By David N. Leff
Editor¿s note: Science Scan is a roundup of recently published biotechnology-relevant research.
Traveler¿s diarrhea, a.k.a. Montezuma¿s revenge, is perpetrated by a bacterium known to research as friend rather than foe ¿ namely, Escherichia coli. It wreaks this tummy upset by secreting a heat-stable enterotoxin into the gut ¿ from duodenum to rectum ¿ and also afflicts people in underdeveloped countries, as well as farm animals.
Gastroenterolgists know this abdominal menace, E. coli heat-stable enterotoxin, by its shorthand name ¿ ST. ST triggers intestinal secretion by binding to a single transmembrane protein, guanylyl cyclase C (GC-C). This enzyme operates only in the brush border (microvilli) zone of the intestine¿s inner-wall epithelial cells. It precipitates secretory diarrhea by a domino effect: First, ST interacts with the catalytic domain of GC-C, which accumulates guanosine 5¿-monophosphate, a major component of ribonucleic acids. This in turn phosphorylates, of all things, the cystic fibrosis transmembrane conductance regulator, thereby increasing chloride transport. Whereupon, the dire bowel symptom kicks in.
STs are an example of molecular mimicry, in which E. coli has developed an evolutionarily advantageous strategy that subverts the machinery of normal intestinal physiology. A paper in the Proceedings of the National Academy of Sciences (PNAS), dated July 3, 2001, explores this grim versatility. Its title: ¿Guanylyl cyclase C agonists regulate progression through the cell cycle of human colon carcinoma cells.¿ Its authors are clinical pharmacologists at Thomas Jefferson University in Philadelphia.
They report finding that an ST receptor on the surface of metastatic colorectal cancer cells controlled the growth of those cells. What¿s more, that hookup between receptor and tumor slowed considerably the malignant growth and spread. It did so by delaying progression of the cell cycle. The authors suggest that this effect could be harnessed to develop novel therapeutic agents against colorectal tumors, such that, ¿Cancer then becomes a chronic disease.¿
APP Amyloid Precursor Protein, Parent Of Alzheimer¿s Plaques, Closely Holds Secrets Of Its Normal Day Jobs
Senile neuritic plaques are the sine qua non of Alzheimer¿s disease (AD). Unlike the other typical sign of AD, neurofibrillary tangles, which bunch up inside the dying AD neurons, plaques build up around those nerve cells externally. They are formed by short amyloid-beta (A-b) peptides, spin-offs of the much larger amyloid precursor proteins (APP).
Neuroscientists know a lot about where APP is, but next to nothing as to what it does ¿ besides parting with a piece of itself in the form of A-b. APP is a widely expressed cell-surface protein in the human body. Besides that plaque-forming A-b, it also releases a stubby tail fragment into the cell¿s cytoplasm.
A paper in Science dated July 6, 2001, relates what happens next. It bears the title: ¿A transcriptively active complex of APP with Fe65 and histone acetyltransferase Tip60.¿ The article¿s authors are neuroscientists at the University of Texas Southwestern Medical Center in Dallas.
They report that the APP tail forms a multimolecular complex with Fe65, the nuclear adapter protein, and Tip60, the histone acetyltransferase. This combo potently stimulates transcription, suggesting that APP¿s release of its mysterious cytoplasmic tail may have a hand in gene expression.
The authors came up with not one but two mutually exclusive models of putative APP functions in Fe65-dependent transcriptional activation: ¿The first model suggests that the physiological role of APP is to keep Fe65 out of the nucleus, and that APP cleavage liberates it for a nuclear function. The second model, by contrast, suggests that the cytoplasmic tail of APP normally functions in transcription when it is bound to nuclear Fe65.¿
The paper concludes: ¿The approaches and observations described in the current study may be helpful for these objectives that will not only be important for knowledge of the normal functions of APP, but also for the pathogenesis of Alzheimer¿s disease.¿ Elsewhere, it raises the possibility of unwanted side effects from drugs intended to prevent formation of neuritic plaques.
Third Wave Technologies Presents All-Purpose RNA Detection/Quantitation ¿Invader¿ Assay
RNA (ribonucleic acid) is the body¿s medium for translating an individual¿s genetic blueprint ¿ found in one¿s DNA ¿ into all the proteins in the body. Many gene families, notably that of cytochrome 450, which is responsible for metabolizing drugs and toxic compounds, generate nearly indistinguishable RNAs.
An experiment described in Nature Biotechnology for July 2001 tested four rat cytochrome 450s, using total RNA prepared from four differentially treated samples. The journal¿s ¿Technical report¿ is titled: ¿An invasive cleavage assay for direct quantitation of specific RNAs.¿ Its co-authors comprise 15 scientists from Third Wave Technologies, of Madison, Wis., plus one from Merck Research Laboratories in West Point, Pa. Third Wave calls its copyrighted invasive cleavage RNA-detection method its ¿Invader¿ assay. (See BioWorld Today, Feb. 13, 2001, p. 1.)
¿The analytical sensitivity of the cytochrome P450 mRNA assays,¿ their paper stated, ¿ranges from 6,000 to 30,000 molecules per reaction, which is sufficient for analyzing changes in expression levels for many different genes.¿ Moreover, an analytical sensitivity of fewer than 100 copies of HIV-1 RNA was demonstrated.
The authors make the point that ¿quantitation of viral RNAs can predict disease progression and therapeutic efficacy,¿ and that ¿gene expression analysis of diseased vs. normal, or untreated vs. treated, tissue can identify relevant biological responses, or assess the effects of pharmacological agents. The Invader assay should be useful,¿ the article concluded, ¿in any area requiring RNA quantitation, such as high-throughput screening in drug discovery research, monitoring of drug metabolism and safety in clinical trials, and clinical detection of viral RNA load.¿ They add that their Invader technology will ultimately allow health care professionals to personalize diagnosis and treatment of infectious disease based on the unique genetic makeup of each patient.