The serologically defined colon cancer antigen-3 (SDCCAG3) is involved in the regulation of ciliogenesis

DLG1 functions upstream of SDCCAG3 and IFT20 to control ciliary targeting of polycystin-2

Polarized vesicular trafficking directs specific receptors and ion channels to cilia, but the underlying mechanisms are poorly understood. Here we describe a role for DLG1, a core component of the Scribble polarity complex, in regulating ciliary protein trafficking in kidney epithelial cells. Conditional knockout of Dlg1 in mouse kidney causes ciliary elongation and cystogenesis, and cell-based proximity labeling proteomics and fluorescence microscopy show alterations in the ciliary proteome upon loss of DLG1. Specifically, the retromer-associated protein SDCCAG3, IFT20, and polycystin-2 (PC2) are reduced in the cilia of DLG1-deficient cells compared to control cells. This phenotype is recapitulated in vivo and rescuable by re-expression of wild-type DLG1, but not a Congenital Anomalies of the Kidney and Urinary Tract (CAKUT)-associated DLG1 variant, p.T489R. Finally, biochemical approaches and Alpha Fold modelling suggest that SDCCAG3 and IFT20 form a complex that associates, at least indirectly, with DLG1. Our work identifies a key role for DLG1 in regulating ciliary protein composition and suggests that ciliary dysfunction of the p.T489R DLG1 variant may contribute to CAKUT.

DLG1 functions upstream of SDCCAG3 and IFT20 to control ciliary targeting of polycystin-2

Polarized vesicular trafficking directs specific receptors and ion channels to cilia, but the underlying mechanisms are poorly understood. Here we describe a role for DLG1, a core component of the Scribble polarity complex, in regulating ciliary protein trafficking in kidney epithelial cells. Conditional knockout of Dlg1 in mouse kidney caused ciliary elongation and cystogenesis, and cell-based proximity labelling proteomics and fluorescence microscopy showed alterations in the ciliary proteome upon loss of DLG1. Specifically, the retromer-associated protein SDCCAG3, IFT20 and polycystin-2 (PC2) were reduced in cilia of DLG1 deficient cells compared to control cells. This phenotype was recapitulated in vivo and rescuable by re-expression of wildtype DLG1, but not a Congenital Anomalies of the Kidney and Urinary Tract (CAKUT)-associated DLG1 variant, p.T489R. Finally, biochemical approaches and Alpha Fold modelling suggested that SDCCAG3 and IFT20 form a complex that associates, at least indirectly, with DLG1. Our work identifies a key role for DLG1 in regulating ciliary protein composition and suggests that ciliary dysfunction of the p.T489R DLG1 variant may contribute to CAKUT.

Long non-coding RNA lncHUPC1 induced by FOXA1 promotes tumor progression by inhibiting apoptosis via miR-133b/SDCCAG3 in prostate cancer

Long non-coding RNAs (lncRNAs) were confirmed to be involved in regulating various malignant behaviors of tumor cells in prostate cancer (PCa). Using The Cancer Genome Atlas (TCGA) prostate adenocarcinoma datasets, several endogenous competing RNA (ceRNA) networks of lncRNA/miRNA/mRNA associated with the progression-free survival (PFS) and Gleason score (GS) were identified using bioinformatics analysis. lncRNA AC004447.2 (lncHUPC1, ENSG00000269131)/miR-133b/serologically defined colon cancer antigen-3 (SDCCAG3) was a newly identified ceRNA network that affected cell growth and apoptosis in PCa. Using q-PCR, lncHUPC1 and SDCCAG3 were found to be up-regulated in PCa cells, while miR-133b was down-regulated. The same results were found in tissue samples from 70 PCa cases. It was confirmed that the knockdown of lncHUPC1 increased the expression of miR-133b and decreased that of SDCCAG3, which further increased apoptosis and inhibited cell growth, while the miR-133b inhibitor partially reversed these effects. After transfection with miR-133b mimic after lncHUPC1-knockdown, the expression of miR-133b increased while that of SDCCAG3 reduced, and the apoptosis of the cells was more obvious and the growth of the cells was slower. Therefore, lncHUPC1 was confirmed to regulate SDCCAG3 by binding to miR-133b. Additionally, we found that the transcription factor Forkhead Box A1 (FOXA1) directly bound to the promoter of lncHUPC1 to activate it. In conclusion, the ceRNA network of lncHUPC1/miR-133b/SDCCAG3 affected the growth and apoptosis of PCa cells, and FOXA1 may be involved in the process as a transcription factor of lncHUPC1.

The retromer complex regulates C. elegans development and mammalian ciliogenesis

The mammalian retromer consists of subunits VPS26 (either VPS26A or VPS26B), VPS29 and VPS35, and a loosely associated sorting nexin (SNX) heterodimer or a variety of other SNX proteins. Despite involvement in yeast and mammalian cell trafficking, the role of retromer in development is poorly understood, and its impact on primary ciliogenesis remains unknown. Using CRISPR/Cas9 editing, we demonstrate that vps-26-knockout worms have reduced brood sizes, impaired vulval development and decreased body length, all of which have been linked to ciliogenesis defects. Although preliminary studies did not identify worm ciliary defects, and impaired development limited additional ciliogenesis studies, we turned to mammalian cells to investigate the role of retromer in ciliogenesis. VPS35 localized to the primary cilium of mammalian cells, and depletion of VPS26, VPS35, VPS29, SNX1, SNX2, SNX5 or SNX27 led to decreased ciliogenesis. Retromer also coimmunoprecipitated with the centriolar protein, CP110 (also known as CCP110), and was required for its removal from the mother centriole. Herein, we characterize new roles for retromer in C. elegans development and in the regulation of ciliogenesis in mammalian cells, suggesting a novel role for retromer in CP110 removal from the mother centriole.

Regulation of polycystin expression, maturation and trafficking

The major autosomal dominant polycystic kidney disease (ADPKD) genes, PKD1 and PKD2, are wildly expressed at the organ and tissue level. PKD1 encodes polycystin 1 (PC1), a large membrane associated receptor-like protein that can complex with the PKD2 product, PC2. Various cellular locations have been described for both PC1, including the plasma membrane and extracellular vesicles, and PC2, especially the endoplasmic reticulum (ER), but compelling evidence indicates that the primary cilium, a sensory organelle, is the key site for the polycystin complex to prevent PKD. As with other membrane proteins, the ER biogenesis pathway is key to appropriately folding, performing quality control, and exporting fully folded PC1 to the Golgi apparatus. There is a requirement for binding with PC2 and cleavage of PC1 at the GPS for this folding and export to occur. Six different monogenic defects in this pathway lead to cystic disease development, with PC1 apparently particularly sensitive to defects in this general protein processing pathway. Trafficking of membrane proteins, and the polycystins in particular, through the Golgi to the primary cilium have been analyzed in detail, but at this time, there is no clear consensus on a ciliary targeting sequence required to export proteins to the cilium. After transitioning though the trans-Golgi network, polycystin-bearing vesicles are likely sorted to early or recycling endosomes and then transported to the ciliary base, possibly via docking to transition fibers (TF). The membrane-bound polycystin complex then undergoes facilitated trafficking through the transition zone, the diffusion barrier at the base of the cilium, before entering the cilium. Intraflagellar transport (IFT) may be involved in moving the polycystins along the cilia, but data also indicates other mechanisms. The ciliary polycystin complex can be ubiquitinated and removed from cilia by internalization at the ciliary base and may be sent back to the plasma membrane for recycling or to lysosomes for degradation. Monogenic defects in processes regulating the protein composition of cilia are associated with syndromic disorders involving many organ systems, reflecting the pleotropic role of cilia during development and for tissue maintenance. Many of these ciliopathies have renal involvement, likely because of faulty polycystin signaling from cilia. Understanding the expression, maturation and trafficking of the polycystins helps understand PKD pathogenesis and suggests opportunities for therapeutic intervention.

Sdccag3 Promotes Implant Osseointegration during Experimental Hyperlipidemia

Hyperlipidemia adversely affects bone metabolism, often resulting in compromised osseointegration and implant loss. In addition, genetic networks associated with osseointegration have been proposed. Serologically defined colon cancer antigen 3 (Sdccag3) is a novel endosomal protein that functions in actin cytoskeleton remodeling, protein trafficking and secretion, cytokinesis, and apoptosis, but its roles in the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) and in implant osseointegration under hyperlipidemic conditions have not been uncovered. Here, we performed microarray and RNA sequencing analysis to determine the differential expression of the Sdccag3 gene and related noncoding RNAs (ncRNAs) and to assess the long noncoding RNA (lncRNA) MSTRG.97162.4-miR-193a-3p-Sdccag3 coexpression network in bone tissues within the region 0.5 mm around implants in hyperlipidemic rats. In this experiment, we found that Sdccag3 and the previously uncharacterized lncRNA-MSTRG.97162.4 were downregulated during hyperlipidemia, while miR-193a-3p was upregulated. Sdccag3 overexpression increased new trabecular formation, the bone volume/total volume (BV/TV) (1.24-fold), and bone-implant combination ratio (BIC%) (1.26-fold). An RNA pulldown experiment revealed that Sdccag3 protein targeted lncRNA-MSTRG.97162.4 nucleotides 361 to 389. In addition, lncRNA-MSTRG.97162.4 overexpression significantly enhanced Sdccag3 (2.78-fold) expression and increased BV/TV (1.45-fold) and BIC% (1.07-fold) at the bone-implant interface. Taken together, these findings indicate that Sdccag3 overexpression enhances implant osseointegration under hyperlipidemic conditions by binding to lncRNA-MSTRG.97162.4. Furthermore, miR-193a-3p overexpression inhibited lncRNA-MSTRG.97162.4 (0.63-fold) and Sdccag3 (0.88-fold) expression and induced poor implant osseointegration (BV/TV, 0.86-fold; BIC%, 0.82-fold), while miR-193a-3p downregulation produced the opposite results (lncRNA-MSTRG.97162.4, 10.69-fold; Sdccag3, 6.96-fold; BV/TV, 1.20-fold; BIC%, 1.26-fold). Therefore, our findings show that Sdccag3 promotes implant osseointegration, and its related lncRNA-MSTRG.97162.4 and miR-193a-3p play an important role in osseointegration during hyperlipidemia, which might be a promising therapeutic target for improving dental implantation success rates.

Apoptotic signalling targets the post-endocytic sorting machinery of the death receptor Fas/CD95

Fas plays a major role in regulating ligand-induced apoptosis in many cell types. It is well known that several cancers demonstrate reduced cell surface levels of Fas and thus escape a potential control system via ligand-induced apoptosis, although underlying mechanisms are unclear. Here we report that the endosome associated trafficking regulator 1 (ENTR1), controls cell surface levels of Fas and Fas-mediated apoptotic signalling. ENTR1 regulates, via binding to the coiled coil domain protein Dysbindin, the delivery of Fas from endosomes to lysosomes thereby controlling termination of Fas signal transduction. We demonstrate that ENTR1 is cleaved during Fas-induced apoptosis in a caspase-dependent manner revealing an unexpected interplay of apoptotic signalling and regulation of endolysosomal trafficking resulting in a positive feedback signalling-loop. Our data provide insights into the molecular mechanism of Fas post-endocytic trafficking and signalling, opening possible explanations on how cancer cells regulate cell surface levels of death receptors.

Fas is a death receptor that regulates apoptosis in many cell types and is downregulated on the cell surface in many cancers. Here, Sharma et al. show that endosome associated trafficking regulator ENTR1 regulates delivery of Fas to lysosomes, thereby controlling its degradation and signalling.

Analysis of primary cilia in renal tissue and cells

Primary cilia are singular, sensory organelles that extend from the plasma membrane of most quiescent mammalian cells. These slender, microtubule-based organelles receive and transduce extracellular cues and regulate signaling pathways. Primary cilia are critical to the development and function of many tissue types, and mutation of ciliary genes causes multi-system disorders, termed ciliopathies. Notably, renal cystic disease is one of the most common clinical features of ciliopathies, highlighting a central role for primary cilia in the kidney. Additionally, acute kidney injury and chronic kidney disease are associated with altered primary cilia lengths on renal epithelial cells, suggesting ciliary dynamics and renal physiology are linked. Here we describe methods to examine primary cilia in kidney tissue and in cultured renal cells. We include immunofluorescence and scanning electron microscopy to determine ciliary localization of proteins and cilia structure. Further, we detail cellular assays to measure cilia assembly and disassembly, which regulate cilia length.

Retromer associates with the cytoplasmic amino-terminus of polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic human disease, with around 12.5 million people affected worldwide. ADPKD results from mutations in either PKD1 or PKD2, which encode the atypical G-protein coupled receptor polycystin-1 (PC1) and the transient receptor potential channel polycystin-2 (PC2), respectively. Although altered intracellular trafficking of PC1 and PC2 is an underlying feature of ADPKD, the mechanisms which govern vesicular transport of the polycystins through the biosynthetic and endosomal membrane networks remain to be fully elucidated. Here, we describe an interaction between PC2 and retromer, a master controller for the sorting of integral membrane proteins through the endo-lysosomal network. We show that association of PC2 with retromer occurs via a region in the PC2 cytoplasmic amino-terminal domain, independently of the retromer-binding Wiskott-Aldrich syndrome and scar homologue (WASH) complex. Based on observations that retromer preferentially interacts with a trafficking population of PC2, and that ciliary levels of PC1 are reduced upon mutation of key residues required for retromer association in PC2, our data are consistent with the identification of PC2 as a retromer cargo protein.This article has an associated First Person interview with the first author of the paper.

Golgi bypass of ciliary proteins

Primary cilia represent small, yet distinct compartments of the plasma membrane. They are speculated to exercise chemo- and mechanosensory functions and to serve as signaling hubs for crucial pathways such as the Wnt and hedgehog cascades. It is therefore necessary that specific integral membrane proteins, in particular sensors and receptors, are sorted to the cilium and not to the surrounding somatic plasma membrane upon being synthesized at the rough endoplasmic reticulum. Apparently no singular "zip code" for the primary cilium exists but rather several ciliary targeting signals whose biochemical and cell biological implications are just about being unravelled. Among the better understood proteins residing in the primary cilium is polycystin-2 which is mutated in patients suffering from autosomal-dominant polycystic kidney disease. A special case in the context of this review concerns the connecting cilium which serves as the trafficking pathway for proteins involved in visual sensation of retinal photoreceptor cells. In order to efficiently capture photons, the photopigments are organized in discs or membrane invaginations. Mutations in certain proteins involved in these processes lead to retinal degeneration and ultimately to blindness. One example is peripherin/rds which is mutated in the rds (retinal degeneration slow) mouse. The trafficking of peripherin/rds from the inner to the outer segment of photoreceptor cells by way of the connecting cilium also seems to diverge at the Golgi apparatus, and the routes of polycystin-2 and peripherin/rds may represent paradigms of ciliary proteins for the type IV pathway of unconventional protein "secretion". This review is part of a special issue of Seminars in Cell and Developmental Biology edited by Walter Nickel and Catherine Rabouille.

BLOC-1 is required for selective membrane protein trafficking from endosomes to primary cilia

Monis et al. demonstrate that the BLOC-1 protein complex is required for trafficking polycystin-2, but not fibrocystin or smoothened, from endosomes to primary cilia. This is the first demonstration of a role for BLOC-1 in ciliary assembly and emphasizes the complexity of distinct ciliary membrane protein trafficking routes.

Primary cilia perceive the extracellular environment through receptors localized in the ciliary membrane, but mechanisms directing specific proteins to this domain are poorly understood. To address this question, we knocked down proteins potentially important for ciliary membrane targeting and determined how this affects the ciliary trafficking of fibrocystin, polycystin-2, and smoothened. Our analysis showed that fibrocystin and polycystin-2 are dependent on IFT20, GMAP210, and the exocyst complex, while smoothened delivery is largely independent of these components. In addition, we found that polycystin-2, but not smoothened or fibrocystin, requires the biogenesis of lysosome-related organelles complex-1 (BLOC-1) for ciliary delivery. Consistent with the role of BLOC-1 in sorting from the endosome, we find that disrupting the recycling endosome reduces ciliary polycystin-2 and causes its accumulation in the recycling endosome. This is the first demonstration of a role for BLOC-1 in ciliary assembly and highlights the complexity of pathways taken to the cilium.