Reactive Oxygen Species in Host Defense to
Pathogens and Predators
(re-vamped
from original, by Dillon Sloan)
· Framework
o
ROS
§
Reactive
Oxygen Species (ROS) are oxygen-containing molecules that can participate in
signaling pathways and can cause destruction to DNA, lipids, and proteins. An
overview of ROSs can be found in this
page of the website.
o
Mitochondria
§
The
mitochondria is an endogenous source of ROS.
The production of mitochondrial ROS seems to be caused by the pre-mature
movement of electrons through Complex I of the electron transport chain to
diatomic oxygen, resulting in the production of the superoxide anion. An overview of this process can be found in this page of the website.
o
Host
Defense and ROS
§
Many
organisms depend on the energy generated from other organisms for survival
(Brown et al., 2004). For some organisms,
this results in parasitism and predation, which harm the infected or the preyed
organism, respectively. Many organisms
subject to infection or predation have developed immune and defense responses
to survive and fight back
(Raffel et al.,
2008). Many of these processes rely on the production
of ROS, representing a benefit of ROS production. Two of these processes are explored in detail
below.
·
Overview of Host Immunity and
Defense Topics
o
Innate
Immunity and mROS
§
Pattern recognition receptors (PRRs), the
well-conserved mediators of host immunity to microbes, bind viral and bacterial
biomolecules known as pathogen-associated molecular patterns (PAMPs) (West et al, 2011b). This interaction
can initiate a variety of immune responses in the host, including the
production of pro-inflammatory cytokines, the initiation of phagocytosis, and
the initiation of the complement system (West
et al, 2006). Some responses involve the mitochondria; for
example, in response to viral infection, a mitochondrial protein, MAVS (mitochondrial
antiviral signaling), is necessary for the NF-κB dependent translocation to the nucleus and subsequent immune
response (Seth et al, 2005). Another response to infection includes the
stimulation of the NADPH-oxidase complex (NOX) in phagocytes. The NOX production of ROS, known as the
respiratory burst, destroys invading pathogens (Segal, 2008). Alongside NOX-derived ROS, mitochondrial ROS
(mROS) has shown the ability to help clear pathogens (Arsenijevic et al, 2000).
Further study has since attempted to determine if mROS stimulation could
be a controlled response to PRR signaling (see below).
o
ROS in plant defense against herbivory and pathogens
§
Photosynthetic organisms use light to drive the
catalysis of redox reactions necessary for metabolism and carbon fixation,
resulting in ROS formation via electron transport in the chloroplast and
mitochondria, forming superoxide anion, hydrogen peroxide, and singlet oxygen. Being a part of the plant’s central metabolic
process, this ROS formation is regulated and utilized by many processes in the
cell (Foyer and Noctor, 2003). Such processes include MAPK (cell defense),
abscisic acid (water conservation), and hypersensitivity response (cell death) signaling,
among others (Asai et al., 2008; Pei et al.,
2000; Zago et al., 2006;
Mittler et al., 2011). An increase in ROS can also cause pathogen death
via an NOX-like protein inducing respiratory burst (Marino et al., 2012). It has also
been shown that plants wounded by insect herbivory show increased amounts of
ROS both locally and systemically (Kerchev
et al., 2012).
For example, it has
been shown that wheat herbivory elicited NOX-derived ROS which contributed to
their resistance to infestation (Moloi
and Van der Westhuizen, 2006). Xanthine
oxidase activity has also been shown as a response to infection in bean leaves
(Montalbini, 1992b). Xanthine
oxidase (XO) is responsible for purine catabolism in plants – converting hypoxanthine to xanthine, and xanthine to
uric acid with the production of superoxide anion (Montalbini, 1992a). Thus, there could be a connection
between infestation of RWA with XO-dependent ROS production (see below).
· Host Immunity and Defense Findings and Evidences
§
Finding
– mROS generated by TLR signalling
·
The researchers determined that when engaged by
lipopolysaccharide and lipotechoic acid (PAMPs), cell-surface TLRs (a subset of
PRRs), caused tumor necrosis factor receptor factor 6 (TRAF6) to move to the mitochondria. It interacted with evolutionary conserved
signaling intermediate in Toll pathways (ECSIT), causing ECSIT ubiquitination
and accumulation around the surface of the mitochondria. ECSIT is an important intermediate, being
potentially involved in respiratory chain assembly and direct augmentation (Vogel et al., 2007). ECSIT-TRAF6 interaction caused the recruitment
of mitochondria to PAMP sources and the increase of mROS production.
§
Supporting
Evidence
·
Mitochondrial recruitment to PAMP
sources
·
·
The data above indicates that mitochondria were
localized to the site of intracellular PAMP sources. The researchers loaded PAMP-coated
microscopic latex beads into macrophages, staining them red. Likewise, they stained the mitochondria
green. The figure above is composed of
two three by three grids, the left grid and right grids showing cells after
fifteen minutes and thirty minutes of bead internalization, respectively. Within each grid, three columns show the
different fluorescent techniques with three different latex bead coats: (from left to right) no PAMP (the negative
control), Pam3CSK4 (a synthetic lipopeptide), and LPS. The three rows show different fluorescent
stains: (from top to bottom) overlapping
of the mitochondrial and latex bead stains, mitochondrial staining alone, and
colocalization fluorescence. This graph
demonstrates a greater degree of colocalization when the beads were coated with
PAMP than when they weren’t, leading the researchers to suggest that the
mitochondria were localized to these PAMPs, presumably by PRR signaling.
·
mROS-dependent
clearance of microorganisms
·
·
The data above indicates that mice
under-expressing ECSIT are less able to clear GFP-labeled Salmonella typhimurium, thus establishing the functional
significance of TLR-mediated mROS production.
The chart above is divided into columns based on the amount of time that
elapsed after the initial in vitro infection
with S. typhimurium. The rows show the different mice genotypes
that were used in the study: a wild type
control with a control shRNA followed by a heterozygous ECSIT mutant treated
with ECSIT shRNA beneath it. The
fluorescence is as labeled, with the host cells being Dapi stained and the S. typhimurium being GFP-labeled. It is clear that the ECSIT deficient mutants
were less able to clear S. typhurium
with time, thus confirming that ECSIT-signaled mROS production was necessary
for clearing the pathogen (not shown:
mROS production was significantly decresed without ECSIT).
§
Finding
– Infestation caused ROS-dependent resistance
·
The
researchers in this study showed that reactive oxygen species generation by
xanthine oxidase in response to wheat (Triticum
aestivum L.) infestation by Russian wheat aphids (RWA) (Diuraphis noxia) contributed to their
resistance to infestation. This study utilized
RWA-resistant and RWA-sensitive forms of wheat.
RWA were allowed to infest these plants, and ROS production, superoxide
dismutase, peroxidase, and chitinase activities were measured over time. Peroxidase (POD) and chitinase are both
downstream defense mechanisms for wheat against RWA (Van der Westhuizen et al., 1998), so determining the levels of
their activity over time is one indicator of increased defenses. Furthermore,
allopurinol, an inhibitor of XO (Montalbini,
1992b),
was drenched in the soil of the plants to determine if the inhibition of XO
would cause the plants to lose their previous resistance.
§
Supporting
Evidence
·
RWA infestation causes ROS
generation in resistant wheat
·
·
The
data above shows that RWA resistant wheat demonstrated a significant increase
in hydrogen peroxide production after eight hours of infestation compared to
the controls. The graph above is split
into two sections, the left and right displaying different periods of time that
the plants were assayed. The x and y
axes show the time (in hours) after infestation and the amount H2O2
in the plant, respectively. The symbols
on the line can be interpreted in the legend.
Notice that the infested resistant population had a significantly higher
amount of H2O2 from eight hours on. This means that infestation caused ROS
production in plants capable of resisting infestation.
·
Defense (POD, chitinase) activity
dependent on XO activity
·
·
·
The data above shows that RWA resistant wheat required uninhibited XO activity for
downstream defense activity in POD and chitinase. The x and y axes on the above graphs show the
time (in hours) after infestation and the activity of POD (upper graph) or Chitinase
(lower graph) in the plant, respectively.
The symbols on the line can be interpreted in the legend, with the final
symbol in the legend (denoted by a dark square and the “#” symbol) denoting
infested resistant plants grown in allopurinol drenched soil. Notice that the infested resistant population
had a significantly higher amount of defensive protein activities after a
period of time. This effect was reversed
upon XO inhibition, indicating that XO-dependent H2O2 production
was responsible for the defense induction (not shown: H2O2 production was
significantly reduced with allopurinol drenching).
*Note, the author possesses all articles. In adding more references to the original as suggested, he confesses that that the additional articles were read only enough to support the statements made and understand their connection to the broader context, not fully.
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