Decreased concentration of plasma leptin in periparturient dairy cows is caused by negative energy balance

Dairy cows suffer from an intense energy deficit at parturition due to the onset of copious milk synthesis and depressed appetite. Despite this deficit, maternal metabolism is almost completely devoted to the support of mammary metabolism. Evidence from rodents suggests that, during periods of nutritional insufficiency, a reduction in plasma leptin serves to co-ordinate energy metabolism. As an initial step to determine if leptin plays this role in periparturient dairy cows, changes in the plasma concentration of leptin were measured during the period from 35 days before to 56 days after parturition. The plasma concentration of leptin was reduced by approximately 50% after parturition and remained depressed during lactation despite a gradual improvement in energy balance; corresponding changes occurred in the abundance of leptin mRNA in white adipose tissue. To determine whether negative energy balance caused this reduction in circulating leptin, cows were either milked or not milked after parturition. Absence of milk removal eliminated the energy deficit of early lactation, and doubled the plasma concentration of leptin. The plasma concentration of leptin was positively correlated with plasma concentrations of insulin and glucose, and negatively correlated with plasma concentrations of growth hormone and non-esterified fatty acids. In conclusion, the energy deficit of periparturient cows causes a sustained reduction in plasma leptin. This reduction could benefit early lactating dairy cows by promoting a faster increase in feed intake and by diverting energy from non-vital functions such as reproduction.

Influences on fetal and placental weights during mid to late gestation in prolific ewes well nourished throughout pregnancy

This study investigated associations between fetal and placental weights from 85 to 130 days gestation in 49 fetuses from 21 ewes of a prolific genotype used as an experimental model of intrauterine growth retardation. The proportion of variation in fetal weight explained by placental weight increased from zero at 85 days to 91% (residual standard deviation (RSD) = 260 g) at 130 days. Overall, stage of pregnancy plus placental weight accounted for 96% of fetal weight variation (RSD = 212 g). Litter size and number of fetuses per uterine horn also influenced individual fetal weights. Gestational age, litter size, placental weight per ewe, and liveweight and condition score of ewes during early to mid gestation (initial LW and CS) explained 99.5% of the variation in fetal weight per ewe (RSD = 236 g). Most variation (86%) in placental weight was explained by stage of pregnancy, litter size, number of placentomes, and initial LW and CS (RSD = 53 g). Placental weight per ewe was influenced by stage of pregnancy, litter size and initial ewe LW and CS (R2 = 0.97; RSD = 89 g). The association of fetal and placental weights with initial ewe LW was positive, and with initial CS was negative. The results show that in the absence of overt nutritional restriction of pregnant ewes, fetal and placental weights are tightly coupled during late gestation and ewe fatness during early pregnancy is inversely related to placental and fetal weights. They demonstrate that placental weight explains most of the variation in fetal weight in the present intrauterine growth retardation model.

Maternal leptin is elevated during pregnancy in sheep

Maternal plasma leptin is elevated during pregnancy in several species, but it is unclear to what extent this elevation reflects changes in adiposity or energy balance. Therefore, Karakul ewes (n = 8) were fed to minimize changes in maternal energy status over the pregnancy-lactation cycle. They were studied 20-40 d before breeding and during mid pregnancy (d 50-60 post coitus [PC]), late pregnancy (d 125-135 PC) and early lactation (d 15-22 post partum). Consistent with the maintenance of near energy equilibrium in nongravid maternal tissues, maternal body weight was increased only during late pregnancy when the weight of the conceptus became significant and plasma concentrations of insulin, NEFA and glucose did not vary with physiological state. In contrast, maternal plasma leptin concentration rose from 5.3 to 9.5 ng/mL between prebreeding and mid pregnancy and then declined progressively through late pregnancy and early lactation. Leptin gene expression increased 2.3 fold in maternal white adipose tissue (WAT) from prebreeding to mid pregnancy and declined to prebreeding levels during early lactation. To determine whether tissue response to insulin was involved in this effect, insulin tolerance tests were performed. The maternal plasma glucose response declined from prebreeding to early lactation, but was not correlated with either plasma leptin concentration or WAT leptin mRNA abundance. In conclusion, pregnancy causes an increase in the synthesis of leptin in sheep. This stimulation does not require increases in adiposity or energy balance and is unrelated to the ability of insulin to promote glucose utilization.

Proteomics characterization of abundant Golgi membrane proteins

A mass spectrometric analysis of proteins partitioning into Triton X-114 from purified hepatic Golgi apparatus (84% purity by morphometry, 122-fold enrichment over the homogenate for the Golgi marker galactosyl transferase) led to the unambiguous identification of 81 proteins including a novel Golgi-associated protein of 34 kDa (GPP34). The membrane protein complement was resolved by SDS-polyacrylamide gel electrophoresis and subjected to a hierarchical approach using delayed extraction matrix-assisted laser desorption ionization mass spectrometry characterization by peptide mass fingerprinting, tandem mass spectrometry to generate sequence tags, and Edman sequencing of proteins. Major membrane proteins corresponded to known Golgi residents, a Golgi lectin, anterograde cargo, and an abundance of trafficking proteins including KDEL receptors, p24 family members, SNAREs, Rabs, a single ARF-guanine nucleotide exchange factor, and two SCAMPs. Analytical fractionation and gold immunolabeling of proteins in the purified Golgi fraction were used to assess the intra-Golgi and total cellular distribution of GPP34, two SNAREs, SCAMPs, and the trafficking proteins GBF1, BAP31, and alpha(2)P24 identified by the proteomics approach as well as the endoplasmic reticulum contaminant calnexin. Although GPP34 has never previously been identified as a protein, the localization of GPP34 to the Golgi complex, the conservation of GPP34 from yeast to humans, and the cytosolically exposed location of GPP34 predict a role for a novel coat protein in Golgi trafficking.

Nutritional regulation of the genes encoding the acid-labile subunit and other components of the circulating insulin-like growth factor system in the sheep

In sheep, perinatal maturation of the endocrine arm of the insulin-like growth factor (IGF) system is characterized by two developmental events. First, concentrations of circulating IGF-I increase rapidly after birth and become responsive to changes in nutrition and growth hormone (GH). Second, the liver initiates synthesis of a serum protein called the acidlabile subunit (ALS). The acid-labile subunit promotes the endocrine actions of IGF-I and -II by recruiting them to long-lived complexes of 150 kDa. In this study, we examined the effect of nutrition on hepatic expression of the ALS gene around the time of birth and later in life. Expression of genes encoding other components of the circulating IGF system was also measured. At d 130 of fetal life, fetuses suffering from chronic undernutrition caused by placental insufficiency had lower expression of the ALS and IGF-I genes than well-nourished fetuses, but they did not have any changes in the expression of the IGF-binding protein (IGFBP)-2 or IGFBP-3 genes. In early postnatal life, hepatic gene expression was analyzed between d 12 and 38 in lambs fed a milk replacer at levels sustaining weight gains of 150 or 337 g/d. The lower plane of nutrition decreased the expression of the ALS, IGF-I, and GH receptor genes and increased the expression of the IGFBP-2 gene; expression of the IGFBP-3 gene was not affected by nutrition at this stage of life. Finally, hepatic gene expression was measured in 3-mo-old lambs offered ad libitum levels of a balanced diet or of a diet limiting for both energy and protein. Although the rate of growth of the lambs fed the limiting diet was reduced by 38%, the only effect detected in hepatic gene expression was a ninefold increase in the abundance of IGFBP-2 mRNA. Overall, these results indicate that undernutrition during late fetal and early postnatal life delays hepatic expression of the ALS gene and final maturation of the endocrine IGF system.

Development of a specific radioimmunoassay to measure physiological changes of circulating leptin in cattle and sheep

Studies of leptin in large domestic ruminants have been limited to measurements of gene expression because methods to measure circulating levels are not available. To develop a bovine leptin radioimmunoassay, we produced recombinant bovine leptin and used it to immunize rabbits, and to prepare bovine leptin tracer and standards. A single antiserum with sufficient affinity and titer was identified. Using this antiserum, logit-transformed binding of (125)I-labeled bovine leptin was linearly related (R(2)= 0.99) to the log of added bovine or ovine leptin between 0.1 to 2.0 ng. Serial dilution of bovine and ovine plasma, chicken serum and bovine milk gave displacement curves that were parallel to those of bovine or ovine leptin. Recoveries of external addition of bovine leptin in ewe and cow plasma ranged between 94 and 104%. Plasma leptin concentration measured by this assay was directly related to the plane of! nutrition in growing calves and lambs. At 11-14 weeks of age, ewe lambs had a higher circulating leptin concentration than ram lambs. Finally, plasma leptin concentration was linearly related to the fat content of the empty carcass in growing cattle and to body condition score in lactating dairy cows. We conclude that circulating leptin in sheep and cattle is increased by fatness and plane of nutrition, consistent with results in humans and rodents. This assay provides an important tool to investigate mechanisms that regulate plasma leptin in cattle and sheep.

An ultrasound-guided procedure to administer a label of DNA synthesis into fetal sheep

A novel technique was developed to deliver a bolus dose of a DNA label into the peritoneal cavity of fetal sheep at 85-130 days gestation. Use of markers to identify the site of injection in fetuses from litters up to quadruplets, and immunohistochemistry to detect the DNA label, 5-bromo-2'-deoxyuridine (BrdU), confirmed the procedure was successful in 85% of cases. Duration of the procedure was (mean +/- SD) 44 +/- 16 min, and recovery from anaesthesia was rapid and uneventful in all cases. Fetal weight was estimated with a high degree of accuracy (residual standard deviation (RSD) = 297 g and r2 = 0.93, P<0.001) and the dose of label administered (110 +/- 33 mg BrdU/kg fetal weight) was adequate in all cases. BrdU detected in fetal nuclei following injection into amniotic fluid highlights the need for positive identification of the injection site in timed, short-term studies, and suggests potential to further develop the technique to investigate cellular events in fetal sheep younger than 85 days of gestation. The results demonstrate that the procedure can be used to determine in vivo whether or not nuclei have entered the S-phase of the cell cycle.

Intrauterine growth retardation is associated with reduced cell cycle activity, but not myofibre number, in ovine fetal muscle

Cellular development of muscle was studied in sheep fetuses at 85 days of gestation. Large and small fetuses were compared at 100, 115 and 130 days, and an additional group of large 130-day fetuses were studied following 7 days of maternal undernutrition. Myogenesis in the peroneus longus muscle was completed between 100 and 115 days of gestation, and myofibre number did not differ between small and large fetuses. The proportion of myofibre-related nuclei identified as entering S-phase of the cell cycle was 1.7% per hour in 85-day fetuses. In large fetuses, subsequent rates were relatively constant (approximately 1.5% h(-1)), whereas in small fetuses cell cycle activity declined with age from 1.3 to 0.9% h(-1), and was 0.5% h(-1) in 130-day fetuses of restricted ewes. The constant rate of cell cycle activity in large fetuses was associated with an increasing estimated rate of muscle growth (peroneus longus (mg) = 0.831 x 10(0.024 x age [d]), r2 = 0.98), which contrasted with slow and relatively constant muscle accretion in small fetuses (8.4 mg day(-1)), and slower muscle accretion at 130 days in large fetuses from restricted ewes. Differences in DNA and RNA content in the semimembranosus muscle increased with age, large fetuses having 70% more muscle DNA, 108% more muscle RNA and 104% larger muscles than small fetuses at 130 days (all P<0.001). The results demonstrate that myonuclei accumulation, but not myofibre number, is associated with fetal growth in sheep and, therefore, with fetal nutrition during mid to late gestation.

Organization and regulation of the gene encoding the sheep acid-labile subunit of the 150-kilodalton insulin-like growth factor-binding protein complex

In adult animals, most circulating insulin-like growth factor I (IGF-I) and IGF-II is sequestered in a 150-kDa complex composed of 1 molecule each of IGF, IGF-binding protein-3 or -5, and the acid-labile subunit (ALS). Capture of IGF in ALS-containing complexes increases their circulating half-lives and concentrations and suppresses their hypoglycemic potential. ALS has been studied almost exclusively in rodents and primates, and no information exists in the sheep despite its extensive use to study the circulating IGF system. To initiate studies in the sheep, we isolated the ovine ALS gene and characterized its spatial and developmental regulation. The ALS gene spans about 3.0 kb of chromosomal DNA and consists of 2 exons interrupted by a 977-bp intron. Exon 1 encodes the first 5 amino acids of the signal peptide; exon 2 encodes the remaining 27 amino acids of the signal peptide and the entire mature protein of 579 amino acids. Transcription initiation occurs at nucleotides -58 and -29 (ATG, + 1), 2 sites that are not preceded by TATA or initiator sequences. A DNA fragment extending from -727 to - 11 of the sheep ALS gene directed basal expression of a luciferase reporter plasmid in the rat liver cell line H4-II-E. GH increased promoter activity by 1.8-fold, consistent with conservation in the sheep promoter of the GH response element previously identified in the mouse gene. A survey of adult tissues by Northern analysis revealed the presence of a 2.2-kb transcript only in liver. Weak expression was first detected in liver on day 130 of fetal life, increased suddenly on day 7 of postnatal age, and changed little thereafter. The sheep is a useful model to understand the regulation and role of ALS, particularly around the time of birth, when final maturation of the circulating IGF system occurs.

Placental transport of nutrients and its implications for fetal growth

Placental growth during early and mid-pregnancy has a powerful, constraining influence on fetal growth during late pregnancy. Studies involving surgical and environmental reduction of placental size in sheep have shown an associated reduction in capacity to transport oxygen, glucose and amino acids. Oxygen transport is limited by placental blood flow but transport of glucose and amino acids is determined by the abundance and activity of specific transport proteins. Glucose transporters include the GLUT1 and GLUT3 isoforms previously identified in brain and other tissues; systems for active transport of amino acids have been inferred but not characterized. Placental metabolism of glucose and amino acids has major effects both on the quantity of carbon and nitrogen delivered to the fetus, and on the composition of substrates involved. For example, the uteroplacental tissues consume more than 60% of uterine glucose uptake during late pregnancy, and the placenta substantially modifies the pattern of amino acids delivered to fetal blood. The placenta also participates in the array of metabolic adaptations of maternal and conceptus tissues to altered maternal nutrient supply. Placental capacity for glucose transport in moderately undernourished ewes is upregulated, partly by increased expression of the GLUT3 transport protein. During more severe glucose deprivation, placental transfer and fetal uptake of glucose are constrained in proportion with maternal supply, leading to fetal growth retardation.

Mass spectrometry of native rat amelogenins: primary transcripts, secretory isoforms, and C-terminal degradation

Cloning technologies have established unambiguously that amelogenins always seem larger in molecular weight (Mr) by gel electrophoresis (SDS-PAGE) than by mass spectrometry (MS). This has caused many problems relating cloned versions of amelogenin to proteins actually secreted by ameloblasts in vivo. In this study, discrete protein fractions at 31-20 kDa (Mr(SDS)) were prepared from freeze-dried rat incisor enamel by techniques optimized for preserving protein integrity. N-terminal sequence and amino acid compositional analyses indicated that the major protein forming these fractions was amelogenin. As expected, the molecular weights estimated by matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) MS were significantly less than their apparent molecular weights estimated by SDS-PAGE. Plots of Mr(SDS) vs. Mr(MS) for all fractions showed high linear correlation (r = 0.992). Analysis of MS data further indicated that the major protein in the 27-kDa fraction corresponded to the R180 secretory isoform of rat amelogenin, whereas some minor proteins in the 23-kDa fraction likely corresponded to a R156 secretory isoform. This was in contrast to major proteins forming the 25-, 24-, and 23-kDa fractions (Mr(SDS)), which seemed to represent proteolytic fragments of R180 progressively altered at the P169-A170, P164-L165, and F151-S152 C-terminal cleavage sites, respectively. Proteins in the 20-kDa fraction (Mr(SDS)) most closely matched by ESI-MS fragments of the R156 secretory isoform that were C-terminally-modified at the equivalent P164-L165 site.

Protein nutrition in late pregnancy, maternal protein reserves and lactation performance in dairy cows

Empirical evidence suggests that prolonged underfeeding of protein to late-pregnant dry cows can have modest negative carry-over effects on milk volume and/or protein yield during early lactation, and may also cause increased incidence of metabolic diseases associated with fatty liver. However, assessment of requirements is hampered by lack of information on relationships between dietary intake of crude protein (N x 6.25) and metabolizable protein supply during late pregnancy, and by incomplete understanding of the quantitative metabolism of amino acids in maternal and conceptus tissues. Inability of the postparturient cow to consume sufficient protein to meet mammary and extra-mammary amino acid requirements, including a significant demand for hepatic gluconeogenesis, necessitates a substantial, albeit transient, mobilization of tissue protein during the first 2 weeks of lactation. Ultimately, much of this mobilized protein appears to be derived from peripheral tissues, especially skeletal muscle and, to a lesser extent, skin, through suppression of tissue protein synthesis, and possibly increased proteolysis. In the shorter term, soon after calving, it is likely that amino acids required for hepatic glucose synthesis are diverted from high rates of synthesis of splanchnic tissue and export proteins, including serum albumin. The prevailing endocrine milieu of the periparturient cow, including major reductions in plasma levels of insulin and insulin-like growth factor-I, together with insulin resistance in peripheral tissues, must permissively facilitate, if not actively promote, net mobilization of amino acids from these tissues.

Effects of birth weight and postnatal nutrition on neonatal sheep: II. Skeletal muscle growth and development

This study investigated effects of birth weight and postnatal nutrition on growth and development of skeletal muscles in neonatal lambs. Low (L; mean +/- SD 2.289 +/- .341 kg, n = 28) and high (H; 4.840 +/- .446 kg, n = 20) birth weight male Suffolk x (Finnsheep x Dorset) lambs were individually reared on a liquid diet to grow rapidly (ad libitum fed, ADG 337 g, n = 20) or slowly (ADG 150 g, n = 20) from birth to live weights (LW) up to approximately 20 kg. At birth, weight of semitendinosus (ST) muscle in L lambs was 43% that in H lambs; aggregate weights of ST and seven other dissected muscles were similarly reduced. In ST muscle of L lambs, mass of DNA, RNA, and protein were also significantly reduced to levels 67, 60, and 34%, respectively, of those in H lambs. However, myofiber numbers of ST, tibialis caudalis, or soleus muscles did not differ between the L and H birth weight lambs and did not change during postnatal growth. During postnatal rearing, daily accretion rate of dissected muscle was lower in L than in H lambs. Accretion of muscle per kilogram of gain in empty body weight (EBW) was reduced in the slowly grown L lambs compared with their H counterparts, although the difference was less pronounced between the rapidly grown L and H lambs. Throughout the postnatal growth period, ST muscle of L lambs contained less DNA with a higher protein:DNA ratio at any given muscle weight than that of H lambs. Slowly grown lambs had heavier muscles at any given EBW than rapidly grown lambs. Content of DNA and protein:DNA ratio in ST muscle were unaffected by postnatal nutrition, but RNA content and RNA:DNA were greater and protein:RNA was lower at any given muscle weight in rapidly grown lambs. Results suggest that myofiber number in fetal sheep muscles is established before the presumed, negative effects of inadequate fetal nutrient supply on skeletal muscle growth and development become apparent. However, proliferation of myonuclei may be influenced by fetal nutrition in late pregnancy. Reduced myonuclei number in severely growth-retarded newborn lambs may limit the capacity for postnatal growth of skeletal muscles.

Negative regulation of epidermal growth factor signaling by selective proteolytic mechanisms in the endosome mediated by cathepsin B

We have investigated the relevant protease activity in rat liver, which is responsible for most of the receptor-mediated epidermal growth factor (EGF) degradation in vivo. EGF was sequentially cleaved by endosomal proteases at a limited number of sites, which were identified by high performance liquid chromatography and mass spectrometry. EGF proteolysis is initiated by hydrolysis at the C-terminal Glu(51)-Leu(52) bond. Three additional minor cleavage sites were identified at positions Arg(48)-Trp(49), Trp(49)-Trp(50), and Trp(50)-Glu(51) after prolonged incubation. Using nondenaturating immunoprecipitation and cross-linking procedures, the major proteolytic activity was identified as that of the cysteine protease cathepsin-B. The effect of injected EGF on subsequent endosomal EGF receptor (EGFR) proteolysis was further evaluated by immunoblotting. Using endosomal fractions prepared from EGF-injected rats and incubated in vitro, the EGFR was lost with a time course superimposable with the loss of phosphotyrosine content. The cathepsin-B proinhibitor CA074-Me inhibited both in vivo and in vitro the endosomal degradation of the EGFR and increased the tyrosine phosphorylation states of the EGFR protein and the molecule SHC within endosomes. The data, therefore, describe a unique pathway for the endosomal processing of internalized EGF receptor complexes, which involves the sequential function of cathepsin-B through selective degradation of both the ligand and receptor.

N-terminal sequence analysis of SAA-derivatives purified from murine inflammatory macrophages

The pathogenesis of secondary amyloidosis in vivo is not well-understood. Experimental studies suggest that incomplete degradation of acute phase serum amyloid A (SAA), presumably endocytosed by activated monocytoid cells, may lead to intralysosomal formation of amyloid A (AA). To establish a possible link between these two events, we have carried out partial N-terminal sequence analysis of affinity purified SAA derivatives from peritoneal macrophages isolated at 4 weeks post-infection from alveolar hydatid cyst infected C57BL/6 mice. The macrophage lysates yielded five N-terminally intact SAA derivatives of approximately 5 to approximately 12 kDa which reacted with anti-mouse AA IgG, and contained a mixture of SAA1 and SAA2 isoforms. The SAA2:SAA1 ratio, evaluated from their proportion present in each M(r) SAA derivative, showed a decrease with the decreasing apparent mass of the N-terminally infected SAA material. These results not only confirm that both SAA1 and SAA2 are processed by activated monocytoid cells but, more importantly, establish a plausible link between N-terminally intact SAA derivatives and formation of AA within activated monocytoid cells.

Effects of birth weight and postnatal nutrition on neonatal sheep: I. Body growth and composition, and some aspects of energetic efficiency

We investigated the effects of birth weight and postnatal nutrition on growth characteristics of neonatal lambs. Low- and high-birth-weight male lambs were individually reared on a high-quality liquid diet to grow rapidly (ad libitum access to feed) or slowly (ADG 150 g) to various weights up to 20 kg live weight (LW). Average daily gain tended to be greater in the high- (mean+/-SE 345+/-14 g) than in the low- (329+/-15 g) birth-weight lambs given ad libitum access to feed owing to slower growth by the small newborns during the immediate postpartum period. At birth, on a weight-specific basis, small newborns contained 6.4% less nitrogen and tended to have more ash (8.9%) than the high-birth-weight newborns. Daily rates of fat, ash, and GE accretion were greater, and nitrogen accretion tended to be greater in the rapidly grown large newborns than in their small counterparts. At any given empty body weight (EBW) during rearing, low-birth-weight lambs contained more fat and less ash, resulting in slowly and rapidly grown small newborns containing 39.3 and 42.7 Mcal GE, respectively, at completion of the study (17.5 kg EBW), compared with 34.8 and 40.5 Mcal in their large counterparts. The differences in fatness and energy content between the birth weight categories are attributed to energy requirements for maintenance that were approximately 30% lower, coupled with higher relative intakes in the low-birthweight lambs, during the early postnatal period. At this time, the ability to consume nutrients in excess of lean tissue growth requirements was apparently more pronounced in small than in large newborns, which resulted in lower efficiency of energy utilization for tissue deposition. Furthermore, body composition differences between the slowly and rapidly reared lambs support the notion of a priority of lean tissue over fat when nutrient supply is limited.

Conserved in vivo phosphorylation of calnexin at casein kinase II sites as well as a protein kinase C/proline-directed kinase site

Calnexin is a lectin-like chaperone of the endoplasmic reticulum (ER) that couples temporally and spatially N-linked oligosaccharide modifications with the productive folding of newly synthesized glycoproteins. Calnexin was originally identified as a major type I integral membrane protein substrate of kinase(s) associated with the ER. Casein kinase II (CK2) was subsequently identified as an ER-associated kinase responsible for the in vitro phosphorylation of calnexin in microsomes (Ou, W-J., Thomas, D. Y., Bell, A. W., and Bergeron, J. J. M. (1992) J. Biol. Chem. 267, 23789-23796). We now report on the in vivo sites of calnexin phosphorylation. After 32PO4 labeling of HepG2 and Madin-Darby canine kidney cells, immunoprecipitated calnexin was phosphorylated exclusively on serine residues. Using nonradiolabeled cells, we subjected calnexin immunoprecipitates to in gel tryptic digestion followed by nanoelectrospray mass spectrometry employing selective scans specific for detection of phosphorylated fragments. Mass analyses identified three phosphorylated sites in calnexin from either HepG2 or Madin-Darby canine kidney cells. The three sites were localized to the more carboxyl-terminal half of the cytosolic domain: S534DAE (CK2 motif), S544QEE (CK2 motif), and S563PR. We conclude that CK2 is a kinase that phosphorylates calnexin in vivo as well as in microsomes in vitro. Another yet to be identified kinase (protein kinase C and/or proline-directed kinase) is directed toward the most COOH-terminal serine residue. Elucidation of the signaling cascade responsible for calnexin phosphorylation at these sites in vivo may define a novel regulatory function for calnexin in cargo folding and transport to the ER exit sites.

The upstream regulatory regions of the hepatocyte growth factor gene promoter are essential for its expression in transgenic mice

To understand the molecular mechanisms of hepatocyte growth factor (HGF) gene transcription in vivo, we report the generation and characterization of transgenic mice harboring various lengths of the mouse HGF promoter linked to the chloramphenicol acetyltransferase reporter gene. Analysis of different tissues of the transgenic mouse lines having the 2.7-kilobase (kb) promoter construct revealed a pattern of reporter gene expression in embryonic and adult tissues that paralleled that of endogenous HGF gene expression. A similar expression pattern was observed in the 0.7-kb transgenic lines. However, in contrast to in vitro data, no promoter activity was detected in four independent transgenic lines harboring the 0.1-kb construct. Akin to the activity of the endogenous HGF gene, which is induced in the liver, lung, and spleen in response to 70% partial hepatectomy, the reporter gene driven by the 2.7-kb promoter construct was strongly induced, whereas that driven by the 0.7-kb promoter construct was modestly induced in these organs after partial hepatectomy. Together, these data suggest that the region between -0.1 and -0.7 kb of the HGF gene promoter is essential to drive its expression in vivo and that additional upstream sequences located between -0.7 and -2.7 kb are also necessary for its maximum inducibility in response to cues that stimulate tissue growth and regeneration.

Mouse testicular sulfated glycoprotein-1: sequence analysis of the common backbone structure of prosaposins

We have generated a cDNA encoding the mouse sulfated glycoprotein-1 (SGP-1) by polymerase chain reaction amplification of a mouse testicular Uni-Zap XR cDNA library with two synthetic oligonucleotide primers. A positive signal of 1,959 bases was isolated and subcloned into the pGEM-T. Sequence analysis showed a near identical nucleotide and amino acid similarity to mouse prosaposin cDNA. A few amino acid differences were found, and they may represent strain-specific heterogeneities. The cDNA has 88% amino acid identity to rat SGP-1 and 64% identity to human prosaposin. Prosaposin is the precursor of four lysosomal saposins (A, B, C, and D) that are generated by the proteolytic maturation of the former. Saposins are sphingolipid binding proteins that function as activators of lysosomal enzymes involved in sphingolipid hydrolysis. Northern blot analysis demonstrated that SGP-1 mRNA is transcribed in the seminiferous epithelium by Sertoli cells but not by germinal cells. Our results also demonstrated two forms of alternatively spliced testicular SGP-1 mRNA. This alternative splicing results in the inclusion or exclusion of exon 8, which encodes for three amino acid residues (QDQ) that are implicated in the sphingolipid binding affinity of saposin B. Sequence aligment indicates that all saposins share a common motif characterized by six conserved cysteines, a conserved N-linked glycosylation site, a conserved proline residue, and 15 positions that are characterized by large hydrophobic amino acids. These characteristics, together with similar secondary structure predictions and the predicted similar formation of three disulfide linkages, create a common framework of amino acids of three alpha helices enclosing an internal hydrophobic core for all saposins. The disulfide placement data, the hydropathy profile, and the presence of amphiphatic helices indicate that all saposins are stable proteins sharing similar secondary and tertiary structures.

Nutrition, development and efficacy of growth modifiers in livestock species

Somatotropin (ST) and synthetic beta-adrenergic agonists (beta-AA) are growth-modifying agents that increase the rate and sometimes, the efficiency of protein deposition in lean tissues of livestock species. The ST-induced increase in muscle protein deposition is effected by a relatively modest increase in protein synthetic rate. This is possibly mediated by the endocrine influence of marked increases in circulating IGF (insulin-like growth factor)-I, and other ST-dependent components of the IGF system; mediation by locally expressed IGF-I may also occur. Increased muscle protein accretion in animals treated with beta-AA seems to be directly mediated by binding of the synthetic agonist to muscle beta-1 or beta-2 receptors, leading to increased muscle protein synthesis, possibly accompanied or followed by decreased protein degradation. This response is transient, due to down-regulation of beta-adrenergic receptors. Maximal responses of muscle protein accretion to both ST and beta-AA are attenuated by feeding inadequate levels of total protein or specific, limiting amino acids. For ST, but not beta-AA, this effect in growing pigs is partially offset by increased efficiency of utilization of absorbed amino acids for protein deposition, with predictable consequences for dietary protein and amino acid requirements. Both ST and beta-AA are less efficacious in promoting muscle protein deposition in very young animals. For ST, this is related to postnatal development of the somatotropic axis; a mechanistic explanation for the similar lack of effect of beta-AA is lacking. In both cases, this phenomenon must be considered against the very high inherent capacity and efficiency of lean tissue protein accretion in the neonate.

Developmental increases in glucose transporter concentration in the sheep placenta

To explore the molecular basis for the gestational increase in glucose transport capacity of the sheep placenta in vivo, placentas from twin-pregnant ewes at days 75, 110, and 140 postcoitus (n = 6/group) were analyzed for glucose transporter (GT) concentration. Concentration (pmol/mg protein) of D-glucose-inhibitable binding sites, measured by [3H]cytochalasin B binding analysis, increased 3.4 times from mid- to late pregnancy. Concurrently, abundance of GLUT-1 and GLUT-3 protein, measured by immunoblotting with specific polyclonal antibodies, increased 2.3 and 2.9 times, respectively, while abundance of GLUT-1 and GLUT-3 mRNA, measured by Northern blotting, increased 1.8 and 3.9 times, respectively. GLUT-4 protein was undetectable at all stages of pregnancy. Quantitative immunoblotting indicated that GLUT-1 accounted for 86.8 +/- 1.6% at day 75 and 56.1 +/- 4.1% at day 140 of total cytochalasin B binding sites. Thus increases in GT concentration explain much of the gestational increase in glucose transfer capacity observed in vivo. The gestational decline in relative contribution of GLUT-1 to cytochalasin binding, together with the greater developmental increases in GLUT-3 mRNA and protein, further suggests that the relative importance of GLUT-3 increases with gestational age.

Adaptations of glucose metabolism during pregnancy and lactation

Increased glucose requirements of the gravid uterus during late pregnancy and even greater requirements of the lactating mammary glands necessitate major adjustments in glucose production and utilization in maternal liver, adipose tissue, skeletal muscle, and other tissues. In ruminants, which at all times rely principally on hepatic gluconeogenesis for their glucose supply, hepatic glucose synthesis during late pregnancy and early lactation is increased to accommodate uterine or mammary demands even when the supply of dietary substrate is inadequate. At the same time, glucose utilization by adipose tissue and muscle is reduced. In pregnant animals, these responses are exaggerated by moderate undernutrition and are mediated by reduced tissue sensitivity and responsiveness to insulin, associated with decreased tissue expression of the insulin-responsive facilitative glucose transporter, GLUT4. Peripheral tissue responses to insulin remain severely attenuated during early lactation but recover as the animal progresses through mid lactation. Specific homeorhetic effectors of decreased insulin-mediated glucose metabolism during late pregnancy have yet to be conclusively identified. In contrast, somatotropin is almost certainly a predominant homeorhetic influence during lactation because its exogenous administration causes specific changes in glucose metabolism (and many other functions) of various nonmammary tissues which faithfully mimic normal adaptations to early lactation.

Paradoxical increases of circulating nonesterified fatty acids in somatotropin treated cattle undergoing mild disturbances

Effects of various doses of bovine somatotropin (bST) on plasma concentrations of nonesterified fatty acids (NEFA) were studied in a 14-d Latin square with six Holstein heifers. Animals were given daily injections of excipient or bST at 12:00 p.m. and fed twice daily at 7:00 a.m. and 7:00 p.m. On Day 14, plasma NEFA remained low through the day except around the 7:00 p.m. feeding when they were substantially elevated. The elevation was significantly greater in bST-treated animals and corresponded to the excitement of the animals in anticipation of the evening feeding. To further investigate this phenomenon, a second experiment was conducted in which nine growing Holstein steers were fed hourly and received either daily intramuscular (i.m.) injection of excipient or bST (120 mg/kg BW) for 15 d in a crossover design. Daily profiles of NEFA were obtained under undisturbed conditions or concurrently with intensive handling. Although no elevations could be detected in any case in control animals, bST caused a substantial rise in NEFA concentration only when animals were subjected to intensive handling. This suggested that NEFA peaks noted in bST-treated heifers in the first experiment resulted from increased ability of adipose tissue to respond to adrenergic stimulation associated with the anticipation of feeding. Consistent with this hypothesis, plasma NEFA concentrations in bST-treated steers were increased to a greater extent during a challenge involving i.v. injection of epinephrine. This amplification of adipose tissue response by bST must be considered when conducting intensive studies. Even the minimal excitement associated with blood sampling can confound the results regarding lipid mobilization, and this may have contributed to the notion that ST is a lipolytic hormone.

Identification of the major synaptojanin-binding proteins in brain

Synaptojanin is a nerve-terminal enriched inositol 5-phosphatase thought to function in synaptic vesicle endocytosis, in part through interactions with the Src homology 3 domain of amphiphysin. We have used synaptojanin purified from Sf9 cells after baculovirus mediated expression in overlay assays to identify two major synaptojanin-binding proteins in rat brain. The first, at 125 kDa, is amphiphysin. The second, at 40 kDa, is the major synaptojanin-binding protein detected, is highly enriched in brain, is concentrated in a soluble synaptic fraction, and co-immunoprecipitates with synaptojanin. The 40-kDa protein does not bind to a synaptojanin construct lacking the proline-rich C terminus, suggesting that its interaction with synaptojanin is mediated through an Src homology 3 domain. The 40-kDa synaptojanin-binding protein was partially purified from rat brain cytosol through a three-step procedure involving ammonium sulfate precipitation, sucrose density gradient centrifugation, and DEAE ion-exchange chromatography. Peptide sequence analysis identified the 40-kDa protein as SH3P4, a member of a novel family of Src homology 3 domain-containing proteins. These data suggest an important role for SH3P4 in synaptic vesicle endocytosis.