Dissertation Defense Announcement
To: The George Mason University Community
Candidate: Rashmi Kumar
Program: PhD Bioinformatics & Computational Biology
Date: Thursday August 14, 2012
Time: 11:00 a.m.
Place: George Mason University
Occoquan Bldg., #204
Prince
William Campus
Dissertation Director/Committee Chair: Dr. M. Saleet Jafri
Committee members: Dr. Dmitri Klimov, Dr.
Patrick Gillevet, Dr.
Siddhartha Sikdar
Title: "A Computational Analysis Of Mitochondrial Reactive Oxygen
Species Dynamics In Cardiomyocytes"
The dissertation is on reserve in the Johnson Center Library, Fairfax
campus.
The doctoral project will not be read at the meeting, but should be
read in advance.
All members of the George Mason University community are invited to
attend.
ABSTRACT:
Mitochondria play an important role in the maintenance of ionic
homeostasis and the control of ATP production, cellular redox potential
and reactive oxygen species (ROS) production. Low levels of ROS are
generated as a byproduct of energy metabolism by mitochondria.
Experiments indicate that the two important sites contributing to ROS
production are the Complex I (NADH: ubiquinone oxidoreductase) and
Complex III (Ubiquinol: cytochrome C oxidoreductase) of mitochondrial
respiratory chain. These highly reactive molecules in excess can lead
to oxidative stress and is linked to multiple pathological conditions
like diabetes, neurodegenerative diseases and ischemia reperfusion. To
ameliorate this risk, several antioxidant defenses play an important
role in maintaining the redox balance. To better understand the complex
system regulating ROS, we present a mechanistic model describing ROS
production across the electron transport chain (ETC) as well as
description of the pathways for the dissipation of ROS. The model
incorporates detailed biochemical kinetics for electron fluxes across
tricarboxylic acid cycle (TCA), mitochondrial calcium ([Ca2+]m)
handling, membrane ion transport processes. This is coupled to the
mechanism of electron transfer across respiratory complex I, II and
III, oxidative phosphorylation and generation of ROS by complexes I
and III. Additionally, the computational model presents a very detailed
mechanism of various matrix and extramitochondrial antioxidant defenses
(described by GSH/GSSG and NADPH/NADP+ redox pairs) and regulation of
mitochondrial permeability transition pore (MPTP) dynamics by ROS.
The model enables study of factors regulating ROS production and
provides the first insights into the mechanism of succinate induced ROS
production with increasing membrane potential (ΔѰm) which is quite
distinct from the mechanism of reverse electron transport (RET)
proposed by others. The association of substrate nature and presence of
inhibitors in regulating ROS production is presented in this study.
The model suggests the mechanisms of mitochondrial acidification (pHm)
in response to cytosolic acidification (pHe), and predicts an important
role of mitochondrial respiration driven proton pumps in maintaining
pHm in addition to the role of potassium-hydrogen (KHEm) exchanger
suggested previously by others. Results from model also indicate that
changes in extramitochondrial pH regulate ΔѰm, Ca2+ and ROS. The model
identifies the factors modulating excess ROS formation during elevated
[Na+]i, as observed during conditions of heart failure. The reduction
in glutathione redox couple ratio (GSH/GSSG) is observed in parallel
with progressive ΔѰm depolarization and increase in ROS levels in the
model.
During experiments a transient increase in ROS levels has been observed
concurrent with a decrease in membrane potential, however, the
mechanism behind these remains controversial. Others have proposed
that there is positive feedback of ROS on ROS production (ROS-induced
ROS release). Our mechanistic model, however proposes an alternative
as ROS production must decrease with decreasing membrane potential.
The model simulates ROS generation in the mitochondria due to laser
induced photoactivation of TMRM. Under certain conditions a transient
increase in ROS levels is observed that is termed a ROS burst. It is
hypothesized that this increase in mitochondrial ROS can lead to
opening of MPTP which releases this burst of stored up ROS. The opening
of MPTP leads to the decrease in mitochondrial membrane potential
accompanying the ROS burst. The simulation results are qualitatively
similar to those shown in experiments using isolated adult cardiac
myocytes offering an alternative explanation for experimentally
observed phenomenon of ROS burst in cardiac myocytes.
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