title: Cyclic Mechanical Loading Is Essential for Rac1-Mediated Elongation and Remodeling of the Embryonic Mitral Valve.
abstract: During valvulogenesis, globular endocardial cushions elongate and remodel into highly organized thin fibrous leaflets. Proper regulation of this dynamic process is essential to maintain unidirectional blood flow as the embryonic heart matures. In this study, we tested how mechanosensitive small GTPases, RhoA and Rac1, coordinate atrioventricular valve (AV) differentiation and morphogenesis. RhoA activity and its regulated GTPase-activating protein FilGAP are elevated during early cushion formation but decreased considerably during valve remodeling. In contrast, Rac1 activity was nearly absent in the early cushions but increased substantially as the valve matured. Using gain- and loss-of-function assays, we determined that the RhoA pathway was essential for the contractile myofibroblastic phenotype present in early cushion formation but was surprisingly insufficient to drive matrix compaction during valve maturation. The Rac1 pathway was necessary to induce matrix compaction in vitro through increased cell adhesion, elongation, and stress fiber alignment. Facilitating this process, we found that acute cyclic stretch was a potent activator of RhoA and subsequently downregulated Rac1 activity via FilGAP. On the other hand, chronic cyclic stretch reduced active RhoA and downstream FilGAP, which enabled Rac1 activation. Finally, we used partial atrial ligation experiments to confirm in vivo that altered cyclic mechanical loading augmented or restricted cushion elongation and thinning, directly through potentiation of active Rac1 and active RhoA, respectively. Together, these results demonstrate that cyclic mechanical signaling coordinates the RhoA to Rac1 signaling transition essential for proper embryonic mitral valve remodeling.
authors.affiliation: The Nancy E. and Peter C. Meinig School of Biomedical Engineering, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Russell A
authors.initials: RA
authors.lastname: Gould
authors.affiliation: Qatar Cardiovascular Research Center (QCRC), Sidra Medical and Research Center, Doha, Qatar; Department of Mechanical Engineering, Dogus University, Istanbul 34722, Turkey.
authors.forename: Huseyin C
authors.initials: HC
authors.lastname: Yalcin
authors.affiliation: Department of Bioengineering, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: Joanna L
authors.initials: JL
authors.lastname: MacKay
authors.affiliation: Department of Regenerative Medicine and Cell Biology, School of Medicine, Cardiovascular Developmental Biology Center, Children's Research Institute, Medical University of South Carolina, 171 Ashley Avenue, Charleston, SC 29425, USA.
authors.forename: Kimberly
authors.initials: K
authors.lastname: Sauls
authors.affiliation: Department of Regenerative Medicine and Cell Biology, School of Medicine, Cardiovascular Developmental Biology Center, Children's Research Institute, Medical University of South Carolina, 171 Ashley Avenue, Charleston, SC 29425, USA.
authors.forename: Russell
authors.initials: R
authors.lastname: Norris
authors.affiliation: Department of Bioengineering, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: Sanjay
authors.initials: S
authors.lastname: Kumar
authors.affiliation: The Nancy E. and Peter C. Meinig School of Biomedical Engineering, Cornell University, Ithaca, NY 14853, USA. Electronic address: jtb47@cornell.edu.
authors.forename: Jonathan T
authors.initials: JT
authors.lastname: Butcher
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000502:physiology; D002642:Chick Embryo; D005109:Extracellular Matrix / Q000378:metabolism; D020690:GTPase-Activating Proteins / Q000378:metabolism; D040542:Mechanotransduction, Cellular / Q000502:physiology; D008943:Mitral Valve / Q000196:embryology* / Q000378:metabolism*; D009024:Morphogenesis; D015398:Signal Transduction; D022502:Stress Fibers / Q000378:metabolism; D013314:Stress, Mechanical; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*; D020742:rhoA GTP-Binding Protein / Q000378:metabolism*
chemical_list: D020690:GTPase-Activating Proteins; D020830:rac1 GTP-Binding Protein; D020742:rhoA GTP-Binding Protein
keywords: FilGAP; filamin-A; malformation; mechanotransduction; myofibroblast