Altered Adhesive Structures and Their Relation to RhoGTPase Activation in Merlin-Deficient Schwannoma

Schwannomas are Schwann cell tumors of the nervous system that occur spontaneously and in patients with neurofibromatosis 2 (NF2) and lack the tumor suppressor merlin. Merlin is known to bind paxillin, beta1 integrin and focal adhesion kinase, members of focal contacts, multi-protein complexes that mediate cell adhesion to the extracellular matrix. Moreover, merlin-deficient Schwannomas show pathological adhesion to the extracellular matrix making the characterization of focal contacts indispensable. Using our Schwannoma in vitro model of human primary Schwann and Schwannoma cells, we here show that Schwannoma cells display an increased number of mature and stable focal contacts. In addition to an involvement of RhoA signaling via the Rho kinase ROCK, Rac1 plays a significant role in the pathological adhesion of Schwannoma cells. The Rac1 guanine exchange factor- beta-Pix, localizes to focal contacts in human primary Schwannoma cells, and we show that part of the Rac1 activation, an effect of merlin-deficiency, occurs at the level of focal contacts in human primary Schwannoma cells. Our results help explaining the pathological adhesion of Schwannoma cells, further strengthen the importance of RhoGTPase signaling in Schwannoma development, and suggest that merlin's role in tumor suppression is linked to focal contacts.