Cell Membrane Lecture Notes PDF

Cell membrane, intracellular compartmentalization Tamás Kovács, 2024 Based on the lecture by Prof. György Vereb, 2022 This lecture: Essential Cell Biology: chapters 11-12 All the pages are required, but: ion channels, the pumps an...

Cell membrane, intracellular compartmentalization Tamás Kovács, 2024 Based on the lecture by Prof. György Vereb, 2022 This lecture: Essential Cell Biology: chapters 11-12 All the pages are required, but: ion channels, the pumps and mechanisms relevant to Ca, osmo- and pH regulation, also included in this chapter of the book, will be discussed in other lectures - lecture slides marked by: ! contain basic information absolutely required for tests and exams!  Topics membrane structure membrane alterations in human glycocalyx diseases (examples) cell cortex membrane-bound intracellular compartments synthesis and composition of cellular membranes asymmetry of the lipid bilayer lateral organization of the membrane lipid rafts ceramide platforms protein-mediated domain formation  Revision Why do we study cellular membranes? ! „The cell is all membranes.” „The membrane is all lipids.” (almost…) Membrane structure and function Revision ! Lipid bilayer 5 nm Functions of the Composition: plasmamembrane: 40-60 % lipid barrier 30-50 % protein transport 10 % carbohydrate signal transduction  Membrane structure: carbohydrates Revision the carbohydrate components of the membrane ! form the glycocalyx (on the extracellular side) - glycolipids (rare) - glycoproteins (common) Proteoglycans are large molecules found in the extracellular matrix of connective tissues. They consist of a core protein with one or more glycosaminoglycan (GAG) chains attached  Functions of glycocalyx components ! communication, surface self / non-self cell adhesion, protection recognition signaling pathogen invasion  Glycocalyx-mediated protection of cancer cells against anti-tumor agents doxorubicin N-linked glycans Herceptin HER2 MUC4 Herceptin binding site no MUC4 and N-linked glycans Cell surface expression of MUC4 glycoprotein on cancer cells → binding of anti-tumor antibodies (ex. Herceptin) ↓ → celluar entry of chemotherapeutics (ex. doxorubicin) ↓ Outlook Membrane proteins – function and structure Revision ! GPI-linked protein Inositol GPI = glycosyl- phosphatydilinositol  Paroxysmal nocturnal hemoglobinuria (PNH) PNH defect PNH: defect of enzyme responsible for GPI biosynthesis → no glycolipid (GPI)-anchor of CD55 and CD59 → assembly of membrane attack complex on RBC surface → intravascular hemolytic anemia Outlook  Membrane proteins interact with the Cell Cortex ! (actin meshwork in the cytosol ”below” the membrane) Intracellular side!!! Red blood cell membrane and its connections with the cell cortex Important proteins: actin, ankyrin (anchor to transmembrane proteins) spectrin (dimer, forms a mesh)  Molecular complexity of membrane – cell cortex connection Red blood cell: Outlook Over 50 transmembrane proteins  Cell shape is determined by ! cell cortex cytoplasmic content Hb-S Hb-A ! Defects in the vertical spectrin interactions: spherocytosis Defects in horizontal spectrin interactions: elliptocytosis.  Biological relevance of membrane fluidity ! - Allows diffusion of hydrophobic substances - Renders membranes deformable - Explains their regenerative capacity Deformability – important e.g. in capillaries Experiment demonstrating regenerative capacity outer layer inner layer Right side out 5* RBC ghost Inside out hypotonic isotonic solution solution (→ swelling (→ re-closing) breaks the membrane) Regenerative capacity of lipid vesicles can also be used to fill the vesicles with drugs for safe and/or targeted delivery to cells Membrane-enclosed intracellular compartments ! Lipid synthesis and steady-state composition of various membrane structures mammals Info yeasts Mitochondria Major differences have bacterial ! ! between lipids. organelles Low cholesterol ! in Mitoch., ER, ! nuclear envelope site of synthesis: mostly SER (also Golgi, plasma membrane inner surface) !  Physical and compositional ! variations in subcellular membranes Outlook unique lipid composition in organelles ↓ unique biophysical parameters and lateral organization optimal for protein function BUT! alterations in composition ↓ alterations in membrane biophysical parameters and lateral organization ↓ altered protein function ↓ diseases ! Lysosomal storage disorders Outlook Niemann-Pick A: lysosomal enzyme dysfunction Øacid sphingomyelinase ↓ Gaucher: →sphingomyelin↑ lipid substrate accummulation Øβ-glucocerebrosidase ↓ →glucosylceramide↑ alterations in membrane biophysical parameters and lateral organization ↓ altered protein function ↓ lysosomes: impaired autophagy mitochondria: oxidative stress↑ vesicles: defects in trafficking ↓ Niemann-Pick C: progressive neurological ØNPC1/NPC2 symptoms and cholesterol↑ hepatosplenomegaly The lipid composition of the cytoplasma ! membrane is asymmetric flop Asym- metric curvature compo- sition flip 1. lipid incorporation into cytoplasmic leaflet 2. flippases/floppases (active) > selective retention 3. scramblases (passive), bidirectional random mixing  Lipid shape determines membrane curvature Info cylinders (e.g.: PC) inverted cones (e.g.: lysoPC) cones (e.g.: PE) curvature PC: phosphatidyl choline; PE : phosphatidyl ethanolamine ©2011 by Cold Spring Harbor Laboratory Press ! PS exposure is an „eat-me signal” of apoptotic cells PS exposure on the surface of phosphatidyl- dying cells: ”eat- serine me” signal for the phagocytes (see apoptosis) Outlook PS exposure in cancer cells PS exposure of cancer cells → immunosuppression → tumor growth↑ Anti-PS therapy in cancer: - anti-PS antibodies ± chemotherapeutic drugs - PS extraction - disruption of PS receptor signaling Outlook Cytosolic and exoplasmic surfaces ! Principles of lateral membrane organization: Revision The fluid-mosaic model ! Lateral mobility Frye-Edidin experiment Singer-Nicolson model Principles of lateral membrane organization: Outlook Lipid immiscibility Lipids alone can form complex structures in artifical membranes with distinctive properties Liquid-disordered domains Liquid-ordered domains Relevance of membrane composition: Lipid-protein interactions Lipids can actively influence the structure and function of proteins through: ! direct interactions alterations in membrane biophysical parameters altered lateral distribution of proteins in the membrane Principles of lateral membrane organization: The lipid raft hypothesis ! "Membrane rafts are small (10-200 nm), heterogeneous, highly dynamic, sterol- and sphingolipid-enriched domains that compartmentalize cellular processes. Small rafts can sometimes be stabilized to form larger platforms through protein-protein and protein- lipid interactions." Principles of lateral membrane organization: The lipid raft hypothesis ! Unique properties of rafts (~liquid-ordered domains in model bilayers): membrane fluidity ↓ lipid packing, thickness and stiffness ↑ → modulation of protein function → signaling platforms Cholesterol and cancer Outlook - obesity and hypercholesterolemia are risk factors of certain cancers - altered cholesterol metabolism in tumors (typically cholesterol↑) ! - targeting cholesterol metabolism as therapeutic approach Lipid rafts, cholesterol and Alzheimer’s disease Outlook Amyloid-precursor protein (APP) in Alzheimer’s disease: - nonamyloidogenic pathway in non-raft domains↓ - amyloidogenic processing in rafts ↑ (cholesterol!) → amyloid-β accumulation and neurotoxicity - anti-cholesterol therapy (statins, cyclodextrins, etc.) Lipid rafts and infections Outlook Receptors of pathogen entry are commonly localized in lipid rafts → anti-raft therapy (statins, cyclodextrins, sphingolipid inhibitors) ! Lipid rafts and HIV Outlook Cellular entry of HIV is strongly raft-dependent → anti-raft therapy (statins, cyclodextrins, sphingolipid inhibitors) Lipid rafts and COVID-19 Outlook Cellular entry of SARS-CoV-2 is strongly raft-dependent → anti-raft therapy (statins, cyclodextrins, sphingolipid inhibitors) Principles of lateral membrane organization: Ceramide platforms ! NON-RAFT DOMAIN Ceramide Sphingomyelin Phospholipid Cholesterol Proteins ? LIPID RAFT CERAMIDE PLATFORM Unique properties of ceramide platforms (~gel phase): membrane fluidity ↓↓ lipid packing, thickness and stiffness ↑↑ → modulation of protein function → signaling platforms Ceramide platforms and apoptosis Outlook ceramide production and formation of ceramide platforms are important in apoptosis induction ↑ in Alzheimer’s disease and ↓ in cancers a common mechanism of action of chemotherapeutic agents Apoptosis: see Lecture 23 Ceramide in cancer cells In cancer cells ceramide/sphingosine-1-phosphate (S1P) ratio↓ → proliferation, survival and chemotherapeutic resistance Novel anti-tumor approaches - regulation of enzymes (ceramidase↓, sphingosine-kinase 1 (SK1)↓) - clustering of death receptors via ceramide platforms (ceramides, synthetic lipids, plant-derived compounds) Outlook Principles of lateral membrane organization: Active contribution of proteins ! Clustering / molecular interactions Focal adhesion Signalosome Immune synapse Lectures 8, 9, 22 Lecture 18 Lecture 20 Tight junction Picketed-fence model Lectures 4 and 9  Signalosomes ! Large supramolecular protein complexes that undergo clustering (oligomerization or polymerization) and/or lateral microdomain separation to form biomolecular condensates that increase the local concentration and signaling activity of the individual components.  Keywords ! lipids phosphatidylserine amphiphilic (amphipathic) scramblase compounds flippase lipid bilayer floppase endoplasmic reticulum (= ER) lipid rafts Golgi complex ceramide platforms lysosome signalosome storage disease focal adhesion glycocalyx immune synapse cell cortex tight junction