Fibrosis

Extracellular Matrix (ECM): Critical for Fibrosis

 Composed of proteins, glycans, glycoproteins and proteoglycans synthesized in ER & secreted

 Cells are contained within the matrix like “raisins in pudding”

 Cells bind with ECM via cell adhesion membrane proteins (integrins, cadherins etc.

 Two major functions of ECM most relevant to fibrosis:

 ECM is intrinsically biologically active, capable of stimulating specific functions and activities of cells attached to it

 ECM has specific mechanical properties that contribute to the function of the tissue.

 The composition and properties of ECM vary by tissue

The Basics of Fibrosis

 The formation of excessive and/or non-native fibrous connective tissue in an organ or tissue in a reparative or reactive process

 Fibrous connective tissue is composed primarily of extracellular matrix (ECM) proteins, with relatively few cells

 Normally, ECM composition, mechanical properties and biological properties are tailored and specific for each tissue

 Fibrosis alters the ECM so it doesn’t fit the cell and tissue requirements, which affects cell and tissue function

ECM and Stiffness

 The specific elasticity of a tissue is intrinsic to its function

 Elasticity is determined largely by composition (which proteins, proteoglycans etc.) and organization of the ECM.

 Composition and organization is determined by the native cells of the tissue depositing the ECM & is essential for function of the tissue.

 If non-native cells deposit ECM, it is likely different in composition and mechanical properties

 One of the consequences of fibrosis, is to change the elasticity of tissues, altering their functionality

Wound Healing & ECM Deposition

When wounded, immune cells and injured native cells secrete an ECM that fills the space.

• This “wound” ECM is often different from the original ECM in composition and mechanical properties

• Wound ECM serves two important (but temporary) functions:

• Provides a temporary barrier with mechanical properties (not ideal, but a quick fix)

• Is a scaffold on which cells can migrate into the wound area

• As cells migrate into the wound ECM, they initiate a process to degrade the wound ECM and replace it with native ECM

• When a scar forms, normal tissue doesn’t reform and wound ECM persists

Fibrosis: Formation of excessive or non-supportive connective tissue (ECM) in a reparative or restorative process

Cells responding to an injury or insult, synthesize and deposit ECM molecules at the site

• Often, the composition and quantity of this deposited ECM is not appropriate for long term function of the tissue, but serves a short term structural need.

• In a normal response, this ECM would be remodeled and replaced by ECM fitting the needs of the tissue as the wound heals.

Wound Healing vs. Fibrosis

Wound Healing

 Damaged cells secrete immune-mediator proteins that recruit immune cells and trigger ECM deposition

 ECM is often non-native with different properties

 Serves as temporary structural scaffold

 The would is closed, and native cells infiltrate the wound ECM, remodeling it and replacing it with the native tissue (ECM + cells) Fibrosis

 Chronic inflammation stimulates excessive (in amount and duration) ECM deposition.

 Different in composition and mechanical properties

 Native cells fail to infiltrate and/or remodel the fibrotic/wound ECM

 Continual inflammation keeps piling on the “wound” ECM, creating a fibrotic scar

Fibrosis constitutes overgrowth, hardening, and/or scarring of various tissues and is attributed to an overabundance of non-supportive ECM

Fibrosis: Balance of MMPs & TIMPs

• MMPs chew up and degrade ECM components

• Matrix remodeling is an ongoing process that is finely tuned by regulating the production of matrix molecules, the abundance and location of MMPs and TIMPs

• TIMPs inhibit MMPs by binding within their active site

MMPs Degrade More than just ECM

 There are over 20 MMPs (& 4 TIMPs), each MMP with specificity for a number of biomolecules

 MMPs are autocatalytic, cutting up itself

MMP Specificities

 Of the 20 MMPs, there are different specificities for:

 Fibrotic ECM

 Clearing ECM from the initial wound

 Natural turnover of native ECM

 Specificity occurs as different MMPs will interact and cleave different ECM proteins found in the above scenarios

 While many cells express multiple MMPs, all cells do not express all 20 MMPs

 Cells that are present within the different stages of the response are tuned to present specific MMPs or TIMPs as they are needed

Fibrosis with Acute vs Chronic Liver Injury

HSC = Hepatic stellate cells, which are the main ECM-producing cell type in the liver

• In acute injury, HSCs are minimally and temporarily

activated, inducing primarily MMP production via TNFa inflammatory cytokine

• Native cells degrade and replace the injury ECM

• In chronic injury, HSCs are fully activated and induce more TIMP than MMP, via TGF-B signaling

• Result is wound ECM buildup & fibrosis

Common factors that lead to fibrotic ECM deposition and failure to remodel

 Chronic (too much for too long) inflammation

 Chronic production of ROS and other inflammatory mediators

 Chronic hypoxia

 Aging and/or disease that causes reduction in quantity of cells that would remodel the fibrotic ECM

 Repeated damage to tissue where the tissue is redamaged before the wound healing process can complete.

 The wound is too large for the tissue to heal. Fibrotic ECM becomes a long term structural filler.