Ruffling membrane, stress fiber, cell spreading and proliferation abnormalities in human Schwannoma cells

Abstract

Schwannomas are peripheral nerve tumors that typically have mutations in the NF2 tumor suppressor gene. We compared cultured schwannoma cells with Schwann cells from normal human peripheral nerves (NHSC). Both cell types expressed speci®c antigenic markers, interacted with neurons, and proliferated in response to glial growth factor, con®rming their identity as Schwann cells. Schwannoma cells frequently had elevated basal proliferation compared to NHSC. Schwannoma cells also showed spread areas 5 ± 7-fold greater than NHSC, aberrant membrane ruing and numerous, frequently disorganized stress ®bers. Dominant negative Rac inhibited schwannoma cell ruing but had no apparent eect on NHSC. Schwannoma cell stress ®bers were inhibited by C3 transferase, tyrphostin A25, or dominant negative RhoA. These data suggest that the Rho and Rac pathways are abnormally activated in schwannoma cells. Levels of ezrin and moesin, proteins related to the NF2 gene product, merlin, were unchanged in schwannoma cells compared to NHSC. Our ®ndings demonstrate for the ®rst time that cell proliferation and actin organization are aberrant in schwannoma cells. Because NF2 is mutant in most or all human schwannomas, we postulate that loss of NF2 contributes to the cell growth and cytoskeletal dysfunction reported here.

Merlin: a tumour suppressor with functions at the cell cortex and in the nucleus

“…It has long been known that cultured schwannoma and meningioma cells have abundant lamellipodia and membrane ruffles, which are reminiscent of fibroblasts expressing activated forms of Rac [78][79][80]. Loss of Merlin was later shown to activate Rac signalling, which has been attributed to the release of negative regulation that Merlin exerts on PAK, consistent with the hypothesis that Merlin functions both upstream and downstream from PAK in a positive feedback loop (Fig 3B; [7,[23][24][25]79]).…”

Inhibition of the hyaluronan-CD44 interaction by merlin contributes to the tumor-suppressor activity of merlin

“…Unlike the ERM proteins, merlin lacks the conventional actin-binding region in its COOH-terminus, but it contains an actin-binding site in its NH 2 -terminal domain and interacts indirectly with the actin cytoskeleton through the actin-binding protein, bII-spectrin (Scoles et al, 1998). Association of merlin with the actin cytoskeleton is thought to underlie its effects on actin cytoskeletal organization (Gutmann et al, 1999b), cell spreading (Pelton et al, 1998) and motility (McClatchey et al, 1998). The ERM proteins and merlin likely play different or even opposite roles in organization of the cortical actin and tumorigenesis by differentially affecting the functions of their common binding partners such as CD44.…”

“…Expression of merlinDel-1 alters the interaction between CD44 and ezrin, and between CD44 and the actin cytoskeleton Increased expression of merlin impairs cell adhesion, spreading and motility, possibly because of its interaction with the actin cytoskeleton (Huynh and Pulst, 1996;Deguen et al, 1998;Pelton et al, 1998;Gutmann et al, 1999b). As a receptor for a major ECM component, CD44 plays an important role in regulating cell adhesion and migration (Lesley et al, 1993;Sherman et al, 1994;Stamenkovic 2000).…”

Magicin, a novel cytoskeletal protein associates with the NF2 tumor suppressor merlin and Grb2

“…Merlin binds to Pak and is thought to inhibit the recruitment of Pak to focal adhesions (Kissil et al, 2003). Furthermore, merlin-deficient schwannoma cells exhibit alterations in actin cytoskeleton organization that are reversed by reintroduction of wild-type but not mutant merlin (Pelton et al, 1998;Bashour et al, 2002). Thus, studies performed on merlin collectively suggest that merlin might indeed play a role in modulating many aspects of receptor-cytoskeleton linkage as well as in signaling to the cytoskeleton that is important for cell growth and adhesion.…”