So for these next series of posts I’m going to attempt to explain what’s happening in the video. I should warn you I’m not exactly an expert in inflammation, so if you disagree with an explanation please make yourself known in the comments. I’m going to use time references from this version of the video, and I’ll split the series into four posts corresponding to the four steps the narrator of the explanation video mentioned: Rolling, Adhesion, Activation, and Transendothelial Migration.
First up is Rolling, but before we get to that, let’s talk about what this video is about. Although parts of the video are showing general features and processes of any cell, the story here focuses on a process known as leukocyte extravasation. What does that mean? Well a leukocyte is a white blood cell, essentially the foot soldier of your immune system. Extravasation refers to fluid leaking out of a container. Leukocyte extravasation is literally white blood cells leaking out of your blood vessels so that they can reach damaged tissue. Essentially your circulatory system is a highway, allowing your white blood cells to travel around your whole body. Extravasation is analagous to taking an exit, which is necessarry when you need your immune cells to reach damaged tissue. This whole video is essentially about the mechanics of taking an exit.
While most of the video does talk about taking an exit, first we have to see how this leukocyte car drives along the circulatory system. This is the rolling step. Let’s go frame-by-frame:
0:10-0:14: We’re inside a blood vessel (essentially standing in the middle of the highway). Note that we’re not actually inside the cell yet. A bunch of red blood cells are rushing past us at a fast rate, driven by blood flow. A few white blood cells creep along the side of the vessel.
0:14-0:16: Now we zoom onto one of those white blood cells and look at the interface between it and the endothelial cell that makes up the blood vessel. Notice all the bumps on the cell surfaces – those will be key in understanding why the cells “stick” to one another.
0:17-0:22: An even closer look at this interface. We’ve zoomed in on the surfaces and can now see individual proteins (those bumps we saw before) making contacts. The camera pans back to let us see that the surfaces of both cells are littered with many of these proteins each making contacts.
Here’s what the explanation video has to say about this portion:
While red blood cells are carried away at high velocity by strong blood flow , leukocytes roll slowly on endothelial cells. P-selectins on endothelial cells interact with PSGL1 a glycoprotein on leukocyte microvillae. 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 first sentence should be self explanatory as long as you remember white blood cell = leukocyte and blood vessel cell = endothelial cell. So what exactly is a P-selectin and what’s PSGL1? They’re both proteins, P-selectin (yellow chain) is found on the surface of the endothelial cell and PSGL1 (purple chain) is on the surface of the leukocyte. These two proteins have a certain stickiness with each other (think opposite parts of a velcro band) that lets the leukocyte adhere to the vessel wall. The blood flow then gently pushes the white blood cell along the surface of the vessel causing the PSGL1 molecules to disconnect and then reconnect to downstream P-selectins. This also provides a possible reason why the red blood cells don’t also stick to the walls. Assuming they don’t express PSGL1 (something I don’t know for certain but seems likely), they don’t have any stickiness to the blood vessel.
Ok back to the frame-by-frame:
0:23-0:28: Now we’re looking at an individual cell – we’re probably supposed to be in the white blood cell but the endothelial cell would have the same basic foundation. That foundation is the cell membrane which is a lipid bilayer. Lipids are a class of molecules which stack together to form the membrane which is vizualized here as a sea on which things are floating. The rafts floating on this sea are actually called “lipid rafts” (although some biologists don’t believe these things exist, or that there function is overstated) and they’re ferrying around important receptors and such. Here’s the explanation video:
The outer leaflet of the lipid bilayer is enriched in sphingolipids and phosphatidylcholine. Sphingolipid-rich rafts raised above the rest of 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.
Pretty much what I said above. Sphingolipids and phosphatidylcholine are both types of lipids. The membrane varies in rigidity and flexibility by the presence of cholesterol (unevenly distributed throughout the membrane) and some of these lipids have hydrocarbon tails that are not saturated (and therefore more flexible).
0:29-0:36: Ok now back to our story which is an inflammation story. Our endothelial cell is at an inflammation site so it produces a signal to say “Hey there’s inflammation over here”. That’s visualized by the orange and green structures at the top. These proteins from the endothelial cell recognize a receptor on the leukocyte (purple thing on the bottom). Activating the purple receptor triggers a whole set of pathways within the leukocyte to start the inflammation response (so we leave the cell-cell interface and finally go within the leukocyte). The explanation video says this:
At sites of inflammation, secreted chemokines, 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 heparin sulfate proteoglycan is just another protein (made by the endothelial cell) that with the the chemokine activates the surface receptor on the leukocyte (the 7 in 7-transmembrane refers to the number of loops in the receptor). Activating a receptor in this manner triggers a signaling cascade within the leukocyte. But that’ll be covered in the activation post. Next up is the adhesion post – we’ve touched on the basics here, but in the next post we’ll look within the cell and see how it’s organized and ultimately how we’re going to have to change that organization to get the leukocyte out of the circulatory system.
Update 5/25/09: Part II is up!
Tags: Inner Life of the Cell, rolling
April 19, 2009 at 10:39 am
How does every machine inside the cell know what to do?
May 24, 2009 at 10:00 am
I think the word “know” is strong. I’d argue that every machine doesn’t really know what to do. The ability of macromolecules to form higher order complexes is highly dependent on their structure, localization, and concentration (amongst other things). Figuring out what interactions are important, how they are mediated, and their regulation are all important questions of modern biological research.