Changes in Calcitonin Receptor Binding in the Shell Gland of Laying Hens (Gallus domesticus) During the Oviposition Cycle

Functional Analysis and Tissue-Specific Expression of Calcitonin and CGRP with RAMP-Modulated Receptors CTR and CLR in Chickens

Calcitonin (CT) and calcitonin gene-related peptide (CGRP) are critical regulators of calcium balance and have extensive implications for vertebrate physiological processes. This study explores the CT and CGRP signaling systems in chickens through cloning and characterization of the chicken calcitonin receptor (CTR) and calcitonin receptor-like receptor (CLR), together with three receptor activity-modifying proteins (RAMPs). We illuminated the functional roles for chickens between the receptors examined alone and in RAMP-associated complexes using luciferase reporter assays. Chicken CTRs and CLRs stimulated the cAMP/PKA and MAPK/ERK signaling pathways, signifying their functional receptor status, with CT showing appreciable ligand activity at nanomolar concentrations across receptor combinations. Notably, it is revealed that chicken CLR can act as a functional receptor for CT without or with RAMPs. Furthermore, we uncovered a tissue-specific expression profile for CT, CGRP, CTR, CLR, and RAMPs in chickens, indicating the different physiological roles across various tissues. In conclusion, our data establish a clear molecular basis to reveal information on CT, CGRP, CTR, CLR, and RAMPs in chickens and contribute to understanding the conserved or divergent functions of this family in vertebrates.

Circadian regulation of calcium and phosphorus homeostasis during the oviposition cycle in laying hens

Maintenance of calcium and phosphorus homeostasis in laying hens is crucial for preservation of skeletal integrity and eggshell quality, though physiological regulation of these systems is incompletely defined. To investigate changes in mineral and vitamin D3 homeostasis during the 24-h egg formation cycle, 32-wk-old commercial laying hens were sampled at 1, 3, 4, 6, 7, 8, 12, 15, 18, 21, 23, and 24 h post-oviposition (HPOP; n ≥ 4). Ovum location and egg calcification stage were recorded, and blood chemistry, plasma vitamin D3 metabolites, circulating parathyroid hormone (PTH), and expression of genes mediating uptake and utilization of calcium and phosphorus were evaluated. Elevated levels of renal 25-hydroxylase from 12 to 23 HPOP suggest this tissue might play a role in vitamin D3 25-hydroxylation during eggshell calcification. In shell gland, retinoid-x-receptor gamma upregulation between 6 and 8 HPOP followed by subsequently increased vitamin D receptor indicate that vitamin D3 signaling is important for eggshell calcification. Increased expression of PTH, calcitonin, and fibroblast growth factor 23 (FGF23) receptors in the shell gland between 18 and 24 HPOP suggest elevated sensitivity to these hormones toward the end of eggshell calcification. Shell gland sodium-calcium exchanger 1 was upregulated between 4 and 7 HPOP and plasma membrane calcium ATPase 1 increased throughout eggshell calcification, suggesting the primary calcium transporter may differ according to eggshell calcification stage. Expression in shell gland further indicated that bicarbonate synthesis precedes transport, where genes peaked at 6 to 7 and 12 to 18 HPOP, respectively. Inorganic phosphorus transporter 1 (PiT-1) expression peaked in kidney between 12 and 15 HPOP, likely to excrete excess circulating phosphorus, and in shell gland between 18 and 21 HPOP. Upregulation of FGF23 receptors and PiT-1 during late eggshell calcification suggest shell gland phosphorus uptake is important at this time. Together, these findings identified potentially novel hormonal pathways involved in calcium and phosphorus homeostasis along with associated circadian patterns in gene expression that can be used to devise strategies aimed at improving eggshell and skeletal strength in laying hens.

Physiological regulation of calcium and phosphorus utilization in laying hens

Commercial laying hens can produce one egg approximately every 24 h. During this process, regulatory systems that control vitamin D₃ metabolism, calcium and phosphorus homeostasis, and intestinal uptake of these minerals work in concert to deliver components required for eggshell calcification and bone mineralization. Commercial production cycles have been extended in recent years to last through 100 weeks of age, and older hens often exhibit an increased prevalence of skeletal fractures and poor eggshell quality. Issues such as these arise, in part, through imbalances that occur in calcium and phosphorus utilization as hens age. As a result, an in-depth understanding of the mechanisms that drive calcium and phosphorus uptake and utilization is required to develop solutions to these welfare and economic challenges. This paper reviews factors that influence calcium and phosphorus homeostasis in laying hens, including eggshell formation and development and roles of cortical and medullary bone. Metabolism and actions of vitamin D₃ and physiological regulation of calcium and phosphorus homeostasis in key tissues are also discussed. Areas that require further research in avian species, such as the role of fibroblast growth factor 23 in these processes and the metabolism and action of bioactive vitamin D₃, are highlighted and the importance of using emerging technologies and establishing in vitro systems to perform functional and mechanistic studies is emphasized.

Effect of calcitonin on adrenocorticotropic hormone secretion stimulated by corticotropin-releasing hormone in the hen anterior pituitary

The presence of a receptor for calcitonin (CT) and the effect of chicken CT (cCT) on adrenocorticotropic hormone (ACTH) secretion stimulated by rat/human corticotropin-releasing hormone (rhCRH) in the hen anterior pituitary were studied. The specific [(125)I]cCT binding component was present in the plasma membrane of hen anterior pituitary and this binding component had properties of a receptor which has binding specificity to cCT, reversibility, saturable binding, high affinity and limited capacity. When anterior pituitary cells were incubated in vitro, cCT increased the maximal secretion of chicken ACTH stimulated by rhCRH. These results suggest that CT may act directly on the anterior pituitary via its receptor binding and enhances the ACTH secretion by CRH.

Calcitonin receptor bindings in the hen hypothalamus before and after oviposition

To demonstrate the presence of a receptor for calcitonin (CT) in the hen hypothalamus and to determine when CT acts on this tissue during the oviposition cycle, bindings of (125)I labeled CT in the plasma membrane fraction of the hen hypothalamus were measured by radioligand binding assay. The specific CT binding component in the plasma membrane fraction of the hypothalamus containing the preoptic area (HPOA) possessed properties of a receptor: binding specificity to CT, saturable binding, high affinity, and limited capacity. As for the median eminence area, no specific binding component was found in the present study. Therefore, the binding component for CT in the plasma membrane fraction of HPOA is likely to be a receptor for CT. In laying hens, the binding affinity of CT receptor increased at 30 min before oviposition and the binding capacity was decreased at 30 min before oviposition but not changed in nonlaying hens during a 24-h period. These results suggest that the action of CT on the hen HPOA may increase 30 min before oviposition.

Calcitonin receptor binding in the hen neurohypophysis before and after oviposition

To demonstrate the presence of a receptor for calcitonin (CT) in the hen neurohypophysis and to estimate the time of action of CT on the neurohypophysis during the oviposition cycle in relation to arginine vasotocin (AVT) release, binding of (125)I-labeled chicken CT in plasma membrane fractions of the hen neurohypophysis was measured by the use of a radioligand binding assay. The binding specificity, reversibility, high affinity, and limited capacity are characteristics of a CT receptor. Therefore, it was elucidated that the CT receptor might exist in the plasma membrane of the neurohypophysis of hens. The binding affinity of CT receptor increased at 30 min before oviposition and the binding capacity was decreased at 15 min before oviposition. However, no change was found in non-laying hens during a 24-h period. Such changes in the CT receptor binding were found at 10 min after an i.v. injection of chicken CT into non-laying hens with an increase in the blood level of AVT. The changes in the binding affinity and capacity of CT receptor of the neurohypophysis may be related to AVT release partly at oviposition time in the hen.

Identification of Calcitonin Expression in the Chicken Ovary: Influence of Follicular Maturation and Ovarian Steroids1

Calcitonin (CALCA), a hormone primarily known for its role in calcium homeostasis, has recently been linked to reproduction, specifically as a marker for embryo implantation in the uterus. Although CALCA expression has been documented in several tissues, there has been no report of production of CALCA in the ovary of any vertebrate species. We hypothesized that the Calca gene is expressed in the chicken ovary, and its expression will be altered by follicular maturation or gonadal steroid administration. Using RT-PCR, we detected Calca mRNA and the calcitonin receptor (Calcr) mRNA in the granulosa and theca layers of preovulatory and prehierarchial follicles. Both CALCA and Calca mRNA were localized in granulosa and thecal cells by confocal microscopy. Using quantitative PCR analysis, F1 follicle granulosa layer was found to contain significantly greater Calca mRNA and Calcr mRNA levels compared with those of any other preovulatory or prehierarchial follicle. The granulosa layer contained relatively greater Calca and Calcr mRNA levels compared with the thecal layer in both prehierarchial and preovulatory follicles. Progesterone (P(4)) treatment of sexually immature chickens resulted in a significantly greater abundance of ovarian Calca mRNA, whereas estradiol (E(2)) or P(4) + E(2) treatment significantly reduced ovarian Calca mRNA quantity. Treatment of prehierarchial follicular granulosa cells in vitro with CALCA significantly decreased FSH-stimulated cellular viability. Collectively, our results indicate that follicular maturation and gonadal steroids influence Calca and Calcr gene expression in the chicken ovary. We conclude that ovarian CALCA is possibly involved in regulating follicular maturation in the chicken ovary.

Presence of calcitonin receptors in shell gland of the guineafowl and changes in binding property during an oviposition cycle

Radioligand binding assays of membrane fraction of the shell gland endometrium of guineafowl revealed the presence of receptor for calcitonin. The equilibrium dissociation constant (Kd) obtained by Scatchard analyses was 0.50 to 1.25 nM in laying birds and 1.12 to 1.19 nM in nonlaying birds. The maximum binding capacity (Bmax) per milligram of protein was 33.1 to 107.5 femtomoles in laying birds and 101.4 to 114.9 femtomoles in nonlaying birds. Both Kd and Bmax values changed during eggshell formation in laying birds. The results suggest that calcitonin receptor binding may be related to eggshell formation in guineafowl.