Regulation of osteoclastic acid secretion by cGMP-dependent protein kinase

Functional osteoclast attachment requires inositol-1,4,5-trisphosphate receptor-associated cGMP-dependent kinase substrate

Osteoclast activity is central to balanced bone turnover to maintain normal bone mass. A specialized osteoclast attachment to bone localizes acid secretion to remove bone mineral; in some cases, attachment is functionally impaired despite normal attachment proteins. The inositol-1,4,5-trisphosphate receptor-1 (IP3R1) is an intracellular calcium channel required for regulation of reversible osteoclast attachment by nitric oxide (NO), an important regulator of both normal and pathological bone degradation. In studies using human osteoclasts produced in vitro, we found that IP3R1 binds an endosomal isoform of the IP3R-associated cGMP-dependent kinase substrate (IRAG). IRAG is a substrate of cGMP-dependent kinase-1 (PKG1) and binds the PKG1 isoform PKG1β, which was the predominant form of PKG1 in human osteoclasts. Western blots of IRAG were consistent with NO-dependent serine phosphorylation of IRAG. An additional effect of PKG1β activity in osteoclasts was disassociation of IP3R1-IRAG complexes, as shown by analysis of IP3R1 complexes and by localization of the proteins within cells. IP3R1-IRAG complexes were stabilized by PKG or Src antagonists, Src activity being a requirement for IP3R1 calcium release downstream of PKG. IP3R1-mediated calcium release regulates cellular detachment in part through the calcium-dependent proteinase μ-calpain. In osteoclasts with IRAG suppressed by siRNA, activity of μ-calpain was increased relative to cells with normal IRAG, and regulation of μ-calpain by NO was lost. Furthermore, cells deficient in IRAG detached easily from substrate and had smaller attached diameters and randomly distributed podosomes, although IRAG knockdown did not affect cell viability. Our results indicate that IRAG is required for PKG1β-regulated cyclic calcium release during motility, and that disruption of the IP3R1-IRAG calcium regulation system is a novel cause of dysfunctional osteoclasts unrelated to defects in attachment proteins or acid secretion.

TRPV-5 mediates a receptor activator of NF-kappaB (RANK) ligand-induced increase in cytosolic Ca2+ in human osteoclasts and down-regulates bone resorption

Most of the signaling effectors located downstream of receptor activator of NF-kappaB (RANK) activation are calcium-sensitive. However, the early signaling events that lead to the mobilization of intracellular calcium in human osteoclasts are still poorly understood. The Ca(2+)-sensitive fluorescent probe Fura2 was used to detect changes in the intracellular concentration of Ca(2+) ([Ca(2+)](i)) in a model of human osteoclasts. Stimulating these cells with receptor activator of NF-kappaB ligand (RANKL) induced a rapid and significant increase in [Ca(2+)](i). Adding extracellular Ca(2+) chelators, depleting intracellular stores, and the use of a phospholipase C inhibitor all indicated that the Ca(2+) was of extracellular origin, suggesting the involvement of a Ca(2+) channel. We showed that none of the classical Ca(2+) channels (L-, T-, or R-type) were involved in the RANKL-induced Ca(2+) spike. However, the effect of high doses of Gd(3+) did suggest that TRP family channels were present in human osteoclasts. The TRPV-5 channel was expressed in osteoclasts and was mainly located in the cellular area in contact with the bone surface. Furthermore, the RNA inactivation of TRPV-5 channel completely inhibited the RANKL-induced increase in [Ca(2+)](i), which was accompanied in the long term by marked activation of bone resorption. Overall, our results show that RANKL induced a significant increase in [Ca(2+)](i) of extracellular origin, probably as a result of the opening of TRPV-5 calcium channels on the surface of human osteoclasts. Our findings suggest that TRPV-5 contributes to maintaining the homeostasis of the human skeleton via a negative feedback loop in RANKL-induced bone resorption.

cGMP may have trophic effects on beta cell function comparable to those of cAMP, implying a role for high-dose biotin in prevention/treatment of diabetes

Incretin hormones have trophic effects on beta cell function that can aid prevention and treatment of diabetes. cAMP is the primary mediator of these effects, and has been shown to potentiate glucose-stimulated insulin secretion, promote proper beta cells differentiation by increasing expression of the crucial transcription factor PDX-1, and prevent beta cell apoptosis. cGMP's role in beta cell function has received far less scrutiny, but there is emerging evidence that it may have a trophic impact on beta cell function analogous to that of cAMP. An increase in plasma glucose boosts beta cell production of cGMP, which acts as a feed-forward mediator to enhance glucose-stimulated insulin secretion. cGMP also has an anti-apoptotic effect in beta cells, and there is now indirect evidence that it promotes expression of PDX-1. Supraphysiological concentrations of biotin can directly activate guanylate cyclase, and there is limited evidence that high intakes of this vitamin can be therapeutically beneficial in diabetics and in rodent models of diabetes. Beneficial effects of cGMP on muscle insulin sensitivity and on control of hepatic glucose output may contribute to biotin's utility in diabetes. The fact that nitric oxide/cGMP exert a range of favorable effects on vascular health should further encourage exploration of biotin's preventive and therapeutic potential. If an appropriate high-dose biotin regimen could achieve a modest systemic increase in guanylate cyclase activity, without entailing unacceptable side effects or risks, such a regimen might have considerable potential for promoting vascular health and preventing or managing diabetes.

Citrate reverses cyclosporin A-induced metabolic acidosis and bone resorption in rats

BACKGROUND

Cyclosporine A (CsA) causes distal renal tubular acidosis (RTA) and osteoporosis. We have recently reported that the reduction of nitric oxide (NO) exacerbates this condition. Distal RTA may deplete bone mineral due to the chronic buffering of acid in the blood. The interaction of CsA and NO in causing metabolic acidosis and bone demineralization has not been studied previously. Nor has the salubrious effect of citrate therapy.

PURPOSE

To examine the effect of systemic pH correction by citrate on renal electrolyte (Na, K, Cl, NH3, HCO3) excretion following acute water loading in CsA-treated and NO-reduced rats. We further evaluated femoral bone density and bone demineralization activity after the same treatments.

METHODS

Rats received CsA, L-arginine (L-Arg), or nitro-L-arginine-methyl ester (L-NAME), or a combination of CsA+L-NAME plus or minus citrate. Urine and blood electrolytes were examined, as well as the urine excretion of deoxypyridinoline and the bone density of both femurs.

RESULTS

CsA and L-NAME reduced urine pH and the serum HCO3- concentration, and increased serum K+ and Cl- concentrations. The combination of CsA with L-NAME caused more severe deficits in the serum HCO3- concentration and elevations in serum K+ and Cl- concentrations than either drug alone. Both CsA and L-NAME reduced urinary nitrate excretion, which was reversed by co-administration of L-Arg. Co-administration of citrate or L-Arg improved the CsA- and L-NAME-induced acidosis and hyperkalemia. Bone resorption and density of the femurs were decreased by CsA and L-NAME and were additive for both drugs. Co-administration of citrate or L-Arg restored both bone resorption and density to normal levels.

CONCLUSION

CsA induces a hyperchloremic metabolic acidosis with hyperkalemia and a reduction in NO. The ensuing systemic acidosis causes bone resorption and demineralization. These effects were corrected by co-treatment with citrate. Citrate, at least in part, directly reduces the protonation of bone in animals treated with CsA and is recommended as a potential adjunct drug to prevent bone demineralization in patients chronically receiving CsA.

Osteoclast signalling pathways

The osteoclast is a monocyte-derived cell with complex regulatory control due to its role, balancing calcium homeostasis with skeletal modelling and repair. Normal differentiation requires tyrosine kinase- and tumor necrosis-family receptors, normally fms and RANK. Ligands for these receptors plus unidentified serum or cell-presented factor(s) are needed for in vitro differentiation, possibly signalling via an immune-like tyrosine kinase acceptor molecule. Osteoclast development and activity are increased by cytokines signalling through GP130, such as IL-6, by TGF-beta, and by IL-1, although these cannot replace serum. Other tyrosine kinase receptors including kit and met can augment fms signalling, and TNFs other than RANKL, including TNFalpha and TRAIL, modify RANK signalling, which is also susceptible to interference by interferons. The situation is further complicated by G-protein coupled receptors including the calcitonin receptor, by integrin or calcium-mediated signals, and by estrogen receptors, which operate in bone largely via NO downstream signals. Differentiation, activity, and survival signals merge in intracellular second messengers. These include cytoplasmic kinases of several families; differentiation pathways often terminate in Erk/Jun kinases or NF-kappaB. Key regulatory intermediates include TRAF6, src, Smad3, phosphatidylinositol-3-kinase, Jak/Stat, and the cGMP-dependent protein kinase I. There are substantial uncertainties regarding how intracellular agents connect to primary signals. The frontier includes characterization of how scaffolding/adapter proteins, such as cbl, gab, grb, p130Cas, and shc, as well as itam-containing proteins and nonreceptor tyrosine kinase adapters of the src and syk families, delimit and integrate signals of multiple receptors to bring about specific outcomes.

Cyclic nucleotide-dependent protein kinases: intracellular receptors for cAMP and cGMP action

Intracellular cAMP and cGMP levels are increased in response to a variety of hormonal and chemical stimuli; these nucleotides play key roles as second messenger signals in modulating myriad physiological processes. The cAMP-dependent protein kinase and cGMP-dependent protein kinase are major intracellular receptors for these nucleotides, and the actions of these enzymes account for much of the cellular responses to increased levels of cAMP or cGMP. This review summarizes many studies that have contributed significantly to an improved understanding of the catalytic, regulatory, and structural properties of these protein kinases. These accumulated findings provide insights into the mechanisms by which these enzymes produce their specific physiological effects and are helpful in considering the actions of other protein kinases as well.

Novel mechanisms of calcium handling by the osteoclast: A review-hypothesis

The osteoclast is a cell that is unique in its ability to resorb bone and, in doing so, becomes exposed to unusually high millimolar Ca2+ concentrations. It is generally accepted that, during resorption, osteoclasts can "sense" changes in their ambient Ca2+ concentration. This triggers a sharp cytosolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The change in cytosolic Ca2+ is transduced finally into inhibition of bone resorption. It has been shown that a type 2 ryanodine receptor isoform, expressed uniquely in the plasma membrane, functions as a Ca2+ influx channel and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily Ca2+ release channels that have a microsomal membrane location in a wide variety of eukaryotic cells, including the osteoclasts. However, only recently has it become obvious that ryanodine receptors are also expressed in osteoclast nuclear membranes, at which site they probably gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+ in turn regulates key nuclear processes, including gene expression and apoptosis. Here, we review the potential mechanisms underlying the recognition, movement, and effects of Ca2+ in the osteoclast. We will also speculate on the general biological significance of the unique processes used by the osteoclast to handle high Ca2+ loads during bone resorption.

Tumor necrosis factor-alpha stimulates mucin secretion and cyclic GMP production by guinea pig tracheal epithelial cells in vitro

Tumor necrosis factor (TNF)-alpha, a pluripotent cytokine implicated in the pathogenesis of airway inflammation, has been shown to provoke hypersecretion of mucin by airway epithelial cells in vitro. In this study, we investigated potential signaling pathways mediating TNF-alpha-induced mucin secretion using guinea pig tracheal epithelial (GPTE) cells in air-liquid interface culture. Exogenously applied TNF-alpha (human recombinant) stimulated mucin secretion in a concentration-dependent manner, with maximal effects at 10 to 15 ng/ml (286 to 429 U/ml). The pathway of stimulated secretion appeared to involve generation of intracellular nitric oxide (NO), activation of soluble guanylate cyclase (GC-S), production of cyclic guanosine monophosphate (cGMP), and activation of cGMP-dependent protein kinase (PKG). TNF-alpha increased production of nitrite and nitrate by GPTE cells; both mucin secretion and cGMP production were attenuated by NG-monomethyl-L-arginine (1 mM), a competitive inhibitor of nitric oxide synthase (NOS), or by the GC-S inhibitor LY83583 (50 microM); and mucin secretion in response to TNF-alpha or to the cGMP analogue dibutyryl cGMP (100 and 500 microM) was attenuated by the specific PKG inhibitor KT5823 (1 microM). Increased mucin secretion and increased cGMP production in response to TNF-alpha both appeared to be mediated by a phospholipase C that hydrolyzes phosphatidylcholine (PC-PLC), and by protein kinase C (PKC), since both responses were attenuated by either D609 (10 and 20 microg/ml), a specific PC-PLC inhibitor, or by each of three PKC inhibitors: Calphostin C (0.3 and 0.5 microM), bisindoylmaleimide (GF 109203X, Go 6850; 20 nM), or Ro31-8220 (10 microM). Collectively, the results suggest that TNF-alpha stimulates secretion of mucin by GPTE cells via a mechanism(s) dependent on PC-PLC and PKC, and involving activation of NOS, generation of NO, production of cGMP, and activation of PKG.

Nitric oxide superoxide and peroxynitrite modulate osteoclast activity

The gas radical, nitric oxide (NO), is a key signalling molecule in the cardiovascular, nervous and immune systems. Recently it has been found that it is produced by both the osteoblast and osteoclast and that it has major effects in producing osteoclast detachment and exerting a tonic inhibition of bone resorption. This detaching effect is mediated by a rapid increase in cGMP following calcium-triggered e-NOS activation during normal bone resorption. This effect is not reproduced in vitro by 8-bromo-cGMP but is seen with the newer rapidly permeant 8-pCPT-cGMP. However the inhibition of bone resorption by SIN-1 in vitro is not mediated solely by cGMP but depends on other factors still unidentified. Superoxide anions alone produces both osteoclast detachment and inhibition of resorption. Both of these actions may be mediated at least in part by peroxynitrite which has the same effect as NO alone on osteoclast detachment.

Nitric oxide synthase and cyclic GMP-dependent protein kinase concentrated at the neuromuscular endplate

Nitric oxide mediates diverse functions in development and physiology of vertebrate skeletal muscle. Neuronal type nitric oxide synthase-mu is enriched in fast-twitch fibers and binds to syntrophin, a component of the sarcolemmal dystrophin glycoprotein complex. Here, we show that cyclic GMP-dependent protein kinase type I, a primary effector for nitric oxide, occurs selectively at the neuromuscular junction, in mice and rats, and both neuronal type nitric oxide synthase-mu and cyclic GMP-dependent protein kinase type I remain at skeletal muscle endplates at least two weeks following muscle denervation. Expression of neuronal type nitric oxide synthase-mu and cyclic GMP-dependent protein kinase type I are up-regulated following fusion of cultured primary myotubes. Interestingly, the highest levels of neuronal type nitric oxide synthase-mu in muscle are found complexed with dystrophin at the sarcolemma of intrafusal fibers in muscle spindles. Localization of neuronal type nitric oxide synthase-mu and cyclic GMP-dependent protein kinase type I at the neuromuscular junction suggests functions for nitric oxide and cyclic GMP in the regulation of synaptic actions of intra- and extrafusal muscle fibers.

Membrane binding sites and non-genomic effects of estrogen in cultured human pre-osteoclastic cells

Besides functional estrogen receptors, the presence of signalling cell surface binding sites for 17beta-estradiol (17betaE2) has been reported in osteoblast- and osteoclast-like cells, suggesting that 17betaE2 may influence bone remodelling by a dual mechanism of action: to affect gene expression mediated by the nuclear activity of the steroid-receptor complex, and to initiate rapid responses triggered by a signal-generating receptor on the cell surface. Recently, we demonstrated that the human pre-osteoclastic cell line FLG 29.1 bears functional estrogen receptors. In this study we examined FLG 29.1 cells for the presence of cell surface binding sites for 17betaE2, and whether 17betaE2 could elicit cell signalling. Using a cell-impermeant and fluorescent estrogen conjugate, 17beta-estradiol-6-carboxymethyloxime-bovine serum albumin-fluorescein isothiocyanate, we demonstrated the presence of specific plasma membrane binding sites for 17betaE2. Stimulation of FLG 29.1 cells with low (1 nM) and high (1 microM) doses of 17betaE2 induced a prompt and significant (P < 0.05) increase of cellular pH, as measured in single cells using an image analysis system. In addition, both cAMP and cGMP were significantly increased by 17betaE2 with a dose-dependent response. Finally, a rapid increase of intracellular calcium ion concentration [Ca2+] was also induced by 1 nM 17betaE2, as measured in single cells using an image analysis system. Our findings strongly suggest a non-genomic action of 17betaE2 on osteoclast precursors.