Canonical transient receptor potential 4 and its small molecule modulators

Abstract

Canonical transient receptor potential 4 (TRPC4) forms non-selective cation channels that contribute to phospholipase C-dependent Ca(2+) entry into cells following stimulation of G protein coupled receptors and receptor tyrosine kinases. Moreover, the channels are regulated by pertussis toxin-sensitive Gi/o proteins, lipids, and various other signaling mechanisms. TRPC4-containing channels participate in the regulation of a variety of physiological functions, including excitability of both gastrointestinal smooth muscles and brain neurons. This review is to present recent advances in the understanding of physiology and development of small molecular modulators of TRPC4 channels.

Figure 1

Structural features of TRPC4α, which contains an extra region with 84 amino acids as compared to TRPC4β. Within this region, two calmoudlin (CaM) binding motifs and one IP₃ receptor binding domain had been experimentally identified [15,16]. Homer had been suggested to bind to the proline-rich region of the related TRPC1 [17]. Stathmin was shown to bind to the N-terminal coiled-coil domains (CC-N) of TRPC4 and TRPC5 [18]. Other domains and interacting proteins are described in the text. ANK 1–4, ankyrin-like repeats 1–4; CC-C, C-terminal coiled-coil domains, CIRB, calmodulin/IP₃ receptor binding region; PDZ-B, PDZ binding domain; LFW, amino acid motif conserved in the hydrophobic putative pore region; EWKFAR, sequence of the TRP box. Adapted from [4] with modifications.

Figure 2

Agonist-activated currents in a HEK293 cell that co-expressed mouse TRPC4β and μ-opioid receptor (μOR). A, For whole-cell experiments, the standard extracellular solution contained (in mmol L⁻¹): 140 NaCl, 5 KCl, 2 CaCl₂, 1 MgCl₂, 10 glucose, 10 HEPES, pH 7.4 adjusted with NaOH. The intracellular solution consisted of (in mmol L⁻¹): 140 CsCl, 0.1 EGTA, 1 MgCl₂, 10 HEPES, pH 7.2 adjusted with CsOH. Currents were measured using 500-ms voltage ramps from +100 mV to −100 mV in 2-s intervals with the holding potential of 0 mV. Shown are time courses of currents at +100 mV (open circles) and −100 mV (filled circles), in response to the application of 10 μmol L⁻¹ Oxo-M and 100 nmol L⁻¹ DAMGO to simultaneously activate the muscarinic (endogenously expressed) and μ-opioid (transfected) receptors, respectively. B, Current-voltage relationships recorded from the same cell by the voltage ramp collected before agonist application (basal, blue line) and at the peak of the agonist response (red line).