Laboratory of Iron Homeostasis: Mleczko-Sanecka Laboratory

Maintenance of systemic and cellular iron balance is essential for human health. Over last two decades our understanding of the molecular control of iron homeostasis has progressed enormously. Still, a substantial proportion of the physiological variation in body iron parameters remains unexplained. The main objective of research in the Laboratory of Iron Homeostasis is to identify new molecular mechanisms involved in the control of cellular and systemic iron levels. To this end we will apply cutting-edge functional genomics approaches, involving cell-based large-scale unbiased genetic screens using CRISPR/Cas9 technology. To establish tools for our projects, we will employ gene editing techniques to generate cellular reporters, engineered to monitor endogenous gene expression levels using a fluorescence-based readout.

Appropriate body iron balance is chiefly ensured by the hepcidin-ferroportin (FPN) regulatory axis (Fig. 1). Hepcidin is a small hormone produced by liver hepatocytes that responds to body iron status, erythropoietic activity and inflammatory cues. It binds to the iron exporter FPN to trigger its degradation, inhibiting iron release to the bloodstream from specialized cell types, such as duodenal enterocytes or splenic macrophages. Iron export via FPN determines iron saturation of the plasma protein transferrin and thus adjusts systemic iron requirements to iron availability. Uptake of transferrin-bound iron occurs via the ubiquitously expressed transferrin receptor (TFR) 1, which constitutes a major route of cellular iron acquisition (Fig. 1).

To gain novel insights into the genetic control of iron homeostasis we have previously designed and conducted large-scale RNAi screens for hepcidin regulators (Mleczko-Sanecka et al., 2010, 2014; Fig. 2). This work identified SMAD7 as an important hepcidin inhibitor and linked hepcidin suppression to proliferative and nutrient-dependent signalling. Furthermore, our screens generated comprehensive lists of potential modifiers of iron homeostasis to be tested in prospective studies. Conceptually, in future research we plan to further expand our interests in unbiased functional genomics approaches to increase the understanding of iron sensing and iron uptake.

Bone morphogenetic protein (BMP) signalling is the key pathway that stimulates hepcidin expression. Among all BMPs, BMP6 has emerged as a crucial endogenous angiocrine factor, produced by liver endothelial cells, that maintains body iron homeostasis and stimulates hepcidin synthesis in hepatocytes under iron-rich conditions (Fig. 1). Strikingly, however, it remains completely unknown how iron-related signals may translate into increased Bmp6 mRNA levels. Therefore, one of our projects aims to dissect iron-dependent regulatory mechanisms that control expression of BMP6.

Hereditary hemochromatosis (HH) is a disease of iron homeostasis hallmarked by excessive iron absorption and progression of iron accumulation in vital organs. Most patients suffering from this frequent disorder are homozygous for the HFE(C282Y) mutation, but only few develop overt clinical symptoms. While the HFE genetic defect misregulates the hepcidin/ferroportin axis, genetic and environmental factors are thought to modulate body iron levels and modify disease severity in HH. To uncover modulators of tissue iron loading we have previously performed a targeted RNAi screen for regulators of cellular transferrin uptake (Mleczko-Sanecka, Altamura et al., in preparation). By this means we identified the chemokine CCL2 as a suppressor of iron-transferrin acquisition and we showed that CCL2 modifies iron levels in patients with HH. One line of our current research is aimed to further dissect and better understand TFR1-dependent and TFR1-independent iron uptake mechanisms, using cell-based screening assays, primary macrophage cultures, and ultimately mouse models. 

Fig. 1

Fig. 1. Systemic iron homeostasis is maintained by the hepcidin/ferroportin axis. The BMP6 cytokine produced by the liver endothelium stimulates hepcidin expression in hepatocytes. Ubiquitously expressed TFR1 mediates iron-transferrin uptake into cells, but other TFR1-independet routes of iron acquisition also contribute to tissue iron status.

 

tl_files/Lab/Mleczko-Sanecka/Fig. 2.jpg

Fig. 2. Unbiased genome-wide RNAi screen provided new insights into genes and pathways involved in regulation of hepcidin (Mleczko-Sanecka et al., 2014). Shown is the interaction network of putative hepcidin activators, grouped within functional categories that were enriched in the screening data. 

 

References:

  • Muckenthaler MU, Rivella S, Hentze MW, Galy B. A Red Carpet for Iron Metabolism. Cell. 2017;168(3):344-361.
  • Mleczko-Sanecka K, Roche F, da Silva AR, Call D, D'Alessio F, Ragab A, Lapinski PE, Ummanni R, Korf U, Oakes C, Damm G, D'Alessandro LA, Klingmüller U, King PD, Boutros M, Hentze MW, Muckenthaler MU. (2014) Unbiased RNAi screen for hepcidin regulators links hepcidin suppression to proliferative Ras/RAF and nutrient-dependent mTOR signaling. Blood. 123(10):1574-85
  • Mleczko-Sanecka K, Casanovas G, Ragab A, Breitkopf K, Muller A, Boutros M, Dooley S, Hentze MW, Muckenthaler MU (2010). SMAD7 controls iron metabolism as a potent inhibitor of hepcidin expression. Blood. 115(13):2657-65
  • Mleczko-Sanecka K, Altamura S, Costa Da Silva M, Zhu M, Vujić Spasić M, Guida C, Marques O, Bartz F, Maus R, Gunkel N, Lima M, Pepperkok R, Maus UA, Runz H, Porto G, U. Muckenthaler MU. The chemokine CCL2 is a novel modifier of tissue iron levels. 2017; in preparation.