Module 17: Endocytosis PDF

MODULE 17 Slide 1: Endocytosis The cellular take-up of macromolecules, particulate substances, and cells. The material is enclosed by the plasma membrane, which invaginates and pinches off to form a vesicle. Types of endocytosis: phagocytosis, pinocytosis, receptor-mediated. Slide 2: Compone...

MODULE 17 Slide 1: Endocytosis The cellular take-up of macromolecules, particulate substances, and cells. The material is enclosed by the plasma membrane, which invaginates and pinches off to form a vesicle. Types of endocytosis: phagocytosis, pinocytosis, receptor-mediated. Slide 2: Components of the Endocytic Pathway Early endosomes (EE): pH ~6.3; receive endocytic vesicles with cargo and plasma membrane from the cell surface; endocytosed ligands dissociate from the receptors, some receptors are recycled. Late endosomes (LE): pH ~5.5, on their way to lysosomes, receive material from early endosomes, TGN, and phagosomes; contain lysosomal proteins. Lysosomes: pH ~4.8, with hydrolases; break down fats, carbohydrates, and proteins into simple compounds that are used in the cytoplasm for synthesis. Slide 3: Early and Late Endosomal Compartments In polarized epithelial cells, endocytosis occurs from the basolateral and apical domains. Endocytosed material enters the early endosomes unique to each domain. Endocytosed receptors are recycled to their original membrane domain unless they contain signals for transcytosis. Molecules endocytosed from either domain and not retrieved from the early endosomes end up in a common late endosomal compartment and are degraded in lysosomes. Slide 4: Multivesicular Bodies (MVB) Endosomes enclose invaginated membrane and internally pinch-off vesicles; MVBs fuse with late endosomes or other MVBs to become late endosomes. Transport lipids, proteins, and nucleic acids; the membrane proteins are degraded or recycled. Slide 5: Phagocytosis Ingestion of particles (microorganisms, dead cells) into phagosomes that eventually fuse with lysosomes. Professional phagocytes (macrophages, neutrophils, dendritic cells) and nonprofessional phagocytes exist. Initiated by the binding of phagocyte receptors to distinct molecular patterns on targets; binding induces actin polymerization at the site of ingestion, resulting in pseudopods. Triggers are antibodies bound to microorganisms, oligosaccharides on the surface of microorganisms. Slide 6: Pinocytosis Non-selective uptake of extracellular compounds into uncoated vesicles (macropinosomes) that fuse with lysosomes or do not mature beyond the early endosomal stage, and are recycled to the plasma membrane. Receptors activated by growth factors increase actin polymerization at the cell membrane; resulting in lamellipodia- induced ruffles that fold inwards and fuse with the basal membrane to form macropinosomes. Antigen-presenting cells "look for" antigens through constitutive pinocytosis; after internalization, the antigens are processed into peptides for presentation. Pinocytosis compared to phagocytosis: (1) "eating" versus "drinking", and (2) in phagocytosis, larger particles are engulfed than in pinocytosis. Slide 7: Caveolar Endocytosis Clathrin-independent, involves plasma membrane invaginations called caveolae. Caveolar coat is structured by specific proteins (caveolins, cavins). Caveolae may collect cargo proteins by virtue of the lipid composition of the caveolar membrane, rather than by the protein coat. Caveolae deliver contents to endosome-like compartments or through transcytosis to the plasma membrane on the opposite side of a polarized cell. Slide 8: Receptor-mediated Endocytosis The cargo binds to transmembrane receptors, accumulates in coated pits, and enters the cell in clathrin-coated vesicles. Some (but not all) receptors enter coated pits only if bound to ligands. The pits function as molecular filters, preferentially collecting certain receptors over others. There are more than 25 different receptors known to participate in this type of endocytosis. Examples include iron-bound transferrin recycling and the uptake of low-density lipoprotein (LDL). MODULE 17 Slide 9: Clathrin Vesicles Mediate endocytosis from the plasma membrane to endosomal compartments and the Golgi. Clathrin: each subunit has several polypeptides forming a triskelion; triskelions assemble to form coated pits on the cytosolic face of membranes. Adaptins (adaptor proteins, AP) bind the clathrin coat and receptors of the cargo molecules in the vesicles. Several types of adaptins (AP) exist, each specific for a set of cargo receptors. Slide 10: Forming Clathrin Vesicles Accessory proteins: for clathrin recruitment, membrane-bending, and scission. The membrane is deformed with the aid of curvature-driving molecules. Dynamin, a large GTPase, forms a helical collar around the neck of an invaginating clathrin-coated vesicle. Upon GTP hydrolysis, dynamin pinches off the vesicle from the parent membrane. Slide 11: The Endosome Fate Ligands that remain bound to the receptors share the fate of the receptors: (1) recycled to the plasma membrane; (2) delivered to a different domain of the plasma membrane (transcytosis); and (3) degraded in the lysosomes. Transferrin, a ligand that carries iron, is delivered by its receptor to early endosomes, where low pH releases the iron; the receptor-transferrin complex recycles back to the plasma membrane, where the ligand dissociates from the receptor. Slide 12: Transcytosis The transfer of compounds from one extracellular space to another through polarized epithelial cells. Receptors are endocytosed to reach a different plasma membrane domain. Receptors move from early endosomes to recycling endosomes. In fat and muscle cells, intracellular glucose transporters are in recycling endosomes until stimulated by insulin to increase glucose uptake; transport vesicles from recycling endosomes deliver GLUTs to the plasma membrane. Slide 16: Putting It Together Endocytosis involves ingestion of fluids, molecules, and particles by cells; localized regions of the plasma membrane invaginate and pinch off to form endocytic vesicles. Some endocytosed material ends up in lysosomes for degradation; most endocytosed plasma membrane components are returned to the cell surface by exocytosis. Clathrin-coated vesicles mediate endocytosis from the plasma membrane. Some receptors are localized in clathrin-coated pits (receptor-mediated endocytosis); these vesicles shed their clathrin coats and fuse with early endosomes. Some ligands dissociate from the receptors in the acidic environment of the endosome and end up in lysosomes, whereas most receptors are recycled to the cell surface. Both receptor and ligand could be degraded in lysosomes (receptor downregulation), or both could be transferred to a different plasma membrane domain (transcytosis). Caveolae, plasma membrane invaginations that mediate endocytosis in a clathrin-independent manner, possess a less prominent coat made of caveolins and cavins. Retrograde transport within the Golgi and towards the ER occurs via vesicles surrounded with coat protein I (COPI); in the opposite direction, COPII vesicles bud off from the ER in the secretory or exocytic pathway towards the Golgi.