Ablation of the Semaphorin 3F‐Neuropilin 2 Axis Increases Contractility in Bladder Smooth Muscle

Impairment of the strength or duration of bladder smooth muscle (SM) contraction characterizes a prevalent but poorly understood clinical condition termed detrusor underactivity (DU). DU is a complex disorder that arises from diverse neuromuscular insults. DU reduces voiding efficiency and can lead to significant urologic complications. Despite significant morbidity, no effective treatment exists for this condition. Neuropilins (Nrp) are transmembrane receptors that bind class 3 semaphorin (Sema3) proteins. We identified bladder SM as a major site of expression of Nrp2 in vivo , and showed that bladder SM strips from mice with genetic deletion of Nrp2 displayed increased contractility compared to those from wild‐type littermates, identifying Nrp2 as a negative regulator of SM contractility. In this study we expanded our analysis of this signaling axis by examining the localization of the Nrp2 ligand, Sema3F, in bladder tissue and compared the contractility of bladder SM in Sema3F knockout mice compared to wild‐type mice. We tested the hypothesis that Nrp2 deletion in vivo improves bladder SM function under conditions of DU. Methods Nrp2 and Sema3F protein expression was localized by immunohistochemistry. Partial bladder outlet obstruction (pBOO) was created surgically in male mice at 3 mos of age by urethral occlusion. We used SM22a‐CreER T2 :Nrp2 fl/fl mice to achieve tamoxifen‐inducible SM‐specific Nrp2 knockout either before creation of pBOO or during the decompensation phase following pBOO. Nrp2‐expressing mice ( Nrp2 fl/fl mice) were used as controls and sham surgery was performed to control for pBOO. Bladder SM contractility was assessed by isometric tension testing. Results Sema3F expression was found in the urothelium and Nrp2 expression was highest in the bladder SM, suggesting a paracrine signaling axis. Sema3F deletion resulted in increased contractility of bladder SM similar to that seen in Nrp2 KO mice. After pBOO, bladder weight increased in both Nrp2‐intact and Nrp2‐deleted mice. Bladder SM from mice in which Nrp2 was deleted prior to pBOO displayed increased contractility at all time points after pBOO compared to tissues from Nrp2‐intact mice. Bladder SM from mice in which Nrp2 was deleted during decompensation showed increased force of contraction compared to obstructed mice in which Nrp2 was not deleted (p<0.05). Conclusions These observations demonstrate that downregulation of Nrp2 by genetic deletion promotes increased bladder SM contractility under conditions of chronic bladder outlet obstruction. Importantly, they show that deletion of Nrp2 in mice that have already undergone bladder decompensation can restore SM contractile function, suggesting that Nrp2 may be a therapeutic target in conditions characterized by detrusor underactivity. Support or Funding Information Vascular Biology Program, Boston Children's Hospital; NIDDK DK65298