Paracytophagy
Encyclopedia
Paracytophagy is the cellular
process whereby a cell engulfs a protrusion which extends from a neighboring cell. This protrusion may contain material which is actively transferred between the cells. The process of paracytophagy was first described as a crucial step during cell-to-cell spread of the intracellular bacterial pathogen Listeria monocytogenes, and is also commonly observed in Shigella flexneri
. Paracytophagy allows these intracellular pathogens to spread directly from cell to cell, thus escaping immune
detection and destruction. Studies of this process have contributed significantly to our understanding of the role of the actin
cytoskeleton
in eukaryotic cells.
is one of the main cytoskeletal proteins in eukaryotic cells. The polymerization of actin filaments is responsible for the formation of pseudopods, filopodia
and lamellipodia
during cell motility. Cells actively build actin microfilaments that push the cell membrane towards the direction of advance.
(Wiskott-Aldrich syndrome protein) help to form the seven-protein Arp2/3 nucleation complex
, which resembles two actin monomers and therefore allows for easier formation of the polymerization nucleus. Arp2/3 is able to cap the trailing ("minus") end of the actin filament, allowing for faster polymerization at the "plus" end. It can also bind to the side of existing filaments to promote filament branching.
can manipulate host cell actin polymerization to move through the cytosol and spread to neighboring cells (see below). Studies of these bacteria, especially of Listeria Actin assembly-inducing protein
(ActA), have resulted in further understanding of the actions of WASP. ActA
is a nucleation promoting factor that mimics WASP. It is expressed polarized to the posterior end of the bacterium, allowing Arp2/3-mediated actin nucleation. This pushes the bacterium in the anterior direction, leaving a trailing "comet tail" of actin. In the case of Shigella, which also moves using an actin comet tail, the bacterial factor recruits host cell WASPs in order to promote actin nucleation.
-like protrusions, or tunneling nanotubes directed toward neighboring cells in a culture of rat PC12 cells have been shown to facilitate transport of organelles through transient membrane fusion. In another example, during bone marrow homing, cells of the surrounding bone engulf pieces of bone marrow hematopoietic cells. These osteoblasts make contact with hematopoietic stem-progenitor cells through membrane nanotubes, and pieces of the donor cells are transferred over time to various endocytic compartments of the target osteoblasts.
A distinct process known as trogocytosis
, the exchange of lipid rafts or membrane patches between immune cells, can facilitate response to foreign stimuli. Moreover exosomes have been shown to deliver not only antigens for cross-presentation
, but also MHCII and co-stimulatory molecules for lymphocyte T activation. In non-immune cells, it has been demonstrated that mitochondria can be exchanged intercellularly to rescue metabolically non-viable cells lacking mitochondria. Mitochondrial transfer has also been observed in cancer cells.
, where they act as a vehicle for the spread of molecules through the epithelia of imaginal discs. Melanosomes are also transferred by filopodia from melanocytes to keratinocytes. This transfer involves a classic filopodial forming pathway, with Cdc42 and WASP as key factors.
Argosomes, melanosomes, and other examples of epithelial transfer have been compared with the process of paracytophagy, all of which can be viewed as special cases of intercellular material transfer between epithelial cells.
. In the case of Listeria, the process was first described in detail using electron microscopy and video microscopy. The following is a description of the process of cell-cell transmission of Listeria monocytogenes, primarily based on Robbins et al. (1999):
, which results in formation of the actin
comet tail and movement of the bacterium throughout the cytoplasm
. When the bacterium encounters the donor cell membrane
, it will either ricochet off it or adhere to it and begin to push outwards, distending the membrane and forming a protrusion of 3-18 μm. The close interaction between the bacterium and the host cell membrane is thought to depend on Ezrin, a member of the ERM family of membrane-associated proteins
. Ezrin attaches the actin-propelled bacterium to the plasma membrane by crosslinking the actin comet tail to the membrane, and maintains this interaction throughout the protrusion process.
containing the bacterium forms inside the target cell cytoplasm.
is not required as pre-existing ActA mRNA can be utilized to translate new ActA protein. The bacterium regains motility and the infection proceeds.
, rhombencephalitis, and encephalitis
. These forms of disease are a direct result of Listeria pathogenicity mechanisms at the cellular level. Listerial infection involving the CNS can occur via three known routes: through the blood, through intracellular delivery, or through neuronal intracellular spread. Paracytophagous cell to cell spread offers Listeria access to the CNS by the latter two mechanisms.
via the paracytophagous mode of invasion. Using these phagocytic cells as vectors, Listeria travels throughout the nerves and reaches tissues usually inaccessible to other bacterial pathogens. Similar to the mechanism seen in HIV
, infected leukocytes in the blood cross the blood brain barrier and transport Listeria into the CNS. Once in the CNS, cell to cell spreading causes associated damage leading to brain encephalitis and bacterial meningitis. Listeria uses phagocytic leukocytes as a “Trojan Horse
” to gain access to a greater range of target cells.
In one study, mice treated with gentamycin via infusion pump
displayed CNS and brain involvement during infection with Listeria, indicating that the population of bacteria responsible for severe pathogenesis resided within cells and was protected from the circulating antibiotic
. Macrophages infected with Listeria pass the infection on to neurons more easily through paracytophagy than through extracellular invasion by free bacteria. The mechanism which specifically targets these infected cells to the CNS is currently not known. This Trojan horse function is also observed and thought to be important in early stages of infection where gut-to-lymph node
infection is mediated by infected dendritic cells.
A second mechanism of reaching the brain tissue is achieved through intra-axonal transport. In this mechanism, Listeria travels along the nerves to the brain, resulting in encephalitis or transverse myelitis. In rats, the dorsal root ganglia can be infected directly by Listeria, and the bacteria can move in retrograde as well as anterograde direction through the nerve cells. The specific mechanisms involved in brain disease are not yet known, but paracytophagy is thought to have some role. Bacteria have not been shown to infect neuronal cells directly in an efficient manner, and the previously described macrophage hand-off is thought to be necessary for this mode of spread.
. Some related concepts include:
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
process whereby a cell engulfs a protrusion which extends from a neighboring cell. This protrusion may contain material which is actively transferred between the cells. The process of paracytophagy was first described as a crucial step during cell-to-cell spread of the intracellular bacterial pathogen Listeria monocytogenes, and is also commonly observed in Shigella flexneri
Shigella flexneri
Shigella flexneri is a species of Gram-negative bacteria in the genus Shigella that can cause diarrhea in humans. There are several different serogroups of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become...
. Paracytophagy allows these intracellular pathogens to spread directly from cell to cell, thus escaping immune
Immunity (medical)
Immunity is a biological term that describes a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of wide...
detection and destruction. Studies of this process have contributed significantly to our understanding of the role of the actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
in eukaryotic cells.
Actin cytoskeleton
ActinActin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
is one of the main cytoskeletal proteins in eukaryotic cells. The polymerization of actin filaments is responsible for the formation of pseudopods, filopodia
Filopodia
Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the...
and lamellipodia
Lamellipodia
The lamellipodium is a cytoskeletal protein actin projection on the mobile edge of the cell. It contains a quasi-two-dimensional actin mesh; the whole structure propels the cell across a substrate...
during cell motility. Cells actively build actin microfilaments that push the cell membrane towards the direction of advance.
Nucleation factors and the Arp2/3 complex
Nucleation factors are enhancers of actin polymerization and contribute to the formation of the trimeric polymerization nucleus. This is a structure required to initiate the process of actin filament polymerization in a stable and efficient way. Nucleation factors such as WASPWiskott-Aldrich syndrome protein
The Wiskott–Aldrich Syndrome Protein is a 502-amino acid protein that is expressed in cells of the hematopoietic system. In the inactive state, WASp exists in an auto-inhibited conformation with sequences near its C-terminus binding to a region near its N-terminus...
(Wiskott-Aldrich syndrome protein) help to form the seven-protein Arp2/3 nucleation complex
Arp2/3 complex
Arp2/3 complex is a seven-subunit protein that plays a major role in the regulation of the actin cytoskeleton. It is a major component of the actin cytoskeleton and is found in most in actin cytoskeleton-containing eukaryotic cells....
, which resembles two actin monomers and therefore allows for easier formation of the polymerization nucleus. Arp2/3 is able to cap the trailing ("minus") end of the actin filament, allowing for faster polymerization at the "plus" end. It can also bind to the side of existing filaments to promote filament branching.
WASP analogs used by pathogens for intracellular motility
Certain intracellular pathogens such as the bacterial species Listeria monocytogenes and Shigella flexneriShigella flexneri
Shigella flexneri is a species of Gram-negative bacteria in the genus Shigella that can cause diarrhea in humans. There are several different serogroups of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become...
can manipulate host cell actin polymerization to move through the cytosol and spread to neighboring cells (see below). Studies of these bacteria, especially of Listeria Actin assembly-inducing protein
Actin assembly-inducing protein
The Actin assembly-inducing protein is a protein encoded and used by Listeria monocytogenes to propel itself through a mammalian host cell. ActA is a bacterial surface protein comprising a membrane-spanning region...
(ActA), have resulted in further understanding of the actions of WASP. ActA
Acta
ACTA may refer to:* Administrative Council for Terminal Attachments, a standards organization for terminal equipment such as registered jacks* Alameda Corridor Transportation Authority, in southern California...
is a nucleation promoting factor that mimics WASP. It is expressed polarized to the posterior end of the bacterium, allowing Arp2/3-mediated actin nucleation. This pushes the bacterium in the anterior direction, leaving a trailing "comet tail" of actin. In the case of Shigella, which also moves using an actin comet tail, the bacterial factor recruits host cell WASPs in order to promote actin nucleation.
Exchange of cellular material between adjacent cells
Cells can exchange material through various mechanisms, such as by secreting proteins, releasing exosomes, or more directly engulfing pieces of adjacent cells. In one example, filopodiaFilopodia
Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the...
-like protrusions, or tunneling nanotubes directed toward neighboring cells in a culture of rat PC12 cells have been shown to facilitate transport of organelles through transient membrane fusion. In another example, during bone marrow homing, cells of the surrounding bone engulf pieces of bone marrow hematopoietic cells. These osteoblasts make contact with hematopoietic stem-progenitor cells through membrane nanotubes, and pieces of the donor cells are transferred over time to various endocytic compartments of the target osteoblasts.
A distinct process known as trogocytosis
Trogocytosis
Trogocytosis is a process whereby lymphocytes conjugated to antigen-presenting cells extract surface molecules from these cells and express them on their own surface...
, the exchange of lipid rafts or membrane patches between immune cells, can facilitate response to foreign stimuli. Moreover exosomes have been shown to deliver not only antigens for cross-presentation
Cross-presentation
The term cross-presentation denotes the ability of certain antigen-presenting cells to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells . Cross-priming describes the stimulation of naive cytotoxic CD8+ T cells by this process...
, but also MHCII and co-stimulatory molecules for lymphocyte T activation. In non-immune cells, it has been demonstrated that mitochondria can be exchanged intercellularly to rescue metabolically non-viable cells lacking mitochondria. Mitochondrial transfer has also been observed in cancer cells.
Argosomes and melanosomes
Argosomes are derived from basolateral epithelial membranes and allow communication between adjacent cells. They were first described in Drosophila melanogasterDrosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
, where they act as a vehicle for the spread of molecules through the epithelia of imaginal discs. Melanosomes are also transferred by filopodia from melanocytes to keratinocytes. This transfer involves a classic filopodial forming pathway, with Cdc42 and WASP as key factors.
Argosomes, melanosomes, and other examples of epithelial transfer have been compared with the process of paracytophagy, all of which can be viewed as special cases of intercellular material transfer between epithelial cells.
Role in the life cycle of intracellular pathogens
The two main examples of paracytophagy are the modes of cell-cell transmission of Listeria monocytogenes and Shigella flexneriShigella flexneri
Shigella flexneri is a species of Gram-negative bacteria in the genus Shigella that can cause diarrhea in humans. There are several different serogroups of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become...
. In the case of Listeria, the process was first described in detail using electron microscopy and video microscopy. The following is a description of the process of cell-cell transmission of Listeria monocytogenes, primarily based on Robbins et al. (1999):
Early events
In an already infected "donor" cell, the Listeria bacterium expresses ActAActa
ACTA may refer to:* Administrative Council for Terminal Attachments, a standards organization for terminal equipment such as registered jacks* Alameda Corridor Transportation Authority, in southern California...
, which results in formation of the actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
comet tail and movement of the bacterium throughout the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
. When the bacterium encounters the donor 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...
, it will either ricochet off it or adhere to it and begin to push outwards, distending the membrane and forming a protrusion of 3-18 μm. The close interaction between the bacterium and the host cell membrane is thought to depend on Ezrin, a member of the ERM family of membrane-associated proteins
ERM protein family
The ERM protein family consists of three closely related proteins, ezrin, radixin and moesin. The three paralogs, ezrin, radixin and moesin, are present in vertebrates whereas other species have only one ERM gene. Therefore, in vertebrates these paralogs likely arose by gene duplication.ERM...
. Ezrin attaches the actin-propelled bacterium to the plasma membrane by crosslinking the actin comet tail to the membrane, and maintains this interaction throughout the protrusion process.
Invasion of target cell and secondary vacuole formation
As the normal site of infection is the gut columnar epithelium, cells are packed closely together and a cell protrusion from one cell will easily push into a neighboring "target" cell without rupturing the target cell membrane or the donor protrusion membrane. At this point, the bacterium at the tip of the protrusion will begin to undergo "fitful movement" caused by continuing polymerization of actin at its rear. After 7–15 minutes, the donor cell membrane pinches off and fitful movement ceases for 15–25 minutes due to depletion of ATP. Subsequently, the target membrane pinches off (taking 30–150 seconds) and the secondary vacuoleVacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
containing the bacterium forms inside the target cell cytoplasm.
Secondary vacuole breakdown and target cell infection
Within 5 minutes, the target cell becomes infected when the secondary vacuole begins to acidify and the inner (donor cell-derived) membrane breaks down through the action of bacterial phospholipases (PI-PLC and PC-PLC). Shortly thereafter, the outer membrane breaks down as a result of the actions of the bacterial protein listeriolysin O which punctures the vacuolar membrane. A cloud of residual donor cell-derived actin persists around the bacterium for up to 30 minutes. The bacterial metalloprotease Mpl cleaves ActA in a pH-dependent fashion while the bacterium is still within the acidified secondary vacuole, but new ActA transcriptionTranscription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
is not required as pre-existing ActA mRNA can be utilized to translate new ActA protein. The bacterium regains motility and the infection proceeds.
Impact on disease
The most severe symptoms of Listeriosis result from involvement of the central nervous system (CNS). These severe and often fatal symptoms include meningitisMeningitis
Meningitis is inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges. The inflammation may be caused by infection with viruses, bacteria, or other microorganisms, and less commonly by certain drugs...
, rhombencephalitis, and encephalitis
Encephalitis
Encephalitis is an acute inflammation of the brain. Encephalitis with meningitis is known as meningoencephalitis. Symptoms include headache, fever, confusion, drowsiness, and fatigue...
. These forms of disease are a direct result of Listeria pathogenicity mechanisms at the cellular level. Listerial infection involving the CNS can occur via three known routes: through the blood, through intracellular delivery, or through neuronal intracellular spread. Paracytophagous cell to cell spread offers Listeria access to the CNS by the latter two mechanisms.
Paracytophagy in CNS infection by Listeria
In peripheral tissues, Listeria can invade cells such as monocytes and dendritic cells from infected endothelial cellsEndothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...
via the paracytophagous mode of invasion. Using these phagocytic cells as vectors, Listeria travels throughout the nerves and reaches tissues usually inaccessible to other bacterial pathogens. Similar to the mechanism seen in HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
, infected leukocytes in the blood cross the blood brain barrier and transport Listeria into the CNS. Once in the CNS, cell to cell spreading causes associated damage leading to brain encephalitis and bacterial meningitis. Listeria uses phagocytic leukocytes as a “Trojan Horse
Trojan Horse
The Trojan Horse is a tale from the Trojan War about the stratagem that allowed the Greeks finally to enter the city of Troy and end the conflict. In the canonical version, after a fruitless 10-year siege, the Greeks constructed a huge wooden horse, and hid a select force of men inside...
” to gain access to a greater range of target cells.
In one study, mice treated with gentamycin via infusion pump
Infusion pump
An infusion pump infuses fluids, medication or nutrients into a patient's circulatory system. It is generally used intravenously, although subcutaneous, arterial and epidural infusions are occasionally used....
displayed CNS and brain involvement during infection with Listeria, indicating that the population of bacteria responsible for severe pathogenesis resided within cells and was protected from the circulating antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
. Macrophages infected with Listeria pass the infection on to neurons more easily through paracytophagy than through extracellular invasion by free bacteria. The mechanism which specifically targets these infected cells to the CNS is currently not known. This Trojan horse function is also observed and thought to be important in early stages of infection where gut-to-lymph node
Lymph node
A lymph node is a small ball or an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells. Lymph nodes are found all through the body, and act as...
infection is mediated by infected dendritic cells.
A second mechanism of reaching the brain tissue is achieved through intra-axonal transport. In this mechanism, Listeria travels along the nerves to the brain, resulting in encephalitis or transverse myelitis. In rats, the dorsal root ganglia can be infected directly by Listeria, and the bacteria can move in retrograde as well as anterograde direction through the nerve cells. The specific mechanisms involved in brain disease are not yet known, but paracytophagy is thought to have some role. Bacteria have not been shown to infect neuronal cells directly in an efficient manner, and the previously described macrophage hand-off is thought to be necessary for this mode of spread.
See also
The process of paracytophagy is considered distinct from similar but unrelated processes such as phagocytosis and trogocytosisTrogocytosis
Trogocytosis is a process whereby lymphocytes conjugated to antigen-presenting cells extract surface molecules from these cells and express them on their own surface...
. Some related concepts include:
- Membrane nanotubes
- Intercellular signaling