WEBVTT 00:00:17.220 --> 00:00:18.940 ADAM MARTIN: So I guess I will start by, 00:00:18.940 --> 00:00:21.540 first of all, congratulating you all. 00:00:21.540 --> 00:00:23.640 Since the last time I've seen you, 00:00:23.640 --> 00:00:28.060 you've all regenerated your intestine. 00:00:28.060 --> 00:00:31.170 So it's like you're a brand new person. 00:00:36.600 --> 00:00:38.610 So there's that. 00:00:38.610 --> 00:00:41.910 I want to start the lecture by talking about something 00:00:41.910 --> 00:00:45.040 I've been putting off telling you about, 00:00:45.040 --> 00:00:48.090 which is the Nobel Prize in physiology or medicine 00:00:48.090 --> 00:00:50.460 that was awarded this year. 00:00:50.460 --> 00:00:55.830 And it was awarded to James Allison and Tasuku Honjo 00:00:55.830 --> 00:01:01.140 for their discovery of a way to harness the immune system 00:01:01.140 --> 00:01:03.720 to fight cancer. 00:01:03.720 --> 00:01:06.900 And we're going to talk about the immune system 00:01:06.900 --> 00:01:12.120 later in the course, and it turns out, 00:01:12.120 --> 00:01:13.230 I will lecture on it. 00:01:13.230 --> 00:01:15.480 And I've actually had immunology, 00:01:15.480 --> 00:01:19.140 and in fact, I had immunology with James Allison. 00:01:19.140 --> 00:01:22.260 So I will do my best to channel James Allison when 00:01:22.260 --> 00:01:24.390 I talk about the immune system. 00:01:24.390 --> 00:01:26.850 But what they won the prize for is, 00:01:26.850 --> 00:01:29.520 they figured out a way to essentially release 00:01:29.520 --> 00:01:32.280 the brake on the immune system in order 00:01:32.280 --> 00:01:36.870 to allow the immune system to better fight cancer. 00:01:36.870 --> 00:01:40.110 And this is a technique that's been used in the clinic, 00:01:40.110 --> 00:01:43.800 and there are currently a number of clinical trials 00:01:43.800 --> 00:01:46.860 that are also looking to see whether this 00:01:46.860 --> 00:01:51.100 can be used in a variety of different cancer types. 00:01:51.100 --> 00:01:51.720 OK. 00:01:51.720 --> 00:01:53.370 So I just wanted to point that out now 00:01:53.370 --> 00:01:55.170 because we're talking about cancer, 00:01:55.170 --> 00:01:56.940 and I'll go more into the mechanism 00:01:56.940 --> 00:01:59.220 as how this works when we start talking 00:01:59.220 --> 00:02:02.830 about the immune system. 00:02:02.830 --> 00:02:06.240 So as I mentioned in the last lecture, 00:02:06.240 --> 00:02:09.240 cancer is basically a progressive loss 00:02:09.240 --> 00:02:11.730 of tissue organization. 00:02:11.730 --> 00:02:18.360 And luckily for us, our body has a number 00:02:18.360 --> 00:02:22.680 of different barriers to cells becoming cancerous and forming 00:02:22.680 --> 00:02:23.670 a tumor. 00:02:23.670 --> 00:02:25.290 And so I wanted to start out just 00:02:25.290 --> 00:02:29.490 by having you guys tell me what you feel the barriers would 00:02:29.490 --> 00:02:31.950 be to this process. 00:02:31.950 --> 00:02:35.010 So who has an idea of a barrier? 00:02:35.010 --> 00:02:39.150 What are the barriers in place by your body 00:02:39.150 --> 00:02:43.080 to prevent cancer cells from arising and forming a tumor, 00:02:43.080 --> 00:02:47.100 and for this process to form a malignant tumor? 00:02:53.230 --> 00:02:54.450 What's that? 00:02:54.450 --> 00:02:55.810 Oh, Rachel, sorry. 00:02:55.810 --> 00:02:57.190 STUDENT: Apoptosis. 00:02:57.190 --> 00:03:01.630 ADAM MARTIN: Apoptosis is a good one, right. 00:03:01.630 --> 00:03:06.940 So there's a careful process in your body 00:03:06.940 --> 00:03:12.280 to limit how long certain cells are resident in the body. 00:03:12.280 --> 00:03:15.340 And all of this depends on communication 00:03:15.340 --> 00:03:20.620 between different cell types in your body-- 00:03:20.620 --> 00:03:23.890 so barriers to tumor agenesis. 00:03:27.970 --> 00:03:31.090 And one type of signal is a survival signal. 00:03:31.090 --> 00:03:33.850 And if a cell is not getting a survival signal, 00:03:33.850 --> 00:03:36.910 then the cell will undergo apoptosis, 00:03:36.910 --> 00:03:40.090 which is what Rachel was referring to. 00:03:40.090 --> 00:03:44.950 So one barrier is that there is a highly regulated 00:03:44.950 --> 00:03:48.340 system of determining whether or not cells divide 00:03:48.340 --> 00:03:50.810 and also whether or not cells live. 00:03:50.810 --> 00:03:51.310 OK. 00:03:51.310 --> 00:03:59.410 So there are growth survival signals, 00:03:59.410 --> 00:04:03.790 which means that the decision for a cell to go off its rocker 00:04:03.790 --> 00:04:06.520 and just start dividing uncontrollably 00:04:06.520 --> 00:04:10.120 is not likely for a normal cell. 00:04:10.120 --> 00:04:14.500 Something has to happen to that cell in order to perturb it. 00:04:14.500 --> 00:04:19.420 So what are some other barriers to forming a tumor 00:04:19.420 --> 00:04:22.930 and for that tumor to become invasive? 00:04:22.930 --> 00:04:23.670 Yes, Jeremy? 00:04:23.670 --> 00:04:27.010 STUDENT: You just mentioned it, but the immune system. 00:04:27.010 --> 00:04:27.760 ADAM MARTIN: Yeah. 00:04:27.760 --> 00:04:33.290 So this cancer cell can't activate the immune system. 00:04:33.290 --> 00:04:38.157 So you could have some type of immunosuppression 00:04:38.157 --> 00:04:38.740 of the cancer. 00:04:41.780 --> 00:04:44.080 And I won't talk about that today, 00:04:44.080 --> 00:04:46.630 but maybe we'll come back and talk about it when we 00:04:46.630 --> 00:04:49.340 talk about the immune system. 00:04:49.340 --> 00:04:49.840 OK. 00:04:49.840 --> 00:04:55.210 Looking at this diagram, when the tumor proceeds 00:04:55.210 --> 00:04:57.310 to invasive cancer, what do you think 00:04:57.310 --> 00:04:59.984 are some barriers to that process happening? 00:05:06.150 --> 00:05:10.590 What are cells normally doing in an organ? 00:05:10.590 --> 00:05:13.530 The cells that line your intestine, 00:05:13.530 --> 00:05:16.360 are they invading into the surrounding tissue? 00:05:16.360 --> 00:05:16.860 No. 00:05:16.860 --> 00:05:19.140 Jeremy is shaking his head no, right? 00:05:19.140 --> 00:05:24.060 So normally, cells don't do that, or certain types of cells 00:05:24.060 --> 00:05:26.550 do, but normally, if you have cells 00:05:26.550 --> 00:05:28.440 that are the lining of an epithelium, 00:05:28.440 --> 00:05:32.460 they're not going off to a blood vessel. 00:05:32.460 --> 00:05:33.480 OK. 00:05:33.480 --> 00:05:40.050 So I'm just going to review the structure of the tissue, 00:05:40.050 --> 00:05:42.990 and then we'll talk about some of the barriers that 00:05:42.990 --> 00:05:45.880 prevent this from happening. 00:05:45.880 --> 00:05:50.130 So I'm drawing a tubular organ here. 00:05:50.130 --> 00:05:53.970 This could be the tube of the intestine. 00:05:53.970 --> 00:05:56.280 There'd be a lumen inside here. 00:05:56.280 --> 00:05:58.770 It could be some ductal structure in an organ, 00:05:58.770 --> 00:06:00.570 like a mammary gland. 00:06:00.570 --> 00:06:03.360 It could be that this is the airway of a lung, 00:06:03.360 --> 00:06:07.260 and there's an epithelial lining around that airway. 00:06:07.260 --> 00:06:13.350 So these cells here are forming an epithelial lining. 00:06:13.350 --> 00:06:17.340 And one important aspect of epithelial biology 00:06:17.340 --> 00:06:22.200 is, immature epithelia, they have a floor 00:06:22.200 --> 00:06:26.550 that the cells stand on. 00:06:26.550 --> 00:06:31.470 So this structure that I just drew around these cells 00:06:31.470 --> 00:06:34.020 in green is known as the basement membrane. 00:06:38.270 --> 00:06:41.940 It's the basement because it's the floor that this other thing 00:06:41.940 --> 00:06:43.890 is built on. 00:06:43.890 --> 00:06:46.020 And this basement membrane is made out 00:06:46.020 --> 00:06:49.920 of what is known as the extracellular matrix, which 00:06:49.920 --> 00:06:53.010 I'll just call matrix right now, and the extracellular matrix 00:06:53.010 --> 00:06:55.710 are basically extracellular proteins 00:06:55.710 --> 00:07:01.050 that form a mesh work that forms a rigid structure on which 00:07:01.050 --> 00:07:02.850 epithelial cells can sit on. 00:07:06.020 --> 00:07:11.380 OK, now if we think about what surrounds this organ, 00:07:11.380 --> 00:07:19.395 there's also matrix proteins in the intervening areas. 00:07:22.780 --> 00:07:24.455 And this is known as connective tissue. 00:07:27.920 --> 00:07:30.800 And so if we think about the cells that 00:07:30.800 --> 00:07:34.880 are the epithelial cells here, they are in one state, 00:07:34.880 --> 00:07:38.660 and this state is known as epithelial, as I just said. 00:07:38.660 --> 00:07:41.540 So these are epithelial cells. 00:07:41.540 --> 00:07:45.440 Epithelial cells have a few properties 00:07:45.440 --> 00:07:47.810 that are really distinguishing. 00:07:47.810 --> 00:07:51.200 The first is that they have high intercellular adhesion. 00:07:58.910 --> 00:08:01.370 And the next is that, if you consider 00:08:01.370 --> 00:08:04.520 their relative mobility, they can obviously 00:08:04.520 --> 00:08:07.250 move within the epithelium because we talked 00:08:07.250 --> 00:08:08.930 about the intestine and how cells 00:08:08.930 --> 00:08:12.440 are moving from the base of the crypts up to the villus. 00:08:12.440 --> 00:08:15.050 But in general, these cells aren't moving in and out 00:08:15.050 --> 00:08:17.060 of the organ. 00:08:17.060 --> 00:08:21.260 So I would say that they have a low migratory potential. 00:08:29.420 --> 00:08:32.750 And so these are the epithelial cells that are here. 00:08:32.750 --> 00:08:38.350 Now there are also cells that are in this interstitial space, 00:08:38.350 --> 00:08:41.030 and I'll draw-- they often have this highly elongated 00:08:41.030 --> 00:08:42.770 morphology. 00:08:42.770 --> 00:08:47.570 And these cells are in a fundamentally different state. 00:08:47.570 --> 00:08:55.220 And this state is known as mesenchymal or stromal. 00:08:58.010 --> 00:09:02.960 So this area outside the organ is also known as the stroma. 00:09:02.960 --> 00:09:07.490 So they're known as stromal cells in this location. 00:09:07.490 --> 00:09:11.030 And these cells have very different properties 00:09:11.030 --> 00:09:17.990 than the epithelial cells in that they have low adhesion-- 00:09:17.990 --> 00:09:20.615 I should say low cell-to-cell adhesion. 00:09:25.220 --> 00:09:28.580 And in contrast to these epithelial cells, 00:09:28.580 --> 00:09:32.330 these stromal cells, as you see in this example up here-- 00:09:32.330 --> 00:09:34.290 that's a human neutrophil-- 00:09:34.290 --> 00:09:37.400 these stromal cells are highly migratory. 00:09:41.360 --> 00:09:46.520 So that neutrophil is chasing a bacterium, 00:09:46.520 --> 00:09:49.160 and they'll eventually get it, but the movie will loop, 00:09:49.160 --> 00:09:53.300 and so you'll see it constantly chasing that bacterium. 00:09:53.300 --> 00:09:56.840 So some examples of stromal cells 00:09:56.840 --> 00:09:59.940 would be immune cells, like this human neutrophil. 00:10:03.840 --> 00:10:05.690 And you probably know that immune cells 00:10:05.690 --> 00:10:09.590 have to traverse different organ systems. 00:10:09.590 --> 00:10:12.710 They have to be rapidly recruited to sites of infection 00:10:12.710 --> 00:10:14.390 or sites of injury. 00:10:14.390 --> 00:10:18.320 And so this type of cell is fundamentally 00:10:18.320 --> 00:10:21.710 different from the types of cells that line your organs, 00:10:21.710 --> 00:10:24.200 where you basically need those cells to stay put. 00:10:27.050 --> 00:10:30.320 When considering cancer, you have these different types 00:10:30.320 --> 00:10:31.850 of cells in your body. 00:10:31.850 --> 00:10:36.515 90% of cancer comes from a cell of epithelial origin. 00:10:39.200 --> 00:10:46.700 So 90% of cancers are epithelial in origin. 00:10:53.570 --> 00:10:56.180 Some other examples of stromal cells 00:10:56.180 --> 00:11:00.380 are cells like fibroblasts. 00:11:00.380 --> 00:11:04.250 And fibroblasts are cells that secrete and remodel 00:11:04.250 --> 00:11:07.160 the matrix that's in connective tissue. 00:11:07.160 --> 00:11:10.100 And they're also important for wound healing and secreting 00:11:10.100 --> 00:11:13.270 matrix during the wound-healing process. 00:11:13.270 --> 00:11:15.020 So that's just to give you a few examples. 00:11:20.630 --> 00:11:21.130 OK. 00:11:21.130 --> 00:11:27.040 So now let's come back to this example and talk about what 00:11:27.040 --> 00:11:33.280 happens to get a cell to go all the way from being 00:11:33.280 --> 00:11:38.680 a normal epithelial cell to having this type of behavior, 00:11:38.680 --> 00:11:41.170 where initially you have growth and a loss 00:11:41.170 --> 00:11:44.650 of the control over growth and survival signaling, 00:11:44.650 --> 00:11:47.410 and eventually, the cancer can become 00:11:47.410 --> 00:11:48.760 what's known as malignant? 00:11:52.690 --> 00:11:55.690 So it's a progression, and initially, you might just 00:11:55.690 --> 00:12:00.040 get increased in-cell division and abnormal cell shape, which 00:12:00.040 --> 00:12:03.190 is known as dysplasia, and at that point, 00:12:03.190 --> 00:12:07.150 it's known as something like an adenoma or something that's 00:12:07.150 --> 00:12:09.910 benign. 00:12:09.910 --> 00:12:14.260 But it's this last point here, up here, 00:12:14.260 --> 00:12:18.730 where the cells breach this basement membrane. 00:12:18.730 --> 00:12:22.300 When that happens, then the cancer is known as carcinoma, 00:12:22.300 --> 00:12:23.530 and it's malignant. 00:12:28.000 --> 00:12:40.630 So malignant cancer is cancer that has breached the basement 00:12:40.630 --> 00:12:41.130 membrane. 00:12:58.990 --> 00:13:03.780 So I want to take you through the progression of tumor 00:13:03.780 --> 00:13:05.410 genesis. 00:13:05.410 --> 00:13:09.460 And first, I'm going to start with the breakdown 00:13:09.460 --> 00:13:11.530 of this growth survival signaling. 00:13:14.280 --> 00:13:17.440 And I just wanted to remind you about what 00:13:17.440 --> 00:13:22.220 we talked about with the intestine as our model organ. 00:13:22.220 --> 00:13:25.210 And in the intestine, remember that this is all 00:13:25.210 --> 00:13:33.500 regulated by signals between stem cells and niche cells. 00:13:33.500 --> 00:13:36.580 So the niche cells are sending the stem cells 00:13:36.580 --> 00:13:40.480 signals like Wnts that control their self renewal, 00:13:40.480 --> 00:13:44.860 and then it's the loss of this signaling that 00:13:44.860 --> 00:13:48.550 allows these cells to eventually undergo apoptosis and get 00:13:48.550 --> 00:13:51.610 shed into the lumen of the intestine. 00:13:51.610 --> 00:13:58.180 So one of the first steps in cancer 00:13:58.180 --> 00:14:11.641 is this breakdown in growth survival signaling. 00:14:14.590 --> 00:14:20.170 So this first breakdown is going to enable the cells to overcome 00:14:20.170 --> 00:14:20.980 this first barrier. 00:14:25.050 --> 00:14:29.250 And so as I just pointed out, remember, 00:14:29.250 --> 00:14:31.350 it helps to think of what has to happen 00:14:31.350 --> 00:14:33.960 in a normal tissue or organ. 00:14:33.960 --> 00:14:40.170 Normally, the decision whether or not a cell divides or dies-- 00:14:40.170 --> 00:14:46.860 so cell division and also death-- 00:14:46.860 --> 00:14:50.190 is highly regulated. 00:14:50.190 --> 00:14:52.830 And it's regulated by communication with other cells. 00:14:59.380 --> 00:15:02.025 So in cancer, this regulation goes awry. 00:15:05.380 --> 00:15:07.290 And how would this regulation go awry? 00:15:10.938 --> 00:15:13.062 Yeah, Jeremy? 00:15:13.062 --> 00:15:16.340 STUDENT: Loss of function in a tumor suppressor 00:15:16.340 --> 00:15:20.019 or an over-activation of the oncogene. 00:15:20.019 --> 00:15:20.960 ADAM MARTIN: Mm-hmm. 00:15:20.960 --> 00:15:24.300 So Jeremy suggested that there could be mutations, 00:15:24.300 --> 00:15:27.030 like oncogenic mutations or tumor suppressor 00:15:27.030 --> 00:15:31.960 loss, that lead to abnormal cell division and death. 00:15:31.960 --> 00:15:34.710 And that is exactly right. 00:15:34.710 --> 00:15:40.890 So you could have oncogenic mutations, 00:15:40.890 --> 00:15:45.720 and these oncogenic mutations, they often hyperactivate 00:15:45.720 --> 00:15:50.070 or constitutively activate these growth-signaling pathways that 00:15:50.070 --> 00:15:53.190 are normally downstream of growth factors and receptor 00:15:53.190 --> 00:15:55.590 tyrosine kinases. 00:15:55.590 --> 00:15:59.160 And if you hyperactivate those, you 00:15:59.160 --> 00:16:03.420 reduce the dependency of the cells on these signals. 00:16:03.420 --> 00:16:08.550 So these oncogenic mutations can reduce 00:16:08.550 --> 00:16:15.070 the dependency of cells on growth factors and growth 00:16:15.070 --> 00:16:15.570 signals. 00:16:21.180 --> 00:16:23.370 OK. 00:16:23.370 --> 00:16:29.700 But one important point is that this is not enough. 00:16:29.700 --> 00:16:33.570 And you can think about this in the case of the intestine 00:16:33.570 --> 00:16:37.800 because if you had an oncogenic mutation in one of these cells 00:16:37.800 --> 00:16:40.650 here, it might not be as dependent 00:16:40.650 --> 00:16:43.290 on signals for growth. 00:16:43.290 --> 00:16:45.450 But it doesn't matter because even though you 00:16:45.450 --> 00:16:47.400 had that mutation, it's going to die, 00:16:47.400 --> 00:16:50.790 and it's going to shed out of the tissue. 00:16:50.790 --> 00:16:54.480 And actually what happens in many cases 00:16:54.480 --> 00:16:58.050 where you activate a signaling pathway that 00:16:58.050 --> 00:17:01.290 allows the cell to, in an abnormal way, 00:17:01.290 --> 00:17:03.600 go through the cell cycle, you actually 00:17:03.600 --> 00:17:07.589 induce a failsafe mechanism at the cell which causes the cell 00:17:07.589 --> 00:17:11.069 to undergo apoptosis. 00:17:11.069 --> 00:17:14.040 So often, you get this step happening, 00:17:14.040 --> 00:17:16.410 and the cells just undergo apoptosis 00:17:16.410 --> 00:17:20.010 because you've evolved to protect your organs 00:17:20.010 --> 00:17:22.619 from this type of mutation. 00:17:22.619 --> 00:17:26.520 So it's the oncogenic mutation, in collaboration 00:17:26.520 --> 00:17:36.300 with loss of tumor suppression, and one of the main tumor 00:17:36.300 --> 00:17:41.010 suppressive mechanisms our body has is apoptosis, 00:17:41.010 --> 00:17:43.230 where if a cell is doing something abnormal, 00:17:43.230 --> 00:17:44.415 the cells simply dies. 00:17:48.100 --> 00:17:51.150 So loss of the tumor suppressor could 00:17:51.150 --> 00:17:55.200 be loss of a gene that promotes apoptosis, and in that case, 00:17:55.200 --> 00:17:57.840 if the cell loses that mechanism, 00:17:57.840 --> 00:18:00.270 the cell will avoid apoptosis. 00:18:08.050 --> 00:18:08.550 OK. 00:18:08.550 --> 00:18:14.190 So this is oncogenic mutations and the loss 00:18:14.190 --> 00:18:18.300 of tumor suppressors which subverts 00:18:18.300 --> 00:18:22.170 the normal communication between cells which is required 00:18:22.170 --> 00:18:24.480 for normal tissue homeostasis. 00:18:24.480 --> 00:18:26.670 OK. 00:18:26.670 --> 00:18:30.780 Now another example of this growth and survival signaling 00:18:30.780 --> 00:18:35.760 is not one which involves, necessarily, 00:18:35.760 --> 00:18:40.230 a genetic change, but one that involves changes in expression. 00:18:40.230 --> 00:18:43.620 And it also involves interaction between the tumor 00:18:43.620 --> 00:18:47.130 and the surrounding cells of the body. 00:18:47.130 --> 00:18:49.740 So I'm going to tell you a little bit about tumor 00:18:49.740 --> 00:18:50.940 microenvironment. 00:18:57.490 --> 00:19:00.630 And this is something that's important to consider 00:19:00.630 --> 00:19:06.330 in cancer because a tumor in a body, it's not in isolation. 00:19:06.330 --> 00:19:11.190 It's surrounded by other cells in your body and other things 00:19:11.190 --> 00:19:16.200 in your body, like matrix, and so here is 00:19:16.200 --> 00:19:19.650 a picture showing you a tumor, and the tumor 00:19:19.650 --> 00:19:23.130 is stained with this membrane protein which is shown here 00:19:23.130 --> 00:19:24.360 in the rust color. 00:19:24.360 --> 00:19:26.250 So that's the tumor. 00:19:26.250 --> 00:19:32.520 And what's also stained in this piece of tissue is the DNA. 00:19:32.520 --> 00:19:34.480 It's stained in blue. 00:19:34.480 --> 00:19:38.520 And so you see the nuclei of the tumor cells in there. 00:19:38.520 --> 00:19:42.810 But you see all these blue nuclei surrounding the tumor. 00:19:42.810 --> 00:19:48.270 And these are stromal cells that are around the tumor. 00:19:48.270 --> 00:19:51.210 And they've actually been recruited to the tumor 00:19:51.210 --> 00:19:54.730 by the cancer cells. 00:19:54.730 --> 00:19:58.100 So what I mean by tumor microenvironment 00:19:58.100 --> 00:20:01.320 is just the region around the tumor. 00:20:01.320 --> 00:20:04.650 And so if you have cancer, and you have a tumor, 00:20:04.650 --> 00:20:08.220 the tumor cells can actually secrete signals 00:20:08.220 --> 00:20:12.090 which recruit stromal cells. 00:20:12.090 --> 00:20:18.000 So there can be an interaction where 00:20:18.000 --> 00:20:22.590 the tumor sends recruitment signals that causes 00:20:22.590 --> 00:20:24.270 stromal cells to come by. 00:20:27.570 --> 00:20:32.550 And what the stromal cells can do, what they do for the tumor 00:20:32.550 --> 00:20:35.130 and why this is beneficial for the tumor, 00:20:35.130 --> 00:20:39.510 is that stromal cells can secrete growth signals 00:20:39.510 --> 00:20:44.880 or survival signals that promote the growth of the tumor. 00:20:44.880 --> 00:20:51.570 So there can be a reciprocal interaction here where 00:20:51.570 --> 00:20:57.810 you get an abnormal conversation between cancer 00:20:57.810 --> 00:21:00.690 cells and the surrounding stromal cells. 00:21:00.690 --> 00:21:02.580 But again, this is very similar, if you 00:21:02.580 --> 00:21:06.330 think about it, to the way a normal organ works. 00:21:06.330 --> 00:21:10.290 You often have these signals going between cells. 00:21:10.290 --> 00:21:15.510 And that is involved in normal tissue homeostasis. 00:21:15.510 --> 00:21:17.700 What's happening here, though, is abnormal 00:21:17.700 --> 00:21:20.250 and that the cancer cells are constitutively 00:21:20.250 --> 00:21:22.110 recruiting these stromal cells just 00:21:22.110 --> 00:21:26.940 to get this growth signal that they're addicted to. 00:21:26.940 --> 00:21:30.570 And because you have just the presence 00:21:30.570 --> 00:21:34.680 of such a loop suggests that these cancer cells, 00:21:34.680 --> 00:21:37.305 even if they have oncogenic mutations 00:21:37.305 --> 00:21:40.290 and have lost tumor suppressors, they 00:21:40.290 --> 00:21:43.770 are not totally independent of growth signals. 00:21:43.770 --> 00:21:49.020 They still, to some extent, rely on growth signals. 00:21:49.020 --> 00:21:53.730 So these oncogenic mutations, they reduce the dependency 00:21:53.730 --> 00:21:58.530 on growth signals, but this dependency, 00:21:58.530 --> 00:22:08.850 the dependency on growth signals, is not eliminated. 00:22:18.120 --> 00:22:23.880 And one experiment that showed this was an experiment that 00:22:23.880 --> 00:22:29.040 was done back in the 1950s where patients with a type of skin 00:22:29.040 --> 00:22:33.600 cancer, basal cell carcinoma, were taken, 00:22:33.600 --> 00:22:35.910 and they had their tumors excised 00:22:35.910 --> 00:22:37.950 from one part of their body. 00:22:37.950 --> 00:22:41.730 And then the tumor was grafted back onto them, 00:22:41.730 --> 00:22:45.360 to another part of their body, distant from the original site 00:22:45.360 --> 00:22:47.010 of the tumor. 00:22:47.010 --> 00:22:50.820 And this grafting experiment was done either 00:22:50.820 --> 00:22:53.490 with the stromal cells that surrounded 00:22:53.490 --> 00:22:56.640 the tumor or the stromal cells surrounding the tumor 00:22:56.640 --> 00:22:58.830 were removed, and just the tumor cells 00:22:58.830 --> 00:23:01.830 were grafted back onto the body. 00:23:01.830 --> 00:23:02.700 OK. 00:23:02.700 --> 00:23:04.500 Now this experiment probably would not 00:23:04.500 --> 00:23:09.120 be allowed today, but back in the '50s, I guess it was legal. 00:23:09.120 --> 00:23:12.240 And so the experimental result is 00:23:12.240 --> 00:23:15.300 that if you graft the tumor cells with the surrounding 00:23:15.300 --> 00:23:20.730 stroma, the tumor was able to establish a second tumor, 00:23:20.730 --> 00:23:25.080 or the tumor cells survived this grafting procedure, 00:23:25.080 --> 00:23:29.910 whereas tumor cells that were taken without the stroma 00:23:29.910 --> 00:23:34.140 underwent cell death when they were put in a new location. 00:23:34.140 --> 00:23:38.550 So there's something about this specialized microenvironment 00:23:38.550 --> 00:23:42.090 that the tumor creates for itself that is enabling 00:23:42.090 --> 00:23:45.450 the tumor to grow and survive. 00:23:45.450 --> 00:23:49.440 And the model is that it's because these stromal cells are 00:23:49.440 --> 00:23:52.980 secreting growth factors that these cancer cells are still 00:23:52.980 --> 00:23:53.940 dependent on. 00:23:58.710 --> 00:24:02.700 So this dependency on growth signals is important, 00:24:02.700 --> 00:24:08.100 clinically, because some types of cancer, 00:24:08.100 --> 00:24:11.160 there is an elevation of growth factor receptors 00:24:11.160 --> 00:24:13.650 that are associated with the cancer. 00:24:13.650 --> 00:24:20.130 And the famous example of that is, in 30% of breast cancers, 00:24:20.130 --> 00:24:23.980 there is a growth factor receptor, the HER2 receptor, 00:24:23.980 --> 00:24:28.980 which is over expressed in the cancer cells. 00:24:28.980 --> 00:24:33.750 So I just drew a receptor here, and I'm now talking 00:24:33.750 --> 00:24:35.700 about the HER2 receptor. 00:24:35.700 --> 00:24:43.890 HER2 stands for human epidermal growth factor, EGF, receptor 2. 00:24:47.550 --> 00:24:52.050 So this is a receptor tyrosine kinase, 00:24:52.050 --> 00:24:54.900 which is a transmembrane protein receptor. 00:24:54.900 --> 00:24:57.930 It's expressed on the surface of the cell. 00:24:57.930 --> 00:25:04.800 And so about 30% of human breast cancers 00:25:04.800 --> 00:25:16.860 are HER2 positive, which means that the cancer cells are over 00:25:16.860 --> 00:25:20.940 expressing this receptor tyrosine kinase. 00:25:20.940 --> 00:25:25.350 The fact that you have these cancer cells over expressing 00:25:25.350 --> 00:25:28.230 a growth factor receptor suggests 00:25:28.230 --> 00:25:32.550 that the cancer requires some type 00:25:32.550 --> 00:25:39.660 of growth stimulus in order for the cancer to be growing. 00:25:39.660 --> 00:25:43.890 And the fact that this was discovered, 00:25:43.890 --> 00:25:48.270 that 30% of breast cancers over express HER2, 00:25:48.270 --> 00:25:51.510 has been used by researchers to develop 00:25:51.510 --> 00:25:56.100 a treatment for this type of cancer, this HER2 positive. 00:25:56.100 --> 00:26:01.170 The treatment is known as Herceptin. 00:26:01.170 --> 00:26:04.110 Possibly some of you have heard of this. 00:26:04.110 --> 00:26:07.150 It was developed at Genentech. 00:26:07.150 --> 00:26:12.180 And what Herceptin is, it's an antibody 00:26:12.180 --> 00:26:16.980 that was raised against a human HER2 protein. 00:26:16.980 --> 00:26:17.790 OK. 00:26:17.790 --> 00:26:20.430 So Herceptin is an antibody. 00:26:23.110 --> 00:26:29.080 It recognizes HER2 on the surface of these cancer cells, 00:26:29.080 --> 00:26:33.150 and you can treat patients with this antibody, 00:26:33.150 --> 00:26:37.080 and an antibody binds to the HER2 positive cells. 00:26:37.080 --> 00:26:40.200 And it either blocks the function of HER2 00:26:40.200 --> 00:26:44.070 or recruits immune cells to kill those cells off. 00:26:44.070 --> 00:26:48.010 The exact mechanism, I don't believe, is known. 00:26:48.010 --> 00:26:50.310 But what is known is that Herceptin has really 00:26:50.310 --> 00:26:56.290 changed how we're able to treat HER2 positive breast cancers. 00:26:56.290 --> 00:26:59.490 And it's been a huge success story 00:26:59.490 --> 00:27:00.915 in the fight against cancer. 00:27:09.260 --> 00:27:09.780 All right. 00:27:09.780 --> 00:27:18.040 So we've talked about this first barrier, the barrier to tumor 00:27:18.040 --> 00:27:21.310 cells becoming at least semi-independent 00:27:21.310 --> 00:27:25.060 of these growth factors. 00:27:25.060 --> 00:27:27.880 And so now I want to talk about other barriers 00:27:27.880 --> 00:27:29.960 to tumor genesis. 00:27:29.960 --> 00:27:33.190 And if you consider an epithelial cell here, 00:27:33.190 --> 00:27:35.620 even if these cells are able to grow, 00:27:35.620 --> 00:27:38.500 they won't be able to leave the organ 00:27:38.500 --> 00:27:40.930 until something else happens. 00:27:40.930 --> 00:27:43.570 And one thing that would need to happen for these cells 00:27:43.570 --> 00:27:47.650 to become malignant and to leave the organ that they were 00:27:47.650 --> 00:27:52.570 initially part of is for there to be a breakdown 00:27:52.570 --> 00:27:55.540 in the adhesion between cells. 00:27:55.540 --> 00:27:59.260 So for the next few minutes, I want 00:27:59.260 --> 00:28:03.850 to talk about the breakdown in cell-to-cell adhesion. 00:28:12.280 --> 00:28:17.080 So how would a cancer cell basically 00:28:17.080 --> 00:28:19.510 unstick itself from the cells that 00:28:19.510 --> 00:28:25.150 surround it in order to leave an organ and go somewhere else? 00:28:27.670 --> 00:28:30.520 And for that, to explain that, I have to remind you 00:28:30.520 --> 00:28:35.210 about some of the normal biology of these epithelial cells, 00:28:35.210 --> 00:28:37.180 which we talked about earlier, which 00:28:37.180 --> 00:28:41.200 is that they express these transmembrane proteins 00:28:41.200 --> 00:28:43.570 that are adhesion proteins. 00:28:43.570 --> 00:28:47.230 So normally, epithelial cells have adhesion proteins. 00:28:50.500 --> 00:28:52.690 And the famous one for epithelia, 00:28:52.690 --> 00:28:55.750 or one of the famous ones, is called 00:28:55.750 --> 00:28:58.490 epithelial or E-cadherin. 00:29:03.380 --> 00:29:07.000 And E-cadherin is a transmembrane protein. 00:29:07.000 --> 00:29:10.420 It has an extracellular domain, but rather 00:29:10.420 --> 00:29:13.210 than that extracellular domain binding 00:29:13.210 --> 00:29:17.220 to some secreted ligand, this extracellular domain 00:29:17.220 --> 00:29:21.190 recognizes E-cadherin molecules on other cells, 00:29:21.190 --> 00:29:22.750 and they stick to each other. 00:29:22.750 --> 00:29:26.230 And it essentially functions like cellular Velcro. 00:29:26.230 --> 00:29:30.340 So the cells link together, and they stick to each other such 00:29:30.340 --> 00:29:31.885 that they form a coherent tissue. 00:29:35.960 --> 00:29:43.150 So the way that cancer cells subvert this mechanism 00:29:43.150 --> 00:29:46.480 of adhesion is, they have to do something 00:29:46.480 --> 00:29:49.810 that inhibits E-cadherin. 00:29:49.810 --> 00:29:54.730 And so cancer cells, what they can do 00:29:54.730 --> 00:29:59.380 is to decrease the cell-to-cell adhesion, 00:29:59.380 --> 00:30:01.270 and I'll tell you how in just a minute. 00:30:04.750 --> 00:30:08.290 And in addition to decreasing this cell-to-cell adhesion, 00:30:08.290 --> 00:30:15.220 they can also promote the genes that are involved in motility. 00:30:23.850 --> 00:30:27.510 So we have to understand, then, how 00:30:27.510 --> 00:30:31.620 it is a cancer cell would do both of these things. 00:30:31.620 --> 00:30:35.220 And I'll start with cell-to-cell adhesion. 00:30:35.220 --> 00:30:39.330 And in contrast to what we've talked about with the growth 00:30:39.330 --> 00:30:43.110 and survival signaling, where you have mutations that happen 00:30:43.110 --> 00:30:45.090 in cancer cells, and you basically 00:30:45.090 --> 00:30:49.450 have an irreversible change to the genome of the cell, 00:30:49.450 --> 00:30:56.280 this change in the adhesive properties of the cell, 00:30:56.280 --> 00:30:58.260 and essentially their cell state-- 00:30:58.260 --> 00:31:01.710 because what you see here, if a cancer cell is 00:31:01.710 --> 00:31:05.790 having less adhesion and getting more migratory, 00:31:05.790 --> 00:31:09.930 it's switching from an epithelial type of state 00:31:09.930 --> 00:31:13.080 to a mesenchymal type of state. 00:31:13.080 --> 00:31:23.250 And what this is called is an epithelial-to-mesenchymal 00:31:23.250 --> 00:31:36.080 transition, and that is EMT, for short. 00:31:40.030 --> 00:31:43.210 And this EMT is not a genetic change. 00:31:43.210 --> 00:31:45.940 It's not caused by a genetic change, 00:31:45.940 --> 00:31:50.230 but it appears that EMT results from changes 00:31:50.230 --> 00:31:51.800 in gene regulation. 00:31:51.800 --> 00:31:59.590 So you can think of it as more of an epigenetic change, 00:31:59.590 --> 00:32:07.460 an epigenetic change in gene expression. 00:32:07.460 --> 00:32:11.350 So there's a change in gene expression 00:32:11.350 --> 00:32:13.540 such that this can reverse later on. 00:32:21.730 --> 00:32:25.640 And there are master regulators of this process, 00:32:25.640 --> 00:32:27.560 which alter gene expression. 00:32:27.560 --> 00:32:32.810 And so the type of gene that alters gene expression, 00:32:32.810 --> 00:32:35.365 these are often called transcription factors. 00:32:43.030 --> 00:32:46.150 And there are several transcription factors 00:32:46.150 --> 00:32:49.165 that are master regulators of this EMT process. 00:32:51.670 --> 00:32:57.145 And their names are Twist, Snail-- 00:33:00.640 --> 00:33:03.530 another one's called Slug. 00:33:03.530 --> 00:33:04.630 There's an Escargot. 00:33:04.630 --> 00:33:07.330 You can see this got a little out of hand. 00:33:07.330 --> 00:33:11.140 And you can tell by the names that they probably 00:33:11.140 --> 00:33:13.330 were not discovered in humans. 00:33:13.330 --> 00:33:16.390 And in fact, these genes were discovered 00:33:16.390 --> 00:33:19.600 in the same genetic screen that I outlined before, 00:33:19.600 --> 00:33:21.470 where Hedgehog was discovered. 00:33:21.470 --> 00:33:21.970 OK. 00:33:21.970 --> 00:33:25.750 So it was a genetic screen in the flies, 00:33:25.750 --> 00:33:30.100 and these are genes that affect the embryonic development 00:33:30.100 --> 00:33:32.260 of the fly. 00:33:32.260 --> 00:33:37.610 And I'll show you this view of the fly embryo. 00:33:37.610 --> 00:33:39.760 So this is an embryo here, and you see, 00:33:39.760 --> 00:33:43.690 it's an epithelial sheet surrounding the yolk. 00:33:43.690 --> 00:33:47.230 And what happens is, in early stages of development, 00:33:47.230 --> 00:33:51.100 a population of cells express the Twist gene. 00:33:51.100 --> 00:33:54.130 So that's Twist in the dark here. 00:33:54.130 --> 00:33:58.720 And this Twist gene causes these cells to basically invade 00:33:58.720 --> 00:34:01.480 into the middle of the embryo. 00:34:01.480 --> 00:34:03.430 So here are the Twist cells now, going 00:34:03.430 --> 00:34:06.400 into the inside of the embryo. 00:34:06.400 --> 00:34:08.530 And then these cells undergo EMT, 00:34:08.530 --> 00:34:12.489 and they start to migrate around inside the embryo. 00:34:12.489 --> 00:34:14.770 What this is doing for the fly is 00:34:14.770 --> 00:34:17.139 that these are the cells that are going on 00:34:17.139 --> 00:34:19.889 to form the muscle. 00:34:19.889 --> 00:34:21.395 And if you look at your neighbor, 00:34:21.395 --> 00:34:23.020 you might notice that their muscles are 00:34:23.020 --> 00:34:25.210 on the inside of their body. 00:34:25.210 --> 00:34:31.480 So this process is basically putting these cells, 00:34:31.480 --> 00:34:33.880 by getting them to move, it's putting the cells 00:34:33.880 --> 00:34:35.889 in the right place for what they are 00:34:35.889 --> 00:34:39.460 going to differentiate into during embryonic development. 00:34:42.370 --> 00:34:46.659 But this process, if activated during cancer, 00:34:46.659 --> 00:34:51.310 can allow these cells to be more mobile and to leave one organ 00:34:51.310 --> 00:34:54.010 and go into another organ. 00:34:54.010 --> 00:34:58.030 So cancer isn't inventing anything new here. 00:34:58.030 --> 00:35:01.930 It's corrupting a normal program that cells have and need 00:35:01.930 --> 00:35:04.750 during development, and it's inactivating it 00:35:04.750 --> 00:35:07.870 at an inappropriate time. 00:35:07.870 --> 00:35:10.750 So these are all transcription factors. 00:35:10.750 --> 00:35:14.410 And what they do is, they repress 00:35:14.410 --> 00:35:18.880 the expression or function of cadherin 00:35:18.880 --> 00:35:23.290 such that the cells are no longer as sticky. 00:35:23.290 --> 00:35:25.990 So basically, when these transcription factors 00:35:25.990 --> 00:35:28.120 get turned on, it makes the cells 00:35:28.120 --> 00:35:30.470 less sticky to each other. 00:35:30.470 --> 00:35:32.890 And it also promotes their migration. 00:35:32.890 --> 00:35:35.710 So it turns on genes that are important for migration. 00:35:39.400 --> 00:35:44.200 This process of EMT, as I mentioned, is reversible, 00:35:44.200 --> 00:35:47.410 and it also involves interactions between tumor 00:35:47.410 --> 00:35:49.820 and stromal cells. 00:35:49.820 --> 00:35:52.600 I'll show you an example of-- 00:35:52.600 --> 00:35:57.010 so in the fly embryo, one thing that's nice about the flies 00:35:57.010 --> 00:36:00.790 is, you can actually watch this happen live. 00:36:00.790 --> 00:36:04.150 And so I'm going to show you a movie now, showing you 00:36:04.150 --> 00:36:06.352 the invasion process. 00:36:06.352 --> 00:36:07.810 We're going to have an embryo here. 00:36:13.460 --> 00:36:15.190 And what I'm going to show you is, 00:36:15.190 --> 00:36:18.980 we're looking at the cells that are expressing Twist and Snail, 00:36:18.980 --> 00:36:21.820 and they're on one side of the embryo. 00:36:21.820 --> 00:36:25.360 And I'm going to show you a section of the embryo that 00:36:25.360 --> 00:36:28.480 is analogous to this. 00:36:28.480 --> 00:36:31.360 So when the cells move inside, they're 00:36:31.360 --> 00:36:33.370 going to disappear in this movie. 00:36:33.370 --> 00:36:36.700 So when you see the crease in the embryo, 00:36:36.700 --> 00:36:38.170 that's when the cells are invading 00:36:38.170 --> 00:36:40.540 into the middle of the embryo. 00:36:40.540 --> 00:36:44.110 And so the cell outlines are in magenta here. 00:36:44.110 --> 00:36:48.970 And what's labeled in green is a type of motor protein 00:36:48.970 --> 00:36:52.690 that's involved in the mobility of cells and, in this case, 00:36:52.690 --> 00:36:54.430 is involved in these cells moving 00:36:54.430 --> 00:36:57.800 into the interior of the embryo. 00:36:57.800 --> 00:37:00.040 So here, you see the motor protein appearing, 00:37:00.040 --> 00:37:02.997 and that's when these cells are going to dive into the embryo. 00:37:02.997 --> 00:37:04.330 So it's almost like a waterfall. 00:37:04.330 --> 00:37:08.560 These cells are moving into the inside of the embryo 00:37:08.560 --> 00:37:12.850 through this process of invagination and, subsequently, 00:37:12.850 --> 00:37:14.500 EMT. 00:37:14.500 --> 00:37:15.817 There it goes again. 00:37:15.817 --> 00:37:17.650 You can see the cells are now on the inside. 00:37:20.260 --> 00:37:23.170 So this process of EMT also involves 00:37:23.170 --> 00:37:27.910 interactions between the cancer cells and the stroma. 00:37:27.910 --> 00:37:31.323 And that's illustrated here because what you can see, 00:37:31.323 --> 00:37:32.740 what's being labeled here, you can 00:37:32.740 --> 00:37:37.090 see the nuclei of the cells, but these cells 00:37:37.090 --> 00:37:41.140 at the edge of the tumor are up-regulating this gene, which 00:37:41.140 --> 00:37:45.100 is an alpha-beta integrin, which I'll mention in just a minute. 00:37:45.100 --> 00:37:47.110 And this is getting up-regulated right where 00:37:47.110 --> 00:37:49.360 the tumor contacts the stroma. 00:37:49.360 --> 00:37:51.430 And this gene, alpha-beta integrin, 00:37:51.430 --> 00:37:56.200 is a gene that's associated with motility and EMT. 00:37:56.200 --> 00:38:00.720 So on that slide here, one gene involved in motility 00:38:00.720 --> 00:38:02.820 is a class of genes called integrins. 00:38:06.250 --> 00:38:12.070 And so this also results from the signaling between the tumor 00:38:12.070 --> 00:38:14.390 and the surrounding cells. 00:38:14.390 --> 00:38:17.830 And again, this is not something that cancer created. 00:38:17.830 --> 00:38:20.750 This is what happens during wound healing. 00:38:20.750 --> 00:38:24.500 So during wound healing, if you injure yourself, 00:38:24.500 --> 00:38:27.550 the wound recruits immune cells, and the immune cells 00:38:27.550 --> 00:38:31.300 signal to the surrounding skin or epithelial cells 00:38:31.300 --> 00:38:36.010 to undergo EMT in order to close and fill in the wound. 00:38:36.010 --> 00:38:39.160 So this is a natural process that 00:38:39.160 --> 00:38:41.650 happens in your body that is simply getting 00:38:41.650 --> 00:38:43.690 corrupted by cancer cells. 00:38:47.620 --> 00:38:48.120 All right. 00:38:48.120 --> 00:38:51.990 So now let's talk about this last step of motility. 00:38:54.550 --> 00:39:01.690 So this is now how the cells would break out and move away. 00:39:09.280 --> 00:39:11.570 Let's come back to this movie here. 00:39:11.570 --> 00:39:14.950 There are a few things I want you to notice about this movie. 00:39:14.950 --> 00:39:19.690 The first is, holy shit, that cell is moving. 00:39:19.690 --> 00:39:26.890 And so the first aspect of this is, because the cell is moving, 00:39:26.890 --> 00:39:31.090 it suggests that there's some sort of force being generated. 00:39:31.090 --> 00:39:39.930 So the cell is generating force. 00:39:39.930 --> 00:39:43.100 And I'm not talking about like some type of mystical Jedi 00:39:43.100 --> 00:39:43.880 force. 00:39:43.880 --> 00:39:48.020 I'm talking about the mass times acceleration force, 00:39:48.020 --> 00:39:50.450 so like a physical force, OK? 00:39:50.450 --> 00:39:53.840 The second thing I want to point out about this movie 00:39:53.840 --> 00:39:57.500 is that you see that bacterium, and it's moving around, 00:39:57.500 --> 00:40:00.590 this cell is able to follow that bacterium. 00:40:00.590 --> 00:40:03.650 So the cell, in the force generation process, 00:40:03.650 --> 00:40:06.500 is very responsive to the signals 00:40:06.500 --> 00:40:08.750 that that cell is getting. 00:40:08.750 --> 00:40:12.560 So the cell is generating force, and this force 00:40:12.560 --> 00:40:14.255 is responsive to signals. 00:40:22.790 --> 00:40:24.860 So for the remaining part of the lecture, 00:40:24.860 --> 00:40:26.510 I basically just want to tell you 00:40:26.510 --> 00:40:30.590 about how it is that the cell generates the force required 00:40:30.590 --> 00:40:34.250 to move itself around. 00:40:34.250 --> 00:40:38.580 And it involves a particular type of protein. 00:40:38.580 --> 00:40:40.520 It's a component of the cytoskeleton, 00:40:40.520 --> 00:40:42.650 and we've talked about microtubules 00:40:42.650 --> 00:40:45.020 and the microtubule cytoskeleton and its role 00:40:45.020 --> 00:40:47.060 in segregating the chromosome. 00:40:47.060 --> 00:40:51.470 But there are other cytoskeletal systems that the cell has, 00:40:51.470 --> 00:40:55.280 and one is called the actin cytoskeleton, which 00:40:55.280 --> 00:40:58.940 is shown on the side above. 00:40:58.940 --> 00:41:02.600 And actin, the actin cytoskeleton, 00:41:02.600 --> 00:41:06.030 is a system that is a biopolymer in the cell. 00:41:06.030 --> 00:41:11.870 So this is a biopolymer, like microtubules. 00:41:11.870 --> 00:41:14.130 And actin is a gene. 00:41:14.130 --> 00:41:18.450 And the actin gene encodes for a protein, the actin protein, 00:41:18.450 --> 00:41:21.610 and when the actin protein is made in the cell, 00:41:21.610 --> 00:41:26.780 it starts out as being just a single globular protein. 00:41:26.780 --> 00:41:29.960 And when the actin is in this state, 00:41:29.960 --> 00:41:35.000 it is called globular or G-actin. 00:41:38.510 --> 00:41:41.870 But these subunits, these proteins, 00:41:41.870 --> 00:41:45.410 can come together and form a polymer. 00:41:45.410 --> 00:41:49.640 So they can go from being individual, isolated proteins 00:41:49.640 --> 00:41:53.465 to forming a polymer that forms a long skinny filament. 00:41:59.990 --> 00:42:05.120 And this form of the protein that's forming a biopolymer 00:42:05.120 --> 00:42:08.210 is known as filamentous or F-actin. 00:42:18.130 --> 00:42:20.740 And these are long, skinny filaments. 00:42:20.740 --> 00:42:23.620 They can be hundreds of nanometers, 00:42:23.620 --> 00:42:28.600 even microns, in length, so hundreds of nanometers long. 00:42:28.600 --> 00:42:32.050 And the filament width is about 10 nanometers. 00:42:32.050 --> 00:42:34.540 So I'm just trying to give you a sense of dimensions, 00:42:34.540 --> 00:42:40.120 that this is a long, skinny filament that the cell can 00:42:40.120 --> 00:42:40.930 assemble. 00:42:40.930 --> 00:42:44.260 And it assembles into these dense meshworks, which you 00:42:44.260 --> 00:42:47.230 can see in the slide up here. 00:42:47.230 --> 00:42:49.340 So this is the leading edge of the cell. 00:42:49.340 --> 00:42:51.640 So the cell would be migrating this way. 00:42:51.640 --> 00:42:57.520 And what you see in this cell is this densely branched network, 00:42:57.520 --> 00:43:00.220 and these are all actin filaments. 00:43:00.220 --> 00:43:05.350 So you get this huge dense forest of actin filaments 00:43:05.350 --> 00:43:08.635 that's right at the edge of the cell that is moving forward. 00:43:11.650 --> 00:43:15.370 One thing I want to point out is, like all things in biology, 00:43:15.370 --> 00:43:17.990 this is not a one-way street. 00:43:17.990 --> 00:43:21.740 And so these biopolymers are very dynamic, 00:43:21.740 --> 00:43:24.640 meaning they can undergo assembly and disassembly, 00:43:24.640 --> 00:43:28.960 and they can do so on the timescale of seconds. 00:43:28.960 --> 00:43:32.410 One last thing I want to point out about the actin filament 00:43:32.410 --> 00:43:36.280 is, there's a polarity to the filament such 00:43:36.280 --> 00:43:39.190 that there's an end, known as the plus end, 00:43:39.190 --> 00:43:41.320 where growth is favored, and there's 00:43:41.320 --> 00:43:43.840 an end, known as the minus end, where there's 00:43:43.840 --> 00:43:45.760 often de-polymerization. 00:43:45.760 --> 00:43:49.720 So you can get a directional growth of the filament. 00:43:49.720 --> 00:43:51.130 So this is where growth happens. 00:43:55.370 --> 00:43:58.180 And so in this network that you're looking at, 00:43:58.180 --> 00:44:00.190 the way the actin is oriented-- 00:44:00.190 --> 00:44:03.520 so I'm going to draw just a cell here-- 00:44:03.520 --> 00:44:11.830 you have this dense meshwork of actin, and in this meshwork, 00:44:11.830 --> 00:44:14.590 there's a polarity to the way the actin filaments are 00:44:14.590 --> 00:44:15.740 oriented. 00:44:15.740 --> 00:44:18.340 So the cell is migrating this way, 00:44:18.340 --> 00:44:21.100 and the plus ends of the actin filaments 00:44:21.100 --> 00:44:23.740 are facing out right at the surface. 00:44:23.740 --> 00:44:27.310 And the minus ends are back here. 00:44:27.310 --> 00:44:30.460 And so what you have, when the cell is migrating, 00:44:30.460 --> 00:44:34.390 is you have this biopolymer network that's 00:44:34.390 --> 00:44:38.020 growing on this end, but shrinking on the other end. 00:44:38.020 --> 00:44:42.550 And that allows the cell to generate a constant protrusive 00:44:42.550 --> 00:44:44.290 force. 00:44:44.290 --> 00:44:47.500 So it's the growth of actin filaments, 00:44:47.500 --> 00:44:55.620 the growth of F-actin, which generates a protrusive force. 00:45:04.340 --> 00:45:08.270 Now if we consider the whole process of cell migration, what 00:45:08.270 --> 00:45:10.640 you can see is that initially, you're 00:45:10.640 --> 00:45:14.090 going to push the cell membrane forward. 00:45:14.090 --> 00:45:16.040 So you get a protrusion. 00:45:16.040 --> 00:45:17.690 It pushes it forward. 00:45:17.690 --> 00:45:22.730 That's often called a lamellipodium, or a pseudopod, 00:45:22.730 --> 00:45:26.220 and so this part of the cell moves forward. 00:45:26.220 --> 00:45:30.170 But in order for the cell to have a net motion forward, 00:45:30.170 --> 00:45:32.990 it then has to stabilize that protrusion. 00:45:32.990 --> 00:45:37.498 And it stabilizes the protrusion by adhering to the substrate. 00:45:37.498 --> 00:45:39.290 So right now, we're just considering a cell 00:45:39.290 --> 00:45:43.340 moving on a flat substrate. 00:45:43.340 --> 00:45:46.250 And so the way it attaches to the substrate 00:45:46.250 --> 00:45:48.155 is through cell matrix adhesion. 00:45:51.170 --> 00:45:53.930 And this cell matrix adhesion is mediated 00:45:53.930 --> 00:45:57.740 by another type of adhesion receptor known as an integrin. 00:46:03.860 --> 00:46:07.070 So the cell pushes forward, anchors itself 00:46:07.070 --> 00:46:10.520 on the substrate, pulls its body along, and then just 00:46:10.520 --> 00:46:13.980 repeats that cycle over and over again. 00:46:13.980 --> 00:46:21.560 So the best way I can illustrate this is, if you think about it, 00:46:21.560 --> 00:46:22.940 you get your protrusion. 00:46:22.940 --> 00:46:24.650 You get your elbow out. 00:46:24.650 --> 00:46:25.580 You put down. 00:46:25.580 --> 00:46:27.480 You anchor. 00:46:27.480 --> 00:46:29.870 And it's just a repeated cycle, and it's 00:46:29.870 --> 00:46:33.360 able to migrate across the substrate. 00:46:33.360 --> 00:46:37.010 So it's kind of like a frontal toe mechanism. 00:46:37.010 --> 00:46:39.350 You have cycles of protrusion. 00:46:39.350 --> 00:46:41.780 You generate traction, and then you 00:46:41.780 --> 00:46:44.260 pull, in order to translocate. 00:46:44.260 --> 00:46:49.220 And so that's how cells are migrating intuitively. 00:46:49.220 --> 00:46:54.050 This is just showing you that cells can pull on stuff. 00:46:54.050 --> 00:46:56.780 So not all cells migrate in this way. 00:46:56.780 --> 00:46:58.940 I just wanted to say that. 00:46:58.940 --> 00:47:02.270 So 3D cells have other mechanisms 00:47:02.270 --> 00:47:07.100 to migrate that de-emphasize this traction mechanism 00:47:07.100 --> 00:47:09.290 that cells have. 00:47:09.290 --> 00:47:11.780 So you can get rid of all the integrins 00:47:11.780 --> 00:47:15.680 that a cell has, and they're still able to migrate. 00:47:15.680 --> 00:47:21.060 And that's because 2D emphasizes the role of adhesion, 00:47:21.060 --> 00:47:23.880 but if you can find the cell, then the cells 00:47:23.880 --> 00:47:25.630 are able to migrate. 00:47:25.630 --> 00:47:29.880 So what's shown here are cells that are not confined. 00:47:29.880 --> 00:47:33.150 This is a confined cell in a micropipette. 00:47:33.150 --> 00:47:35.520 And you can see, it's the same type of cell, 00:47:35.520 --> 00:47:38.200 but the cell can migrate in confinement, 00:47:38.200 --> 00:47:42.630 but it can't migrate outside of confinement. 00:47:42.630 --> 00:47:44.700 So in this sense, the cell is migrating 00:47:44.700 --> 00:47:46.410 through a different mode. 00:47:46.410 --> 00:47:49.110 And you can think of it as the cell 00:47:49.110 --> 00:47:51.960 is doing some type of chimney-ing maneuver. 00:47:51.960 --> 00:47:54.990 So it's able to get in a confined environment, push out 00:47:54.990 --> 00:47:57.900 against its surroundings, and that's how the cell then 00:47:57.900 --> 00:48:01.770 is able to generate traction in the absence of an integrin 00:48:01.770 --> 00:48:04.080 molecule. 00:48:04.080 --> 00:48:04.590 All right. 00:48:04.590 --> 00:48:05.640 Great. 00:48:05.640 --> 00:48:10.070 So we are set for now.