title: The transient receptor potential ion channel TRPV6 is expressed at low levels in osteoblasts and has little role in osteoblast calcium uptake.
abstract: BACKGROUND
  TRPV6 ion channels are key mediators of regulated transepithelial absorption of Ca2+ within the small intestine. Trpv6-/- mice were reported to have lower bone density than wild-type littermates and significant disturbances in calcium homeostasis that suggested a role for TRPV6 in osteoblasts during bone formation and mineralization. TRPV6 and molecules related to transepithelial Ca2+ transport have been reported to be expressed at high levels in human and mouse osteoblasts.
  
  
  RESULTS
  Transmembrane ion currents in whole cell patch clamped SaOS-2 osteoblasts did not show sensitivity to ruthenium red, an inhibitor of TRPV5/6 ion channels, and 45Ca uptake was not significantly affected by ruthenium red in either SaOS-2 (P=0.77) or TE-85 (P=0.69) osteoblastic cells. In contrast, ion currents and 45Ca uptake were both significantly affected in a human bronchial epithelial cell line known to express TRPV6. TRPV6 was expressed at lower levels in osteoblastic cells than has been reported in some literature. In SaOS-2 TRPV6 mRNA was below the assay detection limit; in TE-85 TRPV6 mRNA was detected at 6.90±1.9×10(-5) relative to B2M. In contrast, TRPV6 was detected at 7.7±3.0×10(-2) and 2.38±0.28×10(-4) the level of B2M in human carcinoma-derived cell lines LNCaP and CaCO-2 respectively. In murine primary calvarial osteoblasts TRPV6 was detected at 3.80±0.24×10(-5) relative to GAPDH, in contrast with 4.3±1.5×10(-2) relative to GAPDH in murine duodenum. By immunohistochemistry, TRPV6 was expressed mainly in myleocytic cells of the murine bone marrow and was observed only at low levels in murine osteoblasts, osteocytes or growth plate cartilage.
  
  
  CONCLUSIONS
  TRPV6 is expressed only at low levels in osteoblasts and plays little functional role in osteoblastic calcium uptake.
authors.affiliation: Department of Human Metabolism, The Mellanby Centre for Bone Research, The University of Sheffield, Sheffield, United Kingdom.
authors.forename: Robert
authors.initials: R
authors.lastname: Little
authors.forename: Richmond
authors.initials: R
authors.lastname: Muimo
authors.forename: Louise
authors.initials: L
authors.lastname: Robson
authors.forename: Kate
authors.initials: K
authors.lastname: Harris
authors.forename: Peter S
authors.initials: PS
authors.lastname: Grabowski
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D018938:Caco-2 Cells; D002118:Calcium / Q000493:pharmacokinetics*; D002132:Calcium Radioisotopes; D002460:Cell Line; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D020869:Gene Expression Profiling; D006801:Humans; D007150:Immunohistochemistry; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D010006:Osteoblasts / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012430:Ruthenium Red / Q000494:pharmacology; D050916:TRPV Cation Channels / Q000235:genetics* / Q000378:metabolism / Q000502:physiology
chemical_list: D002132:Calcium Radioisotopes; D050916:TRPV Cation Channels; C120776:TRPV6 channel; D012430:Ruthenium Red; D002118:Calcium