Researchers at the University of Oregon claim that their finding has opened an unparalleled pathway to examine the earliest stages of mitochondrial impairments that lead to potentially fatal metabolic disorders.
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 foiled 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.
The comprehensive 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. 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," Guillemin said.
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 behaviour in the zebrafish.
"The unique characteristics of zebrafish make them an ideal model for studying the effects of mitochondrial deficiencies on early development. 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 behaviour," said Baden.
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. Different tissues respond differently to specific losses in mitochondria," she said.
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.
The study has been published online ahead of regular publication by the Journal of Biological Chemistry.