Stretch-regulated Exocytosis/Endocytosis in Bladder Umbrella Cells

Abstract

The epithelium of the urinary bladder must maintain a highly impermeable barrier despite large variations in urine volume during bladder filling and voiding. To study how the epithelium accommodates these volume changes, we mounted bladder tissue in modified Ussing chambers and subjected the tissue to mechanical stretch. Stretching the tissue for 5 h resulted in a 50% increase in lumenal surface area (from approximately 2900 to 4300 microm(2)), exocytosis of a population of discoidal vesicles located in the apical cytoplasm of the superficial umbrella cells, and release of secretory proteins. Surprisingly, stretch also induced endocytosis of apical membrane and 100% of biotin-labeled membrane was internalized within 5 min after stretch. The endocytosed membrane was delivered to lysosomes and degraded by a leupeptin-sensitive pathway. Last, we show that the exocytic events were mediated, in part, by a cyclic adenosine monophosphate, protein kinase A-dependent process. Our results indicate that stretch modulates mucosal surface area by coordinating both exocytosis and endocytosis at the apical membrane of umbrella cells and provide insight into the mechanism of how mechanical forces regulate membrane traffic in non-excitable cells.

NTPDase3 and ecto-5′-nucleotidase/CD73 are differentially expressed during mouse bladder cancer progression

“…Urothelium is a source of ATP release [28,50,51] and it is also an important site of adenosine biosynthesis. Importantly, both ATP and adenosine have functions in exocytosis of umbrella cell layer [52,53], which is the mechanism to increase the luminal surface area when the bladder fills (cytoplasmatic discoidal/fusiform vesicles fuse with the apical plasma membrane) [54]. The ATP released by urothelium is responsible for micturition reflex, through P2X3 from subepithelial nerve fibers [55].…”

Distinct Apical and Basolateral Membrane Requirements for Stretch-induced Membrane Traffic at the Apical Surface of Bladder Umbrella Cells

“…The outermost layer of this tissue is lined by a single layer of polarized umbrella cells, which are known to respond to mechanical stimuli by augmented ion transport and membrane traffic (Lewis and de Moura, 1982;Truschel et al, 2002;Wang et al, 2003b); however, the relationship of these two processes and the mechanical force (stretch and/or pressure) that acts upon the umbrella cell to stimulate these events is not well understood.…”

“…In the subsequent "late" stage, which occurs after the tissue has reached a mechanical equilibrium, the surface area increases slowly over several hours (Balestreire and Apodaca, 2007). The late stage is temperature sensitive and is dependent on purinergic signaling pathways, activation of the EGF receptor, and protein synthesis and secretion (Truschel et al, 2002;Wang et al, 2003aWang et al, , 2005Balestreire and Apodaca, 2007). In contrast, the early stage is largely unexplored but appears to be insensitive to temperature and does not require EGF signaling or protein synthesis (Balestreire and Apodaca, 2007).…”

“…In contrast, the early stage is largely unexplored but appears to be insensitive to temperature and does not require EGF signaling or protein synthesis (Balestreire and Apodaca, 2007). In addition to stimulating apical exocytosis, mechanical stimuli also trigger increased endocytosis, which modulates the increase in apical surface area (Truschel et al, 2002); however, the relationship of the endocytic and exocytic events and the mechanisms by which they are initiated and coordinated is not known. Previous studies showed that the electrophysiological responses of the uroepithelium are dependent on the direction the tissue is bowed (Ferguson et al, 1997); however, the physiological significance of these differences and the role of the distinct surface domains of umbrella cells in mechanotransduction is yet to be defined.…”

Persistent uroplakin expression in advanced urothelial carcinomas: implications in urothelial tumor progression and clinical outcome

“…As demonstrated by chemical crosslinking and in vitro transfection analyses, the four major UPs form two distinct pairs (UPIa with II and UPIb with IIIa), in order for them to exit efficiently from the endoplasmic reticulum during biosynthesis (16,17). Because UPs are highly conserved during mammalian evolution (12), it has been suggested that they play key roles in urothelial functions, including participation in the permeability barrier, adjustment of urothelial surface area, stabilization of the urothelial surface and development of the urinary tract (12,18,19). Consistent with their proposed functions at the urothelial surface, UPs are expressed in a tightly differentiation-dependent manner, being detected ultrastructurally (in the form of AUMs) and immunohistochemically at the apical surface and cytoplasm of the superficial cell layer of human urothelium with little or no detection in the intermediate and basal layers (20,21).…”