Down-regulation of L-type Ca2+ channel transcript levels by 1,25-dihyroxyvitamin D3. Osteoblastic cells express L-type alpha1C Ca2+ channel isoforms

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.

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.

Forskolin Regulates L-Type Calcium Channel through Interaction between Actinin 4 and β3 Subunit in Osteoblasts

Voltage-dependent L-type calcium channels that permit cellular calcium influx are essential in calcium-mediated modulation of cellular signaling. Although the regulation of voltage-dependent L-type calcium channels is linked to many factors including cAMP-dependent protein kinase A (PKA) activity and actin cytoskeleton, little is known about the detailed mechanisms underlying the regulation in osteoblasts. Our present study investigated the modulation of L-type calcium channel activities through the effects of forskolin on actin reorganization and on its functional interaction with actin binding protein actinin 4. The results showed that forskolin did not significantly affect the trafficking of pore forming α_1c subunit and its interaction with actin binding protein actinin 4, whereas it significantly increased the expression of β₃ subunit and its interaction with actinin 4 in osteoblast cells as assessed by co-immunoprecipitation, pull-down assay, and immunostaining. Further mapping showed that the ABD and EF domains of actinin 4 were interaction sites. This interaction is independent of PKA phosphorylation. Knockdown of actinin 4 significantly decreased the activities of L-type calcium channels. Our study revealed a new aspect of the mechanisms by which the forskolin activation of adenylyl cyclase - cAMP cascade regulates the L-type calcium channel in osteoblast cells, besides the PKA mediated phosphorylation of the channel subunits. These data provide insight into the important role of interconnection among adenylyl cyclase, cAMP, PKA, the actin cytoskeleton, and the channel proteins in the regulation of voltage-dependent L-type calcium channels in osteoblast cells.

Vitamin D deficiency impairs spatial learning in adult rats

BACKGROUND

Through its membrane and intracellular receptors, vitamin D regulates many vital functions in the body including its well known actions on musculoskeletal system. Growing body of evidences demonstrate that vitamin D undergoes some of behavioral aspects of neurocognition. The present study was designed to evaluate the effect of food regimens without vitamin D or with a supplement of 1,25(OH)2D3 on spatial performance of adult rats.

METHODS

The animals were trained in the Morris water maze to find a hidden platform. The time spent and the distance traveled to find the platform, speed of navigation and the percentage of unsuccessful trials were considered for assessment of the task learning.

RESULTS

Our findings indicated that the vitamin D-deprived rats had a significant lower performance compared to both the controls and the animals receiving 1,25(OH)2D3 supplementation. Concerning the unsuccessful trials, lack of vitamin D resulted in the highest failures in the maze navigation. The regimen with additional 1,25(OH)2D3 did not considerably influence learning of the maze task.

CONCLUSION

We concluded that while vitamin D deficiency deteriorates the spatial task learning, the 1,25(OH)2D3 supplementation did not effectively underlie the maze performance.

Dynamic interactions between L-type voltage-sensitive calcium channel Cav1.2 subunits and ahnak in osteoblastic cells

Voltage-sensitive Ca(2+) channels (VSCCs) mediate Ca(2+) permeability in osteoblasts. Association between VSCC alpha(1)- and beta-subunits targets channel complexes to the plasma membrane and modulates function. In mechanosensitive tissues, a 700-kDa ahnak protein anchors VSCCs to the actin cytoskeleton via the beta(2)-subunit of the L-type Ca(v)1.2 (alpha(1C)) VSCC complex. Ca(v)1.2 is the major alpha(1)-subunit in osteoblasts, but the cytoskeletal complex and subunit composition are unknown. Among the four beta-subtypes, the beta(2)-subunit and, to a lesser extent, the beta(3)-subunit coimmunoprecipitated with the Ca(v)1.2 subunit in MC3T3-E1 preosteoblasts. Fluorescence resonance energy transfer revealed a complex between Ca(v)1.2 and beta(2)-subunits and demonstrated their association in the plasma membrane and secretory pathway. Western blot and immunohistochemistry showed ahnak association with the channel complex in the plasma membrane via the beta(2)-subunit. Cytochalasin D exposure disrupted the actin cytoskeleton but did not disassemble or disrupt the function of the complex of L-type VSCC Ca(v)1.2 and beta(2)-subunits and ahnak. Similarly, small interfering RNA knockdown of ahnak did not disrupt the actin cytoskeleton but significantly impaired Ca(2+) influx. Collectively, we showed that Ca(v)1.2 and beta(2)-subunits and ahnak form a stable complex in osteoblastic cells that permits Ca(2+) signaling independently of association with the actin cytoskeleton.

The vitamin D receptor agonist elocalcitol upregulates L-type calcium channel activity in human and rat bladder

Human bladder contraction mainly depends on Ca2+ influx via L-type voltage-gated Ca2+ channels and on RhoA/Rho kinase contractile signaling, which is upregulated in overactive bladder (OAB). Elocalcitol is a vitamin D receptor agonist inhibiting RhoA/Rho kinase signaling in rat and human bladder. Since in the normal bladder from Sprague-Dawley rats elocalcitol treatment delayed the carbachol-induced contraction without changing maximal responsiveness and increased sensitivity to the L-type Ca2+ channel antagonist isradipine, we investigated whether elocalcitol upregulated L-type Ca2+ channels in human bladder smooth muscle cells (hBCs). In hBCs, elocalcitol induced a rapid increase in intracellular [Ca2+], which was abrogated by the L-type Ca2+ channel antagonist verapamil. Moreover, hBCs exhibited L-type voltage-activated Ca2+ currents (I Ca), which were selectively blocked by isradipine and verapamil and enhanced by the selective L-type agonist BAY K 8644. Addition of elocalcitol (10(-7) M) increased L-type I Ca size and specific conductance by inducing faster activation and inactivation kinetics than control and BAY K 8644, while determining a significant negative shift of the activation and inactivation curves, comparable to BAY K 8644. These effects were strengthened in long-term treated hBCs with elocalcitol (10(-8) M, 48 h), which also showed increased mRNA and protein expression of pore-forming L-type alpha(1C)-subunit. In the bladder from Sprague-Dawley rats, BAY K 8644 induced a dose-dependent increase in tension, which was significantly enhanced by elocalcitol treatment (30 microg.kg(-1).day(-1), 2 wk). In conclusion, elocalcitol upregulated Ca2+ entry through L-type Ca2+ channels in hBCs, thus balancing its inhibitory effect on RhoA/Rho kinase signaling and suggesting its possible efficacy for the modulation of bladder contractile mechanisms.

Chronic 1α,25-(OH)2vitamin D3 treatment reduces Ca2+-mediated hippocampal biomarkers of aging

Aging in the hippocampus of several species is characterized by alterations in multiple Ca(2+)-mediated processes, including an increase in L-type voltage-gated Ca(2+) channel (L-VGCC) current, an enhanced Ca(2+)-dependent slow afterhyperpolarization (AHP), impaired synaptic plasticity and elevated Ca(2+) transients. Previously, we found that 1alpha,25-dihydoxyvitamin D(3) (1,25VitD), a major Ca(2+) regulating hormone, down-regulates L-VGCC expression in cultured hippocampal neurons. Here, we tested whether in vivo treatment of aged F344 rats with 1,25VitD would reverse some of the Ca(2+) -mediated biomarkers of aging seen in hippocampal CA1 neurons. As previously reported, L-VGCC currents and the AHP were larger in aged than in young neurons. Treatment with 1,25VitD over 7 days decreased L-VGCC activity in aged rats, as well as the age-related increase in AHP amplitude and duration. In addition, reduced L-VGCC activity was correlated with reduced AHPs in the same animals. These data provide direct evidence that 1,25VitD can regulate multiple Ca(2+)-dependent processes in neurons, with particular impact on reducing age-related changes associated with Ca(2+) dysregulation. Thus, these results may have therapeutic implications and suggest that 1,25VitD, often taken to maintain bone health, may also retard some consequences of brain aging.

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.

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 voltage sensitive calcium channel (VSCC) L-type Cav1.2 (α1C) and T-type Cav3.2 (α1H) subunits during mouse bone development

Voltage-sensitive calcium channels (VSCCs) are key regulators of osteoblast plasma membrane Ca(2+) permeability and are under control of calcitropic hormones. Subtype specific antibodies were used to probe L-type Ca(v)1.2 (alpha(1C)) and T-type Ca(v)3.2 (alpha(1H)) subunit expression during mouse skeletal development. Commencing from E14.5 and continuing through skeletal maturity, immunoreactivity of Ca(v)1.2 (alpha(1C)) subunits was evident in regions of rapid long bone growth, including the perichondrium, periosteum, chondro-osseous junction and trabecular bones. Ca(v)3.2 (alpha(1H)) subunits appeared simultaneously and followed a similar distribution pattern. Both subunits were observed in osteoblasts and chondrocytes under high magnification. Interestingly, Ca(v)3.2 (alpha(1H)) subunits were present, but Ca(v)1.2 (alpha(1C)) subunits were absent from osteocytes. Western Blot and immunohistochemical assessment of in vitro cell culture models of osteogenesis and chondrogenesis confirmed the in vivo observations. We conclude that both L-type Ca(v)1.2 (alpha(1C)) and T-type Ca(v)3.2 (alpha(1H)) VSCCs are dynamically regulated in bones and cartilages during endochondral bone development.

The human paracellin-1 gene (hPCLN-1): renal epithelial cell-specific expression and regulation

Tubular reabsorption of Mg2+ is mediated by the tight junction protein paracellin-1, which is encoded by the gene PCLN-1 (CLDN16) and exclusively expressed in the kidney. Tubular Mg2+ reclamation is modulated by many hormones and factors. The aim of this study was to define regulatory elements essential for renal tubular cell-specific expression of human PCLN-1 (hPCLN-1) and to explore the effect of Mg2+ transport modulators on the paracellin-1 gene promoter. Endogenous paracellin-1 mRNA and protein were detected in renal cell lines opossom kidney (OK), HEK293, and MDCT, but not in the fibroblast cell line NIH3T3. A 7.5-kb hPCLN-1 5'-flanking DNA sequence along with seven 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the renal and nonrenal cells. The highest levels of luciferase activity resulted from transfection of a 5'-flanking 2.5-kb fragment (pJ2M). This activity was maximal in OK cells, was orientation dependent, and was absent in NIH3T3 cells. Mg2+ deprivation significantly increased pJ2M-driven activity in transfected OK cells, whereas Mg2+ load decreased it compared with conditions of normal Mg2+. Deletion analysis along with electrophoretic mobility-shift assay demonstrated that OK cells contain nuclear proteins, which bind a 70-bp region between -1633 and -1703 of major functional significance. Deleting this 70-bp segment, which contains a single peroxisome proliferator-response element (PPRE), or mutating the PPRE, caused a 60% reduction in luciferase activity. Stimulating the 70-bp sequence with 1,25(OH)2 vitamin D decreased luciferase activity by 52%. This effect of 1,25(OH)2 vitamin D was abolished in the absence of PPRE or in the presence of mutated PPRE. We conclude that the PPRE within this 70-bp DNA region may play a key role in the cell-specific and regulatory activity of the hPCLN-1 promoter. Ambient Mg2+ concentration and 1,25(OH)2 vitamin D may modulate paracellular, paracellin-1-mediated, Mg2+ transport at the transcriptional level. 1,25(OH)2 vitamin D exerts its activity on the hPCLN-1 promoter likely via the PPRE site.

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.

Extracellular signal-regulated kinases and calcium channels are involved in the proliferative effect of bisphosphonates on osteoblastic cells in vitro

Bisphosphonates (BPs) are analogues of pyrophosphate, which are widely used for the treatment of different pathologies associated with imbalances in bone turnover. Recent evidence suggested that cells of the osteoblastic lineage might be targets of the action of BPs. The objective of this work was to determine whether BPs induce proliferation of osteoblasts and whether this action involves activation of the extracellular signal-regulated kinases (ERKs). We have shown that three different BPs (olpadronate, pamidronate, and etidronate) induce proliferation in calvaria-derived osteoblasts and ROS 17/2.8 as measured by cell count and by [3H]thymidine uptake. Osteoblast proliferation induced by all BPs diminished to control levels in the presence of U0126, a specific inhibitor of the upstream kinase MEK 1 responsible for ERK phosphorylation. Consistent with this, BPs induced ERK activation as assessed by in-gel kinase assays. Phosphorylation of ERK1/2 was induced by the BPs olpadronate and pamidronate within 30 s, followed by rapid dephosphorylation, whereas etidronate induced phosphorylation of ERKs only after 90 s of incubation and returned to basal levels within 15-30 minutes. In addition, both BP-induced cell proliferation and ERK phosphorylation were reduced to basal levels in the presence of nifedipine, an L-type voltage-sensitive calcium channel (VSCC) inhibitor. These results show that BP-induced proliferation of osteoblastic cells is mediated by activation of ERKs and suggest that this effect requires influx of Ca2+ from the extracellular space through calcium channels.

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.

Vitamin D hormone confers neuroprotection in parallel with downregulation of L-type calcium channel expression in hippocampal neurons

Although vitamin D hormone (VDH; 1,25-dihydroxyvitamin D(3)), the active metabolite of vitamin D, is the major Ca(2+)-regulatory steroid hormone in the periphery, it is not known whether it also modulates Ca(2+) homeostasis in brain neurons. Recently, chronic treatment with VDH was reported to protect brain neurons in both aging and animal models of stroke. However, it is unclear whether those actions were attributable to direct effects on brain cells or indirect effects mediated via peripheral pathways. VDH modulates L-type voltage-sensitive Ca(2+) channels (L-VSCCs) in peripheral tissues, and an increase in L-VSCCs appears linked to both brain aging and neuronal vulnerability. Therefore, we tested the hypothesis that VDH has direct neuroprotective actions and, in parallel, targets L-VSCCs in hippocampal neurons. Primary rat hippocampal cultures, treated for several days with VDH, exhibited a U-shaped concentration-response curve for neuroprotection against excitotoxic insults: lower concentrations of VDH (1-100 nm) were protective, but higher, nonphysiological concentrations (500-1000 nm) were not. Parallel studies using patch-clamp techniques found a similar U-shaped curve in which L-VSCC current was reduced at lower VDH concentrations and increased at higher (500 nm) concentrations. Real-time PCR studies demonstrated that VDH monotonically downregulated mRNA expression for the alpha(1C) and alpha(1D) pore-forming subunits of L-VSCCs. However, 500 nm VDH also nonspecifically reduced a range of other mRNA species. Thus, these studies provide the first evidence of (1) direct neuroprotective actions of VDH at relatively low concentrations, and (2) selective downregulation of L-VSCC expression in brain neurons at the same, lower concentrations.

Cardiac L-type calcium channel alpha 1-subunit is increased by cyclic adenosine monophosphate: messenger RNA and protein expression in intact bone

L-type calcium channels have been identified previously in both osteoblast-like osteosarcoma cell lines and primary cultures of osteoblasts using numerous techniques such as patch clamp recording, drug inhibited 45Ca2+ uptake, and Fura-2 measurements, but intact bone has not been investigated. Using reverse-transcription polymerase chain reaction (RT-PCR) we found that the three major isoforms of the alpha 1-subunit of L-type calcium channels, (alpha 1C, alpha 1D, and alpha 1S) are present in RNA extracted from ROS 17/2.8 osteosarcoma cells, rat femur, and rat skull. Sequencing of most of the alpha 1C-subunit from rat femur and ROS cells revealed that the splice variants in osteosarcoma cells and intact bone differ, but there are no unique sequence variations compared with those found in other tissues. Northern blot analysis of ROS cell RNA indicated that cyclic adenosine monophosphate (cAMP), but not 1 alpha, 25-dihydroxyvitamin D3, increased the messenger RNA (mRNA) of the alpha 1C-subunit. Western blot of ROS cell lysates revealed a band of more then 220 kDa, the amount of which increased in cells treated with cAMP. Using confocal microscopy combined with immunohistochemistry in ROS cells, intact bone, and cartilage, we found that the alpha 1C-subunit of this channel is expressed in osteoblasts and chondrocytes suggesting this channel may be a pathway for signal transduction in intact tissue, because it is in osteosarcoma cell lines and primary osteoblasts grown in tissue culture.

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.

Parathyroid hormone-activated volume-sensitive calcium influx pathways in mechanically loaded osteocytes

This paper documents for the first time a volume-sensitive Ca(2+) influx pathway in osteocytes, which transmits loading-induced signals into bone formation. Stretch loading by swelling rat and chicken osteocytes in hypo-osmotic solution induced a rapid and progressive increase of cytosolic calcium concentration, [Ca(2+)](i). The influx of extracellular Ca(2+) explains the increased [Ca(2+)](i) that paralleled the increase in the mean cell volume. Gadolinium chloride (Gd(3+)), an inhibitor of stretch- activated cation channels, blocked the [Ca(2+)](i) increase caused by hypotonic solutions. Also, the expression of alpha1C subunit of voltage-operated L-type Ca(2+) channels (alpha1C) is required for the hypotonicity-induced [Ca(2+)](i) increase judging from the effect of alpha1C antisense oligodeoxynucleotides. Parathyroid hormone (PTH) specifically potentiated the hypotonicity-induced [Ca(2+)](i) increase in a dose-dependent manner through the activation of adenyl cyclase. The increases induced by both PTH and hypotonicity were observed primarily in the processes of the osteocytes. In cyclically stretched osteocytes on flexible-bottomed plates, PTH also synergistically elevated the insulin-like growth factor-1 mRNA level. Furthermore, Gd(3+) and alpha1C antisense significantly inhibited the stretch-induced insulin-like growth factor-1 mRNA elevation. The volume-sensitive calcium influx pathways of osteocytes represent a mechanism by which PTH potentiates mechanical responsiveness, an important aspect of bone formation.

Induction of tissue plasminogen activator secretion from rat heart microvascular cells by fM 1,25(OH)(2)D(3)

We investigated the effects of 1,25-dihydroxyvitamin D(3) [25(OH)(2)D(3)] on tissue plasminogen activator (tPA) secretion from primary cultures of rat heart microvascular cells. After an initial 5-day culture period, cells were treated for 24 h with 1,25(OH)(2)D(3) and several of its analogs. The results showed that 1,25(OH)(2)D(3) induced tPA secretion at 10(-10) to 10(-16) M. A less calcemic analog, Ro-25-8272, and an analog that binds the vitamin D receptor but is ineffective at perturbing Ca(2+) channels, Ro-24-5531, were approximately 10% as active as 1,25(OH)(2)D(3). An analog that binds the vitamin D receptor poorly but is an effective Ca(2+) channel agonist, Ro-24-2287, required approximately 10(-13) M to induce tPA secretion. Combinations of Ro-24-5531 and Ro-24-2287 were approximately as potent as 1,25(OH)(2)D(3). Treatment of the cells with BAY K 8644 or thapsigargin also increased tPA secretion, suggesting that increased cytosolic calcium concentration ([Ca(2+)]) induces tPA secretion. The results suggested that the sensitivity of the tPA secretory response of microvascular cells to 1,25(OH)(2)D(3) was due in part to generation of a vitamin D-depleted state in vitro and in part to synergistic effects of 1,25(OH)(2)D(3) on two different induction pathways of tPA release.

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.

Cloning of a calcium channel alpha1 subunit from the reef-building coral, Stylophora pistillata

While the mechanisms of cellular Ca2+ entry associated with cell activation are well characterized, the pathway of continuous uptake of the large amount of Ca2+ needed in the biomineralization process remains largely unknown. Scleractinian corals are one of the major calcifying groups of organisms. Recent studies have suggested that a voltage-dependent Ca2+ channel is involved in the transepithelial transport of Ca2+ used for coral calcification. We report here the cloning and sequencing of a cDNA coding a coral alpha1 subunit Ca2+ channel. This channel is closely related to the L-type family found in vertebrates and invertebrates. Immunohistochemical analysis shows that this channel is present within the calicoblastic ectoderm, the site involved in calcium carbonate precipitation. These data and previous results provide molecular evidence that voltage-dependent Ca2+ channels are involved in calcification. Cnidarians are the most primitive organisms in which a Ca2+ channel has been cloned up to now; evolutionary perspectives on Ca2+ channel diversity are discussed.

Nongenomic effects of 1alpha,25-dihydroxyvitamin D(3)

The hormonally active form of vitamin D, 1alpha,25-dihydroxyvitamin D(3), is the key molecule of the vitamin D endocrine system, which produces biological effects in about 30 target cell systems. Growing experimental evidence supports the hypothesis that these biological effects can be generated both by a signal transduction mechanism involving a nuclear receptor (nVDR) that modulates gene transcription, and via a nongenomic receptor located in the plasma membrane (mVDR), which modulates a complex signaling system involving the rapid opening of Ca(2+) channels. Some data reviewed herein also indicate that crosstalk between genomic and nongenomic pathways operates in several cell types, and suggest that the physiological role of the rapid, nongenomic actions might involve the regulation of hormone-mediated gene activation.