Haptoglobin (Hp) is an acute phase plasma protein that avidly captures free haemoglobin (Hb) when it is released from erythrocytes during intravascular haemolysis. This physiological phenomenon is found considerably amplified in different inflammatory and immunological disorders. The macrophage scavenger receptor CD163 is capable of binding and internalizing haemoglobin-haptoglobin complex through a receptor mediated-endocytosis. Besides the membrane-bound form, CD163 can be detected as soluble protein, being constantly shed from the membrane because of the proteolytic activity of metalloproteases.
The specific recognition of Hp-Hb complex by CD163 receptor shows potential application in cancer therapy and for the treatment of some infectious diseases. Indeed, it can be exploited to specifically deliver Hp-Hb-bound chemical substances through CD163 gate to macrophages resident in cancerous or infected tissues.
Given the absence of a detailed structural description of the CD163/Hp-Hb ternary complex, the molecular basis behind their interaction is currently unknown.
The proposed project aims at unveiling the structure of the human CD163/Hp-Hb ternary complex at near-to-atomic resolution by means of single particle cryo-electron microscopy.
CD163 is a scavenger receptor that has been described as a multifunctional molecule involved in receptor-mediated endocytosis upon interaction with multiple ligands. The most studied function of CD163 from macrophages is the internalization of haptoglobin complexed with haemoglobin upon erythrocyte clearance [1].
Since the three-dimensional structure of macromolecules is strictly related to their functions, the determination of the ternary complex haptoglobin-haemoglobin-CD163 structure is the best way to unveil the mechanism of action of the receptor in recognising and internalizing the complex. Unfortunately, similarly to the majority of large-size bio-complexes, CD163/Hp-Hb is not amenable to crystallization, therefore structure determination by X-ray crystallography cannot be pursued. However, the size of CD163/Hp-Hb ternary complex is suitable for single particle cryo-electron microscopy, a technique that only recently experienced a "resolution revolution" that now allows to gain information at near-to atomic-resolution on large macromolecular systems working in solution, in native conditions and that, since it represented a real breakthrough in structural biology, it was awarded in 2017 by the Nobel Prize in Chemistry.
The structural characterization of the ternary complex will unravel the key interactions among all partners involved in the complexes and will clarify their mechanisms of action. Moreover, elucidating the mechanism under the Hp-Hb specific recognition by CD163 through its structural characterization can make this system exploitable for therapeutic strategies: Hp-Hb could be used as a drug carrier and its interaction and internalization by the hepatic macrophage CD163 receptor would provide a route for selectively target malignant hepatic tissues in oncological diseases that needs more specific and effective drug delivery systems.
[1] Kristiansen, M. and Graversen, J. H. et al. Nature (2001).