Zebrafish To Shed Light On Human Mitochondrial Diseases
Science Daily <http://www.sciencedaily.com/> — Zebrafish can now be
used to study COX deficiencies in humans, a discovery that gives
scientists an unprecedented window to view the earliest stages of
mitochondrial impairments that lead to potentially fatal metabolic
disorders, according to researchers at the University of Oregon.
[http://www.sciencedaily.com/images/2007/09/070913135009.jpg]
Katy Baden, whose research has established the zebrafish as a model for
studying human diseases linked to COX deficiencies, is shown in the
Zebrafish International Resource Center. (Credit: Photo by Jim Barlow)
COX deficiencies refer to a breakdown of cytochrome coxidase, an enzyme
located in the mitochondrion of every cell. Mitochondria are crucial
cellular workhorses that provide chemical energy. Research of the
deficiency has been stymied by a lack of model organisms, with mice
being introduced as the first model by Japanese researchers just seven
years ago.

COX involves multiple proteins and assembly factors, and deficiencies of
any one of them can negatively affect metabolic tissues, including the
brain, muscle and eyes. Deficiencies during the prenatal period are
considered to be a potential cause of miscarriages and have been led to
prenatal screenings, but scientists still don't understand the metabolic
requirements of tissues and organs during early development.

The case for zebrafish (Danio rerio) as an alternative research model is
described in a paper posted online ahead of regular publication by the
Journal of Biological Chemistry. The comprehensive UO study, led by
doctoral student Katrina N. Baden, could speed research and point to
specific targets to test potential drug therapies, said co-author Karen
Guillemin, a professor of molecular biology and member of the UO
Institute of Molecular Biology.

"Mitochondrial impairments are emerging as important in many human
diseases, but there have been few models for understanding exactly what
is happening during the early development of the diseases," Guillemin
said. "The use of mice is limited, because knocking out protein
expression in mice mitochondria to mimic human-disease states results in
large numbers of deaths in utero. Therefore, the symptoms that
researchers have wanted to study have not been assessable in mice."

Baden, a veterinarian, performed several experiments, using RNA-blocking
reagents known as morpholinos to reduce gene expression of both a
critical COX subunit and Surf1, an assembly-factor protein that when
mutated can lead to Leigh syndrome, a severe neurological disorder. She
targeted a variety of proteins, alone and in combination, and then added
back components to rescue each deficiency. Normal COX activity declined
as much as 50 percent in the experimental conditions and resulted in
developmental defects in endodermal tissue, cardiac function and
swimming behavior in the zebrafish.

"The unique characteristics of zebrafish make them an ideal model for
studying the effects of mitochondrial deficiencies on early
development," said Baden, who earned her doctorate in July and is now
the veterinarian at the UO-based Zebrafish International Resource
Center. "Because they develop outside of a uterus and are transparent in
early stages, I was able to visualize the effects that molecular
alterations have on cell biology, nervous system development, cardiac
function and fish behavior."

The external and transparent embryo, Guillemin said, will allow
scientists to create specific deficits that mirror those in humans. "The
transparency of the embryo will let us see primary defects, what happens
in the earliest stages, rather than having to settle for seeing
secondary downstream defects later in the disease state," she said.

"Different tissues respond differently to specific losses in
mitochondria."

Baden and Guillemin said that the use of zebrafish will improve
scientific understanding of the mechanisms of mitochondrial associated
pathology in people and speed the identification of new treatments for
mitochondrial diseases.

Other researchers who were co-authors on the study were James G. Murray,
director of assay development for Mitosciences Inc. of Eugene and former
post-doctoral researcher in the UO Institute of Molecular Biology (IMB),
and Roderick A. Capaldi, professor of biology, member of the IMB and
chief scientific officer and co-founder of Mitosciences.

The research was funded in part through two grants to Guillemin from the
National Institute of Diabetes and Digestive and Kidney Diseases
(National Institutes of Health) and the Eugene and Clarissa Evonuk
Memorial Graduate Fellowship in Environmental or Stress Physiology to
Baden.

Note: This story has been adapted from a news release issued by
University of Oregon.



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