Prostaglandin E2 activates outwardly rectifying Cl(-) channels via a cAMP-dependent pathway and reduces cell motility in rat osteoclasts

Novel mutations of CLCN7 cause autosomal dominant osteopetrosis type II (ADO-II) and intermediate autosomal recessive osteopetrosis (IARO) in Chinese patients

SUMMARY

Osteopetrosis is a group of genetic bone disorders. Mutations in the chloride channel 7 gene (CLCN7) lead to chloride channel defect, which results in autosomal dominant osteopetrosis type II (ADO-II), autosomal recessive osteopetrosis (ARO), and intermediate autosomal recessive osteopetrosis (IARO). In the present study, we identified seven novel mutations of the CLCN7 gene and reported the first case of IARO with compound heterozygous mutation in Chinese population.

INTRODUCTION

Osteopetrosis is a heritable bone disorder due to the deficiency of or function defect in osteoclasts. Mutations in the CLCN7 lead to chloride channel defects, which result in osteopetrosis with diverse severity ranging from asymptomatic or relatively mild symptoms in ADO-II to the very severe phenotype in ARO. Heterozygous mutations in CLCN7 are associated to ADO-II, while homozygous and compound heterozygous mutations in CLCN7 may result in ARO and IARO. To date, a total of 24 mutations in CLCN7 were identified in ADO-II, and only 3 mutations were identified in IARO. In the present study, we reported seven unrelated ADO-II patients and one IARO patient from Chinese population and elucidated the characteristics of CLCN7 gene mutations in these patients.

METHODS

All 25 CLCN7 exons and exon-intron boundaries from genomic DNA were amplified and sequenced in eight affected individuals suffering from ADO-II/IARO. The clinical, biochemical, and radiographic analysis were evaluated to compare the differences between ADO-II and IARO both in genotype and phenotype.

RESULTS

The results showed that there were seven novel CLCN7 mutations identified in these ADO-II/IARO patients, including six heterozygous missense mutations (p.L224R, p.S290Y, p.R326G, p.G347R, p.S473N, and p.L564P) and a novel splice mutation (p.K691FS).

CONCLUSIONS

The compound heterozygous mutations (p.L224R and p.K691FS) were firstly observed in one IARO patient. The present study would enrich the database of CLCN7 mutations and improve our understanding of this heritable bone disorder.

Extracellular fluid flow and chloride content modulate H(+) transport by osteoclasts

Background

Bone resorption takes place within the basic multicellular units (BMU), and the surface to be resorbed is isolated from adjacent bone surfaces by a sealing zone between osteoclast membrane and bone matrix, which defines the limits of the resorption lacuna. Considering that the extracellular fluid (ECF) in both BMU and the resorption lacuna can be isolated from its surroundings, I hypothesize that flow and ion composition of the bone ECF in these sites might contribute to the regulation of osteoclast H⁺ secretion. To investigate this hypothesis, I evaluated the H⁺ secretion properties of individual osteoclasts and osteoclast-like cells (OCL-cells) and investigated whether changes in flow or chloride content of the extracellular solution modify the H⁺ secretion properties in vitro.

Results

The results show that 1) osteoclasts are unable to secrete H⁺ and regulate intracellular pH (pH_i) under continuous flow conditions and exhibit progressive intracellular acidification; 2) the cessation of flow coincides with the onset of H⁺ secretion and subsequent progressive intracellular alkalinization of osteoclasts and OCL-cells; 3) osteoclasts exhibit spontaneous rhythmic oscillations of pH_i in non-flowing ECF, 4) pH_i oscillations are not abolished by concanamycin, NPPB, or removal of extracellular Na⁺ or Cl⁻; 5) extracellular Cl⁻ removal modifies the pattern of oscillations, by diminishing H⁺ secretion; 6) pH_i oscillations are abolished by continuous flowing of ECF over osteoclasts and OCL-cells.

Conclusions

The data suggest, for the first time, that ECF flow and Cl⁻ content have direct effects on osteoclast H⁺ secretion and could be part of a mechanism determining the onset of osteoclast H⁺ secretion required for bone resorption.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-015-0066-4) contains supplementary material, which is available to authorized users.

Prostaglandin E2 induces spontaneous rhythmic activity in mouse urinary bladder independently of efferent nerves

BACKGROUND AND PURPOSE

The acute effects of PGE(2) on bladder smooth muscle and nerves were examined to determine the origin of PGE(2)-induced spontaneous rhythmic contractions.

EXPERIMENTAL APPROACH

Contraction studies, confocal Ca(2+) imaging and electrophysiological recordings in strips of mouse urinary bladder were used to differentiate the effects of PGE(2) on bladder smooth muscle and efferent nerves.

KEY RESULTS

PGE(2) (50 µM) increased the tone and caused phasic contractions of detrusor smooth muscle strips. Confocal Ca(2+) imaging showed that PGE(2) increased the frequency of whole-cell Ca(2+) transients (WCTs) (72 ± 5%) and intracellular recordings showed it increased the frequency of spontaneous depolarizations, from 0.31·s(-1) to 0.90·s(-1). Non-selective inhibition of EP receptors using SC-51322 and AH-6809 (10 µM), or the L-type Ca(2+) channel blocker nifedipine (1 µM), prevented these phasic contractions and WCTs, and reduced the tone (by 45 ± 7% and 59 ± 6%, respectively). Blocking P2X1 receptors with NF449 (10 µM) caused a small but significant reduction in the frequency of PGE(2)-induced phasic contractions (24 ± 9%) and WCTs (28 ± 17%) but had no significant effect on spontaneous depolarizations or tone. Inhibiting muscarinic receptors with cyclopentolate (1 µM) had no significant effect on these measures. Spontaneous WCTs became synchronous in PGE(2), implying enhanced functional coupling between neighbouring cells. However, the electrical input resistance was unchanged.

CONCLUSIONS AND IMPLICATIONS

It was concluded that depolarization alone is sufficient to explain a functional increase in intercellular coupling and the ability of PGE(2) to increase detrusor spontaneous rhythmic activity does not require parasympathetic nerves.

cGMP and cyclic nucleotide-gated channels participate in mouse sperm capacitation

During capacitation of mammalian sperm intracellular [Ca(2+)] and cyclic nucleotides increase, suggesting that CNG channels play a role in the physiology of sperm. Here we study the effect of capacitation, 8Br-cAMP (8-bromoadenosine 3',5'-cyclic monophosphate) and 8Br-cGMP (8-bromoguanosine 3',5'-cyclic monophosphate) on the macroscopic ionic currents of mouse sperm, finding the existence of different populations of sperm, in terms of the recorded current and its response to cyclic nucleotides. Our results show that capacitation and cyclic nucleotides increase the ionic current, having a differential sensitivity to cGMP (cyclic guanosine monophosphate) and cAMP (cyclic adenosine monophosphate). Using a specific inhibitor we determine the contribution of CNG channels to macroscopic current and capacitation.

The action of prostaglandins on ion channels

Prostaglandins, in particular PGE(2) and prostacyclin PGI(2) have diverse biological effects. Most importantly, they are involved in inflammation and pain. Prostaglandins in nano- and micromolar concentrations sensitize nerve cells, i.e. make them more sensitive to electrical or chemical stimuli. Sensitization arises from the effect of prostaglandins on ion channels and occurs both at the peripheral terminal of nociceptors at the site of tissue injury (peripheral sensitization) and at the synapses in the spinal cord (central sensitization). The first step is the binding of prostaglandins to receptors in the cell membrane, mainly EP and IP receptors. The receptors couple via G proteins to enzymes such as adenylate cyclase and phospholipase C (PLC). Activation of adenylate cyclase leads to increase of cAMP and subsequent activation of protein kinase A (PKA) or PKA-independent effects of cAMP, e.g. mediated by Epac (=exchange protein activated by cAMP). Activation of PLC causes increase of inositol phosphates and increase of cytosolic calcium. This article summarizes the effects of PGE(2), PGE(1), PGI2 and its stable analogues on non-selective cation channels and sodium, potassium, calcium and chloride channels. It describes the mechanism responsible for the facilitatory or inhibitory prostaglandin effects on ion channels. Understanding these mechanisms is essential for the development of useful new analgesics.

Prostaglandins in bone: bad cop, good cop?

Prostaglandins (PGs) are multifunctional regulators of bone metabolism that stimulate both bone resorption and formation. PGs have been implicated in bone resorption associated with inflammation and metastatic bone disease, and also in bone formation associated with fracture healing and heterotopic ossification. Recent studies have identified roles for inducible cyclooxygenase (COX)-2 and PGE(2) receptors in these processes. Although the effects of PGs have been most often associated with cAMP production and protein kinase A activation, PGs can engage an extensive G-protein signaling network. Further analysis of COX-2 and PG receptors and their downstream G-protein signaling in bone could provide important clues to the regulation of skeletal cell growth in both health and disease.

Intracellular ClC-3 chloride channels promote bone resorption in vitro through organelle acidification in mouse osteoclasts

ClC-7 Cl(-) channels expressed in osteoclasts are important for bone resorption since it has been shown that disruption of the ClCN7 gene in mice leads to severe osteopetrosis. We have previously reported that Cl(-) currents recorded from mouse osteoclasts resemble those of ClC-3 Cl(-) channels. The aim of the present study was to determine the expression of ClC-3 channels in mouse osteoclasts and their functional role during bone resorption. We detected transcripts for both ClC-7 and ClC-3 channels in mouse osteoclasts by RT-PCR. The expression of ClC-3 was confirmed by immunocytochemical staining. Mouse osteoclasts lacking ClC-3 Cl(-) channels (ClC-3(-/-) osteoclasts) derived from ClCN3 gene-deficient mice (ClC-3(-/-)) showed lower bone resorption activity compared with ClC-3+/+ osteoclasts derived from wild-type mice (ClC-3+/+). Treatment of ClC-3+/+ osteoclasts with small interfering RNA (siRNA) against ClC-3 also significantly reduced bone resorption activity. Electrophysiological properties of basal and hypotonicity-induced Cl(-) currents in ClC-3(-/-) osteoclasts did not differ significantly from those in ClC-3+/+ osteoclasts. Using immunocytochemistry, ClC-3 was colocalized with lysosome-associated membrane protein 2. Using pH-sensitive dyes, organelle acidification activity in ClC-3(-/-) osteoclasts was weaker than in ClC-3+/+ osteoclasts. Treatment of ClC-3+/+ osteoclasts with siRNA against ClC-3 also reduced the organelle acidification activity. In conclusion, ClC-3 Cl(-) channels are expressed in intracellular organelles of mouse osteoclasts and contribute to osteoclastic bone resorption in vitro through organelle acidification.

Fluid secretion caused by aerolysin-like hemolysin of Aeromonas sobria in the intestines is due to stimulation of production of prostaglandin E2 via cyclooxygenase 2 by intestinal cells

To clarify the mechanisms of diarrheal disease induced by Aeromonas sobria, we examined whether prostaglandin E2 (PGE2) was involved in the intestinal secretory action of A. sobria hemolysin by use of a mouse intestinal loop model. The amount of PGE2 in jejunal fluid and the fluid accumulation ratio were directly related to the dose of hemolysin. The increase over time in the level of PGE2 was similar to that of the accumulated fluid. In addition, hemolysin-induced fluid secretion and PGE2 synthesis were inhibited by the selective cyclooxygenase 2 (COX-2) inhibitor NS-398 but not the COX-1 inhibitor SC-560. Western blot analysis revealed that hemolysin increased the COX-2 protein levels but reduced the COX-1 protein levels in mouse intestinal mucosa in vivo. These results suggest that PGE2 functions as an important mediator of diarrhea caused by hemolysin and that PGE2 is produced primarily through a COX-2-dependent mechanism. Subsequently, we examined the relationship between PGE2, cyclic AMP (cAMP), and cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels in mouse intestinal mucosa exposed to hemolysin. Hemolysin increased the levels of cAMP in the intestinal mucosa. NS-398 inhibited the increase in cAMP production, but SC-560 did not. In addition, H-89, a cAMP-dependent protein kinase A (PKA) inhibitor, and glibenclamide, a CFTR inhibitor, inhibited fluid accumulation. Taken together, these results indicate that hemolysin activates PGE2 production via COX-2 and that PGE2 stimulates cAMP production. cAMP then activates PKA, which in turn stimulates CFTR Cl- channels and finally leads to fluid accumulation in the intestines.

The non-steroidal anti-inflammatory drug diclofenac reduces appearance of osteoblasts in bone defect healing in rats

INTRODUCTION

Non-steroidal anti-inflammatory drug (NSAID) is well known to significantly delay fracture healing. Results from in vitro studies implicate an impairment of osteoblast proliferation due to NSAIDs during the initial stages of healing. We studied whether diclofenac, a non-selective NSAID, also impairs appearance of osteoblasts in vivo during the early phase of healing (at 10 days).

MATERIALS AND METHODS

Two defects (Ø 1.1 mm) were drilled within distal femurs of 20 male Wistar rats. Ten rats received diclofenac continuously; the other obtained a placebo until sacrificing at 10 days. Osteoblast proliferation was assessed by cell counting using light microscopy, and bone mineral density (BMD) was measured using pQCT.

RESULTS

Osteoblast counts from the centre of bone defect were significantly reduced in the diclofenac group (median 73.5 +/- 8.4 cells/grid) compared to animals fed with placebo (median 171.5 +/- 13.9 cells/grid). BMD within the defect showed a significant reduction after diclofenac administration (median 111.5 +/- 9.3 mg/cm(3)) compared to the placebo group (median 177 +/- 45.4 mg/cm(3)).

CONCLUSION

The reduced appearance of osteoblasts in vivo implicates an inhibiting effect of diclofenac on osteoblasts at a very early level of bone healing. The inhibition of proliferation and migration of osteoblasts, or differentiation from progenitor cells, is implicated in the delay of fracture healing after NSAID application.

Prostaglandin E2 receptors EP2 and EP4 are down-regulated during differentiation of mouse osteoclasts from their precursors

Prostaglandin E2 (PGE2) has been proposed to be a potent stimulator of bone resorption. However, PGE2 itself has been shown to directly inhibit bone-resorbing activity of osteoclasts. We examined the role of PGE2 in the function of mouse osteoclasts formed in vitro. Bone marrow macrophage osteoclast precursors expressed PGE2 receptors EP1, EP2, EP3beta, and EP4, and the expression of EP2 and EP4 was down-regulated during osteoclastic differentiation induced by receptor activator of NF-kappaB ligand and macrophage colony-stimulating factor. In contrast, functional EP1 was continuously expressed in mature osteoclasts. PGE2 as well as calcitonin caused intracellular Ca2+ influx in osteoclasts. However, PGE2 and 17-phenyltrinol-PGE2 (an EP1 agonist) failed to inhibit actin-ring formation and pit formation by osteoclasts cultured on dentine slices. When EP4 was expressed in osteoclasts using an adenovirus carrying EP4 cDNA, both actin-ring and pit-forming activities of osteoclasts were inhibited in an infectious unit-dependent manner. Treatment of EP4-expressing osteoclasts with PGE2 further inhibited their actin-ring and pit-forming activities. Such inhibitory effects of EP4-mediated signals on osteoclast function are similar to those that are calcitonin receptor-mediated. Thus, osteoclast precursors down-regulate their own EP2 and EP4 levels during their differentiation into osteoclasts to escape inhibitory effects of PGE2 on bone resorption.