Urothelial ATP exocytosis: regulation of bladder compliance in the urine storage phase

Abstract

The bladder urothelium is more than just a barrier. When the bladder is distended, the urothelium functions as a sensor to initiate the voiding reflex, during which it releases ATP via multiple mechanisms. However, the mechanisms underlying this ATP release in response to the various stretch stimuli caused by bladder filling remain largely unknown. Therefore, the aim of this study was to elucidate these mechanisms. By comparing vesicular nucleotide transporter (VNUT)-deficient and wild-type male mice, we showed that ATP has a crucial role in urine storage through exocytosis via a VNUT-dependent mechanism. VNUT was abundantly expressed in the bladder urothelium, and when the urothelium was weakly stimulated (i.e. in the early filling stages), it released ATP by exocytosis. VNUT-deficient mice showed reduced bladder compliance from the early storage phase and displayed frequent urination in inappropriate places without a change in voiding function. We conclude that urothelial, VNUT-dependent ATP exocytosis is involved in urine storage mechanisms that promote the relaxation of the bladder during the early stages of filling.

Vesicular nucleotide transporter (VNUT): appearance of an actress on the stage of purinergic signaling

“…VNUT is present in either insulin granules of β cells or glucagon granules of α cells [86,102], and glucose-responsive ATP secretion is lost [86]. As discussed later, ATP secretion from dorsal horn neurons and bladder epithelium in VNUT −/− mice also disappear [98,103]. Taken together, it can be concluded that VNUT −/− mice lose vesicular ATP release, and that VNUT is responsible for the vesicular storage and release of ATP.…”

“…Endogenous VNUT is colocalized with Lamp1, a lysosomal marker, in these cells [77], [81,92]. For details, see refs [4,81,86,92,93,96,98] but VNUT in astrocytes is not [75,76]. VNUT-mediated Δψ-driven ATP accumulation has been observed in living COS and HEK293 cells [78].…”

“…Vesicular ATP release is involved in urine storage promoting relaxation of the bladder during an early stage of filling by stimulating P2 purinoceptors on suburotherial sensory nerves. In the bladders of VNUT −/− mice, vesicular ATP release is impaired, and they exhibit reduced bladder compliance from the early storage phase and frequent urination [98]. It is noteworthy that vesicular ATP release occurs upon weak stimulation (weak stretch) at an early phase of filling through TRPV4.…”

“…VNUT expression was also detected in the cultured human corneal limbal epithelial cells and human conjunctival epithelial cells, although the organelles containing VNUT were unidentified [82]. VNUT is also expressed in type II taste cells in taste buds [83], mouse esophageal keratinocytes [84], cat esophageal mucosa [85], glucagon-containing secretory granules in islet α and insulin granules of β cells [86,87], Schwann cells [88], human lung cancer A549 cells [89], human airway epithelial goblet Calu-3 cells [54], zymogen granules of mouse pancreatic acinar cells [90,91], GLP-1-containing vesicles of intestinal L cells [92], rat parotid glands [93], rat liver epithelium [94], biliary epithelial cells [95], platelets [96], odontoblasts [97], and bladder urothelium [98] (Fig. 4) (Table 1).…”

Clock Genes Regulate the Circadian Expression of Piezo1, TRPV4, Connexin26, and VNUT in an Ex Vivo Mouse Bladder Mucosa

“…Bladder extension is sensed by mechanosensor such as transient receptor potential cation channel subfamily V member 4 ( TRPV4) and Piezo1 via intracellular Ca 2+ influx [16–18], which triggers ATP release from the bladder urothelium. Although many pathways are involved in the release of ATP, we identified the vesicular nucleotide transporter ( VNUT) , that mediates exocytosis [19] and conductive release connexin- or pannexin-hemichannels that mediate diffusible ATP release in the mucosa [20–25], particularly Connexin26 ( Cx26 ) [26], as a main ATP release pathway in the bladder mucosa.…”