Side-by-side comparison of published small molecule inhibitors against thapsigargin-induced store-operated Ca2+ entry in HEK293 cells.

Calcium (Ca2+) is a key second messenger in eukaryotes, with store-operated Ca2+ entry (SOCE) being the main source of Ca2+ influx into non-excitable cells. ORAI1 is a highly Ca2+-selective plasma membrane channel that encodes SOCE. It is ubiquitously expressed in mammals and has been implicated in numerous diseases, including cardiovascular disease and cancer. A number of small molecules have been identified as inhibitors of SOCE with a variety of potential therapeutic uses proposed and validated in vitro and in vivo. These encompass both nonselective Ca2+ channel inhibitors and targeted selective inhibitors of SOCE. Inhibition of SOCE can be quantified both directly and indirectly with a variety of assay setups, making an accurate comparison of the activity of different SOCE inhibitors challenging. We have used a fluorescence based Ca2+ addback assay in native HEK293 cells to generate dose-response data for many published SOCE inhibitors. We were able to directly compare potency. Most compounds were validated with only minor and expected variations in potency, but some were not. This could be due to differences in assay setup relating to the mechanism of action of the inhibitors and highlights the value of a singular approach to compare these compounds, as well as the general need for biorthogonal validation of novel bioactive compounds. The compounds observed to be the most potent against SOCE in our study were: 7-azaindole 14d (12), JPIII (17), Synta-66 (6), Pyr 3 (5), GSK5503A (8), CM4620 (14) and RO2959 (7). These represent the most promising candidates for future development of SOCE inhibitors for therapeutic use.

Na+ entry through heteromeric TRPC4/C1 channels mediates (-)Englerin A-induced cytotoxicity in synovial sarcoma cells.

The sesquiterpene (-)Englerin A (EA) is an organic compound from the plant Phyllanthus engleri which acts via heteromeric TRPC4/C1 channels to cause cytotoxicity in some types of cancer cell but not normal cells. Here we identified selective cytotoxicity of EA in human synovial sarcoma cells (SW982 cells) and investigated the mechanism. EA induced cation channel current (Icat) in SW982 cells with biophysical characteristics of heteromeric TRPC4/C1 channels. Inhibitors of homomeric TRPC4 channels were weak inhibitors of the Icat and EA-induced cytotoxicity whereas a potent inhibitor of TRPC4/C1 channels (Pico145) strongly inhibited Icat and cytotoxicity. Depletion of TRPC1 converted Icat into a current with biophysical and pharmacological properties of homomeric TRPC4 channels and depletion of TRPC1 or TRPC4 suppressed the cytotoxicity of EA. A Na+/K+-ATPase inhibitor (ouabain) potentiated EA-induced cytotoxicity and direct Na+ loading by gramicidin-A caused Pico145-resistant cytotoxicity in the absence of EA. We conclude that EA has a potent cytotoxic effect on human synovial sarcoma cells which is mediated by heteromeric TRPC4/C1 channels and Na+ loading.

Picomolar, selective, and subtype-specific small-molecule inhibition of TRPC1/4/5 channels.

The concentration of free cytosolic Ca2+ and the voltage across the plasma membrane are major determinants of cell function. Ca2+-permeable non-selective cationic channels are known to regulate these parameters, but understanding of these channels remains inadequate. Here we focus on transient receptor potential canonical 4 and 5 proteins (TRPC4 and TRPC5), which assemble as homomers or heteromerize with TRPC1 to form Ca2+-permeable non-selective cationic channels in many mammalian cell types. Multiple roles have been suggested, including in epilepsy, innate fear, pain, and cardiac remodeling, but limitations in tools to probe these channels have restricted progress. A key question is whether we can overcome these limitations and develop tools that are high-quality, reliable, easy to use, and readily accessible for all investigators. Here, through chemical synthesis and studies of native and overexpressed channels by Ca2+ and patch-clamp assays, we describe compound 31, a remarkable small-molecule inhibitor of TRPC1/4/5 channels. Its potency ranged from 9 to 1300 pm, depending on the TRPC1/4/5 subtype and activation mechanism. Other channel types investigated were unaffected, including TRPC3, TRPC6, TRPV1, TRPV4, TRPA1, TRPM2, TRPM8, and store-operated Ca2+ entry mediated by Orai1. These findings suggest identification of an important experimental tool compound, which has much higher potency for inhibiting TRPC1/4/5 channels than previously reported agents, impressive specificity, and graded subtype selectivity within the TRPC1/4/5 channel family. The compound should greatly facilitate future studies of these ion channels. We suggest naming this TRPC1/4/5-inhibitory compound Pico145.

2-sec-Butyl-1-(2-hy-droxy-eth-yl)-1H-benzimidazole-5-carboxylic acid.

In the title compound, C(14)H(18)N(2)O(3), the carb-oxy-lic group is tilted by 12.00 (4)° with respect to the mean plane throught the benzimidazole ring system. The alcohol and carboxyl hydroxy groups are involved in intermolecular O-H⋯O and O-H⋯N hydrogen bonds, forming a two-dimensional network extending parallel the ab plane. The network is further stabilized by weak C-H⋯O inter-actions. The sec-butyl group is disordered over two sets of sites with refined occupancies of 0.484 (4) and 0.516 (4).

Ethyl 1-(2-hy-droxy-eth-yl)-2-[2-(methyl-sulfan-yl)eth-yl]-1H-benzimidazole-5-carboxyl-ate.

In the crystal structure of the title compound, C(15)H(20)N(2)O(3)S, the hy-droxy group is involved in the formation of O-H⋯N hydrogen bonds, which link two mol-ecules into a centrosymmetric dimer. Weak C-H⋯O hydrogen bonds further link these dimers into chains propagating along the a axis. The crystal packing exhibits π-π inter-actions between the five- and six-membered rings of neighbouring mol-ecules [centroid-centroid distance = 3.819 (2) Å] and short inter-molecular S⋯S contacts of 3.495 (1) Å.

Ethyl 1-(2-hy-droxy-eth-yl)-2-propyl-1H-benzimidazole-5-carboxyl-ate.

In the title compound, C(15)H(20)N(2)O(3), the benzimidazole ring is essentially planar, with a maximum deviation from the mean plane of 0.012 (1) Å. The crystal structure is stabilized by inter-molecular O-H⋯N hydrogen bonds, forming centrosymmetric dimers, which are connected in the [100] direction through weak C-H⋯O contacts.

Ethyl 1-(2-hy-droxy-eth-yl)-2-phenyl-1H-benzimidazole-5-carboxyl-ate.

There are two mol-ecules in the asymmetric unit of the title compound, C(18)H(18)N(2)O(3). In each one, the benzimidazole ring system is essentially planar, with maximum deviations of 0.027 (1) and 0.032 (1)Å, and makes dihedral angles of 38.64 (6) and 41.48 (6)°, respectively, with the attached benzene rings. An intra-molecular C-H⋯O hydrogen bond is observed in each mol-ecule. The two independent mol-ecules are connected into a dimer by two inter-molecular O-H⋯N hydrogen bonds. In the crystal, mol-ecules form a two-dimensional layers parallel to (012) via weak inter-molecular C-H⋯O hydrogen bonds. In addition, weak π-π stacking inter-actions are observed with centroid-centroid distances of 3.5244 (12) and 3.6189 (12) Å.