One persistent puzzle in the life sciences is the asymmetric lipid composition of the cellular plasma membrane: while the exoplasmic leaflet is enriched in lipids carrying predominantly saturated fatty acids, the cytoplasmic leaflet hosts preferentially lipids with (poly‐)unsaturated fatty acids. Given the high energy requirements necessary for cells to maintain this asymmetry, the question naturally arises regarding its inherent benefits. In this paper, we propose asymmetry to represent a potential solution for harmonizing two conflicting requirements for the (...) class='Hi'>plasma membrane: first, the need to build a barrier for the uncontrolled influx or efflux of substances; and second, the need to form a fluid and dynamic two‐dimensional substrate for signaling processes. We hence view here the plasma membrane as a composite material, where the exoplasmic leaflet is mainly responsible for the functional integrity of the barrier and the cytoplasmic leaflet for fluidity. We reinforce the validity of the proposed mechanism by presenting quantitative data from the literature, along with multiple examples that bolster our model. (shrink)

Microfilament interactions with the plasma membranes of animal cells appear to vary with cell type and localization. In the erythrocyte, actin oligomers are associated with the membrane via spectrin and ankyrin. The ends of stress fibers in cultured cells, such as fibroblasts, are attached to the plasma membrane at focal adhesion sites and may involve the protein vinculin as a linking protein. In intestinal brush border microvilli a 110,000 dalton protein links the microfilament bundles to sites (...) on the microvillus. A transmembrane complex containing actin stably associated with a cell surface glycoprotein can be isolated from ascites tumor cell microvilli and can be obtained still associated with microfilaments by gentle extraction and gradient centrifugation of the microvilli. These varied interaction mechanisms are believed to be needed to satisfy the different structural and dynamic requirements of the particular systems. (shrink)

All the biological membranes contain oxidoreduction systems actively involved in their bioenergetics. Plasma membrane redox systems seem to be ubiquitous and they have been related to several important functions, including not only their role in cell bioenergetics, but also in cell defense through the generation of reactive oxygen species, in iron uptake, in the control of cell growth and proliferation and in signal transduction. In the last few years, an increasing number of mechanistic and molecular studies have deeply (...) widened our knowledge on the function of these plasma membrane redox systems. The aim of this review is to summarize what is currently known about the components and physiological roles of these systems. (shrink)

Extracellular ATP released from necrotic cells in inflamed tissues activates the P2X7 receptor, stimulates the exposure of phosphatidylserine, and causes cell lysis. Recent findings indicated that XK, a paralogue of XKR8 lipid scramblase, forms a complex with VPS13A at the plasma membrane of T cells. Upon engagement by ATP, an unidentified signal(s) from the P2X7 receptor activates the XK‐VPS13A complex to scramble phospholipids, followed by necrotic cell death. P2X7 is expressed highly in CD25+CD4+ T cells but weakly in (...) CD8+ T cells, suggesting a role of this system in the activation of the immune system to prevent infection. On the other hand, a loss‐of‐function mutation in XK or VPS13A causes neuroacanthocytosis, indicating the crucial involvement of XK‐VPS13A‐mediated phospholipid scrambling at plasma membranes in the maintenance of homeostasis in the nervous and red blood cell systems. (shrink)

Membrane proteins possess certain features that make them susceptible to the electric fields generated at the level of the plasma membrane. A reappraisal of cell signalling, taking into account the protein interactions with the membrane electrostatic profile, suggests that an electrical dimension is deeply involved in this fundamental aspect of cell biology. At least three types of potentials can contribute to this dimension: (1) the potential across the compact layer of water adherent to membrane surfaces; (...) this potential is affected by classical inducers of cell differentiation, like dimethylsulfoxide and hexamethylenebisacetamide; (2) the potential across the Gouy‐Chapman double layer, which accounts for the effects of extracellular cations in the modulation of differentiation; and (3) the resting potential. This last potential and its governing ion currents can be exploited in localised mechanisms of cell signalling centred on the functional association of integrin receptors with ion channels. (shrink)

In nature, cells face a variety of stresses that cause physical damage to the plasma membrane and cell wall. It is well established that evolutionarily conserved cell cycle checkpoints monitor various cellular perturbations, including DNA damage and spindle misalignment. However, the ability of these cell cycle checkpoints to sense a damaged plasma membrane/cell wall is poorly understood. To the best of our knowledge, our recent paper described the first example of such a checkpoint, using budding yeast (...) as a model. In this review, we will discuss this important question as well as provide hypothetical explanations to be tested in the future. (shrink)

Neuronal information processing depends on converting membrane depolarizations into compartmentalized biochemical signals that can modify neuronal activity and structure. However, our understanding of how neurons translate electrical signals into specific biochemical responses remains limited, especially in the soma where gene expression and ion channel function are crucial for neuronal activity. Here, I emphasize the importance of physically compartmentalizing action potential‐triggered biochemical reactions within the soma. Emerging evidence suggests that somatic endoplasmic reticulum–plasma membrane (ER‐PM) junctions are specialized organelles (...) that coordinate electrical and biochemical signaling. The juxtaposition of ion channels and signaling proteins at a prominent subset of these sites enables compartmentalized calcium and cAMP‐dependent protein kinase (PKA) signaling. I explore the hypothesis that these PKA‐containing ER‐PM junctions serve as critical sites for translating membrane depolarizations into PKA signals and identify key gaps in knowledge of the assembly, regulation, and neurobiological functions of this somatic signaling system. (shrink)

You are what you eat – this well‐known phrase properly describes the phenomenon of the effects of diet on acute and chronic inflammation. Several lipids and lipophilic compounds that are delivered with food or are produced in situ in pathological conditions exert immunomodulatory activity due to their interactions with the plasma membrane. This group of compounds includes cholesterol and its oxidized derivatives, fatty acids, α‐tocopherol, and polyphenols. Despite their structural heterogeneity, all these compounds ultimately induce changes in (...) class='Hi'>plasma membrane architecture and fluidity. By doing this, they modulate the dynamics of plasma membrane receptors, such as TLR4. This receptor is activated by lipopolysaccharide, triggering acute inflammation during bacterial infection, which often leads to sepsis and is linked with diverse chronic inflammatory diseases. In this review, we discuss how the impact on plasma membrane properties contributes to the immunomodulatory activity of dietary compounds, pointing to the therapeutic potential of some of them.Also watch the Video Abstract. (shrink)

Many enveloped viruses are released from infected cells by maturing and budding at the plasma membrane. During this process, viral core components are incorporated into membrane vesicles that contain viral transmembrane proteins, termed ‘spike’ proteins. For many years these spike proteins, which are required for infectivity, were believed to be incorporated into virions via a direct interaction between their cytoplasmic domains and viral core components. More recent evidence shows that, while such direct interactions drive budding of alphaviruses, (...) this may not be the case for negative strand RNA viruses and retroviruses. These viruses can bud particles in the absence of spike proteins, using only viral core components to drive the process. In some cases the spike proteins, without the viral core, can be released as virus‐like particles. Optimal budding and release may, therefore, depend on a ‘push‐and‐pull’ concerted action of core and spike, where oligomerization of both components plays a crucial role. (shrink)

Plasma membranes are subject to continuous deformations. Strikingly, some of these transient membrane undulations yield membrane‐associated signaling hubs that differ in composition and function, depending on membrane geometry and the availability of co‐factors. Here, recent advancements on this ubiquitous type of receptor‐independent signaling are reviewed, with a special focus on emerging concepts and technical challenges associated with studying these elusive signaling sites.

The fundamental mechanisms of protein and lipid organization at the plasma membrane have continued to engage researchers for decades. Among proposed models, one idea has been particularly successful which assumes that sterol‐dependent nanoscopic phases of different lipid chain order compartmentalize proteins, thereby modulating protein functionality. This model of membrane rafts has sustainably sparked the fields of membrane biophysics and biology, and shifted membrane lipids into the spotlight of research; by now, rafts have become an integral (...) part of our terminology to describe a variety of cell biological processes. But is the evidence clear enough to continue supporting a theoretical concept which has resisted direct proof by observation for nearly twenty years? In this essay, we revisit findings that gave rise to and substantiated the raft hypothesis, discuss its impact on recent studies, and present alternative mechanisms to account for plasma membrane heterogeneity. (shrink)

Since the early nineteenth century a membrane or wall has been central to the cell’s identity as the elementary unit of life. Yet the literally and metaphorically marginal status of the cell membrane made it the site of clashes over the definition of life and the proper way to study it. In this article I show how the modern cell membrane was conceived of by analogy to the first “artificial cell,” invented in 1864 by the chemist Moritz (...) Traube (1826–1894), and reimagined by the plant physiologist Wilhelm Pfeffer (1845–1920) as a precision osmometer. Pfeffer’s artificial cell osmometer became the conceptual and empirical basis for the law of dilute solutions in physical chemistry, but his use of an artificial analogue to theorize the existence of the plasma membrane as distinct from the cell wall prompted debate over whether biology ought to be more closely unified with the physical sciences, or whether it must remain independent as the science of life. By examining how the histories of plant physiology and physical chemistry intertwined through the artificial cell, I argue that modern biology relocated vitality from protoplasmic living matter to nonliving chemical substances—or, in broader cultural terms, that the disenchantment of life was accompanied by the (re)enchantment of ordinary matter. (shrink)

Lysosomes are the site of degradation of obsolete intracellular material during autophagy and of extracellular macromolecules following endocytosis and phagocytosis. The membrane of lysosomes and late endosomes is enriched in highly glycosylated transmembrane proteins of largely unknown function. Significant progress has been made in recent years towards elucidating the pathways by which these lysosomal membrane proteins are delivered to late endosomes and lysosomes. While some lysosomal membrane proteins follow the constitutive secretory pathway and reach lysosomes indirectly via (...) the cell surface and endocytosis, others exit the trans‐Golgi network in clathrin‐coated vesicles for direct delivery to endosomes and lysosomes. Sorting from the Golgi or the plasma membrane into the endosomal system is mediated by signals encoded by the short cytosolic domain of these proteins. This review will discuss the role of lysosomal membrane proteins in the biogenesis of the late endosomal and lysosomal membranes, with particular emphasis on the structural features and molecular mechanisms underlying the intracellular trafficking of these proteins. (shrink)

It has long been known that the red blood cell contains a membrane skeleton that stabilizes the plasma membrane, determines its shape, and regulates the lateral distribution of the membrane glyco‐proteins to which it is attached. The way in which these functions are regulated in other cells has not been understood. It has now been shown that platelets also contain a membrane skeleton. In contrast to the membrane skeleton of the red blood cell, the (...) platelet membrane skeleton has actin‐binding protein, not spectrin, as a major component. The platelet membrane skeleton regulates the same cellular functions as the red blood cell membrane skeleton. Other cells may contain a membrane skeleton that is critical to their viability and normal functioning. (shrink)

Phosphatidylinositol‐3‐kinases (PI3Ks) are lipid kinases that produce 3‐phosphorylated derivatives of phosphatidylinositol upon activation by various cues. These 3‐phosphorylated lipids bind to various protein effectors to control many cellular functions. Lipid phosphatases such as phosphatase and tensin homolog (PTEN) terminate PI3K‐derived signals and are critical to ensure appropriate signaling outcomes. Many lines of evidence indicate that PI3Ks and PTEN, as well as some specific lipid effectors are highly compartmentalized, either in plasma membrane nanodomains or in endosomal compartments. We examine (...) the evidence for specific recruitment of PI3Ks, PTEN, and other related enzymes to membrane nanodomains and endocytic compartments. We then examine the hypothesis that scaffolding of the sources (PI3Ks), terminators (PTEN), and effectors of these lipid signals with a common plasma membrane nanodomain may achieve highly localized lipid signaling and ensure selective activation of specific effectors. This highlights the importance of spatial regulation of PI3K signaling in various physiological and disease contexts. (shrink)

Phosphatidylinositol 5‐phosphate (PtdIns5P), the least characterized among the three phosphatidylinositol monophosphates, is emerging as a bioactive lipid involved in the control of several cellular functions. Similar to PtdIns3P, it is present in low amounts in mammalian cells, and can be detected at the plasma membrane and endomembranes as well as in the nucleus. Changes in PtdIns5P levels are observed in mammalian cells following specific stimuli or stresses, and in human diseases. Recently, the contribution of several enzymes such as (...) PIKfyve, myotubularins, and type II PtdInsP‐kinases to PtdIns5P metabolism has gained a strong experimental support. Here, we provide a picture emerging from recent studies showing how this lipid can be generated and act as a regulator of membrane and cytoskeleton dynamics, and as a modulator of gene expression. We briefly summarize the current methods and tools for studying PtdIns5P, and discuss how PtdIns5P can integrate and coordinate different functions in a spatiotemporal manner. (shrink)

The lateral mobility of membrane lipids and proteins is presumed to play an important functional role in biomembranes. Photobleaching studies have shown that many proteins in the plasma membrane have diffusion coefficients at least an order of magnitude lower than those obtained when the same proteins are reconstituted in artificial bilayer membranes. Depending on the protein, it has been shown that either the cytoplasmic domain or the ectodomain is the key determinant of its lateral mobility. Single particle (...) tracking microscopy, which allows the motions of single or small groups of membrane molecules to be followed, promises not only to reveal new features of membrane dynamics, but also to help explain longstanding puzzles presented by the photobleaching studies, particularly the so‐called immobile fraction. The combination of the two complementary technologies should measurably enhance our understanding of membrane microstructure. (shrink)

Phosphatidylinositol 3‐phosphate (PtdIns3P) is generated on the cytosolic leaflet of cellular membranes, primarily by phosphorylation of phosphatidylinositol by class II and class III phosphatidylinositol 3‐kinases. The bulk of this lipid is found on the limiting and intraluminal membranes of endosomes, but it can also be detected in domains of phagosomes, autophagosome precursors, cytokinetic bridges, the plasma membrane and the nucleus. PtdIns3P controls cellular functions through recruitment of specific protein effectors, many of which contain FYVE or PX domains. Cellular (...) processes known to be controlled by PtdIns3P and its effectors include endosomal fusion, sorting and motility, autophagy, cytokinesis, regulated exocytosis and signal transduction. Here we discuss how Ptdins3P is generated on specific cellular membranes, how its localizations and functions can be studied, and how its effectors serve to control cellular functions.Editor's suggested further reading in BioEssays:Phosphatidylinositol 4,5‐bisphosphate: Targeted production and signalingAbstractHow does SHIP1/2 balance PtdIns(3,4)P2 and does it signal independently of its phosphatase activity?AbstractPhosphatidylinositol‐3,4,5‐trisphosphate: Tool of choice for class I PI 3‐kinasesAbstractPhosphatidylinositol‐4‐phosphate: The Golgi and beyondAbstract. (shrink)

Signal transduction via receptors for N‐formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process which involves participation of cytoskeletal elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton controls the distribution of FPR in the plane of the plasma membrane, thus controlling the accessibility of FPR to different proteins in functionally distinct domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such (...) as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist‐occupied receptor, preventing cell activation. The mechanism of interaction of FPR with the membrane skeleton is poorly understood but evidence is accumulating that suggests a direct binding of FPR (and other receptors) to cytoskeletal proteins such as actin. (shrink)

Cellular differentiation is often accompanied by the expression of specialized plasma membrane proteins which accumulate in discrete regions. The biogenesis of these specialized membrane domains involves the assembly and co‐localisation of a spectrin‐based membrane skeleton. While the constituents of the membrane skeleton in non‐erythroid cells are often immunologically related to erythroid spectrin, ankyrin, and protein 4.1, there are structural and functional differences between the isoforms of these membrane skeleton polypeptides, as well as highly variable (...) patterns of expression during cellular differentiation. We consider this heterogeneity of structure and expression during development in the context of the hypothesis that non‐erythroid spectrin, ankyrin, and protein 4.1 are involved in the formation of specialized membrane domains. (shrink)

Two key questions in the autophagy field are the mechanisms that underlie the signals for autophagy initiation and the source of membrane for expansion of the nascent membrane, the phagophore. In this review, we discuss recent findings highlighting the role of the classical endosomal pathway, from plasma membrane to lysosome, in the formation and expansion of the phagophore and subsequent degradation of the autophagosome contents. We also highlight the striking conservation of regulatory factors between the two (...) pathways, including those regulating membrane budding and fusion, and the role of the lysosome in sensing the nutrient status of the cell, regulating mTORC1 activity, and ultimately the initiation of autophagy.Editor's suggested further reading in BioEssays The evolution of dynamin to regulate clathrin‐mediated endocytosis Abstract. (shrink)

The plasma membrane of polarized epithelial cells is divided into apical and basolateral surfaces, with different compositions. Proteins can be sent directly from the trans‐Golgi network (TGN) to either surface, or can be sent first to one surface and then transcytosed to the other. The glycosyl phosphatidylinositol anchor is a signal for apical targeting. Signals in the cytoplasmic domain containing a β‐turn determine basolateral targeting and retrieval, and are related to other sorting signals. Transcytosed proteins, such as the (...) polymeric immunoglobulin receptor (plgR), are endocytosed from the basolateral surface and then accumulate in a tubular compartment concentrated underneath the apical surface. This compartment, tentatively termed the apical recycling compartment, may be a central sorting station, as it apparently receives material from both surfaces and sorts them for delivery to the correct surface. Delivery to the apical surface from both the TGN and the apical recycling compartment appears to be regulated by protein kinases A and C, and endocytosis from the apical surface is also regulated by kinases. Transcytosis of the plgR is additionally regulated by phosphorylation of the plgR and by ligand binding to the plgR. Regulation of traffic in polarized epithelial cells plays a central role in cellular homeostasis, response to external signals and differentiation. (shrink)

Citrate, an organic trivalent anion, is a major substrate for generation of energy in most cells. It is produced in mitochondria and used either in the Krebs' cycle or released into cytoplasm through a specific mitochondrial carriers. Citrate can also be taken up from blood through different plasma membrane transporters. In the cytoplasm, citrate can be used ultimately for fatty acid synthesis, which is increased in cancer cells. Here, we review the ways in which citrate can be transported (...) and discuss the changes in transport and metabolism that occur in cancer cells. The primary focus is on the prostate gland, which is known to produce and release large amounts of citrate during its normal secretory function. The significant changes that occur in citrate‐related metabolism and transport in prostate cancer are the second focus. This review strives to relate these mechanisms to molecular biology on the one hand and to clinical applications on the other. (shrink)

Gap junctions are ubiquitous plasma membrane specializations that allow cells to exchange small molecules and ions directly. The isolation, biochemical characterization and molecular cloning of the major protein of rat liver gap junctions lead to a clearer view of these membrane zones that allow cells to ‘talk’ to each other and co‐ordinate their activities in tissues and organs.

The direction and specificity of endolysosomal membrane trafficking is tightly regulated by various cytosolic and membrane‐bound factors, including soluble NSF attachment protein receptors (SNAREs), Rab GTPases, and phosphoinositides. Another trafficking regulatory factor is juxta‐organellar Ca2+, which is hypothesized to be released from the lumen of endolysosomes and to be present at higher concentrations near fusion/fission sites. The recent identification and characterization of several Ca2+ channel proteins from endolysosomal membranes has provided a unique opportunity to examine the roles of (...) Ca2+ and Ca2+ channels in the membrane trafficking of endolysosomes. SNAREs, Rab GTPases, and phosphoinositides have been reported to regulate plasma membrane ion channels, thereby suggesting that these trafficking regulators may also modulate endolysosomal dynamics by controlling Ca2+ flux across endolysosomal membranes. In this paper, we discuss the roles of phosphoinositides, Ca2+, and potential interactions between endolysosomal Ca2+ channels and phosphoinositides in endolysosomal dynamics. (shrink)

Intercellular communication is an essential process in all multicellular organisms. During this process, molecules secreted by one cell will bind to a receptor on the cognate cell leading to the subsequent uptake of the receptor‐ligand complex. Once inside, the cell then determines the fate of the receptor‐ligand complex and any other proteins that were endocytosed together. Approximately 80% of endocytosed material is recycled back to the plasma membrane either directly or indirectly via the Golgi apparatus and the remaining (...) 20% is delivered to the lysosome for degradation. Although most pathways have been identified, we still lack understanding on how specificity in sorting of recycling cargos into different pathways is achieved, and how the cell reaches high accuracy of these processes in the absence of clear sorting signals in the bulk of the client proteins. In this review, we will summarize our current understanding of the mechanism behind recycling cargo sorting and propose a model of differential affinities between cargo and cargo receptors/adaptors with regards to iterative sorting in endosomes. (shrink)

In circulation, T cells are spherical with selectin enriched dynamic microvilli protruding from the surface. Following extravasation, these microvilli serve another role, continuously surveying their environment for antigen in the form of peptide‐MHC (pMHC) expressed on the surface of antigen presenting cells (APCs). Upon recognition of their cognate pMHC, the microvilli are initially stabilized and then flatten into F‐actin dependent microclusters as the T cell spreads over the APC. Within 1–5 min, clathrin is recruited by the ESCRT‐0 component Hrs to (...) mediate release of T cell receptor (TCR) loaded vesicles directly from the plasma membrane by clathrin and ESCRT‐mediated ectocytosis (CEME). After 5‐10 min, Hrs is displaced by the endocytic clathrin adaptor epsin‐1 to induce clathrin‐mediated trans‐endocytosis (CMTE) of TCR‐pMHC conjugates. Here we discuss some of the functional properties of the clathrin machinery which enables it to control these topologically opposite modes of membrane transfer at the immunological synapse, and how this might be regulated during T cell activation. (shrink)

An increasing number of studies indicate that changes in cytosolic free Ca2+ ([Ca2+]c) mediate specific types of signal transduction in plant cells. Modulation of [Ca2+]c is likely to be achieved through changes in the activity of Ca2+ channels, which catalyse passive influx of Ca2+ to the cytosol from extracellular and intracellular compartments. Voltage‐sensitive Ca2+ channels have been detected in the plasma membranes of algae, where they control membrane electrical properties and cell turgor. These channels are sensitive to 1,4‐dihydropyridines, (...) which in animal cells specifically affect one class of voltage‐regulated plasma membrane Ca2+ channel. Ca2+‐permeable channels with different pharmacological properties have been found in the plasma membrane of higher plants. Recent evidence suggests the existence of two discrete classes of Ca2+ channel co‐resident in the vacuolar membrane (tonoplast) of higher plants. The first is gated by inositol 1,4,5‐trisphosphate, and bears a number of similarities to its animal counterpart which is located in the endoplasmic reticulum (ER). The second tonoplast Ca2+ channel is voltage‐operated. However, the specific roles of these tonoplast channels in signal transduction have yet to be elucidated. (shrink)

Superoxide is produced by a NADPH oxidase of phagocytic cells and contributes to their microbicidal activities. The oxidase is activated when receptors in the neutrophil plasma membrane bind to the target microbe. These receptors recognise antibodies and complement fragments which coat the target cell. The oxidase electron transport chain, located in the plasma membrane, comprises a low potential cytochrome b heterodimer (gp 91‐phox and p22‐phox) associated with FAD. It is non‐functional until at least three proteins, p67‐phox, (...) p47‐phox and p21rac (and possibly others), move from the cytosol to dock on the cytochrome b. The docking involves the interaction of SH3 domains may become exposed follwoing phosphorylation of p47‐phox by protein kinase C or, in model systems, by addition of arachidonic acid to reconstitution mixtures. Following the docking process the electron‐transporting component is able to transfer electrons from NADPH to oxygen. This electrogenic event is charge‐compensated by the opening of a prton channel. Components of the oxidase are expressed in non‐phagocytes, where their function is uncretain but could be related to some signal function of superoxide. (shrink)

The quantitative study of regulation of cell growth and proliferation began with the development of the technique for monolayer culture of vertebrate cells in the late 1960s. The basic parameters were defined in the early physiological studies, which continued through the next decade. These included specific and non-specific growth factors and the requirement for continuous exposure to such factors through most of the G1 period for progression to S. In the course of this work, the diversity of biochemical responses and (...) the critical role of increased protein synthesis and accumulation for the onset of DNA synthesis were elucidated. In particular, a central role of free cytosolic Mg2+ in direct regulation of protein synthesis and in ancillary processes as a response to membrane perturbation was established. Eventually, the physiological era was superseded by the molecular era beginning in the 1980s. This work focussed on specific receptors for growth factors that entrained a protein kinase cascade, which terminated in a higher frequency of initiation of protein synthesis. However, the molecular studies virtually ignored the key results of the physiological era. Recent studies of the penultimate molecular steps in the regulatory pathway of protein synthesis, however, have supported a model of growth regulation involving membrane perturbation and MgATP2− concentration, results that integrate the findings of the physiological and molecular eras. The resulting relatively simple “membrane, magnesium mitosis” (MMM) model of proliferation control can explain the seeming paradox of the variety of specific and non-specific growth-enhancing treatments that are mediated by the plasma membrane and which bring about a shared, complex but coordinated growth response that drives cell proliferation. BioEssays 27:311–320, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

The precise orchestration of two opposing protein complexes – one in the cytoplasm (β‐catenin destruction complex) and the other at the plasma membrane (LRP6 signaling complex) – is critical for controlling levels of the transcriptional co‐factor β‐catenin, and subsequent activation of the Wnt/β‐catenin signal transduction pathway. The Wnt pathway component Axin acts as an essential scaffold for the assembly of both complexes. How the β‐catenin destruction and LRP6 signaling complexes are modulated following Wnt stimulation remains controversial. A recent (...) study in Science by He and coworkers reveals an underlying logic for Wnt pathway control in which Axin phosphorylation toggles a switch between the active and inactive states. This mini‐review focuses on this and two other recent studies that provide insight into the initial signaling events triggered by Wnt exposure. We emphasize regulation of the β‐catenin destruction and LRP6 signaling complexes and propose a framework for future work in this area. (shrink)

The ability of two or more cells to unite to form a new syncytial cell has been utilized in metazoans throughout evolution to form many complex organs, such as muscles, bones and placentae. This requires migration, recognition and adhesion between cells together with fusion of their plasma membranes and rearrangement of their cytoplasmic contents. Until recently, understanding of the mechanisms of cell fusion was restricted to fusion between enveloped viruses and their target cells. The identification of new factors that (...) take part in developmental cell fusion in C. elegans opens the way to understanding how cells fuse and what the functions of this process are. In this review, we describe current knowledge on the mechanisms and putative roles of developmental cell fusion in C. elegans and how cell fusion is regulated, together with other intercellular processes to promote organogenesis. BioEssays 25:672–682, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Clathrin‐mediated endocytosis is a major route for the retrieval of plasma‐membrane cargoes, and defects of this process can cause catastrophic human dysfunctions. However, the processes governing how a clathrin‐coated profile (ccp) is initiated are still murky. Despite an ever‐growing cast of molecules proposed as triggers of ccp nucleation and increasingly sophisticated bioimaging techniques examining clathrin‐mediated endocytosis, it is yet unknown if ccp formation is governed by a universal mechanism. A recent paper by Cocucci et al. has tracked single‐molecule (...) events to identify that stable accumulation of ccps requires the near‐simultaneous arrival of two AP2 adaptors bridged by one clathrin triskelion. This commentary examines the role of AP2 in cargo‐mediated endocytosis in the light of recent advances in biophotonics, chemical inhibitors and genetics, examines the claims of other molecules to be the initiators of ccp formation and proposes future directions in research into this topic.Editor's suggested further reading in BioEssays: The evolution of dynamin to regulate clathrin‐mediated endocytosis AbstractClathrin‐mediated endocytosis: What works for small, also works for big Abstract. (shrink)

CB1 receptors are functionally present within brain mitochondria, although they are usually considered specifically targeted to plasma membrane. Acute activation of mtCB1 alters mitochondrial ATP generation, synaptic transmission, and memory performance. However, the detailed mechanism linking disrupted mitochondrial metabolism and synaptic transmission is still uncharacterized. CB1 receptors are among the most abundant G protein-coupled receptors in the brain and impact on several processes, including fear coping, anxiety, stress, learning, and memory. Mitochondria perform several key physiological processes for neuronal (...) homeostasis, including production of ATP and reactive oxygen species, calcium buffering, metabolism of neurotransmitters, and apoptosis. It is therefore possible that acute activation of mtCB1 impacts on these different mitochondrial functions to modulate synaptic transmission. In reviewing and integrating across the literature in this area, we describe the possible mechanisms involved in the regulation of brain physiology by mtCB1 receptors. Activation of mitochondrial CB1 but not of plasma membrane CB1 decreases brain mitochondrial activity, glutamatergic synaptic transmission, and memory performance. Several processes can be involved in this alteration of brain physiology, including mitochondrial ATP and ROS production, axonal mitochondrial transport, Ca2+ homeostasis, and/or metabolism of the neurotransmitter glutamate. (shrink)

The macrophage plays an important role in host development and physiology, and in pathogenesis of many infectious, immunologic and degenerative disease processes. It displays marked heterogeneity of phenotype in different tissues, reflecting local interactions with other cell types, and contributes to host homeostasis through a varied repertoire of plasma membrane and secretory molecules. Upon isolation from the body it continues to express special, as well as general, features of cellular organisation and function, which make it a delight to (...) study in cell culture. (shrink)

Vacuolar ATPases (V‐ATPases, V1Vo‐ATPases) are rotary motor proton pumps that acidify intracellular compartments, and, when localized to the plasma membrane, the extracellular space. V‐ATPase is regulated by a unique process referred to as reversible disassembly, wherein V1‐ATPase disengages from Vo proton channel in response to diverse environmental signals. Whereas the disassembly step of this process is ATP dependent, the (re)assembly step is not, but requires the action of a heterotrimeric chaperone referred to as the RAVE complex. Recently, an (...) alternative pathway of holoenzyme disassembly was discovered that involves binding of Oxidation Resistance 1 (Oxr1p), a poorly characterized protein implicated in oxidative stress response. Unlike conventional reversible disassembly, which depends on enzyme activity, Oxr1p induced dissociation can occur in absence of ATP. Yeast Oxr1p belongs to the family of TLDc domain containing proteins that are conserved from yeast to mammals, and have been implicated in V‐ATPase function in a variety of tissues. This brief perspective summarizes what we know about the molecular mechanisms governing both reversible (ATP dependent) and Oxr1p driven (ATP independent) V‐ATPase dissociation into autoinhibited V1 and Vo subcomplexes. (shrink)

Fully grown Xenopus oocytes can remain in their immature state essentially indefinitely, or, in response to the steroid hormone progesterone, can be induced to develop into fertilizable eggs. This process is termed oocyte maturation. Oocyte maturation is initiated by a novel plasma membrane steroid hormone receptor. Progesterone brings about inhibition of adenylate cyclase and activation of the Mos/MEK1/p42 MAP kinase cascade, which ultimately brings about the activation of the universal M phase trigger Cdc2/cyclin B. Oocyte maturation provides an (...) interesting example of how signaling cascades entrain the cell cycle clock to environmental changes. BioEssays 21:833–842, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

Abstractβ1,4‐galactosyltransferase is unusual among the glycosyltransferases in that it is found in two subcellular compartments where it performs two distinct functions. In the trans‐Golgi complex, galactosyltransferase participates in oligosaccharide biosynthesis, as do the other glycosyltransferases. On the cell surface, however, galactosyltransferase associates with the cytoskeleton and functions as a receptor for extracellular oligosaccharide ligands. Although we now know much regarding galactosyltransferase function in these two compartments, little is known about how it is targeted to these different sites. By cloning the (...) galactosyltransferase gene products, certain features of the protein have been identified that may be critical for its expression on the cell surface or retention within the Golgi complex. This article discusses recent studies which suggest that a cytoplasmic sequence unique to one galactosyltransferase isoform is required for targeting a portion of this protein to the plasma membrane, enabling it to function as a cell adhesion molecule. These findings allow one to manipulate surface galactosyltransferase expression, either positively or negatively, and perturb galactosyltransferase‐dependent cellular interactions during fertilization and development. (shrink)

Thy‐1 is a small glycoprotein of 110 amino acids which, folded in the characteristic structure of an immunoglobulin variable domain1, are anchored to the plasma membrane via a glycophosphatidylinositol (GPI) tail(2,3) (Fig. 1). It is a major component of the surface of various cell types, including neurons, at certain stages of their development (4). These qualities doubtlessly appeal to certain cognoscenti, but it is not clear why they would raise Thy‐1 to the status of a favourite molecule. Indeed, (...) few scientists readily admit to having a favourite. We study individual molecules because science is rooted in specific observations; but we do so in order to discover mechanisms of general importance. A molecule's appeal is dependent on its ability to reveal novel aspects of how nature works.Thy‐1 has been unusual in this respect. It was the first lymphocyte surface antigen shown to be restricted to a functional subset of lymphocytes (T cells in the mouse), a finding crucial to the development of cellular immunology(5); it was one of the first cell surface molecules to be sequenced and indicated the importance of immunoglobulin domains and GPI anchors as structural motifs(1–3); it has been pivotal in studies demonstrating that GPI‐anchored molecules are able to signal across the membrane they do not span(6, 7). Thy‐1 has revealed this much, however, with the charm of an adroit stripper: it has always promised glimpses of things more exciting than that displayed. In particular, the function of this molecule has never emerged.Our current work on Thy‐1 in the nervous system is, we believe, finally prizing this last secret into the open. It suggests that Thy‐1 on the neuronal surface interacts with one of the main glial types of adult brain, the astrocyte, to restrict axonal growth(8). There is no current consensus that astrocytes do regulate such growth, and this is the wider question we are addressing from the perspective of Thy‐1. At issue also is how to progress from knowing the nature of a molecule to defining its function. This task has already been achieved with molecules which were harder to characterise than Thy‐1. The problem is a classic Catch 22 situation: Thy‐1 has been readily characterised because it is small and abundant it is ten times more abundant than any other surface glycoprotein on rat thymocytes(9); but since there is so much of it, whatever it does it must do rather badly (or there would be no need to have so much of it!). Defining a function based on low activity or affinity is not trivial.Here I describe two aspects of work on Thy‐1. One is long completed ‐ the research which led to Thy‐1 being isolated and characterised ‐ and included because it demonstrates features which are still applicable to much research using a antibodies today. The other is our ongoing work on the nervous system, which is at that exciting early stage where hypotheses are formed, but the answers are still coming in. (shrink)

The exocyst is a conserved octameric complex that physically tethers a vesicle to the plasma membrane, prior to membrane fusion. It is important not only for secretion and membrane delivery but also, in mammalian cells, for cytokinesis, ciliogenesis, autophagy, tumorigenesis, and host defense. The combination of genome editing and advanced light microscopy of exocyst subunits in living cells has recently shown the complex to be much more dynamic than previously appreciated, and exposed how little we still (...) know about its function and regulation. (shrink)

The recent discovery and structure determination of a novel ubiquitin‐like dimerization domain in protein kinase D (PKD) has significant implications for its activation. PKD is a serine/threonine kinase activated by the lipid second messenger diacylglycerol (DAG). It is an essential and highly conserved protein that is implicated in plasma membrane directed trafficking processes from the trans‐Golgi network. However, many open questions surround its mechanism of activation, its localization, and its role in the biogenesis of cargo transport carriers. In (...) reviewing this field, the focus is primarily on the mechanisms that control the activation of PKD at precise locations in the cell. In light of the new structural findings, the understanding of the mechanisms underlying PKD activation is critically evaluated, with particular emphasis on the role of dimerization in PKD autophosphorylation, and the provenance and recognition of the DAG that activates PKD. (shrink)

Protein ubiquitination constitutes a post‐translational modification mediated by ubiquitin ligases whereby ubiquitinated substrates are degraded through the proteasomal or lysosomal pathways, or acquire novel molecular functions according to their “ubiquitin codes.” Dysfunction of the ubiquitination process in cells causes various diseases such as cancers along with neurodegenerative, auto‐immune/inflammatory, and metabolic diseases. KCTD10 functions as a substrate recognition receptor for cullin‐3 (CUL3), a scaffold protein in RING‐type ubiquitin ligase complexes. Recently, studies by ourselves and others have identified new substrates that are (...) ubiquitinated by the CUL3/KCTD10 ubiquitin ligase complex. Moreover, the type of polyubiquitination (e.g., K27‐, K48‐, or K63‐chain) of various substrates (e.g., RhoB, CEP97, EIF3D, and TRIF) mediated by KCTD10 underlies its divergent roles in endothelial barrier formation, primary cilium formation, plasma membrane dynamics, cell proliferation, and immune response. Here, the physiological functions of KCTD10 are summarized and potential mechanisms are proposed. (shrink)

The occurrence of oscillatory behaviours in living cells can be viewed as a visible consequence of stable, regulatory homeostatic cycles. Therefore, they may be used as experimental windows on the underlying physiological mechanisms. Recent studies show that growing pollen tubes are an excellent biological model for these purposes. They unite experimental simplicity with clear oscillatory patterns of both structural and temporal features, most being measurable during real‐time in live cells. There is evidence that these cellular oscillators involve an integrated input (...) of plasma membrane ion fluxes, and a cytosolic choreography of protons, calcium and, most likely, potassium and chloride. In turn, these can create positive feedback regulation loops that are able to generate and self‐sustain a number of spatial and temporal patterns. Other features, including cell wall assembly and rheology, turgor, and the cytoskeleton, play important roles and are targets or modulators of ion dynamics. Many of these features have similarities with other cell types, notably with apical‐growing cells. Pollen tubes may thus serve as a powerful model for exploring the basis of cell growth and morphogenesis. BioEssays 23:86–94, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

Multidrug resistance (MDR) is developed in a population of somatic mammalian cells in vivo or in vitro when they are selected for resistance to each of a large group of drugs (colchicine, adriamycin, actinomycin D, etc.). It is produced by the amplification of some unknown gene(s) whose product(s) evidently change(s) the plasma membrane permeability of a cell to a selective agent and to other unrelated compounds. A large specific genomic region (150–250 kbp) undergoes amplification in MDR cells selected (...) for the resistance to different drugs. Amplified DNA is located in homogeneously staining regions of chromosomes, double minute chromosomes and small chromatin bodies – episome‐like structures that replicate out of the S‐phase of a cell cycle. The selection of MDR cell variants is accompanied by regular karyotypic evolution specific for this type of drug resistance. (shrink)

Recent findings necessitate revision of the traditional view of G protein‐coupled receptor (GPCR) signaling and expand the diversity of mechanisms by which receptor signaling influences cell behavior in general. GPCRs elicit signals at the plasma membrane and are then rapidly removed from the cell surface by endocytosis. Internalization of GPCRs has long been thought to serve as a mechanism to terminate the production of second messengers such as cAMP. However, recent studies show that internalized GPCRs can continue to (...) either stimulate or inhibit cAMP production in a sustained manner. They do so by remaining associated with their cognate G protein subunit and adenylyl cyclase at endosomal compartments. Once internalized, the GPCRs produce cellular responses distinct from those elicited at the cell surface. (shrink)

Insect and pathogen attacks activate plant defense genes within minutes in nearby cells, and within hours in leaves far distant from the sites of the predator attacks. A search for signal molecules involved in both the localized and distal signalling has resulted in the identification of an 18‐amino‐acid polypeptide, called systemin, that activates defense genes in leaves of tomato plants when supplied at levels as low as fmols/plant. Several lines of evidence support a role for systemin as a wound hormone. (...) As with animal polypeptide hormones, systemin is derived from a larger precursor protein, called prosystemin, by limited proteolysis. Systemin has been shown by autoradiography to be phloemmobile and, by antisense technology, to be an essential component of the wound‐inducible, systemic signal transduction system leading to the transcriptional activation of the defensive genes. A search for the receptor of systemin has led to the identification in plant plasma membranes of a systeminbinding protein. However, this protein has properties not of a receptor, but of a furin‐like proteinase that cleaves systemin into smaller polypeptides. Systemin and its precursor prosystemin provide prototypes for the emerging possibilities that polypeptide hormones may have broad roles in signalling environmental stress responses, and in regulating plant growth and development as well. (shrink)