Study Notes on Cell Coat (With Diagram) | Cell Biology

In this article we will discuss about Cell Coat:- 1. Subject-Matter of Cell Coat 2. Functions of Cell Coat.

Subject-Matter of Cell Coat:

A plasma membrane or plasma lemma is rarely found naked. It is always surrounded by a protective layer. In plant cells a cellulose wall is present, whereas, in animal cells some type of external coating is present but that is not the cell wall. In animal cell the exposed surface of outer leaflet of plasma membrane is surrounded and protected by the cell coat.

It is sometimes also called glycoca­lyx since it contains sugar units in glycoprotein and polysaccharides. The cell coat is generally considered to be equivalent to the oligosaccha­ride side chains of glycolipid, trans membrane glycoprotein and trans membrane proteoglycans that stick out from the outer leaflet of plasma membrane (Fig. 4.1).

In some cells (goblet cell of the small intestine) the glycocalix also con­tains both glycoprotein and proteoglycan that have been secreted and then absorbed on the cell surface.

The oligosaccharides are covalently attached to the protein moieties. There are two distinct ways in which oligosaccharides are attached to the membrane glycoproteins. They may be N-linked to an asparagine residue in the polypeptide chain or O-linked to a serine or threonine residue (Fig. 4.2).

The N-linked oligosaccharide contain about 12 sugars and are built up around a common core of mannose residue whereas the O-linked oligosaccharide tends to be shorter, about 4 sugar long.

Beyond the cell coat of many cells, however, there is a separate ‘fuzzy’ layer which is prin­cipally made of carbohydrates secreted by the cell. Two layers are very difficult to distinguish from each other because the fuzzy layer appears as an extension of the cell coat proper.

The cell coat can be seen by a variety of stains, such as PAS (Periodic Acid-Schiff) or Alcian blue, for light microscope and ruthenium or lanthanum red as well as labelled lectin (carbohydrate-binding protein) for electron mi­croscope.

The cell coat varies in thickness depending on the cell types. In general, it is 10 to 20 nm in thickness. But in case of Amoeba the cell coat is made of fine filaments which are 5 to 8 nm in diameter and 100 to 200 nm in length.

The cell coat can resist vigorous mechanical and chemical attacks to protect the cell. But it is lost by washing with the solution of urea or exposure to enzymes. When the solution of urea or enzyme is withdrawn, the cell coat is re­formed by the continuous secretion of cell coat components from the cell.

The biosynthesis of the glycoproteins forming the glycocalyx takes place in the ribosomes of the endoplasmic retic­ulum and the final binding with the oligosac­charide moiety occurs in the Golgi complex. Therefore, the cell coat is secretory product of the cell and is overlaid on the cell surface and undergoes continuous renewal.

Functions of Cell Coat:

The cell coat performs a variety of functions which are of great significance to the study of cell biology. It may act mechanically, protect­ing the plasma membrane and participate in the filtration and diffusion process.

It contains many enzymes involved in the digestion of carbohydrates and proteins. The glycocalyx makes a kind of microenvironment for the cell. The classical ABO blood groups are based on specific antigens of the red cell coat. It also plays a part in cell-cell and cell-matrix recognition process.

These functions will be briefly discussed:

i. Supportive Function:

The cell coat provides mechanical strength to the cell and protects the cell from external injury.

ii. Filtration:

It acts as a filter and regulates the passage of molecules according to size. This types of function is found in case of many capillaries— specially in the kidney glomerulus.

iii. Diffusion Barrier:

The cell coat may change the concentration of different substances at the cell surface by acting as a diffusion barrier.

iv. Microenvironment:

The cell coat has negatively charged sialic acid termini on the glycoprotein and may bind Ca⁺⁺ and Na⁺ ions. Thus it may change the cationic environment of the cell.

v. Enzymes:

Many enzymes including alkaline phosphatase have been detected in the cell coat. All the enzymes are involved in the terminal digestion of carbohydrates.

vi. ABO Antigens:

The classical ABO blood groups are based on specific antigens of the erythrocyte’s cell coat which are specified by the terminal carbohy­drates. Several other histo-compatible antigens are also present on the cell coat, which allow the recognition of the cell of one organism and the rejection of other cell.

vii. Cell to Cell Recognition and Adhesion:

In a tissue, cells are able to recognise similar cells. This phenomenon leads to adhesion among the similar kinds of cells. This is shown by H.V. Wilson’s experiments (1907) with sponge. Sponges are ideal material for this experiment because they are composed of only a few cell types.

The cells are loosely attached to one another and, once dissociated, they rapidly re-associate in the simple isotonic salt solution. If the mechanically dissociated cells of sponges are allowed to remain together, they aggregate into cell clusters and new sponges are formed.

If the mechanically dissociated cells taken from two different species of sponge are mixed together, cells of each species are grouped with their own original type.

The sponges that re-formed are found to be exclu­sively of one or the other species cells. Since the cells of the two species used are of different colours, one red and the other yellow, it will allow us to see how, in mixed cell mass, only the cells of the same species sort out their own type and aggregate to each other.

If the above experiments are carried out in artificial sea water lacking Ca⁺⁺ and Mg⁺⁺, the cells are not able to re-aggregate. In this case, the sponge cell has lost a huge proteoglycan which is called the aggregation factor. When such factor is added, the cells are able to restore the property of self-recognition and to reproduce the aggregation.

A hypothetical model, has also been put forward to explain the mode of function of the aggregation fac­tor. According to this model proteoglycan or aggregation factor appears as two extracellular bridging molecules which are joined by divalent calcium ions.

The free termini of the fused ag­gregation factors carry the carbohydrate moiety which are recognised by the receptors located at the recognition site or base plate of the adjoining cell surface (Fig. 4.3).

The receptors of the two adjoining cells bind with the free termini of aggregation factors by another set of divalent calcium ions. Therefore, when the Ca⁺⁺ is withdrawn, the two aggregation factors are not able to bind with each other and no longer bridge the two cells.

Similar experiments were carried out by A. A. Moscona in the tissue of chick embryo. These tissues were dissociated by trypsin— an enzyme that removes the cell coat. In this experiment tissue specific recognition and adhesion was determined by a radioactive cell- binding assay.

The rate of cell adhesion can be measured by determining the number of radioactively labelled cell bound to the cell aggregates after various periods of time. Two type of cells, e.g., retina cells and liver cells, were taken. Cells of each tissue were divided into groups—one was un-labelled and other was radioactively labelled (Fig. 4.4).

When un-labelled retinal cells were mixed with labelled retina cells, cell to cell adhesion occurred. Similar results were obtained in case of liver cells. But when radioactively labelled retina cells were mixed with un-labelled liver cells, or vice versa, no cell adhesion was detected.

If cells of the chick and mouse embryo are mixed, they re-aggregate according to the origin of tissue rather than the species. The immunological strategy outlined in Fig. 4.5 has also been used to identify a number of the cell coat glycoproteins involved in cell to cell recognition and adhesion in chick embryo.

First of all, antibodies are made against the cells of interest or against plasma membrane isolated from such cells. In a cell surface all coat proteins are not involved in cell-cell adhesion. But antibodies will be produced against all plasma membrane proteins. Antibodies are always divalent type. Isolated antibodies will specifically bind with cell surface bound pro­tein or antigens.

The coat proteins—which are involved in cell-cell adhesion—will make a cross-link between two cells in their antibody- bound state.

But if the divalent antibody is fragmented by protease digestion into monova­lent antibody, then monovalent antibody bound with plasma membrane proteins (antigen) in­hibits the formation of cross-link between two isolated cell and cell-cell adhesion will not take place because the recognition factor is blocked by the monovalent antibody.

After this any one bound antibody is made free from antigen and the rest bound antibodies remain as before. In this state, cells are allowed to adhere. This experiment is done repeatedly for each and every antibody in similar fashion.

No cell adhesion is found in each case unless and until the specific antibody blocking the protein responsible for cell-cell adhesion is made free. By this elimination test the specific protein re­sponsible for cell-cell adhesion can be identified.

From this immunological assay, at least two families of glycoproteins responsible for cell-cell adhesion have been identified:

(i) One operates early in development and does not depend on extra-cellular Ca⁺⁺.

(ii) Other operates later in development and depends on extracellular Ca⁺⁺.

The first category of cell surface glycopro­tein is known as neural cell adhesion molecule (Fig. 4.6) or N-CAM which operates in the early embryonic development and is found on the surface of nerve cells and glial cells and causes them to stick together by Ca⁺⁺ independent mechanism.

The second category of cell-surface glycoproteins are collectively called Cadherin’s.

Cadherin’s are of three subtypes such as:

(i) E-Cadherin,

(ii) N-Cadherin, and

(iii) P-Cadherin.

E-Cadherin is found on many types of ep­ithelial cells, N-Cadherin on nerve, heart and lens cells, and P-Cadherin on cells in the placenta and epidermis. The three cadherin’s are homologous trans membrane glycoprotein, and are made of 700 amino acid residues.

The large extracellular part of polypeptide chain of Cadherin is folded into three homologous domains. In absence of Ca⁺⁺ the Cadherin’s undergo conformational change and are rapidly degraded by proteolytic enzymes.

Cadherin’s play an important role in later stages of vertebrate development because their appearance and disappearance correlate with major morphogenetic events in which tissues segregate from one another. It is now clear that cell adhesion takes place due to presence of cell surface glycoproteins.

It is also evident that cells stick to other cells by forming high-affinity non-covalent bond be­tween complementary molecules. This finding suggests that there are three mechanisms by which cell surface molecules can mediate cell- cell adhesion.

In cell adhesion, complementary molecules interact by a hemophilic bond or by a heterothallic bond or by binding through an extra-cellular linker molecule (Fig. 4.7). N- CAM generally binds by a hemophilic interac­tion whereas Cadherin may interact by any one of three means of binding.

Cell-cell recognition followed by adhesion is an inherent property in multicellular organ­ism. But how do the cells communicate with each other during their recognition? Is there any signal mechanism between them? This point is not so clear in case of multicellular organisms. But some clues are obtained from the study of peculiar life cycle of the cellular slime mould, Dictyostellium discoideum.

The life cycle of this slime mould consists of three phases (Fig. 4.8). The growth phase is a free living and dividing stage in which the organ­ism is unicellular amoeboid. This stage feeds on bacteria, yeast etc. by phagocytosis and divides by binary fission.

When the organism is starved, several amoeba gather together to form tiny (1-2 mm) multicellular worm-like or slug-like structure. Each slug is formed by the aggregation of up to 100,000 cells and shows a variety of behaviors that are not shown by the free-living amoeba.

As the slug migrates, the cells begin to differentiate. Cells in the front of the slug migrate down to become the stalk, while cells in the middle migrate up and differentiate into the collection of spores that form the fruiting body.

During starvation, the amoeba starts to se­crete cyclic AMP which serves as a chemotactic signal that attracts other amoeba. In this case, cyclic AMP acts as an extracellular signalling molecule. Once the aggregation starts, its area of influence is rapidly expanded because the aggregating cells not only respond to the cyclic AMP signal but relay it from cell to cell.

Besides activating the cyclic AMP signalling system, during the first 8 hours of starvation, cells adhere by a Ca⁺⁺ dependent mechanism involving a cell-cell adhesion molecule called contact site B and, after 8 hours, a second adhesion system starts to play in which cells adhere by a Ca⁺⁺ independent mechanism involving a cell-cell adhesion molecule called contact site A.

Both contact sites A and B are the integral plasma membrane glycoprotein. These findings were later used to identify cell-cell adhesion molecule in other multicellular organisms. In a tissue, cells not only adhere with each other, but the cell surface molecules also adhere with the extracellular matrix that serves many functions.