Fetuins are hepatic serum proteins - they are are made in the liver and circulate in the blood.
Human alpha2-HS-glycoprotein (genetic symbol AHSG) ist synonymous for a2-HS, A2HS, AHS, HSGA and Fetuin-A. The name »Fetuin« alludes to the fact that the highest serum concentrations are attained during fetal life. Fetuin-A is a single copy gene located on chromosome 3 in the human genome and chromosome 16 in the mouse genome. Immediate neighbours of the gene for Fetuin-A »AHSG/FETUA« in the genome are the genes for Fetuin-B »FETUB«, Histidine-rich »HRG« and Kininogen »KNG«. Collectively these genes form a sub-family of secreted Cystatin-domain containing proteins within the Cystatin superfamily.
Fetuins are glycoproteins with about 350 amino acids and - depending on the animal species studied - 2-3 N-glycosyl and 2-3 O-glycosyl carbohydrate side chains (green symbols). The apparent molecular weight on reducing SDS gel electrophoresis is 55 kDa. Fetuin are tightly folded globular proteins with three distinct folding domains termed D1-D3 in the picture above. Two amino-terminal domains are cystatin-like hence the relationship to the Cystatin superfamily of proteins. These domains are stbilized by disulfide bridges between cystein residues (yellow lines). Fetuin-A has binding domains for apatite, a form of calcium phosphate, and for transforming growth factor beta TGF-ß. The carbohydrate side chains bind to lectins. The picture shows the structure of human Fetuin-A, which has 349 amino acids, two N-linked carbohydrate side chains, three O-linked carbohydrate side chains and three Serine phosphorylation sites (red asteriscs).
Fetuin-B, fetuin-A/alpha-2-Heremans Schmid-glycoprotein (AHSG), kininogen (KNG) and histidine-rich glycoprotein (HRG) are members of the cystatin superfamily of cysteine proteinase inhibitors. These so-called type 3 members are blood plasma proteins constitutively synthesized and secreted by the liver. The proteins encoded by one gene each and have a characteristic and conserved structure: they are glycoproteins with at least two aminoterminal cystatin-like domains and disulfide bridges in distinct distances (Fig. 1).
To study the role of fetuin-B in vivo, fetuin-B deficient (Fetub-/-) mice were generated. In 2013 we could show that absent fetuin-B leads to premature zona pellucida hardening and thus infertility (Dietzel et al., 2013). Fetuin-B is an inhibitor of the proteinase ovastacin rendering the zona pellucida, a layer of extracellular matrix surrounding the oocytes, in a hardened state due to proteolytic cleavage of zona pellucida glycoproteins. Fetuin-B as a potent inhibitor of ovastacin prevents premature zona pellucida hardening before fertilization (Fig. 2). Thus fetuin-B keeps the zona pellucida penetrable for sperm until fertilization and maintains female fertility. After fertilization the proteinase ovastacin mediates definitive zona pellucida hardening. It prevents further sperm attachment and penetration, and protects the pre-implantation embryo.
Premature zona pellucida hardening is a common complication in assisted reproduction and leads to a decreased fertilization rate. To examine the effect of fetuin-B during in vitro fertilization (IVF), we produced recombinant fetuin-B and could show that the fertilization rate of mouse oocytes increased by fetuin-B supplementation (Dietzel et al., 2017).
Our research in mice and the gene sequence homology between human and mice (61%) indicated that fetuin-B could also play an important role in idiopathic (unexplained) infertility in women. Infertility is a disease with various causes and leads in about 15 % of couples to an unsatisfied wish for children. First investigations here demonstrate that serum fetuin-B is associated with fertility. In the run-up to successful IVFs serum fetuin-B was increased but remained constant in unsuccessful attempts (Floehr et al., 2017). Thus we propose that serum fetuin-B may be useful in predicting the fertilization rate in IVF and help to make an informed decision whether oocytes should be fertilized by IVF or intracytoplasmatic sperm injection (ICSI) to overcome the zona pellucida as a barrier.
Just as important as the knowledge about the exact mechanisms of fertilization is the development of contraceptive strategies in human reproductive biology. By generating a genetic manipulated fetuin-B deficient mouse with additional ovastacin deficiency (Fetub-/-, Astl-/-) we could confirm the proteinase ovastacin as a prime molecular target for fetuin-B (Floehr et al., 2017). The recovery of fertility in Fetub-/-, Astl-/- females (Fig. 3) underscored the decisive role of fetuin-B in fertilization, rendering fetuin-B a potential target for contraception.
Floehr J, Dietzel E, Schmitz C, Chappell A, Jahnen-Dechent W. Down-regulation of the liver-derived plasma protein fetuin-B mediates reversible female infertility. Mol Hum Reprod. 2017;23:34-44.
Dietzel E, Floehr J, Van de Leur E, Weiskirchen R, Jahnen-Dechent W. Recombinant fetuin-B protein maintains high fertilization rate in cumulus cell-free mouse oocytes. Mol Hum Reprod. 2017;23:25-33.
Dietzel E, Floehr J, Jahnen-Dechent W. The Biological Role of Fetuin-B in Female Reproduction. Annals of Reproductive Medicine and Treatment. 2016;1:1003.
Floehr J, Dietzel E, Neulen J, Rösing B, Weissenborn U, Jahnen-Dechent W. Association of high fetuin-B concentrations in serum with fertilization rate in IVF: a cross-sectional pilot study. Hum Reprod. 2016;31:630-637
Stöcker W, Karmilin K, Hildebrand A, Westphal H, Yiallouros I, Weiskirchen R, Dietzel E, Floehr J, Jahnen-Dechent W. Mammalian gamete fusion depends on the inhibition of ovastacin by fetuin-B. Biol Chem. 2014;395:1195-1199.
Dietzel E, Wessling J, Floehr J, Schäfer C, Ensslen S, Denecke B, Rösing B, Neulen J, Veitinger T, Spehr M, Tropartz T, Tolba R, Renné T, Egert A, Schorle H, Gottenbusch Y, Hildebrand A, Yiallouros I, Stöcker W, Weiskirchen R, Jahnen-Dechent W. Fetuin-B, a liver-derived plasma protein is essential for fertilization. Dev Cell. 2013;25:106–112.