Modulation of Metabotropic Glutamate Receptor mGluR4: Rat PAM Fold-Shift
The primary pathophysiological change giving rise to the symptoms of Parkinson's disease (PD) is a loss of the dopaminergic neurons in the substantia nigra pars compacta (SNc) that are involved in modulating the function of basal ganglia (BG) nuclei. Unfortunately, traditional therapies for treatment of PD based on dopamine replacement strategies eventually fail in most patients and are more ..
BioActive Compounds: 7
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Assay Provider: Colleen Niswender
Assay Provider Affiliation: Vanderbilt University
The primary pathophysiological change giving rise to the symptoms of Parkinson's disease (PD) is a loss of the dopaminergic neurons in the substantia nigra pars compacta (SNc) that are involved in modulating the function of basal ganglia (BG) nuclei. Unfortunately, traditional therapies for treatment of PD based on dopamine replacement strategies eventually fail in most patients and are associated with numerous side effects. A great deal of effort has been focused on developing a detailed understanding of the circuitry and function of the BG to develop novel, nondopaminergic, approaches for restoring normal BG function in PD patients. Exciting advances suggest that metabotropic glutamate receptors (mGluRs), including the group III mGluRs (mGluR4, -7 and -8), play important roles in regulating transmission through the BG and could serve as targets for novel PD therapeutics (Conn et al., 2005). For instance, mGluR4 activation reduces overactive GABA release at a specific inhibitory BG synapse (Macinnes and Duty, 2008; Marino et al., 2003; Valenti et al., 2003) and reverses motor deficits in a variety of rodent PD models (Konieczny et al., 2007; MacInnes et al., 2004; Marino et al., 2003; Ossowska et al., 2007; Valenti et al., 2003).
To more selectively activate mGluR4 and improve upon the pharmacokinetic liabilities of glutamate analogs, we and others have developed novel positive allosteric modulators (PAMs) which potentiate glutamate function at mGluR4 (Engers et al., 2009; Maj et al., 2003; Marino et al., 2003; Niswender et al., 2008a; Niswender et al., 2008b; Williams et al., 2008); several of these tool compounds exhibit antiparkinsonian and neuroprotective effects in multiple rodent PD models (Battaglia et al., 2006; Marino et al., 2003; Niswender et al., 2008a). Unfortunately, many available compounds have lacked pharmacokinetic properties to make them useful tools for study of mGluR4 function via systemic routes of administration. The probe compound developed here exhibits sufficient potency, efficacy, and pharmacokinetic properties, including brain penetration, to make it a useful compound to progress mGluR4 biology, which will undoubtedly allow the intense study of mGluR4 activation in multiple areas of neuroscience such as psychiatric disorders (Stachowicz et al., 2006; Stachowicz et al., 2004), cancer (Iacovelli et al., 2006), and addiction (Blednov et al., 2004).
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Cell culture, plating, and dye loading. HEK/GIRK cells stably expressing the M4 muscarinic receptor were grown in 45 percent Dulbecco's Modified Eagle Media (DMEM), 45 percent Ham's F12, 10 percent fetal bovine serum (FBS), 100 units/ml penicillin/streptomycin, 20 mM HEPES (pH 7.3), 1 mM sodium pyruvate, 2 mM glutamine, and 700 ug/ml G418. The rat mGluR4 cell line was prepared by PCR amplification of the entire coding sequence of each receptor and cloning into pIRES puro 3 (Invitrogen). Cloning sites were BamHI/Not I. HEK/GIRK/M4 cells were transfected with 24 ug of DNA and stable transfectants were selected with puromycin. And a monoclonal cell lines was established. Cells were grown in 45 percent Dulbecco's Modified Eagle Media (DMEM), 45 percent Ham's F12, 10percent fetal bovine serum (FBS), 100 units/ml penicillin/streptomycin, 20 mM HEPES (pH 7.3), 1 mM sodium pyruvate, and 2 mM glutamine (Growth Media). mGluR/GIRK lines were supplemented with 600 ng/ml puromycin dihydrochloride (Sigma-Aldrich) and 700 ug/ml G418 (Mediatech, Inc., Herndon, VA). Cells for experiments were generally maintained for approximately 15-20 passages; this was particularly important for experiments examining the endogenous alpha2C receptor.
Assays were performed within Vanderbilt University's High-Throughput Screening Center. Cells were plated into 384 well, black-walled, clear-bottom poly-D-lysine coated plates (Greiner) at a density of 15,000 cells/20 uL/well in DMEM containing 10 percent dialyzed FBS, 20 mM HEPES, and 100 units/ml penicillin/streptomycin (Assay Media). Plated cells were incubated overnight at 37 degrees C in the presence of 5 percent CO2. The following day, the medium was removed from the cells and 20 uL/well of 1.7 uM concentration of the indicator dye BTC-AM (Invitrogen; prepared as a stock in DMSO and mixed in a 1:1 ratio with pluronic acid F-127) in Assay Buffer (Hanks Balanced Salt Solution (Invitrogen) containing 20 mM HEPES pH 7.3) was added to the plated cells. Cells were incubated for one hour at room temperature and the dye was replaced with 20 uL of Assay Buffer.
Test compound preparation. Glutamate was diluted in Thallium Buffer (125 mM sodium bicarbonate (added fresh the morning of the experiment), 1 mM magnesium sulfate, 1.8 mM calcium sulfate, 5 mM glucose, 12 mM thallium sulfate, 10 mM HEPES, pH 7.3) at 5x the final concentration to be assayed. For fold shift experiments, compounds were diluted to a 60 uM (2x final) concentration in Assay Buffer. Cell plates and compound plates were loaded onto a Hamamatsu FDSS 6000 kinetic imaging plate reader. Appropriate baseline readings were taken (10 images at 1 Hz, excitation, 470+/-20 nm emission, 540+/-30 nm) and test compounds were added. Compounds were added in a 20 uL volume and incubated for approximately 2.5 minutes prior to the addition of 10 ul of Thallium Buffer +/- agonist. After the addition of agonist, data were collected for an additional 2 min.
Thallium sulfate requires special handling and disposal precautions and investigators are cautioned to contact their Environmental Health and Safety Department to ensure proper procedures are followed.
Data analysis. Data were analyzed using usoft Excel. Raw data were opened in Excel and each data point in a given trace was divided by the first data point from that trace (static ratio). For experiments in which antagonists/potentiators were added, data were again normalized by dividing each point by the fluorescence value immediately before the agonist addition to correct for any subtle differences in the baseline traces after the compound incubation period. The slope of the fluorescence increase beginning five seconds after thallium/agonist addition and ending fifteen seconds after thallium/agonist addition was calculated. Curves were fitted using a four point logistical equation using XLfit (IDBS, Bridgewater, NJ). Subsequent confirmations of concentration-response parameters were performed using independent serial dilutions of source compounds and data from multiple days experiments were integrated and fit using a four point logistical equation in GraphPad Prism (GraphPad Software, Inc., La Jolla, CA).
For compounds that changed the EC50 of glutamate 3 fold or less, 'Outcome' was assigned as 'Inactive' and 'Score' was assigned as '1'. For compounds that changed the EC50 of glutamate greater than 3 fold, 'Outcome' was assigned as 'Active'. 'Score' was assigned as '50' for those with a fold-shift less than or equal to 10 or as '100' for those with a fold-shift greater than 10.
* Activity Concentration. ** Test Concentration.
Data Table (Concise)