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Cell Adhesion and Cell Migration

Antonia Jameson Jordan, DVM, Ph.D. November 24, 2008

Outline:

•  Overview of the kinds of adhesions that cells make

•  Anchoring junctions – adherens junctions – desmosomes

•  The organization of adhesions at epithelia •  Tight junctions •  Migration

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Figure 19-2 Molecular Biology of the Cell (© Garland Science 2008)

Four functional classes of cell junctions in animal tissues: •  Anchoring junctions

–  Cell-cell and cell-matrix •  Transmit stresses through tethering to cytoskeleton

•  Occluding junctions –  Seal gaps between cells to make an impermeable barrier

•  Channel-forming junctions (gap junctions) –  Link cytoplasms of adjacent cells

•  Signal-relaying junctions –  Synapses in nervous system, immunological

Figure 19-1 Molecular Biology of the Cell (© Garland Science 2008)

Anchoring junctions transmit stresses and are tethered to the cytoskeletal elements:

•  Connective tissue - –  Main stress-bearing component is the ECM

•  Epithelial tissue –  Cytoskeletons transmit mechanical stresses

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Table 19-2 Molecular Biology of the Cell (© Garland Science 2008)

Anchoring junctions:

Figure 19-4 Molecular Biology of the Cell (© Garland Science 2008)

Transmembrane adhesion proteins link the cytoskeleton to extracellular structures:

•  Cell-cell adhesions usually mediated by cadherins •  Cell-matrix adhesions usually mediated by integrins •  Internal linkage to cytoskeleton is mediated by intracellular

anchor proteins

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Figure 19-7 Molecular Biology of the Cell (© Garland Science 2008)

The cadherin superfamily includes hundreds of different proteins:

•  Take their name from their dependence on calcium

•  Extracellular domain containing multiple copies of the cadherin motif

•  Intracellular portions varied •  Adhesive and signaling functions

Cadherins mediate Ca2+-dependent cell-cell adhesion in all animals:

•  Main adhesion molecules holding cells together in early embryonic tissues

Figure 19-5 Molecular Biology of the Cell (© Garland Science 2008)

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Figure 19-9a Molecular Biology of the Cell (© Garland Science 2008)

Cadherins mediate homophilic adhesion: •  Cadherins of a specific subtype on one cell will bind

cadherins of the same type on another cell

Figure 19-9b Molecular Biology of the Cell (© Garland Science 2008)

In the absence of calcium the structure becomes floppy:

•  Series of compact domains (cadherin repeats) joined by flexible hinges

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Figure 19-9c Molecular Biology of the Cell (© Garland Science 2008)

The “Velcro” principle of adhesion: •  Low-affinity binding to ligand •  Strength comes from multiple bonds in parallel •  Allows for easy disassembly

Figure 19-10 Molecular Biology of the Cell (© Garland Science 2008)

Selective cell-cell adhesion enables dissociated vertebrate cells to reassemble

into organized tissues: •  Homophilic attachment allows for highly selective

recognition •  Cells of similar type stick together and stay segregated

from other cell types

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Figure 19-12a,b Molecular Biology of the Cell (© Garland Science 2008)

Cadherins control the selective assortment of cells:

•  Appearance and disappearance of specific cadherins

•  A. is labeled for E-cadherin •  B. is labeled for N-cadherin

Figure 19-11 Molecular Biology of the Cell (© Garland Science 2008)

Selective dispersal and reassembly of cells to form tissues in a vertebrate embryo:

•  Cells from epithelial neural tube alter their adhesive properties

•  Epithelial-mesenchymal transition

•  Migrate –  Chemotaxis –  Chemorepulsion –  Contact guidance

•  Re-aggregate

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Figure 19-12c Molecular Biology of the Cell (© Garland Science 2008)

Twist is a transcription factor that regulates epithelial-mesenchymal transitions:

•  Epithelial cells can dis-assemble, migrate away from parent tissue as individual cells -- epithelial-mesenchymal transition

•  Part of normal development, e.g., neural crest •  Twist is essential for neural crest cell development in

embryogenesis •  Twist represses transcription of E-cadherin •  Twist contributes to metastasis in human breast cancers

Figure 19-14 Molecular Biology of the Cell (© Garland Science 2008)

Catenins link classical cadherins to the actin cytoskeleton:

•  Intracellular domains of the cadherins provide anchorage for cytoskeletal filaments

•  Intracellular anchor proteins assemble on the tail of the cadherin

•  Catenins –  β, γ, p120-catenin

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Figure 19-15 Molecular Biology of the Cell (© Garland Science 2008)

Adherens junctions coordinate the actin-based motility of adjacent cells:

•  Allow cells to coordinate the activities of their cytoskeletons •  Form a continuous adhesion belt around each of the interacting cells

in a sheet of epithelium •  Network can contract via myosin motor proteins

–  Motile force for folding of epithelial sheets

Figure 19-16 Molecular Biology of the Cell (© Garland Science 2008)

Adherens junctions coordinate the actin-based motility of adjacent cells:

•  Oriented contraction of bundles of actin filaments running along the adhesion belts causes narrowing of the cells at the apex

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Figure 19-17a Molecular Biology of the Cell (© Garland Science 2008)

Desmosome junctions give epithelia mechanical strength:

•  Structurally similar to adherens junctions •  Link to intermediate filaments

Figure 19-17b Molecular Biology of the Cell (© Garland Science 2008)

Molecular components of a desmosome:

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Figure 19-18 Molecular Biology of the Cell (© Garland Science 2008)

Desmosomes, hemidesmosomes, and the intermediate filament network:

•  Form a structural framework of great tensile strength

Desmoplakin mutations: •  Clinical features include varying degrees of keratoderma,

blisters, nail dystrophy, wooly hair, cardiomyopathy

From Clinical and Experimental Dermatology, 30, 261-266 (2005)

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Clinical importance of desmosomal junctions:

•  Pemphigus –  Auto-antibodies against desmosomal cadherins

•  Cells become “unglued” from each other –  Severe blistering of the skin

Pemphigus foliacious – antibodies against desmoglein 1

Cell-cell junctions send signals into the cell interior:

•  Cross-talk between adhesion machinery and cell signaling pathways allows cell to make or break attachments as dictated by circumstances –  Analagous to cross-talk between integrin signaling and other

signaling pathways

•  Contact inhibition –  In general, when cells are attached to other cells, proliferation is

inhibited –  When attachments are broken, proliferation is stimulated

•  Physiologic example –  Repair a breach in the epithelium

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Figure 6.26 The Biology of Cancer (© Garland Science 2007)

Beta-catenin has dual functions: •  Anchor protein at adherens junctions •  Transcription factor •  Location (at adherens junction versus in the nucleus) determines

its function at any given time

Figure 14.14c The Biology of Cancer (© Garland Science 2007)

Effect of epithelial-mesenchymal transition on β-catenin localization:

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Figure 19-3 Molecular Biology of the Cell (© Garland Science 2008)

Organization of cell junctions in epithelia: •  Relative positions of the junctions are the same in all

epithelia

Figure 19-23 and 19-27 Molecular Biology of the Cell (© Garland Science 2008)

Tight junctions form a seal between cells and a fence between membrane domains:

•  Cells need to segregate proteins to appropriate domain (apical or basolateral)

•  Prevent backflow from one side of the epithelium to the other

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Figure 19-24 Molecular Biology of the Cell (© Garland Science 2008)

The role of tight junctions in allowing epithelia to serve as barriers to solute diffusion:

•  A small extracellular tracer molecule added to one side is prevented from diffusing to the other side by tight junctions

•  Epithelial cells can transiently alter tight junctions to increase permeability of the tissue – paracellular transport

Downloaded from: StudentConsult (on 23 November 2008 06:19 PM) © 2005 Elsevier

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http://cellimages.ascb.org/cdm4/item_viewer.php?CISOROOT=/p4041coll12&CISOPTR=79&CISOBOX=1&REC=1&DMROTATE=

90

Electron micrograph of a bile canaliculus

Downloaded from: StudentConsult (on 23 November 2008 06:19 PM) © 2005 Elsevier

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Figure 19-26 Molecular Biology of the Cell (© Garland Science 2008)

How a tight junction works: •  Branching networks of sealing strands encircle the apical

end of cell in the sheet •  Each strand is composed of a long row of

transmembrane adhesion proteins embedded in each of the two interacting plasma membranes

•  Extracellular domains adhere to one another, occluding the intercellular space

Assembly of a junctional complex depends on scaffold proteins:

•  Junctional complex –  Tight junction –  Adherens junction –  Desmosomal junction

•  Intracellular scaffold proteins position and organize the tight junctions into the correct relationship with the other components of the junctional complex

•  Tjp (Tight junction protein) family or ZO (zonula occludens) protein

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Scaffold proteins in junctional complexes play a key part in the control of cell

proliferation: •  Loss of adhesive contacts with neighbors triggers

proliferation –  Means to heal a defect in an epithelium

•  Decreased expression of ZO protein in many tumors

Figure 19-29 Molecular Biology of the Cell (© Garland Science 2008)

Cell-cell junctions and the basal lamina govern apico-basal polarity in epithelia:

•  Cells need to establish polarity in orientation with surroundings •  Protein complexes that regulate polarity assemble at tight junctions

so that neighboring cells are oriented correctly in relation to each other

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The connections between cell adhesion, ECM, and cell migration:

•  To get out of bloodstream to site of inflammation, they need to make an attachment to the endothelium

•  Then they will have to traverse a basement membrane •  Then they will need to navigate through the ECM

•  Cells crawl. They do not swim.

Figure 19-19 Molecular Biology of the Cell (© Garland Science 2008)

Selectins mediate transient cell-cell adhesions in the bloodstream:

•  Selectins are cell-surface carbohydrate-binding proteins that mediate transient cell-cell interactions –  At a site of inflammation, the endothelial cells express selectins that

bind to oligosaccharides on the surface of a leukocyte

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Figure 19-20 Molecular Biology of the Cell (© Garland Science 2008)

Strong integrin-mediated adhesions are required for extravasation of leukocytes:

•  Leukocyte integrins bind endothelial cell proteins to make a stronger attachment –  Members of immunoglobulin superfamily

•  ICAMs (intercellular adhesion molecules) •  VCAMs (vascular cell adhesion molecules)

Bovine leukocyte adhesion deficiency: •  Defect in neutrophil β2 integrin chain •  Neutrophils unable to leave bloodstream •  Clinical consequences: pneumonia, enteritis, stomatitis •  Autosomal recessive

–  Bulls are routinely tested now

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Cells have to be able to degrade matrix: •  Physiologic examples:

–  Leukocytes need to degrade the basal lamina of a blood vessel to escape

–  Fibroblasts that are embedded in connective tissue need to degrade matrix in order to divide

•  Two classes of proteases –  Matrix metalloproteinases

•  Depend on Ca2+ or Zn2+ –  Serine proteases

•  Protease activity must be tightly regulated –  Local activation

•  Synthesized as inactive precursors –  Confinement by cell-surface receptors –  Secretion of inhibitors

•  Tissue inhibitors of metalloproteases (TIMPs) •  Serpins

Figure 16-86 Molecular Biology of the Cell (© Garland Science 2008)

Events necessary for cell motility:

•  Actin polymerization •  Delivery of membrane to

the leading edge •  Formation of attachments

at leading edge to provide traction

•  Contraction at rear •  Disassembly of

attachments in rear of cell

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Figure 19-52a Molecular Biology of the Cell (© Garland Science 2008)

Cell adhesion and traction allow cells to pull themselves forward:

•  Cell forms integrin-mediated attachment sites at the leading edge – focal adhesions – These allow the

cell to generate traction and pull its body forward

Figure 6.24a The Biology of Cancer (© Garland Science 2007)

Integrins recruit intracellular signaling proteins at sites of cell-substratum adhesion:

•  Focal adhesion kinase (FAK)

•  Tyrosine phosphorylation by FAK creates docking sites for other signaling proteins

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Figure 6.24b The Biology of Cancer (© Garland Science 2007)

FAK in “command and control” of cell motility:

Events that need to be coordinated during cell migration:

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