TRANSCYTOSIS

TRANSCYTOSIS

Transcytosis is a transport mechanism in which an extracellular macromolecule enters through one side of a cell, migrates across cytoplasm of the cell and exits through the other side.

Mechanism of Transcytosis

Cell encloses the extracellular substance by invagination of the cell membrane to form a vesicle. Vesicle then moves across the cell and thrown out through opposite cell membrane by means of exocytosis. Transcytosis involves the receptor-coated pits as in receptor-mediated endocytosis. Receptor protein coating the pits in this process is caveolin and not clathrin. Transcytosis is also called, vesicle trafficking or cytopempsis. Transcytosis plays an important role in selectively transporting the substances between two environments across the cells without any distinct change in the composition of these environments. Example of this type of transport is the movement of proteins from capillary blood into interstitial fluid across the endothelial cells of the capillary. Many pathogens like human immuno deficiency virus (HIV) are also transported by this mechanism.

MOLECULAR MOTORS

Molecular motors are the protein-based molecular machines that perform intracellular movements in

response to specific stimuli.

FUNCTIONS OF MOLECULAR MOTORS

1. Transport of synaptic vesicles containing neurotransmitters from the nerve cell body to synaptic

terminal

2. Role in cell division (mitosis and meiosis) by pulling the chromosomes

3. Transport of viruses and toxins to the interior of the cell for its own detriment.

TYPES OF MOLECULAR MOTORS

Molecular motors are classified into three super families:

1. Kinesin

2. Dynein

3. Myosin.

1. Kinesin

Kinesin transports substances by moving over the microtubules. Each kinesin molecule has two heads and a tail portion. One of the heads hydrolyses ATP to obtain energy. By utilizing this energy, the other head swings continuously causing movement of the whole kinesin molecule . End portion of the tail carries the cargo (substances to be transported). Kinesin is responsible for anterograde transport (transport of substances towards the positive end of microtubule).

2. Dynein

Dynein is almost similar to kinesin and transports substances by moving over the microtubules. But it

is responsible for retrograde transport (transport of substances towards the negative end of microtubule).

3. Myosin

Myosin transports substances by moving over micro filaments. Myosins are classified into 18 types according to the amino acid sequence. However, myosin II and V are functionally significant. Myosin II is involved in muscle contraction . Myosin V is involved in transport of vesicles.

ABNORMALITIES OF SODIUMPOTASSIUM PUMP

Abnormalities in the number or function of Na+-K+ pump are associated with several pathological conditions. Important examples are:

1. Reduction in either the number or concentration of Na+-K+ pump in myocardium is associated with

cardiac failure

2. Excess reabsorption of sodium in renal tubules is associated with hypertension.

CHANNELOPATHIES OR ION CHANNEL DISEASES

Channelopathies or ion channel diseases are caused by mutations in genes that encode the ion channels.

1. Sodium Channel Diseases

Dysfunction of sodium channels leads to muscle spasm and Liddle’s syndrome (dysfunction of sodium channels in kidney resulting in increased osmotic pressure in the blood and hypertension).

2. Potassium Channel Diseases

Potassium channel dysfunction causes disorders of heart, inherited deafness and epileptic seizures in

newborn.

3. Chloride Channel Diseases

Dysfunction of chloride channels results in formation of renal stones and cystic fibrosis. Cystic fibrosis is a generalized disorder affecting the functions of many organs such as lungs (due to excessive mucus),

exocrine glands like pancreas, biliary system and immune system.