Reproduction in Domestic Animals

Reprod Dom Anim 43 (Suppl. 2), 186–192 (2008); doi: 10.1111/j.1439-0531.2008.01160.xISSN 0936-6768The Expanding Role of Recombinant Gonadotropins in Assisted ReproductionTE Adams 1 and I Boime 21 Department of Animal Science, University of California, Davis, CA, USA; 2 Department of Molecular Biology and Pharmacology, WashingtonUniversity School of Medicine, St Louis, MO, USAContentsUsing recombinant gonadotropins for assisted reproduction ofdomestic species is still in its infancy. Yet, the purity, potencyand pathogen-free nature of recombinant gonadotropins makethem attractive alternatives to tissue-derived gonadotropicagents. In this study, the authors summarize the work to dateusing recombinant gonadotropins to enhance the - fertility ofdomestic animals and they discussed their recent studiesexamining the biopotency of single chain analogues of humangonadotropins. In these studies, single chain analogues offollicle stimulating hormone (Fca), chorionic gonadotropin(CGba) or a gonadotropin construct with dual activity(FcCGba) were administered to sheep pre-treated with antiseradirected against GnRH. Ovulation was induced 3 days afteranalogue administration using hCG (1000 IU, iv). AlthoughFca or CGba alone induced only modest oestradiol productionduring the pre-hCG period, serum concentrations of oestradiolwere markedly increased (p < 0.05) 3 days after administrationof FcCGba or the Fca +CGba combination. Finalovarian weight was significantly increased (p < 0.05) inanimals receiving Fca, Fca +CGba or FcCGba. Collectively,these observations demonstrate that the single chain analoguesof the human gonadotropins are active in sheep.IntroductionThe multiple ovulation-embryo transfer (MOET) technologyis one of the most important management toolsthat producers can use to accelerate the pace of geneticimprovement in commercial herds and flocks (Galliet al. 2003; Hasler 2003). Yet, the use of the MOETtechnology in the animal industries in North America isapproaching a plateau (Hasler 2003). One critical factorlimiting the more general acceptance of this technologyis its reliance on hormone-induced superovulation andthe variation and inconsistency inherent in that process(Kanitz et al. 2002). Until recently, the exogenoushormones used to induce the superovulatory responsewere gonadotropins derived from pituitary or placentaltissue. The heterogeneous nature of tissue-derivedgonadotropins and intrinsic difference between animalsin follicular development result in marked animal toanimal variation in the magnitude of the superovulatoryresponse (Monniaux et al. 1983). The purity andpotency of recombinant gonadotropins may reduce thevariation associated with hormone-induced superovulationand, thereby, facilitate more general acceptance ofthis valuable tool for genetic improvement.Non-Recombinant GonadotropinsConventional, tissue-derived gonadotropins used in theanimal industries (Boland et al. 1991; Kanitz et al. 2002)include ovine and porcine follicle stimulating hormone(FSH) and equine and human chorionic gonadotropin(eCG and hCG, respectively). Pituitary and placentalgonadotropins are composed of two subunits, commonlydesignated as the a and b subunits (Pierce andParsons 1981; Hearn and Gomme 2000). The a subunitis shared by all gonadotropins produced by a givenspecies, while the b subunit of each gonadotropin isunique. For example, the dimeric (ab) composition ofhuman LH, FSH and CG consists of a hormone specificb subunit and a common a subunit (Fig. 1). Althoughthe a and b subunits are not covalently linked, theconfiguration of each subunit is stabilized by severalintra-chain disulphide bridges.A feature of the gonadotropins that markedly influencespotency and duration of response is the degree ofglycosylation. Indeed, a significant portion of the massof gonadotropins resides in the oligosaccharide chainsadded during co- and post-translational processing.Pituitary-derived gonadotropins contain three or fourasparagine-associated (N-linked) oligosaccharidegroups. The a subunits of human, bovine and ovinegonadotropins carry oligosaccharide chains attached totwo asparagine residues. Similarly, the b subunits carryone (hLHb) or two (hFSHb) N-linked carbohydratechains. Human chorionic gonadotropin, like FSH, hastwo N-linked chains associated with the b subunit. Inaddition, the b subunit of hCG contains four oligosaccharidechains linked to serine residues (O-linkedchains) located in the carboxy-terminal portion of CGb.The N-linked glycans of LH and FSH are a heterogeneousmix of di- and tri-branched oligosaccharidechains (Green and Baenziger 1988a; b). Heterogeneity isalso evident at the branch termini, with terminalsulphate residues common in LH while sialylatedtermini predominate in FSH (Green and Baenziger1988b). The degree of sialylation also varies acrossspecies. For example, 88% of the oligosaccharide chainsof human FSH end with sialic acid, while only 38% oftermini in ovine FSH are sialylated. In contrast, theN-linked oligosaccharide chains in hCG are much moreuniform and generally carry terminal sialic acid residues(Kessler et al. 1979b). The O-linked glycans of hCG arealso highly sialylated (Kessler et al. 1979a).The carbohydrate portion of the gonadotropinsinfluences the folding, assembly, secretion, clearanceand biological activity of the gonadotropins (Thotakuraand Blithe 1995). The liver plays an active role in theclearance and degradation of the gonadotropins (Fieteet al. 1991). Indeed, hepatocytes contain high concentrationsof a receptor that specifically recognizes thesulphated, but not the sialylated, oligosaccharide chainsof the gonadotropins (Roseman and Baenziger 2000).Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag