Compound synthesis
Peptidyl-resin with modified GLP - 1 backbone sequence of Boc-His(Trt) ā aib - Glu(OtBu) ā Gly ā Thr(tBu) ā Phe ā Thr(tBu) ā Ser(tBu) ā Asp(OtBu) ā Val ā Ser(tBu) ā Ser(tBu) ā Tyr(tBu) ā Leu ā Glu(OtBu) ā Glu(OtBu) ā Gln(Trt) ā Ala ā Ala ā Lys(Boc) ā Glu(OtBu) ā Phe ā Ile ā Ala ā Trp ā Leu ā Val ā Lys(Boc) ā Gly ā Gly ā Pro ā Ser(tBu) ā Ser(tBu) ā Gly ā Ala ā Pro ā Pro ā Pro ā Ser(tBu) - Lys(Mtt) ā amide resin was assembled by automated Fmoc/tBu solid-phase chemistry starting with 0.1mmole of H-Rink Amide ChemMatrixĀ® Ā (PCAS BioMatrix Inc, Saint-Jean-sur-Richelieu, Quebec, Canada) on a Symphony Peptide Synthesizer (Peptide Technology, Tucson, AZ). All Fmoc-amino acids were coupled with 6-Cl-HOBt / DIC activation in DMF. The Fmoc were removed by 20% piperidine in DMF. After the Mtt removal by treating the peptidyl-resin with 1 - 2% TFA / 5% Tis in DCM, Fmoc ā Glu - OtBu was coupled with 6-Cl-HOBt / DIC in DMF. Fmoc was deprotected and Tesaglitazar / (S) ā Ā 2 ā Ethoxy ā 3 - (4 - (4 - ((methylsulfonyl)oxy) phenethoxy) phenyl) propanoic acid (Astatech Inc, Bristol, PA) was coupled with 6-Cl-HOBt / DIC in DMF with 3 - fold excess. GLP - 1 ā Tesaglitazar conjugate peptide was cleaved from the resin with 10 ml TFA cleavage cocktail containing 8.5 mlĀ TFA, 0.5 ml water, 0.5 ml Tis (triisopropylsilane), 0.25 g phenol and 0.25 ml 2 - mercaptoethanol for 2h. Peptide was precipitated with cold ether, dissolved in 20% acetonitrile (ACN) containing 2% acetic acid and injected to a Luna 19 x 250nm / 10āĪ¼m C8 column (Phenomenex, Torrance, CA) to purify with 0.1%TFA / ACN eluent solvents in a Waters 2545 preparative HPLC instrument. Peptide molecular weight characterization was measured by liquid chromatographyāmass spectrometry on an Agilent 1260 Infinity/6120 Quadrupole instrument with a Kinetex C8 column with an eluent gradient of 10% ā 80% 0.05% ACN. The purified GLP ā 1 - Tesaglitazar peptide (HPLC > 95% ) was characterized with molecular mass of 1178.5 / [M + 4H]4+ and 1570.8 / [M + 3H]3+ , which consistent with the theoretically calculated molecular weight Ā 4710.1 with formula C214H315N49O69S.
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AnimalsĀ
C57BL/6J mice and Leprdb (db/db) mice were purchased from the Jackson Laboratory. Glp1r-/-Ā mice were kindly provided by Daniel Drucker (University of Toronto, CA). All wt and ko mice used in our studies were bred on a C57BL/6J background. Mice were double-housed and kept in a constant environment with the ambient temperature set to 22 Ā± 2Ā°C with constant humidity (45 ā 65%) and a 12h/12h light/dark cycle.Ā For studies in DIO mice, maleĀ C57BL/6JĀ mice were fed with a high-fat diet (HFD) comprising 58% kcal from fat (D12331; Research Diets, New Brunswick, NJ). db/db mice were fed with a regular chow diet throughout the study. At the beginning of each experiment, mice were equally distributed into experimental groups according to their body weight and body composition. All animal studies were approved by the State of Bavaria, Germany, or the IACUC of the University of Cincinnati, OH, USA.Ā Compounds were dissolved in PBS and were subcutaneously administered with a volume of 5Ā šl/g body weight in the indicated doses.
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Body composition analysis:
Fat and lean tissue mass were measured via Nuclear Magnetic Resonance (NMR) technology (EchoMRI, Houston, TX, USA).Ā
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Glucose- and insulin tolerance tests (GTT/ITT):
Glucose tolerance was assessed in 6 h fasted mice after intraperitoneal (i.p) injection of 1.75 g glucose per kg body weight (DIO mice). Insulin tolerance was assessed in 6 h fasted mice after i.p. injection of either 0.75 U Insulin per gram body weight (DIO mice) or 1 U Insulin per gram body weight (db/db mice) (Humalog, Eli Lilly, Indianapolis, IN, USA). Tail vein blood glucose was subsequently measured using a handheld glucometer (TheraSense Freestyle) at baseline and after 15, 30, 60 and 120 min.
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Plasma creatinineĀ
Plasma creatinine was measured by a fluorometric creatinine assay kit (Cat#: ab65340, Abcam, Cambridge, United Kingdom) based on the manufacturerās instruction.
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Glucose-stimulated insulin secretion (GSIS) in primary murine islets
Isolation of islets was performed by collagenase P digestion of the wt adult pancreas. 3 mL of collagenase P (1 mg/mL) was injected into the bile duct, and the perfused pancreas was consequently dissected and placed into 3 mL collagenase P for 15 min at 37 Ā°C. Then, 10 mL of G-solution (HBSS + 1% BSA) was added, followed by 2 min centrifugation at 1600 rpm at 4 Ā°C. Pellets were re-suspended in 5.5 mL of gradient preparation (5 mL 10% RPMI + 3 mL 40% Optiprep/ per sample) and placed on top of 2.5 mL of the same solution. To form a 3-layers gradient, 6 mL of G-solution was added on the top. Samples were then subjected to 10 min centrifugation at 1700 rpm. Finally, the interphase between the upper and the middle layers of the gradient was harvested and filtered through a 70 Ī¼m Nylon filter. The eluent was washed with G-solution, and the islets were handpicked under the microscope. For GSIS analysis, the isolated islets were cultured overnight before being transferred to a 96-well plate containing modified Krebs Ringer phosphate Hepes (KRPH) buffer with 1 mM glucose for 1 h. Afterwards, islets were exposed to equal concentrations of the compounds (3 nM) at the same time when undergoing glucose concentrations of 16.8 mM (2 h each). Insulin was quantified from the supernatant. Finally, the islets were lysed for DNA extraction. Insulin concentrations were measured using an ultrasensitive insulin ELISA kit (Cristal Chem). The analysis was performed using a standard curve and the data were normalized to total content of DNA.
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Cell culture and transfection
HEK293T cells were cultured in Dulbeccoās Modified Eagle Medium (DMEM, Cat #: 11995073, Life Technologies, Carlsbad, CA, USA) supplemented with 10% heat-inactivated fetal bovine serum (FBS, Cat #: 10500064, Life Technologies, Carlsbad, CA, USA), 100 IU/mL of penicillin, and 100Ā Ī¼g/mL of streptomycin solution (Pen-Strep, Cat #: P4333, SigmaāAldrich, St. Louis, MO, USA). HEK293T cells (700,000/well) were seeded in 6-well plates and incubated to 70% confluency in DMEM (10% FBS, 1% Pen/Strep). Twenty-four hours following seeding, transient transfections were performed using Lipofectamine 2000 (Cat #: 11668019, Invitrogen, Carlsbad, CA, USA) according to the manufacturer's protocol without including additional transformation carrier DNA.
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Ligand-induced BRET assays:Ā
Twenty-four hours following transfection, HEK293T cells were washed with PBS and resuspended in FluoroBrite phenol red-free complete media (Cat #: A1896701, Life Technologies, Carlsbad, CA, USA) containing 5% FBS and 2 mM of L-glutamine (Cat #: 25030081, Life Technologies, Carlsbad, CA, USA). 100,000 cells/well were plated into poly-D-Lysine-coated (Cat #: P6403, SigmaāAldrich, St. Louis, MO, USA) 96-well white polystyrene LumiNunc plates (Cat #: 10072151, Thermo Fisher Scientific, Waltham, MA, USA). After 24 hrs, the media was replaced with PBS (Cat #: 10010056, Gibco, Carlsbad, CA, USA) containing 10Ā mM of coelenterazine-h (Cat #: S2011, Promega, Madison, WI, USA) or 1:500 dilution of NanoGlo (Cat #: N1110, Promega, Madison, WI, USA). BRET measurements were taken every 30 sec ā 2 min using a PHERAstar FS multi-mode microplate reader. Baseline measurements were taken after an initial 5 min incubation with coelenterazine-h or NanoGlo-containing PBS after which cells were then treated with either vehicle (PBS) or respective ligands. The resulting ligand-specific ratiometric BRET signals were normalized to vehicle producing the āligand-induced BRET ratioā21, followed by an additional normalization step to well-specific baseline readings. Ligand-induced measurements on the temporal scale is represented as the subsequent measurement after time point zero. Positive or negative incremental area under the curves (+iAUC/-iAUC) were calculated where noted. Each experiment was independently performed at least three times, each with at least three technical replicates for each group.
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Receptor signaling and trafficking BRET AssaysĀ
Untagged hGLP-1R was purchased from Sino Biological Inc.Ā (Cat #: HG13944-UT, Beijing, China), hGLP-1R-GFP was a kind gift from David Hodson (University of Birmingham, Birmingham, UK), and hGLP-1R-Rluc8 was a kind gift from Patrick Sexton (Monash University, Melbourne, Australia). Gas recruitment to the GLP-1R-GFP was quantified using mini-Gas, a protein probe that translocates to ligand-bound Gas-coupled GPCRs22. NES-Nluc-MiniGas was a gift from Kevin Pfleger (Harry Perkins Institute of Medical Research, Nedlands, WA, Australia). Intracellular cAMP was measured using the YFP-Epac-Rluc CAMYEL sensor23. hGLP-1R-RLUC8 internalization was quantified using the intracellular plasma membrane marker Venus-KRAS24. Venus-KRAS was a kind gift from Kevin Pfleger. GLP-1R-Rluc8 lysosomal co-localization was measured using Lamp1-mNeonGreen. Lamp1-mNeonGreen was a gift from Dorus Gadella (Addgene plasmid # 98882). Time-dependent RXR/PPARgĀ heterodimerization was measured using RXR-Rluc8 and PPARg2-YFP, both of which were kind gifts from Professor Vincent Ollendorf25.Ā
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Gene expression analysis
For assessment of acute drug effects, animals were treated with the respective compounds 4 hrs prior to tissue harvesting. Dissected tissues were frozen immediately on dry ice, and RNA was isolated using RNeasy Mini Kits (Qiagen). mRNA levels were determined using TaqMan probes in custom-made low density array cards (ThermoFisher) or in single assays on a QuantStudio 7 Real-Time PCR system. Target gene expression was normalized to HPRT and fold change was calculated relative to vehicle treated controls.Ā
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Proteomics sample preparation
Frozen tissue samples were homogenized in 0.1 M Tris-HCl, pH 7.6, and 2% SDC, heated for 5 min at 95Ā°C and sonicated (Diagenode Bioruptor at high intensity, 15 x 30s). 50 Āµg of protein was reduced and alkylated with 10 mM TCEP,40 mM CAA for 5 min at 40Ā°C in the dark. Samples were digested with trypsin and LysC 1:50 (enzyme:protein) overnight at 37Ā°C. Digested peptides were acidified to a final concentration of 1% TFA and loaded onto activated triple layer styrenedivinylbenzeneāreversed phase sulfonated (SDB-RPS; 3 M Empore) STAGE tips. STAGE tips were washed with 100 Āµl ethylacetate 1% TFA, 100 Āµl 30% Methanol 1% TFA and 150 Āµl 0.2% TFA. Peptides were eluted with 60 Āµl SDB-RPS elution buffer (80% ACN, 5% NH4OH), concentrated in a SpeedVac for 40 min at 45Ā°C, and dissolved in 10 Āµl MS loading buffer (2% ACN, 0.1% TFA).
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LC-MS/MS analysis
Single-shot measurements were performed with 500 ng of purified peptides, determined by absorbance at 280 nm on a NanoDrop 2000. Peptides were loaded onto a 50-cm column, packed in-house with 1.9 Āµm C18 ReproSil particles (Dr. Maisch GmbH) with an EASY-nLC 1200 system (Thermo Fisher Scientific). Column temperature was kept at 60Ā°C using a column oven. Peptides were eluted over 60 min using a binary buffer system consisting of buffer A (0.1% formic acid) and buffer B (80% ACN, 0.1% formic acid). In brief, the gradient started with 5% buffer B and increased stepwise to 45% over 45min, followed by a wash-out at 95% buffer B, all at a flowrate of 300 nl/min. Peptides were then transferred to the gas phase using electrospray ionization (ESI), pre-filtered by a FAIMS device (CV -50 V) before entering the Orbitrap Exploris 480 (Thermo Fisher Scientific) mass spectrometer. A data-independent (DIA) acquisition method was used, in which one full scan (300-1650 m/z, max. ion fill time of 45ms, normalized AGC target = 300%, R= 120.000 at 200 m/z) was followed by 66 tMS2 fragment scans of unequally-spaced windows with 1 Th overlap, covering the same m/z range (fill time = 22 ms, normalized AGC target = 1000%, normalized HCD collision energy = 30%, R= 15.000).
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Bioinformatic workflow and proteomic data analysis
DIA raw files were processed using Spectronaut version 14.5.200813.4778426 using the directDIA function with default settings. Raw files from samples of the same tissue were processed together and searched against the mouse UniProt FASTA database (September 2014, 51,210 entries) and the provided MaxQuant contaminants list (245 entries). Spectronaut report files were then loaded into Perseus (v.1.6.2.3). In brief, quantified proteins were filtered for ā„ 3 valid values in at least one biological condition and annotations mapped from UniProtKB, Gene Ontology (GO), and the Kyoto Encyclopedia of Genes and Genomes (KEGG). Significantly up- or downregulated proteins between the conditions were determined by ANOVA (FDR 0.025).
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Statistics
Statistical analyses were performed using the statistical tools implemented in GraphPad Prism8 (version 8.3.0). All data are shown as mean Ā± SEM. Differences between groups were assessed by 1-way ANOVA or 2-way ANOVA with time and treatment as co-variants followed by Bonferroniās post-hoc multiple comparison testing as indicated in the figure legends. A p-value < 0.05 was considered statistically significant.