Expression of the neurofibromatosis 2 tumor suppressor gene product, merlin, in Schwann cells

Gene Therapy for Neurofibromatosis Type 2-Related Schwannomatosis: Recent Progress, Challenges, and Future Directions

Neurofibromatosis type 2 (NF2)-related schwannomatosis is a rare autosomal dominant monogenic disorder caused by mutations in the NF2 gene. The hallmarks of NF2-related schwannomatosis are bilateral vestibular schwannomas (VS). The current treatment options for NF2-related schwannomatosis, such as observation with serial imaging, surgery, radiotherapy, and pharmacotherapies, have shown limited effectiveness and serious complications. Therefore, there is a critical demand for novel effective treatments. Gene therapy, which has made significant advancements in treating genetic diseases, holds promise for the treatment of this disease. This review covers the genetic pathogenesis of NF2-related schwannomatosis, the latest progress in gene therapy strategies, current challenges, and future directions of gene therapy for NF2-related schwannomatosis.

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.

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.

Familial Syndromes Involving Meningiomas Provide Mechanistic Insight Into Sporadic Disease

Currently, there is an incomplete understanding of the molecular pathogenesis of meningiomas, the most common primary brain tumor. Several familial syndromes are characterized by increased meningioma risk, and the genetics of these syndromes provides mechanistic insight into sporadic disease. The best defined of these syndromes is neurofibromatosis type 2, which is caused by a mutation in the NF2 gene and has a meningioma incidence of approximately 50%. This finding led to the subsequent discovery that NF2 loss-of-function occurs in up to 60% of sporadic tumors. Other important familial diseases with increased meningioma risk include nevoid basal cell carcinoma syndrome, multiple endocrine neoplasia 1 (MEN1), Cowden syndrome, Werner syndrome, BAP1 tumor predisposition syndrome, Rubinstein-Taybi syndrome, and familial meningiomatosis caused by germline mutations in the SMARCB1 and SMARCE1 genes. For each of these syndromes, the diagnostic criteria, incidence in the population, and frequency of meningioma are presented to review the relevant clinical information for these conditions. The genetic mutations, molecular pathway derangements, and relationship to sporadic disease for each syndrome are described in detail to identify targets for further investigation. Familial syndromes characterized by meningiomas often affect genes and pathways that are also implicated in a subset of sporadic cases, suggesting key molecular targets for therapeutic intervention. Further studies are needed to resolve the functional relevance of specific genes whose significance in sporadic disease remains to be elucidated.

Proximity biotinylation identifies a set of conformation-specific interactions between Merlin and cell junction proteins

Neurofibromatosis type 2 is an inherited, neoplastic disease associated with schwannomas, meningiomas, and ependymomas and that is caused by inactivation of the tumor suppressor gene NF2 The NF2 gene product, Merlin, has no intrinsic catalytic activity; its tumor suppressor function is mediated through the proteins with which it interacts. We used proximity biotinylation followed by mass spectrometry and direct binding assays to identify proteins that associated with wild-type and various mutant forms of Merlin in immortalized Schwann cells. We defined a set of 52 proteins in close proximity to wild-type Merlin. Most of the Merlin-proximal proteins were components of cell junctional signaling complexes, suggesting that additional potential interaction partners may exist in adherens junctions, tight junctions, and focal adhesions. With mutant forms of Merlin that cannot bind to phosphatidylinositol 4,5-bisphosphate (PIP₂) or that constitutively adopt a closed conformation, we confirmed a critical role for PIP₂ binding in Merlin function and identified a large cohort of proteins that specifically interacted with Merlin in the closed conformation. Among these proteins, we identified a previously unreported Merlin-binding protein, apoptosis-stimulated p53 protein 2 (ASPP2, also called Tp53bp2), that bound to closed-conformation Merlin predominately through the FERM domain. Our results demonstrate that Merlin is a component of cell junctional mechanosensing complexes and defines a specific set of proteins through which it acts.

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.

Nf2 Mutation in Schwann Cells Delays Functional Neural Recovery Following Injury

Merlin is the protein product of the NF2 tumor suppressor gene. Germline NF2 mutation leads to neurofibromatosis type 2 (NF2), characterized by multiple intracranial and spinal schwannomas. Patients with NF2 also frequently develop peripheral neuropathies. While the role of merlin in SC neoplasia is well established, its role in SC homeostasis is less defined. Here we explore the role of merlin in SC responses to nerve injury and their ability to support axon regeneration. We performed sciatic nerve crush in wild-type (WT) and in P0SchΔ39-121 transgenic mice that express a dominant negative Nf2 isoform in SCs. Recovery of nerve function was assessed by measuring mean contact paw area on a pressure pad 7, 21, 60, and 90 days following nerve injury and by nerve conduction assays at 90 days following injury. After 90 days, the nerves were harvested and axon regeneration was quantified stereologically. Myelin ultrastructure was analyzed by electron microscopy. Functional studies showed delayed nerve regeneration in Nf2 mutant mice compared to the WT mice. Delayed neural recovery correlated with a reduced density of regenerated axons and increased endoneurial space in mutants compared to WT mice. Nevertheless, functional and nerve conduction measures ultimately recovered to similar levels in WT and Nf2 mutant mice, while there was a small (∼17%) reduction in the percent of regenerated axons in the Nf2 mutant mice. The data suggest that merlin function in SCs regulates neural ultrastructure and facilitates neural regeneration, in addition to its role in SC neoplasia.

Global Proteome and Phospho-proteome Analysis of Merlin-deficient Meningioma and Schwannoma Identifies PDLIM2 as a Novel Therapeutic Target

Loss or mutation of the tumour suppressor Merlin predisposes individuals to develop multiple nervous system tumours, including schwannomas and meningiomas, sporadically or as part of the autosomal dominant inherited condition Neurofibromatosis 2 (NF2). These tumours display largely low grade features but their presence can lead to significant morbidity. Surgery and radiotherapy remain the only treatment options despite years of research, therefore an effective therapeutic is required.

Unbiased omics studies have become pivotal in the identification of differentially expressed genes and proteins that may act as drug targets or biomarkers. Here we analysed the proteome and phospho-proteome of these genetically defined tumours using primary human tumour cells to identify upregulated/activated proteins and/or pathways. We identified over 2000 proteins in comparative experiments between Merlin-deficient schwannoma and meningioma compared to human Schwann and meningeal cells respectively. Using functional enrichment analysis we highlighted several dysregulated pathways and Gene Ontology terms. We identified several proteins and phospho-proteins that are more highly expressed in tumours compared to controls. Among proteins jointly dysregulated in both tumours we focused in particular on PDZ and LIM domain protein 2 (PDLIM2) and validated its overexpression in several tumour samples, while not detecting it in normal cells. We showed that shRNA mediated knockdown of PDLIM2 in both primary meningioma and schwannoma leads to significant reductions in cellular proliferation.

To our knowledge, this is the first comprehensive assessment of the NF2-related meningioma and schwannoma proteome and phospho-proteome. Taken together, our data highlight several commonly deregulated factors, and indicate that PDLIM2 may represent a novel, common target for meningioma and schwannoma.

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Proteome and phosphoproteome of Merlin-deficient schwannomas and meningiomas were analysed.

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Comparative studies highlighted several pathways relevant for therapeutic intervention.

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PDLIM2 was identified as a novel, commonly upregulated protein in both tumours.

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PDLIM2 knockdown led to a significant reduction in proliferation in both cell types.

Loss or mutation of the protein Merlin causes a genetic condition known as Neurofibromatosis 2 (NF2) characterised by the growth of schwannomas and meningiomas.

We analysed several of these tumour samples and identified over 2000 proteins in comparative experiments between Merlin-deficient schwannoma and meningioma compared to normal controls. We identified PDZ and LIM domain protein 2 (PDLIM2) as overexpressed in both tumour types and further showed that knockdown of PDLIM2 leads to significant reductions in cellular proliferation.

Taken together, our data highlight several deregulated signalling pathways, and indicate that PDLIM2 may represent a novel, common target for meningioma and schwannoma.

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.

Merlin isoform 2 in neurofibromatosis type 2-associated polyneuropathy

The autosomal dominant disorder neurofibromatosis type 2 (NF2) is a hereditary tumor syndrome caused by inactivation of the NF2 tumor suppressor gene, encoding merlin. Apart from tumors affecting the peripheral and central nervous systems, most NF2 patients develop peripheral neuropathies. This peripheral nerve disease can occur in the absence of nerve-damaging tumors, suggesting an etiology that is independent of gross tumor burden. We discovered that merlin isoform 2 (merlin-iso2) has a specific function in maintaining axonal integrity and propose that reduced axonal NF2 gene dosage leads to NF2-associated polyneuropathy. We identified a merlin-iso2-dependent complex that promotes activation of the GTPase RhoA, enabling downstream Rho-associated kinase to promote neurofilament heavy chain phosphorylation. Merlin-iso2-deficient mice exhibited impaired locomotor capacities, delayed sensory reactions and electrophysiological signs of axonal neuropathy. Sciatic nerves from these mice and sural nerve biopsies from NF2 patients revealed reduced phosphorylation of the neurofilament H subunit, decreased interfilament spacings and irregularly shaped axons.

The Rho-kinase inhibitor fasudil restores normal motor nerve conduction velocity in diabetic rats by assuring the proper localization of adhesion-related molecules in myelinating Schwann cells

The Rho/Rho-kinase signaling pathway has been shown to be involved in the complications of diabetes. In this study, we found that fasudil, a specific Rho-kinase inhibitor, had a beneficial effect on the motor nerve conduction velocity (MNCV), which is delayed in rats with streptozotocin (STZ)-induced diabetes. Cadherin-dependent adherens junctions (AJs) in myelinating Schwann cells, necessary for proper myelin formation and rapid propagation of action potentials, are regulated by Rho/Rho-kinase signaling. These AJ structures are maintained by E-cadherin and catenin complexes such as β-catenin and p120 catenin. To elucidate the mechanism underlying the effect of fasudil on MNCV, we examined alterations in AJ structure in the peripheral nerves of the experimental rats. Our results showed that the activities of Rho and Rho-kinase increased simultaneously in the sciatic nerves of the diabetic rats. Fasudil restored the MNCV by suppressing the up-regulation of the Rho-kinase. In the diabetic state, enhanced Rho and Rho-kinase activity reduced p120 catenin expression and altered the distribution of p120 catenin and E-cadherin, which are normally localized in the paranodal compartment of the nodes of Ranvier and Schmidt-Lanterman incisures where autotypic AJs stabilize myelin structure. Fasudil restored normal p120 catenin expression and the distribution of p120 catenin and E-cadherin in the myelin sheath. In conclusion, reduced expression and altered distribution of the adhesion molecules in the myelin sheath might contribute to the slowing of the MNCV in the diabetic rats. Fasudil, through its effect on the distribution of the adhesion-related molecules, might prevent slowing of the MNCV.

Neurofibromatosis type 2 (NF2): diagnosis and management

Neurofibromatosis type 2 (NF2) is an autosomal dominant inherited tumor predisposition syndrome caused by mutations in the NF2 gene on chromosome 22. Affected individuals develop schwannomas typically involving both vestibular nerves leading to hearing loss and eventual deafness. Rehabilitation with brainstem implants and in some cases cochlear implants is improving this outcome. Schwannomas also occur on other cranial nerves, on spinal nerve roots and peripheral nerves, and intracutaneously as plaques. Cranial and spinal meningiomas and spinal ependymomas are other common tumors. Fifty to sixty percent of patients represent de novo mutations and as many as 33% of these are mosaic for the underlying disease causing mutation. Truncating mutations (nonsense, frameshift insertions/deletions) are the most frequent germline events and cause the most severe disease, whilst single and multiple exon deletions are common and are usually associated with milder NF2. Neurological deficits are a major feature of the condition and neurologists have a pivotal role in assigning symptoms to lesions and in managing neuropathies. NF2 represents a difficult management problem with most patients facing substantial morbidity and reduced life expectancy. Surgery remains the focus of current management although watchful waiting and occasionally radiation treatment have a role. We are seeing the advent of tailored drug therapies aimed at the genetic level and these are likely to provide huge improvements for this devastating, life-limiting condition.

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.

Essential function of protein 4.1G in targeting of membrane protein palmitoylated 6 into Schmidt-Lanterman incisures in myelinated nerves

Protein 4.1G is a membrane skeletal protein found in specific subcellular structures in myelinated Schwann cells and seminiferous tubules. Here, we show that in the mouse sciatic nerve, protein 4.1G colocalized at Schmidt-Lanterman incisures (SLI) and the paranodes with a member of the membrane-associated guanylate kinase (MAGUK) family, membrane protein palmitoylated 6 (MPP6). Coimmunoprecipitation experiments revealed that MPP6 was interacting with protein 4.1G. In contrast to wild-type nerves, in 4.1G knockout mice, MPP6 was found largely in the cytoplasm near Schwann cell nuclei, indicating an abnormal protein transport. Although the SLI remained in the 4.1G knockout sciatic nerves, as confirmed by E-cadherin immunostaining, their shape was altered in aged 4.1G knockout nerves compared to their shape in wild-type nerves. In the seminiferous tubules, MPP6 was localized similarly to protein 4.1G along cell membranes of the spermatogonium and early spermatocytes. However, in contrast to myelinated peripheral nerves, the specific localization of MPP6 in the seminiferous tubules was unaltered in the absence of protein 4.1G. These results indicate that 4.1G has a specific role in the targeting of MPP6 to the SLI and the assembly of these subcellular structures.

Neurofibromatosis type 2

Neurofibromatosis type 2 is an autosomal-dominant multiple neoplasia syndrome that results from mutations in the NF2 tumour suppressor gene located on chromosome 22q. It has a frequency of one in 25,000 livebirths and nearly 100% penetrance by 60 years of age. Half of patients inherit a germline mutation from an affected parent and the remainder acquire a de novo mutation for neurofibromatosis type 2. Patients develop nervous system tumours (schwannomas, meningiomas, ependymomas, astrocytomas, and neurofibromas), peripheral neuropathy, ophthalmological lesions (cataracts, epiretinal membranes, and retinal hamartomas), and cutaneous lesions (skin tumours). Optimum treatment is multidisciplinary because of the complexities associated with management of the multiple, progressive, and protean lesions associated with the disorder. We review the molecular pathogenesis, genetics, clinical findings, and management strategies for neurofibromatosis type 2.

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.

Tumor suppressor schwannomin/merlin is critical for the organization of Schwann cell contacts in peripheral nerves

Schwannomin/merlin is the product of a tumor suppressor gene mutated in neurofibromatosis type 2 (NF2). Although the consequences of NF2 mutations on Schwann cell proliferation are well established, the physiological role of schwannomin in differentiated cells is not known. To unravel this role, we studied peripheral nerves in mice overexpressing in Schwann cells schwannomin with a deletion occurring in NF2 patients (P0-SCH-Delta39-121) or a C-terminal deletion. The myelin sheath and nodes of Ranvier were essentially preserved in both lines. In contrast, the ultrastructural and molecular organization of contacts between Schwann cells and axons in paranodal and juxtaparanodal regions were altered, with irregular juxtaposition of normal and abnormal areas of contact. Similar but more severe alterations were observed in mice with conditional deletion of the Nf2 gene in Schwann cells. The number of Schmidt-Lanterman incisures, which are cytoplasmic channels interrupting the compact myelin and characterized by distinct autotypic contacts, was increased in the three mutant lines. P0-SCH-Delta39-121 and conditionally deleted mice displayed exuberant wrapping of nonmyelinated fibers and short internodes, an abnormality possibly related to altered control of Schwann cell proliferation. In support of this hypothesis, Schwann cell number was increased along fibers before myelination in P0-SCH-Delta39-121 mice but not in those with C-terminal deletion. Schwann cell numbers were also more numerous in mice with conditional deletion. Thus, schwannomin plays an important role in the control of Schwann cell number and is necessary for the correct organization and regulation of axoglial heterotypic and glio-glial autotypic contacts.

Phosphorylation and activity of the tumor suppressor Merlin and the ERM protein Moesin are coordinately regulated by the Slik kinase

Merlin and Moesin are closely related members of the 4.1 Ezrin/Radixin/Moesin domain superfamily implicated in regulating proliferation and epithelial integrity, respectively. The activity of both proteins is regulated by head to tail folding that is controlled, in part, by phosphorylation. Few upstream regulators of these phosphorylation events are known. In this study, we demonstrate that in Drosophila melanogaster, Slik, a Ste20 kinase, controls subcellular localization and phosphorylation of Merlin, resulting in the coordinate but opposite regulation of Merlin and Moesin. These results suggest the existence of a novel mechanism for coordinate regulation of cell proliferation and epithelial integrity in developing tissues.

Expression of protein 4.1G in Schwann cells of the peripheral nervous system

The membrane-associated cytoskeletal proteins, including protein 4.1 family, play important roles in membrane integrity, protein targeting, and signal transduction. Although protein 4.1G (4.1G) is expressed ubiquitously in mammalian tissues, it can have very discrete distributions within cells. The present study investigated the expression and distributions of 4.1G in rodent sciatic nerve. Northern and Western blot analysis detected abundant 4.1G mRNA and protein in rat sciatic nerve extracts. Immunohistochemical staining with a 4.1G-specific antibody and double immunolabeling with E-cadherin, betaIV spectrin, and connexin 32 detected 4.1G in paranodal loops, Schmidt-Lanterman incisures, and periaxonal, mesaxonal, and abaxonal membranes of rodent sciatic nerve. Immunoelectron microscopy confirmed the immunodistribution of 4.1G in Schwann cells. In developing mouse sciatic nerves, 4.1G was diffusely distributed in immature Schwann cells and gradually localized at paranodes, incisures, and periaxonal and mesaxonal membranes during their maturation. These data support the concept that 4.1G plays an important role in the membrane expansion and specialization that occurs during formation and maintenance of myelin internodes in the peripheral nervous system.

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.

Molecular pathogenesis of meningiomas

Meningiomas are common central nervous system tumors that originate from the meningeal coverings of the brain and the spinal cord. Most meningiomas are slowly growing benign tumors that histologically correspond to World Health Organization (WHO) grade I. However, certain rare histological variants (clear cell, chordoid, papillary, and rhabdoid), as well as atypical (WHO grade II) and anaplastic (WHO grade III) meningiomas show a more aggressive biological behavior and are clinically associated with a high risk of local recurrence and a less favorable prognosis. This review summarizes the most important features of meningioma pathology and provides an up-to-date overview about the molecular mechanisms involved in meningioma initiation and progression. Current data indicate that meningioma initiation is closely linked to the inactivation of one or more members of the highly conserved protein 4.1 superfamily, including the neurofibromatosis type 2 gene product merlin/schwannomin, protein 4.IB (DAL-1) and protein 4.1R. The genetic alterations in atypical meningiomas are complex and involve losses on 1p, 6q, 10, 14q and 18q, as well as gains on multiple chromosomes. The relevant genes are still unknown. Anaplastic meningiomas show even more complex genetic alterations, including frequent alteration of the CDKN2A, p14ARF, and CDKN2B tumor suppressor genes at 9p21, as well as gene amplification on 17q23. A better understanding of the molecular mechanisms involved in meningioma pathogenesis may not only lead to the identification of novel diagnostic and prognostic marker but will also facilitate the development of new pathogenesis-based therapeutic strategies.

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.

Neurofibromatosis 2 (NF2) tumor suppressor merlin inhibits phosphatidylinositol 3-kinase through binding to PIKE-L

Neurofibromatosis 2 (NF2) is a tumor suppressor, although the molecular mechanism accounting for this effect remains unknown. Here, we show that merlin exerts its activity by inhibiting phosphatidylinositol 3-kinase (PI3-kinase), through binding to PIKE-L. Wild-type merlin, but not patient-derived mutant (L64P), binds PIKE-L and inhibits PI3-kinase activity. This suppression of PI3-kinase activity results from merlin disrupting the binding of PIKE-L to PI3-kinase. In addition, merlin suppression of PI3-kinase activity as well as schwannoma cell growth is abrogated by a single PIKE-L point mutation (P187L) that cannot bind merlin but can still activate PI3-kinase. Knocking down PIKE-L with RNA interference abolishes merlin's tumor-suppressive activity. Our data support the hypothesis that PIKE-L is an important mediator of merlin growth suppression.

Characterization of chicken Nf2/merlin indicates regulatory roles in cell proliferation and migration

The neurofibromatosis 2 (NF2) tumor suppressor protein merlin, or schwannomin, functions as a negative growth regulator such that inactivating mutations in Nf2 predispose humans to tumors. In addition, merlin has a critical role during embryonic development. Nf2-deficient mice die early during embryogenesis, with defects in gastrulation and extraembryonic tissues. To investigate the function of Nf2/merlin during embryonic development, we first identified the homologous Nf2 gene in chicken (cNf2) and examined the distribution of chicken merlin (c-merlin) during myogenesis. cNf2 encoded a full-length mRNA of 1,770 nucleotides and a protein of 589 residues. C-merlin shared high sequence homology and common protein motifs with vertebrate and Drosophila merlins. In addition, cNF2 functions as a negative growth regulator similar to human and Drosophila merlin in vitro. In vivo, c-merlin was expressed diffusely in the forming dermomyotome but down-regulated in migratory muscle precursors in the forelimb. As muscle formed in the limb, c-merlin expression was up-regulated. As an initial examination of c-merlin function during myogenesis, c-merlin was ectopically expressed in muscle precursors and the effects on muscle development were examined. We show that ectopic merlin expression reduces the proliferation of muscle precursors as well as their ability to migrate effectively in limb mesoderm. Collectively, these results demonstrate that c-merlin is developmentally regulated in migrating and differentiating myogenic cells, where it functions as a negative regulator of both muscle growth and motility.

Krox-20 inhibits Jun-NH2-terminal kinase/c-Jun to control Schwann cell proliferation and death

The transcription factor Krox-20 controls Schwann cell myelination. Schwann cells in Krox-20 null mice fail to myelinate, and unlike myelinating Schwann cells, continue to proliferate and are susceptible to death. We find that enforced Krox-20 expression in Schwann cells cell-autonomously inactivates the proliferative response of Schwann cells to the major axonal mitogen β–neuregulin-1 and the death response to TGFβ or serum deprivation. Even in 3T3 fibroblasts, Krox-20 not only blocks proliferation and death but also activates the myelin genes periaxin and protein zero, showing properties in common with master regulatory genes in other cell types. Significantly, a major function of Krox-20 is to suppress the c-Jun NH₂-terminal protein kinase (JNK)–c-Jun pathway, activation of which is required for both proliferation and death. Thus, Krox-20 can coordinately control suppression of mitogenic and death responses. Krox-20 also up-regulates the scaffold protein JNK-interacting protein 1 (JIP-1). We propose this as a possible component of the mechanism by which Krox-20 regulates JNK activity during Schwann cell development.

Rho kinase regulates schwann cell myelination and formation of associated axonal domains

The myelin sheath forms by the spiral wrapping of a glial membrane around an axon. The mechanisms involved are poorly understood but are likely to involve coordinated changes in the glial cell cytoskeleton. Because of its key role as a regulator of the cytoskeleton, we investigated the role of Rho kinase (ROCK), a major downstream effector of Rho, in Schwann cell morphology, differentiation, and myelination. Pharmacologic inhibition of ROCK activity results in loss of microvilli and stress fibers in Schwann cell cultures and strikingly aberrant myelination in Schwann cell-neuron cocultures; there was no effect on Schwann cell proliferation or differentiation. Treated Schwann cells branch aberrantly and form multiple, small, independent myelin segments along the length of axons, each with associated nodes and paranodes. This organization partially resembles myelin formed by oligodendrocytes rather than the single long myelin sheath characteristic of Schwann cells. ROCK regulates myosin light chain phosphorylation, which is robustly, but transiently, activated at the onset of myelination. These results support a key role of Rho through its effector ROCK in coordinating the movement of the glial membrane around the axon at the onset of myelination via regulation of myosin phosphorylation and actomyosin assembly. They also indicate that the molecular machinery that promotes the wrapping of the glial membrane sheath around the axon is distributed along the entire length of the internode.

Identification of a third Protein 4.1 tumor suppressor, Protein 4.1R, in meningioma pathogenesis

Meningiomas are common central nervous system tumors; however, the mechanisms underlying their pathogenesis are largely undefined. In this report, we demonstrate that a third Protein 4.1 family member, Protein 4.1R, functions as a meningioma tumor suppressor. We observed loss of Protein 4.1R expression in two meningioma cell lines (IOMM-Lee, CH157-MN) by Western blotting as well as in 6 of 15 sporadic meningiomas by immunohistochemistry and fluorescence in situ hybridization. In support of a meningioma tumor suppressor function, Protein 4.1R overexpression resulted in reduced IOMM-Lee and CH157-MN cell proliferation. Similar to the Protein 4.1B and merlin tumor suppressors, Protein 4.1R membrane localization increased significantly under conditions of growth arrest in vitro. Lastly, we show that Protein 4.1R interacted with a subset of merlin/Protein 4.1B interactors including CD44 and betaII-spectrin. Collectively, these results suggest that Protein 4.1R functions as an important tumor suppressor in the molecular pathogenesis of meningioma.

Association of Caspr/paranodin with tumour suppressor schwannomin/merlin and beta1 integrin in the central nervous system

Caspr/paranodin is an essential neuronal component of paranodal axoglial junctions, associated with contactin/F3. Its short intracellular domain contains a conserved motif (GNP motif) capable of binding protein 4.1 domains [FERM domains (four point one, ezrin, radixin, moesin)]. Schwannomin/merlin is a tumour suppressor expressed in many cell types, including in neurons, the function and partners of which are still poorly characterized. We show that the FERM domain of schwannomin binds to the paranodin GNP motif in glutathione S-transferase (GST)-pull down assays and in transfected COS-7 cells. The two proteins co-immunoprecipitated in brain extracts. In addition, paranodin and schwannomin were associated with integrin beta1 in transfected cells and in brain homogenates. The presence of paranodin increased the association between integrin beta1 and schwannomin or its N-terminal domain, suggesting that the interactions between these proteins are interdependent. In jimpy mutant mice, which display a severe dysmyelination with deficient paranodal junctions, the interactions between paranodin, schwannomin and integrin beta1 were profoundly altered. Our results show that schwannomin and integrin beta1 can be associated with paranodin in the central nervous system. Since integrin beta1 and schwannomin do not appear to be enriched in paranodes they may be quantitatively minor partners of paranodin in these regions and/or be associated with paranodin at other locations.

Protein 4.1 tumor suppressors: getting a FERM grip on growth regulation

Members of the Protein 4.1 superfamily have highly conserved FERM domains that link cell surface glycoproteins to the actin cytoskeleton. Within this large and constantly expanding superfamily, at least five subgroups have been proposed. Two of these subgroups, the ERM and prototypic Protein 4.1 molecules, include proteins that function as tumor suppressors. The ERM subgroup member merlin/schwannomin is inactivated in the tumor-predisposition syndrome neurofibromatosis 2 (NF2), and the prototypic 4.1 subgroup member, Protein 4.1B, has been implicated in the molecular pathogenesis of breast, lung and brain cancers. This review focuses on what is known of mechanisms of action and critical protein interactions that may mediate the unique growth inhibitory signals of these two Protein 4.1 tumor suppressors. On the basis of insights derived from studying the NF2 tumor suppressor, we propose a model for merlin growth regulation in which CD44 links growth signals from plasma membrane to the nucleus by interacting with ERM proteins and merlin.

Paxillin binds schwannomin and regulates its density-dependent localization and effect on cell morphology

Neurofibromatosis type 2 is an autosomal dominant disorder characterized by tumors, predominantly schwannomas, in the nervous system. It is caused by mutations in the gene NF2, encoding the growth regulator schwannomin (also known as merlin). Mutations occur throughout the 17-exon gene, with most resulting in protein truncation and undetectable amounts of schwannomin protein. Pathogenic mutations that result in production of defective schwannomin include in-frame deletions of exon 2 and three independent missense mutations within this same exon. Mice with conditional deletion of exon 2 in Schwann cells develop schwannomas, which confirms the crucial nature of exon 2 for growth control. Here we report that the molecular adaptor paxillin binds directly to schwannomin at residues 50-70, which are encoded by exon 2. This interaction mediates the membrane localization of schwannomin to the plasma membrane, where it associates with beta 1 integrin and erbB2. It defines a pathogenic mechanism for the development of NF2 in humans with mutations in exon 2 of NF2.

Immunohistochemistry study of human vestibular nerve schwannoma differentiation

Differentiation of primary human vestibular nerve schwannomas (VS) caused by mutations of the NF2 gene was evaluated by examining the expression patterns of genes that are specifically expressed in different stages of Schwann cell lineage. In schwannoma cells that are not in contact with an axon, the expression levels of the major myelin sheath proteins, such as protein zero glycoprotein (P0), myelin basic protein (MBP), and peripheral myelin protein 22 (PMP22), were greatly reduced. However, high expression levels of nerve growth factor receptor (NGFR), neural cell adhesion molecule (N-CAM), and cell adhesion molecule L1 (L1) were observed. In addition, expression of transcription factors Krox20, Krox24, and SCIP/Oct6 was also detected in the tumor cells. These results suggest that loss of the NF2 gene was responsible for the transformation of the Schwann cells into a neoplastic stage that has a similar genetic profile to the pro-myelinating stage. Finally, the primary human vestibular schwannoma cells failed to be regulated and redifferentiated by a regenerating axon, when the human tumors were transplanted into sciatic nerve of nude rat. These results suggest that the NF2 gene might be involved in the differentiation of Schwann cells.

The neurofibromatosis type 2 gene product, merlin, reverses the F-actin cytoskeletal defects in primary human Schwannoma cells

Schwannoma tumors, which occur sporadically and in patients with neurofibromatosis, account for 8% of intracranial tumors and can only be treated by surgical removal. Most schwannomas have biallelic mutations in the NF2 tumor suppressor gene. We previously showed that schwannoma-derived Schwann cells exhibit membrane ruffling and aberrant cell spreading when plated onto laminin, indicative of fundamental F-actin cytoskeletal defects. Here we expand these observations to a large group of sporadic and NF2-related tumors and extend them to schwannomatosis-derived tumors. Mutation at NF2 correlated with F-actin abnormalities, but the extent of morphological change did not correlate with the type of NF2 mutation. We used a recently described molecular strategy, TAT-mediated protein transfer, to acutely introduce the NF2 protein, merlin, into primary human schwannoma cells in an attempt to reverse the cytoskeletal phenotype. Abnormal ruffling and cell spreading by cells with identified NF2 mutations were rapidly reversed by introduction of TAT-merlin. The effect is specific to TAT-merlin isoform 1, the growth-suppressive isoform of merlin. TAT-merlin isoform 2, a TAT-merlin mutant (L64P), and merlin lacking TAT were ineffective in reversing the cytoskeletal phenotype. Results show that merlin isoform 1 is sufficient to restore normal actin organization in NF2-deficient human tumor cells, demonstrating a key role for merlin in the NF2 phenotype. These results lay the foundation for epigenetic complementation studies in NF2 mouse models and possibly for experiments to evaluate the utility of merlin transduction into patients as protein therapy.

Regulation of Schwann cell morphology and adhesion by receptor-mediated lysophosphatidic acid signaling

In peripheral nerves, Schwann cells (SCs) form contacts with axons, other SCs, and extracellular matrix components that are critical for their migration, differentiation, and response to injury. Here, we report that lysophosphatidic acid (LPA), an extracellular signaling phospholipid, regulates the morphology and adhesion of cultured SCs. Treatment with LPA induces f-actin rearrangements resulting in a "wreath"-like structure, with actin loops bundled peripherally by short orthogonal filaments. The latter appear to anchor the SC to a laminin substrate, because they colocalize with the focal adhesion proteins, paxillin and vinculin. SCs also respond to LPA treatment by forming extensive cell-cell junctions containing N-cadherin, resulting in cell clustering. Pharmacological blocking experiments indicate that LPA-induced actin rearrangements and focal adhesion assembly involve Rho pathway activation via a pertussis toxin-insensitive G-protein. The transcript encoding LP(A1), the canonical G-protein-coupled receptor for LPA, is upregulated after sciatic nerve transection, and SCs cultured from lp(A1)-null mice exhibit greatly diminished morphological responses to LPA. Cultured SCs can release an LPA-like factor implicating SCs as a potential source of endogenous, signaling LPA. These data, together with the previous demonstration of LPA-mediated SC survival, implicate endogenous receptor-mediated LPA signaling in the control of SC development and function.

Schwannomin isoform-1 interacts with syntenin via PDZ domains

The neurofibromatosis type 2 gene (NF2) is involved in the pathogenesis of benign tumors of the human nervous system. The NF2 protein, called schwannomin or merlin, is inactivated in virtually all schwannomas and meningiomas. The molecular mechanisms by which schwannomin functions as a tumor suppressor is unknown but believed to involve plasma membrane-cytoskeletal interactions. Two major alternatively spliced isoforms of schwannomin differing in their C termini have been reported. Using the yeast two-hybrid system, we have identified syntenin as a binding partner for schwannomin isoform-1 (sch-1). Syntenin is an adapter protein that couples transmembrane proteoglycans to cytoskeletal components and is involved in intracellular vesicle transport. The C terminus 25 amino acids of sch-1 and the two PDZ domains of syntenin mediate their binding, and mutations introduced within the VAFFEEL region of sch-1 defined a sequence crucial for syntenin recognition. We have showed that the two proteins interacted in vitro and in vivo and localized underneath the plasma membrane. Fibroblast cells expressing heterologous antisense syntenin display alterations in the subcellular distribution of sch-1. Together, these results provide the first functional clue to the existence of schwannomin isoforms and could unravel novel pathways for the transport and subcellular localization of schwannomin in vivo.

Ezrin, radixin, and moesin are components of Schwann cell microvilli

Ezrin, radixin, and moesin (ERM proteins), as well as the neurofibromatosis 2 (NF2) tumor suppressor merlin/schwannomin, all belong to the protein 4.1 family, yet only merlin is a tumor suppressor in Schwann cells. To gain insight into the possible functions of ERM proteins in Schwann cells, we examined their localization in peripheral nerve, because we have previously shown that merlin is found in paranodes and in Schmidt-Lanterman incisures. All three ERM proteins were highly expressed in the microvilli of myelinating Schwann cells that surround the nodal axolemma as well as in incisures and cytoplasmic puncta in the vicinity of the node. In all of these locations, ERM proteins were colocalized with actin filaments. In contrast, ERM proteins did not surround nodes in the CNS. The colocalization of ERM proteins with actin indicates that they have functions different from those of merlin in myelinating Schwann cells.

The protein 4.1 tumor suppressor, DAL-1, impairs cell motility, but regulates proliferation in a cell-type-specific fashion

The neurofibromatosis 2 (NF2) tumor suppressor belongs to the Protein 4.1 family of molecules that link the actin cytoskeleton to cell surface glycoproteins. We have previously demonstrated that the NF2 protein, merlin, can suppress cell growth in vitro and in vivo as well as impair actin cytoskeleton-associated processes, such as cell spreading, attachment, and motility. Recently, we determined that expression of a second Protein 4.1 tumor suppressor, DAL-1, was lost in 60% of sporadic meningiomas, but not schwannomas. In this report, we demonstrate that DAL-1 suppresses cell proliferation in meningioma, but not schwannoma cells. Similar to merlin, DAL-1 interacts with other ERM proteins and betaII-spectrin, but not the merlin interactor protein, SCHIP-1. In addition, we report the identification of the full-length DAL-1 tumor suppressor, termed KIAA0987. Collectively, these results suggest that the two Protein 4.1 meningioma tumor suppressors, merlin and DAL-1, may be functionally distinct proteins with different mechanisms of action.

Tumorigenesis in neurofibromatosis: new insights and potential therapies

The neurofibromatoses NF1 and NF2 are inherited cancer predisposition syndromes in which affected individuals are prone to development of mostly benign, but occasionally malignant, tumors. The NF1 and NF2 genes function as tumor suppressor genes (negative growth regulators), such that their loss of expression predisposes to tumor formation. Neurofibromin, the protein product of the NF1 gene, acts as a negative regulator of the ras proto-oncogene, to reduce cell growth. Merlin, the NF2 gene product, is involved in regulating cell proliferation and motility, and probably plays a role in integrating multiple cell-signaling pathways. By understanding the function of these tumor suppressors, we have a unique opportunity to develop targeted pharmacotherapeutic interventions for these disorders.

Evidence for a cytoskeleton attachment domain at the N-terminus of the NF2 protein

Neurofibromatosis type 2 is a hereditary cancer syndrome characterized by the development of bilateral vestibular schwannomas. Underlying the disease are inactivating mutations of the NF2 tumor suppressor gene, located on chromosome 22, encoding a 595-amino-acid protein. The NF2 protein, also known as merlin or schwannomin, is reported to act as a membrane-cytoskeleton linking protein. This assumption is based on the homology of the NF2 protein to a group of band 4.1-related proteins, ezrin, radixin, and moesin. The cytoskeletal association of the NF2 protein has in part been confirmed by its ability to resist extraction from cells by nonionic detergents. We performed detergent extraction on COS cells transfected with NF2 cDNA constructs. The extracts were analyzed by Western blotting and immunofluorescent staining with monoclonal anti-NF2 antibodies. The results provide evidence for a high-affinity cytoskeleton attachment domain at amino acids 29-131 and a putative lower affinity domain between amino acids 321 and 470.

Cloning and characterization of SCHIP-1, a novel protein interacting specifically with spliced isoforms and naturally occurring mutant NF2 proteins

The neurofibromatosis type 2 (NF2) protein, known as schwannomin or merlin, is a tumor suppressor involved in NF2-associated and sporadic schwannomas and meningiomas. It is closely related to the ezrin-radixin-moesin family members, implicated in linking membrane proteins to the cytoskeleton. The molecular mechanism allowing schwannomin to function as a tumor suppressor is unknown. In attempt to shed light on schwannomin function, we have identified a novel coiled-coil protein, SCHIP-1, that specifically associates with schwannomin in vitro and in vivo. Within its coiled-coil region, this protein is homologous to human FEZ proteins and the related Caenorhabditis elegans gene product UNC-76. Immunofluorescent staining of transiently transfected cells shows a partial colocalization of SCHIP-1 and schwannomin, beneath the cytoplasmic membrane. Surprisingly, immunoprecipitation assays reveal that in a cellular context, association with SCHIP-1 can be observed only with some naturally occurring mutants of schwannomin, or a schwannomin spliced isoform lacking exons 2 and 3, but not with the schwannomin isoform exhibiting growth-suppressive activity. Our observations suggest that SCHIP-1 interaction with schwannomin is regulated by conformational changes in schwannomin, possibly induced by posttranslational modifications, alternative splicing, or mutations.

Expression of the neurofibromatosis type 2 gene in human tissues

The neurofibromatosis Type 2 tumor suppressor gene is implicated in the hereditary tumor syndrome NF2, hallmarked by bilateral vestibular schwannomas, meningiomas, and ocular non-neoplastic features. The gene product has characteristics of a membrane cytoskeleton-linking protein but the mechanism of tumor suppression by the NF2 protein remains to be elucidated. The NF2 gene is widely expressed in mouse and rat tissues. In humans, most of the expression data have accumulated through Northern blot analysis, RT-PCR and, more recently, Western blot analysis, providing information on whole tissues and organs rather than on specific cell types. We report here an extensive survey of NF2 gene expression in human tissues using a combination of mRNA in situ hybridization (mRNA ISH) and immunohistochemistry (IH) with a panel of monoclonal antibodies (MAbs) supplemented by tissue immunoprecipitation experiments with affinity-purified polyclonal antibodies. Expression was observed in many different cell types, most of which appear functionally normal in individuals affected by NF2. Surprisingly, expression could not be consistently documented in Schwann cells and arachnoidal cells by IH or by mRNA ISH in formalin-fixed tissue. However, consistent immunostaining of Schwann cells was seen in frozen sections. (J Histochem Cytochem 47:1471-1479, 1999)

Ezrin regulates cell-cell and cell-matrix adhesion, a possible role with E-cadherin/beta-catenin

Ezrin, radixin, moesin and merlin form a subfamily of conserved proteins in the band 4.1 superfamily. The function of these proteins is to link the plasma membrane to the actin cytoskeleton. Merlin is defective or absent in schwannomas and meningiomas and has been suggested to function as a tumour suppressor. In this study, we have examined the role of ezrin as a potential regulator of the adhesive and invasive behaviour of tumour cells. We have shown that following inhibition of ezrin expression in colo-rectal cancer cells using antisense oligonucleotides, these cells displayed a reduced cell-cell adhesiveness together with a gain in their motile and invasive behaviour. These cells also displayed increased spreading over matrix-coated surfaces. Immunofluorescence studies revealed that antisense-treated cells also displayed an increased staining of paxillin in areas representing focal adhesions. Furthermore, coprecipitation studies revealed an association of ezrin with E-cadherin and beta-catenin. Induction of the phosphorylation of ezrin by orthovanadate and hepatocyte growth factor/scatter factor resulted in changes similar to those seen with antisense treatment, together with a marked decrease in the association of ezrin with both beta-catenin and E-cadherin. It is concluded that ezrin regulates cell-cell and cell-matrix adhesion, by interacting with cell adhesion molecules E-cadherin and beta-catenin, and may thus play an important role in the control of adhesion and invasiveness of cancer cells.

Expression level, subcellular distribution and rho-GDI binding affinity of merlin in comparison with Ezrin/Radixin/Moesin proteins

Merlin, a neurofibromatosis type-2 tumor suppressor, shows significant sequence similarity to ERM (Ezrin/Radixin/Moesin) proteins, general actin filament/plasma membrane cross-linkers, which are regulated in a Rho-dependent manner. To understand its physiological functions, we compared merlin with ERM proteins in vivo and in vitro. Quantitative immunoblotting revealed that the molar ratio of merlin/ERM in cultured epithelial or non-epithelial cells was approximately 0.14 or approximately 0.05, respectively. After centrifugation of cell homogenate, merlin was mostly recovered in the insoluble fraction, whereas almost half of ERM proteins were found in the soluble fraction. Merlin and ERM proteins were concentrated at microvilli when introduced into fibroblasts. In contrast, in epithelial cells, introduced merlin was co-distributed with E-cadherin in lateral membranes, whereas ERM proteins were concentrated in apical microvilli. Finally, we examined the binding affinity of merlin to Rho GDP dissociation inhibitor (Rho-GDI), to which N-terminal halves of ERM proteins but not the full-length molecules specifically bind. In vitro binding assays revealed that the N-terminal halves of merlin isoform-I and -II as well as full-length merlin isoform-II bound to Rho-GDI with similar binding affinity to ERM proteins. Immunoprecipitation confirmed these findings in vivo. These findings do not favor the notion that merlin functions simply in a redundant or competitive manner to ERM proteins.

Identification of a 55-kDa ezrin-related protein that induces cytoskeletal changes and localizes to the nucleolus

Normal and transformed human cells when stained for ezrin, an F-actin-binding ERM (ezrin/radixin/moesin) family protein, revealed a faint and intense immunofluorescence, respectively. Surprisingly, nuclear staining that was assigned to the nucleolus by confocal laser and immunoelectron microscopy was detected in both cell types and was more prominent in normal cells due to the absence of glistering cytoplasmic fluorescence. By Western analysis the nuclear fraction was seen to have a 55-kDa ezrin-reactive protein that did not react to the antibodies raised against the C-terminus of the protein, suggesting that it may correspond to an endogenously cleaved N-terminus of the protein. Transfections of cells with a cDNA encoding full-length ezrin tagged with green fluorescent protein (GFP) at its N-terminus indeed resulted in two GFP-tagged products corresponding to full-length and 55-kDa endogenously cleaved forms. Transfection with a cDNA encoding approximately 55 kDa of the ezrin N-terminus (N-ezrin) showed that it can translocate to the nucleus. N-ezrin transfected cells exhibited irregular cell edges and collapse of actin fibers. Similar changes were seen following microinjection of anti-p81/ezrin antibody, suggesting that N-ezrin may function as a dominant negative competitor of ezrin. These data demonstrate the existence of an N-terminal cleavage form of ezrin that localizes to the nucleolus and that its overexpression induces cytoskeletal changes.

The neurobiology of Schwann cells

This selective review of Schwann cell biology focuses on questions relating to the origins, development and differentiation of Schwann cells and the signals that control these processes. The importance of neuregulins and their receptors in controlling Schwann cell precursor survival and generation of Schwann cells, and the role of these molecules in Schwann cell biology is addressed. The reciprocal signalling between peripheral glial cells and neurons in development and adult life revealed in recent years is highlighted, and the profound change in survival regulation from neuron-dependent Schwann cell precursors to adult Schwann cells that depend on autocrine survival signals is discussed. Besides providing neuronal and autocrine signals, Schwann cells signal to mesenchymal cells and influence the development of the connective tissue sheaths of peripheral nerves. The importance of Desert Hedgehog in this process is described. The control of gene expression during Schwann cell development and differentiation by transcription factors is reviewed. Knockout of Oct-6 and Krox-20 leads to delay or absence of myelination, and these results are related to morphological or physiological observations on knockout or mutation of myelin-related genes. Finally, the relationship between selected extracellular matrix components, integrins and the cytoskeleton is explored and related to disease.