Interaction of botulinum toxin with the epithelial barrier

Abstract

Botulinum neurotoxin (BoNT) is a protein toxin (approximately 150 kDa), which possesses a metalloprotease activity. Food-borne botulism is manifested when BoNT is absorbed from the digestive tract to the blood stream and enters the peripheral nerves, where the toxin cleaves core proteins of the neuroexocytosis apparatus and elicits the inhibition of neurotransmitter release. The initial obstacle to orally ingested BoNT entering the body is the epithelial barrier of the digestive tract. Recent cell biology and molecular biology studies are beginning to elucidate the mechanism by which this large protein toxin crosses the epithelial barrier. In this review, we provide an overview of the structural features of botulinum toxins (BoNT and BoNT complex) and the interaction of these toxins with the epithelial barrier.

Figure 1

Botulinum neurotoxin complexes and food-borne botulism. (a) Schematic structure of botulinum neurotoxin (BoNT) complexes. (b) The pathway followed by BoNT complexes from the lumen of the intestinal tract to the cytosol of the peripheral nerve terminal in food-borne botulism. Orally ingested BoNT complexes (12S and 16S toxins) must cross the intestinal epithelial barrier to cause the food-borne botulism. After absorption from the small intestine, the botulinum neurotoxin complexes enter the lymphatic system, then the blood stream [3, 8]. In the lymphatic circulation and blood, BoNT exists as a free form dissociated from the complex [3, 9] and binds specifically to neurons [2, 10]. Inhibition of neurotransmitter release occurs via a four-step mechanism, (1) binding, (2) endocytosis, (3) translocation, and (4) cleavage of the SNARE proteins [2].

Figure 2

Features of types A to G BoNT complexes. Type A (A1) BoNT is produced by C. botulinum in three forms: 12S, 16S, and 19S toxins. Types B, C, and D BoNT are produced in two forms: 16S and 12S toxins. Types E, F, (and A2) BoNT are produced as 12S toxin. Type G BoNT complex is produced as 16S toxin (reviewed in [3, 6, 7]).

Figure 3

Model for penetration of BoNT complexes through the intestinal epithelial barrier. Step 1: HA of the BoNT complex mediates binding and transcytosis of a small amount of luminally located BoNT complexes across the epithelium without disrupting the epithelial barrier (Arrows in yellow, “route i”) [65]. H chain of BoNT also mediates binding and transcytosis of BoNT (Arrows in green, “route ii”) [–66]. Step 2: HA which has translocated on the basolateral surface disrupts the epithelial barrier. Types A and B HA proteins disrupt the paracellular barrier of without causing cytotoxic effects in the epithelial cells of their susceptible hosts [65, 77]. Type C HA proteins possibly evoke cytotoxic-barrier disrupting activity in the epithelial cells of susceptible animals [77]. Step 3: A large amount of the BoNT complexes and BoNT accumulates in the serosal side by passing from the damaged epithelial barrier (Arrows in light green, “route iii”) [65].