Targeted gene disruption in endocrine research--the case of glucagon-like peptide-1 and neuroendocrine function.

In this issue of Endocrinology, MacLusky et al. (1) use mice with targeted gene deletion of the glucagon-like-peptide-1 (GLP-1) receptor to study the role of GLP-1 signaling to control a number of neuroendocrine systems. Such studies highlight the very exciting stage that endocrine and neuroendocrine research has reached. Recent technical advances, and a decade and a half of intense research effort, have the field poised on the brink of what was once an unthinkable position—the dissection and mapping of the specific pathways that regulate complex endocrine processes and endocrine-related behaviors. Breathtaking progress in molecular and cellular biology has led to the identification of many specific receptors and ligands that mediate neuroendocrine signaling, and the ability to localize their function anatomically. A wide range of receptor agonists and antagonists has been developed permitting the experimental coupling of signaling systems and biologic responses. And finally, techniques for genetic manipulation now permit the generation of precise and long-lasting alterations in the expression of specific signaling elements so that their physiology can be deduced in vivo. The application of these new methodologies has made it possible to test more complex hypotheses, and one result is that a large body of information has been amassed from which ever more sophisticated models of neuroendocrine function can be made. The discovery of glucagon-like peptide 1 (GLP-1), a gastrointestinal hormone and central neurotransmitter, provides a tangible example of the recent progress in endocrinology/neuroendocrinology. The existence of GLP-1 was inferred from the cloning of a complementary DNA for preproglucagon, making it one of the first peptides discovered on the basis of a nucleic acid sequence (2, 3). Subsequent studies in humans and animals revealed that GLP-1 is secreted from the gut after meals and augments nutrient-stimulated insulin secretion (4). Thorens cloned a GLP-1 receptor from rat pancreatic islets that was subsequently shown to be expressed in the CNS as well as other tissues (5, 6). Eng and colleagues (7) first demonstrated that several peptides derived from the venom of the gila monster serve as GLP-1 receptor agonists and antagonists, providing powerful tools for further study of the role of GLP-1. Most recently the research group led by Drucker (8) at the University of Toronto produced a line of transgenic mice with a targeted deletion of the GLP-1 receptor, a “knock-out” model that this group has employed in a series of interesting and timely studies on the physiologic role of GLP-1. The initial characterization of the GLP-1 receptor knockout mice focused on glucose homeostasis, the best-established function of GLP-1. Exogenous administration of GLP-1 or GLP-1 receptor agonists is insulinotropic in humans, rodents, and several in vitro models, suggesting at least a pharmacologic effect of GLP-1 on the pancreatic b-cell. Continuous infusion of the GLP-1 receptor antagonist exendin-(9–39), or immunoneutralization of circulating GLP-1 with specific antibodies, impairs insulin secretion in experimental animals during enteral glucose administration (8–10). These data indicate that endogenous GLP-1, released during nutrient absorption, has a physiologic role to promote insulin secretion. Consistent with these findings, the GLP-1 receptor knockout mouse has significantly higher circulating blood glucose levels than wild-type controls following intragastric glucose loading (11). The glucose intolerance is most notable in male knockout mice and is likely caused by diminished early insulin secretion in response to glucose absorbed from the gut. The agreement between studies using acute pharmacologic methods with those using receptor knockouts provides compelling evidence for a physiologic role of GLP-1 as a hormonal mediator of postprandial insulin secretion. Much less certain is the function of GLP-1 as a neurotransmitter although several putative roles have been proposed for this peptide in the CNS. Of great interest is the possibility that GLP-1 contributes to the regulation of food intake. Several agonists of the GLP-1 receptor show prominent effects to decrease food intake when administered directly into the CNS (12–14), while blocking the activation of central GLP-1 receptors with a peptide antagonist increases food intake (12, 15). Based on these studies using acute pharmacologic approaches, a solid case can be made that GLP-1 has some role in modulating food intake. However, in several careful studies, Drucker and colleagues demonstrated that the GLP-1 receptor knockout mice eat similar amounts and follow nearly the identical growth trajectory as wild-type control mice (16). So, unlike the situation with the regulation of postprandial insulin secretion by GLP-1, the pharmacological data and the knockout data conflict with regard to the role of GLP-1 in the control of food intake. Such disagreement between data derived by inducing acute changes in signaling pathways and those based on permanent gene knockout models has now been recognized in several instances (see Refs. 17 and 21). Reconciling these differences is at first glance difficult, but likely to be enlightening, as we discuss below. In this issue of Endocrinology, the Toronto group reports the results of another set of experiments in their GLP-1 receptor knockout mouse, this time testing the role of GLP-1 to mediate signaling through the hypothalamic-pituitary axis. Again, data from pharmacologic studies suggest that GLP-1 may act as a modulator of this system. Administration of GLP-1 to the third Received November 30, 1999. Address all correspondence and requests for reprints to: Randy J. Seeley, Department of Psychiatry, Box 670559, University of Cincinnati, Cincinnati, Ohio 45267-0559. E-mail: seeleyrj@email.uc. edu. 0013-7227/00/$03.00/0 Vol. 141, No. 2 Endocrinology Printed in U.S.A. Copyright © 2000 by The Endocrine Society