Term Paper Topic
Hemoglobin Scavenger Receptor (HbSR): Structure and Functions
Roll no. – A09
Dr. Sanjeev Singh
Hemoglobin Scavenger Receptor (HbSR): Structure and Function
Introduction: The Hemoglobin Scavenger Receptor Protein (HbSR) is an acute-phase plasma glycoprotein having 130-kDa atomic weight (Etzerodt et, al., 2013). It is also known by several names: CD163 Ag (most commonly known name), Hemoglobin -Haptoglobin (Hb-Hp) complex receptor, Macrophage associated receptor, M130Ag precursor (Onofre et, al. ,2009). HbSR has a place with Scavenger Receptor Cysteine-Rich Superfamily i.e. SRCR-SF Class B; the first subgroup since this protein contains 9 SRCR domains in its extracellular space.
The HbSR receptor is expressed on the surfaces of macrophages and monocytes which contributes resolution of inflammation and tissue repair. These macrophages are abundantly found in spleen, placenta, liver (Kupffer cells) lymph nodes, thymus, bone marrow, brain, lungs, and peritoneum; respectively (Onofre et.al.,2009). This receptor is known to perform variety of functions: anti-inflammatory signal cascade initiation (Ijaz Petra et, al., 2017), Hb-Hp Complex removal (Haan Yang et, al.,2017), biomarker for different diseases (Onofre et, al., 2009), regulatory function during erythropoiesis and innate immune sensor for bacteria (Fabriek et.al., 2018). This receptor was firstly discovered in 1987. The HbSR is mapped to chromosome no.12 and is present in region p13(Onofre et, al.,2009).
The HbSR play important role in immunity and host defense mechanisms by the production of inflammatory mediators such as nitric oxide, Tumor Necrosis Factor (TNF-?), Interleukins (IL)-1? etc. All these inflammatory mediators are produced because of crosslinking of the CD163/ HbSR with the monoclonal antibodies which are directed against CD163(Blood, 2009). The HbSR contains five different isoforms differing in the cytoplasmic domain structure and putative phosphorylation sites (Onofre et, al. ,2009). It is involved in the recognition of various types of ligands such as proteins, polysaccharides, polyribosomes , and lipids (Sarrias et, al., 2004).
Structure of the HbSR
Scavenger Receptor Cysteine-Rich (SRCR) – Superfamily (SRCR-SF) is divided into two groups: Group A and Group B. Group A contain SRCR domain with two exons which encode for six cysteine residues and Group B contains SRCR domain with a single exon which encodes for 6-8 cysteine residues. Group B is further divided into two groups: one group is characterized by the presence of extracellular domain other than SRCR domain and other by the absence of extracellular domain other than SRCR domain (Onofre et, al.,2009). The HbSR is the only Group B protein having eight cysteine residues per domain but domain 8 contain six cysteine residues (Calvert et, al.,2010)
The HbSR is structurally a type I transmembrane protein which consists of following three domains like other receptors: the extracellular domain, the transmembrane domain and the cytoplasmic domain(Fig1) (Etzerodt et, al.,2014). The difference between the the HbSR and other receptors is that it contains a single transmembrane domain and a short cytoplasmic tail (Hogger et, al.,1998). The extracellular domain of the HbSR contains nine Scavenger Receptor Cysteine -Rich (SRCR) domains from SRCR 1-9; which are separated by two proline-serine-threonine(PST)- rich domains in the regions between SRCR6 and SRCR7 and the end of extracellular domain respectively (Zang et, al., 2017). The extracellular domain of the HbSR is highly conserved domain and contain 1003 amino-acid residues. The single transmembrane domain of the HbSR consists of 24 amino acid residues and short cytoplasmic domain comprises of 49 amino acid residues (Onofre et, al.,2009). Different isoforms of the HbSR vary only in the length of the cytoplasmic tails.
Fig 1: Schematic representation of various HbSR class B proteins (Fabriek et, al.,2005)
Some HbSR SRCR domain also contains sites for the binding of calcium and binding of Hb-Hp complexes or several other antibodies.
Expression and regulation of HbSR
The HbSR receptor is expressed on the surfaces of macrophages and monocytes which are abundantly found in spleen, placenta, liver (Kupffer cells) lymph nodes, thymus, bone marrow, brain, lungs and peritoneum; respectively (Onofre et.al.,2009). Monocyte-derived cells such as dendritic cells, macrophages of white pulp and Langerhans cells contain very low HbSR or there is no HbSR (Etzerodt et, al.,2014). A no. of factors are responsible for regulation of the HbSR In-Vitro (Table1) . Up-regulation of HbSR is caused by glucocorticoid, interleukin(IL)-6 and Heme/Hb whereas IL-4, IL-10, IFN-?, TFN-?, CXCL4, GM-CSF and Hb causes the downregulation of the HbSR.
Table 1: list of substances regulating the expression of the HbSR in monocytes/macrophages In-Vitro (Etzerodt et, al.,2014)
Compound Up or Downregulation
For macrophage differentiation mouse systems are widely used animal models and there is lack of anti-HbSR antibody for tracking the HbSR expression is still not known.
Functions of the HbSR
Role of the HbSR in clearing the Hb-Hp complex: Hemoglobin (Hb) is the most abundant protein which is found in the blood. Abnormalities in the synthesis and metabolism of Hb corresponds to the occurrence of many diseases and disorders. Haptoglobin (Hp) is an acute-phase protein whose plasma concentration increases or decreases in response to inflammation and is synthesized by the liver (Ijaz Petra ,2017). It protects tissue from Hb-induced oxidative damage by inhibiting the release of heme-iron and thus preserving the integrity of Hb (Koch et, al.,2002). Hp is composed of three ? and ? subunits which are connected to each other by disulfide bonds. Only ?-chain is responsible for recognition of the HbSR and Hb also binds only to this chain. Hp ?-chain needs the minimum of 88 amino acids for Hb binding (Yang et, al.,2017). Due to gene polymorphism, Hp has following three phenotypes: Hp1-1, Hp2-1 , and Hp2-2 (Koch et, al.,2002).
When Hb is released into plasma due to hemolysis, its tetrameric structure and dissociates into ??-Hb dimer and Hp capture it due to irreversible interactions (Anderson et, al.,2012). Hp-Hb complex undergoes internalization by the HbSR through endocytosis and after endocytosis; in lysosome, the protein is degraded by the digestive enzymes and heme group is reduced by heme oxygenase-1 to produce anti-inflammatory metabolites such as Fe2+, CO ,and biliverdin. Biliverdin is then reduced to bilirubin in the presence Biliverdin reductase and thus secreted to cell exterior (Fig 2) (Etzerodt et, al.,2013).
HbSR as a clinical biomarker: Soluble form of the HbSR (sHbSR) is used as a biomarker for the detection of many diseases such as atherosclerosis, lysosomal Gaucher storage disease, cancer, hepatitis, sepsis, inflammation, leishmaniasis etc. Due to intravascular hemolysis and tissue damage, patients are exposed to a large amount of free heme. HbSR removes hemoglobin from the site of inflammation with higher efficiency.
ELISA kit produced by IQ product is highly sensitive for diagnosis of sHbSR and other methods such as flow cytometry, immunoprecipitation and western blot are also used.
HbSR as a sensor for bacteria: The HbSR is used as a sensor for bacteria and recognizes gram-positive and gram-negative bacteria. Cytokines such as TNF-? are produced when bacteria bind to the HbSR. A study reports that the HbSR act as a sensor for Streptococcus mutants, Escherichia coli, and Staphylococcus aureus. Another study showed binding of HbSR to Staphylococcus aureus through fibronectin peptides induces recognition, phagocytosis, and killing of bacteria (Fabriek et, al.,2009)
Some viruses such as Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) and African Swine Fever Virus (ASFV) also bind to the HbSR. The main difference between these viruses is that in PRRSV the HbSR is used during virus uncoating while ASFV uses it for attachment and internalization process.
Fig 2: Diagrammatic representation of scavenger action of HbSR (Etzerodt et, al.,2013).
Conclusion: The present review summarizes the structure and functions of the HbSR. Besides being a scavenger receptor for removing Hp-Hb complex, it is also used as a sensor for bacteria and viruses. The sHbSR is also used as a biomarker for diagnosis of many acute as well as chronic diseases. Many studies reported the presence of high amount of sHbSR in case of inflammatory disorders. In the future soluble form of the HbSR may be used for diagnosis as well as monitoring of the diseases. It is now possible to direct the drug delivery into the cytosol of the macrophage. A report showed that the technology of directing the drug delivery into the cytosol of the macrophage is having high potency for the development of conjugate glucocorticoids with low side effects.