The roles of flotillin microdomains--endocytosis and beyond

Infection by Anaplasma phagocytophilum Requires Recruitment of Low-Density Lipoprotein Cholesterol by Flotillins

Anaplasma phagocytophilum is an obligatory intracellular bacterium that proliferates in membrane-bound inclusions. A. phagocytophilum is dependent on cholesterol and acquire cholesterol from low-density lipoprotein (LDL) endocytosed by mammalian host cells. The mechanism of cholesterol transport to Anaplasma inclusions, however, is not fully understood. Flotillin-1 (FLOT1) and FLOT2 are cholesterol-associated membrane proteins that form a heterodimer and/or oligomer complex. Here, we found that Anaplasma infection was significantly reduced by small interfering RNA (siRNA) knockdown of FLOT1 or FLOT2. Anaplasma inclusions were encircled with small vesicles containing endogenous FLOT1 or FLOT2 or with ectopically expressed FLOT1-mCherry and FLOT2-green fluorescent protein (FLOT2-GFP). FLOT1- and FLOT2-containing vesicles were enriched with unesterified cholesterol, as indicated by labeling with filipin and aminomethyl coumarin acetic acid-conjugated theonellamide. Localization of FLOT2 to Anaplasma inclusions was dependent on cholesterol, as FLOT2-GFP bearing two mutations in the cholesterol recognition/interaction motif could not target the inclusions. The cholesterol-sequestering agent methyl-β-cyclodextrin abrogated FLOT1 localization to Anaplasma inclusions and cleared infection. FLOT2-GFP also localized to fluorescent 3,3'-dioctadecylindocarbocyanine (DiI)-LDL-containing vesicles, including those surrounding Anaplasma inclusions. FLOT2 siRNA knockdown blocked DiI-LDL trafficking to Anaplasma inclusions and reduced bacteria-associated cholesterol amount, and therefore inhibiting Anaplasma infection. Vesicles containing acid lipase, which hydrolyzes LDL cholesterol esters to free cholesterol, colocalized with FLOT2 and encircled Anaplasma inclusions, while the acid lipase inhibitor orlistat significantly inhibited Anaplasma replication. Together, the data revealed that FLOTs are crucial for Anaplasma replication in host cells, likely by aiding vesicular traffic of LDL-derived free cholesterol to Anaplasma inclusions, and suggest a new way of inhibiting Anaplasma infection.IMPORTANCE Cholesterol is essential for animal cells, but most bacteria do not depend on cholesterol and instead lack cholesterol. However, the intracellular Gram-negative bacterium Anaplasma phagocytophilum that causes human granulocytic anaplasmosis (HGA) is unusual, as it contains significant amount of cholesterol and depends on cholesterol for survival and infection. A. phagocytophilum lacks genes for cholesterol biosynthesis or modification but acquire cholesterol from host cells exclusively from the LDL uptake pathway by a yet-to-be defined mechanism. Here, we uncovered a role of cholesterol-binding proteins FLOT1 and FLOT2 in LDL-derived cholesterol trafficking to Anaplasma inclusions and cholesterol acquisition by Anaplasma species. Importantly, we found that FLOTs localize to A. phagocytophilum-containing inclusions and the compartments containing LDL, and the acid lipase inhibitor orlistat significantly inhibits Anaplasma replication. Our data suggest a fundamental role of FLOTs in intracellular vesicular transport of LDL-derived free cholesterol and may provide insight regarding a new therapeutic target for HGA treatment.

Cryopreservation disrupts lipid rafts and heat shock proteins in yellow catfish sperm

Lipid rafts and associated membrane proteins (flotillin, caveolin) play important roles in cell signaling and sperm fertilization while heat shock proteins (Hsp) ensure properly protein folding to fulfill their physiological functions. The markedly reduced fertility in thawed sperm after cryopreservation could result from disrupted membrane lipid rafts and these proteins. To explore the effect of sperm cryopreservation on lipid rafts and heat shock proteins, we compared lipid raft integrity, and the expression levels of lipid raft associated proteins (Flot-1, Flot-2, Cav-1) as well as heat shock proteins (Hsp90, Hsp70) in fresh and thawed sperm cryopreserved under different scenarios in yellow catfish. We found higher lipid raft integrity, higher protein expression levels of Flot-1, Flot-2, Cav-1, Hsp90, and Hsp70 in fresh sperm samples than in thawed sperm samples, in thawed sperm samples cryopreserved with optimal cooling rate than those cryopreserved with sub-optimal cooling rate, and in thawed sperm samples cryopreserved with extenders supplemented with cholesterol than those supplemented with methyl-β-cyclodextrin (for cholesterol removal). Our findings indicate that lipid raft integrity, and expression levels of Flot-1, Flot-2, Cav-1, Hsp90, and Hsp70 are clearly associated with sperm quality, and together they may play a cumulative role in reduced fertility associated with thawed sperm in aquatic species.

Matrix Rigidity-Dependent Regulation of Ca2+ at Plasma Membrane Microdomains by FAK Visualized by Fluorescence Resonance Energy Transfer

The dynamic regulation of signal transduction at plasma membrane microdomains remains poorly understood due to limitations in current experimental approaches. Genetically encoded biosensors based on fluorescent resonance energy transfer (FRET) can provide high spatiotemporal resolution for imaging cell signaling networks. Here, distinctive regulation of focal adhesion kinase (FAK) and Ca2+ signals are visualized at different membrane microdomains by FRET using membrane-targeting biosensors. It is shown that rigidity-dependent FAK and Ca2+ signals in human mesenchymal stem cells (hMSCs) are selectively activated at detergent-resistant membrane (DRM or rafts) microdomains during the cell-matrix adhesion process, with minimal activities at non-DRM domains. The rigidity-dependent Ca2+ signal at the DRM microdomains is downregulated by either FAK inhibition or lipid raft disruption, suggesting that FAK and lipid raft integrity mediate the in situ Ca2+ activation. It is further revealed that transient receptor potential subfamily M7 (TRPM7) participates in the mobilization of Ca2+ signals within DRM regions. Thus, the findings provide insights into the underlying mechanisms that regulate Ca2+ and FAK signals in hMSCs under different mechanical microenvironments.

Contribution of syndecans to cellular internalization and fibrillation of amyloid-β(1-42)

Intraneuronal accumulation of amyloid-β(1-42) (Aβ1-42) is one of the earliest signs of Alzheimer's disease (AD). Cell surface heparan sulfate proteoglycans (HSPGs) have profound influence on the cellular uptake of Aβ1-42 by mediating its attachment and subsequent internalization into the cells. Colocalization of amyloid plaques with members of the syndecan family of HSPGs, along with the increased expression of syndecan-3 and -4 have already been reported in postmortem AD brains. Considering the growing evidence on the involvement of syndecans in the pathogenesis of AD, we analyzed the contribution of syndecans to cellular uptake and fibrillation of Aβ1-42. Among syndecans, the neuron specific syndecan-3 isoform increased cellular uptake of Aβ1-42 the most. Kinetics of Aβ1-42 uptake also proved to be fairly different among SDC family members: syndecan-3 increased Aβ1-42 uptake from the earliest time points, while other syndecans facilitated Aβ1-42 internalization at a slower pace. Internalized Aβ1-42 colocalized with syndecans and flotillins, highlighting the role of lipid-rafts in syndecan-mediated uptake. Syndecan-3 and 4 also triggered fibrillation of Aβ1-42, further emphasizing the pathophysiological relevance of syndecans in plaque formation. Overall our data highlight syndecans, especially the neuron-specific syndecan-3 isoform, as important players in amyloid pathology and show that syndecans, regardless of cell type, facilitate key molecular events in neurodegeneration.

Modifier Gene Candidates in Charcot-Marie-Tooth Disease Type 1A: A Case-Only Genome-Wide Association Study

BACKGROUND

Charcot-Marie-Tooth disease type 1A (CMT1A) is caused by a uniform 1.5-Mb duplication on chromosome 17p, which includes the PMP22 gene. Patients often present the classic neuropathy phenotype, but also with high clinical variability.

OBJECTIVE

We aimed to identify genetic variants that are potentially associated with specific clinical outcomes in CMT1A.

METHODS

We genotyped over 600,000 genomic markers using DNA samples from 971 CMT1A patients and performed a case-only genome-wide association study (GWAS) to identify potential genetic association in a subset of 644 individuals of European ancestry. A total of 14 clinical outcomes were analyzed in this study.

RESULTS

The analyses yielded suggestive association signals in four clinical outcomes: difficulty with eating utensils (lead SNP rs4713376, chr6 : 30773314, P = 9.91×10-7, odds ratio = 3.288), hearing loss (lead SNP rs7720606, chr5 : 126551732, P = 2.08×10-7, odds ratio = 3.439), decreased ability to feel (lead SNP rs17629990, chr4 : 171224046, P = 1.63×10-7, odds ratio = 0.336), and CMT neuropathy score (lead SNP rs12137595, chr1 : 4094068, P = 1.14×10-7, beta = 3.014).

CONCLUSIONS

While the results require validation in future genetic and functional studies, the detected association signals may point to novel genetic modifiers in CMT1A.

Could vesicular transport of Na+ and Cl- be a feature of salt tolerance in halophytes?

Background

Halophytes tolerate external salt concentrations of 200 mm and more, accumulating salt concentrations of 500 mm and more in their shoots; some, recretohalophytes, excrete salt through glands on their leaves. Ions are accumulated in central vacuoles, but the pathway taken by these ions from the outside of the roots to the vacuoles inside the cells is poorly understood. Do the ions cross membranes through ion channels and transporters or move in vesicles, or both? Vesicular transport from the plasma membrane to the vacuole would explain how halophytes avoid the toxicity of high salt concentrations on metabolism. There is also a role for vesicles in the export of ions via salt glands.

Scope and Methods

We have collected data on the fluxes of sodium and chloride ions in halophytes, based on the weight of the transporting organs and on the membrane area across which the flux occurs; the latter range from 17 nmol m-2 s-1 to 4.2 μmol m-2 s-1 and values up to 1 μmol m-2 s-1 need to be consistent with whatever transport system is in operation. We have summarized the sizes and rates of turnover of vesicles in plants, where clathrin-independent vesicles are 100 nm or more in diameter and can merge with the plasma membrane at rates of 100 s-1. We gathered evidence for vesicular transport of ions in halophytes and evaluated whether vesicular transport could account for the observable fluxes.

Conclusions

There is strong evidence in favour of vesicular transport in plants and circumstantial evidence in favour of the movement of ions in vesicles. Estimated rates of vesicle turnover could account for ion transport at the lower reported fluxes (around 20 nmol m-2 s-1), but the higher fluxes may require vesicles of the order of 1 μm or more in diameter. The very high fluxes reported in some salt glands might be an artefact of the way they were measured.

Flotillins in the intercalated disc are potential modulators of cardiac excitability

BACKGROUND

The intercalated disc (ID) is important for cardiac remodeling and has become a subject of intensive research efforts. However, as yet the composition of the ID has still not been conclusively resolved and the role of many proteins identified in the ID, like Flotillin-2, is often unknown. The Flotillin proteins are known to be involved in the stabilization of cadherins and desmosomes in the epidermis and upon cancer development. However, their role in the heart has so far not been investigated. Therefore, in this study, we aimed at identifying the role of Flotillin-1 and Flotillin-2 in the cardiac ID.

METHODS

Location of Flotillins in human and murine cardiac tissue was evaluated by fluorescent immunolabeling and co-immunoprecipitation. In addition, the effect of Flotillin knockout (KO) on proteins of the ID and in electrical excitation and conduction was investigated in cardiac samples of wildtype (WT), Flotillin-1 KO, Flotilin-2 KO and Flotilin-1/2 double KO mice. Consequences of Flotillin knockdown (KD) on cardiac function were studied (patch clamp and Multi Electrode Array (MEA)) in neonatal rat cardiomyocytes (NRCMs) transfected with siRNAs against Flotillin-1 and/or Flotillin-2.

RESULTS

First, we confirmed presence in the ID and mutual binding of Flotillin-1 and Flotillin-2 in murine and human cardiac tissue. Flotillin KO mice did not show cardiac fibrosis, nor hypertrophy or changes in expression of the desmosomal ID proteins. However, protein expression of the cardiac sodium channel Na_V1.5 was significantly decreased in Flotillin-1 and Flotillin-1/2 KO mice compared to WT mice. In addition, sodium current density showed a significant decrease upon Flotillin-1/2 KD in NRCMs as compared to scrambled siRNA-transfected NRCMs. MEA recordings of Flotillin-2 KD NRCM cultures showed a significantly decreased spike amplitude and a tendency of a reduced spike slope when compared to control and scrambled siRNA-transfected cultures.

CONCLUSIONS

In this study, we demonstrate the presence of Flotillin-1, in addition to Flotillin-2 in the cardiac ID. Our findings indicate a modulatory role of Flotillins on Na_V1.5 expression at the ID, with potential consequences for cardiac excitation.

Liposome co-incubation with cancer cells secreted exosomes (extracellular vesicles) with different proteins expressions and different uptake pathways

We recently showed that in vitro incubation of cells with liposomes of varying compositions can increase exosome secretion and increase the yield of harvested exosomes (extracellular vesicles, EVs). This might foster their potential therapeutic implementations. In the current study, we investigated the surface proteins and the uptake of the harvested exosomes (EVs) to see if the incubation of cells with liposomes would change the biological properties of these exosomes (EVs). Interestingly, exosomes (EVs) induced by solid cationic liposomes lacked some major exosome marker proteins such as CD9, flotillin-1, annexin-A2 and EGF, and subsequently had lower levels of cellular uptake upon re-incubation with donor cancer cells. However, exosomes (EVs) induced under normal condition and by fluid cationic liposomes, displayed the entire spectrum of proteins, and exhibited higher uptake by the donor cancer cells. Although endocytosis was the major uptake pathway of exosomes (EVs) by tumor cells, endocytosis could occur via more than one mechanism. Higher exosome uptake was observed in donor B16BL6 cells than in allogeneic C26 cells, indicating that donor cells might interact specifically with their exosomes (EVs) and avidly internalize them. Taken together, these results suggest a technique for controlling the characteristics of secreted exosomes (EVs) by incubating donor cancer cells with liposomes of varying physiochemical properties.

Proteome analysis reveals a role of rainbow trout lymphoid organs during Yersinia ruckeri infection process

Yersinia ruckeri is the causative agent of enteric redmouth disease in salmonids. Head kidney and spleen are major lymphoid organs of the teleost fish where antigen presentation and immune defense against microbes take place. We investigated proteome alteration in head kidney and spleen of the rainbow trout following Y. ruckeri strains infection. Organs were analyzed after 3, 9 and 28 days post exposure with a shotgun proteomic approach. GO annotation and protein-protein interaction were predicted using bioinformatic tools. Thirty four proteins from head kidney and 85 proteins from spleen were found to be differentially expressed in rainbow trout during the Y. ruckeri infection process. These included lysosomal, antioxidant, metalloproteinase, cytoskeleton, tetraspanin, cathepsin B and c-type lectin receptor proteins. The findings of this study regarding the immune response at the protein level offer new insight into the systemic response to Y. ruckeri infection in rainbow trout. This proteomic data facilitate a better understanding of host-pathogen interactions and response of fish against Y. ruckeri biotype 1 and 2 strains. Protein-protein interaction analysis predicts carbon metabolism, ribosome and phagosome pathways in spleen of infected fish, which might be useful in understanding biological processes and further studies in the direction of pathways.

MT1-MMP targeting to endolysosomes is mediated by upregulation of flotillins

Tumor cell invasion and metastasis formation are the major cause of death in cancer patients. These processes rely on extracellular matrix (ECM) degradation mediated by organelles termed invadopodia, to which the transmembrane matrix metalloproteinase MT1-MMP (also known as MMP14) is delivered from its reservoir, the RAB7-containing endolysosomes. How MT1-MMP is targeted to endolysosomes remains to be elucidated. Flotillin-1 and -2 are upregulated in many invasive cancers. Here, we show that flotillin upregulation triggers a general mechanism, common to carcinoma and sarcoma, which promotes RAB5-dependent MT1-MMP endocytosis and its delivery to RAB7-positive endolysosomal reservoirs. Conversely, flotillin knockdown in invasive cancer cells greatly reduces MT1-MMP accumulation in endolysosomes, its subsequent exocytosis at invadopodia, ECM degradation and cell invasion. Our results demonstrate that flotillin upregulation is necessary and sufficient to promote epithelial and mesenchymal cancer cell invasion and ECM degradation by controlling MT1-MMP endocytosis and delivery to the endolysosomal recycling compartment.

The crossroads of receptor-mediated signaling and endocytosis in plants

Plants deploy numerous plasma membrane receptors to sense and rapidly react to environmental changes. Correct localization and adequate protein levels of the cell-surface receptors are critical for signaling activation and modulation of plant development and defense against pathogens. After ligand binding, receptors are internalized for degradation and signaling attenuation. However, one emerging notion is that the ligand-induced endocytosis of receptor complexes is important for the signal duration, amplitude, and specificity. Recently, mutants of major endocytosis players, including clathrin and dynamin, have been shown to display defects in activation of a subset of signal transduction pathways, implying that signaling in plants might not be solely restricted to the plasma membrane. Here, we summarize the up-to-date knowledge of receptor complex endocytosis and its effect on the signaling outcome, in the context of plant development and immunity.

Active 3'-5' cyclic nucleotide phosphodiesterases are present in detergent-resistant membranes of mural granulosa cells

Lipids rafts are specialised membrane microdomains involved in cell signalling that can be isolated as detergent-resistant membranes (DRMs). The second messenger cyclic AMP (cAMP) has a central role in cell signalling in the ovary and its degradation is carried out by the phosphodiesterase (PDE) enzyme family. We hypothesised that PDEs could be functionally present in the lipid rafts of porcine mural granulosa cell membranes. PDE6C, PDE8A and PDE11A were detected by dot blot in the DRMs and the Triton-soluble fraction of the mural granulosa cells membrane and the cytosol. As shown by immunocytochemistry, PDEs showed clear immunostaining in mural granulosa cell membranes and the cytosol. Interestingly, cAMP-PDE activity was 18 times higher in the DRMs than in the Triton-soluble fraction of cell membranes and was 7.7 times higher in the cytosol than in the DRMs. cAMP-PDE activity in mural granulosa cells was mainly contributed by the PDE8 and PDE11 families. This study shows that PDEs from the PDE8 and PDE11 families are present in mural granulosa cells and that the cAMP-PDE activity is mainly contributed by the cytosol. In the cell membrane, the cAMP-PDE activity is mainly contributed by the DRMs. In addition, receptors for prostaglandin E2 and LH, two G-protein-coupled receptors, are present in lipid rafts and absent from the non-raft fraction of the granulosa cell membrane. These results suggest that in these cells, the lipid rafts exist as a cell-signalling platform and PDEs are one of the key enzyme families present in the raft.

Exosomes and their implications in central nervous system tumor biology

Exosomes are 20-100 nm cellular derived vesicles that upon discovery, were thought to be a form of cellular recycling of intracellular contents. More recently, these vesicles are under investigation for their purported significant roles in intercellular communication in both healthy and diseased states. Herein, we focus on the secretion of exosomes associated with glioblastoma, as most exosome studies on brain tumors have been performed in this tumor type. However, we included exosomes secreted from other forms of brain tumors for comparison as available. Exosomes contain intracellular content that can be transferred to other cells in the tumor or to cells of the immune system and endothelial cells. These recipient cells may subsequently take on oncogenic properties, including therapeutic resistance, cancer progression, and angiogenesis. Genetic components (DNA, RNA and miRNA) of the cell of origin may be included in the secreted exosomes. The presence of genetic material in the exosomes could serve as a biomarker for mutations in tumors, potentially leading to novel treatment strategies. In the last decade, exosomes have been identified as having a major impact on multiple aspects of medicine and tumor biology, and appear to be primed for a critical position in cancer diagnosis, prognosis, and treatment.

Flotillin proteins recruit sphingosine to membranes and maintain cellular sphingosine-1-phosphate levels

Sphingosine-1-phosphate (S1P) is an important lipid signalling molecule. S1P is produced via intracellular phosphorylation of sphingosine (Sph). As a lipid with a single fatty alkyl chain, Sph may diffuse rapidly between cellular membranes and through the aqueous phase. Here, we show that the absence of microdomains generated by multimeric assemblies of flotillin proteins results in reduced S1P levels. Cellular phenotypes of flotillin knockout mice, including changes in histone acetylation and expression of Isg15, are recapitulated when S1P synthesis is perturbed. Flotillins bind to Sph in vitro and increase recruitment of Sph to membranes in cells. Ectopic re-localisation of flotillins within the cell causes concomitant redistribution of Sph. The data suggest that flotillins may directly or indirectly regulate cellular sphingolipid distribution and signalling.

Flotillins Regulate Focal Adhesions by Interacting with α-Actinin and by Influencing the Activation of Focal Adhesion Kinase

Cell–matrix adhesion and cell migration are physiologically important processes that also play a major role in cancer spreading. In cultured cells, matrix adhesion depends on integrin-containing contacts such as focal adhesions. Flotillin-1 and flotillin-2 are frequently overexpressed in cancers and are associated with poor survival. Our previous studies have revealed a role for flotillin-2 in cell–matrix adhesion and in the regulation of the actin cytoskeleton. We here show that flotillins are important for cell migration in a wound healing assay and influence the morphology and dynamics of focal adhesions. Furthermore, anchorage-independent growth in soft agar is enhanced by flotillins. In the absence of flotillins, especially flotillin-2, phosphorylation of focal adhesion kinase and extracellularly regulated kinase is diminished. Flotillins interact with α-actinin, a major regulator of focal adhesion dynamics. These findings are important for understanding the molecular mechanisms of how flotillin overexpression in cancers may affect cell migration and, especially, enhance metastasis formation.

Continuous Change in Membrane and Membrane-Skeleton Organization During Development From Proerythroblast to Senescent Red Blood Cell

Within the context of erythropoiesis and the possibility of producing artificial red blood cells (RBCs) in vitro, a most critical step is the final differentiation of enucleated erythroblasts, or reticulocytes, to a fully mature biconcave discocyte, the RBC. Reviewed here is the current knowledge about this fundamental maturational process. By combining literature data with our own experimental evidence we propose that the early phase in the maturation of reticulocytes to RBCs is driven by a membrane raft-based mechanism for the sorting of disposable membrane proteins, mostly the no longer needed transferrin receptor (TfR), to the multivesicular endosome (MVE) as cargo of intraluminal vesicles that are subsequently exocytosed as exosomes, consistently with the seminal and original observation of Johnstone and collaborators of more than 30 years ago (Pan BT, Johnstone RM. Cell. 1983;33:967-978). According to a strikingly selective sorting process, the TfR becomes cargo destined to exocytosis while other molecules, including the most abundant RBC transmembrane protein, band 3, are completely retained in the cell membrane. It is also proposed that while this process could be operating in the early maturational steps in the bone marrow, additional mechanism(s) must be at play for the final removal of the excess reticulocyte membrane that is observed to occur in the circulation. This processing will most likely require the intervention of the spleen, whose function is also necessary for the continuous remodeling of the RBC membrane all along this cell's circulatory life.

Stat3-mediated alterations in lysosomal membrane protein composition

Lysosome function is essential in cellular homeostasis. In addition to its recycling role, the lysosome has recently been recognized as a cellular signaling hub. We have shown in mammary epithelial cells, both in vivo and in vitro, that signal transducer and activator of transcription 3 (Stat3) modulates lysosome biogenesis and can promote the release of lysosomal proteases that culminates in cell death. To further investigate the impact of Stat3 on lysosomal function, we conducted a proteomic screen of changes in lysosomal membrane protein components induced by Stat3 using an iron nanoparticle enrichment strategy. Our results show that Stat3 activation not only elevates the levels of known membrane proteins but results in the appearance of unexpected factors, including cell surface proteins such as annexins and flotillins. These data suggest that Stat3 may coordinately regulate endocytosis, intracellular trafficking, and lysosome biogenesis to drive lysosome-mediated cell death in mammary epithelial cells. The methodologies described in this study also provide significant improvements to current techniques used for the purification and analysis of the lysosomal proteome.

Exosomes: from biology to clinics

Exosomes are extracellular vesicles with the diameter of 30-120 nm, originating from early endosomes. Exosomes have been actively studied in the last decade, and a great amount of data has appeared on their nature and role in the intercellular transport and signaling both in the normal and pathological conditions. A particular interest to exosomes in the clinical practice emerged after the separation of their circulating fraction from the blood and the study of tumor genetic markers in them became possible (so called “liquid biopsy”). The objective of this review is to familiarize clinical specialists with the fundamentals of exosomes' biology and physiology and with the main achievements on their practical application in the medicine, as a natural drug delivery system, as well as for high-precision, early non-invasive differential diagnostics of diseases.

Identification and characterization of the lamprey Flotillin-1 gene with a role in cell adhesion

Flotillin-1 is a kind of localize into specific cholesterol rich microdomains in cellular membranes and highly conserved lipid rafts marker protein widely distributed in animals and plants. It provides a platform for the reaction of many proteins in signal transduction, as scaffolding plays an important role in transmembrane signaling and cell adhesion. Here, Flotillin-1 protein from lamprey was identified and characterized (designated as L-Flotillin-1). After a partial cDNA sequence of L-Flotillin-1 was identified in a lamprey supraneural body cDNA library, the full-length cDNA was obtained using 3'- and 5'-rapid amplification of cDNA ends (RACE). L-Flotillin-1 encodes 424 amino acids and contains a prohibitin domain and a flotillin repetitive area. The L-Flotillin-1 protein was primarily distributed in kidney, supraneural body, gill, heart, liver and intestine via real-time PCR and immunohistochemistry assays. Immunofluorescence and western blot results showed that L-Flotillin-1 was considered to be used as a marker protein of lamprey lipid rafts and exosomes. Furthermore, overexpression of pEGFP-N1-L-Flotillin-1 induced the up-regulation of vascular cell adhesion molecule-1 (VCAM-1) and intercellular cell adhesion molecule-1 (ICAM-1) mRNA levels. These results indicated that the L-Flotillin-1 gene encodes Flotillin-1 protein that was used as a conserved marker protein and may play an important role in cell adhesion, providing clues for understanding the universal functions of Flotillin-1 proteins in other species and suggesting that these proteins could serve as pattern recognition molecules in immunotherapy. We revealed that Flotillin-1 protein of lamprey overexpression in human cells plays a prevalent role in cell migration and provide new thought of treatment to diseases.

Membrane cholesterol depletion in cortical neurons highlights altered NMDA receptor functionality in a mouse model of amyotrophic lateral sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a chronic neurodegenerative disease affecting upper and lower motor neurons, with unknown aetiology. Lipid rafts, cholesterol enriched microdomains of the plasma membrane, have been linked to neurodegenerative disorders like ALS. The NMDA-receptor subcellular localization in lipid rafts is known to play many roles, from modulating memory strength to neurotoxicity. In this study, performed on the widely used G93A mouse model of ALS, we have shown an equal content of total membrane cholesterol in Control and G93A cortical cultures. Moreover, by electrophysiological studies, we have recorded NMDA- and AMPA-evoked currents which were not significantly different between the two neuronal populations. To study the role of membrane cholesterol on glutamate receptor functionality, we have analysed NMDA and AMPA receptors following cholesterol membrane depletion by methyl-β-cyclodextrin (MβCD). Interestingly, MβCD chronic treatment has provoked a significant reduction of NMDA-evoked currents in both cellular populations which was dose- and time-dependent but significantly higher in ALS neurons compared to Control. The different MβCD effect on NMDA-evoked currents was not due to a different membrane receptor subunit composition but seemed to cause in both neuronal populations a NMDA receptor membrane redistribution. MβCD treatment effect was receptor-specific since no alterations in the two neuronal populations were detected on AMPA receptors. These results lead us to speculate for an altered proteomic composition of lipid rafts in cortical mutated neurons and suggest the need for further studies on the lipid rafts composition and on their interaction with membrane receptors in ALS cortices.

Flotillin scaffold activity contributes to type VII secretion system assembly in Staphylococcus aureus

Scaffold proteins are ubiquitous chaperones that promote efficient interactions between partners of multi-enzymatic protein complexes; although they are well studied in eukaryotes, their role in prokaryotic systems is poorly understood. Bacterial membranes have functional membrane microdomains (FMM), a structure homologous to eukaryotic lipid rafts. Similar to their eukaryotic counterparts, bacterial FMM harbor a scaffold protein termed flotillin that is thought to promote interactions between proteins spatially confined to the FMM. Here we used biochemical approaches to define the scaffold activity of the flotillin homolog FloA of the human pathogen Staphylococcus aureus, using assembly of interacting protein partners of the type VII secretion system (T7SS) as a case study. Staphylococcus aureus cells that lacked FloA showed reduced T7SS function, and thus reduced secretion of T7SS-related effectors, probably due to the supporting scaffold activity of flotillin. We found that the presence of flotillin mediates intermolecular interactions of T7SS proteins. We tested several small molecules that interfere with flotillin scaffold activity, which perturbed T7SS activity in vitro and in vivo. Our results suggest that flotillin assists in the assembly of S. aureus membrane components that participate in infection and influences the infective potential of this pathogen.

The recently discovered functional membrane microdomains (FMM) of prokaryotic cells contain a protein homologous to the scaffold protein flotillin found in eukaryotic lipid rafts. It remains to be elucidated whether, like their eukaryotic counterparts, flotillin homolog proteins have a scaffold function in bacteria. Here we show that the Staphylococcus aureus flotillin FloA acts as a scaffold protein, to promote more efficient assembly of membrane-associated protein interacting partners of multi-enzyme complexes. In a case study, we provide biochemical evidence that FloA participates in assembly of the Type VII secretion system and thus contributes to S. aureus infective potential. Targeted dispersion of FMM-related processes using anti-FMM molecules opens up new perspectives for microbial therapies to treat persistent S. aureus infections.

Suppression of Hepatic FLOT1 (Flotillin-1) by Type 2 Diabetes Mellitus Impairs the Disposal of Remnant Lipoproteins via Syndecan-1

OBJECTIVE

Type 2 diabetes mellitus (T2DM) and the atherometabolic syndrome exhibit a deadly dyslipoproteinemia that arises in part from impaired hepatic disposal of C-TRLs (cholesterol- and triglyceride-rich remnant apoB [apolipoprotein B] lipoproteins). We previously identified syndecan-1 as a receptor for C-TRLs that directly mediates endocytosis via rafts, independent from coated pits. Caveolins and flotillins form rafts but facilitate distinct endocytotic pathways. We now investigated their participation in syndecan-1-mediated disposal of C-TRLs and their expression in T2DM liver.

APPROACH AND RESULTS

In cultured liver cells and nondiabetic murine livers, we found that syndecan-1 coimmunoprecipitates with FLOT1 (flotillin-1) but not with CAV1 (caveolin-1). Binding of C-TRLs to syndecan-1 on the surface of liver cells enhanced syndecan-1/FLOT1 association. The 2 molecules then trafficked together into the lysosomes, implying limited if any recycling back to the cell surface. The interaction requires the transmembrane/cytoplasmic region of syndecan-1 and the N-terminal hydrophobic domain of FLOT1. Knockdown of FLOT1 in cultured liver cells substantially inhibited syndecan-1 endocytosis. Livers from obese, T2DM KKA^y mice exhibited 60% to 70% less FLOT1 protein and mRNA than in nondiabetic KK livers. An adenoviral construct to enhance hepatic expression of wild-type FLOT1 in T2DM mice normalized plasma triglycerides, whereas a mutant FLOT1 missing its N-terminal hydrophobic domain had no effect. Moreover, the adenoviral vector for wild-type FLOT1 lowered plasma triglyceride excursions and normalized retinyl excursions in T2DM KKA^y mice after a corn oil gavage, without affecting postprandial production of C-TRLs.

CONCLUSIONS

FLOT1 is a novel participant in the disposal of harmful C-TRLs via syndecan-1. Low expression of FLOT1 in T2DM liver may contribute to metabolic dyslipoproteinemia.

Do microplastic particles affect Daphnia magna at the morphological, life history and molecular level?

Microplastic particles are ubiquitous not only in marine but also in freshwater ecosystems. However, the impacts of microplastics, consisting of a large variety of synthetic polymers, on freshwater organisms remains poorly understood. We examined the effects of two polymer mixtures on the morphology, life history and on the molecular level of the waterflea Daphnia magna (three different clones). Microplastic particles of ~40 μm were supplied at a low concentration (1% of the food particles) leading to an average of ~30 particles in the digestive tract which reflects a high microplastic contamination but still resembles a natural situation. Neither increased mortality nor changes on the morphological (body length, width and tail spine length) or reproductive parameters were observed for adult Daphnia. The analyses of juvenile Daphnia revealed a variety of small and rather subtle responses of morphological traits (body length, width and tail spine length). For adult Daphnia, alterations in expression of genes related to stress responses (i.e. HSP60, HSP70 & GST) as well as of other genes involved in body function and body composition (i.e. SERCA) were observed already 48h after exposure. We anticipate that the adverse effects of microplastic might be influenced by many additional factors like size, shape, type and even age of the particles and that the rather weak effects, as detected in a laboratory, may lead to reduced fitness in a natural multi-stressor environment.

Function of plasma membrane microdomain-associated proteins during legume nodulation

Plasma membrane microdomains are plasma membrane sub-compartments enriched in sphingolipids and sterols, and composed by a specific set of proteins. They are involved in recognizing signal molecules, transducing these signals, and controlling endocytosis and exocytosis processes. In a recent study, applying biochemical and microscopic methods, we characterized the soybean GmFWL1 protein, a major regulator of soybean nodulation, as a new membrane microdomain-associated protein. Interestingly, upon rhizobia inoculation of the soybean root system, GmFWL1 and one of its interacting partners, GmFLOT2/4, both translocate to the root hair cell tip, the primary site of interaction and infection between soybean and Rhizobium. The role of GmFWL1 as a plasma membrane microdomain-associated protein is also supported by immunoprecipitation assays performed on soybean nodules, which revealed 178 GmFWL1 protein partners including a large number of microdomain-associated proteins such as GmFLOT2/4. In this addendum, we provide additional information about the identity of the soybean proteins repetitively identified as GmFWL1 protein partners. Their function is discussed especially in regard to plant-microbe interactions and microbial symbiosis. This addendum will provide new insights in the role of plasma membrane microdomains in regulating legume nodulation.

Super-resolution microscopy reveals functional organization of dopamine transporters into cholesterol and neuronal activity-dependent nanodomains

Dopamine regulates reward, cognition, and locomotor functions. By mediating rapid reuptake of extracellular dopamine, the dopamine transporter is critical for spatiotemporal control of dopaminergic neurotransmission. Here, we use super-resolution imaging to show that the dopamine transporter is dynamically sequestrated into cholesterol-dependent nanodomains in the plasma membrane of presynaptic varicosities and neuronal projections of dopaminergic neurons. Stochastic optical reconstruction microscopy reveals irregular dopamine transporter nanodomains (∼70 nm mean diameter) that were highly sensitive to cholesterol depletion. Live photoactivated localization microscopy shows a similar dopamine transporter membrane organization in live heterologous cells. In neurons, dual-color dSTORM shows that tyrosine hydroxylase and vesicular monoamine transporter-2 are distinctively localized adjacent to, but not overlapping with, the dopamine transporter nanodomains. The molecular organization of the dopamine transporter in nanodomains is reversibly reduced by short-term activation of NMDA-type ionotropic glutamate receptors, implicating dopamine transporter nanodomain distribution as a potential mechanism to modulate dopaminergic neurotransmission in response to excitatory input.The dopamine transporter (DAT) has a crucial role in the regulation of neurotransmission. Here, the authors use super-resolution imaging to show that DAT clusters into cholesterol-dependent membrane regions that are reversibly regulated by ionotropic glutamate receptors activation.

Exosomes: New players in cancer (Review)

The past decade has witnessed an exponential increase in research on exosomes. For many years considered to be extracellular debris, exosomes are now considered important mediators in intercellular communication. The capability of exosomes to transfer proteins, DNA, mRNA, as well as non-coding RNAs has made them an attractive focus of research into the pathogenesis of different diseases, including cancer. Increasing evidence suggests that tumor cells release a large sum of exosomes, which may not only influence proximal tumor cells and stromal cells in local microenvironment, but also can exert systemic effects when participating in blood circulation. In this study, we review the current understanding on this topic. The literature outlines two broad facets of exosomes in cancer: 1) promotion of tumor growth, tumorigenesis, tumor angiogenesis, tumor immune escape, drug resistance, and metastasis and 2) their role as promising biomarkers for cancer diagnosis and even as potential treatment targets for cancer patients.

Attenuating Staphylococcus aureus Virulence by Targeting Flotillin Protein Scaffold Activity

Scaffold proteins are ubiquitous chaperones that bind proteins and facilitate physical interaction of multi-enzyme complexes. Here we used a biochemical approach to dissect the scaffold activity of the flotillin-homolog protein FloA of the multi-drug-resistant human pathogen Staphylococcus aureus. We show that FloA promotes oligomerization of membrane protein complexes, such as the membrane-associated RNase Rny, which forms part of the RNA-degradation machinery called the degradosome. Cells lacking FloA had reduced Rny function and a consequent increase in the targeted sRNA transcripts that negatively regulate S. aureus toxin expression. Small molecules that altered FloA oligomerization also reduced Rny function and decreased the virulence potential of S. aureus in vitro, as well as in vivo, using invertebrate and murine infection models. Our results suggest that flotillin assists in the assembly of protein complexes involved in S. aureus virulence, and could thus be an attractive target for the development of new antimicrobial therapies.

Human Sialidase Neu3 is S-Acylated and Behaves Like an Integral Membrane Protein

Membrane-bound sialidase Neu3 is involved in the catabolism of glycoconjugates, and plays crucial roles in numerous biological processes. Since the mechanism of its association with membranes is still not completely understood, the aim of this work was to provide further information regarding this aspect. Human Neu3 was found to be associated with the plasma membrane and endomembranes, and it was not released from the lipid bilayer under conditions that typically release peripheral membrane proteins. By different experimental approaches, we demonstrated that its C-terminus is exposed to the cytosol while another portion of the protein is exposed to the extracellular space, suggesting that Neu3 possesses the features of a transmembrane protein. However, in silico analysis and homology modeling predicted that the sialidase does not contain any α-helical transmembrane segment and shares the same β-propeller fold typical of viral and bacterial sialidases. Additionally, we found that Neu3 is S-acylated. Since this post-translational modification is restricted to the cytosolic side of membranes, this finding strongly supports the idea that Neu3 may contain a cytosolic-exposed domain. Although it remains to be determined exactly how this sialidase crosses the lipid bilayer, this study provides new insights about membrane association and topology of Neu3.

A Helicobacter pylori Homolog of Eukaryotic Flotillin Is Involved in Cholesterol Accumulation, Epithelial Cell Responses and Host Colonization

The human pathogen Helicobacter pylori acquires cholesterol from membrane raft domains in eukaryotic cells, commonly known as "lipid rafts." Incorporation of this cholesterol into the H. pylori cell membrane allows the bacterium to avoid clearance by the host immune system and to resist the effects of antibiotics and antimicrobial peptides. The presence of cholesterol in H. pylori bacteria suggested that this pathogen may have cholesterol-enriched domains within its membrane. Consistent with this suggestion, we identified a hypothetical H. pylori protein (HP0248) with homology to the flotillin proteins normally found in the cholesterol-enriched domains of eukaryotic cells. As shown for eukaryotic flotillin proteins, HP0248 was detected in detergent-resistant membrane fractions of H. pylori. Importantly, H. pylori HP0248 mutants contained lower levels of cholesterol than wild-type bacteria (P < 0.01). HP0248 mutant bacteria also exhibited defects in type IV secretion functions, as indicated by reduced IL-8 responses and CagA translocation in epithelial cells (P < 0.05), and were less able to establish a chronic infection in mice than wild-type bacteria (P < 0.05). Thus, we have identified an H. pylori flotillin protein and shown its importance for bacterial virulence. Taken together, the data demonstrate important roles for H. pylori flotillin in host-pathogen interactions. We propose that H. pylori flotillin may be required for the organization of virulence proteins into membrane raft-like structures in this pathogen.

Pentabromophenol suppresses TGF-β signaling by accelerating degradation of type II TGF-β receptors via caveolae-mediated endocytosis

Pentabromophenol (PBP), a brominated flame retardant (BFR), is widely used in various consumer products. BFRs exert adverse health effects such as neurotoxic and endocrine-disrupting effects. In this study, we found that PBP suppressed TGF-β response by accelerating the turnover rate of TGF-β receptors. PBP suppressed TGF-β-mediated cell migration, PAI-1 promoter-driven reporter gene activation, and Smad2/3 phosphorylation in various cell types. Furthermore, PBP abolished TGF-β-mediated repression of E-cadherin expression, in addition to the induction of vimentin expression and N-cadherin and fibronectin upregulation, thus blocking TGF-β-induced epithelial–mesenchymal transition in A549 and NMuMG cells. However, this inhibition was not observed with other congeners such as tribromophenol and triiodophenol. TGF-β superfamily members play key roles in regulating various biological processes including cell proliferation and migration as well as cancer development and progression. The results of this in vitro study provide a basis for studies on the detailed relationship between PBP and modulation of TGF-β signalling. Because PBP is similar to other BFRs such as polybrominated diphenyl ethers (PBDEs), additional laboratory and mechanistic studies should be performed to examine BFRs as potential risk factors for tumorigenesis and other TGF-β-related diseases.

Exploring functional membrane microdomains in bacteria: an overview

Recent studies show that internal organization of bacterial cells is more complex than previously appreciated. A clear example of this is the assembly of the nanoscale membrane platforms termed functional membrane microdomains. The lipid composition of these regions differs from that of the surrounding membrane; these domains confine a set of proteins involved in specific cellular processes such as protease secretion and signal transduction. It is currently thought that functional membrane microdomains act as oligomerization platforms and promote efficient oligomerization of interacting protein partners in bacterial membranes. In this review, we highlight the most noteworthy achievements, challenges and controversies of this emerging research field over the past five years.

Poly-lactic acid nanoparticles (PLA-NP) promote physiological modifications in lung epithelial cells and are internalized by clathrin-coated pits and lipid rafts

Background

Poly-lactic acid nanoparticles (PLA-NP) are a type of polymeric NP, frequently used as nanomedicines, which have advantages over metallic NP such as the ability to maintain therapeutic drug levels for sustained periods of time. Despite PLA-NP being considered biocompatible, data concerning alterations in cellular physiology are scarce.

Methods

We conducted an extensive evaluation of PLA-NP biocompatibility in human lung epithelial A549 cells using high throughput screening and more complex methodologies. These included measurements of cytotoxicity, cell viability, immunomodulatory potential, and effects upon the cells’ proteome. We used non- and green-fluorescent PLA-NP with 63 and 66 nm diameters, respectively. Cells were exposed with concentrations of 2, 20, 100 and 200 µg/mL, for 24, 48 and 72 h, in most experiments. Moreover, possible endocytic mechanisms of internalization of PLA-NP were investigated, such as those involving caveolae, lipid rafts, macropinocytosis and clathrin-coated pits.

Results

Cell viability and proliferation were not altered in response to PLA-NP. Multiplex analysis of secreted mediators revealed a low-level reduction of IL-12p70 and vascular epidermal growth factor (VEGF) in response to PLA-NP, while all other mediators assessed were unaffected. However, changes to the cells’ proteome were observed in response to PLA-NP, and, additionally, the cellular stress marker miR155 was found to reduce. In dual exposures of staurosporine (STS) with PLA-NP, PLA-NP enhanced susceptibility to STS-induced cell death. Finally, PLA-NP were rapidly internalized in association with clathrin-coated pits, and, to a lesser extent, with lipid rafts.

Conclusions

These data demonstrate that PLA-NP are internalized and, in general, tolerated by A549 cells, with no cytotoxicity and no secretion of pro-inflammatory mediators. However, PLA-NP exposure may induce modification of biological functions of A549 cells, which should be considered when designing drug delivery systems. Moreover, the pathways of PLA-NP internalization we detected could contribute to the improvement of selective uptake strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-016-0238-1) contains supplementary material, which is available to authorized users.

The Norovirus NS3 Protein Is a Dynamic Lipid- and Microtubule-Associated Protein Involved in Viral RNA Replication

Norovirus (NoV) infections are a significant health burden to society, yet the lack of reliable tissue culture systems has hampered the development of appropriate antiviral therapies. Here we show that the NoV NS3 protein, derived from murine NoV (MNV), is intimately associated with the MNV replication complex and the viral replication intermediate double-stranded RNA (dsRNA). We observed that when expressed individually, MNV NS3 and NS3 encoded by human Norwalk virus (NV) induced the formation of distinct vesicle-like structures that did not colocalize with any particular protein markers to cellular organelles but localized to cellular membranes, in particular those with a high cholesterol content. Both proteins also showed some degree of colocalization with the cytoskeleton marker β-tubulin. Although the distribution of MNV and NV NS3s were similar, NV NS3 displayed a higher level of colocalization with the Golgi apparatus and the endoplasmic reticulum (ER). However, we observed that although both proteins colocalized in membranes counterstained with filipin, an indicator of cholesterol content, MNV NS3 displayed a greater association with flotillin and stomatin, proteins known to associate with sphingolipid- and cholesterol-rich microdomains. Utilizing time-lapse epifluorescence microscopy, we observed that the membrane-derived vesicular structures induced by MNV NS3 were highly motile and dynamic in nature, and their movement was dependent on intact microtubules. These results begin to interrogate the functions of NoV proteins during virus replication and highlight the conserved properties of the NoV NS3 proteins among the seven Norovirus genogroups.

IMPORTANCE

Many mechanisms involved in the replication of norovirus still remain unclear, including the role for the NS3 protein, one of seven nonstructural viral proteins, which remains to be elucidated. This study reveals that murine norovirus (MNV) NS3 is intimately associated with the viral replication complex and dsRNA. We observed that the NS3 proteins of both MNV and Norwalk virus (NV) induce prominent vesicular structures and that this formation is dependent on microtubules and cellular cholesterol. Thus, this study contributes to our understanding of protein function within different Norovirus genogroups and expands a growing knowledge base on the interaction between positive-strand RNA [(+)RNA] viruses and cellular membranes that contribute to the biogenesis of virus-induced membrane organelles. This study contributes to our understanding of viral protein function and the ability of a viral protein to recruit specific cellular organelles and lipids that enable replication.

Endocytic Trafficking at the Mature Podocyte Slit Diaphragm

Endocytic trafficking couples cell signaling with the cytoskeletal dynamics by organizing a crosstalk between protein networks in different subcellular compartments. Proteins residing in the plasma membrane are internalized and transported as cargo in endocytic vesicles (i.e., endocytosis). Subsequently, cargo proteins can be delivered to lysosomes for degradation or recycled back to the plasma membrane. The slit diaphragm is a modified tight junction connecting foot processes of the glomerular epithelial cells, podocytes. Signaling at the slit diaphragm plays a critical role in the kidney while its dysfunction leads to glomerular protein loss (proteinuria), manifesting as nephrotic syndrome, a rare condition with an estimated incidence of 2-4 new cases per 100,000 each year. Relatively little is known about the role of endocytic trafficking in podocyte signaling and maintenance of the slit diaphragm integrity. This review will focus on the role of endocytic trafficking at the mature podocyte slit diaphragm.

A Novel Role for Flotillin-Containing Lipid Rafts in Negative-Feedback Regulation of Thyroid-Specific Gene Expression by Thyroglobulin

BACKGROUND

Thyroglobulin (Tg) stored in thyroid follicles regulates follicular function in thyroid hormone (TH) synthesis by suppressing thyroid-specific gene expression in a concentration-dependent manner. Thus, Tg is an intrinsic negative-feedback regulator that can restrain the effect of thyrotropin (TSH) in the follicle. However, the underlying mechanisms by which Tg exerts its prominent autoregulatory effect following recognition by thyrocytes remains unclear.

METHODS

In order to identify potential proteins that recognize and interact with Tg, mass spectrometry was used to analyze immunoprecipitated Tg-bound proteins derived from Tg-treated rat thyroid FRTL-5 cells.

RESULTS

Flotillin 1 and flotillin 2, two homologs that are integral membrane proteins in lipid rafts, were identified as novel Tg-binding proteins with high confidence. Further studies revealed that flotillins physically interact with endocytosed Tg, and together these proteins redistribute from the cell membrane to cytoplasmic vesicles. Treatment with the lipid raft disrupter methyl-β-cyclodextrin abolished both the endocytosis and the negative-feedback effect of Tg on thyroid-specific gene expression. Meanwhile, siRNA-mediated knockdown of flotillin 1 or flotillin 2 also significantly inhibited Tg effects on gene expression.

CONCLUSION

Together these results indicate that flotillin-containing lipid rafts are essential for follicular Tg to be recognized by thyrocytes and exert its negative-feedback effects in the thyroid.

Functional Membrane Microdomains Organize Signaling Networks in Bacteria

Membrane organization is usually associated with the correct function of a number of cellular processes in eukaryotic cells as diverse as signal transduction, protein sorting, membrane trafficking, or pathogen invasion. It has been recently discovered that bacterial membranes are able to compartmentalize their signal transduction pathways in functional membrane microdomains (FMMs). In this review article, we discuss the biological significance of the existence of FMMs in bacteria and comment on possible beneficial roles that FMMs play on the harbored signal transduction cascades. Moreover, four different membrane-associated signal transduction cascades whose functions are linked to the integrity of FMMs are introduced, and the specific role that FMMs play in stabilizing and promoting interactions of their signaling components is discussed. Altogether, FMMs seem to play a relevant role in promoting more efficient activation of signal transduction cascades in bacterial cells and show that bacteria are more sophisticated organisms than previously appreciated.

Depot specific differences in the adipogenic potential of precursors are mediated by collagenous extracellular matrix and Flotillin 2 dependent signaling

Objective

Adipose tissue shows a high degree of plasticity, and adipocyte hyperplasia is an important mechanism for adipose tissue expansion. Different adipose depots respond differently to an increased demand for lipid storage. Orchestrating cellular expansion in vivo requires extracellular matrix (ECM) remodeling and a high degree of interaction between cells and ECM.

Methods

We studied decellularized primary adipose stromal cell derived ECM of different adipose depots and reseeded them with primary adipose precursors. We tested ECM effect on adipocyte differentiation and analyzed ECM composition using proteomic and immunohistochemical approaches to identify factors in the ECM influencing adipogenesis.

Results

We show that the ECM of an adipose depot is the major determinant for the differentiation capacity of primary preadipocytes. Visceral adipose tissue stromal cells differentiate less than subcutaneous cells, which, in turn, are less adipogenic than BAT-derived cells. This effect is based on the ECM composition of the respective depot and not dependent on the precursor origin. Addition of vitamin C pronounces the pro-adipogenic effects of the ECM, indicating the importance of collagenous ECM in mediating the effect. Using a proteomic global and a targeted downstream analysis, we identify Flotillin 2 as a protein enriched in pro-adipogenic ECM, which is involved in orchestrating ECM to preadipocyte signaling.

Conclusions

We show that adipose tissue SVF secretes collagenous ECM, which directly modulates terminal differentiation of adipocyte precursors in a depot specific manner. These data demonstrate the importance of the tissue microenvironment in preadipocyte differentiation.

•

Different adipose tissue depots have different differentiation capacities.

•

Differentiation of preadipocytes is enhanced by vitamin c through formation of collagenous ECM.

•

Collagenous ECM is the main determinant of adipose depot specific differences in adipogenic potential.

•

FLOT2 plays a role in mediating collagenous-ECM specific effects.

Membrane associated protein flotillin-2 in Litopenaeus vannamei plays a role in WSSV infection

Flotillin-2, an important protein of vesicular endocytosis, plays an essential role in a large number of cellular processes, including viruses and pathogen infection. In the present study, a flotillin-2 homolog in Litopenaeus vannamei, designed as Lvflotillin-2, was cloned and characterized. To analyze the putative role of Lvflotillin-2 during white spot syndrome virus (WSSV) infection, real-time quantitative PCR was performed. The result showed that the transcriptional level of Lvflotillin-2 was up-regulated significantly after virus challenge. Furthermore, upon WSSV stimulation, Lvflotillin-2 in shrimp cells could translocate from the plasma membrane to intracellular compartments, and unexpectedly, also into nucleus. Additionally, depletion of Lvflotillin-2 inhibited WSSV gene ie1 transcription. It suggested that Lvflotillin-2 could be hijacked by WSSV. These observations indicated that Lvflotillin-2 was involved in WSSV infection, and presented here should be useful for gaining insight into shrimp immunity and WSSV pathogenesis.

Increased expression of endocytosis-Related proteins in rat hippocampus following 10-day electroconvulsive seizure treatment

Although electroconvulsive therapy (ECT) is clinically used for severe depression and drug-resistant Parkinson's disease, its exact biological background and mechanism have not yet been fully elucidated. Two potential explanations have been presented so far to explain the increased neuroplastic and resilient profiles of multiple ECT administrations. One is the alteration of central neurotransmitter receptor densities and the other is the expressional upregulation of brain derived neurotrophic factor in various brain regions with enhanced hippocampal neurogenesis and mossy fiber sprouting. In the present report, western blot analyses revealed significantly upregulated expression of various endocytosis-related proteins following 10-day electroconvulsive seizure (ECS) treatment in rat hippocampal homogenates and hippocampal lipid raft fractions extracted using an ultracentrifugation procedure. Upregulated proteins included endocytosis-related scaffolding proteins (caveolin-1, flotillin-1, and heavy and light chains of clathrin) and small GTPases (Rab5, Rab7, Rab11, and Rab4) specifically expressed on various types of endosomes. Two scaffolding proteins, caveolin-1 and flotillin-1, were also increased in the lipid raft fraction. Together with our previous finding of increased autophagy-related proteins in the hippocampal region, the present results suggest membrane trafficking machinery is enhanced following 10-day ECS treatment. We consider that the membrane trafficking machinery that transports functional proteins in the neuronal cells and from or into the synaptic membranes is one of the new candidates supporting the cellular and behavioral neuroplastic profiles of ECS treatments in animal experiments and ECT administrations in clinical settings.

Endocytosis and Endosomal Trafficking in Plants

Endocytosis and endosomal trafficking are essential processes in cells that control the dynamics and turnover of plasma membrane proteins, such as receptors, transporters, and cell wall biosynthetic enzymes. Plasma membrane proteins (cargo) are internalized by endocytosis through clathrin-dependent or clathrin-independent mechanism and delivered to early endosomes. From the endosomes, cargo proteins are recycled back to the plasma membrane via different pathways, which rely on small GTPases and the retromer complex. Proteins that are targeted for degradation through ubiquitination are sorted into endosomal vesicles by the ESCRT (endosomal sorting complex required for transport) machinery for degradation in the vacuole. Endocytic and endosomal trafficking regulates many cellular, developmental, and physiological processes, including cellular polarization, hormone transport, metal ion homeostasis, cytokinesis, pathogen responses, and development. In this review, we discuss the mechanisms that mediate the recognition and sorting of endocytic and endosomal cargos, the vesiculation processes that mediate their trafficking, and their connection to cellular and physiological responses in plants.

Characterization of the NPC1L1 gene and proteome from an exceptional responder to ezetimibe

BACKGROUND

Strategies to reduce LDL-cholesterol involve reductions in cholesterol synthesis or absorption. We identified a familial hypercholesterolemia patient with an exceptional response to the cholesterol absorption inhibitor, ezetimibe. Niemann-Pick C 1-like 1 (NPC1L1) is the molecular target of ezetimibe.

METHODS AND RESULTS

Sequencing identified nucleotide changes predicted to change amino acids 52 (L52P), 300 (I300T) and 489 (S489G) in exceptional NPC1L1. In silico analyses identified increased stability and cholesterol binding affinity in L52P-NPC1L1 versus WT-NPC1L1. HEK293 cells overexpressing WT-NPC1L1 or NPC1L1 harboring amino acid changes singly or in combination (Comb-NPC1L1) had reduced cholesterol uptake in Comb-NPC1L1 when ezetimibe was present. Cholesterol uptake was reduced by ezetimibe in L52P-NPC1L1, I300T-NPC1L1, but increased in S489G-NPC1L1 overexpressing cells. Immunolocalization studies found preferential plasma membrane localization of mutant NPC1L1 independent of ezetimibe. Flotillin 1 and 2 expression was reduced and binding to Comb-NPC1L1 was reduced independent of ezetimibe exposure. Proteomic analyses identified increased association with proteins that modulate intermediate filament proteins in Comb-NPC1L1 versus WT-NPC1L1 treated with ezetimibe.

CONCLUSION

This is the first detailed analysis of the role of NPC1L1 mutations in an exceptional responder to ezetimibe. The results point to a complex set of events in which the combined mutations were shown to affect cholesterol uptake in the presence of ezetimibe. Proteomic analysis suggests that the exceptional response may also lie in the nature of interactions with cytosolic proteins.

The identification of raft-derived tau-associated vesicles that are incorporated into immature tangles and paired helical filaments

AIMS

Neurofibrillary tangles (NFTs), a cardinal pathological feature of neurodegenerative disorders, such as Alzheimer's disease (AD) are primarily composed of hyper-phosphorylated tau protein. Recently, several other molecules, including flotillin-1, phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] and cyclin-dependent kinase 5 (CDK5), have also been revealed as constituents of NFTs. Flotillin-1 and PtdIns(4,5)P2 are considered markers of raft microdomains, whereas CDK5 is a tau kinase. Therefore, we hypothesized that NFTs have a relationship with raft domains and the tau phosphorylation that occurs within NFTs.

METHODS

We investigated six cases of AD, six cases of other neurodegenerative diseases with NFTs and three control cases. We analysed the PtdIns(4,5)P2-immunopositive material in detail, using super-resolution microscopy and electron microscopy to elucidate its pattern of expression. We also investigated the spatial relationship between the PtdIns(4,5)P2-immunopositive material and tau kinases through double immunofluorescence analysis.

RESULTS

Pretangles contained either paired helical filaments (PHFs) or PtdIns(4,5)P2-immunopositive small vesicles (approximately 1 μm in diameter) with nearly identical topology to granulovacuolar degeneration (GVD) bodies. Various combinations of these vesicles and GVD bodies, the latter of which are pathological hallmarks observed within the neurons of AD patients, were found concurrently in neurons. These vesicles and GVD bodies were both immunopositive not only for PtdIns(4,5)P2, but also for several tau kinases such as glycogen synthase kinase-3β and spleen tyrosine kinase.

CONCLUSIONS

These observations suggest that clusters of raft-derived vesicles that resemble GVD bodies are substructures of pretangles other than PHFs. These tau kinase-bearing vesicles are likely involved in the modification of tau protein and in NFT formation.