Control of cytosolic free calcium in rat and chicken osteoclasts. The role of extracellular calcium and calcitonin.

Single cell [Ca2+], studies were performed in chicken and rat osteoclasts loaded with fura-2 and exposed to a variety of treatments. Under resting conditions, basal [Ca2+]i, was 79.2 +/- 47.3 and 84.3 +/- 65.7 nM (averages +/- S.D.; n = 141 and 126) in the osteoclasts of the two species, respectively. Basal [Ca2+]i was stable in all rat and in approximately 80% of chicken osteoclasts. In the remaining 20%, spontaneous, irregular [Ca2+], fluctuations were observed (amplitude range: 50-200 nm over basal values). Increase of [Ca2+]o over the concentration of the Krebs-Ringer incubation medium (2 mM) induced rises of [Ca2+] in almost all cells investigated. [Ca2+] rises were already appreciable with 0.5 mM [Ca2+]o additions and reached high values with 4 mM additions: 390 +/- 113 and 364 +/- 214 nM [Ca2+], in rat and chicken osteoclasts, respectively (n = 122 and 101). Qualitatively, the responses to [Ca2+]o additions consisted of discrete [Ca2+]i transients, biphasic (an initial spike followed by a plateau), or monophasic (either the spike or the plateau). In a few chicken osteoclasts, the [Ca2+]i increase occurring after [Ca2+]o addition consisted of multiple, irregular fluctuations, similar to those observed in 20% of these cells under resting conditions. In individual osteoclasts subsequently exposed to multiple [Ca2+]o increase pulses, the type of the [Ca2+]i transient (mono- or biphasic) was maintained, and the size was dependent on the magnitude of the [Ca2+]o additions. Effects similar to those of [Ca2+]o were induced by the addition of Cd2+ or Ba2+ (but not La3+ or Mg2+) into the medium. The Cd2+ effect was maintained in part even in a Ca2+-free medium. Of various hormones and factors, parathormone, 1,25-dihydroxyvitamin D3, and prostaglandin E2 were inactive. In contrast, calcitonin was active in rat osteoclasts (which express numerous receptors). [Ca2+]i increases were small (19 +/- 17.9 nM; n = 21) when the hormone was administered alone; they were synergistic (severalfold potentiation) when the hormone was administered before or after [Ca2+]o. The [Ca2+]i effects of calcitonin were mimicked by 8Br-cAMP (31 +/- 26 nM; n = 12) when the nucleotide was administered alone; marked synergism when it was administered in combination with [Ca2+]o. This paper demonstrates for the first time that changes of [Ca2+]i are induced in osteoclasts by treatments with [Ca2+]o and calcitonin and can therefore be involved in intracellular mediation of the physiological effects of these two extracellular signals

Dynamic changes in the osteoclast cytoskeleton in response to growth factors and cell attachment are controlled by β3 integrin

The β3 integrin cytoplasmic domain, and specifically S752, is critical for integrin localization and osteoclast (OC) function. Because growth factors such as macrophage colony–stimulating factor and hepatocyte growth factor affect integrin activation and function via inside-out signaling, a process requiring the β integrin cytoplasmic tail, we examined the effect of these growth factors on OC precursors. To this end, we retrovirally expressed various β3 integrins with cytoplasmic tail mutations in β3-deficient OC precursors. We find that S752 in the β3 cytoplasmic tail is required for growth factor–induced integrin activation, cytoskeletal reorganization, and membrane protrusion, thereby affecting OC adhesion, migration, and bone resorption. The small GTPases Rho and Rac mediate cytoskeletal reorganization, and activation of each is defective in OC precursors lacking a functional β3 subunit. Activation of the upstream mediators c-Src and c-Cbl is also dependent on β3. Interestingly, although the FAK-related kinase Pyk2 interacts with c-Src and c-Cbl, its activation is not disrupted in the absence of functional β3. Instead, its activation is dependent upon intracellular calcium, and on the β2 integrin. Thus, the β3 cytoplasmic domain is responsible for activation of specific intracellular signals leading to cytoskeletal reorganization critical for OC function

The V2 receptor antagonist tolvaptan raises cytosolic calcium and prevents AQP2 trafficking and function: an in vitro and in vivo assessment

Tolvaptan, a selective vasopressin V2 receptor antagonist, is a new generation diuretic. Its clinical efficacy is in principle due to impaired vasopressin-regulated water reabsorption via aquaporin-2 (AQP2). Nevertheless, no direct in vitro evidence that tolvaptan prevents AQP2-mediated water transport, nor that this pathway is targeted in vivo in patients with syndrome of inappropriate antidiuresis (SIAD) has been provided. The effects of tolvaptan on the vasopressin-cAMP/PKA signalling cascade were investigated in MDCK cells expressing endogenous V2R and in mouse kidney. In MDCK, tolvaptan prevented dDAVP-induced increase in ser256-AQP2 and osmotic water permeability. A similar effect on ser256-AQP2 was found in V1aR -/- mice, thus confirming the V2R selectively. Of note, calcium calibration in MDCK showed that tolvaptan per se caused calcium mobilization from the endoplasmic reticulum resulting in a significant increase in basal intracellular calcium. This effect was only observed in cells expressing the V2R, indicating that it requires the tolvaptan-V2R interaction. Consistent with this finding, tolvaptan partially reduced the increase in ser256-AQP2 and the water permeability in response to forskolin, a direct activator of adenylyl cyclase (AC), suggesting that the increase in intracellular calcium is associated with an inhibition of the calcium-inhibitable AC type VI. Furthermore, tolvaptan treatment reduced AQP2 excretion in two SIAD patients and normalized plasma sodium concentration. These data represent the first detailed demonstration of the central role of AQP2 blockade in the aquaretic effect of tolvaptan and underscore a novel effect in raising intracellular calcium that can be of significant clinical relevance

M-CSF regulates the cytoskeleton via recruitment of a multimeric signaling complex to c-Fms Tyr-559/697/721.

M-CSF is known to induce cytoskeletal reorganization in macrophages and osteoclasts by activation of phosphatidylinositol 3-kinase (PI3K) and c-Src, but the detailed mechanisms remain unclear. We find, unexpectedly, that tyrosine (Tyr) to phenylalanine (Phe) mutation of Tyr-721, the PI3K binding site in the M-CSF receptor c-Fms, fails to suppress cytoskeletal remodeling or actin ring formation. In contrast, mutation of c-Fms Tyr-559 to Phe blocks M-CSF-induced cytoskeletal reorganization by inhibiting formation of a Src Family Kinase SFK·c-Cbl·PI3K complex and the downstream activation of Vav3 and Rac, two key mediators of actin remodeling. Using an add-back approach in which specific Tyr residues are reinserted into c-Fms inactivated by the absence of all seven functionally important Tyr residues, we find that Tyr-559 is necessary but not sufficient to transduce M-CSF-dependent cytoskeletal reorganization. Furthermore, this same add-back approach identifies important roles for Tyr-697 and Tyr-721 in collaborating with Tyr-559 to recruit a multimeric signaling complex that can transduce signals from c-Fms to the actin cytoskeleton

A 3D in vitro bone organ model using human progenitor cells

Three-dimensional (3D) organotypic culture models based on human cells may reduce the use of complex and costly animal models, while gaining clinical relevance. This study aimed at developing a 3D osteoblastic-osteoclastic-endothelial cell co-culture system, as an in vitro model to mimic the process of bone turnover. Osteoprogenitor and endothelial lineage cells were isolated from the stromal vascular fraction (SVF) of human adipose tissue, whereas CD14+ osteoclast progenitors were derived from human peripheral blood. Cells were co-cultured within 3D porous ceramic scaffolds using a perfusion-based bioreactor device, in the presence of typical osteoclastogenic factors. After 3 weeks, the scaffolds contained cells with endothelial (2.0±0.3%), pre/osteoclastic (14.0±1.4%) and mesenchymal/osteoblastic (44.0±8.4%) phenotypes, along with tartrate-resistant acid phosphatase-positive (TRAP+) osteoclastic cells in contact with deposited bone-like matrix. Supernatant analysis demonstrated sustained matrix deposition (by C-terminus procollagen-I propeptides), resorption (by N-terminus collagen-I telopeptides and phosphate levels) and osteoclastic activity (by TRAP-5b) only when SVF and CD14+ cells were co-cultured. Scanning electron microscopy and magnetic resonance imaging confirmed the pattern of matrix deposition and resorption. The effectiveness of Vitamin D in replacing osteoclastogenic factors indicated a functional osteoblast-osteoclast coupling in the system. The formation of human-origin bone-like tissue, blood vessels and osteoclasts upon ectopic implantation validated the functionality of the developed cell types. The 3D co-culture system and the associated non-invasive analytical tools can be used as an advanced model to capture some aspects of the functional coupling of bone-like matrix deposition and resorption and could be exploited toward the engineering of multi-functional bone substitute implants

Adrenergic stimulation decreases osteoblast oxytocin synthesis

Oxytocin (OT) regulates bone mass by inducing the differentiation of osteoblasts to a mature, mineralizing phenotype. We have shown recently that osteoblasts can synthesize OT. In view of known interactions between OT-ergic and adrenergic neurons in the central nervous system, we questioned whether the negative regulation of osteoblast differentiation by adrenergic nerves was mediated through its suppression of OT synthesis. We first confirmed that α 1b and β 2 adrenergic receptors were expressed on both primary murine osteoblasts and MC3T3-E1 cells. We then showed that α 1 and β 2 adrenergic agonists downregulated OT synthesis, and that the effect of each agonist was reversed by its respective antagonist. The data suggest that the negative effects of adrenergic stimulation on bone mass could, in part, arise from decreased OT synthesis. © 2011 New York Academy of Sciences