Integrin signalling Vytek 2009 Extracellular matrix ECM Network

Integrin signalling Vytášek 2009

Extracellular matrix (ECM) • Network of macromolecules (proteins, glycoproteins, proteoglycans and polysaccharides) • Provides mechanical support for cells • Influences the behavior and differentiation of cells which are in contact with it • The main receptors of cells, mediating the interaction of cells with ECM, are known as integrins

Integrins • Necessary component of all Metazoans (from sponges to humans) • Any member of the large family of transmembrane proteins that acts as receptor for adhesion molecules of extracellular matrix • It plays a role in the attachment of the cell to the ECM and to other cells, and in signal transduction between ECM and the cell. This transduction is signaling from ECM to the cell (outside-in signaling) as well as signaling from the cell to extracellular space (inside-out signaling)

Integrin • Transmembrane protein which extracellular part is receptor for adhesion molecules of extracellular matrix • Heterodimeric molecules in which the and subunits are noncovalently bonded • subunit has four Ca 2+ -binding domain on its extracellular part of polypeptide chain • subunit bears a number of cysteine-rich domains on its extracellular part of polypeptide chain

Adhesive sequences in matrix proteins and their integrin receptor

Outside-in signaling 1) Occupation of receptor site of integrin by appropriate ECM component causes conformational changes 2) Rearrangements also occur in intracellular part of integrin molecule and cause subsequent interactions with intracellular signaling pathways (eg. activation cytoplasmatic TPK (FAK) and serine/threonine kinases , activation of small GTPases, induction of calcium transport or changes of phospholipid synthesis)

Outside-in signaling 3) Conformational alterations also influence integrin clustering which further modify interactions between cytoplasmatic domains of integrin 4) Cellular phenotypes are determined by this type of signaling; integrins serve for cells as mechanosensors transfering information about ECM into cell and then altering cytoskeleton

Focal adhesion kinase (FAK) Protein tyrosine kinase 2 (PTK 2) • cytoplasmic protein nonreceptor tyrosine kinase • concentrated in the focal adhesions • phosphorylated in response to integrin interaction • the first activation step is autophosphorylation at Tyr 397 full activation is achieved after further phosphorylation (Src, Fyn)

Stages of integrin receptor • Low affinity receptor (cells are not activated) • High affinity receptor (activated cells) • High affinity receptor and clustering of integrin (high avidity activation)

Inside-out signaling • Principle of signaling is changing affinity of integrin receptor for ECM • Nonactivated cells with low affinity integrin receptor after activation bound their intracellular part of integrin with integrin activating complex. It leads to conformation change of integrin molecule including extracellular part • Conformation alternation change affinity of receptor part of integrin from low to high afinity and binding to ECM in greater extend

Integrin activation in hematopoietic cells • Typical example is activation of platelets • Platelet integrin is maintained in the inactive state with low affinity for its extracellular ligand – fibrinogen • After platelet stimulation integrin is rapidly activated to high affinity leading to fibrinogen binding

Activation in tissues • Adherent cells (eg. fibroblasts) must be attached to ECM in constant manner to maintain tissue integrity • Inside-out signaling of integrin is very important during tissue remodeling and repairing after tissue damage • Migration of cells is also regulated by this signalling

Summary • Integrins, commonly found in Metazoans, mediate attachment cells to extracellular matrix and to other cells • Outside-in integrin signaling (binding of ligand to extracellular domain of integrin) regulates processes as protein phosphorylation, proliferation, differentiation and apoptosis • Inside-out signaling is caused by changing of affinity of ligand binding domain due allosteric rearrengement after association of cytoplasmatic integrin complex with intracellular part of integrin molecule

Further informations • Goldfinger, L. E. , Ginsberg, M. H. : Integrin signaling in Encyklopedia of Biological Chemistry Vol. 2 (2004) 441 -445 • http: //student. biology. arizona. edu/honors 2001/gro up 08/intro 2. htm • http: //student. biology. arizona. edu/honors 2001/gro up 08/intro 3. htm • http: //www. geocities. com/capecanaveral/9629/ • Trends Cell Biol 14(2004)678 -686 • Schwartz, M. A. : The Force Is with Us. Science 323(2009) 588 -589