Pituicyte stellation is prevented by RhoA-or Cdc42-dependent actin polymerization

Our aim was to shed light on different steps leading from metabotropic receptor activation to changes in cell shape, such as those that characterize the morphological plasticity of neurohypophysial astrocytes (pituicytes). Using explant cultures of adult rat pituicytes, we have previously established that adenosine A1 receptor activation induces stellation via inhibition of RhoA monomeric GTPase and subsequent disruption of actin stress fibers. Here, we rule out RhoA phosphorylation as a mechanism for that inhibition. Rather, our results are more consistent with involvement of a GTPase-activating protein (GAP). siRNA and pull-down experiments suggest that a step downstream of RhoA might involve Cdc42, another GTPase of the Rho family. However, RhoA activation, e.g., in the presence of serum, induces stress fibers, whereas direct Cdc42 activation appears to confine actin within a submembrane - i.e., cortical - network, which also prevents stellation. Therefore, we propose that RhoA may activate Cdc42 in parallel with an effector, such as p160Rho-kinase, that induces and maintains actin stress fibers in a dominant fashion. Rac1 is not involved in the stellation process per se but appears to induce a dendritogenic effect. Ultimately, it may be stated that pituicyte stellation is inducible upon mere actin depolymerization, and preventable upon actin organization, be it in the form of stress fibers or in a cortical configuration.