5. Cell adhesion and the actin cytoskeleton

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The cell cortex

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The cytoskeleton networks

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The actin cytoskeleton

 Adhesion proteins  The extracellular matrix  The cell architecture is regulated by the adhesion zones 1

The cell cortex

Actin microfilaments

0.2 µm

Protein complexes membranes

5 nm

Medalia et al. 2002 Science 298: 1209-1213 100 nm cryoelectron tomography

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The cell cytoskeleton networks

Cells possess two or three cytoskeleton networks made of polymerized proteins : an actin cytoskeleton ( microfilaments ), a tubuline one ( microtubules ) and intermediate filaments

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The cycle of microfilament polymerisation-depolymerisation is coupled to ATP hydrolysis by actin

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The cycle of microtubule polymerisation-depolymerisation is coupled to GTP hydrolysis by tubulin

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Intermediate filaments are controlled by protein phosphorylation and dephosphorylation About hundred proteins control the growth and dissociation rates or are associated to microfilaments and microtubules : actin binding proteins, microtubule associated proteins, bundling proteins, molecular motors ...

Intermediate filaments

3

microfilaments microtubules

The actin cytoskeleton

Microfilaments are actin polymers, an ATP binding and ATP hydrolysing protein. The cycle of microfilament polymerisation-depolymerisation is coupled to ATP hydrolysis by actin Microfilaments constitute the membrane cortex which allows the deformation of the plasma membrane. Stress fibers are actin microfilaments that link adhesion focal points . Actin microfilaments are reversibly linked to the plasma membrane by specific proteins (talin, catenin, ezrin ...) Mechanical forces are exerted by actin polymerization at the plasma membrane and between actin microfilaments by molecular motors reflection interference contrast microscopy actin immunofluorescence

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Structure of actin microfilaments 4 nm barbed end pointed end 5

Actin polymerization kinetics : 1) no hydrolysis At steady state, V = (1/a)dL/dt = k on C – k off

Critical concentration :

C c = k off /k on = K eq C c +end = C c -end V (polymerization rate)

C

c + end - end C (monomer concentration) 6

Actin polymerization kinetics : 2) ATP hydrolysis V (polymerization rate) + end - end + end - end A ADP A ATP C c ATP

ATP hydrolysis

P i 2 sec At steady state, during treadmilling V = V T (+) + V D (-) = k on T .C

T – k off T + k on D .C

D – k off D C c ADP C (monomer concentration) C D ~ 0.1 C T , k off T < k off D and k on D < k on T In steady state C T c  k off D /k on T 7

Experimental polymerization dynamics Fujiwara et al. PNAS 2007 104 : 8827 –8832 C T c  k off D /k on T  0.25/11.6 = 0.02 µM Physiological actin concentration : about 100 µM Most actin is complexed to thymosin  [Actin T ]  10 µM The theoretical actin microfilament elongation rate is : dL/dt = a.V

T = a.k

on T .[Actin T ] < (4 nm).(12 µM -1 .s

-1 ).(10 µM) = 0.48 µm.s

-1 At the pointed end : -dL/dt = a.V

D = a.k

off D = (4 nm).(0.25 s -1 ) = 1 nm.s

-1 To be compared to the actual velocity of cell edge movements : up to 10 µm.s

-1 8

In vivo evidence of microfilament treadmilling by FRAP experiments

Injection of rhodamine actin Localized photobleaching Transport (treadmilling )

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How cytoskeleton molecules exert forces ?

1. Actin polymerization : the elastic brownian ratchet model (Mogilner and Oster 1996 Biophys. J. 71 : 3030-45)

Stall force

Mechanical work = ATP hydrolysis F.

d = D G/N = D G ° + RT ln{[actin ATP ]} = k B T ln{k on [actin ATP ]/k off } actin ATP + filament N  filament N+1 + P i 10

How cytoskeleton molecules exert forces ?

2. Actin microfilament pressure at nucleation zones (C Sykes & J Prost (2005) P.N.A.S., 102, 7847-52) Actin nucleation at the bead surface  Concentric growth of actin microfilaments  Shear stress accumulation  Breaking the actin gel, resulting in asymmetric growth 11

Visualization of F-actin network movement in motile keratocytes with FSM Yam et al. JCB 178:1207-1221, 2007 30x real time

Actin filament dynamics

12 12

How cytoskeleton molecules exert forces ?

3. Molecular motors sliding on microfilaments 13

Regulation of cytoskeleton dynamics

↑ Lamellipodia

Ridley & Hall 1992 Allen et al. 1997

↑ Stress fibers ↑ Filopodia

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Example : microvilli at the plasma membrane Brush border cells of the intestinal epithelium

Plasma membrane Actin cytoskeleton

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Cell adhesion and the actin cytoskeleton

 The cell cortex  The cytoskeleton networks  The actin cytoskeleton 

Adhesion proteins

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The extracellular matrix



The cell architecture is regulated by the adhesion zones

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The extracellular matrix and cell adhesion molecules

The extracellular matrix is made of macromolecules (proteins and polysaccharides) synthesized and secreted by cells. It provides a specific mechanical and chemical environment for the cells.

Cell adhesion molecules are proteins expressed at the surface of cells that mediate binding to other cells or to the extracellular matrix.

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Extracellular matrix proteins have a complex structure  Proteins of the ECM have a very large multi domain structure  They often contain growth factor-like domains or bind growth factors

A) Fibronectin.

Encoded by a single gene but alternatively spliced at three regions [blue circles and box and V (variable) segment] to generate 12 proteins in rodents and 20 in humans. FN3 domains are widespread in ECM proteins. Binding sites for other matrix proteins are marked. The heparan sulfate –binding site can interact with proteoglycans or with syndecan, an integral-membrane proteoglycan. Integrin-binding sites; RGD (indicated by an asterisk) and LDV (Leu-Asp-Val, indicated by a pound sign). FN is a proangiogenic molecule, whose function depends on both the RGD site and the two alternati vely spliced FN3 domains. FN also binds the proangiogenic growth factors VEGF and HGF.

B) Fibrillin-1.

Fibrillins include EGF-like domains, found in many ECM proteins, as well as TB (TGFb-binding, denoted by T) and hybrid (H) domains, specific to fibrillins and LTBPs. Binding sites for other matrix proteins and growth factors are marked.

C) LTBP-1.

Four-gene family with structures related to fibrillins. Known binding sites for TGF-b/LAP latent complex (SLC, blue), fibrillin, and FN are marked. RGD (asterisk) sequences in fibrillins and LTBPs may bind integrins.

D) Thrombospondin-1 (TSP-1).

TSPs contain TSP1 repeats (also found in other ECM proteins), EGF-like repeats, and a VWC domain, known in other proteins to bind BMPs. TSP3 repeats (purple) and C-terminal domains are unique to TSPs and bind multiple Ca 2+ ions. The RGD (asterisk) sequence is known to bind to integrins. TSPs 1 and 2 have the structure shown, and both have antiangiogenic activity located in the TSP1 repeats, which bind to the CD36 receptor (39) RO Hynnes (2009) the extracellular matrix : not just pretty fibrils

Science

326 : 1216-19

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Fibrillin (2 µm) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Cell adhesion molecules and cell adhesion structures 20

Classes of cell adhesion molecules

Some family members Ca2+ or Mg2+ dependence

Cell-Cell Adhesion

Classical cadherins E, N, P, VE Desmosomal cadherins desmoglein yes yes Ig family members Selectins (blood cells and endothelial cells only) Integrins on blood cells N-CAM L-, E-, and P-selectins α L β 2 (LFA-1) no yes yes

Type

homophilic actin filaments (via catenins) homophilic both heterophilic Intermediate filaments (via desmoplakin, plakoglobin, and other proteins) unknown actin filaments heterophilic

Cytoskeleton association

actin filaments

Cell-Matrix Adhesion

Integrins Transmembrane proteoglycans many types α 6 β 4 syndecans yes yes no adherens junctions desomosomes no no no

Cell structures

heterophilic heterophilic heterophilic actin filaments (via talin, filamin, α actinin, and vinculin) intermediate filaments (via plectin) actin filaments focal adhesions hemidesmosome s no 21

Madin-Darby Canine Kidney cells, an epithelium model

Polarized cell : two compartments around the cell

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Tight junctions separates apical from basolateral compartments The sealing strands hold adjacent membranes together. They are composed of transmembrane proteins (claudins, occludins) that make contact across the intercellular space and create a seal.

Electron micrographs of cells in an epithelium in which a small, extracellular, electron-dense tracer molecule has been added to either the apical side (on the left) or the basolateral side (on the right). In both cases, the tracer is stopped by the tight junction. (photo Daniel Friend) 23

The cell architecture is regulated by the adhesion zones

http://www.cytoo.com/

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Anisotropy of cell adhesive microenvironment governs cell internal organization and orientation of polarity.

Thery M, Racine V, Piel M, Pepin A, Dimitrov A, Chen Y, Sibarita JB, Bornens M. Proc Natl Acad Sci U S A 103(52):19771-6. 2006. Golgi apparatus (in red) in RPE1 cells in standard culture conditions Golgi apparatus (in red) in RPE1 cells on 26 fibronectin micropatterns (in green)