Calcitropic peptides: neural perspectives

Parathyroid Hormone (PTH)-Related Peptides Family: An Intriguing Role in the Central Nervous System

Parathyroid Hormone (PTH) plays a crucial role in the maintenance of calcium homeostasis directly acting on bone and kidneys and indirectly on the intestine. However, a large family of PTH-related peptides exists that exerts other physiological effects on different tissues and organs, such as the Central Nervous System (CNS). In humans, PTH-related peptides are Parathyroid Hormone (PTH), PTH-like hormones (PTHrP and PTHLH), and tuberoinfundibular peptide of 39 (TIP39 or PTH2). With different affinities, these ligands can bind parathyroid receptor type 1 (PTH1R) and type 2 (PTH2R), which are part of the type II G-protein-coupled-receptors (GPCRs) family. The PTH/PTHrP/PTH1R system has been found to be expressed in many areas of the brain (hippocampus, amygdala, hypothalamus, caudate nucleus, corpus callosum, subthalamic nucleus, thalamus, substantia nigra, cerebellum), and literature data suggest the system exercises a protective action against neuroinflammation and neurodegeneration, with positive effects on memory and hyperalgesia. TIP39 is a small peptide belonging to the PTH-related family with a high affinity for PTH2R in the CNS. The TIP39/PTH2R system has been proposed to mediate many regulatory and functional roles in the brain and to modulate auditory, nociceptive, and sexual maturation functions. This review aims to summarize the knowledge of PTH-related peptides distribution and functions in the CNS and to highlight the gaps that still need to be filled.

Calcitonin-typical suppression of osteoclastic activity by amphioxus calcitonin superfamily peptides and insights into the evolutionary conservation and diversity of their structures

Calcitonin (CT) is a hormone that decreases serum calcium level by suppressing osteoclastic activity in the vertebrate bone. In vertebrates, the structure-function relationship of CTs has been studied extensively. We recently identified three CT superfamily peptides, Bf-CTFP1 to 3, and clarified the molecular and functional characteristics of their receptor and receptor activity-modifying protein in amphioxus, Branchiostoma floridae. However, the CT activity of Bf-CTFPs has yet to be investigated. In the present study, a functional analysis of Bf-CTFPs was performed using goldfish scales having both osteoclasts and osteoblasts. All Bf-CTFPs suppressed osteoclastic activity via a goldfish CT receptor. Although the primary amino acid sequences of the Bf-CTFPs showed low sequence similarity to vertebrate CTs, Bf-CTFP1 to 3 share three amino acids, Thr²⁵, Thr²⁷, and Pro³²-NH₂, that are required for receptor binding, with salmon CT. Moreover, homology model analysis revealed that the Bf-CTFPs form alpha-helical structures. The alpha-helical position and length of Bf-CTFP1 and 2 were conserved with those of a highly potent ligand, teleost CT. Interestingly, the composition of the alpha-helix of Bf-CTFP3 differed from those of teleost CT, despite that the action of Bf-CTFP3 on goldfish scales was the same as that of Bf-CTFP1 and 2. Collectively, the present study provides new insights into the structure-function relationship of CT and its functional evolution in chordates.

The significance of Ciona intestinalis as a stem organism in integrative studies of functional evolution of the chordate endocrine, neuroendocrine, and nervous systems

Ascidians are the closest phylogenetic neighbors to vertebrates and are believed to conserve the evolutionary origin in chordates of the endocrine, neuroendocrine, and nervous systems involving neuropeptides and peptide hormones. Ciona intestinalis harbors various homologs or prototypes of vertebrate neuropeptides and peptide hormones including gonadotropin-releasing hormones (GnRHs), tachykinins (TKs), and calcitonin, as well as Ciona-specific neuropeptides such as Ciona vasopressin, LF, and YFV/L peptides. Moreover, molecular and functional studies on Ciona tachykinin (Ci-TK) have revealed the novel molecular mechanism of inducing oocyte growth via up-regulation of vitellogenesis-associated protease activity, which is expected to be conserved in vertebrates. Furthermore, a series of studies on Ciona GnRH receptor paralogs have verified the species-specific regulation of GnRHergic signaling including unique signaling control via heterodimerization among multiple GnRH receptors. These findings confirm the remarkable significance of ascidians in investigations of the evolutionary processes of the peptidergic systems in chordates, leading to the promising advance in the research on Ciona peptides in the next stage based on the recent development of emerging technologies including genome-editing techniques, peptidomics-based multi-color staining, machine-learning prediction, and next-generation sequencing. These technologies and bioinformatic integration of the resultant "multi-omics" data will provide unprecedented insights into the comprehensive understanding of molecular and functional regulatory mechanisms of the Ciona peptides, and will eventually enable the exploration of both conserved and diversified endocrine, neuroendocrine, and nervous systems in the evolutionary lineage of chordates.

Evolution of parathyroid hormone receptor family and their ligands in vertebrate

The presence of the parathyroid hormones in vertebrates, including PTH, PTH-related peptide (PTHrP), and tuberoinfundibular peptide of 39 residues (TIP39), has been proposed to be the result of two rounds of whole genome duplication in the beginning of vertebrate diversification. Bioinformatics analyses, in particular chromosomal synteny study and the characterization of the PTH ligands and their receptors from various vertebrate species, provide evidence that strongly supports this hypothesis. In this mini-review, we summarize recent advances in studies regarding the molecular evolution and physiology of the PTH ligands and their receptors, with particular focus on non-mammalian vertebrates. In summary, the PTH family of peptides probably predates early vertebrate evolution, indicating a more ancient existence as well as a function of these peptides in invertebrates.

Arthritis and prolactin: a phylogenetic viewpoint

Arthritic disorders are family of diseases that have existed since vertebrate life began. Their etiology is multifactorial with genetic, environmental, and gender factors driving chronic joint inflammation. Prolactin is a sexually dimorphic hormone in mammals that can act to both promote and ameliorate rheumatic diseases. It is found in all vertebrate groups where it exerts a wide diversity of actions. This review briefly addresses the presence and features of arthritic diseases in vertebrates, the effects of PRL on joint tissues and immune cells, and whether PRL actions could have contributed to the ubiquity of arthritis in nature. This comparative approach highlights the value of PRL as a biologically conserved factor influencing the development and progression of arthritis.

Functional characterization and evolution of PTH/PTHrP receptors: insights from the chicken

BACKGROUND

The parathyroid hormone (PTH)-family consists of a group of structurally related factors that regulate calcium and bone homeostasis and are also involved in development of organs such as the heart, mammary gland and immune system. They interact with specific members of family 2 B1 G-protein coupled receptors (GPCRs), which have been characterised in teleosts and mammals. Two PTH/PTHrP receptors, PTH1R and PTH2R exist in mammals and in teleost fish a further receptor PTH3R has also been identified. Recently in chicken, PTH-family members involved in calcium transport were characterized and specific PTHRs are suggested to exist although they have not yet been isolated or functionally characterized. The aim of this study is to further explore the evolution and function of the vertebrate PTH/PTHrP system through the isolation, phylogenetic analysis and functional characterization of the chicken receptors.

RESULTS

Two PTHRs were isolated in chicken and sequence comparison and phylogenetic analysis indicate that the chicken receptors correspond to PTH1R and PTH3R, which emerged prior to the teleost/tetrapod divergence since they are present in cartilaginous fish. The vertebrate PTH2R receptor and its ligand TIP39 have been lost from bird genomes. Chicken PTH1R and PTH3R have a divergent and widespread tissue expression and are also evident in very early embryonic stages of development. Receptor stimulation studies using HEK293 cells stably expressing the chicken PTH1R and PTH3R and monitoring cAMP production revealed they are activated by chicken 1-34 N-terminal PTH-family peptides in a dose dependent manner. PTH-L and PTHrP were the most effective peptides in activating PTH1R (EC(50) = 7.7 nM and EC(50) = 22.7 nM, respectively). In contrast, PTH-L (100 nM) produced a small cAMP accumulation on activation of PTH3R but PTHrP and PTH (EC(50) = 2.5 nM and EC(50) = 22.1 nM, respectively) readily activated the receptor. PTHrP also stimulated intracellular Ca(2+) accumulation on activation of PTH1R but not PTH3R.

CONCLUSION

Two PTHR homologues of the vertebrate PTH1R and PTH3R were isolated and functionally characterized in chicken. Their distinct pattern of expression during embryo development and in adult tissues, together with their ligand preference, suggests that they have acquired specific functions, which have contributed to their maintenance in the genome. PTH2R and its activating ligand, TIP39, are absent from bird genomes. Nonetheless identification of putative PTH2R and TIP39 in the genome of an ancient agnathan, lamprey, suggests the PTH/PTHrP ligand and receptor family was already present in an early basal paraphyletic group of vertebrates and during the vertebrate radiation diverged via gene/genome duplication and deletion events. Knowledge of the role PTH/PTHrP system in early vertebrates will help to establish evolution of function.

Parathyroid hormone (PTH) rapidly enhances CFTR-mediated HCO₃⁻ secretion in intestinal epithelium-like Caco-2 monolayer: a novel ion regulatory action of PTH

Besides being a Ca²-regulating hormone, parathyroid hormone (PTH) has also been shown to regulate epithelial transport of certain ions, such as Cl, HCO₃, and Na, particularly in the kidney. Although the intestinal epithelium also expressed PTH receptors, little was known regarding its mechanism in the regulation of intestinal ion transport. We investigated the ion regulatory role of PTH in intestinal epithelium-like Caco-2 monolayer by Ussing chamber technique and alternating current impedance spectroscopy. It was found that Caco-2 cells rapidly responded to PTH within 1 min by increasing apical HCO₃- secretion. CFTR served as the principal route for PTH-stimulated apical HCOV efflux, which was abolished by various CFTR inhibitors, namely, NPPB, glycine hydrazide-101 (GlyH-101), and CFTRinh-172, as well as by small interfering RNA against CFTR. Concurrently, the plasma membrane resistance was decreased with no changes in the plasma membrane capacitance or paracellular permeability. HCOV was probably supplied by basolateral uptake via the electrogenic Na⁺-HCO₃⁻ cotransporter and by methazolamide-sensitive carbonic anhydrase, while the resulting intracellular H⁺ might be extruded by both apical and basolateral Na/H exchangers. Furthermore, the PTH-stimulated HCO₃-secretion was markedly reduced by protein kinase A (PKA) inhibitor (PKI 14-22 amide) and phosphoinositide 3-kinase (PI3K) inhibitors (wortmannin and LY-294002), but not by intracellular Ca²⁺ chelator (BAPTA-AM) or protein kinase C inhibitor (GF-109203X). In conclusion, the present study provided evidence that PTH directly and rapidly stimulated apical HCO₃- secretion through CFTR in PKA- and PI3K-dependent manner, which was a novel noncalciotropic, ion regulatory action of PTH in the intestinal epithelium.

Calcitonin in a protochordate, Ciona intestinalis--the prototype of the vertebrate calcitonin/calcitonin gene-related peptide superfamily

The calcitonin (CT)/CT gene-related peptides (CGRPs) constitute a large peptide family in vertebrates. However, no CT/CGRP superfamily members have so far been identified in invertebrates, and the evolutionary process leading to the diverse vertebrate CT/CGRP superfamily members remains unclear. In this study, we have identified an authentic invertebrate CT, Ci-CT, in the ascidian Ciona intestinalis, which is the phylogenetically closest invertebrate chordate to vertebrates. The amino acid sequence of Ci-CT was shown to display high similarity to those of vertebrate CTs and to share CT consensus motifs, including the N-terminal circular region and C-terminal amidated proline. Furthermore, the Ci-CT gene was found to be the only Ciona CT/CGRP superfamily gene. Ci-CT also exhibited less potent, but significant, activation of the human CT receptor, as compared with salmon CT. Physiological analysis revealed that Ci-CT reduced the osteoclastic activity that is specific to vertebrate CTs. CD analysis demonstrated that Ci-CT weakly forms an alpha-helix structure. These results provide evidence that the CT/CGRP superfamily is essentially conserved in ascidians as well as in vertebrates, and indicate that Ci-CT is a prototype of vertebrate CT/CGRP superfamily members. Moreover, expression analysis demonstrated that Ci-CT is expressed in more organs than vertebrate CTs in the cognate organs, suggesting that an original CT/CGRP superfamily member gene was also expressed in multiple organs, and each CT/CGRP superfamily member acquired its current specific tissue distribution and physiological role concomitantly with diversification of the CT/CGRP superfamily during the evolution of chordates. This is the first report on a CT/CGRP superfamily member in invertebrates.

The effects of saltwater acclimation on neurotransmitters in the lingual salt glands of the estuarine crocodile, Crocodylus porosus

INTRODUCTION

Most avian and reptilian salt glands display marked phenotypic plasticity when animals are exposed to hyperosmotic conditions. In addition, the activity of most salt glands is under considerable control by the nervous system and nerves containing cholinergic, adrenergic and peptidergic neurotransmitters have been identified in avian and reptilian salt gland tissues. The present study sought to determine whether the salt glands of the estuarine crocodile, Crocodylus porosus contain the peptidergic neurotransmitters SP, CGRP, VIP, and PACAP and the gaseous neurotransmitter, NO. In addition, we sought to determine whether there was any evidence for the adaptation of the C. porosus salt gland nervous system to hyperosmotic conditions.

METHODS

Salt glands from freshwater- and saltwater-acclimated C. porosus hatchlings were sectioned and examined immunohistochemically for neurotransmitters within the tissue.

RESULTS

Neurons containing SP, CGRP, VIP, PACAP and NO synthase were identified within C. porosus salt glands. There was no difference in the overall number (density) of neurons within SW-acclimated tissues when compared with FW-acclimated animals. However, there was a significant reduction in density of neurons containing SP and PACAP in SW-acclimated animals.

CONCLUSION

C. porosus salt glands display phenotypic plasticity following exposure to hyperosmotic conditions. In addition to cholinergic and adrenergic neurons, they contain a variety of peptidergic neurotransmitters and the gaseous neurotransmitter NO. Additionally, there appears to be some evidence of acclimation of the nervous system of C. porosus to hypersaline conditions, although the functional significance of these changes remains to be determined.

In silico data filtering to identify new angiogenesis targets from a large in vitro gene profiling data set

The objective of this study was to use gene expression data from well-defined cell culture models, in combination with expression data from diagnostic samples of human diseased tissues, to identify potential therapeutic targets and markers of disease. Using Affymetrix oligonucleotide array technology, we identified a common profile of genes upregulated during endothelial morphogenesis into tubelike structures in three in vitro models of angiogenesis. Rigorous data selection criteria were used to identify a list of over 1,000 genes whose expression was increased more than twofold over baseline at either 4, 8, 24, 40 or 50 h. To further refine and prioritize this list, we used standard bioinformatic algorithms to identify potential transmembrane and secreted proteins. We then overlapped this gene set with genes upregulated in colon tumors vs. normal colon, resulting in a subset of 128 genes in common with our endothelial list. We removed from this list those genes expressed in 6 different colon tumor lines, resulting in a list of 24 putative, vascular-specific angiogenesis-associated genes. Three genes, gp34, stanniocalcin-1 (STC-1), and GA733-1, were expressed at levels 10-fold or more in colon tumors compared with normal mucosa. We validated the vascular-specific expression of one of these genes, STC-1, by in situ hybridization. The ability to combine in vitro and in vivo data sets should permit one to identify putative angiogenesis target genes in various tumors, chronic inflammation, and other disorders where therapeutic manipulation of angiogenesis is a desirable treatment modality.

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

The caudal neurosecretory system of the flounder (Platichthys flesus) has been examined by immunocytochemistry and in situ hybridization for the expression of parathyroid hormone-related protein (PTHrP) and calcium-sensing receptors (CaSR). The N-terminus nucleotide and deduced amino acid sequences of flounder PTHrP were determined and used to prepare oligonucleotide probes and homologous antiserum. The Dahlgren cells of the posterior spinal cord and their axons contained PTHrP protein which was also detected around the capillaries of the urophysis. PTHrP gene expression was abundant in the Dahlgren perikarya and axons in the spinal cord, but it was absent from nerve endings in the urophysis. Calcium-sensing receptor protein was present in the Dahlgren perikarya and axons, also with abundant gene expression, but there was neither protein nor mRNA in the urophysis. There were no apparent differences between freshwater- and seawater-adapted fish in either CaSR or PTHrP expression in the caudal neurosecretory system. These observations suggest that Dahlgren cells produce PTHrP which may be released from axons abutting capillaries in the urophysis. However, the sensing of ionic calcium appears to be confined to the perikarya of the Dahlgren cells in the spinal cord neuropil, suggesting that they are responsive to calcium in the central nervous system rather than the general circulation.