Iron traffics in circulation bound to a siderocalin (Ngal)–catechol complex

Abstract

The lipocalins are secreted proteins that bind small organic molecules. Scn-Ngal (also known as neutrophil gelatinase associated lipocalin, siderocalin, lipocalin 2) sequesters bacterial iron chelators, called siderophores, and consequently blocks bacterial growth. However, Scn-Ngal is also prominently expressed in aseptic diseases, implying that it binds additional ligands and serves additional functions. Using chemical screens, crystallography and fluorescence methods, we report that Scn-Ngal binds iron together with a small metabolic product called catechol. The formation of the complex blocked the reactivity of iron and permitted its transport once introduced into circulation in vivo. Scn-Ngal then recycled its iron in endosomes by a pH-sensitive mechanism. As catechols derive from bacterial and mammalian metabolism of dietary compounds, the Scn-Ngal–catechol–Fe(III) complex represents an unforeseen microbial-host interaction, which mimics Scn-Ngal–siderophore interactions but instead traffics iron in aseptic tissues. These results identify an endogenous siderophore, which may link the disparate roles of Scn-Ngal in different diseases.

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Figure 1: Determination of the affinity of catechol–Fe(III) in complex with Scn-Ngal.
Figure 2: UV-visible spectra of complexes of Scn-Ngal, siderophores and iron.
Figure 3: Scn-Ngal binds to catechol–Fe(III) as well as to 4-Methylcatechol–Fe(III).
Figure 4: The formation and distribution of the Scn-Ngal–catechol–Fe(III) complex in vivo.
Figure 5: Release of ligands from Scn-Ngal as a result of acidification.

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Acknowledgements

We thank R. Abergel, A. Zawadzka and Q. Al-Awqati for helpful discussion. We are grateful to the Gordon lab (Washington University) for gnotobiotic urines and E.I. Christensen (Aarhus University) and T.E. Willnow (Max Delbruck Center for Molecular Medicine) for megalin knockout urines. We salute the classes of 2010 and 2013 of the College of Physicians and Surgeons of Columbia University for donating urine for this study. This work was supported by US National Institutes of Health grants AI117448 (K.N.R.), AI59432 (R.K.S.) and the Emerald Foundation, the March of Dimes and US National Institutes of Health grants DK-55388 and DK-58872 (J.B.).

Author information

G.B. identified siderophores, studied the complex in different models and performed catechol chemistry. K.M., A.K. and B.L. initiated these studies. M.C. and R.K.S. identified the structure of Scn-Ngal and the critical sites of molecular recognition defining Scn-Ngal function; T.M.H., X.L., S.-X.D., D.W.L., A.J.R., R.K., J.C.P. and K.N.R. studied catechol and catechol–Fe chemistry, binding affinity and pH sensitivity; N.P., A.Q., T.L., K.M.S.-O. and M.V. designed and performed cell biology experiments; D.W. performed radioautography, J.B. designed and analyzed experiments and, with contributions from all authors, wrote the paper.

Correspondence to Roland K Strong or Jonathan Barasch.

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