Green Plasma, Metals
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Hephaestin, first identified in 1999 is homologous with ceruloplasmin

Hephaestin, also known as HEPH, is a protein which in humans is encoded by the HEPH gene.

Function

Hephaestin is involved in the metabolism and homeostasis of iron and possibly copper.

It is a transmembrane copper-dependent ferroxidase responsible for transporting dietary iron from intestinal enterocytes into the circulatory system. The highest expression of hephaestin is found in small intestine. It is limited to enterocytes of the villi (where the iron absorption takes place), being almost absent in crypt cells. Hephaestin converts iron(II) state, Fe2+, to iron(III) state, Fe3+, and mediates iron efflux most likely in cooperation with the basolateral iron transporter, ferroportin 1. To a lesser extent hephaestin has been detected in colon, spleen, kidney, breast, placenta and bone trabecular cells but its role in these tissues remains to be established. Hephaestin presents homology with ceruloplasmin, a serum dehydrogenase protein involved in copper detoxification and storage.

Hephaestin is a protein of 1135 aminoacids formed from a precursor of 1158 aminoacids and is 130.4 kDa. It is predicted to bind 6 copper ions per monomer.

Discovery

Hephaestin was first identified by Dr. Christopher D. Vulpe of the University of California, Berkeley in 1999.

  • Vulpe CD, Kuo YM, Murphy TL, Cowley L, Askwith C, Libina N, Gitschier J, Anderson GJ (February 1999). “Hephaestin, a ceruloplasmin homologue implicated in intestinal iron transport, is defective in the sla mouse”. Nat. Genet. 21 (2): 195–9. doi:10.1038/5979. PMID 9988272. S2CID 25530044.

They named the newfound protein after Hephaestus, the Greek god of metal working.

Much of what is known about hephaestin comes from studying heritable mutants of murine iron metabolism. The protein was discovered and identified through the study of mice with sex-linked anemia, or sla mice, in which there is normal mucosal uptake of dietary iron but impaired transport of iron from the intestinal enterocytes into the circulation. sla mice harbor a partial deletion mutation of the HEPH gene, resulting in the expression of a hephaestin protein that is truncated by 194 amino acids. Studies suggest that this truncated hephaestin protein still retains a minimal, yet detectable and quantifiable level of ferroxidase activity.

This raises the possibility that alternative factors may contribute to the decreased efflux of iron seen in the sla phenotype.

In addition to the truncation of the original protein, the iron-deficient sla phenotype may also be explained by the intracellular mislocalization of hephaestin. Wild type hephaestin localizes in a supra nuclear compartment as well as the basolateral surface.

In contrast, sla hephaestin seems to localize only in the supranucelar compartment but is largely undetectable in the latter.

Given hephaestin’s established function in facilitating basolateral iron export, this mislocalization may explain the paradoxical intestinal iron accumulation and systemic iron deficiency observed in sla mice.

Human hephaestin, lacking the putative transmembrane domain, was first recombinantly expressed in 2005 by Drs. Tanya Griffiths, Grant Mauk, and Ross MacGillivray at the University of British Columbia.

  • Griffiths TA, Mauk AG, MacGillivray RT (November 2005). “Recombinant expression and functional characterization of human hephaestin: a multicopper oxidase with ferroxidase activity”. Biochemistry. 44 (45): 14725–31. doi:10.1021/bi051559k. hdl:2429/18540. PMID 16274220.

They demonstrated that recombinant human hephaestin (rhHp) bound copper (determined by inductively coupled plasma mass spectrometry) and exhibited an absorption maximum at ~610 nm consistent with other blue multicopper oxidases such as ceruloplasmin. By using ferrous ammonium sulfate as a substrate, rhHp was shown to have ferroxidase activity with a Km of 2.1 μM for Fe(II).

A Few Words About The Word ‘Recombinant’

Recombination – the process of recombining things.

“the fragmentation of the earlier large units and recombination of land under new ownership”

GENETICS – the rearrangement of genetic material, especially by crossing over in chromosomes or by the artificial joining of segments of DNA from different organisms.

Use over time
  • Google Search, Definitions from Oxford Languages 

What is recombination and example?

Recombination occurs when two molecules of DNA exchange pieces of their genetic material with each other. One of the most notable examples of recombination takes place during meiosis (specifically, during prophase I), when homologous chromosomes line up in pairs and swap segments of DNA….Beyond its role in meiosis, recombination is important to somatic cells in eukaryotes because it can be used to help repair broken DNA, even when the break involves both strands of the double helix. These breaks are known as double-stranded breaks, or DSBs…Recombination isn’t limited to eukaryotes. A special type of recombination called conjugation occurs in many prokaryotes, and it has been particularly well studied and characterized in E. coli bacteria. During conjugation, genetic material from one bacterium is transferred to another bacterium, and it is then recombined in the recipient cell. 

And a few words about the word ‘conjugation‘

GRAMMAR – the variation of the form of a verb in an inflected language such as Latin, by which are identified the voice, mood, tense, number, and person.

“it was the conjugation of verbs he found most difficult”

the class in which a verb is put according to the manner of this variation.plural noun: conjugations“a past participle of the first conjugation”

BIOLOGY – the temporary union of two bacteria or unicellularorganisms for the exchange of genetic material.”immunity may be transferable by conjugation to other bacterial strains”

BIOLOGY – the fusion of two gametes, especially when they are of a similar size.

BIOCHEMISTRY – the combination of two substances.”toxic compounds eliminated from the body by conjugation with glutathione”

CHEMISTRY – the sharing of electron density between nearby multiple bonds in a molecule.

MATHEMATICS – the solution of a problem by transforming it into an equivalent problem of a different form, solving this, and then reversing the transformation.

At least four types of naturally occurring recombination have been identified in living organisms: (1) General or homologous recombination, (2) Illegitimate or nonhomologous recombination, (3) Site-specific recombination, and (4) replicative recombination.Jun 20, 2023

Recombination may refer to:

Structure

Hephaestin is a member of the family of copper oxidases that includes mammalian ceruloplasmin, yeast fet3 and fet5, and bacterial ascorbate oxidase, among others. While hephaestin shares 50% amino acid sequence identity with its serum homologue ceruloplasmin, the hephaestin protein includes an additional 86 amino acids at the C-terminus, which code for a single transmembrane domain and a short cytoplasmic tail.

Note: Fet3p is a multicopper oxidase (MCO)2 found in Saccharomyces cerevisiae with a structure consisting of three cupredoxin-like β-barrel domains and four copper ions located in three distinct metal sites (T1 in domain 3, T2, and the binuclear T3 at the interface between domains 1 and 3).[Sedlak, E.; Ziegler, L.; Kosman, D. J.; Wittung-Stafshede, P. (25 November 2008). “In vitro unfolding of yeast multicopper oxidase Fet3p variants reveals unique role of each metal site”. Proceedings of the National Academy of Sciences. 105 (49): 19258–19263. Bibcode:2008PNAS..10519258S. doi:10.1073/pnas.0806431105. PMC 2614749. PMID 19033465][Singh, A. (1 March 2006). “Assembly, Activation, and Trafficking of the Fet3p·Ftr1p High Affinity Iron Permease Complex in Saccharomyces cerevisiae”. Journal of Biological Chemistry. 281 (19): 13355–13364. doi:10.1074/jbc.M512042200. PMID 16522632] Fet3p is a type I membrane protein with an orientation that places the amino-terminal oxidase domain in the exocellular space (Nexo) and the carboxyl terminus in the cytoplasm (Ccyt). Part of the ferroxidase reaction, Fet3p catalyzes the oxidation of Fe(II) to Fe(III) using O2 as substrate. The Fe(III) generated by Fet3p is a ligand for the iron permease, Ftr1p.

Note: The yeast FET3 gene encodes an integral membrane multicopper oxidase required for high-affinity iron uptake. The FET4 gene encodes an Fe(II) transporter required for low-affinity uptake. To identify other yeast genes involved in iron uptake, we isolated genes that could, when overexpressed, suppress the iron-limited growth defect of a fet3 fet4 mutant. The FET5 gene was isolated in this screen and it encodes a multi-copper oxidase closely related to Fet3p. Several observations indicate that Fet5p plays a role analogous to Fet3p in iron transport. Suppression of the fet3 fet4 mutant phenotype by FET5 overexpression required the putative FTR1 transporter subunit of the high-affinity system. Fet5p is an integral membrane protein whose oxidase domain is located on the cell surface or within an intracellular compartment. Oxidase activity measured in cells with altered levels of FET5 expression suggested that Fet5p is a functional oxidase. FET5 overexpression increased the rate of iron uptake by a novel uptake system. Finally, FET5 mRNA levels are regulated by iron and are increased in cells grown in iron-limited media. These results suggest that Fet5p normally plays a role in the transport of iron. [Abstract from Spizzo T, Byersdorfer C, Duesterhoeft S, Eide D. The yeast FET5 gene encodes a FET3-related multicopper oxidase implicated in iron transport. Mol Gen Genet. 1997 Nov;256(5):547-56. doi: 10.1007/pl00008615. PMID: 9413439.]

Note: In enzymology, a L-ascorbate oxidase (EC 1.10.3.3) is an enzyme that catalyzes the chemical reaction2 L-ascorbate + O2 ⇌\rightleftharpoons  2 dehydroascorbate + 2 H2O Thus, the two substrates of this enzyme are L-ascorbate and O2, whereas its two products are dehydroascorbate and H2O.[Mondovì B, Avigliano L (February 1984). “Ascorbate oxidase.”. In Lontie R (ed.). Copper Proteins and Copper Enzymes. Boca Raton: CRC Press. pp. 101–118. doi:10.1201/9781351070898. ISBN 978-1-351-07089-8.] This enzyme belongs to the family of oxidoreductases, specifically those acting on diphenols and related substances as donor with oxygen as acceptor. This enzyme participates in ascorbate metabolism. It employs one cofactor, copper. The systematic name of this enzyme class is L-ascorbate:oxygen oxidoreductase. Other names in common use include ascorbase, ascorbic acid oxidase, ascorbate oxidase, ascorbic oxidase, ascorbate dehydrogenase, L-ascorbic acid oxidase, AAO, L-ascorbate:O2 oxidoreductase, and AA oxidase. [Boyer PD, Lardy H, Myrback K, eds. (1963). The Enzymes. Vol. 8 (2nd ed.). New York: Academic Press. pp. 297–311.]

While the structure and kinetic activity of ceruloplasmin have been studied extensively, hephaestin has yet to be investigated at a similar level.

Comparative models of hephaestin’s structure have been created using established crystallographic data from ceruloplasmin, and these studies suggest that many of the structural features important in the enzymatic function of the latter are also conserved in the former. In particular, these shared features include cysteine residues involved in disulfide bond formation, histidine residues involved in copper binding, and residues involved in the binding of the iron substrate.

Proposed regulation of expression of hephaestin in response to varying iron uptake and stores.[16] Hephaestin is thought to be both structurally modified and mislocalized in sla mice.[9][11]

From the photo:

Proposed regulation of expression of hephaestin in response to varying iron uptake and stores.

Hephaestin is thought to be both structurally modified and mislocalized in sla mice.

Regulation

The regulation of hephaestin expression and the protein’s role in the larger picture of iron metabolism and homeostasis remain an active area of research. Some studies suggest mechanisms for local and systemic control of intestinal iron transport, in which high dietary iron intake and sufficient iron stores lead to down-regulation of DMT1, ferroportin (Ireg1) and hephaestin protein, thus minimizing iron absorption from the enterocytes into the circulation. Conversely, it is suggested that states of low dietary intake and low iron stores induce up-regulation of DMT1 as well as Ireg1 and hephaestin, thus simultaneously increasing the enterocyte’s capacity for dietary iron uptake on the basolateral surface and export into the circulation on the apical surface.

Note: Natural resistance-associated macrophage protein 2 (NRAMP 2), aka divalent metal transporter 1 (DMT1) and divalent cation transporter 1 (DCT1),[“Solute carrier family 11 (proton-coupled divalent metal ion transporters), member 2”. GeneCards. Retrieved 2011-12-16.] is a protein that in humans is encoded by the SLC11A2 (solute carrier family 11, member 2) gene.[Vidal S, Belouchi AM, Cellier M, Beatty B, Gros P (April 1995). “Cloning and characterization of a second human NRAMP gene on chromosome 12q13”. Mammalian Genome. 6 (4): 224–30. doi:10.1007/BF00352405. PMID 7613023. S2CID 22656880] DMT1 represents a large family of orthologous metal ion transporter proteins that are highly conserved from bacteria to humans.[ Au C, Benedetto A, Aschner M (July 2008). “Manganese transport in eukaryotes: the role of DMT1”. Neurotoxicology. 29 (4): 569–76. doi:10.1016/j.neuro.2008.04.022. PMC 2501114. PMID 18565586.] As its name suggests, DMT1 binds a variety of divalent metals including cadmium (Cd2+), copper (Cu2+), and zinc (Zn2+,); however, it is best known for its role in transporting ferrous iron (Fe2+). DMT1 expression is regulated by body iron stores to maintain iron homeostasis. DMT1 is also important in the absorption and transport of manganese (Mn2+).[Gunshin H, Mackenzie B, Berger UV, Gunshin Y, Romero MF, Boron WF, Nussberger S, Gollan JL, Hediger MA (July 1997). “Cloning and characterization of a mammalian proton-coupled metal-ion transporter”. Nature. 388 (6641): 482–8. Bibcode:1997Natur.388..482G. doi:10.1038/41343. PMID 9242408. S2CID 4416482.] In the digestive tract, it is located on the apical membrane of enterocytes, where it carries out H+-coupled transport of divalent metal cations from the intestinal lumen into the cell…Ferrous-oxidase mediated transport systems exist in order to transport specific ions opposed to DMT1, which does not have complete specificity.[Bertini I (2007). Biological inorganic chemistry : structure and reactivity. Sausalito, Calif.: University Science Books. ISBN 978-1891389436. OCLC 65400780.] The Fet3/FTR1 iron uptake pathway is able to achieve complete specificity for iron over other ions due to the multi-step nature of the pathway.[Bertini I (2007). Biological inorganic chemistry : structure and reactivity. Sausalito, Calif.: University Science Books. ISBN 978-1891389436. OCLC 65400780.] Each of the steps involved in the pathway is specific to either ferrous iron or ferric iron.[Bertini I (2007). Biological inorganic chemistry : structure and reactivity. Sausalito, Calif.: University Science Books. ISBN 978-1891389436. OCLC 65400780.] The DMT1 transporter protein does not have specificity over the ions it transports because it is unable to distinguish between Fe2+ and the other divalent metal ions it transfer through the cell membrane.[Bertini I (2007). Biological inorganic chemistry : structure and reactivity. Sausalito, Calif.: University Science Books. ISBN 978-1891389436. OCLC 65400780.] Although, the reason that non-specific ion transporters, such as DMT1, exist is due to their ability to function in anaerobic environments opposed to the Fet3/FTR1 pathway which requires oxygen as a co substrate.[Bertini I (2007). Biological inorganic chemistry : structure and reactivity. Sausalito, Calif.: University Science Books. ISBN 978-1891389436. OCLC 65400780.] So in anaerobic environments the oxidase would not be able to function thus another means of iron uptake is necessary.[Bertini I (2007). Biological inorganic chemistry : structure and reactivity. Sausalito, Calif.: University Science Books. ISBN 978-1891389436. OCLC 65400780.] Toxic accumulation of divalent metals, especially iron and/or manganese, are frequently discussed aetiological factors in a variety of neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, and multiple sclerosis. DMT1 may be the major transporter of manganese across the blood brain barrier and expression of this protein in the nasal epithelium provides a route for direct absorption of metals into the brain.[Aschner M (May 2006). “The transport of manganese across the blood-brain barrier”. Neurotoxicology. 27 (3): 311–4. doi:10.1016/j.neuro.2005.09.002. PMID 16460806] DMT1 expression in the brain may increase with age,[Ke Y, Chang YZ, Duan XL, Du JR, Zhu L, Wang K, Yang XD, Ho KP, Qian ZM (May 2005). “Age-dependent and iron-independent expression of two mRNA isoforms of divalent metal transporter 1 in rat brain”. Neurobiology of Aging. 26 (5): 739–48. doi:10.1016/j.neurobiolaging.2004.06.002. hdl:10397/15266. PMID 15708449. S2CID 21925120.] increasing susceptibility to metal induced pathologies. DMT1 expression is found to be increased in the substantia nigra of Parkinson’s patients and in the ventral mesencephalon of animal models intoxicated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) – a neurotoxin widely used experimentally to produce Parkinsonian symptoms.

Note: Ferroportin-1, also known as solute carrier family 40 member 1 (SLC40A1) or iron-regulated transporter 1 (IREG1), is a protein that in humans is encoded by the SLC40A1 gene, and is part of the Ferroportin (Fpn) Family (TC# 2.A.100).[Donovan A, Brownlie A, Zhou Y, Shepard J, Pratt SJ, Moynihan J, et al. (February 2000). “Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter”. Nature. 403 (6771): 776–781. Bibcode:2000Natur.403..776D. doi:10.1038/35001596. PMID 10693807. S2CID 4429026] Ferroportin is a transmembrane protein that transports iron from the inside of a cell to the outside of the cell. Ferroportin is the only known iron exporter.[Ward DM, Kaplan J (September 2012). “Ferroportin-mediated iron transport: expression and regulation”. Biochimica et Biophysica Acta (BBA) – Molecular Cell Research. 1823 (9): 1426–1433. doi:10.1016/j.bbamcr.2012.03.004. PMC 3718258. PMID 22440327] After dietary iron is absorbed into the cells of the small intestine, ferroportin allows that iron to be transported out of those cells and into the bloodstream. Fpn also mediates the efflux of iron recycled from macrophages resident in the spleen and liver.[Canonne-Hergaux F, Donovan A, Delaby C, Wang HJ, Gros P (January 2006). “Comparative studies of duodenal and macrophage ferroportin proteins”. American Journal of Physiology. Gastrointestinal and Liver Physiology. 290 (1): G156–G163. doi:10.1152/ajpgi.00227.2005. PMID 16081760] Ferroportin is regulated by hepcidin, a hormone produced by the liver; hepcidin binds to Fpn and limits its iron-efflux activity, thereby reducing iron delivery to the blood plasma.[Nemeth E, Tuttle MS, Powelson J, Vaughn MB, Donovan A, Ward DM, et al. (December 2004). “Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization”. Science. 306 (5704): 2090–2093. Bibcode:2004Sci…306.2090N. doi:10.1126/science.1104742. PMID 15514116. S2CID 24035970.] Therefore, the interaction between Fpn and hepcidin controls systemic iron homeostasis. In addition to iron, ferroportin has been shown to transport cobalt & zinc,[ Mitchell CJ, Shawki A, Ganz T, Nemeth E, Mackenzie B (March 2014). “Functional properties of human ferroportin, a cellular iron exporter reactive also with cobalt and zinc”. American Journal of Physiology. Cell Physiology. 306 (5): C450–C459. doi:10.1152/ajpcell.00348.2013. PMC4042619. PMID24304836] as well as nickel.[Deshpande CN, Ruwe TA, Shawki A, Xin V, Vieth KR, Valore EV, et al. (August 2018). “Calcium is an essential cofactor for metal efflux by the ferroportin transporter family”. Nature Communications. 9 (1): 3075. Bibcode:2018NatCo…9.3075D. doi:10.1038/s41467-018-05446-4. PMC 6079014. PMID 30082682] Ferroportin may also function as a manganese exporter.[Madejczyk MS, Ballatori N (March 2012). “The iron transporter ferroportin can also function as a manganese exporter”. Biochimica et Biophysica Acta (BBA) – Biomembranes. 1818 (3): 651–657.] Ferroportin is found on the basolateral membranes of intestinal epithelia of mammals, [Donovan A, Lima CA, Pinkus JL, Pinkus GS, Zon LI, Robine S, Andrews NC (March 2005). “The iron exporter ferroportin/Slc40a1 is essential for iron homeostasis”. Cell Metabolism. 1 (3): 191–200. doi:10.1016/j.cmet.2005.01.003. PMID 16054062.][Delaby C, Pilard N, Puy H, Canonne-Hergaux F (April 2008). “Sequential regulation of ferroportin expression after erythrophagocytosis in murine macrophages: early mRNA induction by haem, followed by iron-dependent protein expression” (PDF). The Biochemical Journal. 411 (1): 123–131. doi:10.1042/BJ20071474. PMID 18072938.] including: Enterocytes in the duodenum, Hepatocytes, Macrophages of the reticuloendothelial system and Adipocytes. Ferroportin-1 plays an important role in neural tube closure and forebrain patterning.[Mao J, McKean DM, Warrier S, Corbin JG, Niswander L, Zohn IE (September 2010). “The iron exporter ferroportin 1 is essential for development of the mouse embryo, forebrain patterning and neural tube closure”. Development. 137 (18): 3079–3088. doi:10.1242/dev.048744. PMC 2926957. PMID 20702562.] Mouse embryos lacking the Slc40a1 gene are aborted before gastrulation occurs, suggesting that the Fpn1 protein encoded is necessary and essential for normal embryonic development.[Donovan A, Lima CA, Pinkus JL, Pinkus GS, Zon LI, Robine S, Andrews NC (March 2005). “The iron exporter ferroportin/Slc40a1 is essential for iron homeostasis”. Cell Metabolism. 1 (3): 191–200. doi:10.1016/j.cmet.2005.01.003. PMID 16054062.] Fpn1 is expressed in the syncytiotrophoblast cells in the placenta and visceral endoderm of mice at E7.5.[Donovan A, Brownlie A, Zhou Y, Shepard J, Pratt SJ, Moynihan J, et al. (February 2000). “Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter”. Nature. 403 (6771): 776–781. Bibcode:2000Natur.403..776D. doi:10.1038/35001596. PMID 10693807. S2CID 4429026][Donovan A, Lima CA, Pinkus JL, Pinkus GS, Zon LI, Robine S, Andrews NC (March 2005). “The iron exporter ferroportin/Slc40a1 is essential for iron homeostasis”. Cell Metabolism. 1 (3): 191–200. doi:10.1016/j.cmet.2005.01.003. PMID 16054062.] Further, several retrospective studies have noted an increased incidence of spina bifida occurring after low maternal intake of iron during embryonic and fetal development.[Felkner MM, Suarez L, Brender J, Scaife B, Hendricks K (December 2005). “Iron status indicators in women with prior neural tube defect-affected pregnancies”. Maternal and Child Health Journal. 9 (4): 421–428. doi:10.1007/s10995-005-0017-3. PMID 16315101. S2CID 13415844][Groenen PM, van Rooij IA, Peer PG, Ocké MC, Zielhuis GA, Steegers-Theunissen RP (June 2004). “Low maternal dietary intakes of iron, magnesium, and niacin are associated with spina bifida in the offspring”. The Journal of Nutrition. 134 (6): 1516–1522. doi:10.1093/jn/134.6.1516. PMID 15173422.] A study examining the consequences of several different mutations of the Slc40a1 mouse gene suggested that several serious neural tube and patterning defects were produced as a result, including spina bifida, exencephaly, and forebrain truncations, among others.[Mao J, McKean DM, Warrier S, Corbin JG, Niswander L, Zohn IE (September 2010). “The iron exporter ferroportin 1 is essential for development of the mouse embryo, forebrain patterning and neural tube closure”. Development. 137 (18): 3079–3088. doi:10.1242/dev.048744. PMC 2926957. PMID 20702562.] Given the findings of studies to date, there appears to be significant evidence that intact iron transport mechanisms are critical to normal neural tube closure. Furthermore, other experiments have suggested that Fpn1 product and activity is required along the entire anterior-posterior axis of the animal to ensure proper closure of the neural tube.[Mao J, McKean DM, Warrier S, Corbin JG, Niswander L, Zohn IE (September 2010). “The iron exporter ferroportin 1 is essential for development of the mouse embryo, forebrain patterning and neural tube closure”. Development. 137 (18): 3079–3088. doi:10.1242/dev.048744. PMC 2926957. PMID 20702562.] It is known that ferroportin (SLC40A1) gene is expressed at a low level in infertile women. Its mRNA levels were discovered to be down-regulated in these women, specifically in granulosa cells. What’s more, low expression of ferroportin is also associated with infertility when some features like age and smoking habits are considered. It is also important to mention that, not only is ferroportin down-regulated in granulosa cells, but also in cervical cells of infertile women, and that the association between infertility and low ferroportin levels in these cells can be seen, again, when mRNA ferroportin levels was adjusted by age and smoking status.[Moreno-Navarrete JM, López-Navarro E, Candenas L, Pinto F, Ortega FJ, Sabater-Masdeu M, et al.Ferroportin mRNA is down-regulated in granulosa and cervical cells from infertile women.Fertil Steril. 2017 Jan;107(1):236-242.] Ferroportin is inhibited by hepcidin, which binds to ferroportin and internalizes it within the cell.[Nemeth E, Tuttle MS, Powelson J, Vaughn MB, Donovan A, Ward DM, et al. (December 2004). “Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization”. Science. 306 (5704): 2090–2093. Bibcode:2004Sci…306.2090N. doi:10.1126/science.1104742. PMID 15514116. S2CID 24035970] This results in the retention of iron within enterocytes, hepatocytes, and macrophages with a consequent reduction in iron levels within the blood serum. This is especially significant with enterocytes which, when shed at the end of their lifespan, results in significant iron loss. Hepcidin is synthesized in response to various cytokines, as described in the Hepcidin article, as well as in this article by Ganz.[Ganz T (April 2011). “Hepcidin and iron regulation, 10 years later”. Blood. 117 (17): 4425–4433. doi:10.1182/blood-2011-01-258467. PMC 3099567. PMID 21346250.] Ferroportin expression is also regulated by the IRP regulatory mechanism. If the iron concentration is too low, the IRP concentration increases, thus inhibiting the ferroportin translation and increasing intracellular iron and ferritin concentrations. The ferroportin translation is also down regulated post-transcriptionally by the micro RNA miR-485-3p, which is produced in response to iron deficiency.[Sangokoya C, Doss JF, Chi JT (April 2013). “Iron-responsive miR-485-3p regulates cellular iron homeostasis by targeting ferroportin”. PLOS Genetics. 9 (4): e1003408. doi:10.1371/journal.pgen.1003408. PMC 3616902. PMID 23593016.] Mutations in the ferroportin gene are known to cause an autosomal dominant form of iron overload known as type IV haemochromatosis or Ferroportin Disease. The effects of the mutations are generally not severe but a spectrum of clinical outcomes are seen with different mutations. Ferroportin is also associated with African iron overload. Ferroportin and hepcidin are critical proteins for the regulation of systemic iron homeostasis.

Relevance in biology and disease

Hephaestin has not yet been linked to a human disease. However, when the protein was ablated in murine models, both intestine-specific and whole-body hephaestin knockout (KO) strains exhibited similarly severe accumulation of iron in the duodenal enterocytes and suffered from microcytic, hypochromic anemia, indicative of systemic iron deficiency. The shared phenotype between the two strains suggests that intestinal hephaestin plays an important role in maintaining whole-body iron homeostasis. However, since both strains were viable, it is likely that hephaestin is not essential and other compensatory mechanisms exist to keep these mice alive.

In addition to the transport of iron from the intestine and into the circulation, ferroxidases also seem to play an important role in facilitating iron export from retinal cells. While deficiency in hephaestin or ceruloplasmin alone do not seem to cause iron buildup in the retina, studies done on murine models suggest that the combined deficiency is sufficient to cause age-dependent retinal pigment epithelium and retinal iron accumulation, with features consistent with macular degeneration.

Hephaestin has been detected in mouse and human RPE (retinal pigment epithelial) cells as well as in rMC-1 cells (a rat Müller glial cell line), with greatest expression in the Müller endnote next to the internal limiting membrane.

See also

References

  1. GRCh38: Ensembl release 89: ENSG00000089472 – Ensembl, May 2017
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Metabolism: Metal metabolism

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