ARTICLES NATURE MEDICINE VOLUME 9 | NUMBER 9 | SEPTEMBER 2003 1173 GLP-1 is a hormone derived from tissue-specific post-translational processing of the proglucagon gene in intestinal L cells. It shares con- siderable amino acid sequence homology with glucagon; this sequence is conserved in multiple vertebrate and invertebrate species, indicating an important role in normal physiology. Indeed, GLP-1 exerts effects on glucose-dependent insulin secretion, insulin biosynthesis, gastroin- testinal motility, islet b-cell neogenesis, energy homeostasis and food intake 1–4 . GLP-1 and GLP-1R are also expressed in the brain, including the hippocampus 5–7 , a structure that shows considerable plasticity and is crucial for several forms of learning and memory 8 . GLP-1R is cou- pled to multiple G-protein signal transduction pathways leading to activation of adenylyl cyclase, protein kinase C (PKC) and mitogen- activated protein (MAP) kinase 9,10 . In the brain, these pathways are implicated in plasticity and learning and represent targets for mem- ory-enhancing drug development. We therefore hypothesized that GLP-1R may act similarly in the brain to influence hippocampal plas- ticity and facilitate learning. In addition, because the hippocampus is particularly vulnerable to neuronal loss associated with epilepsy, stroke and neurodegenerative disorders, and the same GLP-1R G-pro- tein-coupled pathways mediate cellular responses to apoptotic stimuli, we also explored the role of GLP-1R in neuroprotection. RESULTS Generation of a truncated N-terminal GLP-1 analog We investigated both full-length GLP-1 and a novel peptide, HSEGTFTSD, also called [Ser(2)]exendin(1–9). Basic local alignment search tool analysis revealed that these residues are highly conserved within the GLP superfamily in both vertebrates and invertebrates. These peptides include glucagon itself and the GLP-1R agonist, exendin-4. In contrast, N-terminally truncated exendin(9–39) acts as a GLP-1R antagonist 11 (Fig. 1a). In comparison with human GLP-1, [Ser(2)]exendin(1–9) was synthesized with an N-terminal stearic acid residue to improve lipophilicity. A serine was also substituted for glu- tamine in position 2 to improve peptide stability, as this residue is critical for dipeptidyl-peptidase IV–mediated degradation 12 . To con- firm biologic activity of [Ser(2)]exendin(1–9), a rat insulinoma cell line expressing GLP-1R (RINm5f) 13 was incubated with GLP-1 or [Ser(2)]exendin(1–9) in the presence or absence of exendin(9–39). Both GLP-1 and [Ser(2)]exendin(1–9) stimulated insulin release, which was blocked by exendin(9–39) (Supplementary Fig. 1 online). In fasted rats, intraperitoneal [Ser(2)]exendin(1–9) led to a dose- response hypoglycemic effect consistent with agonist activity at β-cell GLP-1R (Supplementary Fig. 2 online). Cognitive effects The effects of centrally administered GLP-1 and [Ser(2)]exendin(1–9) on associative and spatial learning, both of which are hippocampal dependent, were investigated using the passive avoidance 14 and Morris water maze (MWM) paradigms in rats 15 . GLP-1 and [Ser(2)]exendin(1–9) administered i.c.v. enhanced latency in the 1 Department of Molecular Medicine and Pathology, University of Auckland, Private Bag 92019, Auckland 86716, New Zealand. 2 CNS Gene Therapy Center, Department of Neurosurgery, Jefferson Medical College, 1025 Walnut Street, Philadelphia, Pennsylvania 19107, USA. 3 GRECC, Veterans Affairs Medical Center-St. Louis and Division of Geriatrics, Department of Internal Medicine, Saint Louis University School of Medicine, St. Louis, Missouri 63106, USA. 4 Department of Medicine, Banting and Best Diabetes Centre, Toronto General Hospital, University of Toronto, 200 Elizabeth Street CCRW3-838, Toronto, Ontario M5G 2C4, Canada. Correspondence should be addressed to M.J.D. (m.during@auckland.ac.nz). Published online 17 August 2003; doi:10.1038/nm919 Glucagon-like peptide-1 receptor is involved in learning and neuroprotection Matthew J During 1,2 , Lei Cao 2 , David S Zuzga 2 , Jeremy S Francis 1 , Helen L Fitzsimons 1,2 , Xiangyang Jiao 2 , Ross J Bland 2 , Matthias Klugmann 1 , William A Banks 3 , Daniel J Drucker 4 & Colin N Haile 2 Glucagon-like peptide-1 (GLP-1) is a gut peptide that, together with its receptor, GLP-1R, is expressed in the brain. Here we show that intracerebroventricular (i.c.v.) GLP-1 and [Ser(2)]exendin(1–9) (HSEGTFTSD; homologous to a conserved domain in the glucagon/GLP-1 family) enhance associative and spatial learning through GLP-1R. [Ser(2)]exendin(1–9), but not GLP-1, is also active when administered peripherally. GLP-1R-deficient mice have a phenotype characterized by a learning deficit that is restored after hippocampal Glp1r gene transfer. In addition, rats overexpressing GLP-1R in the hippocampus show improved learning and memory. GLP-1R-deficient mice also have enhanced seizure severity and neuronal injury after kainate administration, with an intermediate phenotype in heterozygotes and phenotypic correction after Glp1r gene transfer in hippocampal somatic cells. Systemic administration of [Ser(2)]exendin(1–9) in wild-type animals prevents kainate-induced apoptosis of hippocampal neurons. Brain GLP-1R represents a promising new target for both cognitive-enhancing and neuroprotective agents. © 2003 Nature Publishing Group http://www.nature.com/naturemedicine