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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 
<http://www.gmu.edu/resources/welcome/Directions-to-GMU.html>
 
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 (??m) 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 ??m, 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 ??m 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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