Role of Transforming Growth Factor-β (TGF-β) in Extracellular Matrix Remodelling

00:01 Welcome.

00:02 So we're just about through making a scar in tissues that cannot regenerate.

00:07 The final stages.

00:09 Now that we've laid down new blood vessels is to deposit and remodel the extracellular matrix.

00:18 Here's where we are on our overall roadmap.

00:21 Remember, we've talked about moderating acute inflammation, recruiting and activating macrophages, regenerating the parenchyma, if we can.

00:29 And now these last two topics is our when we cannot completely regenerate.

00:36 So we have to have angiogenesis granulation tissue.

00:39 And now we're going to deposit and remodel the matrix.

00:42 So let's get going.

00:45 Here we are on our sequential curves.

00:49 And really at the peak of the chronic inflammatory response, where the macrophages are peaking at three to five days after the original injury, read the green arrow.

01:00 And at that point, we are starting to lay down granulation tissue, And at that's going to peak at somewhere between seven and 10 days.

01:08 And even with the very earliest stages of new vessel formation, we're also starting to lay down the matrix that's going to be a scar.

01:18 We may also have some degree of regeneration, Remember, the macrophages are still cranking out growth factors.

01:25 But when we're going to have scar, it's going to be mostly that fibrosis.

01:30 Okay, let's get going.

01:33 This is a transverse section of an animal preparation of a heart where the heart in the upper left hand side has had a myocardial infarct induced by tying a suture around the coronary artery.

01:51 This is a five to 10 days.

01:53 And the special stain here is a trichrome stain, where blue is collagen.

02:00 And we can see in the area of the infarct on the upper left hand side that we do have some blue, but it's pretty, it's pretty loose, and it's not well developed.

02:10 Okay, that's it five to 10 days when we're at the peak of granulation tissue.

02:15 If we wait four weeks, wow, we have quite a bit of dark peacock blue.

02:21 And that blueness there represents the deposition of collagen.

02:26 That's where we're going to go for the next few slides.

02:29 We are over the course of four weeks on that provisional matrix of the newly formed blood vessels, the granulation tissue, we're going to lay down extracellular matrix, and then we're going to remodel it.

02:40 So, Factor 5.

02:42 We've had "Factor 1" Interferon gamma.

02:44 Factor 2: Interleukin-1 / TNF.

02:47 Factor 3: Epidermal Growth Factor Factor 4: The angiogenic Factors, basic FGF, and VEGF.

02:53 "Factor 5" in quotes, remember is Transforming Growth Factor-Beta This is actually a pretty important one, and worth remembering, because it really does drive the final stages of scarring.

03:06 So it's important for causing fibroblasts migration chemotaxis and to drive them to synthesize matrix.

03:17 It also will cause the reduced expression of matrix metalloproteinases.

03:24 So there are various ways that we can degrade matrix.

03:26 Matrix metalloproteinases are a major pathway.

03:29 There is several of the matrix metalloproteinases in excess of 20.

03:33 And when they are chewing away, they're degrading matrix.

03:36 But if we turn them off, if we turn off their synthesis, then we can get more matrix accumulate.

03:43 So this is happening also because of TGF-beta.

03:47 An important brake on the entire process, Everything up to this point has been kind of go go, go, go, go, go.

03:53 Well, at some point, we have to turn it off and go back to just being whatever it was to begin with.

03:59 And TGF-beta inhibits the inflammation.

04:02 It's going to be a very important anti-inflammatory break on the entire process.

04:08 TGF-beta is synthesized in an inactive form by activated macrophages and T cells.

04:13 As well as smooth muscle cells and endothelial cells.

04:17 Its importance, they're in an inactive form.

04:19 It's actually secreted into the provisional stroma.

04:23 And it's sitting there, and it doesn't do its thing until we have proteolysis, until we have enzymes proteases elaborated that will digest it.

04:32 So that's a fairly nuanced way to have regulation of this particular activity.

04:40 It's going to be an important mechanism by which we shut off inflammation, and promote scarring.

04:46 So that's what you need to remember that TGF-beta does.

04:53 After we'd lay down the scar, we just don't leave it there.

04:56 But in fact, we remodeled that.

04:58 So let's look at our M1 macrophage again, it's providing a whole bunch of M1 macrophage cytokines such as epidermal growth factor, interleukin1, tumor necrosis factor.

05:09 Those will actually drive fibroblasts to make precursors procollagenases, prostromelysins that when activated by plasmin and other activators that will create matrix metalloproteinases that degrade the matrix.

05:26 That's part of the original cleaning up the debris is to break down all that degenerated extracellular matrix.

05:34 And that's what M1 macrophages do.

05:36 Degrading the matrix.

05:40 M2, mainly through the effects of TGF-beta turn off that process.

05:45 So we don't make those matrix metalloproteinases.

05:48 This will allow us to accumulate extra matrix.

05:52 So part of what TGF-beta is doing, besides turning off the inflammation is inhibiting the breakdown of matrix.

05:59 And that's going to allow us to synthesize more.

06:04 M2 macrophages also make tissue inhibitors of metalloproteinases.

06:09 So, this is getting again into alphabet soup.

06:12 But TIMPs are molecules that inhibit any activated matrix metalloproteinase.

06:20 So not only do M2 macrophages through TGF-beta turn off fibroblasts synthesis of things that would be MMPs that are active.

06:29 Matrix metalloproteinases that are active to cleave matrix.

06:33 There are also banking inhibitors that will turn off any residual MMPs that might be out there.

06:41 Okay.

06:43 Steroids.

06:44 And I mentioning this as a sidelight because steroids are frequently given by plastic surgeons and other people to limit scarring.

06:51 How is that happening? Well, steroids, in fact, inhibit both M1 and M2 macrophages to some extent, that's part of what makes them so potent in terms of anti-inflammatory agents.

07:05 But they turns out, they inhibit M2 macrophages much, much more.

07:10 So they're more effective on that.

07:12 So there is a relative decrease in the amount of TGF-beta.

07:16 So there'll be a relative increase in the amount of matrix metalloproteinases, and a relative increase in the degradation of the matrix, because we are having more of an effect on M2.

07:29 So all things considered if we give steroids, we get less scarring, because there's reduced fibrosis because of less activation of M2.

07:37 And that's why we give that.

07:39 On the other hand, if we really want good scarring to occur, we have a big surgical incision, and we want it to heal up really nicely, we don't want to give steroids.

07:49 And patients who are on steroids for autoimmune disease need to be very carefully monitored, and the steroids adjusted so that we don't have this effect.

07:59 Okay.