title: Focal adhesion kinase plays a role in osteoblast mechanotransduction in vitro but does not affect load-induced bone formation in vivo.
abstract: A healthy skeleton relies on bone's ability to respond to external mechanical forces. The molecular mechanisms by which bone cells sense and convert mechanical stimuli into biochemical signals, a process known as mechanotransduction, are unclear. Focal adhesions play a critical role in cell survival, migration and sensing physical force. Focal adhesion kinase (FAK) is a non-receptor protein tyrosine kinase that controls focal adhesion dynamics and can mediate reparative bone formation in vivo and osteoblast mechanotransduction in vitro. Based on these data, we hypothesized that FAK plays a role in load-induced bone formation. To test this hypothesis, we performed in vitro fluid flow experiments and in vivo bone loading studies in FAK-/- clonal lines and conditional FAK knockout mice, respectively. FAK-/- osteoblasts showed an ablated prostaglandin E(2) (PGE(2)) response to fluid flow shear. This effect was reversed with the re-expression of wild-type FAK. Re-expression of FAK containing site-specific mutations at Tyr-397 and Tyr-925 phosphorylation sites did not rescue the phenotype, suggesting that these sites are important in osteoblast mechanotransduction. Interestingly, mice in which FAK was conditionally deleted in osteoblasts and osteocytes did not exhibit altered load-induced periosteal bone formation. Together these data suggest that although FAK is important in mechanically-induced signaling in osteoblasts in vitro, it is not required for an adaptive response in vivo, possibly due to a compensatory mechanism that does not exist in the cell culture system.
authors.affiliation: Department of Rehabilitation Research and Development, Center for Tissue Regeneration, Repair, and Restoration, Veterans Affairs Palo Alto Health Care System, Palo Alto, California, United States of America. alesha.castillo@stanford.edu
authors.forename: Alesha B
authors.initials: AB
authors.lastname: Castillo
authors.forename: Jennifer T
authors.initials: JT
authors.lastname: Blundo
authors.forename: Julia C
authors.initials: JC
authors.lastname: Chen
authors.forename: Kristen L
authors.initials: KL
authors.lastname: Lee
authors.forename: Nikitha Reddy
authors.initials: NR
authors.lastname: Yereddi
authors.forename: Eugene
authors.initials: E
authors.lastname: Jang
authors.forename: Shefali
authors.initials: S
authors.lastname: Kumar
authors.forename: W Joyce
authors.initials: WJ
authors.lastname: Tang
authors.forename: Sarah
authors.initials: S
authors.lastname: Zarrin
authors.forename: Jae-Beom
authors.initials: JB
authors.lastname: Kim
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Jacobs
mesh_terms: D000220:Adaptation, Biological / Q000235:genetics; D000818:Animals; D001835:Body Weight / Q000235:genetics; D001842:Bone and Bones / Q000378:metabolism; D002460:Cell Line; D015232:Dinoprostone / Q000378:metabolism; D005260:Female; D051418:Focal Adhesion Kinase 2 / Q000378:metabolism; D051416:Focal Adhesion Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D022001:Focal Adhesions / Q000235:genetics; D017353:Gene Deletion; D015870:Gene Expression; D008297:Male; D040542:Mechanotransduction, Cellular / Q000235:genetics; D051379:Mice; D018345:Mice, Knockout; D010006:Osteoblasts / Q000378:metabolism*; D010012:Osteogenesis / Q000235:genetics; D010766:Phosphorylation; D021381:Protein Transport; D014457:Ulna / Q000033:anatomy & histology / Q000378:metabolism
chemical_list: D051418:Focal Adhesion Kinase 2; D051416:Focal Adhesion Protein-Tyrosine Kinases; D015232:Dinoprostone