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Department of Animal Science, 259 Morrison Hall, Cornell University, Ithaca, New York 14853-4801, USA
1 Department of Animal Health, Welfare and Nutrition, Danish Institute of Agricultural Sciences, PO Box 50, 8830 Tjele, Denmark
(Requests for offprints should be addressed to Y R Boisclair; Email: yrb1{at}cornell.edu)
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Abstract |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Decreased hepatic production, however, is unlikely to be the only factor involved. This is because plasma IGF-I almost always circulates in multi-proteins, high molecular weight complexes (Baxter 1994, Boisclair et al. 2001). In adult animals, the predominant complex consists of one molecule each of IGF-I, IGF binding protein-3 or -5 (IGFBP-3 or -5), and the acid labile subunit (ALS) (Baxter 1994, Twigg & Baxter 1998, Boisclair et al. 2001). Incorporation of free IGF-I in these ternary complexes extends its half-life from minutes to over 12 h (Guler et al. 1989, Baxter 1994, Zapf et al. 1995). Adult animals devoid of ALS have accelerated turnover and severely depressed concentration of plasma IGF-I (Ueki et al. 2000, Yakar et al. 2002, Domene et al. 2004). ALS deficiency also results in the near disappearance of plasma IGFBP-3 whereas absence of IGFBP-3 has no effect on plasma ALS (Ueki et al. 2000, Yakar et al. 2005). These observations suggest that variation in plasma ALS could be a second factor contributing to the fall of plasma IGF-I in periparturient dairy cows.
The ALS cDNA and gene were isolated first in human, mouse and rat (Boisclair et al. 2001), and consequently most of the existing information on ALS was derived from these species. These studies have shown that ALS is synthesized primarily in liver in a growth hormone (GH)-dependent manner (Baxter 1990, Baxter & Dai 1994). The effects of GH occur via a stimulation of gene transcription, resulting in increased abundance of ALS mRNA in liver and ALS in plasma (Dai & Baxter 1994, Ooi et al. 1997, Olivecrona et al. 1999). In contrast, ALS has not been studied in dairy cattle, including during the transition from pregnancy to lactation when plasma IGF-I varies dynamically. The major factor contributing to this lack of information is the poor cross-reactivity of antibodies raised against rodent and human ALS with bovine ALS (Baxter 1990, Baxter & Dai 1994).
To address the lack of information on ALS in dairy cattle, we pursued two specific objectives. First, we cloned the bovine ALS cDNA and used this sequence to develop bovine-specific reagents. Secondly, using these reagents, we asked whether ALS is regulated during the transition from pregnancy to lactation in dairy cattle. Our studies show that hepatic ALS expression and production decline in early lactating dairy cows as a consequence of hepatic GH resistance.
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Materials and Methods |
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Cloning of the bovine ALS cDNA
The full-length bovine ALS cDNA was amplified using a forward primer corresponding to nt –57 to –38 of the -ATCCAGAGG ovine ALS cDNA (relative to A+1TG; 5'GCAGGAGCAGC-3') and a poly A tail-specific reverse primer (5'-T30NN-3'; Clontech laboratories, Palo Alto, CA, USA). PCR was performed using a high fidelity polymerase as recommended by the manufacturer (Ex Taq polymerase; Takara Bio Inc, Japan). Products were subcloned into the vector pCR 2.1-Topo (Invitrogen), sequenced and submitted to Genebank (accession number DQ444712 [GenBank] ).
Spatial ALS expression
Tissues were obtained at slaughter from two adult dairy cows that had been lactating for ~300 days (see Fig. 2
for complete listing of tissues). Ovarian follicular cells were isolated from freshly excised dominant and subdominant follicles (
5 mm) and pooled. Mammary epithelial cells were isolated by digesting the parenchyma of a lactating dairy cow with a mixture of a collagenase, hyaluronidase and elastase (Matitashvili & Bauman 1999). Cells were subjected to gradient centrifugation to yield primarily mammary epithelial cells (>95% of isolated cells). All tissues and cells were frozen in liquid N2 immediately upon isolation.
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Production and validation of bovine ALS antisera
A peptide corresponding to the 28 carboxyl-terminal residues of bovine ALS (ALSc28) was conjugated to keyhole limpet hemocyanin (KLH) using sulfhydryl chemistry (Bio-Synthesis Inc, Lewisville, TX, USA). Conjugated ALSC28 (100 µg) was emulsified with an equal volume of complete Freund’s adjuvant and injected subcutaneously into three New Zealand White rabbits. For secondary immunizations, emulsions were prepared with incomplete Freund’s adjuvant and injected on days 14, 42, 63, 84 and 105 (relative to primary immunization). Rabbits were bled on day 115 and serum was stored at –20 °C.
Immunoreactivity of antisera was evaluated with plasma obtained from adult animals of various species (listed in Fig. 3B). In some studies, bovine plasma was deglycosylated. Briefly, plasma (2 µl) was boiled in 0.5% SDS, 1% 3-mercaptoethanol for 10 min. After adding NaHPO4 and 10% NP-40 to final concentrations of 0.05 M and 1% respectively, the samples were incubated at 37 °C in the absence or presence of 500 units of peptide-N-glycosidase F (PNGase F; New England Biolabs, Beverly, MA, USA). After 1 h, reactions were terminated by boiling in 1x Laemmli loading buffer. Native and deglycosylated plasma were electrophoresed under reducing conditions on 10% SDS–polyacrylamide gels, and electroblotted onto nitro-cellulose membranes (Protran; Schleicher & Schuell Bioscience, Keene, NH, USA). The membranes were blocked in Tris-buffered saline with Tween-20 (TBST; 0.05 M Tris, pH 7.4, 0.2 M NaCl, 0.1% Tween-20) containing 5% (w/v) non-fat dried skim milk (NFM) for 1 h at room temperature. Membranes were then incubated with bovine ALS antisera (1:4000 dilution) in blocking buffer overnight at 4 °C. To establish specificity, incubation with the primary antibody was performed in the presence or absence of ALSC28. After incubation, membranes were washed in TBST and incubated with goat anti-rabbit IgG-horseradish peroxidase (HRP) (1:5000 dilution) in blocking buffer for 1 h at room temperature. Signals were detected by LumiGLO western blot chemiluminescence (KPL, Gaithersburg, MD, USA) and analyzed by densitometric scanning and NIH Image 1.63 software.
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Catheters were surgically implanted in the thoracic duct of six dairy calves using the procedures described by Hartmann and Lascelles (1966). They were used to obtain matched jugular blood and thoracic lymph samples. Blood and follicular fluid were obtained from a second group of four cows (Porter et al. 2000). These cows were injected with a luteolytic dose of prostaglandin F2
(PGF2) between days 7 and 14 of the estrous cycle. After the onset of estrus, animals were examined by transrectal ultrasonography to identify the dominant follicle, and then ovaries were removed by colpotomy. The dominant follicle (9 mm) was dissected and its follicular fluid was aspirated using an 18-gauge needle. Finally, blood and colostrum were obtained from six multiparous dairy cows at day 245 of gestation (5 weeks before parturition). Plasma was prepared by heparin addition and centrifugation (3000 g). Colostrum was centrifugated (10 000 g) for 15 min and the aqueous phase was collected. All fluids were frozen at –20 °C until analyzed for ALS abundance by western immunoblotting (as described in previous section).
Regulation of ALS during the transition from pregnancy to lactation
Temporal profiles of plasma ALS abundance and hepatic ALS mRNA were studied in four multiparous cows in the period between 35 days prepartum and 56 days postpartum. Cows were fed a total mixed ration (TMR) according to recommended feeding standards (NRC 2001); cows were fed once daily at 1100 h and were allowed to feed ad libitum. The TMR contained 1.45 Mcal of net energy of lactation (NEL) and 125 g crude protein (CP) per kg of dry matter (DM) between days –35 and 3 21 (relative to parturition on day 0), 1.63 Mcal NEL/kg DM and 143 g CP/kg DM between day –21 and parturition, and 1.76 Mcal NEL/kg DM and 183 g CP/kg DM during lactation (NRC 2001). Cows were milked thrice daily at 0900, 1600 and 2300 h. Blood samples were collected between 0900 and 1000 h on days –35, –7, –1, +1, +3, +7, +10, +21 and +56 by coccygeal venipuncture. Plasma was prepared immediately and frozen at –20 °C. After blood sampling on days –35, +3 and +56, liver biopsies were obtained by a non-invasive percutaneous method (Kim et al. 2004) and snap-frozen in liquid N2.
To investigate the effect of GH on plasma ALS, a second group of 12 cows was used in the period from 5 weeks prepartum to 5 weeks postpartum. They were fed a TMR ad libitum once daily according to feeding standards (NRC 2001). The NEL and CP content of the TMR were 1.36 Mcal/kg DM and 130 g/kg DM during week –5, 1.56 Mcal/kg DM and 141 g/kg DM between week –4 and parturition, and 1.72 Mcal/kg DM and 179 g/kg DM after parturition. Cows were randomly allocated to a control or a GH group. The GH group received a daily injection of recombinant bovine GH (bovine somatotropin (bST), 45 mg i.m., lot 96J-B5128–002; Monsanto, St Louis, MO, USA) on three consecutive days during weeks –5, –2, +1 and +5. This period of GH administration produces a maximal IGF-I response that persists for over 24 h after the last injection (Cohick et al. 1989). Injections were initiated on the second day of each week. Blood samples were obtained by coccygeal venipuncture between 1000 and 1130 h, the day before the first GH injection (day 0) and the day after the last injection (day 4). Control cows were sampled on the same day of each week. Plasma was prepared immediately and frozen at –20 °C.
Abundance of plasma ALS and hepatic ALS mRNA was analyzed by western immunoblotting and RPA, respectively, as described above. Plasma IGF-I was measured by a previously described double antibody RIA (Kim et al. 2004). Bovine recombinant IGF-I was used for iodination and standard (recombinant IGF-I, lot GST-3; Monsanto). Primary antibody against IGF-I (rabbit anti-human IGF-I, lot AFP4892898) was obtained from the National Hormone and Pituitary Program (Bethesda, MD, USA). The secondary antibody was a caprine anti-rabbit IgG (lot 12515; Biotech Source, Inc, Franklin, MA, USA). Inter- and intra-assay coeffcients of variation averaged less than 8% and 9% respectively.
Statistical analysis
Levels of ALS in various fluids were expressed as a percentage of matched plasma levels and analyzed by a model accounting for source (lymph, follicular fluid and colostrum) as the fixed effects and animal as the random effect. Data obtained during the temporal study in transition dairy cows were analyzed by a mixed model with time as the fixed effect and animal as the random effect. When the effect of time was significant (P< 0.05), the variation was partitioned into linear, quadratic and cubic contrasts. For the GH study, the response was calculated for each variable as the difference between day 4 and day 0. Responses were analyzed by a model accounting for GH (control vs GH), time relative to parturition (weeks –5, –2, +1 and +5), and their interaction (GH x Time) as the fixed effects and animal as the random effect.
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Results |
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We isolated a partial 1833 bp bovine ALS cDNA containing an open reading frame of 611 amino acids (Fig. 1). The cDNA has a high degree of identity with its human (>75%) and ovine ALS counterparts (>95%). Computer analysis of the amino acid sequence predicted a signal peptide of 32 residues and a mature protein of 579 residues. The deduced mature protein has all the features previously described for ALS (Boisclair et al. 2001), including 12 cysteine residues clustered predominantly in the amino and carboxyl terminal regions, 20 leucine-rich repeats of 24 residues and 6 potential asparagine-linked glycosylation sites (NXS/T). Identities of mature bovine ALS with chicken, mouse, human, pig and sheep ALS were 60, 73, 76, 82 and 96% respectively.
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). Expression in non-hepatic tissues was obvious in lung, small intestine, adipose tissue, kidney and heart, but nearly absent in brain and muscle. Detection of bovine ALS in plasma and extra-vascular fluid
To raise bovine ALS antisera, 3 rabbits were immunized with a peptide corresponding to the 28 carboxyl-terminal residues of bovine ALS (ALSc28). This peptide is completely conserved in sheep ALS but has less than 80% identity with the carboxyl termini of mouse, chicken, pig and human ALS (Fig. 3A). Antisera were tested by western immunoblotting for their ability to detect a ~90 kDa protein in plasma. All three antisera detected a protein of 84 kDa in cattle, sheep, goat and chicken plasma (Fig. 3B). Antiserum 1085 was also capable of detecting a ~84 kDa signal in pig plasma, but none of these antisera detected ALS in mouse or human plasma. This cross-species reactivity profile suggests that the antisera recognized an epitope defined predominantly by the 4 amino acid residues PPSL present in bovine, sheep and chicken ALS (Fig. 3A).
Additional studies were performed to validate antiserum 1082. First, we assessed its specificity by determining whether the 84 kDa signal present in bovine plasma was eliminated when performing immunoblotting in the presence of the ALSC28 peptide (Fig. 3C). As expected for a specific signal, the ~84 kDa band was completely eliminated in the presence of ALSC28. Secondly, we determined whether the 84 kDa protein is glycosylated. Bovine plasma was incubated in the presence or absence of PNGase F, followed by SDS-PAGE and immunoblotting (Fig. 3D). PNGase F incubation collapsed the immune signal to 60 kDa, the size predicted by the primary amino acid sequence of mature ALS. Overall these studies indicate that ALS is the 84 kDa protein detected in bovine plasma by antiserum 1082.
ALS abundance was compared in matched plasma and lymph samples obtained from growing cattle. As observed in humans, ALS was more abundant in plasma than lymph (Fig. 4A). ALS has also been shown to occur in other fluid compartments (Boisclair et al. 2001). To determine if this occurs in the cow, we sampled the mammary and ovarian follicular fluid compartments (Fig. 4A). Mammary secretions were obtained from dairy cows 5 weeks before parturition and again after 5 weeks of lactation. ALS was present in colostrum, but at only 15% of levels seen in matched plasma. ALS was nearly undetectable in milk (not shown). ALS was also present in fluid obtained from the dominant follicle at levels that were approximately 80% of those seen in matched plasma (Fig. 4A). These data show that ALS occurs at significant levels outside the vascular compartment.
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). ALS mRNA was also present in isolated mammary epithelial cells and complete ovarian follicles. These data suggest that mammary and follicular cells synthesize a portion of the ALS found in associated fluid compartments. Regulation of ALS during the periparturient period
We measured the variation in plasma IGF-I and ALS in dairy cows between day 35 prepartum and 56 postpartum. The concentration of plasma IGF-I dropped by 50% during the first week postpartum, and remained depressed until the end of the study (Fig. 5A). Plasma ALS followed an identical pattern between day 35 prepartum and day 10 postpartum (Fig. 5B). Unlike IGF-I, however, plasma ALS started to recover by day 21 postpartum and returned to prepartum levels by day 56 postpartum (Fig. 5B, quadratic P<0.001). To determine whether this reflected changes in hepatic expression, ALS mRNA was measured in liver biopsies collected on days –35, +3 and +56. ALS mRNA expression was significantly lower on day +3 than on day –35, but recovered completely by day +56 (Fig. 6, P<0.05). We conclude that hepatic ALS gene expression accounts for the variation in plasma ALS during the periparturient period.
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, GH x Time P<0.05). Nearly identical effects of GH were seen when the response considered was plasma IGF-I (Fig. 7B). We conclude that plasma ALS is positively regulated by GH but that this response is abrogated during the week following parturition.
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Discussion |
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The relative abundance of ALS in plasma and other fluid compartments has not been evaluated directly in cattle, reflecting the lack of reactivity of antibodies raised against human or rat ALS with their bovine counterpart (Baxter 1990, Baxter & Dai 1994). Using the COOH-terminal sequence of bovine ALS as an antigen, we obtained antisera detecting bovine, but not human or mouse ALS. As previously shown in humans (Baxter 1990), we observed much higher ALS abundance in plasma than in lymph, consistent with the inability of ALS to cross the endothelial barrier effciently (Binoux & Hossenlopp 1988). We also detected ALS in colostrum and ovarian follicular fluid. The high ALS abundance in the latter is consistent with follicular cell synthesis, otherwise a steep gradient would be expected between plasma and follicular fluid.
Irrespective of its origin, a more important issue is whether or not extra-vascular ALS is functionally important. Both IGF-I and IGF-II (IGFs) are thought to be positive regulators of follicular growth whereas IGFBP-3 is the most abundant IGFBP in follicular fluid (Fortune et al. 2004, Webb et al. 2004). In cattle, total concentrations of IGF-I and IGFBP-3 do not fluctuate significantly in fluid obtained from dominant and subordinate follicles. Rather, IGF-I availability varies as a consequence of changes in the abundance of IGFBP-2, -4 and -5 (Fortune et al. 2004, Webb et al. 2004). Thus, the dominant follicle has lower levels of IGFBP-4 and -5 and higher levels of free IGF-I, whereas the opposite occurs in atretic follicles (Fortune et al. 2004, Webb et al. 2004). The presence of high levels of ALS and IGFBP-3 along with significant levels of IGFBP-5 suggests that IGFs mostly occur as ternary complexes in bovine follicular fluid. This has actually been demonstrated for IGF-I in human follicular fluid (Hughes et al. 1997). IGFs are also important mammary growth factors (Marshman & Streuli 2002, Akers et al. 2005), including during the period of epithelial cell proliferation and differentiation of late pregnancy (Brisken et al. 2002) when we detected significant levels of ALS in colostrum. Late pregnancy is a period when bovine colostrum contains high levels of IGF-I and IGFBP-3 (Baumrucker & Erondu 2000), raising the possibility of significant ternary complex formation. Moreover, IGFBP-5 is expressed at its highest level during lactogenesis in the bovine mammary gland (Plath-Gabler et al. 2001). Additional studies are needed to determine whether ternary complex formation occurs during the different developmental stages of the bovine follicle and mammary gland.
Periparturient dairy cows provide a unique opportunity to examine the regulation of ALS during a physiological condition that results in dynamic changes in plasma IGF-I. Dairy cows experience an abrupt, 60–70% decline in plasma IGF-I and IGFBP-3 between late pregnancy and parturition (Kobayashi et al. 1999, Kim et al. 2004, Boisclair et al. 2006). In agreement with others (Kobayashi et al. 1999, Radcliff et al. 2003), we showed that the plasma IGF-I depression is associated with reduced IGF-I mRNA in liver (Kim et al. 2004). We now show that plasma ALS abundance is also decreased between pregnancy and early lactation, an effect completely accounted for by lower hepatic ALS mRNA levels. ALS has a low affnity for binary IGF:IGFBP-3 complexes and must circulate in excess over IGF-I and IGFBP-3 to effciently recruit these proteins into long-lived ternary complexes (Holman & Baxter 1996, Boisclair et al. 2001). Therefore, reduced ALS synthesis in the days following parturition probably amplifies the effect of decreased hepatic IGF-I production on plasma IGF-I. Within 3 weeks of parturition, however, plasma ALS levels started to recover such that both hepatic ALS expression and plasma ALS had returned to pre-calving levels by day 56 of lactation. Hepatic IGF-I mRNA abundance was also completely normalized in these cows by day 56 (Kim et al. 2004), but the plasma IGF-I concentration was not. These results suggest that ALS plays little or no role in the persisting plasma IGF-I depression.
In rodents and humans, GH is the most potent positive regulator of hepatic ALS mRNA abundance and plasma ALS concentration (Baxter & Dai 1994, Dai & Baxter 1994, Olivecrona et al. 1999). The essential role of GH is illustrated by the absence of ALS and ternary complexes in the plasma of GH-deficient animals (Zapf et al. 1995). We previously showed that these effects of GH are transcriptional and mediated by STAT5 binding to a single GAS element in the proximal promoter of the mouse ALS gene (Ooi et al. 1998). This GAS element is completely conserved in the promoter of the rat, sheep and human ALS gene (Delhanty & Baxter 1997, Rhoads et al. 2000, Suwanichkul et al. 2000). A similar mechanism has recently been shown to explain the positive effects of GH on the transcription of the rat IGF-I gene (Woelfle et al. 2003, Chia et al. 2006). In the present study, GH treatment increased levels of both ALS and IGF-I during pregnancy and week 5 of lactation, but was ineffective 1 week after parturition. In this context, we recently demonstrated that the liver-specific GH receptor transcript and hepatic GH receptor abundance were decreased immediately after parturition but returned to normal within 3 weeks of parturition (Kim et al. 2004). Therefore, loss of hepatic GH action probably accounts for decreased production of both ALS and IGF-I in the immediate postparturient period.
Dairy cattle offer other opportunities to study the regulation of ALS, including other conditions where plasma IGF-I increases (euglycemic–hyperinsulinemic clamp) or decreases rapidly (50–80% decline during under-nutrition) (McGuire et al. 1995a, 1995b). Cattle are also recognized as a valuable model for studying mammary and ovarian follicular development (Fortune et al. 2004, Akers et al. 2005) and have easily accessible fluid compartments associated with these tissues. The reagents we have developed will facilitate studies on the role of ALS and ternary complexes in plasma and other fluid compartments of cattle.
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Acknowledgements |
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Funding |
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References |
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Received in final form 14 March 2006
Accepted 21 March 2006
Made available online as an Accepted Preprint 24 March 2006
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plasma ALS) was calculated as the plasma abundance difference between day 4 and day 0. Bars represent means±S.E. (n=6) of the response in plasma ALS. (B) Plasma IGF-I was measured by RIA. The response in plasma IGF-I (