Vesicle (biology and chemistry)


In cell biology, a vesicle is an organelle within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form naturally during the processes of secretion, uptake, and the transport of materials within the plasma membrane. Alternatively, they may be prepared artificially, in which case they are called liposomes. If there is only one phospholipid bilayer, the vesicles are called unilamellar liposomes; otherwise they are called multilamellar liposomes. The membrane enclosing the vesicle is also a lamellar phase, similar to that of the plasma membrane, and intracellular vesicles can fuse with the plasma membrane to release their contents outside the cell. Vesicles can also fuse with other organelles within the cell. A vesicle released from the cell is known as an extracellular vesicle.
Vesicles perform a variety of functions. Because it is separated from the cytosol, the inside of the vesicle can be made to be different from the cytosolic environment. For this reason, vesicles are a basic tool used by the cell for organizing cellular substances. Vesicles are involved in metabolism, transport, buoyancy control, and temporary storage of food and enzymes. They can also act as chemical reaction chambers.
The 2013 Nobel Prize in Physiology or Medicine was shared by James Rothman, Randy Schekman and Thomas Südhof for their roles in elucidating the makeup and function of cell vesicles, especially in yeasts and in humans, including information on each vesicle's parts and how they are assembled. Vesicle dysfunction is thought to contribute to Alzheimer's disease, diabetes, some hard-to-treat cases of epilepsy, some cancers and immunological disorders and certain neurovascular conditions.

Types of vesicular structures

Vacuoles

are cellular organelles that contain mostly water.
  • Plant cells have a large central vacuole in the center of the cell that is used for osmotic control and nutrient storage.
  • Contractile vacuoles are found in certain protists, especially those in Phylum Ciliophora. These vacuoles take water from the cytoplasm and excrete it from the cell to avoid bursting due to osmotic pressure.

    Lysosomes

  • Lysosomes are involved in cellular digestion. Food can be taken from outside the cell into food vacuoles by a process called endocytosis. These food vacuoles fuse with lysosomes which break down the components so that they can be used in the cell. This form of cellular eating is called phagocytosis.
  • Lysosomes are also used to destroy defective or damaged organelles in a process called autophagy. They fuse with the membrane of the damaged organelle, digesting it.

    Transport vesicles

  • Transport vesicles can move molecules between locations inside the cell, e.g., proteins from the rough endoplasmic reticulum to the Golgi apparatus.
  • Membrane-bound and secreted proteins are made on ribosomes found in the rough endoplasmic reticulum. Most of these proteins mature in the Golgi apparatus before going to their final destination, which may be to lysosomes, peroxisomes, or outside of the cell. These proteins travel within the cell inside transport vesicles.

    Secretory vesicles

contain materials that are to be excreted from the cell. Cells have many reasons to excrete materials.
One reason is to dispose of wastes.
Another reason is tied to the function of the cell. Within a larger organism, some cells are specialized to produce certain chemicals. These chemicals are stored in secretory vesicles and released when needed.

Types

  • Synaptic vesicles are located at presynaptic terminals in neurons and store neurotransmitters call quanta. When a signal comes down an axon, the synaptic vesicles fuse with the cell membrane releasing the neurotransmitter so that it can be detected by receptor molecules on the next nerve cell.
  • In animals, endocrine tissues release hormones into the bloodstream. These hormones are stored within secretory vesicles. A good example is an endocrine tissue found in the islets of Langerhans in the pancreas. This tissue contains many cell types that are defined by which hormones they produce.
  • Secretory vesicles hold the enzymes that are used to make the cell walls of plants, protists, fungi, bacteria and archaea cells, as well as the extracellular matrix of animal cells.
  • Bacteria, archaea, fungi and parasites release membrane vesicles containing varied but specialized toxic compounds and biochemical signal molecules, which are transported to target cells to initiate processes in favour of the microbe, which include invasion of host cells and killing of competing microbes in the same niche.

    Extracellular vesicles

are lipid bilayer-delimited particles produced by all cells including bacteria.

Types

  • Ectosomes/microvesicles are shed directly from the plasma membrane and can range in size from around 30 nm to larger than a micron in diameter). These may include large particles such as apoptotic blebs released by dying cells, large oncosomes released by some cancer cells, or "exophers," released by nematode neurons and mouse cardiomyocytes.
  • Exosomes: membranous vesicles of endocytic origin.
Different types of EVs may be separated based on density, size, or surface markers. However, EV subtypes have an overlapping size and density ranges, and subtype-unique markers must be established on a cell-by-cell basis. Therefore, it is difficult to pinpoint the biogenesis pathway that gave rise to a particular EV after it has left the cell.
In humans, endogenous extracellular vesicles likely play a role in coagulation, intercellular signaling and waste management. They are also implicated in the pathophysiological processes involved in multiple diseases, including cancer. Extracellular vesicles have raised interest as a potential source of biomarker discovery because of their role in intercellular communication, release into easily accessible body fluids and the resemblance of their molecular content to that of the releasing cells. The extracellular vesicles of stem cells, also known as the secretome of stem cells, are being researched and applied for therapeutic purposes, predominantly degenerative, auto-immune and/or inflammatory diseases.
In Gram-negative bacteria, EVs are produced by the pinching off of the outer membrane; however, how EVs escape the thick cell walls of Gram-positive bacteria, mycobacteria and fungi is still unknown. These EVs contain varied cargo, including nucleic acids, toxins, lipoproteins and enzymes and have important roles in microbial physiology and pathogenesis. In host–pathogen interactions, gram negative bacteria produce vesicles which play roles in establishing a colonization niche, carrying and transmitting virulence factors into host cells and modulating host defense and response.
Ocean cyanobacteria have been found to continuously release vesicles containing proteins, DNA and RNA into the open ocean. Vesicles carrying DNA from diverse bacteria are abundant in coastal and open-ocean seawater samples.

Protocells

The RNA world hypothesis assumes that the first self-replicating genomes were strands of RNA. This hypothesis contains the idea that RNA strands formed ribozymes capable of catalyzing RNA replication. These primordial biological catalysis were considered to be contained within vesicles with membranes composed of fatty acids and related amphiphiles. Template-directed RNA synthesis by the copying of RNA templates inside fatty acid vesicles has been demonstrated by Adamata and Szostak.

Other types

Gas vesicles are used by archaea, bacteria and planktonic microorganisms, possibly to control vertical migration by regulating the gas content and thereby buoyancy, or possibly to position the cell for maximum solar light harvesting. These vesicles are typically lemon-shaped or cylindrical tubes made out of protein; their diameter determines the strength of the vesicle with larger ones being weaker. The diameter of the vesicle also affects its volume and how efficiently it can provide buoyancy. In cyanobacteria, natural selection has worked to create vesicles that are at the maximum diameter possible while still being structurally stable. The protein skin is permeable to gases but not water, keeping the vesicles from flooding.
Matrix vesicles are located within the extracellular space, or matrix. Using electron microscopy, they were discovered independently in 1967 by H. Clarke Anderson and Ermanno Bonucci. These cell-derived vesicles are specialized to initiate biomineralisation of the matrix in a variety of tissues, including bone, cartilage and dentin. During normal calcification, a major influx of calcium and phosphate ions into the cells accompanies cellular apoptosis and matrix vesicle formation. Calcium-loading also leads to formation of phosphatidylserine:calcium:phosphate complexes in the plasma membrane mediated in part by a protein called annexins. Matrix vesicles bud from the plasma membrane at sites of interaction with the extracellular matrix. Thus, matrix vesicles convey to the extracellular matrix calcium, phosphate, lipids and the annexins which act to nucleate mineral formation. These processes are precisely coordinated to bring about, at the proper place and time, mineralization of the tissue's matrix unless the Golgi are non-existent.
Multivesicular body, or MVB, is a membrane-bound vesicle containing a number of smaller vesicles.

Formation and transport

Some vesicles are made when part of the membrane pinches off the endoplasmic reticulum or the Golgi complex. Others are made when an object outside of the cell is surrounded by the cell membrane.

Vesicle coat and cargo molecules

The vesicle "coat" is a collection of proteins that serve to shape the curvature of a donor membrane, forming the rounded vesicle shape. Coat proteins can also function to bind to various transmembrane receptor proteins, called cargo receptors. These receptors help select what material is endocytosed in receptor-mediated endocytosis or intracellular transport.
There are three types of vesicle coats: clathrin, COPI and COPII. The various types of coat proteins help with sorting of vesicles to their final destination. Clathrin coats are found on vesicles trafficking between the Golgi and plasma membrane, the Golgi and endosomes and the plasma membrane and endosomes. COPI coated vesicles are responsible for retrograde transport from the Golgi to the ER, while COPII coated vesicles are responsible for anterograde transport from the ER to the Golgi.
The clathrin coat is thought to assemble in response to regulatory G protein. A protein coat assembles and disassembles due to an ADP ribosylation factor protein.