TRPM4 Channels are Promising Novel Molecular Targets for the Pharmacological Treatment of Overactive Bladder

DOI: 10.5176/2345-783X_PHARMA15.50

Authors: Professor Dr. Georgi V. Petkov

Abstract:

Recently, members of Transient Receptor Potential (TRP) superfamily of ion channels have been implicated in normal and pathologic bladder function. The TRP channels are the most recently discovered ion channel superfamily which currently includes 28 mammalian members subdivided into six subfamilies based on their sequence homology (TRPC, TRPV, TRPM, TRPA, TRPP and TRPML). TRPM4 channels are Ca2+-activated non-selective cation channels that have been recently identified as regulators of detrusor smooth muscle (DSM) function in rodents. However, their expression and function in human DSM remains unexplored. Here, we provide insights into the functional role of TRPM4 channels in human DSM under physiological and pathophysiological conditions of overactive bladder (OAB). We used a multidisciplinary experimental approach including RT-PCR, Western blot, immunohistochemistry and immunocytochemistry, electrophysiology, and functional studies of DSM contractility. Human DSM samples were obtained from patients with or without preoperative OAB symptoms. RT-PCR detected mRNA transcripts for TRPM4 channels in DSM whole tissue and single cells. Western blot and immunohistochemistry with confocal microscopy revealed TRPM4 protein expression in human DSM. Immunocytochemistry further detected TRPM4 protein expression directly in DSM single cells. Patch-clamp experiments showed that 9-phenanthrol, a selective TRPM4 channel inhibitor, significantly decreased the transient inward cation currents (TICCs) and voltage-step-induced whole-cell currents in human DSM cells. Xestospongin C, an inositol 1,4,5-trisphosphate receptor (IP3R) inhibitor, significantly decreased TICCs, suggesting IP3R-mediated regulation of TRPM4 channels. 9-Phenanthrol differentially hyperpolarized the DSM cell membrane potential and inhibited spontaneous phasic contractions in DSM strips isolated from control (non-OAB) and OAB patients. Furthermore, 9-phenanthrol attenuated the carbachol-induced and nerve-evoked contractions in human DSM strips. In conclusion, TRPM4 channels regulate human DSM excitability and contractility and are critical determinants of DSM function in individuals without OAB as well as those with OAB. The study provides evidence for the novel concept that pharmacological or genetic manipulation of TRPM4 channels may prove beneficial for the treatment of OAB

Keywords: TRP channels; 9-phenanthrol; patch-clamp electrophysiology; dtrusor smooth muscle; contractility;