iPS Cell Cultures from a Gerstmann-Sträussler-Scheinker Patient with the Y218N PRNP Mutation Recapitulate tau Pathology.

Gerstmann-Sträussler-Scheinker (GSS) syndrome is a fatal autosomal dominant neurodegenerative prionopathy clinically characterized by ataxia, spastic paraparesis, extrapyramidal signs and dementia. In some GSS familiar cases carrying point mutations in the PRNP gene, patients also showed comorbid tauopathy leading to mixed pathologies. In this study we developed an induced pluripotent stem (iPS) cell model derived from fibroblasts of a GSS patient harboring the Y218N PRNP mutation, as well as an age-matched healthy control. This particular PRNP mutation is unique with very few described cases. One of the cases presented neurofibrillary degeneration with relevant Tau hyperphosphorylation. Y218N iPS-derived cultures showed relevant astrogliosis, increased phospho-Tau, altered microtubule-associated transport and cell death. However, they failed to generate proteinase K-resistant prion. In this study we set out to test, for the first time, whether iPS cell-derived neurons could be used to investigate the appearance of disease-related phenotypes (i.e, tauopathy) identified in the GSS patient.

LRRK2 Expression Is Deregulated in Fibroblasts and Neurons from Parkinson Patients with Mutations in PINK1.

Mutations in PINK1 (PARK6), a serine/threonine kinase involved in mitochondrial homeostasis, are associated with early onset Parkinson's disease. Fibroblasts from Parkinson's disease patients with compound heterozygous mutations in exon 7 (c.1488 + 1G > A; c.1252_1488del) showed no apparent signs of mitochondrial impairment. To elucidate changes primarily caused by lack of functional PINK1, we over-expressed wild-type PINK1, which induced a significant downregulation of LRRK2 (PARK8). Indeed, we found that LRRK2 protein basal levels were significantly higher in the mutant PINK1 fibroblasts. To examine the interaction between the two PARK genes in a disease-relevant cell context, we generated induced pluripotent stem cell (iPSC) lines from mutant, carrier and control fibroblasts by lentiviral-mediated re-programming. Efficiency of neural induction and dopamine differentiation using a floor-plate induction protocol was similar in all genotypes. As observed in fibroblasts, PINK1 mutant neurons showed increased LRRK2 expression both at the RNA and protein level and transient over-expression of wild-type PINK1 efficiently downregulated LRRK2 levels. Additionally, we confirmed a dysregulation of LRRK2 expression in fibroblasts from patients with a different homozygous mutation in PINK1 exon 4, c.926G > A (G309D). Thus, our results identify a novel role of PINK1 modulating the levels of LRRK2 in Parkinson's disease fibroblasts and neurons, suggest a convergent pathway for these PARK genes, and broaden the role of LRRK2 in the pathogenesis of Parkinson's disease.

Mutations in LRRK2 impair NF-κB pathway in iPSC-derived neurons.

BACKGROUND: Mutations in leucine-rich repeat kinase 2 (LRRK2) contribute to both familial and idiopathic forms of Parkinson's disease (PD). Neuroinflammation is a key event in neurodegeneration and aging, and there is mounting evidence of LRRK2 involvement in inflammatory pathways. In a previous study, we described an alteration of the inflammatory response in dermal fibroblasts from PD patients expressing the G2019S and R1441G mutations in LRRK2. METHODS: Taking advantage of cellular reprogramming, we generated induced pluripotent stem cell (iPSC) lines and neurons thereafter, harboring LRRK2G2019S and LRRK2R1441G mutations. We used gene silencing and functional reporter assays to characterize the effect of the mutations. We examined the temporal profile of TNFα-induced changes in proteins of the NF-κB pathway and optimized western blot analysis to capture α-synuclein dynamics. The effects of the mutations and interventions were analyzed by two-way ANOVA tests with respect to corresponding controls. RESULTS: LRRK2 silencing decreased α-synuclein protein levels in mutated neurons and modified NF-κB transcriptional targets, such as PTGS2 (COX-2) and TNFAIP3 (A20). We next tested whether NF-κB and α-synuclein pathways converged and found that TNFα modulated α-synuclein levels, although we could not detect an effect of LRRK2 mutations, partly because of the individual variability. Nevertheless, we confirmed NF-κB dysregulation in mutated neurons, as shown by a protracted recovery of IκBα and a clear impairment in p65 nuclear translocation in the LRRK2 mutants. CONCLUSIONS: Altogether, our results show that LRRK2 mutations affect α-synuclein regulation and impair NF-κB canonical signaling in iPSC-derived neurons. TNFα modulated α-synuclein proteostasis but was not modified by the LRRK2 mutations in this paradigm. These results strengthen the link between LRRK2 and the innate immunity system underscoring the involvement of inflammatory pathways in the neurodegenerative process in PD.

Selection Based on FOXA2 Expression Is Not Sufficient to Enrich for Dopamine Neurons From Human Pluripotent Stem Cells.

Human embryonic and induced pluripotent stem cells are potential cell sources for regenerative approaches in Parkinson disease. Inductive differentiation protocols can generate midbrain dopamine neurons but result in heterogeneous cell mixtures. Therefore, selection strategies are necessary to obtain uniform dopamine cell populations. Here, we developed a selection approach using lentivirus vectors to express green fluorescent protein under the promoter region of FOXA2, a transcription factor that is expressed in the floor plate domain that gives rise to dopamine neurons during embryogenesis. We first validated the specificity of the vectors in human cell lines against a promoterless construct. We then selected FOXA2-positive neural progenitors from several human pluripotent stem cell lines, which demonstrated a gene expression profile typical for the ventral domain of the midbrain and floor plate, but failed to enrich for dopamine neurons. To investigate whether this was due to the selection approach, we overexpressed FOXA2 in neural progenitors derived from human pluripotent stem cell lines. FOXA2 forced expression resulted in an increased expression of floor plate but not mature neuronal markers. Furthermore, selection of the FOXA2 overexpressing fraction also failed to enrich for dopamine neurons. Collectively, our results suggest that FOXA2 is not sufficient to induce a dopaminergic fate in this system. On the other hand, our study demonstrates that a combined approach of promoter activation and lentivirus vector technology can be used as a versatile tool for the selection of a defined cell population from a variety of human pluripotent stem cell lines.

Sustained increase of PKA activity in the postcommissural putamen of dyskinetic monkeys.

Levodopa-induced dyskinesias (LID) are a frequent complication of Parkinson's disease pharmacotherapy that causes significant disability and narrows the therapeutic window. Pharmacological management of LID is challenging partly because the precise molecular mechanisms are not completely understood. Here, our aim was to determine molecular changes that could unveil targetable mechanisms underlying this drug complication. We examined the expression and downstream activity of dopamine receptors (DR) in the striatum of 1-methyl-4-phenyl-1,2,3,6 tetrahydropiridine (MPTP)-lesioned monkeys with and without L-DOPA treatment. Four monkeys were made dyskinetic and other four received a shorter course of L-DOPA and did not develop LID. Our results show that L-DOPA treatment induces an increase in DRD2 and DRD3 expression in the postcommissural putamen, but only DRD3 is correlated with the severity of LID. Dyskinetic monkeys show a hyperactivation of the canonical DRD1-signaling pathway, measured by an increased phosphorylation of protein kinase A (PKA) and its substrates, particularly DARPP32. In contrast, activation of the DRD2-signaling pathway, visible in the levels of Akt phosphorylated on Thr308 and GSK3ß on Ser9, is associated with L-DOPA treatment, independently of the presence of dyskinesias. Our data clearly demonstrate that dyskinetic monkeys present a dysregulation of the DRD3 receptor and the DRD1 pathway with a sustained increase of PKA activity in the postcommissural putamen. Importantly, we found that all signaling changes related to long-term L-DOPA administration are exquisitely restricted to the postcommissural putamen, which may be related to the recurrent failure of pharmacological approaches.

Leucine-rich repeat kinase 2 modulates cyclooxygenase 2 and the inflammatory response in idiopathic and genetic Parkinson's disease.

Inflammatory mechanisms are activated in aging and late-onset neurodegenerative diseases, such as Parkinson's disease (PD). Mutations in leucine-rich repeat kinase 2 (LRRK2) contribute to both idiopathic and familial forms of PD. Here, we investigated the involvement of LRRK2 in inflammatory pathways using primary dermal fibroblasts from patients with 2 common mutations in LRRK2 (G2019S and R1441G), idiopathic PD and age-matched healthy individuals. Basal cyclooxygenase (COX)-2 RNA levels were very high in the fibroblasts of all patients. Remarkably, LRRK2 silencing experiments significantly reduced basal COX-2 levels and COX-2 induction after a pro-inflammatory stimulus. Additionally, in samples from patients with the R1441G mutation and with idiopathic PD, we found a prominent cytoplasmic re-distribution of human antigen R, a protein that, among others, stabilizes COX-2 RNA. Furthermore, the response to lipopolysaccharide was defective in these 2 groups, which showed weak induction of pro-inflammatory cytokines and reduced NFκB transcriptional activation. In summary, we describe multiple defects in inflammatory pathways in which LRRK2 appears to be critically involved. Further studies are required to establish the therapeutic implications of inflammatory dysregulation in the pathophysiology of Parkinson's disease.

The LRRK2 G2019S mutant exacerbates basal autophagy through activation of the MEK/ERK pathway.

Mutations in leucine-rich repeat kinase 2 (LRRK2) are a major cause of familial Parkinsonism, and the G2019S mutation of LRRK2 is one of the most prevalent mutations. The deregulation of autophagic processes in nerve cells is thought to be a possible cause of Parkinson's disease (PD). In this study, we observed that G2019S mutant fibroblasts exhibited higher autophagic activity levels than control fibroblasts. Elevated levels of autophagic activity can trigger cell death, and in our study, G2019S mutant cells exhibited increased apoptosis hallmarks compared to control cells. LRRK2 is able to induce the phosphorylation of MAPK/ERK kinases (MEK). The use of 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (U0126), a highly selective inhibitor of MEK1/2, reduced the enhanced autophagy and sensibility observed in G2019S LRRK2 mutation cells. These data suggest that the G2019S mutation induces autophagy via MEK/ERK pathway and that the inhibition of this exacerbated autophagy reduces the sensitivity observed in G2019S mutant cells.

The MAPK1/3 pathway is essential for the deregulation of autophagy observed in G2019S LRRK2 mutant fibroblasts.

The link between the deregulation of autophagy and cell death processes can be essential in the development of several neurodegenerative diseases, such as Parkinson disease (PD). However, the molecular mechanism of deregulation of this degradative process in PD patients is unknown. The leucine-rich repeat kinase 2 (LRRK2) gene is related to PD and its implication in autophagy regulation has been described. Our recent work shows that the presence of the G2019S LRRK2 mutation, one of the most prevalent in LRRK2, is accompanied by a deregulation of autophagy basal levels dependent on the MAPK1/3 (ERK2/1) pathway.

Rosiglitazone rescues memory impairment in Alzheimer's transgenic mice: mechanisms involving a reduced amyloid and tau pathology.

Clinical studies suggest that agonists at peroxisome proliferator-activated receptor gamma (PPARgamma) may exert beneficial effects in patients with mild-to-moderate Alzheimer's disease (AD), but the mechanism for the potential therapeutic interest of this class of drugs has not yet been elucidated. Here, in mice overexpressing mutant human amyloid precursor protein, we found that chronic treatment with rosiglitazone, a high-affinity agonist at PPARgamma, facilitated beta-amyloid peptide (Abeta) clearance. Rosiglitazone not only reduced Abeta burden in the brain but, importantly, almost completely removed the abundant amyloid plaques observed in the hippocampus and entorhinal cortex of 13-month-old transgenic mice. In the hippocampus, neuropil threads containing phosphorylated tau, probably corresponding to dystrophic neurites, were also decreased by the drug. Rosiglitazone switched on the activated microglial phenotype, promoting its phagocytic ability, reducing the expression of proinflammatory markers and inducing factors for alternative differentiation. The decreased amyloid pathology may account for the reduction of p-tau-containing neuropil threads and for the rescue of impaired recognition and spatial memory in the transgenic mice. This study provides further insights into the mechanisms for the beneficial effect of rosiglitazone in AD patients.

Overexpression of wild-type human APP in mice causes cognitive deficits and pathological features unrelated to Abeta levels.

Transgenic mice expressing mutant human amyloid precursor protein (APP) develop an age-dependent amyloid pathology and memory deficits, but no overt neuronal loss. Here, in mice overexpressing wild-type human APP (hAPP(wt)) we found an early memory impairment, particularly in the water maze and to a lesser extent in the object recognition task, but beta-amyloid peptide (Abeta(42)) was barely detectable in the hippocampus. In these mice, hAPP processing was basically non-amyloidogenic, with high levels of APP carboxy-terminal fragments, C83 and APP intracellular domain. A tau pathology with an early increase in the levels of phosphorylated tau in the hippocampus, a likely consequence of enhanced ERK1/2 activation, was also observed. Furthermore, these mice presented a loss of synapse-associated proteins: PSD95, AMPA and NMDA receptor subunits and phosphorylated CaMKII. Importantly, signs of neurodegeneration were found in the hippocampal CA1 subfield and in the entorhinal cortex that were associated to a marked loss of MAP2 immunoreactivity. Conversely, in mice expressing mutant hAPP, high levels of Abeta(42) were found in the hippocampus, but no signs of neurodegeneration were apparent. The results support the notion of Abeta-independent pathogenic pathways in Alzheimer's disease.

Gonadotropin-releasing hormone analog structural determinants of selectivity for inhibition of cell growth: support for the concept of ligand-induced selective signaling.

GnRH and its receptor are expressed in human reproductive tract cancers, and direct antiproliferative effects of GnRH analogs have been demonstrated in cancer cell lines. The intracellular signaling responsible for this effect differs from that mediating pituitary gonadotropin secretion. The GnRH structure-activity relationship is different for the two effects. Here we report a structure-activity relationship study of GnRH agonist antiproliferative action in model cell systems of rat and human GnRH receptors stably expressed in HEK293 cells. GnRH II was more potent than GnRH I in inhibiting cell growth in the cell lines. In contrast, GnRH I was more potent than GnRH II in stimulating inositol phosphate production, the signaling pathway in gonadotropes. The different residues in GnRH II (His(5), Trp(7), Tyr(8)) were introduced singly or in pairs into GnRH I. Tyr(5) replacement by His(5) produced the highest increase in the antiproliferative potency of GnRH I. Tyr(8) substitution of Arg(8) produced the most selective analog, with very poor inositol phosphate generation but high antiproliferative potency. In nude mice bearing tumors of the HEK293 cell line, GnRH II and an antagonist administration was ineffective in inhibiting tumor growth, but D-amino acid stabilized analogs (D-Lys(6) and D-Arg(6)) ablated tumor growth. Docking of GnRH I and GnRH II to the human GnRH receptor molecular model revealed that Arg(8) of GnRH I makes contact with Asp(302), whereas Tyr(8) of GnRH II appears to make different contacts, suggesting these residues stabilize different receptor conformations mediating differential intracellular signaling and effects on gonadotropin and cell growth. These findings provide the basis for the development of selective GnRH analog cancer therapeutics that directly target tumor cells or inhibit pituitary gonadotropins or do both.

GnRH-mediated DAN production regulates the transcription of the GnRH receptor in gonadotrope cells.

The primary function of gonadotropin-releasing hormone (GnRH) is the regulation of pituitary gonadotropin hormone gene transcription, biosynthesis and release. These effects are mediated through intracellular mobilization of Ca2+ and activation of PKC isoforms and MAP kinases. We show here that DAN (differential screening-selected gene aberrative in neuroblastoma) which is a secreted bone morphogenic protein (BMP) antagonist belonging to the TGFbeta protein superfamily, is controlled by GnRH in murine gonadotrope cells. Acute GnRH stimulation induced a rapid, 27-fold, elevation of DAN mRNA, accompanied by an approximate 3-fold increase in the amount of mature DAN glycoprotein in the cell cytoplasm and in DAN secretion into the culture medium. Incubation of L beta T2 cells in DAN-containing medium altered the levels of a number of cellular proteins. Two of these were identified as the steroidogenic acute regulatory protein (StAR) and the actin-related protein 2/3 complex subunits 2 (p34-ARC) which are primarily involved in steroidogenesis and cytoskeleton remodelling, respectively. DAN caused an approximate 2-fold specific elevation in the cytoplasmic levels of both these proteins in L beta T2 cells. We further tested the effects of DAN on classical GnRH effects viz. gonadotropin and GnRH receptor gene expression. Co-transfection of L beta T2 cells with DAN and gonadotropin subunit promoter luciferase reporter genes had no effect on GnRH stimulation of alpha GSU and LH beta or on the additive GnRH and activin induction of FSH beta subunit transcription. However, co-transfection of DAN markedly inhibited the synergistic activation of GnRH and activin on GnRH receptor gene expression thus implicating DAN as a novel autocrine/paracrine factor that modulates GnRH function in pituitary gonadotropes.

Gonadotropin-releasing hormone functionally antagonizes testosterone activation of the human androgen receptor in prostate cells through focal adhesion complexes involving Hic-5.

Gonadotropin-releasing hormone (GnRH) analogs constitute the most widely employed medical treatment for prostatic cancer. The predominant mechanism of action is presumed to be via the inhibition of gonadotropins and resultant decrease in androgen. However, GnRH analogs have also been shown to directly inhibit prostate cancer cells both in vitro and in vivo through antiproliferative cell cycle arrest and stimulation of apoptosis. Since the GnRH receptor has been shown to affect sex steroid hormone receptor function, we considered that part of GnRH analog actions on prostate cells may be mediated through modulation of the human androgen receptor. Using a model HEK293 cell line expressing the GnRH receptor, we demonstrated a novel signalling pathway of the GnRH receptor that induces nuclear translocation of the androgen receptor that renders it transcriptionally inactive. This mechanism involves the calcium-dependent tyrosine kinase Pyk2, the non-receptor tyrosine kinase c-Src and the focal adhesion protein/steroid receptor co-factor, Hic-5. In this setting there is a GnRH-induced association and nuclear translocation of the androgen receptor with Hic-5. GnRH-induced Pyk2 activation opposed the association of Hic-5 with androgen receptor as overexpression of a dominant negative Pyk2 enhanced the GnRH-induced nuclear translocation of a green fluorescent protein-tagged human androgen receptor. GnRH-induced c-Src activation resulted in the phosphorylation of expressed Hic-5 and promoted its association with the human androgen receptor. In contrast to testosterone, GnRH-induced nuclear translocation did not transcriptionally activate the androgen receptor. We then demonstrated that GnRH can also stimulate androgen receptor mobilization in human prostate PC3, BPH-1 and LNCaP cells, and in cultured rat ventral prostate cells through the same mechanism. To determine if GnRH could antagonize androgen effects in normal tissue, we examined the effect of GnRH on rat ventral prostate organ cultures and demonstrated that GnRH can functionally antagonize the actions of testosterone on prostate cell proliferation and tissue growth. This antagonism of testosterone action by GnRH may underlie in part the capacity of GnRH receptor activation to inhibit prostate tumor growth.

Class II G protein-coupled receptors and their ligands in neuronal function and protection.

G protein-coupled receptors (GPCRs) play pivotal roles in regulating the function and plasticity of neuronal circuits in the nervous system. Among the myriad of GPCRs expressed in neural cells, class II GPCRs which couples predominantly to the Gs-adenylate cyclase-cAMP signaling pathway, have recently received considerable attention for their involvement in regulating neuronal survival. Neuropeptides that activate class II GPCRs include secretin, glucagon-like peptides (GLP-1 and GLP-2), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase activating peptide (PACAP), corticotropin-releasing hormone (CRH), vasoactive intestinal peptide (VIP), parathyroid hormone (PTH), and calcitonin-related peptides. Studies of patients and animal and cell culture models, have revealed possible roles for class II GPCRs signaling in the pathogenesis of several prominent neurodegenerative conditions including stroke, Alzheimer's, Parkinson's, and Huntington's diseases. Many of the peptides that activate class II GPCRs promote neuron survival by increasing the resistance of the cells to oxidative, metabolic, and excitotoxic injury. A better understanding of the cellular and molecular mechanisms by which class II GPCRs signaling modulates neuronal survival and plasticity will likely lead to novel therapeutic interventions for neurodegenerative disorders.

Gonadotropin-releasing hormone (GnRH) antagonists promote proapoptotic signaling in peripheral reproductive tumor cells by activating a Galphai-coupling state of the type I GnRH receptor.

Gonadotropin-releasing hormone (GnRH) receptor agonists are extensively used in the treatment of sex hormone-dependent cancers via the desensitization of pituitary gonadotropes and consequent decrease in steroid sex hormone secretion. However, evidence now points to a direct inhibitory effect of GnRH analogs on cancer cells. These effects appear to be mediated via the Galpha(i)-type G protein, in contrast to the predominant Galpha(q) coupling in gonadotropes. Unlike Galpha(q) coupling, Galpha(i) coupling of the GnRH receptor can be activated by both agonists and antagonists. This unusual pharmacology suggested that the receptor involved in the cancer cells may not be the classical gonadotrope type I GnRH receptor. However, we have previously shown that a functional type II GnRH receptor is not present in man. In the present study, we show that GnRH agonists and selective GnRH antagonists exert potent antiproliferative effects on JEG-3 choriocarcinoma, benign prostate hyperplasia (BPH-1), and HEK293 cells stably expressing the type I GnRH receptor. This antiproliferative action occurs through a Galpha(i)-mediated activation of stress-activated protein kinase pathways, resulting in caspase activation and transmembrane transfer of phosphatidlyserine to the outer membrane envelope. Structurally related antagonistic GnRH analogs displayed divergent antiproliferative efficacies but demonstrated equal efficacies in inhibiting GnRH-induced Galpha(q)-based signaling. Therefore the ability of GnRH receptor antagonists to exert an antiproliferative effect on reproductive tumors may be dependent on ligand-selective activation of the Galpha(i)-coupled form of the type I GnRH receptor.

Met-204 and Tyr-205 are together important for binding GLP-1 receptor agonists but not their N-terminally truncated analogues.

A mutagenesis study to systematically analyse residues spanning the first extracellular loop of the GLP-1 receptor identified a double mutant, Met-204/Tyr-205-Ala/Ala, which displayed: markedly reduced affinity for the natural agonist GLP-1; slightly reduced affinity for its analogue exendin-4; and unaltered affinity for several N-terminally truncated analogues of GLP-1 and exendin-4. This suggests that the locus is important for the formation of the binding site for the N-terminal residues of peptide agonists.

Gonadotropin-releasing hormone-induced activation of diacylglycerol kinase-zeta and its association with active c-src.

Gonadotropin-releasing hormone (GnRH)-induced receptor activation has been demonstrated to entrain a wide variety of signaling modalities. Most signaling pathways are concerned with the control of serine, threonine, or tyrosine-protein kinases, however, in the current article we demonstrate that in both a model cell line and in gonadotropes, GnRH additionally mediates the activation of lipid-directed kinases. We have shown that there is a functional connection between protein-tyrosine kinase modulation and lipid kinase activation. In HEK293 cells stably expressing the Type I mammalian GnRH receptor, we employed a proteomic approach to identify novel protein binding partners for GnRH-activated c-Src. Using matrix-assisted laser desorption ionization time-of-flight mass spectrometry we identified a GnRH-induced association between c-Src and the lipid kinase, diacylglycerol kinase-zeta (DGK-zeta). Using reciprocal co-immunoprecipitation we show that there is a significant elevation of the association between catalytically active c-Src with DGK-zeta in both HEK293 cells and murine gonadotrope LbetaT2 cells. Employing lipid kinase assays we have shown that the catalytic activity of DGK-zeta is significantly heightened in both HEK293 and LbetaT2 cells by GnRH. In addition, we demonstrate that the activation of DGK-zeta exerts a functional role in the murine gonadotrope LbetaT2 cell line. Elevated expression of DGK-zeta resulted in a shortening of the time scale of ERK activation in these cells suggesting a potential role of endogenous DGK-zeta in controlling the induction of LHbeta transcription by ERK1/2.

The isolated N-terminal domain of the glucagon-like peptide-1 (GLP-1) receptor binds exendin peptides with much higher affinity than GLP-1.

Two fragments of the receptor for glucagon-like peptide-1 (GLP-1), each containing the N-terminal domain, were expressed and characterized in either bacterial or mammalian cells. The first fragment, rNT-TM1, included the N-terminal domain and first transmembrane helix and was stably expressed in the membrane of human embryonic kidney 293 cells. The second, 6H-rNT, consisted of only the N-terminal domain of the receptor fused with a polyhistidine tag at its N terminus. The latter fragment was expressed in Escherichia coli in the form of inclusion bodies from which the protein was subsequently purified and refolded in vitro. Although both receptor fragments displayed negligible (125)I-labeled GLP-1(7-36)amide-specific binding, they both displayed high affinity for the radiolabeled peptide antagonist (125)I-exendin-4(9-39). Competition binding studies demonstrated that the N-terminal domain of the GLP-1 receptor maintains high affinity for the agonist exendin-4 as well as the antagonists exendin-4(3-39) and exendin-4(9-39) whereas, in contrast, GLP-1 affinity was greatly reduced. This study shows that although the exendin antagonists are not dependent upon the extracellular loops and transmembrane helices for maintaining their normal high affinity binding, the endogenous agonist GLP-1 requires regions outside of the N-terminal domain. Hence, distinct structural features in exendin-4, between residues 9 and 39, provide additional affinity for the N-terminal domain of the receptor. These data are consistent with a model for the binding of peptide ligands to the GLP-1 receptor in which the central and C-terminal regions of the peptides bind to the N terminus of the receptor, whereas the N-terminal residues of peptide agonists interact with the extracellular loops and transmembrane helices.

The glucagon-like peptide-1 receptor binding site for the N-terminus of GLP-1 requires polarity at Asp198 rather than negative charge.

The mutation of Asp198 to Asn in the receptor for glucagon-like peptide-1(7-36)amide (GLP-1) had no effect upon GLP-1 affinity whereas substitution with Ala greatly reduced affinity, demonstrating the importance of polarity rather than negative charge at Asp198. However, the Asp198-Ala mutation had less effect upon the affinity of Exendin-4, a peptide agonist that has been shown previously not to require its N-terminus for high affinity. Moreover, the affinity of a truncated GLP-1 analogue lacking the first eight residues was not affected by the Asp198-Ala mutation, demonstrating that Asp198 is required for maintaining the binding site of the N-terminal region of GLP-1.