As is often the case at NIH, one institute's loss is another's gain. When James Battey recently became scientific director of NIDCD, NCI lost a top-notch researcher, and NIDCD gained a distinguished leader. Battey came to NCI in 1983, after a postdoc at Harvard Medical School in Boston. In 1988, he moved to NINDS as the chief of the Laboratory of Neurochemistry's Molecular Neuroscience Section. In 1992, Battey returned to NCI to head the Laboratory of Biological Chemistry's Molecular Structure Section. Battey offers this description of his research.
Our laboratory focuses on the structure, function, and regulation of mammalian receptors for bombesin-like peptides. Bombesin is a 14-amino acid peptide originally purified from frog skin by the physiologist Vittorio Erspamer and his colleagues at the University of Rome in 1971. Two homologous mammalian peptides have been purified and sequenced. Gastrin-releasing peptide (GRP) was named for its ability to stimulate the release of gastrin from the G cells in the antral mucosa. Neuromedin B (NMB) was isolated from the spinal cord as an effective agent in inducing smooth muscle contraction in a bioassay. Both peptides show remarkable sequence identity to bombesin over their carboxy-terminal ends - the region essential for high-affinity binding to receptors and all biologic responses attributed to mammalian bombesin-like peptides.
Mammalian bombesin-like peptides mediate a variety of biologic responses, including smooth muscle contraction, secretion, and modulation of neuronal firing rate. In addition, they regulate the growth of cultured fibroblasts expressing bombesin receptors and have been implicated as autocrine growth factors that modulate the growth of some human carcinomas. GRP is transiently expressed in the developing lung, raising the intriguing possibility that such peptides and their receptors are important for establishing patterns of growth and differentiation during fetal development. This wide spectrum of biologic activity raises interesting questions about the receptors for bombesin-like peptides: How many receptors are there? Do all the receptors access the same signal-transduction pathway? Are there specialized receptors for different functions?
To learn more about the molecular mechanisms governing the responses elicited by mammalian bombesin-like peptides, our lab has cloned and characterized cDNAs and then genes that encode three structurally similar, but pharmacologically distinct, receptors for such peptides. All three belong to the growing family of receptors that includes cystic fibrosis transmembrane-conductance regulator and the multidrug-resistance protein. These receptors all possess seven transmembrane-spanning domains, and their responses are coupled through heterotrimeric GTP-binding proteins. The first bombesin receptor, GRP-R, binds GRP with highest affinity; the second, NMB-R, binds NMB with highest affinity; and the third, called bombesin-receptor subtype 3 (BRS-3), binds neither GRP nor NMB with high affinity. The existence of bombesin receptor with no known naturally occurring, high-affinity ligand raises the possibility that there are additional mammalian bombesin-like peptides yet to be discovered. NMB-R and GRP-R are expressed in overlapping but distinct regions of the central and peripheral nervous systems and gastrointestinal tract in adult mammals, whereas BRS-3 shows a different pattern of expression limited to secondary spermatocytes in the testis and the uterus during pregnancy. GRP-R also has a widespread pattern of expression late in embryonic life.
All three receptors appear to activate a similar signal transduction pathway, which involves activating phosphoinositide-specific phospholipase C, elaborating inositol 1,4,5-tris phosphate (IP3), elevating intracellular calcium, and activation of protein kinase C.
With Robert Jensen's lab at NIDDK, we are investigating which segments of bombesin receptors are critical for high affinity binding of agonists and antagonists. Our preliminary studies indicate that multiple regions of the receptor are necessary, that there are residues which determine preferential binding for either NMB or GRP peptides, and that agonist and antagonist binding sites are overlapping but clearly distinct. We have created mutant receptors, which can no longer couple to G-proteins, to help determine if coupling is essential for receptor internalization and other events, such as phosphorylation, that occur when bombesin receptors are activated. To perform these studies, we have generated a number of stably transfected cell lines that express wild-type or mutant receptors at varying levels, as well as specific polyclonal anti-sera suitable for both immunoblotting and immunoprecipitation.
Our studies using bombesin receptors ectopically expressed in Xenopus laevis oocytes suggest that GRP-R and NMB-R may preferentially initiate signal transduction with different heterotrimeric G-proteins. With John Northup at NIMH, we will address the question of receptor-G-protein interactions and establish a biochemical system for studying receptor coupling in vitro, using purified receptors, heterotrimeric G-proteins, and candidate receptor kinases. Using the yeast two-hybrid system with Richard Kahn's lab at NCI, we plan to search for molecules that interact with the bombesin-receptor region that undergoes ligand-activated phosphorylation. This approach has the important advantage of making no initial assumptions about the identity of potential regulatory molecules.
Finally, working with Heinz Arnheiter at NINDS, we are using gene targeting techniques in embryonic stem cells to generate mice that lack a functional allele for GRP-R, NMB-R, or BRS-3. We hope that these knock-outs will reveal the function of the bombesin receptors in the whole animal.