Attenuated Response to In Vivo Mechanical Loading in Mice With Conditional Osteoblast Ablation of the Connexin43 Gene (<i>Gja1</i>)

Grimston, Susan K.; Brodt, Michael D.; Silva, Matthew J.; Civitelli, Roberto

doi:10.1359/jbmr.080222

Cited by 109 publications

(154 citation statements)

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“…As in the case of PTH, stimulation of mineral apposition rate at the endocortical surface by application of a 3-point bending regime to tibiae in vivo was significantly reduced in conditionally Gja1 deleted mice relative to wild type animals [46]. In addition, these mutant mice required approximately 40% more force to generate the same endocortical strain as did wild type mice [46], solidifying the concept that Cx43 plays an instrumental role in adaptive response to osteogenic stimuli of different nature.…”

Section: Connexins In Skeletal Response To Pharmacologic and Physicalmentioning

confidence: 58%

Cell–cell communication in the osteoblast/osteocyte lineage

Civitelli

2008

Archives of Biochemistry and Biophysics

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Skeletal development (bone modeling) and its maintenance in post-natal life in response to local and systemic stimuli (bone remodeling) require coordinated activity among osteoblasts (bone forming cells), osteocytes (cells embedded in bone) and osteoclasts (bone resorbing cells), in order to meet the needs of structural integrity, mechanical competence and maintenance of mineral homeostasis. One mechanism of cell-cell interaction is via direct cell-cell communication via gap junctions. These are transmembrane channels that allow continuity of cytoplasms between communicating cells. The biologic importance of connexin43 (Cx43), the most abundant gap junction protein in the skeleton is demonstrated by the skeletal malformations present in oculodentodigital dysplasia (ODDD), a disease linked to Cx43 gene (GJA1) mutations, and by the low bone mass and osteoblast dysfunction in Gja1 ablated mice. The presence of Cx43 is required for osteoblast differentiation and function, and by forming either gap junctions or "hemichannels" Cx43 allows participation of cell networks to responses to extracellular stimuli, via propagation of specific signals converging upon connexin sensitive transcriptional units. Hence, Cx43 is involved in skeletal responsiveness to anabolic signals, as those provided by parathyroid hormone and physical load, the latter function probably involving osteocyte-osteoblast communication.

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Section: Connexins In Skeletal Response To Pharmacologic and Physicalmentioning

confidence: 58%

Cell–cell communication in the osteoblast/osteocyte lineage

Civitelli

2008

Archives of Biochemistry and Biophysics

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176

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“…Loading was conducted on days 1-5 and 8-12 of a 15-day protocol [29]. Mice were anesthetized (1-3% inhaled isofluorane) and positioned in the loading fixture.…”

Section: In Vivo Loading -Tibial Bendingmentioning

confidence: 99%

Mechanical Stimulation of Bone Formation is Normal in the SAMP6 Mouse

Silva

Brodt

2008

Calcif Tissue Int

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With aging the skeleton may have diminished responsiveness to mechanical stimulation. The senescence accelerated mouse SAMP6 has many features of senile osteoporosis and is thus a useful model to examine how the osteoporotic skeleton responds to mechanical loading. We performed in vivo tibial bending on four-month old SAMP6 (osteoporotic) and SAMR1 (control) mice. Loading was applied daily (60 cycles/day, 5 days/wk) for 2 weeks at peak force levels that produced estimated endocortical strains of 1000 and 2000 με. In a separate group of mice, sham bending was applied. Comparisons were made between right (loaded) and left (non-loaded) tibiae. Tibial bone marrow cells were cultured under osteogenic conditions, and stained for alkaline phosphatase (ALP) and alizarin red (ALIZ) at 14 and 28 days, respectively. Tibiae were then embedded in plastic, sectioned and endocortical bone formation was assessed based on calcein labels. Tibial bending did not alter the osteogenic potential of the marrow as there were no significant differences in ALP or ALIZ staining between loaded and non-loaded bones. Tibial bending activated the formation of endocortical bone in both SAMP6 and SAMR1 mice, whereas sham bending did not elicit an endocortical response. Both groups of mice exhibited bending strain-dependent increases in bone formation rate. We found little evidence of diminished responsiveness to loading in the SAMP6 skeleton. In conclusion, the ability of the SAMP6 mouse to respond normally to an anabolic mechanical stimulus distinguishes it from chronologically aged animals. This finding highlights a limitation of the SAMP6 mouse as a model of senile osteoporosis.

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“…A gene dosage effect is observed using a model of conditional knockout of Cx43 in osteoblasts, as the anabolic response to PTH in heterozygous Cx43 Ϫ/fl mice is intermediate between wild-type and Cx43 conditionally deleted animals (Chung et al, 2006). Similarly, heterozygous Cx43 Ϫ/fl mice demonstrate an intermediate response to mechanical load (Grimston et al, 2008). Gene dosage effects are also apparent in the Cx43 null mouse model.…”

mentioning

confidence: 98%

Connexin43 Potentiates Osteoblast Responsiveness to Fibroblast Growth Factor 2 via a Protein Kinase C-Delta/Runx2–dependent Mechanism

Lima

Niger

Hebert

et al. 2009

MBoC

100

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In this study, we examine the role of the gap junction protein, connexin43 (Cx43), in the transcriptional response of osteocalcin to fibroblast growth factor 2 (FGF2) in MC3T3 osteoblasts. By luciferase reporter assays, we identify that the osteocalcin transcriptional response to FGF2 is markedly increased by overexpression of Cx43, an effect that is mediated by Runx2 via its OSE2 cognate element, but not by a previously identified connexin-responsive Sp1/Sp3-binding element. Furthermore, disruption of Cx43 function with Cx43 siRNAs or overexpression of connexin45 markedly attenuates the response to FGF2. Inhibition of protein kinase C delta (PKC␦) with rottlerin or siRNA-mediated knockdown abrogates the osteocalcin response to FGF2. Additionally, we show that upon treatment with FGF2, PKC␦ translocates to the nucleus, PKC␦ and Runx2 are phosphorylated and these events are enhanced by Cx43 overexpression, suggesting that the degree of activation is enhanced by increased Cx43 levels. Indeed, chromatin immunoprecipitations of the osteocalcin proximal promoter with antibodies against Runx2 demonstrate that the recruitment of Runx2 to the osteocalcin promoter in response to FGF2 treatment is dramatically enhanced by Cx43 overexpression. Thus, Cx43 plays a critical role in regulating the ability of osteoblasts to respond to FGF2 by impacting PKC␦ and Runx2 function. INTRODUCTIONBone formation and remodeling is a tightly organized and dynamic process that requires the coordinated action of osteoblasts, osteocytes, and osteoclasts to maintain bone homeostasis. It is hypothesized that osteoblasts and osteocytes coordinate their activities, at least in part, through direct cell-to-cell communication through gap junctions. Gap junctions are composed of connexins, a family of transmembrane proteins that constitute the intercellular gap junction channels (Beyer et al., 1990). Six connexins assemble to make up the gap junction hemichannel on the plasma membrane of one cell, which docks with a hemichannel on an adjacent cell to form an aqueous gap junction channel. The resultant gap junctions provide direct conduits for the passage of ions and other low molecular weight molecules, including second messengers, among cells.The gap junction protein connexin43 (Cx43) is abundantly expressed in both osteoblasts and osteocytes, where it has been hypothesized to transmit hormonal signals, mechanical load, and growth factor cues among cells in order to coordinate the synthesis of new bone (reviewed in Stains and Civitelli, 2005a;Jiang et al., 2007). Genetic ablation of gja1, the gene encoding Cx43, in mice leads to a severe delay in the ossification of both intramembranous and endochondral derived skeletal elements during embryonic development (Lecanda et al., 2000). The bones of these animals are remarkably brittle, and the osteoblasts isolated from the Cx43 null animals are dysfunctional, with reduced osteogenic and mineralizing capacity (Lecanda et al., 2000). These Cx43-deficient mice die at birth because of a defect in cardiac f...

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Attenuated Response to In Vivo Mechanical Loading in Mice With Conditional Osteoblast Ablation of the Connexin43 Gene (Gja1)

Cited by 109 publications

References 49 publications

Cell–cell communication in the osteoblast/osteocyte lineage

Cell–cell communication in the osteoblast/osteocyte lineage

Mechanical Stimulation of Bone Formation is Normal in the SAMP6 Mouse

Connexin43 Potentiates Osteoblast Responsiveness to Fibroblast Growth Factor 2 via a Protein Kinase C-Delta/Runx2–dependent Mechanism

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