Extracellular matrix, integrins and cell migration (L8) Flashcards
What makes up the extracellular matrix?
The ECM is made up of many components - proteins and other molecules. It is found in between cells. Mainly made up of fibronectin, laminin, collagen and elastin. Fibronectin is very large with lots of domains that have different functions. Laminin is a trimer (alpha, beta and gamma chains) and also have many functions and domains.
What is the structure and functions of integrin?
The integrin family of proteins consists of 18 alpha and 8 beta subtypes which can make 24 different variants of integrins (non covalently associated). They interact with the cytoskeleton and form heterodimers. Each monomer has 1 transmembrane domain, and alpha subunit held by disulphide bridges.
Explain the binding of integrins
Binding of integrins to their matrix ligands is effected by conc of magnesium and calcium in the extracellular medium. The beta subunit contains cystine-rich domains. The intracellular domain binds talin, filamin or alpha-actin. Binding of integrins is a 2-way process. Integrin binding to ECM can be increased by intracellular, and integrin binding to ECM may trigger intracellular signalling. Intracellularly, they bind with vinculin. So the beta subunit does the binding but the dimerisation of the alpha stabilises.
What is FAK and what is its role?
Focal adhesion kinases. Activated on the formation of adhesions, They’re involved in integrin activation. Fibronectin -> integrin -> FAK -> tyrosine phosphorylation -> actin fibres. When integrins cluster at cell-matrix contacts, FAK is reduced to beta subunit by intracellular adaptor proteins like talin. The clustered FAK molecules phosphorylate each other on a specific tyrosine, creating a binding site for Src family (tyrosine kinases). This is important for recycling focal adhesions to enable migration. They act in anoikis (attachment dependent cell death)
How do cells move?
Motility of cells is interlinked with adhesion. In mammals, it’s mainly actin-based. Cells use lamellipodia and actin to move at high speeds. Actin is present at the leading edge and pushes the cell forward.
What is the traditional model of cell movement?
Traditional models of movement suggest that the cell makes new adhesions at the front, and loses ones at the back through recycling of actin. Lamellapodium extend forward, actin polymerisation at the plus end protudes lamellapodium. Signals cause local actin reorganisation. Transmission of tension via cortex. Cell polarity changes and the leading edge moves towards the signal. Myosin II helps retract the tail of the cell. At the leading edge, you get ruffling (caused by capping of actin filaments) and actin transportation. On the electron micrograph you can see lines of actin push in different directions, which is thought to be used in sensing guidance cues.
What drug is used to study actin?
The drug used to study actin is called cytochalasin B - it lands on the plus end of actin and blocks more actin being added
What are APR complexes
APR2/3 resemble actin closely and causes nucleation of actin filaments. ARPs are made up of multiple proteins. They nucleate actin monomers to form a filament (adds to the plus end). They mimic actin to enable the polymerisation. APR complexes branch off of each other at about 70 degrees, giving the ECM more stability and strength. Key kinase Rac stimulates APR2/3
Explain how changes in focal contacts aid motility
Focal contact turnover, actin interactions and microtubules are all important in motility. Adhesions are recycled, leading to a net flow of CAMS. Focal adhesions mature. Roles for actin binding proteins, small G-proteins, myosin generating force.
Low-density focal adhesions (e.g. rac1, cdc42 - dependent (the kinases the focal adhesions require) are immobile in the membrane. Whereas high-density focal adhesions (RhoA, actin-myosin interaction-dependent) can slide around in the membrane. So FAs are stable, and kept, and some only last for a short amount of time.
What are the different signals involved in motility?
Signals can be soluble/diffusable like netrins, or insoluble like CAMs and the ECM which don’t diffuse away. Fibronectin is secreted by a moving cell to lay down a trail for others to follow. Binding antibodies to fibronectin inhibits migration in studies. Also, injections of ARG-GLY-ASP (RGD motif - responsible for cell binding) inhibits migration.
How does membrane cycling work?
The membrane cycling of a moving cell actively participates in migration, it isn’t just dragged along. Done by membrane endocytosis around the cell and exocytosis at the leading edge. There is a net membrane addition. This may generate a force. if the membrane is fixed by focal contacts the cell will move forward. This endocytosis of the ligand helps to establish the gradient more. Or, recycling of receptors can maintain loacl activation (stops saturation)
