Pathological adhesion of primary human schwannoma cells is dependent on altered expression of integrins

Proteomic screening identifies PML/p53 axis as a potential treatment target of facial nerve schwannomas

Facial nerve schwannomas (FNS) represents one of the more difficult treatment paradigms in neurotology. The aim of this study is to investigate the molecular alterations of FNS, thus providing potential targets treatable in the tumour. We for the first time suggest that the deficiency of merlin (the product of NF2 tumour suppressor) is probably one of the key mechanisms underlying FNS tumourigenesis, although no disease-causing NF2 mutations were demonstrated in tumour samples. TMT-labeled spectrometry analysis was used to identify the proteome of FNS relative to nerve controls. Eighty-four significantly deregulated proteins were identified, among which the PML tumour suppressor showed the most significantly increased expression. The PML protein was distributed in the nucleoplasm of non-tumorous Schwann cells, whereas it was preferentially confined to the cytoplasm of FNS cultures. Overexpression of PML and p53, partner proteins positively regulating each other to trigger apoptosis, was further confirmed in FNS tissues/cultures, and this correlated with a significant decrease in the proliferation of FNS cultures in comparison to Schwann cells. It is therefore probable that PML-p53 overexpression may occur as part of protective cellular mechanisms in response to the proliferation signal mediated by loss of merlin in FNS, in accordance with the fact that the tumour is benign slow-growing. This hypothesis was supported by the finding that the p53 activator nutlin-3 could exert dose-dependent inhibitory effects on FNS cultures via a cooperative induction of PML-p53 levels. Thus, the current study may present a potential treatment target directed on the molecular mechanisms of this disease.

A proteasome-resistant fragment of NIK mediates oncogenic NF-κB signaling in schwannomas

Schwannomas are common, highly morbid and medically untreatable tumors that can arise in patients with germ line as well as somatic mutations in neurofibromatosis type 2 (NF2). These mutations most commonly result in the loss of function of the NF2-encoded protein, Merlin. Little is known about how Merlin functions endogenously as a tumor suppressor and how its loss leads to oncogenic transformation in Schwann cells (SCs). Here, we identify nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB)-inducing kinase (NIK) as a potential drug target driving NF-κB signaling and Merlin-deficient schwannoma genesis. Using a genomic approach to profile aberrant tumor signaling pathways, we describe multiple upregulated NF-κB signaling elements in human and murine schwannomas, leading us to identify a caspase-cleaved, proteasome-resistant NIK kinase domain fragment that amplifies pathogenic NF-κB signaling. Lentiviral-mediated transduction of this NIK fragment into normal SCs promotes proliferation, survival, and adhesion while inducing schwannoma formation in a novel in vivo orthotopic transplant model. Furthermore, we describe an NF-κB-potentiated hepatocyte growth factor (HGF) to MET proto-oncogene receptor tyrosine kinase (c-Met) autocrine feed-forward loop promoting SC proliferation. These innovative studies identify a novel signaling axis underlying schwannoma formation, revealing new and potentially druggable schwannoma vulnerabilities with future therapeutic potential.

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.

Cellular prion protein (PrPC) in the development of Merlin-deficient tumours

Loss of function mutations in the neurofibromatosis Type 2 (NF2) gene, coding for a tumour suppressor, Merlin, cause multiple tumours of the nervous system such as schwannomas, meningiomas and ependymomas. These tumours may occur sporadically or as part of the hereditary condition neurofibromatosis Type 2 (NF2). Current treatment is confined to (radio) surgery and no targeted drug therapies exist. NF2 mutations and/or Merlin inactivation are also seen in other cancers including some mesothelioma, breast cancer, colorectal carcinoma, melanoma and glioblastoma. To study the relationship between Merlin deficiency and tumourigenesis, we have developed an in vitro model comprising human primary schwannoma cells, the most common Merlin-deficient tumour and the hallmark for NF2. Using this model, we show increased expression of cellular prion protein (PrP^C) in schwannoma cells and tissues. In addition, a strong overexpression of PrP^C is observed in human Merlin-deficient mesothelioma cell line TRA and in human Merlin-deficient meningiomas. PrP^C contributes to increased proliferation, cell-matrix adhesion and survival in schwannoma cells acting via 37/67 kDa non-integrin laminin receptor (LR/37/67 kDa) and downstream ERK1/2, PI3K/AKT and FAK signalling pathways. PrP^C protein is also strongly released from schwannoma cells via exosomes and as a free peptide suggesting that it may act in an autocrine and/or paracrine manner. We suggest that PrP^C and its interactor, LR/37/67 kDa, could be potential therapeutic targets for schwannomas and other Merlin-deficient tumours.

The scaffold protein KSR1, a novel therapeutic target for the treatment of Merlin-deficient tumors

Merlin has broad tumor-suppressor functions as its mutations have been identified in multiple benign tumors and malignant cancers. In all schwannomas, the majority of meningiomas and 1/3 of ependymomas Merlin loss is causative. In neurofibromatosis type 2, a dominantly inherited tumor disease because of the loss of Merlin, patients suffer from multiple nervous system tumors and die on average around age 40. Chemotherapy is not effective and tumor localization and multiplicity make surgery and radiosurgery challenging and morbidity is often considerable. Thus, a new therapeutic approach is needed for these tumors. Using a primary human in vitro model for Merlin-deficient tumors, we report that the Ras/Raf/mitogen-activated protein, extracellular signal-regulated kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) scaffold, kinase suppressor of Ras 1 (KSR1), has a vital role in promoting schwannomas development. We show that KSR1 overexpression is involved in many pathological phenotypes caused by Merlin loss, namely multipolar morphology, enhanced cell-matrix adhesion, focal adhesion and, most importantly, increased proliferation and survival. Our data demonstrate that KSR1 has a wider role than MEK1/2 in the development of schwannomas because adhesion is more dependent on KSR1 than MEK1/2. Immunoprecipitation analysis reveals that KSR1 is a novel binding partner of Merlin, which suppresses KSR1's function by inhibiting the binding between KSR1 and c-Raf. Our proteomic analysis also demonstrates that KSR1 interacts with several Merlin downstream effectors, including E3 ubiquitin ligase CRL4(DCAF1). Further functional studies suggests that KSR1 and DCAF1 may co-operate to regulate schwannomas formation. Taken together, these findings suggest that KSR1 serves as a potential therapeutic target for Merlin-deficient tumors.

The p53/mouse double minute 2 homolog complex deregulation in merlin-deficient tumours

Deficiency of the tumour suppressor merlin leads to the development of schwannomas, meningiomas and ependymomas occurring spontaneously or as a part of the hereditary disease Neurofibromatosis type 2 (NF2). Merlin loss is also found in a proportion of other cancers like mesothelioma, melanoma, breast cancer and glioblastoma. The tumour suppressor/transcription factor p53 regulates proliferation, survival and differentiation and its deficiency plays a role in the development of many tumours. 53 can be negatively regulated by FAK, PI3K/AKT and MDM2 and possibly positively regulated by merlin in different cell lines. In this study we investigated the role of p53 in merlin-deficient tumours. Using our in vitro model of primary human schwannoma cells we have previously demonstrated that FAK is overexpressed/activated and localises into the nucleus of schwannoma cells increasing proliferation. AKT is strongly activated via platelet-derived growth factor (PDGF) - and insulin-like growth factor 1 (IGF1) - receptors increasing survival. Here we investigated p53 regulation and its role in proliferation and survival of human primary schwannoma cells using western blotting, immunocytochemistry, immunohistochemistry and proliferation, survival and transcription factor assays. In human primary schwannoma cells p53 was found to be downregulated while MDM2 was upregulated leading to increased cell proliferation and survival. p53 is regulated by merlin involving FAK, AKT and MDM2. Merlin reintroduction into schwannoma cells increased p53 levels and activity, and treatment with Nutlin-3, a drug which increases p53 stability by disrupting the p53/MDM2 complex, decreased tumour growth and reduced cell survival. These findings are important to dissect the mechanisms responsible for the development of merlin-deficient tumours and to identify new therapeutic targets. We suggest that Nutlin-3, possibly in combination with FAK or PI3K inhibitors, can be employed as a novel treatment for schwannoma and other merlin-deficient tumours.

Schwannomas and their pathogenesis

Schwannomas may occur spontaneously, or in the context of a familial tumor syndrome such as neurofibromatosis type 2 (NF2), schwannomatosis and Carney's complex. Schwannomas have a variety of morphological appearances, but they behave as World Health Organization (WHO) grade I tumors, and only very rarely undergo malignant transformation. Central to the pathogenesis of these tumors is loss of function of merlin, either by direct genetic change involving the NF2 gene on chromosome 22 or secondarily to merlin inactivation. The genetic pathways and morphological features of schwannomas associated with different genetic syndromes will be discussed. Merlin has multiple functions, including within the nucleus and at the cell membrane, and this review summarizes our current understanding of the mechanisms by which merlin loss is involved in schwannoma pathogenesis, highlighting potential areas for therapeutic intervention.

Loss of SOX10 function contributes to the phenotype of human Merlin-null schwannoma cells

Loss of the Merlin tumour suppressor causes abnormal de-differentiation and proliferation of Schwann cells and formation of schwannoma tumours in patients with neurofibromatosis type 2. Within the mature peripheral nerve the normal development, differentiation and maintenance of myelinating and non-myelinating Schwann cells is regulated by a network of transcription factors that include SOX10, OCT6 (now known as POU3F1), NFATC4 and KROX20 (also known as Egr2). We have examined for the first time how their regulation of Schwann cell development is disrupted in primary human schwannoma cells. We find that induction of both KROX20 and OCT6 is impaired, whereas enforced expression of KROX20 drives both myelin gene expression and cell cycle arrest in Merlin-null cells. Importantly, we show that human schwannoma cells have reduced expression of SOX10 protein and messenger RNA. Analysis of mouse SOX10-null Schwann cells shows they display many of the characteristics of human schwannoma cells, including increased expression of platelet derived growth factor receptor beta (PDGFRB) messenger RNA and protein, enhanced proliferation, increased focal adhesions and schwannoma-like morphology. Correspondingly, reintroduction of SOX10 into human Merlin-null cells restores the ability of these cells to induce KROX20 and myelin protein zero (MPZ), localizes NFATC4 to the nucleus, reduces cell proliferation and suppresses PDGFRB expression. Thus, we propose that loss of the SOX10 protein, which is vital for normal Schwann cell development, is also key to the pathology of Merlin-null schwannoma tumours.

Axl/Gas6/NFκB signalling in schwannoma pathological proliferation, adhesion and survival

TAM family receptor tyrosine kinases comprising Tyro3 (Sky), Axl, and Mer are overexpressed in some cancers, correlate with multidrug resistance and contribute to tumourigenesis by regulating invasion, angiogenesis, cell survival and tumour growth. Mutations in the gene coding for a tumour suppressor merlin cause development of multiple tumours of the nervous system such as schwannomas, meningiomas and ependymomas occurring spontaneously or as part of a hereditary disease neurofibromatosis type 2. The benign character of merlin-deficient tumours makes them less responsive to chemotherapy. We previously showed that, amongst other growth factor receptors, TAM family receptors (Tyro3, Axl and Mer) are significantly overexpressed in schwannoma tissues. As Axl is negatively regulated by merlin and positively regulated by E3 ubiquitin ligase CRL4DCAF1, previously shown to be a key regulator in schwannoma growth we hypothesized that Axl is a good target to study in merlin-deficient tumours. Moreover, Axl positively regulates the oncogene Yes-associated protein, which is known to be under merlin regulation in schwannoma and is involved in increased proliferation of merlin-deficient meningioma and mesothelioma. Here, we demonstrated strong overexpression and activation of Axl receptor as well as its ligand Gas6 in human schwannoma primary cells compared to normal Schwann cells. We show that Gas6 is mitogenic and increases schwannoma cell-matrix adhesion and survival acting via Axl in schwannoma cells. Stimulation of the Gas6/Axl signalling pathway recruits Src, focal adhesion kinase (FAK) and NFκB. We showed that NFκB mediates Gas6/Axl-mediated overexpression of survivin, cyclin D1 and FAK, leading to enhanced survival, cell-matrix adhesion and proliferation of schwannoma. We conclude that Axl/FAK/Src/NFκB pathway is relevant in merlin-deficient tumours and is a potential therapeutic target for schwannoma and other merlin-deficient tumours.

Merlin, a multi-suppressor from cell membrane to the nucleus

Recent evidence suggests that the neurofibromatosis type 2 (NF2) gene encoded protein merlin suppresses mitogenic signalling not only at the cell membrane but also in the nucleus. At the membrane, merlin inhibits signalling by integrins and tyrosine receptor kinases (RTKs) and the activation of downstream pathways, including the Ras/Raf/MEK/ERK, FAK/Src, PI3K/AKT, Rac/PAK/JNK, mTORC1, and Wnt/β-catenin pathways. In the nucleus, merlin suppresses the E3 ubiquitin ligase CRL4(DCAF1) to inhibit proliferation. Gene expression analysis suggested that CRL4(DCAF1) could also regulate the expression of integrins and RTKs. In this review, we explore the links between merlin function at the membrane and in the nucleus, and discuss the potential of targeting the master regulator CRL4 (DCAF1) to treat NF2 and other merlin-deficient tumours.

Molecular mechanisms promoting the pathogenesis of Schwann cell neoplasms

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

Insulin-like growth factor-binding protein-1 (IGFBP-1) regulates human schwannoma proliferation, adhesion and survival

Merlin is a tumour suppressor involved in the development of a variety of tumours including mesotheliomas. Neurofibromatosis type 2 (NF2), a dominantly inherited tumour disease, is also caused by loss of merlin. NF2 patients suffer from multiple genetically well-defined tumours, schwannomas are most frequent among those. Using our in vitro model for human schwannoma, we found that schwannoma cells display enhanced proliferation because of the overexpression/activation of platelet-derived growth factor receptor and ErbB2/3, increased cell-matrix adhesion because of the overexpression of integrins, and decreased apoptosis. Mechanisms underlying schwannomas basal proliferation and cell-matrix adhesion are not understood. Here, we investigated insulin-like growth factor-binding protein-1 (IGFBP-1), which is expressed and released from central nervous system tumours and strongly overexpressed in schwannoma at the mRNA level. IGFBP-1 acts via β1-integrin and focal-adhesion-kinase (FAK), which are strongly overexpressed and basally activated in schwannoma. Using short hairpin RNA knockdown, small inhibitors and recombinant IGFBP-1, we demonstrate that schwannoma cells, in contrast to Schwann cells, release IGFBP-1 that activates the Src/FAK pathway, via integrin β1, potentiating schwannoma's proliferation and cell-matrix adhesion. We show that FAK localizes to the nucleus and Src triggers IGFBP-1 production. Further, we observed downregulation of the tumour-suppressor phosphatase and tensin homolog in schwannoma cells leading to increased activity of anti-apoptotic AKT. Thus, IGFBP-1/integrin β1/Src/FAK pathway has a crucial role in merlin-related tumourigenesis and therefore represents an important therapeutic target in the treatment of merlin-deficient tumours.

PAK kinase regulates Rac GTPase and is a potential target in human schwannomas

Merlin loss causes benign tumours of the nervous system, mainly schwannomas and meningiomas. Schwannomas show enhanced Rac1 and Cdc42 activity, the p21-activated kinase 2 (PAK2) activation and increased ruffling and cell adhesion. PAK regulates activation of merlin. PAK has been proposed as a potential therapeutic target in schwannomas. However where PAK stands in the Rac pathway is insufficiently characterised. We used a novel small-molecule PAK inhibitor, IPA-3, to investigate the role of PAK activation on Rac1/Cdc42 activity, cell spreading and adhesion in human primary schwannoma and Schwann cells. We show that IPA-3 blocks activation of PAK2 at Ser192/197 that antagonises PAK's interaction with Pix. Accordingly, Pix-mediated Rac1 activation is decreased in IPA-3 treated schwannoma cells, indicating that PAK acts upstream of Rac. We show that this Rac activation at the level of focal adhesions in schwannoma cells is essential for cell spreading and adhesion in Schwann and schwannoma cells.

Dissecting and targeting the growth factor-dependent and growth factor-independent extracellular signal-regulated kinase pathway in human schwannoma

Schwannomas are tumors of the nervous system that occur sporadically and in patients with the cancer predisposition syndrome neurofibromatosis type 2 (NF2). Schwannomas and all NF2-related tumors are caused by loss of the tumor suppressor merlin. Using our human in vitro model for schwannoma, we analyzed extracellular signal-regulated kinase 1/2 (ERK1/2) and AKT signaling pathways, their upstream growth factor receptors, and their role in schwannoma cell proliferation and adhesion to find new systemic therapies for these tumors that, to date, are very difficult to treat. We show here that human primary schwannoma cells show an enhanced basal Raf/mitogen-activated protein/ERK kinase/ERK1/2 pathway activity compared with healthy Schwann cells. Due to a strong and prolonged activation of platelet-derived growth factor receptor beta (PDGFRbeta), which is highly overexpressed, ERK1/2 and AKT activation was further increased in schwannoma, leading to increased proliferation. Using specific inhibitors, we discovered that ERK1/2 activation involves the integrin/focal adhesion kinase/Src/Ras signaling cascades and PDGFRbeta-mediated ERK1/2 activation is triggered through the phosphatidylinositol 3-kinase/protein kinase C/Src/c-Raf pathway. Due to the complexity of signals leading to schwannoma cell proliferation, potential new therapeutic agents should target several signaling pathways. The PDGFR and c-Raf inhibitor sorafenib (BAY 43-9006; Bayer Pharmaceuticals), currently approved for treatment of advanced renal cell cancer, inhibits both basal and PDGFRbeta-mediated ERK1/2 and AKT activity and decreases cell proliferation in human schwannoma cells, suggesting that this drug constitutes a promising tool to treat schwannomas. We conclude that our schwannoma in vitro model can be used to screen for new therapeutic targets in general and that sorafenib is possible candidate for future clinical trials.

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

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

Altered adhesive structures and their relation to RhoGTPase activation in merlin-deficient Schwannoma

Schwannomas are Schwann cell tumors of the nervous system that occur spontaneously and in patients with neurofibromatosis 2 (NF2) and lack the tumor suppressor merlin. Merlin is known to bind paxillin, beta1 integrin and focal adhesion kinase, members of focal contacts, multi-protein complexes that mediate cell adhesion to the extracellular matrix. Moreover, merlin-deficient Schwannomas show pathological adhesion to the extracellular matrix making the characterization of focal contacts indispensable. Using our Schwannoma in vitro model of human primary Schwann and Schwannoma cells, we here show that Schwannoma cells display an increased number of mature and stable focal contacts. In addition to an involvement of RhoA signaling via the Rho kinase ROCK, Rac1 plays a significant role in the pathological adhesion of Schwannoma cells. The Rac1 guanine exchange factor- beta-Pix, localizes to focal contacts in human primary Schwannoma cells, and we show that part of the Rac1 activation, an effect of merlin-deficiency, occurs at the level of focal contacts in human primary Schwannoma cells. Our results help explaining the pathological adhesion of Schwannoma cells, further strengthen the importance of RhoGTPase signaling in Schwannoma development, and suggest that merlin's role in tumor suppression is linked to focal contacts.

Neuregulin and laminin stimulate phosphorylation of the NF2 tumor suppressor in Schwann cells by distinct protein kinase A and p21-activated kinase-dependent pathways

Mutations in the neurofibromatosis type 2 (NF2) gene cause formation of schwannomas and other tumors in the nervous system. The NF2 protein, Schwannomin/Merlin, is a cytoskeleton-associated tumor suppressor regulated by phosphorylation at serine 518 (S518). Unphosphorylated Schwannomin restricts cell proliferation in part by inhibiting Rac- and p21-activated kinase (Pak). In a negative-feedback loop, Pak phosphorylates Schwannomin inactivating its ability to inhibit Pak. Little is known about receptor mechanisms that promote Pak activity and Schwannomin phosphorylation. Here we demonstrate in primary Schwann cells (SCs) that Schwannomin is rapidly phosphorylated on S518 by Pak following laminin-1 binding to beta1 integrin, and by protein kinase A following neuregulin-1beta (NRG1beta) binding to ErbB2/ErbB3 receptors. These receptors, together with phosphorylated Schwannomin, P-Pak, Cdc42 and paxillin are enriched at the distal tips of SC processes, and can be isolated as a complex using beta1 integrin antibody. Dual stimulation with laminin-1 and NRG1beta does not synergistically increase Schwannomin phosphorylation because ErbB2 kinase partially antagonizes integrin-dependent activation of Pak. These results identify two parallel, but interactive pathways that inactivate the tumor suppressor activity of Schwannomin to allow proliferation of subconfluent SCs. Moreover, they identify ErbB2, ErbB3 and beta1 integrins as potential therapeutic targets for NF2.

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

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

News on the genetics, epidemiology, medical care and translational research of Schwannomas

Recent years have seen substantial news and updates in the genetics and diagnosis of schwannomas, even a new hereditary disease with schwannomas; Schwannomatosis has been defined. These developments have consequently led to better evaluation of the incidence of schwannomas. Although there has also been progress in the treatment of schwannomas especially in the field of radiation therapy, hereditary diseases with multiple tumours still represent a therapeutic dilemma. NF2 in particular still causes major morbidity and mortality owing to the neurological deficit of multiple tumour disease and deafness caused by vestibular nerve involvement. Thus there has been great enthusiasm about disease models in the hope that translational research will give rise to new therapies.

PTHrP increases transcriptional activity of the integrin subunit alpha5

Increasing evidence is emerging highlighting the role of parathyroid hormone-related protein (PTHrP) during metastasis by regulating cell adhesion. The current study demonstrated that modulation of PTHrP expression by PTHrP overexpression and small interfering RNA-induced silencing resulted in changes in cell adhesion and integrin expression. RNA interference of endogenous PTHrP caused a significant reduction in cell adhesion of a breast cancer cell line to collagen type I, fibronectin and laminin (P<0.05) and of a colon cancer cell to collagen type I and fibronectin (P<0.05). Overexpression of PTHrP induced a significant increase in cell adhesion of colon (P<0.0001) and breast (P<0.05) cancer cells to the same extracellular matrix proteins. These PTHrP-mediated effects were attributed to changes in integrin expression as the differences in adhesion profile correlated with the integrin expression profile. In an attempt to elucidate the mechanism whereby PTHrP regulates integrin expression, promoter activity of the integrin α₅ subunit was analysed and significant increases in transcriptional activity were observed in PTHrP overexpressing cells (P<0.0001), which was dependent on nuclear localisation. These results indicate that modulation of cell adhesion is a normal physiological action of PTHrP, mediated by increasing integrin gene transcription.

Differential gene expression between human schwannoma and control Schwann cells

The NF2 gene encodes the tumour suppressor protein merlin. The mutation of a single allele of this gene causes the autosomal dominantly inherited disease neurofibromatosis type 2 (NF2), which is characterized mainly by vestibular schwannoma carrying a second hit mutation. Complete lack of merlin is also found in spontaneous schwannomas and meningiomas. As the events leading to schwannoma development are largely unknown we investigated the differences in gene expression between schwannoma cells from NF2 patients and normal human primary Schwann cells by cDNA array analysis. We identified 41 genes whose expression levels differed by more than factor 2. Most of these clones were corroborated by real-time reverse transcription polymerase chain reaction analysis. By this method a total of seven genes with increased and seven genes with decreased mRNA levels in schwannoma compared with normal Schwann cells could be identified. Regulated clones, some of which not been described in Schwann cells earlier, included matrix metalloproteinase's, growth factors, growth factor receptors and tyrosine kinases.

Sensitive detection of deletions of one or more exons in the neurofibromatosis type 2 (NF2) gene by multiplexed gene dosage polymerase chain reaction

Mutation detection in the neurofibromatosis type 2 (NF2) gene is challenging because when combining mutation detection methods such as single-strand conformational polymorphism and heteroduplex analysis, denaturing gradient gel electrophoresis, and direct sequencing of aberrant polymerase chain reaction (PCR) fragments only 30 to 60% of the constitutional mutations are detected. Because large deletions and complete chromosome rearrangements are also described methods such as microarray-comparative genomic hybridization and fluorescence in situ hybridization are also used. The one type of mutation often missed corresponds to deletions encompassing one or few exons. To detect this type we have developed a swift and reliable method. We perform a gene dosage analysis with two fluorescent multiplex PCR assays that amplify 15 of the 17 NF2 exons. The labeled PCR products are quantified and gene dose is calculated with respect to controls. We tested the reliability of this method with DNA from eight NF2 patients with known heterozygous NF2 deletions, eight controls and four unknown NF2 patients. In all of the patients with known heterozygous deletions we found in several exons a reduction of gene dosage to 50 to 69%. In one NF2 patient with previously unknown mutation and a severe phenotype we found the gene dosage of two exons reduced by 50% indicating a deletion of these two exons on one allele. This finding was validated by reverse transcriptase-PCR on fibroblast and schwannoma cell cultures of this patient and cDNA sequencing. Our gene dosage assay will detect deletions of one or more exons as well as gross deletions of the whole coding region of the gene. It can complement the existing screening methods because it is faster and easier.

Rearrangements of the intermediate filament GFAP in primary human schwannoma cells

Loss of the tumor suppressor protein merlin causes a variety of benign tumors such as schwannomas, meningiomas, and gliomas in man. We previously reported primary human schwannoma cells to show enhanced integrin-dependent adhesion and a hyperactivation of the small RhoGTPase Rac1. Here we show that the main intermediate filament protein of Schwann cells, the glial fibrillary acidic protein, is collapsed to the perinuclear region instead of being well-spread from the nucleus to the cell periphery. This cytoskeletal reorganization is accompanied by changes in cell shape and increased cell motility. Moreover, we report tyrosine phosphorylation to be enhanced in schwannoma cells, already described earlier in intermediate filament breakdown. Thus, we believe that Rac activation via tyrosine kinase stimulation leads to GFAP collapse in human schwannoma cells, and suggest that this process plays an important role in vivo where schwannoma cells become motile, unspecifically ensheathing extracellular matrix and forming pseudomesaxons.

Reduced apoptosis rates in human schwannomas

Schwannomas, tumors originating from Schwann cells, represent a frequent neurological tumor and can occur both in a genetic disorder called neurofibromatosis type 2 (NF2) and sporadically. In both cases the genetic background is identical as all schwannomas are caused by biallelic mutations in the tumor suppressor gene NF2 coding for merlin. Mutations in this gene have also been found to be responsible for 50% to 60% of spontaneous and 100% of the NF2 associated meningiomas. The NF2 gene product, merlin, links transmembrane proteins to the cytoskeleton and is involved in intracellular signaling processes. It has previously been shown that reexpression of wild-type merlin in primary human schwannoma cells leads to an increase in the number of apoptotic cells. Here, we report in vivo and in vitro evidence that the basal apoptosis rate of primary human schwannoma cells is reduced in comparison to that of normal Schwann cells, supporting the idea that in this benign tumor type, apoptosis has a role in tumorigenesis.