Immunohistochemical localization of a calcium pump and calbindin-D28k in the oviduct of the laying hen

A synthetic review: natural history of amniote reproductive modes in light of comparative evolutionary genomics

There is a current lack of consensus on whether the ancestral parity mode was oviparity (egg-laying) or viviparity (live-birth) in amniotes and particularly in squamates (snakes, lizards, and amphisbaenids). How transitions between parity modes occur at the genomic level has primary importance for how science conceptualises the origin of amniotes, and highly variable parity modes in Squamata. Synthesising literature from medicine, poultry science, reproductive biology, and evolutionary biology, I review the genomics and physiology of five broad processes (here termed the 'Main Five') expected to change during transitions between parity modes: eggshell formation, embryonic retention, placentation, calcium transport, and maternal-fetal immune dynamics. Throughout, I offer alternative perspectives and testable hypotheses regarding proximate causes of parity mode evolution in amniotes and squamates. If viviparity did evolve early in the history of lepidosaurs, I offer the nucleation site hypothesis as a proximate explanation. The framework of this hypothesis can be extended to amniotes to infer their ancestral state. I also provide a mechanism and hypothesis on how squamates may transition from viviparity to oviparity and make predictions about the directionality of transitions in three species. After considering evidence for differing perspectives on amniote origins, I offer a framework that unifies (i) the extended embryonic retention model and (ii) the traditional model which describes the amniote egg as an adaptation to the terrestrial environment. Additionally, this review contextualises the origin of amniotes and parity mode evolution within Medawar's paradigm. Medawar posited that pregnancy could be supported by immunosuppression, inertness, evasion, or immunological barriers. I demonstrate that this does not support gestation or gravidity across most amniotes but may be an adequate paradigm to explain how the first amniote tolerated internal fertilization and delayed egg deposition. In this context, the eggshell can be thought of as an immunological barrier. If serving as a barrier underpins the origin of the amniote eggshell, there should be evidence that oviparous gravidity can be met with a lack of immunological responses in utero. Rare examples of two species that differentially express very few genes during gravidity, suggestive of an absent immunological reaction to oviparous gravidity, are two skinks Lampropholis guichenoti and Lerista bougainvillii. These species may serve as good models for the original amniote egg. Overall, this review grounds itself in the historical literature while offering a modern perspective on the origin of amniotes. I encourage the scientific community to utilise this review as a resource in evolutionary and comparative genomics studies, embrace the complexity of the system, and thoughtfully consider the frameworks proposed.

Heat Stress in Japanese Quails (Coturnix japonica): Benefits of Phytase Supplementation

In tropical and subtropical climate regions, heat stress is one of the main causes of production losses in laying quails, aggravated by the antinutritional effects of the phytate in diet ingredients, which negatively affect the bioavailability of minerals, especially calcium and phosphorus. This situation results in a reduction in production and the quality of eggs from commercial laying quails. Several nutritional strategies are utilized to reduce the adverse effects of high temperatures and antinutritional factors such as phytate. Among these strategies, the use of exogenous enzymes, such as phytase, stands out as a viable alternative. Phytase breaks down phytate molecules, optimizing the absorption of essential minerals and improving productive performance and egg quality under unfavorable conditions. Specifically, it is recommended to use 1500 FTU of phytase, as it not only reduces the effects of thermal stress but also enhances eggshell thickness and calcium absorption. In this context, this bibliographic review sought to produce a document demonstrating the beneficial effects of the phytase enzyme on the hydrolysis of the phytate molecule, the availability of calcium for Japanese quails (Coturnix japonica), and its implications in thermal stress due to heat.

TMT-based quantitative proteomic analysis unveils uterine fluid difference in hens producing normal and pimpled eggs

Eggshell is a crucial indicator of egg quality. Pimpled eggs (PE) a type of eggshell defect are characterized by low eggshell strength, leading to substantial financial losses. Eggshell formation occurs in the uterine fluid (UF), which contains the required ions and matrix proteins However, the underlying mechanisms of PE formation remain poorly understood. In this study, we analyzed the egg quality of PE, and normal eggs (NE) by examining the differences in UF from hens producing PE and NE (n = 6 each). This 2-wk-long assessment involved histomorphological and proteomics analyses. The results showed that NE had better eggshell quality compared to PE, and the uterus structure in PE hens was conducive to the formation of PE. Using quantitative proteomic analysis, we identified 68 differential abundance proteins (DAPs) in the UF of PE hens, including 9 key proteins related to ion transport, protein synthesis and folding, and immunity. Downregulation of CALM1 and SCNN1G proteins in PE hens might have negatively affected the calcium signaling pathway, decreasing the calcium amount in UF. Additionally, the PHB1 and TSN proteins may affect eggshell formation by regulating immune responses. Taken together, our results provide insights into the mechanism of PE production, with potential applications for enhancing eggshell quality.

Pathogenicity comparison between QX-type and Mass-type infectious bronchitis virus to different segments of the oviducts in laying phase

Background

The QX-type infectious bronchitis virus (IBV) has become the predominant genotype worldwide in recent years and has caused serious economic losses to the chicken industry. The most significant feature of QX IBV is that its infection in the early growing stage can cause abnormal oviduct development, resulting in a high proportion of ‘false layers’ in poultry flocks of laying hens and breeders. However, few studies have evaluated whether infections of QX-type IBV in laying stages can also cause severe pathological changes in the oviduct.

Methods

In this study, 300-day-old specific-pathogen-free chickens were infected either with the QX-type strain QXL or Massachusetts (Mass)-type strain M41 to compare their pathogenicity on different segments of the oviduct.

Results

Both the QXL and M41 strains successfully replicated in all segments of the oviduct; however, the QXL strain was more highly distributed in mucosal layer and caused severe lesions in the lamina propria, including interstitial dilation, inflammatory cell infiltration, and distinct expansion of tubular glands. Moreover, the QXL strain induced high expression of proinflammatory cytokines and cytotoxic molecules in the majority of segments in the oviduct. Further research found that the QXL strain may affected the formation of shell membranes and eggshells by inhibiting the expression of type I collagen and CaBP-D28k.

Conclusions

Our results indicate that the QX-type IBV is more pathogenic than Mass-type IBV to oviduct in laying phase. Collectively, these findings provide detailed information on the pathological changes in different segments of the oviduct in laying phase, which could offer a better understanding about the pathogenicity of IBV.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-022-01788-0.

Dietary mulberry-leaf flavonoids improve the eggshell quality of aged breeder hens

Eggshell quality is subject to a significant decline in the late laying period, which results in huge economic losses. The purpose of this study was to investigate the effects of dietary mulberry-leaf flavonoids (MF) on the eggshell quality of aged breeder hens. A total of 270 (60-week-old) Qiling breeder hens were randomly assigned to 3 treatments with supplemental dietary MF doses (0, 30, and 60 mg/kg). The results showed that dietary MF improved the eggshell thickness and strength, following the reduced broken egg ratio (P < 0.05). Histological analysis showed that dietary MF increased glandular density and luminal epithelium height in the shell gland (P < 0.05). MF treatment reduced the apoptotic index of the shell gland, following by improved antioxidant capacity (P < 0.05). The protein expression of Caspase 3 was down-regulated, and Nrf2 was up-regulated by dietary MF (P < 0.05). Furthermore, calcium (Ca) content in the serum and shell gland, as well as the activity of Ca²⁺-ATPase in the shell gland were increased by dietary MF (P < 0.05). Ca transport-related genes (ESRα, ESRβ, KCNA1, OPN, CABP-28K and CDH6) in the shell gland were upregulated by dietary MF treatment (P < 0.05). In conclusion, dietary MF could ameliorate the eggshell quality of aged hens by improving antioxidative capability and Ca deposition in the shell gland of uterus.

Tri-ortho-cresyl phosphate exposure leads to low egg production and poor eggshell quality via disrupting follicular development and shell gland function in laying hens

Tri-ortho-cresyl phosphate (TOCP) has been used commercially as a plasticizer and a flame retardant, which has been reported to cause multiple toxicities in humans and other animals. However, the effect of TOCP on female reproductive system is still unclear. The aim of this investigation was to evaluate the reproductive toxicity of TOCP in female avian and investigate its molecular mechanism. In the current study, 50 adult hens were given a single oral dose of TOCP (750 mg/kg). Egg laid by the hens were harvested and counted. Egg quality is assessed by determining the shell strength and thickness. Samples of ovary, shell gland, and serum were collected on day 0, 2, 7, and 21 after the administration. The morphological and pathological changes in tissues were examined. Cell death, follicular development, and steroidogenesis were determined to assess the toxicity of TOCP on laying hens. The results showed that egg production, egg weight, and eggshell strength significantly decreased after TOCP exposure. The calcium levels in serum and eggshell decreased and the expression levels of the eggshell formation-related genes calbindin-D28k (CaBP-D28k) and carbonic anhydrase 2 (CA2) were downregulated. The inhibitory effects of TOCP on follicular development and steroidogenesis were observed with changes in the levels of the related proteins such as forkhead box O1 (FoxO1) and mothers against decapentaplegic homolog 2/3 (Smad2/3). Cell death was identified, which might lead to follicular development disorder. Taken together, TOCP reduced the quantity and quality of the eggs laid by the hens through disrupting follicular development, steroidogenesis, and shell gland function.

Circadian miR-449c-5p regulates uterine Ca2+ transport during eggshell calcification in chickens

Background

miRNAs regulate circadian patterns by modulating the biological clocks of animals. In our previous study, we found that the clock gene exhibited a cosine expression pattern in the fallopian tube of chicken uterus. Clock-controlled miRNAs are present in mammals and Drosophila; however, whether there are clock-controlled miRNAs in the chicken uterus and, if so, how they regulate egg-laying rhythms is unclear. In this study, we selected 18 layer hens with similar ovipositional rhythmicity (each of three birds were sacrificed for study per 4 h throughout 24 h); their transcriptomes were scanned to identify the circadian miRNAs and to explore regulatory mechanisms within the uterus of chickens.

Results

We identified six circadian miRNAs that are mainly associated with several biological processes including ion trans-membrane transportation, response to calcium ion, and enrichment of calcium signaling pathways. Verification of the experimental results revealed that miR-449c-5p exhibited a cosine expression pattern in the chicken uterus. Ca²⁺-transporting ATPase 4 (ATP2B4) in the plasma membrane is the predicted target gene of circadian miR-449c-5p and is highly enriched in the calcium signaling pathway. We speculated that clock-controlled miR-449c-5p regulated Ca²⁺ transportation during eggshell calcification in the chicken uterus by targeting ATP2B4. ATP2B4 mRNA and protein were rhythmically expressed in the chicken uterus, and dual-luciferase reporter gene assays confirmed that ATP2B4 was directly targeted by miR-449c-5p. The expression of miR-449c-5p showed an opposite trend to that of ATP2B4 within a 24 h cycle in the chicken uterus; it inhibited mRNA and protein expression of ATP2B4 in the uterine tubular gland cells. In addition, overexpression of ATP2B4 significantly decreased intracellular Ca²⁺ concentration (P < 0.05), while knockdown of ATP2B4 accelerated intracellular Ca²⁺ concentrations. We found similar results after ATP2B4 knockdown by miR-449c-5p. Taken together, these results indicate that ATP2B4 promotes uterine Ca²⁺ trans-epithelial transport.

Conclusions

Clock-controlled miR-449c-5p regulates Ca²⁺ transport in the chicken uterus by targeting ATP2B4 during eggshell calcification.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08074-3.

Avian eggshell biomineralization: an update on its structure, mineralogy and protein tool kit

BACKGROUND

The avian eggshell is a natural protective envelope that relies on the phenomenon of biomineralization for its formation. The shell is made of calcium carbonate in the form of calcite, which contains hundreds of proteins that interact with the mineral phase controlling its formation and structural organization, and thus determine the mechanical properties of the mature biomaterial. We describe its mineralogy, structure and the regulatory interactions that integrate the mineral and organic constituents during eggshell biomineralization. Main Body. We underline recent evidence for vesicular transfer of amorphous calcium carbonate (ACC), as a new pathway to ensure the active and continuous supply of the ions necessary for shell mineralization. Currently more than 900 proteins and thousands of upregulated transcripts have been identified during chicken eggshell formation. Bioinformatic predictions address their functionality during the biomineralization process. In addition, we describe matrix protein quantification to understand their role during the key spatially- and temporally- regulated events of shell mineralization. Finally, we propose an updated scheme with a global scenario encompassing the mechanisms of avian eggshell mineralization.

CONCLUSION

With this large dataset at hand, it should now be possible to determine specific motifs, domains or proteins and peptide sequences that perform a critical function during avian eggshell biomineralization. The integration of this insight with genomic data (non-synonymous single nucleotide polymorphisms) and precise phenotyping (shell biomechanical parameters) on pure selected lines will lead to consistently better-quality eggshell characteristics for improved food safety. This information will also address the question of how the evolutionary-optimized chicken eggshell matrix proteins affect and regulate calcium carbonate mineralization as a good example of biomimetic and bio-inspired material design.

The shell gland in laying and natural moulting commercial egg-type chickens: A histomorphological and ultrastructural study

The purpose of the present study was to determine the histological and ultrastructural changes in the luminal epithelium of the shell gland associated with natural moulting. Samples of the shell gland from laying (32 weeks old) and moulting (75 weeks old) hens were studied using histological, histochemical and electron microscopic techniques. In addition, TUNEL was used to demonstrate the distribution of apoptotic cells in the luminal epithelium of the shell gland. Autophagy, characterized by the presence of autophagosomes and autolysosomes, was evident in the early stages of degeneration in non-ciliated, ciliated and mitochondrial cells. The intermediate and advanced stages of regression in non-ciliated as well as mitochondrial cells occurred via apoptosis, while both apoptotic and necrotic ciliated cells were observed during the later stages of degeneration. The results of the present study suggest that a synergy of autophagy, apoptosis and necrosis is involved in the involution of the shell gland during natural moulting.

Age-related changes in the shell gland and duodenum in relation to shell quality and bone strength in commercial laying hen hybrids

Background

During the production period of laying hens, the number of cracked eggshells increases and the skeleton becomes brittle. Both these problems are related to ageing of the hen and cause economic problems for egg producers and impaired animal welfare. This study investigated key factors in the shell gland and duodenum related to eggshell quality and bone strength in laying hens during the production period. Five Lohmann Selected Leghorn (LSL) and five Lohmann Brown (LB), common hybrids in commercial egg production, were euthanized at 21, 29, 49 and 70 weeks (wk) of age. Blood samples for analysis of total calcium were taken at euthanization. Right femur and humerus were used for bone strength measurements and tissue samples from shell gland and duodenum were processed for morphology, immunohistochemical localisation of oestrogen receptors (ERα, ERβ), plasma membrane calcium ATPase (PMCA) and histochemical localisation of carbonic anhydrases (CA). Eggs were collected for shell quality measurements.

Results

At age 49 week, shell and bone strength had both deteriorated, but the hens were then able to maintain the level until 70 week of age and femur bone strength even improved. The main physiological findings associated with the effects seen at 49 week were reduced gland density and a shift in balance between ERα and ERβ in the shell gland, which coincided with a reduction in CA activity in the duodenum. Somewhat surprisingly, capillary density and capillaries with CA activity both increased in the shell gland over time, the latter possibly mediated via ERβ. These findings were independent of hybrid. PMCA was found in both shell gland and duodenum, but appeared unrelated to the age-related changes in shell and bone quality.

Conclusions

In hens around half-way through the production period, both shell quality and bone strength had deteriorated. Decreased gland density and a shift in the balance between ERα and ERβ in the shell gland, co-occurring with a dramatic drop in duodenal CA activity, are suggested as possible factors involved in age-related changes in shell and bone quality.

Electronic supplementary material

The online version of this article (10.1186/s13028-019-0449-1) contains supplementary material, which is available to authorized users.

Effects of dietary Mn-methionine supplementation on the egg quality of laying hens

This study was conducted to investigate the effects of dietary manganese-methionine (Mn-Met) supplementation on the egg quality of laying hens. A total of 480 Jinghong-1 strain layers aged 53 wk were divided into 5 groups with 6 replicates of 16 layers. Birds in the control group were fed a diet supplemented with 60 mg Mn/kg in the form of MnSO4; the birds in other 4 experimental groups were fed a diet supplemented with 20, 40, 60, and 80 mg Mn/kg as Mn-Met, respectively. Dietary Mn-Met treatments significantly affected (P < 0.05) the albumen height, yolk color, and Haugh unit compared to those of the control diet. The Mn contents in the eggshell increased (P < 0.01) significantly by increasing the Mn-Met supplementation, whereas Mn content in eggshell was triple that in the yolk or albumen. Compared with the 60 mg/kg Mn-Met group, the transverse surface in the control group had (P < 0.01) a greater width of mammillary cones, and there were obvious cracks on the outer surface in the control. There was no difference (P > 0.05) in the eggshell gland (ESG) in the expression of calbindin-D28k (CaBP-D28k) mRNA in response to any diet treatment. In conclusion, dietary Mn-Met supplementation increased internal egg quality and the ultrastructure of the eggshell. Compared to the control, 60 mg/kg Mn-Met treatment resulted in improving egg quality, and 20 mg/kg Mn-Met treatment had similar effects the control treatment had on the egg quality. This indicates that the inorganic Mn can be replaced by the lower concentration of Mn-Met.

Homeostasis and secretion of calcium in the oviductal mucosa of toad Rhinella arenarum

The presence of a calcium pump, calbindin D-28KD, and calmodulin in the secretory cells (SC) of the oviductal pars convoluta (PC) of Rhinella arenarum was established for the first time in amphibians using immunohistochemical techniques. Marked variations were observed in the localization and degree of expression of these proteins according to the duct segment and the period of the sexual cycle analyzed. During the preovulatory and ovulatory periods the calcium pump colocalized with calbindin D-28KD can be seen mainly in the apical border of the SC, which are located in the first zones of PC and synthesize and secrete the components of the inner jelly coat layers. These envelopes, which surround the oocytes, contain the molecules indispensable for fertilization, probably inducing the sperm acrosome reaction (AR). Our results suggest that calmodulin, colocalized with the calcium pump at the SC cytoplasmic level, would be involved in the active transport of the cation inside the secretory granules, maintaining adequate levels of intracellular Ca(2+) . During the postreproductive period, a calcium pump colocalized with calbindin D-28KD appears for the first time in the cycle in the basal zones of the SC. This system may be related to the replenishing of intracellular Ca(2+) stores. In contrast, in R. arenarum the Ca(2+) present in the jelly coats that surround the oocytes participates in the AR during fertilization, suggesting that this secretion system of the cation provided by the oviductal mucosa is functionally more active during the reproductive period of this species.

Exogenous estradiol improves shell strength in laying hens at the end of the laying period

Background

Cracked shells, due to age related reduction of shell quality, are a costly problem for the industry. Parallel to reduced shell quality the skeleton becomes brittle resulting in bone fractures. Calcium, a main prerequisite for both eggshell and bone, is regulated by estrogen in a complex manner. The effects of estrogen, given in a low continuous dose, were studied regarding factors involved in age related changes in shell quality and bone strength of laying hens. A pellet containing 0.385 mg estradiol 3-benzoate (21-day-release) or placebo was inserted subcutaneously in 20 birds each of Lohmann Selected Leghorn (LSL) and Lohmann Brown (LB) at 70 weeks of age. Eggs were collected before and during the experiment for shell quality measurements. Blood samples for analysis of total calcium were taken three days after the insertion and at sacrifice (72 weeks). Right femur was used for bone strength measurements and tissue samples from duodenum and shell gland were processed for morphology, immunohistochemical localization of estrogen receptors (ERα, ERβ), plasma membrane calcium ATPase (PMCA) and histochemical localization of carbonic anhydrase (CA).

Results

Estrogen treatment increased shell thickness of both hybrids. In addition, shell weight and shell deformation improved in eggs from the brown hybrids. The more pronounced effect on eggs from the brown hybrid may be due to a change in sensitivity to estrogen, especially in surface epithelial cells of the shell gland, shown as an altered ratio between ERα and ERβ. A regulatory effect of estrogen on CA activity, but not PMCA, was seen in both duodenum and shell gland, and a possible connection to shell quality is discussed. Bone strength was unaffected by treatment, but femur was stronger in LSL birds suggesting that the hybrids differ in calcium allocation between shell and bone at the end of the laying period. Plasma calcium concentrations and egg production were unaffected.

Conclusions

A low continuous dose of estrogen improves shell strength but not bone strength in laying hens at the end of the laying period.

Identification of uterine ion transporters for mineralisation precursors of the avian eggshell

BACKGROUND

In Gallus gallus, eggshell formation takes place daily in the hen uterus and requires large amounts of the ionic precursors for calcium carbonate (CaCO3). Both elements (Ca2+, HCO3-) are supplied by the blood via trans-epithelial transport. Our aims were to identify genes coding for ion transporters that are upregulated in the uterine portion of the oviduct during eggshell calcification, compared to other tissues and other physiological states, and incorporate these proteins into a general model for mineral transfer across the tubular gland cells during eggshell formation.

RESULTS

A total of 37 candidate ion transport genes were selected from our database of overexpressed uterine genes associated with eggshell calcification, and by analogy with mammalian transporters. Their uterine expression was compared by qRTPCR in the presence and absence of eggshell formation, and with relative expression levels in magnum (low Ca2+/HCO3- movement) and duodenum (high rates of Ca2+/HCO3- trans-epithelial transfer). We identified overexpression of eleven genes related to calcium movement: the TRPV6 Ca2+ channel (basolateral uptake of Ca2+), 28 kDa calbindin (intracellular Ca2+ buffering), the endoplasmic reticulum type 2 and 3 Ca2+ pumps (ER uptake), and the inositol trisphosphate receptors type 1, 2 and 3 (ER release). Ca2+ movement across the apical membrane likely involves membrane Ca2+ pumps and Ca2+/Na+ exchangers. Our data suggests that Na+ transport involved the SCNN1 channel and the Na+/Ca2+ exchangers SLC8A1, 3 for cell uptake, the Na+/K+ ATPase for cell output. K+ uptake resulted from the Na+/K+ ATPase, and its output from the K+ channels (KCNJ2, 15, 16 and KCNMA1).We propose that the HCO3- is mainly produced from CO2 by the carbonic anhydrase 2 (CA2) and that HCO3- is secreted through the HCO3-/Cl- exchanger SLC26A9. HCO3- synthesis and precipitation with Ca2+ produce two H+. Protons are absorbed via the membrane's Ca2+ pumps ATP2B1, 2 in the apical membrane and the vacuolar (H+)-atpases at the basolateral level. Our model incorporate Cl- ions which are absorbed by the HCO3-/Cl- exchanger SLC26A9 and by Cl- channels (CLCN2, CFTR) and might be extruded by Cl-/H+ exchanger (CLCN5), but also by Na+ K+ 2 Cl- and K+ Cl- cotransporters.

CONCLUSIONS

Our Gallus gallus uterine model proposes a large list of ion transfer proteins supplying Ca2+ and HCO3- and maintaining cellular ionic homeostasis. This avian model should contribute towards understanding the mechanisms and regulation for ionic precursors of CaCO3, and provide insight in other species where epithelia transport large amount of calcium or bicarbonate.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

Calcium transport in strongly calcifying laying birds: mechanisms and regulation

Birds that lay long clutches (series of eggs laid sequentially before a "pause day"), among them the high-producing, strongly-calcifying Gallus gallus domesticus (domestic hen) and Coturnix coturnix japonica (Japanese quail), transfer about 10% of their total body calcium daily. They appear, therefore, to be the most efficient calcium-transporters among vertebrates. Such intensive transport imposes severe demands on ionic calcium (Ca2+) homeostasis, and activates at least two extremely effective mechanisms for Ca2+ transfer from food and bone to the eggshell. This review focuses on the development, action and regulation of the mechanisms associated with paracellular and transcellular Ca2+ transport in the intestine and the eggshell gland (ESG); it also considers some of the proteins (calbindin, Ca2+ATPase, Na+/Ca2+ exchange, epithelial calcium channels (TRPVs), osteopontin and carbonic anhydrase (CA) associated with this phenomenon. Calbindins are discussed in some detail, as they appear to be a major component of the transcellular transport system, and as only they have been studied extensively in birds. The review aims to gather old and new knowledge, which could form a conceptual basis, albeit not a completely accepted one, for our understanding of the mechanisms associated with this phenomenon. In the intestine, the transcellular pathway appears to compensate for low Ca2+ intake, but in birds fed adequate calcium the major drive for calcium absorption remains the electrochemical potential difference (ECPD) that facilitates paracellular transport. However, the mechanisms involved in Ca2+ transport into the ESG lumen are not yet established. In the ESG, the presence of Ca2+-ATPase and calbindin--two components of the transcellular transport pathway--and the apparently uphill transport of Ca2+ support the idea that Ca2+ is transported via the transcellular pathway. However, the positive (plasma with respect to mucosa) electrical potential difference (EPD) in the ESG, among other findings, indicates that there may be major alternative or complementary paracellular passive transport pathways. The available evidence hints that the flow from the gut to the ESG, which occurs during a relatively short period (11 to 14 h out the 24- to 25.5-h egg cycle), is primarily driven by carbonic anhydrase (CA) activity in the ESG, which results in high HCO3(-) content that, in turn, "sucks out" Ca2+ from the intestinal lumen via the blood and ESG cells, and deposits it in the shell crystals. The increased CA activity appears to be dependent on energy input, whereas it seems most likely that the Ca2+ movement is secondary, that it utilizes passive paracellular routes that fluctuate in accordance with the appearance of the energy-dependent CA activity, and that the level of Ca2+ movement mimics that of the CA activity. The on-off signals for the overall phenomenon have not yet been identified. They appear to be associated with the circadian cycle of gonadal hormones, coupled with the egg cycle: it is most likely that progesterone acts as the "off" signal, and that the "on" signal is provided by the combined effect of an as-yet undefined endocrine factor associated with ovulation and with the mechanical strain that results from "egg white" formation and "plumping". This strain may initially trigger the formation of the mammillae and the seeding of shell calcium crystals in the isthmus, and thereafter initiate the formation of the shell in the ESG.

Calcium ATPase expression in the oviducts of the skink, Lampropholis guichenoti

Lampropholis guichenoti is an oviparous lizard that lays eggs with a calcareous outer shell. We used immunofluorescence microscopy to describe the occurrence and distribution of Ca2+ ATPase pumps in the uterus of L. guichenoti at different stages of the reproductive and egg-shelling cycles. Ca2+ ATPase pumps were not demonstrated by immunofluorescent techniques in any uterine tissue until egg-shelling had commenced and at least partly calcified eggs were in the uterus. During egg-shelling, Ca2+ ATPase pumps occur on the apical and baso-lateral surfaces of uterine epithelial cells, and those of associated shell glands in the stroma of the uterus. We conclude that Ca2+ ATPase pumps provide a major mechanism for deposition of the calcareous eggshell of L. guichenoti and that the pumps are up-regulated when required in the reproductive cycle. Furthermore, it is likely that specific calcium glands in the stroma of the uterus are involved in the rapid transport required for egg-shelling, but the differential contribution of luminal and glandular epithelial cells is not known.

Expression of calbindin-D28k (CaBP28k) in trophoblasts from human term placenta

Calbindin-D28k (CaBP28k) belongs to a large class of eucaryotic proteins that bind calcium (Ca2+) to a specific helix-loop-helix structure. To date, this protein was mainly linked to brain, kidneys, and pancreas. Here, we demonstrate for the first time the existence of CaBP8k in the human placental trophoblasts of the human term placenta. Placental Ca2+ transfer from maternal to fetus is crucial for fetal development, although the biochemical mechanisms responsible for this process are largely unknown. In the current study, we have investigated the 45Ca2+ uptake by human trophoblast cells in correlation with the expression CaBP28k. The expression of CaBP28k was determined by Northern blot analysis, reverse transcriptase polymerase chain reaction (RT-PCR), immunochemistry, and Western blot analysis. Indeed, Northern blot analysis revealed the presence of a CaBP28k transcript in syncytiotrophoblasts, cytotrophoblast cells, and HEK-293 cells. This was further confirmed by RT-PCR analysis followed by sequencing. In addition, anti-CaBP28k labeling was associated with cytotrophoblast and syncytiotrophoblast tissues in placental tissue sections and in vitro cultured cells. The presence of CaBP28k protein in these cells was confirmed by Western blotting. Cytotrophoblast cells isolated from human term placenta showed differentiation into syncytiotrophoblasts in culture according to the increase in hCG secretion. Both Ca2+ uptake and hCG secretion by trophoblasts increased gradually and were high at Day 4. Taken together, these data suggest that CaBP28k may play a role in Ca2+ transport or cell development in human trophoblast possibly trough Ca2+ buffering.

Na(+)-K(+)-ATPase gene expression in the avian eggshell gland: distinct regulation in different cell types

The avian eggshell gland (ESG) is a tissue specialized in transporting the Ca(2+) required for eggshell formation and represents a unique biological system in which the calcification process takes place in a circadian fashion. With the use of RNA fingerprinting, a set of genes differentially induced at the time of calcification was detected, one of which was identified as the alpha(1)-subunit of Na(+)-K(+)-ATPase. The gene was expressed in a circadian manner in both cell types populating the ESG, but in different temporal patterns, suggesting distinct mechanisms of regulation. Ca(2+) flux and mechanical strain were found to regulate gene expression in the inner glandular epithelium and the pseudostratified epithelium facing the lumen, respectively. Mechanical strain also affected gene expression in cell layers facing the lumen in other parts of the oviduct. Only the alpha(1)-isoform, not the alpha(2)- or alpha(3)-isoform, of Na(+)-K(+)-ATPase was expressed in the ESG. In summary, we demonstrate that the alpha(1)-subunit Na(+)-K(+)-ATPase gene is expressed in different epithelial cell types in the ESG and is regulated by various mechanisms, which may reflect the disparity in the physiological roles of the cells in the process of eggshell formation.

New insight in eggshell formation

The matrix proteins that participate in crystalization fulfill important functions during the formation of the calcified tissues and contribute to the biomechanical properties of the mature product. We suggest that osteopontin (OPN) is part of an array of macromolecules synthesized and secreted by the cells adjacent to the mineralization front that self-assemble outside the cell and direct crystal formation. The OPN meets the theoretical requirements for involvement in the mineralization process. The phosphorylated residues of acidic phosphoprotein have been shown to exist in the protein as reactive monoesters that are available for interaction with other ions, among them crystal constituents such as calcium ions. In addition, sulfation of OPN was also found to be associated with mineralization of other tissues. In contrast to the calbindin gene, whose expression is dependent on the calcium flux, the regulation of OPN synthesis is at least in part dependent on the mechanical strain imposed by the resident egg. These results demonstrate the complexity of the regulation of the matrix genes governing eggshell formation.

Precursor matrix proteins in the uterine fluid change with stages of eggshell formation in hens

Organic constituents of the uterine fluid, the acellular milieu in which the eggshell is mineralized, were biochemically characterized at initial, mid and final stages of shell calcification in hens. The electrophoretic protein profiles changed at the different stages of shell mineralization. Two major bands (80-kDa and 43-kDa glycoproteins) with calcium affinity were specific to the initial stage. Four protein bands of 180, 150, 116 and 32 kDa, present at the phase of rapid shell formation, coprecipitated with calcium carbonate in vitro. At this stage were also present a calcium-binding glycoprotein of 36-kDa and a 20-kDa protein. Uterine fluid of the final stage was characterized by a darker intensity of the 66-kDa band, which showed calcium-binding ability and by the presence of three additional proteins (72, 13 and 6 kDa). At least seven bands of the uterine fluid showed similar migration patterns to those of eggshell extracts. Western blotting with ovocleidin and ovalbumin antisera demonstrated the presence of these matrix proteins in uterine fluid collected at initial and mid phase, respectively. Total uterine fluid collected at the end of calcification and dialyzed uterine fluid from the various stages delayed the rate of calcium precipitation in vitro. These observations demonstrate the presence of precursors of eggshell matrix in the uterine fluid and support the hypothesis of their involvement in the process of eggshell mineralization.

DDE-induced eggshell thinning in birds: effects of p,p'-DDE on the calcium and prostaglandin metabolism of the eggshell gland

1. The focus of this review is the effects and mechanism of action of p,p'-DDE on eggshell formation in birds. Inhibition of prostaglandin synthesis in the eggshell gland mucosa is a probable mechanism for p,p'-DDE-induced eggshell thinning. 2. The duck is sensitive to p,p'-DDE-induced eggshell thinning but the domestic fowl is not, and studies comparing the two species in regard to the calcium and prostaglandin metabolism of the eggshell gland have shown that eggshell thinning induced by p,p'-DDE in ducks is accompanied by reduced activity of prostaglandin synthetase, reduced levels of prostaglandin E2, and reduced uptake of 45Ca by the eggshell gland mucosa. The content of calcium, bicarbonate, chloride, sodium, and potassium are also reduced in the eggshell gland lumen in ducks exhibiting eggshell thinning. None of these effects are seen in the domestic fowl. 3. Inhibition of prostaglandin synthesis is a specific effect of p,p'-DDE. The detrimental effects of p,p'-DDE on the eggshell gland seem to be unique when comparing the compound with structurally related substances, i.e., similar treatment regimens with o,p'-DDE, p,p'-DDT, o,p'-DDT, and p,p'-DDD do not cause eggshell thinning in ducks. Neither do they inhibit prostaglandin synthesis in the eggshell gland mucosa. 4. Administration of other compounds that do inhibit prostaglandin synthesis, e.g., indomethacin, does cause the same effects as those seen with p,p'-DDE, i.e., eggshell thinning and the described effects on the calcium and prostaglandin metabolism of the eggshell gland.

Protein kinase C and calmodulin effects on the plasma membrane Ca2+-ATPase from excitable and nonexcitable cells

We have purified Ca2+-ATPase from synaptosomal membranes (SM)1 from rat cerebellum by calmodulin affinity chromatography. The enzyme was identified as plasma membrane Ca2+-ATPase by its interaction with calmodulin and monoclonal antibodies produced against red blood cell (RBC) Ca2+-ATPase, and by thapsigargin insensitivity. The purpose of the study was to establish whether two regulators of the RBC Ca2+-ATPase, calmodulin and protein kinase C (PKC), affect the Ca2+-ATPase isolated from excitable cells and whether their effects are comparable to those on the RBC Ca2+-ATPase. We found that calmodulin and PKC activated both enzymes. There were significant quantitative differences in the phosphorylation and activation of the SM versus RBC Ca2+-ATPase. The steady-state Ca2+-ATPase activity of SM Ca2+-ATPase was approximately 3 fold lower and significantly less stimulated by calmodulin. The initial rate of PKC catalyzed phosphorylation (in the presence of 12-myristate 13-acetate phorbol) was approximately two times slower for SM enzyme. While phosphorylation of RBC Ca2+-ATPase approached maximum level at around 5 min, comparable level of phosphorylation of SM Ca2+-ATPase was observed only after 30 min. The PKC-catalyzed phosphorylation resulted in a statistically significant increase in Ca2+-ATPase activity of up to 20-40%, higher in the SM Ca2+-ATPase. The differences may be associated with diversities in Ca2+-ATPase function in erythrocytes and neuronal cells and different isoforms composition.

Genetic regulation of placental function: a quantitative in situ hybridization study of calcium binding protein (calbindin-D9k) and calcium ATPase mRNAs in sheep placenta

The calcium requirement of the ovine fetus increases progressively throughout pregnancy. The 9-kDa calcium binding protein (calbindin-D9k; 9CBP) is considered to be a reliable marker for epithelia mediating calcium transport. This quantitative in situ hybridization study shows that the levels of 9CBP mRNA show a pregnancy stage-related increase which correlates with fetal calcium demand only in maternal endometrial gland and fetal interplacentomal trophoblast epithelia. Levels of 9CBP mRNA in the placentome, which has by far the greater area of maternofetal contact, show no changes during pregnancy. mRNA for the CaATPase enzyme, a second requirement for calcium transport, is shown to be present in epithelia in interplacentomal and placentomal regions but shows no change in concentration as pregnancy progresses. Results with the 9CBP and CaATPase mRNAs confirm our recent immunocytochemical results with ruminant placenta and indicate the basis for a cellular calcium transport system analogous to that in the enterocyte. The interplacentomal trophoblast system appears to be eminently suitable for investigations of details of the cellular mechanism and control of epithelial calcium transport.

Involvement of osteopontin in egg shell formation in the laying chicken

Expression of the osteopontin (OPN) gene in the oviduct of the laying hen was studied. It was detected only in the egg shell gland (ESG), where massive calcification occurs. No OPN gene expression was detected in any other part of the oviduct, such as the magnum and isthmus. The OPN gene was expressed in a circadian fashion during the daily egg cycle only during the period of egg shell calcification. No OPN gene expression was detected in the ESG of a pre-laying hen before the onset of reproduction, or after forced removal of the egg close to its entrance into the ESG. OPN was found to be synthesized by the epithelial cells of the ESG lining the lumen. Upon synthesis, OPN is immediately secreted out of cells and accumulates in the egg shell. These findings demonstrate for the first time temporal and spatial association of OPN with egg shell calcification. OPN, which was found to be part of the organic matrix of the egg shell, may play an important role in egg shell calcification.

Purification and immunochemistry of a soluble matrix protein of the chicken eggshell (ovocleidin 17)

The protein components of biomineralized structures (matrix proteins) are believed to modulate crystal nucleation and growth, and thereby influence the shape and strength of the final structure. The chicken eggshell contains a complex array of distinct matrix proteins. The most abundant of these was purified to homogeneity by a combination of anionic exchange and hydroxyapatite chromatographies. Antibodies to this protein were raised in rabbit, and utilized for Western blotting and immunohistochemistry. These studies indicated that the 17 kDa antigen (ovocleidin 17, OC-17) is found in the shell gland mucosa, and that only the tubular gland cells were positive. Immunohistochemistry with decalcified shell indicated that OC-17 is uniformly distributed throughout the shell matrix, but concentrated in the mammillary bodies. Our results indicate that this protein is secreted during shell formation and becomes incorporated into this structure. It may therefore play a role in the crystallization process and influence the properties of the resulting eggshell.

Distribution of calbindin-D28K in the brain of the fetal sheep in late gestation

Immunocytochemical distribution of calbindin-D28K was examined in the brains of fetal sheep at 100-105 and 120-125 days of gestation (dGA) and at term in labor (term approximately 150 dGA) and compared to adult sheep. Brains were perfused, cut frozen and immunostained as freely floating sections with a polyclonal anti-calbindin-D28K antibody. Light microscopy revealed that calbindin-D28K immunogenicity in fetal cerebellum, rhombencephalon, mesencephalon, diencephalon and telencephalon was overall very similar to that seen in the brains of adult sheep. In addition even though fetal sheep peripheral plasma cortisol concentrations increase 10-20 times over the last 3-4 weeks of gestation to peak during labor and delivery, no glucocorticoid dependent increases in calbindin-D28K in the hippocampal formation could be seen in the sheep fetus as have been demonstrated for the adult rat given exogenous glucocorticoids. It is concluded that: (1) the basic pattern of calbindin-D28K distribution seen in adult sheep brains is established by at least 100 dGA in the sheep fetus; and (2) the pattern of calbindin-D28K expression in fetal sheep hippocampal formation is not related to peripheral plasma glucocorticoid concentrations.

Immunocytochemical localization of the plasma membrane calcium pump, calbindin-D28k, and parvalbumin in Purkinje cells of avian and mammalian cerebellum

A monoclonal antibody produced against the human erythrocyte plasma membrane calcium pump (PMCA) was shown to react immunohistochemically with an epitope of the PMCA in avian and mammalian cerebellum. Western blot analysis of purified synaptosomes and homogenates from avian cerebellum revealed major immunoreactive proteins with molecular masses (130 kDa and 138 kDa) similar to those of purified erythrocyte PMCA. Dual-imaging confocal immunofluorescence microscopy of avian cerebellum showed that the PMCA antibody stained the periphery of the soma whereas calbindin-D28k was located in the cytosol. PMCA heavily stained the more distal dendrites of the Purkinje cells and, within the resolution of the fluorescence procedure, colocalized with calbindin-D28k. By using alkaline phosphatase-conjugated second antibody, PMCA was again localized to the peripheral soma, to a segmental pattern in dendrites, and to presumed spiny elements. The soma periphery and dendrites of Purkinje cells of the rat cerebellum were also prominently stained with anti-PMCA antibody and compared to parvalbumin localization. Dendritic depolarization and dendritic spiking behavior are significant Ca(2+)-dependent events of Purkinje cells. The rapid decline of intracellular free Ca2+ after the rapid rise time of Ca2+ transients is considered to be due to sequestration by Ca2+ buffers, uptake by intracellular stores, and Ca2+ extrusion mechanisms, the latter a function of PMCA now shown immunohistochemically to be a prominent feature of Purkinje cell dendrites.

Molecular cloning of a plasma membrane calcium pump from human osteoblasts

The osteoblast plays a critical role in bone formation, bone remodelling, bone matrix formation, and matrix calcification. To better understand the process of osteoblast-controlled bone formation, we determined the structure and isoform types of the plasma membrane calcium pump from normal human osteoblasts. A complementary DNA library from normal human osteoblasts was screened for plasma membrane calcium pump clones. Sequencing and analysis of cDNA clones revealed the presence of a 3986 base pair cDNA that encoded a 1220 amino acid protein that was similar to the human plasma membrane calcium pump isoform 1. Polyadenylated RNA from human osteoblast cells contains bands of RNA approximately 5050 and 6750 bases long. Reverse transcription of polyadenylated RNA from human osteoblasts followed by amplification of the RNA-DNA duplex with calcium pump isoform-specific primers revealed the presence of isoforms 1 and 2 of the calcium pump. Isoform 4 was not detected. We conclude that normal adult human osteoblasts contain a plasma membrane calcium pump that is similar to the human plasma membrane calcium pump isoform 1. It is likely that this pump plays an important role in the cell biology of the human osteoblast.

Calcium pump of the plasma membrane is localized in caveolae

The Ca2+ pump in the plasma membrane plays a key role in the fine control of the cytoplasmic free Ca2+ concentration. In the present study, its subcellular localization was examined with immunocytochemical techniques using a specific antibody generated against the erythrocyte membrane Ca2+ pump ATPase. By immunofluorescence microscopy of cultured cells, the labeling with the antibody was seen as numerous small dots, often distributed in linear arrays or along cell edges. Immunogold EM of cryosections revealed that the dots correspond to caveolae, or smooth invaginations of the plasma membrane. The same technique applied to mouse tissues in vivo showed that the Ca2+ pump is similarly localized in caveolae of endothelial cells, smooth muscle cells, cardiac muscle cells, epidermal keratinocytes and mesothelial cells. By quantitative analysis of the immunogold labeling, the Ca2+ pump in capillary endothelial cells and visceral smooth muscle cells was found to be concentrated 18-25-fold in the caveolar membrane compared with the noncaveolar portion of the plasma membrane. In renal tubular and small intestinal epithelial cells, which have been known to contain the Ca2+ pump but do not have many caveolae, most of the labeling was randomly distributed in the basolateral plasma membrane, although caveolae were also positively labeled. The results demonstrate that the caveolae in various cells has the plasmalemmal Ca2+ pump as a common constituent. In conjunction with our recent finding that an inositol 1,4,5-trisphosphate receptor-like protein exists in the caveolae (Fujimoto, T., S. Nakade, A. Miyawaki, K. Mikoshiba, and K. Ogawa. 1992. J. Cell Biol. 119:1507-1513), it is inferred that the smooth plasmalemmal invagination is an apparatus specialized for Ca2+ intake and extrusion from the cytoplasm.