Defining protein expression in the urothelium: a problem of more than transitional interest

The transitional epithelium of the bladder, the urothelium, is a challenging tissue to study due to its fragility, complex cellular makeup, stratified composition, and intimate connections to both neural and connective tissue elements. With the increasing focus on the urothelium as a mechanosensory tissue with complex autocrine and paracrine signaling activities, there have arisen a number of unresolved controversies in the urothelial literature regarding whether certain important sensory and signaling proteins are expressed by the urothelium. Prominent examples of this include the transient receptor potential (TRP) family member TRPV1 and the purinergic receptor P2X(3). The problem is more than one of scientific bookkeeping since studies utilizing genetic models (primarily knockout mice) claim additional credibility for urothelial functions when phenotypes are discovered. Furthermore, both of the above-mentioned receptors are important therapeutic targets for various bladder disorders including inflammatory and neuropathic pain. The reasons for the confusion about urothelial expression are manifold, but they likely include low expression levels in some cases, poor specificity of antibodies (sometimes lacking adequate controls), the presence of nonurothelial cells resident within the urothelium, and the fact that the urothelium is particularly prone to aspecific adsorption of antibodies. In this review, we attempt to summarize some of the pitfalls with currently accepted practices in this regard, as well as to describe a set of guidelines which will improve the reliability of conclusions related to urothelial expression. It is hoped that this will be of value to investigators studying the urothelium, to those attempting to interpret conflicts in the literature, and hopefully also those charged with reviewing unpublished work. These recommendations will outline a set of "baseline" and "best practice" guidelines by which both researchers and reviewers will be able to evaluate the evidence presented.