The NF2 tumor suppressor Merlin and the ERM proteins interact with N-WASP and regulate its actin polymerization function.

The function of the NF2 tumor suppressor merlin has remained elusive despite increasing evidence for its role in actin cytoskeleton reorganization. The closely related ERM proteins (ezrin, radixin, and moesin) act as linkers between the cell membrane and cytoskeleton, and have also been implicated as active actin reorganizers. We report here that merlin and the ERMs can interact with and regulate N-WASP, a critical regulator of actin dynamics. Merlin and moesin were found to inhibit N-WASP-mediated actin assembly in vitro, a function that appears independent of their ability to bind actin. Furthermore, exogenous expression of a constitutively active ERM inhibits N-WASP-dependent Shigella tail formation, suggesting that the ERMs may function as inhibitors of N-WASP function in vivo. This novel function of merlin and the ERMs illustrates a mechanism by which these proteins directly exert their effects on actin reorganization and also provides new insight into N-WASP regulation.

A NHERF binding site links the betaPDGFR to the cytoskeleton and regulates cell spreading and migration.

The Na(+)/H(+) exchanger regulatory factor, NHERF, is a multifunctional adapter protein involved in a wide range of physiological activities. NHERF associates with merlin and the ezrin/radixin/moesin (MERM) family of membrane-actin cytoskeletal linker proteins through its C-terminus and is capable of interacting via its PDZ1 domain to the betaPDGF receptor (betaPDGFR). Thus, NHERF, potentially links the betaPDGFR to the actin cytoskeleton through its interaction with MERM proteins. In the present study, we have examined whether abolishing the interaction of betaPDGFR with NHERF results in actin cytoskeletal rearrangements. We have stably expressed a wild-type betaPDGFR, a mutant betaPDGFR (L1106A) that is incapable of interacting with NHERF, as well as a kinase defective mutant receptor (K634R), in PDGFR-deficient mouse embryonic fibroblasts. Our observations indicate that cells expressing betaPDGFR (L1106A) were impaired in their ability to spread and migrate on fibronectin compared with wild-type and K634R cells. L1106A mutant cells also revealed an increased number of focal adhesions, a condensed F-actin ring at the cell periphery and a decrease in total focal adhesion kinase (FAK) tyrosine phosphorylation. Further, we show that NHERF and MERM proteins could act as intermediary bridging proteins between betaPDGFR and FAK. Thus, the interaction of betaPDGFR with NHERF may provide an essential link between the cell membrane and the cortical actin cytoskeleton independent of receptor activity.

HMGA2 expression in uterine leiomyomata and myometrium: quantitative analysis and tissue culture studies.

The high mobility group gene, HMGA2, is frequently expressed in uterine leiomyomata (UL) with chromosomal rearrangements of 12q15. In contrast, HMGA2 expression has not been detected in karyotypically normal UL or in myometrium, but has been detected in these tissues after culture. To characterize further the expression pattern of HMGA2, we assessed HMGA2 expression by RT-PCR followed by Southern blot hybridization, and by real-time PCR in three tissue panels: (1) primary myometrial cultures, (2) uncultured tissue from 15 karyotypically normal samples consisting of eleven 46,XX UL and four matched myometrial specimens, and (3) uncultured tissue from ten UL with 12q15 rearrangements and three matched myometrial specimens. HMGA2 expression was detected in all samples from the three panels. The level of HMGA2 expression in karyotypically normal UL was similar to the level of expression in myometrium; however, it was significantly less than the level measured in UL with 12q15 rearrangements. This expression analysis by use of detection methods of different sensitivities underscores the importance of studies of HMGA2 expression in uncultured tissues and of careful interpretation of results from experiments on cultured cells. Moreover, detection of HMGA2 expression in myometrium and in UL without 12q15 rearrangements, tissues previously thought not to express HMGA2, suggests that HMGA2 expression is required in normal adult myometrial physiology.