Evidence for stanniocalcin and a related receptor in annelids

Responses of Corpuscles of Stannius to intra-peritoneal vitamin-D3 administration in teleost Labeo rohita (Hamilton, 1822) reared in water with two different levels of calcium concentration

This study assessed the responses of vitamin-D3 intraperitoneally injected to Rohu, Labeo rohita @ of 0 IU/kg bw (only solvent), 100 IU/kg bw and 500 IU/kg bw reared in 20 and 40 ppm of calcium (Ca) enriched water. The cellular changes in Corpuscles of Stannius (CS) gland, serum Ca, and inorganic phosphate (Pi) level were analysed up to the 60th day. Rohu administered with 100 IU/kg bw D3 and exposed to 40 ppm Ca-rich water exhibited notable hyperplasia of CS compared with their control groups. Notable changes with high serum Ca level (13.87 ± 0.3 mg/dl) was detected on the 5th day in fish exposed to 40 ppm Ca-rich water, while related values attained (13.74 ± 0.1 mg/dl) only after 7 days in 20 ppm Ca-rich water of 500 IU/kg bw vitamin D3 injection. Similarly, high serum Pi level (7.66 ± 0.2 mg/dl) in 40 ppm Ca injected with D3 at 500 IU/kg bw. The results demonstrated that the Ca homeostasis of Labeo rohita is influenced by intra-peritoneal vitamin D3. Progressive studies should be conducted by increasing the dose of vitamin D3 to investigate optimum dose/supplement in feed for commercially important aquaculture teleost Labeo rohita for maximum and sustainable absorption of Ca from the variable water Calcium levels to maintain Ca2+ homeostasis.

Four stanniocalcin genes in teleost fish: structure, phylogenetic analysis, tissue distribution and expression during hypercalcemic challenge

Stanniocalcin (STC), first isolated from the corpuscles of Stannius (CS) of teleost fishes and a systemic regulator of mineral metabolism, is present in all vertebrates as two isoforms, STC1 and STC2, encoded by separate genes. Here we show that the genome of Tetraodon nigroviridis, and other teleosts, possess duplicate genes for each STC isoform, designated stc1-a and -b, and stc2-a and -b. Stc1-a was cloned from CS, stc2-a from muscle and the two novel cDNAs, stc1-b and stc2-b, from brain. However, stc2-b was isolated as a conjoined (read-through) transcript with bod1 (bi-orientation defective 1, or FAM44B), and two additional alternative conjoined transcripts were also isolated. The predicted STC products shared the typical vertebrate 10 conserved cysteine residues and N-linked glycosylation motifs, in addition to specific features. Gene structure was generally conserved with four exons and three introns with the exception of stc1-a which gained an extra intron in exon three, originating one extra exon. Gene order and synteny is also maintained across vertebrates and the cpeb4 gene identified in the homologue region of the chordate Ciona was linked to vertebrate stc2 but not stc1. Immunohistochemistry in different species revealed that STC1-A was found only in CS and in a few cells in kidney. STC1-B had a restricted expression and was more prominent in the gills. STC2-A was detected in a variety of tissues, including pituitary, with most abundant immunoreaction in kidney cells and gill rakers and the CS was negative. Expression of stc1-a in CS of Tetraodon was 15-fold (p<0.05) up-regulated 2 h after transfer from 2.9 mM Ca(2+) to 10 mM Ca(2+) water and down-regulated after 12 hours to 11-fold lower than 2.9 mM Ca(2+) fish (p<0.05). With the exception of stc1-a in CS, low expression levels and high individual variation were generally found for the expression of stc transcripts in kidney and gills, with no statistically significant changes in response to the hypercalcemic shock. In conclusion, both stc1 and stc2 genes are represented by paralogues in teleosts genomes and the analysis performed suggests that only stc1-a in the CS is involved in extracellular calcium regulation. The widespread distribution of stcs in fish tissues supports pleiotropic roles.

Stanniocalcin has deep evolutionary roots in eukaryotes

Vertebrates have a large glycoprotein hormone, stanniocalcin, which originally was shown to inhibit calcium uptake from the environment in teleost fish gills. Later, humans, other mammals, and teleost fish were shown to have two forms of stanniocalcin (STC1 and STC2) that were widely distributed in many tissues. STC1 is associated with calcium and phosphate homeostasis and STC2 with phosphate, but their receptors and signaling pathways have not been elucidated. We undertook a phylogenetic investigation of stanniocalcin beyond the vertebrates using a combination of BLAST and HMMER homology searches in protein, genomic, and expressed sequence tag databases. We identified novel STC homologs in a diverse array of multicellular and unicellular organisms. Within the eukaryotes, almost all major taxonomic groups except plants and algae have STC homologs, although some groups like echinoderms and arthropods lack STC genes. The critical structural feature for recognition of stanniocalcins was the conserved pattern of ten cysteines, even though the amino acid sequence identity was low. Signal peptides in STC sequences suggest they are secreted from the cell of synthesis. The role of glycosylation signals and additional cysteines is not yet clear, although the 11th cysteine, if present, has been shown to form homodimers in some vertebrates. We predict that large secreted stanniocalcin homologs appeared in evolution as early as single-celled eukaryotes. Stanniocalcin's tertiary structure with five disulfide bonds and its primary structure with modest amino acid conservation currently lack an established receptor-signaling system, although we suggest possible alternatives.

Low-resolution structural studies of human Stanniocalcin-1

BACKGROUND

Stanniocalcins (STCs) represent small glycoprotein hormones, found in all vertebrates, which have been functionally implicated in Calcium homeostasis. However, recent data from mammalian systems indicated that they may be also involved in embryogenesis, tumorigenesis and in the context of the latter especially in angiogenesis. Human STC1 is a 247 amino acids protein with a predicted molecular mass of 27 kDa, but preliminary data suggested its di- or multimerization. The latter in conjunction with alternative splicing and/or post-translational modification gives rise to forms described as STC50 and "big STC", which molecular weights range from 56 to 135 kDa.

RESULTS

In this study we performed a biochemical and structural analysis of STC1 with the aim of obtaining low resolution structural information about the human STC1, since structural information in this protein family is scarce. We expressed STC1 in both E. coli and insect cells using the baculo virus system with a C-terminal 6 x His fusion tag. From the latter we obtained reasonable amounts of soluble protein. Circular dichroism analysis showed STC1 as a well structured protein with 52% of alpha-helical content. Mass spectroscopy analysis of the recombinant protein allowed to assign the five intramolecular disulfide bridges as well as the dimerization Cys202, thereby confirming the conservation of the disulfide pattern previously described for fish STC1. SAXS data also clearly demonstrated that STC1 adopts a dimeric, slightly elongated structure in solution.

CONCLUSION

Our data reveal the first low resolution, structural information for human STC1. Theoretical predictions and circular dichroism spectroscopy both suggested that STC1 has a high content of alpha-helices and SAXS experiments revealed that STC1 is a dimer of slightly elongated shape in solution. The dimerization was confirmed by mass spectrometry as was the highly conserved disulfide pattern, which is identical to that found in fish STC1.

The respiratory effects of stanniocalcin-1 (STC-1) on intact mitochondria and cells: STC-1 uncouples oxidative phosphorylation and its actions are modulated by nucleotide triphosphates

Stanniocalcin-1 (STC-1) is one of only a handful of hormones that are targeted to mitochondria. High affinity receptors for STC-1 are present on cytoplasmic membranes and both the outer and inner mitochondrial membranes of nephron cells and hepatocytes. In both cell types, STC-1 is also present within the mitochondrial matrix and receptors presumably enable its sequestration. Furthermore, studies in bovine heart sub-mitochondrial particles have shown that STC-1 has concentration-dependent stimulatory effects on electron transport chain activity. The aim of the present study was to determine if the same effects could be demonstrated in intact, respiring mitochondria. At the same time, we also sought to demonstrate the functionality, if any, of an ATP binding cassette that has only recently been identified within the N-terminus of STC-1 by Prosite analysis. Intact, respiring mitochondria were isolated from rat muscle and liver and exposed to increasing concentrations of recombinant human STC-1 (STC-1). Following a 1h exposure to 500 nM STC-1, mitochondria from both organs displayed significant increases in respiration rate as compared to controls. Moreover, STC-1 uncoupled oxidative phosphorylation as ADP:O ratios were significantly reduced in mitochondria from both tissues. The resulting uncoupling was correlated with enhanced mitochondrial (45)Ca uptake in the presence of hormone. Respiratory studies were also conducted on a mouse inner medullary collecting cell line, where STC-1 had time and concentration-dependent stimulatory effects within the physiological range. In the presence of nucleotide triphosphates such as ATP and GTP (5mM) the respiratory effects of STC-1 were attenuated or abolished. Receptor binding studies revealed that this was due to a four-fold decrease in binding affinity (KD) between ligand and receptor. The results suggest that STC-1 stimulates mitochondrial electron transport chain activity and calcium transport, and that these effects are negatively modulated by nucleotide triphosphates.

Neural calcitropic peptides: immunoreactive characterization in fish and invertebrates

Parathyroid hormone (PTH) and stanniocalcin (STC) are primarily produced by the parathyroid glands and corpuscles of Stannius in tetrapods and fish, respectively. However, it is now known that both calcitropic peptides are also synthesized outside of these specialized endocrine glands. The current study employed Western blot analysis to characterize PTH and STC in neural tissues of high- (rats) and low- (hagfish, dogfish, rockfish, trout and skate) vertebrates and invertebrates (starfish, squid, cuttlefish, snails, prawns). Immunoreactive PTH-like peptides, comparable in size to PTH 1-84, were readily detectable in brains of vertebrates lacking (fish) and possessing (rat) parathyroid glands and in invertebrate (snail) ganglia. Immunoreactive STC-like peptides of varying size were similarly detected in brains of vertebrates lacking (rat) and possessing (fish) corpuscles of Stannius and in invertebrate (snail, prawn) ganglia. STC and PTH may thus have evolved as ancestral neuropeptides.

The stanniocalcin family of proteins

Stannniocalcin (STC) is a polypeptide hormone that was originally identified in bony fishes as a systemic regulator of mineral metabolism, and is best known for its regulatory effects on calcium/phosphate transport by the gills, gut and kidneys. The mammalian homolog to fish STC was discovered in 1995 and has resulted in progressively growing interest ever since as to its possible role in humans. Moreover, new discoveries in the mammalian STC field are resulting in significant reappraisals as to its role in fishes. Perhaps the most significant of these has been the discovery of a second gene encoding stanniocalcin-related protein, or STC-2, first in mammals and subsequently in fish. This review covers the comparative endocrinology of the STCs in fishes and mammals from the perspectives of structure, function and regulation. It then delves into some of the newer aspects of STC-1/STC-2 biology that have been uncovered using both classical and transgenic approaches. Of these, one of the most intriguing discoveries relates to the receptor-mediated sequestration of STC by target cell organelles. The functions of other newly discovered mammalian and fish STC variants are also discussed, as is the recent discovery of STC-related homologs in invertebrates. Based on our current state of knowledge, it is apparent that STC has an ancient lineage and that the STC family of proteins is proving to have significant roles in metabolism, reproduction and development.

Stanniocalcin: no longer just a fish tale

Stanniocalcin was originally described as a hormone with calcitonin-like actions in fish. During the last decade, mammalian forms of stanniocalcin have been identified, and this discovery has led to important advances in our understanding of this enigmatic polypeptide hormone. This review briefly covers some early studies on stanniocalcin in fish and then provides a more in-depth look at some of the more intriguing, new aspects of its functions in mammals. The roles of stanniocalcin in renal function, metabolism, angiogenesis, pregnancy and lactation, bone formation, and neural protection are discussed, along with new information relating to its receptor-mediated sequestration and accumulation in target cell organelles.