qHTS Assay to Find Inhibitors of T. brucei phosphofructokinase: hit validation
Various species of the protozoan family Trypanosomatidae are responsible for a range of serious human diseases in tropical and subtropical areas of the world. The subspecies Trypanosoma brucei is one of three known to cause sleeping sickness in sub-Saharan Africa, significantly contributing to the millions of people worldwide who are infected by these parasites and endangering hundreds of millions more. Many of the disorders caused by trypanosomatids are fatal if left untreated, but most currently used drugs are inefficient and toxic. ..more
BioActive Compounds: 18
NIH Molecular Libraries Probe Production Network [MLPCN]
NIH Chemical Genomics Center [NCGC]
Grant: 1 R03 MH092153-01
PI Name: Malcolm Walkinshaw, University of Edinburgh
NCGC Assay Overview:
Various species of the protozoan family Trypanosomatidae are responsible for a range of serious human diseases in tropical and subtropical areas of the world. The subspecies Trypanosoma brucei is one of three known to cause sleeping sickness in sub-Saharan Africa, significantly contributing to the millions of people worldwide who are infected by these parasites and endangering hundreds of millions more. Many of the disorders caused by trypanosomatids are fatal if left untreated, but most currently used drugs are inefficient and toxic.
Of many possible drug targets in trypanosomatid parasites, the carbohydrate metabolism pathway is seen as potentially one of the most selective, as T. brucei, when in the bloodstream of its mammalian host, is entirely dependent on the conversion of the blood sugar glucose into pyruvate for its ATP supply . Oxidative metabolism involving the mitochondrial tricarboxylic acid cycle and oxidative phosphorylation are repressed in these parasites, and recent RNA interference (RNAi) experiments have shown that even partial depletion of certain individual glycolytic enzymes can lead to the death of cultured parasites . One such glycolytic enzyme, phosphofructokinase (PFK), catalyzes the formation of fructose-1,6-bisphosphate (F1,6BP) and ADP from fructose-6-phosphate (F6P) and ATP, and in many metabolic circumstances makes an important contribution to the control of flux through the glycolytic pathway. As a specific glycolytic target, PFK is particularly attractive as it catalyzes the first irreversible step in glycolysis, and structural and kinetic studies have shown very substantial and essential differences from corresponding host enzymes , allowing for the discovery of parasite-selective inhibitors.
The goal of this screen is to find small-molecule inhibitors of T. brucei PFK using a biochemical assay that measures PFK activity as a function of ADP production. A luminescence-based assay for T. brucei PFK activity has been developed which uses luciferase to detect production of ADP. Traditional luciferase-based nucleotide detection reagents rely on ATP production as a direct substrate for the luciferase reaction; however, PFK catalyzes the transfer of a phosphate from ATP to F6P, resulting in the net depletion of ATP. To directly measure the ADP product of the PFK reaction, we used the ADP-Glo detection kit (Promega), which utilizes two separate reactions to 1) deplete all remaining uncatalyzed ATP and 2) convert all remaining ADP to ATP, which is then detected through a traditional luciferase-coupled reaction. Compounds were screened as a concentration-titration series that ranged from 57 uM to 0.7 nM.
NCGC Assay Protocol Summary:
Substrate buffer was dispensed into white, solid 1536-well plates at 3uL/well in 0.1M triethanolamine (TEA) buffer, pH 8.0, containing final concentrations of 5 mM MgCl2, 0.01% Tween20, 0.5 mM fructose-6-phosphate (F6P) and 0.1mM ATP. Then, 23 nL of compounds or DMSO were delivered to each well using a pin tool. One uL T. brucei PFK (0.1M TEA, 0.01% Tween20, 0.1% bovine serum albumin (BSA) and 1.25nM PFK, final concentrations) was then dispensed, and plates were incubated at room temperature for 45 min. The luminescent detection reagent, ADP-Glo (Promega), was then added in two steps: the first reagent, ADP-Glo (which converts remaining uncatalyzed ATP), was added at 2.5uL/well and incubated for ten minutes at room temperature, and the second component, Kinase Detection reagent (which converts ADP to ATP as a substrate for luciferase-based luminescence), was added immediately after at 5uL/well, followed by one final ten minute room temperature incubation. The plates were measured on a ViewLux plate reader for luminescent signal using a clear filter with a one second exposure. The %Activity was determined from the corrected luminescence values. As no specific T. brucei PFK inhibitors have been identified in the literature, 1x (1.25nM) and 0x PFK enzyme controls (untreated) were included to normalize %Activity of identified inhibitors; 0x enzyme values corresponded to 100%Activity (full inhibition), while 1x PFK enzyme values were used to normalize 0%Activity (no inhibition).
Concentration-response curves were fitted to the signals arising from the resulting luminescence. The concentration-effect curves were then classified based on curve quality (r2), response magnitude and degree of measured activity, and compounds were subsequently categorized based on their curve class. Active inhibitors showed concentration-dependent decreases in luminescence, concordant with a decrease in PFK activity and ADP production. Inversely, active activators showed a concentration-dependent increase in luminescence signal above 1x PFK control levels, suggestive of an increase in ADP production. Inactive compounds showed no effect on luminescence signal.
Keywords: T. brucei, phosphofructokinase, PFK, glycolysis, luminescence, MLSMR, MLPCN, NIH Roadmap, qHTS, NCGC
1. Compounds are first classified as having full titration curves, partial modulation, partial curve (weaker actives), single point activity (at highest concentration only), or inactive. See data field "Curve Description". For this assay, apparent inhibitors are ranked higher than compounds that showed apparent activation.
2. For all inactive compounds, PUBCHEM_ACTIVITY_SCORE is 0. For all active compounds, a score range was given for each curve class type given above. Active compounds have PUBCHEM_ACTIVITY_SCORE between 40 and 100. Inconclusive compounds have PUBCHEM_ACTIVITY_SCORE between 1 and 39. Fit_LogAC50 was used for determining relative score and was scaled to each curve class' score range.
Categorized Comment - additional comments and annotations
* Activity Concentration. ** Test Concentration.
Data Table (Concise)