Transcriptional and post-transcriptional regulation of prolactin during the turkey reproductive cycle

Evidence for the Existence of Two Prolactin Isoforms in the Developing Pituitary Gland of the Goose (Anser cygnoides)

Compared to Galliformes such as chicken and turkey, very little is known about the existence and expression of isoforms of prolactin (PRL) in the pituitary glands of Anseriformes. In this study, by generating a rabbit-anti-goose (Anser cygnoides) PRL polyclonal antibody, we analysed the expression patterns of goose PRL isoforms in the embryonic and post-hatch development of the pituitary gland. Our results showed that two immunoreactive bands with molecular weights of about 23 and 26 kDa were detected using the Western blot technique, corresponding to the non-glycosylated (NG-) and the glycosylated (G-) isoform of PRL, respectively. The protein levels of the total PRL in a goose increased gradually from the embryonic day (ED) 22 to the post-hatch day (PD) 28, with a non-significant decrease on PD6. Furthermore, the percentage of G-PRL in the pituitary gland of the goose fluctuated from about 30.3% to 54.7% throughout the embryonic and post-hatch development. At the mRNA level, the expression of PRL increased steadily during the development and reached the highest levels on PD12, but later showed a non-significant decrease on PD28. The inconsistent expression patterns between the PRL mRNA and protein during the stages from PD6 to PD28 indicated that the PRL gene expression involves both transcriptional and post-translational regulation. Taken together, our data unequivocally demonstrated the existence of NG- and G-PRL in the pituitary gland of a goose and that the expression of the total PRL as well as the percentage of G-PRL significantly changed during embryonic and post-hatch development, indicating that the versatile biological functions of PRL during the ontogenesis of a goose could be closely related to changes in both its total expression and the degree of glycosylation in the pituitary gland.

Development of a sandwich ELISA for determining plasma prolactin concentration in domestic birds

The present study was conducted to establish a sandwich ELISA for the determination of prolactin (PRL) concentrations in the plasma of domestic fowls. The assay uses a recombinant goose PRL as the reference standard, expressed in a eukaryotic system, and as the antigen for raising a polyclonal antibody in rabbit. This rabbit anti-goose PRL polyclonal antibody was used for coating the wells of the ELISA plate, and its biotinylated form served as the detection antibody. An avidin-conjugated horseradish peroxidase was used to bind the detection antibody and to catalyze the chromogenic reaction using 3,3',5,5'-tetramethylbenzidine as the substrate. The assay showed a linear relationship between the optical density and concentration of the standard PRL in the 0 to 12.5 ng/mL range, and the assay was sensitive to a concentration as low as 0.39 ng/mL. The intra- and inter-assay CVs were <7% and 11%, respectively. The response curves of the serially diluted plasma samples from goose, duck, and chicken exhibited similar parallel relationships to that observed for the reference standards. Consistent with previous findings, the assay effectively detected differences in PRL concentration in plasma samples from chicken, duck, and goose at various reproductive stages.

Presence of prolactin mRNA in extra-pituitary brain areas in the domestic turkey

It is well known that prolactin plays diverse roles in vertebrate reproduction. Besides expression in the pituitary, prolactin is also found in extra-pituitary tissues. In the present study, prolactin mRNA expression was studied utilizing in situ hybridization histochemistry. Prolactin mRNA, while found throughout the turkey brain, was predominantly localized within the pituitary, confirming a pivotal role of prolactin in turkey reproduction. The expression of prolactin mRNA was also observed within extra-pituitary brain areas including the cerebellum, nucleus accumbens, lateral septum, anterior hypothalamic nucleus, lateral hypothalamus, paraventricular nucleus, ventromedial nucleus, and infundibular nuclear complex. In the hypothalamus, an abundance of prolactin mRNA-expressing cells was observed in the anterior hypothalamic nucleus, lateral hypothalamus, and ventromedial nucleus. Cells expressing the least prolactin mRNA were found in the lateral septum, paraventricular nucleus, and the infundibular nuclear complex. This study reveals, for the first time, that prolactin mRNA was expressed in extra-pituitary brain areas in birds. In addition, the diverse expression of prolactin mRNA in the brain areas suggests that prolactin plays various physiological roles in birds.

Functional genomics of the chicken--a model organism

Since the sequencing of the genome and the development of high-throughput tools for the exploration of functional elements of the genome, the chicken has reached model organism status. Functional genomics focuses on understanding the function and regulation of genes and gene products on a global or genome-wide scale. Systems biology attempts to integrate functional information derived from multiple high-content data sets into a holistic view of all biological processes within a cell or organism. Generation of a large collection ( approximately 600K) of chicken expressed sequence tags, representing most tissues and developmental stages, has enabled the construction of high-density microarrays for transcriptional profiling. Comprehensive analysis of this large expressed sequence tag collection and a set of approximately 20K full-length cDNA sequences indicate that the transcriptome of the chicken represents approximately 20,000 genes. Furthermore, comparative analyses of these sequences have facilitated functional annotation of the genome and the creation of several bioinformatic resources for the chicken. Recently, about 20 papers have been published on transcriptional profiling with DNA microarrays in chicken tissues under various conditions. Proteomics is another powerful high-throughput tool currently used for examining the dynamics of protein expression in chicken tissues and fluids. Computational analyses of the chicken genome are providing new insight into the evolution of gene families in birds and other organisms. Abundant functional genomic resources now support large-scale analyses in the chicken and will facilitate identification of transcriptional mechanisms, gene networks, and metabolic or regulatory pathways that will ultimately determine the phenotype of the bird. New technologies such as marker-assisted selection, transgenics, and RNA interference offer the opportunity to modify the phenotype of the chicken to fit defined production goals. This review focuses on functional genomics in the chicken and provides a road map for large-scale exploration of the chicken genome.

Environmental regulation of the reproductive system in a flexibly breeding Sonoran Desert bird, the Rufous-winged Sparrow, Aimophila carpalis

We investigated reproductive regulation in male Rufous-winged Sparrows, Aimophila carpalis, a Sonoran Desert passerine that breeds after irregular summer rains. Field and captive data demonstrate that increased photoperiod stimulates testicular development in March and maintains it until early September. Free-living birds caught in July and placed on captive long days (16L: 8D) maintained developed testes for up to 7 months, and free-living birds caught in September, during testicular regression, redeveloped testes when placed on captive long days, indicating that these birds were still photosensitive. Captive birds on long days maintained testicular development when exposed to temperatures mimicking those occurring during regression in free-living birds. In free-living birds, testicular development was observed during spring and summer, but unless this was associated with rainfall, breeding (indicated by juveniles) did not occur. Large increases in plasma luteinizing hormone (LH) in free-living males were correlated with heavy rainfall in July/August, when the birds bred, and in November, when they did not breed. In captive birds, plasma LH concentrations were unresponsive to photoperiodic changes, but may have responded to social cues. Plasma prolactin concentrations were directly correlated with photoperiod in free-living birds, but an effect of photoperiod on prolactin secretion was not seen in captive birds. It is concluded that male Rufous-winged Sparrows use long photoperiods to stimulate and maintain testicular development, but exposure to long photoperiods does not terminate breeding by inducing absolute photorefractoriness. The specific timing of reproductive behaviors is apparently determined by elevated plasma LH coinciding with long day stimulated gonad development.

The increase in prolactin-secreting cells in incubating chicken hens can be mimicked by extended treatment of pituitary cells in vitro with vasoactive intestinal polypeptide (VIP)

Incubation of eggs by birds and lactation in mammals are regulated by pituitary prolactin (PRL) and associated with an increase in pituitary PRL-producing cells or lactotrophs. However, the mechanisms controlling this increase in lactotroph numbers are not known. PRL secretion in birds is regulated by vasoactive intestinal polypeptide (VIP). This study was designed to determine whether VIP treatment could modulate lactotroph abundance in culture. Anterior pituitary cells were isolated from laying Japanese White Silkie hens and cultured for 2 or 6 days in the absence or presence of VIP. PRL-secreting cells were identified by reverse hemolytic plaque assay. Treatment with VIP for 6 days substantially increased the abundance of PRL-secreting cells from 47.5% under basal conditions to 70.6% of all pituitary cells following VIP stimulation. However, 2-day VIP treatment had no effect. Furthermore, the extent to which the hens were allowed to accumulate eggs in a clutch prior to isolation of the pituitaries did not affect the lactotroph response to VIP in vitro. These findings indicate that chronic VIP stimulation may be responsible for the increased abundance of lactotrophs found in the pituitary glands of incubating hens.

Turkey prolactin gene regulation by VIP through 35-bp cis-acting element in the proximal promoter

Vasoactive intestinal peptide (VIP) has been shown to increase prolactin (PRL) gene expression and secretion in turkey primary anterior pituitary cells. To characterize cis-acting elements involved in stimulation of PRL gene expression by VIP, 5'-flanking deletions and/or mutations of the turkey PRL promoter fused to the luciferase (Luc) reporter gene have been constructed for use in transient transfection assays. Deletion analysis of the turkey PRL promoter (tPRLP) indicated that the VIP-stimulated tPRLP activity was controlled by three major positive regulatory regions and two negative regions. The -74/+40 Luc construct exhibited a 7- to 8-fold increase in promoter activity in response to VIP treatment. Deletion of the 35-bp segment (-74/-40) or fusion of this sequence to the SV40 promoter demonstrated that a VIP response element (VRE) was present in this region. Functional analysis of this VRE (-74/-40) was performed by mutation of core sequences (TGAATGTATGCA, -61/-50) or deletion of a 35-bp segment and a Decoy assay. Electrophoretic mobility shift assays revealed the presence of three DNA-protein complexes bound to the region -73 to -41. The results of the present study demonstrated that VRE (35-bp) in the proximal PRL promoter is an important cis-acting element for VIP-stimulated PRL gene expression in turkey primary anterior pituitary cells.

Expression of vasoactive intestinal peptide receptor messenger RNA in the hypothalamus and pituitary throughout the turkey reproductive cycle

Vasoactive intestinal peptide (VIP) has been implicated in the regulation of avian reproductive activity and appears to act at the level of the hypothalamus and pituitary. This in situ hybridization histochemistry study describes the distribution of VIP receptor mRNA expression in the hypothalamus and the pituitary of reproductively active (laying) and quiescent (nonphotostimulated, incubating, and photorefractory) female turkeys and characterizes the differences observed in VIP receptor gene expression. VIP receptor mRNA, while expressed throughout the hypothalamus, was specifically expressed in areas known to contain GnRH-I neurons in the chicken, i.e., the lateral septum, medial preoptic area, anterior hypothalamus, and paraventricular nucleus. Significant differences in VIP receptor mRNA expression between different reproductive states was observed only within the infundibular nuclear complex. VIP receptor mRNA was markedly less in nonphotostimulated and photorefractory hens as compared with laying and incubating hens. The most dense VIP receptor mRNA was found in the anterior pituitary, where it was 2.4- and 3.0-fold greater in laying and incubating hens, respectively, as compared with that in nonphotostimulated ones. Hens that stopped incubating and became photorefractory displayed pituitary VIP receptor mRNA levels similar to those of nonphotostimulated birds. The changes in pituitary VIP receptor mRNA expression were positively correlated with known changes in pituitary prolactin (PRL) mRNA expression and PRL content and release. These findings indicate that the variations in PRL secretion observed across the turkey reproductive cycle are, in part, regulated by changes in VIP receptors at the pituitary level.

Influence of VIP on prolactinemia in turkey anterior pituitary cells: role of cAMP second messenger in VIP-induced prolactin gene expression

Vasoactive intestinal peptide (VIP) is the avian prolactin (PRL)-releasing factor. In the turkey, hypothalamic VIP immunoreactivity and mRNA content, as well as VIP levels in hypophyseal portal blood, are closely related to the state of prolactinemia and the reproductive stage. The present study investigated the role of VIP on prolactinemia in turkey anterior pituitary (AP) cells through PRL gene expression and the role of a cAMP second messenger system on VIP-induced PRL expression. In primary AP cells harvested from hens in different prolactinemic states, steady state promoter activities were positively correlated with secreted PRL levels. VIP increased PRL promoter activities in AP cells from hens with intermediate PRL levels (laying), but not in AP cells from hypoprolactinemic hens (nonphotostimulated reproductively quiescent). However, in AP cells from hyperprolactinemic hens (incubating), PRL promoter activity was down-regulated by VIP. PRL mRNA steady state levels were significantly decreased by the cAMP analogue, 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), and PRL secretion was down-regulated by the phosphodiesterase blocker, 3-isobutyl-1-methylxanthine (IBMX) in a dose-dependent manner, suggesting that the cAMP second messenger system might be involved in the inhibitory action of dopamine upon VIP-stimulated PRL secretion and gene expression at the pituitary level. In a study of VIP immediate and long-term effects on c-fos expression in relation to PRL expression, VIP dramatically induced c-fos mRNA expression within 5 min, suggesting that VIP-induced c-fos expression might be involved in VIP-stimulated PRL secretion and gene expression. These results provide additional evidence of the functional significance of VIP in PRL gene expression and suggest that changes in PRL promoter activity by VIP may be one of the important inductive mechanisms leading to prolactinemia.

Photostimulated prolactin release in the Turkey hen: effect of ovariectomy and environmental temperature

Ambient temperature modulates prolactin (PRL) secretion in birds. It is not known whether this modulation directly affects the PRL controlling mechanism(s) or whether it indirectly influences them through the onset of sexual maturity and/or the onset of incubation behavior. These experiments were designed to investigate the effect of elevated (32 degrees ) and reduced (10 degrees ) ambient temperatures on PRL secretion. Somatically mature, ovariectomized female turkeys were used to avoid the confounding effects of reproductive stage, nesting, and egg stimuli on PRL secretion. Hens were ovariectomized 5 weeks before, on the day of, or 10 days after the inception of photostimulation. Temperature treatments included chronic exposure (5 weeks) to 32 or 10 degrees and acute exposure (i.e., temperature was reversed from 32 to 10 degrees or from 10 to 32 degrees on or after the day of photostimulation). Chronic exposure to either 32 or 10 degrees had no effect on the rise in serum PRL that followed photostimulation in both sham-operated controls and ovariectomized hens. Acute exposure to 10 or 32 degrees altered the photoperiodically stimulated rise in plasma PRL. Birds switched from 10 to 32 degrees showed a significantly greater PRL increase than birds shifted from 32 to 10 degrees. Ovariectomy enhanced the PRL response to the gonadal stimulating photoperiod. The effect was most pronounced in hens photostimulated prior to ovariectomy. These findings suggest that ambient temperature and/or ovariectomy have a modulating effect on the PRL response to long days.

Molecular cloning and expression analysis of the turkey vasoactive intestinal peptide receptor

Vasoactive intestinal peptide (VIP) is a prolactin (PRL)-releasing factor whose activity in avian species is believed to be mediated by a specific VIP receptor (VIP-R). Circulating PRL levels are closely related to hypothalamic VIP immunoreactivity, hypothalamic VIP mRNA content, and hypophysial-portal blood VIP concentrations in turkeys. In the present study, a turkey VIP-R (tVIP-R) cDNA was cloned and its mRNA abundance was quantified in various tissues during different reproductive stages. The 2347-bp tVIP-R cDNA encoded a 457 amino acid protein, with a predicted Mr of 52 kDa. The full-length cDNA shares approximately 55% similarity with the mammalian VIP receptor-1. Northern blot analysis revealed that a major 2.7-kb transcript was expressed in laying hen pituitaries. Furthermore, two minor tVIP-R transcripts of 3.7 and 3.4 kb were observed. Semiquantitative reverse transcription polymerase chain reaction (RT-PCR) was performed using RNA from various turkey brain and peripheral tissues throughout the reproductive cycle. The steady-state levels of pituitary tVIP-R mRNA changed during the reproductive cycle, whereas mRNA expression in other tissues was not affected. The steady-state levels of tVIP-R mRNA were only affected in the pituitary, whereas mRNA expression in any of the other tissues examined following the immunization of turkeys against VIP were not affected.

Active immunization with vasoactive intestinal peptide in turkey hens

Active immunization of turkey hens against vasoactive intestinal peptide (VIP) has been shown to inhibit incubation behavior and to increase egg production in second-cycle hens. The objective of this study was to compare the effect of VIP immunization on first- and second-cycle turkey hens during a 27-wk production period. First- (25-wk-old) and second- (54-wk-old) cycle hens were intermixed, distributed among 16 pens, and subjected to a photoperiod of 6 h of light and 18 h of darkness for 10 wk. The first-cycle hens were divided into two groups: keyhole limpet hemocyanin (KLH)-immunized controls (n = 16) and VIP-immunized (n = 18). Second-cycle hens were divided into four groups: 1) unimmunized controls (n = 19), 2) KLH-immunized controls (n = 18), 3) VIP-immunized (n = 19), and 4) VIP-preimmunized (immunized during first cycle; n = 16). Each hen received four antigen injections beginning the day of photostimulation (4-wk intervals), except for the preimmunized hens, which received three injections beginning 4 wk after photostimulation. The maximum titer of VIP antibodies in first-cycle, second-cycle, and preimmunized hens was 17.2+/-2.2, 20.9+/-2.9, and 21.7+/-3.2%, respectively. After photostimulation, plasma prolactin of first- and second-cycle control hens peaked between 484 +/-105 and 630+/-118 ng/mL. In contrast, prolactin changed very little in VIP-immunized turkeys. The average number of daily nest visits was less in first- and second-cycle VIP-immunized hens (1.68+/-0.23 and 1.09+/-0.15 visits per hen per day, respectively) than in their respective KLH-immunized controls (2.47+/-0.36 and 2.65+/-0.45 visits per hen per day). Expression of incubation behavior was 50.0 and 52.6% in first- and second-cycle control hens, respectively, upon termination of the study. In contrast, only 11.1% first-cycle and 5.2% second-cycle VIP-immunized turkeys exhibited the hormonal and behavioral characteristics of incubating hens. Average weekly egg production of first- and second-cycle VIP-immunized turkeys was similar (3.58+/-0.19 vs. 3.63+/-0.14 eggs per hen per wk). First- and second-cycle control hens laid 2.63+/-0.25 and 2.41+/-0.20 eggs per hen per wk, respectively. The present results show that comparable egg production was attained in first- and second-cycle hens by active immunization with VIP.

Effects of vasoactive intestinal peptide on plasma prolactin in passerines

Vasoactive intestinal peptide (VIP) is a potent releaser of prolactin (PRL) in domestic fowl, turkey, and ring doves. However, few comparative studies have investigated this in wild species. We tested the effects of intravenously administered chicken VIP on plasma PRL concentrations in four passerine species: the white-crowned sparrow (Zonotrichia leucophrys gambelii), the dark-eyed junco (Junco hyemalis), the Florida scrub-jay (Aphelocoma coerulescens), and the western scrub-jay (A. californica). In the white-crowned sparrow, junco, and Florida scrub-jay, which were tested during the breeding season, VIP induced a rapid increase in plasma PRL. Serial plasma samples taken after VIP injection in the white-crowned sparrow show a 10-fold increase in PRL within 2 min of treatment, followed by a gradual decline. Effects of VIP, as compared to saline, remained significant for at least 20 min after treatment. Western scrub-jays did not respond to intravenous VIP with a significant rise in PRL secretion, possibly because they were tested after termination of the breeding season. This study indicates that VIP control of PRL release may be widespread among avian species, and that seasonal changes in plasma PRL may be mediated in part at the level of the pituitary. In addition, analysis of the control data revealed no increase in plasma PRL as a result of injection or restraint, suggesting that unlike in mammals, PRL is not released during acute stress in passerines.

In vitro release of isoforms of prolactin from pituitary glands of turkey hens at different physiological stages

To study the in vitro release of PRL isoforms, anterior pituitary glands from medium white turkeys at various physiological stages were stimulated by cVIP in a perifusion system. Pituitaries were cut into hemi-pituitaries after collection and placed into separate perifusion chambers. Medium (M199) was continuously perifused through the system and pituitaries were stimulated with cVIP (10(-7) M). Total PRL content was monitored by RIA and, the ratio of immunoreactive PRL isoforms in the perifusate was estimated by Western blotting. After exposure to X-ray film for autoradiography, the relative intensity of the bands was analyzed by densitometry. All the perifused pituitaries responded to cVIP stimulation by increasing the release of PRL. Two immunoreactive bands with relative molecular weights of 24 and 27 kDa were detected by Western blotting. The immunoreactive band corresponding to the glycosylated isoforms of PRL (27 kDa) was predominant in samples from egg-laying and incubating hens and the band corresponding to the nonglycosylated isoform (24 kDa) was predominant in samples from out-of-lay and molting stages. No changes in the ratio of isoform released were detected during cVIP stimulation. Our data clearly show that glycosylated and nonglycosylated PRL isoforms are released by the pituitary gland in vitro in the same relative proportion that was previously observed in pituitary extracts and thus are likely to reflect the secreted forms of PRL in the blood during various physiological stages. In addition, the PRL-releasing activity of VIP does not affect the ratio of isoforms secreted by the pituitary gland in vitro.

Changes in levels of immunoreactive prolactin isoforms during a reproductive cycle in turkey hens

Changes in the ratio between immunoreactive isoforms of prolactin using Western blotting and in the total prolactin content using radioimmunoassay were measured in pituitary glands from turkey hens at different physiological stages. The type of glycosylation (N- or O-linked carbohydrates) was determined using endoglycosidase digestion (N-glycosidase F, O-glycosidase, and neuraminidase). Low levels of prolactin were observed in pituitary glands from sexually immature, out-of-lay, and molting hens. Higher levels were present during the egg-laying period and the highest levels were detected in hens which expressed incubation behavior. Two immunoreactive bands of apparent molecular weights of 24 and 27 kDa were visualized on Western blots, corresponding to the nonglycosylated and glycosylated forms of prolactin, respectively. In pituitary glands from incubating turkey hens, about 70% of the prolactin was glycosylated (27-kDa isoforms), whereas about 60% was glycosylated in immature and in hens during the first egg-laying period. In pituitaries from out-of-lay and molting hens the percentage of glycosylated prolactin was 38 and 33%, respectively. Thus, higher percentages of glycosylated isoforms (27 kDa) were associated with high levels of total prolactin and lower percentages were associated with low levels of prolactin content in the pituitary gland. Digestion of the isoforms with N-glycosidase F resulted in a single band with an apparent molecular weight of 24 kDa. Partial deglycosylation was achieved using neuraminidase, whereas digestion with O-glycosidase had no apparent effect on the isoforms. Thus it appears that the glycosylated isoforms of prolactin have N-linked carbohydrates containing sialic acid.