Regional distribution and molecular interaction of caveolins in bladder smooth muscle.

UNLABELLED: What's known on the subject? and What does the study add? Caveolae are specialised regions of bladder smooth muscle (BSM) cell membranes where specific signalling pathways are regulated. Caveolin proteins are involved in caveolar biogenesis and function as signal transduction regulators. Expression of caveolin-1, -2, and -3 has been previously identified in the bladder; however, the distribution and relative expression of these proteins have not been defined. The present data show significant differences in the spatial distribution of caveolin proteins throughout the bladder wall. Region dependent variations in the co-localisation of caveolin subtypes in detrusor SM were also detected. These findings support the premise that the unique spatial pattern of caveolin proteins associated with BSM cells may enable regionally distinct functional responses to common stimuli. OBJECTIVE: • To determine the regional expression profile of caveolin isoforms (integral membrane proteins abundant in caveolae), the spatial relationships among caveolin proteins within specific smooth muscle (SM) regions and the extent of their molecular interactions in bladder SM (BSM). MATERIALS AND METHODS: • Regional differences in the expression of caveolin family members were determined by quantitative reverse transcriptase-polymerase chain reaction and Western blot of RNA and protein extracted from the base, body and dome of rat bladders. • To evaluate the distribution of caveolin-1 (Cav-1), Cav-2 and Cav-3 within the bladder, longitudinal tissue sections from the base to dome were processed for confocal microscopy and quantified for intensity of immunoreactivity (IR) and extent of co-localisation. • Interactions among Cav-1, Cav-2 and Cav-3 were determined by co-immunoprecipitation. RESULTS: • Differential expression of Cav-1 and Cav-3 was detected among bladder regions, with lowest expression in the bladder base relative to the dome. • Cav-1 was highly expressed in all regions, although an increase in IR from submucosa to serosa was detected in each region. • The distribution of Cav-2 IR generally paralleled Cav-1, but progressively decreased from submucosa to serosa in each region. • Cav-3 expression predominated in the medial region of BSM increasing progressively from base to dome, but was poorly expressed in the outer SM layer particularly in the dome. • Cav-1 co-precipitated extensively with both Cav-2 and Cav-3. Co-precipitation between Cav-3 and Cav-2 was also detected. CONCLUSIONS: • The isoform-specific spatial distribution and distinct molecular interactions among caveolins in BSM may contribute to the contractile heterogeneity of BSM cells and facilitate differential modulation of responses to local stimuli. • As BSM caveolae regulate key signalling processes involved in contraction, altered expression of caveolin proteins may generate a regional imbalance in contraction/relaxation responses, thus leading to bladder dysfunction.