Voltage-gated sodium and calcium currents in rat osteoblasts

CACNA1C-Related Channelopathies

The CACNA1C gene encodes the pore-forming subunit of the CaV1.2 L-type Ca2+ channel, a critical component of membrane physiology in multiple tissues, including the heart, brain, and immune system. As such, mutations altering the function of these channels have the potential to impact a wide array of cellular functions. The first mutations identified within CACNA1C were shown to cause a severe, multisystem disorder known as Timothy syndrome (TS), which is characterized by neurodevelopmental deficits, long-QT syndrome, life-threatening cardiac arrhythmias, craniofacial abnormalities, and immune deficits. Since this initial description, the number and variety of disease-associated mutations identified in CACNA1C have grown tremendously, expanding the range of phenotypes observed in affected patients. CACNA1C channelopathies are now known to encompass multisystem phenotypes as described in TS, as well as more selective phenotypes where patients may exhibit predominantly cardiac or neurological symptoms. Here, we review the impact of genetic mutations on CaV1.2 function and the resultant physiological consequences.

Large-Conductance Calcium-Activated Potassium Channels and Voltage-Dependent Sodium Channels in Human Cementoblasts

Cementum, which is excreted by cementoblasts, provides an attachment site for collagen fibers that connect to the alveolar bone and fix the teeth into the alveolar sockets. Transmembrane ionic signaling, associated with ionic transporters, regulate various physiological processes in a wide variety of cells. However, the properties of the signals generated by plasma membrane ionic channels in cementoblasts have not yet been described in detail. We investigated the biophysical and pharmacological properties of ion channels expressed in human cementoblast (HCEM) cell lines by measuring ionic currents using conventional whole-cell patch-clamp recording. The application of depolarizing voltage steps in 10 mV increments from a holding potential (Vh) of −70 mV evoked outwardly rectifying currents at positive potentials. When intracellular K⁺ was substituted with an equimolar concentration of Cs⁺, the outward currents almost disappeared. Using tail current analysis, the contributions of both K⁺ and background Na⁺ permeabilities were estimated for the outward currents. Extracellular application of tetraethylammonium chloride (TEA) and iberiotoxin (IbTX) reduced the densities of the outward currents significantly and reversibly, whereas apamin and TRAM-34 had no effect. When the Vh was changed to −100 mV, we observed voltage-dependent inward currents in 30% of the recorded cells. These results suggest that HCEM express TEA- and IbTX-sensitive large-conductance Ca²⁺-activated K⁺ channels and voltage-dependent Na⁺ channels.

Skeletal Functions of Voltage Sensitive Calcium Channels

Voltage-sensitive calcium channels (VSCCs) are ubiquitous multimeric protein complexes that are necessary for the regulation of numerous physiological processes. VSCCs regulate calcium influx and various intracellular processes including muscle contraction, neurotransmission, hormone secretion, and gene transcription, with function specificity defined by the channel's subunits and tissue location. The functions of VSCCs in bone are often overlooked since bone is not considered an electrically excitable tissue. However, skeletal homeostasis and adaptation relies heavily on VSCCs. Inhibition or deletion of VSCCs decreases osteogenesis, impairs skeletal structure, and impedes anabolic responses to mechanical loading. RECENT FINDINGS: While the functions of VSCCs in osteoclasts are less clear, VSCCs have distinct but complementary functions in osteoblasts and osteocytes. PURPOSE OF REVIEW: This review details the structure, function, and nomenclature of VSCCs, followed by a comprehensive description of the known functions of VSCCs in bone cells and their regulation of bone development, bone formation, and mechanotransduction.

Voltage-Gated Calcium Channels in Nonexcitable Tissues

The identification of a gain-of-function mutation in CACNA1C as the cause of Timothy syndrome, a rare disorder characterized by cardiac arrhythmias and syndactyly, highlighted roles for the L-type voltage-gated Ca2+ channel CaV1.2 in nonexcitable cells. Previous studies in cells and animal models had suggested that several voltage-gated Ca2+ channels (VGCCs) regulated critical signaling events in various cell types that are not expected to support action potentials, but definitive data were lacking. VGCCs occupy a special position among ion channels, uniquely able to translate membrane excitability into the cytoplasmic Ca2+ changes that underlie the cellular responses to electrical activity. Yet how these channels function in cells not firing action potentials and what the consequences of their actions are in nonexcitable cells remain critical questions. The development of new animal and cellular models and the emergence of large data sets and unbiased genome screens have added to our understanding of the unanticipated roles for VGCCs in nonexcitable cells. Here, we review current knowledge of VGCC regulation and function in nonexcitable tissues and cells, with the goal of providing a platform for continued investigation.

Biophysical characterization of nanostructured TiO2 as a good substrate for hBM-MSC adhesion, growth and differentiation

Mesenchymal stem cells from human bone marrow (hBM-MSC) are widely utilized for clinical applications involving bone healing. In this context, their use has been often optimized in association to variously designed titanium substrates, being this material of great use in orthopaedic implants. According to recent findings, the ability of hBM-MSC to differentiate towards a specific lineage is not only driven by biochemical signals, but physical stimuli, such as rigidity or roughness of the substrate, can also support a commitment towards osteogenic differentiation. Moreover, the presence of features with defined dimensional scales, in particular nanometer-size, also proved to elicit specific biological effects. Here we evaluated the effectiveness of a nano-patterned titanium surface in sustaining hBM-MSC adhesion, growth and differentiation by means of a panel of biophysical tools: morphometry, electrophysiology, intracellular calcium measurements and immunocytochemistry. The results substantiate the idea that this micro-textured titanium dioxide is a good surface for growth and differentiation of hBM-MSC and it exhibits a stimulating action mainly in the initial period of differentiation. Moreover, the basal concentration of free cytosolic Calcium [Ca²⁺]_i is confirmed to be a good hallmark of the hBM-MSC maturation stage. The study could provide relevant hints to help improving the biocompatibility and osteointegration potential of clinical titanium implants.

Roles of L-type calcium channels (CaV1.2) and the distal C-terminus (DCT) in differentiation and mineralization of rat dental apical papilla stem cells (rSCAPs)

OBJECTIVE

Voltage-gated inward Ca²⁺ currents (ICa) are triggered by cell depolarization and commonly produce transient increases in the cytoplasmic free Ca²⁺ concentration. The Ca_V1.2 distal C-terminus is susceptible to proteolytic cleavage, which yields a truncated Ca_V1.2 subunit and a cleaved C-terminal fragment (CCt or DCT). Stem cells from the apical papilla (SCAPs) has a capacity for differentiation into the odontoblastic-like cells in vitro and dentin forming in vivo, which makes SCAPs advantages in tissue engineering and regenerative endodontic. The aim of this study was to investigate the effect of Ca_V1.2 and its distal C-terminal fragment in the odontoblastic differentiation of rat SCAPs (stem cells from the apical papilla).

DESIGN

In this study, we generated stable Ca_V1.2 knockdown and DCT over-expressed rSCAPs using short hairpin RNA and DCT gene containing Lentivirus vectors, respectively. The transfected apical papilla cells were induced to differentiate into the odontoblast-like cells, and the expression of markers for odontoblastic differentiation were analyzed by alizarin red staining, Real-time Polymerase chain reaction (RT-PCR), and Western blot analysis.

RESULTS

The knockdown of Ca_V1.2 and excess expression of DCT both suppressed the expression of DSPP, ALP in mRNA level and the formation of calcium nodules.

CONCLUSIONS

Our results suggest that Ca_V1.2 and DCT play important roles in the differentiation of rSCAPs, DCT might act as a transcription factor and regulate the differentiation of rSCAPs.

Membrane depolarization regulates intracellular RANKL transport in non-excitable osteoblasts

Parathyroid hormone (PTH) and 1α,25-dihydroxyvitamin D3 (VD3) are important factors in Ca(2+) homeostasis, and promote osteoclastogenesis by modulating receptor activator of nuclear factor kappa-B ligand (RANKL) mRNA expression. However, their contribution to RANKL intracellular transport (RANKLiT), including the trigger for RANKL lysosomal vesicle (RANKL-lv) fusion to the cell membrane, is unclear. In neurons, depolarization of membrane potential increases the intracellular Ca(2+) level ([Ca(2+)]i) and promotes neurotransmitter release via fusion of the synaptic vesicles to the cell membrane. To determine whether membrane depolarization also regulates cellular processes such as RANKLiT in MC3T3-E1 osteoblasts (OBs), we generated a light-sensitive OB cell line and developed a system for altering their membrane potential via delivery of a blue light stimulus. In the membrane fraction of RANKL-overexpressing OBs, PTH and VD3 increased the membrane-bound RANKL (mbRANKL) level at 10 min after application without affecting the mRNA expression level, and depolarized the cell membrane while transiently increasing [Ca(2+)]i. In our novel OB line stably expressing the channelrhodopsin-wide receiver, blue light-induced depolarization increased the mbRANKL level, which was reversed by treatment of blockers for L-type voltage-gated Ca(2+) channels and Ca(2+) release from the endoplasmic reticulum. In co-cultures of osteoclast precursor-like RAW264.7 cells and light-sensitive OBs overexpressing RANKL, light stimulation induced an increase in tartrate-resistant acid phosphatase activity and promoted osteoclast differentiation. These results indicate that depolarization of the cell membrane is a trigger for RANKL-lv fusion to the membrane and that membrane potential contributes to the function of OBs. In addition, the non-genomic action of VD3-induced RANKL-lv fusion included the membrane-bound VD3 receptor (1,25D3-MARRS receptor). Elucidating the mechanism of RANKLiT regulation by PTH and VD3 will be useful for the development of drugs to prevent bone loss in osteoporosis and other bone diseases.

Electro-magnetic field promotes osteogenic differentiation of BM-hMSCs through a selective action on Ca(2+)-related mechanisms

Exposure to Pulsed Electromagnetic Field (PEMF) has been shown to affect proliferation and differentiation of human mesenchymal stem cells derived from bone marrow stroma (BM-hMSC). These cells offer considerable promise in the field of regenerative medicine, but their clinical application is hampered by major limitations such as poor availability and the time required to differentiate up to a stage suitable for implantation. For this reason, several research efforts are focusing on identifying strategies to speed up the differentiation process. In this work we investigated the in vitro effect of PEMF on Ca²⁺-related mechanisms promoting the osteogenic differentiation of BM-hMSC. Cells were daily exposed to PEMF while subjected to osteogenic differentiation and various Ca²⁺-related mechanisms were monitored using multiple approaches for identifying functional and structural modifications related to this process. The results indicate that PEMF exposure promotes chemically induced osteogenesis by mechanisms that mainly interfere with some of the calcium-related osteogenic pathways, such as permeation and regulation of cytosolic concentration, leaving others, such as extracellular deposition, unaffected. The PEMF effect is primarily associated to early enhancement of intracellular calcium concentration, which is proposed here as a reliable hallmark of the osteogenic developmental stage.

Smart electroconductive bioactive ceramics to promote in situ electrostimulation of bone

Biomaterials can still be reinvented to become simple and universal bone regeneration solutions. Following this roadmap, conductive CNT-based "smart" materials accumulate exciting grafting qualities for tuning the in vitro cellular phenotype. Biphasic electrical stimulation of human osteoblastic cells was performed in vitro on either dielectric bioactive bone grafts or conductive CNT-reinforced composites. The efficiency of the electrical stimuli delivery, as well as the effect of stimulation on cellular functions were investigated. Conductive substrates boosted the local culture medium conductivity and the confinement of the exogenous electrical fields. Hence, bone cell proliferation, DNA content and mRNA expression were maximized on the conductive substrates yielding superior stimuli delivering efficiency over dielectric ones. These findings are suggestive that bioactive bone grafts with electrical conductivity are capable of high spatial and temporal control of bone cell stimulation.

L-type calcium channels play a crucial role in the proliferation and osteogenic differentiation of bone marrow mesenchymal stem cells

L-type voltage-dependent Ca(2+) channels (VDCC(L)) play an important role in the maintenance of intracellular calcium homeostasis, and influence multiple cellular processes. They have been confirmed to contribute to the functional activities of osteoblasts. Recently, VDCC(L) expression was reported in mesenchymal stem cells (MSCs), but the role of VDCC(L) in MSCs is still undetermined. The aim of this study was to determine whether VDCC(L) may be regarded as a new regulator in the proliferation and osteogenic differentiation of rat MSC (rMSCs). In this study, we examined functional Ca(2+) currents (I(Ca)) and mRNA expression of VDCC(L) in rMSCs, and then suppressed VDCC(L) using nifedipine (Nif), a VDCC(L) blocker, to investigate its role in rMSCs. The proliferation and osteogenic differentiation of MSCs were analyzed by MTT, flow cytometry, alkaline phosphatase (ALP), Alizarin Red S staining, RT-PCR, and real-time PCR assays. We found that Nif exerts antiproliferative and apoptosis-inducing effects on rMSCs. ALP activity and mineralized nodules were significantly decreased after Nif treatment. Moreover, the mRNA levels of the osteogenic markers, osteocalcin (OCN), bone sialoprotein (BSP), and runt-related transcription factor 2 (Runx2), were also down-regulated. In addition, we transfected α1C-siRNA into the cells to further confirm the role of VDCC(L) in rMSCs, and a similar effect on osteogenesis was found. These results suggest that VDCC(L) plays a crucial role in the proliferation and osteogenic differentiation of rMSCs.

Voltage-dependent calcium and chloride currents in S17 bone marrow stromal cell line

The bone marrow stromal cell line S17 has been used to study hematopoiesis in vitro. In this study, we demonstrate the presence of calcium and chloride currents in cultured S17 cells. Calcium currents were of low amplitude or barely detectable (50-100 pA). Hence to amplify the currents, we have used barium as a charge carrier. Barium currents were identified based on their distinct voltage-dependence, and sensitivity to dihydropyridines. S17 cells also exhibited a slowly activating outward current without inactivation, most commonly seen when the sodium of the extracellular solution was replaced either by TEA (TEA/Cs saline) or NMDG (NMDG saline), or by addition of amiloride to the extracellular solution. This current was abolished either by 500 microM SITS (4,4'-diisothiocyanatostilbene-2-2'-disulfonic acid) or 500 microM DPC (diphenylamine-2-carboxylic acid) a cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel blocker, identifying it as a Cl(-) current. RT-PCR identified the presence of ENaC and CFTR transcripts. CFTR blockade reduced cell proliferation, suggesting that this channel plays a physiological role in regulation of S17 cell proliferation.

VOCCs and TREK-1 ion channel expression in human tenocytes

Mechanosensitive and voltage-gated ion channels are known to perform important roles in mechanotransduction in a number of connective tissues, including bone and muscle. It is hypothesized that voltage-gated and mechanosensitive ion channels also may play a key role in some or all initial responses of human tenocytes to mechanical stimulation. However, to date there has been no direct investigation of ion channel expression by human tenocytes. Human tenocytes were cultured from patellar tendon samples harvested from five patients undergoing routine total knee replacement surgery (mean age: 66 yr; range: 63-73 yr). RT-PCR, Western blotting, and whole cell electrophysiological studies were performed to investigate the expression of different classes of ion channels within tenocytes. Human tenocytes expressed mRNA and protein encoding voltage-operated calcium channel (VOCC) subunits (Ca alpha(1A), Ca alpha(1C), Ca alpha(1D), Ca alpha(2)delta(1)) and the mechanosensitive tandem pore domain potassium channel (2PK(+)) TREK-1. They exhibit whole cell currents consistent with the functional expression of these channels. In addition, other ionic currents were detected within tenocytes consistent with the expression of a diverse array of other ion channels. VOCCs and TREK channels have been implicated in mechanotransduction signaling pathways in numerous connective tissue cell types. These mechanisms may be present in human tenocytes. In addition, human tenocytes may express other channel currents. Ion channels may represent potential targets for the pharmacological management of chronic tendinopathies.

Characterizing the efficacy of calcium channel agonist-release strategies for bone tissue engineering applications

We have previously reported on the use of Bay K8644-release strategies in combination with perfusion-compression bioreactor systems for up regulating bone formation in three-dimensional PLLA scaffolds. Here we report on the analysis of Bay activity following its release from our PLLA scaffolds over the culture period imposed in our tissue engineering protocol using UV spectroscopy in combination with whole cell patch clamping techniques. Bay was released continually from scaffolds within the physiological range required for agonist activity (1-10 microM). Patch clamping allowed for the effects of Bay released from scaffolds to be monitored directly with respect to osteoblast electrophysiology. A characteristic shift in the current-voltage (I-V) relationship of L-type VOCC currents was observed in rat osteoblast sarcoma (ROS) cells patched in a solution with Bay released from scaffolds following 14 and 28 days incubation, with statistically significant differences observed in peak currents compared to non-Bay controls. An increase in the magnitude of the peak inward currents was also noted. The electrophysiological response of osteoblasts in the presence of Bay released from scaffolds demonstrates that the released Bay is stable and maintains its bioactivity following culture of up to 28 days.

Calcium/calmodulin signaling controls osteoblast growth and differentiation

Ca2+ is a ubiquitous intracellular messenger responsible for controlling numerous cellular processes including fertilization, mitosis, neuronal transmission, contraction and relaxation of muscles, gene transcription, and cell death. At rest, the cytoplasmic Ca2+ concentration [Ca2+]i is approximately 100 nM, but this level rises to 500-1,000 nM upon activation. In osteoblasts, the elevation of [Ca2+]i is a result of an increase in the release of Ca2+ from endoplasmic reticulum and/or extracellular Ca2+ influx through voltage gated Ca2+ channels. Many of the cellular effects of Ca2+ are mediated by the Ca2+ binding protein, calmodulin (CaM). Upon binding up to four calcium ions, CaM undergoes a conformational change, which enables it to bind to specific proteins eliciting a specific response. Calmodulin kinase II (CaMKII) is a major target of the Ca(2+)/CaM second messenger system. Once bound to Ca(2+)/CaM, the multimeric CaMKII is released from its autoinhibitory status and maximally activated, which then leads to an intraholoenzyme autophosphorylation reaction. Calcineurin (Cn) is another major target protein that is activated by Ca(2+)/CaM. Cn is a serine-threonine phosphatase that consists of a heterodimeric protein complex composed of a catalytic subunit (CnA) and a regulatory subunit (CnB). Upon activation, Cn directly binds to, and dephosphorylates nuclear factor of activated T cells (NFAT) transcription factors within the cytoplasm allowing them to translocate to the nucleus and participate in the regulation of gene expression. This review will examine the potential mechanisms by which calcium, CaM, CaMKII, and Cn/NFAT control osteoblast proliferation and differentiation.

1alpha,25(OH)2 vitamin D3 actions on ion channels in osteoblasts

Membrane-initiated cellular responses to steroids include modulation of ion channel activities via signal transduction pathways. However, the molecular mechanisms involved in nongenomic actions remain only partially understood. Our research has focused on the rapid effects of 1alpha,25(OH)(2) Vitamin D(3) [1,25D] on L-type Ca(2+) [L-Ca] and DIDS-sensitive Cl(-) channels in osteoblasts. Physiological nanomolar concentrations of hormonally active 1,25D promote rapid (1-5 min) potentiation of outward Cl(-) currents in osteosarcoma ROS 17/2.8 cells and mouse primary osteoblasts. In addition, 1,25D increases inward barium currents through L-Ca channels at low depolarizing potentials within seconds in a fashion similar to the 1,4-dihydropyridine [DHP] agonist Bay K8644. We found that second messenger cAMP is involved in 1,25D potentiation of Cl(-) and Ca(2+) channels. Nongenomic 1,25D effects on ion channel activities in osteoblasts appear to involve different mechanisms that include a possible direct interaction with the L-Ca channel molecule, on one hand, and signaling through the cAMP pathway, on the other. Rapid 1,25D actions on Cl(-) and Ca(2+) currents seem to couple to secretory activities in osteoblasts, thus contributing to bone mass formation.

Bone cell proliferation on carbon nanotubes

We explored the use of carbon nanotubes (CNTs) as suitable scaffold materials for osteoblast proliferation and bone formation. With the aim of controlling cell growth, osteosarcoma ROS 17/2.8 cells were cultured on chemically modified single-walled (SW) and multiwalled (MW) CNTs. CNTs carrying neutral electric charge sustained the highest cell growth and production of plate-shaped crystals. There was a dramatic change in cell morphology in osteoblasts cultured on MWNTs, which correlated with changes in plasma membrane functions.

Magnetic micro- and nanoparticle mediated activation of mechanosensitive ion channels

Most cells are known to respond to mechanical cues, which initiate biochemical signalling pathways and play a role in cell membrane electrodynamics. These cues can be transduced either via direct activation of mechanosensitive (MS) ion channels or through deformation of the cell membrane and cytoskeleton. Investigation of the function and role of these ion channels is a fertile area of research and studies aimed at characterizing and understanding the mechanoactive regions of these channels and how they interact with the cytoskeleton are fundamental to discovering the specific role that mechanical cues play in cells. In this review, we will focus on novel techniques, which use magnetic micro- and nanoparticles coupled to external applied magnetic fields for activating and investigating MS ion channels and cytoskeletal mechanics.

Osteoblast Ca(2+) permeability and voltage-sensitive Ca(2+) channel expression is temporally regulated by 1,25-dihydroxyvitamin D(3)

The cardiac subtype of the L-type voltage-sensitive Ca(2+) channel (VSCC) Cav1.2 (alpha(1C)) is the primary voltage-sensitive channel responsible for Ca(2+) influx into actively proliferating osteoblasts. This channel also serves as the major transducer of Ca(2+) signals in growth-phase osteoblasts in response to hormone treatment. In this study, we have demonstrated that 24-h treatment of MC3T3-E1 preosteoblasts with 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], a coupling factor for bone resorption, coordinately downregulates Cav1.2 (alpha(1C)) and uniquely upregulates T-type channel Cav3.2 (alpha(1H)). No other voltage-sensitive channel alpha-subunit of the 10 that were surveyed was upregulated by 1,25(OH)(2)D(3). The shift from predominantly L-type to T-type channel expression has been demonstrated to occur at both mRNA and protein levels detected using quantitative PCR and immunohistochemistry with antibodies specific for each channel type. Functional and pharmacological studies using specific inhibitors have revealed that treatment with 1,25(OH)(2)D(3) also alters the Ca(2+) permeability properties of the osteoblast membrane from a state of primarily L-current sensitivity to T-current sensitivity. We conclude that the L-type channel is likely to support proliferation of osteoblast cells, whereas T-type channels are more likely to be involved in supporting differentiated functions after 1,25(OH)(2)D(3)-mediated reversal of remodeling has occurred. This latter observation is consistent with the unique expression of the T-type VSCC Cav3.2 (alpha(1H)) in terminally differentiated osteocytes as we recently reported.

Expression of the mechanosensitive 2PK+ channel TREK-1 in human osteoblasts

TREK-1 is a mechanosensitive member of the two-pore domain potassium channel family (2PK+) that is also sensitive to lipids, free fatty acids (including arachidonic acid), temperature, intracellular pH, and a range of clinically relevant compounds including volatile anaesthetics. TREK-1 is known to be expressed at high levels in excitable tissues, such as the nervous system, the heart and smooth muscle, where it is believed to play a prominent role in controlling resting cell membrane potential and electrical excitability. In this report, we use RT-PCR, Western blotting and immunohistochemistry to confirm that human derived osteoblasts and MG63 cells express TREK-1 mRNA and protein. In addition, we show gene expression of TREK2c and TRAAK channels. Furthermore, whole cell patch clamp electrophysiology demonstrates that these cells express a spontaneously active, outwardly rectifying potassium "background leak" current that shares many similarities to TREK-1. The outward current is largely insensitive to TEA and Ba2+, and is sensitive to application of lysophosphatidylcholine (LPC). In addition, blocking TREK-1 channel activity is shown to upregulate bone cell proliferation. It is concluded that human osteoblasts functionally express TREK-1 and that these channels contribute, at least in part, to the resting membrane potential of human osteoblast cells. We hypothesise a possible role for TREK-1 in mechanotransduction, leading to bone remodelling.

Multiple molecular mechanisms of 1 alpha,25(OH)2-vitamin D3 rapid modulation of three ion channel activities in osteoblasts

Rapid nongenomic responses to steroids include modulation of ion channel activities on the cell membrane of target cells, but little is known about the molecular mechanisms involved. In this paper we investigate the mechanisms underlying the combined action of the secosteroid hormone 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)(2)D3] on three different ion channel types in rat osteoblasts, which include a voltage-gated L-type Ca(2+) channel, a mechanosensitive Cl(-) channel, and a stretch-activated cation (SA-Cat) channel. We found that physiological nanomolar concentrations of 1alpha,25(OH)(2)D3 rapidly modify the overall electrical activity of the membrane in ROS 17/2.8 cells. 1alpha,25(OH)(2)D3 increases the osteoblast L-type Ca(2+) channel activity at low depolarizing voltages in a fashion similar to the 1,4-dihydropyridine (DHP) agonist Bay K8644. At highly depolarizing potentials 1alpha,25(OH)(2)D3 potentiates volume-sensitive Cl(-) currents through mechanisms that may involve a putative membrane receptor. We show for the first time that 1alpha,25(OH)(2)D3 also increases inward currents through SA-Cat channels at positive membrane voltages in a dose-dependent manner. Contrary to our expectations, the stereoisomer 1beta,25(OH)(2)D3, which suppresses 1alpha,25(OH)(2)D3 activation of osteoblast Cl(-) currents, mimicked 1alpha,25(OH)(2)D3 agonist effects on Ca(2+) and SA-Cat channel activities. Cyclic AMP is involved in 1alpha,25(OH)(2)D3 effects on both Ca(2+) and SA-Cat channels, but not in Cl(-) channels. We conclude that 1alpha,25(OH)(2)D3 rapid effects on ion channel activities in ROS 17/2.8 cells occur through multiple mechanisms that, on the one hand, involve a possible direct interaction with the L-type Ca(2+) channel molecule and, on the other hand, molecular pathways that may include a putative membrane receptor.

Identification and localization of T-type voltage-operated calcium channel subunits in human male germ cells. Expression of multiple isoforms

Low voltage activated, voltage-operated Ca(2+) channels are expressed in rodent male germ cells and are believed to be pivotal in induction of the acrosome reaction in mouse spermatozoa. However, in humans, very little is known about expression of voltage-operated Ca(2+) channels in male germ cells or their function. We have used reverse transcription-polymerase chain reaction, in situ hybridization, and patch clamp recording to investigate the expression of low voltage activated voltage-operated Ca(2+) channels in human male germ cells. We report that full-length transcripts for both alpha(1G) and alpha(1H) low voltage activated channel subunits are expressed in human testis. Multiple isoforms of alpha(1G) are present in the testis and at least two isoforms of alpha(1H), including a splice variant not previously described in the human. Transcripts for all the isoforms of both alpha(1G) and alpha(1H) were detected by reverse transcription-polymerase chain reaction on mRNA isolated from human spermatogenic cells. In situ hybridization for alpha(1G) and alpha(1H) localized transcripts both in germ cells and in other cell types in the testis. Within the seminiferous tubules, alpha(1H) was detected primarily in germ cells. Using the whole cell patch clamp technique, we detected T-type voltage-operated Ca(2+) channel currents in isolated human male germ cells, although the current amplitude and frequency of occurrence were low in comparison to the occurrence of T-currents in murine male germ cells. We conclude that low voltage activated voltage-operated Ca(2+) channels are expressed in cells of the human male germ line.

Electrophysiology of mammalian Schwann cells

Schwann cells are the satellite cell of the peripheral nervous system, and they surround axons and motor nerve terminals. The review summarises evidence for the ion channels expressed by mammalian Schwann cells, their molecular nature and known or speculated functions. In addition, the recent evidence for gap junctions and cytoplasmic diffusion pathways within the myelin and the functional consequences of a lower-resistance myelin sheath are discussed. The main types of ion channel expressed by Schwann cells are K(+) channels, Cl(-) channels, Na(+) channels and Ca(2+) channels. Each is represented by a variety of sub-types. The molecular and biophysical characteristics of the cation channels expressed by Schwann cells are closely similar or identical to those of channels expressed in peripheral axons and elsewhere. In addition, Schwann cells express P(2)X ligand-gated ion channels. Possible in vivo roles for each ion channel type are discussed. Ion channel expression in culture could have a special function in driving or controlling cell proliferation and recent evidence indicates that some Ca(2+) channel and Kir channel expression in culture is dependent upon the presence of neurones and local electrical activity.

Potentiation by stannous chloride of calcium entry into osteoblastic MC3T3-E1 cells through voltage-dependent L-type calcium channels

The present study was undertaken to confirm that L-type Ca(2+) channels are involved in Ca(2+) entry into osteoblastic MC3T3-E1 cells and to examine the effect of SnCl2, a Ca(2+)]-channel activator, on the intracellular Ca(2+)concentration ([Ca(2+)]i). High K(+)concentration-dependently raised the [Ca(2+)]i. All of the L-type Ca(2+)channel blockers used here, such as nifedipine, nicardipine, verapamil, and diltiazem, and CdCl2 (a non-selective blocker) inhibited the high K(+)-induced [Ca(2+)]i rise, but v-conotoxin GVIA (an N-type blocker) and NiCl2(a T-type blocker) had no effect. Application of SnCl2 alone did not change the [Ca(2+)]i. However, in the presence of high K(+), SnCl2 enhanced the high K(+)-induced [Ca(2+)]i rise, which was inhibited by Ca(2+)]-free medium or nifedipine. In the case where high K(+)was applied prior to SnCl2, SnCl2 alone raised the [Ca(2+)]i by itself. In conclusion, MC3T3-E1 cells possess the voltage-dependent L-type Ca(2+)] channels and SnCl2 facilitates the Ca(2+) entry through the L-type ones under the condition of the membrane depolarization. There is the possibility that Ca(2+) release from intracellular Ca(2+) stores is involved in the action of SnCl2.

Hormonally-regulated expression of voltage-operated Ca(2+) channels in osteocytic (MLO-Y4) cells

Voltage operated calcium channels (VOCCs) are important in stimulus-response coupling in osteoblasts. We have investigated the expression of VOCCs in the mouse osteocyte cell line, MLO-Y4. Using the whole-cell patch clamp technique we were unable to detect any VOCC currents (n = 436) even in the presence of the L-type VOCC agonist Bay K 8644 (n = 350). Reverse transcription polymerase chain reaction (RT-PCR), using primers to detect alpha(1C), alpha(1D), and alpha(1G) VOCC subunits (all of which are expressed in primary osteoblasts), did not generate detectable products with mRNA from MLO-Y4 cells. However, after treatment with physiological levels of hormones, VOCC alpha(1) subunit mRNAs were detected in MLO-Y4 cells. PTH, 17beta-estradiol, and dexamethasone-treatment induced expression of L-type (alpha(1C), alpha(1D)) subunit transcripts. ATP-treatment induced expression of T-type (alpha(1G)) transcripts. Using whole-cell patch clamp we detected VOCC currents in 5-10% of cells after treatment. Current characteristics (L- or T-type) were consistent with the transcript expressed.

Parathyroid hormone enhances fluid shear-induced [Ca2+]i signaling in osteoblastic cells through activation of mechanosensitive and voltage-sensitive Ca2+ channels

Osteoblasts respond to both fluid shear and parathyroid hormone (PTH) with a rapid increase in intracellular calcium concentration ([Ca2+]i). Because both stimuli modulate the kinetics of the mechanosensitive cation channel (MSCC), we postulated PTH would enhance the [Ca2+]i response to fluid shear by increasing the sensitivity of MSCCs. After a 3-minute preflow at 1 dyne/cm2, MC3T3-E1 cells were subjected to various levels of shear and changes in [Ca2+]i were assessed using Fura-2. Pretreatment with 50 nM bovine PTH(1-34) [bPTH(1-34)] significantly enhanced the shear magnitude-dependent increase in [Ca2+]i. Gadolinium (Gd3+), an MSCC blocker, significantly inhibited the mean peak [Ca2+]i response to shear and shear + bPTH(1-34). Nifedipine (Nif), an L-type voltage-sensitive Ca2+ channel (VSCC) blocker, also significantly reduced the [Ca2+]i response to shear + bPTH(1-34), but not to shear alone, suggesting VSCC activation plays an interactive role in the action of these stimuli together. Activation of either the protein kinase C (PKC) or protein kinase A (PKA) pathways with specific agonists indicated that PKC activation did not alter the Ca2+ response to shear, whereas PKA activation significantly increased the [Ca2+]i response to lower magnitudes of shear. bPTH(1-34), which activates both pathways, induced the greatest [Ca2+]i response at each level of shear, suggesting an interaction of these pathways in this response. These data indicate that PTH significantly enhances the [Ca2+]i response to shear primarily via PKA modulation of the MSCC and VSCC.

Three types of K(+) currents in murine osteocyte-like cells (MLO-Y4)

Osteocytes play an important role in signaling within bone. Communication of osteocytes with each other and with bone lining cells may have a function in mineral homeostasis and mechanotransduction. However, very little is known of the expression of ion channels in these cells. Using the whole-cell patch-clamp technique, we have detected three types of K(+) currents in the mouse osteocyte-like cell line MLO-Y4. The most commonly observed current (48% of cells) activated rapidly (20 msec) in response to depolarizing steps from -40 mV and exhibited voltage-dependent inactivation. The current was inhibited by 20 mmol/L tetraethyl ammonium (TEA) and abolished by intracellular 2 mmol/L 4-aminopyridine (4-AP). Biophysical and pharmacological characteristics of the current differed from those of inactivating K(+) currents in osteoblastic cells. In 22% of cells, a slowly activating, voltage-activated current was observed (threshold at 20-30 mV). This current was TEA insensitive, was abolished by intracellular application of 2 mmol/L 4-AP, and was strongly inhibited by apamin, a selective inhibitor of small conductance (SK) Ca(2+)-activated K(+) channels. A third current developed during whole-cell dialysis (37% of cells). This current showed little voltage sensitivity. It was abolished by intracellular application of 2 mmol/L 4-AP, high-extracellular Ba(2+) (108 mmol/L), or by inclusion of ATP in the intracellular solution, but was insensitive to TEA, apamin, Cs(+), and glibenclamide. None of these currents was affected by replacement of chloride with acetate in the bath or pipette salines. Reverse-transcription polymerase chain reaction confirmed the presence of mRNA for the types 1 and 2 SK channels, but message for the large conductance (BK) Ca(2+)-activated K(+) channel was not detected in these cells. Message for the sulphonylurea receptor SUR2, a subunit of glibenclamide-insensitive ATP-dependent K(+) channels (K(ATP)), was also detected, but the glibenclamide-sensitive SUR1 subunit was not. These data are the first descriptions of SK- and ATP-sensitive, glibenclamide-insensitive channels in cells of osteoblastic lineage. Our findings are consistent with a change in K(+) channel expression during differentiation from osteoblast to osteocyte. K(+) channels of osteocytes will contribute to maintenance of the cell membrane potential and thus may participate in mechanosensitivity and osteocyte intercellular communication. In addition, they may be involved in homeostatic maintenance of the extracellular fluid occupying the periosteocytic space.

Calcium-channel activation and matrix protein upregulation in bone cells in response to mechanical strain

Femur-derived osteoblasts cultured from rat femora were loaded with Fluo-3 using the AM ester. A quantifiable stretch was applied and [Ca(2+)]i levels monitored by analysis of fluorescent images obtained using an inverted microscope and laser scanning confocal imaging system. Application of a single pulse of tensile strain via an expandable membrane resulted in immediate increase in [Ca(2+)]i in a proportion of the cells, followed by a slow and steady decrease to prestimulation levels. Application of parathyroid hormone (10(-6) M) prior to mechanical stimulation potentiated the load-induced elevation of [Ca(2+)]i. Mechanically stimulating osteoblasts in Ca(2+)-free media or in the presence of either nifedipine (10 microM; L-type Ca(2+)-channel blocker) or thapsigargin (1 microM; depletes intracellular Ca(2+) stores) reduced strain-induced increases in [Ca(2+) ]i. Furthermore, strain-induced increases in [Ca(2+)]i were enhanced in the presence of Bayer K 8644 (500 nm), an agonist of L-type calcium channels. The effects of mechanical strain with and without inhibitors and agonists are described on the total cell population and on single cell responses. Application of strain and strain in the presence of the calcium-channel agonist Bay K 8644 to periosteal-derived osteoblasts increased levels of the extracellular matrix proteins osteopontin and osteocalcin within 24 h postload. This mechanically induced increase in osteopontin and osteocalcin was inhibited by the addition of the calcium-channel antagonist, nifedipine. Our results suggest an important role for L-type calcium channels and a thapsigargin-sensitive component in early mechanical strain transduction pathways in osteoblasts.

Altered electrophysiological responses to mechanical stimulation and abnormal signalling through alpha5beta1 integrin in chondrocytes from osteoarthritic cartilage

OBJECTIVE

To establish whether chondrocytes from normal and osteoarthritic human articular cartilage recognize and respond to pressure induced mechanical strain in a similar manner.

DESIGN

Chondrocytes, extracted from macroscopically normal and osteoarthritic human articular cartilage obtained from knee joints at autopsy, were grown in monolayer culture and subjected to cyclical pressure-induced strain (PIS) in the absence or presence of anti-integrin antibodies, agents known to block ion channels and inhibitors of key molecules involved in the integrin-associated signalling pathways. The response of the cells to mechanical stimulation was assessed by measuring changes in membrane potential.

RESULTS

Unlike chondrocytes from normal articular cartilage, which showed a membrane hyperpolarization response to PIS, chondrocytes from osteoarthritic cartilage responded by membrane depolarization. The mechanotransduction pathway involves alpha5beta1 integrins, stretch-activated ion channels, tyrosine kinases and phospholipase C but the actin cytoskeleton and protein kinase C, which are important in the membrane hyperpolarization response in normal chondrocytes, are not necessary for membrane depolarization in osteoarthritic chondrocytes in response to PIS.

CONCLUSION

Chondrocytes derived from osteoarthritic cartilage show a different signalling pathway via alpha5beta1 integrin in response to mechanical stimulation which may be of importance in the production of phenotypic changes recognized to be present in diseased cartilage.

Ca(2+) regulates fluid shear-induced cytoskeletal reorganization and gene expression in osteoblasts

Osteoblasts subjected to fluid shear increase the expression of the early response gene, c-fos, and the inducible isoform of cyclooxygenase, COX-2, two proteins linked to the anabolic response of bone to mechanical stimulation, in vivo. These increases in gene expression are dependent on shear-induced actin stress fiber formation. Here, we demonstrate that MC3T3-E1 osteoblast-like cells respond to shear with a rapid increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) that we postulate is important to subsequent cellular responses to shear. To test this hypothesis, MC3T3-E1 cells were grown on glass slides coated with fibronectin and subjected to laminar fluid flow (12 dyn/cm(2)). Before application of shear, cells were treated with two Ca(2+) channel inhibitors or various blockers of intracellular Ca(2+) release for 0. 5-1 h. Although gadolinium, a mechanosensitive channel blocker, significantly reduced the [Ca(2+)](i) response, neither gadolinium nor nifedipine, an L-type channel Ca(2+) channel blocker, were able to block shear-induced stress fiber formation and increase in c-fos and COX-2 in MC3T3-E1 cells. However, 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, an intracellular Ca(2+) chelator, or thapsigargin, which empties intracellular Ca(2+) stores, completely inhibited stress fiber formation and c-fos/COX-2 production in sheared osteoblasts. Neomycin or U-73122 inhibition of phospholipase C, which mediates D-myo-inositol 1,4,5-trisphosphate (IP(3))-induced intracellular Ca(2+) release, also completely suppressed actin reorganization and c-fos/COX-2 production. Pretreatment of MC3T3-E1 cells with U-73343, the inactive isoform of U-73122, did not inhibit these shear-induced responses. These results suggest that IP(3)-mediated intracellular Ca(2+) release is required for modulating flow-induced responses in MC3T3-E1 cells.

Hormonal regulation of [Ca(2+)](i) in periosteal-derived osteoblasts: effects of parathyroid hormone, 1,25(OH)(2)D(3) and prostaglandin E(2)

The effects of hormonal modulators of osteoblast function, parathyroid hormone, 1,25(OH)(2)D(3) and prostaglandins on [Ca(2+)](i) in periosteal-derived osteoblasts from rat femurs have been investigated. Our results show that application of parathyroid hormone PTH (10(-5) M) and prostaglandin E(2) (PGE(2)) (4 microM) result in a rapid heterogeneous elevation in [Ca(2+)](i) that, in the case of PTH, is dependent on both extracellular and intracellular sources of calcium. Variable responses to treatments have been found within populations of cells. The PGE(2) response is dose dependent. Treatment with 1,25(OH)(2)D(3) (10(-8) M) induces a brief (60-90 sec) elevation in [Ca(2+)](i) that is almost totally abolished in EGTA-buffered Ca(2+)-free medium. Interactive effects of multiple hormone treatments have been observed. Pretreatment with 1,25(OH)(2)D(3) results in near-total inhibition of the PTH and PGE(2) responses. In conclusion, modulation of [Ca(2+)](i) appears to play a role not only in the direct effects of osteotropic hormones on osteoblasts but also in the synergistic and antagonistic effects between circulating hormones.

Ribozyme ablation demonstrates that the cardiac subtype of the voltage-sensitive calcium channel is the molecular transducer of 1, 25-dihydroxyvitamin D(3)-stimulated calcium influx in osteoblastic cells

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) stimulates transmembrane influx of Ca(2+) through L-type voltage-sensitive Ca(2+) channels (VSCCs) in ROS 17/2.8 osteoblastic cells. Ca(2+) influx modulates osteoblastic activities including matrix deposition, hormone responsiveness, and Ca(2+)-dependent signaling. 1, 25(OH)(2)D(3) also regulates transcript levels encoding VSCCs. L-type VSCCs are multisubunit complexes composed of a central pore-forming alpha(1) subunit and four additional subunits. The alpha(1) subunit is encoded by one gene in a multimember family, defining tissue-specific subtypes. Osteoblasts synthesize two splice variants of the alpha(1C) cardiac VSCC subtype; however, the molecular identity of the 1,25(OH)(2)D(3)-regulated VSCC remained unknown. We created a ribozyme specifically cleaving alpha(1C) mRNA. To increase target ablation efficiency, the ribozyme was inserted into U1 small nuclear RNA (snRNA) by engineering the U1 snRNA expression cassette, conferring the ribozyme transcript with stabilizing stem-loops at both sides and the Sm binding site that facilitates localization into nucleoplasm. After transfection of ROS 17/2.8 cells with U1 ribozyme-encoding vector, stable clonal cells were selected in which the expression of alpha(1C) transcript and protein were strikingly reduced. Ca(2+) influx assays in ribozyme transfectants showed selective attenuation of depolarization and 1, 25(OH)(2)D(3)-regulated Ca(2+) responses. We conclude that the cardiac subtype of the L-type VSCC is the transducer of stimulated Ca(2+) influx in ROS 17/2.8 osteoblastic cells.

Mechanotransduction pathways in bone: calcium fluxes and the role of voltage-operated calcium channels

Changes in strain distribution across the vertebrate skeleton induce modelling and remodelling of bone structure. This relationship, like many in biomedical science, has been recognised since the 1800s, but it is only the recent development of in vivo and in vitro models that is allowing detailed investigation of the cellular mechanisms involved. A number of secondary messenger pathways have been implicated in load transduction by bone cells, and many of these pathways are similar to those proposed for other load-responsive cell types. It appears that load transduction involves interaction between several messenger pathways, rather than one specific switch. Interaction between these pathways may result in a cascade of responses that promote and maintain bone cell activity in remodelling of bone. The paper outlines research on the early rapid signals for load transduction and, in particular, activation of membrane channels in osteoblasts. The involvement of calcium channels in the immediate load response and the modulation of intracellular calcium as an early signal are discussed. These membrane channels present a possible target for manipulation in the engineering of bone tissue repair.

Molecular characterization of the alpha1 subunit of the L type voltage calcium channel expressed in rat calvarial osteoblasts

Voltage-activated calcium channels (VACCs) regulate extracellular calcium influx in many cells. VACCs are composed of five subunits. The alpha1 subunit is considered the most important in regulating channel function. Three isoforms of this subunit have been described: skeletal, cardiac, and neuroendocrine. It was the purpose of the present study to determine the molecular identity of the alpha1 subunit of the VACCs in rat calvarial osteoblasts and to study the nature of the regulation of these channels as a function of cellular growth. We also attempted to identify which isoform of the alpha1 subunit of the VACCs mediates the effects of epidermal growth factor (EGF) on osteoblastic cell proliferation. Reverse transcription-polymerase chain reaction was used to detect the isoforms of the VACCs that are expressed in osteoblastic cells. These analyses showed that the proliferative state of the cell and the time in culture influence RNA expression. The only alpha1 subunit detected in osteoblasts corresponds to the cardiac isoform. In additional experiments, the effects of EGF on cytosolic calcium and osteoblast proliferation were determined. For these experiments, the synthesis of the different isoforms of the VACCs was selectively blocked by antisense oligonucleotides prior to EGF stimulation. These studies showed that the cardiac isoform mediates the effects of EGF on cytosolic calcium and cellular proliferation in rat calvarial osteoblasts.

Electrophysiological characterization of voltage-gated Na+ current expressed in the highly metastatic Mat-LyLu cell line of rat prostate cancer

Voltage-gated Na+ channels, classically associated with impulse conduction in excitable tissues, are also found in a variety of epithelial cell types where their possible functions are not known so well. We have previously reported expression of a voltage-gated Na+ channel specifically in the highly metastatic Mat-LyLu rat prostate cancer cell line; blockage of the current with tetrodotoxin (TTX) significantly reduced the invasiveness of the cells in vitro, suggesting that the channel may have a functional role in metastasis. The aim of the present study was to characterize this current using the whole-cell patch clamp recording technique, and compare it to Na+ currents found in various other tissues. The inward current of the Mat-LyLu cells was abolished completely, but reversibly, in Na+-free solution, confirming that Na+ was indeed the permeant ion. Activation occurred at -40 mV and currents reached a maximal amplitude at around 6 mV. Boltzmann fits to current activation and steady-state inactivation revealed that the currents were half activated at about -15 mV and half inactivated at -80 mV. Both current inactivation and recovery from inactivation followed a double-exponential time course with fast and slow components. The Na+ currents were highly sensitive to block by TTX (IC50 approximately 18 nM), whilst 1 microM mu-conotoxin GIIIA mostly had no effect. 100 microM Cd2+ also had no effect on the current, whilst 2.5 mM Cd2+, Mn2+, and Co2+ each caused a depolarizing shift in activation and a reduction in peak conductance of around 20%. In conclusion, the Na+ channel expressed in the highly metastatic Mat-LyLu cell line appeared to have electrophysiological and pharmacological properties of TTX-sensitive channels. Further work is needed, however, to elucidate the exact nature of the channel protein and the mechanism(s) of its involvement in cellular invasiveness.

Stimulation by 1alpha,25(OH)2-vitamin D3 of whole cell chloride currents in osteoblastic ROS 17/2.8 cells. A structure-function study

1alpha,25-Dihydroxyvitamin D3 (1alpha,25(OH)2D3) can generate biological responses via genomic and nongenomic mechanisms. This article reports for the first time the effects of 1alpha,25(OH)2D3 and structurally related analogs on whole cell chloride currents in osteoblastic cells. 1alpha,25(OH)2D3 promoted the rapid enhancement of outwardly rectifying Cl- currents in 93% of the osteoblasts in a concentration-dependent manner, with a maximal increase of about 4-fold between 0.5 and 5 nM. This effect of 1alpha,25(OH)2D3 was blocked by 1 nM stereoisomer 1beta,25(OH)2D3 when added to the bath before 1alpha,25(OH)2D3. On the other hand, 1 nM of the 6-s-cis locked analog 1alpha,25(OH)2-lumisterol3 significantly increased by about 2.2-fold outward Cl- currents in the ROS 17/2.8 cells, whereas the increase promoted by same concentration of the 6-s-trans locked analog 1alpha,25(OH)2-tachysterol (0.8-fold) was significantly lower, suggesting that the 6-s-cis locked or steroid-like form was preferred over the extended 6-s-trans conformer to promote these rapid effects of the hormone. We conclude that the agonist effects of 1alpha,25(OH)2D3 in osteoblasts at the cellular membrane level seem to be determined by some structural features of the molecule which may be crucial for its interaction with a putative membrane receptor in the cell surface.

1,25 (OH)2D3 enhances PTH-induced Ca2+ transients in preosteoblasts by activating L-type Ca2+ channels

We previously demonstrated electrophysiologically that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] shifts the activation threshold of L-type Ca2+ channels in osteoblasts toward the resting potential and prolongs mean open time. Presently, we used single-cell Ca2+ imaging to study the combined effects of 1,25(OH)2D3 and parathyroid hormone (PTH) during generation of Ca2+ transients in fura 2-loaded MC3T3-E1 cells. Pretreatment with 1,25(OH)2D3 concentrations, which alone did not produce Ca2+ transients, consistently enhanced Ca2+ responses to PTH. Enhancement was dose dependent over the range of 1 to 10 nM and was blocked by pretreatment with 5 microM nitrendipine during pretreatment. A 1,25(OH)2D3 analog that activates L-type channels and shifts their activation threshold also enhanced PTH responses. In contrast, an analog devoid of membrane Ca2+ effects did not enhance PTH-induced Ca2+ transients. The PTH-induced Ca2+ transient involved activation of a dihydropyridine-insensitive cation channel that was inhibited by Gd3+. Together, these data suggest that 1,25(OH)2D3 increases osteoblast responsiveness to PTH through rapid modification of L-type Ca2+ channel gating properties, whose activation enhances Ca2+ entry through other channels such as the PTH-responsive, Gd(3+)-sensitive cation channel.

The functional matrix hypothesis revisited. 2. The role of an osseous connected cellular network

Intercellular gap junctions permit bone cells to intercellularly transmit, and subsequently process, periosteal functional matrix information, after its initial intracellular mechanotransduction. In addition, gap junctions, as electrical synapses, underlie the organization of bone tissue as a connected cellular network, and the fact that all bone adaptation processes are multicellular. The structural and operational characteristics of such biologic networks are outlined and their specific bone cell attributes described. Specifically, bone is "tuned" to the precise frequencies of skeletal muscle activity. The inclusion of the concepts and databases that are related to the intracellular and intercellular bone cell mechanisms and processes of mechanotransduction and the organization of bone as a biologic connected cellular network permit revision of the functional matrix hypothesis, which offers an explanatory chain, extending from the epigenetic event of muscle contraction hierarchically downward to the regulation of the bone cell genome.

Electrophysiological responses of human bone cells to mechanical stimulation: evidence for specific integrin function in mechanotransduction

Bone cells respond to mechanical stimuli, but the transduction mechanisms responsible are not fully understood. Integrins, a family of heterodimeric transmembrane glycoproteins, which link components of the extracellular matrix with the actin cytoskeleton, have been implicated as mechanoreceptors. We have assessed the roles of integrins in the transduction of cyclical mechanical stimuli to human bone cells (HBCs), which results in changes in membrane potential. HBC showed membrane depolarization following 0.104 Hz mechanical stimulation and membrane hyperpolarization following stimulation at 0.33 Hz. The membrane depolarization response involved tetrodotoxin-sensitive sodium channels and could be inhibited by antibodies against alpha V, beta 1, and beta 5 integrins. In contrast, the hyperpolarization response was inhibited by gadolinium and antibodies to the integrin-associated protein (CD47), alpha 5 and beta 1 integrin. Both responses could be abrogated by ARg-Gly-Asp (RGD)-containing peptides, inhibition of tyrosine kinase activity, and disruption of the cytoskeleton. These results demonstrate differential electrophysiological responses of HBC to different frequencies of mechanical strain. Furthermore, they suggest that integrins act as HBC mechanoreceptors with distinct signaling pathways being activated by different frequencies of mechanical stimuli.

24R,25-(OH)2 vitamin D3 inhibits 1alpha,25-(OH)2 vitamin D3 and testosterone potentiation of calcium channels in osteosarcoma cells

Calcium influx via L-type calcium channels in osteoblast cells causes a rapid (in seconds) elevation in intracellular calcium initiated by plasma membrane receptors for 1alpha, 25-dihydroxyvitamin D3 (1alpha,25-D3). 24R,25-Dihydroxyvitamin D3 (24,25-D3) alone, in concentrations up to 200 nM, does not cause potentiation of calcium currents in osteoblasts, but it does inhibit the current potentiation by 1alpha,25-D3. To determine how various steroids interact in their potentiation of calcium channels, the action of vitamin D3 analogues and testosterone with calcium channels in the rat osteoblast-like cell line ROS 17/2.8 was investigated. Bath additions of both 1alpha,25-D3 and testosterone at doses below K1/2 (the dose causing 50% left shift in the current-voltage relationship) are additive in their ability to potentiate calcium channels. When 1alpha,25-D3 and testosterone are added together at concentrations that would cause a maximal shift in the current-voltage relationship by each agent alone (Vmax), the effect of these steroids is not additive. Taken together these data suggest one population of calcium channels is activated by 1alpha, 25-D3 or testosterone. The shift in the current-voltage relationship caused by 1alpha,25-D3 is reduced by 1beta,25-dihydroxyvitamin D3 (1beta,25-D3), an agent which is thought to act specifically on the plasma membrane receptor for 1alpha,25-D3, but the potentiation caused by testosterone is not blocked by 1beta,25-D3. However, 24, 25-D3 inhibits the left shift in the peak current-voltage relationship mediated by either 1alpha,25-D3 and testosterone. This result implies that 1) 1beta,25-D3 directly displaces 1alpha,25-D3 but not testosterone from its plasma membrane receptor, and 2) the rapid (in seconds) stimulatory effects of 1alpha,25-D3 and testosterone on calcium channels are mediated by separate plasma membrane receptors for testosterone and 1alpha,25-D3, which are blocked by another receptor for 24,25-D3. The interaction of these three receptors with L-type calcium channels is pertussis toxin-sensitive.

Voltage-gated ionic channels in cultured rabbit articular chondrocytes

The membrane properties of cultured cells of rabbit articular chondrocytes were studied using the whole-cell patch clamp technique. The average cell capacitance was 37.9 +/- 9.0 pF (n = 13), and the cell resting potential was -41.0 +/- 7.0 mV (n = 11). We were unable to induce an action potential by applying a depolarizing current. Upon step depolarization, under voltage clamp conditions, one kind of inward and two kinds of outward currents were elicited. The inward current was initially observed at around -30 mV, peaked at 0 mV, and reversed at around +90 mV. Tetrodotoxin (TTX; 1 microM) was shown to completely block this inward current. At steady state, the inward current was half-inactivated at -51 mV, with a slope factor of 6.3 mV. Two outward currents were determined from measurements of activation threshold, reversal potential, and pharmacological responses. One was observed at around -30 mV, and its amplitude increased with membrane depolarization. Extracellularly applied 4-aminopyridine (4 AP) (1 mM) and tetraethyl ammonium chloride (TEA) (5 mM) blocked this current. The other outward current was observed at around +10 mV, and its direction reversed at a potential close to that predicted by the Nernst equation for a Cl- selective channel. This current fluctuated markedly, and the fluctuation did not decline throughout the 100 ms of the step pulse. Extracellularly applied 4-acetamido-4'-isothiocyanostilbenezene-2,2-disulfonic acid (SITS) (0.25 mM) blocked this current, but the same dose of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) had little effect. These results suggest the presence of TTX-sensitive Na+, 4-AP- and TEA-sensitive K+, and SITS-sensitive Cl- channels in rabbit articular chondrocyte membrane. The functional significance of these channels is discussed.

Action potential-like responses in B104 cells with low Na+ channel densities

Action potential generation and Na+ currents were studied in B104 neuroblastoma cells in vitro using the whole-cell patch-clamp method in voltage-clamp and current-clamp mode. Action potential-like responses were elicited in 38 of 42 cells, with a threshold close to -55 mV for depolarizing stimuli, and -56 mV for anode-break stimuli. Response amplitudes were larger when cells were held at more negative prepulse potentials, and were well fit by a Boltzmann distribution with a midpoint of approx. -75 mV, close to the V1/2 for Na+ current steady-state inactivation in these cells. Cells displaying action potential-like responses exhibited a peak Na+ current density of 133 +/- 0.14 pA/pF (range, 10.2-296.2 pA/pF) and a low gK:gNa ratio (0.0067 +/- 0.0023). Exposure to 0.1 mM Cd2+ did not block the generation of action potential-like responses in B104 cells, while 1 microM TTX abolished the responses. We conclude that low densities of Na+ channels (< 3/microns2, and < 1/micron2 in some cells) can support the generation of action potential-like responses in B104 cells if they are held at hyperpolarized levels to remove inactivation. The low leak and K+ conductance of these cells may contribute to their ability to generate action potential-like responses under these circumstances.

Expression of voltage-operated Ca2+ channels in rat bone marrow stromal cells in vitro

The expression of voltage-operated Ca2+ currents (VOCCs) in bone marrow stromal cells cultured for 3-30 days has been studied by the use of the whole-cell patch-clamp technique. Both low-voltage-activated (LVA) and high-voltage-activated (HVA) VOCCs were recorded. LVA currents were first detectable after 6-7 days in culture and reached a peak of expression at 8 days, after which both the amplitude and frequency of expression of the current fell rapidly. The current was virtually undetectable in cells cultured for more than 15 days. The HVA current was detectable after 3 days in culture and reached a peak of both amplitude and frequency of expression after 1-2 weeks. This current was expressed consistently throughout the remaining culture period. In cultures treated with dexamethasone (10(-8) mol/L) peak expression of LVA currents still occurred at 7-8 days, but currents were enhanced approximately threefold. Expression of LVA currents was maintained to the end of the culture period. Expression of HVA currents was not significantly modified by treatment of cultures with dexamethasone. Examination of the biophysical and pharmacological (blockade by Ni2+ and diphenylhydantoin) properties of the LVA current in these cells suggests that they may have similarities with the LVA T currents of neuronal cells.

Voltage-dependent potentiation of low-voltage-activated Ca2+ channel currents in cultured rat bone marrow cells

1. The whole-cell patch-clamp technique was used to study Ca2+ channel currents in stromal cells of 7-10 day dexamethasone-treated and control rat bone marrow cultures. In saline containing either 108 mM Ba2+ or a 2.5 mM Ca(2+)-1 mM Mg2+ mixture, most cells expressed both fast-inactivating, low-voltage-activated (LVA) and slow-inactivating, high-voltage-activated (HVA) currents. 2. Repeated application of 400 ms voltage steps to 60 mV above the holding potential (Vh, -90 mV in Ca(2+)-Mg2+ mixture and -60 mV in Ba2+) at a frequency > or = 0.1 Hz resulted in a potentiation of the LVA component of the 2nd and subsequent currents. 3. LVA current potentiation was examined using a two-pulse (prepulse-test pulse) method. Prepulses to Vh + 150 mV induced an 80-100% increase in the amplitude of the LVA component of Ca2+ channel currents in saline containing either Ba2+ or Ca(2+)-Mg2+. This effect was also seen in non-dexamethasone-treated cultures. 4. Potentiation was not modified by omission of ATP and GTP from the pipette saline, and was not inhibited by extracellular application of the broad spectrum kinase inhibitors H-7 or RK252-a. 5. Potentiation was dependent on the amplitude and duration of the prepulse. Using the standard protocol, the threshold for potentiation was approximately Vh + 45 mV and saturation occurred at Vh + 150-180 mV. Further increases in prepulse amplitude did not modify potentiation. With a prepulse to +10 mV (Ba2+ saline) potentiation was half-maximal with a prepulse duration of 250-300 ms duration and saturated at 750-1000 ms. 6. Peak potentiation occurred 1-2 s after the prepulse. The time for total decay of potentiation varied from 10 to 90 s. 7. Voltage dependency of prepulse-induced potentiation did not resemble that of inactivation induced by similar prepulses. 8. Current kinetics, I-V relationship and sensitivity to blockade by Ni2+ and diphenylhydantoin of prepulse-recruited current resembled those of control LVA current. 9. The amplitude of prepulse-recruited current was positively correlated with control LVA current amplitude. 10. LVA currents supported regenerative potentials under current clamp. Repeated activation reduced spike latency. 11. It is suggested that current potentiation may be recruited physiologically, possibly in association with activation of stretch-sensitive channels, causing enhanced activation of HVA Ca2+ currents.

Type II brain sodium channel expression in non-neuronal cells: embryonic rat osteoblasts

Although voltage-sensitive sodium channels play a central role in electrogenesis in neurons, rat brain sodium channels are also present in some glial cells. To determine whether rat brain sodium channel alpha-subunit isotypes are expressed in other cell types, we examined osteoblasts within the embryonic day 17 (E17) vertebral column with in situ hybridization and immunocytochemical methods. For in situ hybridization studies, riboprobes hybridizing to isoform-specific sequences in the 3'-noncoding region of sodium channel mRNAs (NCI, NCII and NCIII) were utilized. Sodium channel mRNA I and III were not detectable in osteoblasts of the vertebra centrum or neural arches in E17 rats. In contrast, sodium channel mRNA II was moderately expressed by osteoblasts in the developing vertebral column of E17 rats. In immunocytochemical experiments, antipeptide antibodies directed against conserved and isotype-specific regions of the sodium channel alpha-subunit were used. Antibody SP20, which recognizes a conserved region of the sodium channel, intensely stains osteoblasts in both the vertebra centrum and neural arches. Antibody SP11-I, which recognizes sodium channel I, exhibited negligible-to-low levels of immunostaining in vertebral column osteoblasts. Osteoblasts reacted with antibody SP11-II, which recognizes sodium channel II, displayed moderate levels of immunostaining. Antibody SP32-III, which recognizes sodium channel III, displayed negligible levels of staining in osteoblasts within vertebra centrum and neural arches. These results demonstrate that osteoblasts in situ within E17 vertebral columns express sodium channel II mRNA and protein. Together with previous electrophysiological observations, the present results suggest that functional sodium channels are expressed in osteoblasts in vivo. These results extend the range of non-neuronal cells known to express rat brain sodium channels.

Multiple calcium channel transcripts in rat osteosarcoma cells: selective activation of alpha 1D isoform by parathyroid hormone

Osteoblasts express calcium channels that are thought to be involved in the transduction of extracellular signals regulating bone metabolism. The molecular identity of the pore-forming subunit (alpha 1) of L-type calcium channel(s) was determined in rat osteosarcoma UMR-106 cells, which express an osteoblast phenotype. A homology-based reverse transcriptase-polymerase chain reaction cloning strategy was employed that used primers spanning the fourth domain. Three types of cDNAs were isolated, corresponding to the alpha 1S (skeletal), alpha 1C (cardiac), and alpha 1D (neuroendocrine) isoforms. In the transmembrane segment IVS3 and the extracellular loop formed by the IVS3-S4 linker, a single pattern of mRNA splicing was found that occurs in all three types of calcium channel transcripts. Northern blot analysis revealed an 8.6-kb mRNA that hybridized to the alpha 1C probe and 4.8- and 11.7-kb mRNAs that hybridized to the alpha 1S and alpha 1D probes. Antisense oligonucleotides directed to the calcium channel alpha 1D transcript, but not those directed to alpha 1S or alpha 1C transcripts, inhibited the rise of intracellular calcium induced by parathyroid hormone. However, alpha 1D antisense oligonucleotides had no effect on the accumulation of cAMP induced by parathyroid hormone. When L-type calcium channels were activated with Bay K 8644, antisense oligonucleotides to each of the three isoforms partially inhibited the rise of intracellular calcium. The present results provide evidence for the expression of three distinct calcium channel alpha 1-subunit isoforms in an osteoblast-like cell line. We conclude that the alpha 1D isoform is selectively activated by parathyroid hormone.

Tetrodotoxin-sensitive fast Na+ current in embryonic chicken osteoclasts

A voltage-dependent, fast, transient inward current was characterized in embryonic chicken osteoclasts using the permeabilized patch configuration of the patch-clamp technique. The current was activated by depolarizations to higher than -28 +/- 4 mV from a holding potential of -80 mV. It peaked within 1-1.5 ms, and inactivated within 3.3-6.9 ms. The 50% inactivation voltage was -59 +/- 6 mV with a steepness factor of 0.11 +/- 0.06. The current disappeared with the removal of extracellular Na+ and was reversibly blocked by tetrodotoxin (K0.5 < 15 nM) but not by verapamil (< or = 100 microM). We conclude that this new current in embryonic chicken osteoclasts is a sodium current known from excitable cells.

Calcium currents in osteoblastic cells: dependence upon cellular growth stage

Patch clamp physiological techniques were used to characterize the voltage-activated calcium currents (VACC) expressed in the plasma membrane of osteoblastic cells as a function of time in culture and proliferative state of the cell. Osteoblast-enriched preparations were isolated by collagenase digestions of newborn rat calvaria and cultured under different conditions which affected cell proliferation (i.e., low serum in the media to arrest proliferation). VACC were isolated by replacing the intracellular potassium with cesium, and adding 1 microM tetrodotoxin to the bath. Under conditions that favored cell proliferation, low cell density, and media supplemented with 10% fetal calf serum (FCS), a transient calcium current was not expressed until day 3 in culture. There was a statistically significant relationship between the percentage of cells expressing this current and the time in culture. The magnitude of the current significantly increased as days in culture increased. Under the same conditions, the sustained VACC was detected after 7 or 8 days in culture. However, arresting cell proliferation after 2 days in culture by reducing the FCS concentration to 0.01% induced the expression of the sustained VACC the next day. The data suggest that the expression of VACC in the plasma membrane of rat calvarial osteoblasts depends on the time in culture and the state of proliferation of the cells. These results should prove to be valuable in studying the functional significance of VACC in osteoblastic cells and their regulation by various bone regulatory agents.

Dual modulation of the L-type calcium current of rat osteoblastic cells by parathyroid hormone: opposite effects of protein kinase C and cyclic nucleotides

Using whole-cell voltage-clamp recording of rat osteoblastic cells, we show that PTH-(1-34), known to stimulate protein kinase C (PKC) and adenylate cyclase, has a dual effect on the L-type calcium current. It induces a long-lasting increase and a superimposed reversible decrease, which can be separated by repeating hormone applications. The stimulatory effect is the only effect induced by the (3-34) fragment, able to stimulate PKC but unable to stimulate adenylate cyclase. The L current is stimulated by an active phorbol ester and is reduced by permeable analogues of cyclic AMP. Thus, the effect of PTH-(1-34) can be explained by the opposite effects of PKC and cyclic AMP. Dibutyryl cyclic GMP reduces the L current even more potently than dibutyryl cyclic AMP. The above modulations are all voltage-insensitive. These results led us to reinvestigate the effects of some vitamin D3 metabolites known to stimulate PKC and/or guanylate cyclase, and previously reported to affect the voltage-sensitivity of the L current. We only detected voltage-insensitive effects.