Botulinum Toxin Type A Inhibits Submandibular Secretion via the ERK/miR-124-3p/Specificity Protein 1/Claudin-1 Axis

Botulinum toxin type A (BTXA) is widely used for the treatment of sialorrhea; however, its mechanism remains unclear. Tight junctions (TJs) are limiting factors for salivary secretion through the paracellular pathway in the salivary gland, among which claudin-1 (Cldn1) is a TJ protein that mainly plays a barrier role. This study observed that Cldn1 was upregulated in BTXA-treated rats' submandibular glands and SMG-C6 cells. Knockdown of Cldn1 reversed the BTXA-induced reduction in paracellular permeability. The transcription factor specificity protein-1 (Sp1), which binds to the Cldn1 promoter, was also upregulated by BTXA, and its expression was linked to the ERK1/2 pathway. Inhibition of ERK1/2 by U0126 reversed the BTXA-induced upregulation of Sp1 and Cldn1, as well as the reduction in paracellular permeability. MiR-124-3p, which directly targets Sp1, was downregulated by BTXA, but its overexpression counteracted Sp1 and Cldn1 upregulation. Although miR-124-3p did not affect ERK1/2 phosphorylation, ERK1/2 inhibition reversed the BTXA-induced decrease in miR-124-3p expression. These findings reveal a regulatory pathway through which BTXA reduces paracellular permeability in SMG-C6 cells via the ERK1/2/miR-124-3p/Sp1/Cldn1 axis.