Endocytosis and Physiology: Insights from Disabled-2 Deficient Mice

dab2 is required for the scavenging function of lymphatic endothelial cells in the zebrafish meninges

To date it is only partially understood how the brain is cleared of waste products resulting from its high metabolic activity, although this process has important implications for the development and progression of neurodegenerative diseases. Lymphatic vessels play a central role in maintaining fluid and tissue homeostasis, and the recent description of meningeal lymphatic vessels within the dura mater of mice, human and zebrafish has raised considerable interest in unraveling the function of these vessels. In zebrafish, brain lymphatic endothelial cells (BLECs) constitute an additional meningeal lymphatic endothelial cell population. These highly endocytically active cells contribute to the clearance of the brain, but the molecular basis of this scavenging activity is only poorly understood. Here, we report on the characterization of zebrafish disabled 2 (dab2) mutants. Embryos lacking maternally provided dab2 show defective venous sprouting from the caudal vein plexus at 26hpf. Furthermore, we show that the cargo-specific adaptor protein is specifically expressed in BLECs, and that BLECs are significantly impeded in their capacity to internalize specific substrates injected into the cerebrospinal fluid upon loss of zygotic dab2. Our work therefore identifies Dab2 as an important member of the molecular machinery mediating the scavenging function of BLECs in the meninges.

Disabled-2, a versatile tissue matrix multifunctional scaffold protein with multifaceted signaling: Unveiling its potential in the cancer battle

A multifunctional scaffold protein termed Disabled-2 (Dab2) has recently gained attention in the scientific community and has emerged as a promising candidate in the realm of cancer research. Dab2 protein is involved in a variety of signaling pathways, due to which its significance in the pathogenesis of several carcinomas has drawn considerable attention. Dab2 is essential for controlling the advancement of cancer because it engages in essential signaling pathways such as the Wnt/β-catenin, epidermal growth factor receptor (EGFR), and transforming growth factor-beta (TGF-β) pathways. Dab2 can also repress epithelial-mesenchymal transition (EMT) which is involved in tumor progression with metastatic expansion and adds another layer of significance to its possible impact on cancer spread. Furthermore, the role of Dab2 in processes such as cell growth, differentiation, apoptosis, invasion, and metastasis has been explored in certain investigative studies suggesting its significance. The present review examines the role of Dab2 in the pathogenesis of various cancer subtypes including breast cancer, ovarian cancer, gastric cancer, prostate cancer, and bladder urothelial carcinoma and also sheds some light on its potential to act as a therapeutic target and a prognostic marker in the treatment of various carcinomas. By deciphering this protein's diverse signaling, we hope to provide useful insights that may pave the way for novel therapeutic techniques and tailored treatment approaches in cancer management. Preclinical and clinical trial data on the impact of Dab2 regulation in cancer have also been included, allowing us to delineate role of Dab2 in tumor suppressor function, as well as its correlation with disease stage classification and potential therapy options. However, we observed that there is very scarce data in the form of studies on the evaluation of Dab2 role and treatment function in carcinomas, and further research into this matter could prove beneficial in the generation of novel therapeutic agents for patient-centric and tailored therapy, as well as early prognosis of carcinomas.

The scaffold protein disabled 2 (DAB2) and its role in tumor development and progression

BACKGROUND

Disabled 2 (DAB2) is a multifunctional protein that has emerged as a critical component in the regulation of tumor growth. Its dysregulation is implicated in various types of cancer, underscoring its importance in understanding the molecular mechanisms underlying tumor development and progression. This review aims to unravel the intricate molecular mechanisms by which DAB2 exerts its tumor-suppressive functions within cancer signaling pathways.

METHODS AND RESULTS

We conducted a comprehensive review of the literature focusing on the structure, expression, physiological functions, and tumor-suppressive roles of DAB2. We provide an overview of the structure, expression, and physiological functions of DAB2. Evidence supporting DAB2's role as a tumor suppressor is explored, highlighting its ability to inhibit cell proliferation, induce apoptosis, and modulate key signaling pathways involved in tumor suppression. The interaction between DAB2 and key oncogenes is examined, elucidating the interplay between DAB2 and oncogenic signaling pathways. We discuss the molecular mechanisms underlying DAB2-mediated tumor suppression, including its involvement in DNA damage response and repair, regulation of cell cycle progression and senescence, and modulation of epithelial-mesenchymal transition (EMT). The review explores the regulatory networks involving DAB2, covering post-translational modifications, interactions with other tumor suppressors, and integration within complex signaling networks. We also highlight the prognostic significance of DAB2 and its role in pre-clinical studies of tumor suppression.

CONCLUSION

This review provides a comprehensive understanding of the molecular mechanisms by which DAB2 exerts its tumor-suppressive functions. It emphasizes the significance of DAB2 in cancer signaling pathways and its potential as a target for future therapeutic interventions.

New insights of platelet endocytosis and its implication for platelet function

Endocytosis constitutes a cellular process in which cells selectively encapsulate surface substances into endocytic vesicles, also known as endosomes, thereby modulating their interaction with the environment. Platelets, as pivotal hematologic elements, play a crucial role not only in regulating coagulation and thrombus formation but also in facilitating tumor invasion and metastasis. Functioning as critical components in the circulatory system, platelets can internalize various endosomal compartments, such as surface receptors, extracellular proteins, small molecules, and pathogens, from the extracellular environment through diverse endocytic pathways, including pinocytosis, phagocytosis, and receptor-mediated endocytosis. We summarize recent advancements in platelet endocytosis, encompassing the catalog of cargoes, regulatory mechanisms, and internal trafficking routes. Furthermore, we describe the influence of endocytosis on platelet regulatory functions and related physiological and pathological processes, aiming to offer foundational insights for future research into platelet endocytosis.

Genetic polymorphism of the Dab2 gene and its association with Type 2 Diabetes Mellitus in the Chinese Uyghur population

Objective

The human Disabled-2 (Dab2) protein is an endocytic adaptor protein, which plays an essential role in endocytosis of transmembrane cargo, including low-density lipoprotein cholesterol (LDL-C). As a candidate gene for dyslipidemia, Dab2 is also involved in the development of type 2 diabetes mellitus(T2DM). The aim of this study was to investigate the effects of genetic variants of the Dab2 gene on the related risk of T2DM in the Uygur and Han populations of Xinjiang, China.

Methods

A total of 2,157 age- and sex-matched individuals (528 T2DM patients and 1,629 controls) were included in this case-control study. Four high frequency SNPs (rs1050903, rs2255280, rs2855512 and rs11959928) of the Dab2 gene were genotyped using an improved multiplex ligation detection reaction (iMLDR) genotyping assay, and the forecast value of the SNP for T2DM was assessed by statistical analysis of clinical data profiles and gene frequencies.

Results

We found that in the Uygur population studied, for both rs2255280 and rs2855512, there were significant differences in the distribution of genotypes (AA/CA/CC), and the recessive model (CC vs. CA + AA) between T2DM patients and the controls (P < 0.05). After adjusting for confounders, the recessive model (CC vs. CA + AA) of both rs2255280 and rs2855512 remained significantly associated with T2DM in this population (rs2255280: OR = 5.303, 95% CI [1.236 to -22.755], P = 0.025; rs2855512: OR = 4.892, 95% CI [1.136 to -21.013], P = 0.033). The genotypes (AA/CA/CC) and recessive models (CC vs. CA + AA) of rs2855512 and rs2255280 were also associated with the plasma glucose and HbA1c levels (all P < 0.05) in this population. There were no significant differences in genotypes, all genetic models, or allele frequencies between the T2DM and control group in the Han population group (all P > 0.05).

Conclusions

The present study suggests that the variation of the Dab2 gene loci rs2255280 and rs2855512 is related to the incidence of T2DM in the Uygur population, but not in the Han population. In this study, these variations in Dab2 were an independent predictor for T2DM in the Uygur population of Xinjiang, China.

Expression Pattern of DAB Adaptor Protein 2 in Left- and Right-Side Colorectal Carcinoma

Left-sided and right-sided colorectal cancer (L-CRC and R-CRC) have relatively different clinical pictures and pathophysiological backgrounds. The aim of this study was to investigate the presence of DAB adapter protein 2 (DAB2) as a potential molecular mechanism that contributes to this diversity in terms of malignancy and responses to therapy. The expression of the suppressor gene DAB2 in colon cancer has already been analyzed, but its significance has not been fully elucidated. Archived samples from 34 patients who underwent colon cancer surgery were included in this study, with 13 patients with low-grade CRC and 21 with high-grade CRC. Twenty of the tumors were R-CRC, while 14 were L-CRC. DAB2 expression was analyzed immunohistochemically in the tumor tissue and the colon resection margin was used as a control. Tumors were divided into L-CRC and R-CRC, with splenic flexure as the cutoff point for each side. The results showed that R-CRC had lower DAB2 protein expression compared to L-CRC (p = 0.01). High-grade tumors had reduced DAB2 expression compared to low-grade tumors (p = 0.02). These results are consistent with the analysis of DAB2 gene expression data that we exported from the TCGA Colon and Rectal Cancer Study (COADREAD). In 736 samples of colon cancer, lower DAB2 gene expression was found in R-CRC compared to L-CRC (p < 0.0001). DAB2 gene expression was significantly higher in the sigmoid colon than in the cecum and ascending colon (p < 0.01). The analysis confirmed a lower expression of the DAB2 in tumors with positive microsatellite instability (p < 0.001). In conclusion, DAB2 has a role in the biological differences between R-CRC and L-CRC and its therapeutic and diagnostic potential needs to be further examined.

Non-inositol 1,4,5-trisphosphate (IP3) receptor IP3-binding proteins

Conventionally, myo-D-inositol 1, 4,5-trisphosphate (IP₃) is thought to exert its second messenger effects through the gating of IP₃R Ca²⁺ release channels, located in Ca²⁺-storage organelles like the endoplasmic reticulum. However, there is considerable indirect evidence to support the concept that IP₃ might interact with other, non-IP₃R proteins within cells. To explore this possibility further, the Protein Data Bank was searched using the term "IP3". This resulted in the retrieval of 203 protein structures, the majority of which were members of the IP₃R/ryanodine receptor superfamily of channels. Only 49 of these structures were complexed with IP₃. These were inspected for their ability to interact with the carbon-1 phosphate of IP₃, since this is the least accessible phosphate group of its precursor, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). This reduced the number of structures retrieved to 35, of which 9 were IP₃Rs. The remaining 26 structures represent a diverse range of proteins, including inositol-lipid metabolizing enzymes, signal transducers, PH domain containing proteins, cytoskeletal anchor proteins, the TRPV4 ion channel, a retroviral Gag protein and fibroblast growth factor 2. Such proteins may impact on IP₃ signalling and its effects on cell-biology. This represents an area open for exploration in the field of IP₃ signalling.

Insights of Endocytosis Signaling in Health and Disease

Endocytosis in mammalian cells is a fundamental cellular machinery that regulates vital physiological processes, such as the absorption of metabolites, release of neurotransmitters, uptake of hormone cellular defense, and delivery of biomolecules across the plasma membrane. A remarkable characteristic of the endocytic machinery is the sequential assembly of the complex proteins at the plasma membrane, followed by internalization and fusion of various biomolecules to different cellular compartments. In all eukaryotic cells, functional characterization of endocytic pathways is based on dynamics of the protein complex and signal transduction modules. To coordinate the assembly and functions of the numerous parts of the endocytic machinery, the endocytic proteins interact significantly within and between the modules. Clathrin-dependent and -independent endocytosis, caveolar pathway, and receptor mediated endocytosis have been attributed to a greater variety of physiological and pathophysiological roles such as, autophagy, metabolism, cell division, apoptosis, cellular defense, and intestinal permeabilization. Notably, any defect or alteration in the endocytic machinery results in the development of pathological consequences associated with human diseases such as cancer, cardiovascular diseases, neurological diseases, and inflammatory diseases. In this review, an in-depth endeavor has been made to illustrate the process of endocytosis, and associated mechanisms describing pathological manifestation associated with dysregulated endocytosis machinery.

Albumin Uptake and Processing by the Proximal Tubule: Physiologic, Pathologic and Therapeutic Implications

For nearly 50 years the proximal tubule (PT) has been known to reabsorb, process, and either catabolize or transcytose albumin from the glomerular filtrate. Innovative techniques and approaches have provided insights into these processes. Several genetic diseases, nonselective PT cell defects, chronic kidney disease (CKD), and acute PT injury lead to significant albuminuria, reaching nephrotic range. Albumin is also known to stimulate PT injury cascades. Thus, the mechanisms of albumin reabsorption, catabolism, and transcytosis are being reexamined with the use of techniques that allow for novel molecular and cellular discoveries. Megalin, a scavenger receptor, cubilin, amnionless, and Dab2 form a nonselective multireceptor complex that mediates albumin binding and uptake and directs proteins for lysosomal degradation after endocytosis. Albumin transcytosis is mediated by a pH-dependent binding affinity to the neonatal Fc receptor (FcRn) in the endosomal compartments. This reclamation pathway rescues albumin from urinary losses and cellular catabolism, extending its serum half-life. Albumin that has been altered by oxidation, glycation, or carbamylation or because of other bound ligands that do not bind to FcRn traffics to the lysosome. This molecular sorting mechanism reclaims physiological albumin and eliminates potentially toxic albumin. The clinical importance of PT albumin metabolism has also increased as albumin is now being used to bind therapeutic agents to extend their half-life and minimize filtration and kidney injury. The purpose of this review is to update and integrate evolving information regarding the reabsorption and processing of albumin by proximal tubule cells including discussion of genetic disorders and therapeutic considerations.

Cubilin, the intrinsic factor-vitamin B12 receptor

Cubilin (CUBN), the intrinsic factor-vitamin B12 receptor is a large endocytic protein involved in various physiological functions: vitamin B12 uptake in the gut; reabsorption of albumin and maturation of vitamin D in the kidney; nutrient delivery during embryonic development. Cubilin is an atypical receptor, peripherally associated to the plasma membrane. The transmembrane proteins amnionless (AMN) and Lrp2/Megalin are the currently known molecular partners contributing to plasma membrane transport and internalization of Cubilin. The role of Cubilin/Amn complex in the handling of vitamin B12 in health and disease has extensively been studied and so is the role of the Cubilin-Lrp2 tandem in renal pathophysiology. Accumulating evidence strongly supports a role of Cubilin in some developmental defects including impaired closure of the neural tube. Are these defects primarily caused by the dysfunction of a specific Cubilin ligand or are they secondary to impaired vitamin B12 or protein uptake? We will present the established Cubilin functions, discuss the developmental data and provide an overview of the emerging implications of Cubilin in the field of cardiovascular disease and cancer pathogenesis.

A network pharmacology-based approach to explore mechanism of action of medicinal herbs for alopecia treatment

Hair loss is one of the most common skin problems experienced by more than half of the world's population. In East Asia, medicinal herbs have been used widely in clinical practice to treat hair loss. Recent studies, including systematic literature reviews, indicate that medicinal herbs may demonstrate potential effects for hair loss treatment. In a previous study, we identified medical herbs used frequently for alopecia treatment. Herein, we explored the potential novel therapeutic mechanisms of 20 vital medicinal herbs for alopecia treatment that could distinguish them from known mechanisms of conventional drugs using network pharmacology analysis methods. We determined the herb-ingredient–target protein networks and ingredient-associated protein (gene)-associated pathway networks and calculated the weighted degree centrality to define the strength of the connections. Data showed that 20 vital medicinal herbs could exert therapeutic effects on alopecia mainly mediated via regulation of various target genes and proteins, including acetylcholinesterase (AChE), phospholipase A2 (PLA2) subtypes, ecto-5-nucleotidase (NTE5), folate receptor (FR), nicotinamide N-methyltransferase (NNMT), and quinolinate phosphoribosyltransferase (QPRT). Findings regarding target genes/proteins and pathways of medicinal herbs associated with alopecia treatment offer insights for further research to better understand the pathogenesis and therapeutic mechanism of medicinal herbs for alopecia treatment with traditional herbal medicine.

Cubilin-, megalin-, and Dab2-dependent transcription revealed by CRISPR/Cas9 knockout in kidney proximal tubule cells

The multiligand receptors megalin (Lrp2) and cubilin (Cubn) and their endocytic adaptor protein Dab2 (Dab2) play essential roles in maintaining the integrity of the apical endocytic pathway of proximal tubule (PT) cells and have complex and poorly understood roles in the development of chronic kidney disease. Here, we used RNA-sequencing and CRISPR/Cas9 knockout (KO) technology in a well-differentiated cell culture model to identify PT-specific transcriptional changes that are directly consequent to the loss of megalin, cubilin, or Dab2 expression. KO of Lrp2 had the greatest transcriptional effect, and nearly all genes whose expression was affected in Cubn KO and Dab2 KO cells were also changed in Lrp2 KO cells. Pathway analysis and more granular inspection of the altered gene profiles suggested changes in pathways with immunomodulatory functions that might trigger the pathological changes observed in KO mice and patients with Donnai-Barrow syndrome. In addition, differences in transcription patterns between Lrp2 and Dab2 KO cells suggested the possibility that altered spatial signaling by aberrantly localized receptors contributes to transcriptional changes upon the disruption of PT endocytic function. A reduction in transcripts encoding sodium-glucose cotransporter isoform 2 was confirmed in Lrp2 KO mouse kidney lysates by quantitative PCR analysis. Our results highlight the role of megalin as a master regulator and coordinator of ion transport, metabolism, and endocytosis in the PT. Compared with the studies in animal models, this approach provides a means to identify PT-specific transcriptional changes that are directly consequent to the loss of these target genes.NEW & NOTEWORTHY Megalin and cubilin receptors together with their adaptor protein Dab2 represent major components of the endocytic machinery responsible for efficient uptake of filtered proteins by the proximal tubule (PT). Dab2 and megalin expression have been implicated as both positive and negative modulators of kidney disease. We used RNA sequencing to knock out CRISPR/Cas9 cubilin, megalin, and Dab2 in highly differentiated PT cells to identify PT-specific changes that are directly consequent to knockout of each component.

Short chain fatty acids delay the development of hepatocellular carcinoma in HBx transgenic mice

Chronic infection with hepatitis B virus (HBV) is a major risk factor for the development of hepatocellular carcinoma (HCC). The HBV encoded oncoprotein, HBx, alters the expression of host genes and the activity of multiple signal transduction pathways that contribute to the pathogenesis of HCC by multiple mechanisms independent of HBV replication. However, it is not clear which pathways are the most relevant therapeutic targets in hepatocarcinogenesis. Short chain fatty acids (SCFAs) have strong anti-inflammatory and anti-neoplastic properties, suggesting that they may block the progression of chronic liver disease (CLD) to HCC, thereby identifying the mechanisms relevant to HCC development. This hypothesis was tested in HBx transgenic (HBxTg) mice fed SCFAs. Groups of HBxTg mice were fed with SCFAs or vehicle from 6 to 9 months of age and then assessed for dysplasia, and from 9 to 12 months of age and then assessed for HCC. Livers from 12 month old mice were then analyzed for changes in gene expression by mass spectrometry-based proteomics. SCFA-fed mice had significantly fewer dysplastic and HCC nodules compared to controls at 9 and 12 months, respectively. Pathway analysis of SCFA-fed mice showed down-regulation of signaling pathways altered by HBx in human CLD and HCC, including those involved in inflammation, phosphatidylinositol 3-kinase, epidermal growth factor, and Ras. SCFA treatment promoted increased expression of the tumor suppressor, disabled homolog 2 (DAB2). DAB2 depresses Ras pathway activity, which is constitutively activated by HBx. SCFAs also reduced cell viability in HBx-transfected cell lines in a dose-dependent manner while the viability of primary human hepatocytes was unaffected. These unique findings demonstrate that SCFAs delay the pathogenesis of CLD and development of HCC, and provide insight into some of the underlying mechanisms that are relevant to pathogenesis in that they are responsive to treatment.

Dynamic conversion of cell sorting patterns in aggregates of embryonic stem cells with differential adhesive affinity

BACKGROUND

Mammalian early development comprises the proliferation, differentiation, and self-assembly of the embryonic cells. The classic experiment undertaken by Townes and Holtfreter demonstrated the ability of dissociated embryonic cells to sort and self-organize spontaneously into the original tissue patterns. Here, we further explored the principles and mechanisms underlying the phenomenon of spontaneous tissue organization by studying aggregation and sorting of mouse embryonic stem (ES) cells with differential adhesive affinity in culture.

RESULTS

As observed previously, in aggregates of wild-type and E-cadherin-deficient ES cells, the cell assemblies exhibited an initial sorting pattern showing wild-type cells engulfed by less adhesive E-cadherin-deficient ES cells, which fits the pattern predicted by the differential adhesive hypothesis proposed by Malcom Steinberg. However, in further study of more mature cell aggregates, the initial sorting pattern reversed, with the highly adhesive wild-type ES cells forming an outer shell enveloping the less adhesive E-cadherin-deficient cells, contradicting Steinberg's sorting principle. The outer wild-type cells of the more mature aggregates did not differentiate into endoderm, which is known to be able to sort to the exterior from previous studies. In contrast to the naive aggregates, the mature aggregates presented polarized, highly adhesive cells at the outer layer. The surface polarity was observed as an actin cap contiguously spanning across the apical surface of multiple adjacent cells, though independent of the formation of tight junctions.

CONCLUSIONS

Our experimental findings suggest that the force of differential adhesive affinity can be overcome by even subtle polarity generated from strong bilateral ligation of highly adhesive cells in determining cell sorting patterns.

Endocytic Adaptors in Cardiovascular Disease

Endocytosis is the process of actively transporting materials into a cell by membrane engulfment. Traditionally, endocytosis was divided into three forms: phagocytosis (cell eating), pinocytosis (cell drinking), and the more selective receptor-mediated endocytosis (clathrin-mediated endocytosis); however, other important endocytic pathways (e.g., caveolin-dependent endocytosis) contribute to the uptake of extracellular substances. In each, the plasma membrane changes shape to allow the ingestion and internalization of materials, resulting in the formation of an intracellular vesicle. While receptor-mediated endocytosis remains the best understood pathway, mammalian cells utilize each form of endocytosis to respond to their environment. Receptor-mediated endocytosis permits the internalization of cell surface receptors and their ligands through a complex membrane invagination process that is facilitated by clathrin and adaptor proteins. Internalized vesicles containing these receptor-ligand cargoes fuse with early endosomes, which can then be recycled back to the plasma membrane, delivered to other cellular compartments, or destined for degradation by fusing with lysosomes. These intracellular fates are largely determined by the interaction of specific cargoes with adaptor proteins, such as the epsins, disabled-homolog 2 (Dab2), the stonin proteins, epidermal growth factor receptor substrate 15, and adaptor protein 2 (AP-2). In this review, we focus on the role of epsins and Dab2 in controlling these sorting processes in the context of cardiovascular disease. In particular, we will focus on the function of epsins and Dab2 in inflammation, cholesterol metabolism, and their fundamental contribution to atherogenicity.

The role of disabled-2 (Dab2) in diseases

Disabled-2 (Dab2/DOC-2) is a mitogen-responsive adaptor protein required for multiple cellular functions. It is involved in many signaling pathways and plays an integral role in vesicular uptake and trafficking, modulating immune function, protein-protein interactions, cellular homeostasis and differentiation, oncogenesis, and inflammatory processes in organ systems. It contains domains for binding to NPXY motif-containing and SH3 domain-containing adapter proteins, phosphoinositides, glycoprotein 100 (gp100, or megalin), integrins, clathrin, and myosin VI. However, the molecular mechanism(s) of Dab2's biological function still remain to be elucidated. In this review, we provide an extensive up-to-date understanding of the function of Dab2 and its regulation in cardiovascular diseases, immune disorders, tumorigenesis, and central nervous system disorders.

Disabled-2: a positive regulator of the early differentiation of myoblasts

Dab2 is an adaptor protein and a tumor suppressor. Our previous study has found that Dab2 was expressed in early differentiating skeletal muscles in mouse embryos. In this study, we determined the role of Dab2 in the skeletal muscle differentiation using C2C12 myoblasts in vitro and Xenopus laevis embryos in vivo. The expression of Dab2 was increased in C2C12 myoblasts during the formation of myotubes in vitro. Knockdown of Dab2 expression in C2C12 myoblasts resulted in a reduction of myotube formation, whereas the myotube formation was enhanced upon overexpression of Dab2. Re-expression of Dab2 in C2C12 myoblasts with downregulated expression of Dab2 restored their capacity to form myotubes. Microarray profiling and subsequent network analyses on the 155 differentially expressed genes after Dab2 knockdown showed that Mef2c was an important myogenic transcription factor regulated by Dab2 through the p38 MAPK pathway. It was also involved in other pathways that are associated with muscular development and functions. In Xenopus embryos developed in vivo, XDab2 was expressed in the myotome of somites where various myogenic markers were also expressed. Knockdown of XDab2 expression with antisense morpholinos downregulated the expression of myogenic markers in somites. In conclusion, this study is the first to provide solid evidence to show that Dab2 is a positive regulator of the early myoblast differentiation.

Epstein-Barr virus miR-BART1-3p suppresses apoptosis and promotes migration of gastric carcinoma cells by targeting DAB2

Although Epstein-Barr virus (EBV) is known to encode over 40 different miRNAs of its own, the roles of most EBV miRNAs remain unknown. Disabled homolog 2 (DAB2) is a putative tumor suppressor, but its role in gastric carcinoma (GC), especially in EBV-associated GC, needs to be clarified. Our qRT-PCR and mRNA microarray results showed that DAB2 expression was down-regulated in EBV-positive GC cells compared to EBV-negative cells. Four BART miRNAs that might target DAB2 were predicted, and we found, using a luciferase reporter assay, that miR-BART1-3p directly targeted the 3'-UTR of DAB2. The miR-BART1-3p transfection decreased DAB2 expression at both mRNA and protein levels, while transfection of an inhibitor of miR-BART1-3p, miR-BART1-3p(i), increased DAB2 expression. In addition, miR-BART1-3p as well as siDAB2 increased migration and decreased apoptosis. Meanwhile, miR-BART1-3p(i) or pcDNA3.1-DAB2 transfection decreased migration and increased apoptosis in EBV-infected GC cells. Furthermore, decreased migration by miR-BART1-3p(i) was abrogated by co-transfected siDAB2, while decreased migration by miR-BART1-3p(i) was further suppressed by a co-transfected DAB2 over-expression vector. Our data suggest that miR-BART1-3p plays an important role in the tumorigenesis of EBV-associated GC by directly targeting DAB2.

Activation and suppression of hematopoietic integrins in hemostasis and immunity

Nolte and Margadant review the current understanding of the activation and inactivation of integrin receptors expressed by hematopoietic cells and the role of these conformational changes in modulating platelet and leukocyte function.

Phosphorylation of Dab2 is involved in inhibited VEGF-VEGFR-2 signaling induced by downregulation of syndecan-1 in glomerular endothelial cell

Disabled-2 (Dab2) and PAR-3 (partitioning defective 3) are reported to play critical roles in maintaining retinal microvascular endothelial cells biology by regulating VEGF-VEGFR-2 signaling. The role of Dab2 and PAR-3 in glomerular endothelial cell (GEnC) is unclear. In this study, we found that, no matter whether with vascular endothelial growth factor (VEGF) treatment or not, decreased expression of Dab2 could lead to cell apoptosis by preventing activation of VEGF-VEGFR-2 signaling in GEnC, accompanied by reduced membrane VEGFR-2 expression. And silencing of PAR-3 gene expression caused increased apoptosis of GEnC by inhibiting activation of VEGF-VEGFR-2 signaling and membrane VEGFR-2 expression. In our previous research, we found that the silencing of syndecan-1 gene expression inhibited VEGF-VEGFR-2 signaling by modulating internalization of VEGFR-2. And our further research demonstrated that downregulation of syndecan-1 lead to no significant change in the expression of Dab2 and PAR-3 both at messenger RNA and protein levels in GEnC, while phosphorylation of Dab2 was significantly increased in GEnC transfected with Dab2 small interfering RNA (siRNA) compared with control siRNA. Atypical protein kinase C (aPKC) could induce phosphorylation of Dab2, thus negatively regulating VEGF-VEGFR-2 signaling. And we found that decreased expression of syndecan-1 lead to activation of aPKC, and aPKC inhibitor treatment could block phosphorylation of Dab2 in GEnC. Besides, aPKC inhibitor treatment could activate VEGF-VGEFR-2 signaling in GEnC transfected with syndecan-1 siRNA in a dose-dependent manner. In conclusion, we speculated that phosphorylation of Dab2 is involved in preventing activation of VEGF-VEGFR-2 signaling in GEnC transfected with syndecan-1 siRNA. This provides a new target for the therapy of GEnC injury and kidney disease.

Endocytic Adaptor Proteins in Health and Disease: Lessons from Model Organisms and Human Mutations

Cells need to exchange material and information with their environment. This is largely achieved via cell-surface receptors which mediate processes ranging from nutrient uptake to signaling responses. Consequently, their surface levels have to be dynamically controlled. Endocytosis constitutes a powerful mechanism to regulate the surface proteome and to recycle vesicular transmembrane proteins that strand at the plasma membrane after exocytosis. For efficient internalization, the cargo proteins need to be linked to the endocytic machinery via adaptor proteins such as the heterotetrameric endocytic adaptor complex AP-2 and a variety of mostly monomeric endocytic adaptors. In line with the importance of endocytosis for nutrient uptake, cell signaling and neurotransmission, animal models and human mutations have revealed that defects in these adaptors are associated with several diseases ranging from metabolic disorders to encephalopathies. This review will discuss the physiological functions of the so far known adaptor proteins and will provide a comprehensive overview of their links to human diseases.

Renal compartment-specific genetic variation analyses identify new pathways in chronic kidney disease

Chronic kidney disease (CKD), a condition in which the kidneys are unable to clear waste products, affects 700 million people globally. Genome-wide association studies (GWASs) have identified sequence variants for CKD; however, the biological basis of these GWAS results remains poorly understood. To address this issue, we created an expression quantitative trait loci (eQTL) atlas for the glomerular and tubular compartments of the human kidney. Through integrating the CKD GWAS with eQTL, single-cell RNA sequencing and regulatory region maps, we identified novel genes for CKD. Putative causal genes were enriched for proximal tubule expression and endolysosomal function, where DAB2, an adaptor protein in the TGF-β pathway, formed a central node. Functional experiments confirmed that reducing Dab2 expression in renal tubules protected mice from CKD. In conclusion, compartment-specific eQTL analysis is an important avenue for the identification of novel genes and cellular pathways involved in CKD development and thus potential new opportunities for its treatment.

FGFR1-mediated protocadherin-15 loading mediates cargo specificity during intraflagellar transport in inner ear hair-cell kinocilia

The mechanosensory hair cells of the inner ear are required for hearing and balance and have a distinctive apical structure, the hair bundle, that converts mechanical stimuli into electrical signals. This structure comprises a single cilium, the kinocilium, lying adjacent to an ensemble of actin-based projections known as stereocilia. Hair bundle polarity depends on kinociliary protocadherin-15 (Pcdh15) localization. Protocadherin-15 is found only in hair-cell kinocilia, and is not localized to the primary cilia of adjacent supporting cells. Thus, Pcdh15 must be specifically targeted and trafficked into the hair-cell kinocilium. Here we show that kinocilial Pcdh15 trafficking relies on cell type-specific coupling to the generic intraflagellar transport (IFT) transport mechanism. We uncover a role for fibroblast growth factor receptor 1 (FGFR1) in loading Pcdh15 onto kinociliary transport particles in hair cells. We find that on activation, FGFR1 binds and phosphorylates Pcdh15. Moreover, we find a previously uncharacterized role for clathrin in coupling this kinocilia-specific cargo with the anterograde IFT-B complex through the adaptor, DAB2. Our results identify a modified ciliary transport pathway used for Pcdh15 transport into the cilium of the inner ear hair cell and coordinated by FGFR1 activity.

Primitive Endoderm Differentiation: From Specification to Epithelialization

At the time of implantation, the mouse blastocyst has developed three cell lineages: the epiblast (Epi), the primitive endoderm (PrE), and the trophectoderm (TE). The PrE and TE are extraembryonic tissues but their interactions with the Epi are critical to sustain embryonic growth, as well as to pattern the embryo. We review here the cellular and molecular events that lead to the production of PrE and Epi lineages and discuss the different hypotheses that are proposed for the induction of these cell types. In the second part, we report the current knowledge about the epithelialization of the PrE.

The ins and outs of endocytic trafficking in platelet functions

PURPOSE OF REVIEW

Although platelet endocytosis has been recognized in granule cargo loading and the trafficking of several platelet surface receptors, its acute physiological relevance is poorly understood as is its mechanism. The present review discusses the current understanding of platelet endocytosis and its implications for platelet function.

RECENT FINDINGS

Recent studies are beginning to identify and define the proteins that mediate platelet endocytosis. These studies have shown that platelets contain different endosomal compartments and may use multiple endocytic routes to take in circulating molecules and surface proteins. The studies have also shown that platelet endocytosis is involved in several aspects of platelet function such as signaling, spreading, and granule cargo loading.

SUMMARY

Mechanistic studies of platelet endocytosis have shown it to be not only involved in granule cargo loading but also in various other platelet functions important for hemostasis and beyond.

LRP1 regulates peroxisome biogenesis and cholesterol homeostasis in oligodendrocytes and is required for proper CNS myelin development and repair

Low-density lipoprotein receptor-related protein-1 (LRP1) is a large endocytic and signaling molecule broadly expressed by neurons and glia. In adult mice, global inducible (Lrp1^flox/flox;CAG-CreER) or oligodendrocyte (OL)-lineage specific ablation (Lrp1^flox/flox;Pdgfra-CreER) of Lrp1 attenuates repair of damaged white matter. In oligodendrocyte progenitor cells (OPCs), Lrp1 is required for cholesterol homeostasis and differentiation into mature OLs. Lrp1-deficient OPC/OLs show a strong increase in the sterol-regulatory element-binding protein-2 yet are unable to maintain normal cholesterol levels, suggesting more global metabolic deficits. Mechanistic studies revealed a decrease in peroxisomal biogenesis factor-2 and fewer peroxisomes in OL processes. Treatment of Lrp1^−/− OPCs with cholesterol or activation of peroxisome proliferator-activated receptor-γ with pioglitazone alone is not sufficient to promote differentiation; however, when combined, cholesterol and pioglitazone enhance OPC differentiation into mature OLs. Collectively, our studies reveal a novel role for Lrp1 in peroxisome biogenesis, lipid homeostasis, and OPC differentiation during white matter development and repair.

News on the molecular regulation and function of hepatic low-density lipoprotein receptor and LDLR-related protein 1

PURPOSE OF REVIEW

Clearing of atherogenic lipoprotein particles by the liver requires hepatic low-density lipoprotein receptor (LDLR) and LDLR-related protein 1 (LRP1). This review highlights recent studies that have expanded our understanding of the molecular regulation and metabolic functions of LDLR and LRP1 in the liver.

RECENT FINDINGS

Various proteins orchestrate the intracellular trafficking of LDLR and LRP1. After internalization, the receptors are redirected via recycling endosomes to the cell surface. Several new endocytic proteins that facilitate the endosomal trafficking of LDLR and consequently the clearance of circulating LDL cholesterol have recently been reported. Mutations in some of these proteins cause hypercholesterolemia in human. In addition, LRP1 controls cellular cholesterol efflux by modulating the expression of ABCA1 and ABCG1, and hepatic LRP1 protects against diet-induced hepatic insulin resistance and steatosis through the regulation of insulin receptor trafficking.

SUMMARY

LDLR and LRP1 have prominent roles in cellular and organismal cholesterol homeostasis. Their functioning, including their trafficking in the cell, is controlled by numerous proteins. Comprehensive studies into the molecular regulation of LDLR and LRP1 trafficking have advanced our fundamental understanding of cholesterol homeostasis, and these insights may lead to novel therapeutic strategies for atherosclerosis, hyperlipidemia and insulin resistance in the future.