Differential effects of the 3',5'-cyclic adenosine monophosphate and protein kinase C pathways on the response of isolated rat osteoclasts to calcitonin

Physiological consequences of space flight, including abnormal bone metabolism, space radiation injury, and circadian clock dysregulation: Implications of melatonin use and regulation as a countermeasure

Exposure to the space environment induces a number of pathophysiological outcomes in astronauts, including bone demineralization, sleep disorders, circadian clock dysregulation, cardiovascular and metabolic dysfunction, and reduced immune system function. A recent report describing experiments aboard the Space Shuttle mission, STS-132, showed that the level of melatonin, a hormone that provides the biochemical signal of darkness, was decreased during microgravity in an in vitro culture model. Additionally, abnormal lighting conditions in outer space, such as low light intensity in orbital spacecraft and the altered 24-h light-dark cycles, may result in the dysregulation of melatonin rhythms and the misalignment of the circadian clock from sleep and work schedules in astronauts. Studies on Earth have demonstrated that melatonin regulates various physiological functions including bone metabolism. These data suggest that the abnormal regulation of melatonin in outer space may contribute to pathophysiological conditions of astronauts. In addition, experiments with high-linear energy transfer radiation, a ground-based model of space radiation, showed that melatonin may serve as a protectant against space radiation. Gene expression profiling using an in vitro culture model exposed to space flight during the STS-132 mission, showed that space radiation alters the expression of DNA repair and oxidative stress response genes, indicating that melatonin counteracts the expression of these genes responsive to space radiation to promote cell survival. These findings implicate the use of exogenous melatonin and the regulation of endogenous melatonin as countermeasures for the physiological consequences of space flight.

Prolonged calcitonin receptor signaling by salmon, but not human calcitonin, reveals ligand bias

Salmon calcitonin (sCT) and human calcitonin (hCT) are pharmacologically distinct. However, the reason for the differences is unclear. Here we analyze the differences between sCT and hCT on the human calcitonin receptor (CT_(a)R) with respect to activation of cAMP signaling, β-arrestin recruitment, ligand binding kinetics and internalization. The study was conducted using mammalian cell lines heterologously expressing the human CT_(a) receptor. CT_(a)R downstream signaling was investigated with dose response profiles for cAMP production and β-arrestin recruitment for sCT and hCT during short term (<2 hours) and prolonged (up to 72 hours) stimulation. CT_(a)R kinetics and internalization was investigated with radio-labeled sCT and hCT ligands on cultured cells and isolated membrane preparations from the same cell line. We found that sCT and hCT are equipotent during short-term stimulations with differences manifesting themselves only during long-term stimulation with sCT inducing a prolonged activation up to 72 hours, while hCT loses activity markedly earlier. The prolonged sCT stimulation of both cAMP accumulation and β-arrestin recruitment was attenuated, but not abrogated by acid wash, suggesting a role for sCT activated internalized receptors. We have demonstrated a novel phenomenon, namely that two distinct CT_(a)R downstream signaling activation patterns are activated by two related ligands, thereby highlighting qualitatively different signaling responses in vitro that could have implications for sCT use in vivo.

Loss of protein kinase C-δ protects against LPS-induced osteolysis owing to an intrinsic defect in osteoclastic bone resorption

Bone remodeling is intrinsically regulated by cell signaling molecules. The Protein Kinase C (PKC) family of serine/threonine kinases is involved in multiple signaling pathways including cell proliferation, differentiation, apoptosis and osteoclast biology. However, the precise involvement of individual PKC isoforms in the regulation of osteoclast formation and bone homeostasis remains unclear. Here, we identify PKC-δ as the major PKC isoform expressed among all PKCs in osteoclasts; including classical PKCs (-α, -β and -γ), novel PKCs (-δ, -ε, -η and -θ) and atypical PKCs (-ι/λ and -ζ). Interestingly, pharmacological inhibition and genetic ablation of PKC-δ impairs osteoclastic bone resorption in vitro. Moreover, disruption of PKC-δ activity protects against LPS-induced osteolysis in mice, with osteoclasts accumulating on the bone surface failing to resorb bone. Treatment with the PKC-δ inhibitor Rottlerin, blocks LPS-induced bone resorption in mice. Consistently, PKC-δ deficient mice exhibit increased trabeculae bone containing residual cartilage matrix, indicative of an osteoclast-rich osteopetrosis phenotype. Cultured ex vivo osteoclasts derived from PKC-δ null mice exhibit decreased CTX-1 levels and MARKS phosphorylation, with enhanced formation rates. This is accompanied by elevated gene expression levels of cathepsin K and PKC -α, -γ and -ε, as well as altered signaling of pERK and pcSrc416/527 upon RANKL-induction, possibly to compensate for the defects in bone resorption. Collectively, our data indicate that PKC-δ is an intrinsic regulator of osteoclast formation and bone resorption and thus is a potential therapeutic target for pathological osteolysis.

Hormone-stimulated modulation of endocytic trafficking in osteoclasts

A precise control of vesicular trafficking is crucial not only for osteoclastic bone resorption, but also for the crosstalk between osteoclasts and osteoblasts, which regulates bone homeostasis. In addition to the release of growth factors and modulators, such as glutamate, flux through the intracellular trafficking routes could also provide the osteoclast with a monitoring function of its resorption activity. To establish the signaling pathways regulating trafficking events in resorbing osteoclasts, we used the bone conserving hormone calcitonin, which has the unique property of inducing osteoclast quiescence. Calcitonin acts through the calcitonin receptor and activates multiple signaling pathways. By monitoring trafficking of a fluorescent low molecular weight probe in mature, bone resorbing osteoclasts we show for the first time that calcitonin blocks endocytosis from the ruffled border by phospholipase C (PLC) activation. Furthermore, we identify a requirement for polyunsaturated fatty acids in endocytic trafficking in osteoclasts. Inhibition of PLC prior to calcitonin treatment restores endocytosis to 75% of untreated rates. This effect is independent of protein kinase C activation and can be mimicked by an increase in intracellular calcium. We thus define an essential role for intracellular calcium levels in the maintenance of endocytosis in osteoclasts.

Identification of the calcitonin receptor in osteoarthritic chondrocytes

Background

Preclinical and clinical studies have shown that salmon calcitonin has cartilage protective effects in joint degenerative diseases, such as osteoarthritis (OA). However, the presence of the calcitonin receptor (CTR) in articular cartilage chondrocytes is yet to be identified. In this study, we sought to further investigate the expression of the CTR in naïve human OA articular chondrocytes to gain further confirmation of the existents of the CTR in articular cartilage.

Methods

Total RNA was purified from primary chondrocytes from articular cartilage biopsies from four OA patients undergoing total knee replacement. High quality cDNA was produced using a dedicated reverse transcription polymerase chain reaction (RT-PCR) protocol. From this a nested PCR assay amplifying the full coding region of the CTR mRNA was completed. Western blotting and immunohistochemistry were used to characterize CTR protein on protein level in chondrocytes.

Results

The full coding transcript of the CTR isoform 2 was identified in all four individuals. DNA sequencing revealed a number of allelic variants of the gene including two potentially novel polymorphisms: a frame shift mutation, +473del, producing a shorter form of the receptor protein, and a single nucleotide polymorphism in the 3' non coding region of the transcript, +1443 C>T. A 53 kDa protein band, consistent with non-glycosylated CTR isoform 2, was detected in chondrocytes with a similar size to that expressed in osteoclasts. Moreover the CTR was identified in the plasma membrane and the chondrocyte lacuna of both primary chondrocytes and OA cartilage section.

Conclusions

Human OA articular cartilage chondrocytes do indeed express the CTR, which makes the articular a pharmacological target of salmon calcitonin. In addition, the results support previous findings suggesting that calcitonin has a direct anabolic effect on articular cartilage.

Calcitonin induces expression of the inducible cAMP early repressor in osteoclasts

The cAMP response element modulator gene (Crem) encodes a variety of transcriptional regulators including the inducible cAMP early repressor, ICER. We previously showed that Crem knockout mice, which are deficient in CREM and ICER factors, display slightly increased long bone mass and decreased osteoclast number. These data are consistent with the notion that Crem regulates bone mass in part through an effect on osteoclast formation and/or function. Since ICER is strongly induced by cAMP, we asked whether the calcium-regulating hormone calcitonin, which stimulates cAMP production and inhibits osteoclastic bone resorption, could induce ICER in osteoclasts. The monocytic cell line RAW264.7 was treated with receptor activator of NF-kappaB ligand (RANKL) to induce osteoclast formation. Calcitonin caused a time- and dose-dependent induction of ICER mRNA and an increase in ICER protein abundance in RANKL-treated RAW264.7 cells. Calcitonin also induced ICER mRNA and protein in osteoclasts derived from primary mouse bone marrow cell cultures. Calcitonin-treated osteoclasts showed immunoreactivity with an anti-CREM antibody. Calcitonin decreased the activity of wild-type and Crem knockout osteoclasts in vitro, and this inhibitory effect was greater in Crem knockout osteoclasts. Furthermore, calcitonin decreased calcitonin receptor mRNA expression in wildtype osteoclasts, but not in Crem knockout osteoclasts. These data suggest that calcitonin induction of ICER in osteoclasts might regulate osteoclast activity.

The effect of orchidectomy on thyroid C cells and bone histomorphometry in middle-aged rats

This study was to evaluate the effect of androgen deficiency on thyroid immunoreactive C-cells and bone structure and function in a male orchidectomized middle-aged rat model. Fifteen-month-old male Wistar rats were divided into orchidectomized (Orx) and the sham-operated control (Sham) group. In the Orx group significant decreases (P < 0.05) were found in the volume of C cells (by 14%), their relative volume density (by 13%) and serum calcitonin concentration (by 54%) compared to the controls. Analyses of trabecular microarchitecture of the proximal tibia metaphysis showed that Orx induced marked decreases of cancellous bone area, trabecular thickness and trabecular number (by 52, 20 and 19% respectively; P < 0.05), whereas trabecular separation was increased by 27% (P < 0.05). In Orx rats, serum osteocalcin concentration was increased by 119% (P < 0.05), while serum calcium and phosphorus were 6 and 14% (P < 0.05) lower, respectively, compared to the levels in the Sham. In addition, urine calcium content was considerably higher (by 129%; P < 0.05) in Orx animals. These findings indicate that the androgen deficiency caused by Orx in middle-aged rats modulated the structure of C cells and diminished secretion of calcitonin. Histomorphometrical and biochemical analyses demonstrated a decrease of cancellous bone mass and increased bone turnover.

Calpain is required for normal osteoclast function and is down-regulated by calcitonin

Osteoclast motility is thought to depend on rapid podosome assembly and disassembly. Both mu-calpain and m-calpain, which promote the formation and disassembly of focal adhesions, were observed in the podosome belt of osteoclasts. Calpain inhibitors disrupted the podosome belt, blocked the constitutive cleavage of the calpain substrates filamin A, talin, and Pyk2, which are enriched in the podosome belt, induced osteoclast retraction, and reduced osteoclast motility and bone resorption. The motility and resorbing activity of mu-calpain(-/-) osteoclast-like cells were also reduced, indicating that mu-calpain is required for normal osteoclast activity. Histomorphometric analysis of tibias from mu-calpain(-/-) mice revealed increased osteoclast numbers and decreased trabecular bone volume that was apparent at 10 weeks but not at 5 weeks of age. In vitro studies suggested that the increased osteoclast number in the mu-calpain(-/-) bones resulted from increased osteoclast survival, not increased osteoclast formation. Calcitonin disrupted the podosome ring, induced osteoclast retraction, and reduced osteoclast motility and bone resorption in a manner similar to the effects of calpain inhibitors and had no further effect on these parameters when added to osteoclasts pretreated with calpain inhibitors. Calcitonin inhibited the constitutive cleavage of a fluorogenic calpain substrate and transiently blocked the constitutive cleavage of filamin A, talin, and Pyk2 by a protein kinase C-dependent mechanism, demonstrating that calcitonin induces the inhibition of calpain in osteoclasts. These results indicate that calpain activity is required for normal osteoclast activity and suggest that calcitonin inhibits osteoclast bone resorbing activity in part by down-regulating calpain activity.

The delta e13 isoform of the calcitonin receptor forms a six-transmembrane domain receptor with dominant-negative effects on receptor surface expression and signaling

The CTRdelta e13 splice variant of the rabbit calcitonin receptor, which lacks the 14 amino acids of the seventh transmembrane domain (TMD) that are encoded by exon 13, is poorly expressed on the cell surface, fails to mobilize intracellular calcium or activate Erk, and inhibits the cell surface expression of the full-length C1a isoform. Nuclear magnetic resonance- and fluorescence-activated cell sorter-based experiments showed that the residual seventh TMD of CTRdelta e13 fails to partition into the lipid bilayer, resulting in an extracellular C terminus. Truncating the receptor after residue 397 to delete the cytoplasmic tail resulted in reduced cell surface expression and an inability to mobilize intracellular calcium or activate Erk, but the truncated receptor did not inhibit C1a cell surface expression. In contrast, when the receptor was truncated after residue 374 to eliminate the entire seventh TMD domain and the C-terminal domain, the resulting receptor reduced the cell surface expression of C1a in a manner similar to that of CTRdelta e13. Thus, normal cell surface expression, mobilization of intracellular calcium, and Erk activation requires the cytoplasmic C-terminal tail of the CTR, whereas the absence of the seventh TMD in the transmembrane helical bundle causes the dominant-negative effect on the surface expression of C1a.

Amylin inhibits bone resorption while the calcitonin receptor controls bone formation in vivo

Amylin is a member of the calcitonin family of hormones cosecreted with insulin by pancreatic β cells. Cell culture assays suggest that amylin could affect bone formation and bone resorption, this latter function after its binding to the calcitonin receptor (CALCR). Here we show that Amylin inactivation leads to a low bone mass due to an increase in bone resorption, whereas bone formation is unaffected. In vitro, amylin inhibits fusion of mononucleated osteoclast precursors into multinucleated osteoclasts in an ERK1/2-dependent manner. Although Amylin +/− mice like Amylin-deficient mice display a low bone mass phenotype and increased bone resorption, Calcr +/− mice display a high bone mass due to an increase in bone formation. Moreover, compound heterozygote mice for Calcr and Amylin inactivation displayed bone abnormalities observed in both Calcr +/− and Amylin +/− mice, thereby ruling out that amylin uses CALCR to inhibit osteoclastogenesis in vivo. Thus, amylin is a physiological regulator of bone resorption that acts through an unidentified receptor.

12-O-tetradecanoylphorbol-13-acetate (TPA) inhibits osteoclastogenesis by suppressing RANKL-induced NF-kappaB activation

The mechanism by which TPA-induced PKC activity modulates osteoclastogenesis is not clear. Using a RAW(264.7) cell culture system and assays for NF-kappaB nuclear translocation, NF-kappaB reporter gene activity, and MAPK assays, we demonstrated that TPA inhibits osteoclastogenesis through the suppression of RANKL-induced NF-kappaB activation.

INTRODUCTION

The protein kinase C (PKC) pathway has been suggested to be an important regulator of osteoclastic bone resorption. The role of PKC in RANKL-induced osteoclastogenesis, however, is not clear. In this study, we examined the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA), a PKC activator, on osteoclastogenesis and studied its role in RANKL-induced signaling.

MATERIALS AND METHODS

RANKL-induced RAW(264.7) cell differentiation into osteoclast-like cells was used to assess the effect of TPA on osteoclastogenesis. Assays for NF-kappaB nuclear translocation, NF-kappaB reporter gene activity, protein kinase activity, and Western blotting were used to examine the effects of TPA on RANKL-induced NF-kappaB, c-Jun N-terminal kinase (JNK), and MEK/ERK and p38 signal transduction pathways.

RESULTS

We found that TPA inhibited RANKL-induced RAW(264.7) cell differentiation into osteoclasts in a dose-dependent manner. Time course analysis showed that the inhibitory effect of TPA on RANKL-induced osteoclastogenesis occurs predominantly at an early stage of osteoclast differentiation. TPA alone had little effect on NF-kappaB activation in RAW(264.7) cells, but it suppresses the RANKL-induced NF-kappaB activation in a dose-dependent fashion. Interestingly, the suppressive effect of TPA on RANKL-induced NF-kappaB activation was prevented by a conventional PKC inhibitor, Go6976. Supershift studies revealed that the RANKL-induced DNA binding of NF-kappaB complexes consisted of C-Rel, NF-kappaB1 (p50), and RelA (p65). In addition, TPA induced the activation of JNK in RAW(264.7) cells but had little effect on RANKL-induced activation of JNK. TPA also inhibited RANKL-induced activation of ERK but had little effect on p38 activation.

CONCLUSION

Given that NF-kappaB activation is obligatory for osteoclast differentiation, our studies imply that inhibition of osteoclastogenesis by TPA is, at least in part, caused by the suppression of RANKL-induced activation of NF-kappaB during an early stage of osteoclastogenesis. Selective modulation of RANKL signaling pathways by PKC activators may have important therapeutic implications for the treatment of bone diseases associated with enhanced bone resorption.

Regulatory mechanisms and physiological relevance of a voltage-gated H+ channel in murine osteoclasts: phorbol myristate acetate induces cell acidosis and the channel activation

The voltage-gated H+ channel is a powerful H+ extruding mechanism of osteoclasts, but its functional roles and regulatory mechanisms remain unclear. Electrophysiological recordings revealed that the H+ channel operated on activation of protein kinase C together with cell acidosis.

INTRODUCTION

H+ is a key signaling ion in bone resorption. In addition to H+ pumps and exchangers, osteoclasts are equipped with H+ conductive pathways to compensate rapidly for pH imbalance. The H+ channel is distinct in its strong H+ extrusion ability and voltage-dependent gatings.

METHODS

To investigate how and when the H+ channel is available in functional osteoclasts, the effects of phorbol 12-myristate 13-acetate (PMA), an activator for protein kinase C, on the H+ channel were examined in murine osteoclasts generated in the presence of soluble RANKL (sRANKL) and macrophage-colony stimulating factor (M-CSF).

RESULTS AND CONCLUSIONS

Whole cell recordings clearly showed that the H+ current was enhanced by increasing the pH gradient across the plasma membrane (delta(pH)), indicating that the H+ channel changed its activity by sensing delta(pH). The reversal potential (V(rev)) was a valuable tool for the real-time monitoring of delta(pH) in clamped cells. In the permeabilized patch, PMA (10 nM-1.6 microM) increased the current density and the activation rate, slowed decay of tail currents, and shifted the threshold toward more negative voltages. In addition, PMA caused a negative shift of V(rev), suggesting that intracellular acidification occurred. The PMA-induced cell acidosis was confirmed using a fluorescent pH indicator (BCECF), which recovered quickly in a K(+)-rich alkaline solution, probably through the activated H+ channel. Both cell acidosis and activation of the H+ channel by PMA were inhibited by staurosporine. In approximately 80% of cells, the PMA-induced augmentation in the current activity remained after compensating for the delta(pH) changes, implying that both delta(pH)-dependent and -independent mechanisms mediated the channel activation. Activation of the H+ channel shifted the membrane potential toward V(rev). These data suggest that the H+ channel may contribute to regulation of the pH environments and the membrane potential in osteoclasts activated by protein kinase C.

The alternatively spliced deltae13 transcript of the rabbit calcitonin receptor dimerizes with the C1a isoform and inhibits its surface expression

Numerous alternatively spliced transcripts are generated from the gene for the G protein-coupled calcitonin receptor, and some of the splice variants show differences in receptor-mediated signaling events. This study showed that the deltae13 splice variant of the rabbit calcitonin receptor is expressed together with the more common C1a in osteoclast-like cells. Since other G protein-coupled receptors form homo- or heterodimers, we examined whether heterodimerization of the calcitonin receptor splice variants occurs and, if so, whether it affects the function of the receptor. Homodimers of both isoforms and deltae13/C1a heterodimers were detected by co-immunoprecipitation and fluorescence resonance energy transfer analysis. In contrast to the C1a isoform, the deltae13 isoform was not efficiently transported to the cell surface. When co-expressed with the C1a splice variant, the deltae13 isoform colocalized with the C1a isoform within the cell but not at the cell surface. Furthermore, the overexpression of the deltae13 variant led to a significant reduction of the C1a surface expression and consequently a reduction of the cAMP response and Erk phosphorylation after ligand stimulation. We therefore suggest that the deltae13 variant of the rabbit calcitonin receptor acts to regulate the surface expression of the C1a isoform.

Calcitonin inhibits proton extrusion in resorbing rat osteoclasts via protein kinase A

Although calcitonin is well known to be a potent inhibitor of bone resorption, it remains unknown how it regulates osteoclastic H(+) transport. In this study, we examined the effects of calcitonin on H(+) extrusion in cultured rat resorbing osteoclasts using an intracellular pH (pHi) indicator, BCECF [2'7'-bis-(2-carboxyethyl)- 5-carboxyfluorescein]. Resorbing osteoclasts were identified by their formation of resorbing pits on calcium phosphate-coated quartz coverslips. Both basal pHi and H(+) extrusion activity were significantly higher compared to non-resorbing osteoclasts. Two types of H(+)-extruding systems were identified by pharmacological and immunocytochemical means: a bafilomycin-A(1)-sensitive and an amiloride-sensitive system [H(+) extrusion mediated by a vacuolar type proton pump (V-ATPase) and by a Na(+)/H(+) exchanger (NHE), respectively]. Calcitonin inhibited both H(+) extrusion activities in a dose-dependent manner and this action was mimicked by protein kinase A (PKA) activators, but not by protein kinase C (PKC) activators. Pretreatment with PKA inhibitors completely suppressed calcitonin-induced inhibition, whereas neither PKC inhibitors nor calcium chelators suppressed it. These results indicate that calcitonin inhibits H(+) extrusion generated by V-ATPase and NHE via PKA activation. These inhibitory mechanisms of H(+) transport by calcitonin are important for the regulation of bone resorption.

Inhibitory effect of menaquinone-7 (vitamin K2) on osteoclast-like cell formation and osteoclastic bone resorption in rat bone tissues in vitro

The effect of menaquinone-7 (MK-7; vitamin K2) on osteoclast-like cell formation and osteoclastic bone resorption in rat femoral tissues in vitro was investigated. The bone marrow cells were cultured for 7 days in a a-minimal essential medium (alpha-MEM) containing a well-known bone resorbing agent [parathyroid hormone (1-34) (PTH) or prostaglandin E2 (PGE2)] with an effective concentration. Osteoclast-like cells were estimated by staining for tartrate-resistant acid phosphatase (TRACP), a marker enzyme of osteoclasts. The presence of PTH (10(-8) M) or PGE2 (10(-6) M) induced a remarkable increase in osteoclast-like multinucleated cells. These increases were significantly inhibited by MK-7 (10(-8) - 10(-5) M). MK-7 (10(-7) and 10(-6) M) significantly inhibited phorbol 12-myristate 13-acetate-induced osteoclast-like cell formation, whereas MK-7 did not inhibit dibutyryl cyclic adenosine monophosphate (DcAMP) (10(-5) M)-induced osteoclast-like cell formation. These results suggest that the inhibitory action of MK-7 is partly involved in protein kinase C signaling. The bone cells isolated from rat femoral tissues were cultured for 48 h in an alpha-MEM containing either vehicle or MK-7 (10(-8) - 10(-5) M). The presence of MK-7 (10(-6) and 10(-5) M) caused a significant decrease in the number of mature osteoclasts. Such a decrease was also seen in the presence of calcitonin (10(-10) - 10(-8) M), DcAMP (10(-6) and 10(-5) M), or calcium chloride (10(-4) and 10(-3) M). The effect of MK-7 (10(-6) M) in decreasing the number of osteoclasts was not further enhanced in the presence of calcitonin (10(-8) M), DcAMP (10(-5) M), or calcium chloride (10(-3) M), and was completely abolished by the presence of dibucaine (10(-6) M) or staurosporine (10(-7) M), which are inhibitors of Ca2+-dependent protein kinases. These results suggested that MK-7 has a suppressive effect on osteoclasts. Moreover, the femoral-metaphyseal tissues obtained from rats were cultured for 48 h in Dulbecco's modified Eagle's medium containing either vehicle, PTH (10(-7) M), orPGE2 (10(-5) M) in the absence or presence of MK-7 (10(-7) - 10(-5) M). The presence of PTH or PGE2 induced a significant decrease in bone calcium content. These decreases were significantly blocked by MK-7 (10(-7) - 10(-5) M). This study demonstrates that MK-7 has an inhibitory effect on osteoclastic bone resorption in vitro.

Carbonic anhydrase II is an AP-1 target gene in osteoclasts

c-Fos, a member of the AP-1 family of transcription factors, is necessary for osteoclast differentiation but to date, none of the osteoclast-phenotypic markers have been identified as AP-1 target genes. Here, we demonstrate that carbonic anhydrase II (CA II), an enzyme necessary for osteoclast activity, is transcriptionally upregulated by c-Fos/AP-1. A functional AP-1 binding site is present in the CA II promoter and is necessary for this regulation. Furthermore, we show that AP-1 binding activity, mainly composed of Fra-2 and JunD, is induced by treatment of bone marrow cultures with the osteoclastogenic hormone 1,25 dihydroxyvitamin D(3). Fra-2 and JunD are found in mature osteoclasts as well. Thus, our data demonstrate that cFos/AP-1 can directly regulate the expression of this osteoclast marker and that AP-1 activity is upregulated in osteoclast progenitors in response to osteoclastogenic signals.

Protein kinase C antagonizes pertussis-toxin-sensitive coupling of the calcitonin receptor to adenylyl cyclase

The calcitonin receptor is known to couple to Gs and Gq, activating adenylyl cyclase and phospholipase C, respectively. The observation of pertussis-toxin-sensitive responses to calcitonin suggests that the receptor is capable of coupling to Gi/o as well. However, the calcitonin-dependent activation of adenylyl cyclase in HEK-293 cells that stably express the cloned rabbit calcitonin receptor, as in many other cells that express calcitonin receptors, shows little pertussis toxin sensitivity. Calcitonin treatment of these cells stimulates protein kinase C, which is reported to antagonize the receptor-dependent activation of Gi. The possibility that protein kinase C could be antagonizing Galphai-adenylyl cyclase coupling was tested by examining the effects of protein kinase C inhibitors (chelerythrine chloride and sphingosine) or of chronic treatment with phorbol ester to deplete protein kinase C. All three treatments led to a reduction of calcitonin-induced adenylyl cyclase activity that was reversed by pertussis toxin. Inhibiting or depleting protein kinase C had no effect on the activation of adenylyl cyclase by cholera toxin, indicating that Gs and adenylyl cyclase were not affected by these treatments. Calcitonin treatment of HEK-293 cells, that stably express a myc-tagged rabbit calcitonin receptor, induced the formation of complexes of the receptor and Galphai subunits, confirming that the calcitonin receptor interacts with Gi. Thus, the calcitonin receptor can couple to Gi, but the inhibition of adenylyl cyclase by Galphai is negatively regulated by protein kinase C.

A new method to isolate large numbers of rabbit osteoclasts and osteoclast-like cells: application to the characterization of serum response element binding proteins during osteoclast differentiation

We have developed a new method that allows the purification of large numbers of both authentic osteoclasts (OCs) and in vitro differentiated osteoclast-like cells (OCLs) from rabbits. We characterized the OCLs in terms of the expression of different phenotypic markers of OC differentiation and their ability to resorb bone. The method provides a system for performing biochemical and molecular studies of OC differentiation and function in a single species. We used this system to characterize the effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on the expression of proteins that bind to the serum response element (SRE) of the c-fos promoter. We found that OCLs and OCs displayed similar SRE-binding activities, including the serum response factor (SRF). This pattern is established in a time-dependent and cell-specific manner in response to long-term treatment of rabbit bone marrow by 1,25(OH)2D3. Thus, 1,25(OH)2D3 can modulate SRF and/or SRF-related protein. This finding may contribute to understanding the role of c-Fos in the regulation of OC differentiation.

Calcitonin responsiveness and receptor expression in porcine and murine osteoclasts: a comparative study

The presence of the calcitonin (CT) receptor is a distinguishing characteristic of osteoclasts; however, species variability exists with respect to functional responsiveness to CT. In the present study, CT responsiveness and temporal expression of the CT receptor in differentiating cultures of porcine osteoclasts was examined and compared to murine osteoclasts. In vitro porcine osteoclast differentiation was evaluated using bone marrow cultures from neonatal pigs. Murine osteoclast differentiation was studied using cocultures of murine bone marrow and BALC cells, a calvarial-derived cell line. In the presence of 1,25 (OH)2D3, a time-dependent increase in osteoclast differentiation was observed in porcine and murine cultures. Salmon CT (sCT) and porcine CT (pCT) inhibited 1,25 (OH)2D3-stimulated porcine osteoclast differentiation at 10(-8) and 10(-7) mol/L (60% with 10(-7) mol/L sCT and 85% inhibition with 10(-7) mol/L pCT). Treatment of murine cocultures with sCT (10(-17)-10(-7) mol/L) resulted in a concentration-dependent decrease in osteoclast differentiation with a maximal inhibition of 70%. Osteoclast differentiation was inhibited in a concentration-dependent manner by recombinant human transforming growth factor-beta1 (rhTGF-beta1) in both species. The effects of CT on resorption lacunae formation were determined by culturing in vitro generated porcine or murine osteoclasts on bovine cortical bone slices for 18 h in the presence or absence of CT. With both porcine and murine osteoclasts, a concentration-dependent decrease in resorption lacunae formation was observed between 10(-13) and 10(-7) mol/L sCT with the highest concentrations completely abolishing resorption. However, pCT only inhibited porcine osteoclastic resorption at 10(-7) mol/L. CT receptor messenger ribonucleic acid (mRNA) expression was determined at different time points during in vitro osteoclast differentiation. In porcine cultures, expression of CT receptor mRNA correlated with the presence of osteoclasts. In murine cocultures, mRNA for the CT receptor was observed at each time point examined and was independent of the presence of multinucleated osteoclasts. Thus, porcine and murine differentiating osteoclast cultures express CT receptor mRNA; however, receptor expression correlates with osteoclast formation only in the porcine cultures. In summary, porcine and murine osteoclasts express CT receptor mRNA and functional responsiveness to CT. These findings suggest that the effects of sCT on osteoclast resorption are similar in murine and porcine cells, but that sCT is a less potent inhibitor of porcine than murine osteoclast differentiation.

The calcitonin receptor stimulates Shc tyrosine phosphorylation and Erk1/2 activation. Involvement of Gi, protein kinase C, and calcium

While it is well established that adenylyl cyclase and phospholipase C-beta are two proximal signal effectors for the calcitonin receptor, the more distal signaling pathways are less well characterized. G protein-coupled receptors can activate Erk1/2 by Gs-, Gi-, or Gq-dependent signaling pathways, depending on the specific receptor and cell type examined. Since the calcitonin receptor can couple to all three of these G proteins, the ability of calcitonin to activate Erk1/2 was investigated. Calcitonin induced time- and concentration-dependent increases in Shc tyrosine phosphorylation, Shc-Grb2 association and Erk1/2 phosphorylation and activation in a HEK 293 cell line that stably expresses the rabbit calcitonin receptor C1a isoform. Pertussis toxin, which inactivates Gi, and calphostin C, a protein kinase C inhibitor, each partially inhibited calcitonin-induced Shc tyrosine phosphorylation, Shc-Grb2 association, and Erk1/2 phosphorylation. In contrast, neither forskolin nor H89, a protein kinase A inhibitor, had a significant effect on basal or calcitonin-stimulated Erk1/2 phosphorylation. Our results suggest that the calcitonin receptor induces Shc phosphorylation and Erk1/2 activation in HEK293 cells by parallel Gi- and PKC-dependent mechanisms. The calcitonin-induced elevation of cytosolic free Ca2+ was required for Erk1/2 phosphorylation, since preventing any change in cytosolic free Ca2+ by chelating both cytosolic and extracellular Ca2+ abolished the response. However, the change in Ca2+ that is induced by calcitonin is not sufficient to account for the calcitonin-induced Erk1/2 phosphorylation, since treatment with 100 nM ionomycin or 10 microM thapsigargin, each of which induced elevations of Ca2+ comparable to those induced by calcitonin, induced significantly less Erk1/2 phosphorylation than that induced by calcitonin. Erk1/2 may have important roles as downstream effectors mediating cellular responses to calcitonin stimulation.

Phospholipase D- and protein kinase C isoenzyme-dependent signal transduction pathways activated by the calcitonin receptor

The calcitonin receptor expressed by the porcine LLC-PK1 renal tubule cells is a seven-transmembrane domain, G protein-coupled receptor activating adenylyl-cyclase and phospholipase C. Salmon calcitonin stimulated dose- and time-dependent release of the phospholipase D-dependent phosphatidylcholine product [3H] choline with an EC50 = 2.5 +/-0.3 x 10(-8) M, similar to that determined for phosphoinositide metabolism (EC50 = 4.5 +/-1.0 x 10(-8)M). The hormone failed to induce release of [3H]phosphocholine and [3H]glycerophosphocholine, ruling out activation of phosphatydilcholine-specific phospholipase C and phospholipase A. Calcitonin stimulated phosphatidic acid, a product of phospholipase D-dependent phosphatydilcholine hydrolysis. Activation of phospholipase D was confirmed by release of [3H]phosphatydilethanol, a specific and stable product in the presence of a primary alcohol. Activation of calcitonin receptor induced diacylglycerol formation, with a rapid peak followed by a prolonged increase, due to activation of phospholipase C and of phospholipase D. Consequently, the protein kinase-C alpha, but not the delta isoenzyme, was cytosol-to-membrane translocated by approximately 50% after 20 min exposure to calcitonin, whereas protein kinase-C zeta, which was approximately 40% membrane-linked in unstimulated cells, translocated by approximately 19%. The human calcitonin receptor expressed by BIN-67 ovary tumor cells, although displaying higher affinity for calcitonin, failed to activate phospholipase D and protein kinase-C in response to the hormone. This receptor lacks the G protein binding consensus site due to the presence of a 48-bp cassette encoding for a 16-amino acid insert in the predicted first intracellular loop. This modification is likely to prevent the calcitonin receptor from associating to phospholipase-coupled signaling.

Colony-stimulating factor-1 induces cytoskeletal reorganization and c-src-dependent tyrosine phosphorylation of selected cellular proteins in rodent osteoclasts

Colony-stimulating factor-1 (CSF-1) stimulates motility and cytoplasmic spreading in mature osteoclasts. Therefore, we examined the cellular events and intracellular signaling pathways that accompany CSF-1-induced spreading in normal osteoclasts. To explore the role c-src plays in these processes, we also studied osteoclasts prepared from animals with targeted disruption of the src gene. In normal osteoclasts, CSF-1 treatment induces rapid cytoplasmic spreading, with redistribution of F-actin from a well-delineated central attachment ring to the periphery of the cell. CSF-1 increases membrane phosphotyrosine staining in osteoclasts and induces the phosphorylation of several cellular proteins in cultured, osteoclast-like cells, including c-fms, c-src, and an 85-kD Grb2-binding protein. Src kinase activity is increased threefold after CSF-1 treatment. In src- cells, no attachment ring is present, and CSF-1 fails to induce spreading or a change in the pattern of F-actin distribution. Although c-fms becomes phosphorylated after CSF-1 treatment, the 85-kD protein is significantly less phosphorylated in src- osteoclast-like cells. These results indicate that c-src is critical for the normal cytoskeletal architecture of the osteoclast, and, in its absence, the spreading response induced by CSF-1 is abrogated, and downstream signaling from c-fms is altered.

Ca2+ or phorbol ester but not inflammatory stimuli elevate inducible nitric oxide synthase messenger ribonucleic acid and nitric oxide (NO) release in avian osteoclasts: autocrine NO mediates Ca2+-inhibited bone resorption

Osteoclast bone resorption is essential for normal calcium homeostasis and is therefore tightly controlled by calciotropic hormones and local modulatory cytokines and factors. Among these is nitric oxide (NO), a multifunctional free radical that potently inhibits osteoclast bone resorption in vitro and in vivo. Previous findings led us to propose that NO might serve as an autocrine, as well as paracrine, regulator of osteoclast function. This premise was investigated using isolated bone-resorptive avian osteoclasts and focusing on the inducible isoform of NO synthase (iNOS) responsible for inflammatory stimulated high-level NO synthesis in other cells. Avian osteoclasts expressed both iNOS messenger RNA (mRNA) and protein. However, inflammatory cytokines that induce iNOS mRNA, protein, and NO in other cells did not do so in avian osteoclasts, consistent with the known role of inflammatory stimuli in promoting osteoclast resorption and localized bone loss. In searching for potential modulators of osteoclast iNOS, protein kinase C activation [by phorbol 12-myristate 13-acetate (PMA)] and intracellular Ca2+ rises (A23187) were each found to elevate osteoclast iNOS mRNA and protein levels, while increasing NO release and reducing osteoclast bone resorption. The iNOS selective inhibitor aminoguanidine suppressed stimulated osteoclast NO production elicited by either signal, but reversed only the resorption inhibition due to raised Ca2+. Thus, whereas additional inhibitory signals are presumably coproduced in osteoclasts treated with PMA, osteoclast iNOS-derived NO may act as an autocrine signal to mediate Ca2+-inhibited bone resorption. These findings document for the first time an iNOS whose mRNA levels are regulated by Ca2+ or PMA, but not inflammatory stimuli, and the autocrine production of NO as a Ca2+ sensing signal to suppress osteoclast bone resorption. The unusual regulation of osteoclast iNOS makes it a potentially attractive target for designing novel therapeutic agents to alleviate excessive bone loss.

The deletion of 14 amino acids in the seventh transmembrane domain of a naturally occurring calcitonin receptor isoform alters ligand binding and selectively abolishes coupling to phospholipase C

The cDNA that encodes the rabbit calcitonin receptor was cloned by screening a rabbit osteoclast library. Reverse transcription-polymerase chain reaction amplification of calcitonin receptor sequences from rabbit osteoclast RNA yielded cDNAs that encode two isoforms of the calcitonin receptor. One isoform is homologous to the C1a isoform previously identified in multiple cell types and species, while the second, designated CTRDeltae13, is a previously unidentified isoform that is apparently generated by alternative splicing during mRNA processing that deletes exon 13, resulting in the absence of 14 amino acids in the predicted seventh transmembrane domain. Expression of mRNA transcripts encoding the two isoforms varies in a tissue-specific manner, with CTRDeltae13 accounting for less than 15% of the total calcitonin receptor mRNA in osteoclasts, kidney, and brain, but comprising at least 50% of the transcripts in skeletal muscle and lung. The two isoforms were expressed, and the ligand binding and signal transduction properties were characterized. Deletion of the residues in the seventh transmembrane domain in CTRDeltae13 reduced the binding affinity for salmon and human calcitonin by more than 10-fold and approximately 2-fold, respectively, resulting in a receptor that failed to discriminate between the two forms of calcitonin. Both isoforms activated adenylyl cyclase, with EC50 values consistent with the difference in ligand affinities. In contrast, only the C1a isoform, but not the CTRDeltae13 isoform, activated phospholipase C. Thus, while the CTRDeltae13 remains active despite the deletion of a significant portion of its seventh transmembrane domain, it has significantly altered ligand recognition and signal transduction properties.

Zinc compounds inhibit osteoclast-like cell formation at the earlier stage of rat marrow culture but not osteoclast function

The effect of zinc compounds on osteoclast-like cell formation in rat marrow culture in vitro was investigated. The bone marrow cells were cultured for 7 days in alpha-minimal essential medium containing a well-known bone resorbing hormone (1, 25-dihydroxyvitamin D3 and parathyroid hormone [1-34]). Osteoclast-like cell formation was estimated by staining for tartrate-resistant acid phosphatase (TRACP), a marker enzyme of osteoclasts. The presence of 1, 25-dihydroxyvitamin D3 (10(-8) M) or parathyroid hormone (PTH; 10(-8) M) induced a remarkable increase in osteoclast-like multinucleated cells (MNC). These increases were clearly inhibited by the presence of zinc sulfate or zinc-chelating dipeptide (beta-alanyl-L-histidinato zinc; AHZ) in the concentration range of 10(-7) to 10(-5) M. The inhibitory effect was seen at the earlier stage of osteoclast-like MNC formation. However, zinc compounds (10(-6) M) did not have an effect on PTH (10(-8) M)-induced osteoclast-like cell formation in the presence of EGTA (5 x 10(-4) M), dibucaine (10(-5) M) or staurosporine (10(-9) M). Moreover, when osteoclasts isolated from rat femoral-diaphyseal tissues were cultured for 24 h in the presence of zinc compounds (10(-7) to 10(-5) M), the compounds did not have an effect on cell numbers or lysosomal enzymes activity (acid phosphatase and beta-glucuronidase) in the cells. The present study clearly demonstrates that zinc compounds inhibit osteoclast-like cell formation at the earlier stage with differentiation of marrow cells.

Calcitonin receptor mRNA expression in the human prostate

OBJECTIVES

A subpopulation of prostate neuroendocrine (NE) cells contain calcitonin (CT). It has been postulated that CT-producing cells in the prostate account for the high CT level in the semen, and may be involved in the regulation of other epithelial cells via a paracrine mechanism. The presence of CT binding sites in the plasma membrane fraction of prostate tissue has been demonstrated by radioligand binding assay. In the present study, we investigated the CT receptor gene expression in the human prostate, a key component of the autocrine/paracrine loop in the CT functional pathway.

METHODS

Reverse transcription polymerase chain reaction (RT-PCR) was carried out to evaluate the CT receptor mRNA expression in normal prostate tissue. Subsequent DNA sequencing was used to verify RT-PCR amplified products and to determine the isoform of the receptor. To define the location of the CT receptor expression, nonradioactive in situ hybridization was performed with a digoxigenin-labeled probe complementary to the coding region of the CT receptor mRNA. A polyclonal antibody against CT was used to reveal the CT-secreting cells in the prostate.

RESULTS

CT receptor MRNA expression was detected in the prostate tissue. Further analysis of the DNA sequence showed that CT receptor expressed in the prostate was the isoform without a 16-amino acid insert in the first intracellular domain. In situ hybridization revealed that CT receptor was present in the prostate NE cells. Immunocytochemical staining of mirror image sections showed that some CT-secreting cells also expressed CT receptor.

CONCLUSIONS

CT receptor expression in the prostate, a key component in the CT functional pathway, is located in subsets of dispersed NE cells (CT secreting and CT nonsecreting), which indicates that prostate CT may play an important role in the autocrine/paracrine regulation of the prostate NE system.

Cytoskeletal changes in osteoclasts during the resorption cycle

Osteoclasts are large, multinucleated cells which change their shape and polarity according to their resorptive activity. At least in vitro, nonresorbing osteoclasts move on the bone surface and do not show clear evidence of apical-basolateral polarity. When stimulated for resorption, osteoclasts undergo a rapid reorganization of the cytoskeleton and appear clearly polarized. The detailed nature of different membrane domains in polarized osteoclasts is still far from clear, but a remarkable feature is the formation of a tight sealing zone between the ruffled border and the rest of the cell membrane. Characteristic organization of F-actin into a belt or ring-like structure with a double circle of vinculin around it is needed for the formation of the sealing zone. This type of microfilament organization is typical only for resorbing osteoclasts and can thus be used as a marker for resorbing cells. These characteristic changes in the molecular organization of the cytoskeleton in osteoclasts during the resorption cycle offer several potential targets to inhibit bone resorption, perhaps cell-specific.

Platelet-activating factor induces pseudopod formation in calcitonin-treated rabbit osteoclasts

We demonstrated previously that platelet-activating factor (PAF), a potent inflammatory mediator, acts on osteoclasts to elevate cytosolic [Ca2+] and stimulate resorption. However, it is not clear whether the effects of PAF on resorptive activity are direct or indirect. In the present study, we investigated the effects of PAF on osteoclast motility. Osteoclasts were isolated from the long bones of neonatal rabbits, and cell motility and morphology were monitored using time-lapse video microscopy. Calcitonin, a hormone known to induce retraction of pseudopods and inhibit resorptive activity, was used to render osteoclasts quiescent. Within 10 minutes of calcitonin treatment (100 ng/ml, final), pronounced retraction of pseudopods was observed in 68 of 112 cells tested. When PAF (200 nM, final) was added 10 minutes after calcitonin treatment, pseudopods were evident 1 h later in 15 of 37 calcitonin-responsive cells tested. In contrast, pseudopods were evident in only 4 of 31 calcitonin-responsive cells treated with control solutions (PAF-vehicle or S-PAF, the biologically inactive stereoisomer of PAF). Pseudopod formation was quantified by measuring the planar area of pseudopods with a computer-based video analysis system. When assessed 60 minutes following PAF treatment, the pseudopod area was significantly greater in PAF-treated cells than in control cells. In some calcitonin-treated osteoclasts, PAF induced pseudopod formation when applied focally using an extracellular micropipette, consistent with a direct action of PAF. We conclude that PAF directly induces pseudopod formation in calcitonin-inhibited osteoclasts, a morphologic response indicative of osteoclast activation.

Cell cycle-dependent and kinase-specific regulation of the apical Na/H exchanger and the Na,K-ATPase in the kidney cell line LLC-PK1 by calcitonin

Calcitonin (CT), which regulates serum calcium through its actions in bone and the kidney tubule, also has a potent natriuretic effect in vivo. Na reabsorption in the proximal kidney tubule is mostly dependent on the activity of the Na,K-ATPase and the apical Na/H exchanger. We have previously shown that CT regulates the activity of the Na,K-ATPase in the proximal kidney tubule cell line LLC-PK1 in a cell cycle-dependent manner. We report here that, in the same cells, CT also regulates the Na/H exchanger through a cell cycle-specific activation of the Ca/calmodulin-dependent protein kinase II. In G2 phase, no changes in ethylisopropyl amiloride-sensitive 22Na uptake is observed, despite an increase in cAMP. In contrast, the hormone inhibits the apical exchanger when the cells are in S phase, resulting in an 80% inhibition of 22Na uptake. These results demonstrate that CT affects the activity of the two major proximal tubule Na transport systems and may help clarify the mechanisms by which CT regulates Na+ reabsorption.

Macrophage colony-stimulating factor release and receptor expression in bone cells

Colony-stimulating factors (CSF) may play a role in bone resorption. To examine whether osteoblasts secrete colony-stimulating activity (CSA) in response to parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP), conditioned medium (CM) from ROS 17/2.8 cells and primary rat osteoblasts were examined for induction of clonal growth of cultured rat bone marrow cells. Untreated cells constitutively secreted CSA, which increased with PTH and PTHrP treatment. The colonies formed were principally comprised of macrophages, and preincubation of CM with antiserum to murine macrophage colony-stimulating factor (M-CSF) neutralized most of the CSA, suggesting that the osteoblast-derived CSA was predominantly due to M-CSF. PTHrP treatment upregulated steady-state M-CSF mRNA levels. To investigate a paracrine role for M-CSF in bone we examined bone tissue and cells for the M-CSF receptor c-fms using immunohistochemical techniques and demonstrated staining of mature osteoclasts both in situ and after isolation. We conclude that M-CSF is responsible for the majority of the CSA released by PTH- and PTHrP-treated rat osteoblasts. In addition we identified CSF-1 receptor expression in mature osteoclasts. These data suggest that M-CSF is a mediator of osteoblast-osteoclast interaction in PTH- and PTHrP-induced bone resorption.

Immediate cell signal by bone-related peptides in human osteoclast-like cells

We tested whether recognition of bone-related peptides regulates intracellular Ca2+ concentration ([Ca2+]i) of giant cell tumor of bone (GCT). [Ca2+]i was measured in single cells by fura 2 fluorometry. GCT cells were sensitive to bone sialoprotein-II (BSP-II), osteopontin (OPN), and related fragments. Responses consisted of a prompt increase of [Ca2+]i, mostly transient, with a peak followed by a rapid return toward baseline. Responses were not mimicked by bovine plasma fibronectin. Sensitivity of GCT cells to bone peptides was specific, since BALB/3T3 fibroblasts and U-937 histiocytic lymphoma cells with monocytic phenotype failed to respond to BSP-II and OPN fragments. GRGDSP synthetic esapeptide, carrying the Arg-Gly-Asp adhesive motif, and GRGESP (Asp replaced by Glu), but not the GRADSP (Gly replaced by Ala), were active in inducing [Ca2+]i transients as well. Responses were observed also in cells treated with the BSP-II 1C fragment, lacking any known adhesive sequence, indicating that the active peptides inducing [Ca2+]i increments may be multiple. Sensitivity to extracellular matrix peptides was present in a variable fraction of the cells and was downregulated on long-term culture. The mechanism inducing [Ca2+]i elevations was mostly related to Ca2+ release from thapsigargin-sensitive intracellular pools.

Calcitonin induces a decreased Na+ conductance in identified neurons of Aplysia

The ionic mechanism of the effect of extracellularly ejected calcitonin (CT) on the membrane of identified neurons R9 and R10 of Aplysia was investigated with voltage-clamp, micropressure ejection, and ion substitution techniques. Micropressure-ejected CT caused a marked hyperpolarization in the unclamped neuron. Heat-inactivated CT was without effect. Clamping the same neuron at its resting potential level (-60 mV) and re-ejecting CT with the same dose produced a slow outward current (Io(CT), 30-40 sec in duration, 4-6 nA in amplitude) associated with a decrease in input membrane conductance. Io(CT) was decreased by depolarization and increased by hyperpolarization. The extrapolated reversal potential of Io(CT) was approximately +10 mV. Io(CT) was sensitive to changes in the external Na+ concentration but not to changes in K+, Ca2+, and Cl- concentrations. Micropressure-ejected forskolin produced a slow outward current, which, like the current to CT, was associated with a decrease in input membrane conductance, and was sensitive to changes in the external Na+ concentration. Io(CT) was prolonged by bath-applied isobutylmethylxanthine (IBMX) but was not affected by 1-oleoyl-2-acetylglycerol (OAG) and calphostin C. Neither superfusion of the neuron with nordihydroguaiaretic acid (NDGA) nor superfusion with indomethacin caused any changes in Io(CT). These results suggest that extracellular CT can induce a slow outward current associated with a decrease in Na+ conductance, mediated by a receptor-controlled increase in intracellular cyclic adenosine 3',5'-monophosphate.

Osteoclasts express high levels of pp60c-src in association with intracellular membranes

Deletion of the c-src gene in transgenic mice by homologous recombination leads to osteopetrosis, a skeletal defect characterized by markedly deficient bone resorption (Soriano, P., C. Montgomery, R. Geske, and A. Bradley. 1991. Cell. 64:693-702), demonstrating a critical functional role of pp60c-src in osteoclast activity. Since decreased bone resorption could result from a defect either within the osteoclast or within other cells present in its environment, indirectly affecting osteoclast functions, we determined which cell(s) in bone expressed high levels of pp60c-src Measuring pp60c-src protein and kinase activities in osteoclasts and immunolocalizing pp60c-src in bone, we find that expression of pp60c-src is nearly as high in osteoclasts as in brain and platelets. In contrast, other bone cells contain only very low levels of the protein. In addition, expression of the c-src gene product increases when bone marrow cells are induced to express an osteoclast-like phenotype by 1,25-dihydroxy-vitamin D3, further suggesting that high expression of pp60c-src is part of the osteoclast phenotype. Three other src-like kinases, c-fyn, c-yes, and c-lyn, are also expressed in osteoclasts at ratios to pp60c-src similar to what is found in platelets. These src-related proteins do not, however, compensate for the absence of pp60c-src in the src- mice, thereby suggesting that pp60c-src may have a specific function in osteoclasts. Although further work is necessary to elucidate what the critical role of pp60c-src in osteoclasts is, our observation that the protein is associated mostly with the membranes of intracellular organelles suggests the possibility that this role might be at least in part related to the targeting or fusion of membrane vesicles.