Plasma Membranes: Intercellular Contact and Communication

In this article we will discuss about the Intercellular Contact and Communication of Plasma Membranes.

Cells have developed specialized regions on their membranes for intercellular communication. Gap junctions mediate and regulate the passage of ions and small molecules through a narrow hydrophilic pore connecting the cytoplasm’s of adjacent cells.

These pores are composed of subunits called “connexions”. Connexions consist of 6 protein subunits that span the membrane and connect with the analogous structures on the adjacent cells.

In response to specific chemical stimuli, the subunits rearrange themselves relative to one another to pro­vide a tangential central opening about 2 nm in diameter and through this central opening, ions and small molecules can pass from one cytoplasm to another in a regulated manner.

Signal Transmission:

Neurotransmitters, hormones, and immunoglobulin’s bind to specific receptors (integral proteins) exposed to the outside of cellular membranes and trans­mit information through these membranes to the cytoplasm.

The β-adrenergic receptor, which binds catecholamine’s stereo-specifically, is asym­metrically located on the outer aspect of plasma membranes of target cells such as the erythrocytes. The binding of the cat­echolamine on the outside stimulates the catalytic activity of adenylate cyclase which is asymmetrically located on the inside of the membrane.

Adenylate cy­clase generates cAMP from ATP. Thus, the information on the outside of the second messenger cAMP.

The catecholamine when binds to the beta receptor, the latter by conformational change activates phospholipid methyl-transferase I (an integral enzyme). This enzyme generates phosphatidyl-N-mono-methyl ethanolamine from phosphatidyl-ethanolamine.

The phosphatidyl-N-mono-methyl-ethanolamine flip-flops to­ward the outside of the membrane and is further methylated to phosphatidyl-N-di-methyl-ethanolamine and subsequently to phosphatidylcholine by the integral enzyme phospholipid methyltransferase II, located toward the outside of the mem­brane.

The increased local concentration of phosphatidylcholine enhances the lo­cal membrane fluidity. The increased flu­idity exposes more pre-existing beta-receptor molecules to the outside membrane surface.

Phospholipid methylation serves as an ini­tial common pathway for the transmission of many receptor-mediated biologic sig­nals through membranes. In some systems, the increased fluidity due to phospholi­pid methylation allows the rapid influx of calcium which can bind to calmodulin and activate specific enzymes including phospholipases.

The phospholipases from phosphatidylcholine can in turn generate free fatty acids including arachidonic acid which is an immediate precursor of prostaglandins. The prostaglandins are involved in signal transmission in several trans-membranes, hormonally responsive systems.

Endocytosis:

1. Endocytosis is a transport process which allows cells to internalize extracellular material and forms endocytotic vesicles during that process. Endocytotic vesicles are formed when segments of the plasma membrane invaginate enclosing a volume of extracellular fluid and pinch off as the fusion of plasma membranes seals the neck of the vesicle on the original site of invagination.

Subsequent fusion of the endocytotic vesicle with other membrane structures performs the transport of its con­tents to other cellular compartments or even back to the cells exterior.

2. Endocytosis are of two types-Phagocyto­sis and Pinocytosis. Phagocytosis occurs only in specialized phagocytic cells present in blood. Pinocytosis leads to all cellular uptake of fluid and fluid con­tents. The adsorptive pinocytosis in a receptor-mediated, selective process pri­marily responsible for the uptake of macromolecules for which there is finite number of binding sites on the plasma membrane.

These high affinity receptors permit pinocytosis to concentrate ligands from the medium and to minimize the up­take of fluid or soluble unbound macro-molecules.

3. Several hormones, the other macromolecules, are subject to adsorptive pinocy­tosis and form receptosomes. vesicles which avoid lysosomes and deliver their contents to the Golgi system.

4. Adsorptive pinocytosis of extracellular glycoproteins requires glycoproteins that carry specific carbohydrate recognition signals. These recognition signals are bound by membrane receptor molecules which play a role analogous to that of the LDL receptor.

Acid hydrolases taken up by adsorptive pinocytosis in fibroblasts are recognized by their mannose-6-phosphate moieties. The mannose-6-phosphate moiety also plays an important role in the intracellular segregation of the acid hydrolases to the lysosomes of the cells in which they are synthesized.

5. The other type of pinocytosis is a fluid phase pinocytosis which is a nonselective fluid phase process in which the uptake of a solute is simply proportionate to its con­centration in the surrounding medium.

This type of pinocytosis forms small vesi­cles and it is an extraordinarily active proc­ess utilizing up to 50 per cent of the plasma membrane per hour in some cells. The components of the membrane must be recycled to maintain cellular integrity.