[I denne prisberømte animasjonen blir vi vitne til de fantastiske prosessene når hvite blodlegemer blir aktivert. Du vil kanskje kjenne igjen en god del av celleorganellene, reseptorer og proteiner på cellemembranen mm fra nyere ME-forskning]
The Inner Life of the Cell is an 8.5-minute 3D computer graphics animation illustrating the molecular mechanisms that occur when a white blood cell in the blood vessels of the human body is activated by inflammation (Leukocyte extravasation). It shows how a white blood cell rolls along the inner surface of the capillary, flattens out, and squeezes through the cells of the capillary wall to the site of inflammation where it contributes to the immune reaction. (wikipedia).
Harvard University selected XVIVO to develop an animation that would take their cellular biology students on a journey through the microscopic world of a cell, illustrating mechanisms that allow a white blood cell to sense its surroundings and respond to an external stimulus.
This award winning piece was the first topic in a series of animations XVIVO is creating for Harvard’s educational website BioVisions at Harvard.
While red blood cells are carried away at high velocity by a strong blood flow, leukocytes roll slowly on endothelial cells. P-selectins on endothelial cells interact with PSGL1, a glycoprotein on leukocyte microvilli. Leukocytes pushed by the blood flow adhere and roll on endothelial cells because existing interactions are broken while new ones are formed.
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. The outer leaflet of the lipid bilayer is enriched in sphingolipids, and phosphatidyl choline.
Sphingolipid rich rafts raised above the rest to the leaflet recruit specific membrane proteins. Rafts rigidity is caused by the tight packing of cholesterol molecules against the straight sphingolipids hydrocarbon chains. Outside the rafts, kinks in unsaturated hydrocarbon chains and lower cholesterol concentration result in increased fluidity.
At sites of inflammation, secreted chemokine’s, bound to heparin sulfate proteoglycan on endothelial cells, are presented to leukocyte-7 transmembrane receptors. The binding stimulates leukocytes and triggers an intracellular cascade of signaling reactions.
The inner leaflet of the bilayer has a very different composition then that of the outer leaflet. While some proteins traverse the membrane, others are either anchored into the inner leaflet by covalently attached fatty acid chains or are recruited through non-covalent interactions with membrane proteins.
The membrane bound protein complexes are critical for the transmission of signals across the plasma membrane. Beneath the lipid bilayer, Spectrin tetramers, arranged into a hexagonal network, are anchored by membrane proteins. This network forms the membrane skeleton that contributes to membrane stability, and membrane protein distribution.
The cytoskeleton is comprised of networks of filamentous proteins that are responsible for the special organization of cytosolic components. Inside microvilli, actin filaments form tight parallel bundles which are stabilized by cross-linking proteins. While deeper in the cytosol, the actin network adopts a gel like structure stabilized by a variety of actin-binding proteins.
Filaments, capped at their minus ends by a protein complex, grow away from the plasma membrane by the addition of actin monomers to their plus end. The actin network is a very dynamic structure, with continuous directional polymerization and dis-assembly.
Severing proteins induce kinks in the filament and lead to the formation of short fragments that rapidly depolymerize, or give rise to new filaments. The cytoskeleton includes a network of microtubules created by the lateral association of proto-filaments formed by the polymerization of tubulin dimers.
While the plus ends of some microtubules extend towards the plasma membrane, proteins stabilize the curved confirmation of proto-filaments from other microtubules causing their rapid plus end depolymerisation. Microtubules provide tracks along which membrane-bound vesicles travel to and from the plasma membrane.
The directional movement of these cargo vesicles is due to a family of motor proteins linking vesicles and microtubules. Membrane-bound organelles like mitochondria are loosely trapped by the cytoskeleton. Mitochondria change shape continuously and their orientation is partly dictated by their interaction with microtubules.
All the microtubules originate from the centrosome, a discreet fibrous structure containing 2 orthogonal centrioles and located near the cell nucleus. Pores in the nuclear envelope allow the import of particles containing mRNA and proteins into the cytosol.
Here, free ribosomes translate the mRNA molecules into proteins. Some of these proteins will reside in the cytosol while others will associate with specialized cytosolic proteins and be imported into mitochondria or other organelles. The synthesis of cell secreted and integral membrane proteins is initiated by free ribosomes, which then dock to protein translocators at the surface of the endoplasmic reticulum.
Nascent proteins pass through an aqueous pore in the translocator. Cell secreted proteins accumulate in the lumen of the endoplasmic reticulum, while integral membrane proteins become embedded in the endoplasmic reticulum membrane.
Proteins are transported from the endoplasmic reticulum to the Golgi apparatus by vesicles traveling along the microtubules. Protein glycosylation, initiated in the endoplasmic reticulum, is completed inside the lumen of the Golgi apparatus. Fully glycosylated proteins are transported from the Golgi apparatus to the plasma membrane.
When a vesicle fuses with the plasma membrane, proteins contained in the vesicles lumen are secreted, and proteins imbedded in the vesicles membrane defuse in the cell membrane.
At sites of inflammation, Chemokine secreted by endothelial cells, bind to the extracellular domains of G-protein coupled membrane receptors. This binding causes a conformational change in the cytosolic portion of the receptor, and the consequent activation the sub unit of the G-protein.
The activation of the G-proteins sub unit triggers a cascade of protein activation, which in turn leads to the activation and clustering of integrins inside lipid rafts. A dramatic conformational change occurs in the extracellular domain of the activated integrins. This now allows for their interaction with I-Cam proteins displayed at the surface of the endothelial cells.
These strong interactions immobilize the rolling leukocyte at the site of inflammation. Additional signaling events cause a profound reorganization of the cytoskeleton, resulting in the spreading of one edge of the leukocyte. The leading edge of the leukocyte inserts itself between endothelial cells and the leukocyte migrates through the blood vessel wall into the inflamed tissue.
Rolling, activation, adhesion, and trans-endothelial migration are the four steps of the process called leukocyte extravasation.
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