Calcitonin gene-related peptide elevates calcium and polarizes membrane potential in MG-63 cells by both cAMP-independent and -dependent mechanisms

Vibration, a treatment for migraine, linked to calpain driven changes in actin cytoskeleton

Understanding how a human cell reacts to external physical stimuli is essential to understanding why vibration can elicit localized pain reduction. Stimulation of epithelial cells with external vibration forces has been shown to change cell shape, particularly in regards to structures involved in non-muscle cell motility. We hypothesized that epithelial cells respond to vibration transduction by altering proteins involved in remodeling cytoskeleton. Epithelial cells were exposed to vibration and assessed by microscopy, cytoskeletal staining, immunoblotting and quantitative RT-PCR. Here, we report that epithelial cell lines exposed to 15 minutes of vibration retract filopodia and concentrate actin at the periphery of the cell. In particular, we show an increased expression of the calcium-dependent, cysteine protease, calpain. The discovery that cell transitions are induced by limited exposure to natural forces, such as vibration, provides a foundation to explain how vibrational treatment helps migraine patients.

Calcitonin gene-related peptide induces the histone H3 lysine 9 acetylation in astrocytes associated with neuroinflammation in rats with neuropathic pain

Aims

Calcitonin gene‐related peptide (CGRP) as a regulator of astrocyte activation may facilitate spinal nociceptive processing. Histone H3 lysine 9 acetylation (H3K9ac) is considered an important regulator of cytokine and chemokine gene expression after peripheral nerve injury. In this study, we explored the relationship between CGRP and H3K9ac in the activation of astrocytes, and elucidated the underlying mechanisms in the pathogenesis of chronic neuropathic pain.

Methods

Astroglial cells (C6) were treated with CGRP and differentially enrichments of H3K9ac on gene promoters were examined using ChIP‐seq. A chronic constriction injury (CCI) rat model was used to evaluate the role of CGRP on astrocyte activation and H3K9ac signaling in CCI‐induced neuropathic pain. Specific inhibitors were employed to delineate the involved signaling.

Results

Intrathecal injection of CGRP and CCI increased the number of astrocytes displaying H3K9ac in the spinal dorsal horn of rats. Treatment of CGRP was able to up‐regulate H3K9ac and glial fibrillary acidic protein (GFAP) expression in astroglial cells. ChIP‐seq data indicated that CGRP significantly altered H3K9ac enrichments on gene promoters in astroglial cells following CGRP treatment, including 151 gaining H3K9ac and 111 losing this mark, which mostly enriched in proliferation, autophagy, and macrophage chemotaxis processes. qRT‐PCR verified expressions of representative candidate genes (ATG12, ATG4C, CX3CR1, MMP28, MTMR14, HMOX1, RET) and RTCA verified astrocyte proliferation. Additionally, CGRP treatment increased the expression of H3K9ac, CX3CR1, and IL‐1β in the spinal dorsal horn. CGRP antagonist and HAT inhibitor attenuated mechanical and thermal hyperalgesia in CCI rats. Such analgesic effects were concurrently associated with the reduced levels of H3K9ac, CX3CR1, and IL‐1β in the spinal dorsal horn of CCI rats.

Conclusion

Our findings highly indicate that CGRP is associated with the development of neuropathic pain through astrocytes‐mediated neuroinflammatory responses via H3K9ac in spinal dorsa horn following nerve injury.

This study found that CGRP act on their astrocytic receptors and lead to H3K9 acetylation (H3K9ac), which are mainly associated with proliferation‐, autophagy‐, and inflammation‐related gene expression. The number of astrocytes with H3K9ac expression is increased after nerve injury. Inhibition of CGRP attenuates the development of neuropathic pain, which was accompanied by the suppression of H3K9ac, CX3CR1, and IL‐1β expression in CCI rats.

Calcitonin and Bone Physiology: In Vitro, In Vivo, and Clinical Investigations

Calcitonin was discovered as a peptide hormone that was known to reduce the calcium levels in the systemic circulation. This hypocalcemic effect is produced due to multiple reasons such as inhibition of bone resorption or suppression of calcium release from the bone. Thus, calcitonin was said as a primary regulator of the bone resorption process. This is the reason why calcitonin has been used widely in clinics for the treatment of bone disorders such as osteoporosis, hypercalcemia, and Paget's disease. However, presently calcitonin usage is declined due to the development of efficacious formulations of new drugs. Calcitonin gene-related peptides and several other peptides such as intermedin, amylin, and adrenomedullin (ADM) are categorized in calcitonin family. These peptides are known for the structural similarity with calcitonin. Aside from having a similar structure, these peptides have few overlapping biological activities and signal transduction action through related receptors. However, several other activities are also present that are peptide specific. In vitro and in vivo studies documented the posttreatment effects of calcitonin peptides, i.e., positive effect on bone osteoblasts and their formation and negative effect on osteoclasts and their resorption. The recent research studies carried out on genetically modified mice showed the inhibition of osteoclast activity by amylin, while astonishingly calcitonin plays its role by suppressing osteoblast and bone turnover. This article describes the review of the bone, the activity of the calcitonin family of peptides, and the link between them.

The Activity of Peptides of the Calcitonin Family in Bone

Calcitonin was discovered over 50 yr ago as a new hormone that rapidly lowers circulating calcium levels. This effect is caused by the inhibition of calcium efflux from bone, as calcitonin is a potent inhibitor of bone resorption. Calcitonin has been in clinical use for conditions of accelerated bone turnover, including Paget's disease and osteoporosis; although in recent years, with the development of drugs that are more potent inhibitors of bone resorption, its use has declined. A number of peptides that are structurally similar to calcitonin form the calcitonin family, which currently includes calcitonin gene-related peptides (αCGRP and βCGRP), amylin, adrenomedullin, and intermedin. Apart from being structurally similar, the peptides signal through related receptors and have some overlapping biological activities, although other activities are peptide specific. In bone, in vitro studies and administration of the peptides to animals generally found inhibitory effects on osteoclasts and bone resorption and positive effects on osteoblasts and bone formation. Surprisingly, studies in genetically modified mice have demonstrated that the physiological role of calcitonin appears to be the inhibition of osteoblast activity and bone turnover, whereas amylin inhibits osteoclast activity. The review article focuses on the activities of peptides of the calcitonin family in bone and the challenges in understanding the relationship between the pharmacological effects and the physiological roles of these peptides.

CGRP may regulate bone metabolism through stimulating osteoblast differentiation and inhibiting osteoclast formation

Calcitonin-gene-related peptide (CGRP) is a neuropeptide, which is widely distributed throughout the central and peripheral nervous systems. Numerous mechanisms underlying the action of CGRP in osteoblast-associated cells have been suggested for bone growth and metabolism. The present study was designed to closely investigate the osteoblast‑ and osteoclast-associated mechanisms of the effect of CGRP administration on bone metabolism in primary osteoblasts. Primary osteoblasts were obtained from newborn rabbit calvaria and incubated with different concentrations of human CGRP (hCGRP), hCGRP and hCGRP (8‑37), or without treatment as a control. Intracellular calcium (Ca2+) and cyclic adenosine monophosphate (cAMP) were detected following treatment, as well as the expression levels of osteoblast differentiation markers, including activating transcription factor‑4 (ATF4) and osteocalcin (OC), and receptor activator of nuclear factor κB ligand (RANKL) and osteoprotegerin (OPG). The isolated primary osteoblasts were found to stain positively for ALP. hCGRP treatment had no significant effect on transient intracellular Ca2+ in the osteoblasts. Treatment of the osteoblasts with hCGRP led to elevations in the expression levels of cAMP, ATF4 and OPG, and downregulation in the expression of RANKL, in a dose‑dependent manner. These effects were markedly reversed by the addition of hCGRP (8‑37). The results of the present study demonstrated that CGRP administration not only stimulated osteoblast differentiation, as demonstrated by upregulated expression levels of ATF4 and OC in the hCGRP‑treated osteoblasts, but also inhibited OPG/RANKL‑regulated osteoclastogenesis. CGRP may act as a modulator of bone metabolism through osteoblast and osteoclast-associated mechanisms, which result in osteoblast formation with subsequent activation of bone formation.

Calcitonin gene-related peptide modulates the production of pro-inflammatory cytokines associated with periprosthetic osteolysis by THP-1 macrophage-like cells

OBJECTIVE

An anti-resorptive impact of the neuropeptide calcitonin gene-related peptide (CGRP) on periprosthetic osteolysis, the leading cause of early prosthesis loosening, has been shown previously. In this study, the impact of CGRP on pro-inflammatory cytokine production associated with periprosthetic osteolysis was analysed using THP-1 macrophage-like cells.

METHODS

Cells were stimulated with ultra-high-molecular-weight polyethylene (UHMWPE) particles (cell-to-particle ratios of 1:100 and 1:500) and lipopolysaccharides (LPS; 1 µg/ml) to establish osteolytic conditions, and simultaneously treated with CGRP (10(-8)M). Receptor activator of nuclear factor-κB (RANK), RANK ligand (RANKL) and tumour necrosis factor (TNF)-α mRNA expression were measured by quantitative RT-PCR. RANK protein was detected by Western blot. Secreted protein levels of TNF-α as well as interleukin (IL)-1β and IL-6 were quantified in cell culture supernatants by ELISA and Bio-Plex cytokine assay, respectively.

RESULTS

Activation of macrophage-like cells failed to enhance the production of RANK but led to a dose- and time-dependent increase of TNF-α mRNA and secreted protein levels of TNF-α, IL-1β and IL-6. Application of CGRP time-dependently suppressed TNF-α mRNA expression induced by low-particle concentrations and LPS, while both particle- and LPS-induced secretion of TNF-α was inhibited. A pronounced inhibitory effect of CGRP on LPS-induced cytokine production at 24 h of incubation was also observed with IL-1β and IL-6.

CONCLUSIONS

CGRP shows a time-dependent inhibitory effect on the secretion of osteolysis-associated pro-inflammatory cytokines, indicating an indirect anti-resorptive influence of the neuropeptide on both aseptic prosthesis loosening and bacterially induced bone resorption which might enhance the life time of total joint replacements.

Calcitonin gene-related peptide stimulates BMP-2 expression and the differentiation of human osteoblast-like cells in vitro

AIM

To investigate whether bone morphogenic protein-2 (BMP-2) expression was involved in calcitonin gene-related peptide (CGRP)-induced osteogenesis in human osteoblast-like cells in vitro.

METHODS

MG-63 osteogenic human osteosarcoma cells were treated with CGRP (10-8 mol/L) for 48 h. Cell cycle phases were determined using flow cytometry assay. The protein levels of BMP-2, ALP, Osteocalcin, ColIa1, CREB, and pCREB were measured with Western blotting, while the mRNA level of BMP-2 was measured with qR-T PCR. The expression of ALP in MG-63 cells was also studied using immunofluorescence staining. The level of cAMP was measured with ELISA assay.

RESULTS

CGRP treatment significantly stimulated proliferation of MG-63 cells, and increased the expression of BMP-2 and the osteogenic proteins ALP, Osteocalcin and ColIa1. Pretreatment with the BMP signaling inhibitor Noggin (100 ng/mL) did not affect CGRP-stimulated proliferation and BMP-2 expression, but abolished the CGRP-induced increases of the osteogenic proteins ALP, Osteocalcin and ColIa1. Furthermore, CGRP treatment markedly increased cAMP level in MG-63 cells, whereas pretreatment with the cAMP pathway inhibitor H89 (5 μmol/L) abolished the CGRP-induced increases of cAMP level and BMP-2 expression.

CONCLUSION

In MG-63 cells, the BMP pathway is involved in CGRP-induced osteogenic differentiation but not in proliferation, whereas the cAMP/pCREB pathway is involved in the expression of BMP-2.

Endothelin-converting enzyme-1 gene ablation attenuates pulmonary fibrosis via CGRP-cAMP/EPAC1 pathway

Endothelin-1 (ET-1) has been shown to be involved in human pulmonary fibrosis. However, recent clinical trials targeting the ET-1 pathway with ET-1 receptor antagonists failed to achieve beneficial outcomes. Another strategy opposing the actions of ET-1 involves the inhibition of endothelin-converting enzyme-1 (ECE-1). We hypothesize that ECE-1 inhibition exerts beneficial effects on pulmonary fibrosis. Pulmonary fibrosis was induced by instilling bleomycin intratracheally into ECE-1 heterozygous knockout mice (ECE-1(+/-)) and their wild-type control mice (ECE-1(+/+)). Lung inflammation and fibrosis were assessed on Days 7, 14, and 28 after bleomycin instillation. The activity of ECE-1 and the concentrations of its related peptides, ET-1, bradykinin, atrial natriuretic peptide (ANP), and calcitonin gene-related peptide (CGRP), were determined. ECE-1(+/-) mice demonstrated less lung inflammation and limited fibrosis compared with control mice. ECE-1 activity was half-reduced in ECE-1(+/-) mice, and this activity also altered ET-1 and CGRP concentrations, but not concentrations of bradykinin and ANP. ET-1 concentrations were found to be lower in ECE-1(+/-) mice after the development of fibrosis, in contrast to the unaltered concentrations during inflammation. Reduced ECE-1 activity resulted in higher CGRP concentrations, which altered the pathological functionality of the lung, indicating the activation of the CGRP pathway involving cyclic adenosine monophosphate (cAMP)/exchange protein directly activated by cAMP and cAMP/protein kinase A in ECE-1(+/-) mice. Bleomycin instillation on Day 14 induced the accumulation of M2 macrophages expressing CGRP receptors in ECE-1(+/-) mice. Our results emphasize that the in vivo ECE-1-mediated degradation of CGRP promotes the transition from lung inflammation to fibrosis. Further, our study identified M2 macrophages as the target cells of CGRP action during this transition.

The antimicrobial peptide LL-37 alters human osteoblast Ca2+ handling and induces Ca2+-independent apoptosis

The human antimicrobial peptide cathelicidin LL-37 has, besides its antimicrobial properties, also been shown to regulate apoptosis in a cell type-specific manner. Mechanisms involved in LL-37-regulated apoptotic signaling are not identified. Here, we show that LL-37 reduces the human osteoblast-like MG63 cell number and cell viability in the micromolar concentration range with an IC50 value of about 5 µM. Treatment with 4 µM LL-37 increased the number of annexin V-positive cells and stimulated activation of caspase 3 showing that LL-37 promotes apoptosis. Treatment with 4 µM LL-37 caused an acute and sustained rise in intracellular Ca(2+) concentration assessed by laser-scanning confocal microscopy of Fluo-4-AM-loaded MG63 cells. LL-37 increased Ca(2+) also in the presence of the respective L- and T-type voltage-sensitive Ca(2+) channel blockers nifedipine and NiCl2. LL-37 had no effect on Ca(2+) in cells incubated with Ca(2+)-free solution. LL-37 (4 and 8 µM) reduced the MG63 cell number both in the presence and absence of Ca(2+) in the medium. In conclusion, LL-37 reduces the osteoblast cell number by promoting apoptosis, and furthermore, LL-37 stimulates Ca(2+) inflow via a mechanism independent of voltage-sensitive Ca(2+) channels. Interestingly, LL-37-induced lowering of the cell number seems to be mediated via a mechanism independent of Ca(2+).

Effect of calcitonin gene-related peptide on nitric oxide production in osteoblasts: an experimental study

The aim of this study was to investigate the in vitro effects and regulatory mechanism of CGRP (calcitonin gene-related peptide) on NO (nitric oxide) production in osteoblasts. MOB (primary human mandibular osteoblasts) and osteoblast-like cells (MG-63) were either cultured with CGRP or co-incubated with inhibitors targeting eNOS (endothelial nitric oxide synthase), iNOS (inducible nitric oxide synthase), nNOS (neuronal nitric oxide synthase) and [Ca2+]i (intracellular Ca2+). The NO concentration in cell culture supernatants was measured during the first 24 h using the Griess test; cellular NO was marked with the fluorescent marker DAF-FM, DA (3-amino, 4-aminomethyl-2',7'-difluorescein; diacetate) and measured by fluorescence microscopy from 1 to 4 h after treatment. eNOS and iNOS mRNA expression levels were measured by quantitative RT-PCR during the first 24 h after treatment. CGRP-induced NO production in the supernatants was high between 1 to 12 h, while cellular NO was highest between 1 to 2 h after treatment and returned to basal levels by 3 h. Both in MG-63 cells and MOBs, the most effective CGRP concentration was 10 nM with a peak time of 1 h. CGRP-induced NO production decreased when eNOS activity was inhibited or when voltage-dependent L-type Ca2+ channels were blocked at 4 h. CGRP was not able to induce changes in iNOS or eNOS mRNA levels and had no effect on the cytokine-induced increase of iNOS expression. Our results suggest that CGRP transiently induces NO production in osteoblasts by elevating intracellular Ca2+ to stimulate the activity of eNOS in vitro.

Timing, rather than the concentration of cyclic AMP, correlates to osteogenic differentiation of human mesenchymal stem cells

Previously, we demonstrated that protein kinase A (PKA) activation using dibutyryl-cAMP in human mesenchymal stem cells (hMSCs) induces in vitro osteogenesis and bone formation in vivo. To further investigate the physiological role of PKA in hMSC osteogenesis, we tested a selection of G-protein-coupled receptor ligands which signal via intracellular cAMP production and PKA activation. Treatment of hMSCs with parathyroid hormone, parathyroid hormone-related peptide, melatonin, epinephrine, calcitonin or calcitonin gene-related peptide did not result in accumulation of cAMP or induction of alkaline phosphatase (ALP) expression. The only ligand that did induce cAMP, prostaglandin E2, even inhibited ALP expression and mineralization, suggesting that physiological levels of cAMP may inhibit osteogenesis. Furthermore, intermittent exposure of hMSCs to dibutyryl-cAMP inhibited ALP expression, whereas we did not observe an inhibitive effect at low dibutyryl-cAMP concentrations. Taken together, our results demonstrate that cAMP can either stimulate or inhibit osteogenesis in hMSCs, depending on the duration, rather than the strength, of the signal provided.

Calcitonin gene-related peptide (CGRP) inhibits apoptosis in human osteoblasts by β-catenin stabilization

Transgenic mice over-expressing calcitonin gene-related peptide (CGRP) in osteoblasts have increased bone density due to increased bone formation, thus suggesting that CGRP plays a role in bone metabolism. In this study we determined the relationship between CGRP, the canonical Wnt signaling and apoptosis in human osteoblasts (hOBs) in consideration of the well-documented involvement of this pathway in bone cells. Primary cultures of hOBs were treated with CGRP 10(-8) M. Levels of β-catenin, which is the cytoplasmic protein mediator of canonical Wnt signaling, and mRNA were determined. CGRP increases both the expression and the levels of cytoplasmic β-catenin by binding to its receptor, as this effect is blocked by the antagonist CGRP(8-37). This facilitatory action on β-catenin appears to be mediated by the inhibition of the enzyme GSK-3β via protein kinase A (PKA) activation. GSK-3β is a glycogen synthase kinase that, by phosphorylating β-catenin, promotes its degradation by the proteosomal machinery. Moreover, the peptide is able to inhibit hOBs apoptosis stimulated by dexamethasone or by serum deprivation, possibly through the accumulation of β-catenin, since the inhibitor of PKA activity H89 partially prevents the antiapoptotic effect of the peptide. In conclusion CGRP, released by nerve fibers, exerts its anabolic action on bone cells by stimulating canonical Wnt signaling and by inhibiting hOBs apoptosis, thus favoring local bone regeneration.

Calcitonin gene-related peptide (CGRP) triggers Ca2+ responses in cultured astrocytes and in Bergmann glial cells from cerebellar slices

The neuropeptide calcitonin gene-related peptide (CGRP) is transiently expressed in cerebellar climbing fibers during development while its receptor is mainly expressed in astrocytes, in particular Bergmann glial cells. Here, we analyzed the effects of CGRP on astrocytic calcium signaling. Mouse cultured astrocytes from cerebellar or cerebral cortex as well as Bergmann glial cells from acutely isolated cerebellar slices were loaded with the Ca(2+) sensor Fura-2. CGRP triggered transient increases in intracellular Ca(2+) in astrocytes in culture as well as in acute slices. Responses were observed in the concentration range of 1 nm to 1 mm, in both the cell body and its processes. The calcium transients were dependent on release from intracellular stores as they were blocked by thapsigargin but not by the absence of extracellular calcium. In addition, after CGRP application a further delayed transient increase in calcium activity could be observed. Finally, cerebellar astrocytes from neonatal mice expressed receptor component protein, a component of the CGRP receptor, as revealed by immunofluorescence and confocal microscopy. It is thus proposed that the CGRP-containing afferent fibers in the cerebellum (the climbing fibers) modulate calcium in astrocytes by releasing the neuropeptide during development and hence possibly influence the differentiation of Purkinje cells.

Oscillating cortical thick ascending limb cells at the juxtaglomerular apparatus

While studying the intracellular calcium dynamics in cells of the macula densa, the observation was made that tubular epithelial cells located near the macula densa and associated with the renal arterioles exhibit spontaneous Ca2+ oscillations. In this study, the cortical thick ascending limb-distal tubule, with attached glomerulus, was isolated and perfused. At a low luminal sodium chloride concentration, Ca2+ oscillations at a frequency of 63 mHz were observed in tubular cells that were within 100 microm of the macula densa plaque using four-dimensional multiphoton microscopy and wide-field fluorescence microscopy with fura-2. The Ca2+ oscillations were absent in the macula densa cells. Spontaneous oscillations in basolateral membrane potential suggested that Ca2+ oscillations occurred, at least in part, through depolarization-induced increases in Ca2+ entry. The amplitude of these Ca2+ oscillations was significantly enhanced by the activation of the Ca2+-sensing receptor. Increasing the luminal sodium chloride concentration or luminal flow resulted in a significant increase in both the amplitude of Ca2+ oscillations and the intracellular Ca2+ concentration in perimacular cortical thick ascending limb cells. In addition, luminal furosemide attenuated the [NaCl]L-dependent changes in intracellular Ca2+ concentration, but hydrochlorothiazide had no effect. These findings demonstrate that tubular epithelial cells at the perimeter of the macula densa exhibit spontaneous oscillations in intracellular Ca2+ concentration, enhanced by tubular flow and luminal sodium chloride. These oscillatory patterns may play a role in juxtaglomerular signaling.

Capsaicin-induced increase of intestinal cefazolin absorption in rats

The effect of capsaicin on intestinal cefazolin absorption was examined by means of an in situ closed loop method in rats to clarify whether the vanilloid receptor (TRPV1) is involved in drug absorption driven by passive diffusion. In control experiments with 1 mg/mL cefazolin, the amount of cefazolin absorbed from the closed loop was 15.3+/-1.5 microg/cm in the rat jejunum. The absorption amount was increased to 22.8+/-0.9 and 23.4+/-2.4 microg/cm when capsaicin was applied with cefazolin at concentrations of 10 and 400 microM, respectively. The enhancing effect of capsaicin on cefazolin absorption was suppressed when ruthenium red, a non-selective inhibitor of transient receptor potential (TRP) cation channels, was intravenously infused into the rat during the experiment. Cefazolin accumulation in the intestinal tissue was not altered in the presence of capsaicin. Collectively, the mechanism accounting for the capsaicin-induced increase in the intestinal cefazolin absorption is probably that capsaicin associating with TRPV1 increases the intrinsic permeability of cefazolin in intestine.

Capacitative calcium entry and proliferation of human osteoblast-like MG-63 cells

Adult bone tissue is continuously being remodelled and bone mass is maintained by a balance between osteoclastic bone resorption and osteoblastic bone formation. Alteration of osteoblastic cell proliferation may account in part for lack of balance between these two processes in bone loss of osteoporosis. There is calcium (Ca2+) control in numerous cellular functions; however, involvement of capacitative Ca2+ entry (CCE) in proliferation of bone cells is less well investigated.

OBJECTIVES

The study described here was aimed to investigate roles of CCE in the proliferation of osteoblast-like MG-63 cells.

MATERIALS AND METHODS

Pharmacological characterizations of CCE were undertaken in parallel, with evaluation of the expression of transient receptor potential canonical (TRPC) channels and of cell proliferation.

RESULTS

Intracellular Ca2+ store depletion by thapsigargin induced CCE in MG-63 cells; this was characterized by a rapid transient increase of intracellular Ca2+ followed by significant CCE, induced by conditions that stimulated cell proliferation, namely serum and platelet-derived growth factor. Inhibitors of store-operated Ca2+ channels (2-APB and SKF-96365) prevented CCE, while voltage-dependent Ca2+ channel blockers had no effect. Expression of various TRPC channels was shown in the cells, some having been shown to be responsible for CCE. Voltage-dependent Ca2+ channel blockers had no effect on osteoblast proliferation while thapsigargin, 2-APB and SKF-96395, inhibited it. Cell cycle analysis showed that 2-APB and SKF-96395 lengthen the S and G2/M phases, which would account for the reduction in cell proliferation.

CONCLUSIONS

Our results indicate that CCE, likely attributed to the activation of TRPCs, might be the main route for Ca2+ influx involved in osteoblast proliferation.

BG60S dissolution interferes with osteoblast calcium signals

We investigated the influence of extracellular calcium concentration, caused by the dissolution of a bioactive glass with 60% of silicon (BG60S), on intracellular calcium (Ca(i) (2 +)) signals and expression of inositol 1, 4, 5-triphosphate receptors (InsP(3)R) in primary culture of osteoblasts. We found that BG60S caused an increase in Ca(i) (2 +) signals in this cell type. Additionally, osteoblasts pre-incubated in the presence of BG60S showed an increase in Ca(i) (2 +) when cells were stimulated with vasopressin. On the other hand, a decrease in Ca(i) (2 +) signals were observed in osteoblasts pre-treated with BG60S and stimulated with KCl. We furher found that in osteoblasts, the type I InsP(3)R is preferentially distributed in the nucleus while the type II InsP(3)R in the cytoplasm. Preincubation of osteoblasts with BG60S altered the receptor expression level, increasing the type I InsP(3)R in the nucleus and decreasing type II InsP(3)R in the cytosol. Together, our results showed that in osteoblasts, BG60S increased Ca(i) (2 +)signals and altered Ca(i) (2 +) machinery.

Calcitonin forms oligomeric pore-like structures in lipid membranes

Calcitonin is a polypeptidic hormone involved in calcium metabolism in the bone. It belongs to the amyloid protein family, which is characterized by the common propensity to aggregate acquiring a beta-sheet conformation and include proteins associated with important neurodegenerative diseases. Here we show for the first time, to our knowledge, by transmission electron microscopy (TEM) that salmon-calcitonin (sCT) forms annular oligomers similar to those observed for beta-amyloid and alpha-sinuclein (Alzheimer's and Parkinson's diseases). We also investigated the interaction between sCT and model membranes, such as liposomes, with particular attention to the effect induced by lipid "rafts" made of cholesterol and G(M1). We observed, by TEM immunogold labeling of sCT, that protein binding is favored by the presence of rafts. In addition, we found by TEM that sCT oligomers inserted in the membrane have the characteristic pore-like morphology of the amyloid proteins. Circular dichroism experiments revealed an increase in beta-content in sCT secondary structure when the protein was reconstituted in rafts mimicking liposomes. Finally, we showed, by spectrofluorimetry experiments, that the presence of sCT allowed Ca(2+) entry in rafts mimicking liposomes loaded with the Ca(2+)-specific fluorophore Fluo-4. This demonstrates that sCT oligomers have ion-channel activity. Our results are in good agreement with recent electrophysiological studies reporting that sCT forms Ca(2+)-permeable ion channels in planar model membranes. It has been proposed that, beyond the well-known interaction of the monomer with the specific receptor, the formation of Ca(2+) channels due to sCT oligomers could represent an extra source of Ca(2+) entry in osteoblasts. Structural and functional data reported here support this hypothesis.

CGRP inhibits osteoprotegerin production in human osteoblast-like cells via cAMP/PKA-dependent pathway

The osteoprotegerin (OPG)/receptor activator of nuclear factor-kappaB ligand (RANKL)/receptor activator of nuclear factor-kappaB (RANK) system was evaluated as a potential target of CGRP anabolic activity on bone. Primary cultures of human osteoblast-like cells (hOB) express calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1, and, because CGRP stimulates cAMP (one of the modulators of OPG production in osteoblasts), it was investigated whether it affects OPG secretion and expression in hOB. CGRP treatment of hOB (10(-11) M-10(-7) M) dose-dependently inhibited OPG secretion with an EC(50) of 1.08 x 10(-10) M, and also decreased its expression. This action was blocked by the antagonist CGRP(8-37). Forskolin, a stimulator of cAMP production, and dibutyryl cAMP also reduced the production of OPG. CGRP (10(-8) M) enhanced protein kinase A (PKA) activity in hOB, and hOB exposure to the PKA inhibitor, H89 (2 x 10(-6) M), abolished the inhibitory effect of CGRP on OPG secretion. Conditioned media from CGRP-treated hOB increased the number of multinucleated tartrate-resistant acid phosphatase-positive cells and the secretion of cathepsin K in human peripheral blood mononuclear cells compared with the conditioned media of untreated hOB. These results show that the cAMP/PKA pathway is involved in the CGRP inhibition of OPG mRNA and protein secretion in hOB and that this effect favors osteoclastogenesis. CGRP could thus modulate the balance between osteoblast and osteoclast activity, participating in the fine tuning of all of the bone remodeling phases necessary for the subsequent anabolic effect.

PDF receptor signaling in Drosophila contributes to both circadian and geotactic behaviors

The neuropeptide Pigment-Dispersing Factor (PDF) is a principle transmitter regulating circadian locomotor rhythms in Drosophila. We have identified a Class II (secretin-related) G protein-coupled receptor (GPCR) that is specifically responsive to PDF and also to calcitonin-like peptides and to PACAP. In response to PDF, the PDF receptor (PDFR) elevates cAMP levels when expressed in HEK293 cells. As predicted by in vivo studies, cotransfection of Neurofibromatosis Factor 1 significantly improves coupling of PDFR to adenylate cyclase. pdfr mutant flies display increased circadian arrhythmicity, and also display altered geotaxis that is epistatic to that of pdf mutants. PDFR immunosignals are expressed by diverse neurons, but only by a small subset of circadian pacemakers. These data establish the first synapse within the Drosophila circadian neural circuit and underscore the importance of Class II peptide GPCR signaling in circadian neural systems.

Effect of calcitonin gene-related peptide on gonadotrophin-releasing hormone mRNA expression in GT1-7 cells

Recent evidence has shown calcitonin gene-related peptide (CGRP) to be a key mediator of stress-induced suppression of the gonadotrophin-releasing hormone (GnRH) pulse generator, although little is known about the neural pathways involved. In the present study, we investigated the potential direct action of CGRP on GnRH neurones using GT1-7 cells, an established GnRH cell line. First, we detected expression of the CGRP receptor subunits, calcitonin receptor-like receptor and receptor activity-modifying protein-1 in the GT1-7 cells by reverse transcriptase-polymerase chain reaction. Second, we have shown that CGRP inhibits GnRH mRNA expression in the GT1-7 cells, which was effectively reversed by the CGRP receptor antagonist, CGRP8-37. These results suggest that CGRP down regulates expression of GnRH mRNA, via CGRP receptors in the GT1-7 cell, thus implying that a potential direct action of CGRP may mediate a suppressive effect on the GnRH neural network.

Immature osteoblastic MG63 cells possess two calcitonin gene-related peptide receptor subtypes that respond differently to [Cys(Acm)(2,7)] calcitonin gene-related peptide and CGRP(8-37)

Calcitonin gene-related peptide (CGRP) is clearly an anabolic factor in skeletal tissue, but the distribution of CGRP receptor (CGRPR) subtypes in osteoblastic cells is poorly understood. We previously demonstrated that the CGRPR expressed in osteoblastic MG63 cells does not match exactly the known characteristics of the classic subtype 1 receptor (CGRPR1). The aim of the present study was to further characterize the MG63 CGRPR using a selective agonist of the putative CGRPR2, [Cys(Acm)(2,7)]CGRP, and a relatively specific antagonist of CGRPR1, CGRP(8-37). [Cys(Acm)(2,7)]CGRP acted as a significant agonist only upon ERK dephosphorylation, whereas this analog effectively antagonized CGRP-induced cAMP production and phosphorylation of cAMP response element-binding protein (CREB) and p38 MAPK. Although it had no agonistic action when used alone, CGRP(8-37) potently blocked CGRP actions on cAMP, CREB, and p38 MAPK but had less of an effect on ERK. Schild plot analysis of the latter data revealed that the apparent pA2 value for ERK is clearly distinguishable from those of the other three plots as judged using the 95% confidence intervals. Additional assays using 3-isobutyl-1-methylxanthine or the PKA inhibitor N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H-89) indicated that the cAMP-dependent pathway was predominantly responsible for CREB phosphorylation, partially involved in ERK dephosphorylation, and not involved in p38 MAPK phosphorylation. Considering previous data from Scatchard analysis of [125I]CGRP binding in connection with these results, these findings suggest that MG63 cells possess two functionally distinct CGRPR subtypes that show almost identical affinity for CGRP but different sensitivity to CGRP analogs: one is best characterized as a variation of CGRPR1, and the second may be a novel variant of CGRPR2.