Mechanisms of lipid regulation and lipid gating in TRPC channels

TRPC proteins form cation channels that integrate and relay cellular signals by mechanisms involving lipid recognition and lipid-dependent gating. The lipohilic/amphiphilic molecules that function as cellular activators or modulators of TRPC proteins span a wide range of chemical structures. In this context, cellular redox balance is likely linked to the lipid recognition/gating features of TRPC channels. Both classical ligand-protein interactions as well as indirect and promiscuous sensory mechanisms have been proposed. Some of the recognition processes are suggested to involve ancillary lipid-binding scaffolds or regulators as well as dynamic protein–protein interactions determined by bilayer architecture. A complex interplay of protein–protein and protein-lipid interactions is likely to govern the gating and/or plasma membrane recruitment of TRPC channels, thereby providing a distinguished platform for signal integration and coincident signal detection. Both the primary molecular event(s) of lipid recognition by TRPC channels as well as the transformation of these events into distinct gating movements is poorly understood at the molecular level, and it remains elusive whether lipid sensing in TRPCs is conferred to a distinct sensor domain. Recent structural information on the molecular action of lipophilic activators in distantly related members of the TRP superfamily encourages speculations on TRPC gating mechanisms involved in lipid recognition/gating. This review aims to provide an update on the current understanding of the lipid–dependent control of TRPC channels with focus on the TRPC lipid sensing, signal-integration hub and a short discussion of potential links to redox signaling.     

ABCA1 mutants reveal an interdependency between lipid export function, apoA-I binding activity, and Janus kinase 2 activation

ABCA1 exports cholesterol and phospholipids from cells by a multistep pathway that involves forming cell surface lipid domains, solubilizing these lipids by apolipoproteins, binding of apolipoproteins to ABCA1, and activating signaling processes. Here we used a mutational analysis approach to evaluate the relationship between these events. We prepared seven naturally occurring mutants and one artificial missense mutant of ABCA1 with varying degrees of impaired function, expressed them to similar levels as wild-type ABCA1 on the cell surface of BHK cells, and measured ABCA1-dependent lipid export, apolipoprotein A-I (apoA-I) binding, and signaling activities. Linear regression analyses showed that cholesterol and phospholipid efflux and cellular apoA-I binding correlated significantly with the ability of ABCA1 to form cell surface lipid domains. Lipid export and cellular apoA-I binding activities and formation of lipid domains also correlated with the amount of apoA-I that could be cross-linked to ABCA1. Moreover, each of these lipid export and apoA-I binding activities correlated with apoA-I-induced Janus kinase 2 (JAK2) activation. Thus, these missense mutations in ABCA1 impair lipid export, apoA-I binding, and apoA-I-stimulated JAK2 activities to similar extents, indicating that these processes are highly interactive components of a pathway that functions to export lipids from cells.     

Information storing by biomagnetites

Since the discovery of the presence of biogenic magnetites in living organisms, there have been speculations on the role that these biomagnetites play in cellular processes. It seems that the formation of biomagnetite crystals is a universal phenomenon and not an exception in living cells. Many experimental facts show that features of organic and inorganic processes could be indistinguishable at nanoscale levels. Living cells are quantum “devices” rather than simple electronic devices utilizing only the charge of conduction electrons. In our opinion, due to their unusual biophysical properties, special biomagnetites must have a biological function in living cells in general and in the brain in particular. In this paper, we advance a hypothesis that while biomagnetites are developed jointly with organic molecules and cellular electromagnetic fields in cells, they can record information about the Earth’s magnetic vector potential of the entire flight in migratory birds.     

Psychological essentialism from first principles

Cognitive scientists have documented the existence of “essentialist” intuitions in humans: from a very early age, we assume that things have deep unobserved properties that make them what they are. I provide a sketch of an adaptationist explanation of psychological essentialism, arguing that these intuitions are the unsurprising output of adaptations for inductive inference. Variations on this insight have previously been used mostly as after-the-fact speculations, yet theories of adaptive function should ideally have a primary role in informing psychological research. Here I propose that viewing essentialist intuitions through an adaptationist lens has implications for some widespread assumptions about the phenomenon. Notably, researchers' focus on “higher-level” processes like categorization has led them to assume that essentialism is restricted to a few cognitive processes, but the ubiquity of inductive inference problems in cognition suggests otherwise. Additionally, because essentialist intuitions are the output of mechanisms solving related but distinct inference problems, it is unlikely that a single mechanistic theory can account for them all.     

Thermodynamics,the structure of integral membrane proteins,and transport

Membranes are structures whose lipid and protein components are at, or close to, equilibrium in the plane of the membrane, but are not at equilibrium across the membrane. The thermodynamic tendency of ionic and highly polar molecules to be in contact with water rather than with nonpolar media (hydrophilic interactions) is important in determining these equilibrium and nonequilibrium states. In this paper, we speculate about the structures and orientations of integral proteins in a membrane, and about how the equilibrium and nonequilibrium features of such structures and orientations might be influenced by the special mechanisms of biosynthesis, processing, and membrane insertion of these proteins. The relevance of these speculations to the mechanisms of the translocation event in membrane transport is discussed, and specific protein models of transport that have been proposed are analyzed.     

Deconvoluting the biology and druggability of protein lipidation using chemical proteomics

Lipids are indispensable cellular building blocks, and their post-translational attachment to proteins makes them important regulators of many biological processes. Dysfunction of protein lipidation is also implicated in many pathological states, yet its systematic analysis presents significant challenges. Thanks to innovations in chemical proteomics, lipidation can now be readily studied by metabolic tagging using functionalized lipid analogs, enabling global profiling of lipidated substrates using mass spectrometry. This has spearheaded the first deconvolution of their full scope in a range of contexts, from cells to pathogens and multicellular organisms. Protein N-myristoylation, S-acylation, and S-prenylation are the most well-studied lipid post-translational modifications because of their extensive contribution to the regulation of diverse cellular processes. In this review, we focus on recent advances in the study of these post-translational modifications, with an emphasis on how novel mass spectrometry methods have elucidated their roles in fundamental biological processes.     

Detergent-insoluble glycosphingolipid/cholesterol-rich membrane domains, lipid rafts and caveolae (review).

Within the cell membrane glycosphingolipids and cholesterol cluster together in distinct domains or lipid rafts, along with glycosyl-phosphatidylinositol (GPI)-anchored proteins in the outer leaflet and acylated proteins in the inner leaflet of the bilayer. These lipid rafts are characterized by insolubility in detergents such as Triton X-100 at 4 degrees C. Studies on model membrane systems have shown that the clustering of glycosphingolipids and GPI-anchored proteins in lipid rafts is an intrinsic property of the acyl chains of these membrane components, and that detergent extraction does not artefactually induce clustering. Cholesterol is not required for clustering in model membranes but does enhance this process. Single particle tracking, chemical cross-linking, fluorescence resonance energy transfer and immunofluorescence microscopy have been used to directly visualize lipid rafts in membranes. The sizes of the rafts observed in these studies range from 70-370 nm, and depletion of cellular cholesterol levels disrupts the rafts. Caveolae, flask-shaped invaginations of the plasma membrane, that contain the coat protein caveolin, are also enriched in cholesterol and glycosphingolipids. Although caveolae are also insoluble in Triton X-100, more selective isolation procedures indicate that caveolae do not equate with detergent-insoluble lipid rafts. Numerous proteins involved in cell signalling have been identified in caveolae, suggesting that these structures may function as signal transduction centres. Depletion of membrane cholesterol with cholesterol binding drugs or by blocking cellular cholesterol biosynthesis disrupts the formation and function of both lipid rafts and caveolae, indicating that these membrane domains are involved in a range of biological processes.     

A study of the chorion and the follicle cells in relation to the sperm-egg interaction in the ascidian,Ciona intestinalis

We present ultrastructural observations on the processes underlying interaction between the spermatozoon and the egg envelopes in Ciona intestinalis. The morphological and cytochemical basis of sperm binding to the chorion and the subsequent steps such as the acrosome reaction and penetration of the spermatozoon through the chorion are described. Some speculations are also presented on the role of the follicle cells in fertilization.     

Mammalian Conception, Sev Differentiation, and Hermaphroditism as Viewed in Historical Perspective

This paper surveys in broad perspective some of the steps inthe development of theones of mammalian sex differentiationfrom antiquity to the present. The questions of why and howmales and females differentnte and why there are intersexeshave been answered by conjectures, speculations, and theones.In antiquity, when the existence of mimimhan egg and sperm wasunknownsex determination was con]ec tuicd to depend upon such factorsas which testis contiibuted the semen or which sideof the uterusreceived it. These fanciful notions peisisted until the middleof the 18th centuiy. Discovery of mammalian sperm in the samecentury and the ovum in the 10th centuiy led to obseivationsof fertilization. With advances in genetics andthe identificationot sex chromosomes, the 20th century was prepared to constructtheories foi normal sev differentiation and inteisexuality basedon experimental observation. The bovine freemartin—a naturalexperiment—prompted the hounonal theory leseaich on frogsled to a coittco medullaiy inductor theory, cellular cmmerismin fteemaitin and co tvun was the basis for a cellular theory.Of these, the hormonal theory in particular, provided some verifiablespeculations. However, there is no completely acceptable theoryand new lines of research are badly needed     

Mitochondrial fatty acid synthesis,fatty acids and mitochondrial physiology

Mitochondria and fatty acids are tightly connected to a multiplicity of cellular processes that go far beyond mitochondrial fatty acid metabolism. In line with this view, there is hardly any common metabolic disorder that is not associated with disturbed mitochondrial lipid handling. Among other aspects of mitochondrial lipid metabolism, apparently all eukaryotes are capable of carrying out de novo fatty acid synthesis (FAS) in this cellular compartment in an acyl carrier protein (ACP)-dependent manner. The dual localization of FAS in eukaryotic cells raises the questions why eukaryotes have maintained the FAS in mitochondria in addition to the “classic” cytoplasmic FAS and what the products are that cannot be substituted by delivery of fatty acids of extramitochondrial origin. The current evidence indicates that mitochondrial FAS is essential for cellular respiration and mitochondrial biogenesis. Although both β-oxidation and FAS utilize thioester chemistry, CoA acts as acyl-group carrier in the breakdown pathway whereas ACP assumes this role in the synthetic direction. This arrangement metabolically separates these two pathways running towards opposite directions and prevents futile cycling. A role of this pathway in mitochondrial metabolic sensing has recently been proposed. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.     

Cooperative elastic stresses, the hydrophobic effect, and lipid tilt in membrane remodeling

One of the fundamental properties of biological membranes is the high lateral integrity provided by the lipid bilayer, the structural core and the foundation of their barrier function. This tensile strength is due to the intrinsic properties of amphiphilic lipid molecules, which spontaneously self-assemble into a stable bilayer structure due to the hydrophobic effect. In the highly dynamic life of cellular membranes systems, however, this integrity has to be regularly compromised. One of the emerging puzzles is the mechanism of localized rupture of lipid monolayer, the formation of tiny hydrophobic patches and flipping of lipid tails between closely apposed monolayers. The energy cost of such processes is prohibitively high, unless cooperative deformations in a small membrane patch are carefully organized. Here we review the latest experimental and theoretical data on how such deformations can be conducted, specifically describing how elastic stresses yield tilting of lipids leading to cooperative restructuring of lipid monolayers. Proteins specializing in membrane remodeling assemble into closely packed circular complexes to arrange these deformations in time and space.     

Use of transgenic plants and mutants to study the regulation and function of lipid composition

Mutants and transgenic plants with altered expression of genes implicated in lipid metabolism are providing fresh insights into the regulation and function of lipid composition. To date, several genes encoding fatty acid desaturases, acyltransferases, a thioesterase, a lipid transfer protein and an isoform of acyl-carrier protein have been introduced into transgenic plants. Despite the fact that some of these transgenic plants had large alterations in lipid composition, they showed surprisingly little phenotypic variation from wild-type plants. Although detailed analyses of these plants are just beginning, several theories regarding the roles of particular genes in various plant processes, such as cold tolerance and transfer of lipids between membranes, have been either substantiated or discarded on the basis of the data already obtained. In addition, constructs that contain the promoter regions of genes implicated in lipid metabolism fused to reporter genes have been introduced into transgenic plants and are providing some clues as to how lipid composition is regulated.     

Yeast Integral Membrane Proteins Apq12, Brl1, and Brr6 Form a Complex Important for Regulation of Membrane Homeostasis and Nuclear Pore Complex Biogenesis

Proper functioning of intracellular membranes is critical for many cellular processes. A key feature of membranes is their ability to adapt to changes in environmental conditions by adjusting their composition so as to maintain constant biophysical properties, including fluidity and flexibility. Similar changes in the biophysical properties of membranes likely occur when intracellular processes, such as vesicle formation and fusion, require dramatic changes in membrane curvature. Similar modifications must also be made when nuclear pore complexes (NPCs) are constructed within the existing nuclear membrane, as occurs during interphase in all eukaryotes. Here we report on the role of the essential nuclear envelope/endoplasmic reticulum (NE/ER) protein Brl1 in regulating the membrane composition of the NE/ER. We show that Brl1 and two other proteins characterized previously—Brr6, which is closely related to Brl1, and Apq12—function together and are required for lipid homeostasis. All three transmembrane proteins are localized to the NE and can be coprecipitated. As has been shown for mutations affecting Brr6 and Apq12, mutations in Brl1 lead to defects in lipid metabolism, increased sensitivity to drugs that inhibit enzymes involved in lipid synthesis, and strong genetic interactions with mutations affecting lipid metabolism. Mutations affecting Brl1 or Brr6 or the absence of Apq12 leads to hyperfluid membranes, because mutant cells are hypersensitive to agents that increase membrane fluidity. We suggest that the defects in nuclear pore complex biogenesis and mRNA export seen in these mutants are consequences of defects in maintaining the biophysical properties of the NE.     

Tunable cell-surface mimetics as engineered cell substrates

Most recent breakthroughs in understanding cell adhesion, cell migration, and cellular mechanosensitivity have been made possible by the development of engineered cell substrates of well-defined surface properties. Traditionally, these substrates mimic the extracellular matrix (ECM) environment by the use of ligand-functionalized polymeric gels of adjustable stiffness. However, such ECM mimetics are limited in their ability to replicate the rich dynamics found at cell-cell contacts. This review focuses on the application of cell surface mimetics, which are better suited for the analysis of cell adhesion, cell migration, and cellular mechanosensitivity across cell-cell interfaces. Functionalized supported lipid bilayer systems were first introduced as biomembrane-mimicking substrates to study processes of adhesion maturation during adhesion of functionalized vesicles (cell-free assay) and plated cells. However, while able to capture adhesion processes, the fluid lipid bilayer of such a relatively simple planar model membrane prevents adhering cells from transducing contractile forces to the underlying solid, making studies of cell migration and cellular mechanosensitivity largely impractical. Therefore, the main focus of this review is on polymer-tethered lipid bilayer architectures as biomembrane-mimicking cell substrate. Unlike supported lipid bilayers, these polymer-lipid composite materials enable the free assembly of linkers into linker clusters at cellular contacts without hindering cell spreading and migration and allow the controlled regulation of mechanical properties, enabling studies of cellular mechanosensitivity. The various polymer-tethered lipid bilayer architectures and their complementary properties as cell substrates are discussed.     

Localization of Banana bunchy top virus and cellular compartments in gut and salivary gland tissues of the aphid vector Pentalonia nigronervosa

Banana bunchy top virus (BBTV) (Nanoviridae: Babuvirus) is transmitted by aphids of the genus Pentalonia in a circulative manner. The cellular mechanisms by which BBTV translocates from the anterior midgut to the salivary gland epithelial tissues are not understood. Here, we used multiple fluorescent markers to study the distribution and the cellular localization of early and late endosomes, macropinosomes, lysosomes, microtubules, actin filaments, and lipid raft subdomains in the gut and principal salivary glands of Pentalonia nigronervosa. We applied colabeling assays, to colocalize BBTV viral particles with these cellular compartments and structures. Our results suggest that multiple potential cellular processes, including clathrin‐ and caveolae‐mediated endocytosis and lipid rafts, may not be involved in BBTV internalization.     

The SPFH domain-containing proteins: more than lipid raft markers

Membrane microdomains with distinct lipid compositions, called lipid rafts, represent a potential mechanism for compartmentalizing cellular functions within the plane of biological membranes. SPFH domain-containing proteins are found in lipid raft microdomains in diverse cellular membranes. The functions of these proteins are just beginning to be elucidated. Recent advances in the understanding of structural features and their roles within lipid rafts include a potential function for SPFH proteins in the formation of membrane microdomains and lipid raft-associated processes, such as endocytosis and mechanosensation.     

The life of lipid droplets

Lipid droplets are the least characterized of cellular organelles. Long considered simple lipid storage depots, these dynamic and remarkable organelles have recently been implicated in many biological processes, and we are only now beginning to gain insights into their fascinating lives in cells. Here we examine what we know of the life of lipid droplets. We review emerging data concerning their cellular biology and present our thoughts on some of the most salient questions for investigation.     

The role of lipins in innate immunity and inflammation

Lipins are phosphatidic acid phosphatase enzymes whose cellular function in regulating lipid metabolism has been known for decades, particularly in metabolically active tissues such as adipose tissue or liver. In recent years evidence is accumulating for key regulatory roles of the lipin family in innate immune cells. Lipins may help regulate signaling through relevant immune receptors such as Toll-like receptors, and are also integral part of the cellular machinery for lipid storage in these cells, thereby modulating certain inflammatory processes. Mutations in genes that encode for members of this family produce autoinflammatory hereditary diseases or diseases with an important inflammatory component in humans. In this review we summarize recent findings on the role of lipins in cells of the innate immune system and in the onset and progress of inflammatory processes.     

Lipid rafts,caveolae, caveolin-1, and entry by Chlamydiae into host cells

Obligate intracellular bacterial pathogens of the genus Chlamydia are reported to enter host cells by both clathrin-dependent and clathrin-independent processes. C. trachomatis serovar K recently was shown to enter cells via caveolae-like lipid raft domains. We asked here how widespread raft-mediated entry might be among the Chlamydia. We show that C. pneumoniae, an important cause of respiratory infections in humans that additionally is associated with cardiovascular disease, and C. psittaci, an important pathogen in domestic mammals and birds that also infects humans, each enter host cells via cholesterol-rich lipid raft microdomains. Further, we show that C. trachomatis serovars E and F also use these domains to enter host cells. The involvement of these membrane domains in the entry of these organisms was indicated by the sensitivity of their entry to the raft-disrupting agents Nystatin and filipin, and by their intracellular association with caveolin-1, a 22-kDa protein associated with the formation of caveolae in rafts. In contrast, caveolin-marked lipid raft domains do not mediate entry of C. trachomatis serovars A, 36B, and C, nor of LGV serovar L2 and MoPn. Finally, we show that entry of each of these chlamydial strains is independent of cellular expression of caveolin-1. Thus, entry via the Nystatin and filipin-sensitive pathway is dependent on lipid rafts containing cholesterol, rather than invaginated caveolae per se.