Aquaporin 5 (AQP5) is a water channel protein that modulates transcellular water permeability and plays an important role in salivary secretion [1]. In salivary glands, AQP5 translocates from intracellular vesicles to the apical membrane of acinar cells upon stimulation, thereby increasing water permeability. The effective trafficking of AQP5 is critical for salivary secretion, while its abnormal trafficking is associated with hyposalivation [2]. Therefore, understanding the molecular mechanisms of AQP5 trafficking is essential for a better understanding of the pathophysiology of related disorders and identify potential new therapeutic targets. AQP5 trafficking is a process that involves membrane fusion between the vesicles and apical plasma membranes. Membrane fusion requires the interplay of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins on the vesicles (v-SNAREs) and target membranes (t-SNAREs) [3,4]. One of the best-studied models of membrane fusion is synaptic vesicle exocytosis, a process that depends on the formation of SNARE complex composed of one v-SNARE, namely vesicle-associated membrane protein (VAMP) 2 and two t-SNAREs, namely syntaxin 1 and synaptosomal-associated protein (SNAP) 25 [5]. However, the role of SNARE proteins in AQP5 trafficking remains unknown.

The SNAP25 family consists of SNAP25, SNAP23, SNAP29, and SNAP47 [6]. Among them, only SNAP25 and SNAP23 have been regarded as regulators in exocytosis. SNAP25 regulates synaptic vesicle exocytosis in the nervous system, whereas SNAP23 drives exocytosis events in diverse nonneuronal cells, including dense core granule release in platelets [7], degranulation in mast cells [8], and GLUT4 transport in adipocytes [9]. In salivary gland acinar cells, SNAP23 promotes the fusion of secretory granules to the plasma membrane [10]. In addition, the interaction of SNAP23 with VAMP2 plays a key role in cAMP-mediated amylase release from acinar cells in rat parotid glands [11]. SNAP23 also plays an important role in regulated exocytosis by forming a complex with syntaxin 4 and VAMP8 in mice salivary gland acinar cells [12]. Based on these findings, we hypothesize that SNAP23 may be one of the t-SNAREs involved in AQP5 trafficking in acinar cells of salivary glands.

Botulinum toxin type A (BoNT/A), a neurotoxin in widespread clinical use, effectively treats sialorrhea, but its mechanism remains unclear [13,14]. Traditionally, the mechanism of action of BoNT/A is to cleave SNAP25, a t-SNARE necessary for synaptic vesicle exocytosis, thereby inhibiting neurotransmitter release from nerve endings and leading to chemical denervation [15]. However, several studies have shown direct inhibitory effects of BoNT/A on target tissues, including muscle and bladder [16,17]. Furthermore, recent studies have shown that BoNT/A regulates glutamate release and alleviates neuralgia and neuroinflammation by decreasing SNAP23 expression [[18], [19], [20]], suggesting that BoNT/A has other mechanisms of action by targeting SNAP23. Our previous work investigated the effect of BoNT/A on the rat submandibular gland (SMG) and found that BoNT/A decreased salivary flow rate by cleaving SNAP25 at the neuroglandular junction in SMGs [21]. Interestingly, we also demonstrated that BoNT/A induced redistribution of AQP5 in cultured acinar cells [21]. If SNAP23 plays a key role in AQP5 trafficking as hypothesized above, BoNT/A may directly affect AQP5 trafficking by targeting SNAP23 in addition to causing chemical denervation of SMGs. Further experiments are needed to test this hypothesis.

The present study was designed to explore the potential role of SNAP23 in modulating AQP5 trafficking in acinar cells and to elucidate the effect of BoNT/A on AQP5 trafficking and its mechanism. The results should help to demonstrate the critical role of SNAP23 in AQP5 trafficking and provide a deeper understanding of the mechanism of BoNT/A in the treatment of sialorrhea.