Tumor-suppression functions of merlin are independent of its role as an organizer of the actin cytoskeleton in Schwann cells

Morphological control of merlin-Rac antagonism in proliferation-promoting signaling

The extension of lamellipodia, which are thin, fanlike projections at the cell periphery, requires the assembly of branched actin networks under the control of the small GTPase Rac1. In melanoma, a hyperactive P29S Rac1 mutant is associated with resistance to inhibitors that target the kinases BRAF and MAPK and with more aggressive disease because it sequesters and inactivates the tumor suppressor merlin (encoded by NF2) inside abnormally large lamellipodia. Here, we investigated how these merlin-inactivating lamellipodia are maintained using quantitative, live cell imaging of cell morphology and signaling dynamics. We showed that Rac1 and merlin activity were regulated in spatially confined regions or microdomains within the lamellipodium. The role of merlin as a proliferation-limiting tumor suppressor required its ability to inhibit lamellipodial extension and to locally inhibit Rac1 signaling. Conversely, local inactivation of merlin in lamellipodia released these restraints on morphology and signaling, leading to enhanced proliferation. Merlin and Rac1 are thus in a morphologically and dynamically regulated double-negative feedback loop, a signaling motif that can amplify and stabilize modest stimuli of lamellipodia extensions that enable melanoma to sustain mitogenic signaling under growth challenge. This represents an example of how acute oncogenicity is promoted by collaborations between cell morphological programs and biochemical signaling.

Interactions among Merlin, Arkadia, and SKOR2 mediate NF2-associated human Schwann cell proliferation

BACKGROUND

NF2-related Schwannomatosis (previously referred to as Neurofibromatosis Type 2, or NF2) is a genetic-associated disease resulting from mutations in the gene, NF2. NF2 encodes the Merlin protein, which acts as a tumor suppressor. Bilateral vestibular schwannoma (VS) is a hallmark of NF2. Although the exactly molecular mechanism mediating NF2-driven schwannomatosis is not fully understood, it is known that defective Merlin protein functionality leads to abnormal cell proliferation.

METHODS

Herein, we utilized a human induced pluripotent stem cell (hiPSC)-based Schwann cell (SC) model to investigate the role of Merlin in human SCs. SCs were derived from hiPSCs carrying a NF2 mutation (c.191 T > C; p. L64P), its isogenic wild-type control cell line, and a NF2 patient-derived hiPSC line. Phenotypes were determined via immunocytochemistry and various bioassays. Different proteins interacting with Merlin in wild-type and NF2 mutation SCs were identified using co-immunoprecipitation followed by mass spectrometry.

RESULTS

SC derived from NF2L64P hiPSCs showed significantly higher proliferation and abnormal morphology compared to NF2WT SCs. Phenotypes that could be restored by wildtype NF2 overexpression. Interactome profiling of Merlin (NF2) in SCs derived from NF2WT- and NF2L64P- hiPCSs identified differential protein binding levels. Among identified proteins, we validated the interaction among Merlin, an E3 ubiquitin ligase (Arkadia), and a SKI family co-repressor (SKOR2). This complex plays a significant role for this interaction in SC proliferation. Our findings were further validated by SCs derived from the patient-derived hiPSCs carrying a deletion in the chromosome 22 which spans the NF2 gene.

CONCLUSIONS

Our results presented a hiPSC-derived SC system for SC-related disease modeling and established a new model in which Merlin interacts with Arkadia and SKOR2. This interaction is required for the proper cell proliferation in human SCs.

PI4P-mediated solid-like Merlin condensates orchestrate Hippo pathway regulation

Despite recent studies implicating liquid-like biomolecular condensates in diverse cellular processes, many biomolecular condensates exist in a solid-like state, and their function and regulation are less understood. We show that the tumor suppressor Merlin, an upstream regulator of the Hippo pathway, localizes to both cell junctions and medial apical cortex in Drosophila epithelia, with the latter forming solid-like condensates that activate Hippo signaling. Merlin condensation required phosphatidylinositol-4-phosphate (PI4P)-mediated plasma membrane targeting and was antagonistically controlled by Pez and cytoskeletal tension through plasma membrane PI4P regulation. The solid-like material properties of Merlin condensates are essential for physiological function and protect the condensates against external perturbations. Collectively, these findings uncover an essential role for solid-like condensates in normal physiology and reveal regulatory mechanisms for their formation and disassembly.

Antagonistic actions of PAK1 and NF2/Merlin drive myelin membrane expansion in oligodendrocytes

In the central nervous system, the formation of myelin by oligodendrocytes (OLs) relies on the switch from the polymerization of the actin cytoskeleton to its depolymerization. The molecular mechanisms that trigger this switch have yet to be elucidated. Here, we identified P21-activated kinase 1 (PAK1) as a major regulator of actin depolymerization in OLs. Our results demonstrate that PAK1 accumulates in OLs in a kinase-inhibited form, triggering actin disassembly and, consequently, myelin membrane expansion. Remarkably, proteomic analysis of PAK1 binding partners enabled the identification of NF2/Merlin as its endogenous inhibitor. Our findings indicate that Nf2 knockdown in OLs results in PAK1 activation, actin polymerization, and a reduction in OL myelin membrane expansion. This effect is rescued by treatment with a PAK1 inhibitor. We also provide evidence that the specific Pak1 loss-of-function in oligodendroglia stimulates the thickening of myelin sheaths in vivo. Overall, our data indicate that the antagonistic actions of PAK1 and NF2/Merlin on the actin cytoskeleton of the OLs are critical for proper myelin formation. These findings have broad mechanistic and therapeutic implications in demyelinating diseases and neurodevelopmental disorders.

The molecular biology of NF2/Merlin on tumorigenesis and development

The neurofibromatosis type 2 (NF2) gene, known for encoding the tumor suppressor protein Merlin, is central to the study of tumorigenesis and associated cellular processes. This review comprehensively examines the multifaceted role of NF2/Merlin, detailing its structural characteristics, functional diversity, and involvement in various signaling pathways such as Wnt/β-catenin, Hippo, TGF-β, RTKs, mTOR, Notch, and Hedgehog. These pathways are crucial for cellular growth, proliferation, and differentiation. NF2 mutations are specifically linked to the development of schwannomas, meningiomas, and ependymomas, although the precise mechanisms of tumor formation in these specific cell types remain unclear. Additionally, the review explores Merlin's role in embryogenesis, highlighting the severe developmental defects and embryonic lethality caused by NF2 deficiency. The potential therapeutic strategies targeting these genetic aberrations are also discussed, emphasizing inhibitors of mTOR, HDAC, and VEGF as promising avenues for treatment. This synthesis of current knowledge underscores the necessity for ongoing research to elucidate the detailed mechanisms of NF2/Merlin and develop effective therapeutic strategies, ultimately aiming to improve the prognosis and quality of life for individuals with NF2 mutations.

Molecular neuropathology of brain-invasive meningiomas

Invasion of brain tissue by meningiomas has been identified as one key factor for meningioma recurrence. The identification of meningioma tumor tissue surrounded by brain tissue in neurosurgical samples has been touted as a criterion for atypical meningioma (CNS WHO grade 2), but is only rarely seen in the absence of other high-grade features, with brain-invasive otherwise benign (BIOB) meningiomas remaining controversial. While post-surgery irradiation therapy might be initiated in brain-invasive meningiomas to prevent recurrences, specific treatment approaches targeting key molecules involved in the invasive process are not established. Here we have compiled the current knowledge about mechanisms supporting brain tissue invasion by meningiomas and summarize preclinical models studying targeted therapies with potential inhibitory effects.

Potential Molecular Biomarkers of Vestibular Schwannoma Growth: Progress and Prospects

Vestibular schwannomas (VSs, also known as acoustic neuromas) are relatively rare benign brain tumors stem from the Schwann cells of the eighth cranial nerve. Tumor growth is the paramount factor for neurosurgeons to decide whether to choose aggressive treatment approach or careful follow-up with regular magnetic resonance imaging (MRI), as surgery and radiation can introduce significant trauma and affect neurological function, while tumor enlargement during long-term follow-up will compress the adjacent nerves and tissues, causing progressive hearing loss, tinnitus and vertigo. Recently, with the deepening research of VS biology, some proteins that regulate merlin conformation changes, inflammatory cytokines, miRNAs, tissue proteins and cerebrospinal fluid (CSF) components have been proposed to be closely related to tumor volume increase. In this review, we discuss advances in the study of biomarkers that associated with VS growth, providing a reference for exploring the growth course of VS and determining the optimal treatment strategy for each patient.

Conformational flexibility determines the Nf2/merlin tumor suppressor functions

The Neurofibromatosis type 2 gene encodes the Nf2/merlin tumor suppressor protein that is responsible for the regulation of cell proliferation. Once activated, Nf2/merlin modulates adhesive signaling pathways and thereby inhibits cell growth. Nf2/merlin controls oncogenic gene expression by modulating the Hippo pathway. By responding to several physical and biochemical stimuli, Hippo signaling determines contact inhibition of proliferation as well as organ size. The large tumor suppressor (LATS) serine/threonine-protein kinase is the key enzyme in the highly conserved kinase cascade that negatively regulates the activity and localization of the transcriptional coactivators Yes-associated protein (YAP) and its paralogue transcriptional coactivator with PDZ-binding motif (TAZ). Nf2/merlin belongs to the band 4.1, ezrin, radixin, moesin (FERM) gene family that links the actin cytoskeleton to adherens junctions, remodels adherens junctions during epithelial morphogenesis and maintains organized apical surfaces on the plasma cell membrane. Nf2/merlin and ERM proteins have a globular N-terminal cloverleaf head domain, the FERM domain, that binds to the plasma membrane, a central α-helical domain, and a tail domain that binds to its head domain. Here we present the high-resolution crystal structure of Nf2/merlin bound to LATS1 which shows that LATS1 binding to Nf2/merlin displaces the Nf2/merlin tail domain and causes an allosteric shift in the Nf2/merlin α-helix that extends from its FERM domain. This is consistent with the fact that full-length Nf2/merlin binds LATS1 ~10-fold weaker compared to LATS1 binding to the Nf2/merlin-PIP₂ complex. Our data increase our understanding of Nf2/merlin biology by providing mechanistic insights into the Hippo pathway that are relevant to several diseases in particular oncogenic features that are associated with cancers.

The tumor suppressor LATS2 reduces v-Src-induced membrane blebs in a kinase activity-independent manner

When cells with excess DNA, such as tetraploid cells, undergo cell division, it can contribute to cellular transformation via asymmetrical chromosome segregation-generated genetic diversity. Cell cycle progression of tetraploid cells is suppressed by large tumor suppressor 2 (LATS2) kinase-induced inhibitory phosphorylation of the transcriptional coactivator Yes-associated protein (YAP). We recently reported that the oncogene v-Src induces tetraploidy and promotes cell cycle progression of tetraploid cells by suppressing LATS2 activity. We explore here the mechanism by which v-Src suppresses LATS2 activity and the role of LATS2 in v-Src-expressing cells. LATS2 was directly phosphorylated by v-Src and the proto-oncogene c-Src, resulting in decreased LATS2 kinase activity. This kinase-deficient LATS2 accumulated in a YAP transcriptional activity-dependent manner, and knockdown of either LATS2 or the LATS2-binding partner moesin-ezrin-radixin-like protein (Merlin) accelerated v-Src-induced membrane bleb formation. Upon v-Src expression, the interaction of Merlin with LATS2 was increased possibly due to a decrease in Merlin phosphorylation at Ser518, the dephosphorylation of which is required for the open conformation of Merlin and interaction with LATS2. LATS2 was colocalized with Merlin at the plasma membrane in a manner that depends on the Merlin-binding region of LATS2. The bleb formation in v-Src-expressing and LATS2-knockdown cells was rescued by the reexpression of wild-type or kinase-dead LATS2 but not the LATS2 mutant lacking the Merlin-binding region. These results suggest that the kinase-deficient LATS2 plays a role with Merlin at the plasma membrane in the maintenance of cortical rigidity in v-Src-expressing cells, which may cause tumor suppression.

The Potential of MLN3651 in Combination with Selumetinib as a Treatment for Merlin-Deficient Meningioma

Meningioma is the most common primary intracranial tumour, and surgical resection is the main therapeutic option. Merlin is a tumour suppressor protein that is frequently mutated in meningioma. The activity of the E3 ubiquitin ligase complex, CRL4-DCAF1, and the Raf/MEK/ERK scaffold protein Kinase suppressor of Ras 1 (KSR1) are upregulated in Merlin-deficient tumours, which drives tumour growth. Identifying small molecules that inhibit these key pathways may provide an effective treatment option for patients with meningioma. We used meningioma tissue and primary cells derived from meningioma tumours to investigate the expression of DDB1 and Cullin 4-associated factor 1 (DCAF1) and KSR1, and confirmed these proteins were overexpressed. We then used primary cells to assess the therapeutic potential of MLN3651, a neddylation inhibitor which impacts the activity of the CRL family of E3 ubiquitin ligases and the MAPK/ERK kinase (MEK1/2) inhibitor selumetinib. MLN3651 treatment reduced proliferation and activated apoptosis, whilst increasing Raf/MEK/ERK pathway activation. The combination of MLN3651 and the MEK1/2 inhibitor selumetinib prevented the increase in Raf/MEK/ERK activity, and had an additive effect compared with either treatment alone. Therefore, the combined targeting of CRL4-DCAF1 and Raf/MEK/ERK activity represents an attractive novel strategy in the treatment of Merlin-deficient meningioma.

Emergence and Evolution of ERM Proteins and Merlin in Metazoans

Ezrin, radixin, moesin, and merlin are cytoskeletal proteins, whose functions are specific to metazoans. They participate in cell cortex rearrangement, including cell-cell contact formation, and play an important role in cancer progression. Here, we have performed a comprehensive phylogenetic analysis of the proteins spanning 87 species. The results describe a possible mechanism for the protein family origin in the root of Metazoa, paralogs diversification in vertebrates, and acquisition of novel functions, including tumor suppression. In addition, a merlin paralog, present in most vertebrates but lost in mammals, has been described here for the first time. We have also highlighted a set of amino acid variations within the conserved motifs as the candidates for determining physiological differences between ERM paralogs.

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.

Peroxynitrite supports a metabolic reprogramming in merlin-deficient Schwann cells and promotes cell survival

Neurofibromatosis type 2 (NF2) is an autosomal-dominant disorder characterized by the development of bilateral vestibular schwannomas. The NF2 gene encodes the tumor suppressor merlin, and loss of merlin activity promotes tumorigenesis and causes NF2. Cellular redox signaling has been implicated in different stages of tumor development. Among reactive nitrogen species, peroxynitrite is the most powerful oxidant produced by cells. We recently showed that peroxynitrite-mediated tyrosine nitration down-regulates mitochondrial metabolism in tumor cells. However, whether peroxynitrite supports a metabolic shift that could be exploited for therapeutic development is unknown. Here, we show that vestibular schwannomas from NF2 patients and human, merlin-deficient (MD) Schwann cells have high levels of endogenous tyrosine nitration, indicating production of peroxynitrite. Furthermore, scavenging or inhibiting peroxynitrite formation significantly and selectively decreased survival of human and mouse MD-Schwann cells. Using multiple complementary methods, we also found that merlin deficiency leads to a reprogramming of energy metabolism characterized by a peroxynitrite-dependent decrease of oxidative phosphorylation and increased glycolysis and glutaminolysis. In MD-Schwann cells, scavenging of peroxynitrite increased mitochondrial oxygen consumption and membrane potential, mediated by the up-regulation of the levels and activity of mitochondrial complex IV. This increase in mitochondrial activity correlated with a decrease in the glycolytic rate and glutamine dependence. This is the first demonstration of a peroxynitrite-dependent reprogramming of energy metabolism in tumor cells. Oxidized proteins constitute a novel target for therapeutic development not only for the treatment of NF2 schwannomas but also other tumors in which peroxynitrite plays a regulatory role.

MicroRNA‑92a promotes cell proliferation, migration and survival by directly targeting the tumor suppressor gene NF2 in colorectal and lung cancer cells

Inactivation of the tumor suppressor protein Merlin leads to the development of benign nervous system tumors in neurofibromatosis type 2 (NF2). Documented causes of Merlin inactivation include deleterious mutations in the encoding neurofibromin 2 gene (NF2) and aberrant Merlin phosphorylation leading to proteasomal degradation. Rare somatic NF2 mutations have also been detected in common human malignancies not associated with NF2, including colorectal and lung cancer. Furthermore, tumors without NF2 mutations and with unaltered NF2 transcript levels, but with low Merlin expression, have been reported. The present study demonstrated that NF2 is also regulated by microRNAs (miRNAs) through direct interaction with evolutionarily conserved miRNA response elements (MREs) within its 3′-untranslated region (3′UTR). Dual-Luciferase assays in human colorectal carcinoma (HCT116) and lung adenocarcinoma (A549) cells revealed downregulation of NF2 by miR-92a-3p via its wild-type 3′UTR, but not NF2−3′UTR with mutated miR-92a-3p MRE. HCT116 cells overexpressing miR-92a-3p exhibited significant downregulation of endogenous NF2 mRNA and protein levels, which was rescued by co-transfection of a target protector oligonucleotide specific for the miR-92a-3p binding site within NF2−3′UTR. miR-92a-3p overexpression in HCT116 and A549 cells promoted migration, proliferation and resistance to apoptosis, as well as altered F-actin organization compared with controls. Knockdown of NF2 by siRNA phenocopied the oncogenic effects of miR-92a overexpression on HCT116 and A549 cells. Collectively, the findings of the present study provide functional proof of the unappreciated role of miRNAs in NF2 regulation and tumor progression, leading to enhanced oncogenicity.

Lipid binding promotes the open conformation and tumor-suppressive activity of neurofibromin 2

Neurofibromatosis type 2 (NF2) is a tumor-forming disease of the nervous system caused by deletion or by loss-of-function mutations in NF2, encoding the tumor suppressing protein neurofibromin 2 (also known as schwannomin or merlin). Neurofibromin 2 is a member of the ezrin, radixin, moesin (ERM) family of proteins regulating the cytoskeleton and cell signaling. The correlation of the tumor-suppressive function and conformation (open or closed) of neurofibromin 2 has been subject to much speculation, often based on extrapolation from other ERM proteins, and controversy. Here we show that lipid binding results in the open conformation of neurofibromin 2 and that lipid binding is necessary for inhibiting cell proliferation. Collectively, our results provide a mechanism in which the open conformation is unambiguously correlated with lipid binding and localization to the membrane, which are critical for the tumor-suppressive function of neurofibromin 2, thus finally reconciling the long-standing conformation and function debate.

Neurofibromin 2 (NF2) is a tumour suppressor that inhibits cell growth. Here the authors combine functional, biochemical, and structural studies and show that lipid-bound NF2 adopts an open conformation and that NF2 lipid binding is required for inhibition of cell proliferation.

NF2/Merlin Inactivation and Potential Therapeutic Targets in Mesothelioma

The neurofibromatosis type 2 (NF2) gene encodes merlin, a tumor suppressor protein frequently inactivated in schwannoma, meningioma, and malignant mesothelioma (MM). The sequence of merlin is similar to that of ezrin/radixin/moesin (ERM) proteins which crosslink actin with the plasma membrane, suggesting that merlin plays a role in transducing extracellular signals to the actin cytoskeleton. Merlin adopts a distinct closed conformation defined by specific intramolecular interactions and regulates diverse cellular events such as transcription, translation, ubiquitination, and miRNA biosynthesis, many of which are mediated through Hippo and mTOR signaling, which are known to be closely involved in cancer development. MM is a very aggressive tumor associated with asbestos exposure, and genetic alterations in NF2 that abrogate merlin’s functional activity are found in about 40% of MMs, indicating the importance of NF2 inactivation in MM development and progression. In this review, we summarize the current knowledge of molecular events triggered by NF2/merlin inactivation, which lead to the development of mesothelioma and other cancers, and discuss potential therapeutic targets in merlin-deficient mesotheliomas.

The conformation change and tumor suppressor role of Merlin are both independent of Serine 518 phosphorylation

Merlin functions as a tumor suppressor and suppresses malignant activity of cancer cells through multiple mechanisms. However, whether Serine 518 phosphorylation regulates the conformation of Merlin as well as the open-closed conformational changes affect Merlin's tumor inhibitory activity remain controversial. In this study, we used different mutants to mimic related conformational states of Merlin and investigated its physiological functions. Our results showed that the phosphorylation at Serine 518 has no influence on Merlin's conformation, subcellular localization, or cell proliferation inhibitory activity. As a fully closed conformational state, the A585W mutant loses the ability to recruit Lats2 to the cell membrane, but it does not affect its subcellular distribution or cell proliferation inhibitory activity. As a fully open conformational state, mimicking the conformation of Merlin isoform II, the ΔEL mutant has the same physiological function as the wild type Merlin isoform I. Collectively, we provide for the first time in vivo evidence that the function of Merlin, as a tumor suppressor is independent of its conformational change.

Childhood neurofibromatosis type 2 (NF2) and related disorders: from bench to bedside and biologically targeted therapies

Neurofibromatosis type 2 [NF2; MIM # 101000] is an autosomal dominant disorder characterised by the occurrence of vestibular schwannomas (VSs), schwannomas of other cranial, spinal and cutaneous nerves, cranial and spinal meningiomas and/or other central nervous system (CNS) tumours (e.g., ependymomas, astrocytomas). Additional features include early onset cataracts, optic nerve sheath meningiomas, retinal hamartomas, dermal schwannomas (i.e., NF2-plaques), and (few) café-au-lait spots. Clinically, NF2 children fall into two main groups: (1) congenital NF2 - with bilateral VSs detected as early as the first days to months of life, which can be stable/asymptomatic for one-two decades and suddenly progress; and (2) severe pre-pubertal (Wishart type) NF2- with multiple (and rapidly progressive) CNS tumours other-than-VS, which usually present first, years before VSs [vs. the classical adult (Gardner type) NF2, with bilateral VSs presenting in young adulthood, sometimes as the only disease feature]. Some individuals can develop unilateral VS associated with ipsilateral meningiomas or multiple schwannomas localised to one part of the peripheral nervous system [i.e., mosaic NF2] or multiple non-VS, non-intradermal cranial, spinal and peripheral schwannomas (histologically proven) [schwannomatosis]. NF2 is caused by mutations in the NF2 gene at chromosome 22q12.1, which encodes for a protein called merlin or schwannomin, most similar to the exrin-readixin-moesin (ERM) proteins; mosaicNF2 is due to mosaic phenomena for the NF2 gene, whilst schwannomatosis is caused by coupled germ-line and mosaic mutations either in the SMARCB1 gene [SWNTS1; MIM # 162091] or the LZTR1 gene [SWNTS2; MIM # 615670] both falling within the 22q region and the NF2 gene. Data driven from in vitro and animal studies on the merlin pathway [e.g., post-translational and upstream/downstream regulation] allowed biologically targeted treatment strategies [e.g., Lapatinib, Erlotinib, Bevacizumab] aimed to multiple tumour shrinkage and/or regression and tumour arrest of progression with functional improvement.

An Essential Role for the Tumor-Suppressor Merlin in Regulating Fatty Acid Synthesis

Neurofibromatosis type 2 (NF2) is an autosomal dominant disorder characterized by the development of multiple tumors in the central nervous system, most notably schwannomas, and meningiomas. Mutational inactivation of the NF2 gene encoding the protein Merlin is found in most sporadic and inherited schwannomas, but the molecular mechanisms underlying neoplastic changes in schwannoma cells remain unclear. We report here that Nf2-deficient cells display elevated expression levels of key enzymes involved in lipogenesis and that this upregulation is caused by increased activity of Torc1. Inhibition or knockdown of fatty acid synthase (FASN), the enzyme that catalyzes the formation of palmitic acid from malonyl-CoA, drove NF2-deficient cells into apoptosis. Treatment of NF2-mutant cells with agents that inhibit the production of malonyl-CoA reduced their sensitivity to FASN inhibitors. Collectively, these results suggest that the altered lipid metabolism found in NF2-mutant cells renders them sensitive to elevated levels of malonyl-CoA, as occurs following blockade of FASN, suggesting new targeted strategies in the treatment of NF2-deficient tumors. Cancer Res; 77(18); 5026-38. ©2017 AACR.

E-cadherin expression is correlated with focal adhesion kinase inhibitor resistance in Merlin-negative malignant mesothelioma cells

Malignant mesothelioma (MM) is an aggressive tumor commonly caused by asbestos exposure after a long latency. Focal adhesion kinase (FAK) inhibitors inhibit the cell growth of Merlin-deficient MM cells; however, their clinical efficacy has not been clearly determined. The aim of this study was to evaluate the growth inhibitory effect of the FAK inhibitor VS-4718 on MM cell lines and identify biomarkers for its efficacy. Although most Merlin-deficient cell lines were sensitive to VS-4718 compared with control MeT-5A cells, a subset of these cell lines exhibited resistance to this drug. Microarray and qRT-PCR analyses using RNA isolated from Merlin-deficient MM cell lines revealed a significant correlation between E-cadherin mRNA levels and VS-4718 resistance. Merlin- and E-cadherin-negative Y-MESO-22 cells underwent apoptosis upon treatment with a low concentration of VS-4718, whereas Merlin-negative, E-cadherin-positive Y-MESO-9 cells did not undergo VS-4718-induced apoptosis. Furthermore, E-cadherin knockdown in Merlin-negative MM cells significantly sensitized cells to VS-4718 and induced apoptotic cell death upon VS-4718 treatment. Together, our results suggest that E-cadherin serves as a predictive biomarker for molecular target therapy with FAK inhibitors for patients with mesothelioma and that its expression endows MM cells with resistance to FAK inhibitors.

15α-methoxypuupehenol Induces Antitumor Effects In Vitro and In Vivo against Human Glioblastoma and Breast Cancer Models

Studies with 15α-methoxypuupehenol (15α-MP), obtained from the extracts of Hyrtios species, identified putative targets that are associated with its antitumor effects against human glioblastoma and breast cancer. In the human glioblastoma (U251MG) or breast cancer (MDA-MB-231) cells, treatment with 15α-MP repressed pY705Stat3, pErk1/2, pS147CyclinB1, pY507Alk (anaplastic lymphoma kinase), and pY478ezrin levels and induced pS10merlin, without inhibiting pJAK2 (Janus kinase) or pAkt induction. 15α-MP treatment induced loss of viability of breast cancer (MDA-MB-231, MDA-MB-468) and glioblastoma (U251MG) lines and glioblastoma patient-derived xenograft cells (G22) that harbor aberrantly active Stat3, with only moderate or little effect on the human breast cancer, MCF7, colorectal adenocarcinoma Caco-2, normal human lung fibroblast, WI-38, or normal mouse embryonic fibroblast (MEF Stat3^fl/fl) lines that do not harbor constitutively active Stat3 or the Stat3-null (Stat3^-/-) mouse astrocytes. 15α-MP-treated U251MG cells have severely impaired F-actin organization and altered morphology, including the cells rounding up, and undergo apoptosis, compared with a moderate, reversible morphology change observed for similarly treated mouse astrocytes. Treatment further inhibited U251MG or MDA-MB-231 cell proliferation, anchorage-independent growth, colony formation, and migration in vitro while only moderately or weakly affecting MCF7 cells or normal mouse astrocytes. Oral gavage delivery of 15α-MP inhibited the growth of U251MG subcutaneous tumor xenografts in mice, associated with apoptosis in the treated tumor tissues. Results together suggest that the modulation of Stat3, CyclinB1, Alk, ezrin, merlin, and Erk1/2 functions contributes to the antitumor effects of 15α-MP against glioblastoma and breast cancer progression. Mol Cancer Ther; 16(4); 601-13. ©2017 AACR.

Ezrin: a regulator of actin microfilaments in cell junctions of the rat testis

Ezrin, radixin, moesin and merlin (ERM) proteins are highly homologous actin-binding proteins that share extensive sequence similarity with each other. These proteins tether integral membrane proteins and their cytoplasmic peripheral proteins (e.g., adaptors, nonreceptor protein kinases and phosphatases) to the microfilaments of actin-based cytoskeleton. Thus, these proteins are crucial to confer integrity of the apical membrane domain and its associated junctional complex, namely the tight junction and the adherens junction. Since ectoplasmic specialization (ES) is an F-actin-rich testis-specific anchoring junction-a highly dynamic ultrastructure in the seminiferous epithelium due to continuous transport of germ cells, in particular spermatids, across the epithelium during the epithelial cycle-it is conceivable that ERM proteins are playing an active role in these events. Although these proteins were first reported almost 25 years and have since been extensively studied in multiple epithelia/endothelia, few reports are found in the literature to examine their role in the actin filament bundles at the ES. Studies have shown that ezrin is also a constituent protein of the actin-based tunneling nanotubes (TNT) also known as intercellular bridges, which are transient cytoplasmic tubular ultrastructures that transport signals, molecules and even organelles between adjacent and distant cells in an epithelium to coordinate cell events that occur across an epithelium. Herein, we critically evaluate recent data on ERM in light of recent findings in the field in particular ezrin regarding its role in actin dynamics at the ES in the testis, illustrating additional studies are warranted to examine its physiological significance in spermatogenesis.

Tumor suppressor Nf2/merlin drives Schwann cell changes following electromagnetic field exposure through Hippo-dependent mechanisms

Previous evidence showed mutations of the neurofibromin type 2 gene (Nf2), encoding the tumor suppressor protein merlin, in sporadic and vestibular schwannomas affecting Schwann cells (SCs). Accordingly, efforts have been addressed to identify possible factors, even environmental, that may regulate neurofibromas growth. In this context, we investigated the exposure of SC to an electromagnetic field (EMF), which is an environmental issue modulating biological processes. Here, we show that SC exposed to 50 Hz EMFs changes their morphology, proliferation, migration and myelinating capability. In these cells, merlin is downregulated, leading to activation of two intracellular signaling pathways, ERK/AKT and Hippo. Interestingly, SC changes their phenotype toward a proliferative/migrating state, which in principle may be pathologically relevant for schwannoma development.

Merlin status regulates p75(NTR) expression and apoptotic signaling in Schwann cells following nerve injury

After nerve injury, Schwann cells (SCs) dedifferentiate, proliferate, and support axon regrowth. If axons fail to regenerate, denervated SCs eventually undergo apoptosis due, in part, to increased expression of the low-affinity neurotrophin receptor, p75(NTR). Merlin is the protein product of the NF2 tumor suppressor gene implicated in SC tumorigenesis. Here we explore the contribution of merlin to SC responses to nerve injury. We find that merlin becomes phosphorylated (growth permissive) in SCs following acute axotomy and following gradual neural degeneration in a deafness model, temporally correlated with increased p75(NTR) expression. p75(NTR) levels are elevated in P0SchΔ39-121 transgenic mice that harbor an Nf2 mutation in SCs relative to wild-type mice before axotomy and remain elevated for a longer period of time following injury. Replacement of wild-type, but not phospho-mimetic (S518D), merlin isoforms suppresses p75(NTR) expression in primary human schwannoma cultures which otherwise lack functional merlin. Despite elevated levels of p75(NTR), SC apoptosis following axotomy is blunted in P0SchΔ39-121 mice relative to wild-type mice suggesting that loss of functional merlin contributes to SC resistance to apoptosis. Further, cultured SCs from mice with a tamoxifen-inducible knock-out of Nf2 confirm that SCs lacking functional merlin are less sensitive to p75(NTR)-mediated cell death. Taken together these results point to a model whereby loss of axonal contact following nerve injury results in merlin phosphorylation leading to increased p75(NTR) expression. Further, they demonstrate that merlin facilitates p75(NTR)-mediated apoptosis in SCs helping to explain how neoplastic SCs that lack functional merlin survive long-term in the absence of axonal contact.

Tumor Suppressor NF2 Blocks Cellular Migration by Inhibiting Ectodomain Cleavage of CD44

Ectodomain cleavage (shedding) of transmembrane proteins by metalloproteases (MMP) generates numerous essential signaling molecules, but its regulation is not totally understood. CD44, a cleaved transmembrane glycoprotein, exerts both antiproliferative or tumor-promoting functions, but whether proteolysis is required for this is not certain. CD44-mediated contact inhibition and cellular proliferation are regulated by counteracting CD44 C-terminal interacting proteins, the tumor suppressor protein merlin (NF2) and ERM proteins (ezrin, radixin, moesin). We show here that activation or overexpression of constitutively active merlin or downregulation of ERMs inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced [as well as serum, hepatocyte growth factor (HGF), or platelet-derived growth factor (PDGF)] CD44 cleavage by the metalloprotease ADAM10, whereas overexpressed ERM proteins promoted cleavage. Merlin- and ERM-modulated Ras or Rac activity was not required for this function. However, latrunculin (an actin-disrupting toxin) or an ezrin mutant which is unable to link CD44 to actin, inhibited CD44 cleavage, identifying a cytoskeletal C-terminal link as essential for induced CD44 cleavage. Cellular migration, an important tumor property, depended on CD44 and its cleavage and was inhibited by merlin. These data reveal a novel function of merlin and suggest that CD44 cleavage products play a tumor-promoting role. Neuregulin, an EGF ligand released by ADAM17 from its pro-form NRG1, is predominantly involved in regulating cellular differentiation. In contrast to CD44, release of neuregulin from its pro-form was not regulated by merlin or ERM proteins. Disruption of the actin cytoskeleton however, also inhibited NRG1 cleavage. This current study presents one of the first examples of substrate-selective cleavage regulation.

IMPLICATIONS

Investigating transmembrane protein cleavage and their regulatory pathways have provided new molecular insight into their important role in cancer formation and possible treatment.

Sensing the local environment: actin architecture and Hippo signalling

The Hippo network is a major conserved growth suppressor pathway that participates in organ size control during development and prevents tumour formation during adult homeostasis. Recent evidence has implicated the actin cytoskeleton as a link between tissue architecture and Hippo signalling. In this review, we will consider the evidence and models proposed for the regulation of Hippo signalling by actin dynamics and structure. We cover aspects of signalling regulation by mechanotransduction, cytoskeletal tethering and the spatial reorganization of signalling components. We also examine the physiological and pathological contexts in which these mechanisms are relevant.

Molecular insights into NF2/Merlin tumor suppressor function

The FERM domain protein Merlin, encoded by the NF2 tumor suppressor gene, regulates cell proliferation in response to adhesive signaling. The growth inhibitory function of Merlin is induced by intercellular adhesion and inactivated by joint integrin/receptor tyrosine kinase signaling. Merlin contributes to the formation of cell junctions in polarized tissues, activates anti-mitogenic signaling at tight-junctions, and inhibits oncogenic gene expression. Thus, inactivation of Merlin causes uncontrolled mitogenic signaling and tumorigenesis. Merlin's predominant tumor suppressive functions are attributable to its control of oncogenic gene expression through regulation of Hippo signaling. Notably, Merlin translocates to the nucleus where it directly inhibits the CRL4(DCAF1) E3 ubiquitin ligase, thereby suppressing inhibition of the Lats kinases. A dichotomy in NF2 function has emerged whereby Merlin acts at the cell cortex to organize cell junctions and propagate anti-mitogenic signaling, whereas it inhibits oncogenic gene expression through the inhibition of CRL4(DCAF1) and activation of Hippo signaling. The biochemical events underlying Merlin's normal function and tumor suppressive activity will be discussed in this Review, with emphasis on recent discoveries that have greatly influenced our understanding of Merlin biology.

A deletion causing NF2 exon 9 skipping is associated with familial autosomal dominant intramedullary ependymoma

BACKGROUND

Intramedullary ependymomas are rare and benign tumors in the adult. Little is known about their physiopathology, but the implication of the NF2 gene is suspected because of their presence in a third of patients with type 2 neurofibromatosis (NF2), a disorder caused by mutation of the NF2 gene.

METHODS

We conducted a clinical and genetic study of a family in which 5 of 9 members suffered from intramedullary ependymoma. Karyotyping and CGH array analysis were performed on DNA from peripheral blood lymphocytes from affected participants. The NF2 gene sequences were then determined in DNA from 3 nonaffected and all 5 affected members of the family.

RESULTS

Karyotype and CGH array findings were normal. Sequencing of NF2 revealed a heterozygous deletion, c.811-39_841del69bp, at the intron 8/exon 9 junction, in all affected members that was absent from all nonaffected members. RT-PCR analysis and sequencing revealed a novel NF2 transcript characterized by a skipping of exon 9 (75 bp). This deletion is predicted to result in a 25-amino acid deletion in the N-terminal FERM domain of neurofibromin 2. Modeling of this mutant domain suggests possible disorganization of the subdomain C.

CONCLUSION

We report the first family with an NF2 mutation associated with intramedullary ependymomas without other features of NF2 syndrome. This mutation, which has not been described previously, may particularly affect the function of neurofibromin 2 in ependymocytes leading to the development of intramedullary WHO grade II ependymomas. We propose that sporadic intramedullary ependymomas should also be analyzed for this region of NF2 gene.

Activated ERM protein plays a critical role in drug resistance of MOLT4 cells induced by CCL25

We have previously demonstrated that the CCR9/CCL25 signaling pathway plays an important role in drug resistance in human acute T-lymphocytic leukemia (T-ALL) by inducing activation of ERM protein with polarized distribution in T-ALL cell line MOLT4. However, the mechanism of action of the activated ERM protein in the drug resistance of MOLT4 cells induced by CCL25 remains uncharacterized. Here we investigated the mechanism of CCR9/CCL25-initiated drug resistance in CCR9-high-expressing T-ALL cells. Our results showed that 1) the function of P-gp was increased after treatment with CCL25; 2) P-gp colocalized and co-immunoprecipitated with p-ERM and F-actin in CCL25 treated cells; and 3) ERM-shRNA conferred drug sensitivity coincident with release of ERM interactions with P-gp and F-actin after treatment with CCL25. These data suggest it is pivotal that P-gp associate with the F-actin cytoskeleton through p-ERM in CCR9/CCL25 induced multidrug resistance of T-ALL cells. Strategies aimed at inhibiting P-gp-F-actin cytoskeleton association may be helpful in increasing the efficiency of therapies in T-ALL.

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.

Stability of the tumor suppressor merlin depends on its ability to bind paxillin LD3 and associate with β1 integrin and actin at the plasma membrane

The NF2 gene encodes a tumor suppressor protein known as merlin or schwannomin whose loss of function causes Neurofibromatosis Type 2 (NF2). NF2 is characterized by the development of benign tumors, predominantly schwannomas, in the peripheral nervous system. Merlin links plasma membrane receptors with the actin cytoskeleton and its targeting to the plasma membrane depends on direct binding to the paxillin scaffold protein. Exon 2 of NF2, an exon mutated in NF2 patients and deleted in a mouse model of NF2, encodes the merlin paxillin binding domain (PBD1). Here, we sought to determine the role of PBD1 in regulation of merlin stability and association with plasma membrane receptors and the actin cytoskeleton in Schwann cells. Using a fluorescence-based pulse-chase technique, we measured the half-life of Halo-tagged merlin variants carrying PBD1, exon 2, and exons 2 and 3 deletions in transiently transfected Schwann cells. We found that PBD1 alone was necessary and sufficient to increase merlin's half-life from approximately three to eleven hours. Merlin lacking PBD1 did not form a complex with surface β1 integrins or associate with the actin cytoskeleton. In addition, direct binding studies using purified merlin and paxillin domains revealed that merlin directly binds paxillin LD3 (leucine-aspartate 3) domain as well as the LD4 and LD5 domains. Together these results demonstrate that a direct interaction between merlin PBD1 and the paxillin LD3-5 domains targets merlin to the plasma membrane where it is stabilized by its association with surface β1 integrins and cortical actin.

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

Inhibition of proliferation by cell-to-cell contact is essential for tissue organization, and its disruption contributes to tumorigenesis. The FERM domain protein Merlin, encoded by the NF2 tumour suppressor gene, is an important mediator of contact inhibition. Merlin was thought to inhibit mitogenic signalling and activate the Hippo pathway by interacting with diverse target-effectors at or near the plasma membrane. However, recent studies highlight that Merlin pleiotropically affects signalling by migrating into the nucleus and inducing a growth-suppressive programme of gene expression through its direct inhibition of the CRL4DCAF1 E3 ubiquitin ligase. In addition, Merlin promotes the establishment of epithelial adhesion and polarity by recruiting Par3 and aPKC to E-cadherin-dependent junctions, and by ensuring the assembly of tight junctions. These recent advances suggest that Merlin acts at the cell cortex and in the nucleus in a similar, albeit antithetic, manner to the oncogene β-catenin.

The tumor suppressor DiRas3 forms a complex with H-Ras and C-RAF proteins and regulates localization, dimerization, and kinase activity of C-RAF

The maternally imprinted Ras-related tumor suppressor gene DiRas3 is lost or down-regulated in more than 60% of ovarian and breast cancers. The anti-tumorigenic effect of DiRas3 is achieved through several mechanisms, including inhibition of cell proliferation, motility, and invasion, as well as induction of apoptosis and autophagy. Re-expression of DiRas3 in cancer cells interferes with the signaling through Ras/MAPK and PI3K. Despite intensive research, the mode of interference of DiRas3 with the Ras/RAF/MEK/ERK signal transduction is still a matter of speculation. In this study, we show that DiRas3 associates with the H-Ras oncogene and that activation of H-Ras enforces this interaction. Furthermore, while associated with DiRas3, H-Ras is able to bind to its effector protein C-RAF. The resulting multimeric complex consisting of DiRas3, C-RAF, and active H-Ras is more stable than the two protein complexes H-Ras·C-RAF or H-Ras·DiRas3, respectively. The consequence of this complex formation is a DiRas3-mediated recruitment and anchorage of C-RAF to components of the membrane skeleton, suppression of C-RAF/B-RAF heterodimerization, and inhibition of C-RAF kinase activity.

Unfurling of the band 4.1, ezrin, radixin, moesin (FERM) domain of the merlin tumor suppressor

The merlin-1 tumor suppressor is encoded by the Neurofibromatosis-2 (Nf2) gene and loss-of-function Nf2 mutations lead to nervous system tumors in man and to several tumor types in mice. Merlin is an ERM (ezrin, radixin, moesin) family cytoskeletal protein that interacts with other ERM proteins and with components of cell-cell adherens junctions (AJs). Merlin stabilizes the links of AJs to the actin cytoskeleton. Thus, its loss destabilizes AJs, promoting cell migration and invasion, which in Nf2(+/-) mice leads to highly metastatic tumors. Paradoxically, the "closed" conformation of merlin-1, where its N-terminal four-point-one, ezrin, radixin, moesin (FERM) domain binds to its C-terminal tail domain, directs its tumor suppressor functions. Here we report the crystal structure of the human merlin-1 head domain when crystallized in the presence of its tail domain. Remarkably, unlike other ERM head-tail interactions, this structure suggests that binding of the tail provokes dimerization and dynamic movement and unfurling of the F2 motif of the FERM domain. We conclude the "closed" tumor suppressor conformer of merlin-1 is in fact an "open" dimer whose functions are disabled by Nf2 mutations that disrupt this architecture.

FERM domain phosphoinositide binding targets merlin to the membrane and is essential for its growth-suppressive function

The neurofibromatosis type 2 tumor suppressor protein, merlin, is related to the ERM (ezrin, radixin, and moesin) family of plasma membrane-actin cytoskeleton linkers. For ezrin, phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding to the amino-terminal FERM domain is required for its conformational activation, proper subcellular localization, and function, but less is known about the role of phosphoinositide binding for merlin. Current evidence indicates that association with the membrane is important for merlin to function as a growth regulator; however, the mechanisms by which merlin localizes to the membrane are less clear. Here, we report that merlin binds phosphoinositides, including PIP(2), via a conserved binding motif in its FERM domain. Abolition of FERM domain-mediated phosphoinositide binding of merlin displaces merlin from the membrane and releases it into the cytosol without altering the folding of merlin. Importantly, a merlin protein whose FERM domain cannot bind phosphoinositide is defective in growth suppression. Retargeting the mutant merlin into the membrane using a dual-acylated amino-terminal decapeptide from Fyn is sufficient to restore the growth-suppressive properties to the mutant merlin. Thus, FERM domain-mediated phosphoinositide binding and membrane association are critical for the growth-regulatory function of merlin.