Gap junctions and fluid flow response in MC3T3-E1 cells

Mechanical stimulation effects on functional end effectors in osteoblastic MG-63 cells

Receptor activator of Nf-kappaB ligand (RANKL) and osteoprotegerin (OPG) have been implicated in bone metabolism. Specifically, the balance of these factors in conjunction with receptor activator of Nf-kappaB (RANK) is believed to be key in determining the rate of osteoclastogenesis and the net outcome of bone formation/resorption. While it is well accepted that mechanical loading in vivo affects bone formation/resorption and that alterations in the responsiveness of bone cells to mechanical loading have been implicated in metabolic bone diseases, the effect of in vitro mechanical loading on osteoblastic production of OPG and RANKL has not been extensively studied. Thus, in the current study, we developed an in vitro model to load human osteoblasts and studied levels of OPG, RANKL, PGE(2) and macrophage colony stimulating factor (M-CSF). We hypothesized that stimulating osteoblastic cells would increase the release of soluble OPG relative to RANKL favoring a bone-forming (and resorption-inhibiting) event. To accomplish this, we developed a small-scale loading machine that imparts via bending, well-defined substrate deformation to bone cells cultured on artificial substrates. Following 2h of loading and a 1h incubation period, media was collected and levels of soluble OPG, RANKL, PGE(2) and M-CSF were quantified using ELISA and western blotting. We found that mechanical loading significantly increased soluble OPG levels relative to RANKL at this 3h time point. Levels of soluble and cellular RANKL detected were not significantly affected by mechanical stimulation. The relative shift in abundance of OPG over RANKL associated with applied mechanical stimulation suggests the soluble OPG:RANKL ratio may be important in load-induced coupling mechanisms of bone cells.

Cell-to-cell interactions in bone

Bone development (modeling) occurs by migration, aggregation, and condensation of immature osteo/chondroprogenitor cells to form the cartilaginous anlage. This process requires precisely controlled cell-cell interactions. Likewise, bone remodeling in the adult skeleton is a dynamic process that requires coordinated cellular activities among osteoblasts, osteocytes, and osteoclasts to maintain bone homeostasis. The cooperative nature of both bone modeling and remodeling requires tightly regulated mechanisms of intercellular recognition and communication that permit the cells to sort and migrate, synchronize activity, equalize hormonal responses, and diffuse locally generated signals. Osteoblasts and osteocytes achieve these interactions through cadherin-based adherens junctions as well as by formation of communicating junctions, gap junctions. This review examines the current knowledge of how direct cell-to-cell interactions modulate osteoblast function.

Fluid shear-induced ATP secretion mediates prostaglandin release in MC3T3-E1 osteoblasts

ATP is rapidly released from osteoblasts in response to mechanical load. We examined the mechanisms involved in this release and established that shear-induced ATP release was mediated through vesicular fusion and was dependent on Ca2+ entry into the cell through L-type voltage-sensitive Ca2+ channels. Degradation of secreted ATP by apyrase prevented shear-induced PGE2 release.

INTRODUCTION

Fluid shear induces a rapid rise in intracellular calcium ([Ca2+]i) in osteoblasts that mediates many of the cellular responses associated with mechanotransduction in bone. A potential mechanism for this increase in [Ca2+]i is the activation of purinergic (P2) receptors resulting from shear-induced extracellular release of ATP. This study was designed to determine the effects of fluid shear on ATP release and the possible mechanisms associated with this release.

MATERIALS AND METHODS

MC3T3-E1 preosteoblasts were plated on type I collagen, allowed to proliferate to 90% confluency, and subjected to 12 dynes/cm2 laminar fluid flow using a parallel plate flow chamber. ATP release into the flow media was measured using a luciferin/luciferase assay. Inhibitors of channels, gap junctional intercellular communication (GJIC), and vesicular formation were added before shear and maintained in the flow medium for the duration of the experiment.

RESULTS AND CONCLUSIONS

Fluid shear produced a transient increase in ATP release compared with static MC3T3-E1 cells (59.8 +/- 15.7 versus 6.2 +/- 1.8 nM, respectively), peaking within 1 minute of onset. Inhibition of calcium entry through the L-type voltage-sensitive Ca2+ channel (L-VSCC) with nifedipine or verapamil significantly attenuated shear-induced ATP release. Channel inhibition had no effect on basal ATP release in static cells. Ca(2+)-dependent ATP release in response to shear seemed to result from vesicular release and not through gap hemichannels. Vesicle disruption with N-ethylmaleimide, brefeldin A, or monensin prevented increases in flow-induced ATP release, whereas inhibition of gap hemichannels with either 18alpha-glycyrrhetinic acid or 18beta-glycyrrhetinic acid did not. Degradation of extracellular ATP with apyrase prevented shear-induced increases in prostaglandin E2 (PGE2) release. These data suggest a time line of mechanotransduction wherein fluid shear activates L-VSCCs to promote Ca2+ entry that, in turn, stimulates vesicular ATP release. Furthermore, these data suggest that P2 receptor activation by secreted ATP mediates flow-induced prostaglandin release.

Oscillatory fluid flow affects human marrow stromal cell proliferation and differentiation

Mechanical loading is an important regulator of bone formation and bone loss. Decreased osteoblast number and function are important cellular mechanisms by which mechanical disuse leads to decreased bone formation. Decreased osteoblast number may be a result of decreased osteoprogenitor proliferation, differentiation, or both. However, the effects of cellular level physical signals on osteoprogenitors are not well understood. In this study, we examined the effects of loading induced oscillatory fluid flow (OFF), a potent regulator of osteoblastic cell function, on marrow stromal cells (MSCs). MSCs subjected to OFF exhibited increased intracellular Ca2+ mobilization. In addition, MSCs exhibited increased proliferation and increased mRNA levels for osteopontin and osteocalcin genes. Collagen I and core binding factor 1 mRNA levels did not change. MSCs subjected to OFF also exhibited decreased alkaline phosphatase activity. These results suggest that MSCs are mechanosensitive and that Ca2+ may play a role in the signaling pathway.

Mechanical regulation of HB-GAM expression in bone cells

Bone adaption upon mechanical stimulation is accompanied by changes in gene expression. In this context we investigated the influence of mechanical loading on heparin binding growth associated molecule (HB-GAM) expression, an extracellular matrix molecule which in cell culture has been shown to stimulate the differentiation of osteoblasts. We obtained information on the participating signal transduction pathways using a mitogenic loading regimen. Specific inhibitors of various signal transduction pathways were added to loaded cells and to unloaded controls. By semi-quantitative PCR studies we demonstrated a rapid decrease of HB-GAM expression in primary osteoblasts and SaOs-2 cells by 20-30% upon mechanical loading within 30min. We showed that the RGD-integrin interaction is involved in the regulation of HB-GAM expression. Furthermore, integrity of the cytoskeleton, stretch-activated, and voltage-sensitive Ca(2+) channels as well as gap junctional communication are necessary for the downregulation of HB-GAM expression by mechanical loading.

Mechanosensitivity of bone cells to oscillating fluid flow induced shear stress may be modulated by chemotransport

Fluid flow has been shown to be a potent physical stimulus in the regulation of bone cell metabolism. In addition to membrane shear stress, loading-induced fluid flow will enhance chemotransport due to convection or mass transport thereby affecting the biochemical environment surrounding the cell. This study investigated the role of oscillating fluid flow induced shear stress and chemotransport in cellular mechanotransduction mechanisms in bone. Intracellular calcium mobilization and prostaglandin E(2) (PGE(2)) production were studied with varying levels of shear stress and chemotransport. In this study MC3T3-E1 cells responded to oscillating fluid flow with both an increase in intracellular calcium concentration ([Ca(2+)](i)) and an increase in PGE(2) production. These fluid flow induced responses were modulated by chemotransport. The percentage of cells responding with an [Ca(2+)](i) oscillation increased with increasing flow rate, as did the production of PGE(2). In addition, depriving the cells of nutrients during fluid flow resulted in an inhibition of both [Ca(2+)](i) mobilization and PGE(2) production. These data suggest that depriving the cells of a yet to be determined biochemical factor in media affects the responsiveness of bone cells even at a constant peak shear stress. Chemotransport alone will not elicit a response, but it appears that sufficient nutrient supply or waste removal is needed for the response to oscillating fluid flow induced shear stress.

Fluid flow-induced prostaglandin E2 response of osteoblastic ROS 17/2.8 cells is gap junction-mediated and independent of cytosolic calcium

It has been well demonstrated that bone adapts to mechanical loading. To accomplish this at the cellular level, bone cells must be responsive to mechanical loading (mechanoresponsive). This can occur via such mechanisms as direct cell deformation or signal transduction via complex pathways involving chemotransport, hormone response, and/or gene expression, to name a few. Mechanotransduction is the process by which a bone cell senses a biophysical signal and elicits a response. While it has been demonstrated that bone cells can respond to a wide variety of biophysical signals including fluid flow, stretch, and magnetic fields, the exact pathways and mechanisms involved are not clearly understood. We postulated that gap junctions may play an important role in bone cell responsiveness. Gap junctions (GJ) are membrane-spanning channels that physically link cells and support the transport of small molecules and ions in the process of gap junctional intercellular communication (GJIC). In this study we examined the role of GJ and GJIC in mechanically stimulated osteoblastic cells. Following fluid flow stimulation, we quantified prostaglandin E(2) (PGE(2)) (oscillatory flow) and cytosolic calcium (Ca(2+)) (oscillatory and steady flow) responses in ROS 17/2.8 cells and a derivative of these cells expressing antisense cDNA for the gap junction protein connexin 43 (RCx16) possessing significantly different levels of GJIC. We found that the ROS17/2.8 cells possessing increased GJIC also exhibited increased PGE(2) release to the supernatant following oscillatory fluid flow stimulation in comparison to coupling-decreased RCx16 cells. Interestingly, we found that neither osteoblastic cell line responded to oscillatory or steady fluid flow stimulation with an increase in Ca(2+). Thus, our results suggest that GJ and GJIC may be important in the mechanotransduction mechanisms by which PGE(2) is mechanically induced in osteoblastic cells independent of Ca(2+).

Intercellular calcium communication regulates platelet aggregation and thrombus growth

The ability of platelets to form stable adhesion contacts with other activated platelets (platelet cohesion or aggregation) at sites of vascular injury is essential for hemostasis and thrombosis. In this study, we have examined the mechanisms regulating cytosolic calcium flux during the development of platelet-platelet adhesion contacts under the influence of flow. An examination of platelet calcium flux during platelet aggregate formation in vitro demonstrated a key role for intercellular calcium communication (ICC) in regulating the recruitment of translocating platelets into developing aggregates. We demonstrate that ICC is primarily mediated by a signaling mechanism operating between integrin alpha IIb beta 3 and the recently cloned ADP purinergic receptor P2Y12. Furthermore, we demonstrate that the efficiency by which calcium signals are propagated within platelet aggregates plays an important role in dictating the rate and extent of thrombus growth.

Cell density and growth-dependent down-regulation of both intracellular calcium responses to agonist stimuli and expression of smooth-surfaced endoplasmic reticulum in MC3T3-E1 osteoblast-like cells

A two-dimensional intracellular Ca(2+) ([Ca(2+)](i)) imaging system was used to examine the relationship between [Ca(2+)](i) handling and the proliferation of MC3T3-E1 osteoblast-like cells. The resting [Ca(2+)](i) level in densely cultured cells was 1.5 times higher than the [Ca(2+)](i) level in sparsely cultured cells or in other cell types (mouse fibroblasts, rat vascular smooth muscle cells, and bovine endothelial cells). A high resting [Ca(2+)](i) level may be specific for MC3T3-E1 cells. MC3T3-E1 cells were stimulated with ATP (10 microM), caffeine (10 mM), thapsigargin (1 microM), or ionomycin (10 microM), and the effect on the [Ca(2+)](i) level of MC3T3-E1 cells was studied. The percentage of responding cells and the degree of [Ca(2+)](i) elevation were high in the sparsely cultured cells and low in densely cultured cells. The rank order for the percentage of responding cells and magnitude of the Ca(2+) response to the stimuli was ionomycin > thapsigargin = ATP > caffeine and suggests the existence of differences among the various [Ca(2+)](i) channels. All Ca(2+) responses in the sparsely cultured MC3T3-E1 cells, unlike in other cell types, disappeared after the cells reached confluence. Heptanol treatment of densely cultured cells restored the Ca(2+) response, suggesting that cell-cell contact is involved with the confluence-dependent disappearance of the Ca(2+) response. Immunohistological analysis of type 1 inositol trisphosphate receptors and electron microscopy showed distinct expression of inositol trisphosphate receptor proteins and smooth-surfaced endoplasmic reticulum in sparsely cultured cells but reduced levels in densely cultured cells. These results indicate that the underlying basis of confluence-dependent [Ca(2+)](i) regulation is down-regulation of smooth-surfaced endoplasmic reticulum by cell-cell contacts.

Osteopontin-deficient bone cells are defective in their ability to produce NO in response to pulsatile fluid flow

Osteopontin (OPN) is a noncollagenous component of bone matrix. It mediates cell attachment and activates signal transduction pathways. In this work, bone cells, cultured from fragments of long bones derived from wild-type and OPN-/- ("knock-out") mice, were exposed to pulsatile fluid flow (PFF) over a 60-min period. The medium was assayed periodically for nitric oxide (NO) and prostaglandin E(2) (PGE(2)) release. OPN+/+ cells exhibited a peak of NO production 5-10 min after the onset of PFF, decreasing to a stable plateau at 15 min; much less NO was produced by the OPN-/- cells. PFF resulted in reduced PGE(2) release by both cell types, although the reduction was less for the OPN-/- cells in the 15-30 min window. Both cell types exhibited a similar enhancement of cyclooxygenase2 mRNA levels 60 min after initiation of PFF. These results suggest that bone cells require OPN to respond fully to PFF as assessed by increased NO and reduced PGE(2) synthesis.