An anti-Ras function of neurofibromatosis type 2 gene product (NF2/Merlin)

NF2: An underestimated player in cancer metabolic reprogramming and tumor immunity

Neurofibromatosis type 2 (NF2) is a tumor suppressor gene implicated in various tumors, including mesothelioma, schwannomas, and meningioma. As a member of the ezrin, radixin, and moesin (ERM) family of proteins, merlin, which is encoded by NF2, regulates diverse cellular events and signalling pathways, such as the Hippo, mTOR, RAS, and cGAS-STING pathways. However, the biological role of NF2 in tumorigenesis has not been fully elucidated. Furthermore, cross-cancer mutations may exert distinct biological effects on tumorigenesis and treatment response. In addition to the functional inactivation of NF2, the codeficiency of other genes, such as cyclin-dependent kinase inhibitor 2A/B (CDKN2A/B), BRCA1-associated protein-1 (BAP1), and large tumor suppressor 2 (LATS2), results in unique tumor characteristics that should be considered in clinical treatment decisions. Notably, several recent studies have explored the metabolic and immunological features associated with NF2, offering potential insights into tumor biology and the development of innovative therapeutic strategies. In this review, we consolidate the current knowledge on NF2 and examine the potential connection between cancer metabolism and tumor immunity in merlin-deficient malignancies. This review may provide a deeper understanding of the biological roles of NF2 and guide possible therapeutic avenues.

Merlin tumor suppressor function is regulated by PIP2-mediated dimerization

Neurofibromatosis Type 2 is an inherited disease characterized by Schwann cell tumors of cranial and peripheral nerves. The NF2 gene encodes Merlin, a member of the ERM family consisting of an N-terminal FERM domain, a central α-helical region, and a C-terminal domain. Changes in the intermolecular FERM-CTD interaction allow Merlin to transition between an open, FERM accessible conformation and a closed, FERM-inaccessible conformation, modulating Merlin activity. Merlin has been shown to dimerize, but the regulation and function Merlin dimerization is not clear. We used a nanobody based binding assay to show that Merlin dimerizes via a FERM-FERM interaction, orientated with each C-terminus close to each other. Patient derived and structural mutants show that dimerization controls interactions with specific binding partners, including HIPPO pathway components, and correlates with tumor suppressor activity. Gel filtration experiments showed that dimerization occurs after a PIP2 mediated transition from closed to open conformation monomers. This process requires the first 18 amino acids of the FERM domain and is inhibited by phosphorylation at serine 518. The discovery that active, open conformation Merlin is a dimer represents a new paradigm for Merlin function with implications for the development of therapies designed to compensate for Merlin loss.

KRAS Pathway Alterations in Malignant Pleural Mesothelioma: An Underestimated Player

Simple Summary

Malignant pleural mesothelioma (MPM) is a rare, incurable cancer. KRAS pathway alterations are frequent in human MPM but have been likely underestimated by next generation sequencing studies.

Abstract

Malignant pleural mesothelioma (MPM) is a rare, incurable cancer of the mesothelial cells lining the lungs and the chest wall that is mainly caused by asbestos inhalation. The molecular mechanisms of mesothelial carcinogenesis are still unclear despite comprehensive studies of the mutational landscape of MPM, and the most frequently mutated genes BAP1, NF2, CDKN2A, TP53, and TSC1 cannot cause MPM in mice in a standalone fashion. Although KRAS pathway alterations were sporadically detected in older studies employing targeted sequencing, they have been largely undetected by next generation sequencing. We recently identified KRAS mutations and copy number alterations in a significant proportion of MPM patients. Here, we review and analyze multiple human datasets and the published literature to show that, in addition to KRAS, multiple other genes of the KRAS pathway are perturbed in a significant proportion of patients with MPM.

KRAS signaling in malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) arises from mesothelial cells lining the pleural cavity of asbestos-exposed individuals and rapidly leads to death. MPM harbors loss-of-function mutations in BAP1, NF2, CDKN2A, and TP53, but isolated deletion of these genes alone in mice does not cause MPM and mouse models of the disease are sparse. Here, we show that a proportion of human MPM harbor point mutations, copy number alterations, and overexpression of KRAS with or without TP53 changes. These are likely pathogenic, since ectopic expression of mutant KRASG12D in the pleural mesothelium of conditional mice causes epithelioid MPM and cooperates with TP53 deletion to drive a more aggressive disease form with biphasic features and pleural effusions. Murine MPM cell lines derived from these tumors carry the initiating KRASG12D lesions, secondary Bap1 alterations, and human MPM-like gene expression profiles. Moreover, they are transplantable and actionable by KRAS inhibition. Our results indicate that KRAS alterations alone or in accomplice with TP53 alterations likely play an important and underestimated role in a proportion of patients with MPM, which warrants further exploration.

Contribution of mTOR and PTEN to Radioresistance in Sporadic and NF2-Associated Vestibular Schwannomas: A Microarray and Pathway Analysis

The use of radiation treatment has increased for both sporadic and neurofibromatosis type 2 (NF2)-associated vestibular schwannoma (VS). However, there are a subset of radioresistant tumors and systemic treatments that are seldom used in these patients. We investigated molecular alterations after radiation in three NF2-associated and five sporadically operated recurrent VS after primary irradiation. We compared these findings with 49 non-irradiated (36 sporadic and 13 NF2-associated) VS through gene-expression profiling and pathway analysis. Furthermore, we stained the key molecules of the distinct pathway by immunohistochemistry. A total of 195 differentially expressed genes in sporadic and NF2-related comparisons showed significant differences based on the criteria of p value < 0.05 and a two-fold change. These genes were involved in pathways that are known to be altered upon irradiation (e.g., mammalian target of rapamycin (mTOR), phosphatase and tensin homolog (PTEN) and vascular endothelial growth factor (VEGF) signaling). We observed a combined downregulation of PTEN signaling and an upregulation of mTOR signaling in progressive NF2-associated VS after irradiation. Immunostainings with mTOR and PTEN antibodies confirmed the respective molecular alterations. Taken together, mTOR inhibition might be a promising therapeutic strategy in NF2-associated VS progress after irradiation.

The NF2 tumor suppressor merlin interacts with Ras and RasGAP, which may modulate Ras signaling

Inactivation of the tumor suppressor NF2/merlin underlies neurofibromatosis type 2 (NF2) and some sporadic tumors. Previous studies have established that merlin mediates contact inhibition of proliferation; however, the exact mechanisms remain obscure and multiple pathways have been implicated. We have previously reported that merlin inhibits Ras and Rac activity during contact inhibition, but how merlin regulates Ras activity has remained elusive. Here we demonstrate that merlin can directly interact with both Ras and p120RasGAP (also named RasGAP). While merlin does not increase the catalytic activity of RasGAP, the interactions with Ras and RasGAP may fine-tune Ras signaling. In vivo, loss of RasGAP in Schwann cells, unlike the loss of merlin, failed to promote tumorigenic growth in an orthotopic model. Therefore, modulation of Ras signaling through RasGAP likely contributes to, but is not sufficient to account for, merlin’s tumor suppressor activity. Our study provides new insight into the mechanisms of merlin-dependent Ras regulation and may have additional implications for merlin-dependent regulation of other small GTPases.

Phosphorylation of Merlin by Aurora A kinase appears necessary for mitotic progression

Although Merlin's function as a tumor suppressor and regulator of mitogenic signaling networks such as the Ras/rac, Akt, and Hippo pathways is well-documented, in mammals as well as in insects, its role during cell cycle progression remains unclear. In this study, using a combination of approaches, including FACS analysis, time-lapse imaging, immunofluorescence microscopy, and co-immunoprecipitation, we show that Ser-518 of Merlin is a substrate of the Aurora protein kinase A during mitosis and that its phosphorylation facilitates the phosphorylation of a newly discovered site, Thr-581. We found that the expression in HeLa cells of a Merlin variant that is phosphorylation-defective on both sites leads to a defect in centrosomes and mitotic spindles positioning during metaphase and delays the transition from metaphase to anaphase. We also show that the dual mitotic phosphorylation not only reduces Merlin binding to microtubules but also timely modulates ezrin interaction with the cytoskeleton. Finally, we identify several point mutants of Merlin associated with neurofibromatosis type 2 that display an aberrant phosphorylation profile along with defective α-tubulin-binding properties. Altogether, our findings of an Aurora A-mediated interaction of Merlin with α-tubulin and ezrin suggest a potential role for Merlin in cell cycle progression.

Mutual regulation of the Hippo/Wnt/LPA/TGF‑β signaling pathways and their roles in glaucoma (Review)

Glaucoma is the leading cause of irreversible blindness worldwide and there is no effective treatment thus far. The trabecular meshwork has been identified as the major pathological area involved. Certain signaling pathways in the trabecular meshwork, including the Wnt, lysophosphatidic acid and transforming growth factor-β pathways, have been identified as novel therapeutic targets in glaucoma treatment. Meanwhile, it has been reported that key proteins in these pathways, particularly the primary transcription regulator Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), exhibit interactions with the Hippo pathway. The Hippo pathway, which was first identified in Drosophila, has drawn great focus with regard to various aspects of studies in recent years. One role of the Hippo pathway in the regulation of organ size was indicated by more recent evidence. Defining the relevant physiological function of the Hippo pathway has proven to be extremely complicated. Studies have ascribed a role for the Hippo pathway in an overwhelming number of processes, including cell proliferation, cell death and cell differentiation. Therefore, the present review aimed to unravel the roles of YAP and TAZ in the Hippo pathway and the pathogenesis of glaucoma. Furthermore, a new and creative study for the treatment of glaucoma is provided.

The Hippo Pathway: A Master Regulatory Network Important in Development and Dysregulated in Disease

The Hippo Pathway is a master regulatory network that regulates proliferation, cell growth, stemness, differentiation, and cell death. Coordination of these processes by the Hippo Pathway throughout development and in mature organisms in response to diverse external and internal cues plays a role in morphogenesis, in controlling organ size, and in maintaining organ homeostasis. Given the importance of these processes, the Hippo Pathway also plays an important role in organismal health and has been implicated in a variety of diseases including eye disease, cardiovascular disease, neurodegeneration, and cancer. This review will focus on Drosophila reports that identified the core components of the Hippo Pathway revealing specific downstream biological outputs of this complicated network. A brief description of mammalian reports will complement review of the Drosophila studies. This review will also survey upstream regulation of the core components with a focus on feedback mechanisms.

Role of Merlin/NF2 inactivation in tumor biology

Merlin (Moesin-ezrin-radixin-like protein, also known as schwannomin) is a tumor suppressor protein encoded by the neurofibromatosis type 2 gene NF2. Loss of function mutations or deletions in NF2 cause neurofibromatosis type 2 (NF2), a multiple tumor forming disease of the nervous system. NF2 is characterized by the development of bilateral vestibular schwannomas. Patients with NF2 can also develop schwannomas on other cranial and peripheral nerves, as well as meningiomas and ependymomas. The only potential treatment is surgery/radiosurgery, which often results in loss of function of the involved nerve. There is an urgent need for chemotherapies that slow or eliminate tumors and prevent their formation in NF2 patients. Interestingly NF2 mutations and merlin inactivation also occur in spontaneous schwannomas and meningiomas, as well as other types of cancer including mesothelioma, glioma multiforme, breast, colorectal, skin, clear cell renal cell carcinoma, hepatic and prostate cancer. Except for malignant mesotheliomas, the role of NF2 mutation or inactivation has not received much attention in cancer, and NF2 might be relevant for prognosis and future chemotherapeutic approaches. This review discusses the influence of merlin loss of function in NF2-related tumors and common human cancers. We also discuss the NF2 gene status and merlin signaling pathways affected in the different tumor types and the molecular mechanisms that lead to tumorigenesis, progression and pharmacological resistance.

Inhibition of Cdc42-mediated activation of mixed lineage kinase 3 by the tumor suppressor protein merlin

Mammalian mitogen-activated protein kinase (MAPK) signaling pathways respond to diverse extracellular signals and coordinate a range of cellular responses. Mixed lineage kinase 3 (MLK3) is a member of the mixed lineage kinase family of MAPK kinase kinases (MAP3Ks) that functions to regulate multiple MAPK signaling pathways. Activated forms of the Rho GTP ases, Rac and Cdc42, interact with MLK3 through the Cdc42/Rac-interactive binding (CRIB) motif and promote MLK3 catalytic activity. Our recent findings demonstrate that merlin, the product of the neurofibromatosis type 2 (NF2) tumor suppressor gene, is a physiological inhibitor of MLK3. Our results suggest that merlin inhibits MLK3 activity by blocking the Cdc42-MLK3 interaction. In this commentary, the effect of merlin on Cdc42-mediated activation of MLK3 and MAPK signaling will be discussed.

Merlin, the NF2 gene product

Merlin, the protein product of NF2 gene, is one of the most versatile tumor suppressors capable of integrating different mechanisms that regulate cell proliferation, motility, survival and signaling pathways underlying and governing those mechanisms. Merlin is considered a member of the band 4.1 families of cytoskeleton-associated proteins also called ERM family and acts as tumor suppressor. The main cause for transformation of Schwann cells into schwannomas is credited to the inactivation of the neurofibromin 2 (NF2) gene and the consecutive loss of its protein merlin. Recent scientific advances improved our understanding of pathogenic mechanisms involving NF2 gene. The present review brings genetic properties of NF2 gene, molecular characteristics of merlin, summarizes mutational spectra and explains merlin's multifunctional roles regarding its involvement in neurofibromatosis associated tumorigenesis.

Silencing of CD44 gene expression in human 143-B osteosarcoma cells promotes metastasis of intratibial tumors in SCID mice

Osteosarcoma (OS) is the most frequent primary malignant bone cancer in children and adolescents with a high propensity for lung metastasis. Therefore, it is of great importance to identify molecular markers leading to increased metastatic potential in order to devise more effective therapeutic strategies that suppress metastasis, the major cause of death in OS. CD44, the principal receptor for the extracellular matrix component hyaluronan (HA), is frequently found overexpressed in tumor cells and has been implicated in metastatic spread in various cancer types. Here, we investigated the effects of stable shRNA-mediated silencing of CD44 gene products on in vitro and in vivo metastatic properties of the highly metastatic human 143-B OS cell line. In vitro, CD44 knockdown resulted in a 73% decrease in the adhesion to HA, a 57% decrease in the migration rate in a trans-filter migration assay, and a 28% decrease in the cells' capacity for anchorage-independent growth in soft agar compared to the control cells, implicating that CD44 expression contributes to the metastatic activity of 143-B cells. However, making use of an orthotopic xenograft OS mouse model, we demonstrated that reduced CD44 expression facilitated primary tumor growth and formation of pulmonary metastases. The enhanced malignant phenotype was associated with decreased adhesion to HA and reduced expression of the tumor suppressor merlin in vivo. In conclusion, our study identified CD44 as a metastasis suppressor in this particular experimental OS model.

Distinct overlapping sequences at the carboxy-terminus of merlin regulate its tumour suppressor and morphogenic activity

The Neurofibromatosis 2 (NF2) gene product merlin is a tumour suppressor, which in addition to inhibiting cell proliferation regulates cell morphology. The morphogenic properties of merlin may play a role in tumour suppression, as patient-derived tumour cells demonstrate cytoskeletal abnormalities. However, it is still unclear how these functions are linked. The N-terminal FERM-domain of merlin is highly homologous to the oncogenic protein ezrin, while the C-termini are less conserved, suggesting that the opposite effect of the proteins on proliferation could be mediated by their distinct C-terminal regions. In this study we characterize the role of the most C-terminal residues of merlin in the regulation of proliferation, cytoskeletal organization, phosphorylation and intramolecular associations. In addition to the two full-length merlin isoforms and truncating mutations found in patients, we focused on the evolutionally conserved C-terminal residues 545-547, also harbouring disease-causing mutations. We demonstrate that merlin induces cell extensions, which result from impaired retraction of protrusions rather than from increased formation of filopodia. The residues 538-568 were found particularly important for this morphogenic activity. The results further show that both merlin isoforms are able to equally inhibit proliferation, whereas C-terminal mutants affecting residues 545-547 are less effective in growth suppression. This study demonstrates that the C-terminus contains distinct but overlapping functional domains important for regulation of the morphogenic activity, intramolecular associations and cell proliferation.

Inhibition of MEK pathway in vestibular schwannoma cell culture

OBJECTIVES/HYPOTHESIS

The purpose of this study was to evaluate the Ras GTPase (Ras) to extracellular signal-regulated kinase (ERK) pathway in vestibular schwannoma (VS) cell cultures and patient excised schwannoma tumors. Mitogen-activated protein kinase kinase (MEK) inhibitor CI-1040 (PD184352) was utilized to evaluate the effect of specific MEK inhibition on benign schwannoma cell culture proliferation and apoptosis.

STUDY DESIGN

Prospective evaluation of human schwannoma cell lines and tumors.

METHODS

Western blotting was completed with phospho-antibodies for proteins in the Ras-ERK pathway. Increasing concentrations of CI-1040 were utilized in schwannoma cell cultures to evaluate cell proliferation and apoptosis. Proliferation was measured with the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide proliferation assay, and apoptosis was monitored with flow cytometry of annexin V/propidium iodide-stained cells.

RESULTS

The most consistent Ras-ERK pathway alterations were found in phospho-MEK and ERK. Phospho-MEK was not overexpressed in the schwannoma cell lines, but six out of 10 VS showed significant increases compared to benign Schwann cell controls. Similarly, nine of 10 VS tumors showed increased phospho-ERK expression. CI-1040 showed significantly reduced schwannoma cell proliferation at the 50 and 100 μM (IC(50) 20 μM and 30 μM) concentrations when compared to carrier only controls in two out three schwannoma cell lines. The remaining schwannoma cell line was relatively refractory to the antiproliferative effects of CI-1040 at doses up to 100 μM (IC(50) 58 μM). Cumulative data of four separate schwannoma cell lines demonstrated that apoptosis was increased in treated schwannoma cells at CI-1040 concentrations of 50 and 100 μM at 72 hours.

CONCLUSIONS

There is overexpression of phosphorylated (activated) proteins in the Ras-ERK pathway in schwannoma cultures and tumors as compared to benign human Schwann cell culture controls. MEK inhibitor, CI-1040, created significantly decreased schwannoma cell proliferation and increased apoptosis in cell culture. These data justify the use of MEK inhibitors in animal treatment studies of VS.

Merlin: the wizard requires protein stability to function as a tumor suppressor

Neurofibromatosis type 2 (NF2), characterized by tumors of the nervous system, is a result of functional loss of the NF2 gene. The NF2 gene encodes Merlin (moesin-ezrin-radixin-like protein), an ERM (Ezrin, Radixin, Moesin) protein family member. Merlin functions as a tumor suppressor through impacting mechanisms related to proliferation, apoptosis, survival, motility, adhesion, and invasion. Several studies have summarized the tumor intrinsic mutations in Merlin. Given the fact that tumor cells are not in isolation, but rather in an intricate, mutually sustaining synergy with their surrounding stroma, the dialog between the tumor cells and the stroma can potentially impact the molecular homeostasis and promote evolution of the malignant phenotype. This review summarizes the epigenetic modifications, transcript stability, and post-translational modifications that impact Merlin. We have reviewed the role of extrinsic factors originating from the tumor milieu that influence the availability of Merlin inside the cell. Information regarding Merlin regulation could lead to novel therapeutics by stabilizing Merlin protein in tumors that have reduced Merlin protein expression without displaying any NF2 genetic alterations.

Molecular mechanisms promoting the pathogenesis of Schwann cell neoplasms

Neurofibromas, schwannomas and malignant peripheral nerve sheath tumors (MPNSTs) all arise from the Schwann cell lineage. Despite their common origin, these tumor types have distinct pathologies and clinical behaviors; a growing body of evidence indicates that they also arise via distinct pathogenic mechanisms. Identification of the genes that are mutated in genetic diseases characterized by the development of either neurofibromas and MPNSTs [neurofibromatosis type 1 (NF1)] or schwannomas [neurofibromatosis type 2 (NF2), schwannomatosis and Carney complex type 1] has greatly advanced our understanding of these mechanisms. The development of genetically engineered mice with ablation of NF1, NF2, SMARCB1/INI1 or PRKAR1A has confirmed the key role these genes play in peripheral nerve sheath tumorigenesis. Establishing the functions of the NF1, NF2, SMARCB1/INI1 and PRKAR1A gene products has led to the identification of key cytoplasmic signaling pathways promoting Schwann cell neoplasia and identified new therapeutic targets. Analyses of human neoplasms and genetically engineered mouse models have established that interactions with other tumor suppressors such as TP53 and CDKN2A promote neurofibroma-MPNST progression and indicate that intratumoral interactions between neoplastic and non-neoplastic cell types play an essential role in peripheral nerve sheath tumorigenesis. Recent advances have also provided new insights into the identity of the neural crest-derived populations that give rise to different types of peripheral nerve sheath tumors. Based on these findings, we now have an initial outline of the molecular mechanisms driving the pathogenesis of neurofibromas, MPNSTs and schwannomas. However, this improved understanding in turn raises a host of intriguing new questions.

Regulation of Drosophila glial cell proliferation by Merlin-Hippo signaling

Glia perform diverse and essential roles in the nervous system, but the mechanisms that regulate glial cell numbers are not well understood. Here, we identify and characterize a requirement for the Hippo pathway and its transcriptional co-activator Yorkie in controlling Drosophila glial proliferation. We find that Yorkie is both necessary for normal glial cell numbers and, when activated, sufficient to drive glial over-proliferation. Yorkie activity in glial cells is controlled by a Merlin-Hippo signaling pathway, whereas the upstream Hippo pathway regulators Fat, Expanded, Crumbs and Lethal giant larvae have no detectable role. We extend functional characterization of Merlin-Hippo signaling by showing that Merlin and Hippo can be physically linked by the Salvador tumor suppressor. Yorkie promotes expression of the microRNA gene bantam in glia, and bantam promotes expression of Myc, which is required for Yorkie and bantam-induced glial proliferation. Our results provide new insights into the control of glial growth, and establish glia as a model for Merlin-specific Hippo signaling. Moreover, as several of the genes we studied have been linked to human gliomas, our results suggest that this linkage could reflect their organization into a conserved pathway for the control of glial cell proliferation.

Radiobiology of vestibular schwannomas: mechanisms of radioresistance and potential targets for therapeutic sensitization

Vestibular schwannomas (VS) are benign tumors arising from the Schwann cells of cranial nerve VIII. Historically the prevailing therapy for patients with VS has been microsurgical resection. More recently, stereotactic radiosurgery (SRS) and fractionated stereotactic radiotherapy have gained acceptance as effective alternatives. Although the side effect profile and rates of tumor control appear to be favorable for SRS, there is a subset of radioresistant tumors that continue to progress despite properly administered radiation treatment. In this review, the authors summarize what is known about the mechanism of radioresistance in VS at the clinical and molecular level. An improved understanding of the radiobiological behavior of VS may help guide appropriate patient selection for SRS and potentially aid in the design of novel therapies to treat radioresistant tumors.

Molecular biology of familial and sporadic vestibular schwannomas: implications for novel therapeutics

Vestibular schwannomas (VSs) are benign tumors arising from the sheath of cranial nerve VIII. The pathogenesis underlying most familial and sporadic VSs has been linked to a mutation in a single gene, the neurofibromin 2 (NF2) gene located on chromosome 22, band q11-13.1. In this review, the authors summarized what is known about the epidemiology of NF2 mutations and patients with VSs. The authors also discuss the function of the NF2 gene product, merlin, and describe the known and hypothetical effects of genetic mutations that lead to merlin dysfunction on a broad variety of cellular and histological end points. A better understanding of the molecular pathobiology of VSs may lead to novel therapeutics to augment current modalities of treatment while minimizing morbidity.

Mammalian NDR/LATS protein kinases in hippo tumor suppressor signaling

The NDR/LATS family of kinases is a subgroup of the AGC group of protein kinases and is conserved from lower eukaryotes to humans. Like other AGC kinases, NDR/LATS kinases require phosphorylation of conserved Ser/Thr residues for activation. On the one hand, binding of the coactivator MOB to NDR/LATS allows autophosphorylation. On the other hand, MST kinases directly phosphorylate NDR/LATS kinases. In addition to our understanding of the molecular activation mechanisms, recent studies have shown that LATS kinases play a central role in Hippo/SWH (Salvador/Warts/Hippo) tumor suppressor pathways, which coordinate cell proliferation and apoptosis by regulating proto-oncogenes, such as YAP and TAZ. In this review, we summarize current knowledge of Merlin/MST/SAV/MOB/LATS/NDR/YAP/TAZ networks (also termed mammalian Hippo signaling) and their roles in mammalian cellular transformation.

Hereditary tumor syndromes and gliomas

Several congenital syndromes caused by germline mutations in tumor suppressor genes predispose to the development of glial tumors. In the last few decades our knowledge about the molecular functions of these genes and the pathogenesis of hereditary tumor syndromes has greatly increased. The most common syndromes are the neurofibromatoses (type 1 and type 2) and the tuberous scleroses complex. There are interesting overlaps in the molecular pathogen-esis. Deregulation of Ras or downstream Ras pathways including MEK/ERK and AKT/ mTOR plays an important role in these three syndromes. Other rare syndromes include Li-Fraumeni, melanoma-astrocytoma, and Turcot syndrome involving cell cycle regulators and DNA repair genes. The genes and pathways involved in the pathogenesis of these syndromes also play an important role in the development of sporadic tumors. Therefore research on hereditary syndromes contributes substantially to our understanding of tumor formation.

The phosphorylation status of merlin in sporadic vestibular Schwannomas

The events leading to Schwannomas development are still largely unknown. Some studies have demonstrated that merlin acts as a tumor suppressor by blocking Ras-mediated signaling. In this study, we analyze the clinical and biological behaviors of seven randomly selected sporadic vestibular Schwannomas removed from the patients. We find that merlin was commonly lost in these Schwannomas, due to loss of merlin expression or phosphorylation status of merlin expression. Heightened CDKs/cyclins signal transduction concomitant with loss of p27 was well correlated with loss of functional merlin in Schwannomas. More, we show that phosphorylated merlin Schwannomas exhibited increased Ras/Rac/PAK signal transduction. That was in agreement with the severe clinical behaviors, i.e., phosphorylation status of merlin increased tumor size in sporadic vestibular Schwannomas. These results led us to suggest that phosphorylated merlin, a kind of type of mutation merlin, is involved in tumorigenesis of sporadic vestibular Schwannomas.

The neurofibromatosis 2 protein, merlin, regulates glial cell growth in an ErbB2- and Src-dependent manner

Individuals with the inherited cancer predisposition syndrome neurofibromatosis 2 (NF2) develop several central nervous system (CNS) malignancies, including glial cell neoplasms (ependymomas). Recent studies have suggested that the NF2 protein, merlin (or schwannomin), may regulate receptor tyrosine kinase signaling, intracellular mitogenic growth control pathways, or adherens junction organization in non-nervous-system cell types. For this report, we used glial fibrillary acidic protein conditional knockout mice and derivative glia to determine how merlin regulates CNS glial cell proliferation. We show that the loss of merlin in glial cells results in increased proliferation in vitro and in vivo. Merlin regulation of glial cell growth reflects deregulated Src activity, such that pharmacologic or genetic inhibition of Src activation reduces Nf2(-/-) glial cell growth to wild-type levels. We further show that Src regulates Nf2(-/-) glial cell growth by sequentially regulating FAK and paxillin phosphorylation/activity. Next, we demonstrate that Src activation results from merlin regulation of ErbB2 activation and that genetic or pharmacologic ErbB2 inhibition reduces Nf2(-/-) glial cell Src/Src effector activation and proliferation to wild-type levels. Lastly, we show that merlin competes with Src for direct binding to ErbB2 and present a novel molecular mechanism for merlin regulation of ErbB2-dependent Src signaling and growth control.

The neurofibromatosis 2 tumor suppressor gene product, merlin, regulates human meningioma cell growth by signaling through YAP

Neurofibromatosis type 2 (NF2) is an autosomal dominant disorder characterized by the occurrence of schwannomas and meningiomas. Several studies have examined the ability of the NF2 gene product, merlin, to function as a tumor suppressor in diverse cell types; however, little is known about merlin growth regulation in meningiomas. In Drosophila, merlin controls cell proliferation and apoptosis by signaling through the Hippo pathway to inhibit the function of the transcriptional coactivator Yorkie. The Hippo pathway is conserved in mammals. On the basis of these observations, we developed human meningioma cell lines matched for merlin expression to evaluate merlin growth regulation and investigate the relationship between NF2 status and Yes-associated protein (YAP), the mammalian homolog of Yorkie. NF2 loss in meningioma cells was associated with loss of contact-dependent growth inhibition, enhanced anchorage-independent growth and increased cell proliferation due to increased S-phase entry. In addition, merlin loss in both meningioma cell lines and primary tumors resulted in increased YAP expression and nuclear localization. Finally, siRNA-mediated reduction of YAP in NF2-deficient meningioma cells rescued the effects of merlin loss on cell proliferation and S-phase entry. Collectively, these results represent the first demonstration that merlin regulates cell growth in human cancer cells by suppressing YAP.

Impaired intercellular adhesion and immature adherens junctions in merlin-deficient human primary schwannoma cells

Schwannomas that occur spontaneously or in patients with neurofibromatosis Type 2, lack both alleles for the tumor suppressor and plasma membrane-cytoskeleton linker merlin. We have shown that human primary schwannoma cells display activation of the RhoGTPases Rac1 and Cdc42 which results in highly dynamic and ongoing protrusive activity like ruffling. Ruffling is an initial and temporally limited step in the formation of intercellular contacts like adherens junctions that are based on the cadherin-catenin system. We tested if there is a connection between Rac1-induced ongoing ruffling and the maintenance, stabilization and functionality of adherens junctions and if this is of relevance in human, merlin-deficient schwannoma cells. We show intense ongoing ruffling is not limited to membranes of single human primary schwannoma cells, but occurs also in membranes of contacting cells, even when confluent. Live cell imaging shows that newly formed contacts are released after a short time, suggesting disturbed formation or stabilization of adherens junctions. Morphology, high phospho-tyrosine levels and cortactin staining indicate that adherens junctions are immature in human primary schwannoma cells, whereas they display characteristics of mature adherens junctions in human primary Schwann cells. When merlin is reintroduced, human primary schwannoma cells show only initial ruffling in contacting cells and adherens junctions appear more mature. We therefore propose that ongoing Rac-induced ruffling causes immature adherens junctions and leads to impaired, nonfunctional intercellular adhesion in aggregation assays in merlin-deficient schwannoma cells that could be an explanation for increased proliferation rates due to loss of contact inhibition or tumor development in general.

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.

A conditional mouse model for malignant mesothelioma

Malignant mesothelioma is a devastating disease that has been associated with loss of Neurofibromatosis type 2 (NF2) and genetic lesions affecting RB and P53 pathways. We introduced similar lesions in the mesothelial lining of the thoracic cavity of mice. Mesothelioma developed at high incidence in Nf2;Ink4a/Arf and Nf2;p53 conditional knockout mice with median survival times of approximately 30 and 20 weeks, respectively. Murine mesothelioma closely mimicked human malignant mesothelioma. Conditional Nf2;Ink4a/Arf mice showed increased pleural invasion compared to conditional Nf2;p53 mice. Interestingly, upon Ink4a loss in the latter mice median survival was significantly reduced and all tumors were highly invasive, suggesting that Ink4a loss substantially contributes to the poor clinical outcome of malignant mesothelioma.

The merlin interacting proteins reveal multiple targets for NF2 therapy

The neurofibromatosis 2 (NF2) tumor suppressor protein merlin is commonly mutated in human benign brain tumors. The gene altered in NF2 was located on human chromosome 22q12 in 1993 and the encoded protein named merlin and schwannomin. Merlin has homology to ERM family proteins, ezrin, radixin, and moesin, within the protein 4.1 superfamily. In efforts to determine merlin function several groups have discovered 34 merlin interacting proteins, including ezrin, radixin, moesin, CD44, layilin, paxillin, actin, N-WASP, betaII-spectrin, microtubules, TRBP, eIF3c, PIKE, NHERF, MAP, RalGDS, RhoGDI, EG1/magicin, HEI10, HRS, syntenin, caspr/paranodin, DCC, NGB, CRM1/exportin, SCHIP1, MYPT-1-PP1delta, RIbeta, PKA, PAK (three types), calpain and Drosophila expanded. Many of the proteins that interact with the merlin N-terminal domain also bind ezrin, while other merlin interacting proteins do not bind other members of the ERM family. Merlin also interacts with itself. This review describes these proteins, their possible roles in NF2, and the resultant hypothesized merlin functions. Review of all of the merlin interacting proteins and functional consequences of losses of these interactions reveals multiple merlin actions in PI3-kinase, MAP kinase and small GTPase signaling pathways that might be targeted to inhibit the proliferation of NF2 tumors.

Actin-Rich Protrusions and Nonlocalized GTPase Activation in Merlin-Deficient Schwannomas

Schwannomas lack both alleles for the tumor suppressor Merlin, a cytoskeleton-membrane linker. Previous results showed increased cell spreading of schwannoma cells, but little is known about the underlying mechanisms. Electron microscopy reveals that schwannoma cells not only show more lamellipodia/ruffles but also multiple filopodia. We show that Cdc42, important in filopodia formation, is activated. Both Rac1 and Cdc42 are found all around the cell periphery and in colocalization with their effector phospho-p21 activated kinase in human schwannoma cells. We therefore claim that Rac1 and Cdc42 are activated in a nonlocalized manner, which explains the disperse distribution of lamellipodia/ruffles and filopodia. Using live cell imaging, we further demonstrate continuous remodeling of the many actin-rich protrusions in schwannoma cells. The underlying cytoskeleton of these structures is thin and extensively branched. The actin-related protein 2/3 complex, a major regulator of actin branching, is enriched in the many lamellipodia and ruffles of human primary schwannoma cells. We suggest that the Merlin deficiency in human primary schwannoma cells leads to a random, nonlocalized activation of Rac1 and Cdc42, inducing many actin-rich protrusion zones, not only at the leading edge but also all around the cell periphery. Their nondirectional occurrence together with the continuous and highly dynamic actin remodeling results in the dedifferentiation of these tumor cells.

Experimental therapeutic approaches to peripheral nerve tumors

✓Discovery that the Schwann cell is the primary cell type responsible for both the neurofibroma as well as the schwannoma has proven to represent a crucial milestone in understanding the pathogenesis of peripheral nerve tumor development. This information and related findings have served as a nidus for research aimed at more fully characterizing this family of conditions. Recent discoveries in the laboratory have clarified an understanding of the molecular mechanisms underlying the pathogenesis of benign peripheral nerve tumors. Similarly, the mechanisms whereby idiopathic and syndromic (NF1- andNF2-associated) nerve sheath tumors progress to malignancy are being elucidated. This detailed understanding of the molecular pathogenesis of peripheral nerve tumors provides the information necessary to create a new generation of therapies tailored specifically to the prevention, cessation, or reversal of pathological conditions at the fundamental level of dysfunction. The authors review the data that have helped to elucidate the molecular pathogenesis of this category of conditions, explore the current progress toward exploitation of these findings, and discuss potential therapeutic avenues for future research.

The molecular biology of vestibular schwannomas: dissecting the pathogenic process at the molecular level

OBJECTIVE

The goal of this article was to review concisely what is currently known about the tumorigenesis of vestibular schwannomas.

BACKGROUND

Recent advances in molecular biology have led to a better understanding of the cause of vestibular schwannomas. Mutations in the neurofibromatosis type 2 tumor suppressor gene (NF2) have been identified in these tumors. In addition, the interactions of merlin, the protein product of the NF2 gene, and other cellular proteins are beginning to give us a better idea of NF2 function and the pathogenesis of vestibular schwannomas.

METHODS

Review of the relevant basic science studies at our institution as well as the basic science and clinical literature.

RESULTS

The clinical characteristics of vestibular schwannomas and neurofibromatosis type 2 syndromes are reviewed and related to alterations in the NF2 gene. Studies demonstrating our current understanding of tumor developmental pathways are highlighted. In addition, methods of clinical and genetic screening for neurofibromatosis type 2 disease are outlined. Avenues for the development of potential future research and therapies are discussed.

CONCLUSION

Great strides have been made to identify why vestibular schwannomas develop at the molecular level. Continued research is needed to find targeted therapies with which to treat these tumors.

NF2 gene expression in sporadic meningiomas: Relation to grades or histotypes real time-PCR study

One of the most common regions involved in the meningiomas tumorigenesis is chromosome 22q where the NF2 gene resides. The deficiency or loss of the NF2 gene product, merlin/schwannomin, plays a role in tumor development and metastatization. Conflicting results have been reported on the prognostic value of merlin in meningiomas. Several studies have indicated NF2 gene inactivation as an early tumorigenic event unrelated to the histological grade or clinical behavior. On the contrary, the NF2 gene alteration rate differs between the different histotypes. A pathogenesis independent from the NF2 gene has been suggested in meningothelial meningiomas. In the present work, we studied the NF2 gene expression through real time-PCR (RT-PCR) in 30 meningiomas. The average of the NF2 gene expression of all meningiomas was considered as reference value. The average of expression of WHO grade I and II meningiomas was higher than the average of all meningiomas, whereas that of WHO grade III meningiomas was lower. When we compared the NF2 gene expression in the different meningioma grades we did not note a significant difference (P = 0.698) despite the tendency to decrease from grade I to grade III. The average expression of meningothelial meningiomas was higher than the reference value, and that of non-meningothelial meningiomas was lower. The difference in NF2 gene expression between meningothelial and non-meningothelial meningiomas was statistically significant (P = 0.013). Our data supports the finding that alterations in NF2 gene alteration are histotype related but not grade related.

Conditional N-rasG12V expression promotes manifestations of neurofibromatosis in a mouse model

Human clinical neurofibromatosis type 1 (NF1) and type 2 (NF2) result from mutations and inactivation of neurofibromin and merlin genes, respectively, which negatively regulate Ras pathways. To evaluate the contribution of N-Ras activity to the development of NF, we generated a novel transgenic mouse expressing oncogenic N-ras specifically in central nerve cells, neural crest-derived cells and lens epithelial cells. Soon after birth, the mouse skin showed hyperpigmentation of the epidermis and melanin-laden macrophages in the dermis, as observed in the café-au-lait spots of human cases. At 3 months of age, all the mice had neurofibromas in the skin and neurofibroma-like tumors with structure similar to Wagner-Meissner bodies in the adrenal medulla. At 4 months of age, all the mice developed subcapsular cataract. In the 5th month, some developed protruding dermal neurofibromas involving subcutaneous fat. However, plexiform neurofibroma, schwannoma, astrocytoma and pheochromocytoma were not observed in the mice, suggesting a requirement for signal(s) other than the activated N-Ras pathway to induce these tumors. Thus, the activated N-Ras signal may be a main pathway for the development of the disease phenotypes characteristic of NF.

Merlin/Neurofibromatosis Type 2 Suppresses Growth by Inhibiting the Activation of Ras and Rac

The small G-protein Ras is a tightly controlled regulator of cell fate. Prolonged or persistent arrest in the activated GTP-loaded state by mutation of Ras as in lung cancer or in a Ras-GTPase-activating protein as in neurofibromatosis type 1 promotes tumorigenesis. We now show that the tumor-suppressor protein merlin (mutated in neurofibromatosis type 2) also controls Ras activity. Systematic analysis of growth factor signaling located the step of merlin interference to the activation of Ras and Rac. Merlin independently uncouples both Ras and Rac from growth factor signals. In the case of Ras, merlin acts downstream of the receptor tyrosine kinase-growth factor receptor binding protein 2 (Grb2)-SOS complex. However, merlin does not bind either SOS or Ras, but it counteracts the ERM (ezrin, radixin, moesin)-dependent activation of Ras, which correlates with the formation of a complex comprising ERM proteins, Grb2, SOS, Ras, and filamentous actin. Because efficient signaling from Ras requires Rac-p21-activated kinase-dependent phosphorylations of Raf and mitogen-activated protein/extracellular signal-regulated kinase kinase, merlin can also inhibit signal transfer from dominantly active Ras mutants. We propose that the interference of merlin with Ras- and Rac-dependent signal transfer represents part of the tumor-suppressive action of merlin.

Identification and characterization of putative tumor suppressor NGB, a GTP-binding protein that interacts with the neurofibromatosis 2 protein

Mutations of the neurofibromatosis 2 (NF2) tumor suppressor gene have frequently been detected not only in schwannomas and other central nervous system tumors of NF2 patients but also in their sporadic counterparts and malignant tumors unrelated to the NF2 syndrome such as malignant mesothelioma, indicating a broader role for the NF2 gene in human tumorigenesis. However, the mechanisms by which the NF2 product, merlin or schwannomin, is regulated and controls cell proliferation remain elusive. Here, we identify a novel GTP-binding protein, dubbed NGB (referring to NF2-associated GTP binding protein), which binds to merlin. NGB is highly conserved between Saccharomyces cerevisiae, Caenorhabditis elegans, and human cells, and its GTP-binding region is very similar to those found in R-ras and Rap2. However, ectopic expression of NGB inhibits cell growth, cell aggregation, and tumorigenicity in tumorigenic schwanomma cells. Down-regulation and infrequent mutation of NGB were detected in human glioma cell lines and primary tumors. The interaction of NGB with merlin impairs the turnover of merlin, yet merlin does not affect the GTPase nor GTP-binding activity of NGB. Finally, the tumor suppressor functions of NGB require merlin and are linked to its ability to suppress cyclin D1 expression. Collectively, these findings indicate that NGB is a tumor suppressor that regulates and requires merlin to suppress cell proliferation.

Regulation of the Neurofibromatosis 2 gene promoter expression during embryonic development

Mutations in the Neurofibromatosis 2 (NF2) gene are associated with predisposition to vestibular schwannomas, spinal schwannomas, meningiomas, and ependymomas. Presently, how NF2 is expressed during embryonic development and in the tissues affected by neurofibromatosis type 2 (NF2) has not been well defined. To examine NF2 expression in vivo, we generated transgenic mice carrying a 2.4-kb NF2 promoter driving beta-galactosidase (beta-gal) with a nuclear localization signal. Whole-mount embryo staining revealed that the NF2 promoter directed beta-gal expression as early as embryonic day E5.5. Strong expression was detected at E6.5 in the embryonic ectoderm containing many mitotic cells. beta-gal staining was also found in parts of embryonic endoderm and mesoderm. The beta-gal staining pattern in the embryonic tissues was corroborated by in situ hybridization analysis of endogenous Nf2 RNA expression. Importantly, we observed strong NF2 promoter activity in the developing brain and in sites containing migrating cells including the neural tube closure, branchial arches, dorsal aorta, and paraaortic splanchnopleura. Furthermore, we noted a transient change of NF2 promoter activity during neural crest cell migration. While little beta-gal activity was detected in premigratory neural crest cells at the dorsal ridge region of the neural fold, significant activity was seen in the neural crest cells already migrating away from the dorsal neural tube. In addition, we detected considerable NF2 promoter activity in various NF2-affected tissues such as acoustic ganglion, trigeminal ganglion, spinal ganglia, optic chiasma, the ependymal cell-containing tela choroidea, and the pigmented epithelium of the retina. The NF2 promoter expression pattern during embryogenesis suggests a specific regulation of the NF2 gene during neural crest cell migration and further supports the role of merlin in cell adhesion, motility, and proliferation during development.

Constitutive neuregulin-1/ErbB signaling contributes to human vestibular schwannoma proliferation

Vestibular schwannomas (VSs) are benign tumors that arise from the Schwann cells (SCs) lining the vestibular nerve. VS cells survive and proliferate far from neurons and axonally derived growth factors. We have previously shown that VSs produce the glial growth factor, neuregulin-1 (NRG1), and its receptors, ErbB2 and ErbB3. In the present work, we explore the contribution of constitutive NRG1:ErbB signaling to human VS cell proliferation. We confirm that human VSs, which express markers of immature and denervated SCs, also express endogenous NRG1 and activated ErbB2. We find that a blocking anti-NRG1 antibody and trastuzumab (Herceptin, HCN), a humanized anti-ErbB2 inhibitory monoclonal antibody, effectively inhibit NRG1 induced SC proliferation. Treatment of primary VS cultures with anti-NRG1 or HCN reduces cell proliferation in the absence of exogenous NRG1. Furthermore, conditioned medium from VS cell cultures contains NRG1 and stimulates SC proliferation in SC cultures, an effect that is inhibited by anti-NRG1 and HCN. These data suggest an autocrine pathway of VS growth stimulation involving NRG and ErbB receptors. Inhibition of constitutive NRG:ErbB signaling reduces VS cell proliferation in vitro and may have therapeutic potential for patients with VSs.

The genetic and molecular pathogenesis of NF1 and NF2

Neurofibromatosis types 1 and 2 (NF1 and NF2) are autosomal dominant phakomatoses. The NF1 and NF2 genes encode for neurofibromin and merlin, respectively. These 2 functionally unrelated proteins both act as tumor suppressor genes, possibly through modulation of the RAS/RAC oncogenic pathways. Improved understanding of the mechanisms by which these tumor suppressors act may allow for medical therapies for neurofibromatosis and may offer insights for cancer therapeutics.

Merlin inhibits growth hormone-regulated Raf–ERKs pathways by binding to Grb2 protein

Numerous studies have suggested that the NF2 protein merlin is involved in the regulation of abnormal cell growth and proliferation. In this study, to better understand the merlin's mechanisms that contribute to the inhibition of tumorigenesis, we examined the potential action of merlin on the cell proliferative signaling pathways in response to growth hormone (GH). Merlin effectively attenuated the GH-induced serum response element (SRE) and Elk-1-mediated transcriptional activation, as well as the endogenous SRE-regulated gene c-fos expression in NIH3T3 cells. In addition, merlin prevented the Raf-1 complex activation process, which resulted in the suppression of MAP kinase/ERK, extracellular signal-regulated kinase (ERKs), and Elk-1 phosphorylation, which are the downstream signals of Raf-1. Moreover, it was shown that merlin interacted with endogenous growth factor receptor bound 2 (Grb2) protein and inhibited its expression. These results suggest that merlin contributes, via its protein-to-protein interaction with Grb2 and consequent inhibition of the MAPK pathways, to the regulation of the abnormal cell proliferation, and this provides a further mechanism underlying the tumor suppressor function of merlin.

Advanced Glycation End Products Inhibit Tubulogenesis and Migration of Kidney Epithelial Cells in an Ezrin-Dependent Manner

Nonenzymatic glycation of proteins to form advanced glycation end products (AGE) is implicated in diabetic complications, including nephropathy. It was shown recently that AGE bind to the ERM (ezrin, radixin, and moesin) family of membrane-cytoskeletal linker proteins in renal homogenates. Herein is reported the effects of AGE-BSA on ezrin-dependent LLC-PK1 kidney epithelial cellular functions: migration and hepatocyte growth factor (HGF)-induced tubulogenesis. LLC-PK1 cells were stably transfected with cDNA for ezrin sense, ezrin antisense, and N-ezrin. Transfection of LLC-PK1 cells with ezrin antisense and dominant negative N-ezrin decreased basal tubulogenesis and migration relative to vector-only transfection, establishing the ezrin dependency of these processes. AGE-BSA (20 or 40 microM) significantly decreased HGF-induced tubulogenesis and basal migration in two vector control lines relative to BSA-treated cells. However, AGE-BSA inhibition of both HGF-induced tubulogenesis and migration was overcome by overexpressing ezrin. These results demonstrate that the AGE-ezrin interaction significantly alters cellular function. These changes may be relevant to detrimental renal consequences as a result of diabetes.

Membrane organization and tumorigenesis--the NF2 tumor suppressor, Merlin

The NF2 tumor-suppressor gene was cloned more than a decade ago, but the function of its encoded protein, Merlin, remains elusive. Merlin, like the closely related ERM proteins, appears to provide regulated linkage between membrane-associated proteins and the actin cytoskeleton and is therefore poised to function in receiving and interpreting signals from the extracellular milieu. Recent studies suggest that Merlin may coordinate the processes of growth-factor receptor signaling and cell adhesion. Varying use of this organizing activity by different types of cells could provide an explanation for the unique spectrum of tumors associated with NF2 deficiency in mammals.

The merlin tumor suppressor interacts with Ral guanine nucleotide dissociation stimulator and inhibits its activity

Neurofibromatosis type 2 (NF2) is the most commonly mutated gene in benign tumors of the human nervous system such as schwannomas and meningiomas. The NF2 gene encodes a protein called schwannomin or merlin, which is involved in regulating cell growth and proliferation through protein-protein interactions with various cellular proteins. In order to better understand the mechanism by which merlin exerts its function, yeast two-hybrid screening was performed and Ral guanine nucleotide dissociation stimulator (RalGDS), a downstream molecule of Ras, was identified as a merlin-binding protein. The direct interaction between merlin and RalGDS was confirmed both in vitro and in the NIH3T3 cells. The domain analyses revealed that the broad C-terminal region of merlin (aa 141-595) is necessary for the interaction with the C-terminal Ras-binding domain (RBD) of RalGDS. Functional studies showed that merlin inhibits the RalGDS-induced RalA activation, the colony formation and the cell migration in mammalian cells. These results suggest that merlin can function as a tumor suppressor by inhibiting the RalGDS-mediated oncogenic signals.

The NF2 tumor suppressor gene product, merlin, inhibits cell proliferation and cell cycle progression by repressing cyclin D1 expression

Inactivation of the NF2 tumor suppressor gene has been observed in certain benign and malignant tumors. Recent studies have demonstrated that merlin, the product of the NF2 gene, is regulated by Rac/PAK signaling. However, the mechanism by which merlin acts as a tumor suppressor has remained obscure. In this report, we show that adenovirus-mediated expression of merlin in NF2-deficient tumor cells inhibits cell proliferation and arrests cells at G1 phase, concomitant with decreased expression of cyclin D1, inhibition of CDK4 activity, and dephosphorylation of pRB. The effect of merlin on cell cycle progression was partially overridden by ectopic expression of cyclin D1. RNA interference experiments showed that silencing of the endogenous NF2 gene results in upregulation of cyclin D1 and S-phase entry. Furthermore, PAK1-stimulated cyclin D1 promoter activity was repressed by cotransfection of NF2, and PAK activity was inhibited by expression of merlin. Interestingly, the S518A mutant form of merlin, which is refractory to phosphorylation by PAK, was more efficient than the wild-type protein in inhibiting cell cycle progression and in repressing cyclin D1 promoter activity. Collectively, our data indicate that merlin exerts its antiproliferative effect, at least in part, via repression of PAK-induced cyclin D1 expression, suggesting a unifying mechanism by which merlin inactivation might contribute to the overgrowth seen in both noninvasive and malignant tumors.

Erbin Regulates Mitogen-activated Protein (MAP) Kinase Activation and MAP Kinase-dependent Interactions between Merlin and Adherens Junction Protein Complexes in Schwann Cells

Biallelic mutations in the neurofibromatosis 2 (NF2) gene are linked to schwannoma and meningioma tumorigenesis. Cells with NF2 mutations exhibit elevated levels of phosphorylated extracellular signal-regulated kinase (ERK) and aberrant cell-cell and cell-matrix contacts. The NF2 gene product, merlin, associates with adherens junction protein complexes, suggesting that part of its function as a tumor suppressor involves regulating cell junctions. Here, we find that a novel PDZ protein, called erbin, binds directly to the merlin-binding partner, EBP0, and regulates adherens junction dissociation through a MAP kinase-dependent mechanism. Reducing erbin expression using a targeted siRNA in primary cultures of Schwann cells results in altered cell-cell interactions, disruption of E-cadherin adherens junctions, increased cell proliferation, and elevated levels of phosphorylated ERK, all phenotypes observed in cells that lack merlin. Reduction of erbin expression also results in the dissociation of merlin from adherens junction proteins and an increase in the levels of phosphorylated merlin. These phenotypes can be rescued if cells with reduced levels of erbin are treated with a pharmacological inhibitor of ERK kinase. Collectively, these data indicate that erbin regulates MAP kinase activation in Schwann cells and suggest that erbin links merlin to both adherens junction protein complexes and the MAP kinase signaling pathway.

MAP, a protein interacting with a tumor suppressor, merlin, through the run domain

Merlin (or schwannomin) is a tumor suppressor encoded by the neurofibromatosis type 2 gene. Many studies have suggested that merlin is involved in the regulation of cell growth and proliferation through interactions with various cellular proteins. To better understand the function of merlin, we tried to identify the proteins that bind to merlin using the yeast two-hybrid screening. Characterization of the positive clones revealed a protein of 749 amino acids named merlin-associated protein (MAP), which showed wide tissue distribution in Northern blot analysis. Sequence analysis revealed that MAP is a potential homologue of a yeast check-point protein, BUB2, and contains TBC, SH3, and RUN domains, thereby implicating its role in the Ras-like GTPase signal pathways. MAP and merlin were directly associated in vitro and in vivo, and colocalized in NIH3T3 cells. The RUN domain of MAP and the C-terminus of merlin appeared to be responsible for their interaction. MAP decreased the AP-1-dependent promoter activity additively with merlin in NIH3T3 cells. In addition, merlin and MAP synergistically reduced the colony formation of NIH3T3 cells. These results suggest that MAP may play a cooperative role in the merlin-mediated growth suppression of cells.

A clue to the therapy of neurofibromatosis type 2: NF2/merlin is a PAK1 inhibitor

BACKGROUND

Neurofibromatosis type 2 is a group of tumors caused by loss-of-function mutations of a tumor suppressor gene encoding NF2/merlin. Development of chemotherapeutics for this disease, which often threatens the life of young children, has been hampered by a limited information on the signaling function of NF2. NF2 can inhibit Ras-induced malignant transformation. However, the primary (signaling) target of NF2 in the oncogenic pathway has not been previously identified.

RESULTS

Here, using a series of NF2 constructs, we show that NF2 inhibits directly the Rac/CDC42-dependent Ser/Thr kinase PAK1, which is essential for both Ras transformation and neurofibromatosis type 1 (NF1), through two separate domains. A mutant of NF2, that lacks the PAK1-inhibiting domain of 78 amino acids (NF78C, residues 447-524), fails to suppress Ras transformation. Furthermore, PAK1-specific inhibitors CEP-1347 and WR-PAK18 selectively inhibit the growth of NF2-deficient cancer cells, but not NF2-positive cells.

CONCLUSIONS

These results suggest that PAK1 is essential for the malignant growth of NF2-deficient cells, and that PAK1-blocking drugs could be potentially useful forthe treatment of neurofibromatosis types 2, in addition to Ras-induced cancers and neurofibromatosis type 1.