Abstract
BackgroundIn 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.ResultsA 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.ConclusionsOur 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.
Highlights
In Gallus gallus, eggshell formation takes place daily in the hen uterus and requires large amounts of the ionic precursors for calcium carbonate (CaCO3)
Use of a recent transcriptomic study revealing uterine genes related to eggshell calcification [21] and of the analogies with transporters previously described in mammalian tissues transferring large quantities of ions allows the identification of putative genes encoding proteins involved in uterine trans-epithelial ion transports
Identification of uterine ion transporters The first step of this work was to establish a list of ion transporters potentially involved in supplying eggshell minerals
Summary
In Gallus gallus, eggshell formation takes place daily in the hen uterus and requires large amounts of the ionic precursors for calcium carbonate (CaCO3). In Gallus gallus, eggshell formation takes place daily in the hen uterus and is one of the most rapid mineralisation processes [1] It requires large amount of calcium carbonate (CaCO3) as the hen exports the equivalent of her body weight as eggshell in one year of egg production (>1.5 kg). Both elements (Ca2+ and HCO3- ) are not stored in the uterus but are continuously supplied during eggshell formation by the blood plasma via trans-epithelial transport taking place across the uterine glandular cells [2,3,4]. The 28 kDa calbindin expression is greatly upregulated during eggshell formation and falls after suppression of calcification (by premature egg expulsion), suggesting a very close relationship between
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