HER2 expression defines unique requirements for flotillin and c-Src in EGFR signaling.

The epidermal growth factor receptor (EGFR) controls many cellular functions. Upon binding its ligand, the receptor undergoes dimerization, phosphorylation and activation of signals including the phosphoinositide-3-kinase (PI3K)-Akt pathway. Although some studies have indicated that EGFR signaling may be controlled by signal enrichment within various membrane rafts, such as flotillin nanodomains, others have found a limited effect of disruption of these nanodomains on EGFR signaling, suggesting that specific factors may define context-specific control of EGFR signaling. Ligand-bound EGFR can homodimerize or instead undergo heterodimerization with the related receptor HER2 (also known as ERBB2) when the latter is expressed. We examined how EGFR signaling in the presence of HER2 distinctly requires flotillin nanodomains. Induction of HER2 expression altered EGFR signaling duration, which is consistent with EGFR-HER2 heterodimer formation. EGFR and c-Src (also known as SRC) localized within plasma membrane structures demarked by flotillin-1 more prominently in HER2-expressing cells. Consistently, HER2-expressing cells, but not cells lacking HER2, were dependent on flotillin-1 and c-Src for EGFR signaling leading to Akt activation and cell proliferation. Hence, HER2 expression establishes a requirement for flotillin membrane rafts and c-Src in EGFR signaling.

Apolipoprotein A1 and high-density lipoprotein limit low-density lipoprotein transcytosis by binding SR-B1.

Atherosclerosis results from the deposition and oxidation of LDL and immune cell infiltration in the sub-arterial space leading to arterial occlusion. Studies have shown that transcytosis transports circulating LDL across endothelial cells lining blood vessels. LDL transcytosis is initiated by binding to either scavenger receptor B1 (SR-B1) or activin A receptor-like kinase 1 on the apical side of endothelial cells leading to its transit and release on the basolateral side. HDL is thought to partly protect individuals from atherosclerosis due to its ability to remove excess cholesterol and act as an antioxidant. Apolipoprotein A1 (APOA1), an HDL constituent, can bind to SR-B1, raising the possibility that APOA1/HDL can compete with LDL for SR-B1 binding, thereby limiting LDL deposition in the sub-arterial space. To examine this possibility, we used in vitro approaches to quantify the internalization and transcytosis of fluorescent LDL in coronary endothelial cells. Using microscale thermophoresis and affinity capture, we find that SR-B1 and APOA1 interact and that binding is enhanced when using the cardioprotective variant of APOA1 termed Milano (APOA1-Milano). In male mice, transiently increasing the levels of HDL reduced the acute deposition of fluorescently labeled LDL in the atheroprone inner curvature of the aorta. Reduced LDL deposition was also observed when increasing circulating wild-type APOA1 or the APOA1-Milano variant, with a more robust inhibition from the APOA1-Milano. The results suggest that HDL may limit SR-B1-mediated LDL transcytosis and deposition, adding to the mechanisms by which it can act as an atheroprotective particle.

FASN inhibitor TVB-3166 prevents S-acylation of the spike protein of human coronaviruses.

The spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other coronaviruses mediates host cell entry and is S-acylated on multiple phylogenetically conserved cysteine residues. Multiple protein acyltransferase enzymes have been reported to post-translationally modify spike proteins; however, strategies to exploit this modification are lacking. Using resin-assisted capture MS, we demonstrate that the spike protein is S-acylated in SARS-CoV-2-infected human and monkey epithelial cells. We further show that increased abundance of the acyltransferase ZDHHC5 associates with increased S-acylation of the spike protein, whereas ZDHHC5 knockout cells had a 40% reduction in the incorporation of an alkynyl-palmitate using click chemistry detection. We also found that the S-acylation of the spike protein is not limited to palmitate, as clickable versions of myristate and stearate were also labelled the protein. Yet, we observed that ZDHHC5 was only modified when incubated with alkyne-palmitate, suggesting it has specificity for this acyl-CoA, and that other ZDHHC enzymes may use additional fatty acids to modify the spike protein. Since multiple ZDHHC isoforms may modify the spike protein, we also examined the ability of the FASN inhibitor TVB-3166 to prevent S-acylation of the spike proteins of SARS-CoV-2 and human CoV-229E. We show that treating cells with TVB-3166 inhibited S-acylation of expressed spike proteins and attenuated the ability of SARS-CoV-2 and human CoV-229E to spread in vitro. Our findings further substantiate the necessity of CoV spike protein S-acylation and demonstrate that de novo fatty acid synthesis is critical for the proper S-acylation of the spike protein.

Mesoscale organization of domains in the plasma membrane - beyond the lipid raft.

The plasma membrane is compartmentalized into several distinct regions or domains, which show a broad diversity in both size and lifetime. The segregation of lipids and membrane proteins is thought to be driven by the lipid composition itself, lipid-protein interactions and diffusional barriers. With regards to the lipid composition, the immiscibility of certain classes of lipids underlies the "lipid raft" concept of plasmalemmal compartmentalization. Historically, lipid rafts have been described as cholesterol and (glyco)sphingolipid-rich regions of the plasma membrane that exist as a liquid-ordered phase that are resistant to extraction with non-ionic detergents. Over the years the interest in lipid rafts grew as did the challenges with studying these nanodomains. The term lipid raft has fallen out of favor with many scientists and instead the terms "membrane raft" or "membrane nanodomain" are preferred as they connote the heterogeneity and dynamic nature of the lipid-protein assemblies. In this article, we will discuss the classical lipid raft hypothesis and its limitations. This review will also discuss alternative models of lipid-protein interactions, annular lipid shells, and larger membrane clusters. We will also discuss the mesoscale organization of plasmalemmal domains including visible structures such as clathrin-coated pits and caveolae.

7-Ketocholesterol impairs phagocytosis and efferocytosis via dysregulation of phosphatidylinositol 4,5-bisphosphate.

The plasma membrane is inhomogeneously organized containing both highly ordered and disordered nanodomains. 7-Ketocholesterol (7KC), an oxysterol formed from the nonenzymatic oxidation of cholesterol, is a potent disruptor of membrane order. Importantly, 7KC is a component of oxidized low-density lipoprotein and accumulates in macrophage and foam cells found in arterial plaques. Using a murine macrophage cell line, J774, we report that both IgG-mediated and phosphatidylserine-mediated phagocytic pathways are inhibited by the accumulation of 7KC. Examination of the well-studied Fcγ receptor pathway revealed that the cell surface receptor abundance and ligand binding are unaltered while downstream signaling and activation of spleen tyrosine kinase is not affected. However, while the recruitment of phospholipase Cγ1 was unaffected its apparent enzymatic activity was compromised resulting in sustained phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2 ] levels and polymerized actin at the base of the phagocytic cup. Additionally, we found that 7KC prevented the activation of PLCß downstream of the P2Y6 G-protein coupled receptor and that 7KC impaired PLCγ activity in response to a direct elevation of cytosolic calcium induced by ionomycin. Finally, we demonstrate that 7KC partly attenuates the activity of rapamycin recruitable constitutively active PLCß3. Together, our results demonstrate that the accumulation of 7KC impairs macrophage function by altering PtdIns(4,5)P2 catabolism and, thus, impairing actin depolymerization required for the completion of phagocytosis.

Transcellular vesicular transport in epithelial and endothelial cells: Challenges and opportunities.

Vesicle-mediated transcellular transport or simply "transcytosis" is a cellular process used to shuttle macromolecules such as lipoproteins, antibodies, and albumin from one surface of a polarized cell to the other. This mechanism is in contrast to the transit of small molecules such as anions, cations and amino acids that occur via uptake, diffusion through the cytosol and release and is also distinct from paracellular leak between cells. Importantly, transcytosis has evolved as a process to selectively move macromolecules between 2 neighboring yet unique microenvironments within a multicellular organism. Examples include the movement of lipoproteins out of the circulatory system and into tissues and the delivery of immunoglobulins to mucosal surfaces. Regardless of whether the transport is conducted by endothelial or epithelial cells, the process often involves receptor-mediated uptake of a ligand into an endocytic vesicle, regulated transit of the carrier through the cytoplasm and release of the cargo via an exocytic event. While transcytosis has been examined in detail in epithelial cells, for both historical and technical reasons, the process is less understood in endothelial cells. Here, we spotlight aspects of epithelial transcytosis including recent findings and review the comparative dearth of knowledge regarding the process in endothelial cells highlighting the opportunity for further study.

Integrity under stress: Host membrane remodelling and damage by fungal pathogens.

Membrane bilayers of eukaryotic cells are an amalgam of lipids and proteins that distinguish organelles and compartmentalise cellular functions. The mammalian cell has evolved mechanisms to sense membrane tension or damage and respond as needed. In the case of the plasma membrane and phagosomal membrane, these bilayers act as a barrier to microorganisms and are a conduit by which the host interacts with pathogens, including fungi such as Candida, Cryptococcus, Aspergillus, or Histoplasma species. Due to their size, morphological flexibility, ability to produce long filaments, secrete pathogenicity factors, and their potential to replicate within the phagosome, fungi can assault host membranes in a variety of physical and biochemical ways. In addition, the recent discovery of a fungal pore-forming peptide toxin further highlights the importance of membrane biology in the outcomes between host and fungal cells. In this review, we discuss the apparent "stretching" of membranes as a sophisticated biological response and the role of vesicular transport in combating membrane stress and damage. We also review the known pathogenicity factors and physical properties of fungal pathogens in the context of host membranes and discuss how this may contribute to pathogenic interactions between fungal and host cells.

Phospholipid subcellular localization and dynamics.

Membrane biology seeks to understand how lipids and proteins within bilayers assemble into large structures such as organelles and the plasma membranes. Historically, lipids were thought to merely provide structural support for bilayer formation and membrane protein function. Research has now revealed that phospholipid metabolism regulates nearly all cellular processes. Sophisticated techniques helped identify >10,000 lipid species suggesting that lipids support many biological processes. Here, we highlight the synthesis of the most abundant glycerophospholipid classes and their distribution in organelles. We review vesicular and nonvesicular transport pathways shuttling lipids between organelles and discuss lipid regulators of membrane trafficking and second messengers in eukaryotic cells.

CD36 mediates albumin transcytosis by dermal but not lung microvascular endothelial cells: role in fatty acid delivery.

In healthy blood vessels, albumin crosses the endothelium to leave the circulation by transcytosis. However, little is known about the regulation of albumin transcytosis or how it differs in different tissues; its physiological purpose is also unclear. Using total internal reflection fluorescence microscopy, we quantified transcytosis of albumin across primary human microvascular endothelial cells from both lung and skin. We then validated our in vitro findings using a tissue-specific knockout mouse model. We observed that albumin transcytosis was saturable in the skin but not the lung microvascular endothelial cells, implicating a receptor-mediated process. We identified the scavenger receptor CD36 as being both necessary and sufficient for albumin transcytosis across dermal microvascular endothelium, in contrast to the lung where macropinocytosis dominated. Mutations in the apical helical bundle of CD36 prevented albumin internalization by cells. Mice deficient in CD36 specifically in endothelial cells exhibited lower basal permeability to albumin and less basal tissue edema in the skin but not in the lung. Finally, these mice also exhibited a smaller subcutaneous fat layer despite having identical total body weights and circulating fatty acid levels as wild-type animals. In conclusion, CD36 mediates albumin transcytosis in the skin but not the lung. Albumin transcytosis may serve to regulate fatty acid delivery from the circulation to tissues.

Salmonella exploits host Rho GTPase signalling pathways through the phosphatase activity of SopB.

Salmonella uses Type 3 secretion systems (T3SSs) to deliver virulence factors, called effectors, into host cells during infection. The T3SS effectors promote invasion into host cells and the generation of a replicative niche. SopB is a T3SS effector that plays an important role in Salmonella pathogenesis through its lipid phosphatase activity. Here, we show that SopB mediates the recruitment of Rho GTPases (RhoB, RhoD, RhoH, and RhoJ) to bacterial invasion sites. RhoJ contributes to Salmonella invasion, and RhoB and RhoH play an important role in Akt activation. R-Ras1 also contributes to SopB-dependent Akt activation by promoting the localised production of PI(3,4)P2 /PI(3,4,5)P3 . Our studies reveal new signalling factors involved in SopB-dependent Salmonella infection.

Distribution, dynamics and functional roles of phosphatidylserine within the cell.

Phosphatidylserine (PtdSer), an essential constituent of eukaryotic membranes, is the most abundant anionic phospholipid in the eukaryotic cell accounting for up to 10% of the total cellular lipid. Much of what is known about PtdSer is the role exofacial PtdSer plays in apoptosis and blood clotting. However, PtdSer is generally not externally exposed in healthy cells and plays a vital role in several intracellular signaling pathways, though relatively little is known about the precise subcellular localization, transmembrane topology and intracellular dynamics of PtdSer within the cell. The recent development of new, genetically-encoded probes able to detect phosphatidylserine is leading to a more in-depth understanding of the biology of this phospholipid. This review aims to give an overview of recent developments in our understanding of the role of PtdSer in intracellular signaling events derived from the use of these recently developed methods of phosphatidylserine detection.

Cresyl violet: a superior fluorescent lysosomal marker.

We have characterized cresyl violet as a membrane-permeant fluorophore that localizes to lysosomes and acidic vacuoles of budding yeast, Drosophila, human, murine and canine cells. An acidotropic weak base, cresyl violet is shown to be virtually insensitive to physiological alkali and divalent cations. Because of its unique spectral properties, it can be used in combination with green, red and far-red fluorophores, is less susceptible to photobleaching than alternative acidotropic probes, and does not undergo photoconversion. At concentrations that yield bright labeling of acidic compartments, cresyl violet does not alter the organellar pH nor does it affect the buffering capacity. Its affordability, together with its chemical and spectral properties, make cresyl violet a superior lysosomal marker devoid of many of the negative characteristics associated with other lysosomal probes.

Phosphatidylserine dictates the assembly and dynamics of caveolae in the plasma membrane.

Caveolae are bulb-shaped nanodomains of the plasma membrane that are enriched in cholesterol and sphingolipids. They have many physiological functions, including endocytic transport, mechanosensing, and regulation of membrane and lipid transport. Caveola formation relies on integral membrane proteins termed caveolins (Cavs) and the cavin family of peripheral proteins. Both protein families bind anionic phospholipids, but the precise roles of these lipids are unknown. Here, we studied the effects of phosphatidylserine (PtdSer), phosphatidylinositol 4-phosphate (PtdIns4P), and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) on caveolar formation and dynamics. Using live-cell, single-particle tracking of GFP-labeled Cav1 and ultrastructural analyses, we compared the effect of PtdSer disruption or phosphoinositide depletion with caveola disassembly caused by cavin1 loss. We found that PtdSer plays a crucial role in both caveola formation and stability. Sequestration or depletion of PtdSer decreased the number of detectable Cav1-GFP puncta and the number of caveolae visualized by electron microscopy. Under PtdSer-limiting conditions, the co-localization of Cav1 and cavin1 was diminished, and cavin1 degradation was increased. Using rapamycin-recruitable phosphatases, we also found that the acute depletion of PtdIns4P and PtdIns(4,5)P2 has minimal impact on caveola assembly but results in decreased lateral confinement. Finally, we show in a model of phospholipid scrambling, a feature of apoptotic cells, that caveola stability is acutely affected by the scrambling. We conclude that the predominant plasmalemmal anionic lipid PtdSer is essential for proper Cav clustering, caveola formation, and caveola dynamics and that membrane scrambling can perturb caveolar stability.

Both the PH domain and N-terminal region of oxysterol-binding protein related protein 8S are required for localization to PM-ER contact sites.

Oxysterol-binding protein-related proteins are implicated in the sensing and transporting lipids at the membrane contact sites. One of the members of the mammalian ORP family, ORP8, is thought to transport lipids through directly tethering both ER and PM membranes. Targeting to PM is thought to be mediated by N-terminal pleckstrin homology domain via binding to phosphoinositides. Sequence alignments and NMR structural determination revealed that the PH domain of ORP8 is atypical and contains an insertion of 20 amino acids in an unstructured loop region that may potentially block interactions with ligands. Using standard lipid-protein overlay assays or liposomal binding assays we could not detect binding of a recombinant version of the PH domain. Examination of a series of deletion constructs demonstrated that both the N-terminal polybasic region and the PH domain are required for proper targeting of the short splice variant ORP8S to the PM-ER contact site in Chinese hamster ovary cells.

CD44 Antibody Inhibition of Macrophage Phagocytosis Targets Fcγ Receptor- and Complement Receptor 3-Dependent Mechanisms.

Targeting CD44, a major leukocyte adhesion molecule, using specific Abs has been shown beneficial in several models of autoimmune and inflammatory diseases. The mechanisms contributing to the anti-inflammatory effects of CD44 Abs, however, remain poorly understood. Phagocytosis is a key component of immune system function and can play a pivotal role in autoimmune states where CD44 Abs have shown to be effective. In this study, we show that the well-known anti-inflammatory CD44 Ab IM7 can inhibit murine macrophage phagocytosis of RBCs. We assessed three selected macrophage phagocytic receptor systems: Fcγ receptors (FcγRs), complement receptor 3 (CR3), and dectin-1. Treatment of macrophages with IM7 resulted in significant inhibition of FcγR-mediated phagocytosis of IgG-opsonized RBCs. The inhibition of FcγR-mediated phagocytosis was at an early stage in the phagocytic process involving both inhibition of the binding of the target RBC to the macrophages and postbinding events. This CD44 Ab also inhibited CR3-mediated phagocytosis of C3bi-opsonized RBCs, but it did not affect the phagocytosis of zymosan particles, known to be mediated by the C-type lectin dectin-1. Other CD44 Abs known to have less broad anti-inflammatory activity, including KM114, KM81, and KM201, did not inhibit FcγR-mediated phagocytosis of RBCs. Taken together, these findings demonstrate selective inhibition of FcγR and CR3-mediated phagocytosis by IM7 and suggest that this broadly anti-inflammatory CD44 Ab inhibits these selected macrophage phagocytic pathways. The understanding of the immune-regulatory effects of CD44 Abs is important in the development and optimization of therapeutic strategies for the potential treatment of autoimmune conditions.

Complementary probes reveal that phosphatidylserine is required for the proper transbilayer distribution of cholesterol.

Cholesterol is an essential component of metazoan cellular membranes and it helps to maintain the structural integrity and fluidity of the plasma membrane. Here, we developed a cholesterol biosensor, termed D4H, based on the fourth domain of Clostridium perfringens theta-toxin, which recognizes cholesterol in the cytosolic leaflet of the plasma membrane and organelles. The D4H probe disassociates from the plasma membrane upon cholesterol extraction and after perturbations in cellular cholesterol trafficking. When used in combination with a recombinant version of the biosensor, we show that plasmalemmal phosphatidylserine is essential for retaining cholesterol in the cytosolic leaflet of the plasma membrane. In vitro experiments reveal that 1-stearoy-2-oleoyl phosphatidylserine can induce phase separation in cholesterol-containing lipid bilayers and shield cholesterol from cholesterol oxidase. Finally, the altered transbilayer distribution of cholesterol causes flotillin-1 to relocalize to endocytic organelles. This probe should be useful in the future to study pools of cholesterol in the cytosolic leaflet of the plasma membrane and organelles.

Is basic science disappearing from medicine? The decline of biomedical research in the medical literature.

Explosive growth in our understanding of genomics and molecular biology have fueled calls for the pursuit of personalized medicine, the notion of harnessing biologic variability to provide patient-specific care. This vision will necessitate a deep understanding of the underlying pathophysiology in each patient. Medical journals play a pivotal role in the education of trainees and clinicians, yet we suspected that the amount of basic science in the top medical journals has been in decline. We conducted an automated search strategy in PubMed to identify basic science articles and calculated the proportion of articles dealing with basic science in the highest impact journals for 8 different medical specialties from 1994 to 2013. We observed a steep decline (40-60%) in such articles over time in almost all of the journals examined. This rapid decline in basic science from medical journals is likely to affect practitioners' understanding of and interest in the basic mechanisms of disease and therapy. In this Life Sciences Forum, we discuss why this decline may be occurring and what it means for the future of science and medicine.

Molecular probes to visualize the location, organization and dynamics of lipids.

Cellular lipids play crucial roles in the cell, including in energy storage, the formation of cellular membranes, and in signaling and vesicular trafficking. To understand the functions and characteristics of lipids within cells, various methods to image lipids have been established. In this Commentary, we discuss the four main types of molecular probes that have significantly contributed to our understanding of the cell biology of lipids. In particular, genetically encoded biosensors and antibodies will be discussed, and how they have been used extensively with traditional light and electron microscopy to determine the subcellular localization of lipids and their spatial and temporal regulation. We highlight some of the recent studies that have investigated the distribution of lipids and their ability to cluster using super-resolution and electron microscopy. We also examine methods for analyzing the movement and dynamics of lipids, including single-particle tracking (SPT), fluorescence recovery after photobleaching (FRAP) and fluorescence correlation spectroscopy (FCS). Although the combination of these lipid probes and the various microscopic techniques is very powerful, we also point out several potential caveats and limitations. Finally, we discuss the need for new probes for a variety of phospholipids and cholesterol.

Bem3, a Cdc42 GTPase-activating protein, traffics to an intracellular compartment and recruits the secretory Rab GTPase Sec4 to endomembranes.

Cell polarity is essential for many cellular functions including division and cell-fate determination. Although RhoGTPase signaling and vesicle trafficking are both required for the establishment of cell polarity, the mechanisms by which they are coordinated are unclear. Here, we demonstrate that the yeast RhoGAP (GTPase activating protein), Bem3, is targeted to sites of polarized growth by the endocytic and recycling pathways. Specifically, deletion of SLA2 or RCY1 led to mislocalization of Bem3 to depolarized puncta and accumulation in intracellular compartments, respectively. Bem3 partitioned between the plasma membrane and an intracellular membrane-bound compartment. These Bem3-positive structures were polarized towards sites of bud emergence and were mostly observed during the pre-mitotic phase of apical growth. Cell biological and biochemical approaches demonstrated that this intracellular Bem3 compartment contained markers for both the endocytic and secretory pathways, which were reminiscent of the Spitzenkörper present in the hyphal tips of growing fungi. Importantly, Bem3 was not a passive cargo, but recruited the secretory Rab protein, Sec4, to the Bem3-containing compartments. Moreover, Bem3 deletion resulted in less efficient localization of Sec4 to bud tips during early stages of bud emergence. Surprisingly, these effects of Bem3 on Sec4 were independent of its GAP activity, but depended on its ability to efficiently bind endomembranes. This work unveils unsuspected and important details of the relationship between vesicle traffic and elements of the cell polarity machinery: (1) Bem3, a cell polarity and peripherally associated membrane protein, relies on vesicle trafficking to maintain its proper localization; and (2) in turn, Bem3 influences secretory vesicle trafficking.