Polarized sorting in epithelial cells: raft clustering and the biogenesis of the apical membrane

Cholesterol-dependent Nsp5-endosomes co-trafficking to lysosomes facilitates porcine reproductive and respiratory syndrome virus replication by activating autophagy

Our previous studies showed that intracellular endosomal vesicles participated in PRRS virions trafficking in the early stage of viral infection, and cholesterol retention in endosomal vesicles disturbed viral replication via blocking PRRSV-endosomal vesicles membrane fusion. However, whether endosomal vesicles were associated with PRRSV protein(s) trafficking and the role of cholesterol in this process was still unclarity. In this study, we sought to elucidate the mechanism of cholesterol in endosomal vesicles-mediated viral protein transportation. The results showed that endosomal vesicles participated in trafficking of PRRSV Nsp5 protein. After being synthesized in endoplasmic reticulum (ER) and Golgi apparatus, Nsp5 was trafficked to early endosomes (EEs), but not endocytic recycling compartments (ERCs), then to late endosomes (LEs), and eventually reached lysosomal compartments, whereas disruption of cholesterol flux or LEs function led to the inability of Nsp5 arriving at lysosomes, where Nsp5 activated cellular autophagy to promote PRRSV replication. Molecular docking predictions revealed that cholesterol could form two hydrogen bonds with 74 alanine and 78 asparagine of Nsp5. After mutating the aforementioned binding sites, the replication efficiency of PRRSV decreased. Subsequently, the role of cholesterol in PRRSV replication was explored. Blocking of cholesterol flux significantly inhibited PRRSV replication. Single virus infection cycle analysis showed that cholesterol flux disorder did not affect virus adsorption, but could inhibit virus entry into host cells and block EEs-LEs-lysosomes mediated trafficking of virions, leading to virions retention in endosomal compartments. The present studies suggest that cholesterol and endosomal vesicles synergistically participate in Nsp5 trafficking to promote PRRSV replication, which may provide new insights for the development of novel antiviral drugs targeting cholesterol metabolism pathways or the improvement of commercial vaccines.

Sorting of complex sphingolipids within the cellular endomembrane systems

Cells contain a plethora of structurally diverse lipid species, which are unevenly distributed across the different cellular membrane compartments. Some of these lipid species require vesicular trafficking to reach their subcellular destinations. Here, we review recent advances made in the field that contribute to understanding lipid sorting during endomembrane trafficking.

Stage-specific function of sphingolipid synthases in African trypanosomes

The protozoan parasite Trypanosoma brucei is the only known eukaryote capable of synthesizing the three main phosphosphingolipids: sphingomyelin (SM), inositol phosphorylceramide (IPC), and ethanolamine phosphorylceramide (EPC). It has four paralogous genes encoding sphingolipid synthases (TbSLS1-4). TbSLS1 is a dedicated IPC synthase, TbSLS2 is a dedicated EPC synthase, and TbSLS3 and TbSLS4 are bifunctional SM/EPC synthases. IPC synthesis occurs exclusively in the procyclic insect stage (PCF), EPC is limited to the mammalian bloodstream form (BSF), and SM is synthesized throughout the life cycle. TbSLSs are indispensable for the viability of BSF and are, thus, potential drug targets. The relative stage-specific expression of each TbSLS paralog was compared, and the results match phosphosphingolipid content. Induction of pan-specific RNAi silencing was lethal in both BSF and PCF. To investigate individual TbSLS functions, separate HA-tagged genes, recoded to be RNAi-resistant (RNAiR), were engineered to replace a single allele of the entire TbSLS locus within parental BSF and PCF RNAi cell lines. RNAiR TbSLS3 and TbSLS4 both rescued BSF growth under silencing. Expression of RNAiR TbSLS1, normally repressed in BSF, did not rescue BSF viability but was not detrimental to normal in vitro growth. RNAiR TbSLS1, TbSLS3, and TbSLS4 were each sufficient to rescue PCF growth, indicating IPC is not essential for PCF viability in vitro. All TbSLSs localize to distal Golgi compartments in both BSF and PCF cells. These findings raise interesting questions about the roles of individual phosphosphingolipids in in vivo infection of the mammalian and tsetse hosts.

IMPORTANCE

African trypanosomes are eukaryotic pathogens that cause human and veterinary African trypanosomaisis. Uniquely, they synthesize all three major phosphosphingolipid species using four distinct sphingolipid synthases (SLS). This work details the function of each SLS in both bloodstream and insect form parasites. Novel and unexpected sphingolipid dependences are found in each stage. These results are consistent with this metabolic pathway being a valid target for chemotherapeutic intervention.

Diversification of sphingolipid synthase activities in kinetoplastid protozoa

Phosphosphingolipids (PSL) are essential components of eukaryotic membranes. The major PSL in fungi and protists is inositol phosphorylceramide (IPC), while sphingomyelin (SM), and to a lesser extent ethanolamine phosphorylceramide (EPC) predominate in mammals. Most kinetoplastid protozoa have a syntenic locus that encodes a single sphingolipid synthase (SLS) gene. Uniquely, among the kinetoplastids, the salivarian (African) trypanosomes have expanded this locus from a single gene in Trypanosoma vivax (TvSLS) to four genes in T. brucei (TbSLS1-4). We have previously shown that one of these is an IPC synthase, while the others are SM/EPC synthases, and that specificity is controlled by a single signature residue (IPC, serine; SM/EPC, phenylalanine). This residue is serine in T. cruzi and Leishmania major SLSs, both of which are demonstrated IPC synthases. However, T. vivax has a tyrosine at this residue raising the issue of specificity. Using a liposome-supplemented in vitro translation system we now show that T. vivax SLS is an SM/EPC synthase, and that the basal kinetoplastid Bodo saltans SLS is an IPC synthase (serine). We use these data, and a multiple alignment of available sequences, to discuss the evolution of kinetoplastid SLSs and their unique expansion in T. brucei and related salivarian trypanosomes.

Partitioning into ER membrane microdomains impacts autophagic protein turnover during cellular aging

Eukaryotic membranes are compartmentalized into distinct micro- and nanodomains that rearrange dynamically in response to external and internal cues. This lateral heterogeneity of the lipid bilayer and associated clustering of distinct membrane proteins contribute to the spatial organization of numerous cellular processes. Here, we show that membrane microdomains within the endoplasmic reticulum (ER) of yeast cells are reorganized during metabolic reprogramming and aging. Using biosensors with varying transmembrane domain length to map lipid bilayer thickness, we demonstrate that in young cells, microdomains of increased thickness mainly exist within the nuclear ER, while progressing cellular age drives the formation of numerous microdomains specifically in the cortical ER. Partitioning of biosensors with long transmembrane domains into these microdomains increased protein stability and prevented autophagic removal. In contrast, reporters with short transmembrane domains progressively accumulated at the membrane contact site between the nuclear ER and the vacuole, the so-called nucleus-vacuole junction (NVJ), and were subjected to turnover via selective microautophagy occurring specifically at these sites. Reporters with long transmembrane domains were excluded from the NVJ. Our data reveal age-dependent rearrangement of the lateral organization of the ER and establish transmembrane domain length as a determinant of membrane contact site localization and autophagic degradation.

Human growth factor-mediated signalling through lipid rafts regulates stem cell proliferation, development and survival of Schistosoma mansoni

Although the mechanisms by which schistosomes grow and develop in humans are poorly defined, their unique outer tegument layer, which interfaces with host blood, is considered vital to homeostasis of the parasite. Here, we investigated the importance of tegument lipid rafts to the biology of Schistosoma mansoni in the context of host-parasite interactions. We demonstrate the temporal clustering of lipid rafts in response to human epidermal growth factor (EGF) during early somule development, concomitant with the localization of anteriorly orientated EGF receptors (EGFRs) and insulin receptors, mapped using fluorescent EGF/insulin ligand. Methyl-β-cyclodextrin (MβCD)-mediated depletion of cholesterol from lipid rafts abrogated the EGFR/IR binding at the parasite surface and led to modulation of protein kinase C, extracellular signal-regulated kinase, p38 mitogen-activated protein kinase and Akt signalling pathways within the parasite. Furthermore, MβCD-mediated lipid raft disruption, and blockade of EGFRs using canertinib, profoundly reduced somule motility and survival, and attenuated stem cell proliferation and somule growth and development particularly to the fast-growing liver stage. These findings provide a novel paradigm for schistosome development and vitality in the host, driven through host-parasite interactions at the tegument, that might be exploitable for developing innovative therapeutic approaches to combat human schistosomiasis.

Applications of phase-separating multi-bilayers in protein-membrane domain interactions

Synthetic model membranes are important tools to elucidate lipid domain and protein interactions due to predefined lipid compositions and characterizable biophysical properties. Here, we introduce a model membrane with multiple lipid bilayers (multi-bilayers) stacked on a mica substrate that is prepared through a spin-coating technique. The spin-coated multi-bilayers are useful in the study of phase separated membranes with a high cholesterol content, mobile lipids, microscopic and reversible phase separation, and easy conjugation with proteins, which make them a good model to study interactions between proteins and membrane domains.

Very-long-chain fatty acids are crucial to neuronal polarity by providing sphingolipids to lipid rafts

Fatty acids have long been considered essential to brain development; however, the involvement of their synthesis in nervous system formation is unclear. We generate mice with knockout of GPSN2, an enzyme for synthesis of very-long-chain fatty acids (VLCFAs) and investigate the effects. Both GPSN2-/- and GPSN2+/- mice show abnormal neuronal networks as a result of impaired neuronal polarity determination. Lipidomics of GPSN2-/- embryos reveal that ceramide synthesis is specifically inhibited depending on FA length; namely, VLCFA-containing ceramide is reduced. We demonstrate that lipid rafts are highly enriched in growth cones and that GPSN2+/- neurons lose gangliosides in their membranes. Application of C24:0 ceramide, but not C16:0 ceramide or C24:0 phosphatidylcholine, to GPSN2+/- neurons rescues both neuronal polarity determination and lipid-raft density in the growth cone. Taken together, our results indicate that VLCFA synthesis contributes to physiological neuronal development in brain network formation, in particular neuronal polarity determination through the formation of lipid rafts.

The Membrane-Mediated Interaction of Liquid-Ordered Lipid Domains in the Presence of Amphipathic Peptides

The lipid membranes of living cells are composed of a large number of lipid types and can undergo phase separation with the formation of nanometer-scale liquid-ordered lipid domains, also called rafts. Raft coalescence, i.e., the fusion of lipid domains, is involved in important cell processes, such as signaling and trafficking. In this work, within the framework of the theory of elasticity of lipid membranes, we explore how amphipathic peptides adsorbed on lipid membranes may affect the domain-domain fusion processes. We show that the elastic deformations of lipid membranes drive amphipathic peptides to the boundary of lipid domains, which leads to an increase in the average energy barrier of the domain-domain fusion, even if the surface concentration of amphipathic peptides is low and the domain boundaries are only partially occupied by the peptides. This inhibition of the fusion of lipid domains may lead to negative side effects of using amphipathic peptides as antimicrobial agents.

A CRISPR screen in intestinal epithelial cells identifies novel factors for polarity and apical transport

Epithelial polarization and polarized cargo transport are highly coordinated and interdependent processes. In our search for novel regulators of epithelial polarization and protein secretion, we used a genome-wide CRISPR/Cas9 screen and combined it with an assay based on fluorescence-activated cell sorting (FACS) to measure the secretion of the apical brush-border hydrolase dipeptidyl peptidase 4 (DPP4). In this way, we performed the first CRISPR screen to date in human polarized epithelial cells. Using high-resolution microscopy, we detected polarization defects and mislocalization of DPP4 to late endosomes/lysosomes after knockout of TM9SF4, anoctamin 8, and ARHGAP33, confirming the identification of novel factors for epithelial polarization and apical cargo secretion. Thus, we provide a powerful tool suitable for studying polarization and cargo secretion in epithelial cells. In addition, we provide a dataset that serves as a resource for the study of novel mechanisms for epithelial polarization and polarized transport and facilitates the investigation of novel congenital diseases associated with these processes.

Exploring the Link between Vacuolar-Type Proton ATPase and Epithelial Cell Polarity

Vacuolar-type H⁺-ATPase (V-ATPase) was first identified as an electrogenic proton pump that acidifies the lumen of intracellular organelles. Subsequently, it was observed that the proton pump also participates in the acidification of extracellular compartments. V-ATPase plays important roles in a wide range of cell biological processes and physiological functions by generating an acidic pH; therefore, it has attracted much attention not only in basic research but also in pathological and clinical aspects. Emerging evidence indicates that the luminal acidic endocytic organelles and their trafficking may function as important hubs that connect and coordinate various signaling pathways. Various pharmacological analyses have suggested that acidic endocytic organelles are important for the maintenance of cell polarity. Recently, several studies using genetic approaches have revealed the involvement of V-ATPase in the establishment and maintenance of apico-basal polarity. This review provides a brief overview of the relationship between the polarity of epithelial cells and V-ATPase as well as V-ATPase driven luminal acidification.

Segregation of the membrane cargoes, BACE1 and amyloid precursor protein (APP) throughout the Golgi apparatus

The intracellular trafficking of β‐site amyloid precursor protein (APP) cleaving enzyme (BACE1) and APP regulates amyloid‐β production. Our previous work demonstrated that newly synthesized BACE1 and APP are segregated into distinct trafficking pathways from the trans‐Golgi network (TGN), and that alterations in their trafficking lead to an increase in Aβ production in non‐neuronal and neuronal cells. However, it is not known whether BACE1 and APP are transported through the Golgi stacks together and sorted at the TGN or segregated prior to arrival at the TGN. To address this question, we have used high‐resolution Airyscan technology followed by Huygens deconvolution to quantify the overlap of BACE1 and APP in Golgi subcompartments in HeLa cells and primary neurons. Here, we show that APP and BACE1 are segregated, on exit from the endoplasmic reticulum and in the cis‐Golgi and throughout the Golgi stack. In contrast, the transferrin receptor, which exits the TGN in AP‐1 mediated transport carriers as for BACE1, colocalizes with BACE1, but not APP, throughout the Golgi stack. The segregation of APP and BACE1 is independent of the Golgi ribbon structure and the cytoplasmic domain of the cargo. Overall, our findings reveal the segregation of different membrane cargoes early in the secretory pathway, a finding relevant to the regulation of APP processing events.

Hydrophobic Mismatch Controls the Mode of Membrane-Mediated Interactions of Transmembrane Peptides

Various cellular processes require the concerted cooperative action of proteins. The possibility for such synchronization implies the occurrence of specific long-range interactions between the involved protein participants. Bilayer lipid membranes can mediate protein-protein interactions via relatively long-range elastic deformations induced by the incorporated proteins. We considered the interactions between transmembrane peptides mediated by elastic deformations using the framework of the theory of elasticity of lipid membranes. An effective peptide shape was assumed to be cylindrical, hourglass-like, or barrel-like. The interaction potentials were obtained for membranes of different thicknesses and elastic rigidities. Cylindrically shaped peptides manifest almost neutral average interactions-they attract each other at short distances and repel at large ones, independently of membrane thickness or rigidity. The hourglass-like peptides repel each other in thin bilayers and strongly attract each other in thicker bilayers. On the contrary, the barrel-like peptides repel each other in thick bilayers and attract each other in thinner membranes. These results potentially provide possible mechanisms of control for the mode of protein-protein interactions in membrane domains with different bilayer thicknesses.

MAL2 mediates the formation of stable HER2 signaling complexes within lipid raft-rich membrane protrusions in breast cancer cells

The lipid raft-resident protein, MAL2, has been implicated as contributing to the pathogenesis of several malignancies, including breast cancer, but the underlying mechanism for its effects on tumorigenesis is unknown. Here, we show that MAL2-mediated lipid raft formation leads to HER2 plasma membrane retention and enhanced HER2 signaling in breast cancer cells. We demonstrate physical interactions between HER2 and MAL2 in lipid rafts using proximity ligation assays. Super-resolution structured illumination microscopy imaging displays the structural organization of the HER2/Ezrin/NHERF1/PMCA2 protein complex. Formation of this protein complex maintains low intracellular calcium concentrations in the vicinity of the plasma membrane. HER2/MAL2 protein interactions in lipid rafts are enhanced in trastuzumab-resistant breast cancer cells. Our findings suggest that MAL2 is crucial for lipid raft formation, HER2 signaling, and HER2 membrane stability in breast cancer cells, suggesting MAL2 as a potential therapeutic target.

Jeong et al. show that the formation of MAL2-mediated lipid raft-rich membrane protrusions is crucial for HER2 signaling in breast cancer cells. MAL2 is required for the formation of HER2/Ezrin/NHERF1/PMCA2 protein complexes. Formation of these protein complexes leads to a low calcium environment in the plasma membrane

Alkyl-Terminated Gold Nanoparticles as a Self-Therapeutic Treatment for Psoriasis

We present a self-therapeutic nanoparticle for topical delivery to epidermal keratinocytes to prevent and treat psoriasis. Devoid of known chemical or biological antipsoriatic drugs, this sub-15 nm nanoparticle contains a 3 nm gold core and a shell of 1000 Da polyethylene glycol strands modified with 30% octadecyl chains. When it is applied to imiquimod-induced psoriasis mice without an excipient, the nanoparticle can cross the stratum corneum and preferentially enter keratinocytes. Applying the nanoparticles concurrently with imiquimod prevents psoriasis and downregulates genes that are enriched in the downstream of the interleukin-17 signaling pathway and linked to epidermis hyperproliferation and inflammation. Applying the nanoparticles after psoriasis is established treats the psoriatic skin as effectively as standard steroid and vitamin D analog-based therapy but without hair loss and skin wrinkling. The nanoparticles do not accumulate in major organs or induce long-term toxicity. Our nanoparticle offers a simple, safe, and effective alternative for treating psoriasis.

Amphipathic Peptides Impede Lipid Domain Fusion in Phase-Separated Membranes

Cell membranes are heterogeneous in lipid composition which leads to the phase separation with the formation of nanoscopic liquid-ordered domains, also called rafts. There are multiple cell processes whereby the clustering of these domains into a larger one might be involved, which is responsible for such important processes as signal transduction, polarized sorting, or immune response. Currently, antimicrobial amphipathic peptides are considered promising antimicrobial, antiviral, and anticancer therapeutic agents. Here, within the framework of the classical theory of elasticity adapted for lipid membranes, we investigate how the presence of the peptides in a phase-separated membrane influences the fusion of the domains. We show that the peptides tend to occupy the boundaries of liquid-ordered domains and significantly increase the energy barrier of the domain-domain fusion, which might lead to misregulation of raft clustering and adverse consequences for normal cell processes.

Should I stay or should I go? Golgi membrane spatial organization for protein sorting and retention

The Golgi complex is the membrane-bound organelle that lies at the center of the secretory pathway. Its main functions are to maintain cellular lipid homeostasis, to orchestrate protein processing and maturation, and to mediate protein sorting and export. These functions are not independent of one another, and they all require that the membranes of the Golgi complex have a well-defined biochemical composition. Importantly, a finely-regulated spatiotemporal organization of the Golgi membrane components is essential for the correct performance of the organelle. In here, we review our current mechanistic and molecular understanding of how Golgi membranes are spatially organized in the lateral and axial directions to fulfill their functions. In particular, we highlight the current evidence and proposed models of intra-Golgi transport, as well as the known mechanisms for the retention of Golgi residents and for the sorting and export of transmembrane cargo proteins. Despite the controversies, conflicting evidence, clashes between models, and technical limitations, the field has moved forward and we have gained extensive knowledge in this fascinating topic. However, there are still many important questions that remain to be completely answered. We hope that this review will help boost future investigations on these issues.

Amyloid β production along the neuronal secretory pathway: Dangerous liaisons in the Golgi?

β-amyloid peptides (Aβ) are generated in intracellular compartments of neurons and secreted to form cytotoxic fibrils and plaques. Dysfunctional membrane trafficking contributes to aberrant Aβ production and Alzheimer's disease. Endosomes represent one of the major sites for Aβ production and recently the Golgi has re-emerged also as a major location for amyloid precursor protein (APP) processing and Aβ production. Based on recent findings, here we propose that APP processing in the Golgi is finely tuned by segregating newly-synthesised APP and the β-secretase BACE1 within the Golgi and into distinct trans-Golgi network transport pathways. We hypothesise that there are multiple mechanisms responsible for segregating APP and BACE1 during transit through the Golgi, and that perturbation in Golgi morphology associated with Alzheimer's disease, and or changes in cholesterol metabolism associated with Alzheimer's disease risk factors, may lead to a loss of partitioning and enhanced Aβ production.

Double emulsion-pretreated microwell culture for the in vitro production of multicellular spheroids and their in situ analysis

Multicellular spheroids have served as a promising preclinical model for drug efficacy testing and disease modeling. Many microfluidic technologies, including those based on water-oil-water double emulsions, have been introduced for the production of spheroids. However, sustained culture and the in situ characterization of the generated spheroids are currently unavailable for the double emulsion-based spheroid model. This study presents a streamlined workflow, termed the double emulsion-pretreated microwell culture (DEPMiC), incorporating the features of (1) effective initiation of uniform-sized multicellular spheroids by the pretreatment of double emulsions produced by microfluidics without the requirement of biomaterial scaffolds; (2) sustained maintenance and culture of the produced spheroids with facile removal of the oil confinement; and (3) in situ characterization of individual spheroids localized in microwells by a built-in analytical station. Characterized by microscopic observations and Raman spectroscopy, the DEPMiC cultivated spheroids accumulated elevated lipid ordering on the apical membrane, similar to that observed in their Matrigel counterparts. Made possible by the proposed technological advancement, this study subsequently examined the drug responses of these in vitro-generated multicellular spheroids. The developed DEPMiC platform is expected to generate health benefits in personalized cancer treatment by offering a pre-animal tool to dissect heterogeneity from individual tumor spheroids.

Muscarinic Receptors and BK Channels Are Affected by Lipid Raft Disruption of Salivary Gland Cells

Activity-dependent fluid secretion is the most important physiological function of salivary glands and is regulated via muscarinic receptor signaling. Lipid rafts are important for G-protein coupled receptor (GPCR) signaling and ion channels in plasma membranes. However, it is not well understood whether lipid raft disruption affects all membrane events or only specific functions in muscarinic receptor-mediated water secretion in salivary gland cells. We investigated the effects of lipid raft disruption on the major membrane events of muscarinic transcellular water movement in human salivary gland (HSG) cells. We found that incubation with methyl-β-cyclodextrin (MβCD), which depletes lipid rafts, inhibited muscarinic receptor-mediated Ca²⁺ signaling in HSG cells and isolated mouse submandibular acinar cells. However, MβCD did not inhibit a Ca²⁺ increase induced by thapsigargin, which activates store-operated Ca²⁺ entry (SOCE). Interestingly, MβCD increased the activity of the large-conductance Ca²⁺-activated K⁺ channel (BK channel). Finally, we found that MβCD did not directly affect the translocation of aquaporin-5 (AQP5) into the plasma membrane. Our results suggest that lipid rafts maintain muscarinic Ca²⁺ signaling at the receptor level without directly affecting the activation of SOCE induced by intracellular Ca²⁺ pool depletion or the translocation of AQP5 into the plasma membrane.

Role of Lipids in Morphogenesis of T-Cell Microvilli

T cells communicate with the environment via surface receptors. Cooperation of surface receptors regulates T-cell responses to diverse stimuli. Recently, finger-like membrane protrusions, microvilli, have been demonstrated to play a role in the organization of receptors and, hence, T-cell activation. However, little is known about the morphogenesis of dynamic microvilli, especially in the cells of immune system. In this review, I focus on the potential role of lipids and lipid domains in morphogenesis of microvilli. Discussed is the option that clustering of sphingolipids with phosphoinositides at the plasma membrane results in dimpling (curved) domains. Such domains can attract phosphoinositide-binding proteins and stimulate actin cytoskeleton reorganization. This process triggers cortical actin opening and bundling of actin fibres to support the growing of microvilli. Critical regulators of microvilli morphogenesis in T cells are unknown. At the end, I suggest several candidates with a potential to organize proteins and lipids in these structures.

Infection of polarized bovine respiratory epithelial cells by bovine viral diarrhea virus (BVDV)

Bovine viral diarrhea virus (BVDV) is affecting cattle populations all over the world causing acute disease, immunosuppressive effects, respiratory diseases, gastrointestinal, and reproductive failure in cattle. The virus is taken up via the oronasal route and infection of epithelial and immune cells contributes to the dissemination of the virus throughout the body. However, it is not known how the virus gets across the barrier of epithelial cells encountered in the airways. Here, we analyzed the infection of polarized primary bovine airway epithelial cells (BAEC). Infection of BAEC by a non-cytopathogenic BVDV was possible via both the apical and the basolateral plasma membrane, but the infection was most efficient when the virus was applied to the basolateral plasma membrane. Irrespective of the site of infection, BVDV was efficiently released to the apical site, while only minor amounts of virus were detected in the basal medium. This indicates that the respiratory epithelium can release large amounts of BVDV to the environment and susceptible animals via respiratory fluids and aerosols, but BVDV cannot cross the airway epithelial cells to infect subepithelial cells and establish systemic infection. Further experiments showed that the receptor, bovine CD46, for BVDV is expressed predominantly on the apical membrane domain of the polarized epithelial cells. In a CD46 blocking experiment, the addition of an antibody directed against CD46 almost completely inhibited apical infection, whereas basolateral infection was not affected. While CD46 serves as a receptor for apical infection of BAEC by BVDV, the receptor for basolateral infection remains to be elucidated.

Distinct roles for luminal acidification in apical protein sorting and trafficking in zebrafish

Epithelial cell physiology critically depends on the asymmetric distribution of channels and transporters. However, the mechanisms targeting membrane proteins to the apical surface are still poorly understood. Here, we performed a visual forward genetic screen in the zebrafish intestine and identified mutants with defective apical targeting of membrane proteins. One of these mutants, affecting the vacuolar H⁺-ATPase gene atp6ap1b, revealed specific requirements for luminal acidification in apical, but not basolateral, membrane protein sorting and transport. Using a low temperature block assay combined with genetic and pharmacologic perturbation of luminal pH, we monitored transport of newly synthesized membrane proteins from the TGN to apical membrane in live zebrafish. We show that vacuolar H⁺-ATPase activity regulates sorting of O-glycosylated proteins at the TGN, as well as Rab8-dependent post-Golgi trafficking of different classes of apical membrane proteins. Thus, luminal acidification plays distinct and specific roles in apical membrane biogenesis.

Membrane Heterogeneity Beyond the Plasma Membrane

The structure and organization of cellular membranes have received intense interest, particularly in investigations of the raft hypothesis. The vast majority of these investigations have focused on the plasma membrane of mammalian cells, yielding significant progress in understanding membrane heterogeneity in terms of lipid composition, molecular structure, dynamic regulation, and functional relevance. In contrast, investigations on lipid organization in other membrane systems have been comparatively scarce, despite the likely relevance of membrane domains in these contexts. In this review, we summarize recent observations on lipid organization in organellar membranes, including endoplasmic reticulum, Golgi, endo-lysosomes, lipid droplets, and secreted membranes like lung surfactant, milk fat globule membranes, and viral membranes. Across these non-plasma membrane systems, it seems that the biophysical principles underlying lipid self-organization contribute to lateral domains.

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

Lipid accumulation controls the balance between surface connection and scission of caveolae

Caveolae are bulb-shaped invaginations of the plasma membrane (PM) that undergo scission and fusion at the cell surface and are enriched in specific lipids. However, the influence of lipid composition on caveolae surface stability is not well described or understood. Accordingly, we inserted specific lipids into the cell PM via membrane fusion and studied their acute effects on caveolae dynamics. We demonstrate that sphingomyelin stabilizes caveolae to the cell surface, whereas cholesterol and glycosphingolipids drive caveolae scission from the PM. Although all three lipids accumulated specifically in caveolae, cholesterol and sphingomyelin were actively sequestered, whereas glycosphingolipids diffused freely. The ATPase EHD2 restricts lipid diffusion and counteracts lipid-induced scission. We propose that specific lipid accumulation in caveolae generates an intrinsically unstable domain prone to scission if not restrained by EHD2 at the caveolae neck. This work provides a mechanistic link between caveolae and their ability to sense the PM lipid composition.

Lipid rafts and pathogens: the art of deception and exploitation

Lipid rafts, solid regions of the plasma membrane enriched in cholesterol and glycosphingolipids, are essential parts of a cell. Functionally, lipid rafts present a platform that facilitates interaction of cells with the outside world. However, the unique properties of lipid rafts required to fulfill this function at the same time make them susceptible to exploitation by pathogens. Many steps of pathogen interaction with host cells, and sometimes all steps within the entire lifecycle of various pathogens, rely on host lipid rafts. Such steps as binding of pathogens to the host cells, invasion of intracellular parasites into the cell, the intracellular dwelling of parasites, microbial assembly and exit from the host cell, and microbe transfer from one cell to another all involve lipid rafts. Interaction also includes modification of lipid rafts in host cells, inflicted by pathogens from both inside and outside the cell, through contact or remotely, to advance pathogen replication, to utilize cellular resources, and/or to mitigate immune response. Here, we provide a systematic overview of how and why pathogens interact with and exploit host lipid rafts, as well as the consequences of this interaction for the host, locally and systemically, and for the microbe. We also raise the possibility of modulation of lipid rafts as a therapeutic approach against a variety of infectious agents.

Lipid Rafts: Controversies Resolved, Mysteries Remain

The lipid raft hypothesis postulates that lipid-lipid interactions can laterally organize biological membranes into domains of distinct structures, compositions, and functions. This proposal has in equal measure exhilarated and frustrated membrane research for decades. While the physicochemical principles underlying lipid-driven domains has been explored and is well understood, the existence and relevance of such domains in cells remains elusive, despite decades of research. Here, we review the conceptual underpinnings of the raft hypothesis and critically discuss the supporting and contradicting evidence in cells, focusing on why controversies about the composition, properties, and even the very existence of lipid rafts remain unresolved. Finally, we highlight several recent breakthroughs that may resolve existing controversies and suggest general approaches for moving beyond questions of the existence of rafts and towards understanding their physiological significance.

Mechanisms behind the polarized distribution of lipids in epithelial cells

Epithelial cells are polarized cells and typically display distinct plasma membrane domains: basal plasma membrane domains face the underlying tissue, lateral domains contact adjacent cells and apical domains face the exterior lumen. Each membrane domain is endowed with a specific macromolecular composition that constitutes the functional identity of that domain. Defects in apical-basal plasma membrane polarity altogether or more subtle defects in the composition of either apical or basal plasma membrane domain can give rise to severe diseases. Lipids are the main component of cellular membranes and mechanisms that control their polarized distribution in epithelial cells are emerging. In particular sphingolipids and phosphatidylinositol lipids have taken center stage in the organization of the apical and basolateral plasma membrane domain. This short review article discusses mechanisms that contribute to the polarized distribution of lipids in epithelial cells.

Acid Sphingomyelinase regulates the localization and trafficking of palmitoylated proteins

In human, loss of acid sphingomyelinase (ASM/SMPD1) causes Niemann–Pick disease, type A. ASM hydrolyzes sphingomyelins to produce ceramides but protein targets of ASM remain largely unclear. Our mass spectrometry-based proteomic analyses have identified >100 proteins associated with the ASM-dependent, detergent-resistant membrane microdomains (lipid rafts), with >60% of these proteins being palmitoylated, including SNAP23, Src-family kinases Yes and Lyn, and Ras and Rab family small GTPases. Inactivation of ASM abolished the presence of these proteins in the plasma membrane, with many of them trapped in the Golgi. While palmitoylation inhibitors and palmitoylation mutants phenocopied the effects of ASM inactivation, we demonstrated that ASM is required for the transport of palmitoylated proteins, such as SNAP23 and Lyn, from the Golgi to the plasma membrane without affecting palmitoylation directly. Importantly, ASM delivered extracellularly can regulate the trafficking of SNAP23 from the Golgi to the plasma membrane. Our studies suggest that ASM, acting at the plasma membrane to produce ceramides, regulates the localization and trafficking of the palmitoylated proteins.

Summary: Acid sphingomyelinase (ASM) regulates palmitoylated protein trafficking and localization.

Apical Trafficking Pathways of Influenza A Virus HA and NA via Rab17- and Rab23-Positive Compartments

The envelope proteins of influenza A virus, hemagglutinin (HA) and neuraminidase (NA), play critical roles in viral entry to host cells and release from the cells, respectively. After protein synthesis, they are transported from the trans-Golgi network (TGN) to the apical plasma membrane (PM) and assembled into virus particles. However, the post-TGN transport pathways of HA and NA have not been clarified. Temporal study by confocal microscopy revealed that HA and NA colocalized soon after their synthesis, and relocated together from the TGN to the upper side of the cell. Using the Rab family protein, we investigated the post-TGN transport pathways of HA and NA. HA partially colocalized with AcGFP-Rab15, Rab17, and Rab23, but rarely with AcGFP-Rab11. When analyzed in cells stably expressing AcGFP-Rab, HA/NA colocalized with Rab15 and Rab17, markers of apical sorting and recycling endosomes, and later colocalized with Rab23, which distributes to the apical PM and endocytic vesicles. Overexpression of the dominant-negative (DN) mutants of Rab15 and Rab17, but not Rab23, significantly delayed HA transport to the PM. However, Rab23DN impaired cell surface expression of HA. Live-cell imaging revealed that NA moved rapidly with Rab17 but not with Rab15. NA also moved with Rab23 in the cytoplasm, but this motion was confined at the upper side of the cell. A fraction of HA was localized to Rab17 and Rab23 double-positive vesicles in the cytoplasm. Coimmunoprecipitation indicated that HA was associated with Rab17 and Rab23 in lipid raft fractions. When cholesterol was depleted by methyl-β-cyclodextrin treatment, the motion of NA and Rab17 signals ceased. These results suggest that HA and NA are incorporated into lipid raft microdomains and are cotransported to the PM by Rab17-positive and followed by Rab23-positive vesicles.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Cholesterol Binding to the Transmembrane Region of a Group 2 Hemagglutinin (HA) of Influenza Virus Is Essential for Virus Replication, Affecting both Virus Assembly and HA Fusion Activity

Hemagglutinin (HA) of influenza virus is incorporated into cholesterol-enriched nanodomains of the plasma membrane. Phylogenetic group 2 HAs contain the conserved cholesterol consensus motif (CCM) YKLW in the transmembrane region. We previously reported that mutations in the CCM retarded intracellular transport of HA and decreased its nanodomain association. Here, we analyzed whether cholesterol interacts with the CCM. Incorporation of photocholesterol into HA was significantly reduced if the whole CCM is replaced by alanine, both using immunoprecipitated HA and when HA is embedded in the membrane. We next used reverse genetics to investigate the significance of the CCM for virus replication. No virus was rescued if the whole motif is exchanged (YKLW4A); singly (LA) or doubly (YK2A and LW2A) mutated virus showed decreased titers and a comparative fitness disadvantage. In polarized cells, transport of HA mutants to the apical membrane was not disturbed. Reduced amounts of HA and cholesterol were incorporated into the viral membrane. Mutant viruses exhibit a decrease in hemolysis, which is only partially corrected if the membrane is replenished with cholesterol. More specifically, viruses have a defect in hemifusion, as demonstrated by fluorescence dequenching. Cells expressing HA YKLW4A fuse with erythrocytes, but the number of events is reduced. Even after acidification unfused erythrocytes remain cell bound, a phenomenon not observed with wild-type HA. We conclude that cholesterol binding to a group 2 HA is essential for virus replication. It has pleiotropic effects on virus assembly and membrane fusion, mainly on lipid mixing and possibly a preceding step.IMPORTANCE The glycoprotein HA is a major pathogenicity factor of influenza viruses. Whereas the structure and function of HA's ectodomain is known in great detail, similar data for the membrane-anchoring part of the protein are missing. Here, we demonstrate that the transmembrane region of a group 2 HA interacts with cholesterol, the major lipid of the plasma membrane and the defining element of the viral budding site nanodomains of the plasma membrane. The cholesterol binding motif is essential for virus replication. Its partial removal affects various steps of the viral life cycle, such as assembly of new virus particles and their subsequent cell entry via membrane fusion. A cholesterol binding pocket in group 2 HAs might be a promising target for a small lipophilic drug that inactivates the virus.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

Spatial Overlap of Claudin- and Phosphatidylinositol Phosphate-Binding Sites on the First PDZ Domain of Zonula Occludens 1 Studied by NMR

Background: The tight junction is an intercellular adhesion complex composed of claudins (CLDs), occludin, and the scaffolding proteins zonula occludens 1 (ZO-1) and its two paralogs ZO-2 and ZO-3. ZO-1 is a multifunctional protein that contains three PSD95/Discs large/ZO-1(PDZ) domains. A key functional domain of ZO-1 is the first PDZ domain (ZO-1(PDZ1)) that recognizes the conserved C-termini of CLDs. Methods: In this study, we confirmed that phosphoinositides bound directly to ZO-1(PDZ1) by biochemical and solution NMR experiments. We further determined the solution structure of mouse ZO-1(PDZ1) by NMR and mapped the phosphoinositide binding site onto its molecular surface. Results: The phosphoinositide binding site was spatially overlapped with the CLD-binding site of ZO-1(PDZ1). Accordingly, inositol-hexaphosphate (phytic acid), an analog of the phosphoinositide head group, competed with ZO-1(PDZ)-CLD interaction. Conclusions: The results suggested that the PDZ domain–phosphoinositide interaction plays a regulatory role in biogenesis and homeostasis of the tight junction.

Roles of membrane lipids in the organization of epithelial cells: Old and new problems

Epithelial cells have characteristic membrane domains. Identification of membrane proteins playing an important role in these membrane domains has progressed and numerous studies have been performed on the functional analysis of these membrane proteins. On the other hand, the precise roles of membrane lipids in the organization of these membrane domains are largely unknown. Historically, the concept of lipid raft arose from the analysis of lipid composition of the apical membrane, and it can be said that epithelial cells are an optimal experimental model for elucidating the functions of lipids. In this review, I discuss the role of lipids in the formation of epithelial polarity and in the formation of cell membrane structures of epithelial cells such as microvilli in the apical domain, cell-cell adhesion apparatus in the lateral domain and cell-matrix adhesion in the basal domain.

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkörper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkörper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.

Distribution of Glycan Motifs at the Surface of Midgut Cells in the Cotton Leafworm (Spodoptera littoralis) Demonstrated by Lectin Binding

Glycans are involved in many biological phenomena, including signal transduction, cell adhesion, immune response or differentiation. Although a few papers have reported on the role of glycans in the development and proper functioning of the insect midgut, no data are available regarding the localization of the glycan structures on the surface of the cells in the gut of insects. In this paper, we analyzed the spatial distribution of glycans present on the surface of the midgut cells in larvae of the cotton leafworm Spodoptera littoralis, an important agricultural pest insect worldwide. For this purpose, we established primary midgut cell cultures, probed these individual cells that are freely suspended in liquid medium with a selection of seven fluorescently labeled lectins covering a range of different carbohydrate binding specificities [mannose oligomers (GNA and HHA), GalNAc/Gal (RSA and SSA), GlcNAc (WGA and Nictaba) and Neu5Ac(α-2,6)Gal/GalNAc (SNA-I)], and visualized the interaction of these lectins with the different zones of the midgut cells using confocal microscopy. Our analysis focused on the typical differentiated columnar cells with a microvillar brush border at their apical side, which are dominantly present in the Lepidopteran midgut and function in food digestion and absorption, and as well as on the undifferentiated stem cells that are important for midgut development and repair. Confocal microscopy analyses showed that the GalNAc/Gal-binding lectins SSA and RSA and the terminal GlcNAc-recognizing WGA bound preferentially to the apical microvillar zone of the differentiated columnar cells as compared to the basolateral pole. The reverse result was observed for the mannose-binding lectins GNA and HHA, as well as Nictaba that binds preferentially to GlcNAc oligomers. Furthermore, differences in lectin binding to the basal and lateral zones of the cell membranes of the columnar cells were apparent. In the midgut stem cells, GNA and Nictaba bound more strongly to the membrane of these undifferentiated cells compared to the microvillar pole of the columnar cells, while SSA, HHA, WGA, and SNA-I showed stronger binding to the microvilli. Our results indicated that polarization of the midgut cells is also reflected by a specific distribution of glycans, especially between the basal and microvillar pole. The data are discussed in relation to the functioning and development of the insect midgut.

Vibrio cholerae OmpU induces IL-8 expression in human intestinal epithelial cells

Although Vibrio cholerae colonizes the small intestine and induces acute inflammatory responses, less is known about the molecular mechanisms of V. cholerae-induced inflammatory responses in the intestine. We recently reported that OmpU, one of the most abundant outer membrane proteins of V. cholerae, plays an important role in the innate immunity of the whole bacteria. In this study, we evaluated the role of OmpU in induction of IL-8, a representative chemokine that recruits various inflammatory immune cells, in the human intestinal epithelial cell (IEC) line, HT-29. Recombinant OmpU (rOmpU) of V. cholerae induced IL-8 expression at the mRNA and protein levels in a dose- and time-dependent manner. Interestingly, IL-8 was secreted through both apical and basolateral sides of the polarized HT-29 cells upon apical exposure to rOmpU but not upon basolateral exposure. rOmpU-induced IL-8 expression was inhibited by interference of lipid raft formation with nystatin, but not by blocking the formation of clathrin-coated pits with chlorpromazine. In addition, rOmpU-induced IL-8 expression was mediated via ERK1/2 and p38 kinase pathways, but not via JNK signaling pathway. Finally, V. cholerae lacking ompU elicited decreased IL-8 expression and adherence to HT-29 cells compared to the parental strain. Collectively, these results suggest that V. cholerae OmpU might play an important role in intestinal inflammation by inducing IL-8 expression in human IECs.

Membrane Transport across Polarized Epithelia

Polarized epithelial cells line diverse surfaces throughout the body forming selective barriers between the external environment and the internal milieu. To cross these epithelial barriers, large solutes and other cargoes must undergo transcytosis, an endocytic pathway unique to polarized cell types, and significant for the development of cell polarity, uptake of viral and bacterial pathogens, transepithelial signaling, and immunoglobulin transport. Here, we review recent advances in our knowledge of the transcytotic pathway for proteins and lipids. We also discuss briefly the promise of harnessing the molecules that undergo transcytosis as vehicles for clinical applications in drug delivery.

Sterol targeting drugs reveal life cycle stage-specific differences in trypanosome lipid rafts

Cilia play important roles in cell signaling, facilitated by the unique lipid environment of a ciliary membrane containing high concentrations of sterol-rich lipid rafts. The African trypanosome Trypanosoma brucei is a single-celled eukaryote with a single cilium/flagellum. We tested whether flagellar sterol enrichment results from selective flagellar partitioning of specific sterol species or from general enrichment of all sterols. While all sterols are enriched in the flagellum, cholesterol is especially enriched. T. brucei cycles between its mammalian host (bloodstream cell), in which it scavenges cholesterol, and its tsetse fly host (procyclic cell), in which it both scavenges cholesterol and synthesizes ergosterol. We wondered whether the insect and mammalian life cycle stages possess chemically different lipid rafts due to different sterol utilization. Treatment of bloodstream parasites with cholesterol-specific methyl-β-cyclodextrin disrupts both membrane liquid order and localization of a raft-associated ciliary membrane calcium sensor. Treatment with ergosterol-specific amphotericin B does not. The opposite results were observed with ergosterol-rich procyclic cells. Further, these agents have opposite effects on flagellar sterol enrichment and cell metabolism in the two life cycle stages. These findings illuminate differences in the lipid rafts of an organism employing life cycle-specific sterols and have implications for treatment.

An Integrated Approach to Explore Composition and Dynamics of Cholesterol-rich Membrane Microdomains in Sexual Stages of Malaria Parasite

Membrane microdomains that include lipid rafts, are involved in key physiological and pathological processes and participate in the entry of endocellular pathogens. These assemblies, enriched in cholesterol and sphingolipids, form highly dynamic, liquid-ordered phases that can be separated from the bulk membranes thanks to their resistance to solubilization by nonionic detergents. To characterize complexity and dynamics of detergent-resistant membranes of sexual stages of the rodent malaria parasite Plasmodium berghei, here we propose an integrated study of raft components based on proteomics, lipid analysis and bioinformatics. This analysis revealed unexpected heterogeneity and unexplored pathways associated with these specialized assemblies. Protein-protein relationships and protein-lipid co-occurrence were described through multi-component networks. The proposed approach can be widely applied to virtually every cell type in different contexts and perturbations, under physiological and/or pathological conditions.

Crosstalk of cell polarity signaling pathways

Cell polarity, the asymmetric organization of cellular components along one or multiple axes, is present in most cells. From budding yeast cell polarization induced by pheromone signaling, oocyte polarization at fertilization to polarized epithelia and neuronal cells in multicellular organisms, similar mechanisms are used to determine cell polarity. Crucial role in this process is played by signaling lipid molecules, small Rho family GTPases and Par proteins. All these signaling circuits finally govern the cytoskeleton, which is responsible for oriented cell migration, cell shape changes, and polarized membrane and organelle trafficking. Thus, typically in the process of cell polarization, most cellular constituents become polarized, including plasma membrane lipid composition, ion concentrations, membrane receptors, and proteins in general, mRNA, vesicle trafficking, or intracellular organelles. This review gives a brief overview how these systems talk to each other both during initial symmetry breaking and within the signaling feedback loop mechanisms used to preserve the polarized state.

Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence It

Sphingolipids are structural components in the plasma membranes of eukaryotic cells. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The segregation of sphingolipids into distinct membrane domains is likely essential for cellular function. This review presents the early studies of sphingolipid distribution in the plasma membranes of mammalian cells that shaped the most popular current model of plasma membrane organization. The results of traditional imaging studies of sphingolipid distribution in stimulated and resting cells are described. These data are compared with recent results obtained with advanced imaging techniques, including super-resolution fluorescence detection and high-resolution secondary ion mass spectrometry (SIMS). Emphasis is placed on the new insight into the sphingolipid organization within the plasma membrane that has resulted from the direct imaging of stable isotope-labeled lipids in actual cell membranes with high-resolution SIMS. Super-resolution fluorescence techniques have recently revealed the biophysical behaviors of sphingolipids and the unhindered diffusion of cholesterol analogs in the membranes of living cells are ultimately in contrast to the prevailing hypothetical model of plasma membrane organization. High-resolution SIMS studies also conflicted with the prevailing hypothesis, showing sphingolipids are concentrated in micrometer-scale membrane domains, but cholesterol is evenly distributed within the plasma membrane. Reductions in cellular cholesterol decreased the number of sphingolipid domains in the plasma membrane, whereas disruption of the cytoskeleton eliminated them. In addition, hemagglutinin, a transmembrane protein that is thought to be a putative raft marker, did not cluster within sphingolipid-enriched regions in the plasma membrane. Thus, sphingolipid distribution in the plasma membrane is dependent on the cytoskeleton, but not on favorable interactions with cholesterol or hemagglutinin. The alternate views of plasma membrane organization suggested by these findings are discussed.

Dynamics of intracellular processes in live-cell systems unveiled by fluorescence correlation microscopy

Fluorescence fluctuation-based methods are non-invasive microscopy tools especially suited for the study of dynamical aspects of biological processes. These methods examine spontaneous intensity fluctuations produced by fluorescent molecules moving through the small, femtoliter-sized observation volume defined in confocal and multiphoton microscopes. The quantitative analysis of the intensity trace provides information on the processes producing the fluctuations that include diffusion, binding interactions, chemical reactions and photophysical phenomena. In this review, we present the basic principles of the most widespread fluctuation-based methods, discuss their implementation in standard confocal microscopes and briefly revise some examples of their applications to address relevant questions in living cells. The ultimate goal of these methods in the Cell Biology field is to observe biomolecules as they move, interact with targets and perform their biological action in the natural context. © 2016 IUBMB Life, 69(1):8-15, 2017.

Molecular and cellular basis for the unique functioning of Nrf1, an indispensable transcription factor for maintaining cell homoeostasis and organ integrity

The consensuscis-regulatory AP-1 (activator protein-1)-like AREs (antioxidant-response elements) and/or EpREs (electrophile-response elements) allow for differential recruitment of Nrf1 [NF-E2 (nuclear factor-erythroid 2)-related factor 1], Nrf2 and Nrf3, together with each of their heterodimeric partners (e.g. sMaf, c-Jun, JunD or c-Fos), to regulate different sets of cognate genes. Among them, NF-E2 p45 and Nrf3 are subject to tissue-specific expression in haemopoietic and placental cell lineages respectively. By contrast, Nrf1 and Nrf2 are two important transcription factors expressed ubiquitously in various vertebrate tissues and hence may elicit putative combinational or competitive functions. Nevertheless, they have de facto distinct biological activities because knockout of their genes in mice leads to distinguishable phenotypes. Of note, Nrf2 is dispensable during development and growth, albeit it is accepted as a master regulator of antioxidant, detoxification and cytoprotective genes against cellular stress. Relative to the water-soluble Nrf2, less attention has hitherto been drawn to the membrane-bound Nrf1, even though it has been shown to be indispensable for embryonic development and organ integrity. The biological discrepancy between Nrf1 and Nrf2 is determined by differences in both their primary structures and topovectorial subcellular locations, in which they are subjected to distinct post-translational processing so as to mediate differential expression of ARE-driven cytoprotective genes. In the present review, we focus on the molecular and cellular basis for Nrf1 and its isoforms, which together exert its essential functions for maintaining cellular homoeostasis, normal organ development and growth during life processes. Conversely, dysfunction of Nrf1 results in spontaneous development of non-alcoholic steatohepatitis, hepatoma, diabetes and neurodegenerative diseases in animal models.