1 Laboratory of Biological Chemistry, University of Ioannina Medical School, Ioannina 45110 and 2 Department of Obstetrics and Gynecology, University of Thessalia Medical School, Larissa 41222, Greece
3 To whom correspondence should be addressed. e-mail: efriligo{at}cc.uoi.gr
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Abstract |
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Key words: albumin/GnRH/GnSAF/LH/Pichia pastoris
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Introduction |
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Several attempts have been made to purify and identify the bioactive GnSAF molecule(s), and five putative GnSAF/IF sequences have been proposed (Tio et al., 1994; Danforth and Cheng, 1995
; Pappa et al., 1999a
; Fowler et al., 2002
). The five sequences have no apparent consensus and refer to proteins ranging in size from 12 to 69 kDa. Among them, only one displays substantial homology to a known portion of the human genome (Pappa et al., 1999a
); this sequence has been identified as a 12.5 kDa protein corresponding to the C-terminal 95peptide of human serum albumin (HSA), raising the exciting possibility that a bioactive C-terminal domain of HSA may participate in the regulation of LH secretion (Pappa et al., 1999a
). In the progress of the present study, we have employed in silico analysis and found out that this 95peptide corresponds to HSA subdomain IIIB (He and Carter, 1992
); therefore, we refer to this sequence with the alternative name subdomain IIIB or DIIIB throughout this manuscript.
To date, none of the five putative GnSAF/IF sequences has been confirmed as pertinent to a genuine GnSAF molecule, and the possibility that other, unrelated co-purifying proteins may contribute to the above findings cannot be ruled out in each case. In general, the relevant types of isolation studies have major drawbacks related to the low concentration of GnSAF in biological fluids, high bioactivity and a background of large numbers and abundance of other proteins. To overcome such problems and provide more clear evidence, we have produced the C-terminal 95peptide of HSA in recombinant form using the heterologous expressionsecretion system of the methylotrophic yeast Pichia pastoris and subjected it to GnSAF bioassay. In parallel experiments, we have analysed the full-length HSA molecule or other domains of HSA, engineered and produced in recombinant form in the same system. Pichia pastoris was selected because it is a low-complexity expressionsecretion system that has also been employed in the past to study domains of HSA (Dockal et al., 1999).
Our data show that, consistent with the proposal of Pappa et al. (1999a), the C-terminal polypeptide of HSA (subdomain IIIB) possesses specific GnSAF bioactivity, that is not reproducible with larger domains of HSA or intact HSA itself.
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Materials and methods |
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Bacterial and yeast strains and plasmids
Escherichia coli TOP10F' [F'{proAB, lacIq, lacZM15, Tn10(TetR)}mcrA,
(mrr-hsdRMS-mcrBC),
80lacZÄM15,
lac X74, deoR, recA1, araD139,
(ara-leu)7697, galU, galK, rpsL(StrR), endA1, nupG
] was used for initial propagation and verification of the recombinant plasmids. Pichia pastoris GS115 [his4] (Invitrogen, Groningen, The Netherlands) was used as a host strain for the heterologous expression of HSA domains. Strain GS115/albumin (Invitrogen), having the full-length cDNA of HSA incorporated into the genome under control of a methanol-inducible promoter (AOX1 locus), was used as a control for secreted expression and production of recombinant HSA. Novel GS115 strains harbouring specified domains of HSA were engineered in the course of this study. Plasmid pPICZaA (Invitrogen) was used for cloning and for expressionsecretion of the selected recombinant sequences in the P.pastoris system, in each case. Selection of GS115 harbouring pPICZaA-based episomes was carried out on growth media containing zeocin (0.1 mg/ml).
Design and construction of recombinant P.pastoris strains
DNA encoding selected HSA domains was amplified by PCR using Pfu polymerase and chromosomal DNA of GS115/albumin as a template. The sense and antisense primers used had been designed to carry non-annealing overhangs at their 5' end containing an EcoRI and a NotI restriction site, respectively (see Table I). The pair of primers DIIIB-(EcoRI)-sense and (NotI)-antisense or (NotI)-His6-antisense was used to amplify the sequence of HSA codons 490585 (corresponding to subdomain IIIB); the pair of primers DIII-(EcoRI)-sense and (NotI)-antisense or (NotI)-His6-antisense was used to amplify the sequence of HSA codons 381585 (domain III). In both cases, the (NotI)-His6-antisense primer was employed to engineer a C-terminal sequence tag of 2.5 kDa containing the c-myc epitope (Glu-Gln-Lys-Leu-Ile-Ser-Glu-Glu-Asp-Leu-Asn) and a His6 tail, for optimal detection and/or purification of the recombinant polypeptides. Two additional constructs were made by using primers DI-(EcoRI)-sense and DI-(NotI)-antisense to amplify the sequence of HSA codons 1197 (domain I), or primers DII-(EcoRI)-sense and DII-(NotI)-antisense to amplify the sequence of HSA codons 189385 (domain II). Due to the engineered EcoRI site (Table I), a short extension by two amino acid residues (Glu-Phe) is introduced at the N-terminus of the processed recombinant polypeptides, in all cases. The PCR products were digested with EcoRI and NotI and ligated to a similarly treated vector pPICZaA (Invitrogen); this subcloning step places the desired HSA sequences at the AOX1 locus to allow efficient incorporation into the P.pastoris genome and methanol-inducible expression/secretion of the products.
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Expression of recombinant peptides
All incubations were carried out at 30°C on an orbital shaker at 250 r.p.m. Recombinant P.pastoris clones were grown on buffered glycerol-complex medium [BMGY: 1% (w/v) yeast extract, 2% (w/v) peptone, 0.1 mol/l KPi pH 6.0, 1.34% (w/v) yeast nitrogen base, 4 x 105% (w/v) biotin, 1% (v/v) glycerol] for 20 h, normalized to an OD600 of 1.0 and transferred to buffered methanol complex medium [BMMY: 1% (w/v) yeast extract, 2% (w/v) peptone, 0.1 mol/l KPi pH 6.0, 1.34% (w/v) yeast nitrogen base, 4 x 105% (w/v) biotin, 0.5% (v/v) methanol] for induction of recombinant coding sequences through the AOX1 promoter. Growth on BMMY was maintained for an additional 72 h. Subsequently, culture supernatants were collected, filter sterilized, snap-frozen in liquid nitrogen and stored at 80°C until use. Pelleted cells were also harvested for total protein determination.
Immunoblot analysis
Culture supernatants from clones expressing recombinant domains of HSA were analysed by 15% or 18% SDSPAGE (Laemmli, 1970). Visualization of protein bands was performed with both Coomassie brilliant blue and silver staining. Proteins were electroblotted to polyvinylidene difluoride membranes (BioTrace-PVDF; Pall Corporation, Ann Arbor, MI) and probed with monoclonal anti-myc 9E10 antibody (at a dilution of 1:2000) or polyclonal anti-HSA antibody (at a dilution of 1:15 000). Immunoreactive bands were detected with enhanced chemiluminescence (Amersham Pharmacia Biotech) following incubation with anti-mouse IgG or anti-rabbit IgGHRP conjugate, respectively (at a dilution of 1:5000 in either case).
Rat pituitary cell cultures
Pituitaries were excised from 2-month-old female Wistar rats that had been sacrificed by stunning and cervical dislocation. The anterior pituitary tissue was separated and used to prepare primary cell cultures, as described (Pappa et al., 1999a,b) with minor modifications. Anterior pituitary cells (200 000 per well) were spread on 24-well tissue culture plates. Cells were cultured for 48 h in SFDM (Dulbeccos modified Eagles medium nutrient mixture F-12 Ham supplemented with10 µg/ml insulin, 5 µg/ml transferrin, 100 U/ml penicillin, 100 mg/ml streptomycin, 10% (v/v) fetal calf serum (FCS) and 2.5 mmol/l L-glutamine) and then subjected to GnSAF bioassay.
GnSAF bioassay
Primary pituitary cells pre-incubated for 48 h in SFDM were transferred to SFDM without FCS (SFDMminus) (0.5 ml per well) for another 48 h. During this second period, cells were incubated in triplicate in the presence of test substances (25 µl per well, unless otherwise indicated); at least six control wells receiving SFDMminus without other additions were included in all culture plates. Subsequently, cells were washed with medium and incubated with SFDMminus containing GnRH (0.1 µmol/l) together with test substances (25 µl per well, unless otherwise indicated), for another 4 h. During this final period, three of the control wells were incubated with GnRH alone to measure optimal GnRH-induced LH levels and three of them received SFDMminus without additions to measure basal LH secretion. Cell-conditioned media were collected and stored at 20°C until assay. Concentrations of LH in the culture media were determined with competitive enzyme-linked immunosorbent assay (ELISA) as described (Pappa et al., 1999b) using hormone preparations NIDDK-rLH-RP-3 and rLH-I-10 and antiserum NIDDK-anti-rLH-S-11. The useful range of the assay was from 0.5 to 50 ng/ml (NIDDK-rLH-RP-3). Intra- and inter-assay coefficients of variation were, for basal secretion, 11 and 8%, respectively, and for GnRH-induced LH secretion, 11 and 16%, respectively.
GnSAF bioactivity is calculated according to the formula [1 A/B] (%) where A = LH(ng/ml)[GnRH + SAMPLE] LH(ng/ml)BASAL, B = LH(ng/ml)GnRH LH(ng/ml)BASAL, LH(ng/ml)BASAL = the basal secretion level of LH (3.0 ± 0.4 ng LH per well, n = 12), LH(ng/ml)GnRH = the GnRH-induced LH secretion (46.5 ± 8.5 ng LH per well, n = 12) and LH(ng/ml)[GnRH + SAMPLE] = the GnRH-induced LH secretion attained in the presence of the sample tested.
In each calculation, the values LH(ng/ml)BASAL, LH(ng/ml)GnRH and LH(ng/ml)[GnRH + SAMPLE] are determined from wells of the same culture dish.
Bioactivity-blocking effect
To determine whether anti-HSA polyclonal antibody blocks GnSAF activity of IIIB polypeptide, supernatants from GS115/DIIIB or control supernatants (25 µl) were pre-incubated on an orbital shaker with anti-HSA (Abcam) (10 µl; at the desired dilution) in SFDMminus (final volume, 0.5 ml) at 37°C for 5 min. These samples subsequently were subjected to GnSAF bioassay, as described above.
Effects on basal LH and basal FSH levels
To assess the effect of P.pastoris supernatants on basal LH and basal FSH secretion, pituitary cell-conditioned media collected after 48 h of incubation in the absence or presence of test substances were stored at 20°C and used to determine concentrations of LH and FSH. Concentrations of FSH were determined with competitive ELISA (Pappa et al., 1999b) using hormone preparations NIDDK-rFSH-RP-2 and rFSH-I-9 and antiserum NIDDK-anti-rFSH-S-11. The useful range of the assay was from 1.25 to 40 ng/ml (NIDDK-rFSH-RP-2). The basal secretion level of FSH after 48 h was 10.9 ± 3.8 ng FSH per well (n = 6). Concentrations of LH were determined with competitive ELISA, as described above. The basal secretion level of LH after 48 h was 13.7 ± 1.3 ng LH per well (n = 6).
Blue Sepharose chromatography
Pichia pastoris culture supernatants were harvested by centrifugation, filtered and dialysed against 100 vols of binding buffer (20 mmol/l NaPi pH 7.5, 0.15 mol/l NaCl) with three changes. Dialysed samples (1 ml) were incubated with Blue Sepharose 6 Fast Flow (gel volume, 50 µl) pre-equilibrated in binding buffer. After washing with 30 vols of binding buffer, elution was performed twice with a 100 µl volume of 20 mmol/l NaPi pH 7.5, 2 mol/l NaCl. All of the above steps were performed at 4°C. Samples subjected to chromatography, flow-through fractions and eluting protein fractions were analysed by SDSPAGE (18%) followed by silver staining.
Protein determinations
Protein concentrations were determined with the bicinchoninic acid (BCA) method (Pierce, Rockford, IL) using bovine serum albumin as a standard. Assessment of total intracellular protein in cell lysates of P.pastoris (yielding 0.570.65 µg of extracted protein per ml of cell culture extract, under the conditions employed) was used to normalize the volumes of supernatants analysed by SDSPAGE and assayed for GnSAF activity. The P.pastoris lysates were prepared using glass beads (0.5 mm diameter) and a Mini-BeadBeater cell disrupter (Biospec Products, Bartlesvill, OK).
Bionformatics
To estimate domain boundaries, the full-length HSA sequence was subjected to in silico analysis using CATH (Class-Architecture-Topology-Homologous superfamily) (Pearl et al., 2000).
Statistical analysis
Differences between treatments and controls were tested by Students t-test. Results are presented as means ± SEM.
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Results |
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The transformed P.pastoris clones were grown on the appropriate medium for induction with methanol. After induction of expression, supernatants were analysed by SDSPAGE (15 or 18%) followed by silver staining and immunoblotting. Silver staining of the electrophoresed proteins revealed the presence of major protein bands at the approximate sizes expected for DIIIB (12 kDa) and DIIIB-myc-His6 (14 kDa), respectively (Figure 1A). Clearly, these bands comprise the vast majority of total protein in the medium, while they are absent from equivalent electropherograms of P.pastoris transformed with empty vector alone (Figure 1A). Immunoreactivity of these protein bands with anti-HSA antibody (for either DIIIB or DIIIB-myc-His6 versions) (Figure 1B) and/or anti-myc antibody 9E10 (not shown) identifies the proteins as corresponding to the designed recombinant polypeptides of HSA (DIIIB and DIIIB-myc-His6, respectively). The heterologous expression of these polypeptides is not accompanied by over-representation of other bands corresponding to native secreted proteins of P.pastoris, as evidenced from the silver staining picture (Figure 1A).
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Intact HSA does not display GnSAF bioactivity
Supernatants from induced P.pastoris GS115/albumin were analysed by SDSPAGE and Coomassie brilliant blue staining (Figure 4A) and assayed for GnSAF activity (Figure 4B). Clearly, under the conditions employed, culture supernatants from GS115/albumin appear to contain an overexpressed protein corresponding to recombinant HSA (Figure 4A) but fail to display any detectable GnSAF bioactivity, indistinguishable from supernatants of GS115 transformed with empty vector (Figure 4B and Table II). To reinforce the significance of these findings further, pure HSA (Sigma A3782) was also assayed in the same system and found to display negligible GnSAF bioactivity at doses of 0.125, 0.25, 1 or 2 µg (Figure 4C); the doses of full-length HSA applied (0.12.0 µg, or 230 pmol) were equivalent to or higher, on a molar basis, than doses of HSA DIIIB estimated to be active on the GnSAF bioassay (see Pappa et al., 1999a).
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Discussion |
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Interestingly, HSA belongs to a multi-gene protein family that also includes -fetoprotein and vitamin D-binding protein (Gibbs and Dugaiczyk, 1987
); the molecule displays binding capacity for several ligands, including long chain fatty acids, drugs and peptide hormones (Peters, 1996
). Domain analysis of the sequence of HSA in silico predicts three major structurally similar domains, with approximate sequences of codons 1197 (domain I), 189385 (domain II) and 381585 (domain III), each of which is composed of two subdomains (IA and IB, IIA and IIB, IIIA and IIIB, respectively); this characteristic domain architecture of HSA has been emphasized in the past (Gibbs and Dugaiczyk, 1987
; He and Carter, 1992
; Peters, 1996
), while clear experimental evidence that domains I, II and III are expressible as structurally independent proteins in the P.pastoris system has been provided by Dockal et al. (1999
). In our present study, subdomain IIIB is also shown to be expressible in secreted form in the same system, providing an important experimental tool to study the mechanism of this putative GnSAF biomolecule.
The observed GnSAF bioactivity is not attributable to endogenous secreted factors of P.pastoris. It is also not derived from intact HSA (residues 1585) or larger HSA domains such as domain III (residues 381585) that are inactive when used as the recombinant expressed entities. Similarly, the two other major HSA domains, I and II, are inactive. Taken together, these findings indicate that the C-terminal polypeptide corresponding to subdomain IIIB of HSA possesses inherent GnSAF-like activity that is cryptic in the structural context of either full-length albumin or domain III and released only when the active moiety is separated from HSA with an as yet unidentified mechanism. This might be due to a specific proteolytic cleavage event, as postulated by Pappa et al. (1999a), or even to alternative splicing of the HSA transcript. In any case, the results are reminiscent of other examples where specific polypeptide portions exert different biological effects from their corresponding parental proteins, such as the angiogenesis inhibitors endostatin, a 20 kDa C-terminal portion of collagen XVII (OReilly et al., 1997
), and angiostatin, a 38 kDa internal sequence of plasminogen (OReilly et al., 1994
).
Although it remains unclear whether the endogenous human GnSAF bioactivity derives from one molecular entity or a number of different contributing molecules, as discussed below, the observation that a C-terminal subdomain of HSA can act as a GnSAF raises very important questions (i) on the in vivo mechanism of production of this putative GnSAF moiety in the ovarian granulosa cells and/or the FF; and (ii) on the mechanism of attenuation of the GnRH-induced LH surge in the pituitary gonadotrophs. The availability of recombinant GnSAF-like molecules from heterologous, simple biological systems such as the one presented here will greatly facilitate studies on the molecular details of the GnSAF mechanism. In this respect, studies have already been initiated to examine the effect of the relevant HSA domains on the clonal gonadotrophic cell line LT2, an in vitro model for GnRH-mediated signalling leading to pulsatile LH secretion (Turgeon et al., 1996
; Liu et al., 2002
; Kakar et al., 2003
).
Much research has been carried out in the past decade to identify molecule(s) responsible for GnSAF bioactivity. Purification, sequencing and structurefunctional analysis of such molecule(s) would be of major importance in the field of human fertility and provide crucial improvements in the application of IVF programmes (Messinis and Templeton, 1989, 1990a,
b, 1991). Originally, Fowler et al. (1992
) analysed human FF and recovered GnSAF/IF bioactivity in ultrafiltration fractions of 1030 kDa without proceeding with further purification. Since then, four systematic identification trials have been described with varying results and no apparent consensus, yielding proteins between 12 and 69 kDa in size (Tio et al., 1994
; Danforth and Cheng, 1995
; Pappa et al., 1999a
; Fowler et al., 2002
). Tio et al. (1994
) reported the isolation from rat Sertoli cell-conditioned medium of a 37 kDa protein with a partial N-terminal sequence of NH2-SDXXPQL; this molecule displayed GnSAF bioactivity but was not completely devoid of inhibin-like activity since it inhibited basal FSH secretion to some extent. Danforth and Cheng (1995
) isolated a candidate GnSAF/IF of 69 kDa with a partial N-terminal sequence of NH2-SKPLAE from porcine FF. By using antibodies raised against the 69 kDa porcine protein, Mroueh et al. (1996
) identified two putative GnSAF/IF-like homologues in human FF with molecular sizes of 63 and 59 kDa. More recently, Fowler et al. (2002
) showed that GnSAF/IF activity associated with proteins of 6066 kDa and a pI of 5.75.8 is present in culture supernatants from human granulosa/luteal cells of superovulated ovaries as well as from granulosa cells of unstimulated ovaries, and provided two candidate GnSAF/IF sequences (EPQVYVHAP and NH2-XVPQGNAGN).
None of the sequences of the aforementioned studies shared any significant homology with known proteins in computer-assisted database searches; furthermore, none has been confirmed as GnSAF thus far, and it remains unclear whether they represent different parts of one novel molecule, partial sequences of different bioactive molecules, or even unrelated co-purifying proteins or mixtures thereof. In a separate study, Pappa et al. (1999a) isolated from human FF a candidate GnSAF molecule of 12.5 kDa with a sequence corresponding to the C-terminal 95peptide of HSA, on the basis of mass spectrometric analysis. Although this work is also not exempt from criticism concerning putative co-purification contaminants, it is unique because (i) it yields a candidate sequence with homology to a known DNA portion of the human genome; (ii) it agrees with the original fractionation results of Fowler et al. (1992
); and (iii) it is strongly supported by our present finding that the relevant polypeptide still behaves as a GnSAF when produced in recombinant form from the heterologous expressionsecretion system of the yeast P.pastoris, and anti-HSA antibodies can block the GnSAF-like activity associated with this polypeptide in the P.pastoris supernatants.
GnSAF is different from inhibin (Ying, 1988; Fowler et al., 1990
). Although production of GnSAF, as well as inhibin, is considered to be stimulated by FSH in both rats and humans (Koppenaal et al., 1992
; Messinis et al., 1993
), it has been questioned if GnSAF is regulated by FSH in the rat ovarian cycle (Tio et al., 1998
). In any case, unequivocal distinction between GnSAF and inhibin can be made on the basis that the latter suppresses the basal secretion level of FSH, an activity that is not observed with GnSAF itself. As shown here, the bioactivity of the C-terminal 95peptide of HSA (subdomain IIIB) is free from contributions of inhibin-like activity, indicating that the corresponding moiety represents a genuine GnSAF-like molecule.
In conclusion, using the expressionsecretion system of P.pastoris, we have confirmed that subdomain IIIB of HSA possesses GnSAF-like activity. Taken together with the results of Pappa et al. (1999a), and in the light of the other findings on candidate GnSAF molecules (Mroueh et al., 1996
; Fowler et al., 2002
), the data favor the interpretation that GnSAF may involve the coordinated action of more than one polypeptide species in the regulation of GnRH-induced LH secretion from pituitary gonadotrophs.
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Acknowledgements |
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Submitted on November 25, 2003; accepted on January 9, 2004.