Pre- and Post-synaptic Effects of Botulinum Toxin A on Submandibular Glands

Area of Science:

• Salivary gland physiology
• Neurotoxin research
• Molecular biology

Background:

• Botulinum toxin type A (BoNT/A) is clinically used for sialorrhea (excessive salivation).
• Previous research focused on BoNT/A's effect on saliva secretion, but its direct impact on salivary gland cells and molecular mechanisms remained unclear.
• The role of synaptosomal-associated protein 25 (SNAP-25) proteolysis and aquaporin 5 (AQP5) distribution in BoNT/A-induced salivary inhibition needed further investigation.

Purpose of the Study:

• To investigate the molecular mechanisms underlying BoNT/A-induced inhibition of saliva secretion in rat submandibular glands (SMG).
• To confirm SNAP-25 cleavage in SMG after BoNT/A injection.
• To determine if BoNT/A has direct effects on acinar cells, specifically on AQP5 expression and localization.

Main Methods:

• Intraglandular injection of BoNT/A into rat SMG.
• Western blotting to detect SNAP-25 cleavage and AQP5 protein levels.
• In vitro studies using GFP-AQP5-transfected SMG-C6 cells to assess direct BoNT/A effects.
• Cell surface biotinylation and immunofluorescence to analyze AQP5 distribution.

Main Results:

• BoNT/A injection into rat SMG dose-dependently decreased salivary flow for at least 4 weeks, with recovery by 12 weeks.
• SNAP-25 cleavage was observed in SMG neurons and synapses, indicating presynaptic inhibition.
• BoNT/A treatment led to AQP5 downregulation and redistribution from the cell membrane to the cytoplasm in SMG acinar cells, suggesting postsynaptic effects.

Conclusions:

• BoNT/A reversibly inhibits saliva secretion in rat SMGs via presynaptic SNAP-25 cleavage and postsynaptic AQP5 redistribution.
• This study provides the first evidence for a direct effect of BoNT/A on salivary gland acinar cells, specifically impacting AQP5 localization.
• Understanding these dual mechanisms enhances knowledge of BoNT/A's therapeutic action in sialorrhea.