The SH3, HOOK and guanylate kinase-like domains of hDLG are important for its cytoplasmic localization

Evidence for a nuclear role for Drosophila Dlg as a regulator of the NURF complex

Scribble (Scrib), Discs-large (Dlg), and Lethal giant larvae (Lgl) are basolateral regulators of epithelial polarity and tumor suppressors whose molecular mechanisms of action remain unclear. We used proximity biotinylation to identify proteins localized near Dlg in the Drosophila wing imaginal disc epithelium. In addition to expected membrane- and cytoskeleton-associated protein classes, nuclear proteins were prevalent in the resulting mass spectrometry dataset, including all four members of the nucleosome remodeling factor (NURF) chromatin remodeling complex. Subcellular fractionation demonstrated a nuclear pool of Dlg and proximity ligation confirmed its position near the NURF complex. Genetic analysis showed that NURF activity is also required for the overgrowth of dlg tumors, and this growth suppression correlated with a reduction in Hippo pathway gene expression. Together, these data suggest a nuclear role for Dlg in regulating chromatin and transcription through a more direct mechanism than previously thought.

Whole-Exome Sequencing Identified DLG1 as a Candidate Gene for Familial Exudative Vitreoretinopathy

Purpose: Familial exudative vitreoretinopathy (FEVR) is a blinding retinal vascular disease. Clinically, FEVR is characterized by incomplete vascularization of the peripheral retina and pathological neovascularization. Only about 50% of FEVR cases can be explained by known FEVR disease gene variations. This study aimed to identify novel genes associated with the FEVR phenotype and explore their pathogenic mechanisms. Materials and Methods: Exome sequencing analyses were conducted on one Chinese family with FEVR whose affected members did not exhibit pathogenic variants in the known FEVR genes (verified using Sanger sequencing analysis). Functions of the affected proteins were evaluated using reporter assays. Western blot analysis was used to detect mutant protein expression and the genes' pathogenic mechanisms. Results: A rare novel heterozygous variant in DLG1 (c.1792A>G; p.S598G) was identified. The amino acid residues surrounding the identified variant are highly conserved among vertebrates. A luciferase reporter assay revealed that the mutant DLG1 protein DLG1-S598G lost its ability to activate Wnt signaling. Moreover, a knockdown (KD) of DLG1 in human primary retinal endothelial cells impaired tube formation. Mechanistically, DLG1 KD led to a reduction in phosphorylated VEGFR2, an essential receptor for the angiogenic potency that signals the vascular endothelial growth factor molecule. Conclusions: The data reported here demonstrate that DLG1 is a novel candidate gene for FEVR.

Long-distance signaling from synapse to nucleus via protein messengers

The communication between synapses and the cell nucleus has attracted considerable interest for many years. This interest is largely fueled by the idea that synapse-to-nucleus signaling might specifically induce the expression of genes that make long-term memory "stick." However, despite many years of research, it is still essentially unclear how synaptic signals are conveyed to the nucleus, and it remains to a large degree enigmatic how activity-induced gene expression feeds back to synaptic function. In this chapter, we will focus on the activity-dependent synapto-nuclear trafficking of protein messengers and discuss the underlying mechanisms of their retrograde transport and their supposed functional role in neuronal plasticity.

Nucleocytoplasmic protein shuttling: the direct route in synapse-to-nucleus signaling

In neurons multiple signaling pathways converge in the nucleus to regulate the expression of genes associated with long-term structural changes of synapto-dendritic input. Of pivotal importance for this type of transcriptional regulation is synapse-to-nucleus communication. Several studies suggest that the nuclear transport of proteins from synapses is involved in this signaling process, including evidence that synapses contain proteins with nuclear localization sequences and components of the nuclear import machinery. Here, we review the evidence for synapse-to-nucleus signaling by means of retrograde transport of proteins from distal processes. We discuss the mechanisms involved in their translocation and their role in the control of nuclear gene expression. Finally, we summarize the current thinking regarding the functional implications of nuclear signaling and address open questions in this evolving area of neuroscience.

Oncogenic function for the Dlg1 mammalian homolog of the Drosophila discs-large tumor suppressor

The fact that several different human virus oncoproteins, including adenovirus type 9 E4-ORF1, evolved to target the Dlg1 mammalian homolog of the membrane-associated Drosophila discs-large tumor suppressor has implicated this cellular factor in human cancer. Despite a general belief that such interactions function solely to inactivate this suspected human tumor suppressor protein, we demonstrate here that E4-ORF1 specifically requires endogenous Dlg1 to provoke oncogenic activation of phosphatidylinositol 3-kinase (PI3K) in cells. Based on our results, we propose a model wherein E4-ORF1 binding to Dlg1 triggers the resulting complex to translocate to the plasma membrane and, at this site, to promote Ras-mediated PI3K activation. These findings establish the first known function for Dlg1 in virus-mediated cellular transformation and also surprisingly expose a previously unrecognized oncogenic activity encoded by this suspected cellular tumor suppressor gene.

Cdc42 and Par6-PKCzeta regulate the spatially localized association of Dlg1 and APC to control cell polarization

Cell polarization is essential in a wide range of biological processes such as morphogenesis, asymmetric division, and directed migration. In this study, we show that two tumor suppressor proteins, adenomatous polyposis coli (APC) and Dlg1-SAP97, are required for the polarization of migrating astrocytes. Activation of the Par6–PKCζ complex by Cdc42 at the leading edge of migrating cells promotes both the localized association of APC with microtubule plus ends and the assembly of Dlg-containing puncta in the plasma membrane. Biochemical analysis and total internal reflection fluorescence microscopy reveal that the subsequent physical interaction between APC and Dlg1 is required for polarization of the microtubule cytoskeleton.

The mammalian homolog of the Drosophila discs large tumor suppressor protein up-regulates expression of the ELR+ CXC chemokine Scyb5

The mammalian homolog of the Drosophila discs large tumor suppressor protein Dlg functions as a scaffolding protein that facilitates the transmission of diverse signals. In the present study, we attempted to identify the downstream target genes of Dlg, and found that Dlg up-regulates expression of the ELR+ CXC chemokine Scyb5, which has been implicated in the immune system. Our finding suggests that Scyb5 may play an important role in the tumor suppressor function of Dlg.

PDZ domain-binding motif of human T-cell leukemia virus type 1 Tax oncoprotein augments the transforming activity in a rat fibroblast cell line

While human T-cell leukemia virus type 1 (HTLV-1) is associated with the development of adult T-cell leukemia (ATL), HTLV-2 has not been reported to be associated with such malignant leukemias. HTLV-1 Tax1 oncoprotein transforms a rat fibroblast cell line (Rat-1) to form multiple large colonies in soft agar, and this activity is much greater than that of HTLV-2 Tax2. We have demonstrated here that the increased number of transformed colonies induced by Tax1 relative to Tax2 was mediated by a PDZ domain-binding motif (PBM) in Tax1, which is absent in Tax2. Tax1 PBM mediated the interaction of Tax1 with the discs large (Dlg) tumor suppressor containing PDZ domains, and the interaction correlated well with the transforming activities of Tax1 and the mutants. Through this interaction, Tax1 altered the subcellular localization of Dlg from the detergent-soluble to the detergent-insoluble fraction in a fibroblast cell line as well as in HTLV-1-infected T-cell lines. These results suggest that the interaction of Tax1 with PDZ domain protein(s) is critically involved in the transforming activity of Tax1, the activity of which may be a crucial factor in malignant transformation of HTLV-1-infected cells in vivo.

Different isoforms of synapse-associated protein, SAP97, are expressed in the heart and have distinct effects on the voltage-gated K+ channel Kv1.5

The SAP97 isoforms differ by alternatively spliced insertion domains that regulate protein localization and oligomerization. We used reverse transcription-PCR to identify SAP97 isoforms of human and rat myocardium. In Chinese hamster ovary cells, cloned protein expression was studied using Western blot, confocal imaging of green fluorescent protein-tagged proteins, and patch clamp technique. The two main cardiac SAP97 isoforms contained both I3 and I1B inserts and differed by the I1A insert. Both isoforms co-precipitated with hKv1.5 channels. Only the isoform lacking I1A increased the current (by 215 +/- 22%), whatever the level of channel expression. To examine the involvement of the proline-rich I1A insert in the effect of SAP97, a W623F mutation in the Src homology 3 domain was created, and that restored the effect of the SAP97 on current. SAP97 isoform containing an I1A and I2 domain instead of the I3 domain stimulated the current, whereas SAP97 after deletion of the Src homology 3 and guanylate kinase-like domains did not. In cells co-expressing I3(+I1A) or I3(-I1A), green fluorescent protein-tagged Kv1.5 channels were organized in plaque-like structures at the plasma membrane level, whereas intracellular aggregates of channels predominated with the I2 isoform. The two cardiac SAP97 isoforms have different effects on the hKv1.5 current, depending on their capacity to form channel clusters.

Protein 4.1-mediated membrane targeting of human discs large in epithelial cells

Human discs large (hDlg) protein binds to protein 4.1R via a motif encoded by an alternatively spliced exon located between the SH3 and the C-terminal guanylate kinase-like domains. To evaluate the functional significance of protein 4.1R binding for subcellular localization of hDlg in vivo, we expressed full-length recombinant constructs of two naturally occurring isoforms of hDlg termed hDlg-I2 and hDlg-I3. The hDlg-I3 but not the hDlg-I2 isoform binds to the FERM (Four.1-Ezrin-Radixin-Moesin) domain of protein 4.1R in vitro. Upon transient transfection into subconfluent Madine-Darby canine kidney (MDCK) epithelial cells, the hDlg-I3 fused with the green fluorescent protein accumulated predominantly at the plasma membrane of cell-cell contact sites, whereas the hDlg-I2 fusion protein distributed in the cytoplasm. In contrast, in stably transfected confluent MDCK cells, both hDlg-I2 and -I3 isoforms localized efficiently to the lateral membrane, consistent with the previous notion that the N-terminal domain of hDlg mediates its membrane targeting in polarized epithelial cells. We introduced a double mutation (I38A/I40A) into the N-terminal domain of hDlg, which disrupted its interaction with DLG2, a key event in the membrane targeting of hDlg. Interestingly, the hDlg-I2 isoform harboring the I38A/I40A mutation mislocalized from the membrane into cytoplasm. Importantly, the hDlg-I3 isoform with the same mutation localized efficiently to the membrane of confluent MDCK cells. Together, our results demonstrate that in addition to the N-terminal targeting domain, the alternatively spliced I3 insertion plays a critical role in recruiting hDlg to the lateral membrane in epithelial cells via its interaction with protein 4.1R.

Binding of the human homolog of the Drosophila discs large tumor suppressor protein to the mitochondrial ribosomal protein MRP-S34

The human homolog of the Drosophila discs large tumor suppressor protein (hDLG) functions as a scaffolding protein that facilitates the transmission of diverse downstream signals. Here we show that hDLG interacts through its PDZ domains with the carboxy-terminal S/TXV motif of the mitochondrial ribosomal protein S-34 (MRP-S34). Our results suggest that hDLG interacts with MRP-S34 prior to entry of MRP-S34 into the mitochondria and may be involved in the trafficking of MRP-S34.