Cellular Adhesion and Tissue Ahmed A. Mirza, PhD, MT(ASCP)

Outline  Introduction  Importance of adhesion in  Tissue development Differential cell hypothesis  Cell junctions  Anchoring molecules  Cadherins  Integrins  Cell migration  CAMs and Selectins  Clinical relevance of adhesion Homeostasis Infection

Cellular Adhesion Lecture Objectives By the end of this lecture the student should be able to: – Define the Differential cell affinity hypothesis – Give an example to describe the hypothesis at work – List the four types of junctions in the cell and discuss their functions – Describe the function of their components – Know the difference between the junctions and be able to identify them visually from a figure – Define CAMs and their superfamilies – Discuss the functions and the role of Integrins, cadherens, and selectins – name examples of: Integrins, cadherens, selectins – The role of CAMs in cell migration – List possible pathologies of defection in CAMs

Introduction Varying degrees of cell-to-cell adhesion  Very strong (found in tissue) to very transient (found in WBC)  Different molecules with different tasks  Found everywhere in the body  Changing level of expression based on: Cell cycle and differentiation Outside factors and ligands

Examples... Platelets aggregation at a wound site Migration of white cells or monocytes into a site of infection Migration of osteoblasts to repair bone

Importance of Adhesion

Tissue formation  In order for tissue to be formed, early cells of similar type must come together to develop into a functional tissue  Different tissue forms due to a process explained by the Differential cell affinity hypothesis or Differential Adhesion hypothesis.

What is this hypothesis? According to this hypothesis, cells congregate near other similar cells with similar adhesion molecules to maximize the bonding (adhesion) strength between cells by creating a more hermodynamically stable (less energy costing) tissue or structure.

 And even if we purposely mix the cells of different germ layers (epidermal and neural in the figure below), they will eventually find their like and congregate to for two separate entities

In other words: Cells could have the same type of “glue” on their surface. The amount of “glue” or its different distribution on the cell could cause differences in the number of stable connections between cells and their different types. Accordingly, the differences in strength of cohesion between cells is caused by the types and numbers of adhesion molecules on expressed on cell surfaces.

Example: Tissue of early stages of embryo development – the separation of the ectoderm from the neural tube and formation of the neural tube. Cells expressing certain molecules on their surface come together.

Cell junctions Four functional classes of cell junctions in animal tissues: Anchoring junctions Provide structure and strength to epithelial cells Transmit stresses by pulling on the cytoskeleton Some can stand higher stress on the tissue (e.g., connective tissue)

Two types: cell-to-cell and cell-to-matrix Cell-to-cell: two structures Adherens: forms anchoring points that connects the cell wall of one cell to that of the adjacent one Desmosomes: forms anchoring points that connects components of the cytosceleton of the cell to the adjacent one Cell-to-matrix: actin-linked and hemidesmosomes Both anchor the cell filements to the extracellular matrix Important molecule families: cadherins, integrins

Occluding junctions (tight junctions) Seal gaps between cells to make them impermeable (e.g., intestine) Establishes polarity (apical/basal) Prevent backflow from one side of the epithelial layer to the next Important molecules : Channel-forming junctions (gap junctions) Link cytoplasms of adjacent cells Signal-relaying junctions Synapses in nervous system

Junction summary figure…

Adhesion Molecules Transmembrane adhesion proteins link the cytoskeleton to extracellular structures: Cell-cell adhesions (cadherins) Cell-matrix adhesions (integrins) Internal linkage to cytoskeleton (intracellular anchor proteins)

Cadherins The cadherin superfamily includes hundreds of different proteins: Take their name from their dependence on calcium Extracellular domain has multiple copies of the cadherin motif Intracellular domain that varies Adhesive and signaling functions Forms strong binding from many repeats attaching at the same time – like “Velcro”

Cadherins Velcro

Integrins Involved in cell-extracellular matrix adhesion and cell-cell adhesion Structure: heterodimer consisting of two transmembrane glycoprotein subunits (α and β), which are non-covalently bound. Both subunits contribute to ligand binding. About 18 α subunits and 8 β subunits have been identified, giving ~24 unique integrins. Integrins are necessary during early development such as fertilization, gastrulation, implantation, vessel formation and other processes, but also during migration (discussed later)

Anchoring molecules summary…

Cell Migration

Cellular migration is a very complex process

CAMs Cell adhesion molecules (CAMs) are proteins on the surfaces of cells (such as WBC) to endothelial cells, to each other or to the extracellular matrix. Three superfamilies: Immunoglobulin, integrin and selectin families The extracellular domains of these molecules trigger a reaction inside the cell that will lead to an immune response or regulation of one Examples: ICAM (intercellular), VCAM (vascular), P-selectin

Selectins Selectins play a role in WBC migration from blood vessels into the tissues during inflammation. Different adhesion molecules are involved at each stage. At the inflammation site, endothelial cells express seletins on the surface, which bind to molecules on the WBC and bring it down to a slow role, but not stopping. Eventually stronger binding by specifically expressed integrins on the WBC will bring it to a stop and attach it to cells at the inflammation site Finally the WBC traverse into the tissue

Clinical Relevance

Cell-cell junction problems… Desmoplakin mutation (in the demosome) Keratoderma, nail dystrophy cardiomyopathy

Integrin problems… Human Luekocyte adhesion deficiency Lack of integrin beta-2 (CD18) Increased bacterial infection after birth Glanzmann’s thrombasthenia defected integrin αIIbβ-3 Increased mucosal bleeding

References Molecular Biology of the Cell (Garland science 2008) migration. Accessed on 10/20/14 AdhesionMolI.aspx. Accessed on 10/23/14