cell interactions and extracellular matrix 3

cell junctions

• permanent anchorage

• stability

• communication

cell adhesion

• transient anchorage

• cell-cell recognition

• often before cell junctions are established

• partially overlapping molecules

neural tube development

1. neural crest cells exit from neural tube

2. migration of neural crest cells

3. aggregation of neural crest cells

4. differentiation of neural crest cells into nerve cells and satellite cells (peripheral ganglia)

cell dissociation experiments

• embryonic tissue: easy dissociation

  • trypsin (protease)

  • ethylenediamine-tetraacetic acid (EDTA)

• mix dissociated cells from two different embryonic origins (liver and retina)

• cells sort by cadherin type and level

cadherin-dependent cell sorting

• different types

  • L cells (fibroblasts) don’t express cadherins

  • cells with added E-cadherin sort to center

  • cells with added N-cadherin sort to outside

• different levels

  • cells with high levels of E-cadherin sort to center

  • cells with low levels of E-cadherin sort to outside

mechanisms of cell-cell adhesion

• preferred

  • homophilic binding (most frequently)

  • heterophilic binding

• occasionally possible

  • binding through an extracellular linker molecule

cell adhesion molecules

• cadherins

  • has Ca²⁺ binding sites

• Ig-superfamily CAMs

  • Ig domains

  • type III fibronectin repeats

• mucin-like CAMs

  • carbohydrate region

  • lectin domain

• selectins

  • binds calcium

• integrin

  • binds fibronectin

  • binds calcium

cadherin superfamily structure

• variable

• intracellular C-terminus

  • except T-cadherin

    • extracellular

    • linked to membrane by glycosylphosphatidylinositol (GPI) anchor

• extracellular N-terminus

• extracellular cadherin domain

classical cadherin (E-cadherin) structure

• five cadherin repeats outside of cell

  • similar to immunoglobin fold

• three calcium molecules between final cadherin repeats

• two calcium molecules between other cadherin repeats

• C-terminus in cytosol

• N-terminus outside of cell

function of calcium in cadherins

• 2 molecules of Ca²⁺ between cadherin repeats other than tip protects against degradation by proteases and allows dimerization

• 3 molecules of Ca²⁺ before tip cadherin repeat allows tetramer formation between cells

affinity

describes interaction of two components (relatively low of cadherins)

avidity

accumulated strength of multiple affinities (high for cadherins)

mouse embryo compaction

in part due to presence of E-cadherin around 3 days post fertilization

cadherin diversity in the central nervous system

• classical cadherins

  • E-cadherin, R-cadherin, cadherin-6 found in different regions of embryonic mouse brain

• protocadherin gene (non-classical)

  • contains variable region exons for extracellular domains (V1-V15) and constant regions (C1-C3)

  • after transcription and RNA splicing, only one variable exon remains

linkage of classical cadherins in cell adhesion

• p120 bound to intracellular cadherin domain

• β catenin bound to intracellular cadherin domain and p120

• α catenin bound to β catenin and actin filament

• → 2 cadherin proteins, each bound to an actin filament

signaling of classical cadherins in cell adhesion

1. Rac1 (GTP binding signaling molecule) bound to GDP and GDI (GDP dissociation inhibitor Rho)

2. binding between two cadherin dimers (between cells) recruits Rac1 to membrane and GDI dissociates

3. cadherin binding activates phosphoinositide 3-kinsase (PI3K)

4. PI3K phosphorylates and activates GEF Tiam1 (guanine nucleotide exchange factor)

5. GEF Tiam1 exchanges GDP bound to Rac1 for GTP (→ Rac1-GTP)

6. Rac1-GTP initiates actin assembly

selectins

• oligosaccharide (carbohydrate) binding proteins

• calcium-dependent

• function in inflammation

L-selectin

expressed in lymphocytes

P-selectin

expressed in platelets and endothelial cells

E-selectin

expressed in activated endothelial cells

inflammation

• at inflammation site, endothelial cells express selectins, and oligosaccharides are located on lymphocytes

• selectin-dependent weak adhesion and rolling of lymphocytes on endothelial cells

• integrin-dependent strong adhesion and emigration of lymphocytes into tissue

Ig-like cell adhesion proteins (N-CAM)

• homophilic interaction

• not dependent on calcium

• 20 different forms through alternative splicing

• coexpressed with cadherins

• functions in fine-tuning of interactions during development and regeneration

• disulfide bonds in Ig-like domain made by protein disulfide isomerase

  • non-spontaneous

  • happens during secretion onto plasma membrane

• can have extracellular C-terminus, linked to membrane by GPI anchor or transmembrane with C-terminus in cytosol

N-CAM

neural cell adhesion molecule

I-CAM

intercellular adhesion molecules

V-CAM

vascular cell adhesion molecules