1 00:00:02,350 --> 00:00:06,809 While red blood cells are carried away at high velocity by a strong blood flow, 2 00:00:07,450 --> 00:00:10,089 leukocytes roll slowly on endothelial cells. 3 00:00:10,730 --> 00:00:14,970 P-selectins on endothelial cells interact with PSGL1, 4 00:00:14,970 --> 00:00:18,050 a glycoprotein on leukocyte microvilli. 5 00:00:19,149 --> 00:00:23,609 Leukocytes pushed by the blood flow adhere and roll on endothelial cells 6 00:00:23,609 --> 00:00:27,449 because existing interactions are broken while new ones are formed. 7 00:00:28,210 --> 00:00:40,189 These interactions are possible because the extended extracellular domains of both proteins emerge from the extracellular matrix, which covers the surface of both cell types. 8 00:00:46,740 --> 00:00:51,700 The outer leaflet of the lipid bilayer is enriched in sphingolipids and phosphatidylcholine. 9 00:00:52,679 --> 00:00:57,960 Sphingolipid-rich rafts raised above the rest of the leaflet recruit specific membrane proteins. 10 00:00:57,960 --> 00:01:05,959 Rats' rigidity is caused by the tight packing of cholesterol molecules against the straight sphingolipid's hydrocarbon chains. 11 00:01:05,959 --> 00:01:14,959 Outside the rats, kinks in unsaturated hydrocarbon chains and lower cholesterol concentration result in increased fluidity. 12 00:01:14,959 --> 00:01:26,959 At sites of inflammation, secreted chemokines bound to heparin sulfate proteoglycan on endothelial cells are presented to leukocyte 7 transmembrane receptors. 13 00:01:26,959 --> 00:01:33,159 The binding stimulates leukocytes and triggers an intracellular cascade of signaling reactions. 14 00:01:34,959 --> 00:01:40,120 The inner leaflet of the bilayer has a very different composition than that of the outer leaflet. 15 00:01:41,140 --> 00:01:48,920 While some proteins traverse the membrane, others are either anchored into the inner leaflet by covalently attached fatty acid chains, 16 00:01:49,299 --> 00:01:53,480 or are recruited through non-covalent interactions with membrane proteins. 17 00:01:53,480 --> 00:02:00,159 The membrane-bound protein complexes are critical for the transmission of signals across the plasma membrane. 18 00:02:01,019 --> 00:02:08,000 Beneath the lipid bilayer, spectrum tetramers arranged into a hexagonal network are anchored by membrane proteins. 19 00:02:08,639 --> 00:02:15,300 This network forms the membrane skeleton that contributes to membrane stability and membrane protein distribution. 20 00:02:15,300 --> 00:02:24,460 The cytoskeleton is comprised of networks of filamentous proteins that are responsible for the spatial organization of cytosolic components. 21 00:02:25,620 --> 00:02:32,360 Inside microvilli, actin filaments form tight parallel bundles, which are stabilized by cross-linking proteins. 22 00:02:33,219 --> 00:02:40,300 While deeper in the cytosol, the actin network adopts a gel-like structure, stabilized by a variety of actin-binding proteins. 23 00:02:40,300 --> 00:02:47,280 proteins. Filaments, kept at their minus ends by a protein complex, grow away from the plasma 24 00:02:47,280 --> 00:02:53,759 membrane by the addition of actin monomers to their plus end. The actin network is a 25 00:02:53,759 --> 00:03:03,280 very dynamic structure with continuous directional polymerization and disassembly. Severing proteins 26 00:03:03,280 --> 00:03:08,199 induce kinks in the filament and lead to the formation of short fragments that rapidly 27 00:03:08,199 --> 00:03:11,439 depolymerize, or give rise to new filaments. 28 00:03:13,460 --> 00:03:20,080 The cytoskeleton includes a network of microtubules created by the lateral association of protofilaments, 29 00:03:20,520 --> 00:03:23,120 formed by the polymerization of tubulin dimers. 30 00:03:23,919 --> 00:03:30,020 While the plus ends of some microtubules extend toward the plasma membrane, proteins stabilize 31 00:03:30,020 --> 00:03:35,360 the curved conformation of protofilaments from other microtubules, causing their rapid 32 00:03:35,360 --> 00:03:36,939 plus-end depolymerization. 33 00:03:38,199 --> 00:03:45,199 Microtubules provide tracks along which membrane-bound vesicles travel to and from the plasma membrane. 34 00:03:45,199 --> 00:03:56,699 The directional movement of these cargo vesicles is due to a family of motor proteins linking vesicles and microtubules. 35 00:03:56,699 --> 00:04:03,699 Membrane-bound organelles like mitochondria are loosely trapped by the cytoskeleton. 36 00:04:03,699 --> 00:04:10,939 Mitochondria change shape continuously, and their orientation is partly dictated by their interaction with microtubules. 37 00:04:12,620 --> 00:04:22,779 All the microtubules originate from the centrosome, a discrete fiber structure containing two orthogonal centrioles and located near the cell nucleus. 38 00:04:28,040 --> 00:04:35,220 Pores in the nuclear envelope allow the import of particles containing mRNA and proteins into the cytosol. 39 00:04:42,660 --> 00:04:50,259 Here, free ribosomes translate the mRNA molecules into proteins. 40 00:04:50,259 --> 00:04:54,920 Some of these proteins will reside in the cytosol. 41 00:04:54,920 --> 00:05:00,180 Others will associate with specialized cytosolic proteins and be imported into mitochondria 42 00:05:00,180 --> 00:05:03,720 or other organelles. 43 00:05:03,720 --> 00:05:10,040 The synthesis of cell-secreted and integral membrane proteins is initiated by free ribosomes, 44 00:05:10,040 --> 00:05:14,540 which then dock to protein translocators at the surface of the endoplasmic reticulum. 45 00:05:15,180 --> 00:05:18,459 Nascent proteins pass through an aqueous pore in the translocator. 46 00:05:19,279 --> 00:05:23,500 Cell-secreted proteins accumulate in the lumen of the endoplasmic reticulum, 47 00:05:23,500 --> 00:05:28,879 while integral membrane proteins become embedded in the endoplasmic reticulum membrane. 48 00:05:37,100 --> 00:05:41,420 Proteins are transported from the endoplasmic reticulum to the Golgi apparatus 49 00:05:41,420 --> 00:05:44,319 by vesicles traveling along the microtubules. 50 00:05:57,259 --> 00:06:04,620 Protein glycosylation, initiated in the endoplasmic reticulum, is completed inside the lumen of the Golgi apparatus. 51 00:06:06,300 --> 00:06:16,569 Fully glycosylated proteins are transported from the Golgi apparatus to the plasma membrane. 52 00:06:17,629 --> 00:06:27,040 When a vesicle fuses with the plasma membrane, proteins contained in the vesicle's lumen are secreted, 53 00:06:27,040 --> 00:06:31,800 and proteins embedded in the vesicle's membrane diffuse in the cell membrane. 54 00:06:31,800 --> 00:06:42,800 At sites of inflammation, chemokines secreted by endothelial cells bind to the extracellular domains of G-protein coupled membrane receptors. 55 00:06:42,800 --> 00:06:51,800 This binding causes a conformational change in the cytosolic portion of the receptor and the consequent activation of a subunit of the G-protein. 56 00:06:51,800 --> 00:07:03,800 The activation of the G protein subunit triggers a cascade of protein activation, which in turn leads to the activation and clustering of integrins inside lipid rafts. 57 00:07:03,800 --> 00:07:09,800 A dramatic conformational change occurs in the extracellular domain of the activated integrins. 58 00:07:09,800 --> 00:07:16,800 This now allows for their interaction with ICAM proteins displayed at the surface of the endothelial cells. 59 00:07:16,800 --> 00:07:22,800 These strong interactions immobilize the rolling leukocyte at the site of inflammation. 60 00:07:22,800 --> 00:07:27,800 Additional signaling events cause a profound reorganization of the cytoskeleton, 61 00:07:27,800 --> 00:07:30,800 resulting in the spreading of one edge of the leukocyte. 62 00:07:30,800 --> 00:07:35,800 The leading edge of the leukocyte inserts itself between endothelial cells, 63 00:07:35,800 --> 00:07:40,800 and the leukocyte migrates through the blood vessel wall into the inflamed tissue. 64 00:07:40,800 --> 00:07:52,139 Rolling, activation, adhesion, and transendothelial migration are the four steps of a process called leukocyte extravasation.