Sorting nexin 9 interacts with dynamin 1 and N-WASP and coordinates synaptic vesicle endocytosis

Out, in and back again: PtdIns(4,5)P(2) regulates cadherin trafficking in epithelial morphogenesis

The morphogenesis of epithelial cells in the tissue microenvironment depends on the regulation of the forces and structures that keep cells in contact with their neighbours. The formation of cell-cell contacts is integral to the establishment and maintenance of epithelial morphogenesis. In epithelial tissues, the misregulation of the signalling pathways that control epithelial polarization induces migratory and invasive cellular phenotypes. Many cellular processes influence cadherin targeting and function, including exocytosis, endocytosis and recycling. However, the localized generation of the lipid messenger PtdIns(4,5)P(2) is emerging as a fundamental signal controlling all of these processes. The PtdIns(4,5)P(2)-generating enzymes, PIPKs (phosphatidylinositol phosphate kinases) are therefore integral to these pathways. By the spatial and temporal targeting of PIPKs via the actions of its functional protein associates, PtdIns(4,5)P(2) is generated at discrete cellular locales to provide the cadherin-trafficking machinery with its required lipid messenger. In the present review, we discuss the involvement of PtdIns(4,5)P(2) and the PIPKs in the regulation of the E-cadherin (epithelial cadherin) exocytic and endocytic machinery, the modulation of actin structures at sites of adhesion, and the direction of cellular pathways which determine the fate of E-cadherin and cell-cell junctions. Recent work is also described that has defined phosphoinositide-mediated E-cadherin regulatory pathways by the use of organismal models.

Differential effects of ethanol in the nucleus accumbens shell of alcohol-preferring (P), alcohol-non-preferring (NP) and Wistar rats: a proteomics study

The objective of this study was to determine the effects of ethanol injections on protein expression in the nucleus accumbens shell (ACB-sh) of alcohol-preferring (P), alcohol-non-preferring (NP) and Wistar (W) rats. Rats were injected for 5 consecutive days with either saline or 1 g/kg ethanol; 24 h after the last injection, rats were killed and brains obtained. Micro-punch samples of the ACB-sh were homogenized; extracted proteins were subjected to trypsin digestion and analyzed with a liquid chromatography-mass spectrometer procedure. Ethanol changed expression levels (1.15-fold or higher) of 128 proteins in NP rats, 22 proteins in P, and 28 proteins in W rats. Few of the changes observed with ethanol treatment for NP rats were observed for P and W rats. Many of the changes occurred in calcium-calmodulin signaling systems, G-protein signaling systems, synaptic structure and histones. Approximately half the changes observed in the ACB-sh of P rats were also observed for W rats. Overall, the results indicate a unique response to ethanol of the ACB-sh of NP rats compared to P and W rats; this unique response may reflect changes in neuronal function in the ACB-sh that could contribute to the low alcohol drinking behavior of the NP line.

Structure and plasticity of Endophilin and Sorting Nexin 9

Endophilin and Sorting Nexin 9 (Snx9) play key roles in endocytosis by membrane curvature sensing and remodeling via their Bin/Amphiphysin/Rvs (BAR) domains. BAR and the related F-BAR domains form dimeric, crescent-shaped units that occur N- or C-terminally to other lipid-binding, adaptor, or catalytic modules. In crystal structures, the PX-BAR unit of Snx9 (Snx9(PX-BAR)) adopts an overall compact, moderately curved conformation. SAXS-based solution studies revealed an alternative, more curved state of Snx9(PX-BAR) in which the PX domains are flexibly connected to the BAR domains, providing a model for how Snx9 exhibits lipid-dependent curvature preferences. In contrast, Endophilin appears to be rigid in solution, and the SH3 domains are located at the distal tips of a BAR domain dimer with fixed curvature. We also observed tip-to-tip interactions between the BAR domains in a trigonal crystal form of Snx9(PX-BAR) reminiscent of functionally important interactions described for F-BAR domains.

SNX9 regulates tubular invagination of the plasma membrane through interaction with actin cytoskeleton and dynamin 2

Dynamic membrane remodeling during intracellular trafficking is controlled by the intricate interplay between lipids and proteins. BAR domains are modules that participate in endocytic processes by binding and deforming the lipid bilayer. Sorting nexin 9 (SNX9), which functions in clathrin-mediated endocytosis, contains a BAR domain, however, the properties of this domain are not well understood. Here we show that SNX9 shares many properties with other BAR domain-containing proteins, such as amphiphysin and endophilin. SNX9 is able to deform the plasma membrane, as well as liposomes, into narrow tubules and recruit N-WASP and dynamin 2 to these tubules via its SH3 domain. SNX9-induced tubulation is antagonized by N-WASP and dynamin 2 while it is enhanced by perturbation of actin dynamics. However, SNX9 also has several unique properties. The tubulating activity requires the BAR and PX domains, as well as the low-complexity (LC) domain, which binds the Arp2/3 complex. SNX9 also binds to PtdIns(4)P-5-kinases via its PX domain and its tubulating activity is regulated by phosphoinositides. In addition, the kinase activity of PtdIns(4)P-5-kinases is stimulated by interaction with SNX9, suggesting a positive feedback interaction between SNX9 and PtdIns(4)P-5-kinases. These results suggest that SNX9 functions in the coordination of membrane remodeling and fission via interactions with actin-regulating proteins, endocytic proteins and PtdIns(4,5)P2-metabolizing enzymes.

The novel sorting nexin SNX33 interferes with cellular PrP formation by modulation of PrP shedding

The cellular prion protein (PrP(c)) is a glycosyl-phosphatidylinositol (GPI)-anchored protein trafficking in the secretory and endocytic pathway and localized mainly at the plasma membrane. Conversion of PrP(c) into its pathogenic isoform PrP(Sc) is associated with pathogenesis and transmission of prion diseases. Intramolecular cleavage in the middle, the extreme C-terminal part or within the GPI anchor and shedding of PrP(c) modulate this conversion process by reducing the substrate for prion formation. These phenomena provide similarities with the processing of amyloid precursor protein in Alzheimer's disease. Sorting nexins are a family of proteins with important functions in protein trafficking. In this study, we investigated the role of the newly described sorting nexin 33 (SNX33) in trafficking and processing of PrP(c). We found that overexpression of SNX33 in neuronal and non-neuronal cell lines resulted in increased shedding of full-length PrP(c) from the plasma membrane and modulated the rate of PrP(c) endocytosis. This was paralleled by reduction of PrP(Sc) formation in persistently and newly infected cells. Using deletion mutants, we demonstrate that production of PrP fragment N1 is not influenced by SNX33. Our data provide new insights into the cellular mechanisms of PrP(c) shedding and show how this can affect cellular PrP(Sc) conversion.

SNX18 is an SNX9 paralog that acts as a membrane tubulator in AP-1-positive endosomal trafficking

SNX9, SNX18 and SNX30 constitute a separate subfamily of PX-BAR-containing sorting nexin (SNX) proteins. We show here that most tissues express all three paralogs, and immunoprecipitation and immunofluorescence experiments demonstrated that the SNX9-family proteins act as individual entities in cells. Their SH3 domains displayed a high selectivity for dynamin 2, and the PX-BAR units had the capacity to tubulate membranes when expressed in HeLa cells. As previously described for the PX-BAR domain of SNX9 (SNX9-PX-BAR), purified SNX18-PX-BAR caused liposome tubulation in vitro and had a binding preference for PtdIns(4,5)P(2). However, contrary to SNX9, which primarily acts in clathrin-mediated endocytosis at the plasma membrane, endogenous SNX18 localized to AP-1- and PACS1-positive endosomal structures, which were devoid of clathrin and resistant to Brefeldin A. Moreover, a gamma-adaptin recognition motif was defined in a low-complexity region of SNX18, and a complex of endogenous SNX18 and AP-1 could be immunoprecipitated after Brefeldin A treatment. Overexpression of SNX18 sequestered AP-1 from peripheral endosomes and resulted in the formation of short SNX18-decorated tubes with distinct dynamin puncta. The results indicate that SNX9-family members make up discrete membrane-scission units together with dynamin, and suggest that SNX18 mediates budding of carriers for AP-1-positive endosomal trafficking.

A novel sorting nexin modulates endocytic trafficking and alpha-secretase cleavage of the amyloid precursor protein

Ectodomain shedding of the amyloid precursor protein (APP) by the two proteases alpha- and beta-secretase is a key regulatory event in the generation of the Alzheimer disease amyloid beta peptide (Abeta). beta-Secretase catalyzes the first step in Abeta generation, whereas alpha-secretase cleaves within the Abeta domain, prevents Abeta generation, and generates a secreted form of APP with neuroprotective properties. At present, little is known about the cellular mechanisms that control APP alpha-secretase cleavage and Abeta generation. To explore the contributory pathways, we carried out an expression cloning screen. We identified a novel member of the sorting nexin (SNX) family of endosomal trafficking proteins, called SNX33, as a new activator of APP alpha-secretase cleavage. SNX33 is a homolog of SNX9 and was found to be a ubiquitously expressed phosphoprotein. Exogenous expression of SNX33 in cultured cells increased APP alpha-secretase cleavage 4-fold but surprisingly had little effect on beta-secretase cleavage. This effect was similar to the expression of the dominant negative dynamin-1 mutant K44A. SNX33 bound the endocytic GTPase dynamin and reduced the rate of APP endocytosis in a dynamin-dependent manner. This led to an increase of APP at the plasma membrane, where alpha-secretase cleavage mostly occurs. In summary, our study identifies SNX33 as a new endocytic protein, which modulates APP endocytosis and APP alpha-secretase cleavage, and demonstrates that the rate of APP endocytosis is a major control factor for APP alpha-secretase cleavage.

Mimicking small G-proteins: an emerging theme from the bacterial virulence arsenal

The identification of the Ras superfamily of small molecular weight GTPases (G-proteins) has opened up new fields in cancer biology, immunity and infectious disease research. Because of their ubiquitous role in cellular homeostasis, small G-proteins are common targets for several pathogens, including bacteria. It is well known that pathogenic bacteria have evolved virulence factors that chemically modify GTPases or directly mimic the activities of key regulatory proteins. However, recent studies now suggest that bacterial 'effector' proteins can also mimic the activities of Ras small G-proteins despite their lack of guanine nucleotide binding or GTPase enzymatic activity. The study of these unique pathogenic strategies continues to reveal novel mechanistic insights into host cellular communication networks and the role of small G-protein signalling during human infectious disease.

SNX9 activities are regulated by multiple phosphoinositides through both PX and BAR domains

Sorting nexin 9 (SNX9) functions at the interface between membrane remodeling and the actin cytoskeleton. In particular, SNX9 links membrane binding to potentiation of N-WASP and dynamin GTPase activities. SNX9 is one of a growing number of proteins that contain two lipid-binding domains, a phox homology (PX) and a Bin1/Amphiphysin/RVS167 (BAR) domain, and localizes to diverse membranes that are enriched in different phosphoinositides. Here, we investigate the mechanism by which SNX9 functions at these varied membrane environments. We show that SNX9 has low-lipid-binding affinity and harnesses a broad range of phosphoinositides to synergistically enhance both dynamin and N-WASP activities. We introduced point mutations in either the PX domain, BAR domain or both that are predicted to disrupt their functions and examined their respective roles in lipid-binding, and dynamin and N-WASP activation. We show that the broad lipid specificity of SNX9 is not because of independent and additive contributions by individual domains. Rather, the two domains appear to function in concert to confer lipid-binding and SNX9's membrane active properties. We also demonstrate that the two domains are differentially required for full SNX9 activity in N-WASP and dynamin regulation, and for localization of SNX9 to clathrin-coated pits and dorsal ruffles. In total, our results suggest that SNX9 can integrate signals from varied lipids through two domains to direct membrane remodeling events at multiple cellular locations.