SEARCH

SEARCH BY CITATION

Sir,

We read with interest the comprehensive review [1] compiling the evidence on the efficacy of botulinum neurotoxin A (BoNT/A) in the treatment of either neurogenic or idiopathic detrusor overactivity. In light of the recent advances in the understanding of the actions of BoNT/A, we summarize its molecular mechanisms. Considerable debate continues on the interaction of BoNT/A at cellular and subcellular levels, the final event often being the inhibition of release of pre-synaptic vesicular acetylcholine. The putative actions of BoNT/A involve inhibition of vesicular release of ATP, substance P, and at the cellular level, the decreased expression of the capsaicin TPRV1, purinergic P2X3 receptors on the urothelium and suburothelial nerve endings, and P2Y receptors on bladder myofibroblasts [2,3]. Acetylcholine is packed in vesicles that interact with the neuronal membrane at the presynaptic junction during exocytosis via intracellular SNARE proteins such as SNAP-25, VAMP (also known as synaptobrevin) and syntaxin. BoNT/A, because of its proteolytic action on SNAP-25, destabilizes the interaction between synaptic vesicles and the pre-synaptic plasma membrane, thereby inhibiting the exocytosis of acetylcholine-containing synaptic vesicles. In addition, the cleavage of SNAP-25 by BoNT/A probably results in decreased expression of TPRV1 [4] and perhaps even the P2X3 receptors modulating the sensory pathways in the micturition reflex [3]. This is the most likely explanation for its remarkable effectiveness in relieving urgency.

Although the mechanisms of entry of the BoNT/A into the synaptic terminal via clathrin-mediated endocytosis had been described, the prime event of interaction of BoNT/A at a specific receptor was not known until the recent experiments conducted by Dong et al.[5]. Using cultured rat hippocampal neurones it was shown that BoTN/A was taken up at the presynaptic terminal in an activity-dependent manner, confirming the existence of BoNT/A receptors at the level of synaptic vesicle. Immunochemistry confirmed the receptor as SV2 (three isoforms exist) which formed a stable complex with BoNT/A. BoNT/A binding to SV2A and SV2B ‘knockout’ hippocampal neurones was abolished and was restored by expressing SV2A, SV2B or SV2C. Mice that lacked an SV2 isoform (SV2B) had reduced sensitivity to BoNT/A, providing further evidence that SV2 acts as the protein receptor for BoNT/A. The SV2 receptor is a conserved integral protein localized to synaptic and endocrine vesicles, with the BoNT/A binding moiety localized in the luminal surface of the vesicle. During exocytosis the active region of the SV2 receptor is exposed to BoNT/A followed by the internalization of the toxin, destabilizing the SNARE complex and thus inhibiting the release of acetylcholine. Ca2+ is instrumental to the process of synaptic transmission and it was suggested that the SV2 receptor as a transporter of cations, including Ca2+, by virtue of the presence of paired negative charges in its transmembrane domain, might be instrumental in facilitating this process. The presence of SV2 receptors in rat suburothelial nerve endings was reported by Hansen et al.[6]. Although there is a clear relationship between purinergic receptors and SV2 receptors in the suburothelium, there has been very little emphasis on the functional and/or anatomical role of these receptors in the bladder. As the efficiency of BoNT/A has been well documented in various clinical trials, cited by the authors, the next step should be directed towards exploring the interaction between SV2 receptor and BoNT/A in patients with overactive bladders, which might open the gateway for new pharmacological interventions. Assuming the effects of BoNT/A are mainly, if not exclusively, via the SV2 receptor, it might be possible to evaluate patients not responding to BoNT/A for the absence of SV2 receptors, thus avoiding unnecessary intervention and perhaps looking for alternative therapies.