Source (MLPCN Center Name): The Scripps Research Institute Molecular Screening Center (SRIMSC)
Center Affiliation: The Scripps Research Institute (TSRI)
Assay Provider: Stefan Heller, Stanford University
Network: Molecular Libraries Probe Production Centers Network (MLPCN)
Grant Proposal Number 1 R03 MH083077-01
Grant Proposal PI: Stefan Heller , Stanford University
External Assay ID: TRPN1_AG_Calcium_1536_3XEC50 CS

Name: Fluorescence counterscreen assay for TRPML3 agonists: dose response cell-based high-throughput screening assay to identify agonists of the transient receptor potential channel N1 (TRPN1)

Description:

Cell signaling pathways that mediate osmosensation, photosensation, and thermosensation depend on a family of diverse transient receptor potential (TRP) cation channels, which are activated by agonist-receptor coupling (1-5). A role for these channels in inner ear hair cell mechanotransduction was gleaned from TRP channel mutations identified in flies, worms, and lower vertebrates with defective balance and impaired sensitivity to touch (1-5). TRPML3 (mucolipin 3; MCOLN3) is a TRP channel expressed in inner ear hair cells and stereocilia (5-7), suggesting it may play a role in hearing and mechanotransduction. Reports that mice with mutations in TRPML3 (known as varitint-waddler mutants) exhibit early-onset hearing loss accompanied by head-bobbing and circling behaviors (8-10), provided further support for a role of TRPML3 in hearing and vestibular function. As a result, the identification of selective probes for TRPML3 would be useful to investigate the function of TRPML3 in inner ear mechanotransduction and hearing biology.

References:

1. Clapham, D.E., TRP channels as cellular sensors. Nature. 2003. 426(6966): p. 517-24.
2. Cuajungco, M.P., C. Grimm, and S. Heller, TRP channels as candidates for hearing and balance abnormalities in vertebrates. Biochim Biophys Acta. 2007. 1772(8): p. 1022-7.
3. Gillespie, P.G. and R.G. Walker. Molecular basis of mechanosensory transduction. Nature. 2001. 413(6852): p. 194-202.
4. Eberl, D.F., R.W. Hardy, and M.J. Kernan. Genetically similar transduction mechanisms for touch and hearing in Drosophila. J Neurosci. 2000. 20(16): p. 5981-8.
5. Qian F, Noben-Trauth K. Cellular and molecular function of mucolipins (TRPML) and polycystin 2 (TRPP2). Pflugers Arch. 2005 Oct;451(1):277-85.
6. Atiba-Davies M, Noben-Trauth K. TRPML3 and hearing loss in the varitint-waddler mouse. Biochim Biophys Acta. 2007 Aug;1772(8):1028-31.
7. Gong, Z., W. Son, Y.D. Chung, J. Kim, D.W. Shin, C.A. McClung, Y. Lee, H.W. Lee, D.J. Chang, B.K. Kaang, H. Cho, U. Oh, J. Hirsh, M.J. Kernan, and C. Kim. Two interdependent TRPV channel subunits, inactive and Nanchung, mediate hearing in Drosophila. J Neurosci. 2004. 24(41): p. 9059-66.
8. Di Palma, F.; Belyantseva, I. A.; Kim, H. J.; Vogt, T. F.; Kachar, B.; Noben-Trauth, K. Mutations in Mcoln3 associated with deafness and pigmentation defects in varitint-waddler (Va) mice. Proc. Nat. Acad. Sci. 99: 14994-14999, 2002.
9. Nagata K, Zheng L, Madathany T, Castiglioni AJ, Bartles JR, Garc#a-A#overos J. Proc Natl Acad Sci U S A. 2008 Jan 8;105(1):353-8. Epub 2007 Dec 27. The varitint-waddler (Va) deafness mutation in TRPML3 generates constitutive, inward rectifying currents and causes cell degeneration.
10. van Aken AF, Atiba-Davies M, Marcotti W, Goodyear RJ, Bryant JE, Richardson GP, Noben-Trauth K, Kros CJ. J Physiol. 2008 Sep 18. TRPML3 mutations cause impaired mechano-electrical transduction and depolarization by an inward-rectifier cation current in auditory hair cells of varitint-waddler mice.

Keywords:

TRPML3, TRP cation channel, HEK 293, HTS assay, 1536, dose response, EC50, counterscreen, agonist, activator, deafness, fluorescence, calcium, Fluo-8 dye, Scripps, Scripps Florida, Scripps Research Institute Molecular Screening Center, SRIMSC, Molecular Libraries Probe Production Centers Network, MLPCN.

Assay Overview:

The purpose of this assay is to determine dose response curves for compounds identified as active in a previous set of experiments entitled, "Primary cell-based high-throughput screening assay to identify agonists of the transient receptor potential channel ML3" (PubChem AID 1448), inactive in a set of experiments entitled, "Primary cell-based high-throughput screening assay to identify agonists of the transient receptor potential channel N1 (TRPN1)" (PubChem AID 1424), and that confirmed activity in a set of experiments entitled, "Confirmation cell-based high-throughput screening assay to identify agonists of the transient receptor potential channel ML3" (PubChem AID 1526). This assay also serves as a counterscreen to determine whether compounds act as nonselective agonists due to activation of TRPN1. This assay employs a HEK293 cell line that stably expresses the zebrafish TRPN1-YFP cation channel. The cells are treated with test compounds followed by measurement of intracellular calcium as monitored by a fluorescent, cell permeable calcium indicator dye. As designed, compounds that act as TRPN1 agonists will increase calcium mobilization, resulting in increased relative fluorescence of the indicator dye and increased well fluorescence. Compounds were tested in triplicate using a 10-point, 1:3 dilution series starting at a nominal concentration of 29.9 uM.

Protocol Summary:

The TRPN1 HEK293 cell line was routinely cultured in T-175 sq cm flasks at 37 degrees C and 95% relative humidity (RH). The growth media consisted of Minimum Essential Medium with GlutaMAX and supplemented with 10% v/v heat-inactivated qualified fetal bovine serum, 800 micrograms/mL Geneticin, and 1X antibiotic mix (penicillin, streptomycin, and neomycin). The day before the assay 1500 cells in 3 ul of growth media were seeded into each well of 1536 well microtiter plates and allowed to incubate at 37 degrees C, 5% CO2, and 95 % RH for 23 hours. Next, 2 ul of the fluorogenic Fluo-8 intracellular calcium indicator mixture with 1 mM trypan red plus (prepared according to the manufacturer's protocol) was added to each well. After a 1 hour incubation at 37 degrees C, 5% CO2, and 95 % RH followed by a 30 minute incubation at room temperature, the assay was started by performing a basal read of plate fluorescence (470-495 nm excitation and 515-575 nm emission) for 5 seconds on the FLIPR Tetra (Molecular Devices). Next, 15 nL of test compound in DMSO, DMSO alone (0.3% final concentration), or the cholinergic agonist carbachol (87 uM final concentration) in DMSO were dispensed to the appropriate wells. Then a real time fluorescence measurement was immediately performed for the remaining 120 seconds of the assay.

A ratio for each well was calculated to normalize assay data, according to the following mathematical expression:
Ratio = I_Max / I_Min
Where I_Max represents the maximum measured fluorescence emission intensity over the 125 second read and I_Min represents the minimum (basal) measured fluorescence emission intensity before compound was added.

Percent activation was calculated from the median ratio as follows:
%Activation = ((Ratio_Test_Compound- Median_Ratio_Low_Control) / (Median_Ratio_High_Control - Median_Ratio_ Low_Control)) *100
Where:
Test_Compound is defined as wells containing test compound.
High_Control is defined as wells containing carbachol.
Low_Control is defined as wells containing DMSO.

For each test compound, percent activation was plotted against compound concentration. A four parameter equation describing a sigmoidal dose-response curve was then fitted with adjustable baseline using Assay Explorer software (MDL Information Systems). The reported EC50 values were generated from fitted curves by solving for the X-intercept value at the 50% activation level of the Y-intercept value. In cases where the highest concentration tested (i.e. 29.9 uM) did not result in greater than 50% activation, the EC50 was determined manually depending on the observed activation at the individual concentrations. Compounds with EC50 values greater than 10 uM were considered inactive. Compounds with EC50 values equal to or less than 10 uM were considered active.

Any compound with a percent activation value <50% at all test concentrations was assigned an activity score of zero. Any compound with a percent activation value >50% at any test concentration was assigned an activity score greater than zero. Activity score was then ranked by potency, with the most potent compounds assigned the highest activity scores.

List of Reagents:

TRPN1 HEK293 cell line (provided by Prof. Stefan Heller)
Fluo-8 No Wash Calcium Assay Kit (ABD Bioquest, part 36316)
Trypan red plus (ABD Bioquest, part 2456)
MEM with GlutaMAX (Invitrogen, part 41090-101)
Geneticin (Invitrogen, part 10131-027)
Trypsin-EDTA solution (Invitrogen, part 25200-056)
Fetal Bovine Serum (Invitrogen, part 16140-071)
Carbachol (Sigma, part C4382)
100X Penicillin-Streptomycin-Neomycin mix (Invitrogen, part 15640-055)
T-175 tissue culture flasks (Corning, part 431080)
1536-well plates (Greiner, part 789072)

Due to the increasing size of the MLPCN compound library, this assay may have been run as two or more separate campaigns, each campaign testing a unique set of compounds. All data reported were normalized on a per-plate basis. In this assay carbachol had an approximate EC50 of 1.5 uM. Possible artifacts of this assay can include, but are not limited to: dust or lint located in or on wells of the microtiter plate, compounds that non-specifically modulate calcium levels or channel activity, and compounds that quench or emit fluorescence within the well. All test compound concentrations reported are nominal; the specific concentration for a particular test compound may vary based upon the actual sample provided by the MLSMR. The MLSMR was not able to provide all compounds selected for testing in this AID.

Grant Number: 1 R03 MH083077-01