Genetic tools that target mechanoreceptors produce reliable labeling of bladder afferents and altered mechanosensation.

Abstract

Mechanosensitive neurons are important sensors of bladder distention, but their role in urologic disease remains unclear. Our current knowledge about how disease alters bladder sensation comes from studies that focus primarily on peptidergic nociceptors, leaving our understanding of neuropeptide-negative mechanoreceptors incomplete. In this study, we found that a substantial proportion of neurofilament heavy (NFH)-positive A fibers innervating the bladder were calcitonin gene-related peptide (CGRP)-negative, potentially representing uncharacterized mechanoreceptors. We then identified two genetic strategies that label mechanoreceptors in mouse skin and confirmed that they likewise label bladder afferents. Cre-mediated tdTomato reporter expression driven by tyrosine kinase B (TrkB), which labels A-delta mechanoreceptors in skin, successfully labeled bladder nerve terminals. The majority of TrkB bladder afferents were CGRP-negative and NFH-positive, with more characteristic staining patterns seen at the level of the cell body. The Ret proto-oncogene also produced robust labeling of bladder afferents, where colocalization with CGRP and NFH was consistent with multiple afferent subtypes. Because TrkB labeling was more specific for putative mechanoreceptors, we directly tested the role of TrkB neurons in bladder mechanosensation in vivo. Using an intersectional genetic strategy, we selectively ablated TrkB afferents and measured bladder responses to mechanical distention using anesthetized cystometry. Compared to controls, mice with ablated TrkB afferents required higher distention pressure to elicit voids. Interestingly, after ablation, distention also increased the frequency of non-voiding contractions, a poorly understood phenotype of several urologic diseases. These genetic strategies comprise critical new tools to advance the study of mechanoreceptors in bladder function and urologic disease pathophysiology.