In experiments with mice, a research team led by Johns Hopkins scientists has discovered an unusual protein pair that stops blood vessels' growth in the developing back. Results of the studies, published today in the express online edition of Science, are of special interest to researchers trying to prevent blood flow that nourishes tumors or exploit the signals vessels emit during growth to help regrow damaged nerves.
During an animal's prenatal development, protein "signs" tell growing blood vessels which way to go and when to stop or turn back. Scientists already knew that one big family of "stop" proteins works by binding to two proteins, called receptors, on the leading edge of a budding blood vessel. In new experiments, the Hopkins-led team reports on one member of this family of proteins that works differently from the others.
"Unlike all of the others in this group, called semaphorins, this protein only needs one protein receptor partner," says lead author Chenghua Gu, D.V.M., Ph.D., a postdoctoral fellow in neuroscience in Hopkins' Institute for Basic Biomedical Sciences. "It's a totally new observation of blood vessel growth in development, and it has made us rethink how the semaphorins control this process."
Semaphorins float freely in tissues adjacent to blood vessels and nerves and stop them from migrating into inappropriate areas. Although the protein the team studied, known as Sema3E, belongs to this family, its binding partners and exact job were unclear until now.
Gu and others from the laboratories of Hopkins neuroscience professors Alex Kolodkin, Ph.D., and David Ginty, Ph.D., engineered a version of Sema3E that colors its binding partner blue. They found that the resulting blue pattern on the developing mice looked suspiciously like the pattern of plexin-D1, a previously described protein found in blood vessels and nerves.
To prove that this was indeed Sema3E's binding partner, the researchers inserted the plexin-D1 protein into cultured monkey cells that don't naturally contain it. They discovered that the Sema3E protein bound tightly to the monkey cells and created a signal. Surprisingly, the two proteins didn't need a third that is known to work with semaphorins in other situations.
To see how each of these three proteins affects blood vessel migration, Gu and colleagues from Hopkins, Yutaka Yoshida, Ph.D., and other collaborators from Columbia University and the Developmental Biology Institute of Marseille in France engineered mice to lack each of the three proteins. In mice without Sema3E or plexin-D1, blood vessels along a particular part of the back were disorganized. In contrast, mice lacking the third protein (called neuropilin) grew the vessels normally.
"We know that these two proteins are crucial for the growth of blood vessels in the back, but now we're investigating whether the pair controls blood vessels and nerves in other parts of the developing mouse, too," says Ginty, also a Howard Hughes Medical Institute investigator.
Sema3E was originally isolated from an invasive tumor cell line, so it's thought to be associated with progression of cancer. Both plexin-D1 and Sema3E are found in many species, including humans, and when disrupted could contribute to vascular birth defects, coronary heart disease, and adult nerve regeneration problems, say the researchers.
This research was supported by grants from the National Institutes of Health (NIH), Howard Hughes Medical Institute (HHMI), the Christopher Reeve Paralysis Foundation, the Packard Center for ALS Research at Johns Hopkins, the Institut National de la Santé et de la Recherche Médical (INSERM), Centre National de la Recherche Scientifique (CNRS), Association Française contre les Myopathies (AFM) and a European Commission contract.
Authors on the paper are Gu, Ginty, Kolodkin, Dorothy Reimert and Janna Merte from Hopkins; Yoshida and Thomas Jessell from HHMI at Columbia University, New York; and Jean Livet, Fanny Mann and Christopher Henderson from the Developmental Biology Institute of Marseille (IBDM), France.
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