Absorption and Transport of Botulinum Neurotoxins

Abstract

Botulinum neurotoxin (BoNT) is a potent toxin, which blocks the neurotransmitter release at neuromuscular junctions. BoNT can be acquired from the digestive tract (food-borne botulism, Clostridium botulinum intestinal colonization), respiratory tract (inhalational botulism), or wound (wound botulism). BoNT associates to nontoxic proteins (ANTPs), which have a main role in toxin protection against acidic pH and proteases, especially in the gastrointestinal tract. BoNT, which enters through the digestive or respiratory tract, has to first cross the epithelial barrier. This is achieved by a receptor-mediated transcytosis, which delivers the whole and active toxin at the basolateral side of epithelial cells. ANTPs containing hemagglutinins (HAs) may have an additional role in altering the intercellular junctions and facilitating toxin passage through the paracellular way. Then, BoNT disseminates locally and at distance via the blood/lymph circulation and possibly via a retrograde axonal transport to the target motor neuron endings, where the toxin uses an endocytic pathway permitting the release of the light (L)-chain into the cytosol and its subsequent proteolytic activity towards the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins involved in the neurotransmitter exocytosis.

Translocation and dissemination to target neurons of botulinum neurotoxin type B in the mouse intestinal wall

“…Minimum amounts of fluorescent HCcB yielding detectable signal in intestinal tissues were around 10 μg per intestinal loop (Fig. Moreover, it has to be taken into account that BoNT/B is less active than BoNT/A, about 10-fold less (Foran et al, 2003;Rasetti-Escargueil et al, 2009), and that only a low toxin fraction passes through the intestinal barrier based on in vivo experiments (reviewed in Popoff and Connan, 2014), therefore justifying the amounts used in this study. We selected to use higher amounts (100 μg/intestinal loop) to more efficiently visualize the cells mediating toxin trafficking.…”

“…In contrast to BoNT/A, which has been found to enter the intestinal mucosa preferentially via crypt enteroendocrine cells (Couesnon et al, 2012), no HCcB or BoNT/B labelling was detected in epithelial crypt cells, or enteroendocrine cells using anti-chromogranin A antibodies. In vivo experiments showed that less than 0.01% to 0.1% of intraduodenally administrated BoNT can be recovered in lymph of blood circulation (reviewed in Popoff and Connan, 2014). devoid of apical brush border).…”

“…However, we were unable to reproduce the results with 0.6 μg BoNT/A per mouse. Moreover, it has to be taken into account that BoNT/B is less active than BoNT/A, about 10-fold less (Foran et al, 2003;Rasetti-Escargueil et al, 2009), and that only a low toxin fraction passes through the intestinal barrier based on in vivo experiments (reviewed in Popoff and Connan, 2014), therefore justifying the amounts used in this study. However, one cannot fully exclude that in the present work; HCcB or BoNT/B uses different entry pathways than in the in vivo situation.…”

“…The absorption of BoNT from the digestive tract is a very low efficient process. In vivo experiments showed that less than 0.01% to 0.1% of intraduodenally administrated BoNT can be recovered in lymph of blood circulation (reviewed in Popoff and Connan, 2014). Thus, a low-toxin passage through the enterocytes or at least some of them may support the weak intestinal absorption of toxin, which has been observed in vivo.…”

Neuronal selectivity of botulinum neurotoxins

“…BoNT complexes containing hemagglutinins (HA) can be transported through M cells and since HAs are able to disrupt the E-cadherin intercellular junctions between intestinal epithelial cells, BoNT can pass the intestinal barrier via the paracellular pathway. In addition, BoNT free of HAs can be transported by transcytosis through certain intestinal epithelial cells (Fujinaga and Popoff, 2018;Lee et al, 2014;Matsumura et al, 2008;Matsumura et al, 2015;Popoff and Connan, 2014;Sugawara et al, 2010). Using fluorescent BoNT/A and BoNT/B or their corresponding receptor binding domains, it has been found that these molecules interact with intestinal epithelial cell surface, but with a much lower affinity than on neuronal cells.…”