By David N. Leff
Editor's note: Science Scan is a roundup of recently published biotechnology-relevant research.
City folks who've rarely - if ever - seen a working farm freely quote the rustic expression, "Locking the barn door after the horse is stolen." This metaphor neatly fits the dilemma of current treatment for HIV infection and AIDS.
That triple-drug cocktail, of course, is HAART - highly active antiretroviral therapy. While it's proven to offer HIV-positive people facing AIDS longer life, and a better life, HAART's own life looks limited. Its prime enzymic ingredients - which inhibit reverse transcriptase and protease - confront the growing drug-resistance potential of the virus. While new and better agents are in the works, the fundamental drawback of HAART is that it locks the door giving access to HIV's target T-helper cells and macrophages after the virus has made good its entry.
Virologists and drug designers are looking for a strategy to scotch the virus before it penetrates those immune-system cells. What they've learned in recent years is just how HIV choreographs its break-and-enter light fantastic. When the glycoprotein 160 (gp160) that envelopes HIV-1 virions approaches its intended T-cell victim's port of entry, it cleaves into two sub-particles: gp120 (the surface segment) and gp41 (the transmembrane segment).
While gp120 sidles up to its target, and binds to receptors on that cell's surface, the gp41 protein sequence goes through kinky contortions of shape, preliminary to fusing the virion and target-cell membranes. This brief exposure of gp41's soft underbelly - featuring an evanescent deep cavity, or pocket, has the drug designers scrambling to craft inhibitors of that protein fragment. Their secret weapon is X-ray crystallography, which lays bare the agile sub-particle's briefly vulnerable fusion site.
This month, in closely parallel research accounts, two leading scientific journals simultaneously reveal that occult bull's-eye. One is the Oct. 1, 1999 issue of Cell, which reports: "Inhibiting HIV-1 entry: Discovery of D-peptide inhibitors that target the gp41 coiled-coil pocket."
The other article, in the October 1999 issue of Nature Structural Biology, bears the title: "Selection of gp41-mediated HIV-1 cell entry inhibitors from biased combinatorial libraries of non-natural binding elements."
The senior author of Cell's report is structural biologist Peter Kim at the Whitehead Institute of Biomedical Research in Cambridge, Mass. He first identified gp41's fleetingly exposed pocket two years ago as a potential oral drug target. Now Kim and his co-authors have identified compounds that bind to this cavity. "Those compounds," Kim pointed out, "can serve as starting points for developing small-molecule drugs that stop HIV entry into cells. Or," he went on, "scientists can use our 'pocket' for high-throughput screening to identify new candidate drugs by their ability to disrupt the interactions between the pocket and the binding molecules." To this end, Whitehead will offer non-exclusive licenses of the technology to pharmaceutical and biotech companies worldwide.
Harvard University molecular and cell biologist Stephen Harrison is senior author of the Nature Structural Biology paper. As did the Cell article, he and his co-authors found, "The structure of the gp41 ectodomain shows that there is a deep cavity on the surface of the inner core, which might be targeted by small-molecule inhibitors." They screened 61,275 potential drug-candidate ligands in a combinatorial chemical library of binding elements targeted to the gp41 core.
First Cloned Sheep Acquired Her Mitochondria Exclusively From Maternal Progenitor Cell
Dolly, the trailblazing Scottish cloned sheep, is back in the news. She was created by "nuclear transfer," in which nuclear donor cells are fused with recipient egg cells from which the nuclei have been extracted. This process merges the cytoplasms of both cells - both containing mitochondria - organelles of which the genes are inherited solely from mothers. So Dolly inherited genomes from two cellular sources - nuclear (donor only) and mitochondrial (donor, recipient or both).
Whence the controversy: To which parental cell does she owe her mitochondrial endowment? It's resolved by research reported in the September 1999 Nature Genetics, cryptically titled: "Mitochondrial DNA genotypes in nuclear transfer-derived cloned sheep." The article's senior author is molecular geneticist Eric Schon of Columbia University in New York, with cloner Ian Wilmut, one of Dolly's creators at the Roslin Institute in Scotland. They scoped Dolly's inheritance pattern, plus that of nine other cloned sheep, and determined, "The mitochondrial DNA of each of the 10 nuclear-transfer sheep was derived exclusively from recipient enucleated oocytes, with no detectable contribution from the respective somatic donor cells. "
The co-authors cited, in conclusion, "implications for future attempts to correct maternally inherited mitochondrial genetic disorders."
In Effect Of Age On Male Sexual Performance, Elderly Rats Quantify Key Role Of Nitric Oxide
Now that a former U.S. presidential candidate has let penile erectility dysfunction out of the closet, diminished male sexual performance is fair game for open analysis. The psychophysiological culprit, it seems, is not advancing age alone but a gas - nitric oxide - which influences the effect of superannuation on penile activity. Nitric oxide (NO), the agent through which the drug Viagra works, is a vasodilator, and regulator of blood flow and blood pressure.
A research report in the current Proceedings of the National Academy of Sciences (PNAS), dated Sept. 28, 1999, deals with this issue experimentally. Its title: "Gene transfer of endothelial nitric oxide synthase [eNOS] to the penis augments erectile responses in the aged rat." (The enzyme eNOS generates NO.)
The PNAS paper's co-authors, at Tulane University School of Medicine in New Orleans, transferred the gene for eNOS, via an adenovirus vector, into the corpora cavernosum penis of 40-week-old anesthetized rats. The enzyme abundantly expressed nitric oxide in that erectile tissue.
One day later, the team applied electric nerve stimulation to the rodents' members, and determined "the magnitude of the increase in cavernosal pressure in response to nerve stimulation was increased significantly." Their overall findings "suggest that adenoviral transfer of the eNOS gene enhances erectile function in the rat."