Differential activation of Rac1 and RhoA in neuroblastoma cell fractions.

The Rho guanine nucleotide triphosphatases (GTPases) Rac1 and RhoA are important regulators of axon growth. However, the specific roles each plays are complicated by implications that each is involved in promoting and inhibiting neurite outgrowth. Differential regulation of Rac1 and RhoA activation in cell bodies and growth cones may be important in directing axon growth. To test this, we separated neuroblastoma cells into growth cone and cell body fractions and assessed Rac1 and RhoA activation in response to outgrowth promoters, serum withdrawal and 8-bromoadeosine-5',3'-cyclic monophosphate (8-Br-cAMP), and outgrowth inhibitors, chondroitin sulfate proteoglycans (CSPGs) or semaphorin 3A (Sema 3A). In whole cell lysates, serum withdrawal decreased and CSPGs or Sema 3A increased RhoA activity, but no treatments affected Rac1 activity. In growth cones, serum withdrawal or 8-Br-cAMP increased Rac1 activation and serum withdrawal decreased RhoA activation. Conversely, outgrowth inhibitors decreased Rac1 activity. Additionally, 8-Br-cAMP reversed increases in RhoA activity induced by Sema 3A in whole cell lysates and CSPGs in growth cones. These data suggest that activation of RhoA and Rac1 is differentially regulated in specific cellular regions, perhaps contributing to the complexity of Rho GTPase-mediated axon growth.