Introduction to Molecular and Cell Biology, Biol. 220 Lecture 33: Cell Adhesion Molecules
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Cell-adhesion molecules (CAMs) enable like cells to associate with each other and segregate into distinct tissues (Fig 22.1).

Fig. 22.1

Following aggregation, cells form specialized cell junctions that stablilize cell interactions and promote communication between cells (Fig 22.1).

The extracellular matrix (ECM) is a complex network of proteins and carbohydrates residing between cells that helps bind tissues together and is a reservoir for many hormones controlling cell growth and differentiation (Fig 22.1).  Plants have cell walls which are very different from the extracellular matrix of animals but serve a similar function .

There are five classes major families of CAMs (Fig 22.2).

• Cadherins
• Immunoglobulin (Ig) superfamily
• Selectins
• Mucins
• Integrins
  Fig. 22.2
  
  

Cadherins
There are over 40 different cadherins coded for by separate genes, the majority of the different types are located in brain tissue.

Each cadherin is 720-750 amino acids in length and has the C-termini located in the cytoplasm while the N-terminus is extracellular and is composed of several repeats that bind to Ca²⁺.  Cell-cell adhesion is therefore dependent on Ca²⁺.

During the metastasis of tumor cells, there is a correlation with the loss of cadherins on the cell surface and increased cell mobility.

E-cadherins hold most epithelial cell sheets together.  Epithelial cells line the small intestine and kidney tubules.

Cadherin-containing junctions connect cells to one another (Fig. 22.5).

Fig. 22.5

N-CAM
Nerve-cell adhesion molecules (N-CAMs) belong to the Ig superfamily of CAMs.

Mediate cell adhesion by binding like N-CAMs on adjacent cells.

Coded for by a single gene, the diversity is generated by alternative mRNA splicing and differences in glycosylation (Fig. 22.3)

Adhesive properties are down modulated by addition of long chains of sialic acid (Fig. 17.31).  Highest concentrations of sialic acid found in embryonic tissue.  Thelow adhesion in embryonic nerve cells allows development of nervous system.  One third as much is found in adult tissues.

Fig. 17.31

Selectins
Selectins aid in leukocyte extravasation (Fig. 22.4):
Fig. 22.4

• Endothelial activation causes P-selectin to be displayed on the cell surface.
• Leukocytes are trapped by the P-selectin.
• Endothelial cell signals leukocyte through platelet-activating factor (PAF) to activate alphaL-beta2 integrins.
• AlphaL-beta2 integrins on the leukocyte bind to I-CAM on the epithelial cell and activate the loosening of the tight junctions as leukocyte passes between epithelial cells.

Gap Junctions
Gap junctions are channels between cells (Fig. 22.8) that permit metabolic components to be exchanged between cells.

Fig. 22.8

They are constructed from 12 copies of a single-transmembrane protein, connexin.
.

Integrins
Integrins are hetero dimers of alpha and beta subunits.

There are at least 22 integrin heterodimers composed of 17 types of alpha subunits and 8 types of beta subunits.

Combinations of alpha an beta subunits have specific ligands (Table 22.2).

Table 22.2

Integrins primarily mediqte cell-matrix interactions (Fig. 22.9).

Fig. 22.9

 

Created 2004 by CA Rinehart• email CA Rinehart

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