Cell Motility Learning Objectives

Cell Motility Learning Objectives

1. Be able to define cell motility and give four examples of the process within the body.

Cell motility is the directed movement of a cell. It is important four four reasons: wandering cells must get to sites of infections, cells must migrate during embryology and normal development, cell motility is involved in wound healing, and is involved in the spread of cancer throughout the body.

2. Be able to describe and relate the components and the process that enables a cell to exit through a vessel wall (extravasation). Specifically a leukocyte leaving a blood capillary to go after the site of an infection.

Extravasation is the process of attaching to the endothelial lining followed by migration of the cell through the vessel wall. The process begins with activation of the endothelial cells. This is performed by cytokines from the antibody/antigen interaction or released by mast cells. This causes exocytosis of P-selectin within seconds. Also production of PAF (platelet activating factor) by the endothelium. Trapping is next. This involves the binding and rolling to a stop. The P-selectin temporarily binds to selectin receptor on the leukocyte. This is not very tightly bound, the bonds break easily and the cell rolls. PAF activates PAF receptor on leukocyte that causes conformational change in integrin on the surface of leukocyte. Next we move on to Adhesion. Here a tight adhesion of the leukocyte to wall occurs when integrin of the leukocyte binds to ICAM (intracellular adhesion molecule) of the endothelial cell. The leukocyte is now tightly attached and the cell thins and spreads out over the capillary surface. The next step is Migration through the wall. The cell has lamellapodia or pseudopodia that push between the adjacent cells. Metalloproteases secreted by the leukocyte breaks the junctional complex (E-cadherin). The pseudopodia grabs on to the basement membrane and pulls the cell through into extracellular connective tissue space. Focal contacts are the leading edge of pseudopodia which bind to the basement membrane. Integrin is a transmembrane protein involved in the binding. The cystolic side links to cytoskeleton filaments of actin. The exterior side attaches to laminin, collagen IV, or fibronectin to pull itself through wall.

Integrin is a cell surface adhesion molecule that acts to bind cell to cell and cell to extracellular matrix. It is a transmembrane protein consisting of alpha and beta heterodimer subunits. Different mixes of subunits bind to different extracellular materials to give it specificity for only certain substrates. Typically the various integrins have relatively low binding affinities for their ligand. It takes the weak bindings of numerous integrin molecules to give the cell a firm hold onto the extracellular matrix.

3. Be able to describe and relate the process and structures needed for a cell to move through the connective tissue space (diapedesis).

Diapedisis is cell walking. There are two types. Slow moving occurs in fibroblasts and the growth cones of neurons. Fast moving occur in leukocytes and macrophages. We can see “cytoplasmic streaming.” Both types of motility have the same basic steps, differing only in speed.

The first major step is the extension of the lamellapodia (podosomes). Next is adhesion by focal attachments to adhesion molecules and collagen fibers in the extracellular matrix. These focal adhesions attach the cell to substratum, give cell a point to pull against or move toward, and prevent the leading edge of lamella or pseudopodia from retracting. Next the cytoplasm flows forward. The actin fiber extension occurs and the cytoskeletal framework shifts forward. The cytoplasm of the cell flows into lamellapodia in the direction of the chemotaxic signals released by the mast cells. The cytoplasm changes form (gel like to solution) to readily flow into the expanding process. Finally there is retraction with a footprint. The trailing edge thins and forms retraction fiber. This “snaps” leaving behind a footprint or a bit of membrane and cytoplasm at focal attachment point.

Movement of the cell membrane forward occurs in one of three mechanisms. First option is that cytoskeletal elements, actin filaments, are bound to membrane wall by myosin I. The microfilaments grow, polymerize under the influence of profilin to push membrane ahead of it. Another option is myosin I, with plasma membrane attached to it, extends the microfilaments without profilin but still push the membrane forward. The final way is myosin I attaches plasma membrane to cytoskeleton and myosin I moves forward within the membrane to shift microfilaments forward. This rapid shift of cytoplasm from gel to solution thought to be caused by increased osmotic pressure in forward edge. In each of the mechinisms is the idea that the nucleus and all other organelles are attached or trapped within the cytoskeleton and are moved forward with the cytoskelton framework.

4. Be able to relate how a cell knows where to exit a vessel wall to enter the connective tissue space.

The cell is directed by chemotaxtic factors (cytokines) and calcium concentration gradients. Different integrin subunit combinations give it specificity for only certain substrates. Some are for ECM, some for type IV collagen, laminin, fibronectin, leukocytes...

5. Be able to relate and describe how different cell junctions, basement membranes and adhesion molecules interact during cell motility.

Cell Junctions are specific morphologically defined regions of the cell membrane. Cell-to-cell adhesions include tight junctions (zonula occludens), adhesion belts (zonula adherens(cadherins)), desmosomes (macula adherens(cadherins)), and gap junctions (nexus/communicating junctions). Cell-to-matrix adhesion includes hemidesmosomes (integrins) and focal attachments (integrin).

Non-junctional adhesion occurs where no structural entity is seen in the membranes but physiologically there is adherence by the membranes and biochemical adhesion molecules are present. Cell-to-cell adhesions come under the four families of cell surface molcules: cadherins, Ig-like CAMs (cell adhesion molecules), integrins, and selectins (bind to carbohydrates on cell surface). Cell-to-matrix adhesion most often is integrins but sometimes uses proteoglycan surface molecules. Specifically transmembrane surface molcules like syndycan and fibroglycan.

Most cell-to-cell junctional and non-junctional adhesion involves cadherin. Unsually mediates homophilic interactions between like cells. Most cell-to-matrix junctional and non-junctional adhesions use integrin. This usually mediates heterophilic interactions (cells to matrix). The interaction of non-junctional cell surface molecules is not enough to ensure cell adhesion. A reasonable hypothesis is that non-junctional, cell-to-cell adhesion molecules initiate tissue specific cell-cell adhesions, which are then stabilized by the assembly of full-blown intercellular junctions.

6. Describe how cell motility is important during infection, embryological development, repair, and the spread of cancer.

The way to stop metastisis of cancer is to phosphorylate integrins in the inactivation stage. Phosphoyrylation of integrin on the cystolic side is thought to prevent integrin from binding to fibronectin thus allowing the cells to round up and undergo mitosis. In organ formation, the basal lamina is essential for differentiation and formation fo the salivary gland. Cells hellp create order in the extracellular matrix by integrins. When a cell is attached to ECM it forms stress fibers which disappear when cells become motile. Integrins mediate the interaction between the cytoskeleton and the ECM. Cells in the organized matrix very quickly align their cytoskeleton and the arrangement is propagated throughout the forming tissue. They are also involved in reconstruction of regenerating myoneural junctions dependent of basal lamina. Lamin has been shown to regulate neuronal outgrowth. Fibronectin promotes migration and helps with blood clots.

7. Be able to relate and describe the processes ocurring inside the cell as it pulls itself along.

8. Be able to describe and discuss the mechanisms of junctional and non-junctional cell adhesion.

Leukocyte Adhesion Deficiency is improperly produced integrin and at this stage the leukocytes cannot effectively migrate out of the blood vessel. A lack of beta 3 integrin causes excessive bleeding due to lack of clotting called Glanzmann's disease.