Transcript expression of the tuberoinfundibular peptide (TIP)39/PTH2 receptor system and non-PTH1 receptor-mediated tonic effects of TIP39 and other PTH2 receptor ligands in renal vessels

Silencing of parathyroid hormone (PTH) receptor 1 in T cells blunts the bone anabolic activity of PTH

Intermittent parathyroid hormone (iPTH) treatment stimulates T-cell production of the osteogenic Wnt ligand Wnt10b, a factor required for iPTH to activate Wnt signaling in osteoblasts and stimulate bone formation. However, it is unknown whether iPTH induces Wnt10b production and bone anabolism through direct activation of the parathyroid hormone (PTH)/PTH-related protein receptor (PPR) in T cells. Here, we show that conditional silencing of PPR in T cells blunts the capacity of iPTH to induce T-cell production of Wnt10b; activate Wnt signaling in osteoblasts; expand the osteoblastic pool; and increase bone turnover, bone mineral density, and trabecular bone volume. These findings demonstrate that direct PPR signaling in T cells plays an important role in PTH-induced bone anabolism by promoting T-cell production of Wnt10b and suggest that T cells may provide pharmacological targets for bone anabolism.

Evolution of the vertebrate pth2 (tip39) gene family and the regulation of PTH type 2 receptor (pth2r) and its endogenous ligand pth2 by hedgehog signaling in zebrafish development

In mammals, parathyroid hormone (PTH), secreted by parathyroid glands, increases calcium levels in the blood from reservoirs in bone. While mammals have two PTH receptor genes, PTH1R and PTH2R, zebrafish has three receptors, pth1r, pth2r, and pth3r. PTH can activate all three zebrafish Pthrs while PTH2 (alias tuberoinfundibular peptide 39, TIP39) preferentially activates zebrafish and mammalian PTH2Rs. We know little about the roles of the PTH2/PTH2R system in the development of any animal. To determine the roles of PTH2 and PTH2R during vertebrate development, we evaluated their expression patterns in developing zebrafish, observed their phylogenetic and conserved synteny relationships with humans, and described the genomic organization of pth2, pth2r, and pth2r splice variants. Expression studies showed that pth2 is expressed in cells adjacent to the ventral part of the posterior tuberculum in the diencephalon, whereas pth2r is robustly expressed throughout the central nervous system. Otic vesicles express both pth2 and pth2r, but heart expresses only pth2. Analysis of mutants showed that hedgehog (Hh) signaling regulates the expression of pth2 transcripts more than that of nearby gnrh2-expressing cells. Genomic analysis showed that a lizard, chicken, and zebra finch lack a PTH2 gene, which is associated with an inversion breakpoint. Likewise, chickens lack PTH2R, while humans lack PTH3R, a case of reciprocally missing ohnologs (paralogs derived from a genome duplication). The considerable evolutionary conservation in genomic structure, synteny relationships, and expression of zebrafish pth2 and pth2r provides a foundation for exploring the endocrine roles of this system in developing vertebrate embryos.

Disruption of PTH receptor 1 in T cells protects against PTH-induced bone loss

Background

Hyperparathyroidism in humans and continuous parathyroid hormone (cPTH) treatment in mice cause bone loss by regulating the production of RANKL and OPG by stromal cells (SCs) and osteoblasts (OBs). Recently, it has been reported that T cells are required for cPTH to induce bone loss as the binding of the T cell costimulatory molecule CD40L to SC receptor CD40 augments SC sensitivity to cPTH. However it is unknown whether direct PTH stimulation of T cells is required for cPTH to induce bone loss, and whether T cells contribute to the bone catabolic activity of PTH with mechanisms other than induction of CD40 signaling in SCs.

Methodology/Principal Findings

Here we show that silencing of PTH receptor 1 (PPR) in T cells blocks the bone loss and the osteoclastic expansion induced by cPTH, thus demonstrating that PPR signaling in T cells is central for PTH-induced reduction of bone mass. Mechanistic studies revealed that PTH activation of the T cell PPR stimulates T cell production of the osteoclastogenic cytokine tumor necrosis factor α (TNF). Attesting to the relevance of this effect, disruption of T cell TNF production prevents PTH-induced bone loss. We also show that a novel mechanism by which TNF mediates PTH induced osteoclast formation is upregulation of CD40 expression in SCs, which increases their RANKL/OPG production ratio.

Conclusions/Significance

These findings demonstrate that PPR signaling in T cells plays an essential role in PTH induced bone loss by promoting T cell production of TNF. A previously unknown effect of TNF is to increase SC expression of CD40, which in turn increases SC osteoclastogenic activity by upregulating their RANKL/OPG production ratio. PPR-dependent stimulation of TNF production by T cells and the resulting TNF regulation of CD40 signaling in SCs are potential new therapeutic targets for the bone loss of hyperparathyroidism.

The TIP39-PTH2 receptor system: unique peptidergic cell groups in the brainstem and their interactions with central regulatory mechanisms

Tuberoinfundibular peptide of 39 residues (TIP39) is the recently purified endogenous ligand of the previously orphan G-protein coupled parathyroid hormone 2 receptor (PTH2R). The TIP39-PTH2R system is a unique neuropeptide-receptor system whose localization and functions in the central nervous system are different from any other neuropeptides. TIP39 is expressed in two brain regions, the subparafascicular area in the posterior thalamus, and the medial paralemniscal nucleus in the lateral pons. Subparafascicular TIP39 neurons seem to divide into a medial and a lateral cell population in the periventricular gray of the thalamus, and in the posterior intralaminar complex of the thalamus, respectively. Periventricular thalamic TIP39 neurons project mostly to limbic brain regions, the posterior intralaminar thalamic TIP39 neurons to neuroendocrine brain areas, and the medial paralemniscal TIP39 neurons to auditory and other brainstem regions, and the spinal cord. The widely distributed axon terminals of TIP39 neurons have a similar distribution as the PTH2R-containing neurons, and their fibers, providing the anatomical basis of a neuromodulatory action of TIP39. Initial functional studies implicated the TIP39-PTH2R system in nociceptive information processing in the spinal cord, in the regulation of different hypophysiotropic neurons in the hypothalamus, and in the modulation of affective behaviors. Recently developed novel experimental tools including mice with targeted mutations of the TIP39-PTH2R system and specific antagonists of the PTH2R will further facilitate the identification of the specific roles of TIP39 and the PTH2R.

Class II G protein-coupled receptors and their ligands in neuronal function and protection

G protein-coupled receptors (GPCRs) play pivotal roles in regulating the function and plasticity of neuronal circuits in the nervous system. Among the myriad of GPCRs expressed in neural cells, class II GPCRs which couples predominantly to the Gs-adenylate cyclase-cAMP signaling pathway, have recently received considerable attention for their involvement in regulating neuronal survival. Neuropeptides that activate class II GPCRs include secretin, glucagon-like peptides (GLP-1 and GLP-2), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase activating peptide (PACAP), corticotropin-releasing hormone (CRH), vasoactive intestinal peptide (VIP), parathyroid hormone (PTH), and calcitonin-related peptides. Studies of patients and animal and cell culture models, have revealed possible roles for class II GPCRs signaling in the pathogenesis of several prominent neurodegenerative conditions including stroke, Alzheimer's, Parkinson's, and Huntington's diseases. Many of the peptides that activate class II GPCRs promote neuron survival by increasing the resistance of the cells to oxidative, metabolic, and excitotoxic injury. A better understanding of the cellular and molecular mechanisms by which class II GPCRs signaling modulates neuronal survival and plasticity will likely lead to novel therapeutic interventions for neurodegenerative disorders.

Identification and characterization of the zebrafish and fugu genes encoding tuberoinfundibular peptide 39

Although the PTH type 2 receptor (PTH2R) has been isolated from mammals and zebrafish, only its mammalian agonist, tuberoinfundibular peptide 39 (TIP39), has been characterized thus far. To determine whether zebrafish TIP39 (zTIP39) functions similarly with the zebrafish PTHR (zPTH2R) and human PTH2Rs and to determine its tissue-specific expression, fugu (Takifugu rubripes) and zebrafish (Danio rerio) genomic databases were screened with human TIP39 (hTIP39) sequences. A single TIP39 gene was identified for each fish species, which showed significant homology to mammalian TIP39. Using standard molecular techniques, we isolated cDNA sequences encoding zTIP39. The fugu TIP39 precursor was encoded by a gene comprising at least three exons. It contained a hydrophobic signal sequence and a predicted prosequence with a dibasic cleavage site, similar to that found in mammalian TIP39 ligands. Phylogenetic analyses suggested that TIP39 forms the basal group from which PTH and PTHrP have been derived. Functionally, subtle differences in potency could be discerned between hTIP39 and zTIP39. The human PTH2R and zPTH2R were stimulated slightly better by both hTIP39 and zTIP39, whereas zTIP39 had a higher potency at a previously isolated zPTH2R splice variant. Whole-mount in situ hybridization of zebrafish revealed strong zTIP39 expression in the region of the hypothalamus and in the heart of 24- and 48-h-old embryos. Similarly, zPTH2R expression was highly expressed throughout the brain of 48- and 72-h-old embryos. Because the mammalian PTH2R was also most abundantly expressed in these tissues, the TIP39-PTH2R system may serve conserved physiological roles in mammals and fishes.

Identification and characterization of two parathyroid hormone-like molecules in zebrafish

Zebrafish (Danio rerio) have receptors homologous to the human PTH (hPTH)/PTHrP receptor (PTH1R) and PTH-2 receptor (PTH2R) and an additional receptor (PTH3R) with high homology to the PTH1R. To find natural ligands for zPTH1R and zPTH3R, we searched the zebrafish genomic database and discovered two distinct regions that, when translated (zPTH1 and zPTH2), showed high homology to hPTH. Isolation of cDNAs and determination of the intron/exon boundaries revealed genomic structures which were similar to known PTHs. Peptides consisting of the first 34 amino acids after the pre- and prosequences of the zebrafish PTHs (zPTHs) were synthesized and were shown to be fully active at the hPTH1R. zPTH2(1-34) was, however, approximately 30-fold less potent at the zPTH1R than hPTH(1-34), hPTHrP(1-36), and zPTH1(1-34). When tested with zPTH3R, zPTH1(1-34) and hPTHrP(1-36) showed similar potencies, whereas the potency of zPTH2(1-34) was moderately (3-fold) reduced. To determine whether other fishes have multiple PTHs, we searched the genomic database of the Japanese pufferfish (Takifugu rubripes) and identified zPTH1 and zPTH2 homologs. Phylogenetic analysis showed that PTHs from zebrafish and pufferfish are more closely related to each other than to known mammalian PTH homologs or to PTHrP and tuberoinfundibular peptide of 39 residues. This is consistent with evolution of two teleost PTH-like peptides occurring after the evolutionary divergence between fishes and mammals. Overall, the PTH system appears more complex in fishes than in mammals, providing evidence of continued evolution in nontetrapod species. The availability of multiple forms of fish PTH and their receptors provide additional tools for PTH ligand/receptor structure-function studies.

Centrally administered tuberoinfundibular peptide of 39 residues inhibits arginine vasopressin release in conscious rats

Tuberoinfundibular peptide of 39 residues (TIP39) is a recently discovered neuropeptide identified on the basis of its ability to activate the PTH2 receptor, and it is thought to be the brain PTH2 receptor's endogenous ligand. The PTH2 receptor is highly expressed in the hypothalamus, suggesting a role in the modulation of neuroendocrinological functions. PTHrP, which also belongs to the PTH-related peptides family, stimulates arginine vasopressin (AVP) release. In the present study, therefore, we investigated the effect of centrally administered TIP39 on AVP release in conscious rats. Intracerebroventricular administration of TIP39 (10-500 pmol/rat) significantly suppressed the plasma AVP concentration in dehydrated rats, and the maximum effect was obtained 5 min after administration (dehydration with 100 pmol/rat TIP39, 4.32 +/- 1.17 pg/ml; vs. control, 8.21 +/- 0.70 pg/ml). The plasma AVP increase in response to either hyperosmolality [ip injection of hypertonic saline (HS), 600 mosmol/kg] or hypovolemia [ip injection of polyethylene glycol (PEG)] was also significantly attenuated by an intracerebroventricular injection of TIP39 (HS with 100 pmol/rat TIP39, 2.65 +/- 0.52 pg/ml; vs. HS alone, 4.69 +/- 0.80 pg/ml; PEG with 100 pmol/rat TIP39, 4.10 +/- 0.79 pg/ml; vs. PEG alone, 6.19 +/- 0.34 pg/ml). Treatment with naloxone [1.5 mg/rat, sc injection], a nonselective opioid receptor antagonist, significantly reversed the inhibitory effects of TIP39 on AVP release. These results suggest that central TIP39 plays an inhibitory role in the osmoregulation and baroregulation of AVP release and that intrinsic opioid systems are involved in its mechanism.