Extra-cellular matrix, fibroblasts and fibrosis Flashcards
3 classes of molecules in ECM
- Structural proteins (collagens and elastins)
- Adhesive glycoproteins to attach cells to matrix
(fibronectins and laminins) - Proteoglycans (protein-polysaccharide complexes)
to embed the structural proteins
Main structural protein in the heart
Collagen
How does collagen act?
Scaffold for cells during the force of contraction
Prevents myocyte slippage
Provides elasticity
Main types of collagen in the heart
Collagen type I and III
Composition of collagen type I
two alpha1- chains and one alpha
What is collagen produced in
Fibroblasts
Single collagen pro-peptide has>
N terminus and C terminus and in centre has a glycine followed by 2 different repeats (different collagens may have different - in collagen type I - two alpa1 and 1 alpha2)
What happens once pro peptides synthesised and where
Coil into a tight right handed triple helix of 3 alpha chains. In RER
What happens after triple helix of collagen forms
Excreted into extracellular space
What happens after excretion of collagen?
Converted into collagen from procollagen by the enzymatic cleavage of the N- and C-propeptides
What do adhesive glycoproteins consist of
Adhesive glycoprotein consisting of 3 chains: alpha beta and gamma
What do adhesive glycoproteins proteins do?
Bridges between structural ECM molecules to reinforce this network, as well as to connect the ECM to cells within the extracellular space
Main glycoprotein in the heart and what is it produced by?
Laminin
produced by cardiac myocytes, vascular smooth muscle cells and endothelial cells.
What do proteoglycans do?
Bind everything together more strongly - higher order structur
Primary function of proteoglycans. Properties for this
Bind water to provide hydration and compressive resistance.
biochemical and hydrodynamic characteristics due to presence of glycosaminoglycans (GAGs)
Structure of proteoglycan
Core protein (long part), covalently linked to GAGs
What are GAGs
long, negatively charged, linear chains of disaccharide repeats
Cell type associated with regulating ECM
Fibroblasts
Highest cell number in heart vs volume
Fibroblasts
Development of fibroblasts
Epicardial and endocardial layers can undergo mesenchymal transformation - produce resident fibroblats
Impact of injury on fibroblasts
BM derived cells can be recruited and transformed into cardiac fibroblasts
Functions of fibroblasts
- Primary function is to maintain and regulate ECM
- Can produce a number of active peptides (e.g.
cytokines, growth factors, peptides) - Contribute to production of ECM proteins (collagens,
elastin, fibronectin) - Produce ECM-regulatory proteins, matrix
metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases (TIMPs)
What do MMPs do?
degrade ECM protein
What do TIMPs do
inhibit MMP
How do MMPs affect electrophysiology
Non-excitable, do not produce APs
BUT can couple with myocytes to transfer current
Resting Vm of fibroblasts
-30mV - less negative than cardiomyocyte
Currents in fibroblasts
K, Na, and other minor currents
Do fibroblasts couple with myocytes
in vitro culture leading to a higher probability of spontaneous depolarisation.
Not clear whether this occurs in intact tissue
Well exercised heart - impact on structure
Myocyte hypertropy
No fibrosis
Increased vasculature density.
Does increase some of ECM
Pathology impact
Myocyte death
Increased fibroblast proliferation/fibrosis
Decreased vasculature density
Impact of disease on ECM
Either excess ECM
or
ECM breakdown
Consequence of increased ECM breakdown in disease
cardiac tissue destruction → High levels of MMP1 results in collagen loss and reduced contractility in cardiomyopathies
Consequence of excess ECM in disease
pathological fibrosis following tissue injury
Leads to areas of inactivity, plus ↑cardiac stiffness, ↑rigidity
Impact of fibrosis
areas of inactivity as well as ↑ cardiac stiffness
How is scar classified?
Degrees of density
Interstitial – fibrosis along the fibril bundles Deposition in the direction of the collagen orientation
Diffuse - everywhere
Patchy - larger areas of red, the other unaffected areas
(Collagen in the wall of the blood vessel is normal) Compact → around an infarct area
Consequence of scar
separation of strands of myocardium - this forces excitation waves to take anisotropic, circuitous paths - may low re-entry excitation
What mediates fibroblasts modulation of cardiomyocyte electrical activity by fibroblasts
Voltage-gated Na+
THEORY - ONLY APPLIES IF COUPLED
How do fibroblasts affect AP in myocytes
Whenever the cardiomyocyte membrane potential is different from -30mV, current flows through gap junctions between the cells to pull myocytes voltage towards -30, leading to shortened APD
Talk through AP change if fibroblast coupled to myocyte
myocyte-fibroblast coupling leads to slow conduction and shorter APDs, which can result in re-entry
What are myofibroblasts?
Remodelled fibroblasts become these from injury
Properties of myofibroblasts
More mobile
Contractile
Protective mechanism
What is alphaSMA
Smooth muscle cell markers that are not typically expressed in quiescent fibroblasts but are in myofibroblasts
How do myofibroblasts increase contractility
Connect to fibronectin via specialized adhesion complexes called fibronexus
What is another effect of increased contractile force by myofibroblasts?
mediates further fibroblast recruitment to site of injury
What are MMPs a therapeutic target?
They control ECM remodelling
Effect of broad spectrum MMP inhibition
attenuate the degree of post-MI LV dilation and expansion of the infarct in a pig model – but broad spectrum is feared to have systemic side effects
Specific MMP inhibitor
PG116800 inhibits MMP-1 and MMP-7
Early clinical trials in premier study failed to reduce LV remodelling or improve clinical outcomes after MI
Other therapeutic targets - RAS
Modulation of the renin-angiotensin (Ang) system using Ang converting enzyme (ACE) inhibitors or Ang receptor antagonists (e.g. losartan) appear to be effective in reducing cardiac fibrosis in a variety of models in animals and humans
Other therapeutic targets: TGF-b
plays a central role in fibroblast activation inducing ECM deposition by supressing MMP and inducing TIMP expression
Wounds treated with anti-TGF-b showed reduced ECM synthesis and scarring
Inhibitors of TGF-b receptors are also considered as potential anti-fibrotic compounds
Other therapeutic targets: Endothelin
Production of ECM and fibroblast differentiatio. Endothelin receptor antagonism may be considered as an appropriate therapy for fibrosis
Currently endothelin receptor blockers (bosentan) are used for the treatment of pulmonary arterial hypertensionn
Other therapeutic targets: Platelet derived growth factor
(PDGF) is responsible for inflammatory and smooth muscle cell migration and proliferation to the site of injury.
PDGF receptor inhibitor Imantinib mesylate delays wound closure together with reduction in myofibroblast number and fibronectin and collagen I
Other therapeutic targets: TNF-a
TNF-a is a pro-inflammatory cytokine expressed at the site of injury could also be a potential target since its overexpression in animal models develop heart failure
