Vessels that reciprocate

© D. Melton

Organs need oxygen and nutrients for growth, so during development they release factors encouraging the generation of healthy blood vessels. Researchers have now found that the reciprocal relationship also occurs: growing blood vessels influence the development and maturation of the pancreas and liver. Douglas Melton and colleagues at the Howard Hughes Medical Institute, Harvard University (Boston, MA), discovered that developing aorta encouraged the differentiation of islet cells in the pancreas (Science, 27 September 2001; 10.1126/science.1064344). In vitro, cultured endodermal cells from Xenopus embryos did not differentiate into islet cells without the co-culture of aorta. In mice overexpressing vascular endothelial growth factor (VEGF-164), blood vessels were generated along with insulin-secreting cells. Kenneth Zaret and colleagues at the Fox Chase Cancer Center took an alternative strategy and showed that mouse livers failed to grow in mice lacking flk-1, which encodes the VEGF receptor (Science, 27 September 2001; 10.1126/science.1063889). Although liver tissue grows in the absence of flk-1, it contains connective tissue rather than liver cells. The influence of endothelial cells occurred even before they formed functional blood vessels, suggesting that the factor was not derived from the blood. Zaret is now interested in identifying “the epithelial signals that promote endothelial cells to come to their neighborhood.” Together, the data provide further clues in the environment needed for tissue development, and will be of value for tissue engineers. LF

Hepatitis hot potato

Cheaper alternatives to recombinant hepatitis B vaccines might be plant-based vaccines, but these are often degraded in the gut. To address this problem, Yasmin Thanavala and fellow researchers at the Roswell Park Cancer Institute (Buffalo, NY) and the Boyce Thompson Institute for Plant Research (Ithaca, NY) have developed a transgenic potato expressing an effective hepatitis vaccine that is protected from digestion (Proc. Natl. Acad. Sci. USA 98, 11539–11544, 2001). The potato plants were transformed using a hepatitis B surface antigen (HBsAg) linked to the plasmid vector, pHB114, previously shown to optimally stabilize the messenger RNA (Nat. Biotechnol. 18, 1167–1171, 2000). Mice fed raw potato expressing HBsAg mounted a strong immune response against hepatitis B, and generated a secondary response to a booster shot. Electron micrographs of transgenic potato cells suggest the basis for the vaccine's efficacy: the HBsAg accumulates in high concentrations as viruslike particles within membrane-bound vesicles in tuber cells. The authors speculate that this “bioencapsulation” protects the antigen from digestion in the gut, and may offer the slow release of antigen needed for long-term immune protection. JJ

Antisense targets pathogens

Various genomics technologies, including gene knockouts and point mutations, have been of value for elucidating the role of genes in bacterial growth and pathogenesis. However, technical constraints mean that it can be difficult to retrieve genes that are essential for growth—lethal mutants. For the first time, Martin Rosenberg and colleagues at GlaxoSmithKline (Philadelphia, PA) have used antisense technology to identify such genes in the bacterium Staphylococcus aureus (Proc. Natl. Acad. Sci. USA 98, 12103–12108, 2001). The researchers generated a library of constructs containing fragments of the S. aureus genome under the control of an antibiotic-induced promoter; S. aureus bacteria were then transformed with the library. Colonies of the bacteria growing in the absence of the antibiotic, but not its presence, were assumed to express antisense molecules for genes critical for growth; of these genes around 30% were novel. Next, Rosenberg and colleagues tested the function of some of the inducible antisense vectors in vivo, infecting mice with S. aureus and determining whether growth-modifying antisense molecules could influence bacterial growth in a physiological model of infection. The authors claimed that this technique could be used to “examine the importance of the gene product when it is switched off after infection has been established.” An inducible antisense-based system would provide a convenient means to look for antibiotics capable of beating infections in situ. LF

Tracking a sugar high

For decades, scientists have sought the molecular switch for gluconeogenesis, the generation of glucose by the liver. Now, J. Cliff Yoon and colleagues from the Dana Farber Cancer Institute in Boston (MA) have discovered a switch, so-called PGC-1 (Nature 413, 131–138, 2001). PGC-1 is a transcriptional co-activator that is known to play a role in cellular respiration and adaptive thermogenesis. However, researchers did not believe that PGC-1 was expressed predominantly in the liver. Yoon et al. performed experiments both in vitro and in vivo, finding that activation of key gluconeogenic enzymes hinged on PGC-1 triggering the transcription of the glucocorticoid receptor and hepatic nuclear factor-4α. Insight into the molecular controls of the production of glucose in the liver could lead to novel and effective drug targets for the treatment of diabetes. Bruce Spiegelman says that the team will “genetically knockout” PGC-1 and determine how it couples with other factors in the liver. The discovery further raises the possibility that other coactivators of transcription play a role in other disease processes. CM

Gene tagging

Pinpointing genes within the morass of genetic sequence is no easy task, but a novel gene-tagging strategy could assist. Andrew Simpson and colleagues at research institutes in Brazil have used open reading frame expressed tags, so-called ORESTES, to tag genes switched on in healthy and diseased human tissues (Proc. Natl. Acad. Sci. USA 98, 12103–12108, 2001). Normally, genes are hunted down using expressed sequence tags (ESTs)—DNA sequences complementary to messenger RNA (usually biased toward the 3′ end). Here, the researchers use partial sequences from more central regions of the cDNA derived using PCR, providing more valuable information about the gene's function and generating a scaffold from which to create a complete transcript sequence. In total, the researchers generated 700,000 ORESTES sequences in 24 healthy and diseased human tissues, around 20% of all ESTs in public databases, and representing around 60% of all genes in the draft of the human genome. Because of its sensitivity, ORESTES can detect rare gene transcripts as well as those in abundance, although it is vulnerable to PCR artifacts. Simpson says he is now following up on estimates of the number of genes in the human genome, predicting between 50,000 and 60,000—higher than earlier estimates of 30,000. LF

Research News Briefs written by Liz Fletcher, Judy Jamison, and Christopher Martino.