Hormonal, pH, and calcium regulation of connexin 43-mediated dye transfer in osteocytes in chick calvaria.
Gap junctional intercellular communication among osteocytes in chick calvaria, their natural 3D environment, was examined using FRAP analysis. Cell-cell communication among osteocytes in chick calvaria was mediated by Cx43 and was regulated by extracellular pH, extracellular calcium ion concentration, and PTH.


BACKGROUND
The intercellular network of communication among osteocytes is mediated by gap junctions. Gap junctional intercellular communication (GJIC) is thought to play an important role in integration and synchronization of bone remodeling. We hypothesized that extracellular pH (pH(o)) and extracellular calcium ion concentration ([Ca2+](e)), both of which are dynamically altered by osteoclasts during bone remodeling, affect GJIC among osteocytes. Using fluorescence replacement after photobleaching (FRAP) analysis, we examined the effect of changes in pH(o) and [Ca2+](e) and addition of PTH on GJIC in osteocytes in chick calvaria. Additionally, we examined the role of intracellular calcium on the regulation of GJIC among osteocytes.


METHODS
Anti-Connexin43 (Cx43) immunolabeling was used to localize gap junctions in chick calvaria. GJIC among osteocytes in chick calvariae was assessed using FRAP.


RESULTS
Cx43 immunoreactivity was detected in most of the osteocyte processes. FRAP analysis showed dye-coupling among osteocytes in chick calvariae. In untreated osteocytes, fluorescence intensity recovered 43.7 +/- 2.2% within 5 min after photobleaching. Pretreatment of osteocytes with 18 alpha-GA, a reversible inhibitor of GJIC, significantly decreased fluorescence recovery to 10.7 +/- 2.2%. When pH(o) was decreased from 7.4 to 6.9, fluorescence recovery significantly decreased from 43.3 +/- 2.9% to 19.7 +/- 2.3%. Conversely, when pH(o) was increased from 7.4 to 8.0, fluorescence recovery was significantly increased to 61.9 +/- 4.5%. When [Ca2+](e) was increased from 1 to 25 mM, fluorescence recovery was significantly decreased from 47.0 +/- 6.1% to 16.1 +/- 2.1%. In bone fragments exposed to 1.0-10 nM rPTH for 3 h, replacement of fluorescence was significantly increased to 60.7 +/- 7.2%. Chelating intracellular calcium ions affected GJIC regulation by [Ca2+](e) and PTH.


CONCLUSIONS
Our study of cell-cell communication between osteocytes in chick calvaria showed for the first time that GJIC among osteocytes is regulated by the extracellular environment and by hormonal stimulation during bone remodeling. This method may be more biologically relevant to living bone than current methods.
Ishihara
Yoshihito
Y
Department of Orthodontics and Dentofacial Orthopedics, Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences, Okayama, Japan.
Kamioka
Hiroshi
H
Honjo
Tadashi
T
Ueda
Hirotaka
H
Takano-Yamamoto
Teruko
T
Yamashiro
Takashi
T
D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D016723:Bone Remodeling / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D002450:Cell Communication / Q000187:drug effects / Q000502:physiology*; D002642:Chick Embryo; D002645:Chickens; D018031:Connexin 43 / Q000378:metabolism*; D005110:Extracellular Space; D005456:Fluorescent Dyes / Q000494:pharmacology; D017629:Gap Junctions / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D010011:Osteocytes / Q000166:cytology / Q000378:metabolism*; D010281:Parathyroid Hormone / Q000378:metabolism* / Q000494:pharmacology; D012886:Skull / Q000166:cytology / Q000378:metabolism*
D018031:Connexin 43; D005456:Fluorescent Dyes; D010281:Parathyroid Hormone; D002118:Calcium