Proteins in higher organisms, from fruit flies to humans, which are secreted or displayed on cell surfaces must have sugars attached to them in order to function properly. This glycosylation process is necessary for protein stability, may modulate protein activity and provides recognition sites on cell surface proteins, which is necessary for cell-to-cell interactions.
The glycosylation process fascinates Palter, an associate professor of biology in Temple's College of Science and Technology, who has been studying its biological roles for the past six years.
One project involves growing insect cells outside of the organism and using them to produce human proteins that can be used therapeutically, such as clotting factors.
"When I first became involved in this project, the objective was a bio-engineering one," said Palter, a geneticist. "We wanted to produce an insect cell line that basically could produce the exact glycosylation pattern that would occur in humans. Because insect cells are missing the last two steps in the glycosylation process, you can't use them to generate therapeutic proteins, even though they are easier and less expensive to use, compared with mammalian cell lines."
Palter, who came to Temple in 1988, says the insect cells must be genetically altered to produce human proteins that have a glycosylation pattern typical of human proteins, and not insect proteins; otherwise, the proteins will be destroyed in the liver or recognized as foreign by the immune system.
"When we started, the objective was to look at the genomes of insects, which just started being sequenced, and figure out what glycosylation enzymes they had and what enzymes they were missing so that we could either add on or subtract an enzyme to duplicate the human pathway," she said.
Palter was intrigued by a paper from Jurgen Roth and his colleagues published in 1992, which reported detecting the complex sugars typical of human proteins in insects, but found they were restricted to the cells of the central nervous system.
"Unfortunately, researchers did not believe or follow up on Roth's initial study," she said. "However, I became interested in what the role of complex glycosylation might be in the central nervous system. As a geneticist, I thought we could use the sophisticated genetic approaches available in the fruit fly to understand this."
This led to a second research project, which involves studying mutant flies that lack the ability to add the terminal sugar, sialic acid, to specific proteins of the central nervous system. Mutant flies exhibit abnormal mating behavior, have defects in their ability to fly and climb, and as they age they experience neurodegeneration resulting in seizures and paralysis.
"The nerves of the mutant flies are defective in electrical signaling, which impairs their ability to communicate with one another and muscle cells," Palter said. "Our mutant flies may provide a model system to study a class of human diseases whose symptoms include memory loss, ataxia, epilepsy and neurodegeneration."
Both of these studies are being done in collaboration with scientists at Johns Hopkins University, the University of Iowa and New York University Medical School, and have been funded through support from the National Institutes of Health. The researchers have published some of their early findings in prestigious journals, including the Journal of Biological Chemistry.
"I'm collaborating with the groups at Johns Hopkins to do the engineering and chemical analysis part of this research, but because I'm a developmental biologist and a geneticist, I'm more interested in what the normal role of glycosylation is in the central nervous system," Palter said, emphasizing that most of the applicable parts of major medical discoveries can trace their roots to studies in basic science.
"Their expertise is in cell culture, chemical analysis and molecular modeling, whereas I'm the molecular geneticist in this collaboration, trying to figure out what is the biological significance of these modifications."