Calcium-sensing receptors and parathyroid hormone-related protein in the caudal neurosecretory system of the flounder (Platichthys flesus)

[The caudal neurosecretory system, the other "neurohypophysial" system in fish]

The caudal neurosecretory system (CNSS) is a neuroendocrine complex whose existence is specific to fishes. Structurally, it has many similarities with the hypothalamic-neurohypophyseal complex of other vertebrates. However, it differs regarding its position at the caudal end of the spinal cord and the nature of the hormones it secretes, the most important being urotensins. The CNSS was first described more than 60 years ago, but its embryological origin is totally unknown and its role is still poorly understood. Paradoxically, it is almost no longer studied today. Recent developments in imaging and genome editing could make it possible to resume investigations on CNSS in order to solve the mysteries that still surround it.

Role of PTHrP nuclear localization and carboxyl terminus sequences in postnatal spinal cord development

Parathyroid hormone-related peptide (PTHrP) acts under physiological conditions to regulate normal development of several tissues and organs. The role of PTHrP in spinal cord development has not been characterized. Pthrp knock in (Pthrp KI) mice were genetically modified to produce PTHrP in which there is a deficiency of the nuclear localization sequence (NLS) and C-terminus. Using this genetically modified mouse model, we have characterized its effect on spinal cord development early postnatally. The spinal cords from Pthrp KI mice displayed a significant reduction in its length, weight, and cross-sectional area compared to wild-type controls. Histologically, there was a decreased development of neurons and glial cells that caused decreased cell proliferation and increased apoptosis. The neural stem cells (NSCs) cultures also revealed decreased cell proliferation and differentiation and increased apoptosis. The proposed mechanism of delayed spinal cord development in Pthrp KI mice may be due to alteration in associated pathways in regulation of cell-division cycles and apoptosis. There was significant downregulation of Bmi-1 and upregulation of cyclin-dependent kinase inhibitors p27, p21, and p16 in Pthrp KI animals. We conclude that NLS and C-terminus peptide segments of PTHrP play an important role in inhibiting cell apoptosis and stimulation of cellular proliferation necessary for normal spinal cord development.

Hormonal axes in Drosophila: regulation of hormone release and multiplicity of actions

Hormones regulate development, as well as many vital processes in the daily life of an animal. Many of these hormones are peptides that act at a higher hierarchical level in the animal with roles as organizers that globally orchestrate metabolism, physiology and behavior. Peptide hormones can act on multiple peripheral targets and simultaneously convey basal states, such as metabolic status and sleep-awake or arousal across many central neuronal circuits. Thereby, they coordinate responses to changing internal and external environments. The activity of neurosecretory cells is controlled either by (1) cell autonomous sensors, or (2) by other neurons that relay signals from sensors in peripheral tissues and (3) by feedback from target cells. Thus, a hormonal signaling axis commonly comprises several components. In mammals and other vertebrates, several hormonal axes are known, such as the hypothalamic-pituitary-gonad axis or the hypothalamic-pituitary-thyroid axis that regulate reproduction and metabolism, respectively. It has been proposed that the basic organization of such hormonal axes is evolutionarily old and that cellular homologs of the hypothalamic-pituitary system can be found for instance in insects. To obtain an appreciation of the similarities between insect and vertebrate neurosecretory axes, we review the organization of neurosecretory cell systems in Drosophila. Our review outlines the major peptidergic hormonal pathways known in Drosophila and presents a set of schemes of hormonal axes and orchestrating peptidergic systems. The detailed organization of the larval and adult Drosophila neurosecretory systems displays only very basic similarities to those in other arthropods and vertebrates.

Molecular characterisation and expression of parathyroid hormone-related protein in the caudal neurosecretory system of the euryhaline flounder, Platichthys flesus

Parathyroid hormone-related protein (PTHrP) is a hypercalcemic factor in fish, but the source of circulating PTHrP remains unclear. In this study investigation of the caudal neurosecretory system (CNSS), considered one of major sources of PTHrP in fish, provided valuable insights into this regulatory system. We report pthrpa and pthrpb gene cloning, characterization, expression, and responses to low salinity and hypocalcemia challenge in flounder. The pthrpa and pthrpb precursors, isolated from a European flounder CNSS library, consist of 166 and 192 amino acid residues, respectively, with an overall homology of approximately 59.2%. Both precursors contain a signal peptide and a mature peptide with cleavage and amidation sites. The flounder PTHrPA and PTHrPB peptides share only 41% sequence identity with human PTHrPA. Quantitative PCR analysis demonstrated that the bone and bladder, are respectively major sites of pthrpa and pthrpb expression in flounder. Urophysectomy confirmed the CNSS as a likely contributor to circulating PTHrP peptides. There were no significant differences in CNSS pthrpa and pthrpb mRNA expression or plasma PTHrP levels between seawater (SW) and freshwater (FW)-adapted fish, though plasma total calcium concentrations were higher in FW animals. The intraperitonial administration of EGTA rapidly induced hypocalcemia and concomitant elevation in plasma PTHrP accompanied by increases in both pthrpa and pthrpb expression in the CNSS. Together, these findings support an evolutionary conserved role for PTHrP in the endocrine regulation of calcium.

Neuropeptides isotocin and arginine vasotocin in urophysis of three fish species

In this study, for the first time, both neuropeptides isotocin (IT) and arginine vasotocin (AVT) have been identified and measured in urophysis, the neurohaemal organ of the caudal neurosecretory system of teleost fish. So far, AVT, but not IT, was quantified by radioimmunoassay (RIA) in urophysis of several fish species. We have used high-performance liquid chromatographic assay with fluorescence detection (HPLC-FL) preceded by solid-phase extraction (SPE) and liquid chromatography-electrospray ionization triple-quadrupole tandem mass spectrometry (LC-ESI MS/MS) technique to determine both neuropeptides in urophysis of three fish species. The efficiency of peptide's SPE extraction was 79-85%. In HPLC-FL method, the limits of detection (LOD) and quantification (LOQ) were estimated as 1.0 and 3.4 pmol/mL for IT and 0.25 and 2.20 pmol/mL for AVT. In LC-MS/MS method, LOD and LOQ were estimated as 0.4 and 1.2 pmol/mL for IT and 0.06 and 0.2 pmol/mL for AVT. The chromatographic methods are good alternative for RIA, because enable to measure both nonapeptides simultaneously in one sample. In round goby (Neogobius melanostomus), three-spined stickleback (Gasterosteus aculeatus) and sea bream (Sparus aurata), urophysial IT concentrations ranged between 0.056 and 0.678 pmol/mg tissue and AVT concentrations ranged between 0.0008 (or even below detection threshold) and 0.084 pmol/mg tissue.

Daily rhythms of urotensin I and II gene expression and hormone secretion in the caudal neurosecretory system of the euryhaline flounder (Platichthys flesus)

The caudal neurosecretory system (CNSS) is a unique neuroendocrine structure for environmental adaptation in fish, and is the major site of expression and secretion of urotensin I (UI) and II (UII). This study examined daily changes in mRNA expression and the secretion profile of UI and UII in the CNSS. Daily rhythms were observed in mRNA level of CNSS UI, urophysis UI, plasma UII, glucose, potassium and sodium. No statistically significant (Cosinor, P>0.05) diel rhythmicity in mRNA level of CNSS UII, urophysis UII, cortisol, lactate, osmolality and chloride were detected. The calculated acrophase of sodium, cortisol, plasma UII, urophysis UII, urophysis UI and mRNA level of CNSS UI rhythms were recorded at 13:04 h, 13:39 h, 14:45 h, 15:27 h, 14:41 h and 14:39 h, respectively and a positive relationship was evident among them. The acrophase of glucose and potassium rhythms were recorded at 18:57 h and 22:35 h, respectively. The glucose levels increased progressively at the onset of the UII surge at 15:00 h and reached peak values at dusk. The results support the hypothesis that the CNSS may play a role in the control of co-ordinated daily changes in energy mobilization, nutritional behavior and osmoregulatory systems in euryhaline flounder. Our findings described for the first time the existence of daily rhythms of CNSS hormone expression and secretion in Platichthys flesus. These results reveal the importance of taking into account the time of day when assessing stress responses and evaluating UI and UII as physiological indicators of stress in this species.

Duplicated zebrafish co-orthologs of parathyroid hormone-related peptide (PTHrP, Pthlh) play different roles in craniofacial skeletogenesis

In mammals, parathyroid hormone-related peptide (PTHrP, alias PTH-like hormone (Pthlh)) acts as a paracrine hormone that regulates the patterning of cartilage, bone, teeth, pancreas, and thymus. Beyond mammals, however, little is known about the molecular genetic mechanisms by which Pthlh regulates early development. To evaluate conserved pathways of craniofacial skeletogenesis, we isolated two Pthlh co-orthologs from the zebrafish (Danio rerio) and investigated their structural, phylogenetic, and syntenic relationships, expression, and function. Results showed that pthlh duplicates originated in the teleost genome duplication. Zebrafish pthlha and pthlhb were maternally expressed and showed overlapping and distinct zygotic expression patterns during skeletal development that mirrored mammalian expression domains. To explore the regulation of duplicated pthlh genes, we studied their expression patterns in mutants and found that both sox9a and sox9b are upstream of pthlha in arch and fin bud cartilages, but only sox9b is upstream of pthlha in the pancreas. Morpholino antisense knockdown showed that pthlha regulates both sox9a and sox9b in the pharyngeal arches but not in the brain or otic vesicles and that pthlhb does not regulate either sox9 gene, which is likely related to its highly degraded nuclear localization signal. Knockdown of pthlha but not pthlhb caused runx2b overexpression in craniofacial cartilages and premature bone mineralization. We conclude that in normal cartilage development, sox9 upregulates pthlh, which downregulates runx2, and that the duplicated nature of all three of these genes in zebrafish creates a network of regulation by different co-orthologs in different tissues.

The Combinatorial Nature of Osmosensing in Fishes

Organisms exposed to altered salinity must be able to perceive osmolality change because metabolism has evolved to function optimally at specific intracellular ionic strength and composition. Such osmosensing comprises a complex physiological process involving many elements at organismal and cellular levels of organization. Input from numerous osmosensors is integrated to encode magnitude, direction, and ionic basis of osmolality change. This combinatorial nature of osmosensing is discussed with emphasis on fishes.

Calcium-sensing receptor: a high-affinity presynaptic target for aminoglycoside-induced weakness

Administration of aminoglycoside antibiotics can precipitate sudden, profound bouts of weakness that have been attributed to block of presynaptic voltage-activated calcium channels (VACCs) and failure of neuromuscular transmission. This serious adverse drug reaction is more likely in neuromuscular diseases such as myasthenia gravis. The relatively low affinity of VACC for aminoglycosides prompted us to explore alternative mechanisms. We hypothesized that the presynaptic Ca(2+)-sensing receptor (CaSR) may contribute to aminoglycoside-induced weakness due to its role in modulating synaptic transmission and its sensitivity to aminoglycosides in heterologous expression systems. We have previously shown that presynaptic CaSR controls a non-selective cation channel (NSCC) that regulates nerve terminal excitability and transmitter release. Using direct, electrophysiological recording, we report that neuronal VACCs are inhibited by neomycin (IC(50) 830 +/- 110 microM) at a much lower affinity than CaSR-modulated NSCC currents recorded from acutely isolated presynaptic terminals (synaptosomes; IC(50) 20 +/- 1 microM). Thus, at clinically relevant concentrations, aminoglycoside-induced weakness is likely precipitated by enhanced CaSR activation and subsequent decrease in terminal excitability rather than through direct inhibition of VACCs themselves.

The corpuscles of Stannius, calcium-sensing receptor, and stanniocalcin: responses to calcimimetics and physiological challenges

This study has examined whether the calcium-sensing receptor (CaSR) plays a role in control of stanniocalcin-1 (STC-1), the dominant calcium regulatory hormone of fish, comparable with that demonstrated for CaSR in the mediation of ionized calcium regulation of PTH secretion in mammals. In a previous study, we have cloned flounder STC-1 from the corpuscles of Stannius (CS). Here, we report the cloning and characterization of the CS CaSR, and the in vivo responses of this system to altered salinity, EGTA induced hypocalcemia, and calcimimetic administration. Quantitative PCR analysis demonstrated, for the first time, that the CS are major sites of CaSR expression in flounder. Immunoblot analysis of CS proteins with CaSR-specific antibodies revealed a broad band of approximately 215-300 kDa under nonreducing conditions, and bands of approximately 215-300 kDa and approximately 120-150 kDa under reducing conditions. There were no differences in CS CaSR mRNA expression or plasma STC-1 levels between seawater and freshwater (FW)-adapted fish, although CS STC-1 mRNA expression was lower in FW animals. Immunoblots showed that glycosylated monomeric forms of the CaSR migrated at a lower molecular mass in CS samples from FW animals. The ip administration of EGTA rapidly induced hypocalcemia, and a concomitant lowering of plasma STC-1. Calcimimetic administration (1 mg/kg R-568) rapidly increased plasma STC-1 levels, and reduced plasma concentrations of calcium, phosphate, and magnesium when compared with S-568-treated controls. Together, these findings support an evolutionary conserved role for the CaSR in the endocrine regulation of calcium before the appearance of parathyroid glands in tetrapods.

Extracellular calcium-sensing receptor distribution in osmoregulatory and endocrine tissues of the tilapia

The extracellular calcium-sensing receptor (CaSR) serves an important detector function in vertebrate Ca(2+) homeostasis. In this study, we surveyed using immunohistochemistry the tissue and cellular distribution of the CaSR protein in the Mozambique tilapia (Oreochromis mossambicus) and the Japanese eel (Anguilla japonica). Specifically, we examined receptor expression in ion-transporting barrier tissues that may be directly responsive to extracellular Ca(2+) levels, and in tissues that are implicated in endocrine signaling to homeostatic effectors such as Ca(2+)-transporting epithelia. In tilapia osmoregulatory tissues, CaSR protein is strongly expressed in proximal segments of renal tubule, but not in distal segments (where Na(+),K(+)-ATPase is prominently expressed) or in glomeruli. The receptor was also localized in the ion-transporting mitochondria-rich cells of gill and in ion- and nutrient-transporting epithelia of middle and posterior intestine. Consistent with our earlier RT-PCR assessment of mRNA expression in tilapia, CaSR protein expression was salinity dependent in some osmoregulatory tissues. In tilapia pituitary gland, CaSR expression was observed in the rostral pars distalis (containing prolactin-secreting cells, and in the pars intermedia (containing somatolactin-secreting and melanocyte-stimulating hormone-secreting cells), with notably greater expression in the latter. In the eel, weak immunostaining was seen in the stanniocalcin-secreting cells of the corpuscles of Stannius. Olfactory lobe CaSR expression suggests an environment-sensing role for the receptor. Altogether, these findings support the involvement of CaSR in piscine Ca(2+) homeostasis at the levels of environmental sensing, of integrative endocrine signaling through both hypercalcemic (prolactin, and perhaps somatolactin) and hypocalcemic (stanniocalcin) hormones, and of direct local regulation of Ca(2+)-transporting tissues.

Melatonin synthesis under calcium constraint in gilthead sea bream (Sparus auratus L.)

Brain or blood plasma melatonin was analysed as a measure for pineal melatonin production in sea bream. Access to calcium was limited by diluting the seawater to 2.5 per thousand and removing calcium from the diet or by prolonged feeding of vitamin D-deficient diet. Interactions/relations between melatonin and calcium balance and the hypercalcemic endocrines PTHrP and calcitriol were assessed. Restricting calcium availability in both water and diet had no effect on plasma melatonin, but when calcium was low in the water or absent from food, increased and decreased plasma melatonin was observed, respectively. Fish on a vitamin D-deficient diet (D- fish) showed decreased plasma calcitriol levels and remained normocalcemic. Decreased brain melatonin was found at all sampling times (10-22 weeks) in the D- fish compared to the controls. A positive correlation between plasma Ca2+ and plasma melatonin was found (R(2)=0.19; N=41; P <0.01) and brain melatonin was negatively correlated with plasma PTHrP (R(2)=0.78; N=4; P <0.05). The positive correlation between plasma levels of melatonin and Ca2+ provides evidence that melatonin synthesis is influenced by plasma Ca2+. The decreased melatonin production in the D- fish points to direct or indirect involvement of calcitriol in melatonin synthesis by the pineal organ in teleosts. The hypercalcemic factors PTHrP and calcitriol appeared to be negatively correlated with melatonin and this substantiates an involvement of melatonin in modulating the endocrine response to cope with hypocalcemia. It further points to the importance of Ca2+ in melatonin physiology.

Seasonal changes in peptide, receptor and ion channel mRNA expression in the caudal neurosecretory system of the European flounder (Platichthys flesus)

The caudal neurosecretory system (CNSS) of the euryhaline flounder Platichthys flesus has suggested roles in osmoregulatory, reproductive and nutritional adaptation, as fish migrate between seawater (winter) and brackish/freshwater (summer) environments. This study examined seasonal changes in mRNA expression profile of functionally important genes in the CNSS. cDNAs encoding neuropeptides, receptors and ion channels were cloned by reverse transcriptase polymerase chain reaction (RT-PCR) and screening of a flounder CNSS cDNA library. The expression profile of cloned genes was determined by real-time RT-PCR at 2-month intervals throughout the year in CNSS from seawater-adapted fish. Plasma cortisol (measured by radioimmunoassay) showed a peak in April, the time of spawning. Expression levels of mRNA for peptides urotensins I and II (UI, UII) and corticotropin releasing factor (CRF) all showed a seasonal cycle, with lowest expression in April and highest in August-October. The expression of CRF2(UI), UT(UII) and CRF1 receptors was not correlated with corresponding peptide expression. Receptors for potential neuromodulators of CNSS activity also displayed a seasonal mRNA expression profile. Glucocorticoid, 5-hydroxytryptamine, kappa-opioid and glutamate receptor expression peaked around April, suggesting that modulation of electrical activity of the neurosecretory Dahlgren cells is of particular importance at this time. Expression of mRNA for L-type Ca(2+) and Ca-activated K(+) channels was lower during the summer months. These channels underlie electrical bursting activity in Dahlgren cells. Ion channel mRNA expression was also lower in CNSS from flounder fully adapted to freshwater as opposed to seawater, consistent with previously reported observations of reduced bursting activity in Dahlgren cells from freshwater-adapted CNSS. These findings support the hypothesis that the CNSS is functionally reprogrammed to cope with changes in physiological challenge as fish migrate between sea and estuaries in winter and spring.

Fish caudal neurosecretory system: a model for the study of neuroendocrine secretion

The caudal neurosecretory system (CNSS) is unique to fish and has suggested homeostatic roles in osmoregulation and reproduction. Magnocellular neuroendocrine Dahlgren cells, located in the terminal segments of the spinal cord, project to a neurohaemal organ, the urophysis, from which neuropeptides are released. In the euryhaline flounder Platichthys flesus Dahlgren cells synthesise at least four peptides, including urotensins I and II and CRF. These peptides are differentially expressed with co-localisation of up to three in a single cell. Dahlgren cells display a range of electrical firing patterns, including characteristic bursting activity, which is dependent on L-type Ca(2+) and Ca-activated K(+)channels. Activity is modulated by a range of extrinsic and intrinsic neuromodulators. This includes autoregulation by the secreted peptides themselves, leading to enhanced bursting. Electrophysiological and mRNA expression studies have examined changes in response to altered physiological demands. Bursting activity is more robust and more Dahlgren cells are recruited in seawater compared to freshwater adapted fish and this is mirrored by a reduction in mRNA expression for L-type Ca(2+) and Ca-activated K(+) channels. Acute seawater/freshwater transfer experiments support a role for UII in adaptation to hyperosmotic conditions. Responses to stress suggest a shared role for CRF and UI, released from the CNSS. We hypothesise that the Dahlgren cell population is reprogrammed, both in anticipation of and in response to changed physiological demands, and this is seen as changes in gene expression profile and electrical activity. The CNSS shows striking parallels with the hypothalamic-neurohypophysial system, providing a highly accessible system for studies of neuroendocrine mechanisms. Furthermore, the presence of homologues of urotensins throughout the vertebrates has sparked new interest in these peptides and their functional evolution.

Parathyroid hormone-related protein in teleost fish

A brief description is given of the discovery of PTHrP and the roles of the peptide in mammalian physiology. Next, the occurrence of PTHrP in the earliest vertebrates, sharks, skates and fishes, is reviewed and the calciotropic functions of PTHrP are addressed more specifically in fishes. Parathyroid hormone-related protein (PTHrP) is a hypercalcemic hormone in teleostean fishes, but also has para- and autocrine functions. After the isolation and identification of fish PTHrP and PTHrP receptors and the subsequent development of recombinant protein and a real-time quantitative PCR, a calciotropic role of PTHrP in fish physiology could be assessed. PTHrP influences calcium physiology via regulation of calcium mobilisation from internal sources (bone and scales) and via calcium uptake from the environment (water and diet). Continuous variations in the need for calcium and in the availability of environmental calcium require fast calciotropes to guarantee calcium balance, in which PTHrP is pivotal for the fish. PTHrP is essential in fish bone physiology, e.g. in mineralisation and calcium reabsorption from the scales. Moreover, PTHrP plays a role in vitellogenesis, cortisol production, regulation of renal Mrp2 activity and melatonin synthesis. The plethora of functions of PTHrP in fish concern endocrine, paracrine and autocrine (and possibly intracrine) functions; calciotropic actions of PTHrP at the organismal and cellular level are prominent in fish. The strong conservation of the pthrp gene in the vertebrate lineage and the N-terminal similarity of the coded proteins relates to the important role of PTHrP in calcium physiology that is of paramount importance to all physiological processes. Recent and ongoing studies will contribute to our rapidly expanding knowledge of the original physiological functions of PTHrP in teleost fish.

The parathyroid hormone family of peptides: structure, tissue distribution, regulation, and potential functional roles in calcium and phosphate balance in fish

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) are two factors that share amino acid sequence homology and act via a common receptor. In tetrapods, PTH is the main endocrine factor acting in bone and kidney to regulate calcium and phosphate. PTHrP is an essential paracrine developmental factor present in many tissues and is involved in the regulation of ossification, mammary gland development, muscle relaxation, and other functions. Fish apparently lack an equivalent of the parathyroid gland and were long thought to be devoid of PTH. Only in recent years has the existence of PTH-like peptides and their receptors in fish been firmly established. Two forms of PTH, two of PTHrP, and a protein with intermediate characteristics designated PTH-L are encoded by separate genes in teleost fish. Three receptors encoded by separate genes in fish mediate PTH/PTHrP actions, whereas only two receptors have so far been found in terrestrial vertebrates. PTHrP has been more intensively studied than PTH, from lampreys to advanced teleosts. It is expressed in many tissues and is present in high concentration in fish blood. Administration of this peptide alters calcium metabolism and has marked effects on associated gene expression and enzyme activity in vivo and in vitro. This review provides a comprehensive overview of the physiological roles, distribution, and molecular relationships of the piscine PTH-like peptides.

cDNA Cloning and Functional Expression of a Ca2+-sensing Receptor with Truncated C-terminal Tail from the Mozambique Tilapia (Oreochromis mossambicus)

The complete cDNA sequence of the tilapia extracellular Ca(2+)-sensing receptor (CaR) was determined. The transcript length of tilapia CaR (tCaR) is 3.4 kbp and encodes a 940-amino acid, 7-transmembrane domain protein that is consistent in its structural features with known mammalian and piscine CaRs. The tCaR extracellular domain includes a characteristic hydrophobic segment, conserved cysteine residues that are implicated in receptor dimerization (Cys(129) and Cys(131)) and in coupling to the transmembrane domain (nine conserved cysteine residues), and conserved serine residues (Ser(147) and Ser(169-171)) that are linked to receptor binding of Ca(2+) and L-amino acid-mediated potentiation of function. mRNA expression of tCaR was strong in kidney, brain, and gill. Weaker expression was observed in pituitary, stomach, intestine, urinary bladder, and heart. This distribution is consistent with possible physiological roles in endocrine cells, excitable tissues, and ion-transporting barrier epithelia. Expression of tCaR mRNA in kidney and intestine was salinity-dependent, suggesting a role for the receptor in iono-/osmoregulation in this euryhaline teleost species. Human embryonic kidney-293 cells transiently transfected with tCaR cDNA demonstrated dose-dependent phospholipase C activation in response to elevations in the extracellular Ca(2+) concentration ([Ca(2+)](o)). Functional activation of the mitogen-activated protein kinase cascade by high [Ca(2+)](o) was also confirmed in these cells despite the naturally occurring truncation of the receptor's intracellular tail, which removes segments variably linked in mammalian CaRs to filamin-coupled activation of mitogen-activated protein kinase cascades. Sensitivity of phospholipase C activation to [Ca(2+)](o) was dependent on the ionic strength of the bathing medium, supporting a role in salinity sensing.

Determination of tissue and plasma concentrations of PTHrP in fish: development and validation of a radioimmunoassay using a teleost 1-34 N-terminal peptide

A specific and sensitive radioimmunoassay (RIA) for the N-terminus of sea bream (Sparus auratus) and flounder (Platichthys flesus) parathyroid hormone-related protein (PTHrP) was developed. A (1-34) amino-terminal sequence of flounder PTHrP was synthesized commercially and used as the antigen to generate specific antiserum. The same sequence with an added tyrosine (1-35(Tyr)) was used for iodination. Human (1-34) parathyroid hormone (PTH), human (1-34) PTHrP, and rat (1-34) PTHrP did not cross-react with the antiserum or displace the teleost peptide. Measurement of PTHrP in fish plasma was only possible after denaturing by heat treatment due to endogenous plasma binding activity. The minimum detectable concentration of (1-34) PTHrP in the assay was 2.5 pg/tube. The level of immunoreactive (1-34) PTHrP in plasma was 5.2+/-0.44 ng/ml (mean+/-SEM, n=20) for flounder and 2.5+/-0.29 ng/ml (n=64) for sea bream. Dilution curves of denatured fish plasma were parallel to the assay standard curve, indicating that the activity in the samples was indistinguishable immunologically from (1-34) PTHrP. Immunoreactivity was present, in order of abundance, in extracts of pituitary, oesophagus, kidney, head kidney, gills, intestine, skin, muscle, and liver. The pituitary gland and oesophagus contained the most abundant levels of PTHrP, 37.7+/-6.1 ng/g wet tissue and 2.3+/-0.7 ng/g wet tissue, respectively. The results suggest that in fish PTHrP may act in a paracrine and/or autocrine manner but may also be a classical hormone with the pituitary gland as a potential major source of the protein.

Extracellular calcium-sensing receptors in the parathyroid gland, kidney, and other tissues

PURPOSE OF REVIEW

The discovery of the extracellular calcium-sensing receptor, CasR has broadened our understanding of calcium homeostasis and led to the development of new pharmacological agents, calcimimetics, for treating hyperparathyroidism. In the present review, I discuss the function of CasR as well as provide evidence for the presence of additional calcium-sensing mechanisms in the skeleton and possibly other tissues.

RECENT FINDINGS

Inactivating and activating mutations of the CasR respectively cause hereditary hyperparathyroidism, and demonstrate the predominant role of the CasR in controlling parathyroid gland function. Calcimimetics, which increase the sensitivity of CasR to extracellular calcium have been developed to treat secondary and primary hyperparathyroidism. In recent clinical trials in patients with end stage kidney disease, the calcimimetic cinacalcet suppressed parathyroid hormone to a greater degree than conventional therapy with vitamin D analogues without causing hypercalcemia or hyperphosphatemia. CasR receptor also has functions in other tissues, including regulation of renal calcium excretion and calcitonin secretion by thyroidal C-cells, but the presence of redundant sensing mechanisms for extracellular calcium in other tissues, including bone, confounds the assessment of the receptor's function at these sites. Mouse genetic approaches have so far failed to identify any essential, non-redundant role for the calcium-sensing receptor in regulating chondrogenesis or osteogenesis, and have failed to establish a function for the protein outside of the parathyroid gland, kidney, and thyroidal C-cells. Rather, there is evidence for other putative calcium sensing receptor-like mechanisms in osteoblasts that remain to be identified.

SUMMARY

Sensing of extracellular calcium by CasR is important in regulating calcium homeostasis, but CasR may have vestigial function in various tissues where it is expressed in low abundance. The relative importance of CasR and the novel calcium-sensing mechanisms in mediating response to extracellular calcium in many of these tissues remain to be determined.

Extracellular calcium sensing and signalling

Ca2+ is well established as an intracellular second messenger. However, the molecular identification of a detector for extracellular Ca2+--the extracellular calcium-sensing receptor--has opened up the possibility that Ca2+ might also function as a messenger outside cells. Information about the local extracellular Ca2+ concentration is conveyed to the interior of many cell types through this unique G-protein-coupled receptor. Here, we describe new emerging concepts concerning the signalling function of extracellular Ca2+, with particular emphasis on the extracellular calcium-sensing receptor.

Application of comparative genomics in fish endocrinology

This review discusses the ways in which comparative genomics can contribute to the study of fish endocrinology. First, the phylogenetic position of fish and an overview of their specific endocrine systems are presented. The emphasis will be on teleosts because they are the most abundant fishes and because most data are available for this group. Second, the complexity of fish genomics is reviewed. With the vast array of genome sizes and ploidy levels, assignment of gene orthology is more difficult in fish, but this is an absolute prerequisite in functional analysis and it is important to be aware of such genome plasticity when cloning genes. The ease with which a gene is cloned at the genomic level is directly related to genome size and complexity, a factor that is not known in the majority of fish species. Finally, the methodology is presented along with specific examples of parathyroid hormone-related protein (PTHrP) (a previously unidentified hormone in fish), calcium-sensing receptor, and calcitonin (with a duplication of this particular ligand in Fugu rubripes). Preliminary data also suggest that there are further duplicated genes in the calcium regulatory system. Comparative genomics has provided a valuable approach for isolating and characterizing a range of fish genes involved in calcium regulation. However, for understanding the physiology and endocrine regulation of this system, particularly with regard to gene duplication, an alternative approach is required in which conventional endocrinology techniques will play a significant role.