Light Chain Diversity among the Botulinum Neurotoxins

Abstract

Botulinum neurotoxins (BoNT) are produced by several species of clostridium. There are seven immunologically unique BoNT serotypes (A–G). The Centers for Disease Control classifies BoNTs as ‘Category A’ select agents and are the most lethal protein toxins for humans. Recently, BoNT-like proteins have also been identified in several non-clostridia. BoNTs are di-chain proteins comprised of an N-terminal zinc metalloprotease Light Chain (LC) and a C-terminal Heavy Chain (HC) which includes the translocation and receptor binding domains. The two chains are held together by a disulfide bond. The LC cleaves Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). The cleavage of SNAREs inhibits the fusion of synaptic vesicles to the cell membrane and the subsequent release of acetylcholine, which results in flaccid paralysis. The LC controls the catalytic properties and the duration of BoNT action. This review discusses the mechanism for LC catalysis, LC translocation, and the basis for the duration of LC action. Understanding these properties of the LC may expand the applications of BoNT as human therapies.

AbobotulinumtoxinA (Dysport®), OnabotulinumtoxinA (Botox®), and IncobotulinumtoxinA (Xeomin®) Neurotoxin Content and Potential Implications for Duration of Response in Patients

“…The ability of the BoNT-A to bind, internalise, and deliver the LC into the target neuron plays a role, as does the proteolytic activity of the LC (the rate at which it cleaves its SNARE (Soluble NSF Attachment Protein) REceptor) protein target and thus blocks neurotransmitter release). The quantity of BoNT-A available also significantly impacts therapeutic effectiveness: the greater the number of 150 kDa BoNT-A molecules, the greater the ability to cleave SNARE substrates [21]. The total Clostridial protein content of the three main commercially available BoNT-A products—Dysport ® , Botox ® , and Xeomin ® —has been previously reported in the literature, giving values of 5 ng per 100 U vial of Botox, 4.35 ng per 500 U vial of Dysport (correcting an earlier publication stating 12.5 ng per 500 U vial), and Xeomin containing 0.6 ng per 100 U vial [13,22,23].…”

Botulinum Neurotoxin Light Chains Expressed by Defective Herpes Simplex Virus Type-1 Vectors Cleave SNARE Proteins and Inhibit CGRP Release in Rat Sensory Neurons

“…In addition, variability between the different BoNTs could be due at least in part to differences at the level of toxin binding and penetration, which relies on the properties of the heavy chain. It should be noted however that exogenously administrated holotoxins and vector-induced synthesis of transgenic LC could differ in several quantitative and qualitative ways, including expression, concentration, localization, trafficking, solubility, and ubiquitin-mediated degradation [ 47 ]. Indeed these differences could explain some discrepancies between our results and those obtained using holotoxins [ 47 ].…”

“…It should be noted however that exogenously administrated holotoxins and vector-induced synthesis of transgenic LC could differ in several quantitative and qualitative ways, including expression, concentration, localization, trafficking, solubility, and ubiquitin-mediated degradation [ 47 ]. Indeed these differences could explain some discrepancies between our results and those obtained using holotoxins [ 47 ]. Botulinum neurotoxins, both when secreted by bacteria through natural infection or injected for clinical or cosmetic indications, attach to the synaptic membrane of neurons and are internalized into endosomes.…”

Emerging Opportunities in Human Pluripotent Stem-Cells Based Assays to Explore the Diversity of Botulinum Neurotoxins as Future Therapeutics

“…The HC is divided into two sub-domains including a C-terminal specific neuronal receptor binding domain and a translocation domain. The LC is a zinc endopeptidase that can cleave one or more specific proteins of the neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex [ 24 , 25 , 30 , 31 ]. Neuronal SNARE proteins include syntaxin, synaptosomal-associated protein 25 KDa (SNAP25), and vesicle-associated membrane protein (VAMP, also called synaptobrevin).…”