Phosphatidylinositol 3-kinase and the control of endosome dynamics: new players defined by structural motifs

Intermolecular functional coupling between phosphoinositides and the potassium channel KcsA

Biological membranes are composed of a wide variety of lipids. Phosphoinositides (PIPns) in the membrane inner leaflet only account for a small percentage of the total membrane lipids but modulate the functions of various membrane proteins, including ion channels, which play important roles in cell signaling. KcsA, a prototypical K⁺ channel that is small, simple, and easy to handle, has been broadly examined regarding its crystallography, in silico molecular analysis, and electrophysiology. It has been reported that KcsA activity is regulated by membrane phospholipids, such as phosphatidylglycerol. However, there has been no quantitative analysis of the correlation between direct lipid binding and the functional modification of KcsA, and it is unknown whether PIPns modulate KcsA function. Here, using contact bubble bilayer recording, we observed that the open probability of KcsA increased significantly (from about 10% to 90%) when the membrane inner leaflet contained only a small percentage of PIPns. In addition, we found an increase in the electrophysiological activity of KcsA correlated with a larger number of negative charges on PIPns. We further analyzed the affinity of the direct interaction between PIPns and KcsA using microscale thermophoresis and observed a strong correlation between direct lipid binding and the functional modification of KcsA. In conclusion, our approach was able to reconstruct the direct modification of KcsA by PIPns, and we propose that it can also be applied to elucidate the mechanism of modification of other ion channels by PIPns.

The Nero Lucano Pig Breed: Recovery and Variability

Simple Summary

The reduction of biodiversity determines the loss of species and breeds, with the consequent disappearance of production systems, knowledge, cultures and local traditions. The Nero Lucano pig is a native breed of Southern Italy (Basilicata region) recovered, starting from 2001, because of the high quality of its cured meat products. This study gives a picture of the low genetic variability of this breed. Knowledge of individual inbreeding levels allows for planning of interventions to reduce the negative effects of the low effective population size and, then, improve the efficiency of the actual recovery project.

Abstract

The Nero Lucano (NL) pig is a black coat colored breed characterized by a remarkable ability to adapt to the difficult territory and climatic conditions of Basilicata region in Southern Italy. In the second half of the twentieth century, technological innovation, agricultural evolution, new breeding methods and the demand for increasingly lean meat brought the breed almost to extinction. Only in 2001, thanks to local institutions such as: the Basilicata Region, the University of Basilicata, the Regional Breeders Association and the Medio Basento mountain community, the NL pig returned to populate the area with the consequent possibility to appreciate again its specific cured meat products. We analyzed the pedigrees recorded by the breeders and the Illumina Porcine SNP60 BeadChip genotypes in order to obtain the genetic structure of the NL pig. Results evidenced that this population is characterized by long mean generation intervals (up to 3.5 yr), low effective population size (down to 7.2) and high mean inbreeding coefficients (F_MOL = 0.53, F_ROH = 0.39). This picture highlights the low level of genetic variability and the critical issues to be faced for the complete recovery of this population.

A role for Regulator of G protein Signaling-12 (RGS12) in the balance between myoblast proliferation and differentiation

Regulators of G Protein Signaling (RGS proteins) inhibit G protein-coupled receptor (GPCR) signaling by accelerating the GTP hydrolysis rate of activated Gα subunits. Some RGS proteins exert additional signal modulatory functions, and RGS12 is one such protein, with five additional, functional domains: a PDZ domain, a phosphotyrosine-binding domain, two Ras-binding domains, and a Gα·GDP-binding GoLoco motif. RGS12 expression is temporospatially regulated in developing mouse embryos, with notable expression in somites and developing skeletal muscle. We therefore examined whether RGS12 is involved in the skeletal muscle myogenic program. In the adult mouse, RGS12 is expressed in the tibialis anterior (TA) muscle, and its expression is increased early after cardiotoxin-induced injury, suggesting a role in muscle regeneration. Consistent with a potential role in coordinating myogenic signals, RGS12 is also expressed in primary myoblasts; as these cells undergo differentiation and fusion into myotubes, RGS12 protein abundance is reduced. Myoblasts isolated from mice lacking Rgs12 expression have an impaired ability to differentiate into myotubes ex vivo, suggesting that RGS12 may play a role as a modulator/switch for differentiation. We also assessed the muscle regenerative capacity of mice conditionally deficient in skeletal muscle Rgs12 expression (via Pax7-driven Cre recombinase expression), following cardiotoxin-induced damage to the TA muscle. Eight days post-damage, mice lacking RGS12 in skeletal muscle had attenuated repair of muscle fibers. However, when mice lacking skeletal muscle expression of Rgs12 were cross-bred with mdx mice (a model of human Duchenne muscular dystrophy), no increase in muscle degeneration was observed over time. These data support the hypothesis that RGS12 plays a role in coordinating signals during the myogenic program in select circumstances, but loss of the protein may be compensated for within model syndromes of prolonged bouts of muscle damage and repair.

VEGF-A Regulates Cellular Localization of SR-BI as Well as Transendothelial Transport of HDL but Not LDL

Objective—

Low- and high-density lipoproteins (LDL and HDL) must pass the endothelial layer to exert pro- and antiatherogenic activities, respectively, within the vascular wall. However, the rate-limiting factors that mediate transendothelial transport of lipoproteins are yet little known. Therefore, we performed a high-throughput screen with kinase drug inhibitors to identify modulators of transendothelial LDL and HDL transport.

Approach and Results—

Microscopy-based high-content screening was performed by incubating human aortic endothelial cells with 141 kinase-inhibiting drugs and fluorescent-labeled LDL or HDL. Inhibitors of vascular endothelial growth factor (VEGF) receptors (VEGFR) significantly decreased the uptake of HDL but not LDL. Silencing of VEGF receptor 2 significantly decreased cellular binding, association, and transendothelial transport of 125 I-HDL but not 125 I-LDL. RNA interference with VEGF receptor 1 or VEGF receptor 3 had no effect. Binding, uptake, and transport of HDL but not LDL were strongly reduced in the absence of VEGF-A from the cell culture medium and were restored by the addition of VEGF-A. The restoring effect of VEGF-A on endothelial binding, uptake, and transport of HDL was abrogated by pharmacological inhibition of phosphatidyl-inositol 3 kinase/protein kinase B or p38 mitogen-activated protein kinase, as well as silencing of scavenger receptor BI. Moreover, the presence of VEGF-A was found to be a prerequisite for the localization of scavenger receptor BI in the plasma membrane of endothelial cells.

Conclusions—

The identification of VEGF as a regulatory factor of transendothelial transport of HDL but not LDL supports the concept that the endothelium is a specific and, hence, druggable barrier for the entry of lipoproteins into the vascular wall.

Lipid - Motor Interactions: Soap Opera or Symphony?

Intracellular transport of organelles can be driven by multiple motor proteins that bind to the lipid membrane of the organelle and work as a team. We review present knowledge on how lipids orchestrate the recruitment of motors to a membrane. Looking beyond recruitment, we also discuss how heterogeneity and local mechanical properties of the membrane may influence function of motor-teams. These issues gain importance because phagocytosed pathogens use lipid-centric strategies to manipulate motors and survive in host cells.

Pro-inflammatory properties of shark cartilage supplement

The erosion and breakdown of cartilage is generally recognized to be an integral manifestation of arthritic disease, which is often accompanied by the development and progression of inflammation associated with it. Commercial shark cartilage (SC) is a popular dietary supplement taken for the prevention and/or control of chronic disease, including arthritis. The efficacy of SC in maintaining joint health remains questionable; there is a lack of sufficient reliable information on its effect on immunocompetent cells, and the potential health risks involved have not been adequately assessed. Our earlier in vitro studies showed that SC extracts induce a Th1-type inflammatory cytokine response in human leucocytes, and collagen type II alpha 1 protein was shown to be an active cytokine-inducing component in SC. In this study, we further define the cellular response to SC stimulation by classifying leucocytes into primary and secondary responders employing enriched leucocyte subpopulations. Inhibitors of specific signaling pathways were used to verify the functional effect of SC on specific pathway(s) utilized. Results indicate the monocyte/macrophage as the initially responding cell, followed by lymphocytes and the production of interferon-γ. Chemokines, MCP-1 and RANTES, were produced at significant levels in stimulated leucocyte cultures. Initial cellular activation is likely followed by activation of Jun Kinase and p38 mitogen-activated protein kinase signal transduction pathways. This study presents evidence of significant immunological reactivity of components of commercial SC supplement, which could pose a potential health risk for consumers, particularly those with underlying inflammatory disease such as irritable bowel syndrome and arthritis.

Potential role of ATM in hepatocyte endocytosis of ApoE-deficient, ApoB48-containing lipoprotein in ApoE-deficient mice

Individuals carrying mutations at both ataxia telangiectasia mutated (ATM) gene alleles reportedly have increased plasma cholesterol and triglyceride levels. Previous studies have demonstrated that defective ATM function promotes atherosclerosis. We previously demonstrated that ATM facilitates the clearance of plasma apolipoprotein (Apo)E-deficient, ApoB48-containing (E⁻/B⁴⁸) lipoproteins in ApoE-deficient mice (ApoE^−/− mice). However, to date there is no exact explanation available as to the mechanism(s) through which ATM is involved in the removal of E⁻/B⁴⁸ lipoprotein in ApoE^−/− mice. In this study, to our knowledge, we demonstrate for the first time that heterozygous ATM mutation reduces the hepatocyte uptake of E⁻/B⁴⁸ lipoproteins in ApoE^−/− mice; however, heterozygous ATM mutation did not affect hepatocyte binding to E⁻/B⁴⁸ lipoproteins. Moreover, our results revealed that ATM proteins were localized in the nucleus, early endosomes and late endosomes, but not in the plasma membrane in the hepatocytes of ApoE^−/− mice. In addition, following treatment with the ATM activator, chloroquine, and E⁻/B⁴⁸ lipoproteins, ATM interacted with class III phosphatidylinositol-3-kinases (PI3Ks) and the activated ATM protein enhanced class III PI3K activity. Furthermore, treatment with a class III PI3K inhibitor (LY290042 and 3-MA) attenuated the intracellular total cholesterol accumulation induced by ATM activation. These results provide insight into the mechanisms behind the involvment of ATM in the process of endocytosis of E⁻/B⁴⁸ lipoprotein in ApoE^−/− mice, demonstrating the role of class III PI3K protein.

Organelle membrane proteomics reveals differential influence of mycobacterial lipoglycans on macrophage phagosome maturation and autophagosome accumulation

The mycobacterial cell wall component lipoarabinomannan (LAM) has been described as one of the key virulence factors of Mycobacterium tuberculosis. Modification of the terminal arabinan residues of this lipoglycan with mannose caps in M. tuberculosis or with phosphoinositol caps in Mycobacterium smegmatis results in distinct host immune responses. Given that M. tuberculosis typically persists in the phagosomal vacuole after being phagocytosed by macrophages, we performed a proteomic analysis of that organelle after treatment of macrophages with LAMs purified from the two mycobacterial species. The quantitative changes in phagosomal proteins suggested a distinct role for mannose-capped LAM in modulating protein trafficking pathways that contribute to the arrest of phagosome maturation. Enlightened by our proteomic data, we performed further experiments to show that only the LAM from M. tuberculosis inhibits accumulation of autophagic vacuoles in the macrophage, suggesting a new function for this virulence-associated lipid.

LAM has been described as one of the key virulence factors of Mycobacterium tuberculosis. Modification of the terminal arabinan residues of this lipoglycan with mannose caps in M. tuberculosis or with phosphoinositol caps in Mycobacterium smegmatis results in distinct host immune responses. Given that M. tuberculosis typically persists in the phagosomal vacuole after being phagocytosed by macrophages, we performed a proteomic analysis of that organelle after treatment of macrophages with LAMs purified from the two mycobacterial species.

Internalization of swine vesicular disease virus into cultured cells: a comparative study with foot-and-mouth disease virus

We performed a comparative analysis of the internalization mechanisms used by three viruses causing important vesicular diseases in animals. Swine vesicular disease virus (SVDV) internalization was inhibited by treatments that affected clathrin-mediated endocytosis and required traffic through an endosomal compartment. SVDV particles were found in clathrin-coated pits by electron microscopy and colocalized with markers of early endosomes by confocal microscopy. SVDV infectivity was significantly inhibited by drugs that raised endosomal pH. When compared to foot-and-mouth disease virus (FMDV), which uses clathrin-mediated endocytosis, the early step of SVDV was dependent on the integrity of microtubules. SVDV-productive endocytosis was more sensitive to plasma membrane cholesterol extraction than that of FMDV, and differential cell signaling requirements for virus infection were also found. Vesicular stomatitis virus, a model virus internalized by clathrin-mediated endocytosis, was included as a control of drug treatments. These results suggest that different clathrin-mediated routes are responsible for the internalization of these viruses.

A Trypanosoma cruzi phosphatidylinositol 3-kinase (TcVps34) is involved in osmoregulation and receptor-mediated endocytosis

Trypanosoma cruzi, the etiological agent of Chagas disease, has the ability to respond to a variety of environmental changes during its life cycle both in the insect vector and in the vertebrate host. Because regulation of transcription initiation seems to be nonfunctional in this parasite, it is important to investigate other regulatory mechanisms of adaptation. Regulatory mechanisms at the level of signal transduction pathways involving phosphoinositides are good candidates for this purpose. Here we report the identification of the first phosphatidylinositol 3-kinase (PI3K) in T. cruzi, with similarity with its yeast counterpart, Vps34p. TcVps34 specifically phosphorylates phosphatidylinositol to produce phosphatidylinositol 3-phosphate, thus confirming that it belongs to class III PI3K family. Overexpression of TcVps34 resulted in morphological and functional alterations related to vesicular trafficking. Although inhibition of TcVps34 with specific PI3K inhibitors, such as wortmannin and LY294,000, resulted in reduced regulatory volume decrease after hyposmotic stress, cells overexpressing this enzyme were resistant to these inhibitors. Furthermore, these cells were able to recover their original volume faster than wild type cells when they were submitted to severe hyposmotic stress. In addition, in TcVps34-overexpressing cells, the activities of vacuolar-H+-ATPase and vacuolar H+-pyrophosphatase were altered, suggesting defects in the acidification of intracellular compartments. Furthermore, receptor-mediated endocytosis was partially blocked although fluid phase endocytosis was not affected, confirming a function for TcVps34 in membrane trafficking. Taken together, these results strongly support that TcVps34 plays a prominent role in vital processes for T. cruzi survival such as osmoregulation, acidification, and vesicular trafficking.

Syk associates with clathrin and mediates phosphatidylinositol 3-kinase activation during human rhinovirus internalization

Human rhinovirus (HRV) causes the common cold. The most common acute infection in humans, HRV is a leading cause of exacerbations of asthma and chronic obstruction pulmonary disease because of its ability to exacerbate airway inflammation by altering epithelial cell biology upon binding to its receptor, ICAM-1. ICAM-1 regulates not only viral entry and replication but also signaling pathways that lead to inflammatory mediator production. We recently demonstrated the Syk tyrosine kinase to be an important mediator of HRV-ICAM-1 signaling: Syk regulates replication-independent p38 MAPK activation and IL-8 expression. In leukocytes, Syk regulates receptor-mediated internalization via PI3K. Although PI3K has been shown to regulate HRV-induced IL-8 expression and clathrin-mediated endocytosis of HRV, the role of airway epithelial Syk in this signaling pathway is not known. We postulated that Syk regulates PI3K activation and HRV endocytosis in the airway epithelium. Using confocal microscopy and immunoprecipitation, we demonstrated recruitment of the normally cytosolic Syk to the plasma membrane upon HRV16-ICAM-1 binding, along with Syk-clathrin coassociation. Subsequent incubation at 37 degrees C to permit internalization revealed redistribution of Syk to punctate structures resembling endosomes and colocalization with HRV16. Internalized HRV was not detected in cells overexpressing the kinase inactive Syk(K396R) mutant, indicating that kinase activity was necessary for endocytosis. HRV-induced PI3K activation was dependent on Syk; Syk knockdown by small interfering RNA significantly decreased phosphorylation of the PI3K substrate Akt. Together, these data reveal Syk to be an important mediator of HRV endocytosis and HRV-induced PI3K activation.

Core protein machinery for mammalian phosphatidylinositol 3,5-bisphosphate synthesis and turnover that regulates the progression of endosomal transport. Novel Sac phosphatase joins the ArPIKfyve-PIKfyve complex

Perturbations in phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2)-synthesizing enzymes result in enlarged endocytic organelles from yeast to humans, indicating evolutionarily conserved function of PtdIns(3,5)P2 in endosome-related events. This is reinforced by the structural and functional homology of yeast Vac14 and human Vac14 (ArPIKfyve), which activate yeast and mammalian PtdIns(3,5)P2-producing enzymes, Fab1 and PIKfyve, respectively. In yeast, PtdIns(3,5)P2-specific phosphatase, Fig4, in association with Vac14, turns over PtdIns(3,5)P2, but whether such a mechanism operates in mammalian cells and what the identity of mammalian Fig4 may be are unknown. Here we have identified and characterized Sac3, a Sac domain phosphatase, as the Fig4 mammalian counterpart. Endogenous Sac3, a widespread 97-kDa protein, formed a stable ternary complex with ArPIKfyve and PIKfyve. Concordantly, Sac3 cofractionated and colocalized with ArPIKfyve and PIKfyve. The intrinsic Sac3(WT) phosphatase activity preferably hydrolyzed PtdIns(3,5)P2 in vitro, although the other D5-phosphorylated polyphosphoinositides were also substrates. Ablation of endogenous Sac3 by short interfering RNAs elevated PtdIns(3,5)P2 in (32)P-labeled HEK293 cells. Ectopically expressed Sac3(WT) in COS cells colocalized with and dilated EEA1-positive endosomes, consistent with the PtdIns(3,5)P2 requirement in early endosome dynamics. In vitro reconstitution of carrier vesicle formation from donor early endosomes revealed a gain of function upon Sac3 loss, whereas PIKfyve or ArPIKfyve protein depletion produced a loss of function. These data demonstrate a coupling between the machinery for PtdIns(3,5)P2 synthesis and turnover achieved through a physical assembly of PIKfyve, ArPIKfyve, and Sac3. We suggest that the tight regulation in PtdIns(3,5)P2 homeostasis is mechanistically linked to early endosome dynamics in the course of cargo transport.

Wortmannin delays transfer of human rhinovirus serotype 2 to late endocytic compartments

Human rhinovirus 2 (HRV2) is internalized by members of the low-density lipoprotein receptor family into early endosomes (pH 6.2-6.0) where it dissociates from its receptors. After transfer into late endosomes, the virus undergoes a conformational change and RNA uncoating solely induced by pH < 5.6. Finally, virus capsids are degraded in lysosomes. To investigate the role of phosphatidylinositol 3-kinases (PI3K) in the HRV2 entry route, we used the inhibitor wortmannin. Although virus internalization was not altered by wortmannin, virus accumulated in enlarged early endosomes. Furthermore, the drug delayed HRV2 degradation and viral protein synthesis. Consequently, wortmannin-sensitive PI3K are involved in HRV2 transport from early to late compartments. However, wortmannin had no effect on the titer of infectious virus produced. Our data therefore suggest that virus retained in early endosomes for prolonged time periods can undergo the conformational change that otherwise occurs at pH < or = 5.6 in late endosomes.

Regulation of the desensitization and ion selectivity of ATP-gated P2X2 channels by phosphoinositides

Phosphoinositides (PIP(n)s) are known to regulate the activity of some ion channels. Here we determined that ATP-gated P2X(2) channels also are regulated by PIP(n)s, and investigated the structural background and the unique features of this regulation. We initially used two-electrode voltage clamp to analyse the electrophysiological properties of P2X(2) channels expressed in Xenopus oocytes, and observed that preincubation with wortmannin or LY294002, two PI3K inhibitors, accelerated channel desensitization. K365Q or K369Q mutation of the conserved, positively charged, amino acid residues in the proximal region of the cytoplasmic C-terminal domain also accelerated desensitization, whereas a K365R or K369R mutation did not. We observed that the permeability of the channel to N-methyl-d-glucamine (NMDG) transiently increased and then decreased after ATP application, and that the speed of the decrease was accelerated by K365Q or K369Q mutation or PI3K inhibition. Using GST-tagged recombinant proteins spanning the proximal C-terminal region, we then analysed their binding of the P2X(2) cytoplasmic domain to anionic lipids using PIP(n)s-coated nitrocellulose membranes. We found that the recombinant proteins that included the positively charged region bound to PIPs and PIP(2)s, and that this binding was eliminated by the K365Q and K369Q mutations. We also used a fluorescence assay to confirm that fusion proteins comprising the proximal C-terminal region of P2X(2) with EGFP expressed in COS-7 cells closely associated with the membrane. Taken together, these results show that membrane-bound PIP(n)s play a key role in maintaining channel activity and regulating pore dilation through electrostatic interaction with the proximal region of the P2X(2) cytoplasmic C-terminal domain.

Phosphatidylinositol-3-kinase regulates scavenger receptor class B type I subcellular localization and selective lipid uptake in hepatocytes

OBJECTIVE

The high-density lipoprotein (HDL) receptor scavenger receptor Class B type I (SR-BI) plays a key role in mediating the final step of reverse cholesterol transport. This study examined the possible regulation of hepatic SR-BI by phosphatidylinositol-3-kinase (PI3K), a well known regulator of endocytosis and membrane protein trafficking.

METHODS AND RESULTS

SR-BI-dependent HDL selective cholesterol ester uptake in human HepG2 hepatoma cells was decreased (approximately 50%) by the PI3K inhibitors wortmannin and LY294002. Insulin increased selective uptake (approximately 30%), and this increase was blocked by PI3K inhibitors. Changes in SR-BI activity could be accounted for by pronounced changes in the subcellular localization and cell surface expression of SR-BI as determined by HDL cell surface binding, receptor biotinylation studies, and confocal fluorescence microscopy of HepG2 cells expressing green fluorescent protein-tagged SR-BI. Thus, under conditions of PI3K activation by insulin, and to a lesser extent by the SR-BI ligand HDL, cell surface expression of SR-BI was promoted, resulting in increased SR-BI-mediated HDL selective lipid uptake.

CONCLUSIONS

Our data indicate that PI3K activation stimulates hepatic SR-BI function post-translationally by regulating the subcellular localization of SR-BI in a P13K-dependent manner. Decreased hepatocyte PI3K activity in insulin-resistant states, such as type 2 diabetes, obesity, or metabolic syndrome, may impair reverse cholesterol transport by reducing cell surface expression of SR-BI.

Gene silencing reveals a specific function of hVps34 phosphatidylinositol 3-kinase in late versus early endosomes

The human type III phosphatidylinositol 3-kinase, hVps34, converts phosphatidylinositol (PtdIns) to phosphatidylinositol 3-phosphate [PtdIns(3)P]. Studies using inhibitors of phosphatidylinositide 3-kinases have indicated that production of PtdIns(3)P is important for a variety of vesicle-mediated trafficking events, including endocytosis, sorting of receptors in multivesicular endosomes, and transport of lysosomal enzymes from the trans-Golgi network (TGN) to the endosomes and lysosomes. This study utilizes small interfering (si)RNA-mediated gene silencing to define the specific trafficking pathways in which hVps34 functions in human U-251 glioblastoma cells. Suppression of hVps34 expression reduced the cellular growth rate and caused a striking accumulation of large acidic phase-lucent vacuoles that contain lysosomal membrane proteins LAMP1 and LGP85. Analysis of these structures by electron microscopy suggests that they represent swollen late endosomes that have lost the capacity for inward vesiculation but retain the capacity to fuse with lysosomes. Morphological perturbation of the late endosome compartment was accompanied by a reduced rate of processing of the endosomal intermediate form of cathepsin D to the mature lysosomal form. There was also a reduction in the rate of epidermal growth factor receptor (EGFR) dephosphorylation and degradation following ligand stimulation, consistent with the retention of the EGFR on the limiting membranes of the enlarged late endosomes. By contrast, the suppression of hVps34 expression did not block trafficking of cathepsin D between the TGN and late endosomes, or endocytic uptake of fluid-phase markers, or association of a PtdIns(3)P-binding protein, EEA1, with early endosomes. LAMP1-positive vacuoles were depleted of PtdIns(3)P in the hVps34-knockdown cells, as judged by their inability to bind the PtdIns(3)P probe GFP-2xFYVE. By contrast, LAMP1-negative vesicles continued to bind GFP-2xFYVE in the knockdown cells.

Overall, these findings indicate that hVps34 plays a major role in generating PtdIns(3)P for internal vesicle formation in multivesicular/late endosomes. The findings also unexpectedly suggest that other wortmannin-sensitive kinases and/or polyphosphoinositide phosphatases may be able to compensate for the loss of hVps34 and maintain PtdIns(3)P levels required for vesicular trafficking in the early endocytic pathway or the TGN.

Role of lipids in the MAPK signaling pathway

The mitogen-activated protein kinase (MAPK) signaling pathway is activated in response to a variety of extracellular stimuli such as growth factor stimulation. The best-characterized MAPK pathway involves the sequential activation of Raf, MEK and ERK proteins, capable of regulating the gene expression required for cell proliferation. Binding to specific lipids can regulate both the subcellular localization of these MAPK signaling proteins as well as their kinase activities. More recently it has become increasingly clear that the majority of MAPK signaling takes place intracellularly on endosomes and that the perturbation of endocytic pathways has dramatic effects on the MAPK pathway. This review highlights the direct effects of lipids on the localization and regulation of MAPK pathway proteins. In addition, the indirect effects lipids have on MAPK signaling via their regulation of endocytosis and the biophysical properties of different membrane lipids as a result of growth factor stimulation are discussed. The ability of a protein to bind to both lipids and proteins at the same time may act like a "ZIP code" to target that protein to a highly specific microlocation and could also allow a protein to be "handed off" to maintain tight control over its binding partners and location.

Functional specificity of the mammalian Beclin-Vps34 PI 3-kinase complex in macroautophagy versus endocytosis and lysosomal enzyme trafficking

Beclin 1 was originally identified as a novel Bcl-2-interacting protein, but co-immunoprecipitation studies suggest that the major physiological partner for Beclin 1 is the mammalian class III phosphatidylinositol 3-kinase (PI 3-kinase) Vps34. Beclin 1 has been proposed to function as a tumor suppressor by promoting cellular macroautophagy, a process that is known to depend on Vps34. However, an alternative role for Beclin 1 in modulating normal Vps34-dependent protein trafficking pathways has not been ruled out. This possibility was examined in U-251 glioblastoma cells. Immunoprecipitates of endogenous Beclin 1 contained human Vps34 (hVps34), but not Bcl-2. Suppression of Beclin 1 expression by short interfering (si)RNA-mediated gene silencing blunted the autophagic response of the cells to nutrient deprivation or C2-ceramide. However, other PI 3-kinase-dependent trafficking pathways, such as the post-endocytic sorting of the epidermal growth factor receptor (EGFR) or the proteolytic processing of procathepsin D en route from the trans-Golgi network (TGN) to lysosomes, were not affected. Depletion of Beclin 1 did not reduce endocytic internalization of a fluid phase marker (horseradish peroxidase, HRP) or cause swelling of late endosomal compartments typically seen in cells where the function of hVps34 is impaired. These findings argue against a role for Beclin 1 as an essential chaperone or adaptor for hVps34 in normal vesicular trafficking, and they support the hypothesis that Beclin 1 functions mainly to engage hVps34 in the autophagic pathway.

D-3 phosphoinositides of the ciliate Tetrahymena: Characterization and study of their regulatory role in lysosomal enzyme secretion

Phosphatidylinositol 3-phosphate, PtdIns3P, is a phosphoinositide which is implicated in regulating membrane trafficking in both mammalian and yeast cells. It also serves as a precursor for the synthesis of phosphatidylinositol 3,5-bisphosphate, PtdIns3,5P2, a phosphoinositide, the exact functions of which remain unknown. In this report, we show that these two phosphoinositides are constitutive lipid components of the ciliate Tetrahymena. Using HPLC analysis, PtdIns3P and PtdIns3,5P2 were found to comprise 16% and 30-40% of their relevant phosphoinositide pools, respectively. Treatment of Tetrahymena cells with wortmannin (0.1-10 microM) resulted in the depletion of PtdIns3P and PtdIns3,5P2 without any effect on D-4 phosphoinositides. Wortmannin was further used for the investigation of D-3 phosphoinositide involvement in the regulation of lysosomal vesicular trafficking. Incubation of Tetrahymena cells with wortmannin resulted in enhanced secretion of two different lysosomal enzymes without any change in their total activities. Experiments performed with a T. thermophila secretion mutant strain verified that the wortmannin-induced secretion is specific and it is not due to a diversion of lysosomal enzymes to other secretory pathways. Moreover, experiments performed with a phagocytosis-deficient T. thermophila strain showed that a substantial fraction of wortmannin-induced secretion was dependent on the presence of functional phagosomes/phagolysosomes.

The ubiquitous phosphoinositides

There are now known to exist seven phosphoinositides all derived through various metabolic routes from the parent lipid phosphatidylinositol. With one additional metabolite, diacylglycerol, these represent a rich resource of bioactive lipids responsible for recruiting protein effectors and marking membrane compartments. The metabolic map of this pathway and the nature of the binding partner interactions are reviewed.

Acquisition of Unprecedented Phosphatidylinositol 3,5-Bisphosphate Rise in Hyperosmotically Stressed 3T3-L1 Adipocytes, Mediated by ArPIKfyve-PIKfyve Pathway

Unlike yeast, where hyperosmotic stress induces a dramatic increase in phosphatidylinositol 3,5-bisphosphate (PtdIns 3,5-P(2)) synthesis, in mammalian cells, although activating a complex array of signaling events, hyperosmotic stress fails to up-regulate PtdIns 3,5-P(2), indicating the PtdIns 3,5-P(2) pathway is not involved in mammalian osmo-protective responses. Here we report an unexpected and marked PtdIns 3,5-P(2) increase in response to hyperosmotic stress in differentiated 3T3-L1 adipocytes. Because this effect was not observed in the precursor preadipocytes, a specific role during acquisition of the adipocyte phenotype and transition into insulin-responsive cells could be suggested. However, acute insulin action did not result in a measurable PtdIns 3,5-P(2) rise, indicating the PtdIns 3,5-P(2) pathway is a specific hyperosmotically activated signaling cascade selectively operating in differentiated 3T3-L1 adipocytes. Hyperosmolarity activates different components of several kinase cascades, including p38 mitogen-activated protein and tyrosine kinases, but these appear to be separate from the activated PtdIns 3,5-P(2) pathway. Because PtdIns 3,5-P(2) is primarily produced by PIKfyve-catalyzed synthesis and requires the upstream activator hVac14 (called herein ArPIKfyve) that physically associates with and activates PIKfyve, we examined the contribution of ArPIKfyve-PIKfyve for the hyperosmotic stress-induced rise in PtdIns 3,5-P(2). Small interfering RNA-directed gene silencing to selectively deplete ArPIKfyve or PIKfyve in 3T3-L1 adipocytes determined the ArPIKfyve-PIKfyve axis fully accountable for the hyperosmotically activated PtdIns 3,5-P(2). Together these results reveal a previously uncharacterized PtdIns 3,5-P(2) pathway activated selectively in hyperosmotically stressed 3T3-L1 adipocytes and suggest a plausible role for PtdIns 3,5-P(2) in the osmo-protective response mechanism in this cell type.

3,3′,5-Triiodo-l-thyronine Up-regulation of Na,K-ATPase Activity and Cell Surface Expression in Alveolar Epithelial Cells Is Src Kinase- and Phosphoinositide 3-Kinase-dependent

We previously reported that thyroid hormone, 3,3',5-triiodo-l-thyronine (T3), increased Na,K-ATPase activity of adult rat alveolar epithelial cells in a transcription-independent manner via increased cell surface expression of the alpha(1) and beta(1) subunits of Na,K-ATPase. Now we sought to identify signaling molecules necessary for T3 stimulation of Na,K-ATPase activity in alveolar epithelial cells. Whereas protein kinase A inhibitor H-8 and protein kinase C inhibitor bisindolymaleimide did not block the T3-induced increase in Na,K-ATPase activity, two inhibitors of phosphoinositide 3-kinase (PI3K), wortmannin and Ly294002, and two Src kinase inhibitors, PP1 and PP2, blocked the T3-induced Na,K-ATPase activity. T3 stimulated the activity of PI3K as measured by phosphatidylinositol 3-phosphate. T3 also stimulated the serine 473 phosphorylation of the PI3K downstream molecule PKB/Akt in a dose-dependent manner. Transient expression of a constitutively active mutant of the PI3K catalytic subunit p110 augmented Na,K-ATPase activity and increased the amount of cell surface Na,K-ATPase alpha(1) subunit protein. T3 also stimulated Src family kinase activity. Transient expression of a constitutively active Src kinase increased Na,K-ATPase activity, PI3K activity, and phosphorylation of PKB/Akt at serine 473. PP1 or PP2 blocked T3-stimulated PKB/Akt phosphorylation at serine 473 and PI3K activity that was activated by an active mutant of Src; however, wortmannin did not inhibit the T3-stimulated Src kinase activity. Although PP1 and wortmannin abolished the increase in Na,K-ATPase activity induced by the active mutant of Src, PP1 did not inhibit the active mutant of PI3K-up-regulated Na,K-ATPase activity. In summary, T3 stimulates the PI3K/PKB pathway via the Src family of tyrosine kinases, and activation of both the Src family kinases and PI3K is required for the T3-induced stimulation of Na,K-ATPase activity and its cell surface expression in adult rat alveolar epithelial cells.

Sorting Nexin-1 Mediates Tubular Endosome-to-TGN Transport through Coincidence Sensing of High- Curvature Membranes and 3-Phosphoinositides

BACKGROUND

Sorting nexins (SNXs) are phox homology (PX) domain-containing proteins thought to regulate endosomal sorting of internalized receptors. The prototypical SNX is sorting nexin-1 (SNX1), a protein that through its PX domain binds phosphatidylinositol 3-monophosphate [PtdIns(3)P] and phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P(2)]. SNX1 is associated with early endosomes, from where it has been proposed to regulate the degradation of internalized epidermal growth factor (EGF) receptors through modulating endosomal-to-lysosomal sorting.

RESULTS

We show here that SNX1 contains a BAR (Bin/Amphiphysin/Rvs) domain, a membrane binding domain that endows SNX1 with the ability to form dimers and to sense membrane curvature. We present evidence that through coincidence detection, the BAR and PX domains efficiently target SNX1 to a microdomain of the early endosome defined by high curvature and the presence of 3-phosphoinositides. In addition, we show that the BAR domain endows SNX1 with an ability to tubulate membranes in-vitro and drive the tubulation of the endosomal compartment in-vivo. Using RNA interference (RNAi), we establish that SNX1 does not play a role in EGF or transferrin receptor sorting; rather it specifically perturbs endosome-to-trans Golgi network (TGN) transport of the cation-independent mannose-6-phosphate receptor (CI-MPR). Our data support an evolutionarily conserved function for SNX1 from yeast to mammals and provide functional insight into the molecular mechanisms underlying lipid-mediated protein targeting and tubular-based protein sorting.

CONCLUSIONS

We conclude that through coincidence detection SNX1 associates with a microdomain of the early endosome-characterized by high membrane curvature and the presence of 3-phosphoinositides-from where it regulates tubular-based endosome-to-TGN retrieval of the CI-MPR.

Aquaporin-2 is retrieved to the apical storage compartment via early endosomes and phosphatidylinositol 3-kinase-dependent pathway

Aquaporin-2 (AQP2) is one of the water-channel proteins expressed in principal cells of kidney collecting ducts, where it is stored in the intracellular compartment. Previous studies have demonstrated that AQP2 vesicles constitute a distinct intracellular compartment partially overlapping with early endosomes. In this report, we performed in vitro experiments using the renal epithelial cell line, Madin-Darby canine kidney (MDCK) cells, stably expressing AQP2 (MDCK-hAQP2). In nonpolarized cells, AQP2 vesicles were scattered in the cytoplasm and did not colocalize with Golgi 58K or TGN38. Small portions of AQP2 vesicles were positive for the lysosome marker cathepsin D. An early endosome antigen (EEA1) localized around AQP2 vesicles in close proximity, suggesting involvement of the endosomal system in the trafficking of AQP2. AQP2 vesicles are distinct from other recycling molecules, such as glucose transporter 4 (GLUT4) and endocytosed transferrin. In polarized MDCK-hAQP2 cells, AQP2 vesicles were localized in the subapical recycling compartment and distinct from the Golgi apparatus, trans-Golgi network, lysosome, and early endosome in the nonstimulated state. When the cells were treated with forskolin, translocation of AQP2 to the apical membrane was observed. Washout of forskolin induced retrieval of AQP2 into the cytoplasm, and AQP2 was transiently colocalized with EEA1-positive endosomes. Then, AQP2 moved from EEA1-positive endosomes to the subapical AQP2-storage compartment, which is sensitive to wortmannin and LY294002. These results suggest that AQP2 resides in a recycling compartment at the apical side in polarized MDCK-hAQP2 cells, and its retrieval uses the apical endosomal system and the phosphatidylinositol 3-kinase-dependent pathway.

Macrophage colony-stimulating factor differentially regulates low density lipoprotein and transferrin receptors

Endocytosis mediated by both LDL receptors (LDLRs) and transferrin receptors (TfRs) occurs in clathrin-coated pits and requires specific tyrosine-based internalization sequences located in the cytoplasmic domain of these receptors. Internalization of these receptors is mediated by endocytic proteins that interact with the internalization domains. We previously showed that macrophage colony-stimulating factor (M-CSF) rapidly increases LDLR-dependent uptake and metabolism of LDL. To study the mechanism by which M-CSF regulates LDL uptake, we compared the effect of M-CSF on the internalization of LDL and transferrin (Tf). Our results show that M-CSF substantially increased the rate of LDLR internalization without increasing LDLR localization on the cell surface. In contrast, M-CSF treatment of macrophages rapidly increased the localization of TfR to the cell surface but did not alter the relative rate of Tf internalization. Moreover, M-CSF regulated TfR and LDLR via the activation of distinct signaling pathways. Recruitment of TfR to the cell surface was attenuated by phosphatidylinositol 3-kinase inhibitors, whereas stimulated LDL uptake was inhibited by the serine/threonine phosphatase inhibitor okadaic acid. Taken together, our results indicate that M-CSF differentially regulates receptors that undergo endocytosis and that increased LDL uptake results from a selective increase in the rate of LDLR internalization.

Phosphoinositide Signaling

Lipid signaling by phosphoinositides (PIP(n)s) involves an array of proteins with lipid recognition, kinase, phosphatase, and phospholipase functions. Understanding PIP(n) pathway signaling requires identification and characterization of PIP(n)-interacting proteins. Moreover, spatiotemporal localization and physiological function of PIP(n)-protein complexes must be elucidated in cellular and organismal contexts. For protein discovery to functional elucidation, reporter-linked phosphoinositides or tethered PIP(n)s have been essential. The phosphoinositide 3-kinase (PI 3-K) signaling pathway has recently emerged as an important source of potential "druggable" therapeutic targets in human pathophysiology in both academic and pharmaceutical environments. This review summarizes the chemistry of PIP(n) affinity probes and their use in identifying macromolecular targets. The process of target validation will be described, i.e., the use of tethered PIP(n)s in determining PIP(n) selectivity in vitro and in establishing the function of PIP(n)-protein complexes in living cells.

HIP1 and HIP1r stabilize receptor tyrosine kinases and bind 3-phosphoinositides via epsin N-terminal homology domains

Huntingtin-interacting protein 1-related (HIP1r) is the only known mammalian relative of huntingtin-interacting protein 1 (HIP1), a protein that transforms fibroblasts via undefined mechanisms. Here we demonstrate that both HIP1r and HIP1 bind inositol lipids via their epsin N-terminal homology (ENTH) domains. In contrast to other ENTH domain-containing proteins, lipid binding is preferential to the 3-phosphate-containing inositol lipids, phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,5-bisphosphate. Furthermore, the HIP1r ENTH domain, like that of HIP1, is necessary for lipid binding, and expression of an ENTH domain-deletion mutant, HIP1r/deltaE, induces apoptosis. Consistent with the ability of HIP1r and HIP1 to affect cell survival, full-length HIP1 and HIP1r stabilize pools of growth factor receptors by prolonging their half-life following ligand-induced endocytosis. Although HIP1r and HIP1 display only a partially overlapping pattern of protein interactions, these data suggest that both proteins share a functional homology by binding 3-phosphorylated inositol lipids and stabilizing receptor tyrosine kinases in a fashion that may contribute to their ability to alter cell growth and survival.

Characterization of the Drosophila sphingosine kinases and requirement for Sk2 in normal reproductive function

Sphingosine kinase is a highly conserved enzyme that catalyzes the synthesis of sphingosine 1-phosphate and reduces cellular levels of sphingosine and ceramide. Although ceramide is pro-apoptotic and sphingosine is generally growth-inhibitory, sphingosine 1-phosphate signaling promotes cell proliferation, survival, and migration. Sphingosine kinase is thus in a strategic position to regulate important cell fate decisions which may contribute to normal animal development. To facilitate studies examining the potential role of sphingosine kinase and long chain base metabolism in Drosophila development, we characterized two putative Drosophila sphingosine kinase genes, Sk1 and Sk2. Both genes functionally and biochemically complement a yeast sphingosine kinase mutant, express predominantly cytosolic activities, and are capable of phosphorylating a range of endogenous and non-endogenous sphingoid base substrates. The two genes demonstrate overlapping but distinct temporal and spatial expression patterns in the Drosophila embryo, and timing of expression is consistent with observed changes in long chain base levels throughout development. A null Sk2 transposon insertion mutant demonstrated elevated long chain base levels, impaired flight performance, and diminished ovulation. This is the first reported mutation of a sphingosine kinase in an animal model; the associated phenotypes indicate that Sk1 and Sk2 are not redundant in biological function and that sphingosine kinase is essential for diverse physiological functions in this organism.

Roles of PI3K in neutrophil function

Neutrophils are terminally differentiated cells that play a vital role in host defense. It has recently become evident that phospholipid regulation plays an import role in many neutrophil functions. We review the regulation of neutrophil functions such as chemotaxis, superoxide production, and phagocytosis by phosphatidylinositol-3,4,5-trisphosphate (PIP3), which is generated in neutrophils by PI3K(gamma). Several lines of evidence are presented demonstrating the importance of this kinase in regulating chemotaxis, in particular the directionality of chemotactic migration. Evidence suggesting that this kinase is important for phagocytosis, especially during engulfment and the internalization of large particles, is also reviewed. Finally, it is suggested that PI3K is important for superoxide production and neutrophil priming. The common link between these seemingly diverse functions is that PI3K(gamma), via its phospholipid products, appears to be providing spatial-temporal cues for the binding of actin-organizing proteins.

PIKfyve controls fluid phase endocytosis but not recycling/degradation of endocytosed receptors or sorting of procathepsin D by regulating multivesicular body morphogenesis

The mammalian phosphatidylinositol (PtdIns) 5-P/PtdIns 3,5-P2-producing kinase PIKfyve has been implicated in maintaining endomembrane homeostasis in mammalian cells. To address the role of PIKfyve in trafficking processes, we examined the functioning of the biosynthetic, endocytic, and recycling pathways in stable human embryonic kidney 293 cell lines inducibly expressing the wild-type or kinase-defective dominant-negative form. PIKfyveWT or PIKfyveK1831E expression did not affect the processing and lysosomal targeting of newly synthesized procathepsin D. Likewise the rates of transferrin uptake/recycling or epidermal growth factor receptor degradation were not altered upon expression of either protein. In contrast, PIKfyveK1831E but not PIKfyveWT expression markedly impaired the late uptake of fluid phase marker horseradish peroxidase. Inspection of the organelle morphology by confocal microscopy with specific markers in COS cells transiently expressing PIKfyveK1831E showed the Golgi apparatus, end lysosomes, and the recycling compartment indistinguishable from nontransfected cells, despite the dramatic PIKfyveK1831E-induced endomembrane vacuolation. In contrast, we observed a striking effect on the late endocytic compartment, marked by disruption of the dextran-labeled perinuclear endosomal compartment and formation of dispersed enlarged vesicles. Electron microscopy identified the cytoplasmic vacuoles in the PIKfyveK1831E-expressing human embryonic kidney 293 cells as enlarged multivesicular body-like structures with substantially lower number of internal vesicles and membrane whorls. Together, these data indicate that PIKfyve selectively regulates the sorting and traffic of peripheral endosomes containing lysosomaly directed fluid phase cargo through controlling the morphogenesis and function of multivesicular bodies.

Dynamics of phosphoinositides in membrane retrieval and insertion

The phosphoinositides PtdIns(4,5)P2 and PtdIns(3,4,5)P3 are concentrated in plasma membranes of eukaryotic cells, and excluded from endosomes, whereas PtdIns(3)P is formed in these latter intracellular membranes and is apparently excluded from the plasma membrane. The logic of this asymmetric disposition is now revealed by the nature of the effector proteins that selectively bind these lipids through specific modules and by the processes that they catalyze. PtdIns(3,4,5)P3 has a role in directing exocytosis, in addition to many other signaling events, whereas PtdIns(4,5)P2 directs endocytosis through its ability to anchor several coat proteins to the plasma membrane. Remarkably, the elimination of PtdIns(4,5)P2 from forming endosomes may be required for membrane fission to occur. Thus membrane insertion and retrieval can be regulated by plasma membrane concentrations of PtdIns(3,4,5)P3 and PtdIns(4,5)P2, whereas PtdIns(3)P directs the downstream trafficking and recycling of intracellular membranes through its attraction of proteins that catalyze these processes. The phosphoinositides thereby control many cell features that depend upon protein sorting, including the composition of the plasma membrane itself, which in turn determines the cell's responses to its environment.

Phosphatidylinositol 3-kinases in carcinoembryonic antigen-related cellular adhesion molecule-mediated internalization of Neisseria gonorrhoeae

Neisseria gonorrhoeae can be internalized by mammalian cells through interactions between bacterial opacity-associated (Opa) adhesins and members of the human carcinoembryonic antigen-related cellular adhesion molecule (CEACAM) family. We examined the role of phosphatidylinositol 3-kinases (PI3Ks) in gonococcal invasion of epithelial cell lines expressing either CEACAM1 or CEACAM3. CEACAM3-mediated internalization, but not that mediated by CEACAM1, was accompanied by localized and transient accumulation of the class I PI3K product phosphatidylinositol 3,4,5-trisphosphate at sites of bacterial engulfment. Inhibition of phosphatidylinositol 3-kinases reduced CEACAM3-mediated uptake but, paradoxically, led to an increase in intracellular survival of bacteria internalized via either CEACAM1 or CEACAM3, suggesting additional roles for PI3K products. Consistent with this finding, the class III PI3K product phosphatidylinositol 3-phosphate accumulated and persisted in the membrane of gonococcal phagosomes after internalization. Inhibition of PI3K blocked phagosomal acquisition of the late endosomal marker lysosome-associated membrane protein 2 and reduced phagosomal acidification. Inhibiting phagosomal acidification with concanamycin A also increased survival of intracellular gonococci. These results suggest two modes of action of phosphatidylinositol 3-kinases during internalization of gonococci: synthesis of phosphatidylinositol 3,4,5-trisphosphate is important for CEACAM3-mediated uptake, while phosphatidylinositol 3-phosphate is needed for phagosomal maturation and acidification, which are required for optimal bacterial killing.

Phosphoinositide recognition domains

Domains or modules known to bind phosphoinositides have increased dramatically in number over the past few years, and are found in proteins involved in intracellular trafficking, cellular signaling, and cytoskeletal remodeling. Analysis of lipid binding by these domains and its structural basis has provided significant insight into the mechanism of membrane recruitment by the different cellular phosphoinositides. Domains that target only the rare (3-phosphorylated) phosphoinositides must bind with very high affinity, and with exquisite specificity. This is achieved solely by headgroup interactions in the case of certain pleckstrin homology (PH) domains [which bind PtdIns(3,4,5)P3 and/or PtdIns(3,4)P2], but requires an additional membrane-insertion and/or oligomerization component in the case of the PtdIns(3)P-targeting phox homology (PX) and FYVE domains. Domains that target PtdIns(4,5)P2, which is more abundant by some 25-fold, do not require the same stringent affinity and specificity characteristics, and tend to be more diverse in structure. The mode of phosphoinositide binding by different domains also appears to reflect their distinct functions. For example, pleckstrin homology domains that serve as simple targeting domains recognize only phosphoinositide headgroups. By contrast, certain other domains, notably the epsin ENTH domain, appear to promote bilayer curvature by inserting into the membrane upon binding.

Genome-wide analysis of signaling domain function

Approximately 2.5% of human gene products contain one or more small domains that drive interactions between proteins and other cellular components in cell signaling processes. The many interactions driven by these relatively simple domains are thought to cooperate with one another to yield complex signaling networks that allow very fine control of cell function. In principle, if we can understand all domain-mediated interactions it should be possible to model these networks. Genome-wide analysis of signaling domain interactions represents a first step in this direction, and several advances of this sort in yeast have been reported over the past year. These reports suggest, for some domains at least, that the prospect of generating 'wiring diagrams' with this simple approach is feasible.

Intracellular trafficking of human CXCR1 and CXCR2: regulation by receptor domains and actin-related kinases

In this study we investigated the regulation of CXCR1 and CXCR2 intracellular trafficking. First, we produced a chimeric CXCR2 receptor that contained the internalization motifs of both CXCR2 and CXCR1 (CXCR2: LLKIL sequence; CXCR1: C-terminal phosphorylation sites). Elevated levels of internalization were induced by different ELR-expressing CXC chemokines on the chimeric receptor, as compared to wild-type CXCR2. Analysis of inter-relationships between CXCR1 and CXCR2 during internalization indicated that the exposure of cells that expressed both CXCR1 and CXCR2 to CXCL8 or CXCL6 resulted in decreased levels of CXCR1 internalization as compared to those in cells that expressed only CXCR1. To characterize the role of actin-related components in CXCR1 and CXCR2 trafficking, wortmannin, a potent inhibitor of phosphatidylinositol kinases, was used. The presence of wortmannin during receptor recycling inhibited CXCR1 and CXCR2 re-expression following CXCL8-induced internalization, and resulted in a marked disruption of the proper organization of actin filaments. The kinase-dependent recycling process required CXCR2 C-terminal phosphorylation sites. Our results suggest that actin-related kinases are required for the proper functionality of actin filaments, which are the instrumental factors needed for receptor recycling. In all, CXCR1 and CXCR2 internalization and recycling are tightly regulated by receptor domains and by actin-related kinases.

TGF beta receptor internalization into EEA1-enriched early endosomes: role in signaling to Smad2

Transforming growth factor (TGF)beta is an important physiological regulator of cellular growth and differentiation. It activates a receptor threonine/serine kinase that phosphorylates the transcription factor Smad2, which then translocates into the nucleus to trigger specific transcriptional events. Here we show that activated type I and II TGF beta receptors internalize into endosomes containing the early endosomal protein EEA1. The extent of TGF beta-stimulated Smad2 phosphorylation, Smad2 nuclear translocation, and TGF beta-stimulated transcription correlated closely with the extent of internalization of the receptor. TGF beta signaling also requires SARA (Smad anchor for receptor activation), a 135-kD polypeptide that contains a FYVE Zn(++) finger motif. Here we show that SARA localizes to endosomes containing EEA1, and that disruption of this localization inhibits TGF beta-induced Smad2 nuclear translocation. These results indicate that traffic of the TGF beta receptor into the endosome enables TGF beta signaling, revealing a novel function for the endosome as a compartment specialized for the amplification of certain extracellular signals.

Dissecting virus entry via endocytosis

Numerous virus families utilize endocytosis to infect host cells, mediating virus internalization as well as trafficking to the site of replication. Recent research has demonstrated that viruses employ the full endocytic capabilities of the cell. The endocytic pathways utilized include clathrin-mediated endocytosis, caveolae, macropinocytosis and novel non-clathrin, non-caveolae pathways. The tools to study endocytosis and, consequently, virus entry are becoming more effective and specific as the amount of information on endocytic component structure and function increases. The use of inhibitory drugs, although still quite common, often leads to non-specific disruptions in the cell. Molecular inhibitors in the form of dominant-negative proteins have surpassed the use of chemical inhibitors in terms of specificity to individual pathways. Dominant-negative molecules are derived from both structural proteins of endocytosis, such as dynamin and caveolin, and regulatory proteins, primarily small GTPases and kinases. This review focuses on the experimental approaches taken to examine virus entry and provides both classic examples and recent research on a variety of virus families.

A monoclonal antibody to visualize PtdIns(3,4,5)P(3) in cells

Phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] is a second messenger produced in response to agonist stimulation. Traditionally, visualization of phosphoinositide polyphosphates (PtdInsP(n)) in living cells is accomplished using chimeric green fluorescent protein (GFP)-pleckstrin homology (PH) domain proteins, while PtdInsP(n) quantitation is accomplished by extraction and separation of radiolabeled cellular PtdInsP(n)s. Here we describe preparation of a covalent protein-PtdIns(3,4,5)P(3) immunogen, characterization of binding selectivity of an anti-PtdIns(3,4,5)P(3) IgM, and immunodetection of PtdIns(3,4,5)P(3) in stimulated mammalian cells. This antibody has greater than three orders of magnitude selectivity for binding PtdIns(3,4,5)P(3) relative to its precursor, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), and is therefore optimal for studies of cell function. The immunodetection in platelet-derived growth factor (PDGF)-stimulated NIH 3T3 cells was benchmarked against HPLC analysis of [3H]-myo-inositol-labeled cellular PtdInsP(n)s. In addition, the changes in subcellular amounts and localizations of both PtdIns(3,4,5)P(3) and PtdIns(4,5)P(2) in stimulated NIH 3T3 fibroblasts and human neutrophils were observed by immunofluorescence. In insulin- or PDGF-stimulated fibroblasts, PtdIns(3,4,5)P(3) levels increased in the cytoplasm, peaking at 10 min. In contrast, increases in the PtdIns(4,5)P(2) levels were detected in nuclei, corresponding to the production of new substrate following depletion by phosphoinositide (PI) 3-kinase.