Pertussis toxin
Encyclopedia
Pertussis toxin is a protein-based AB5-type
exotoxin
produced by the bacterium Bordetella pertussis
, which causes whooping cough. PT is involved in the colonization of the respiratory tract
and the establishment of infection. Research suggests PT may have a therapeutic role in treating a number of common human ailments including hypertension, viral inhibition, and autoimmune inhibition.
of pertussis
. However, this was only discovered in the early 1980s. The appearance of pertussis is quite recent, compared with other epidemic infectious diseases. The earliest mention of pertussis, or whooping cough, is of an outbreak in Paris in 1414. This was published in Moulton’s The Mirror of Health, in 1640. Another epidemic of pertussis took place in Paris in 1578 and was described by a contemporary observer, Guillaume de Baillou. Pertussis was well known throughout Europe by the middle of the eighteenth century. Jules Bordet and Octave Gengou described in 1900 the finding of a new “ovoid bacillus” in the sputum of a 6-month-old infant with whooping cough. They were also the first to cultivate the Bordetella pertussis at the Pasteur Institute
in Brussels in 1906.
B. pertussis is a species of the genus Bordetella
. One difference between the different species of Bordetella is that B. pertussis produces PT and the other species do not. Bordetella parapertussis
shows the most similarity to B. pertussis and was therefore used for research determining the role of PT in causing the typical symptoms of whooping cough. Rat studies showed that the development of paroxysmal coughing, a characteristic for whooping cough, occurred in rats infected with B. pertussis. Rats infected with B. parapertussis or a PT-deficient mutant of B. pertussis did not show this symptom; neither of these two strains produced PT.
s are referred to as "A/B toxins", essentially because they are formed from two subunits. The "A" subunit possesses enzyme activity, and is transferred to the host cell following a conformational change in the membrane-bound transport "B" subunit. Pertussis toxin is an exotoxin with six subunits (named S1 through S5—each complex contains two copies of S4). The subunits are arranged in a A-B structure: the A component is enzymatically active
and is formed from the S1 subunit, while the B component is the receptor
-binding portion and is made up of subunits S2–S5. The subunits are encoded by ptx genes encoded on a large PT operon
that also includes additional genes which encode Ptl proteins: Together these proteins form the PT secretion complex.
, it is taken up in an endosome
, after which it undergoes retrograde transport to the trans-Golgi network and endoplasmic reticulum
. At some point during this transport, the A subunit (or protomer) becomes activated, perhaps through the action of glutathione
and ATP
. PT catalyzes the ADP-ribosylation
of the αi subunits of the heterotrimeric G protein
. This prevents the G proteins from interacting with G protein-coupled receptor
s on the cell membrane
, thus interfering with intracellular communication. The Gi subunits remain locked in their GDP-bound, inactive state, thus unable to inhibit adenyl cyclase activity, leading to increased cellular concentrations of cAMP.
Increased intracellular cAMP affects normal biological signaling. The toxin causes several systemic effects, among which is an increased release of insulin
, causing hypoglycemia
. Whether the effects of pertussis toxin are responsible for the paroxysmal cough remains unknown.
As a result of this unique mechanisms, PT has also become widely used as a biochemical tool to ADP-ribosylate GTP-binding proteins in the study of signal transduction. It has also become an essential component of new acellular vaccines.
production and the inhibition of the neutrophil chemotaxis
. Chemokines are signaling molecules produced by infected cells and attract neutrophils and macrophages. It is thought that neutrophil chemotaxis is disrupted by inhibiting G-protein-coupled chemokine receptors by the ADP-ribosylation of Gi proteins.
Because of the disrupted signaling pathways, synthesis of chemokines will be affected. This will prevent the infected cell from producing them and thereby inhibiting recruitment of neutrophils. Under normal circumstances alveolar macrophages and other lung cells produce a variety of chemokines. It has been found that PT inhibits the early transcription of keratinocyte-derived chemokine, macrophage inflammatory protein 2 and LPS-induced CXC chemokine. Eventually, PT causes lymphocytosis
, one of the systemic manifestations of whooping cough.
PT is a decisive virulence determinant of B. pertussis that is able to cross the blood brian barrier by increasing its permeability. As a result, PT can cause severe neurological complications . However, recently it has been found that the medicinal usage of Pertussis toxin can promote the development of regulatory T cells and prevent central nervous system autoimmune disease such as multiple sclerosis.
. However, the A subunit lacks lysine
residues, which are essential for ubiquitin
-dependent degradation. Therefore PT subunit A will not be metabolized like most other proteins.
PT is heat-stable and protease-resistant, but once the A and B are separated these properties change. The B subunit will stay heat-stable at temperatures up to 60 °C, but it is susceptible to protein degradation. PT subunit A on the other hand is less susceptible to ubiquitin-dependent degradation, but is unstable at temperature of 37 °C. This facilitates unfolding of the protein in the ER and tricks the cell into transporting the A subunit to the cytosol where normally unfolded proteins will be marked for degradation. So, the unfolded conformation will stimulate the ERAD
-mediated translocation of PT A into the cytosol. Once in the cytosol, it can bind to NAD and form a stable folded protein again. Being thermally unstable is also the Achilles heel of PT subunit A. As always there is an equilibrium between the folded and unfolded state. When the protein is unfolded, it is susceptible to degradation by the 20S proteasome, which can only degrade unfolded proteins.
AB5 toxin
The AB5 toxins are six-component protein complexes secreted by a number of pathogenic bacteria. All share a similar structure and mechanism for entering targeted host cells.- Structure and mechanism :...
exotoxin
Exotoxin
An exotoxin is a toxin excreted by a microorganism, like bacteria, fungi, algae, and protozoa. An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. They are highly potent and can cause major damage to the host...
produced by the bacterium Bordetella pertussis
Bordetella pertussis
Bordetella pertussis is a Gram-negative, aerobic coccobacillus of the genus Bordetella, and the causative agent of pertussis or whooping cough. Unlike B. bronchiseptica, B. pertussis is non-motile. Its virulence factors include pertussis toxin, filamentous hæmagglutinin, pertactin, fimbria, and...
, which causes whooping cough. PT is involved in the colonization of the respiratory tract
Respiratory tract
In humans the respiratory tract is the part of the anatomy involved with the process of respiration.The respiratory tract is divided into 3 segments:*Upper respiratory tract: nose and nasal passages, paranasal sinuses, and throat or pharynx...
and the establishment of infection. Research suggests PT may have a therapeutic role in treating a number of common human ailments including hypertension, viral inhibition, and autoimmune inhibition.
History
Today it is clear that PT plays a central role in the pathogenesisPathogenesis
The pathogenesis of a disease is the mechanism by which the disease is caused. The term can also be used to describe the origin and development of the disease and whether it is acute, chronic or recurrent...
of pertussis
Pertussis
Pertussis, also known as whooping cough , is a highly contagious bacterial disease caused by Bordetella pertussis. Symptoms are initially mild, and then develop into severe coughing fits, which produce the namesake high-pitched "whoop" sound in infected babies and children when they inhale air...
. However, this was only discovered in the early 1980s. The appearance of pertussis is quite recent, compared with other epidemic infectious diseases. The earliest mention of pertussis, or whooping cough, is of an outbreak in Paris in 1414. This was published in Moulton’s The Mirror of Health, in 1640. Another epidemic of pertussis took place in Paris in 1578 and was described by a contemporary observer, Guillaume de Baillou. Pertussis was well known throughout Europe by the middle of the eighteenth century. Jules Bordet and Octave Gengou described in 1900 the finding of a new “ovoid bacillus” in the sputum of a 6-month-old infant with whooping cough. They were also the first to cultivate the Bordetella pertussis at the Pasteur Institute
Pasteur Institute
The Pasteur Institute is a French non-profit private foundation dedicated to the study of biology, micro-organisms, diseases, and vaccines. It is named after Louis Pasteur, who made some of the greatest breakthroughs in modern medicine at the time, including pasteurization and vaccines for anthrax...
in Brussels in 1906.
B. pertussis is a species of the genus Bordetella
Bordetella
Bordetella is a genus of small , Gram-negative coccobacilli of the phylum proteobacteria. Bordetella species, with the exception of B. petrii, are obligate aerobes as well as highly fastidious, or difficult to culture. Three species are human pathogens ; one of these Bordetella is a genus of small...
. One difference between the different species of Bordetella is that B. pertussis produces PT and the other species do not. Bordetella parapertussis
Bordetella parapertussis
Bordetella parapertussis is a small Gram-negative bacterium of the genus Bordetella which is adapted to colonise the mammalian respiratory tract. Pertussis caused by B. parapertussis manifests with similar symptoms to B. pertussis-derived disease but tends to be generally less severe. Immunity...
shows the most similarity to B. pertussis and was therefore used for research determining the role of PT in causing the typical symptoms of whooping cough. Rat studies showed that the development of paroxysmal coughing, a characteristic for whooping cough, occurred in rats infected with B. pertussis. Rats infected with B. parapertussis or a PT-deficient mutant of B. pertussis did not show this symptom; neither of these two strains produced PT.
Structure
A large group of bacterial exotoxinExotoxin
An exotoxin is a toxin excreted by a microorganism, like bacteria, fungi, algae, and protozoa. An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. They are highly potent and can cause major damage to the host...
s are referred to as "A/B toxins", essentially because they are formed from two subunits. The "A" subunit possesses enzyme activity, and is transferred to the host cell following a conformational change in the membrane-bound transport "B" subunit. Pertussis toxin is an exotoxin with six subunits (named S1 through S5—each complex contains two copies of S4). The subunits are arranged in a A-B structure: the A component is enzymatically active
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
and is formed from the S1 subunit, while the B component is the receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
-binding portion and is made up of subunits S2–S5. The subunits are encoded by ptx genes encoded on a large PT operon
Operon
In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...
that also includes additional genes which encode Ptl proteins: Together these proteins form the PT secretion complex.
Mechanism of pathogenesis
PT is released from B. pertussis in an inactive form. Following PT binding to a cell membrane receptorReceptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
, it is taken up in an endosome
Endosome
In biology, an endosome is a membrane-bound compartment inside eukaryotic cells. It is a compartment of the endocytic membrane transport pathway from the plasma membrane to the lysosome. Molecules internalized from the plasma membrane can follow this pathway all the way to lysosomes for...
, after which it undergoes retrograde transport to the trans-Golgi network and endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
. At some point during this transport, the A subunit (or protomer) becomes activated, perhaps through the action of glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
and ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
. PT catalyzes the ADP-ribosylation
ADP-ribosylation
ADP-ribosylation is the addition of one or more ADP-ribose moieties to a protein. These reactions are involved in cell signaling and the control of many cell processes, including DNA repair and apoptosis.-ADP-ribosylation enzymes:...
of the αi subunits of the heterotrimeric G protein
G protein
G proteins are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from many hormones, neurotransmitters, and other signaling factors. G protein-coupled receptors are transmembrane receptors...
. This prevents the G proteins from interacting with G protein-coupled receptor
G protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
s on the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
, thus interfering with intracellular communication. The Gi subunits remain locked in their GDP-bound, inactive state, thus unable to inhibit adenyl cyclase activity, leading to increased cellular concentrations of cAMP.
Increased intracellular cAMP affects normal biological signaling. The toxin causes several systemic effects, among which is an increased release of insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
, causing hypoglycemia
Hypoglycemia
Hypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...
. Whether the effects of pertussis toxin are responsible for the paroxysmal cough remains unknown.
As a result of this unique mechanisms, PT has also become widely used as a biochemical tool to ADP-ribosylate GTP-binding proteins in the study of signal transduction. It has also become an essential component of new acellular vaccines.
Effects on the Immune system
It has been shown that PT affects the innate immune response. It inhibits the early recruitment of neutrophils and macrophages. PT interferes with the early chemokineChemokine
Chemokines are a family of small cytokines, or proteins secreted by cells. Their name is derived from their ability to induce directed chemotaxis in nearby responsive cells; they are chemotactic cytokines...
production and the inhibition of the neutrophil chemotaxis
Chemotaxis
Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...
. Chemokines are signaling molecules produced by infected cells and attract neutrophils and macrophages. It is thought that neutrophil chemotaxis is disrupted by inhibiting G-protein-coupled chemokine receptors by the ADP-ribosylation of Gi proteins.
Because of the disrupted signaling pathways, synthesis of chemokines will be affected. This will prevent the infected cell from producing them and thereby inhibiting recruitment of neutrophils. Under normal circumstances alveolar macrophages and other lung cells produce a variety of chemokines. It has been found that PT inhibits the early transcription of keratinocyte-derived chemokine, macrophage inflammatory protein 2 and LPS-induced CXC chemokine. Eventually, PT causes lymphocytosis
Lymphocytosis
Lymphocytosis is an increase in the number or proportion of lymphocytes in the blood, usually detected when a complete blood count is routinely obtained. Lymphocytes normally represent 20 to 40% of circulating white blood cells...
, one of the systemic manifestations of whooping cough.
PT is a decisive virulence determinant of B. pertussis that is able to cross the blood brian barrier by increasing its permeability. As a result, PT can cause severe neurological complications . However, recently it has been found that the medicinal usage of Pertussis toxin can promote the development of regulatory T cells and prevent central nervous system autoimmune disease such as multiple sclerosis.
Metabolism
PT is known to dissociate into two parts in the ER: the enzymatic active A subunit (S1) and the cell-binding B subunit. The two subunits are separated by proteolic cleavage. The B subunit will undergo ubiquitin-dependent degradation by the 26S proteasomeProteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
. However, the A subunit lacks lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
residues, which are essential for ubiquitin
Ubiquitin
Ubiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling.Ubiquitin can be attached to proteins and label them for destruction...
-dependent degradation. Therefore PT subunit A will not be metabolized like most other proteins.
PT is heat-stable and protease-resistant, but once the A and B are separated these properties change. The B subunit will stay heat-stable at temperatures up to 60 °C, but it is susceptible to protein degradation. PT subunit A on the other hand is less susceptible to ubiquitin-dependent degradation, but is unstable at temperature of 37 °C. This facilitates unfolding of the protein in the ER and tricks the cell into transporting the A subunit to the cytosol where normally unfolded proteins will be marked for degradation. So, the unfolded conformation will stimulate the ERAD
ERAD
Endoplasmic-reticulum-associated protein degradation designates a cellular pathway which targets misfolded proteins of the endoplasmic reticulum for ubiquitination and subsequent degradation by a protein-degrading complex, called the proteasome....
-mediated translocation of PT A into the cytosol. Once in the cytosol, it can bind to NAD and form a stable folded protein again. Being thermally unstable is also the Achilles heel of PT subunit A. As always there is an equilibrium between the folded and unfolded state. When the protein is unfolded, it is susceptible to degradation by the 20S proteasome, which can only degrade unfolded proteins.