BERKELEY, CA (UroToday.com) - This published abstract concerns the sensory function of transient receptor potential vanilloid subtype 4 (TRPV4) in the human bladder.
The bladder urothelium has 2 main functions. Firstly, it has to create a barrier against toxic waste products in urine. Secondly, it has an important function in sensory signaling. For the latter function, cumulative research over the last 2 decades has shown that the urothelial sensory pathway is one of the key components in the pathophysiology of overactive bladder (OAB). Hence, there is a big focus on receptors in the urothelium as potential new targets for pharmacological treatment of OAB. This is, however, a complicated matter since the urothelium and suburothelium express a wide range of receptors (muscarinic, purinergic and prostaglandin receptors, TRP channels etc.), whose functions and interactions have not been fully clarified.
Since the introduction of resiniferatoxin and capsaicin treatments (TRPV1 agonists) for the treatment of OAB, there has been increased interest in the TRP channels of the urinary bladder. This family of cation channels is expressed on cell membranes. They have a wide variety of functions in the human body, such as heat and cold sensation, stretch sensation, and calcium maintenance. A recent study by Everaerts et al. showed that TRPV4 is by far the most abundantly expressed TRP channel in urothelium cells. TRPV4 channels are activated by stimuli, such as mechanical stretch, which provokes an ATP release in urothelial cells. Combined with the voiding dysfunction that is observed in the transgenic TRPV4 -/- mouse phenotype, this implies that TRPV4 channels are involved in mechanosensory (afferent) signaling in the bladder. In animal studies, blocking of TRPV4 leads to an inhibition of (hyper) contractility in inflamed bladders. For that reason, it may be a suitable target for OAB treatment.
Our study was conducted to investigate the exact location of TRPV4 channels in the human bladder urothelium, and to investigate a possible mechanism for stretch activation of TRPV4 in the urothelium. Since the shape and volume of urothelial cells is very flexible in order to cope with large bladder volume changes, detecting stretch by the stretch receptors most likely has to be done at specific locations. We hypothesized that rigid cell junctions (like adherence junctions and tight junctions) are the most suitable sites for stretch activation of TRPV4 channels, because they 1) are also located on the cell membrane, 2) interconnect neighboring cells, and 3) are linked to the intracellular cytoskeleton. The rigid cell junctions are crucial for barrier function, and form the proverbial connective chains in a necklace.
This morphological study was conducted using immunofluorescence, double-labeling assays, confocal microscopy, Western blotting and immunoprecipitation assays on human and mouse bladders, and a well differentiated urothelial cell line. To enhance reliability of our experiments, we used TRPV4 -/- mouse tissues as negative control, as well as multiple antibodies against bordering proteins, to exactly pinpoint the molecular connections of TRPV4. The use of more than 1 technique to prove this connection is, in our opinion, essential, especially since there is much debate about the specificity of antibodies and the value of immunofluorescence assays as definitive evidence.
Our results demonstrated a molecular connection between TRPV4 and the α-catenin proteins of the adherence junction. The α-catenin protein is part of the intracellular complex of the adherence junction, and it is connected to the actin cytoskeleton. These results do not only clarify where TRPV4 is located, but also provide insight in potentially new ways of specifically blocking the stretch activation pathway of TRPV4 channels. Apart from this, these results offer clues that perhaps clarify that the 2 main functions of the urothelium (barrier and sensation), do not use separate mechanisms, but instead are closely linked together.
Various TRPV4 antagonists are being investigated now. The efficacy and safety of these compounds still need to undergo further preclinical work-up. There is also limited data on the involvement of TRPV4 channels in the pathophysiology of OAB. Despite this, TRPV4 channels seem to be important for bladder function and are very interesting to investigate.
In summary, pharmacological treatment of OAB is changing. Development of novel conservative therapies is essential to decrease the burden of this widespread disease. Understanding how TRPV4 and other TRP channels function in the bladder may help improve the treatment of this condition.
Dick AW Janssen1 as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
1MD Researcher, Dept. of Urology, Radboud University Nijmegen Medical Centre (RUNMC), The Netherlands