The exon junction complex regulates the splicing of cell polarity gene dlg1 to control Wingless signaling in development

Diverse Roles of the Exon Junction Complex Factors in the Cell Cycle, Cancer, and Neurodevelopmental Disorders-Potential for Therapeutic Targeting

The exon junction complex (EJC) plays a crucial role in regulating gene expression at the levels of alternative splicing, translation, mRNA localization, and nonsense-mediated decay (NMD). The EJC is comprised of three core proteins: RNA-binding motif 8A (RBM8A), Mago homolog (MAGOH), eukaryotic initiation factor 4A3 (eIF4A3), and a peripheral EJC factor, metastatic lymph node 51 (MLN51), in addition to other peripheral factors whose structural integration is activity-dependent. The physiological and mechanistic roles of the EJC in contribution to molecular, cellular, and organismal level function continue to be explored for potential insights into genetic or pathological dysfunction. The EJC’s specific role in the cell cycle and its implications in cancer and neurodevelopmental disorders prompt enhanced investigation of the EJC as a potential target for these diseases. In this review, we highlight the current understanding of the EJC’s position in the cell cycle, its relation to cancer and developmental diseases, and potential avenues for therapeutic targeting.

The Physiological Roles of the Exon Junction Complex in Development and Diseases

The exon junction complex (EJC) becomes an increasingly important regulator of early gene expression in the central nervous system (CNS) and other tissues. The EJC is comprised of three core proteins: RNA-binding motif 8A (RBM8A), Mago homolog (MAGOH), eukaryotic initiation factor 4A3 (EIF4A3), and a peripheral EJC factor, metastatic lymph node 51 (MLN51), together with various auxiliary factors. The EJC is assembled specifically at exon-exon junctions on mRNAs, hence the name of the complex. The EJC regulates multiple levels of gene expression, from splicing to translation and mRNA degradation. The functional roles of the EJC have been established as crucial to the normal progress of embryonic and neurological development, with wide ranging implications on molecular, cellular, and organism level function. Dysfunction of the EJC has been implicated in multiple developmental and neurological diseases. In this review, we discuss recent progress on the EJC’s physiological roles.

Eukaryotic initiation factor 4A3 inhibits Wnt/β-catenin signaling and regulates axis formation in zebrafish embryos

A key step in the activation of canonical Wnt signaling is the interaction between β-catenin and Tcf/Lefs that forms the transcription activation complex and facilitates the expression of target genes. Eukaryotic initiation factor 4A3 (EIF4A3) is an ATP-dependent DEAD box-family RNA helicase and acts as a core subunit of the exon junction complex (EJC) to control a series of RNA post-transcriptional processes. In this study, we uncover that EIF4A3 functions as a Wnt inhibitor by interfering with the formation of β-catenin/Tcf transcription activation complex. As Wnt stimulation increases, accumulated β-catenin displaces EIF4A3 from a transcriptional complex with Tcf/Lef, allowing the active complex to facilitate the expression of target genes. In zebrafish embryos, eif4a3 depletion inhibited the development of the dorsal organizer and pattern formation of the anterior neuroectoderm by increasing Wnt/β-catenin signaling. Conversely, overexpression of eif4a3 decreased Wnt/β-catenin signaling and inhibited the formation of the dorsal organizer before gastrulation. Our results reveal previously unreported roles of EIF4A3 in the inhibition of Wnt signaling and the regulation of embryonic development in zebrafish.

A Genetic Screen Links the Disease-Associated Nab2 RNA-Binding Protein to the Planar Cell Polarity Pathway in Drosophila melanogaster

Mutations in the gene encoding the ubiquitously expressed RNA-binding protein ZC3H14 result in a non-syndromic form of autosomal recessive intellectual disability in humans. Studies in Drosophila have defined roles for the ZC3H14 ortholog, Nab2 (aka Drosophila Nab2 or dNab2), in axon guidance and memory due in part to interaction with a second RNA-binding protein, the fly Fragile X homolog Fmr1, and coregulation of shared Nab2-Fmr1 target mRNAs. Despite these advances, neurodevelopmental mechanisms that underlie defective axonogenesis in Nab2 mutants remain undefined. Nab2 null phenotypes in the brain mushroom bodies (MBs) resemble defects caused by alleles that disrupt the planar cell polarity (PCP) pathway, which regulates planar orientation of static and motile cells via a non-canonical arm of the Wnt/Wg pathway. A kinked bristle phenotype in surviving Nab2 mutant adults additionally suggests a defect in F-actin polymerization and bundling, a PCP-regulated processes. To test for Nab2-PCP genetic interactions, a collection of PCP mutant alleles was screened for modification of a rough-eye phenotype produced by Nab2 overexpression in the eye (GMR> Nab2) and, subsequently, for modification of a viability defect among Nab2 nulls. Multiple PCP alleles dominantly modify GMR> Nab2 eye roughening and a subset rescue low survival and thoracic bristle kinking in Nab2 zygotic nulls. Collectively, these genetic interactions identify the PCP pathway as a potential target of the Nab2 RNA-binding protein in developing eye and wing tissues and suggest that altered PCP signaling could contribute to neurological defects that result from loss of Drosophila Nab2 or its vertebrate ortholog ZC3H14.

The exon junction complex is required for stem and progenitor cell maintenance in planarians

Named for its assembly near exon-exon junctions during pre-mRNA splicing, the exon junction complex (EJC) regulates multiple aspects of RNA biochemistry, including export of spliced mRNAs from the nucleus and translation. Transcriptome analyses have revealed broad EJC occupancy of spliced metazoan transcripts, yet inhibition of core subunits has been linked to surprisingly specific phenotypes and a growing number of studies support gene-specific regulatory roles. Here we report results from a classroom-based RNAi screen revealing the EJC is necessary for regeneration in the planarian flatworm Schmidtea mediterranea. RNAi animals rapidly lost the stem and progenitor cells that drive formation of new tissue during both regeneration and cell turnover, but exhibited normal amputation-induced changes in gene expression in differentiated tissues. Together with previous reports that partial loss of EJC function causes stem cell defects in Drosophila and mice, our observations implicate the EJC as a conserved, posttranscriptional regulator of gene expression in stem cell lineages. This work also highlights the combined educational and scientific impacts of discovery-based research in the undergraduate biology curriculum.

Dlg1 activates beta-catenin signaling to regulate retinal angiogenesis and the blood-retina and blood-brain barriers

Beta-catenin (i.e., canonical Wnt) signaling controls CNS angiogenesis and the blood-brain and blood-retina barriers. To explore the role of the Discs large/membrane-associated guanylate kinase (Dlg/MAGUK) family of scaffolding proteins in beta-catenin signaling, we studied vascular endothelial cell (EC)-specific knockout of Dlg1/SAP97. EC-specific loss of Dlg1 produces a retinal vascular phenotype that closely matches the phenotype associated with reduced beta-catenin signaling, synergizes with genetically-directed reductions in beta-catenin signaling components, and can be rescued by stabilizing beta-catenin in ECs. In reporter cells with CRISPR/Cas9-mediated inactivation of Dlg1, transfection of Dlg1 enhances beta-catenin signaling ~4 fold. Surprisingly, Frizzled4, which contains a C-terminal PDZ-binding motif that can bind to Dlg1 PDZ domains, appears to function independently of Dlg1 in vivo. These data expand the repertoire of Dlg/MAGUK family functions to include a role in beta-catenin signaling, and they suggest that proteins other than Frizzled receptors interact with Dlg1 to enhance beta-catenin signaling.

Fruit fly research in China

Served as a model organism over a century, fruit fly has significantly pushed forward the development of global scientific research, including in China. The high similarity in genomic features between fruit fly and human enables this tiny insect to benefit the biomedical studies of human diseases. In the past decades, Chinese biologists have used fruit fly to make numerous achievements on understanding the fundamental questions in many diverse areas of biology. Here, we review some of the recent fruit fly studies in China, and mainly focus on those studies in the fields of stem cell biology, cancer therapy and regeneration medicine, neurological disorders and epigenetics.

Regulation of cellular and PCP signalling by the Scribble polarity module

Since the first identification of the Scribble polarity module proteins as a new class of tumour suppressors that regulate both cell polarity and proliferation, an increasing amount of evidence has uncovered a broader role for Scribble, Dlg and Lgl in the control of fundamental cellular functions and their signalling pathways. Here, we review these findings as well as discuss more specifically the role of the Scribble module in PCP signalling.

Membrane Targeting of Disheveled Can Bypass the Need for Arrow/LRP5

The highly conserved Wnt signaling pathway regulates cell proliferation and differentiation in vertebrates and invertebrates. Upon binding of a Wnt ligand to a receptor of the Fz family, Disheveled (Dsh/Dvl) transduces the signal during canonical and non-canonical Wnt signaling. The specific details of how this process occurs have proven difficult to study, especially as Dsh appears to function as a switch between different branches of Wnt signaling. Here we focus on the membrane-proximal events that occur once Dsh is recruited to the membrane. We show that membrane-tethering of the Dsh protein is sufficient to induce canonical Wnt signaling activation even in the absence of the Wnt co-receptor Arrow/LRP5/6. We map the protein domains required for pathway activation in membrane tethered constructs finding that both the DEP and PDZ domains are dispensable for canonical signaling only in membrane-tethered Dsh, but not in untethered/normal Dsh. These data lead to a signal activation model, where Arrow is required to localize Dsh to the membrane during canonical Wnt signaling placing Dsh downstream of Arrow.

Multifaceted Regulation of Gene Expression by the Apoptosis- and Splicing-Associated Protein Complex and Its Components

The differential deposition of RNA-binding proteins (RBPs) on pre-mRNA mediates the processes of gene expression. One of the complexes containing RBPs that play a crucial part in RNA metabolism is the apoptosis-and splicing-associated protein (ASAP) complex. In this review, we present a summary of the structure of ASAP complex and its localization. Also, we discuss the findings by different groups on various functions of the subunits of the ASAP complex in RNA metabolism. The subunits of the ASAP complex are RNPS1, Acinus and SAP18. Originally, the ASAP complex was thought to link RNA processing with apoptosis. Further studies have shown the role of these components in RNA metabolism of cells, including transcription, splicing, translation and nonsense-mediated mRNA decay (NMD). In transcription, RNPS1 is involved in preventing the formation of R-loop, while Acinus and SAP18 suppress transcription with the help of histone deacetylase. On the one hand, individual components of the ASAP complex, namely RNPS1 and Acinus act as splicing activators, whereas on the other hand, in-vitro assay shows that the ASAP complex behaves as splicing repressor. In addition, the individual members of the ASAP complex associates with the exon junction complex (EJC) to play roles in splicing and translation. RNPS1 increases the translation efficiency by participating in the 3'end processing and polysome association of mRNAs. Similarly, during NMD RNPS1 aids in the recruitment of decay factors by interacting with EJC.