Frabin and other related Cdc42-specific guanine nucleotide exchange factors couple the actin cytoskeleton with the plasma membrane

Oct4 is a gatekeeper of epithelial identity by regulating cytoskeletal organization in skin keratinocytes

Oct4 is a pioneer transcription factor regulating pluripotency. However, it is not well known whether Oct4 has an impact on epidermal cells. We generated OCT4 knockout clonal cell lines using immortalized human skin keratinocytes to identify a functional role for the protein. Here, we report that Oct4-deficient cells transitioned into a mesenchymal-like phenotype with enlarged size and shape, exhibited accelerated migratory behavior, decreased adhesion, and appeared arrested at the G2/M cell cycle checkpoint. Oct4 absence had a profound impact on cortical actin organization, with loss of microfilaments from the cell membrane, increased puncta deposition in the cytoplasm, and stress fiber formation. E-cadherin, β-catenin, and ZO1 were almost absent from cell-cell contacts, while fibronectin deposition was markedly increased in the extracellular matrix (ECM). Mapping of the transcriptional and chromatin profiles of Oct4-deficient cells revealed that Oct4 controls the levels of cytoskeletal, ECM, and differentiation-related genes, whereas epithelial identity is preserved through transcriptional and non-transcriptional mechanisms.

A Novel Mutation in Frabin (FGD4) Causing a Mild Phenotype of CMT4H in an Indian Patient

Charcot-Marie-Tooth disease 4H(CMT4H) is an autosomal recessive demyelinating form of CMT caused by FGD4/FRABIN mutations. CMT4H is characterized by early onset and slowly progressing motor and sensory deficits in the distal extremities, along with foot deformities. We describe a patient with CMT4H who presented with rapidly progressing flaccid quadriparesis during the postpartum period, which improved significantly with steroid therapy. Magnetic resonance imaging and ultrasonography demonstrated considerable nerve thickening with increased cross-sectional area in the peripheral nerves. A nerve biopsy revealed significant demyelination and myelin outfolding. This is the first report of an Indian patient with a novel homozygous nonsense c.1672C>T (p.Arg558Ter) mutation in the FGD4 gene, expanding the mutational and phenotypic spectrum of this disease.

FGD3 binds with HSF4 to suppress p65 expression and inhibit pancreatic cancer progression

Pancreatic cancer is regarded as the most lethal solid tumor worldwide. Deregulated and constitutively activated NF-κB signaling is one of the major characteristics of pancreatic cancer. The total expression level and subcellular localization of RelA/p65 have been shown to determine the activation of canonical NF-κB signaling in pancreatic cancer. FGD3, which is involved in regulating the actin cytoskeleton and cell shape, has been reported to inhibit cancer cell migration and predict a favorable prognosis in multiple types of cancer. However, the specific role of FGD3 in pancreatic cancer is still unknown. In this study, we conducted a systematic investigation of the cancer-related role of FGD3 in pancreatic cancer. We demonstrated that FGD3 was abnormally downregulated in pancreatic cancer tissues and that low expression of FGD3 was associated with unfavorable prognosis in patients with pancreatic cancer. Then, we showed that FGD3 inhibited pancreatic cancer cell proliferation, invasion and metastasis in vivo and in vitro. Moreover, we revealed that FGD3 silencing activated the NF-κB signaling pathway by promoting HSF4 nuclear translocation and increasing p65 expression in pancreatic cancer cells. Therefore, our results identified a novel and targetable FGD3/HSF4/p65 signaling axis in pancreatic cancer cells.

A familiar study on self-limited childhood epilepsy patients using hIPSC-derived neurons shows a bias towards immaturity at the morphological, electrophysiological and gene expression levels

Background

Self-limited Childhood Epilepsies are the most prevalent epileptic syndrome in children. Its pathogenesis is unknown. In this disease, symptoms resolve spontaneously in approximately 50% of patients when maturity is reached, prompting to a maturation problem. The purpose of this study was to understand the molecular bases of this disease by generating and analyzing induced pluripotent stem cell-derived neurons from a family with 7 siblings, among whom 4 suffer from this disease.

Methods

Two affected siblings and, as controls, a healthy sister and the unaffected mother of the family were studied. Using exome sequencing, a homozygous variant in the FYVE, RhoGEF and PH Domain Containing 6 gene was identified in the patients as a putative genetic factor that could contribute to the development of this familial disorder. After informed consent was signed, skin biopsies from the 4 individuals were collected, fibroblasts were derived and reprogrammed and neurons were generated and characterized by markers and electrophysiology. Morphological, electrophysiological and gene expression analyses were performed on these neurons.

Results

Bona fide induced pluripotent stem cells and derived neurons could be generated in all cases. Overall, there were no major shifts in neuronal marker expression among patient and control-derived neurons. Compared to two familial controls, neurons from patients showed shorter axonal length, a dramatic reduction in synapsin-1 levels and cytoskeleton disorganization. In addition, neurons from patients developed a lower action potential threshold with time of in vitro differentiation and the amount of current needed to elicit an action potential (rheobase) was smaller in cells recorded from NE derived from patients at 12 weeks of differentiation when compared with shorter times in culture. These results indicate an increased excitability in patient cells that emerges with the time in culture. Finally, functional genomic analysis showed a biased towards immaturity in patient-derived neurons.

Conclusions

We are reporting the first in vitro model of self-limited childhood epilepsy, providing the cellular bases for future in-depth studies to understand its pathogenesis. Our results show patient-specific neuronal features reflecting immaturity, in resonance with the course of the disease and previous imaging studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02658-2.

Held Up in Traffic-Defects in the Trafficking Machinery in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth disease (CMT), also known as motor and sensory neuropathy, describes a clinically and genetically heterogenous group of disorders affecting the peripheral nervous system. CMT typically arises in early adulthood and is manifested by progressive loss of motor and sensory functions; however, the mechanisms leading to the pathogenesis are not fully understood. In this review, we discuss disrupted intracellular transport as a common denominator in the pathogenesis of different CMT subtypes. Intracellular transport via the endosomal system is essential for the delivery of lipids, proteins, and organelles bidirectionally to synapses and the soma. As neurons of the peripheral nervous system are amongst the longest neurons in the human body, they are particularly susceptible to damage of the intracellular transport system, leading to a loss in axonal integrity and neuronal death. Interestingly, defects in intracellular transport, both in neurons and Schwann cells, have been found to provoke disease. This review explains the mechanisms of trafficking and subsequently summarizes and discusses the latest findings on how defects in trafficking lead to CMT. A deeper understanding of intracellular trafficking defects in CMT will expand our understanding of CMT pathogenesis and will provide novel approaches for therapeutic treatments.

Fgd5 is a Rac1-specific Rho GEF that is selectively inhibited by aurintricarboxylic acid

Rho proteins are signalling molecules that control cellular dynamics, movement and morphological changes. They are activated by Rho guanine-nucleotide exchange factors (Rho GEFs) that transduce upstream signals into Rho-mediated activation of downstream processes. Fgd5 is a Rho GEF involved in angiogenesis and its target Rho protein for this process has been linked to Cdc42 activation. Here, we examined the function of purified Fgd5, specifically, which Rho proteins it activates and pinpoint the structural domains required for enzymatic activity. Using a GEF enzyme assay, we found that purified Fgd5 showed preferential activation of Rac1 and direct binding of Rac1 in pull-down and co-immunoprecipitation assays. Structural comparisons showed that the Fgd5 DH domain is highly similar to the Rac1 GEF, TrioN, supporting a role for Fgd5 as a Rac1 GEF. Compounds that bind to purified Fgd5 DH-PH protein were identified by screening a small molecule library via surface plasmon resonance. The effects of eleven ligands were further examined for their ability to inhibit the Fgd5 GEF enzymatic activity and Rac1 interaction. From these studies, we found that the compound aurintricarboxylic acid, and to a lesser extent mitoxantrone dihydrochloride, inhibited both Fgd5 GEF activation of Rac1 and their interaction. Aurintricarboxylic acid had no effect on the activity or binding of the Rac1 GEF, TrioN, thus demonstrating the feasibility of selectively disrupting Rho GEF activators. Abbreviations: a.a.: amino acid; ATA: aurintricarboxylic acid; DH: Dbl homology; DOCK: dictator of cytokinesis; Fgd: faciogenital dysplasia; GEF: guanine-nucleotide exchange factor; GST: glutathione S-transferase; LOPAC: library of pharmacologically active compounds; PH: pleckstrin homology; PDB: protein data bank; s.e.m.: standard error of the mean; SPR: surface plasmon resonance.

EPEC Recruits a Cdc42-Specific GEF, Frabin, To Facilitate PAK Activation and Host Cell Colonization

Enteropathogenic Escherichia coli (EPEC) is a leading cause of diarrhea in children, especially in the developing world. EPEC initiates infection by attaching to cells in the host intestine, triggering the formation of actin-rich “pedestal” structures directly beneath the adherent pathogen. These bacteria inject their own receptor into host cells, which upon binding to a protein on the pathogen surface triggers pedestal formation. Multiple other proteins are also delivered into the cells of the host intestine, which work together to hijack host signaling pathways to drive pedestal production. Here we show how EPEC hijacks a host protein, Frabin, which creates the conditions in the cell necessary for the pathogen to manipulate a specific pathway that promotes pedestal formation. This provides new insights into this essential early stage in disease caused by EPEC.

Enteropathogenic Escherichia coli (EPEC) is an extracellular pathogen that tightly adheres to host cells by forming “actin pedestals” beneath the bacteria, a critical step in pathogenesis. EPEC injects effector proteins that manipulate host cell signaling cascades to trigger pedestal assembly. We have recently shown that one such effector, EspG, hijacks p21-activated kinase (PAK) and sustains its activated state to drive the cytoskeletal changes necessary for attachment of the pathogen to target cells. This EspG subversion of PAK required active Rho family small GTPases in the host cell. Here we show that EPEC itself promotes the activation of Rho GTPases by recruiting Frabin, a host guanine nucleotide exchange factor (GEF) for the Rho GTPase Cdc42. Cells devoid of Frabin showed significantly lower EPEC-induced PAK activation, pedestal formation, and bacterial attachment. Frabin recruitment to sites of EPEC attachment was driven by EspG and required localized enrichment of phosphatidylinositol 4,5-bisphosphate (PIP2) and host Arf6. Our findings identify Frabin as a key target for EPEC to ensure the activation status of cellular GTPases required for actin pedestal formation.

Characterization of guanine nucleotide exchange activity of DH domain of human FGD2

FGD2, a member of FGD family, contains a Dbl homology domain (DH) and two pleckstrin homology domains segregated by a FYVE domain. The DH domain has been deduced to be responsible for guanine nucleotide exchange of CDC42 to activate downstream factors. Our aim was to build a prokaryotic expression system for the DH domain and to examine its guanine nucleotide exchange activity toward CDC42 in vitro. A recombinant vector, which was successfully constructed based on pGEX-6P-1, was employed to express the DH domain of human FGD2 (FGD2-DH) in E. coli BL21 (DE3). Purified FGD2-DH behaved as a homogeneous monomer with an estimated molecular weight that corresponded to the theoretical molecular weight and was predicted to be an α-helix protein by circular dichroism spectroscopy. FGD2-DH displayed weak guanine nucleotide exchange activity in vitro and very weak interactions with CDC42 following glutaraldehyde cross-linking.

A Genome-Wide Screen in Mice To Identify Cell-Extrinsic Regulators of Pulmonary Metastatic Colonisation

Metastatic colonization, whereby a disseminated tumor cell is able to survive and proliferate at a secondary site, involves both tumor cell-intrinsic and -extrinsic factors. To identify tumor cell-extrinsic (microenvironmental) factors that regulate the ability of metastatic tumor cells to effectively colonize a tissue, we performed a genome-wide screen utilizing the experimental metastasis assay on mutant mice. Mutant and wildtype (control) mice were tail vein-dosed with murine metastatic melanoma B16-F10 cells and 10 days later the number of pulmonary metastatic colonies were counted. Of the 1,300 genes/genetic locations (1,344 alleles) assessed in the screen 34 genes were determined to significantly regulate pulmonary metastatic colonization (15 increased and 19 decreased; P < 0.005 and genotype effect <-55 or >+55). While several of these genes have known roles in immune system regulation (Bach2, Cyba, Cybb, Cybc1, Id2, Igh-6, Irf1, Irf7, Ncf1, Ncf2, Ncf4 and Pik3cg) most are involved in a disparate range of biological processes, ranging from ubiquitination (Herc1) to diphthamide synthesis (Dph6) to Rho GTPase-activation (Arhgap30 and Fgd4), with no previous reports of a role in the regulation of metastasis. Thus, we have identified numerous novel regulators of pulmonary metastatic colonization, which may represent potential therapeutic targets.

An RNAi screen unravels the complexities of Rho GTPase networks in skin morphogenesis

During mammalian embryogenesis, extensive cellular remodeling is needed for tissue morphogenesis. As effectors of cytoskeletal dynamics, Rho GTPases and their regulators are likely involved, but their daunting complexity has hindered progress in dissecting their functions. We overcome this hurdle by employing high throughput in utero RNAi-mediated screening to identify key Rho regulators of skin morphogenesis. Our screen unveiled hitherto unrecognized roles for Rho-mediated cytoskeletal remodeling events that impact hair follicle specification, differentiation, downgrowth and planar cell polarity. Coupling our top hit with gain/loss-of-function genetics, interactome proteomics and tissue imaging, we show that RHOU, an atypical Rho, governs the cytoskeletal-junction dynamics that establish columnar shape and planar cell polarity in epidermal progenitors. Conversely, RHOU downregulation is required to remodel to a conical cellular shape that enables hair bud invagination and downgrowth. Our findings underscore the power of coupling screens with proteomics to unravel the physiological significance of complex gene families.

Lowe syndrome-linked endocytic adaptors direct membrane cycling kinetics with OCRL in Dictyostelium discoideum

Mutations of the inositol 5-phosphatase OCRL cause Lowe syndrome (LS), characterized by congenital cataract, low IQ, and defective kidney proximal tubule resorption. A key subset of LS mutants abolishes OCRL’s interactions with endocytic adaptors containing F&H peptide motifs. Converging unbiased methods examining human peptides and the unicellular phagocytic organism Dictyostelium discoideum reveal that, like OCRL, the Dictyostelium OCRL orthologue Dd5P4 binds two proteins closely related to the F&H proteins APPL1 and Ses1/2 (also referred to as IPIP27A/B). In addition, a novel conserved F&H interactor was identified, GxcU (in Dictyostelium) and the Cdc42-GEF FGD1-related F-actin binding protein (Frabin) (in human cells). Examining these proteins in D. discoideum, we find that, like OCRL, Dd5P4 acts at well-conserved and physically distinct endocytic stations. Dd5P4 functions in coordination with F&H proteins to control membrane deformation at multiple stages of endocytosis and suppresses GxcU-mediated activity during fluid-phase micropinocytosis. We also reveal that OCRL/Dd5P4 acts at the contractile vacuole, an exocytic osmoregulatory organelle. We propose F&H peptide-containing proteins may be key modifiers of LS phenotypes.

Evolutionary Loss of Genomic Proximity to Conserved Noncoding Elements Impacted the Gene Expression Dynamics During Mammalian Brain Development

Conserved noncoding elements (CNEs) have a significant regulatory influence on their neighboring genes. Loss of proximity to CNEs through genomic rearrangements can, therefore, impact the transcriptional states of the cognate genes. Yet, the evolutionary implications of such chromosomal alterations have not been studied. Through genome-wide analysis of CNEs and the cognate genes of representative species from five different mammalian orders, we observed a significant loss of genes' linear proximity to CNEs in the rat lineage. The CNEs and the genes losing proximity had a significant association with fetal, but not postnatal, brain development as assessed through ontology terms, developmental gene expression, chromatin marks, and genetic mutations. The loss of proximity to CNEs correlated with the independent evolutionary loss of fetus-specific upregulation of nearby genes in the rat brain. DNA breakpoints implicated in brain abnormalities of germline origin had significant representation between a CNE and the gene that exhibited loss of proximity, signifying the underlying developmental tolerance of genomic rearrangements that allowed the evolutionary splits of CNEs and the cognate genes in the rodent lineage. Our observations highlighted a nontrivial impact of chromosomal rearrangements in shaping the evolutionary dynamics of mammalian brain development and might explain the loss of brain traits, like cerebral folding of the cortex, in the rodent lineage.

Expression of FGD4 positively correlates with the aggressive phenotype of prostate cancer

BACKGROUND

FGD4 (Frabin) is an F-actin binding protein with GTP/GDP exchange activity specific for CDC42. It is involved in reorganization of the actin cytoskeleton, which requires both actin binding and CDC42 activating function of FGD4. Expression of FGD4 is altered in patients with heterogeneous hereditary motor and sensory neuropathies as a result of demyelination of peripheral nerves.

METHODS

In this study, we examined the expression of FGD4 in prostate cancer specimens using immunohistochemistry and studied the function of FGD4 in maintaining cell phenotype, behavior and drug sensitivity using overexpression and siRNA-based silencing approaches. We used Mann-Whitney test for comparative analysis of FGD4 expression.

RESULTS

Our results show that the expression of FGD4 is upregulated in cancerous prostates compared to the luminal cells in benign prostatic hyperplasia, although the basal cells showed high staining intensities. We noted a gradual increase in the staining intensity of FGD4 with increasing aggressiveness of the disease. Inhibition of expression of FGD4 using siRNAs showed reduced proliferation and cell cycle arrest in G2/M phase of androgen dependent LNCaP-104S and androgen refractory PC-3 cells. Inhibition of FGD4 also resulted in reduced cell migration and CDC42 activities in PC-3 cells whereas, ectopic expression of FGD4 induced cell migration, altered expression of mesenchymal and epithelial markers and activation of CDC42/PAK signaling pathway. Reduced expression of FGD4 improved sensitivity of LNCaP-104S cells to the anti-androgen drug Casodex and PC-3 cells to the microtubule stabilizing drug docetaxel.

CONCLUSIONS

Our data demonstrate a tumor promoting and a cell migratory function of FGD4 in prostate cancer cells and that inhibition of FGD4 expression enhances the response for both androgen-dependent and independent prostate cancer cells towards currently used prostate cancer drugs.

The Hippo-YAP Pathway Regulates 3D Organ Formation and Homeostasis

The vertebrate body shape is formed by the specific sizes and shapes of its resident tissues and organs, whose alignments are essential for proper functioning. To maintain tissue and organ shape, and thereby function, it is necessary to remove senescent, transformed, and/or damaged cells, which impair function and can lead to tumorigenesis. However, the molecular mechanisms underlying three-dimensional (3D) organ formation and homeostasis are not fully clear. Yes-associated protein (YAP) is a transcriptional co-activator that is involved in organ size control and tumorigenesis. Recently, we reported that YAP is essential for proper 3D body shape through regulation of cell tension by using a unique medaka fish mutant, hirame (hir). In Madin&ndash;Darby canine kidney (MDCK) epithelial cells, active YAP-transformed cells are eliminated apically when surrounded by normal cells. Furthermore, in a mosaic mouse model, active YAP-expressing damaged hepatocytes undergo apoptosis and are eliminated from the liver. Thus, YAP functions in quantitative and quality control in organogenesis. In this review, we describe the various roles of YAP in vertebrates, including in the initiation of liver cancer.

Multiple Rap1 effectors control Epac1-mediated tightening of endothelial junctions

Epac1 and Rap1 mediate cAMP-induced tightening of endothelial junctions. We have previously found that one of the mechanisms is the inhibition of Rho-mediated tension in radial stress fibers by recruiting the RhoGAP ArhGAP29 in a complex containing the Rap1 effectors Rasip1 and Radil. However, other mechanisms have been proposed as well, most notably the induction of tension in circumferential actin cables by Cdc42 and its GEF FGD5. Here, we have investigated how Rap1 controls FGD5/Cdc42 and how this interconnects with Radil/Rasip1/ArhGAP29. Using endothelial barrier measurements, we show that Rho inhibition is not sufficient to explain the barrier stimulating effect of Rap1. Indeed, Cdc42-mediated tension is induced at cell-cell contacts upon Rap1 activation and this is required for endothelial barrier function. Depletion of potential Rap1 effectors identifies AF6 to mediate Rap1 enhanced tension and concomitant Rho-independent barrier function. When overexpressed in HEK293T cells, AF6 is found in a complex with FGD5 and Radil. From these results we conclude that Rap1 utilizes multiple pathways to control tightening of endothelial junctions, possibly through a multiprotein effector complex, in which AF6 functions to induce tension in circumferential actin cables.

YAP determines the cell fate of injured mouse hepatocytes in vivo

The presence of senescent, transformed or damaged cells can impair tissue function or lead to tumorigenesis; therefore, organisms have evolved quality control mechanisms to eliminate them. Here, we show that YAP activation induced by inactivation of the Hippo pathway specifically in damaged hepatocytes promotes their selective elimination by using in vivo mosaic analysis in mouse liver. These damaged hepatocytes migrate into the hepatic sinusoids, undergo apoptosis and are engulfed by Kupffer cells. In contrast, YAP activation in undamaged hepatocytes leads to proliferation. Cellular stresses such as ethanol that damage both liver sinusoidal endothelial cells and hepatocytes switch cell fate from proliferation to migration/apoptosis in the presence of activated YAP. This involves the activation of CDC42 and Rac that regulate cell migration. Thus, we suggest that YAP acts as a stress sensor that induces elimination of injured cells to maintain tissue and organ homeostasis.

Senescent and injured cells affect tissue functions and can drive tumorigenesis. Thus, efficient elimination of these cells is pivotal for tissue integrity. Here Miyamura et al. show that YAP acts as a cellular stress sensor and promotes the elimination of damaged cells to maintain tissue homeostasis.

The preterm cervix reveals a transcriptomic signature in the presence of premature prelabor rupture of membranes

BACKGROUND

Premature prelabor rupture of fetal membranes accounts for 30% of all premature births and is associated with detrimental long-term infant outcomes. Premature cervical remodeling, facilitated by matrix metalloproteinases, may trigger rupture at the zone of the fetal membranes overlying the cervix. The similarities and differences underlying cervical remodeling in premature prelabor rupture of fetal membranes and spontaneous preterm labor with intact membranes are unexplored.

OBJECTIVES

We aimed to perform the first transcriptomic assessment of the preterm human cervix to identify differences between premature prelabor rupture of fetal membranes and preterm labor with intact membranes and to compare the enzymatic activities of matrix metalloproteinases-2 and -9 between premature prelabor rupture of fetal membranes and preterm labor with intact membranes.

STUDY DESIGN

Cervical biopsies were collected following preterm labor with intact membranes (n = 6) and premature prelabor rupture of fetal membranes (n = 5). Biopsies were also collected from reference groups at term labor (n = 12) or term not labor (n = 5). The Illumina HT-12 version 4.0 BeadChips microarray was utilized, and a novel network graph approach determined the specificity of changes between premature prelabor rupture of fetal membranes and preterm labor with intact membranes. Quantitative reverse transcription-polymerase chain reaction and Western blotting confirmed the microarray findings. Immunofluorescence was used for localization studies and gelatin zymography to assess matrix metalloproteinase activity.

RESULTS

PML-RARA-regulated adapter molecule 1, FYVE-RhoGEF and PH domain-containing protein 3 and carcinoembryonic antigen-ralated cell adhesion molecule 3 were significantly higher, whereas N-myc downstream regulated gene 2 was lower in the premature prelabor rupture of fetal membranes cervix when compared with the cervix in preterm labor with intact membranes, term labor, and term not labor. PRAM1 and CEACAM3 were localized to immune cells at the cervical stroma and NDRG2 and FGD3 were localized to cervical myofibroblasts. The activity of matrix metalloproteinase-9 was higher (1.22 ± 4.403-fold, P < .05) in the cervix in premature prelabor rupture of fetal membranes compared with preterm labor with intact membranes.

CONCLUSION

We identified 4 novel proteins with a potential role in the regulation of cervical remodeling leading to premature prelabor rupture of fetal membranes. Our findings contribute to the studies dissecting the mechanisms underlying premature prelabor rupture of fetal membranes and inspire further investigations toward the development of premature prelabor rupture of fetal membranes therapeutics.

Mechanisms of Impaired Neutrophil Migration by MicroRNAs in Myelodysplastic Syndromes

In myelodysplastic syndromes (MDS), functional defects of neutrophils result in high mortality because of infections; however, the molecular basis remains unclear. We recently found that miR-34a and miR-155 were significantly increased in MDS neutrophils. To clarify the effects of the aberrant microRNA expression on neutrophil functions, we introduced miR-34a, miR-155, or control microRNA into neutrophil-like differentiated HL60 cells. Ectopically introduced miR-34a and miR-155 significantly attenuated migration toward chemoattractants fMLF and IL-8, but enhanced degranulation. To clarify the mechanisms for inhibition of migration, we studied the effects of miR-34a and miR-155 on the migration-regulating Rho family members, Cdc42 and Rac1. The introduced miR-34a and miR-155 decreased the fMLF-induced active form of Cdc42 to 29.0 ± 15.9 and 39.7 ± 4.8% of that in the control cells, respectively, although Cdc42 protein levels were not altered. miR-34a decreased a Cdc42-specific guanine nucleotide exchange factor (GEF), dedicator of cytokinesis (DOCK) 8, whereas miR-155 reduced another Cdc42-specific GEF, FYVE, RhoGEF, and PH domain-containing (FGD) 4. The knockdown of DOCK8 and FGD4 by small interfering RNA suppressed Cdc42 activation and fMLF/IL-8-induced migration. miR-155, but not miR-34a, decreased Rac1 protein, and introduction of Rac1 small interfering RNA attenuated Rac1 activation and migration. Neutrophils from patients showed significant attenuation in migration compared with healthy cells, and protein levels of DOCK8, FGD4, and Rac1 were well correlated with migration toward fMLF (r = 0.642, 0.686, and 0.436, respectively) and IL-8 (r = 0.778, 0.659, and 0.606, respectively). Our results indicated that reduction of DOCK8, FGD4, and Rac1 contributes to impaired neutrophil migration in MDS.

Dynamin Binding Protein (Tuba) Deficiency Inhibits Ciliogenesis and Nephrogenesis in Vitro and in Vivo

Dysfunction of renal primary cilia leads to polycystic kidney disease. We previously showed that the exocyst, a protein trafficking complex, is essential for ciliogenesis and regulated by multiple Rho and Rab family GTPases, such as Cdc42. Cdc42 deficiency resulted in a disruption of renal ciliogenesis and a polycystic kidney disease phenotype in zebrafish and mice. Here we investigate the role of Dynamin binding protein (also known as Tuba), a Cdc42-specific guanine nucleotide exchange factor, in ciliogenesis and nephrogenesis using Tuba knockdown Madin-Darby canine kidney cells and tuba knockdown in zebrafish. Tuba depletion resulted in an absence of cilia, with impaired apical polarization and inhibition of hepatocyte growth factor-induced tubulogenesis in Tuba knockdown Madin-Darby canine kidney cell cysts cultured in a collagen gel. In zebrafish, tuba was expressed in multiple ciliated organs, and, accordingly, tuba start and splice site morphants showed various ciliary mutant phenotypes in these organs. Co-injection of tuba and cdc42 morpholinos at low doses, which alone had no effect, resulted in genetic synergy and led to abnormal kidney development with highly disorganized pronephric duct cilia. Morpholinos targeting two other guanine nucleotide exchange factors not known to be in the Cdc42/ciliogenesis pathway and a scrambled control morpholino showed no phenotypic effect. Given the molecular nature of Cdc42 and Tuba, our data strongly suggest that tuba and cdc42 act in the same ciliogenesis pathway. Our study demonstrates that Tuba deficiency causes an abnormal renal ciliary and morphogenetic phenotype. Tuba most likely plays a critical role in ciliogenesis and nephrogenesis by regulating Cdc42 activity.

Suppression of chemotaxis by SSeCKS via scaffolding of phosphoinositol phosphates and the recruitment of the Cdc42 GEF, Frabin, to the leading edge

Chemotaxis is controlled by interactions between receptors, Rho-family GTPases, phosphatidylinositol 3-kinases, and cytoskeleton remodeling proteins. We investigated how the metastasis suppressor, SSeCKS, attenuates chemotaxis. Chemotaxis activity inversely correlated with SSeCKS levels in mouse embryo fibroblasts (MEF), DU145 and MDA-MB-231 cancer cells. SSeCKS loss induced chemotactic velocity and linear directionality, correlating with replacement of leading edge lamellipodia with fascin-enriched filopodia-like extensions, the formation of thickened longitudinal F-actin stress fibers reaching to filopodial tips, relative enrichments at the leading edge of phosphatidylinositol (3,4,5)P3 (PIP3), Akt, PKC-ζ, Cdc42-GTP and active Src (Src^poY416), and a loss of Rac1. Leading edge lamellipodia and chemotaxis inhibition in SSeCKS-null MEF could be restored by full-length SSeCKS or SSeCKS deleted of its Src-binding domain (ΔSrc), but not by SSeCKS deleted of its three MARCKS (myristylated alanine-rich C kinase substrate) polybasic domains (ΔPBD), which bind PIP2 and PIP3. The enrichment of activated Cdc42 in SSeCKS-null leading edge filopodia correlated with recruitment of the Cdc42-specific guanine nucleotide exchange factor, Frabin, likely recruited via multiple PIP2/3-binding domains. Frabin knockdown in SSeCKS-null MEF restores leading edge lamellipodia and chemotaxis inhibition. However, SSeCKS failed to co-immunoprecipitate with Rac1, Cdc42 or Frabin. Consistent with the notion that chemotaxis is controlled by SSeCKS-PIP (vs. -Src) scaffolding activity, constitutively-active phosphatidylinositol 3-kinase could override the ability of the Src inhibitor, SKI-606, to suppress chemotaxis and filopodial enrichment of Frabin in SSeCKS-null MEF. Our data suggest a role for SSeCKS in controlling Rac1 vs. Cdc42-induced cellular dynamics at the leading chemotactic edge through the scaffolding of phospholipids and signal mediators, and through the reorganization of the actin cytoskeleton controlling directional movement.

Cdc42 regulates Schwann cell radial sorting and myelin sheath folding through NF2/merlin-dependent and independent signaling

The Rho family GTPase Cdc42 has been implicated in developmental Schwann cell (SC) proliferation, providing sufficient SCs for radial sorting of axons preceding SC differentiation in the peripheral nervous system. We generated Cdc42 conditional knockout (Cdc42-CKO) mice and confirmed aberrant axon sorting in Cdc42-CKO nerves. In adult Cdc42-CKO nerves, blood vessels were enlarged, and mature Remak bundles containing small axons were absent. Abnormal infoldings and outfoldings of myelin sheaths developed in Cdc42-CKO nerves, mimicking pathological features of Charcot-Marie-Tooth (CMT) disease. The NF2/merlin tumor suppressor has been implicated up- and down-stream of Cdc42. In Cdc42-CKO;NF2-del double mutant mice, radial sorting defects seen in Cdc42-CKO nerves were rescued, while changes in myelin sheaths in Cdc42-CKO nerves were not. Phosphorylation of Focal adhesion kinase (FAK) and P-GSK3β, as well as expression of β-catenin were decreased in Cdc42-CKO nerves, and these changes were rescued by NF2/merlin mutation in Cdc42-CKO;NF2-del double mutant mice. Thus, Cdc42 regulates SC radial sorting in vivo through NF2/merlin dependent signaling pathways, while Cdc42 modulation of myelin sheath folding is NF2/merlin independent.

PtdIns5P: news and views of its appearance, disappearance and deeds

Accumulated evidence indicates that PtdIns5P, one of the seven phosphoinositides, found now to be constitutively present in yeast, plants and metazoa, serves as a signaling molecule to modulate pleiotropic cellular functions in both the nucleus and the cytoplasm. The enzymatic routes in biogenesis of basal PtdIns5P have remained incompletely understood. The role for candidate kinase PIKfyve that is principally involved in PtdIns(3,5)P2 production, has been questioned. In this review article we scrutinize the past obstacles that prevented the definitive implication of PIKfyve in PtdIns5P biosynthesis from PtdIns and focus on the recent pharmacological and genetic advancements that now make this conclusion well supported. We further summarize our current knowledge of the diverse stimuli modulating PtdIns5P levels, binding partners and regulated cellular process, with particular reference to the available mechanistic insights for the relevant signaling pathways.

p63RhoGEF-mediated formation of a single polarized lamellipodium is required for chemotactic migration in breast carcinoma cells

Short hairpin RNAs targeting 66 Rho-GEFs were screened for inhibition of chemotaxis. Six Rho-GEFs (p63RhoGEF, Trio, Duet, Net1, Frabin/Fgd4, and AAH33666) were found to be required for the serum-induced chemotactic migration of MDA-MB-231 human breast carcinoma cells. Knockdown of p63RhoGEF suppressed serum-induced RhoA activation and chemotaxis and caused the aberrant formation of multiple lamellipodial protrusions after serum stimulation while control cells formed a single polarized lamellipodium. These results indicate that p63RhoGEF plays a crucial role in serum-induced chemotaxis by limiting lamellipodial protrusion to one direction via RhoA activation.

Myelin is dependent on the Charcot-Marie-Tooth Type 4H disease culprit protein FRABIN/FGD4 in Schwann cells

Studying the function and malfunction of genes and proteins associated with inherited forms of peripheral neuropathies has provided multiple clues to our understanding of myelinated nerves in health and disease. Here, we have generated a mouse model for the peripheral neuropathy Charcot-Marie-Tooth disease type 4H by constitutively disrupting the mouse orthologue of the suspected culprit gene FGD4 that encodes the small RhoGTPase Cdc42-guanine nucleotide exchange factor Frabin. Lack of Frabin/Fgd4 causes dysmyelination in mice in early peripheral nerve development, followed by profound myelin abnormalities and demyelination at later stages. At the age of 60 weeks, this was accompanied by electrophysiological deficits. By crossing mice carrying alleles of Frabin/Fgd4 flanked by loxP sequences with animals expressing Cre recombinase in a cell type-specific manner, we show that Schwann cell-autonomous Frabin/Fgd4 function is essential for proper myelination without detectable primary contributions from neurons. Deletion of Frabin/Fgd4 in Schwann cells of fully myelinated nerve fibres revealed that this protein is not only required for correct nerve development but also for accurate myelin maintenance. Moreover, we established that correct activation of Cdc42 is dependent on Frabin/Fgd4 function in healthy peripheral nerves. Genetic disruption of Cdc42 in Schwann cells of adult myelinated nerves resulted in myelin alterations similar to those observed in Frabin/Fgd4-deficient mice, indicating that Cdc42 and the Frabin/Fgd4-Cdc42 axis are critical for myelin homeostasis. In line with known regulatory roles of Cdc42, we found that Frabin/Fgd4 regulates Schwann cell endocytosis, a process that is increasingly recognized as a relevant mechanism in peripheral nerve pathophysiology. Taken together, our results indicate that regulation of Cdc42 by Frabin/Fgd4 in Schwann cells is critical for the structure and function of the peripheral nervous system. In particular, this regulatory link is continuously required in adult fully myelinated nerve fibres. Thus, mechanisms regulated by Frabin/Fgd4-Cdc42 are promising targets that can help to identify additional regulators of myelin development and homeostasis, which may crucially contribute also to malfunctions in different types of peripheral neuropathies.

Immune and inflammatory pathways are involved in inherent bone marrow ossification

BACKGROUND

Bone marrow plays a key role in bone formation and healing. Although a subset of marrow explants ossifies in vitro without excipient osteoinductive factors, some explants do not undergo ossification. The disparity of outcome suggests a significant heterogeneity in marrow tissue in terms of its capacity to undergo osteogenesis.

QUESTIONS/PURPOSES

We sought to identify: (1) proteins and signaling pathways associated with osteogenesis by contrasting the proteomes of ossified and poorly ossified marrow explants; and (2) temporal changes in proteome and signaling pathways of marrow ossification in the early and late phases of bone formation.

METHODS

Explants of marrow were cultured. Media conditioned by ossified (n = 4) and poorly ossified (n = 4) subsets were collected and proteins unique to each group were identified by proteomic analysis. Proteomic data were processed to assess proteins specific to the early phase (Days 1-14) and late phase (Days 15-28) of the culture period. Pathways involved in bone marrow ossification were identified through bioinformatics.

RESULTS

Twenty-eight proteins were unique to ossified samples and eight were unique to poorly ossified ones. Twelve proteins were expressed during the early phase and 15 proteins were specific to the late phase. Several identified pathways corroborated those reported for bone formation in the literature. Immune and inflammatory pathways were specific to ossified samples.

CONCLUSIONS

The marrow explant model indicates the inflammatory and immune pathways to be an integral part of the osteogenesis process.

Epstein-Barr virus-encoded LMP1 interacts with FGD4 to activate Cdc42 and thereby promote migration of nasopharyngeal carcinoma cells

Epstein-Barr virus (EBV) is closely associated with nasopharyngeal carcinoma (NPC), a human malignancy notorious for its highly metastatic nature. Among EBV-encoded genes, latent membrane protein 1 (LMP1) is expressed in most NPC tissues and exerts oncogenicity by engaging multiple signaling pathways in a ligand-independent manner. LMP1 expression also results in actin cytoskeleton reorganization, which modulates cell morphology and cell motility— cellular process regulated by RhoGTPases, such as Cdc42. Despite the prominent association of Cdc42 activation with tumorigenesis, the molecular basis of Cdc42 activation by LMP1 in NPC cells remains to be elucidated. Here using GST-CBD (active Cdc42-binding domain) as bait in GST pull-down assays to precipitate active Cdc42 from cell lysates, we demonstrated that LMP1 acts through its transmembrane domains to preferentially induce Cdc42 activation in various types of epithelial cells, including NPC cells. Using RNA interference combined with re-introduction experiments, we identified FGD4 (FYVE, RhoGEF and PH domain containing 4) as the GEF (guanine nucleotide exchange factor) responsible for the activation of Cdc42 by LMP1. Serial deletion experiments and co-immunoprecipitation assays further revealed that ectopically expressed FGD4 modulated LMP1-mediated Cdc42 activation by interacting with LMP1. Moreover, LMP1, through its transmembrane domains, directly bound FGD4 and enhanced FGD4 activity toward Cdc42, leading to actin cytoskeleton rearrangement and increased motility of NPC cells. Depletion of FGD4 or Cdc42 significantly reduced (∼50%) the LMP1-stimulated cell motility, an effect that was partially reversed by expression of a constitutively active mutant of Cdc42. Finally, quantitative RT-PCR and immunohistochemistry analyses showed that FGD4 and LMP1 were expressed in NPC tissues, supporting the potential physiologically relevance of this mechanism in NPC. Collectively, our results not only uncover a novel mechanism underlying LMP1-mediated Cdc42 activation, namely LMP1 interaction with FGD4, but also functionally link FGD4 to NPC tumorigenesis.

Epstein-Barr virus (EBV) is closely associated with human malignancies, including nasopharyngeal carcinoma (NPC). Among EBV-expressed genes, latent membrane protein 1 (LMP1) has been detected in most NPC tissues and has the ability to transform cell growth and drive cell migration, both of which are highly associated with tumorigenesis and tumor progression. Previous reports have demonstrated that cell migration primarily involves cytoskeleton rearrangement, and the RhoGTPase Cdc42 is known to actively mediate such rearrangement processes. Using LMP1-expressing NPC cells, we discovered that LMP1 induces Cdc42 activation by directly binding to FGD4, a positive regulator of Cdc42, thereby promoting motility of NPC cells. The observed correlation between FGD4 and LMP1 expression in NPC tissues provides support of physiological relevance. Notably, FGD4 has recently been shown to be responsible for a type of inherited neural disease. Our findings not only provide a novel insight into EBV pathogenesis, but also suggest a role for FGD4 in tumorigenesis.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

FRET imaging and statistical signal processing reveal positive and negative feedback loops regulating the morphology of randomly migrating HT-1080 cells

Cell migration plays an important role in many physiological processes. Rho GTPases (Rac1, Cdc42, RhoA) and phosphatidylinositols have been extensively studied in directional cell migration. However, it remains unclear how Rho GTPases and phosphatidylinositols regulate random cell migration in space and time. We have attempted to address this issue using fluorescence resonance energy transfer (FRET) imaging and statistical signal processing. First, we acquired time-lapse images of random migration of HT-1080 fibrosarcoma cells expressing FRET biosensors of Rho GTPases and phosphatidyl inositols. We developed an image-processing algorithm to extract FRET values and velocities at the leading edge of migrating cells. Auto- and cross-correlation analysis suggested the involvement of feedback regulations among Rac1, phosphatidyl inositols and membrane protrusions. To verify the feedback regulations, we employed an acute inhibition of the signaling pathway with pharmaceutical inhibitors. The inhibition of actin polymerization decreased Rac1 activity, indicating the presence of positive feedback from actin polymerization to Rac1. Furthermore, treatment with PI3-kinase inhibitor induced an adaptation of Rac1 activity, i.e. a transient reduction of Rac1 activity followed by recovery to the basal level. In silico modeling that reproduced the adaptation predicted the existence of a negative feedback loop from Rac1 to actin polymerization. Finally, we identified MLCK as the probable controlling factor in the negative feedback. These findings quantitatively demonstrate positive and negative feedback loops that involve actin, Rac1 and MLCK, and account for the ordered patterns of membrane dynamics observed in randomly migrating cells.

The Aarskog-Scott syndrome protein Fgd1 regulates podosome formation and extracellular matrix remodeling in transforming growth factor β-stimulated aortic endothelial cells

Podosomes are dynamic actin-rich adhesion plasma membrane microdomains endowed with extracellular matrix-degrading activities. In aortic endothelial cells, podosomes are induced by transforming growth factor β (TGF-β), but how this occurs is largely unknown. It is thought that, in endothelial cells, podosomes play a role in vessel remodeling and/or in breaching anatomical barriers. We demonstrate here that, in bovine aortic endothelial cells, that the Cdc42-specific guanine exchange factor (GEF) Fgd1 is expressed and regulated by TGF-β to induce Cdc42-dependent podosome assembly. Within 15 min of TGF-β stimulation, Fgd1, but none of the other tested Cdc42 GEFs, undergoes tyrosine phosphorylation, associates with Cdc42, and translocates to the subcortical cytoskeleton via a cortactin-dependent mechanism. Small interfering RNA-mediated Fgd1 knockdown inhibits TGF-β-induced Cdc42 activation. Fgd1 depletion also reduces podosome formation and associated matrix degradation and these defects are rescued by reexpression of Fgd1. Although overexpression of Fgd1 does not promote podosome formation per se, it enhances TGF-β-induced matrix degradation. Our results identify Fgd1 as a TGF-β-regulated GEF and, as such, the first GEF to be involved in the process of cytokine-induced podosome formation. Our findings reveal the involvement of Fgd1 in endothelial cell biology and open up new avenues to study its role in vascular pathophysiology.

Familial syndrome resembling Aarskog syndrome

Aarskog(-Scott) syndrome (AAS) is characterized by short stature, and facial, limb, and genital anomalies. AAS can be an X-linked condition caused by mutations in the FGD1 gene, but there is evidence that an autosomal dominant or recessive form also exists. We report on a Chinese family in whom several members have manifestations of AAS, but differ in limb anomalies and show additional characteristics. FGD1 sequencing and linkage analysis excluded FGD1 as the cause in this family. A common known submicroscopic chromosome imbalance is less likely. Both autosomal dominant and recessive patterns of inheritance remain possible.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Cell adhesion molecules nectins and associating proteins: Implications for physiology and pathology

Nectins have recently been identified as new cell adhesion molecules (CAMs) consisting of four members. They show immunoglobulin-like structures and exclusively localize at adherens junctions (AJs) between two neighboring cells. During the formation of cell-cell junctions, nectins function in cooperation with or independently of cadherins, major CAMs at AJs. Similar to cadherins, which are linked to the actin cytoskeleton by binding to catenins, nectins also bind to afadin through their C-terminal region and are linked to the actin cytoskeleton. In addition to nectins, there are nectin-like molecules (Necls), which resemble nectins in their structures and consist of five members. Nectins and Necls are involved in the formation of various kinds of cell-cell adhesion, and also play key roles in diverse cellular functions including cell movement, proliferation, survival, and differentiation. Thus, nectins and Necls are crucial for physiology and pathology of multicellular organisms.(Communicated by Shigetada NAKANISHI, M.J.A.).

Homocysteine induces hypophosphorylation of intermediate filaments and reorganization of actin cytoskeleton in C6 glioma cells

In this study, we investigated the actions of high homocysteine (Hcy) levels (100 and 500 microM) on the cytoskeleton of C6 glioma cells. Results showed that the predominant cytoskeletal response was massive formation of actin-containing filopodia at the cell surface that could be related with Cdc42 activation and increased vinculin immunocontent. In cells treated with 100 microM Hcy, folic acid, trolox, and ascorbic acid, totally prevented filopodia formation, while filopodia induced by 500 microM Hcy were prevented by ascorbic acid and attenuated by folic acid and trolox. Moreover, competitive NMDA ionotropic antagonist DL-AP5 totally prevented the formation of filopodia in both 100 and 500 microM Hcy treated cells, while the metabotropic non-selective group I/II antagonist MCPG prevented the effect of 100 microM Hcy but only slightly attenuated the effect induced by of 500 microM Hcy on actin cytoskeleton. The competitive non-NMDA ionotropic antagonist CNQX was not able to prevent the effects of Hcy on the reorganization of actin cytoskeleton in the two concentrations used. Also, Hcy-induced hypophosphorylation of vimentin and glial fibrillary acidic protein (GFAP) and this effect was prevented by DL-AP5, MCPG, and CNQX. In conclusion, our results show that Hcy target the cytoskeleton of C6 cells probably by excitoxicity and/or oxidative stress mechanisms. Therefore, we could propose that the dynamic restructuring of the actin cytoskeleton of glial cells might contribute to the response to the injury provoked by elevated Hcy levels in brain.

The amino acid motif L/IIxxFE defines a novel actin-binding sequence in PDZ-RhoGEF

PDZ-RhoGEF is a member of the regulator family of G protein signaling (RGS) domain-containing RhoGEFs (RGS-RhoGEFs) that link activated heterotrimeric G protein alpha subunits of the G12 family to activation of the small GTPase RhoA. Unique among the RGS-RhoGEFs, PDZ-RhoGEF contains a short sequence that localizes the protein to the actin cytoskeleton. In this report, we demonstrate that the actin-binding domain, located between amino acids 561 and 585, directly binds to F-actin in vitro. Extensive mutagenesis identifies isoleucine 568, isoleucine 569, phenylalanine 572, and glutamic acid 573 as being necessary for binding to actin and for colocalization with the actin cytoskeleton in cells. These results define a novel actin-binding sequence in PDZ-RhoGEF with a critical amino acid motif of IIxxFE. Moreover, sequence analysis identifies a similar actin-binding motif in the N-terminus of the RhoGEF frabin, and as with PDZ-RhoGEF, mutagenesis and actin interaction experiments demonstrate an LIxxFE motif, consisting of the key amino acids leucine 23, isoleucine 24, phenylalanine 27, and glutamic acid 28. Taken together, results with PDZ-RhoGEF and frabin identify a novel actin-binding sequence. Lastly, inducible dimerization of the actin-binding region of PDZ-RhoGEF revealed a dimerization-dependent actin bundling activity in vitro. PDZ-RhoGEF exists in cells as a dimer, raising the possibility that PDZ-RhoGEF could influence actin structure in a manner independent of its ability to activate RhoA.

A novel Frabin (FGD4) nonsense mutation p.R275X associated with phenotypic variability in CMT4H

BACKGROUND

Charcot Marie Tooth (CMT) disease is a heterogeneous group of inherited peripheral motor and sensory neuropathies. CMT4H is an early onset autosomal recessive demyelinating neuropathy. The locus responsible for CMT4H was assigned to chromosome 12p11.21-q13.11 by homozygosity mapping and mutations in the Frabin gene (FGD4 Rho GDP/GTP exchange factor) were subsequently identified in six families.

METHODS

We sequenced the Frabin gene in a cohort of 12 UK CMT families with clinically defined autosomal recessive demyelinating neuropathy.

RESULTS

We identified a novel homozygous Frabin p.R275X mutation in a family from Northern Ireland. The two affected cases in this family had a very slowly progressive neuropathy with both cases remaining ambulant into middle age. Examination of mRNA from lymphoblasts showed that this stop mutation caused very little nonsense mediated mRNA decay and the predominant mRNA species was the mutant form that is likely to be translated into a truncated protein.

CONCLUSIONS

This work extends the understanding of the pathogenesis of Frabin mutation-associated Charcot Marie Tooth (CMT) 4H and suggests that mutations in Frabin should also be considered in ambulant adults with CMT1.