By David N. Leff
Editor's note: Science Scan is a roundup of recently published biotechnology-relevant research.
A spare-parts catalog listing available replacements for lost or broken components of the human anatomy would run from artificial limbs and joints to transplanted organs to porcine and plastic heart valves to voice boxes, dentures and scalp hair. Still on the drawing board are prosthetic brain cells to make good the neuronal losses incurred in diseases such as Alzheimer's, Parkinson's, Huntington's and multiple sclerosis.
More than 200 victims of Parkinson's disease have received brain transplants of human fetal nerve cells to restore their brains' output of dopamine, with good to fair to poor clinical results. Moreover, the scarcity of such tissues - plus ethical and legal objections to their use - compromises the extension of this still-experimental approach.
The recent advent of neural stem cells, which can in principle inspire any brain cell to proliferate, has opened a new vista on adding missing brain neurons to that spare-parts list some day. A team of Swedish and American neuroscientists that teamed with a U.S. biotech company - CytoTherapeutics Inc, of Lincoln, R.I. - report progress in this endeavor. Their paper in the July 15, 1999, issue of The Journal of Neuroscience bears the title: "Site-specific migration and neuronal differentiation of human neural progenitor cells after transplantation in the adult rat brain."
As their starting material, the co-authors obtained human forebrain tissue from two embryos, one in the sixth week of gestation, the other in the ninth week. From these specimens they incubated neural stem cells in the presence of a spectrum of growth factors and, in 21 passages, expanded these continuously dividing cultures to yield at least 10 million cells. Then, by site-directed stereotaxic surgery, the team implanted 100,000-cell aliquots into the brains of 20 adult rats. The several cerebral targets of these insertions included areas of the hippocampus and corpus striatum, which are considered central to loss of motor and cognitive functions in Parkinson's and other neurodegenerative diseases. (See BioWorld Today, July 19, 1999, p. 1.)
After giving the implants several weeks to settle in, differentiate and proliferate, the co-authors examined the rodent recipients' brains microscopically and histochemically. They found that "both cultures showed a growth rate that was similar to . . . other human progenitor cell cultures derived from different gestational ages," the article states. Moreover, "Expression of neuronal markers in the striatal transplants indicate that a substantial fraction of the grafted human progenitors had developed toward a neuronal phenotype."
The co-authors cautioned, "It remains to be demonstrated, however, to what extent these newly formed neurons can undergo complete maturation." They did, however, conclude "this culture system may provide an almost unlimited source of human neural progenitors . . . as an alternative to primary embryonic brain tissues for intracerebral transplantation."
Multi-Threat Gene Therapy Strategy Takes On Question Of Optimal Vectors, Sites, Regulation
In its current growing pains, the art and science of gene therapy has a number of hurdles to vault. One is finding or making the ideal DNA-delivery vector, with adenovirus (AV) and adeno-associated virus (AAV) front-running for this title. Then there's the question of the preferred site - systemic, regional or local - for introducing the gene construct. Among the prime competing sites: bloodstream circulation, the abdomen and the muscle. Assuming the solution of these problems, there would still remain the key challenge of regulating the expression of the protein encoded by the gene of interest - turning it on or off, up or down.
Gene therapy pioneer James Wilson, at the University of Pennsylvania-affiliated Institute for Human Gene Therapy, has been tackling all of this unfinished business in a single strategy. His latest findings appear in the current Proceedings of the National Academy of Sciences (PNAS), dated July 20, 1999. The PNAS report is titled: "Long-term regulated expression of growth hormone in mice after intramuscular gene transfer."
In a series of in vivo experiments, Wilson and his co-authors compared the effectiveness of AV and AAV viral vectors in delivering the human growth hormone gene (hGH) to nude mice, and the efficiency of a regulatory transcription factor, rapamycin, which is a proprietary, orally available, molecule developed by Ariad Pharmaceuticals Inc., of Cambridge, Mass. (See BioWorld Today, Jan. 7, 1999, p. 1.)
"In mice transduced [intramuscularly] with AAV or adenovirus vectors," their paper reported, "basal human growth hormone expression is undetectable, and is induced to high levels by a single administration of rapamycin." When they withdrew the drug, hGH levels dropped back to baseline in about 1.5 days. As for duration of the therapeutic effect, "In adenovirus-infected cells, peak hGH levels began to decrease after 50 days, whereas AAV-infected cells show no reduction in peak levels over at least a 10-month period."