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

A novel induced pluripotent stem cell model of Schwann cell differentiation reveals NF2 - related gene regulatory networks of the extracellular matrix

Schwann cells are vital to development and maintenance of the peripheral nervous system and their dysfunction has been implicated in a range of neurological and neoplastic disorders, including NF2 -related schwannomatosis. We developed a novel human induced pluripotent stem cell (hiPSC) model to study Schwann cell differentiation in health and disease. We performed transcriptomic, immunofluorescence, and morphological analysis of hiPSC derived Schwann cell precursors (SPCs) and terminally differentiated Schwann cells (SCs) representing distinct stages of development. To validate our findings, we performed integrated, cross-species analyses across multiple external datasets at bulk and single cell resolution. Our hiPSC model of Schwann cell development shared overlapping gene expression signatures with human amniotic mesenchymal stem cell (hAMSCs) derived SCs and in vivo mouse models, but also revealed unique features that may reflect species-specific aspects of Schwann cell biology. Moreover, we identified gene co-expression modules that are dynamically regulated during hiPSC to SC differentiation associated with ear and neural development, cell fate determination, the NF2 gene, and extracellular matrix (ECM) organization. By cross-referencing results between multiple datasets, we identified new genes potentially associated with NF2 expression. Our hiPSC model further provides a tractable platform for studying Schwann cell development in the context of human disease.

A pilot investigation of differential hydroxymethylation levels in patient-derived neural stem cells implicates altered cortical development in bipolar disorder

Introduction

Bipolar disorder (BD) is a chronic mental illness characterized by recurrent episodes of mania and depression and associated with social and cognitive disturbances. Environmental factors, such as maternal smoking and childhood trauma, are believed to modulate risk genotypes and contribute to the pathogenesis of BD, suggesting a key role in epigenetic regulation during neurodevelopment. 5-hydroxymethylcytosine (5hmC) is an epigenetic variant of particular interest, as it is highly expressed in the brain and is implicated in neurodevelopment, and psychiatric and neurological disorders.

Methods

Induced pluripotent stem cells (iPSCs) were generated from the white blood cells of two adolescent patients with bipolar disorder and their same-sex age-matched unaffected siblings (n = 4). Further, iPSCs were differentiated into neuronal stem cells (NSCs) and characterized for purity using immuno-fluorescence. We used reduced representation hydroxymethylation profiling (RRHP) to perform genome-wide 5hmC profiling of iPSCs and NSCs, to model 5hmC changes during neuronal differentiation and assess their impact on BD risk. Functional annotation and enrichment testing of genes harboring differentiated 5hmC loci were performed with the online tool DAVID.

Results

Approximately 2 million sites were mapped and quantified, with the majority (68.8%) located in genic regions, with elevated 5hmC levels per site observed for 3' UTRs, exons, and 2-kb shorelines of CpG islands. Paired t-tests of normalized 5hmC counts between iPSC and NSC cell lines revealed global hypo-hydroxymethylation in NSCs and enrichment of differentially hydroxymethylated sites within genes associated with plasma membrane (FDR = 9.1 × 10-12) and axon guidance (FDR = 2.1 × 10-6), among other neuronal processes. The most significant difference was observed for a transcription factor binding site for the KCNK9 gene (p = 8.8 × 10-6), encoding a potassium channel protein involved in neuronal activity and migration. Protein-protein-interaction (PPI) networking showed significant connectivity (p = 3.2 × 10-10) between proteins encoded by genes harboring highly differentiated 5hmC sites, with genes involved in axon guidance and ion transmembrane transport forming distinct sub-clusters. Comparison of NSCs of BD cases and unaffected siblings revealed additional patterns of differentiation in hydroxymethylation levels, including sites in genes with functions related to synapse formation and regulation, such as CUX2 (p = 2.4 × 10-5) and DOK-7 (p = 3.6 × 10-3), as well as an enrichment of genes involved in the extracellular matrix (FDR = 1.0 × 10-8).

Discussion

Together, these preliminary results lend evidence toward a potential role for 5hmC in both early neuronal differentiation and BD risk, with validation and more comprehensive characterization to be achieved through follow-up study.

Review of Cervical Schwannomas: Presentation, Pathogenesis, Management, Complications and Future Directions

Cervical schwannomas are benign peripheral nerve sheath tumors, relatively uncommon pathologies. The purpose of this review is to summarize and expand on the existing literature on cervical schwannomas, focusing on clinical presentation, pathogenesis, surgical and radiologic management and innovative therapies including ultrasound-guided techniques. Pubmed and SCOPUS databases were searched using combinations of terms including "cervical schwannoma", "surgery", "fusion", "complications", "radiosurgery", and others. The findings regarding these unique clinical entities are presented below.

PLEK2, RRM2, GCSH: A Novel WWOX-Dependent Biomarker Triad of Glioblastoma at the Crossroads of Cytoskeleton Reorganization and Metabolism Alterations

Glioblastoma is one of the deadliest human cancers. Its malignancy depends on cytoskeleton reorganization, which is related to, e.g., epithelial-to-mesenchymal transition and metastasis. The malignant phenotype of glioblastoma is also affected by the WWOX gene, which is lost in nearly a quarter of gliomas. Although the role of WWOX in the cytoskeleton rearrangement has been found in neural progenitor cells, its function as a modulator of cytoskeleton in gliomas was not investigated. Therefore, this study aimed to investigate the role of WWOX and its collaborators in cytoskeleton dynamics of glioblastoma. Methodology on RNA-seq data integrated the use of databases, bioinformatics tools, web-based platforms, and machine learning algorithm, and the obtained results were validated through microarray data. PLEK2, RRM2, and GCSH were the most relevant WWOX-dependent genes that could serve as novel biomarkers. Other genes important in the context of cytoskeleton (BMP4, CCL11, CUX2, DUSP7, FAM92B, GRIN2B, HOXA1, HOXA10, KIF20A, NF2, SPOCK1, TTR, UHRF1, and WT1), metabolism (MTHFD2), or correlation with WWOX (COL3A1, KIF20A, RNF141, and RXRG) were also discovered. For the first time, we propose that changes in WWOX expression dictate a myriad of alterations that affect both glioblastoma cytoskeleton and metabolism, rendering new therapeutic possibilities.

Schwannoma of right cerebellopontine angle. A cytologic diagnosis

Schwannomas affect mainly head and neck peripheral nerves, are benign tumors and derive from Schwann cells. Schwannoma of right cerebellopontine angle is extremely rare to diagnose by cytology. We report one such rare case presenting the cytological features in material obtained during the resection of the tumor. Case report: A 47-year-old female was diagnosed by MRI with a tumor of right cerebellopontine angle.. Cytologic material from the tumor was obtained intraoperatively and diagnosed cytologically as a neurilemoma. Conclusion: This case is presented here to focus the ability of cytology in diagnosis of schwannoma in intraoperative material of the tumor, using immunohistochemistry and confirmed by histology-immunohistochemistry. (www.actabiomedica.it)

Syndromic Hearing Loss: A Brief Review of Common Presentations and Genetics

Congenital hearing loss is one of the most common birth defects worldwide, with around 1 in 500 people experiencing some form of severe hearing loss. While over 400 different syndromes involving hearing loss have been described, it is important to be familiar with a wide range of syndromes involving hearing loss so an early diagnosis can be made and early intervention can be pursued to maximize functional hearing and speech-language development in the setting of verbal communication. This review aims to describe the presentation and genetics for some of the most frequently occurring syndromes involving hearing loss, including neurofibromatosis type 2, branchio-oto-renal syndrome, Treacher Collins syndrome, Stickler syndrome, Waardenburg syndrome, Pendred syndrome, Jervell and Lange-Nielsen syndrome, Usher syndromes, Refsum disease, Alport syndrome, MELAS, and MERRF.

Role of Merlin/NF2 inactivation in tumor biology

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

NF2 Loss Promotes Oncogenic RAS-Induced Thyroid Cancers via YAP-Dependent Transactivation of RAS Proteins and Sensitizes Them to MEK Inhibition

Ch22q LOH is preferentially associated with RAS mutations in papillary and in poorly differentiated thyroid cancer (PDTC). The 22q tumor suppressor NF2, encoding merlin, is implicated in this interaction because of its frequent loss of function in human thyroid cancer cell lines. Nf2 deletion or Hras mutation is insufficient for transformation, whereas their combined disruption leads to murine PDTC with increased MAPK signaling. Merlin loss induces RAS signaling in part through inactivation of Hippo, which activates a YAP-TEAD transcriptional program. We find that the three RAS genes are themselves YAP-TEAD1 transcriptional targets, providing a novel mechanism of promotion of RAS-induced tumorigenesis. Moreover, pharmacologic disruption of YAP-TEAD with verteporfin blocks RAS transcription and signaling and inhibits cell growth. The increased MAPK output generated by NF2 loss in RAS-mutant cancers may inform therapeutic strategies, as it generates greater dependency on the MAPK pathway for viability.

SIGNIFICANCE

Intensification of mutant RAS signaling through copy-number imbalances is commonly associated with transformation. We show that NF2/merlin inactivation augments mutant RAS signaling by promoting YAP/TEAD-driven transcription of oncogenic and wild-type RAS, resulting in greater MAPK output and increased sensitivity to MEK inhibitors.

LIM domain kinases as potential therapeutic targets for neurofibromatosis type 2

Neurofibromatosis type 2 (NF2) is caused by mutations in the NF2 gene that encodes a tumor-suppressor protein called merlin. NF2 is characterized by formation of multiple schwannomas, meningiomas and ependymomas. Merlin loss-of-function is associated with increased activity of Rac and p21-activated kinases (PAKs) and deregulation of cytoskeletal organization. LIM domain kinases (LIMK1 and 2) are substrate for Cdc42/Rac-PAK and modulate actin dynamics by phosphorylating cofilin at serine-3. This modification inactivates the actin severing and depolymerizing activity of cofilin. LIMKs also translocate into the nucleus and regulate cell cycle progression. Significantly, LIMKs are overexpressed in several tumor types, including skin, breast, lung, liver and prostate. Here we report that mouse Schwann cells (MSCs) in which merlin function is lost as a result of Nf2 exon2 deletion (Nf2(ΔEx2)) exhibited increased levels of LIMK1, LIMK2 and active phospho-Thr508/505-LIMK1/2, as well as phospho-Ser3-cofilin, compared with wild-type normal MSCs. Similarly, levels of LIMK1 and 2 total protein and active phosphorylated forms were elevated in human vestibular schwannomas compared with normal human Schwann cells (SCs). Reintroduction of wild-type NF2 into Nf2(ΔEx2) MSC reduced LIMK1 and LIMK2 levels. We show that pharmacological inhibition of LIMK with BMS-5 decreased the viability of Nf2(ΔEx2) MSCs in a dose-dependent manner, but did not affect viability of control MSCs. Similarly, LIMK knockdown decreased viability of Nf2(ΔEx2) MSCs. The decreased viability of Nf2(ΔEx2) MSCs was not due to caspase-dependent or -independent apoptosis, but rather due to inhibition of cell cycle progression as evidenced by accumulation of cells in G2/M phase. Inhibition of LIMKs arrests cells in early mitosis by decreasing aurora A activation. Our results suggest that LIMKs are potential drug targets for NF2 and tumors associated with merlin deficiency.

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.

mTORC1 inhibition delays growth of neurofibromatosis type 2 schwannoma

BACKGROUND

Neurofibromatosis type 2 (NF2) is a rare autosomal dominant genetic disorder, resulting in a variety of neural tumors, with bilateral vestibular schwannomas as the most frequent manifestation. Recently, merlin, the NF2 tumor suppressor, has been identified as a novel negative regulator of mammalian target of rapamycin complex 1 (mTORC1); functional loss of merlin was shown to result in elevated mTORC1 signaling in NF2-related tumors. Thus, mTORC1 pathway inhibition may be a useful targeted therapeutic approach.

METHODS

We studied in vitro cell models, cohorts of mice allografted with Nf2(-/-) Schwann cells, and a genetically modified mouse model of NF2 schwannoma in order to evaluate the efficacy of the proposed targeted therapy for NF2.

RESULTS

We found that treatment with the mTORC1 inhibitor rapamycin reduced the severity of NF2-related Schwann cell tumorigenesis without significant toxicity. Consistent with these results, in an NF2 patient with growing vestibular schwannomas, the rapalog sirolimus induced tumor growth arrest.

CONCLUSIONS

Taken together, these results constitute definitive evidence that justifies proceeding with clinical trials using mTORC1-targeted agents in selected patients with NF2 and in patients with NF2-related sporadic tumors.

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

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

In vitro antisense therapeutics for a deep intronic mutation causing Neurofibromatosis type 2

Neurofibromatosis type 2 (NF2) is an autosomal-dominant disorder affecting about 1:33 000 newborns, mainly characterized by the development of tumors of the nervous system and ocular abnormalities. Around 85% of germline NF2 mutations are point mutations. Among them, ∼25% affect splicing and are associated with a variable disease severity. In the context of our NF2 Multidisciplinary Clinics, we have identified a patient fulfilling clinical criteria for the disease and exhibiting a severe phenotype. The patient carries a deep intronic mutation (g. 74409T>A, NG_009057.1) that produces the insertion of a cryptic exon of 167pb in the mature mRNA between exons 13 and 14, resulting in a truncated merlin protein (p.Pro482Profs*39). A mutation-specific antisense phosphorodiamidate morpholino oligomer was designed and used in vitro to effectively restore normal NF2 splicing in patient-derived primary fibroblasts. In addition, merlin protein levels were greatly recovered after morpholino treatment, decreasing patient's fibroblasts in vitro proliferation capacity and restoring cytoeskeleton organization. To our knowledge, this is the first NF2 case caused by a deep intronic mutation in which an in vitro antisense therapeutic approximation has been tested. These results open the possibility of using this approach in vivo for this type of mutation causing NF2.

Rac1 is required for Prkar1a-mediated Nf2 suppression in Schwann cell tumors

Schwannomas are peripheral nerve sheath tumors that often occur in the setting of an inherited tumor predisposition syndrome, including Neurofibromatosis Types 1 (NF1) and 2 (NF2), Familial Schwannomatosis (FS) and Carney Complex (CNC). Loss of the NF2 tumor suppressor (encoding NF2, or Merlin) is associated with upregulation of the Rac1 small GTPase, which is thought to play a key role in mediating tumor formation. In prior studies, we generated a mouse model of schwannomas by performing tissue-specific knockout of the CNC gene Prkar1a, which encodes the type 1A regulatory subunit of Protein Kinase A. These tumors exhibited down-regulation of Nf2 protein and an increase in activated Rac1. To assess the requirement for Rac1 in schwannoma formation, we generated a double knockout of Prkar1a and Rac1 in Schwann cells and monitored tumor formation. Loss of Rac1 reduced tumor formation by reducing proliferation and enhancing apoptosis. Surprisingly, the reduction of tumor formation was accompanied by re-expression of the Nf2 protein. Furthermore, activated Rac1 was able to downregulate Nf2 in vitro in a Pak-dependent manner. These in vivo data indicate that activation of Rac1 is responsible for suppression of Nf2 protein production; deficiency of Nf2 in Schwann cells leads to loss of cellular growth control and tumor formation.. Further, PKA activation through mutation in Prkar1a is sufficient to initiate Rac1 signaling, with subsequent reduction of Nf2 and schwannomagenesis. Although in vitro evidence has shown that loss of Nf2 activates Rac1, our data indicates that signaling between Nf2 and Rac1 occurs in a bidirectional fashion, and these interactions are modulated by PKA.

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.

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.

p21-Activated kinases are required for transformation in a cell-based model of neurofibromatosis type 2

Background

NF2 is an autosomal dominant disease characterized by development of bilateral vestibular schwannomas and other benign tumors in central nervous system. Loss of the NF2 gene product, Merlin, leads to aberrant Schwann cell proliferation, motility, and survival, but the mechanisms by which this tumor suppressor functions remain unclear. One well-defined target of Merlin is the group I family of p21-activated kinases, which are allosterically inhibited by Merlin and which, when activated, stimulate cell cycle progression, motility, and increased survival. Here, we examine the effect of Pak inhibition on cells with diminished Merlin function.

Methodology/Principal Findings

Using a specific peptide inhibitor of group I Paks, we show that loss of Pak activity restores normal cell movement in cells lacking Merlin function. In addition, xenografts of such cells form fewer and smaller tumors than do cells without Pak inhibition. However, in tumors, loss of Pak activity does not reduce Erk or Akt activity, two signaling proteins that are thought to mediate Pak function in growth factor pathways.

Conclusions/Significance

These results suggest that Pak functions in novel signaling pathways in NF2, and may serve as a useful therapeutic target in this disease.

Merlin, a "magic" linker between extracellular cues and intracellular signaling pathways that regulate cell motility, proliferation, and survival

Genetic alterations of neurofibromatosis type 2 (NF2) gene lead to the development of schwannomas, meningiomas, and ependymomas. Mutations of NF2 gene were also found in thyroid cancer, mesothelioma, and melanoma, suggesting that it functions as a tumor suppressor in a wide spectrum of cells. The product of NF2 gene is merlin (moesin-ezrin-radixin-like protein), a member of the Band 4.1 superfamily proteins. Merlin shares significant sequence homology with the ERM (Ezrin-Radixin-Moesin) family proteins and serves as a linker between transmembrane proteins and the actin-cytoskeleton. Merlin is a multifunctional protein and involved in integrating and regulating the extracellular cues and intracellular signaling pathways that control cell fate, shape, proliferation, survival, and motility. Recent studies showed that merlin regulates the cell-cell and cell-matrix adhesions and functions of the cell surface adhesion/extracellular matrix receptors including CD44 and that merlin and CD44 antagonize each other's function and work upstream of the mammalian Hippo signaling pathway. Furthermore, merlin plays important roles in stabilizing the contact inhibition of proliferation and in regulating activities of several receptor tyrosine kinases. Accumulating data also suggested an emerging role of merlin as a negative regulator of growth and progression of several non-NF2 associated cancer types. Together, these recent advances have improved our basic understanding about merlin function, its regulation, and the major signaling pathways regulated by merlin and provided the foundation for future translation of these findings into the clinic for patients bearing the cancers in which merlin function and/or its downstream signaling pathways are impaired or altered.

Proteomics: a strategy to understand the novel targets in protein misfolding and cancer therapy

Proteins carry out important functions as they fold themselves. Protein misfolding occurs during different biochemical processes and may lead to the development of diseases such as cancer, which is characterized by genetic instability. The cancer microenvironment exposes malignant cells to a variety of stressful conditions that may further promote protein misfolding. Tumor development and progression often arises from mutations that interfere with the appropriate function of tumor-suppressor proteins and oncogenes. These may be due to alteration of catalytic activity of the protein, loss of binding sites for effector proteins or alterations of the native folded protein conformation. Src family kinases, p53, mTOR and C-terminus of HSC70 interacting protein (CHIPs) are some examples associated with protein misfolding and tumorigenesis. Molecular chaperones, such as heat-shock protein (HSP)70 and HSP90, assist protein folding and recognize target misfolded proteins for degradation. It is likely that this misfolding in cancer is linked by common principles, and may, therefore, present an exciting possibility to identify common targets for therapeutic intervention. Here we aim to review a number of examples that show how alterations in the folding of tumor-suppressor proteins or oncogenes lead to tumorigenesis. The possibility of targeting the targets to repair or degrade protein misfolding in cancer therapy is discussed.

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

Merlin is the product of the Nf2 tumor-suppressor gene, and inactivation of Nf2 leads to the development of neural tumors such as schwannomas and meningiomas in humans and mice. Merlin is a member of the ERM (ezrin, radixin and moesin) family of proteins that function as organizers of the actin cytoskeleton. Merlin structure is thought to be similar to that of the ERM proteins, and is held in a closed clamp conformation via intramolecular interactions of its N-terminal FERM (four-point-one, ERM) domain with an alpha-helical C-terminal domain. Like ERMs, merlin can remodel actin-rich cortical structures, yet merlin uniquely inhibits the proliferation of many different cell types. Here, we report that the F2 subdomain of the FERM domain and a domain close to the C-terminus that is defined by residues 532-579 are essential for merlin-mediated inhibition of primary Schwann cell proliferation. Furthermore, we demonstrate that the F1 subdomain of the merlin FERM domain is required for actin colocalization, proper regulation of merlin C-terminal phosphorylation and for remodeling the cytoskeleton, yet is not required for the inhibition of Schwann cell proliferation. Thus, tumor suppression by merlin is independent of its role as an organizer of the actin cytoskeleton in Schwann cells.

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

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

Creation of a bi-directional protein transduction system for suppression of HIV-1 expression by p27SJ

p27SJ is a novel protein from a callus culture of St. John's wort that modulates transcription of the HIV-1 promoter in several mammalian cells [Darbinian-Sarkissian, N., Darbinyan, A., Otte, J., Radhakrishnan, S., Sawaya, B.E., Arzumanyan, A., Chipitsyna, G., Popov, Y., Rappaport, J., Amini, S., Khalili, K., 2006. p27(SJ), a novel protein from St. John's wort, that suppresses expression of HIV-1 genome. Gene Ther. 13, 288-295]. Here, we armed p27SJ with signals from Ig-kappa light chain that allow its efficient excretion from the cells, and from HIV-1 Tat that facilitates its uptake by other cells for its utilization by a protein transduction method. We demonstrate that treatment of cells containing the HIV-1 LTR with conditioned media from cells expressing the armed p27SJ ((exc)p27SJ(upt)) results in suppression of the viral activation by the C/EBPbeta transcription factor. Once imported into the cells, (exc)p27SJ(upt) impacts the nuclear localization of C/EBPbeta and by retaining the protein in the cytoplasm affects its DNA binding and hence transcriptional activity. The armed p27SJ also inhibits Tat-induced activation of the LTR and decreases the level of viral replication in promonocytic cells including U-937 and T-lymphocytic cells. Our observations introduce a new bi-directional protein transduction system with a broad spectrum of applications for manufacturing therapeutic peptides by a specific group of cells called donor, and delivery to the target cells named recipient. Furthermore, our results support the utility of soluble p27SJ in suppressing transcription and replication of HIV-1 by interfering with the function of cellular proteins such as C/EBPbeta and viral activators including Tat.

Protein transduction: generation of full-length transducible proteins using the TAT system

This unit describes the technology that allows an investigator to transduce full-length proteins by utilizing a minimal, eleven-amino acid, HIV-TAT transduction domain that can be fused to a protein of choice using the pTAT or pTAT-HA protein expression plasmids. Bacterial expression, followed by solubilization of protein aggregates with a denaturing agent, affords high yields of transducible fusion protein. The fusion protein, once added to the culture medium, can cross the cell membrane and then be degraded or refolded by the cellular machinery. Correct targeting and function of the fusion protein can be easily examined by fluorescent microscopy or immunohistochemistry. This strategy was established and improved to its current state by the purification and transduction of a multitude of fusion proteins. Because the pool of fusion proteins spans many different functions, the protocols cover a wide variety of commonly used protein isolation and characterization methods.

Modeling NF2 with human arachnoidal and meningioma cell culture systems: NF2 silencing reflects the benign character of tumor growth

Meningiomas, common tumors arising from arachnoidal cells of the meninges, may occur sporadically, or in association with the inherited disorder, neurofibromatosis 2 (NF2). Most sporadic meningiomas result from NF2 inactivation, resulting in loss of tumor suppressor merlin, implicated in regulating membrane-cytoskeletal organization. To investigate merlin function in an authentic target cell type for NF2 tumor formation, we established primary cultures from genetically-matched meningioma and normal arachnoidal tissues. Our studies revealed novel and distinct cell biological and biochemical properties unique to merlin-deficient meningioma cells compared to merlin-expressing arachnoidal and meningioma cells, and other NF2-deficient cell types. Merlin-deficient meningioma cells displayed cytoskeletal and cell contact defects, altered cell morphology and growth properties, most notably cell senescence, implicating the activation of senescence pathways in limiting benign meningioma growth. Merlin suppression by RNAi in arachnoidal cells replicated merlin-deficient meningioma features, thus establishing these cell systems as disease-relevant models for studying NF2 tumorigenesis.

Point mutation in the NF2 gene of HEI-193 human schwannoma cells results in the expression of a merlin isoform with attenuated growth suppressive activity

Neurofibromatosis type 2 (NF2) is a genetic disorder characterized by the formation of bilateral schwannomas of the eighth cranial nerve. Although the protein product of the NF2 gene (merlin) is a classical tumor suppressor, the mechanism by which merlin suppresses cell proliferation is not fully understood. The availability of isolated tumor cells would facilitate a better understanding of the molecular function of merlin, but primary schwannoma cells obtained from patients grow slowly and do not yield adequate numbers for biochemical analysis. In this study, we have examined the NF2 mutation in HEI-193 cells, an immortalized cell line derived from the schwannoma of an NF2 patient. Previous work showed that the NF2 mutation in HEI-193 cells causes a splicing defect in the NF2 transcript. We have confirmed this result and further identified the resultant protein product as an isoform of merlin previously designated as isoform 3. The level of isoform 3 proteins in HEI-193 cells is comparable to the levels of merlin isoforms 1 and 2 in normal human Schwann cells and several other immortalized cell lines. In contrast to many mutant forms of merlin, isoform 3 is as resistant to proteasomal degradation as isoforms 1 and 2 and can interact with each of these isoforms in vivo. Cell proliferation assays showed that, in NF2(-/-) mouse embryonic fibroblasts, exogenously expressed merlin isoform 3 does exhibit growth suppressive activity although it is significantly lower than that of identically expressed merlin isoform 1. These results indicate that, although HEI-193 cells have undetectable levels of merlin isoforms 1 and 2, they are, in fact, not a merlin-null model because they express the moderately active growth suppressive merlin isoform 3.

Nf2/Merlin: a coordinator of receptor signalling and intercellular contact

This review explores possible mechanisms by which the neurofibromatosis type-2 tumour suppressor Merlin regulates contact-dependent inhibition of proliferation. Starting from an evolutionary perspective, the concurrent emergence of intercellular contacts and proliferation control in multicellular organisms is first considered. Following a brief survey of the molecular and subcellular milieus in which merlin performs its function, the importance of different cellular and biological contexts in defining the function of merlin is discussed. Finally, an integrated model for merlin and the Ezrin, Radixin, and Moesin (ERM) proteins functioning in the regulation of cellular interfaces is proposed.

The merlin interacting proteins reveal multiple targets for NF2 therapy

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

Experimental therapeutic approaches to peripheral nerve tumors

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

Targeting APL fusion proteins by peptide interference

A significant barrier to experimental therapeutics is the ability to identify and specifically target oncogenic proteins involved in the molecular pathogenesis of disease. In acute promyelocytic leukemia (APL), aberrant transcription factors and their associated machinery play a central role in mediating the malignant phenotype. The mechanism of action of APL chimeric fusion proteins involves their ability to either self-associate or interact with different partner proteins. Thus, targeting protein-protein interactions could have a significant impact in blocking the activity of APL oncoproteins. As therapeutic targets, the interface between interacting proteins may not always be amenable to highly specific small molecule blockade. In contrast, peptides are well-suited to this purpose and can be reliably delivered when fused to cell-permeable peptide domains. Therapeutic peptides can be designed to directly target APL fusion proteins, their downstream effectors, or other potentially synergistic oncogenic mechanisms of importance in APL blasts. In addition to serving as potential therapeutic agents, such reagents could serve as powerful reagents to dissect the molecular pathogenesis of APL.

Phosphorylation of the NF2 tumor suppressor in Schwann cells is mediated by Cdc42-Pak and requires paxillin binding

Mutations in the Neurofibromatosis type 2 tumor suppressor gene that encodes Schwannomin causes formation of benign schwannomas. Schwannoma cells lose their characteristic bipolar shape and become rounded with excessive ruffling membranes. Schwannomin is phosphorylated at serine 518 (S518) by p21 activated kinase (Pak). Unphosphorylated schwannomin is associated with growth inhibition but little is known about the function of the phosphorylated form, or the molecular events leading to its phosphorylation. Here, we report in SCs that schwannomin S518 phosphorylation requires binding to paxillin and targeting to the plasma membrane. Phospho-S518-schwannomin is enriched in the peripheral-most aspects of membrane specializations where paxillin, activated Pak, Cdc42 but not Rac are highly expressed. Schwannomin and Pak phosphorylation levels are not reduced in response to lowering Rac-GTP levels with NSC23766. Expression of schwannomin S518A/D-GFP variants each distinctively altered Schwann cell shape and polarity. These results are consistent with tight spatial regulation of S518 phosphorylation at the plasma membrane in a paxillin and Cdc42-Pak dependent manner that leads to local reorganization of the SC cytoskeleton.

Influence of protein transduction domains on intracellular delivery of macromolecules

As the plasma membrane and blood-brain barrier selectively restrict the entry of most compounds into cells to < 500 Da, delivering macromolecules into cells was, until recently, little more than a goal. However, with significant effort to capitalise on therapeutic targets available in the post-genomic era, novel approaches for delivering therapeutic macromolecules are being rapidly developed. The discovery of small cationic peptides, termed peptide/protein transduction domains or cell-penetrating peptides, which cross biological membranes, has emerged as a venerable Trojan horse to transport large, biologically active molecules, such as peptides, proteins and oligonucleotides, into mammalian cells in vitro, as well as in preclinical models and clinical trials in vivo. This review discusses the implications of peptide/protein transduction domain-mediated delivery of macromolecules and their possible uses as important primary drug delivery agents.

Temporal control of Rac in Schwann cell-axon interaction is disrupted in NF2-mutant schwannoma cells

Schwann cell-axon interaction is the hallmark feature of peripheral nerves, yet the intracellular signals underlying this interaction are unknown. Schwann cells extend processes and migrate on developing axons before differentiation, requiring coordinated regulation of the Schwann cell cytoskeleton. Small GTPases of the Rho family, including Rho, Rac, and cell division cycle 42, regulate the actin cytoskeleton. The neurofibromatosis type 2 (NF2) gene is commonly mutated in schwannomas, Schwann cell tumors that contain cells lacking axon interaction. NF2 is involved in suppression of Rac signaling, and cultured schwannoma cells contain elevated, GTP-bound, active Rac. Despite these previous studies, a causal relationship between Rac activation and the abnormal cellular morphology of schwannoma is unknown. We used fluorescence resonance energy transfer to follow Rac activity in normal human Schwann cells and schwannoma cells during interaction with neurons. Normal Schwann cells elongated processes along neurites under low Rac activity. Schwannoma cells showed high Rac activity at distal regions of the cells and failed to align processes with neurites. Application of a Rac-specific inhibitor, the chemical compound NSC23766, to schwannoma cells restored neuronal interaction. The data support the significance of regulated Rac signaling in mediating Schwann cell-axon interaction and suggest that controlling Rac activity as a possible therapy for schwannomas.

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

Mutations in the tumor suppressor gene coding for merlin cause Neurofibromatosis type 2 (NF2), all spontaneous schwannomas, and a majority of meningiomas. Merlin links transmembrane proteins to the cytoskeleton. Accordingly, primary human schwannoma cells lacking merlin show an increased number of lamellipodia and filopodia as well as increased cell spreading. We show enhanced adhesion in primary human schwannoma cells and present evidence that this is dependent on the integrin chains alpha6beta1 and alpha6beta4. We further demonstrate that the integrin chains beta1 and beta4 are upregulated in schwannomas using different complementary methods, and report higher expression of these integrins per schwannoma cell by fluorescence assisted cell sorting (FACS). Finally we report clustering of the integrin chains alpha6, beta1, and beta4 on schwannoma cells. Our findings fit well into recent data on the role of merlin in signaling cascades connected to integrins and help explain pathological ensheathment of extracellular matrix or pseudomesaxon formation which is a hallmark of schwannoma histopathology.

Coronin 3 and its role in murine brain morphogenesis

Coronins belong to the fundamental WD40-repeat proteins. They are mainly found at the submembraneous area, they bind F-actin in vitro, and most of the seven mammalian coronins have unclear roles. Coronin 3 is abundantly expressed in the adult CNS. All murine brain areas express coronin 3 during embryogenesis and the first postnatal stages. Expression in grey matter decreases postnatally, except for hippocampal pyramidal and dentate gyrus neurons, and cerebellar Purkinje cells, while levels in white matter increase in the course of myelination. Consistently, coronin 3 is abundant in differentiating neuro-2a and PC-12 cells and in primary oligodendrocytes. Treatment with PKC activator PMA reduced coronin 3 protein levels. To address its functions, neuro-2a and PC-12 cells were transfected with GFP-tagged coronin 3 versions. Full-length coronin 3 among other areas localized to outgrowing neurites, whereas truncated proteins efficiently suppressed neurite formation. Our results favour a role for coronin 3 in neuron morphogenesis and possibly migration.

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

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

Magicin, a novel cytoskeletal protein associates with the NF2 tumor suppressor merlin and Grb2

Neurofibromatosis 2 (NF2) is a dominantly inherited disorder characterized by bilateral vestibular schwannomas and meningiomas. Merlin, the neurofibromatosis 2 tumor suppressor protein, is related to the ERM (ezrin, radixin, moesin) proteins and, like its family members, is thought to play a role in plasma membrane-cytoskeletal interactions. We report a novel protein as a merlin-specific binding partner that we have named magicin (merlin and Grb2 interacting cytoskeletal protein) and show that the two proteins interact in vitro and in vivo as well as colocalize beneath the plasma membrane. Magicin is a 24 kDa protein that is expressed in many cell lines and tissues. Magicin, similar to merlin, associates with the actin cytoskeleton as determined by cofractionation, immunofluorescence and electron microscopy. Analysis of the magicin sequence reveals binding motifs for the adaptor protein Grb2. Employing affinity binding, blot overlay and co-immunoprecipitation assays, we demonstrate an interaction between Grb2 and magicin. In addition, merlin is capable of forming a ternary complex with magicin and Grb2. These results support a role for merlin in receptor-mediated signaling at the cell surface, and may have implications in the regulation of cytoskeletal reorganization.

Molecular pathogenesis of skull base tumors

OBJECTIVE

To review contemporary molecular biological literature related to skull base tumor biology and tumorigenesis.

DATA SOURCES

PUBMED and Ovid literature searches were performed using keyword search. Only English language articles published between 1965 and December 4, 2003 were chosen.

STUDY SELECTION AND DATA EXTRACTION

All relevant articles from the past 8 years, as well as landmark articles in years before 1995, were retrieved and reviewed.

CONCLUSION

Consistent progress is being made toward the molecular genetic and biological basis of the most common skull base tumors. An understanding of these mechanisms will aid the neurotologist in future diagnosis and management of the lesions.

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

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

Radixin: cytoskeletal adopter and signaling protein

Radixin functions as a membrane-cytoskeletal crosslinkers in actin-rich cell surface structures and is thereby thought to be essential for cortical cytoskeleton organization, cell motility, adhesion and proliferation. This modular polypeptide consists of a long, central helix, termed the alpha-domain, which connects an N-terminal 4.1/ezrin/radixin/moesin (FERM) domain required for membrane binding and a C-terminal region that contains a major actin-binding motif. Conformational regulation of radixin protein function occurs by association of the FERM and C-terminal domains, whereby the membrane- and actin-binding activities are mutually suppressed and the protein is thought to take an inactive 'closed' form. Further analyses of radixin and its family members have also revealed associations with human disease. With the rudimentary state of our present knowledge and the pivotal roles these proteins play, studies on this protein family are sure to continue to attract considerable interest.

Activation of the tumor suppressor merlin modulates its interaction with lipid rafts

Neurofibromatosis type 2 (NF2) is a genetic disorder characterized by bilateral schwannomas of the eighth cranial nerve. The NF2 tumor suppressor protein, merlin, is related to the ERM (ezrin, radixin, and moesin) family of membrane/F-actin linkers. Merlin resists solubilization by the detergent Triton X-100 (TX-100), a property commonly attributed to association with the cytoskeleton. Accordingly, NF2 patient mutations that encode merlins with enhanced TX-100 solubility have been explained previously in terms of loss of cytoskeletal attachment. However, here we present data to suggest that the detergent resistance of merlin is a result of its constitutive residence in lipid rafts. Furthermore, when cells are grown to high density, merlin shifts to a more buoyant lipid raft fraction in a density gradient. This shift is mimicked in subconfluent cells treated with cytochalasin D, suggesting that the shift results from merlin dissociation from the actin cytoskeleton, but not from lipid rafts. Intramolecular NH(2)- and COOH-terminal binding, which occurs when merlin transitions to the growth-suppressive form, also brings about a similar change in buoyant density. Our results suggest that constitutive residence of merlin in lipid rafts is crucial for its function and that as merlin becomes growth suppressive in vivo, one significant molecular event may be the loss of interaction with the actin cytoskeleton. To our knowledge, merlin is the first tumor suppressor known to reside within lipid rafts, and the significance of this finding is underscored by known loss-of-function NF2 patient mutations that encode merlins with enhanced TX-100 solubility.

Cyclic AMP-dependent protein kinase phosphorylates merlin at serine 518 independently of p21-activated kinase and promotes merlin-ezrin heterodimerization

Mutations in the NF2 tumor suppressor gene encoding merlin induce the development of tumors of the nervous system. Merlin is highly homologous to the ERM (ezrin-radixin-moesin) family of membrane/cytoskeleton linker proteins. However, the mechanism for the tumor suppressing activity of merlin is not well understood. Previously, we characterized a novel role for merlin as a protein kinase A (PKA)-anchoring protein, which links merlin to the cAMP/PKA signaling pathway. In this study we show that merlin is also a target for PKA-induced phosphorylation. In vitro [gamma-(33)P]ATP labeling revealed that both the merlin N and C termini are phosphorylated by PKA. Furthermore, both in vitro and in vivo phosphorylation studies of the wild-type and mutated C termini demonstrated that PKA can phosphorylate merlin at serine 518, a site that is phosphorylated also by p21-activated kinases (PAKs). Merlin was phosphorylated by PKA in cells in which PAK activity was suppressed, indicating that the two kinases function independently. Both in vitro and in vivo interaction studies indicated that phosphorylation of serine 518 promotes heterodimerization between merlin and ezrin, an event suggested to convert merlin from a growth-suppressive to a growth-permissive state. This study provides further evidence on the connection between merlin and cAMP/PKA signaling and suggests a role for merlin in the cAMP/PKA transduction pathway.

Voltage-Gated ion currents of schwann cells in cell culture models of human neurofibromatosis

K(+) (K) channels play a role in the proliferation of many cell types in normal cells and certain disease states. Several laboratories have studied K currents in cultured Schwann cells from models of the human diseases, neurofibromatosis type 1 (NF1) and neurofibromatosis type 2 (NF2). These diseases are characterized by the growth of Schwann cell tumors. In all cell culture NF models the K current properties differ in tumor-derived and normal Schwann cells. Depending on the model however, the type of K channel abnormality differs. K channels appear to play a role in the proliferation of Schwann cell cultures of these disease models, because a link has been established between K current blockade and the inhibition of Schwann cell proliferation in NF1 and NF2. Differences in the proliferation response of normal Schwann cells to K channel blockers suggest that in vitro regulation of proliferation in neoplastic and normal Schwann cells is complex.

NF2: The wizardry of merlin

Neurofibromatosis type II (NF2) is an autosomal dominant cancer syndrome characterized by the formation of tumors of the nervous system, particularly schwannomas and meningiomas. The NF2 gene is also implicated in the development of sporadic schwannomas and meningiomas, as well as tumor types seemingly unrelated to the NF2 disorder, such as malignant mesotheliomas. Inactivation of NF2 occurs by a "two-hit" mechanism, as proposed by Al Knudson, and the NF2 gene behaves as a classical tumor suppressor gene. The NF2 gene product, merlin, exhibits homology with the ezrin-radixin-moesin family of membrane-cytoskeleton-linking proteins. During the past several years, there has been intensive investigation aimed at elucidating the mechanisms underlying merlin's functions. In this review, we summarize the involvement of NF2 inactivation in tumorigenesis. We also discuss observations implicating merlin in cell motility and cell proliferation, with a focus on recent findings linking merlin to Rac signaling.

Evidence of protein transduction but not intercellular transport by proteins fused to HIV tat in retinal cell culture and in vivo

The human immunodeficiency virus type-1 Tat protein is known to exit virally infected cells and enter the nucleus of adjacent uninfected cells. This property has been mapped to an 11-amino-acid protein transduction domain (PTD). When the PTD of Tat is fused to heterologous proteins and added exogenously to cells, the fusion peptide is able to demonstrate protein transduction across plasma membranes. Recent reports indicate that endogenously expressed Tat fusion peptides can demonstrate intercellular transport and improve biodistribution of therapeutic protein in the context of adenovirus vectors. Intercellular transport and protein transduction have not been observed in some studies and in the former have been attributed to an artifact of fixation. We have attempted to resolve these studies using an approach that unambiguously distinguishes cells that express Tat fusion protein from those that receive it from their environment. We find no evidence of intercellular transport in the context of an adenovirus vector in cell culture or in vivo. Instead, we find that Tat fusion peptides are down regulated in terms of expression not only in the context of adenovirus vectors, but also when expressed from transfected plasmid DNA. However, when Tat fusion peptides are released from cells by degradation of the plasma membrane, the fusion peptides demonstrate protein transduction without the need for cell fixation, indicating a unidirectional transport of Tat fusion proteins across the plasma membrane. Our data are consistent with previously reported studies and help to explain the apparently different results obtained from several different laboratories.