Bacterial Pathogens: Strategies Used in Manipulating Host Cells
Cell manipulation is a process through which pathogens such as bacteria take over the normal functioning of host cells and reprogram the functions of these units forcing them to work differently. Different pathogens use different manipulation strategies. Some pathogens secrete toxins that induce hypoglycemia in host cells while others manipulate the immune response. This paper aims at explaining how pathogens use the strategies of phagosome maturation to manipulate host cells. Additionally, manipulation of cell signaling pathway and manipulation of cytoskeleton evident in Bacillus anthracis and Salmonella enterica respectively have been discussed.
Phagosome maturation is the process in which intracellular vacuoles or phagosomes go through fission and fusion to manipulate the composition of their limiting membranes and contents through a process that looks like the progression of the endocytic pathway. Phagosome maturation gives the vacuole degradative properties. It is part of the phagocytosis process, a fundamental process used by cells to capture and clear foreign particles including in pathogenic microorganisms. Phagosome maturation forms an important step in a series of steps that result in the eventual destruction of pathogens during phagocytosis. However, several pathogens have evolved effective strategies of inhibiting phagosome maturation, thereby preventing the normal process of phagocytosis. This advantage enables the pathogens to continue their replication and perpetuate infection. One such pathogen is Mycobacterium tuberculosis. One of the earlier features of phagosome maturation is the rapid and gradual acidification of the phagosome (Uribe-Querol & Rosales 3). The process is accompanied by changes in the membrane composition and contents of the phagosome to form microbial vacuole. According to Uribe-Querol and Rosales, the process of phagosome maturation may be divided into three distinct stages: early phagosome, late phagosome, and phagolysosome (3). The first stage is characterized by the formation of small GTPase Rab5, the development of acidic conditions as a result of the accumulation of V-ATPase, and early endosome antigen 1 (EEA1). The low pH of the phagosome produces a direct impact on many pathogens (Uribe-Querol & Rosales 3). Moreover, the acidic conditions are required for the activation of a wide range of hydrolytic enzymes.
Mycobacterium tuberculosis inhibits phagosome maturation by blocking V-ATPase accumulation of the phagosome membrane (Uribe-Querol & Rosales 3). In part, this is achieved through protein tyrosine phosphatase action (PtpA) and nucleoside diphosphate kinase (Ndk). The Nkd is a GTPase-activating protein (GAP) for Rab5. By inactivating this particular GTPase, it effectively blocks any recruitment and delivery of EEA1 to the membrane. The process is described in the figure below.
Additionally, the blockage involves mannose-capped lipoarabinomannan (ManLAM) in a process that results in its binding to mannose receptors. The lipoprotein LprG serves to increase ManLAM’s surface-expression (Uribe-Querol & Rosales 8). Moreover, its binding to lysosomal-associated membrane proteins serves to limit a cell’s traffic machinery. Both adhesin PstS-1 and ManLAM bind mannose receptor that plays a role in lysosome fusion machinery. The secretory acid phosphatase attaches itself to Rab7, thereby blocking autophagosome-lysosome fusion. In summary, Mycobacterium tuberculosis manipulates cells by preventing the delivery of EEA1 to ensure that the phagosomes that contain the pathogen are maintained within a friendly environment.Bacterial Pathogens: Strategies Used in Manipulating Host Cells
In the case of Legionella pneumophila, the pathogen intercepts endoplasmic reticulum’s vesicular traffic to form an organelle that gives it access to cysteine. It forms an intracellular niche referred to as Legionella-containing vacuole by manipulating specific components of the host cell (Pike et al 1). When Legionella pneumophila is phagocytized, the phagosome gets remodeled quickly through the formation of compartments that resembles endoplasmic reticulum. At the same time, secretory vesicle and tubular endoplasmic reticulum are directly towards the phagosome. The Legionella pneumophila promotes the fusion of Legionella-containing vacuole membrane with the endoplasmic reticulum. By manipulating the host cell this way, the pathogen avoids fusing with the endosomal compartment, which guarantees its survival.
Another common strategy employed by pathogens to enhance infections is manipulating host cell signaling pathways. During an infection, pathogenic bacteria alter several eukaryotic signaling pathways using specific toxins and effectors. Pathogens target signaling molecules because of their involvement in the regulation of cellular processes. Several pathways involved in regulating the survival of the host cell tend to be manipulated and controlled by bacteria and viruses. They include the G-protein signaling, Mitogen-activated protein kinase (MAPK) signaling, and signals involved in controlling innate immune responses (Alto & Orth 3). The pathogens produce certain virulence factors such as proteins or peptides that play a role in achieving their survival and replication goals.
Manipulation of cell signaling pathways is most evident in MAPK pathways. They are made up of cascades of kinases that rely on phosphorylation, a biochemical in which phosphate is added, to activate each other. A mitogen-activated protein kinase kinase kinase (MAPKKK) is responsible for activating a mitogen-activated protein kinase kinase (MAPKK). In turn, a MAPKK activates MAPK. Each of these stages has multiple family members. The signaling pathways are responsible for controlling different cellular behaviors such as cell migration, cell proliferation, autophagy, and apoptosis (Alto & Orth 4). Therefore, they are a major target by pathogens.
Bacillus anthracis, the bacteria that causes Anthrax is an example of pathogens that manipulate cell signaling pathways of the host cell to survive and replicate. It produces anthrax toxin, a multisubunit complex that contains edema factor, lethal factor (LF), and protective antigen. The toxin uses a protective antigen to attach itself to either of the two anthrax toxin receptors on the host cell surface. The edema factor and lethal factor and edema factors are delivered into the cytoplasm of the cell following their uptake during endocytosis, a process that is receptor-mediated. Calmodulin, a calcium-binding protein involved in calcium cell signaling pathways binds to the cytoplasmic edema factor. The change in conformation that is created yields an active enzyme. This particular by-product is responsible for generating cyclic adenosine monophosphate (cAMP) from cellular Adenosine triphosphate (ATP). When in excess, cAMP binds and activates downstream effectors, thereby effectively disrupting signaling. One such effector is the cAMP-dependent protein kinase. On the other hand, the cytoplasmic lethal factor is an active protease enzyme that requires a metal in its catalytic mechanism. It effectively produces kinases that lack the ability to interact with respective substrates to produce a proliferation response by splitting the amino-terminal extensions from mitogen-activated protein kinase kinases, MKL! And MKK2. The effect produced by both cytoplasmic edema factor and lethal factor on the infected cells is irreversible.
Figure illustrating the action of bacteria effectors on signaling pathways
In the case of Yersinia ssp, the pathogens use the molecule YopJ to manipulate cell signaling pathways, interfere with innate immune response, and encourage apoptosis, programed cell death, in infected cells. The YopJ effector is delivered directly into the cytoplasm of the host cell. It then prevents the activation of all mitogen-activated protein kinase kinases MAPKKs and the enzyme complex IKKβ, thereby effectively blocking the activation of all of mitogen-activated protein kinase (MAPK) pathways. Rather than split MAPKKs as is the case with Bacillus anthracis, YopJ uses acetyl moiety to modify MAPKKs. The process uses acetyl-CoA to modify either threonine or serine, two amino acids present in IKKβ and MAPKKs activation loops. The product of this translation is biochemical modification that effectively strives with phosphorylation. An outbreak of an infection is followed by inducements of host survival signals and apoptosis. The movement YopJ molecules have been delivered and are present in the cell, death becomes the default pathway. This is because YopJ effectively blocks inhibits signaling pathways by blocking NF-κB survival pathway. The signaling pathway meant to communicate with the immune response for the purpose of inducing survival signal becomes inhibited. Therefore, YopJ effectively reduces the effect of immune response during an infection involving Yersinia ssp.
Manipulation of cytoskeleton is another mechanism used by intracellular bacterial pathogen to infect and replicate within the host cell. According to Bhavsar, Guttman, and Finlay (827), different intracellular microorganisms take advantage of the components found within cytoskeleton to gain entry into host cells. Eukaryotic cells have cytoskeleton consisting of intermediate and actin filaments in addition to microtubes. Studies conducted over the years on action of bacterial pathogens in actin filaments reveal that bacterial pathogens modulate cellular regulators such as G proteins used in the process of actin filaments polymerization by using delivered effectors. Eventually, these pathogens gain control over the entire process of polymerization. My manipulating the cytoskeleton, the pathogens benefits from structural support provided to bacteria-containing vacuoles and internalization of bacteria. Moreover, cytoskeleton rearrangement aids in pathogen dissemination and achieving actin-dependent bacterial movements.Bacterial Pathogens: Strategies Used in Manipulating Host Cells
The use of effectors by bacterial pathogens to manipulate normal cellular functions is exemplified in the invasion of mammalian cells by the bacteria Salmonella enterica. During the process, two T3SS effector proteins namely SopE and SopE2 are delivered by S. enterica into the host cell. The two effectors work as guanine-nucleotide-exchange, impacting G proteins directly. Guanine-nucleotide-exchange may be defined as protein domains that function to stimulate the production of guanosine diphosphate (GDP) that serve to activate monomeric GTPases thereby enabling guanosine triphosphate to bind (Zhang et al. 2566). The functioning of the effectors as guanine-nucleotide-exchange in the present case result in the G-protein activation. According to Bhavsar, Guttman, and Finlay, G protein CDC42 and RAC family of proteins found within the target host cell are activated (828). This in turn results in significant structural changes in cells through creation of motile cell surface formed by polymerized actin-rich filaments. It is these structures that engulf the bacteria and internalize them.
Figure: Salmonella invasion mechanism. Retrieved from Wiedemann et al (5).
After the bacterial pathogens have escaped from actin-rich and membrane-enclosed vesicles and invaded their cytosol, they manage to move within the host cell by manipulating the dynamics of actin-filament. During the nucleation of actin, a bacterial-protein-mediated process, these pathogens add acting to one of the poles (Bhavsar et al 828). This is evident in Shigella flexneri. The capability of movement of this particular bacteria is brought about by IcsA. This bacterial effector works with N-WASP, the host protein to recruit Arp2/3 complex that polymerizes actin filaments at one of the poles of the advancing bacterium.
One of the most vital functions of living organisms’ innate defenses is to eliminate pathogens. Various phagocytic leukocytes use the process of phagocytosis to clear pathogenic bacteria. However, pathogens have evolved a mechanism to counter the effect of phagocytosis or prevent them altogether. As a result, pathogens pose a major health threat. In this paper, three main mechanisms used by some pathogens to manipulate host cell or specific steps of the phagocytic process have been discussed. While some pathogens have already been studied by scientists in detail, others such as the novel COVID-19 seem to be resisting phagocytosis in ways that scientists have not completely understood. With modern technical advances, however, it is possible that novel therapeutic approaches could be developed and increase the susceptibility of these pathogens to phagocyte attack Bacterial Pathogens: Strategies Used in Manipulating Host Cells
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