Preformed reggie/flotillin caps: stable priming platforms for macrodomain assembly in T cells

Pre-organized landscape of T cell surface

T cell activation is initiated by the recognition of specific antigenic peptides and subsequently accomplished by complex signaling cascades. These aspects have been extensively studied for decades as pivotal factors in the establishment of adaptive immunity. However, how receptors or signaling molecules are organized in the resting state prior to encountering antigens has received less attention. Recent advancements in super-resolution microscopy techniques have revealed topographically controlled pre-formed organization of key molecules involved in antigen recognition and signal transduction on microvillar projections of T cells before activation and substantial effort has been dedicated to characterizing the topological structure of resting T cells over the past decade. This review will summarize our current understanding of how key surface receptors are pre-organized on the T-cell plasma membrane and discuss the potential role of these receptors, which are preassembled prior to ligand binding in the early activation events of T cells.

Molecular characterization and function of the lipid raft protein Lvflotillin-1A from Litopenaeus vannamei

White spot syndrome virus (WSSV) can cause a contagious, high virulent and pandemic disease for crustaceans, especially shrimps. However, the molecular mechanism of WSSV pathogenesis remains unclear. Flotillins are lipid raft-associated proteins, which mainly include flotillin-1 and flotillin-2. They are involved in the formation of large heteromeric protein complexes engaged in diverse signalling pathways at the membrane-cytosol interface. They defined a clathrin-independent endocytic pathway in mammalian cells. Our previous studies suggested that shrimp flotillin-2 might mediate endocytosis involved in WSSV infection. To further explore the function of shrimp flotillin, a flotillin-1 homologous, Lvflotillin-1A was identified and characterized in Litopenaeus vanamei. The transcription of Lvflotillin-1A showed a significant decline at 12h post-infection, followed by complete recovery and a slight up-regulation after the WSSV challenge. Gene silencing revealed that inhibition of Lvflotillin-1A raised the virus infection, suggesting Lvflotillin-1A might play an important role in shrimp immunity. Furthermore, co-immunoprecipitation and immunofluorescence illustrated that Lvflotillin-1A and Lvflotillin-2 could form hetero-oligomers, and co-expression promoted the accumulation of intracellular vesicles. The study revealed that WSSV might up-regulate Lvflotillin-2 expression and alter the subcellular location of Lvflotillin-1 protein to facilitate virus infection. These results will provide information for understanding the interaction between WSSV and shrimp.

MPP1 Determines the Mobility of Flotillins and Controls the Confinement of Raft-Associated Molecules

MPP1 (membrane palmitoylated protein 1) belongs to the MAGUK (membrane-associated guanylate kinase homologs) scaffolding protein family. These proteins organize molecules into complexes, thereby maintaining the structural heterogeneity of the plasma membrane (PM). Our previous results indicated that direct, high-affinity interactions between MPP1 and flotillins (raft marker proteins) display dominant PM-modulating capacity in erythroid cells. In this study, with high-resolution structured illuminated imaging, we investigated how these complexes are organized within erythroid cells on the nanometer scale. Furthermore, using other spectroscopic techniques, namely fluorescence recovery after photobleaching (FRAP) and spot-variation fluorescence correlation spectroscopy (svFCS), we revealed that MPP1 acts as a key raft-capturing molecule, regulating temporal immobilization of flotillin-based nanoclusters, and controls local concentration and confinement of sphingomyelin and Thy-1 in raft nanodomains. Our data enabled us to uncover molecular principles governing the key involvement of MPP1-flotillin complexes in the dynamic nanoscale organization of PM of erythroid cells.

Case Report: Anti-flotillin 1/2 Autoantibody-Associated Atypical Dementia

Flotillin proteins are involved in neurodegeneration and T-cell immunity. Here, we report the case of 65-year-old woman who presented with dementia, depressive symptoms, and a patient history involving speech problems. As diagnostics methods we applied magnetic resonance imaging, clinical examination, extensive neuropsychological testing, and cerebrospinal fluid analysis. Neuropsychological testing revealed major cognitive decline in attentional, executive, and memory functions together with impaired activities of daily living. The cerebrospinal fluid showed elevated phosphorylated tau protein 181. We identified serum autoantibodies against the flotillin 1/2 complex. Immunotherapy entailing four cycles of high-dose steroids resulted in less cognitive dysfunction along with reduced depressive symptoms in the second follow-up after starting steroids. In conclusion: probable autoimmune-mediated dementia associated with anti-flotillin 1/2 complex autoantibodies expands the phenotypic spectrum of anti-flotillin 1/2 antibody disease.

Caveolin-1, tetraspanin CD81 and flotillins in lymphocyte cell membrane organization, signaling and immunopathology

The adaptive immune system relies on B and T lymphocytes to ensure a specific and long-lasting protection of an individual from a wide range of potential pathogenic hits. Lymphocytes are highly potent and efficient in eliminating pathogens. However, lymphocyte activation must be tightly regulated to prevent incorrect activity that could result in immunopathologies, such as autoimmune disorders or cancers. Comprehensive insight into the molecular events underlying lymphocyte activation is of enormous importance to better understand the function of the immune system. It provides the basis to design therapeutics to regulate lymphocyte activation in pathological scenarios. Most reported defects in immunopathologies affect the regulation of intracellular signaling pathways. This highlights the importance of these molecules, which control lymphocyte activation and homeostasis impacting lymphocyte tolerance to self, cytokine production and responses to infections. Most evidence for these defects comes from studies of disease models in genetically engineered mice. There is an increasing number of studies focusing on lymphocytes derived from patients which supports these findings. Many indirectly involved proteins are emerging as unexpected regulators of the immune system. In this mini-review, we focus in proteins that regulate plasma membrane (PM) compartmentalization and thereby impact the steady state and the activation of immunoreceptors, namely the T cell antigen receptor (TCR) and the B cell antigen receptor (BCR). Some of these membrane proteins are shown to be involved in immune abnormalities; others, however, are not thoroughly investigated in the context of immune pathogenesis. We aim to highlight them and stimulate future research avenues.

A sensitive S-Trap-based approach to the analysis of T cell lipid raft proteome

The analysis of T cell lipid raft proteome is challenging due to the highly dynamic nature of rafts and the hydrophobic character of raft-resident proteins. We explored an innovative strategy for bottom-up lipid raftomics based on suspension-trapping (S-Trap) sample preparation. Mouse T cells were prepared from splenocytes by negative immunoselection, and rafts were isolated by a detergent-free method and OptiPrep gradient ultracentrifugation. Microdomains enriched in flotillin-1, LAT, and cholesterol were subjected to proteomic analysis through an optimized protocol based on S-Trap and high pH fractionation, followed by nano-LC-MS/MS. Using this method, we identified 2,680 proteins in the raft-rich fraction and established a database of 894 T cell raft proteins. We then performed a differential analysis on the raft-rich fraction from nonstimulated versus anti-CD3/CD28 T cell receptor (TCR)-stimulated T cells. Our results revealed 42 proteins present in one condition and absent in the other. For the first time, we performed a proteomic analysis on rafts from ex vivo T cells obtained from individual mice, before and after TCR activation. This work demonstrates that the proposed method utilizing an S-Trap-based approach for sample preparation increases the specificity and sensitivity of lipid raftomics.

Stomatin is highly expressed in exosomes of different origin and is a promising candidate as an exosomal marker

Proteins involved in the organizing of lipid rafts can be found in exosomes, as shown for caveolin-1, and they could contribute to exosomal cargo sorting, as shown for flotillins. Stomatin belongs to the same stomatin/prohibitin/flotillin/HflK/C family of lipid rafts proteins, but it has never been studied in exosomes except for extracellular vesicles (EVs) originating from blood cells. Here we first show the presence of stomatin in exosomes produced by epithelial cancer cells (non-small cell lung cancer, breast, and ovarian cancer cells) as well as in EVs from biological fluids, including blood plasma, ascitic fluids, and uterine flushings. A high abundance of stomatin in EVs of various origins and its enrichment in exosomes make stomatin a promising exosomal marker. Comparison with other lipid raft proteins and exosomal markers showed that the level of stomatin protein in exosomes from different sources corresponds well to that of CD9, while it differs essentially from flotillin-1 and flotillin-2 homologs, which in turn are present in exosomes in nearly equal proportions. In contrast, the level of vesicular caveolin-1 as well as its EV-to-cellular ratio vary drastically depending on cell type.

Membrane Compartmentalization and Scaffold Proteins in Leukocyte Migration

Leukocyte migration across vessels into and within peripheral and lymphoid tissues is essential for host defense against invading pathogens. Leukocytes are specialized in sensing a variety of guidance cues and to integrate environmental stimuli to navigate in a timely and spatially controlled manner. These extracellular signals must be transmitted across the leukocyte’s plasma membrane in a way that intracellular signaling cascades enable directional cell movement. Therefore, the composition of the membrane in concert with proteins that influence the compartmentalization of the plasma membrane or contribute to delineate intracellular signaling molecules are key in controlling leukocyte navigation. This becomes evident by the fact that mislocalization of membrane proteins is known to deleteriously affect cellular functions that may cause diseases. In this review we summarize recent advances made in the understanding of how membrane cholesterol levels modulate chemokine receptor signaling and hence leukocyte trafficking. Moreover, we provide an overview on the role of membrane scaffold proteins, particularly tetraspanins, flotillins/reggies, and caveolins in controlling leukocyte migration both in vitro and in vivo.

Lipid Rafts in Exosome Biogenesis

Exosomes (secreted extracellular vesicles formed in the intracellular vesicular transport system) play a crucial role in distant cell-cell communication. Exosomes transfer active forms of various biomolecules; the molecular composition of the exosomal cargo is a result of targeted selection and depends on the type of producer cells. The mechanisms underlying exosome formation and cargo selection are poorly understood. It is believed that there are several pathways for exosome biogenesis, although the questions about their independence and simultaneous coexistence in the cell still remain open. The least studied topic is the recently discovered mechanism of exosome formation associated with lipid rafts, or membrane lipid microdomains. Here, we present modern concepts and basic hypotheses on the mechanisms of exosome biogenesis and secretion and summarize current data on the involvement of lipid rafts and their constituent molecules in these processes. Special attention is paid to the analysis of possible role in the exosome formation of raft-forming proteins of the SPFH family, components of planar rafts, and caveolin, the main component of caveolae.

Flotillins: At the Intersection of Protein S-Palmitoylation and Lipid-Mediated Signaling

Flotillin-1 and flotillin-2 are ubiquitously expressed, membrane-associated proteins involved in multifarious cellular events from cell signaling, endocytosis, and protein trafficking to gene expression. They also contribute to oncogenic signaling. Flotillins bind the cytosolic leaflet of the plasma membrane and endomembranes and, upon hetero-oligomerization, serve as scaffolds facilitating the assembly of multiprotein complexes at the membrane-cytosol interface. Additional functions unique to flotillin-1 have been discovered recently. The membrane-binding of flotillins is regulated by S-palmitoylation and N-myristoylation, hydrophobic interactions involving specific regions of the polypeptide chain and, to some extent, also by their oligomerization. All these factors endow flotillins with an ability to associate with the sphingolipid/cholesterol-rich plasma membrane domains called rafts. In this review, we focus on the critical input of lipids to the regulation of the flotillin association with rafts and thereby to their functioning. In particular, we discuss how the recent developments in the field of protein S-palmitoylation have contributed to the understanding of flotillin1/2-mediated processes, including endocytosis, and of those dependent exclusively on flotillin-1. We also emphasize that flotillins affect directly or indirectly the cellular levels of lipids involved in diverse signaling cascades, including sphingosine-1-phosphate and PI(4,5)P₂. The mutual relations between flotillins and distinct lipids are key to the regulation of their involvement in numerous cellular processes.

Flotillins promote T cell receptor sorting through a fast Rab5-Rab11 endocytic recycling axis

The targeted endocytic recycling of the T cell receptor (TCR) to the immunological synapse is essential for T cell activation. Despite this, the mechanisms that underlie the sorting of internalised receptors into recycling endosomes remain poorly understood. To build a comprehensive picture of TCR recycling during T cell activation, we developed a suite of new imaging and quantification tools centred on photoactivation of fluorescent proteins. We show that the membrane-organising proteins, flotillin-1 and -2, are required for TCR to reach Rab5-positive endosomes immediately after endocytosis and for transfer from Rab5- to Rab11a-positive compartments. We further observe that after sorting into in Rab11a-positive vesicles, TCR recycles to the plasma membrane independent of flotillin expression. Our data suggest a mechanism whereby flotillins delineate a fast Rab5-Rab11a endocytic recycling axis and functionally contribute to regulate the spatial organisation of these endosomes.

Internalized receptors are recycled back to the cell surface, but their precise mechanisms are unclear. Here, the authors show that the flotillin membrane proteins may regulate the transfer of internalized T cell receptors into Rab5 and Rab11-positive endosomes to support its rapid recycling.

In Vivo Function of the Lipid Raft Protein Flotillin-1 during CD8+ T Cell-Mediated Host Surveillance

Flotillin-1 (Flot1) is an evolutionary conserved, ubiquitously expressed lipid raft-associated scaffolding protein. Migration of Flot1-deficient neutrophils is impaired because of a decrease in myosin II-mediated contractility. Flot1 also accumulates in the uropod of polarized T cells, suggesting an analogous role in T cell migration. In this study, we analyzed morphology and migration parameters of murine wild-type and Flot1^-/- CD8⁺ T cells using in vitro assays and intravital two-photon microscopy of lymphoid and nonlymphoid tissues. Flot1^-/- CD8⁺ T cells displayed significant alterations in cell shape and motility parameters in vivo but showed comparable homing to lymphoid organs and intact in vitro migration to chemokines. Furthermore, their clonal expansion and infiltration into nonlymphoid tissues during primary and secondary antiviral immune responses was comparable to wild-type CD8⁺ T cells. Taken together, Flot1 plays a detectable but unexpectedly minor role for CD8⁺ T cell behavior under physiological conditions.

The Virtuous Cycle of Axon Growth: Axonal Transport of Growth-Promoting Machinery as an Intrinsic Determinant of Axon Regeneration

Injury to the brain and spinal cord has devastating consequences because adult central nervous system (CNS) axons fail to regenerate. Injury to the peripheral nervous system (PNS) has a better prognosis, because adult PNS neurons support robust axon regeneration over long distances. CNS axons have some regenerative capacity during development, but this is lost with maturity. Two reasons for the failure of CNS regeneration are extrinsic inhibitory molecules, and a weak intrinsic capacity for growth. Extrinsic inhibitory molecules have been well characterized, but less is known about the neuron-intrinsic mechanisms which prevent axon re-growth. Key signaling pathways and genetic/epigenetic factors have been identified which can enhance regenerative capacity, but the precise cellular mechanisms mediating their actions have not been characterized. Recent studies suggest that an important prerequisite for regeneration is an efficient supply of growth-promoting machinery to the axon; however, this appears to be lacking from non-regenerative axons in the adult CNS. In the first part of this review, we summarize the evidence linking axon transport to axon regeneration. We discuss the developmental decline in axon regeneration capacity in the CNS, and comment on how this is paralleled by a similar decline in the selective axonal transport of regeneration-associated receptors such as integrins and growth factor receptors. In the second part, we discuss the mechanisms regulating selective polarized transport within neurons, how these relate to the intrinsic control of axon regeneration, and whether they can be targeted to enhance regenerative capacity. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

Roles of flotillins in tumors

The identification and use of molecular biomarkers have greatly improved the diagnosis and treatment of malignant tumors. However, a much deeper understanding of oncogenic proteins is needed for the benefit to cancer patients. The lipid raft marker proteins, flotillin-1 and flotillin-2, were first found in goldfish retinal ganglion cells during axon regeneration. They have since been found in a variety of cells, mainly on the inner surface of cell membranes, and not only act as a skeleton to provide a platform for protein-protein interactions, but also are involved in signal transduction, nerve regeneration, endocytosis, and lymphocyte activation. Previous studies have shown that flotillins are closely associated with tumor development, invasion, and metastasis. In this article, we review the functions of flotillins in relevant cell processes, their underlying mechanisms of action in a variety of tumors, and their potential applications to tumor molecular diagnosis and targeted therapy.

MPP1 directly interacts with flotillins in erythrocyte membrane - Possible mechanism of raft domain formation

Flotillins are prominent, oligomeric protein components of erythrocyte (RBC) membrane raft domains and are considered to play an important structural role in lateral organization of the plasma membrane. In our previous work on erythroid membranes and giant plasma membrane vesicles (GPMVs) derived from them we have shown that formation of functional domains (resting state rafts) depends on the presence of membrane palmitoylated protein 1 (MPP1/p55), pointing to its new physiological role. Exploration of the molecular mechanism of MPP1 function in organizing membrane domains described here, through searching for its molecular partners in RBC membrane by using different methods, led to the identification of the raft-marker proteins, flotillin 1 and flotillin 2, as hitherto unreported direct MPP1 binding-partners in the RBC membrane. These proteins are found in high molecular-weight complexes in native RBC membrane and, significantly, their presence was shown to be separate from the well-known protein 4.1-dependent interactions of MPP1 with membrane proteins. Furthermore, FLIM analysis revealed that loss of the endogenous MPP1-flotillins interactions resulted in significant changes in RBC membrane-fluidity, emphasizing the physiological importance of such interactions in vivo. Therefore, our data establish a new perspective on the role of MPP1 in erythroid cells and suggests that direct MPP1-flotillins interactions could be the major driving-force behind the formation of raft domains in RBC.

Orchestrating Lymphocyte Polarity in Cognate Immune Cell-Cell Interactions

The immune synapse (IS) is a specialized structure established between different immune cells that fulfills several functions, including a role as a communication bridge. This intimate contact between a T cell and an antigen-presenting cell promotes the proliferation and differentiation of lymphocytes involved in the contact. T-cell activation requires the specific triggering of the T-cell receptor (TCR), which promotes the activation of different signaling pathways inducing the polarization of the T cell. During this process, different adhesion and signaling receptors reorganize at specialized membrane domains, concomitantly to the polarization of the tubulin and actin cytoskeletons, forming stable polarization platforms. The centrosome also moves toward the IS, driving the movement of different organelles, such as the biosynthetic, secretory, degrading machinery, and mitochondria, to sustain T-cell activation. A proper orchestration of all these events is essential for T-cell effector functions and the accomplishment of a complete immune response.

The prion protein inhibits monocytic cell migration by stimulating β1 integrin adhesion and uropod formation

The broad tissue distribution and evolutionary conservation of the glycosylphosphatidylinositol (GPI)-anchored prion protein (PrP, also known as PRNP) suggests that it plays a role in cellular homeostasis. Given that integrin adhesion determines cell behavior, the proposed role of PrP in cell adhesion might underlie the various in vitro and in vivo effects associated with PrP loss-of-function, including the immune phenotypes described in PrP(-/-) mice. Here, we investigated the role of PrP in the adhesion and (transendothelial) migration of human (pro)monocytes. We found that PrP regulates β1-integrin-mediated adhesion of monocytes. Additionally, PrP controls the cell morphology and migratory behavior of monocytes: PrP-silenced cells show deficient uropod formation on immobilized VCAM and display bleb-like protrusions on the endothelium. Our data further show that PrP regulates ligand-induced integrin activation. Finally, we found that PrP controls the activation of several proteins involved in cell adhesion and migration, including RhoA and its effector cofilin, as well as proteins of the ERM family. We propose that PrP modulates β1 integrin adhesion and migration of monocytes through RhoA-induced actin remodeling mediated by cofilin, and through the regulation of ERM-mediated membrane-cytoskeleton linkage.

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.

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.

The actin-bundling protein L-plastin supports T-cell motility and activation

Tight regulation of actin dynamics is essential for T-cell trafficking and activation. Recent studies in human and murine T cells reveal that T-cell motility and full T-cell activation require the hematopoietic-specific, actin-bundling protein L-plastin (LPL). T cells lacking LPL do not form fully mature synapses and thus demonstrate reduced cytokine production and proliferation. Reduction or loss of LPL expression also reduces the velocity of T cells and impairs thymic egress and intranodal motility. Whereas dispensable for proximal T-cell receptor and chemokine receptor signaling, LPL is critical to the later stages of synapse maturation and cellular polarization. Serine phosphorylation, calcium, and calmodulin binding regulate the bundling activity and localization of LPL following T-cell receptor and chemokine receptor engagement. However, the interaction between these regulatory domains and resulting changes in local control of actin cytoskeletal structures has not been fully elucidated. Circumstantial evidence suggests a function for LPL in either the formation or maintenance of integrin-associated adhesion structures. As LPL may be a target of the commonly used immunosuppressive agent dexamethasone, full elucidation of the regulation and function of LPL in T-cell biology may illuminate new pathways for clinically useful immunotherapeutics.

Flotillin microdomains stabilize cadherins at cell-cell junctions

Cadherins are essential in many fundamental processes and assemble at regions of cell-cell contact in large macromolecular complexes named adherens junctions. We have identified flotillin 1 and 2 as new partners of the cadherin complexes. We show that flotillins are localised at cell-cell junctions (CCJs) in a cadherin-dependent manner. Flotillins and cadherins are constitutively associated at the plasma membrane and their colocalisation at CCJ increases with CCJ maturation. Using three-dimensional structured illumination super-resolution microscopy, we found that cadherin and flotillin complexes are associated with F-actin bundles at CCJs. The knockdown of flotillins dramatically affected N- and E-cadherin recruitment at CCJs in mesenchymal and epithelial cell types and perturbed CCJ integrity and functionality. Moreover, we determined that flotillins are required for cadherin association with GM1-containing plasma membrane microdomains. This allows p120 catenin binding to the cadherin complex and its stabilization at CCJs. Altogether, these data demonstrate that flotillin microdomains are required for cadherin stabilization at CCJs and for the formation of functional CCJs.

The roles of flotillin microdomains--endocytosis and beyond

Flotillins are membrane proteins that form microdomains in the plasma membrane of all mammalian cell types studied to date. They span the evolutionary spectrum, with proteins related to flotillins present in bacteria, fungi, plants and metazoans, which suggests that they perform important, and probably conserved, functions. Flotillins have been implicated in myriad processes that include endocytosis, signal transduction and regulation of the cortical cytoskeleton, yet the molecular mechanisms that underlie flotillin function in these different cases are still poorly understood. In this Commentary, we will provide an introduction to these intriguing proteins, summarise their proposed functions and discuss in greater detail some recent insights into the role of flotillin microdomains in endocytosis that have been provided by several independent studies. Finally, we will focus on the questions that are raised by these new experiments and their implications for future studies.

Flotillin-1/reggie-2 protein plays dual role in activation of receptor-tyrosine kinase/mitogen-activated protein kinase signaling

Our previous work has shown that the membrane microdomain-associated flotillin proteins are potentially involved in epidermal growth factor (EGF) receptor signaling. Here we show that knockdown of flotillin-1/reggie-2 results in reduced EGF-induced phosphorylation of specific tyrosines in the EGF receptor (EGFR) and in inefficient activation of the downstream mitogen-activated protein (MAP) kinase and Akt signaling. Although flotillin-1 has been implicated in endocytosis, its depletion affects neither the endocytosis nor the ubiquitination of the EGFR. However, EGF-induced clustering of EGFR at the cell surface is altered in cells lacking flotillin-1. Furthermore, we show that flotillins form molecular complexes with EGFR in an EGF/EGFR kinase-independent manner. However, knockdown of flotillin-1 appears to affect the activation of the downstream MAP kinase signaling more directly. We here show that flotillin-1 forms a complex with CRAF, MEK1, ERK, and KSR1 (kinase suppressor of RAS) and that flotillin-1 knockdown leads to a direct inactivation of ERK1/2. Thus, flotillin-1 plays a direct role during both the early phase (activation of the receptor) and late (activation of MAP kinases) phase of growth factor signaling. Our results here unveil a novel role for flotillin-1 as a scaffolding factor in the regulation of classical MAP kinase signaling. Furthermore, our results imply that other receptor-tyrosine kinases may also rely on flotillin-1 upon activation, thus suggesting a general role for flotillin-1 as a novel factor in receptor-tyrosine kinase/MAP kinase signaling.

Research advances on flotillins

The proteins of flotillin-1 and flotillin-2 were originally discovered in axon regeneration of goldfish retinal ganglion cells. They are generally used as marker proteins of lipid rafts and considered to be scaffolding proteins of lipid microdomains. Although they are ubiquitously expressed and well-conserved from fly to man, their exact functions remain controversial. In this review, we summarize the structure of flotillins and some functions of them, such as regulating axon regeneration, endocytosis, T cell activation, insulin signaling, membrane protein recruitment, roles in the progression of some diseases and so on.

Dynamic reorganization of flotillins in chemokine-stimulated human T-lymphocytes

Background

Different types of membrane microdomains (rafts) have been postulated to be present in the rear and front of polarized migrating T-lymphocytes. Disruption of rafts by cholesterol sequestration prevents T-cell polarization and migration. Reggie/flotillin-1 and -2 are two highly homologous proteins that are thought to shape membrane microdomains. We have previously demonstrated the enrichment of flotillins in the uropod of human neutrophils. We have now investigated mechanisms involved in chemokine-induced flotillin reorganization in human T-lymphocytes, and possible roles of flotillins in lymphocyte polarization.

Results

We studied flotillin reorganization and lateral mobility at the plasma membrane using immunofluorescence staining and FRAP (fluorescence recovery after photobleaching). We show that flotillins redistribute early upon chemokine stimulation, and form very stable caps in the uropods of human peripheral blood T-lymphocytes, colocalizing with the adhesion molecule PSGL-1 and activated ezrin/radixin/moesin (ERM) proteins. Chemokine-induced formation of stable flotillin caps requires integrity and dynamics of the actin cytoskeleton, but is not abolished by inhibitors suppressing Rho-kinase or myosin II activity. Tagged flotillin-2 and flotillin-1 coexpressed in T-lymphocytes, but not singly expressed proteins, colocalize in stable caps at the tips of uropods. Lateral mobility of coexpressed flotillins at the plasma membrane is already partially restricted in the absence of chemokine. Incubation with chemokine results in almost complete immobilization of flotillins. Capping is abolished when wild-type flotillin-1 is coexpressed with a mutant of flotillin-2 (G2A) that is unable to interact with the plasma membrane, or with a deletion mutant of flotillin-2 that lacks a putative actin-binding domain. Wild-type flotillin-2 in contrast forms caps when coexpressed with a mutant of flotillin-1 unable to interact with membranes. Transfection of T-lymphocytes with flotillin-2-G2A reduces cell polarization and uropod recruitment of endogenous flotillin-1 and PSGL-1.

Conclusions

Our data suggest that stable flotillin cap formation in the rear of polarized T-lymphocytes requires flotillin heterooligomer formation, as well as direct F-actin interactions of flotillin-2 and raft/membrane association of flotillin-2, but not -1. Our data also implicate flotillin-rich actin-dependent membrane microdomains in T-lymphocyte uropod formation.

Intoxication of zebrafish and mammalian cells by cholera toxin depends on the flotillin/reggie proteins but not Derlin-1 or -2

Cholera toxin (CT) causes the massive secretory diarrhea associated with epidemic cholera. To induce disease, CT enters the cytosol of host cells by co-opting a lipid-based sorting pathway from the plasma membrane, through the trans-Golgi network (TGN), and into the endoplasmic reticulum (ER). In the ER, a portion of the toxin is unfolded and retro- translocated to the cytosol. Here, we established zebrafish as a genetic model of intoxication and examined the Derlin and flotillin proteins, which are thought to be usurped by CT for retro-translocation and lipid sorting, respectively. Using antisense morpholino oligomers and siRNA, we found that depletion of Derlin-1, a component of the Hrd-1 retro-translocation complex, was dispensable for CT-induced toxicity. In contrast, the lipid raft-associated proteins flotillin-1 and -2 were required. We found that in mammalian cells, CT intoxication was dependent on the flotillins for trafficking between plasma membrane/endosomes and two pathways into the ER, only one of which appears to intersect the TGN. These results revise current models for CT intoxication and implicate protein scaffolding of lipid rafts in the endo-somal sorting of the toxin-GM1 complex.

Microdomain-forming proteins and the role of the reggies/flotillins during axon regeneration in zebrafish

The two proteins reggie-1 and reggie-2 (flotillins) were identified in axon-regenerating neurons in the central nervous system and shown to be essential for neurite growth and regeneration in fish and mammals. Reggies/flotillins are microdomain scaffolding proteins sharing biochemical properties with lipid raft molecules, form clusters at the cytoplasmic face of the plasma membrane and interact with signaling molecules in a cell type specific manner. In this review, reggie microdomains, lipid rafts, related scaffolding proteins and caveolin-which, however, are responsible for their own microdomains and functions-are introduced. Moreover, the function of the reggies in axon growth is demonstrated: neurons fail to extend axons after reggie knockdown. Furthermore, our current concept of the molecular mechanism underlying reggie function is presented: the association of glycosyl-phophatidyl inositol (GPJ)-anchored surface proteins with reggie microdomains elicits signals which activate src tyrosine and mitogen-activated protein kinases, as well as small guanosine 5'-triphosphate-hydrolyzing enzymes. This leads to the mobilization of intracellular vesicles and to the recruitment of bulk membrane and specific cargo proteins, such as cadherin, to specific sites of the plasma membrane such as the growth cone of elongating axons. Thus, reggies regulate the targeted delivery of cargo-a process which is required for process extension and growth. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

[Molecular principles of tau-induced toxicity: new experimental therapy strategies for treatment of Alzheimer's disease]

Neurofibrillary tangles are the hallmark of Alzheimer's disease together with amyloid plaques. They are composed of hyperphosphorylated and aggregated Tau proteins. Consequently, experimental disease modifying approaches include kinase and aggregation inhibitors as well as substances which increase degradation of Tau proteins.

Reggies/flotillins regulate retinal axon regeneration in the zebrafish optic nerve and differentiation of hippocampal and N2a neurons

The reggies/flotillins--proteins upregulated during axon regeneration in retinal ganglion cells (RGCs)--are scaffolding proteins of microdomains and involved in neuronal differentiation. Here, we show that reggies regulate axon regeneration in zebrafish (ZF) after optic nerve section (ONS) in vivo as well as axon/neurite extension in hippocampal and N2a neurons in vitro through signal transduction molecules modulating actin dynamics. ZF reggie-1a, -2a, and -2b downregulation by reggie-specific morpholino (Mo) antisense oligonucleotides directly after ONS significantly reduced ZF RGC axon regeneration: RGC axons from reggie Mo retinas were markedly reduced. Moreover, the number of axon-regenerating RGCs, identified by insertion of A488-coupled dextran, decreased by 69% in retinas 7 d after Mo application. At 10 and 14 d, RGCs decreased by 53 and 33%, respectively, in correlation with the gradual inactivation of the Mos. siRNA-mediated knockdown of reggie-1 and -2 inhibited the differentiation and axon/neurite extension in hippocampal and N2a neurons. N2a cells had significantly shorter filopodia, more cells had lamellipodia and fewer neurites, defects which were rescued by a reggie-1 construct without siRNA-binding sites. Furthermore, reggie knockdown strongly perturbed the balanced activation of the Rho family GTPases Rac1, RhoA, and cdc42, influenced the phosphorylation of cortactin and cofilin, the formation of the N-WASP, cortactin and Arp3 complex, and affected p38, Ras, ERK1/2 (extracellular signal-regulated kinases 1 and 2), and focal adhesion kinase activation. Thus, as suggested by their prominent re-expression after lesion, the reggies represent neuron-intrinsic factors for axon outgrowth and regeneration, being crucial for the coordinated assembly of signaling complexes regulating cytoskeletal remodeling.

Flotillins interact with PSGL-1 in neutrophils and, upon stimulation, rapidly organize into membrane domains subsequently accumulating in the uropod

Background

Neutrophils polarize and migrate in response to chemokines. Different types of membrane microdomains (rafts) have been postulated to be present in rear and front of polarized leukocytes and disruption of rafts by cholesterol sequestration prevents leukocyte polarization. Reggie/flotillin-1 and -2 are two highly homologous proteins that are ubiquitously enriched in detergent resistant membranes and are thought to shape membrane microdomains by forming homo- and hetero-oligomers. It was the goal of this study to investigate dynamic membrane microdomain reorganization during neutrophil activation.

Methodology/Principal Findings

We show now, using immunofluorescence staining and co-immunoprecipitation, that endogenous flotillin-1 and -2 colocalize and associate in resting spherical and polarized primary neutrophils. Flotillins redistribute very early after chemoattractant stimulation, and form distinct caps in more than 90% of the neutrophils. At later time points flotillins accumulate in the uropod of polarized cells. Chemotactic peptide-induced redistribution and capping of flotillins requires integrity and dynamics of the actin cytoskeleton, but does not involve Rho-kinase dependent signaling related to formation of the uropod. Both flotillin isoforms are involved in the formation of this membrane domain, as uropod location of exogenously expressed flotillins is dramatically enhanced by co-overexpression of tagged flotillin-1 and -2 in differentiated HL-60 cells as compared to cells expressing only one tagged isoform.

Flotillin-1 and -2 associate with P-selectin glycoprotein ligand 1 (PSGL-1) in resting and in stimulated neutrophils as shown by colocalization and co-immunoprecipitation. Neutrophils isolated from PSGL-1-deficient mice exhibit flotillin caps to the same extent as cells isolated from wild type animals, implying that PSGL-1 is not required for the formation of the flotillin caps. Finally we show that stimulus-dependent redistribution of other uropod-located proteins, CD43 and ezrin/radixin/moesin, occurs much slower than that of flotillins and PSGL-1.

Conclusions/Significance

These results suggest that flotillin-rich actin-dependent membrane microdomains are importantly involved in neutrophil uropod formation and/or stabilization and organize uropod localization of PSGL-1.

Localization of low-density detergent-resistant membrane proteins in intact and acrosome-reacted mouse sperm

Mammalian sperm become fertile after completing capacitation, a process associated with cholesterol loss and changes in the biophysical properties of the sperm membranes that prepares the sperm to undergo the acrosome reaction. Different laboratories have hypothesized that cholesterol efflux can influence the extent and/or movement of lipid raft microdomains. In a previous study, our laboratory investigated the identity of sperm proteins putatively associated with rafts. After extraction with Triton X-100 and ultracentrifugation in sucrose gradients, proteins distributing to the light buoyant-density fractions were cored from polyacrylamide gels and microsequenced. In this study, a subset of these proteins (TEX101, basigin, hexokinase 1, facilitated glucose transporter 3, IZUMO, and SPAM1) and other molecules known to be enriched in membrane rafts (caveolin 2, flotillin 1, flotillin 2, and the ganglioside GM3) were selected to investigate their localization in the sperm and their behavior during capacitation and the acrosome reaction. These molecules localize to multiple sperm domains, including the acrosomal cap (IZUMO, caveolin 2, and flotillin 2), equatorial segment (GM3), cytoplasmic droplet (TEX101), midpiece (basigin, facilitated glucose transporter 3, and flotillin 2), and principal piece (facilitated glucose transporter 3). Some of these markers modified their immunofluorescence pattern after sperm incubation under capacitating conditions, and these changes correlated with the occurrence of the acrosome reaction. While GM3 and caveolin 2 were not detected after the acrosome reaction, flotillin 2 was found in the equatorial segment of acrosome-reacted sperm, and IZUMO distributed along the sperm head, reaching the post- and para-acrosomal areas. Taking into consideration the requirement of the acrosome reaction for sperm to become fusogenic, these results suggest that membrane raft dynamics may have a role in sperm-egg membrane interaction.

Regulation of focal adhesion formation and filopodia extension by the cellular prion protein (PrPC)

While the prion protein (PrP) is clearly involved in neuropathology, its physiological roles remain elusive. Here, we demonstrate PrP functions in cell-substrate interaction in Drosophila S2, N2a and HeLa cells. PrP promotes cell spreading and/or filopodia formation when overexpressed, and lamellipodia when downregulated. Moreover, PrP normally accumulates in focal adhesions (FAs), and its downregulation leads to reduced FA numbers, increased FA length, along with Src and focal adhesion kinase (FAK) activation. Furthermore, its overexpression elicits the formation of novel FA-like structures, which require intact reggie/flotillin microdomains. Altogether, PrP modulates process formation and FA dynamics, possibly via signal transduction involving FAK and Src.

Trafficking of the microdomain scaffolding protein reggie-1/flotillin-2

The reggie/flotillin proteins oligomerize and associate into clusters which form scaffolds for membrane microdomains. Besides their localization at the plasma membrane, the reggies/flotillins reside at various intracellular compartments; however, the trafficking pathways used by reggie-1/flotillin-2 remain unclear. Here, we show that trafficking of reggie-1/flotillin-2 is BFA sensitive and that deletion mutants of reggie-1/flotillin-2 accumulate in the Golgi complex in HeLa, Jurkat and PC12 cells, suggesting Golgi-dependent trafficking of reggie-1/flotillin-2. Using total internal reflection fluorescence microscopy, we observed fast cycling of reggie-1/flotillin-2-positive vesicles at the plasma membrane, which engaged in transient interactions with the plasma membrane only. Reggie-1/flotillin-2 cycling was independent of clathrin, but was inhibited by cholesterol depletion and microtubule disruption. Cycling of reggie-1/flotillin-2 was negatively correlated with cell-cell contact formation but was stimulated by serum, epidermal growth factor and by cholesterol loading mediated by low density lipoproteins. However, reggie-1/flotillin-2 was neither involved in endocytosis of the epidermal growth factor itself nor in endocytosis of GPI-GFPs or the GPI-anchored cellular prion protein (PrP(c)). Reggie-2/flotillin-1 and stomatin-1 also exhibited cycling at the plasma membrane similar to reggie-1/flotillin-2, but these vesicles and microdomains only partially co-localized with reggie-2/flotillin-1. Thus, regulated vesicular cycling might be a general feature of SPFH protein-dependent trafficking.

Reggie-1/flotillin-2 promotes secretion of the long-range signalling forms of Wingless and Hedgehog in Drosophila

The lipid-modified morphogens Wnt and Hedgehog diffuse poorly in isolation yet can spread over long distances in vivo, predicting existence of two distinct forms of these mophogens. The first is poorly mobile and activates short-range target genes. The second is specifically packed for efficient spreading to induce long-range targets. Subcellular mechanisms involved in the discriminative secretion of these two forms remain elusive. Wnt and Hedgehog can associate with membrane microdomains, but the function of this association was unknown. Here we show that a major protein component of membrane microdomains, reggie-1/flotillin-2, plays important roles in secretion and spreading of Wnt and Hedgehog in Drosophila. Reggie-1 loss-of-function results in reduced spreading of the morphogens, while its overexpression stimulates secretion of Wnt and Hedgehog and expands their diffusion. The resulting changes in the morphogen gradients differently affect the short- and long-range targets. In its action reggie-1 appears specific for Wnt and Hedgehog. These data suggest that reggie-1 is an important component of the Wnt and Hedgehog secretion pathway dedicated to formation of the mobile pool of these morphogens.

Coassembly of flotillins induces formation of membrane microdomains, membrane curvature, and vesicle budding

Endocytosis has a crucial role in many cellular processes. The best-characterized mechanism for endocytosis involves clathrin-coated pits [1], but evidence has accumulated for additional endocytic pathways in mammalian cells [2]. One such pathway involves caveolae, plasma-membrane invaginations defined by caveolin proteins. Plasma-membrane microdomains referred to as lipid rafts have also been associated with clathrin-independent endocytosis by biochemical and pharmacological criteria [3]. The mechanisms, however, of nonclathrin, noncaveolin endocytosis are not clear [4, 5]. Here we show that coassembly of two similar membrane proteins, flotillin1 and flotillin2 [6-8], is sufficient to generate de novo membrane microdomains with some of the predicted properties of lipid rafts [9]. These microdomains are distinct from caveolin1-positive caveolae, are dynamic, and bud into the cell. Coassembly of flotillin1 and flotillin2 into microdomains induces membrane curvature, the formation of plasma-membrane invaginations morphologically similar to caveolae, and the accumulation of intracellular vesicles. We propose that flotillin proteins are defining structural components of the machinery that mediates a clathrin-independent endocytic pathway. Key attributes of this machinery are the dependence on coassembly of both flotillins and the inference that flotillin microdomains can exist in either flat or invaginated states.

Linking membrane microdomains to the cytoskeleton: Regulation of the lateral mobility of reggie-1/flotillin-2 by interaction with actin

The reggies/flotillins are oligomeric scaffolding proteins for membrane microdomains. We show here that reggie-1/flotillin-2 microdomains are organized along cortical F-actin in several cell types. Interaction with F-actin is mediated by the SPFH domain as shown by in vivo co-localization and in vitro binding experiments. Reggie-1/flotillin-2 microdomains form independent of actin, but disruption or stabilization of the actin cytoskeleton modulate the lateral mobility of reggie-1/flotillin-2 as shown by FRAP. Furthermore, reggie/flotillin microdomains can efficiently be immobilized by actin polymerisation, while exchange of reggie-1/flotillin-2 molecules between microdomains is enhanced by actin disruption as shown by tracking of individual microdomains using TIRF microscopy.

Contributions to our understanding of T cell physiology through unveiling the T cell proteome

Since the sequencing of the human genome was completed, attention has turned to examining the functionality of the molecular machinery, in particular of protein expression. Differential proteome analysis by two-dimensional electrophoresis has been adopted to study changes in T cell proteomes during T cell activation, and this work is increasing our understanding of the complexity of signals elicited across multiple pathways. The purpose of this review is to summarize the available evidence in the application of proteomic techniques and methodologies to understand T cell receptor activation from lipid raft and cytoskeletal rearrangements, through to signalling cascades, transcription factor modulation and changes in protein expression patterns. These include post-translational modifications, which are not encoded by the genome.

Reggie/flotillin proteins are organized into stable tetramers in membrane microdomains

Reggie-1 and -2 proteins (flotillin-2 and -1 respectively) form their own type of non-caveolar membrane microdomains, which are involved in important cellular processes such as T-cell activation, phagocytosis and signalling mediated by the cellular prion protein and insulin; this is consistent with the notion that reggie microdomains promote protein assemblies and signalling. While it is generally known that membrane microdomains contain large multiprotein assemblies, the exact organization of reggie microdomains remains elusive. Using chemical cross-linking approaches, we have demonstrated that reggie complexes are composed of homo- and hetero-tetramers of reggie-1 and -2. Moreover, native reggie oligomers are indeed quite stable, since non-cross-linked tetramers are resistant to 8 M urea treatment. We also show that oligomerization requires the C-terminal but not the N-terminal halves of reggie-1 and -2. Using deletion constructs, we analysed the functional relevance of the three predicted coiled-coil stretches present in the C-terminus of reggie-1. We confirmed experimentally that reggie-1 tetramerization is dependent on the presence of coiled-coil 2 and, partially, of coiled-coil 1. Furthermore, since depletion of reggie-1 by siRNA (small interfering RNA) silencing induces proteasomal degradation of reggie-2, we conclude that the protein stability of reggie-2 depends on the presence of reggie-1. Our data indicate that the basic structural units of reggie microdomains are reggie homo- and hetero-tetramers, which are dependent on the presence of reggie-1.

The lipid raft proteins flotillins/reggies interact with Galphaq and are involved in Gq-mediated p38 mitogen-activated protein kinase activation through tyrosine kinase

The heterotrimeric G protein alpha q subunit (Galphaq) mediates a variety of cell functions by activating the effector molecule phospholipase Cbeta. Galphaq activity is regulated by G protein betagamma subunits, G protein-coupled receptors, RGS proteins, and Ric-8. In this study, we identified the lipid raft resident proteins, flotillin-1/reggie-2 and flotillin-2/reggie-1, as Galphaq-binding proteins. The interactions of Galphaq and flotillins were independent of the nucleotide-binding state of Galphaq, and the N-terminal portion of flotillins was critical for the interaction. A short interfering RNA-mediated knockdown of flotillins, particularly flotillin-2, attenuated the UTP-induced activation of p38 mitogen-activated protein kinase (MAPK) but not that of ERK1/2. The activation of p38 MAPK was inhibited by the Src family tyrosine kinase inhibitor PP2 and the cholesterol-depleting agent methyl-beta-cyclodextrin, which is generally used for the disruption of lipid rafts. In contrast, the activation of ERK1/2 was not inhibited by these compounds. These lines of evidence suggested that a Gq-coupled receptor activates specifically p38 MAPK through lipid rafts and Src kinase activation, in which flotillins positively modulate the Gq signaling.