Likening his discovery to a paleontologist unearthing a new dinosaur species, Vladimir Kapitonov, a staff scientist at the Genetic Information Research Institute, recently revealed a new class of transposable elements in eukaryotes. These jumping genes use rolling circle replication--an ancient process characteristic of some plasmid replication in bacteria--to copy and insert itself throughout entire genomes (V.V. Kapitonov, J. Jurka, "Rolling circle transposons in eukaryotes," Proceedings of the National Academy of Sciences, 98[15]:8714-9 July 17, 2001.) Found in the genomes of Arabidopsis thaliana, Oriza sativa, and Caenorhabditis elegans, these elements have been dubbed helitrons because nearly always they code for a helicase, which unwinds the double helix at both the original transposon and the integration site. Through computational analysis of variation in transposons, which in Arabidopsis and nematodes constitute roughly 2 percent of genomic DNA, Kapitonov estimates that helitrons are tens of millions years old. The origin and purpose, if any, of helitrons and other transposable elements still baffles scientists. It is known that they exist (or at least existed) to replicate and propagate. Though hypothesizing a viral origin would perhaps be easiest, the exon-intron structure and sequence of the genes that helitrons encode indicate that they were captured. "The transposon takes something from the host and uses it for its own sake," Kapitonov says. If a gene taken provides no selective advantage, he suggests that it is "destroyed by multiple mutations." In this way, transposons may help power evolution. (See also, B.A. Palevitz, "Genetic parasites and a whole lot more," The Scientist, 14[20]:13, Oct. 16, 2000.)
Boosting Nerve Cell Regeneration
Most attempts to jumpstart regeneration in the central nervous system (CNS) change the chemical milieu. Maureen Condic, an assistant professor of neurobiology and anatomy, University of Utah School of Medicine, Salt Lake City, is working on a more targeted way to stimulate CNS regeneration by increasing expression of a single gene. The research focuses on a type of integrin protein that links the cytoskeleton to the extracellular matrix. The expression declines from embryohood onward. "In the embryo, integrins extend axons," says Condic. "In the adult, they probably also have a structural role, but at synapses, to strengthen transmission important in learning and memory." Condic based her comments on the fates of mice and flies that have had integrin genes knocked out. Condic introduced extra integrin genes in an adenovirus vector into rat sensory neurons in culture (M.L. Condic, "Adult neuronal regeneration induced by transgenic integrin expression," Journal of Neuroscience, 21[13]:4782-8, July 1, 2001). She measured the percentage of cells with neurites, and the number and length of these extensions. "The first time I did the experiment, I was stunned. There are more axons, and more cells that make them," she recounts. Says Condic: "An advantage of focusing on the neuron is that we have better spatial and temporal control of the manipulation than changing the environment or implanting stem cells," which could avoid unexpected effects on bystander areas.
