# EPO

## Overview
Erythropoietin (EPO) is a gene that encodes the hormone erythropoietin, a critical glycoprotein involved in the regulation of red blood cell production (erythropoiesis). The hormone is primarily produced in the kidneys and, to a lesser extent, in the liver. Erythropoietin functions by binding to its receptor on erythroid progenitor cells in the bone marrow, initiating a cascade of intracellular signaling that promotes the survival, proliferation, and differentiation of these cells into mature red blood cells. The gene's expression is tightly regulated by oxygen levels in the body, with hypoxia (low oxygen conditions) triggering an increase in EPO production to enhance the oxygen-carrying capacity of the blood (Fandrey2004Oxygen-dependent; Jelkmann2007Control). Additionally, erythropoietin has roles beyond erythropoiesis, including protective effects in the nervous system and responses to neuronal injury (Chateauvieux2011Erythropoietin).

## Structure
Erythropoietin (EPO) is a glycoprotein hormone with a molecular structure characterized by a four-helical bundle topology, which is a common structural motif among long-chain helical cytokines. This structure includes long helix lengths, specific interhelical angles, and long crossover loops (Cheetham1998NMR). The primary structure of EPO consists of 165 amino acids, with a notable presence of three N-linked glycosylation sites at asparagine residues 24, 38, and 83, and one O-linked glycosylation site at serine 126 (Hedayati2017Molecular). These glycosylation sites are crucial for the protein's plasma stability and function (Hedayati2017Molecular).

The secondary structure of EPO includes four amphipathic alpha-helical bundles, which are typical of cytokines (Wen1994Erythropoietin). The tertiary structure is further defined by the arrangement of these helices along with two long and one short loop structure, contributing to its functional configuration (Tilbrook1999Erythropoietin). The quaternary structure details are not specified in the provided sources.

EPO's interaction with its receptor involves key residues from the helix D and the AB loop, forming a hydrophobic pocket and a positively charged area crucial for receptor binding (Cheetham1998NMR). The presence of antiparallel β-strands in the long crossover loops is unique to EPO within its cytokine family, highlighting its distinct structural features (Cheetham1998NMR).

## Function
Erythropoietin (EPO) is a glycoprotein hormone encoded by the EPO gene, primarily produced in the kidneys and to a lesser extent in the liver. The mature form of the hormone consists of 165 amino acids and is heavily glycosylated, which is crucial for its stability and function in the bloodstream (Fisher1997Erythropoietin:). EPO plays a pivotal role in erythropoiesis, the process of red blood cell production, by binding to the erythropoietin receptor (EPO-R) on erythroid progenitor cells in the bone marrow. This interaction triggers a cascade of intracellular signaling events, including the activation of JAK2 kinase and other pathways that lead to the survival, proliferation, and maturation of these progenitor cells into functional red blood cells (Fisher1997Erythropoietin:).

The expression of the EPO gene is highly sensitive to oxygen levels in the body. Under hypoxic conditions, or low oxygen availability, the gene's expression is upregulated, mediated by the hypoxia-inducible factor-1 (HIF-1). This regulatory mechanism ensures an increase in EPO production, thereby stimulating the bone marrow to produce more red blood cells to enhance the body's oxygen-carrying capacity (Fandrey2004Oxygen-dependent; Jelkmann2007Control).

Additionally, EPO has non-erythropoietic functions, including neuroprotection and aiding in brain development, which highlights its role in other physiological processes beyond red blood cell production (Fandrey2004Oxygen-dependent).

## Clinical Significance
Mutations in the EPO gene have been linked to various forms of erythrocytosis, a condition characterized by an increased red blood cell mass. One notable mutation involves a single base deletion in exon 2 of the EPO gene, leading to a frameshift that truncates the EPO signal peptide and results in the production of a novel peptide. This mutation, surprisingly, leads to the production of erythropoietin through alternative mRNA transcripts originating from an intron 1 promoter, causing autosomal dominant erythrocytosis (Lappin2019Update; Gangat2021JAK2). Another mutation in the 5′UTR of the EPO gene enhances its interaction with HIF2, leading to increased EPO production and familial erythrocytosis (Gangat2021JAK2).

Alterations in EPO gene expression are also significant in clinical settings. For instance, overexpression of EPO has been observed in renal cell carcinoma (RCC), particularly clear cell RCC, which is associated with paraneoplastic polycythemia. This overexpression is linked to the activation of hypoxia-inducible factors (HIF), particularly HIF-2a, often due to mutations in the von Hippel-Lindau (VHL) gene (Wiesener2007Erythropoietin).

These genetic and expression alterations in the EPO gene underscore its crucial role in erythropoiesis and highlight the complex regulatory mechanisms that can lead to erythrocytosis when dysregulated.

## Interactions
Erythropoietin (EPO) interacts with the erythropoietin receptor (EPOR) on the surface of erythroid cells, initiating a cascade of intracellular signaling. This interaction involves EPO binding to EPOR homodimers, which triggers conformational changes in the receptor's extracellular domain, leading to the activation of the associated Janus Kinase (JAK)-2 through autophosphorylation (Chateauvieux2011Erythropoietin). Activated JAK2 phosphorylates multiple tyrosine residues on EPOR, facilitating the recruitment of various Src homology-2 (SH2) domain-containing proteins that initiate further signaling pathways, such as the phosphatidylinositol-3 kinase (PI3K)/protein kinase B (AKT) pathway and the RAS/RAF/mitogen-activated protein kinase (MAPK)/MEK/ERK1/2 pathway (Chateauvieux2011Erythropoietin).

Additionally, EPO has been shown to interact with specific S100 proteins, such as S100A2, S100A6, and S100P. These interactions, which are dependent on the Ca2+-loaded forms of the S100 proteins, suggest a conformation-dependent binding that could potentially regulate the activity of EPO, similar to how certain S100 proteins regulate other cytokines (Kazakov2022Erythropoietin).

Despite some studies suggesting potential interactions with the beta-common receptor (βc receptor), there is no direct evidence supporting a direct interaction between EPO and the βc receptor in the presence of EPO (Cheung2018EPO).


## References


[1. (Tilbrook1999Erythropoietin) Peta A. Tilbrook and S. Peter Klinken. Erythropoietin and erythropoietin receptor. Growth Factors, 17(1):25–35, January 1999. URL: http://dx.doi.org/10.3109/08977199909001060, doi:10.3109/08977199909001060. (58 citations) 10.3109/08977199909001060](https://doi.org/10.3109/08977199909001060)

[2. (Lappin2019Update) Terence R. Lappin and Frank S. Lee. Update on mutations in the hif: epo pathway and their role in erythrocytosis. Blood Reviews, 37:100590, September 2019. URL: http://dx.doi.org/10.1016/j.blre.2019.100590, doi:10.1016/j.blre.2019.100590. (71 citations) 10.1016/j.blre.2019.100590](https://doi.org/10.1016/j.blre.2019.100590)

[3. (Kazakov2022Erythropoietin) Alexey S. Kazakov, Evgenia I. Deryusheva, Andrey S. Sokolov, Maria E. Permyakova, Ekaterina A. Litus, Victoria A. Rastrygina, Vladimir N. Uversky, Eugene A. Permyakov, and Sergei E. Permyakov. Erythropoietin interacts with specific s100 proteins. Biomolecules, 12(1):120, January 2022. URL: http://dx.doi.org/10.3390/biom12010120, doi:10.3390/biom12010120. (11 citations) 10.3390/biom12010120](https://doi.org/10.3390/biom12010120)

[4. (Jelkmann2007Control) Wolfgang Jelkmann. Control of Erythropoietin Gene Expression and its Use in Medicine, pages 179–197. Elsevier, 2007. URL: http://dx.doi.org/10.1016/s0076-6879(07)35010-6, doi:10.1016/s0076-6879(07)35010-6. (90 citations) 10.1016/s0076-6879(07)35010-6](https://doi.org/10.1016/s0076-6879(07)35010-6)

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[6. (Wiesener2007Erythropoietin) Michael S. Wiesener, Philine Münchenhagen, Markus Gläser, Bettina A. Sobottka, Karl X. Knaup, Katrin Jozefowski, Jan Steffen Jürgensen, Jan Roigas, Christina Warnecke, Hermann‐Josef Gröne, Patrick H. Maxwell, Carsten Willam, and Kai‐Uwe Eckardt. Erythropoietin gene expression in renal carcinoma is considerably more frequent than paraneoplastic polycythemia. International Journal of Cancer, 121(11):2434–2442, July 2007. URL: http://dx.doi.org/10.1002/ijc.22961, doi:10.1002/ijc.22961. (44 citations) 10.1002/ijc.22961](https://doi.org/10.1002/ijc.22961)

[7. (Gangat2021JAK2) Naseema Gangat, Natasha Szuber, Animesh Pardanani, and Ayalew Tefferi. Jak2 unmutated erythrocytosis: current diagnostic approach and therapeutic views. Leukemia, 35(8):2166–2181, May 2021. URL: http://dx.doi.org/10.1038/s41375-021-01290-6, doi:10.1038/s41375-021-01290-6. (37 citations) 10.1038/s41375-021-01290-6](https://doi.org/10.1038/s41375-021-01290-6)

[8. (Cheetham1998NMR) Janet C. Cheetham, Duncan M. Smith, Kenneth H. Aoki, Janice L. Stevenson, Thomas J. Hoeffel, Rashid S. Syed, Joan Egrie, and Timothy S. Harvey. Nmr structure of human erythropoietin and a comparison with its receptor bound conformation. Nature Structural Biology, 5(10):861–866, October 1998. URL: http://dx.doi.org/10.1038/2302, doi:10.1038/2302. (227 citations) 10.1038/2302](https://doi.org/10.1038/2302)

[9. (Hedayati2017Molecular) Mohammad Hossein Hedayati, Dariush Norouzian, Mahdi Aminian, Shahram Teimourian, Reza Ahangari Cohan, Soroush Sardari, and M. Reza Khorramizadeh. Molecular design, expression and evaluation of pasylated human recombinant erythropoietin with enhanced functional properties. The Protein Journal, 36(1):36–48, February 2017. URL: http://dx.doi.org/10.1007/s10930-017-9699-9, doi:10.1007/s10930-017-9699-9. (34 citations) 10.1007/s10930-017-9699-9](https://doi.org/10.1007/s10930-017-9699-9)

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[12. (Cheung2018EPO) Karen S. Cheung Tung Shing, Sophie E. Broughton, Tracy L. Nero, Kevin Gillinder, Melissa D. Ilsley, Hayley Ramshaw, Angel F. Lopez, Michael D. W. Griffin, Michael W. Parker, Andrew C. Perkins, and Urmi Dhagat. Epo does not promote interaction between the erythropoietin and beta-common receptors. Scientific Reports, August 2018. URL: http://dx.doi.org/10.1038/s41598-018-29865-x, doi:10.1038/s41598-018-29865-x. (29 citations) 10.1038/s41598-018-29865-x](https://doi.org/10.1038/s41598-018-29865-x)

[13. (Fisher1997Erythropoietin:) J. W. Fisher. Erythropoietin: physiologic and pharmacologic aspects. Experimental Biology and Medicine, 216(3):358–369, December 1997. URL: http://dx.doi.org/10.3181/00379727-216-44183, doi:10.3181/00379727-216-44183. (183 citations) 10.3181/00379727-216-44183](https://doi.org/10.3181/00379727-216-44183)