Cdc42 Mediates Nucleus Movement and MTOC Polarization in Swiss 3T3 Fibroblasts under Mechanical Shear Stress

Jerry S.H. Lee,* Yiider Tseng,* and Denis Wirtz* ${dagger}$ ${ddagger}$ ${sect}$

Departments of ^*Chemical and Biomolecular Engineering and Materials Science and Engineering and Graduate Program in Molecular Biophysics, The Johns Hopkins University, Baltimore, MD 21218

Monitoring Editor: Jennifer Lippincott-Schwartz

Nucleus movementis essential during nucleus positioning for tissue growth anddevelopment in eukaryotic cells. However, molecular regulatorsof nucleus movement in interphase fibroblasts have yet to beidentified. Here, we report that nuclei of Swiss 3T3 fibroblastsundergo enhanced movement when subjected to shear flows. Suchmovement includes both rotation and translocation and is dependenton microtubule, not F-actin, structure. Through inactivationof Rho GTPases, well-known mediators of cytoskeleton reorganization,we demonstrate that Cdc42, not RhoA or Rac1, controls the extentof nucleus translocation, and more importantly, of nucleus rotationin the cytoplasm. In addition to generating nuclei movement,we find that shear flows also causes repositioning of the nearbyMTOC in the direction of flow. This behavior is also controlledby Cdc42 via the Par6/protein kinase C ${zeta}$ pathway. These resultsare the first to establish Cdc42 as a molecular regulator ofnot only shear-induced MTOC polarization in Swiss 3T3 fibroblasts,but also of shear-induced microtubule-dependent nucleus movement.We propose that the movements of MTOC and nucleus are coupledchemically, as they are both regulated by Cdc42 and dependentupon microtubule structure, and physically, possibly via Hook/SUNfamily homologues similar to those found in C. elegans.

Corresponding author. E-mail: wirtz{at}jhu.edu