1 00:00:17,220 --> 00:00:18,940 ADAM MARTIN: So I guess I will start by, 2 00:00:18,940 --> 00:00:21,540 first of all, congratulating you all. 3 00:00:21,540 --> 00:00:23,640 Since the last time I've seen you, 4 00:00:23,640 --> 00:00:28,060 you've all regenerated your intestine. 5 00:00:28,060 --> 00:00:31,170 So it's like you're a brand new person. 6 00:00:36,600 --> 00:00:38,610 So there's that. 7 00:00:38,610 --> 00:00:41,910 I want to start the lecture by talking about something 8 00:00:41,910 --> 00:00:45,040 I've been putting off telling you about, 9 00:00:45,040 --> 00:00:48,090 which is the Nobel Prize in physiology or medicine 10 00:00:48,090 --> 00:00:50,460 that was awarded this year. 11 00:00:50,460 --> 00:00:55,830 And it was awarded to James Allison and Tasuku Honjo 12 00:00:55,830 --> 00:01:01,140 for their discovery of a way to harness the immune system 13 00:01:01,140 --> 00:01:03,720 to fight cancer. 14 00:01:03,720 --> 00:01:06,900 And we're going to talk about the immune system 15 00:01:06,900 --> 00:01:12,120 later in the course, and it turns out, 16 00:01:12,120 --> 00:01:13,230 I will lecture on it. 17 00:01:13,230 --> 00:01:15,480 And I've actually had immunology, 18 00:01:15,480 --> 00:01:19,140 and in fact, I had immunology with James Allison. 19 00:01:19,140 --> 00:01:22,260 So I will do my best to channel James Allison when 20 00:01:22,260 --> 00:01:24,390 I talk about the immune system. 21 00:01:24,390 --> 00:01:26,850 But what they won the prize for is, 22 00:01:26,850 --> 00:01:29,520 they figured out a way to essentially release 23 00:01:29,520 --> 00:01:32,280 the brake on the immune system in order 24 00:01:32,280 --> 00:01:36,870 to allow the immune system to better fight cancer. 25 00:01:36,870 --> 00:01:40,110 And this is a technique that's been used in the clinic, 26 00:01:40,110 --> 00:01:43,800 and there are currently a number of clinical trials 27 00:01:43,800 --> 00:01:46,860 that are also looking to see whether this 28 00:01:46,860 --> 00:01:51,100 can be used in a variety of different cancer types. 29 00:01:51,100 --> 00:01:51,720 OK. 30 00:01:51,720 --> 00:01:53,370 So I just wanted to point that out now 31 00:01:53,370 --> 00:01:55,170 because we're talking about cancer, 32 00:01:55,170 --> 00:01:56,940 and I'll go more into the mechanism 33 00:01:56,940 --> 00:01:59,220 as how this works when we start talking 34 00:01:59,220 --> 00:02:02,830 about the immune system. 35 00:02:02,830 --> 00:02:06,240 So as I mentioned in the last lecture, 36 00:02:06,240 --> 00:02:09,240 cancer is basically a progressive loss 37 00:02:09,240 --> 00:02:11,730 of tissue organization. 38 00:02:11,730 --> 00:02:18,360 And luckily for us, our body has a number 39 00:02:18,360 --> 00:02:22,680 of different barriers to cells becoming cancerous and forming 40 00:02:22,680 --> 00:02:23,670 a tumor. 41 00:02:23,670 --> 00:02:25,290 And so I wanted to start out just 42 00:02:25,290 --> 00:02:29,490 by having you guys tell me what you feel the barriers would 43 00:02:29,490 --> 00:02:31,950 be to this process. 44 00:02:31,950 --> 00:02:35,010 So who has an idea of a barrier? 45 00:02:35,010 --> 00:02:39,150 What are the barriers in place by your body 46 00:02:39,150 --> 00:02:43,080 to prevent cancer cells from arising and forming a tumor, 47 00:02:43,080 --> 00:02:47,100 and for this process to form a malignant tumor? 48 00:02:53,230 --> 00:02:54,450 What's that? 49 00:02:54,450 --> 00:02:55,810 Oh, Rachel, sorry. 50 00:02:55,810 --> 00:02:57,190 STUDENT: Apoptosis. 51 00:02:57,190 --> 00:03:01,630 ADAM MARTIN: Apoptosis is a good one, right. 52 00:03:01,630 --> 00:03:06,940 So there's a careful process in your body 53 00:03:06,940 --> 00:03:12,280 to limit how long certain cells are resident in the body. 54 00:03:12,280 --> 00:03:15,340 And all of this depends on communication 55 00:03:15,340 --> 00:03:20,620 between different cell types in your body-- 56 00:03:20,620 --> 00:03:23,890 so barriers to tumor agenesis. 57 00:03:27,970 --> 00:03:31,090 And one type of signal is a survival signal. 58 00:03:31,090 --> 00:03:33,850 And if a cell is not getting a survival signal, 59 00:03:33,850 --> 00:03:36,910 then the cell will undergo apoptosis, 60 00:03:36,910 --> 00:03:40,090 which is what Rachel was referring to. 61 00:03:40,090 --> 00:03:44,950 So one barrier is that there is a highly regulated 62 00:03:44,950 --> 00:03:48,340 system of determining whether or not cells divide 63 00:03:48,340 --> 00:03:50,810 and also whether or not cells live. 64 00:03:50,810 --> 00:03:51,310 OK. 65 00:03:51,310 --> 00:03:59,410 So there are growth survival signals, 66 00:03:59,410 --> 00:04:03,790 which means that the decision for a cell to go off its rocker 67 00:04:03,790 --> 00:04:06,520 and just start dividing uncontrollably 68 00:04:06,520 --> 00:04:10,120 is not likely for a normal cell. 69 00:04:10,120 --> 00:04:14,500 Something has to happen to that cell in order to perturb it. 70 00:04:14,500 --> 00:04:19,420 So what are some other barriers to forming a tumor 71 00:04:19,420 --> 00:04:22,930 and for that tumor to become invasive? 72 00:04:22,930 --> 00:04:23,670 Yes, Jeremy? 73 00:04:23,670 --> 00:04:27,010 STUDENT: You just mentioned it, but the immune system. 74 00:04:27,010 --> 00:04:27,760 ADAM MARTIN: Yeah. 75 00:04:27,760 --> 00:04:33,290 So this cancer cell can't activate the immune system. 76 00:04:33,290 --> 00:04:38,157 So you could have some type of immunosuppression 77 00:04:38,157 --> 00:04:38,740 of the cancer. 78 00:04:41,780 --> 00:04:44,080 And I won't talk about that today, 79 00:04:44,080 --> 00:04:46,630 but maybe we'll come back and talk about it when we 80 00:04:46,630 --> 00:04:49,340 talk about the immune system. 81 00:04:49,340 --> 00:04:49,840 OK. 82 00:04:49,840 --> 00:04:55,210 Looking at this diagram, when the tumor proceeds 83 00:04:55,210 --> 00:04:57,310 to invasive cancer, what do you think 84 00:04:57,310 --> 00:04:59,984 are some barriers to that process happening? 85 00:05:06,150 --> 00:05:10,590 What are cells normally doing in an organ? 86 00:05:10,590 --> 00:05:13,530 The cells that line your intestine, 87 00:05:13,530 --> 00:05:16,360 are they invading into the surrounding tissue? 88 00:05:16,360 --> 00:05:16,860 No. 89 00:05:16,860 --> 00:05:19,140 Jeremy is shaking his head no, right? 90 00:05:19,140 --> 00:05:24,060 So normally, cells don't do that, or certain types of cells 91 00:05:24,060 --> 00:05:26,550 do, but normally, if you have cells 92 00:05:26,550 --> 00:05:28,440 that are the lining of an epithelium, 93 00:05:28,440 --> 00:05:32,460 they're not going off to a blood vessel. 94 00:05:32,460 --> 00:05:33,480 OK. 95 00:05:33,480 --> 00:05:40,050 So I'm just going to review the structure of the tissue, 96 00:05:40,050 --> 00:05:42,990 and then we'll talk about some of the barriers that 97 00:05:42,990 --> 00:05:45,880 prevent this from happening. 98 00:05:45,880 --> 00:05:50,130 So I'm drawing a tubular organ here. 99 00:05:50,130 --> 00:05:53,970 This could be the tube of the intestine. 100 00:05:53,970 --> 00:05:56,280 There'd be a lumen inside here. 101 00:05:56,280 --> 00:05:58,770 It could be some ductal structure in an organ, 102 00:05:58,770 --> 00:06:00,570 like a mammary gland. 103 00:06:00,570 --> 00:06:03,360 It could be that this is the airway of a lung, 104 00:06:03,360 --> 00:06:07,260 and there's an epithelial lining around that airway. 105 00:06:07,260 --> 00:06:13,350 So these cells here are forming an epithelial lining. 106 00:06:13,350 --> 00:06:17,340 And one important aspect of epithelial biology 107 00:06:17,340 --> 00:06:22,200 is, immature epithelia, they have a floor 108 00:06:22,200 --> 00:06:26,550 that the cells stand on. 109 00:06:26,550 --> 00:06:31,470 So this structure that I just drew around these cells 110 00:06:31,470 --> 00:06:34,020 in green is known as the basement membrane. 111 00:06:38,270 --> 00:06:41,940 It's the basement because it's the floor that this other thing 112 00:06:41,940 --> 00:06:43,890 is built on. 113 00:06:43,890 --> 00:06:46,020 And this basement membrane is made out 114 00:06:46,020 --> 00:06:49,920 of what is known as the extracellular matrix, which 115 00:06:49,920 --> 00:06:53,010 I'll just call matrix right now, and the extracellular matrix 116 00:06:53,010 --> 00:06:55,710 are basically extracellular proteins 117 00:06:55,710 --> 00:07:01,050 that form a mesh work that forms a rigid structure on which 118 00:07:01,050 --> 00:07:02,850 epithelial cells can sit on. 119 00:07:06,020 --> 00:07:11,380 OK, now if we think about what surrounds this organ, 120 00:07:11,380 --> 00:07:19,395 there's also matrix proteins in the intervening areas. 121 00:07:22,780 --> 00:07:24,455 And this is known as connective tissue. 122 00:07:27,920 --> 00:07:30,800 And so if we think about the cells that 123 00:07:30,800 --> 00:07:34,880 are the epithelial cells here, they are in one state, 124 00:07:34,880 --> 00:07:38,660 and this state is known as epithelial, as I just said. 125 00:07:38,660 --> 00:07:41,540 So these are epithelial cells. 126 00:07:41,540 --> 00:07:45,440 Epithelial cells have a few properties 127 00:07:45,440 --> 00:07:47,810 that are really distinguishing. 128 00:07:47,810 --> 00:07:51,200 The first is that they have high intercellular adhesion. 129 00:07:58,910 --> 00:08:01,370 And the next is that, if you consider 130 00:08:01,370 --> 00:08:04,520 their relative mobility, they can obviously 131 00:08:04,520 --> 00:08:07,250 move within the epithelium because we talked 132 00:08:07,250 --> 00:08:08,930 about the intestine and how cells 133 00:08:08,930 --> 00:08:12,440 are moving from the base of the crypts up to the villus. 134 00:08:12,440 --> 00:08:15,050 But in general, these cells aren't moving in and out 135 00:08:15,050 --> 00:08:17,060 of the organ. 136 00:08:17,060 --> 00:08:21,260 So I would say that they have a low migratory potential. 137 00:08:29,420 --> 00:08:32,750 And so these are the epithelial cells that are here. 138 00:08:32,750 --> 00:08:38,350 Now there are also cells that are in this interstitial space, 139 00:08:38,350 --> 00:08:41,030 and I'll draw-- they often have this highly elongated 140 00:08:41,030 --> 00:08:42,770 morphology. 141 00:08:42,770 --> 00:08:47,570 And these cells are in a fundamentally different state. 142 00:08:47,570 --> 00:08:55,220 And this state is known as mesenchymal or stromal. 143 00:08:58,010 --> 00:09:02,960 So this area outside the organ is also known as the stroma. 144 00:09:02,960 --> 00:09:07,490 So they're known as stromal cells in this location. 145 00:09:07,490 --> 00:09:11,030 And these cells have very different properties 146 00:09:11,030 --> 00:09:17,990 than the epithelial cells in that they have low adhesion-- 147 00:09:17,990 --> 00:09:20,615 I should say low cell-to-cell adhesion. 148 00:09:25,220 --> 00:09:28,580 And in contrast to these epithelial cells, 149 00:09:28,580 --> 00:09:32,330 these stromal cells, as you see in this example up here-- 150 00:09:32,330 --> 00:09:34,290 that's a human neutrophil-- 151 00:09:34,290 --> 00:09:37,400 these stromal cells are highly migratory. 152 00:09:41,360 --> 00:09:46,520 So that neutrophil is chasing a bacterium, 153 00:09:46,520 --> 00:09:49,160 and they'll eventually get it, but the movie will loop, 154 00:09:49,160 --> 00:09:53,300 and so you'll see it constantly chasing that bacterium. 155 00:09:53,300 --> 00:09:56,840 So some examples of stromal cells 156 00:09:56,840 --> 00:09:59,940 would be immune cells, like this human neutrophil. 157 00:10:03,840 --> 00:10:05,690 And you probably know that immune cells 158 00:10:05,690 --> 00:10:09,590 have to traverse different organ systems. 159 00:10:09,590 --> 00:10:12,710 They have to be rapidly recruited to sites of infection 160 00:10:12,710 --> 00:10:14,390 or sites of injury. 161 00:10:14,390 --> 00:10:18,320 And so this type of cell is fundamentally 162 00:10:18,320 --> 00:10:21,710 different from the types of cells that line your organs, 163 00:10:21,710 --> 00:10:24,200 where you basically need those cells to stay put. 164 00:10:27,050 --> 00:10:30,320 When considering cancer, you have these different types 165 00:10:30,320 --> 00:10:31,850 of cells in your body. 166 00:10:31,850 --> 00:10:36,515 90% of cancer comes from a cell of epithelial origin. 167 00:10:39,200 --> 00:10:46,700 So 90% of cancers are epithelial in origin. 168 00:10:53,570 --> 00:10:56,180 Some other examples of stromal cells 169 00:10:56,180 --> 00:11:00,380 are cells like fibroblasts. 170 00:11:00,380 --> 00:11:04,250 And fibroblasts are cells that secrete and remodel 171 00:11:04,250 --> 00:11:07,160 the matrix that's in connective tissue. 172 00:11:07,160 --> 00:11:10,100 And they're also important for wound healing and secreting 173 00:11:10,100 --> 00:11:13,270 matrix during the wound-healing process. 174 00:11:13,270 --> 00:11:15,020 So that's just to give you a few examples. 175 00:11:20,630 --> 00:11:21,130 OK. 176 00:11:21,130 --> 00:11:27,040 So now let's come back to this example and talk about what 177 00:11:27,040 --> 00:11:33,280 happens to get a cell to go all the way from being 178 00:11:33,280 --> 00:11:38,680 a normal epithelial cell to having this type of behavior, 179 00:11:38,680 --> 00:11:41,170 where initially you have growth and a loss 180 00:11:41,170 --> 00:11:44,650 of the control over growth and survival signaling, 181 00:11:44,650 --> 00:11:47,410 and eventually, the cancer can become 182 00:11:47,410 --> 00:11:48,760 what's known as malignant? 183 00:11:52,690 --> 00:11:55,690 So it's a progression, and initially, you might just 184 00:11:55,690 --> 00:12:00,040 get increased in-cell division and abnormal cell shape, which 185 00:12:00,040 --> 00:12:03,190 is known as dysplasia, and at that point, 186 00:12:03,190 --> 00:12:07,150 it's known as something like an adenoma or something that's 187 00:12:07,150 --> 00:12:09,910 benign. 188 00:12:09,910 --> 00:12:14,260 But it's this last point here, up here, 189 00:12:14,260 --> 00:12:18,730 where the cells breach this basement membrane. 190 00:12:18,730 --> 00:12:22,300 When that happens, then the cancer is known as carcinoma, 191 00:12:22,300 --> 00:12:23,530 and it's malignant. 192 00:12:28,000 --> 00:12:40,630 So malignant cancer is cancer that has breached the basement 193 00:12:40,630 --> 00:12:41,130 membrane. 194 00:12:58,990 --> 00:13:03,780 So I want to take you through the progression of tumor 195 00:13:03,780 --> 00:13:05,410 genesis. 196 00:13:05,410 --> 00:13:09,460 And first, I'm going to start with the breakdown 197 00:13:09,460 --> 00:13:11,530 of this growth survival signaling. 198 00:13:14,280 --> 00:13:17,440 And I just wanted to remind you about what 199 00:13:17,440 --> 00:13:22,220 we talked about with the intestine as our model organ. 200 00:13:22,220 --> 00:13:25,210 And in the intestine, remember that this is all 201 00:13:25,210 --> 00:13:33,500 regulated by signals between stem cells and niche cells. 202 00:13:33,500 --> 00:13:36,580 So the niche cells are sending the stem cells 203 00:13:36,580 --> 00:13:40,480 signals like Wnts that control their self renewal, 204 00:13:40,480 --> 00:13:44,860 and then it's the loss of this signaling that 205 00:13:44,860 --> 00:13:48,550 allows these cells to eventually undergo apoptosis and get 206 00:13:48,550 --> 00:13:51,610 shed into the lumen of the intestine. 207 00:13:51,610 --> 00:13:58,180 So one of the first steps in cancer 208 00:13:58,180 --> 00:14:11,641 is this breakdown in growth survival signaling. 209 00:14:14,590 --> 00:14:20,170 So this first breakdown is going to enable the cells to overcome 210 00:14:20,170 --> 00:14:20,980 this first barrier. 211 00:14:25,050 --> 00:14:29,250 And so as I just pointed out, remember, 212 00:14:29,250 --> 00:14:31,350 it helps to think of what has to happen 213 00:14:31,350 --> 00:14:33,960 in a normal tissue or organ. 214 00:14:33,960 --> 00:14:40,170 Normally, the decision whether or not a cell divides or dies-- 215 00:14:40,170 --> 00:14:46,860 so cell division and also death-- 216 00:14:46,860 --> 00:14:50,190 is highly regulated. 217 00:14:50,190 --> 00:14:52,830 And it's regulated by communication with other cells. 218 00:14:59,380 --> 00:15:02,025 So in cancer, this regulation goes awry. 219 00:15:05,380 --> 00:15:07,290 And how would this regulation go awry? 220 00:15:10,938 --> 00:15:13,062 Yeah, Jeremy? 221 00:15:13,062 --> 00:15:16,340 STUDENT: Loss of function in a tumor suppressor 222 00:15:16,340 --> 00:15:20,019 or an over-activation of the oncogene. 223 00:15:20,019 --> 00:15:20,960 ADAM MARTIN: Mm-hmm. 224 00:15:20,960 --> 00:15:24,300 So Jeremy suggested that there could be mutations, 225 00:15:24,300 --> 00:15:27,030 like oncogenic mutations or tumor suppressor 226 00:15:27,030 --> 00:15:31,960 loss, that lead to abnormal cell division and death. 227 00:15:31,960 --> 00:15:34,710 And that is exactly right. 228 00:15:34,710 --> 00:15:40,890 So you could have oncogenic mutations, 229 00:15:40,890 --> 00:15:45,720 and these oncogenic mutations, they often hyperactivate 230 00:15:45,720 --> 00:15:50,070 or constitutively activate these growth-signaling pathways that 231 00:15:50,070 --> 00:15:53,190 are normally downstream of growth factors and receptor 232 00:15:53,190 --> 00:15:55,590 tyrosine kinases. 233 00:15:55,590 --> 00:15:59,160 And if you hyperactivate those, you 234 00:15:59,160 --> 00:16:03,420 reduce the dependency of the cells on these signals. 235 00:16:03,420 --> 00:16:08,550 So these oncogenic mutations can reduce 236 00:16:08,550 --> 00:16:15,070 the dependency of cells on growth factors and growth 237 00:16:15,070 --> 00:16:15,570 signals. 238 00:16:21,180 --> 00:16:23,370 OK. 239 00:16:23,370 --> 00:16:29,700 But one important point is that this is not enough. 240 00:16:29,700 --> 00:16:33,570 And you can think about this in the case of the intestine 241 00:16:33,570 --> 00:16:37,800 because if you had an oncogenic mutation in one of these cells 242 00:16:37,800 --> 00:16:40,650 here, it might not be as dependent 243 00:16:40,650 --> 00:16:43,290 on signals for growth. 244 00:16:43,290 --> 00:16:45,450 But it doesn't matter because even though you 245 00:16:45,450 --> 00:16:47,400 had that mutation, it's going to die, 246 00:16:47,400 --> 00:16:50,790 and it's going to shed out of the tissue. 247 00:16:50,790 --> 00:16:54,480 And actually what happens in many cases 248 00:16:54,480 --> 00:16:58,050 where you activate a signaling pathway that 249 00:16:58,050 --> 00:17:01,290 allows the cell to, in an abnormal way, 250 00:17:01,290 --> 00:17:03,600 go through the cell cycle, you actually 251 00:17:03,600 --> 00:17:07,589 induce a failsafe mechanism at the cell which causes the cell 252 00:17:07,589 --> 00:17:11,069 to undergo apoptosis. 253 00:17:11,069 --> 00:17:14,040 So often, you get this step happening, 254 00:17:14,040 --> 00:17:16,410 and the cells just undergo apoptosis 255 00:17:16,410 --> 00:17:20,010 because you've evolved to protect your organs 256 00:17:20,010 --> 00:17:22,619 from this type of mutation. 257 00:17:22,619 --> 00:17:26,520 So it's the oncogenic mutation, in collaboration 258 00:17:26,520 --> 00:17:36,300 with loss of tumor suppression, and one of the main tumor 259 00:17:36,300 --> 00:17:41,010 suppressive mechanisms our body has is apoptosis, 260 00:17:41,010 --> 00:17:43,230 where if a cell is doing something abnormal, 261 00:17:43,230 --> 00:17:44,415 the cells simply dies. 262 00:17:48,100 --> 00:17:51,150 So loss of the tumor suppressor could 263 00:17:51,150 --> 00:17:55,200 be loss of a gene that promotes apoptosis, and in that case, 264 00:17:55,200 --> 00:17:57,840 if the cell loses that mechanism, 265 00:17:57,840 --> 00:18:00,270 the cell will avoid apoptosis. 266 00:18:08,050 --> 00:18:08,550 OK. 267 00:18:08,550 --> 00:18:14,190 So this is oncogenic mutations and the loss 268 00:18:14,190 --> 00:18:18,300 of tumor suppressors which subverts 269 00:18:18,300 --> 00:18:22,170 the normal communication between cells which is required 270 00:18:22,170 --> 00:18:24,480 for normal tissue homeostasis. 271 00:18:24,480 --> 00:18:26,670 OK. 272 00:18:26,670 --> 00:18:30,780 Now another example of this growth and survival signaling 273 00:18:30,780 --> 00:18:35,760 is not one which involves, necessarily, 274 00:18:35,760 --> 00:18:40,230 a genetic change, but one that involves changes in expression. 275 00:18:40,230 --> 00:18:43,620 And it also involves interaction between the tumor 276 00:18:43,620 --> 00:18:47,130 and the surrounding cells of the body. 277 00:18:47,130 --> 00:18:49,740 So I'm going to tell you a little bit about tumor 278 00:18:49,740 --> 00:18:50,940 microenvironment. 279 00:18:57,490 --> 00:19:00,630 And this is something that's important to consider 280 00:19:00,630 --> 00:19:06,330 in cancer because a tumor in a body, it's not in isolation. 281 00:19:06,330 --> 00:19:11,190 It's surrounded by other cells in your body and other things 282 00:19:11,190 --> 00:19:16,200 in your body, like matrix, and so here is 283 00:19:16,200 --> 00:19:19,650 a picture showing you a tumor, and the tumor 284 00:19:19,650 --> 00:19:23,130 is stained with this membrane protein which is shown here 285 00:19:23,130 --> 00:19:24,360 in the rust color. 286 00:19:24,360 --> 00:19:26,250 So that's the tumor. 287 00:19:26,250 --> 00:19:32,520 And what's also stained in this piece of tissue is the DNA. 288 00:19:32,520 --> 00:19:34,480 It's stained in blue. 289 00:19:34,480 --> 00:19:38,520 And so you see the nuclei of the tumor cells in there. 290 00:19:38,520 --> 00:19:42,810 But you see all these blue nuclei surrounding the tumor. 291 00:19:42,810 --> 00:19:48,270 And these are stromal cells that are around the tumor. 292 00:19:48,270 --> 00:19:51,210 And they've actually been recruited to the tumor 293 00:19:51,210 --> 00:19:54,730 by the cancer cells. 294 00:19:54,730 --> 00:19:58,100 So what I mean by tumor microenvironment 295 00:19:58,100 --> 00:20:01,320 is just the region around the tumor. 296 00:20:01,320 --> 00:20:04,650 And so if you have cancer, and you have a tumor, 297 00:20:04,650 --> 00:20:08,220 the tumor cells can actually secrete signals 298 00:20:08,220 --> 00:20:12,090 which recruit stromal cells. 299 00:20:12,090 --> 00:20:18,000 So there can be an interaction where 300 00:20:18,000 --> 00:20:22,590 the tumor sends recruitment signals that causes 301 00:20:22,590 --> 00:20:24,270 stromal cells to come by. 302 00:20:27,570 --> 00:20:32,550 And what the stromal cells can do, what they do for the tumor 303 00:20:32,550 --> 00:20:35,130 and why this is beneficial for the tumor, 304 00:20:35,130 --> 00:20:39,510 is that stromal cells can secrete growth signals 305 00:20:39,510 --> 00:20:44,880 or survival signals that promote the growth of the tumor. 306 00:20:44,880 --> 00:20:51,570 So there can be a reciprocal interaction here where 307 00:20:51,570 --> 00:20:57,810 you get an abnormal conversation between cancer 308 00:20:57,810 --> 00:21:00,690 cells and the surrounding stromal cells. 309 00:21:00,690 --> 00:21:02,580 But again, this is very similar, if you 310 00:21:02,580 --> 00:21:06,330 think about it, to the way a normal organ works. 311 00:21:06,330 --> 00:21:10,290 You often have these signals going between cells. 312 00:21:10,290 --> 00:21:15,510 And that is involved in normal tissue homeostasis. 313 00:21:15,510 --> 00:21:17,700 What's happening here, though, is abnormal 314 00:21:17,700 --> 00:21:20,250 and that the cancer cells are constitutively 315 00:21:20,250 --> 00:21:22,110 recruiting these stromal cells just 316 00:21:22,110 --> 00:21:26,940 to get this growth signal that they're addicted to. 317 00:21:26,940 --> 00:21:30,570 And because you have just the presence 318 00:21:30,570 --> 00:21:34,680 of such a loop suggests that these cancer cells, 319 00:21:34,680 --> 00:21:37,305 even if they have oncogenic mutations 320 00:21:37,305 --> 00:21:40,290 and have lost tumor suppressors, they 321 00:21:40,290 --> 00:21:43,770 are not totally independent of growth signals. 322 00:21:43,770 --> 00:21:49,020 They still, to some extent, rely on growth signals. 323 00:21:49,020 --> 00:21:53,730 So these oncogenic mutations, they reduce the dependency 324 00:21:53,730 --> 00:21:58,530 on growth signals, but this dependency, 325 00:21:58,530 --> 00:22:08,850 the dependency on growth signals, is not eliminated. 326 00:22:18,120 --> 00:22:23,880 And one experiment that showed this was an experiment that 327 00:22:23,880 --> 00:22:29,040 was done back in the 1950s where patients with a type of skin 328 00:22:29,040 --> 00:22:33,600 cancer, basal cell carcinoma, were taken, 329 00:22:33,600 --> 00:22:35,910 and they had their tumors excised 330 00:22:35,910 --> 00:22:37,950 from one part of their body. 331 00:22:37,950 --> 00:22:41,730 And then the tumor was grafted back onto them, 332 00:22:41,730 --> 00:22:45,360 to another part of their body, distant from the original site 333 00:22:45,360 --> 00:22:47,010 of the tumor. 334 00:22:47,010 --> 00:22:50,820 And this grafting experiment was done either 335 00:22:50,820 --> 00:22:53,490 with the stromal cells that surrounded 336 00:22:53,490 --> 00:22:56,640 the tumor or the stromal cells surrounding the tumor 337 00:22:56,640 --> 00:22:58,830 were removed, and just the tumor cells 338 00:22:58,830 --> 00:23:01,830 were grafted back onto the body. 339 00:23:01,830 --> 00:23:02,700 OK. 340 00:23:02,700 --> 00:23:04,500 Now this experiment probably would not 341 00:23:04,500 --> 00:23:09,120 be allowed today, but back in the '50s, I guess it was legal. 342 00:23:09,120 --> 00:23:12,240 And so the experimental result is 343 00:23:12,240 --> 00:23:15,300 that if you graft the tumor cells with the surrounding 344 00:23:15,300 --> 00:23:20,730 stroma, the tumor was able to establish a second tumor, 345 00:23:20,730 --> 00:23:25,080 or the tumor cells survived this grafting procedure, 346 00:23:25,080 --> 00:23:29,910 whereas tumor cells that were taken without the stroma 347 00:23:29,910 --> 00:23:34,140 underwent cell death when they were put in a new location. 348 00:23:34,140 --> 00:23:38,550 So there's something about this specialized microenvironment 349 00:23:38,550 --> 00:23:42,090 that the tumor creates for itself that is enabling 350 00:23:42,090 --> 00:23:45,450 the tumor to grow and survive. 351 00:23:45,450 --> 00:23:49,440 And the model is that it's because these stromal cells are 352 00:23:49,440 --> 00:23:52,980 secreting growth factors that these cancer cells are still 353 00:23:52,980 --> 00:23:53,940 dependent on. 354 00:23:58,710 --> 00:24:02,700 So this dependency on growth signals is important, 355 00:24:02,700 --> 00:24:08,100 clinically, because some types of cancer, 356 00:24:08,100 --> 00:24:11,160 there is an elevation of growth factor receptors 357 00:24:11,160 --> 00:24:13,650 that are associated with the cancer. 358 00:24:13,650 --> 00:24:20,130 And the famous example of that is, in 30% of breast cancers, 359 00:24:20,130 --> 00:24:23,980 there is a growth factor receptor, the HER2 receptor, 360 00:24:23,980 --> 00:24:28,980 which is over expressed in the cancer cells. 361 00:24:28,980 --> 00:24:33,750 So I just drew a receptor here, and I'm now talking 362 00:24:33,750 --> 00:24:35,700 about the HER2 receptor. 363 00:24:35,700 --> 00:24:43,890 HER2 stands for human epidermal growth factor, EGF, receptor 2. 364 00:24:47,550 --> 00:24:52,050 So this is a receptor tyrosine kinase, 365 00:24:52,050 --> 00:24:54,900 which is a transmembrane protein receptor. 366 00:24:54,900 --> 00:24:57,930 It's expressed on the surface of the cell. 367 00:24:57,930 --> 00:25:04,800 And so about 30% of human breast cancers 368 00:25:04,800 --> 00:25:16,860 are HER2 positive, which means that the cancer cells are over 369 00:25:16,860 --> 00:25:20,940 expressing this receptor tyrosine kinase. 370 00:25:20,940 --> 00:25:25,350 The fact that you have these cancer cells over expressing 371 00:25:25,350 --> 00:25:28,230 a growth factor receptor suggests 372 00:25:28,230 --> 00:25:32,550 that the cancer requires some type 373 00:25:32,550 --> 00:25:39,660 of growth stimulus in order for the cancer to be growing. 374 00:25:39,660 --> 00:25:43,890 And the fact that this was discovered, 375 00:25:43,890 --> 00:25:48,270 that 30% of breast cancers over express HER2, 376 00:25:48,270 --> 00:25:51,510 has been used by researchers to develop 377 00:25:51,510 --> 00:25:56,100 a treatment for this type of cancer, this HER2 positive. 378 00:25:56,100 --> 00:26:01,170 The treatment is known as Herceptin. 379 00:26:01,170 --> 00:26:04,110 Possibly some of you have heard of this. 380 00:26:04,110 --> 00:26:07,150 It was developed at Genentech. 381 00:26:07,150 --> 00:26:12,180 And what Herceptin is, it's an antibody 382 00:26:12,180 --> 00:26:16,980 that was raised against a human HER2 protein. 383 00:26:16,980 --> 00:26:17,790 OK. 384 00:26:17,790 --> 00:26:20,430 So Herceptin is an antibody. 385 00:26:23,110 --> 00:26:29,080 It recognizes HER2 on the surface of these cancer cells, 386 00:26:29,080 --> 00:26:33,150 and you can treat patients with this antibody, 387 00:26:33,150 --> 00:26:37,080 and an antibody binds to the HER2 positive cells. 388 00:26:37,080 --> 00:26:40,200 And it either blocks the function of HER2 389 00:26:40,200 --> 00:26:44,070 or recruits immune cells to kill those cells off. 390 00:26:44,070 --> 00:26:48,010 The exact mechanism, I don't believe, is known. 391 00:26:48,010 --> 00:26:50,310 But what is known is that Herceptin has really 392 00:26:50,310 --> 00:26:56,290 changed how we're able to treat HER2 positive breast cancers. 393 00:26:56,290 --> 00:26:59,490 And it's been a huge success story 394 00:26:59,490 --> 00:27:00,915 in the fight against cancer. 395 00:27:09,260 --> 00:27:09,780 All right. 396 00:27:09,780 --> 00:27:18,040 So we've talked about this first barrier, the barrier to tumor 397 00:27:18,040 --> 00:27:21,310 cells becoming at least semi-independent 398 00:27:21,310 --> 00:27:25,060 of these growth factors. 399 00:27:25,060 --> 00:27:27,880 And so now I want to talk about other barriers 400 00:27:27,880 --> 00:27:29,960 to tumor genesis. 401 00:27:29,960 --> 00:27:33,190 And if you consider an epithelial cell here, 402 00:27:33,190 --> 00:27:35,620 even if these cells are able to grow, 403 00:27:35,620 --> 00:27:38,500 they won't be able to leave the organ 404 00:27:38,500 --> 00:27:40,930 until something else happens. 405 00:27:40,930 --> 00:27:43,570 And one thing that would need to happen for these cells 406 00:27:43,570 --> 00:27:47,650 to become malignant and to leave the organ that they were 407 00:27:47,650 --> 00:27:52,570 initially part of is for there to be a breakdown 408 00:27:52,570 --> 00:27:55,540 in the adhesion between cells. 409 00:27:55,540 --> 00:27:59,260 So for the next few minutes, I want 410 00:27:59,260 --> 00:28:03,850 to talk about the breakdown in cell-to-cell adhesion. 411 00:28:12,280 --> 00:28:17,080 So how would a cancer cell basically 412 00:28:17,080 --> 00:28:19,510 unstick itself from the cells that 413 00:28:19,510 --> 00:28:25,150 surround it in order to leave an organ and go somewhere else? 414 00:28:27,670 --> 00:28:30,520 And for that, to explain that, I have to remind you 415 00:28:30,520 --> 00:28:35,210 about some of the normal biology of these epithelial cells, 416 00:28:35,210 --> 00:28:37,180 which we talked about earlier, which 417 00:28:37,180 --> 00:28:41,200 is that they express these transmembrane proteins 418 00:28:41,200 --> 00:28:43,570 that are adhesion proteins. 419 00:28:43,570 --> 00:28:47,230 So normally, epithelial cells have adhesion proteins. 420 00:28:50,500 --> 00:28:52,690 And the famous one for epithelia, 421 00:28:52,690 --> 00:28:55,750 or one of the famous ones, is called 422 00:28:55,750 --> 00:28:58,490 epithelial or E-cadherin. 423 00:29:03,380 --> 00:29:07,000 And E-cadherin is a transmembrane protein. 424 00:29:07,000 --> 00:29:10,420 It has an extracellular domain, but rather 425 00:29:10,420 --> 00:29:13,210 than that extracellular domain binding 426 00:29:13,210 --> 00:29:17,220 to some secreted ligand, this extracellular domain 427 00:29:17,220 --> 00:29:21,190 recognizes E-cadherin molecules on other cells, 428 00:29:21,190 --> 00:29:22,750 and they stick to each other. 429 00:29:22,750 --> 00:29:26,230 And it essentially functions like cellular Velcro. 430 00:29:26,230 --> 00:29:30,340 So the cells link together, and they stick to each other such 431 00:29:30,340 --> 00:29:31,885 that they form a coherent tissue. 432 00:29:35,960 --> 00:29:43,150 So the way that cancer cells subvert this mechanism 433 00:29:43,150 --> 00:29:46,480 of adhesion is, they have to do something 434 00:29:46,480 --> 00:29:49,810 that inhibits E-cadherin. 435 00:29:49,810 --> 00:29:54,730 And so cancer cells, what they can do 436 00:29:54,730 --> 00:29:59,380 is to decrease the cell-to-cell adhesion, 437 00:29:59,380 --> 00:30:01,270 and I'll tell you how in just a minute. 438 00:30:04,750 --> 00:30:08,290 And in addition to decreasing this cell-to-cell adhesion, 439 00:30:08,290 --> 00:30:15,220 they can also promote the genes that are involved in motility. 440 00:30:23,850 --> 00:30:27,510 So we have to understand, then, how 441 00:30:27,510 --> 00:30:31,620 it is a cancer cell would do both of these things. 442 00:30:31,620 --> 00:30:35,220 And I'll start with cell-to-cell adhesion. 443 00:30:35,220 --> 00:30:39,330 And in contrast to what we've talked about with the growth 444 00:30:39,330 --> 00:30:43,110 and survival signaling, where you have mutations that happen 445 00:30:43,110 --> 00:30:45,090 in cancer cells, and you basically 446 00:30:45,090 --> 00:30:49,450 have an irreversible change to the genome of the cell, 447 00:30:49,450 --> 00:30:56,280 this change in the adhesive properties of the cell, 448 00:30:56,280 --> 00:30:58,260 and essentially their cell state-- 449 00:30:58,260 --> 00:31:01,710 because what you see here, if a cancer cell is 450 00:31:01,710 --> 00:31:05,790 having less adhesion and getting more migratory, 451 00:31:05,790 --> 00:31:09,930 it's switching from an epithelial type of state 452 00:31:09,930 --> 00:31:13,080 to a mesenchymal type of state. 453 00:31:13,080 --> 00:31:23,250 And what this is called is an epithelial-to-mesenchymal 454 00:31:23,250 --> 00:31:36,080 transition, and that is EMT, for short. 455 00:31:40,030 --> 00:31:43,210 And this EMT is not a genetic change. 456 00:31:43,210 --> 00:31:45,940 It's not caused by a genetic change, 457 00:31:45,940 --> 00:31:50,230 but it appears that EMT results from changes 458 00:31:50,230 --> 00:31:51,800 in gene regulation. 459 00:31:51,800 --> 00:31:59,590 So you can think of it as more of an epigenetic change, 460 00:31:59,590 --> 00:32:07,460 an epigenetic change in gene expression. 461 00:32:07,460 --> 00:32:11,350 So there's a change in gene expression 462 00:32:11,350 --> 00:32:13,540 such that this can reverse later on. 463 00:32:21,730 --> 00:32:25,640 And there are master regulators of this process, 464 00:32:25,640 --> 00:32:27,560 which alter gene expression. 465 00:32:27,560 --> 00:32:32,810 And so the type of gene that alters gene expression, 466 00:32:32,810 --> 00:32:35,365 these are often called transcription factors. 467 00:32:43,030 --> 00:32:46,150 And there are several transcription factors 468 00:32:46,150 --> 00:32:49,165 that are master regulators of this EMT process. 469 00:32:51,670 --> 00:32:57,145 And their names are Twist, Snail-- 470 00:33:00,640 --> 00:33:03,530 another one's called Slug. 471 00:33:03,530 --> 00:33:04,630 There's an Escargot. 472 00:33:04,630 --> 00:33:07,330 You can see this got a little out of hand. 473 00:33:07,330 --> 00:33:11,140 And you can tell by the names that they probably 474 00:33:11,140 --> 00:33:13,330 were not discovered in humans. 475 00:33:13,330 --> 00:33:16,390 And in fact, these genes were discovered 476 00:33:16,390 --> 00:33:19,600 in the same genetic screen that I outlined before, 477 00:33:19,600 --> 00:33:21,470 where Hedgehog was discovered. 478 00:33:21,470 --> 00:33:21,970 OK. 479 00:33:21,970 --> 00:33:25,750 So it was a genetic screen in the flies, 480 00:33:25,750 --> 00:33:30,100 and these are genes that affect the embryonic development 481 00:33:30,100 --> 00:33:32,260 of the fly. 482 00:33:32,260 --> 00:33:37,610 And I'll show you this view of the fly embryo. 483 00:33:37,610 --> 00:33:39,760 So this is an embryo here, and you see, 484 00:33:39,760 --> 00:33:43,690 it's an epithelial sheet surrounding the yolk. 485 00:33:43,690 --> 00:33:47,230 And what happens is, in early stages of development, 486 00:33:47,230 --> 00:33:51,100 a population of cells express the Twist gene. 487 00:33:51,100 --> 00:33:54,130 So that's Twist in the dark here. 488 00:33:54,130 --> 00:33:58,720 And this Twist gene causes these cells to basically invade 489 00:33:58,720 --> 00:34:01,480 into the middle of the embryo. 490 00:34:01,480 --> 00:34:03,430 So here are the Twist cells now, going 491 00:34:03,430 --> 00:34:06,400 into the inside of the embryo. 492 00:34:06,400 --> 00:34:08,530 And then these cells undergo EMT, 493 00:34:08,530 --> 00:34:12,489 and they start to migrate around inside the embryo. 494 00:34:12,489 --> 00:34:14,770 What this is doing for the fly is 495 00:34:14,770 --> 00:34:17,139 that these are the cells that are going on 496 00:34:17,139 --> 00:34:19,889 to form the muscle. 497 00:34:19,889 --> 00:34:21,395 And if you look at your neighbor, 498 00:34:21,395 --> 00:34:23,020 you might notice that their muscles are 499 00:34:23,020 --> 00:34:25,210 on the inside of their body. 500 00:34:25,210 --> 00:34:31,480 So this process is basically putting these cells, 501 00:34:31,480 --> 00:34:33,880 by getting them to move, it's putting the cells 502 00:34:33,880 --> 00:34:35,889 in the right place for what they are 503 00:34:35,889 --> 00:34:39,460 going to differentiate into during embryonic development. 504 00:34:42,370 --> 00:34:46,659 But this process, if activated during cancer, 505 00:34:46,659 --> 00:34:51,310 can allow these cells to be more mobile and to leave one organ 506 00:34:51,310 --> 00:34:54,010 and go into another organ. 507 00:34:54,010 --> 00:34:58,030 So cancer isn't inventing anything new here. 508 00:34:58,030 --> 00:35:01,930 It's corrupting a normal program that cells have and need 509 00:35:01,930 --> 00:35:04,750 during development, and it's inactivating it 510 00:35:04,750 --> 00:35:07,870 at an inappropriate time. 511 00:35:07,870 --> 00:35:10,750 So these are all transcription factors. 512 00:35:10,750 --> 00:35:14,410 And what they do is, they repress 513 00:35:14,410 --> 00:35:18,880 the expression or function of cadherin 514 00:35:18,880 --> 00:35:23,290 such that the cells are no longer as sticky. 515 00:35:23,290 --> 00:35:25,990 So basically, when these transcription factors 516 00:35:25,990 --> 00:35:28,120 get turned on, it makes the cells 517 00:35:28,120 --> 00:35:30,470 less sticky to each other. 518 00:35:30,470 --> 00:35:32,890 And it also promotes their migration. 519 00:35:32,890 --> 00:35:35,710 So it turns on genes that are important for migration. 520 00:35:39,400 --> 00:35:44,200 This process of EMT, as I mentioned, is reversible, 521 00:35:44,200 --> 00:35:47,410 and it also involves interactions between tumor 522 00:35:47,410 --> 00:35:49,820 and stromal cells. 523 00:35:49,820 --> 00:35:52,600 I'll show you an example of-- 524 00:35:52,600 --> 00:35:57,010 so in the fly embryo, one thing that's nice about the flies 525 00:35:57,010 --> 00:36:00,790 is, you can actually watch this happen live. 526 00:36:00,790 --> 00:36:04,150 And so I'm going to show you a movie now, showing you 527 00:36:04,150 --> 00:36:06,352 the invasion process. 528 00:36:06,352 --> 00:36:07,810 We're going to have an embryo here. 529 00:36:13,460 --> 00:36:15,190 And what I'm going to show you is, 530 00:36:15,190 --> 00:36:18,980 we're looking at the cells that are expressing Twist and Snail, 531 00:36:18,980 --> 00:36:21,820 and they're on one side of the embryo. 532 00:36:21,820 --> 00:36:25,360 And I'm going to show you a section of the embryo that 533 00:36:25,360 --> 00:36:28,480 is analogous to this. 534 00:36:28,480 --> 00:36:31,360 So when the cells move inside, they're 535 00:36:31,360 --> 00:36:33,370 going to disappear in this movie. 536 00:36:33,370 --> 00:36:36,700 So when you see the crease in the embryo, 537 00:36:36,700 --> 00:36:38,170 that's when the cells are invading 538 00:36:38,170 --> 00:36:40,540 into the middle of the embryo. 539 00:36:40,540 --> 00:36:44,110 And so the cell outlines are in magenta here. 540 00:36:44,110 --> 00:36:48,970 And what's labeled in green is a type of motor protein 541 00:36:48,970 --> 00:36:52,690 that's involved in the mobility of cells and, in this case, 542 00:36:52,690 --> 00:36:54,430 is involved in these cells moving 543 00:36:54,430 --> 00:36:57,800 into the interior of the embryo. 544 00:36:57,800 --> 00:37:00,040 So here, you see the motor protein appearing, 545 00:37:00,040 --> 00:37:02,997 and that's when these cells are going to dive into the embryo. 546 00:37:02,997 --> 00:37:04,330 So it's almost like a waterfall. 547 00:37:04,330 --> 00:37:08,560 These cells are moving into the inside of the embryo 548 00:37:08,560 --> 00:37:12,850 through this process of invagination and, subsequently, 549 00:37:12,850 --> 00:37:14,500 EMT. 550 00:37:14,500 --> 00:37:15,817 There it goes again. 551 00:37:15,817 --> 00:37:17,650 You can see the cells are now on the inside. 552 00:37:20,260 --> 00:37:23,170 So this process of EMT also involves 553 00:37:23,170 --> 00:37:27,910 interactions between the cancer cells and the stroma. 554 00:37:27,910 --> 00:37:31,323 And that's illustrated here because what you can see, 555 00:37:31,323 --> 00:37:32,740 what's being labeled here, you can 556 00:37:32,740 --> 00:37:37,090 see the nuclei of the cells, but these cells 557 00:37:37,090 --> 00:37:41,140 at the edge of the tumor are up-regulating this gene, which 558 00:37:41,140 --> 00:37:45,100 is an alpha-beta integrin, which I'll mention in just a minute. 559 00:37:45,100 --> 00:37:47,110 And this is getting up-regulated right where 560 00:37:47,110 --> 00:37:49,360 the tumor contacts the stroma. 561 00:37:49,360 --> 00:37:51,430 And this gene, alpha-beta integrin, 562 00:37:51,430 --> 00:37:56,200 is a gene that's associated with motility and EMT. 563 00:37:56,200 --> 00:38:00,720 So on that slide here, one gene involved in motility 564 00:38:00,720 --> 00:38:02,820 is a class of genes called integrins. 565 00:38:06,250 --> 00:38:12,070 And so this also results from the signaling between the tumor 566 00:38:12,070 --> 00:38:14,390 and the surrounding cells. 567 00:38:14,390 --> 00:38:17,830 And again, this is not something that cancer created. 568 00:38:17,830 --> 00:38:20,750 This is what happens during wound healing. 569 00:38:20,750 --> 00:38:24,500 So during wound healing, if you injure yourself, 570 00:38:24,500 --> 00:38:27,550 the wound recruits immune cells, and the immune cells 571 00:38:27,550 --> 00:38:31,300 signal to the surrounding skin or epithelial cells 572 00:38:31,300 --> 00:38:36,010 to undergo EMT in order to close and fill in the wound. 573 00:38:36,010 --> 00:38:39,160 So this is a natural process that 574 00:38:39,160 --> 00:38:41,650 happens in your body that is simply getting 575 00:38:41,650 --> 00:38:43,690 corrupted by cancer cells. 576 00:38:47,620 --> 00:38:48,120 All right. 577 00:38:48,120 --> 00:38:51,990 So now let's talk about this last step of motility. 578 00:38:54,550 --> 00:39:01,690 So this is now how the cells would break out and move away. 579 00:39:09,280 --> 00:39:11,570 Let's come back to this movie here. 580 00:39:11,570 --> 00:39:14,950 There are a few things I want you to notice about this movie. 581 00:39:14,950 --> 00:39:19,690 The first is, holy shit, that cell is moving. 582 00:39:19,690 --> 00:39:26,890 And so the first aspect of this is, because the cell is moving, 583 00:39:26,890 --> 00:39:31,090 it suggests that there's some sort of force being generated. 584 00:39:31,090 --> 00:39:39,930 So the cell is generating force. 585 00:39:39,930 --> 00:39:43,100 And I'm not talking about like some type of mystical Jedi 586 00:39:43,100 --> 00:39:43,880 force. 587 00:39:43,880 --> 00:39:48,020 I'm talking about the mass times acceleration force, 588 00:39:48,020 --> 00:39:50,450 so like a physical force, OK? 589 00:39:50,450 --> 00:39:53,840 The second thing I want to point out about this movie 590 00:39:53,840 --> 00:39:57,500 is that you see that bacterium, and it's moving around, 591 00:39:57,500 --> 00:40:00,590 this cell is able to follow that bacterium. 592 00:40:00,590 --> 00:40:03,650 So the cell, in the force generation process, 593 00:40:03,650 --> 00:40:06,500 is very responsive to the signals 594 00:40:06,500 --> 00:40:08,750 that that cell is getting. 595 00:40:08,750 --> 00:40:12,560 So the cell is generating force, and this force 596 00:40:12,560 --> 00:40:14,255 is responsive to signals. 597 00:40:22,790 --> 00:40:24,860 So for the remaining part of the lecture, 598 00:40:24,860 --> 00:40:26,510 I basically just want to tell you 599 00:40:26,510 --> 00:40:30,590 about how it is that the cell generates the force required 600 00:40:30,590 --> 00:40:34,250 to move itself around. 601 00:40:34,250 --> 00:40:38,580 And it involves a particular type of protein. 602 00:40:38,580 --> 00:40:40,520 It's a component of the cytoskeleton, 603 00:40:40,520 --> 00:40:42,650 and we've talked about microtubules 604 00:40:42,650 --> 00:40:45,020 and the microtubule cytoskeleton and its role 605 00:40:45,020 --> 00:40:47,060 in segregating the chromosome. 606 00:40:47,060 --> 00:40:51,470 But there are other cytoskeletal systems that the cell has, 607 00:40:51,470 --> 00:40:55,280 and one is called the actin cytoskeleton, which 608 00:40:55,280 --> 00:40:58,940 is shown on the side above. 609 00:40:58,940 --> 00:41:02,600 And actin, the actin cytoskeleton, 610 00:41:02,600 --> 00:41:06,030 is a system that is a biopolymer in the cell. 611 00:41:06,030 --> 00:41:11,870 So this is a biopolymer, like microtubules. 612 00:41:11,870 --> 00:41:14,130 And actin is a gene. 613 00:41:14,130 --> 00:41:18,450 And the actin gene encodes for a protein, the actin protein, 614 00:41:18,450 --> 00:41:21,610 and when the actin protein is made in the cell, 615 00:41:21,610 --> 00:41:26,780 it starts out as being just a single globular protein. 616 00:41:26,780 --> 00:41:29,960 And when the actin is in this state, 617 00:41:29,960 --> 00:41:35,000 it is called globular or G-actin. 618 00:41:38,510 --> 00:41:41,870 But these subunits, these proteins, 619 00:41:41,870 --> 00:41:45,410 can come together and form a polymer. 620 00:41:45,410 --> 00:41:49,640 So they can go from being individual, isolated proteins 621 00:41:49,640 --> 00:41:53,465 to forming a polymer that forms a long skinny filament. 622 00:41:59,990 --> 00:42:05,120 And this form of the protein that's forming a biopolymer 623 00:42:05,120 --> 00:42:08,210 is known as filamentous or F-actin. 624 00:42:18,130 --> 00:42:20,740 And these are long, skinny filaments. 625 00:42:20,740 --> 00:42:23,620 They can be hundreds of nanometers, 626 00:42:23,620 --> 00:42:28,600 even microns, in length, so hundreds of nanometers long. 627 00:42:28,600 --> 00:42:32,050 And the filament width is about 10 nanometers. 628 00:42:32,050 --> 00:42:34,540 So I'm just trying to give you a sense of dimensions, 629 00:42:34,540 --> 00:42:40,120 that this is a long, skinny filament that the cell can 630 00:42:40,120 --> 00:42:40,930 assemble. 631 00:42:40,930 --> 00:42:44,260 And it assembles into these dense meshworks, which you 632 00:42:44,260 --> 00:42:47,230 can see in the slide up here. 633 00:42:47,230 --> 00:42:49,340 So this is the leading edge of the cell. 634 00:42:49,340 --> 00:42:51,640 So the cell would be migrating this way. 635 00:42:51,640 --> 00:42:57,520 And what you see in this cell is this densely branched network, 636 00:42:57,520 --> 00:43:00,220 and these are all actin filaments. 637 00:43:00,220 --> 00:43:05,350 So you get this huge dense forest of actin filaments 638 00:43:05,350 --> 00:43:08,635 that's right at the edge of the cell that is moving forward. 639 00:43:11,650 --> 00:43:15,370 One thing I want to point out is, like all things in biology, 640 00:43:15,370 --> 00:43:17,990 this is not a one-way street. 641 00:43:17,990 --> 00:43:21,740 And so these biopolymers are very dynamic, 642 00:43:21,740 --> 00:43:24,640 meaning they can undergo assembly and disassembly, 643 00:43:24,640 --> 00:43:28,960 and they can do so on the timescale of seconds. 644 00:43:28,960 --> 00:43:32,410 One last thing I want to point out about the actin filament 645 00:43:32,410 --> 00:43:36,280 is, there's a polarity to the filament such 646 00:43:36,280 --> 00:43:39,190 that there's an end, known as the plus end, 647 00:43:39,190 --> 00:43:41,320 where growth is favored, and there's 648 00:43:41,320 --> 00:43:43,840 an end, known as the minus end, where there's 649 00:43:43,840 --> 00:43:45,760 often de-polymerization. 650 00:43:45,760 --> 00:43:49,720 So you can get a directional growth of the filament. 651 00:43:49,720 --> 00:43:51,130 So this is where growth happens. 652 00:43:55,370 --> 00:43:58,180 And so in this network that you're looking at, 653 00:43:58,180 --> 00:44:00,190 the way the actin is oriented-- 654 00:44:00,190 --> 00:44:03,520 so I'm going to draw just a cell here-- 655 00:44:03,520 --> 00:44:11,830 you have this dense meshwork of actin, and in this meshwork, 656 00:44:11,830 --> 00:44:14,590 there's a polarity to the way the actin filaments are 657 00:44:14,590 --> 00:44:15,740 oriented. 658 00:44:15,740 --> 00:44:18,340 So the cell is migrating this way, 659 00:44:18,340 --> 00:44:21,100 and the plus ends of the actin filaments 660 00:44:21,100 --> 00:44:23,740 are facing out right at the surface. 661 00:44:23,740 --> 00:44:27,310 And the minus ends are back here. 662 00:44:27,310 --> 00:44:30,460 And so what you have, when the cell is migrating, 663 00:44:30,460 --> 00:44:34,390 is you have this biopolymer network that's 664 00:44:34,390 --> 00:44:38,020 growing on this end, but shrinking on the other end. 665 00:44:38,020 --> 00:44:42,550 And that allows the cell to generate a constant protrusive 666 00:44:42,550 --> 00:44:44,290 force. 667 00:44:44,290 --> 00:44:47,500 So it's the growth of actin filaments, 668 00:44:47,500 --> 00:44:55,620 the growth of F-actin, which generates a protrusive force. 669 00:45:04,340 --> 00:45:08,270 Now if we consider the whole process of cell migration, what 670 00:45:08,270 --> 00:45:10,640 you can see is that initially, you're 671 00:45:10,640 --> 00:45:14,090 going to push the cell membrane forward. 672 00:45:14,090 --> 00:45:16,040 So you get a protrusion. 673 00:45:16,040 --> 00:45:17,690 It pushes it forward. 674 00:45:17,690 --> 00:45:22,730 That's often called a lamellipodium, or a pseudopod, 675 00:45:22,730 --> 00:45:26,220 and so this part of the cell moves forward. 676 00:45:26,220 --> 00:45:30,170 But in order for the cell to have a net motion forward, 677 00:45:30,170 --> 00:45:32,990 it then has to stabilize that protrusion. 678 00:45:32,990 --> 00:45:37,498 And it stabilizes the protrusion by adhering to the substrate. 679 00:45:37,498 --> 00:45:39,290 So right now, we're just considering a cell 680 00:45:39,290 --> 00:45:43,340 moving on a flat substrate. 681 00:45:43,340 --> 00:45:46,250 And so the way it attaches to the substrate 682 00:45:46,250 --> 00:45:48,155 is through cell matrix adhesion. 683 00:45:51,170 --> 00:45:53,930 And this cell matrix adhesion is mediated 684 00:45:53,930 --> 00:45:57,740 by another type of adhesion receptor known as an integrin. 685 00:46:03,860 --> 00:46:07,070 So the cell pushes forward, anchors itself 686 00:46:07,070 --> 00:46:10,520 on the substrate, pulls its body along, and then just 687 00:46:10,520 --> 00:46:13,980 repeats that cycle over and over again. 688 00:46:13,980 --> 00:46:21,560 So the best way I can illustrate this is, if you think about it, 689 00:46:21,560 --> 00:46:22,940 you get your protrusion. 690 00:46:22,940 --> 00:46:24,650 You get your elbow out. 691 00:46:24,650 --> 00:46:25,580 You put down. 692 00:46:25,580 --> 00:46:27,480 You anchor. 693 00:46:27,480 --> 00:46:29,870 And it's just a repeated cycle, and it's 694 00:46:29,870 --> 00:46:33,360 able to migrate across the substrate. 695 00:46:33,360 --> 00:46:37,010 So it's kind of like a frontal toe mechanism. 696 00:46:37,010 --> 00:46:39,350 You have cycles of protrusion. 697 00:46:39,350 --> 00:46:41,780 You generate traction, and then you 698 00:46:41,780 --> 00:46:44,260 pull, in order to translocate. 699 00:46:44,260 --> 00:46:49,220 And so that's how cells are migrating intuitively. 700 00:46:49,220 --> 00:46:54,050 This is just showing you that cells can pull on stuff. 701 00:46:54,050 --> 00:46:56,780 So not all cells migrate in this way. 702 00:46:56,780 --> 00:46:58,940 I just wanted to say that. 703 00:46:58,940 --> 00:47:02,270 So 3D cells have other mechanisms 704 00:47:02,270 --> 00:47:07,100 to migrate that de-emphasize this traction mechanism 705 00:47:07,100 --> 00:47:09,290 that cells have. 706 00:47:09,290 --> 00:47:11,780 So you can get rid of all the integrins 707 00:47:11,780 --> 00:47:15,680 that a cell has, and they're still able to migrate. 708 00:47:15,680 --> 00:47:21,060 And that's because 2D emphasizes the role of adhesion, 709 00:47:21,060 --> 00:47:23,880 but if you can find the cell, then the cells 710 00:47:23,880 --> 00:47:25,630 are able to migrate. 711 00:47:25,630 --> 00:47:29,880 So what's shown here are cells that are not confined. 712 00:47:29,880 --> 00:47:33,150 This is a confined cell in a micropipette. 713 00:47:33,150 --> 00:47:35,520 And you can see, it's the same type of cell, 714 00:47:35,520 --> 00:47:38,200 but the cell can migrate in confinement, 715 00:47:38,200 --> 00:47:42,630 but it can't migrate outside of confinement. 716 00:47:42,630 --> 00:47:44,700 So in this sense, the cell is migrating 717 00:47:44,700 --> 00:47:46,410 through a different mode. 718 00:47:46,410 --> 00:47:49,110 And you can think of it as the cell 719 00:47:49,110 --> 00:47:51,960 is doing some type of chimney-ing maneuver. 720 00:47:51,960 --> 00:47:54,990 So it's able to get in a confined environment, push out 721 00:47:54,990 --> 00:47:57,900 against its surroundings, and that's how the cell then 722 00:47:57,900 --> 00:48:01,770 is able to generate traction in the absence of an integrin 723 00:48:01,770 --> 00:48:04,080 molecule. 724 00:48:04,080 --> 00:48:04,590 All right. 725 00:48:04,590 --> 00:48:05,640 Great. 726 00:48:05,640 --> 00:48:10,070 So we are set for now.