistically, TAC and BMP6 increase ALK2 homo-oligomerization and ALK2-ALK3 hetero-oligomerization and the interaction between ALK2 and the type II receptors. By acting on the same receptors, TAC and BMP6 cooperate in BMP pathway activation and hepcidin expression both in vitro and in vivo. Interestingly, the activation state of ALK3 modulates its interaction with FKBP12, which may explain the cell-specific activity of FKBP12. Overall, our results identify the mechanism whereby FKBP12 regulates the BMP-SMAD pathway and hepcidin expression in hepatocytes, and suggest that FKBP12-ALK2 interaction is a potential pharmacologic target in disorders caused by defective BMP-SMAD signaling and characterized by low hepcidin and high BMP6 expression.

The continuum of Drosophila embryonic development at single-cell resolution.

Drosophila melanogaster is a powerful, long-standing model for metazoan development and gene regulation. We profiled chromatin accessibility in almost 1 million and gene expression in half a million nuclei from overlapping windows spanning the entirety of embryogenesis. Leveraging developmental asynchronicity within embryo collections, we applied deep neural networks to infer the age of each nucleus, resulting in continuous, multimodal views of molecular and cellular transitions in absolute time. We identify cell lineages; infer their developmental relationships; and link dynamic changes in enhancer usage, transcription factor (TF) expression, and the accessibility of TFs' cognate motifs. With these data, the dynamics of enhancer usage and gene expression can be explored within and across lineages at the scale of minutes, including for precise transitions like zygotic genome activation.

Hepcidin and the BMP-SMAD pathway: An unexpected liaison.

Hepcidin, the main regulator of iron metabolism, is synthesized and released by hepatocytes in response to increased body iron concentration and inflammation. Deregulation of hepcidin expression is a common feature of genetic and acquired iron disorders: in Hereditary Hemochromatosis (HH) and iron-loading anemias low hepcidin causes iron overload, while in Iron Refractory Iron Deficiency Anemia (IRIDA) and anemia of inflammation (AI), high hepcidin levels induce iron-restricted erythropoiesis. Hepcidin expression in the liver is mainly controlled by the BMP-SMAD pathway, activated in a paracrine manner by BMP2 and BMP6 produced by liver sinusoidal endothelial cells. The BMP type I receptors ALK2 and ALK3 are responsible for iron-dependent hepcidin upregulation and basal hepcidin expression, respectively. Characterization of animal models with genetic inactivation of the key components of the pathway has suggested the existence of two BMP/SMAD pathway branches: the first ALK3 and HH proteins dependent, responsive to BMP2 for basal hepcidin activation, and the second ALK2 dependent, activated by BMP6 in response to increased tissue iron. The erythroid inhibitor of hepcidin Erythroferrone also impacts on the liver BMP-SMAD pathway although its effect is blunted by pathway hyper-activation. The liver BMP-SMAD pathway is required also in inflammation to cooperate with JAK2/STAT3 signaling for full hepcidin activation. Pharmacologic targeting of BMP-SMAD pathway components or regulators may improve the outcome of both genetic and acquired disorders of iron overload and deficiency by increasing or inhibiting hepcidin expression.