The roles of flotillin microdomains--endocytosis and beyond

Midgut proteome of an argasid tick, Ornithodoros erraticus: a comparison between unfed and engorged females

Background

The argasid tick Ornithodoros erraticus is the vector of African swine fever virus and of several Borrelia species that cause human relapsing fever in the Iberian Peninsula. The tick midgut is part of the ectoparasite-host interface and expresses proteins that are vital for the survival of the tick. Midgut proteins are therefore potential targets for drug and/or vaccine design aimed at the development of new strategies for tick control. Thus, the aim of this work was the characterization of the proteome of the O. erraticus midgut before and after a blood meal trying to elucidate the induced changes upon blood feeding.

Methods

Midgut tissues from unfed and engorged O. erraticus females were dissected and proteins were fractionated by centrifugation and SDS-PAGE, and the corresponding gel pieces analysed by LC–MS/MS. The identified proteins were classified according to their Protein Class and Molecular Function and the differences between fed and unfed specimens were analysed.

Results

Overall 555 tick proteins were identified: 414 in the midgut of the unfed specimens and 376 in the fed specimens, of which 235 were present in both groups. The proteins with catalytic, binding and structural functions were the most numerous and abundant, consistent with their role in the intracellular processing of the blood meal. The analysis of some groups of proteins putatively involved directly in blood meal digestion, including protein digestion (peptidase activity), iron metabolism, enzymes involved in oxidative stress and detoxification and membrane traffic and transport proteins, detected some differences between the fed and unfed ticks

Conclusions

This work reports for the first time the collection and analysis of the midgut proteome of an argasid tick species and provides molecular information about the argasid machinery involved in blood digestion. This information represents a starting point for the identification and selection of new targets for the development of alternative control strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1148-z) contains supplementary material, which is available to authorized users.

A novel role for flotillin-1 in H-Ras-regulated breast cancer aggressiveness

Elevated expression and aberrant activation of Ras have been implicated in breast cancer aggressiveness. H-Ras, but not N-Ras, induces breast cell invasion. A crucial link between lipid rafts and H-Ras function has been suggested. This study sought to identify the lipid raft protein(s) responsible for H-Ras-induced tumorigenicity and invasiveness of breast cancer. We conducted a comparative proteomic analysis of lipid raft proteins from invasive MCF10A human breast epithelial cells engineered to express active H-Ras and non-invasive cells expressing active N-Ras. Here, we identified a lipid raft protein flotillin-1 as an important regulator of H-Ras activation and breast cell invasion. Flotillin-1 was required for epidermal growth factor-induced activation of H-Ras, but not that of N-Ras, in MDA-MB-231 triple-negative breast cancer (TNBC) cells. Flotillin-1 knockdown inhibited the invasiveness of MDA-MB-231 and Hs578T TNBC cells in vitro and in vivo. In xenograft mouse tumor models of these TNBC cell lines, we showed that flotillin-1 played a critical role in tumor growth. Using human breast cancer samples, we provided clinical evidence for the metastatic potential of flotillin-1. Membrane staining of flotillin-1 was positively correlated with metastatic spread (p = 0.013) and inversely correlated with patient disease-free survival rates (p = 0.005). Expression of flotillin-1 was associated with H-Ras in breast cancer, especially in TNBC (p < 0.001). Our findings provide insight into the molecular basis of Ras isoform-specific interplay with flotillin-1, leading to tumorigenicity and aggressiveness of breast cancer.

Molecular composition of functional microdomains in bacterial membranes

Membranes of eukaryotic cells organize a number of proteins related to signal transduction and membrane trafficking into microdomains, which are enriched in particular lipids, like cholesterol and sphingolipids and are commonly referred as to lipid rafts or membrane rafts. The existence of this type of signaling platforms was traditionally associated with eukaryotic membranes because prokaryotic cells were considered too simple organisms to require a sophisticated organization of their signaling networks. However, the research that have been performed during last years have shown that bacteria organize many signaling transduction processes in Functional Membrane Microdomains (FMMs), which are similar to the lipid rafts that are found in eukaryotic cells. The current knowledge of the existence of FMMs in bacteria is described in this review and the specific structural and biological properties of these membrane microdomains are introduced. The organization of FMMs in bacterial membranes reveals an unexpected level of sophistication in signaling transduction and membrane organization that is unprecedented in bacteria, suggesting that bacteria as more complex organisms than previously considered.

In vivo characterization of the scaffold activity of flotillin on the membrane kinase KinC of Bacillus subtilis

Scaffold proteins are ubiquitous chaperones that bind to proteins and facilitate the physical interaction of the components of signal transduction pathways or multi-enzymic complexes. In this study, we used a biochemical approach to dissect the molecular mechanism of a membrane-associated scaffold protein, FloT, a flotillin-homologue protein that is localized in functional membrane microdomains of the bacterium Bacillus subtilis. This study provides unambiguous evidence that FloT physically binds to and interacts with the membrane-bound sensor kinase KinC. This sensor kinase activates biofilm formation in B. subtilis in response to the presence of the self-produced signal surfactin. Furthermore, we have characterized the mechanism by which the interaction of FloT with KinC benefits the activity of KinC. Two separate and synergistic effects constitute this mechanism: first, the scaffold activity of FloT promotes more efficient self-interaction of KinC and facilitates dimerization into its active form. Second, the selective binding of FloT to KinC prevents the occurrence of unspecific aggregation between KinC and other proteins that may generate dead-end intermediates that could titrate the activity of KinC. Flotillin proteins appear to play an important role in prokaryotes in promoting effective binding of signalling proteins with their correct protein partners.

The influence of flow, shear stress and adhesion molecule targeting on gold nanoparticle uptake in human endothelial cells

The uptake of nanoparticles by endothelial cells is dependent on shear stress adaptation and flow exposure conditions. Adaptation of primary human umbilical vein endothelial cells (HUVECs) to shear stress for 24 h was associated with reduced internalisation of unmodified 80 nm spherical gold nanoparticles (AuNPs) (mean hydrodynamic size of 99 nm in culture medium) after exposure to flow conditions compared with cells that were cultured and exposed to static conditions. Under static conditions, targeting of 80 nm AuNPs conjugated with antibodies against the intracellular adhesion molecule 1 (ICAM-1) (mean hydrodynamic size of 109 nm in culture medium) markedly increased the internalisation of AuNPs in HUVECs that were activated with the tumour necrosis factor (TNF), a treatment that markedly increased the surface expression of ICAM-1. Shear stress-adapted and TNF-activated HUVECs, which were exposed to flow conditions, had higher association with anti-ICAM-1 AuNPs than cells that were not TNF-activated or exposed to particles under static conditions. Hence, shear stress adaptation reduces the uptake of unmodified AuNPs and increases the association between anti-ICAM-1 AuNPs and TNF-activated HUVECs.

Charming neighborhoods on the cell surface: plasma membrane microdomains regulate receptor tyrosine kinase signaling

Receptor tyrosine kinases (RTK) are an important family of growth factor and hormone receptors that regulate many aspects of cellular physiology. Ligand binding by RTKs at the plasma membrane elicits activation of many signaling intermediates. The spatial and temporal regulation of RTK signaling within cells is an important determinant of receptor signaling outcome. In particular, the compartmentalization of the plasma membrane into a number of microdomains allows context-specific control of RTK signaling. Indeed various RTKs are recruited to and enriched within specific plasma membrane microdomains under various conditions, including lipid-ordered domains such as caveolae and lipid rafts, clathrin-coated structures, tetraspanin-enriched microdomains, and actin-dependent protrusive membrane microdomains such as dorsal ruffles and invadosomes. We examine the evidence for control of RTK signaling by each of these plasma membrane microdomains, as well as molecular mechanisms for how this spatial organization controls receptor signaling.

Formation of Tubulovesicular Carriers from Endosomes and Their Fusion to the trans-Golgi Network

Endosomes undergo extensive spatiotemporal rearrangements as proteins and lipids flux through them in a series of fusion and fission events. These controlled changes enable the concentration of cargo for eventual degradation while ensuring the proper recycling of other components. A growing body of studies has now defined multiple recycling pathways from endosomes to the trans-Golgi network (TGN) which differ in their molecular machineries. The recycling process requires specific sets of lipids, coats, adaptors, and accessory proteins that coordinate cargo selection with membrane deformation and its association with the cytoskeleton. Specific tethering factors and SNARE (SNAP (Soluble NSF Attachment Protein) Receptor) complexes are then required for the docking and fusion with the acceptor membrane. Herein, we summarize some of the current knowledge of the machineries that govern the retrograde transport from endosomes to the TGN.

The myelin proteolipid plasmolipin forms oligomers and induces liquid-ordered membranes in the Golgi complex

Myelin comprises a compactly stacked massive surface area of protein-poor thick membrane that insulates axons to allow fast signal propagation. Increasing levels of the myelin protein plasmolipin (PLLP) were correlated with post-natal myelination; however, its function is unknown. Here, the intracellular localization and dynamics of PLLP were characterized in primary glial and cultured cells using fluorescently labeled PLLP and antibodies against PLLP. PLLP localized to and recycled between the plasma membrane and the Golgi complex. In the Golgi complex, PLLP forms oligomers based on fluorescence resonance energy transfer (FRET) analyses. PLLP oligomers blocked Golgi to plasma membrane transport of the secretory protein vesicular stomatitis virus G protein (VSVG), but not of a VSVG mutant with an elongated transmembrane domain. Laurdan staining analysis showed that this block is associated with PLLP-induced proliferation of liquid-ordered membranes. These findings show the capacity of PLLP to assemble potential myelin membrane precursor domains at the Golgi complex through its oligomerization and ability to attract liquid-ordered lipids. These data support a model in which PLLP functions in myelin biogenesis through organization of myelin liquid-ordered membranes in the Golgi complex.

Exploring the existence of lipid rafts in bacteria

An interesting concept in the organization of cellular membranes is the proposed existence of lipid rafts. Membranes of eukaryotic cells organize signal transduction proteins into membrane rafts or lipid rafts that are enriched in particular lipids such as cholesterol and are important for the correct functionality of diverse cellular processes. The assembly of lipid rafts in eukaryotes has been considered a fundamental step during the evolution of cellular complexity, suggesting that bacteria and archaea were organisms too simple to require such a sophisticated organization of their cellular membranes. However, it was recently discovered that bacteria organize many signal transduction, protein secretion, and transport processes in functional membrane microdomains, which are equivalent to the lipid rafts of eukaryotic cells. This review contains the most significant advances during the last 4 years in understanding the structural and biological role of lipid rafts in bacteria. Furthermore, this review shows a detailed description of a number of molecular and genetic approaches related to the discovery of bacterial lipid rafts as well as an overview of the group of tentative lipid-protein and protein-protein interactions that give consistency to these sophisticated signaling platforms. Additional data suggesting that lipid rafts are widely distributed in bacteria are presented in this review. Therefore, we discuss the available techniques and optimized protocols for the purification and analysis of raft-associated proteins in various bacterial species to aid in the study of bacterial lipid rafts in other laboratories that could be interested in this topic. Overall, the discovery of lipid rafts in bacteria reveals a new level of sophistication in signal transduction and membrane organization that was unexpected for bacteria and shows that bacteria are more complex than previously appreciated.

Oddershede L, Loeschner K, Larsen EH, Loft S, Møller P. Uptake of gold nanoparticles in primary human endothelial cells

Single-particle resolution techniques show that endothelial cells internalise 80 nm unmodified gold nanoparticles by endocytosis with subsequent transport to vesicles.

Flotillins in intercellular adhesion - from cellular physiology to human diseases

Flotillin 1 and 2 are ubiquitous and highly conserved proteins. They were initially discovered in 1997 as being associated with specific caveolin-independent cholesterol- and glycosphingolipid-enriched membrane microdomains and as being expressed during axon regeneration. Flotillins have a role in a large number of physiopathological processes, mainly through their function in membrane receptor clustering and in the regulation of clathrin-independent endocytosis. In this Commentary, we summarize the research performed so far on the role of flotillins in cell-cell adhesion. Recent studies have demonstrated that flotillins directly regulate the formation of cadherin complexes. Indeed, flotillin microdomains are required for the dynamic association and stabilization of cadherins at cell-cell junctions and also for cadherin signaling. Moreover, because flotillins regulate endocytosis and also the actin cytoskeleton, they could have an indirect role in the assembly and stabilization of cadherin complexes. Because it has also recently been shown that flotillins are overexpressed during neurodegenerative diseases and in human cancers, where their upregulation is associated with metastasis formation and poor prognosis, understanding to what extent flotillin upregulation participates in the development of such pathologies is thus of particular interest, as well as how, at the molecular level, it might affect cell adhesion processes.

Lipid rafts, KCa/ClCa/Ca2+ channel complexes and EGFR signaling: Novel targets to reduce tumor development by lipids?

Membrane lipid rafts are distinct plasma membrane nanodomains that are enriched with cholesterol, sphingolipids and gangliosides, with occasional presence of saturated fatty acids and phospholipids containing saturated acyl chains. It is well known that they organize receptors (such as Epithelial Growth Factor Receptor), ion channels and their downstream acting molecules to regulate intracellular signaling pathways. Among them are Ca2+ signaling pathways, which are modified in tumor cells and inhibited upon membrane raft disruption. In addition to protein components, lipids from rafts also contribute to the organization and function of Ca2+ signaling microdomains. This article aims to focus on the lipid raft KCa/ClCa/Ca2+ channel complexes that regulate Ca2+ and EGFR signaling in cancer cells, and discusses the potential modification of these complexes by lipids as a novel therapeutic approach in tumor development. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Unconventional secretion of synuclein-γ promotes tumor cell invasion

Synuclein-γ (SNCG) is a chaperone protein and exists mainly in the cytoplasm. SNCG confers chemoresistance, and is a potential unfavorable biomarker for multiple types of cancer. Our previous work demonstrated that SNCG could be detected in the serum of cancer patients and the medium of cultured cancer cells, but the mechanism of SNCG secretion and its biological roles are unknown. Here, we showed that SNCG levels in the culture medium were positively correlated with cancer cell densities and the concentrations of fetal bovine serum added. SNCG secretion was unaffected by brefeldin A, an inhibitor of the classic protein transport pathway, but was antagonized by exosome inhibitor, lysosome inhibitor, ABC transporter inhibitor, and phosphatidylinositide 3-kinase inhibitor, and knockdown of Rab27a. Ultracentrifugation fractionation revealed that intracellular SNCG was present as both free and vesicle-associated forms, but that the extracellular SNCG was mainly free. The results of reciprocal coimmunoprecipitation experiments showed an interaction between SNCG and flotillin-2, a marker of exosomes and lipid rafts. Moreover, we demonstrated that SNCG, both secreted from tumor cells and exogenously added, markedly promoted cancer cell migration and invasion, but had no effect on noncancerous cells. These findings suggest that SNCG is actively secreted by cancer cells via an unconventional secretion pathway and contributes to aggressive phenotypes of cancer cells.

The dynamics of plant plasma membrane proteins: PINs and beyond

Plants are permanently situated in a fixed location and thus are well adapted to sense and respond to environmental stimuli and developmental cues. At the cellular level, several of these responses require delicate adjustments that affect the activity and steady-state levels of plasma membrane proteins. These adjustments involve both vesicular transport to the plasma membrane and protein internalization via endocytic sorting. A substantial part of our current knowledge of plant plasma membrane protein sorting is based on studies of PIN-FORMED (PIN) auxin transport proteins, which are found at distinct plasma membrane domains and have been implicated in directional efflux of the plant hormone auxin. Here, we discuss the mechanisms involved in establishing such polar protein distributions, focusing on PINs and other key plant plasma membrane proteins, and we highlight the pathways that allow for dynamic adjustments in protein distribution and turnover, which together constitute a versatile framework that underlies the remarkable capabilities of plants to adjust growth and development in their ever-changing environment.

Flotillin-1 facilitates toll-like receptor 3 signaling in human endothelial cells

Endothelial cells are important elements in the vascular response to danger-associated molecules signaling through toll-like receptors (TLRs). Flotillin-1 and -2 are markers of membrane rafts but their true endothelial function is unknown. We hypothesized that flotillins are required for TLR signaling in human umbilical vein endothelial cells (HUVECs). Knockdown of flotillin-1 by shRNA decreased the TLR3-mediated poly-I:C-induced but not the TLR4-mediated LPS-induced inflammatory activation of HUVEC. As TLR3 but not TLR4 signals through the endosomal compartment, flotillin-1 might be involved in the transport of poly-I:C to its receptor. Consistently, uptake of poly-I:C was attenuated by flotillin-1 knockdown and probably involved the scavenger receptor SCARA4 as revealed by knockdown of this receptor. To determine the underlying mechanism, SILAC proteomics was performed. Down-regulation of flotillin-1 led to a reduction of the structural caveolae proteins caveolin-1, cavin-1 and -2, suggesting a role of flotillin-1 in caveolae formation. Flotillin-1 and caveolin-1 colocalized within the cell, and knockdown of flotillin-1 decreased caveolin-1 expression in an endoplasmic reticulum stress-dependent manner. Importantly, downregulation of caveolin-1 also attenuated TLR3-induced signaling. To demonstrate the importance of this finding, cell adhesion was studied. Flotillin-1 shRNA attenuated the poly-I:C-mediated induction of the adhesion molecules VCAM-1 and ICAM-1. As a consequence, the poly-I:C-induced adhesion of peripheral blood mononuclear cells onto HUVECs was significantly attenuated by flotillin-1 shRNA. Collectively, these data suggest that interaction between flotillin-1 and caveolin-1 may facilitate the transport of TLR3-ligands to its intracellular receptor and enables inflammatory TLR3 signaling.

Routes and mechanisms of extracellular vesicle uptake

Extracellular vesicles (EVs) are small vesicles released by donor cells that can be taken up by recipient cells. Despite their discovery decades ago, it has only recently become apparent that EVs play an important role in cell-to-cell communication. EVs can carry a range of nucleic acids and proteins which can have a significant impact on the phenotype of the recipient. For this phenotypic effect to occur, EVs need to fuse with target cell membranes, either directly with the plasma membrane or with the endosomal membrane after endocytic uptake. EVs are of therapeutic interest because they are deregulated in diseases such as cancer and they could be harnessed to deliver drugs to target cells. It is therefore important to understand the molecular mechanisms by which EVs are taken up into cells. This comprehensive review summarizes current knowledge of EV uptake mechanisms. Cells appear to take up EVs by a variety of endocytic pathways, including clathrin-dependent endocytosis, and clathrin-independent pathways such as caveolin-mediated uptake, macropinocytosis, phagocytosis, and lipid raft–mediated internalization. Indeed, it seems likely that a heterogeneous population of EVs may gain entry into a cell via more than one route. The uptake mechanism used by a given EV may depend on proteins and glycoproteins found on the surface of both the vesicle and the target cell. Further research is needed to understand the precise rules that underpin EV entry into cells.

Soluble adenylyl cyclase of sea urchin spermatozoa

Fertilization, a key step in sexual reproduction, requires orchestrated changes in cAMP concentrations. It is notable that spermatozoa (sperm) are among the cell types with extremely high adenylyl cyclase (AC) activity. As production and consumption of this second messenger need to be locally regulated, the discovery of soluble AC (sAC) has broadened our understanding of how such cells deal with these requirements. In addition, because sAC is directly regulated by HCO(3)(-) it is able to translate CO₂/HCO(3)(-)/pH changes into cAMP levels. Fundamental sperm functions such as maturation, motility regulation and the acrosome reaction are influenced by cAMP; this is especially true for sperm of the sea urchin (SU), an organism that has been a model in the study of fertilization for more than 130 years. Here we summarize the discovery and properties of SU sperm sAC, and discuss its involvement in sperm physiology. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Endocytic trafficking of membrane-bound cargo: a flotillin point of view

The ubiquitous and highly conserved flotillin proteins, flotillin-1 and flotillin-2, have been shown to be involved in various cellular processes such as cell adhesion, signal transduction through receptor tyrosine kinases as well as in cellular trafficking pathways. Due to the fact that flotillins are acylated and form hetero-oligomers, they constitutively associate with cholesterol-enriched lipid microdomains. In recent years, such microdomains have been appreciated as platforms that participate in endocytosis and other cellular trafficking steps. This review summarizes the current findings on the role of flotillins in membrane-bound cargo endocytosis and endosomal trafficking events. We will discuss the proposed function of flotillins in endocytosis in the light of recent findings that point towards a role for flotillins in a step that precedes the actual endocytic uptake of cargo molecules. Recent findings have also revealed that flotillins may be important for endosomal sorting and recycling of specific cargo molecules. In addition to these aspects, the cellular trafficking pathway of flotillins themselves as potential cargo in the context of growth factor signaling will be discussed.

Clathrin-independent pathways of endocytosis

There are many pathways of endocytosis at the cell surface that apparently operate at the same time. With the advent of new molecular genetic and imaging tools, an understanding of the different ways by which a cell may endocytose cargo is increasing by leaps and bounds. In this review we explore pathways of endocytosis that occur in the absence of clathrin. These are referred to as clathrin-independent endocytosis (CIE). Here we primarily focus on those pathways that function at the small scale in which some have distinct coats (caveolae) and others function in the absence of specific coated intermediates. We follow the trafficking itineraries of the material endocytosed by these pathways and finally discuss the functional roles that these pathways play in cell and tissue physiology. It is likely that these pathways will play key roles in the regulation of plasma membrane area and tension and also control the availability of membrane during cell migration.

Role of dynamin and clathrin in the cellular trafficking of flotillins

Flotillin-1 and flotillin-2 are highly conserved, membrane-microdomain-associated proteins that have been shown to be involved in signal transduction, membrane trafficking and cell adhesion. Upon growth factor stimulation, flotillins are tyrosine phosphorylated and become endocytosed from the plasma membrane into endosomes from which they are recycled back to the plasma membrane. Although a role for flotillin-1 in the endocytosis of certain cargo proteins has been suggested, it is not known how the growth-factor-induced endocytosis of flotillins is regulated and which endocytosis pathway is used. However, this is likely to be different from the pathway used by flotillin-dependent cargo. In this study, we have addressed the mechanistic details of flotillin trafficking during growth factor signaling. We show that dynamin-2 activity is required for the uptake of flotillins from the plasma membrane upon epidermal growth factor stimulation, and inhibition of dynamin-2 GTPase activity impairs flotillin endocytosis. Surprisingly, recycling of flotillins from endosomes to the plasma membrane appears to require both dynamin-2 and clathrin. Upon overexpression of dynamin-2 mutants or depletion of clathrin heavy chain, flotillins are permanently trapped in endosomes. These data show that clathrin and dynamin are required for the endosomal sorting of flotillins, and the study provides a mechanistic dissection of the thus far poorly characterized endosomal trafficking of flotillins.

EphrinB2 affects apical constriction in Xenopus embryos and is regulated by ADAM10 and flotillin-1

The Eph/ephrin signalling pathways have a critical function in cell adhesion and repulsion, and thus play key roles in various morphogenetic events during development. Here we show that a decrease in ephrinB2 protein causes neural tube closure defects during Xenopus laevis embryogenesis. Such a decrease in ephrinB2 protein levels is observed on the loss of flotillin-1 scaffold protein, a newly identified ephrinB2-binding partner. This dramatic decline in ephrinB2 protein levels on the absence of flotillin-1 expression is specific, and is partly the result of an increased susceptibility to cleavage by the metalloprotease ADAM10. These findings indicate that flotillin-1 regulates ephrinB2 protein levels through ADAM10, and is required for appropriate neural tube morphogenesis in the Xenopus embryo.

Regulation of exosome release by glycosphingolipids and flotillins

Exosomes are released by cells after fusion of multivesicular bodies with the plasma membrane. The molecular mechanism of this process is still unclear. We investigated the role of sphingolipids and flotillins, which constitute a raft-associated family of proteins, in the release of exosomes. Interestingly, our results show that dl-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, an inhibitor of glucosylceramide synthase, seemed to affect the composition of exosomes released from PC-3 cells. However, the inhibition of ceramide formation from the de novo pathway by fumonisin B1 did not affect exosome secretion. Moreover, in contrast to findings obtained with other cell lines published so far, inhibition of neutral sphingomyelinase 2, an enzyme that catalyzes the formation of ceramide from sphingomyelin, did not inhibit the secretion of exosomes in PC-3 cells. Finally, small interfering RNA-mediated downregulation of flotillin-1 and flotillin-2 did not significantly change the levels of released exosomes as such, but seemed to affect the composition of exosomes. In conclusion, our results reveal the involvement of glycosphingolipids and flotillins in the release of exosomes from PC-3 cells, and indicate that the role of ceramide in exosome formation may be cell-dependent.

Molecular paleontology and complexity in the last eukaryotic common ancestor

Eukaryogenesis, the origin of the eukaryotic cell, represents one of the fundamental evolutionary transitions in the history of life on earth. This event, which is estimated to have occurred over one billion years ago, remains rather poorly understood. While some well-validated examples of fossil microbial eukaryotes for this time frame have been described, these can provide only basic morphology and the molecular machinery present in these organisms has remained unknown. Complete and partial genomic information has begun to fill this gap, and is being used to trace proteins and cellular traits to their roots and to provide unprecedented levels of resolution of structures, metabolic pathways and capabilities of organisms at these earliest points within the eukaryotic lineage. This is essentially allowing a molecular paleontology. What has emerged from these studies is spectacular cellular complexity prior to expansion of the eukaryotic lineages. Multiple reconstructed cellular systems indicate a very sophisticated biology, which by implication arose following the initial eukaryogenesis event but prior to eukaryotic radiation and provides a challenge in terms of explaining how these early eukaryotes arose and in understanding how they lived. Here, we provide brief overviews of several cellular systems and the major emerging conclusions, together with predictions for subsequent directions in evolution leading to extant taxa. We also consider what these reconstructions suggest about the life styles and capabilities of these earliest eukaryotes and the period of evolution between the radiation of eukaryotes and the eukaryogenesis event itself.

The role of flotillins in regulating aβ production, investigated using flotillin 1-/-, flotillin 2-/- double knockout mice

Flotillin 1 and flotillin 2 associate in the plasma membrane to form microdomains that have roles in cell signaling, regulation of cell-cell contacts, membrane-cytoskeletal interactions, and endocytosis. They are thought to be involved in the trafficking and hence processing of the Amyloid Precursor Protein, APP. In this study we set out to obtain in vivo confirmation of a link between flotillins and cleavage of APP to release amyloidogenic Aβ peptide, and to generate tools that would allow us to ask whether flotillins are functionally redundant. We used a mouse model for Aβ-dependent cerebral amyloidosis, APPPS1 mice, combined with deletion of either flotillin 1 singly, or flotillin 1 and flotillin 2 together. There was a small but significant reduction in Aβ levels, and the abundance of congo-red stained plaques, in brains of 12 week old mice lacking flotillin 1. A similar reduction in Aβ levels was observed in the flotillin 1-/-, flotillin 2-/- double knockouts. We did not observe large effects on the clustering or endocytosis of APP in flotillin 1-/- mouse embryonic fibroblasts. We conclude that flotillins are likely to play some role in APP trafficking or processing, but the relevant cellular mechanisms require more investigation. The availability of flotillin 1-/-, flotillin 2-/- mice, which have no overt phenotypes, will facilitate research into flotillin function in vivo.

Functional genomic screen identifies novel mediators of collagen uptake

Tissue fibrosis occurs when matrix production outpaces matrix degradation. Degradation of collagen, the main component of fibrotic tissue, is mediated through an extracellular proteolytic pathway and intracellular pathway of cellular uptake and lysosomal digestion. Recent studies demonstrate that disruption of the intracellular pathways can exacerbate fibrosis. These pathways are poorly characterized. Here we identify novel mediators of the intracellular pathway of collagen turnover through a genome-wide RNA interference screen in Drosophila S2 cells. Screening of 7505 Drosophila genes conserved among metazoans identified 22 genes that were required for efficient internalization of type I collagen. These included proteins involved in vesicle transport, the actin cytoskeleton, and signal transduction. We show further that the flotillin genes have a conserved and central role in collagen uptake in Drosophila and human cells. Short hairpin RNA-mediated silencing of flotillins in human monocyte and fibroblasts impaired collagen uptake by promoting lysosomal degradation of the endocytic collagen receptors uPARAP/Endo180 and mannose receptor. These data provide an initial characterization of intracellular pathways of collagen turnover and identify the flotillin genes as critical regulators of this process. A better understanding of these pathways may lead to novel therapies that reduce fibrosis by increasing collagen turnover.

Refurbishing the plasmodesmal chamber: a role for lipid bodies?

Lipid bodies (LBs) are universal constituents of both animal and plant cells. They are produced by specialized membrane domains at the tubular endoplasmic reticulum (ER), and consist of a core of neutral lipids and a surrounding monolayer of phospholipid with embedded amphipathic proteins. Although originally regarded as simple depots for lipids, they have recently emerged as organelles that interact with other cellular constituents, exchanging lipids, proteins and signaling molecules, and shuttling them between various intracellular destinations, including the plasmamembrane (PM). Recent data showed that in plants LBs can deliver a subset of 1,3-β-glucanases to the plasmodesmal (PD) channel. We hypothesize that this may represent a more general mechanism, which complements the delivery of glycosylphosphatidylinositol (GPI)-anchored proteins to the PD exterior via the secretory pathway. We propose that LBs may contribute to the maintenance of the PD chamber and the delivery of regulatory molecules as well as proteins destined for transport to adjacent cells. In addition, we speculate that LBs deliver their cargo through interaction with membrane domains in the cytofacial side of the PM.

Insights into the mechanism for dictating polarity in migrating T-cells

This review is focused on mechanisms of chemokine-induced polarization of T-lymphocytes. Polarization involves, starting from spherical cells, formation of a morphologically and functionally different rear (uropod) and front (leading edge). This polarization is required for efficient random and directed T-cell migration. The addressed topics concern the specific location of cell organelles and of receptors, signaling molecules, and cytoskeletal proteins in chemokine-stimulated polarized T-cells. In chemokine-stimulated, polarized T-cells, specific proteins, signaling molecules and organelles show enrichment either in the rear, the midzone, or the front; different from the random location in spherical resting cells. Possible mechanisms involved in this asymmetric location will be discussed. A major topic is also the functional role of proteins and cell organelles in T-cell polarization and migration. Specifically, the roles of adhesion and chemokine receptors, cytoskeletal proteins, signaling molecules, scaffolding proteins, and membrane microdomains in these processes will be discussed. The polarity which is established during contact formation of T-cells with antigen-presenting cells is not discussed in detail.

Massive endocytosis triggered by surface membrane palmitoylation under mitochondrial control in BHK fibroblasts

Large Ca transients cause massive endocytosis (MEND) in BHK fibroblasts by nonclassical mechanisms. We present evidence that MEND depends on mitochondrial permeability transition pore (PTP) openings, followed by coenzyme A (CoA) release, acyl CoA synthesis, and membrane protein palmitoylation. MEND is blocked by inhibiting mitochondrial Ca uptake or PTP openings, depleting fatty acids, blocking acyl CoA synthesis, metabolizing CoA, or inhibiting palmitoylation. It is triggered by depolarizing mitochondria or promoting PTP openings. After mitochondrial MEND blockade, MEND is restored by cytoplasmic acyl CoA or CoA. MEND is blocked by siRNA knockdown of the plasmalemmal acyl transferase, DHHC5. When acyl CoA is abundant, transient H2O2 oxidative stress or PKC activation initiates MEND, but the immediate presence of H2O2 prevents MEND. The PTP inhibitor, NIM811, significantly increases plasmalemma in normally growing cells. Thus, the MEND pathway may contribute to constitutive as well as pathological plasmalemma turnover in dependence on mitochondrial stress signaling. DOI: http://dx.doi.org/10.7554/eLife.01293.001.

Reggies/flotillins interact with Rab11a and SNX4 at the tubulovesicular recycling compartment and function in transferrin receptor and E-cadherin trafficking

In this study reggie-1/flotillin-2 is identified as a component of the tubulovesicular sorting and recycling compartment, where it interacts with and controls the activity of Rab11a and SNX4. Evidence is given that reggie-1 expression is necessary for the proper recycling of transferrin receptor and E-cadherin in HeLa and A431 cells, respectively.

The lipid raft proteins reggie-1 and -2 (flotillins) are implicated in membrane protein trafficking but exactly how has been elusive. We find that reggie-1 and -2 associate with the Rab11a, SNX4, and EHD1–decorated tubulovesicular recycling compartment in HeLa cells and that reggie-1 directly interacts with Rab11a and SNX4. Short hairpin RNA–mediated down-regulation of reggie-1 (and -2) in HeLa cells reduces association of Rab11a with tubular structures and impairs recycling of the transferrin–transferrin receptor (TfR) complex to the plasma membrane. Overexpression of constitutively active Rab11a rescues TfR recycling in reggie-deficient HeLa cells. Similarly, in a Ca²⁺ switch assay in reggie-depleted A431 cells, internalized E-cadherin is not efficiently recycled to the plasma membrane upon Ca²⁺ repletion. E-cadherin recycling is rescued, however, by overexpression of constitutively active Rab11a or SNX4 in reggie-deficient A431 cells. This suggests that the function of reggie-1 in sorting and recycling occurs in association with Rab11a and SNX4. Of interest, impaired recycling in reggie-deficient cells leads to de novo E-cadherin biosynthesis and cell contact reformation, showing that cells have ways to compensate the loss of reggies. Together our results identify reggie-1 as a regulator of the Rab11a/SNX4-controlled sorting and recycling pathway, which is, like reggies, evolutionarily conserved.

RIP1 expression is necessary for CD30-mediated cell death induction in anaplastic large-cell lymphoma cells

CD30, a member of the tumor necrosis factor receptor (TNFR) superfamily, is consistently expressed by tumor cells of anaplastic large-cell lymphoma (ALCL). CD30 stimulation induces massive caspase-dependent cell death of ALCL cells in case of canonical NFκB inhibition or proteasome inhibition. However, CD30, a TNFR lacking a death domain (DD), is unable to recruit a death inducing complex containing TRADD (TNFR1-associated DD-protein) or FADD (FAS-associated DD-domain protein) together with the receptor-interacting protein 1 (RIP1) and caspase-8. Thus, the mechanism explaining CD30-induced cell death of lymphocytes remains obscure. Here, we demonstrate that blockage of RIP1 by siRNA or pharmacological inhibition of RIP1 by Necrostatin-1 almost completely prevented CD30-induced cell death. In addition, we revealed CD30-induced accumulation of RIP1 at the cytoplasma membrane of NFκB-inhibited ALCL cells by confocal laser scanning microscopy. Finally, primary ALCL cases can be subdivided into two groups based on the presence or absence of RIP1 as revealed by immunohistology. Taken together, our study identified RIP1 as a crucial mediator of CD30-induced cell death that bears features of apoptosis as well as necroptosis. RIP1 expression in ALCL tumor cells might eligible for the therapeutic application of CD30 antibodies in combination with NFκB/proteasome inhibitors that should result in CD30-induced cell death.

Flotillin-1 mediates PrPc endocytosis in the cultured cells during Cu²⁺ stimulation through molecular interaction

Flotillins are membrane association proteins consisting of two homologous members, flotillin-1 (Flot-1) and flotillin-2 (Flot-2). They define a clathrin-independent endocytic pathway in mammal cells, which are also distinct from some other endocytosis mechanisms. The implicated cargoes of the flotillin-dependent pathway are mainly some GPI-anchored proteins, such as CD59 and Thy-1, which positionally colocalize with flotillins at the plasma membrane microdomains. To see whether flotillins are involved in the endocytosis of PrP(C), the potential molecular interaction between PrP(C) and flotillins in a neuroblastoma cell line SK-N-SH was analyzed. Co-immunoprecipitation assays did not reveal a detectable complex in the cell lysates of a normal feeding situation. After stimulation of Cu(2+), PrP(C) formed a clear complex with Flot-1, but not with Flot-2. Immunofluorescent assays illustrated that PrP(C) colocalized well with Flot-1, and the complexes of PrP(C)-Flot-1 shifted from the cell membrane to the cytoplasm along with the treatment of Cu(2+). Down-regulating the expression of Flot-1 in SK-N-SH cells by Flot-1-specific RNAi obviously abolished the Cu(2+)-stimulated endocytosis process of PrP(C). Moreover, we also found that in the cell line human embryonic kidney 293 (HEK293) without detectable PrP(C) expression, the distribution of cellular Flot-1 maintained almost unchanged during Cu(2+) treatment. Cu(2+)-induced PrP(C)-Flot-1 molecular interaction and endocytosis in HEK293 cells were obtained when expressing wild-type human PrP (PrP(PG5)), but not in the preparation expressing octarepeat-deleted PrP (PrP(PG0)). Our data here provide direct evidences for the molecular interaction and endocytosis of PrP(C) with Flot-1 in the presence of copper ions, and the octarepeat region of PrP(C) is critical for this process, which strongly indicates that the Flot-1-dependent endocytic pathway seems to mediate the endocytosis process of PrP(C) in the special situation.

Trafficking of the Menkes copper transporter ATP7A is regulated by clathrin-, AP-2-, AP-1-, and Rab22-dependent steps

ATP7A mediates copper absorption and feeds cuproenzymes in the trans-Golgi network. To regulate copper homeostasis, ATP7A cycles between the TGN and plasma membrane. The roles of clathrin, adaptor complexes, lipid rafts, and Rab22a are assessed in an attempt to decipher the regulatory proteins involved in ATP7A cycling.

The transporter ATP7A mediates systemic copper absorption and provides cuproenzymes in the trans-Golgi network (TGN) with copper. To regulate metal homeostasis, ATP7A constitutively cycles between the TGN and plasma membrane (PM). ATP7A trafficking to the PM is elevated in response to increased copper load and is reversed when copper concentrations are lowered. Molecular mechanisms underlying this trafficking are poorly understood. We assess the role of clathrin, adaptor complexes, lipid rafts, and Rab22a in an attempt to decipher the regulatory proteins involved in ATP7A cycling. While RNA interference (RNAi)–mediated depletion of caveolin 1/2 or flotillin had no effect on ATP7A localization, clathrin heavy chain depletion or expression of AP180 dominant-negative mutant not only disrupted clathrin-regulated pathways, but also blocked PM-to-TGN internalization of ATP7A. Depletion of the μ subunits of either adaptor protein-2 (AP-2) or AP-1 using RNAi further provides evidence that both clathrin adaptors are important for trafficking of ATP7A from the PM to the TGN. Expression of the GTP-locked Rab22a_Q64L mutant caused fragmentation of TGN membrane domains enriched for ATP7A. These appear to be a subdomain of the mammalian TGN, showing only partial overlap with the TGN marker golgin-97. Of importance, ATP7A remained in the Rab22a_Q64L-generated structures after copper treatment and washout, suggesting that forward trafficking out of this compartment was blocked. This study provides evidence that multiple membrane-associated factors, including clathrin, AP-2, AP-1, and Rab22, are regulators of ATP7A trafficking.

Endocytosis of gene delivery vectors: from clathrin-dependent to lipid raft-mediated endocytosis

The ideal nonviral vector delivers its nucleic acid cargo to a specific intracellular target. Vectors enter cells mainly through endocytosis and are distributed to various intracellular organelles. Recent advances in microscopy, lipidomics, and proteomics confirm that the cell membrane is composed of clusters of lipids, organized in the form of lipid raft domains, together with non-raft domains that comprise a generally disordered lipid milieu. The binding of a nonviral vector to either region can determine the pathway for its endocytic uptake and subsequent intracellular itinerary. Given this model of the cell membrane structure, endocytic pathways should be reclassified in relation to lipid rafts. In this review, we attempt to assess the currently recognized endocytic pathways in mammalian cells. The endocytic pathways are classified in relation to the membrane regions that make up the primary endocytic vesicles. This review covers the well-recognized clathrin-mediated endocytosis (CME), phagocytosis, and macropinocytosis in addition to the less addressed pathways that take place in lipid rafts. These include caveolae-mediated, flotillin-dependent, GTPase regulator associated with focal adhesion kinase-1 (GRAF1)-dependent, adenosine diphosphate-ribosylation factor 6 (Arf6)-dependent, and RhoA-dependent endocytic pathways. We summarize the regulators associated with each uptake pathway and methods for interfering with these regulators are discussed. The fate of endocytic vesicles resulting from each endocytic uptake pathway is highlighted.

Flotillins regulate membrane mobility of the dopamine transporter but are not required for its protein kinase C dependent endocytosis

Flotillins were proposed to mediate clathrin-independent endocytosis, and recently, flotillin-1 was implicated in the protein kinase C (PKC)-triggered endocytosis of the dopamine transporter (DAT). Since endocytosis of DAT was previously shown to be clathrin-mediated, we re-examined the role of clathrin coat proteins and flotillin in DAT endocytosis using DAT tagged with the hemagglutinin epitope (HA) in the extracellular loop and a quantitative HA antibody uptake assay. Depletion of flotillin-1, flotillin-2 or both flotillins together by small interfering RNAs (siRNAs) did not inhibit PKC-dependent internalization and degradation of HA-DAT. In contrast, siRNAs to clathrin heavy chain and μ2 subunit of clathrin adaptor complex AP-2 as well as a dynamin inhibitor Dyngo-4A significantly decreased PKC-dependent endocytosis of HA-DAT. Similarly, endocytosis and degradation of DAT that is not epitope-tagged were highly sensitive to the clathrin siRNAs and dynamin inhibition but were not affected by flotillin knockdown. Very little co-localization of DAT with flotillins was observed in cells ectopically expressing DAT and in cultured mouse dopaminergic neurons. Depletion of flotillins increased diffusion rates of HA-DAT in the plasma membrane, suggesting that flotillin-organized microdomains may regulate the lateral mobility of DAT. We propose that clathrin-mediated endocytosis is the major pathway of PKC-dependent internalization of DAT, and that flotillins may modulate functional association of DAT with plasma membrane rafts rather than mediate DAT endocytosis.

Wnt secretion and gradient formation

Concentration gradients formed by the lipid-modified morphogens of the Wnt family are known for their pivotal roles during embryogenesis and adult tissue homeostasis. Wnt morphogens are also implicated in a variety of human diseases, especially cancer. Therefore, the signaling cascades triggered by Wnts have received considerable attention during recent decades. However, how Wnts are secreted and how concentration gradients are formed remains poorly understood. The use of model organisms such as Drosophila melanogaster has provided important advances in this area. For instance, we have previously shown that the lipid raft-associated reggie/flotillin proteins influence Wnt secretion and spreading in Drosophila. Our work supports the notion that producing cells secrete Wnt molecules in at least two pools: a poorly diffusible one and a reggie/flotillin-dependent highly diffusible pool which allows morphogen spreading over long distances away from its source of production. Here we revise the current views of Wnt secretion and spreading, and propose two models for the role of the reggie/flotillin proteins in these processes: (i) reggies/flotillins regulate the basolateral endocytosis of the poorly diffusible, membrane-bound Wnt pool, which is then sorted and secreted to apical compartments for long-range diffusion, and (ii) lipid rafts organized by reggies/flotillins serve as “dating points” where extracellular Wnt transiently interacts with lipoprotein receptors to allow its capture and further spreading via lipoprotein particles. We further discuss these processes in the context of human breast cancer. A better understanding of these phenomena may be relevant for identification of novel drug targets and therapeutic strategies.

Membrane trafficking in podocyte health and disease

Podocytes are highly specialized epithelial cells localized in the kidney glomerulus. The distinct cell signaling events and unique cytoskeletal architecture tailor podocytes to withstand changes in hydrostatic pressure during glomerular filtration. Alteration of glomerular filtration leads to kidney disease and frequently manifests with proteinuria. It has been increasingly recognized that cell signaling and cytoskeletal dynamics are coupled more tightly to membrane trafficking than previously thought. Membrane trafficking coordinates the cross-talk between protein networks and signaling cascades in a spatially and temporally organized fashion and may be viewed as a communication highway between the cell exterior and interior. Membrane trafficking involves transport of cargo from the plasma membrane to the cell interior (i.e., endocytosis) followed by cargo trafficking to lysosomes for degradation or to the plasma membrane for recycling. Yet, recent studies indicate that the conventional classification does not fully reflect the complex and versatile nature of membrane trafficking. While the increasing complexity of elaborate protein scaffolds and signaling cascades is being recognized in podocytes, the role of membrane trafficking is less well understood. This review will focus on the role of membrane trafficking in podocyte health and disease.

Flotillin-2 expression in the human gut: from a cell model to human tissue in health and inflammatory bowel diseases

BACKGROUND AND AIMS

The etiopathogenesis of inflammatory bowel diseases (IBD) remains largely unexplained. Flotillins (flotillin-1 and flotillin-2) are ubiquitous proteins which have been linked to inflammation and regeneration. We hypothesized that alterations in the expression of flotillin-2 in enterocytes may be related to the pathogenesis of IBD as a classical example of an inflammatory disorder of mostly unknown origin.

METHODS

Cell and tissue localization of flotillin-2 (and -1) were investigated by immunofluorescent staining in 1. polarized and unpolarized CaCo-2w cells as a model of human enterocytes (native and after TNFα stimulation) and 2. intestinal biopsies from controls, patients with ulcerative colitis (UC) and patients with Crohn's disease (CD). For quantification of flotillin-2, we analyzed its expression in ileal and colonic biopsies from controls, UC patients and CD patients using real-time RT-PCR, Western blot and indirect immunofluorescence.

RESULTS

In polarized CaCo-2w cells and human enterocytes in biopsies, flotillins were localized at the basolateral membrane and on subapical vesicles, but not in the apical membrane. Flotillin-2 expression did not differ between UC patients, CD patients and controls. However, it was significantly higher in colonic biopsies compared to ileal biopsies in all groups.

CONCLUSIONS

By virtue of its abundant expression in enterocytes, flotillin-2 must have an essential function in intestinal physiology, especially in the colon. Yet our data could not link flotillin-2 to the pathogenesis of IBD.

The HSP90 inhibitor geldanamycin perturbs endosomal structure and drives recycling ErbB2 and transferrin to modified MVBs/lysosomal compartments

The ErbB2 receptor is a validated cancer target whose internalization and trafficking remain poorly understood. The authors propose that ErbB2 internalization upon geldanamycin (GA) occurs predominantly via clathrin-mediated endocytosis and that GA affects endosomal structure and sorting, forcing recycling cargoes toward mixed endo/lysosomal compartments, irrespective of their HSP90 interaction.

The ErbB2 receptor is a clinically validated cancer target whose internalization and trafficking mechanisms remain poorly understood. HSP90 inhibitors, such as geldanamycin (GA), have been developed to target the receptor to degradation or to modulate downstream signaling. Despite intense investigations, the entry route and postendocytic sorting of ErbB2 upon GA stimulation have remained controversial. We report that ErbB2 levels inversely impact cell clathrin-mediated endocytosis (CME) capacity. Indeed, the high levels of the receptor are responsible for its own low internalization rate. GA treatment does not directly modulate ErbB2 CME rate but it affects ErbB2 recycling fate, routing the receptor to modified multivesicular endosomes (MVBs) and lysosomal compartments, by perturbing early/recycling endosome structure and sorting capacity. This activity occurs irrespective of the cargo interaction with HSP90, as both ErbB2 and the constitutively recycled, HSP90-independent, transferrin receptor are found within modified endosomes, and within aberrant, elongated recycling tubules, leading to modified MVBs/lysosomes. We propose that GA, as part of its anticancer activity, perturbs early/recycling endosome sorting, routing recycling cargoes toward mixed endosomal compartments.

Simplified equation to extract diffusion coefficients from confocal FRAP data

Quantitative measurements of diffusion can provide important information about how proteins and lipids interact with their environment within the cell and the effective size of the diffusing species. Confocal fluorescence recovery after photobleaching (FRAP) is one of the most widely accessible approaches to measure protein and lipid diffusion in living cells. However, straightforward approaches to quantify confocal FRAP measurements in terms of absolute diffusion coefficients are currently lacking. Here, we report a simplified equation that can be used to extract diffusion coefficients from confocal FRAP data using the half time of recovery and effective bleach radius for a circular bleach region, and validate this equation for a series of fluorescently labeled soluble and membrane-bound proteins and lipids. We show that using this approach, diffusion coefficients ranging over three orders of magnitude can be obtained from confocal FRAP measurements performed under standard imaging conditions, highlighting its broad applicability.

Flotillin-2 is an acrosome-related protein involved in mouse spermiogenesis

Spermatogenesis is a complex process of terminal differentiation by which mature sperms are generated, and it can be divided into three phases: mitosis, meiosis and spermiogenesis. In a previous study, we established a series of proteomic profiles for spermatogenesis to understand the regulation of male fertility and infertility. Here, we further investigated the localization and the role of flotillin-2 in spermiogenesis. Flotillin-2 expression was investigated in the testis of male CD1 mice at various developmental stages of spermatogenesis by using Western blotting, immunohistochemistry and immunofluorescence. Flotillin-2 was knocked down in vivo in three-week-old male mice using intratesticular injection of small inhibitory RNA (siRNA), and sperm abnormalities were assessed three weeks later. Flotillin-2 was expressed at high levels in male germ cells during spermatogenesis. Flotillin-2 immunoreactivity was observed in pachytene spermatocytes as a strong dot-shaped signal and in round spermatids as a sickle-shaped distribution ahead of the acrosome. Immunofluorescence confirmed flotillin-2 was localized in front of the acrosome in round spermatids, indicating that flotillin-2 was localized to the Golgi apparatus. Knockdown of flotillin-2in vivo led to a significant increase in head sperm abnormalities isolated from the cauda epididymis, compared with control siRNA-injected testes. This study indicates that flotillin-2 is a novel Golgi-related protein involved in sperm acrosome biogenesis.

The regulation of cell-cell adhesion during epithelial-mesenchymal transition, motility and tumor progression

Adherens junctions (AJs) are essential for the maintenance of epithelial homeostasis and a key factor in the regulation of cell migration and tumor progression. AJs maintain cell-cell adhesion by linking transmembrane proteins to the actin cytoskeleton. Additionally, they participate in recruitment of signaling receptors and cytoplasmic proteins to the membrane. During cellular invasion or migration, AJs are dynamically regulated and their composition modified to initiate changes in signaling pathways and cytoskeleton organization involved in cellular motility. Loss of E-cadherin, a key component of AJs, is characteristic of epithelial-mesenchymal-transition (EMT) and is associated with tumor cell invasion. We will review recent findings describing novel mechanisms involved in E-cadherin transcription regulation, endocytosis of E-cadherin and signaling associated with loss of AJs as well as reorganization of the AJ during EMT.

Harnessing the power of the endosome to regulate neural development

Endocytosis and endosomal trafficking play a multitude of roles in cellular function beyond regulating entry of essential nutrients. In this review, we discuss the cell biological principles of endosomal trafficking, the neuronal adaptations to endosomal organization, and the role of endosomal trafficking in neural development. In particular, we consider how cell fate decisions, polarity, migration, and axon outgrowth and guidance are influenced by five endosomal tricks: dynamic modulation of receptor levels by endocytosis and recycling, cargo-specific responses via cargo-specific endocytic regulators, cell-type-specific endocytic regulation, ligand-specific endocytic regulation, and endosomal regulation of ligand processing and trafficking.

Cell surface dynamics – how Rho GTPases orchestrate the interplay between the plasma membrane and the cortical cytoskeleton

Small GTPases are known to regulate hundreds of cell functions. In particular, Rho family GTPases are master regulators of the cytoskeleton. By regulating actin nucleation complexes, Rho GTPases control changes in cell shape, including the extension and/or retraction of surface protrusions and invaginations. Protrusion and invagination of the plasma membrane also involves the interaction between the plasma membrane and the cortical cytoskeleton. This interplay between membranes and the cytoskeleton can lead to an increase or decrease in the plasma membrane surface area and its tension as a result of the fusion (exocytosis) or internalization (endocytosis) of membranous compartments, respectively. For a long time, the cytoskeleton and plasma membrane dynamics were investigated separately. However, studies from many laboratories have now revealed that Rho GTPases, their modulation of the cytoskeleton, and membrane traffic are closely connected during the dynamic remodeling of the cell surface. Arf- and Rab-dependent exocytosis of specific vesicles contributes to the targeting of Rho GTPases and their regulatory factors to discrete sites of the plasma membrane. Rho GTPases regulate the tethering of exocytic vesicles and modulate their subsequent fusion. They also have crucial roles in the different forms of endocytosis, where they participate in the sorting of membrane domains as well as the sculpting and sealing of membrane flasks and cups. Here, we discuss how cell surface dynamics depend on the orchestration of the cytoskeleton and the plasma membrane by Rho GTPases.