A novel role for a YXXPhi motif in directing the caveolin-dependent sorting of membrane-spanning proteins

Urea-modulated UT-B urea transporter internalization is clathrin- and caveolae-dependent in infantile hemangioma-derived vascular endothelial cells

The aim of this study was to investigate the manner of urea-modulated UT-B urea transporter (UT) internalization in infantile hemangioma-derived vascular endothelial cells (HemECs). The immunohistochemistry assay was performed to identify infancy hemangioma-derived endothelial cell line (XPTS-1) cells. Cell toxicity was detected with the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay. Quantitative real-time polymerase chain reaction and Western blot analysis were measured to analyze the expression of UT-B. UT-B internalization was observed by confocal microscopy. The clathrin inhibitor chlorpromazine (CPZ) and caveolin endocytic disrupter methyl-β-cyclodextrin (MβCD) were used in XPTS-1 cells transfected with UT-B-GFP to repress endocytosis. Urea-promoted UT-B expression in a concentration-dependent manner in an infantile XPTS-1 cell line. CPZ and MβCD significantly inhibited UT-B protein internalization. The pretreatment of UT-B-GFP cells with adaptor protein2 (AP2)-μ2-siRNA and caveolin-siRNA significantly inhibited UT-B protein internalization. Our findings suggested that urea-mediated UT-B UT internalization is clathrin and caveolae dependent in infantile HemECs.

The amino acid transporter, SLC1A3, is plasma membrane-localised in adipocytes and its activity is insensitive to insulin

The hormone insulin coordinates the catabolism of nutrients by protein phosphorylation. Phosphoproteomic analysis identified insulin-responsive phosphorylation of the Glu/Asp transporter SLC1A3/EAAT1 in adipocytes. The role of SLC1A3 in adipocytes is not well-understood. We show that SLC1A3 is localised to the plasma membrane and the major regulator of acidic amino acid uptake in adipocytes. However, its localisation and activity were unaffected by insulin or mutation of the insulin-regulated phosphosite. The latter was also observed using a heterologous expression system in Xenopus laevis oocytes. Thus, SLC1A3 maintains a constant import of acidic amino acids independently of nutritional status in adipocytes.

Prospects of osteoactivin in tissue regeneration

INTRODUCTION

Osteoactivin (OA) was first discovered in an osteopetrotic rat model using mRNA differential display a decade ago and has been studied recently. OA in bone tissue can directly or indirectly regulate the differentiation of osteoblasts by influencing cell behaviours, such as proliferation and adhesion, as well as inducing serial signal cascades, which would be of great importance in the field of tissue engineering. The results of recent studies have further demonstrated that OA plays a critical role in the differentiation and function of cells, especially in bone formation and fracture healing. Areas covered: The discovery, structure, and function of OA as well as its therapeutic potential in tissue regeneration of bone defects, kidney injury, liver damage, and muscle atrophy. Expert opinion: OA has great potential in promoting the regeneration of damaged tissues, particularly bone tissue, which is supported by a large body of data. Future studies should focus on exploring the underlying mechanism of OA as well as pursuing the ideal form of OA-related regenerative medicine.

Rab43 regulates the sorting of a subset of membrane protein cargo through the medial Golgi

To evaluate the role of cytoplasmic domains of membrane-spanning proteins in directing trafficking through the secretory pathway, we generated fluorescently tagged VSV G tsO45 with either the native G tail (G) or a cytoplasmic tail derived from the chicken AE1-4 anion exchanger (G(AE)). We previously showed that these two proteins progressed through the Golgi with distinct kinetics. To investigate the basis for the differential sorting of G and G(AE), we analyzed the role of several Golgi-associated small GTP-binding proteins and found that Rab43 differentially regulated their transport through the Golgi. We show that the expression of GFP-Rab43 arrested the anterograde transport of G(AE) in a Rab43-positive medial Golgi compartment. GFP-Rab43 expression also inhibited the acquisition of endoH-resistant sugars and the surface delivery of G(AE), as well as the surface delivery of the AE1-4 anion exchanger. In contrast, GFP-Rab43 expression did not affect the glycosylation or surface delivery of G. Unexpectedly, down-regulation of endogenous Rab43 using small interfering RNA resulted in an increase in the accumulation of G(AE) on the cell surface while having minimal effect on the surface levels of G. Our data demonstrate that Rab43 regulates the sorting of a subset of membrane-spanning cargo as they progress through the medial Golgi.

The miR-199-dynamin regulatory axis controls receptor-mediated endocytosis

Small non-coding RNAs (microRNAs) are important regulators of gene expression that modulate many physiological processes; however, their role in regulating intracellular transport remains largely unknown. Intriguingly, we found that the dynamin (DNM) genes, a GTPase family of proteins responsible for endocytosis in eukaryotic cells, encode the conserved miR-199a and miR-199b family of miRNAs within their intronic sequences. Here, we demonstrate that miR-199a and miR-199b regulate endocytic transport by controlling the expression of important mediators of endocytosis such as clathrin heavy chain (CLTC), Rab5A, low-density lipoprotein receptor (LDLR) and caveolin-1 (Cav-1). Importantly, miR-199a-5p and miR-199b-5p overexpression markedly inhibits CLTC, Rab5A, LDLR and Cav-1 expression, thus preventing receptor-mediated endocytosis in human cell lines (Huh7 and HeLa). Of note, miR-199a-5p inhibition increases target gene expression and receptor-mediated endocytosis. Taken together, our work identifies a new mechanism by which microRNAs regulate intracellular trafficking. In particular, we demonstrate that the DNM, miR-199a-5p and miR-199b-5p genes act as a bifunctional locus that regulates endocytosis, thus adding an unexpected layer of complexity in the regulation of intracellular trafficking.

Degradation mechanism of a Golgi-retained distal renal tubular acidosis mutant of the kidney anion exchanger 1 in renal cells

Distal renal tubular acidosis (dRTA) can be caused by mutations in the SLC4A1 gene encoding the anion exchanger 1 (AE1). Both recessive and dominant mutations result in mistrafficking of proteins, preventing them from reaching the basolateral membrane of renal epithelial cells, where their function is needed. In this study, we show that two dRTA mutants are prematurely degraded. Therefore, we investigated the degradation pathway of the kidney AE1 G701D mutant that is retained in the Golgi. Little is known about degradation of nonnative membrane proteins from the Golgi compartments in mammalian cells. We show that the kidney AE1 G701D mutant is polyubiquitylated and degraded by the lysosome and the proteosome. This mutant reaches the plasma membrane, where it is endocytosed and degraded by the lysosome via a mechanism dependent on the peripheral quality control machinery. Furthermore, we show that the function of the mutant is rescued at the cell surface upon inhibition of the lysosome and incubation with a chemical chaperone. We conclude that modulating the peripheral quality control machinery may provide a novel therapeutic option for treatment of patients with dRTA due to a Golgi-retained mutant.

Involvement of Caveolin-1 in CD83 Internalization in Mouse Dendritic Cells

To become potent T-cell stimulators, DCs need to mature. Treatment with soluble CD83 (sCD83) induces immune tolerance and protects against transplant rejection by maintaining dendritic cells in an immature, tolerogenic state. Until now, the mechanism through which sCD83 keeps DCs immature has not been investigated. The internalizing pathway of CD83 was screened by Western blot, and the direct interactions between internalized proteins were verified through coimmunoprecipitation (co-IP) and transmission electron microscopy (TEM). CD83 plasma membrane levels were detected by Western blot using a plasma membrane protein extraction protocol. The changes in CD83 surface levels in DCs were detected by flow cytometry. Caveolin-1 function was detected in a kidney transplant model. In this study, we demonstrated that caveolin-1 could affect CD83 level during endocytosis in mouse DCs. Caveolin-1 coprecipitates with CD83, as demonstrated by co-IP analysis. TEM morphometric analysis of the entire CD83 distribution associated with internalized caveolin-1 demonstrated a significant interaction in cellular vesicles. sCD83 reduces endogenous CD83 plasma membrane levels, and caveolin-1 knockdown reverts CD83 levels in plasma membrane. sCD83 treatment decreases CD83 surface levels in DCs. siRNA to caveolin-1 in DCs inhibits this effect of sCD83. The effects of sCD83-treated DCs were proved in CD1 mice. Knocking down caveolin-1 in DCs obstructs the effects of sCD83 on kidney transplant. In conclusion, our data indicated that a caveolin-dependent endocytic pathway is involved in CD83 internalization in DCs and that caveolin-1 is involved in the activity of DCs.

Clathrin-mediated endocytosis of mammalian erythroid AE1 anion exchanger facilitated by a YXXΦ or a noncanonical YXXXΦ motif in the N-terminal stretch

To explore the roles of the conserved YXXΦ-type motif in the erythroid-specific N-terminal stretch of anion exchanger 1 (AE1), cell surface expression and internalization of various mutants derived from murine erythroid AE1 tagged with an N-terminal enhanced green fluorescent protein and an extracellular FLAG (EGFP-mAE1Flag) were explored in K562 and HEK293 cells. EGFP-mAE1Flag showed rapid internalization, in association with the internalizations of transferrin and the endogenous AE1 chaperone-like protein glycophorin A in K562 cells. Disruption of the conserved Y72VEL sequence markedly reduced the internalization and increased the relative abundance of cell-surface AE1, whereas substitution of the N-terminal region from bovine AE1 that lacks the relevant motif for the corresponding region had less of an effect on internalization. Deletion or substitution mutations of the Y7EDQL sequence in the bovine N-terminal stretch resulted in the decreased internalization of the AE1 proteins. Cell surface biotinylation and deglycosylation studies showed that approximately 30% of the cell-surface EGFP-mAE1Flag and several other mutants was sorted to the plasma membrane without N-glycan maturation in the Golgi apparatus. These findings indicate that the conserved YXXΦ sequence or a noncanonical YXXXΦ sequence in the N-terminal region facilitates the endocytic recycling of erythroid AE1 through a clathrin-mediated pathway.

Internalization of UT-A1 urea transporter is dynamin dependent and mediated by both caveolae- and clathrin-coated pit pathways

Dynamin is a large GTPase involved in several distinct modes of cell endocytosis. In this study, we examined the possible role of dynamin in UT-A1 internalization. The direct relationship of UT-A1 and dynamin was identified by coimmunoprecipitation. UT-A1 has cytosolic NH(2) and COOH termini and a large intracellular loop. Dynamin specifically binds to the intracellular loop of UT-A1, but not the NH(2) and COOH termini. In cell surface biotinylation experiments, coexpression of dynamin and UT-A1 in HEK293 cells resulted in a decrease of UT-A1 cell surface expression. Conversely, cells expressing dynamin mutant K44A, which is deficient in GTP binding, showed an increased accumulation of UT-A1 protein on the cell surface. Cell plasma membrane lipid raft fractionation experiments revealed that blocking endocytosis with dynamin K44A causes UT-A1 protein accumulation in both the lipid raft and nonlipid raft pools, suggesting that both caveolae- and clathrin-mediated mechanisms may be involved in the internalization of UT-A1. This was further supported by 1) small interfering RNA to knock down either caveolin-1 or μ2 reduced UT-A1 internalization in HEK293 cells and 2) inhibition of either the caveolae pathway by methyl-β-cyclodextrin or the clathrin pathway by concanavalin A caused UT-A1 cell membrane accumulation. Functionally, overexpression of dynamin, caveolin, or μ2 decreased UT-A1 urea transport activity and decreased UT-A1 cell surface expression. We conclude that UT-A1 endocytosis is dynamin-dependent and mediated by both caveolae- and clathrin-coated pit pathways.

Molecular physiology and genetics of Na+-independent SLC4 anion exchangers

Plasmalemmal Cl(-)/HCO(3)(-) exchangers are encoded by the SLC4 and SLC26 gene superfamilies, and function to regulate intracellular pH, [Cl(-)] and cell volume. The Cl(-)/HCO(3)(-) exchangers of polarized epithelial cells also contribute to transepithelial secretion and reabsorption of acid-base equivalents and Cl(-). This review focuses on Na(+)-independent electroneutral Cl(-)/HCO(3)(-) exchangers of the SLC4 family. Human SLC4A1/AE1 mutations cause the familial erythroid disorders of spherocytic anemia, stomatocytic anemia and ovalocytosis. A largely discrete set of AE1 mutations causes familial distal renal tubular acidosis. The Slc4a2/Ae2(-/-) mouse dies before weaning with achlorhydria and osteopetrosis. A hypomorphic Ae2(-/-) mouse survives to exhibit male infertility with defective spermatogenesis and a syndrome resembling primary biliary cirrhosis. A human SLC4A3/AE3 polymorphism is associated with seizure disorder, and the Ae3(-/-) mouse has increased seizure susceptibility. The transport mechanism of mammalian SLC4/AE polypeptides is that of electroneutral Cl(-)/anion exchange, but trout erythroid Ae1 also mediates Cl(-) conductance. Erythroid Ae1 may mediate the DIDS-sensitive Cl(-) conductance of mammalian erythrocytes, and, with a single missense mutation, can mediate electrogenic SO(4)(2-)/Cl(-) exchange. AE1 trafficking in polarized cells is regulated by phosphorylation and by interaction with other proteins. AE2 exhibits isoform-specific patterns of acute inhibition by acidic intracellular pH and independently by acidic extracellular pH. In contrast, AE2 is activated by hypertonicity and, in a pH-independent manner, by ammonium and by hypertonicity. A growing body of structure-function and interaction data, together with emerging information about physiological function and structure, is advancing our understanding of SLC4 anion exchangers.