Merlin and the ERM proteins in Schwann cells, neurons and growth cones

Collaborative Roles for RAC1, ERM Proteins and PTEN During Adult Sensory Axon Regeneration

The role of local of growth cone (GC) manipulation in adult regenerative systems is largely unexplored despite substantial translational importance. Here we investigated collaboration among Rac1 GTPase, its partnering ERM proteins and PTEN in adult sensory neurons and adult nerve regeneration. We confirmed expression of both Rac1 and ERM in adults and noted substantial impacts on neurite outgrowth in naïve and pre-injured adult sensory neurons. PTEN inhibition added to this outgrowth. Rac1 activation acted directly on adult GCs facilitating both attractive turning and advancement. In vivo regeneration indices including electrophysiological recovery, return of sensation, walking, repopulation of myelinated axons and reinnervation of the target epidermis indicated benefits of local Rac1 activation. These indices suggested maximal GC activation whereas local PTEN inhibition offered only limited added improvement. Our findings provide support for the concept of manipulating adult GCs, by emphasizing local Rac1 activation in directing therapy for nerve repair.

Mechanical stimulation and electrophysiological monitoring at subcellular resolution reveals differential mechanosensation of neurons within networks

A growing consensus that the brain is a mechanosensitive organ is driving the need for tools that mechanically stimulate and simultaneously record the electrophysiological response of neurons within neuronal networks. Here we introduce a synchronized combination of atomic force microscopy, high-density microelectrode array and fluorescence microscopy to monitor neuronal networks and to mechanically characterize and stimulate individual neurons at piconewton force sensitivity and nanometre precision while monitoring their electrophysiological activity at subcellular spatial and millisecond temporal resolution. No correlation is found between mechanical stiffness and electrophysiological activity of neuronal compartments. Furthermore, spontaneously active neurons show exceptional functional resilience to static mechanical compression of their soma. However, application of fast transient (∼500 ms) mechanical stimuli to the neuronal soma can evoke action potentials, which depend on the anchoring of neuronal membrane and actin cytoskeleton. Neurons show higher responsivity, including bursts of action potentials, to slower transient mechanical stimuli (∼60 s). Moreover, transient and repetitive application of the same compression modulates the neuronal firing rate. Seemingly, neuronal networks can differentiate and respond to specific characteristics of mechanical stimulation. Ultimately, the developed multiparametric tool opens the door to explore manifold nanomechanobiological responses of neuronal systems and new ways of mechanical control.

Cotargeting Phosphoinositide 3-Kinase and Focal Adhesion Kinase Pathways Inhibits Proliferation of NF2 Schwannoma Cells

Neurofibromatosis Type 2 (NF2) is a tumor predisposition syndrome caused by germline inactivating mutations in the NF2 gene encoding the merlin tumor suppressor. Patients develop multiple benign tumor types in the nervous system including bilateral vestibular schwannomas (VS). Standard treatments include surgery and radiation therapy, which may lead to loss of hearing, impaired facial nerve function, and other complications. Kinase inhibitor monotherapies have been evaluated clinically for NF2 patients with limited success, and more effective nonsurgical therapies are urgently needed. Schwannoma model cells treated with PI3K inhibitors upregulate activity of the focal adhesion kinase (FAK) family as a compensatory survival pathway. We screened combinations of 13 clinically relevant PI3K and FAK inhibitors using human isogenic normal and merlin-deficient Schwann cell lines. The most efficacious combination was PI3K/mTOR inhibitor omipalisib with SRC/FAK inhibitor dasatinib. Sub-GI50 doses of the single drugs blocked phosphorylation of their major target proteins. The combination was superior to either single agent in promoting a G1 cell-cycle arrest and produced a 44% decrease in tumor growth over a 2-week period in a pilot orthotopic allograft model. Evaluation of single and combination drugs in six human primary VS cell models revealed the combination was superior to the monotherapies in 3 of 6 VS samples, highlighting inter-tumor variability between patients consistent with observations from clinical trials with other molecular targeted agents. Dasatinib alone performed as well as the combination in the remaining three samples. Preclinically validated combination therapies hold promise for NF2 patients and warrants further study in clinical trials.

Willin/FRMD6: A Multi-Functional Neuronal Protein Associated with Alzheimer's Disease

The FERM domain-containing protein 6 (FRMD6), also known as Willin, is an upstream regulator of Hippo signaling that has recently been shown to modulate actin cytoskeleton dynamics and mechanical phenotype of neuronal cells through ERK signaling. Physiological functions of Willin/FRMD6 in the nervous system include neuronal differentiation, myelination, nerve injury repair, and vesicle exocytosis. The newly established neuronal role of Willin/FRMD6 is of particular interest given the mounting evidence suggesting a role for Willin/FRMD6 in Alzheimer's disease (AD), including a series of genome wide association studies that position Willin/FRMD6 as a novel AD risk gene. Here we describe recent findings regarding the role of Willin/FRMD6 in the nervous system and its actions in cellular perturbations related to the pathogenesis of AD.

Transcriptomic Profile Reveals Deregulation of Hearing-Loss Related Genes in Vestibular Schwannoma Cells Following Electromagnetic Field Exposure

Hearing loss (HL) is the most common sensory disorder in the world population. One common cause of HL is the presence of vestibular schwannoma (VS), a benign tumor of the VIII cranial nerve, arising from Schwann cell (SC) transformation. In the last decade, the increasing incidence of VS has been correlated to electromagnetic field (EMF) exposure, which might be considered a pathogenic cause of VS development and HL. Here, we explore the molecular mechanisms underlying the biologic changes of human SCs and/or their oncogenic transformation following EMF exposure. Through NGS technology and RNA-Seq transcriptomic analysis, we investigated the genomic profile and the differential display of HL-related genes after chronic EMF. We found that chronic EMF exposure modified the cell proliferation, in parallel with intracellular signaling and metabolic pathways changes, mostly related to translation and mitochondrial activities. Importantly, the expression of HL-related genes such as NEFL, TPRN, OTOGL, GJB2, and REST appeared to be deregulated in chronic EMF exposure. In conclusion, we suggest that, at a preclinical stage, EMF exposure might promote the transformation of VS cells and contribute to HL.

The Autism-Related lncRNA MSNP1AS Regulates Moesin Protein to Influence the RhoA, Rac1, and PI3K/Akt Pathways and Regulate the Structure and Survival of Neurons

Autism spectrum disorder (ASD) is a complex disease involving multiple genes and multiple sites, and it is closely related to environmental factors. It has been gradually revealed that long noncoding RNAs (lncRNAs) may regulate the pathogenesis of ASD at the epigenetic level. In neuronal cells, the lncRNA moesin pseudogene 1 antisense (MSNP1AS) forms a double-stranded RNA with moesin (MSN) to suppress moesin protein expression. MSNP1AS overexpression can activate the RhoA pathway and inhibit the Rac1 and PI3K/Akt pathways; however, the regulation of Rac1 by MSNP1AS is not associated with MSN, and the effect on the RhoA pathway may also be associated with other factors. MSNP1AS can decrease the number and length of neurites, inhibit neuronal cell viability and migration, and promote apoptosis. Downregulation of MSN expression functions similarly to MSNP1AS, and its overexpression can block the above functions of MSNP1AS. In addition, in vivo experiments show that MSN improves social interactions and reduces repetitive behaviors in BTBR mice, decreases the activity of RhoA and restores the activity of PI3K/Akt pathway. Therefore, the abnormal expression of MSNP1AS in ASD patients might influence the structure and survival of neuronal cells through the regulation of moesin protein expression to facilitate the development and progression of ASD. These findings provide new evidence for studying the mechanisms of lncRNAs in ASD. LAY SUMMARY: Autism spectrum disorder (ASD) is a common neurodevelopmental disease and its neurodevelopmental mechanisms have not been elucidated. More and more studies have found that long noncoding RNAs (lncRNAs) can regulate the development of central nervous system in many ways and affect the pathogenic process of ASD. Moesin pseudogene 1 antisense (MSNP1AS) is an up-regulated lncRNA in ASD patients. In-depth functional experiments showed that MSNP1AS inhibited moesin protein expression and regulated the activation of multiple signaling pathways, thus decreasing the number and length of neurites, inhibiting neuronal cell viability and migration, and promoting apoptosis. Therefore, MSNP1AS is an important lncRNA related to ASD and can regulate the biological function of neurons.

Rho-associated protein kinase-dependent moesin phosphorylation is required for PD-L1 stabilization in breast cancer

Expression of programmed cell death ligand (PD-L1) is associated with poor prognosis in breast cancer. Understanding the regulation of PD-L1 expression in breast cancer could provide a new strategy for breast cancer treatment. Here, we demonstrate that moesin (MSN) phosphorylation by Rho-associated protein kinase (ROCK) stabilizes PD-L1 protein levels. Our results indicate that phosphorylated MSN may compete with the E3 ubiquitin ligase SPOP for binding PD-L1. ROCK inhibition via the Y-27632 inhibitor or MSN silencing decreased PD-L1 expression, resulting in T-cell activation both in vitro and in vivo. Administration of Y-27632 into immunocompetent Balb/c mice bearing breast tumors suppressed tumor progression and enhanced CD4+ and CD8+ T-cell infiltration. RNA-seq analysis of Y-27632-treated mouse tumors revealed that ROCK inhibition upregulated several immune response genes. However, the combination of Y-27632 and an anti-PD-1 antibody did not show additive or synergistic effects due to reduced PD-L1 in the presence of Y-27632. Our study unravels a previously unappreciated mechanism of PD-L1 regulation through the ROCK-MSN pathway. Moreover, we found that ROCK inhibitors could be combined with breast cancer immunotherapy.

Perspectives for Ezrin and Radixin in Astrocytes: Kinases, Functions and Pathology

Astrocytes are increasingly perceived as active partners in physiological brain function and behaviour. The structural correlations of the glia–synaptic interaction are the peripheral astrocyte processes (PAPs), where ezrin and radixin, the two astrocytic members of the ezrin-radixin-moesin (ERM) family of proteins are preferentially localised. While the molecular mechanisms of ERM (in)activation appear universal, at least in mammalian cells, and have been studied in great detail, the actual ezrin and radixin kinases, phosphatases and binding partners appear cell type specific and may be multiplexed within a cell. In astrocytes, ezrin is involved in process motility, which can be stimulated by the neurotransmitter glutamate, through activation of the glial metabotropic glutamate receptors (mGluRs) 3 or 5. However, it has remained open how this mGluR stimulus is transduced to ezrin activation. Knowing upstream signals of ezrin activation, ezrin kinase(s), and membrane-bound binding partners of ezrin in astrocytes might open new approaches to the glial role in brain function. Ezrin has also been implicated in invasive behaviour of astrocytomas, and glial activation. Here, we review data pertaining to potential molecular interaction partners of ezrin in astrocytes, with a focus on PKC and GRK2, and in gliomas and other diseases, to stimulate further research on their potential roles in glia-synaptic physiology and pathology.

Merlin modulates process outgrowth and synaptogenesis in the cerebellum

Neurofibromatosis type 2 (NF2) patients are prone to develop glial-derived tumors in the peripheral and central nervous system (CNS). The Nf2 gene product -Merlin is not only expressed in glia, but also in neurons of the CNS, where its function still remains elusive. Here, we show that cerebellar Purkinje cells (PCs) of isoform-specific Merlin-deficient mice were innervated by smaller vGluT2-positive clusters at presynaptic terminals than those of wild-type mice. This was paralleled by a reduction in frequency and amplitude of miniature excitatory postsynaptic currents (mEPSC). On the contrary, in conditional transgenic mice in which Merlin expression was specifically ablated in PCs (L7Cre;Nf2^fl/fl), we found enlarged vGluT2-positive clusters in their presynaptic buttons together with increased amplitudes of miniature postsynaptic currents. The presynaptic terminals of these PCs innervating neurons of the deep cerebellar nuclei were also enlarged. When exploring mice with Merlin-deficient granule cells (GCs) (Math1Cre;Nf2^fl/fl), we found cerebellar extracts to contain higher amounts of vGluT1 present in parallel fiber terminals. In parallel, mEPSC frequency was increased in Math1Cre;Nf2^fl/fl mice. On the contrary, VGluT2 clusters in cerebellar glomeruli composed of NF2-deficient presynaptic Mossy fiber terminals and NF2-deficient postsynaptic GC were reduced in size as shown for isoform-specific knockout mice. These changes in Math1Cre;Nf2^fl/fl-deficient mice were paralleled by an increased activation of Rac1-Cofilin signaling which is known to impact on cytoskeletal reorganization and synapse formation. Consistent with the observed synaptic alterations in these transgenic mice, we observed altered ultrasonic vocalization, which is known to rely on proper cerebellar function. No gross morphological changes or motor coordination deficits were observed in any of these transgenic mice. We therefore conclude that Merlin does not regulate overall cerebellar development, but impacts on pre- and post-synaptic terminal organization.

Multi-dimensional bioinspired tactics using an engineered mussel protein glue-based nanofiber conduit for accelerated functional nerve regeneration

Limited regenerative capacity of the nervous system makes treating traumatic nerve injuries with conventional polymer-based nerve grafting a challenging task. Consequently, utilizing natural polymers and biomimetic topologies became obvious strategies for nerve conduit designs. As a bioinspired natural polymer from a marine organism, mussel adhesive proteins (MAPs) fused with biofunctional peptides from extracellular matrix (ECM) were engineered for accelerated nerve regeneration by enhancing cell adhesion, proliferation, neural differentiation, and neurite formation. To physically promote contact guidance of neural and Schwann cells and to achieve guided nerve regeneration, MAP was fabricated into an electrospun aligned nanofiber conduit by introducing synthetic polymer poly(lactic-co-glycolic acid) (PLGA) to control solubility and mechanical property. In vitro and in vivo experiments demonstrated that the multi-dimensional tactics of combining adhesiveness from MAP, integrin-mediated interaction from ECM peptides (in particular, IKVAV derived from laminin α1 chain), and contact guidance from aligned nanofibers synergistically accelerated functional nerve regeneration. Thus, MAP-based multi-dimensional approach provides new opportunities for neural regenerative applications including nerve grafting. STATEMENT OF SIGNIFICANCE: Findings in neural regeneration indicate that a bioinspired polymer-based nerve conduit design should harmoniously constitute various factors, such as biocompatibility, neurotrophic molecule, biodegradability, and contact guidance. Here, we engineered three fusion proteins of mussel-derived adhesive protein with ECM-derived biofunctional peptides to simultaneously provide biocompatibility and integrin-based interactions. In addition, a fabrication of robust aligned nanofiber conduits containing the fusion proteins realized suitable biodegradability and contact guidance. Thus, our multi-dimensional strategy on conduit design provided outstanding biocompatibility, biodegradability, integrin-interaction, and contact guidance to achieve an accelerated functional nerve regeneration. We believe that our bioengineered mussel adhesive protein-based multi-dimensional strategy would offer new insights into the design of nerve tissue engineering biomaterials.

Neurofibromatosis type 2 tumor suppressor protein is expressed in oligodendrocytes and regulates cell proliferation and process formation

The neurofibromatosis type 2 (NF2) tumor suppressor protein Merlin functions as a negative regulator of cell growth and actin dynamics in different cell types amongst which Schwann cells have been extensively studied. In contrast, the presence and the role of Merlin in oligodendrocytes, the myelin forming cells within the CNS, have not been elucidated. In this work, we demonstrate that Merlin immunoreactivity was broadly distributed in the white matter throughout the central nervous system. Following Merlin expression during development in the cerebellum, Merlin could be detected in the cerebellar white matter tract at early postnatal stages as shown by its co-localization with Olig2-positive cells as well as in adult brain sections where it was aligned with myelin basic protein containing fibers. This suggests that Merlin is expressed in immature and mature oligodendrocytes. Expression levels of Merlin were low in oligodendrocytes as compared to astrocytes and neurons throughout development. Expression of Merlin in oligodendroglia was further supported by its identification in either immortalized cell lines of oligodendroglial origin or in primary oligodendrocyte cultures. In these cultures, the two main splice variants of Nf2 could be detected. Merlin was localized in clusters within the nuclei and in the cytoplasm. Overexpressing Merlin in oligodendrocyte cell lines strengthened reduced impedance in XCELLigence measurements and Ki67 stainings in cultures over time. In addition, the initiation and elongation of cellular projections were reduced by Merlin overexpression. Consistently, cell migration was retarded in scratch assays done on Nf2-transfected oligodendrocyte cell lines. These data suggest that Merlin actively modulates process outgrowth and migration in oligodendrocytes.

Akt regulates neurite growth by phosphorylation-dependent inhibition of radixin proteasomal degradation

Neurite growth is controlled by a complex molecular signaling network that regulates filamentous actin (F-actin) dynamics at the growth cone. The evolutionarily conserved ezrin, radixin, and moesin family of proteins tether F-actin to the cell membrane when phosphorylated at a conserved threonine residue and modulate neurite outgrowth. Here we show that Akt binds to and phosphorylates a threonine 573 residue on radixin. Akt-mediated phosphorylation protects radixin from ubiquitin-dependent proteasomal degradation, thereby enhancing radixin protein stability, which permits proper neurite outgrowth and growth cone formation. Conversely, the inhibition of Akt kinase or disruption of Akt-dependent phosphorylation reduces the binding affinity of radixin to F-actin as well as lowers radixin protein levels, resulting in decreased neurite outgrowth and growth cone formation. Our findings suggest that Akt signaling regulates neurite outgrowth by stabilizing radixin interactions with F-actin, thus facilitating local F-actin dynamics.

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

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

Expression of merlin, NDRG2, ERBB2, and c-MYC in meningiomas: relationship with tumor grade and recurrence

Meningiomas are common, usually benign tumors of the central nervous system that have a high rate of post-surgical recurrence or regrowth. We determined expression of the proteins merlin, NDRG2, ERBB2, and c-MYC in meningiomas using immunohistochemistry and assessed relationships between protein expression and gender, age, tumor grade, and recurrence or regrowth. The study sample comprised 60 patients, (44 women and 16 men) with a mean age of 53.2 ± 12.7 years. Tumors were classified as grade I (n=48) or grades II and III (n=12). Expression of merlin, NDRG2, ERBB2, and c-MYC was not significantly different statistically with relation to gender, age, or meningioma recurrence or regrowth. Merlin was expressed in 100% of the cases. No statistically significant difference between tumor grade and recurrence or regrowth was identified. Statistically significant differences were identified between the mean age of patients with grade I (54.83 ± 11.60) and grades II and III (46.58 ± 15.08) meningiomas (P=0.043), between strong c-MYC expression and grades II and III (P<0.001), and between partial surgical resection and tumor recurrence or regrowth (P<0.001). These findings reveal the lower mean age among grades II and III meningioma patients than grade I patients, the influence of the protein merlin on tumorigenesis, the association of c-MYC with aggressive meningiomas, and that partial surgical resection is associated with tumor recurrence or regrowth.

Substrate Availability of Mutant SPT Alters Neuronal Branching and Growth Cone Dynamics in Dorsal Root Ganglia

Serine palmitoyltransferase (SPT) is a key enzyme in the first step of sphingolipid biosynthesis. Mutations in the SPTLC1 gene that encodes for SPT subunits cause hereditary sensory neuropathy type 1. However, little is understood about how mutant SPT regulates mechanisms of sensory neuron and axonal growth. Using transgenic mice overexpressing the C133W SPT mutant, we found that mutant dorsal root ganglia (DRG) during growth in vitro exhibit increased neurite length and branching, coinciding with elevated expression of actin-cross-linking proteins at the neuronal growth cone, namely phosphorylated Ezrin/Radixin/Moesin. In addition, inhibition of SPT was able to reverse the mutant phenotype. Because mutant SPT preferentially uses l-alanine over its canonical substrate l-serine, we also investigated the effects of substrate availability on DRG neurons. Supplementation with l-serine or removal of l-alanine independently restored normal growth patterns in mutant SPTLC1(C133W) DRG. Therefore, we report that substrate availability and selectivity of SPT influence the regulation of neurite growth in DRG neurons.

SIGNIFICANCE STATEMENT

Hereditary sensory neuropathy type 1 is an autosomal-dominant disorder that leads to a sensory neuropathy due to mutations in the serine palmitoyltransferase (SPT) enzyme. We investigated how mutant SPT and substrate levels regulate neurite growth. Because SPT is an important enzyme in the synthesis of sphingolipids, our data are of broader significance to other peripheral and metabolic disorders.

Antagonistic Effects of BACE1 and APH1B-γ-Secretase Control Axonal Guidance by Regulating Growth Cone Collapse

ΒACE1 is the major drug target for Alzheimer's disease, but we know surprisingly little about its normal function in the CNS. Here, we show that this protease is critically involved in semaphorin 3A (Sema3A)-mediated axonal guidance processes in thalamic and hippocampal neurons. An active membrane-bound proteolytic CHL1 fragment is generated by BACE1 upon Sema3A binding. This fragment relays the Sema3A signal via ezrin-radixin-moesin (ERM) proteins to the neuronal cytoskeleton. APH1B-γ-secretase-mediated degradation of this fragment stops the Sema3A-induced collapse and sensitizes the growth cone for the next axonal guidance cue. Thus, we reveal a cycle of proteolytic activity underlying growth cone collapse and restoration used by axons to find their correct trajectory in the brain. Our data also suggest that BACE1 and γ-secretase inhibition have physiologically opposite effects in this process, supporting the idea that combination therapy might attenuate some of the side effects associated with these drugs.

Dynamic regulation of Schwann cell enhancers after peripheral nerve injury

Myelination of the peripheral nervous system is required for axonal function and long term stability. After peripheral nerve injury, Schwann cells transition from axon myelination to a demyelinated state that supports neuronal survival and ultimately remyelination of axons. Reprogramming of gene expression patterns during development and injury responses is shaped by the actions of distal regulatory elements that integrate the actions of multiple transcription factors. We used ChIP-seq to measure changes in histone H3K27 acetylation, a mark of active enhancers, to identify enhancers in myelinating rat peripheral nerve and their dynamics after demyelinating nerve injury. Analysis of injury-induced enhancers identified enriched motifs for c-Jun, a transcription factor required for Schwann cells to support nerve regeneration. We identify a c-Jun-bound enhancer in the gene for Runx2, a transcription factor induced after nerve injury, and we show that Runx2 is required for activation of other induced genes. In contrast, enhancers that lose H3K27ac after nerve injury are enriched for binding sites of the Sox10 and early growth response 2 (Egr2/Krox20) transcription factors, which are critical determinants of Schwann cell differentiation. Egr2 expression is lost after nerve injury, and many Egr2-binding sites lose H3K27ac after nerve injury. However, the majority of Egr2-bound enhancers retain H3K27ac, indicating that other transcription factors maintain active enhancer status after nerve injury. The global epigenomic changes in H3K27ac deposition pinpoint dynamic changes in enhancers that mediate the effects of transcription factors that control Schwann cell myelination and peripheral nervous system responses to nerve injury.

Anchoring of protein kinase A by ERM (ezrin-radixin-moesin) proteins is required for proper netrin signaling through DCC (deleted in colorectal cancer)

Netrin-1, acting through its principal receptor DCC (deleted in colorectal cancer), serves as an axon guidance cue during neural development and also contributes to vascular morphogenesis, epithelial migration, and the pathogenesis of some tumors. Several lines of evidence suggest that netrin-DCC signaling can regulate and be regulated by the cAMP-dependent protein kinase, PKA, although the molecular details of this relationship are poorly understood. Specificity in PKA signaling is often achieved through differential subcellular localization of the enzyme by interaction with protein kinase A anchoring proteins (AKAPs). Here, we show that AKAP function is required for DCC-mediated activation of PKA and phosphorylation of cytoskeletal regulatory proteins of the Mena/VASP (vasodilator-stimulated phosphoprotein) family. Moreover, we show that DCC and PKA physically interact and that this association is mediated by the ezrin-radixin-moesin (ERM) family of plasma membrane-actin cytoskeleton cross-linking proteins. Silencing of ERM protein expression inhibits DCC-PKA interaction, DCC-mediated PKA activation, and phosphorylation of Mena/VASP proteins as well as growth cone morphology and neurite outgrowth. Finally, although expression of wild-type radixin partially rescued growth cone morphology and tropism toward netrin in ERM-knockdown cells, expression of an AKAP-deficient mutant of radixin did not fully rescue growth cone morphology and switched netrin tropism from attraction to repulsion. These data support a model in which ERM-mediated anchoring of PKA activity to DCC is required for proper netrin/DCC-mediated signaling.

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

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

A neuronal function of the tumor suppressor protein merlin

Mutagenic loss of the NF2 tumor suppressor gene encoded protein merlin is known to provoke the hereditary neoplasia syndrome, Neurofibromatosis type 2 (NF2). In addition to glial cell-derived tumors in the PNS and CNS, disease-related lesions also affect the skin and the eyes. Furthermore, 60% of NF2 patients suffer from peripheral nerve damage, clinically referred to as peripheral neuropathy. Strikingly, NF2-associated neuropathy often occurs in the absence of nerve damaging tumors, suggesting tumor-independent events. Recent findings indicate an important role of merlin in neuronal cell types concerning neuromorphogenesis, axon structure maintenance and communication between axons and Schwann cells. In this review, we compile clinical and experimental evidences for the underestimated role of the tumor suppressor merlin in the neuronal compartment.

Structural basis of DDB1-and-Cullin 4-associated Factor 1 (DCAF1) recognition by merlin/NF2 and its implication in tumorigenesis by CD44-mediated inhibition of merlin suppression of DCAF1 function

Merlin, a tumor suppressor encoded by the neurofibromatosis type 2 gene, has been shown to suppress tumorigenesis by inhibiting the Cullin 4-RING E3 ubiquitin ligase CRL4(DCAF) (1) in the nucleus. This inhibition is mediated by direct binding of merlin to DDB1-and-Cullin 4-associated Factor 1 (DCAF1), yet the binding mode of merlin to DCAF1 is not well defined. Here, we report structural and biophysical studies of the merlin binding to DCAF1 and its interference with CD44 binding. The crystal structure of the merlin FERM domain bound to the DCAF1 C-terminal acidic tail reveals that the hydrophobic IILXLN motif located at the C-terminal end of DCAF1 binds subdomain C of the FERM domain by forming a β-strand. The binding site and mode resemble that of merlin binding to the CD44 cytoplasmic tail. Competition binding assay showed that CD44 and DCAF1 compete for binding to the merlin FERM domain in solution. The CD44 cytoplasmic tail is known to be cleaved for nuclear translocation by regulated intra-membrane proteolysis (RIP). Our structure implies that, in the nucleus, the CD44 cytoplasmic tail cleaved by RIP could release DCAF1 from merlin by competing for binding to the merlin FERM domain, which results in the inhibition of merlin-mediated suppression of tumorigenesis.

The expanding family of FERM proteins

Our understanding of the FERM (4.1/ezrin/radixin/moesin) protein family has been rapidly expanding in the last few years, with the result that many new physiological functions have been ascribed to these biochemically unique proteins. In the present review, we will discuss a number of new FRMD (FERM domain)-containing proteins that were initially discovered from genome sequencing but are now being established through biochemical and genetic studies to be involved both in normal cellular processes, but are also associated with a variety of human diseases.

Mechanisms underlying the initiation and dynamics of neuronal filopodia: from neurite formation to synaptogenesis

Filopodia are finger-like cellular protrusions found throughout the metazoan kingdom and perform fundamental cellular functions during development and cell migration. Neurons exhibit a wide variety of extremely complex morphologies. In the nervous system, filopodia underlie many major morphogenetic events. Filopodia have roles spanning the initiation and guidance of neuronal processes, axons and dendrites to the formation of synaptic connections. This chapter addresses the mechanisms of the formation and dynamics of neuronal filopodia. Some of the major lessons learned from the study of neuronal filopodia are (1) there are multiple mechanisms that can regulate filopodia in a context-dependent manner, (2) that filopodia are specialized subcellular domains, (3) that filopodia exhibit dynamic membrane recycling which also controls aspects of filopodial dynamics, (4) that neuronal filopodia contain machinery for the orchestration of the actin and microtubule cytoskeleton, and (5) localized protein synthesis contributes to neuronal filopodial dynamics.

The role of cell adhesion molecules for navigating axons: density matters

Cell adhesion molecules of the immunoglobulin-super-family (IgSF-CAMs) do not only have a physical effect, mediating merely attachment between cell surfaces. For navigating axons, IgSF-CAMs also exert an instructive impact: Upon activation, they elicit intracellular signalling cascades in the tip of the axon, the growth cone, which regulate in a spatio-temporally concerted action both speed and direction of the axon. Density and distribution of IgSF-CAMs in the growth cone plasma membrane play important roles for the activation of IgSF-CAMs, their clustering, and the adhesive forces they acquire, as well as for the local restriction and effective propagation of their intracellular signals.

Preso regulation of dendritic outgrowth through PI(4,5)P2-dependent PDZ interaction with βPix

In neuronal development, dendritic outgrowth and arborization are important for the establishment of neural circuit formation. A previous study reported that PSD-95-interacting regulator of spine morphogenesis (Preso) formed a complex with PAK-interacting exchange factor-beta (βPix) via PSD-95/Dlg/ZO-1 (PDZ) interaction. Here, we report that Preso and its binding protein, βPix, are localized in dendritic growth cones. Knockdown and dominant-negative inhibition of Preso in cultured neurons markedly reduced the dendritic outgrowth but not branching, and led to a decrease in the intensity of βPix and F-actin in neuronal dendritic tips. Moreover, phosphatidylinositol 4,5-bisphosphate (PIP(2) ) induced a conformational change in Preso toward the open PDZ domain and enhanced the interaction with βPix. In addition, the Preso band 4.1 protein, ezrin, radixin and moesin (FERM) domain mutant is unable to interact with PIP(2) and it did not rescue the Preso-knockdown effect. These results indicate that PIP(2) is a key signalling molecule that regulates dendritic outgrowth through activation of small GTPase signalling via interaction between Preso and βPix.

Excessive expression of hippocampal ezrin is induced by intrastriatal injection of 6-hydroxydopamine

Accumulating evidence in humans demonstrates that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson's disease (PD). Ezrin, radixin, and moesin are collectively known as ERM proteins. Although ERM proteins have important implications in cell-shape determination and relevant signaling pathway, they have not been studied in the hippocampus in association with visuo-spatial memory impairments. The purpose of the present study is to examine whether the expression level of ERM proteins in the hippocampus is changed by an intrastriatal injection of 6-hydroxydopamine (6-OHDA) in mice. The intrastriatal injection of 6-OHDA induced partial dopaminergic deficits and spatial memory impairments. We also found that ezrin was increased in the hippocampus by the microinjection of 6-OHDA. On the other hand, protein levels of radixin and moesin were not influenced by 6-OHDA lesions. These results suggest that excessive ezrin may be related to visuo-spatial memory impairments.

Neurofibromatoses

The studies of familial tumor predisposition syndromes have contributed immensely to our understanding of oncogenesis. Neurofibromatosis 1, neurofibromatosis 2 and schwannomatosis are inherited autosomal dominant neurocutaneous disorders with complete penetrance. They are clinically and genetically distinct and considerable knowledge has been gathered about their pathogenesis. In this chapter, the genetics, molecular mechanism of disease, as well as clinical features, diagnosis and treatment are discussed.

MIG-15 and ERM-1 promote growth cone directional migration in parallel to UNC-116 and WVE-1

Neurons require precise targeting of their axons to form a connected network and a functional nervous system. Although many guidance receptors have been identified, much less is known about how these receptors signal to direct growth cone migration. We used Caenorhabditis elegans motoneurons to study growth cone directional migration in response to a repellent UNC-6 (netrin homolog) guidance cue. The evolutionarily conserved kinase MIG-15 [homolog of Nck-interacting kinase (NIK)] regulates motoneuron UNC-6-dependent repulsion through unknown mechanisms. Using genetics and live imaging techniques, we show that motoneuron commissural axon morphology defects in mig-15 mutants result from impaired growth cone motility and subsequent failure to migrate across longitudinal obstacles or retract extra processes. To identify new genes acting with mig-15, we screened for genetic enhancers of the mig-15 commissural phenotype and identified the ezrin/radixin/moesin ortholog ERM-1, the kinesin-1 motor UNC-116 and the actin regulator WVE-1 complex. Genetic analysis indicates that mig-15 and erm-1 act in the same genetic pathway to regulate growth cone migration and that this pathway functions in parallel to the UNC-116/WVE-1 pathway. Further, time-lapse imaging of growth cones in mutants suggests that UNC-116 might be required to stimulate protrusive activity at the leading edge, whereas MIG-15 and ERM-1 maintain low activity at the rear edge. Together, these results support a model in which the MIG-15 kinase and the UNC-116-WVE-1 complex act on opposite sides of the growth cone to promote robust directional migration.

The activation of ezrin-radixin-moesin proteins is regulated by netrin-1 through Src kinase and RhoA/Rho kinase activities and mediates netrin-1-induced axon outgrowth

The receptor Deleted in Colorectal Cancer (DCC) mediates the attractive response of axons to the guidance cue netrin-1 during development. On netrin-1 stimulation, DCC is phosphorylated and induces the assembly of signaling complexes within the growth cone, leading to activation of cytoskeleton regulators, namely the GTPases Rac1 and Cdc42. The molecular mechanisms that link netrin-1/DCC to the actin machinery remain unclear. In this study we seek to demonstrate that the actin-binding proteins ezrin-radixin-moesin (ERM) are effectors of netrin-1/DCC signaling in embryonic cortical neurons. We show that ezrin associates with DCC in a netrin-1-dependent manner. We demonstrate that netrin-1/DCC induces ERM phosphorylation and activation and that the phosphorylation of DCC is required in that context. Moreover, Src kinases and RhoA/Rho kinase activities mediate netrin-1-induced ERM phosphorylation in neurons. We also observed that phosphorylated ERM proteins accumulate in growth cone filopodia, where they colocalize with DCC upon netrin-1 stimulation. Finally, we show that loss of ezrin expression in cortical neurons significantly decreases axon outgrowth induced by netrin-1. Together, our findings demonstrate that netrin-1 induces the formation of an activated ERM/DCC complex in growth cone filopodia, which is required for netrin-1-dependent cortical axon outgrowth.

A tight junction-associated Merlin-angiomotin complex mediates Merlin's regulation of mitogenic signaling and tumor suppressive functions

The Merlin/NF2 tumor suppressor restrains cell growth and tumorigenesis by controlling contact-dependent inhibition of proliferation. We have identified a tight-junction-associated protein complex comprising Merlin, Angiomotin, Patj, and Pals1. We demonstrate that Angiomotin functions downstream of Merlin and upstream of Rich1, a small GTPase Activating Protein, as a positive regulator of Rac1. Merlin, through competitive binding to Angiomotin, releases Rich1 from the Angiomotin-inhibitory complex, allowing Rich1 to inactivate Rac1, ultimately leading to attenuation of Rac1 and Ras-MAPK pathways. Patient-derived Merlin mutants show diminished binding capacities to Angiomotin and are unable to dissociate Rich1 from Angiomotin or inhibit MAPK signaling. Depletion of Angiomotin in Nf2(-/-) Schwann cells attenuates the Ras-MAPK signaling pathway, impedes cellular proliferation in vitro and tumorigenesis in vivo.

Regulation of mixed lineage kinase 3 is required for Neurofibromatosis-2-mediated growth suppression in human cancer

The Neurofibromatosis-2 (NF2) tumor suppressor merlin negatively regulates cell proliferation in numerous cell types. We have previously shown that the NF2 protein (merlin/schwannomin) associates with mixed lineage kinase 3 (MLK3), a mitogen-activated protein kinase (MAPK) kinase kinase that is required for the proliferation of normal and neoplastic cells. In this study, we show that merlin inhibits MLK3 activity, as well as the activation of its downstream effectors, B-Raf, extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK). The ability of merlin to regulate MLK3 activity requires a direct association between MLK3 and residues in the C-terminal region of merlin. Merlin integrates Rho GTPase family signaling with MAPK activity by inhibiting the binding between MLK3 and its upstream activator, Cdc42. Furthermore, we demonstrate that MLK3 is required for merlin-mediated suppression of cell proliferation and invasion. Collectively, these results establish merlin as a potent inhibitor of MLK3, ERK and JNK activation in cancer, and provide a mechanistic link between deregulated MAPK and Rho GTPase signaling in NF2 growth control.

Cell adhesion molecules in context: CAM function depends on the neighborhood

Cell adhesion molecules (CAMs) are now known to mediate much more than adhesion between cells and between cells and the extracellular matrix. Work by many researchers has illuminated their roles in modulating activation of molecules such as receptor tyrosine kinases, with subsequent effects on cell survival, migration, and process extension. CAMs also are known to serve as substrates for proteases which can create diffusible fragments capable of signaling independently from the CAM. The diversity of interactions is further modulated by membrane rafts, which can co-localize or separate potential signaling partners to affect the likelihood of a given signaling pathway being activated. Given the ever-growing number of known CAMs and the fact that their heterophilic binding in cis or in trans can affect their interactions with other molecules, including membrane-bound receptors, one would predict a wide range of effects attributable to a particular CAM in a particular cell at a particular stage of development. The function(s) of a given CAM must therefore be considered in the context of the history of the cell expressing it and the repertoire of molecules expressed both by that cell and its neighbors.

Dephosphorylation-dependent inhibitory activity of juxtanodin on filamentous actin disassembly

In the vertebrate central nervous system, maturation of oligodendrocytes is accompanied by a dramatic transformation of cell morphology. Juxtanodin (JN) is an actin cytoskeleton-related oligodendroglial protein that promotes arborization of cultured oligodendrocytes. We performed in vitro and in culture experiments to further elucidate the biochemical effects, molecular interactions, and activity regulation of JN. Pulldown and co-sedimentation assays confirmed JN binding to filamentous but not globular beta-actin largely through a C-terminal domain of 14 amino acid residues. JN had much lower affinity to F-alpha-actin than to F-beta-actin. Bundling and actin polymerization assays revealed no JN influence on F-beta-actin cross-linking or G-beta-actin polymerization. Sedimentation assay, however, demonstrated that JN slowed the rate of F-beta-actin disassembly induced by dilution with F-actin depolymerization buffer. JN-S278E mutant, a mimic of phosphorylated JN at serine 278, exhibited a much diminished affinity/stabilizing effect on F-beta-actin. Immunoblotting revealed both phosphorylated and dephosphorylated native JN of the brain, with the former migrating slightly slower than the latter and becoming undetectable when brain lysate was subjected to in vitro dephosphorylation prior to being loaded for electrophoresis. In cultured OLN-93 cells, overexpression of JN promoted the formation of actin fibers and inhibited F-actin disassembly induced by latrunculin A. S278E phosphomimetic mutation resulted in loss of JN activity in cultured cells, whereas S278A, T258A, and T258E dephospho-/phosphomimetic mutations did not. These findings establish JN as an actin cytoskeleton-stabilizing protein that may play active roles in oligodendroglial differentiation and myelin formation. Specific phosphorylation of JN might serve as an important mechanism regulating JN functions.

Glutamate receptor-mediated phosphorylation of ezrin/radixin/moesin proteins is implicated in filopodial protrusion of primary cultured hippocampal neuronal cells

Previously, we reported the phosphorylation of moesin induced by electroconvulsive shock in rat brain and by glutamate in immortalized rat hippocampal cells. However, the function of phosphorylated moesin in differentiated neurons is not well understood. In this study, we observed that glutamate induces phosphorylation of ezrin/radixin/moesin proteins (ERM) in cultured hippocampal cells and that phosphorylated ERM localizes at the newly formed filopodia of neurites. The glutamate-induced phosphorylation of ERM is calcium-dependent, and inhibition of protein kinase C abolishes ERM phosphorylation as well as RhoA activation. The inhibitions of RhoA and RhoA kinase also diminishes the glutamate-induced ERM phosphorylation in cultured hippocampal cells. The knock-down of moesin or the inhibition of ERM phosphorylation results in the reduction of glutamate-induced filopodia protrusion and diminishes the increase in active synaptic boutons induced by glutamate treatment. These results indicate that glutamate-induced phosphorylation of ERM proteins in primary cultured differentiated hippocampal neurons is mediated by calcium-dependent protein kinase C, RhoA and RhoA kinase, and the phosphorylated ERM protein is necessary for the formation of filopodial protrusion and may be involved in pre-synaptic trafficking.

Initiation of malignancy by duodenal contents reflux and the role of ezrin in developing esophageal squamous cell carcinoma

Gastroesophageal reflux has recently been implicated as a causative factor in upper aerodigestive tract carcinogenesis. Esophageal squamous cell carcinomas (ESCCs) have developed in duodenal-content reflux animals without any known carcinogen present. We established a cell line, designated ESCC-DR, from a thoracic metastatic tumor in a reflux animal. To gain insight into the genomic alterations associated with duodenal content reflux-induced carcinogenesis, we first performed comparative genomic hybridization using an Agilent rat 244K array in ESCC-DR and identified many chromosomal gains and losses. Of the many genes identified, we detected an interesting ezrin amplicon that has been recently reported in human ESCC. Ezrin, which cross-links the cytoskeleton and plasma membrane, is involved in the growth and metastatic potential of cancer cells. Overexpression of ezrin protein in ESCC-DR was confirmed by Western blotting. We also compared ezrin protein expression levels and patterns in hyperplastic, dysplastic, ESCC, and metastatic sites developed in two distinct reflux models using immunohistochemistry. Immunohistochemical staining of ezrin revealed overexpression in the nucleus, and the cytoplasm as well as plasma membrane of ESCC cells. Phosphorylated ERM (ezrin, radixin, moesin) was expressed at the leading edge, or invasive front, of larger metastatic sites. Taken together, duodenal reflux has a great potential for initiating malignancy, and thus likely plays a role in development of ESCC. Ezrin probably influences the growth and invasiveness of ESCC cells, and phosphorylation is only required in metastatic behavior of tumor cells at the leading edge and invasive front.

Nanopatterns biofunctionalized with cell adhesion molecule DM-GRASP offered as cell substrate: spacing determines attachment and differentiation of neurons

The density/spacing of plasma membrane proteins is thought to be crucial for their function; clear-cut experimental evidence, however, is still rare. We examined nanopatterns biofunctionalized with cell adhesion molecule DM-GRASP with respect to their impact on neuron attachment and neurite growth. Data analysis/modeling revealed that these cellular responses improve with increasing DM-GRASP density, with the exception of one spacing which does not allow for the anchorage of a cytoskeletal protein (spectrin) to three DM-GRASP molecules.

Phosphorylation of ezrin/radixin/moesin proteins by LRRK2 promotes the rearrangement of actin cytoskeleton in neuronal morphogenesis

Leucine-rich repeat kinase 2 (LRRK2) functions as a putative protein kinase of ezrin, radixin, and moesin (ERM) family proteins. A Parkinson's disease-related G2019S substitution in the kinase domain of LRRK2 further enhances the phosphorylation of ERM proteins. The phosphorylated ERM (pERM) proteins are restricted to the filopodia of growing neurites in which they tether filamentous actin (F-actin) to the cytoplasmic membrane and regulate the dynamics of filopodia protrusion. Here, we show that, in cultured neurons derived from LRRK2 G2019S transgenic mice, the number of pERM-positive and F-actin-enriched filopodia was significantly increased, and this correlates with the retardation of neurite outgrowth. Conversely, deletion of LRRK2, which lowered the pERM and F-actin contents in filopodia, promoted neurite outgrowth. Furthermore, inhibition of ERM phosphorylation or actin polymerization rescued the G2019S-dependent neuronal growth defects. These data support a model in which the G2019S mutation of LRRK2 causes a gain-of-function effect that perturbs the homeostasis of pERM and F-actin in sprouting neurites critical for neuronal morphogenesis.

Preso, a novel PSD-95-interacting FERM and PDZ domain protein that regulates dendritic spine morphogenesis

PSD-95 is an abundant postsynaptic density (PSD) protein involved in the formation and regulation of excitatory synapses and dendritic spines, but the underlying mechanisms are not comprehensively understood. Here we report a novel PSD-95-interacting protein Preso that regulates spine morphogenesis. Preso is mainly expressed in the brain and contains WW (domain with two conserved Trp residues), PDZ (PSD-95/Dlg/ZO-1), FERM (4.1, ezrin, radixin, and moesin), and C-terminal PDZ-binding domains. These domains associate with actin filaments, the Rac1/Cdc42 guanine nucleotide exchange factor betaPix, phosphatidylinositol-4,5-bisphosphate, and the postsynaptic scaffolding protein PSD-95, respectively. Preso overexpression increases the density of dendritic spines in a manner requiring WW, PDZ, FERM, and PDZ-binding domains. Conversely, knockdown or dominant-negative inhibition of Preso decreases spine density, excitatory synaptic transmission, and the spine level of filamentous actin. These results suggest that Preso positively regulates spine density through its interaction with the synaptic plasma membrane, actin filaments, PSD-95, and the betaPix-based Rac1 signaling pathway.

Actin in axons: stable scaffolds and dynamic filaments

Actin filaments are thin polymers of the 42 kD protein actin. In mature axons a network of subaxolemmal actin filaments provide stability for membrane integrity and a substrate for short distance transport of cargos. In developing neurons dynamic regulation of actin polymerization and organization mediates axonal morphogenesis and axonal pathfinding to synaptic targets. Other changes in axonal shape, collateral branching, branch retraction, and axonal regeneration, also depend on actin filament dynamics. Actin filament organization is regulated by a diversity of actin-binding proteins (ABP). ABP are the focus of complex extrinsic and intrinsic signaling pathways, and many neurological pathologies and dysfunctions arise from defective regulation of ABP function.

CHL1 promotes Sema3A-induced growth cone collapse and neurite elaboration through a motif required for recruitment of ERM proteins to the plasma membrane

Close homolog of L1 (CHL1) is a transmembrane cell adhesion molecule with unique developmental functions in cortical neuronal positioning and dendritic projection within the L1 family, as well as shared functions in promotion of integrin-dependent neurite outgrowth and semaphorin3A (Sema3A)-mediated axon repulsion. The molecular mechanisms by which CHL1 mediates these diverse functions are obscure. Here it is demonstrated using a cytofluorescence assay that CHL1 is able to recruit ezrin, a member of the ezrin-radixin-moesin (ERM) family of filamentous actin binding proteins to the plasma membrane, and that this requires a membrane-proximal motif (RGGKYSV) in the CHL1 cytoplasmic domain. This sequence in CHL1 is shown to have novel functions necessary for Sema3A-induced growth cone collapse and CHL1-dependent neurite outgrowth and branching in cortical embryonic neurons. In addition, stimulation of haptotactic cell migration and cellular adhesion to fibronectin by CHL1 depends on the CHL1/ERM recruitment motif. These findings suggest that a direct or indirect interaction between CHL1 and ERM proteins mediates Sema3A-induced growth cone collapse as well as neurite outgrowth and branching, which are essential determinants of axon guidance and connectivity in cortical development.

Contact-dependent inhibition of EGFR signaling by Nf2/Merlin

The neurofibromatosis type 2 (NF2) tumor suppressor, Merlin, is a membrane/cytoskeleton-associated protein that mediates contact-dependent inhibition of proliferation. Here we show that upon cell-cell contact Merlin coordinates the processes of adherens junction stabilization and negative regulation of epidermal growth factor receptor (EGFR) signaling by restraining the EGFR into a membrane compartment from which it can neither signal nor be internalized. In confluent Nf2(-/-) cells, EGFR activation persists, driving continued proliferation that is halted by specific EGFR inhibitors. These studies define a new mechanism of tumor suppression, provide mechanistic insight into the poorly understood phenomenon of contact-dependent inhibition of proliferation, and suggest a therapeutic strategy for NF2-mutant tumors.

Lateral motor column axons execute a ternary trajectory choice between limb and body tissues

Background

Neuronal topographic map formation requires appropriate selection of axonal trajectories at intermediate choice points prior to target innervation. Axons of neurons in the spinal cord lateral motor column (LMC), as defined by a transcription factor code, are thought to innervate limb target tissues exclusively. Axons of the medial and lateral LMC divisions appear to execute a binary decision at the base of the limb as they choose between ventral and dorsal limb trajectories. The cellular logic that guides motor axon trajectory choices into non-limb tissues such as the ventral flank remains unclear.

Results

We determined the spinal cord motor column origin of motor nerves that innervate ventral flank tissues at hindlimb level. We found unexpectedly that a subset of medial LMC axons innervates ventral non-limb mesenchyme at hindlimb level, rather than entering ventral limb mesenchyme. We also found that in a conditional BmprIa mutant where all ventral hindlimb mesenchyme is converted to a dorsal identity, all medial LMC axons are redirected into the ventral flank, while lateral LMC axons innervate the bidorsal limb.

Conclusion

We have found that medial LMC neurons innervate both ventral flank and limb targets. While normally only a subset of medial LMC axons innervate the flank, all are capable of doing so. Furthermore, LMC axons execute a ternary, rather than binary, choice at the base of the limb between ventral flank, ventral limb and dorsal limb trajectories. When making this choice, medial and lateral LMC axons exhibit different and asymmetric relative preferences for these three trajectories. These data redefine the LMC as a motor column that innervates both limb and body tissues.

Safety of radiosurgery applied to conditions with abnormal tumor suppressor genes

OBJECTIVE

To assess the risk of radiosurgery inducing malignancy in neurofibromatosis-2 (NF2) and von Hippel-Lindau disease.

METHODS

A retrospective cohort study of 118 NF2 and 19 von Hippel-Lindau disease patients, totalling 906 and 62 patient-years of follow-up data, respectively.

RESULTS

Two cases of intracranial malignancy were identified, both of which occurred in NF2 patients. One of these was thought to have arisen before the radiosurgery; the other was a glioblastoma diagnosed 3 years after radiosurgery.

CONCLUSION

Because gliomas may occur in as many as 4% of NF2 patients, this may not represent an increased risk. We continue to offer radiosurgery treatment to selected NF2 and von Hippel-Lindau disease patients and consider that the late risk of malignancy arising after irradiation must be put in the context of the condition being treated, the treatment options available to these individuals, and their life expectancy.

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.

Evaluation of NF2 gene deletion in sporadic schwannomas, meningiomas, and ependymomas by chromogenic in situ hybridization

Fluorescence in situ hybridization, loss of heterozygosity testing, and comparative genomic hybridization have been used to detect NF2 gene alterations in both sporadic and neurofibromatosis type 2 (NF2)-associated central nervous system tumors. In this study, we performed chromogenic in situ hybridization (CISH) and immunohistochemistry to evaluate for NF2 gene deletion in a group of sporadic meningiomas, schwannomas, and ependymomas. Twenty-two sporadic tumors, including 9 ependymomas, 10 meningiomas, and 3 schwannomas, were studied. CISH and immunohistochemistry were performed using the NF2 gene deletion probe and NF2 polyclonal antibody. Deletion of the NF2 gene was identified in 11 (50%) tumors, including 60% (6/10) of meningiomas, 33% (3/9) of ependymomas, and 67% (2/3) of schwannomas. The remaining 11 (50%) cases were diploid. Overall, immunoexpression of NF2 protein was observed in 50% (11/22) tumors, and concordance between CISH and immunohistochemistry was observed in 73% of cases. Our results support previous observations that schwannomas and meningiomas, and to a lesser degree, ependymomas, express a high incidence of NF2 gene deletion, which supports the hypothesis that NF2 gene plays an important role in their tumorigenesis. In addition, we have validated CISH as an efficient, economic, and reliable method for routinely assessing NF2 gene deletion in these tumors.

Na+-H+ exchanger regulatory factor 1 is a PDZ scaffold for the astroglial glutamate transporter GLAST

Glutamate is a key neurotransmitter and its levels in the synaptic cleft are tightly regulated by reuptake mechanisms that primarily involve transporters in astrocytes. This requires that the glutamate transporters be spatially constrained to effect maximum glutamate transport. GLAST (EAAT1) is the predominant astroglial transporter and contains a class I PDZ-binding consensus (ETKM) in its C-terminus. The epithelial Na(+)/H(+) exchanger regulatory factors NHERF1 and NHERF2 are PDZ proteins that contain two tandem PDZ domains and a C-terminal domain that binds members of the ERM (ezrin-radixin-moesin) family of membrane-cytoskeletal adaptors. NHERF proteins have been extensively characterized in renal epithelia and their expression in brain has recently been reported; however, their function in the brain remains unknown. The aims of the current study were to (1) determine the distribution of NHERF1/2 in the rodent brain and (2) investigate whether GLAST was a physiological ligand for NHERF1/2. Immunohistochemistry revealed that NHERF1 expression was widespread in rat brain (abundant in cerebellum, cerebral cortex, hippocampus, and thalamus) and primarily restricted to astrocytes whereas NHERF2 expression was primarily restricted to endothelial cells of blood vessels and capillaries. Importantly, NHERF1 distribution closely matched that of GLAST and confocal imaging demonstrated co-localization of the two proteins. Co-immunoprecipitation demonstrated that GLAST, NHERF1, and ezrin associate in vivo. In vitro binding assays showed that GLAST bound directly to the PDZ1 domain of NHERF1 via the C-terminal ETKM motif of GLAST. These findings implicate the GLAST-NHERF1 complex in the regulation of glutamate homeostasis in astrocytes.

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

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

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

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

The distribution of constitutional and somatic mutations in the neurofibromatosis 2 gene

Constitutional heterozygous inactivating mutations in the neurofibromatosis 2 (NF2) tumor suppressor gene cause the autosomal dominant disease NF2, and biallelic inactivating somatic NF2 mutations are found in a high proportion of unilateral sporadic vestibular schwannoma (USVS) and sporadic meningioma. We surveyed the distributions of constitutional NF2 mutations in 823 NF2 families, 278 somatic NF2 mutations in USVS, and 208 somatic NF2 mutations in sporadic meningioma. Based on the available NF2 mutation data, the most dominant influence on the spectra of mutations in exons 1-15 are C>T transitions that change arginine codons (CGA) to stop codons (TGA) due to spontaneous deamination of methylcytosine to thymine in CpG dinucleotides. The paucity of reported mutations in exon 9 and the absence of reported mutations in exons 16 and 17 may be related to structure-function relationships in the NF2 protein.