ArhGEF18 regulated Rho signaling in vertebrate retina development

ARHGEF18 can promote BVDV NS5B activation of the host NF-κB signaling pathway by combining with the NS5B-palm domain

Rho guanine nucleotide exchange factor 18 (ARHGEF18) is a member of the Rho guanine nucleotide exchange factor (RhoGEF) family. RhoGEF plays an important role in the occurrence of tumors and neurological diseases; however, its involvement in host cell resistance against pathogenic microorganisms is mostly unknown. Herein, we report that bovine viral diarrhea virus (BVDV) nonstructural protein 5B (NS5B) can activate the nuclear factor kappa B (NF-κB) signaling pathway to induce an immune response. To clarify the functional domains of NS5B that activate NF-κB signaling, the six structural domains of NS5B were expressed separately: NS5B-core, NS5B-finger, NS5B-palm, NS5B-thumb, NS5B-N and NS5B-c domain. We preliminarily determined that the functional domains of NS5B that activate NF-κB signaling are the finger and palm domains. We used a bovine kidney cell cDNA library and yeast two-hybrid technology to identify that the host protein ARHGEF18 interacts with NS5B. Co-immunoprecipitation assays showed that ARHGEF18 interacts strongly with NS5B-palm. Interestingly ARHGEF18 could promote NF-κB signaling activation by BVDV NS5B. In addition silencing ARHGEF18 significantly inhibited NS5B-palm activation of NF-κB signaling. We concluded that ARHGEF18 can bind to BVDV NS5B through the palm domain to activate the NF-κB pathway. These findings provide direct evidence that BVDV NS5B induces immune responses by activating NF-κB signaling.

Whole-Exome Sequencing of 21 Families: Candidate Genes for Early-Onset High Myopia

High myopia is the most severe and pathological form of myopia. It occurs when the spherical refractive error exceeds -6.00 spherical diopters (SDs) or the axial length (AL) of the eye is greater than 26 mm. This article focuses on early-onset high myopia, an increasingly common condition that affects children under 10 years of age and can lead to other serious ocular pathologies. Through the genetic analysis of 21 families with early-onset high myopia, this study seeks to contribute to a better understanding of the role of genetics in this disease and to propose candidate genes. Whole-exome sequencing studies with a panel of genes known to be involved in the pathology were performed in families with inconclusive results: 3% of the variants found were classified as pathogenic, 6% were likely pathogenic and the remaining 91% were variants of uncertain significance. Most of the families in this study were found to have alterations in several of the proposed genes. This suggests a polygenic inheritance of the pathology due to the cumulative effect of the alterations. Further studies are needed to validate and confirm the role of these alterations in the development of early-onset high myopia and its polygenic inheritance.

The WAVE complex drives the morphogenesis of the photoreceptor outer segment cilium

The photoreceptor outer segment is a modified cilium filled with hundreds of flattened "disc" membranes responsible for efficient light capture. To maintain photoreceptor health and functionality, outer segments are continuously renewed through the addition of new discs at their base. This process is driven by branched actin polymerization nucleated by the Arp2/3 complex. To induce actin polymerization, Arp2/3 requires a nucleation promoting factor. Here, we show that the nucleation promoting factor driving disc morphogenesis is the pentameric WAVE complex and identify all protein subunits of this complex. We further demonstrate that the knockout of one of them, WASF3, abolishes actin polymerization at the site of disc morphogenesis leading to formation of disorganized membrane lamellae emanating from the photoreceptor cilium instead of an outer segment. These data establish that, despite the intrinsic ability of photoreceptor ciliary membranes to form lamellar structures, WAVE-dependent actin polymerization is essential for organizing these membranes into a proper outer segment.

Tumor necrosis factor-induced ArhGEF10 selectively activates RhoB contributing to human microvascular endothelial cell tight junction disruption

Capillary endothelial cells (ECs) maintain a semi-permeable barrier between the blood and tissue by forming inter-EC tight junctions (TJs), regulating selective transport of fluid and solutes. Overwhelming inflammation, as occurs in sepsis, disrupts these TJs, leading to leakage of fluid, proteins, and small molecules into the tissues. Mechanistically, disruption of capillary barrier function is mediated by small Rho-GTPases, such as RhoA, -B, and -C, which are activated by guanine nucleotide exchange factors (GEFs) and disrupted by GTPase-activating factors (GAPs). We previously reported that a mutation in a specific RhoB GAP (p190BRhoGAP) underlays a hereditary capillary leak syndrome. Tumor necrosis factor (TNF) treatment disrupts TJs in cultured human microvascular ECs, a model of capillary leak. This response requires new gene transcription and involves increased RhoB activation. However, the specific GEF that activates RhoB in capillary ECs remains unknown. Transcriptional profiling of cultured tight junction-forming human dermal microvascular endothelial cells (HDMECs) revealed that 17 GEFs were significantly induced by TNF. The function of each candidate GEF was assessed by short interfering RNA depletion and trans-endothelial electrical resistance screening. Knockown of ArhGEF10 reduced the TNF-induced loss of barrier which was phenocopied by RhoB or dual ArhGEF10/RhoB knockdown. ArhGEF10 knockdown also reduced the extent of TNF-induced RhoB activation and disruption at tight junctions. In a cell-free assay, immunoisolated ArhGEF10 selectively catalyzed nucleotide exchange to activate RhoB, but not RhoA or RhoC. We conclude ArhGEF10 is a TNF-induced RhoB-selective GEF that mediates TJ disruption and barrier loss in human capillary endothelial cells.

Rho GTPase regulatory proteins in podocytes

The Rho family of small GTPases (Rho GTPases) are the master regulators of the actin cytoskeleton and consist of 22 members. Previous studies implicated dysregulation of Rho GTPases in podocytes in the pathogenesis of proteinuric glomerular diseases. Rho GTPases are primarily regulated by the three families of proteins; guanine nucleotide exchange factors (GEFs; 82 members), GTPase-activating proteins (GAPs; 69 members), and GDP dissociation inhibitors (GDIs; 3 members). Since the regulatory proteins far outnumber their substrate Rho GTPases and act in concert in a cell/context-dependent manner, the upstream regulatory mechanism directing Rho GTPases in podocytes is largely unknown. In this review, we summarize recent advances in the understanding of the role of Rho GTPase regulatory proteins in podocytes, including the known mutations of these proteins that cause proteinuria in humans. We also provide critical appraisal of the in vivo and in vitro studies and identify the knowledge gap in the field that will require further studies.

Association between a Single Nucleotide Polymorphism in the 3'-UTR of ARHGEF18 and the Risk of Nonidiopathic Pulmonary Arterial Hypertension in Chinese Population

ARHGEF18 has been identified as upregulated in the lung tissues of rat models of pulmonary artery hypertension introduced by hypoxia or monocrotaline (MCT). We used online SNP function prediction tools to screen the candidate SNPs that might be associated with the regulation of the ARHGEF18 expression. The result suggested that rs3745357 located in the 3'-untranslated region of ARHGEF18 is probably a genetic modifier in the process. In the present study, we aimed to investigate the association between ARHGEF18 rs3745357 polymorphism and nonidiopathic pulmonary arterial hypertension susceptibility (niPAH). A total of 293 participants were included in the case-control study (117 patients and 176 healthy controls). The rs3745357 variant was discriminated by using cleaved amplification polymorphism (CAP) sequence-tagged site technology. Although the overall allele and genotype frequencies of rs3745357 in niPAH patients were close to those of the control group, significant differences have been identified when we further divided the niPAH patients into subgroups with or without coronary heart disease (CHD). Rs3745357 C allele frequency was significantly higher in niPAH patients without CHD history (p = 0.001), while the frequency was significantly lower in niPAH patients with CHD history (p = 0.017) when compared to control subjects. The distribution of genotype frequencies was also quite different. After adjustment by gender and age, significant differences were found between patients with CHD history and controls. The results suggest that the ARHGEF18 rs3745357 variant may be used as a marker for the genetic susceptibility to niPAH.

Biallelic Mutation of ARHGEF18, Involved in the Determination of Epithelial Apicobasal Polarity, Causes Adult-Onset Retinal Degeneration

Mutations in more than 250 genes are implicated in inherited retinal dystrophy; the encoded proteins are involved in a broad spectrum of pathways. The presence of unsolved families after highly parallel sequencing strategies suggests that further genes remain to be identified. Whole-exome and -genome sequencing studies employed here in large cohorts of affected individuals revealed biallelic mutations in ARHGEF18 in three such individuals. ARHGEF18 encodes ARHGEF18, a guanine nucleotide exchange factor that activates RHOA, a small GTPase protein that is a key component of tight junctions and adherens junctions. This biological pathway is known to be important for retinal development and function, as mutation of CRB1, encoding another component, causes retinal dystrophy. The retinal structure in individuals with ARHGEF18 mutations resembled that seen in subjects with CRB1 mutations. Five mutations were found on six alleles in the three individuals: c.808A>G (p.Thr270Ala), c.1617+5G>A (p.Asp540Glyfs∗63), c.1996C>T (p.Arg666∗), c.2632G>T (p.Glu878∗), and c.2738_2761del (p.Arg913_Glu920del). Functional tests suggest that each disease genotype might retain some ARHGEF18 activity, such that the phenotype described here is not the consequence of nullizygosity. In particular, the p.Thr270Ala missense variant affects a highly conserved residue in the DBL homology domain, which is required for the interaction and activation of RHOA. Previously, knock-out of Arhgef18 in the medaka fish has been shown to cause larval lethality which is preceded by retinal defects that resemble those seen in zebrafish Crumbs complex knock-outs. The findings described here emphasize the peculiar sensitivity of the retina to perturbations of this pathway, which is highlighted as a target for potential therapeutic strategies.

Amyloid Precursor-Like Protein 2 deletion-induced retinal synaptopathy related to congenital stationary night blindness: structural, functional and molecular characteristics

Background

Amyloid precursor protein knockout mice (APP-KO) have impaired differentiation of amacrine and horizontal cells. APP is part of a gene family and its paralogue amyloid precursor-like protein 2 (APLP2) has both shared as well as distinct expression patterns to APP, including in the retina. Given the impact of APP in the retina we investigated how APLP2 expression affected the retina using APLP2 knockout mice (APLP2-KO).

Results

Using histology, morphometric analysis with noninvasive imaging technique and electron microscopy, we showed that APLP2-KO retina displayed abnormal formation of the outer synaptic layer, accompanied with greatly impaired photoreceptor ribbon synapses in adults. Moreover, APLP2-KO displayed a significant decease in ON-bipolar, rod bipolar and type 2 OFF-cone bipolar cells (36, 21 and 63 %, respectively). Reduction of the number of bipolar cells was accompanied with disrupted dendrites, reduced expression of metabotropic glutamate receptor 6 at the dendritic tips and alteration of axon terminals in the OFF laminae of the inner plexiform layer. In contrast, the APP-KO photoreceptor ribbon synapses and bipolar cells were intact. The APLP2-KO retina displayed numerous phenotypic similarities with the congenital stationary night blindness, a non-progressive retinal degeneration disease characterized by the loss of night vision. The pathological phenotypes in the APLP2-KO mouse correlated to altered transcription of genes involved in pre- and postsynatic structure/function, including CACNA1F, GRM6, TRMP1 and Gα0, and a normal scotopic a-wave electroretinogram amplitude, markedly reduced scotopic electroretinogram b-wave and modestly reduced photopic cone response. This confirmed the impaired function of the photoreceptor ribbon synapses and retinal bipolar cells, as is also observed in congenital stationary night blindness. Since congenital stationary night blindness present at birth, we extended our analysis to retinal differentiation and showed impaired differentiation of different bipolar cell subtypes and an altered temporal sequence of development from OFF to ON laminae in the inner plexiform layer. This was associated with the altered expression patterns of bipolar cell generation and differentiation factors, including MATH3, CHX10, VSX1 and OTX2.

Conclusions

These findings demonstrate that APLP2 couples retina development and synaptic genes and present the first evidence that APLP2 expression may be linked to synaptic disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0245-z) contains supplementary material, which is available to authorized users.

Polarity transitions during neurogenesis and germinal zone exit in the developing central nervous system

During neural development, billions of neurons differentiate, polarize, migrate and form synapses in a precisely choreographed sequence. These precise developmental events are accompanied by discreet transitions in cellular polarity. While radial glial neural stem cells are highly polarized, transiently amplifying neural progenitors are less polarized after delaminating from their parental stem cell. Moreover, preceding their radial migration to a final laminar position neural progenitors re-adopt a polarized morphology before they embarking on their journey along a glial guide to the destination where they will fully mature. In this review, we will compare and contrast the key polarity transitions of cells derived from a neuroepithelium to the well-characterized polarity transitions that occur in true epithelia. We will highlight recent advances in the field that shows that neuronal progenitor delamination from germinal zone (GZ) niche shares similarities to an epithelial-mesenchymal transition. Moreover, studies in the cerebellum suggest the acquisition of radial migration and polarity in transiently amplifying neural progenitors share similarities to mesenchymal-epithelial transitions. Where applicable, we will compare and contrast the precise molecular mechanisms used by epithelial cells and neuronal progenitors to control plasticity in cell polarity during their distinct developmental programs.