Emerging opportunities for serotypes of botulinum neurotoxins

Abstract

Background: Two decades ago, botulinum neurotoxin (BoNT) type A was introduced to the commercial market. Subsequently, the toxin was approved by the FDA to address several neurological syndromes, involving muscle, nerve, and gland hyperactivity. These syndromes have typically been associated with abnormalities in cholinergic transmission. Despite the multiplicity of botulinal serotypes (designated as types A through G), therapeutic preparations are currently only available for BoNT types A and B. However, other BoNT serotypes are under study for possible clinical use and new clinical indications;

Objective: To review the current research on botulinum neurotoxin serotypes A-G, and to analyze potential applications within basic science and clinical settings;

Conclusions: The increasing understanding of botulinal neurotoxin pathophysiology, including the neurotoxin's effects on specific neuronal populations, will help us in tailoring treatments for specific diagnoses, symptoms and patients. Scientists and clinicians should be aware of the full range of available data involving neurotoxin subtypes A-G.

Figure 1

(A) Illustration of molecular structure of Botulinum Neurotoxin type B (BoNT/B). Botulinum neurotoxin is composed of a heavy chain (Hc) attached to a light chain (Lc) by a non-covalent disulfide bridge (Cys⁴⁴⁵–Cys⁴³⁶). Hc is divided in an amino region (Hcn) and a carboxyl region (Hcc). Hcc is the binding domain; Hcn is the translocation domain, and Lc is the catalytic domain [16].The asterisk (*) indicates the helix of Lc containing the motif HEXXH;(B)A simplified illustration of the BoNT/B structure. Figure 1A was modified from the RCSBProtein Data Bank [20].The molecular modeling was refined with PyMOL Molecular Graphics System, Version 1.2r3pre, Schrödinger, LLC.

Figure 2

Commercial preparations of botulinum neurotoxins include the botulinum neurotoxin molecule, and non-toxic haemagglutinin proteins (HP) and non-haemagglutinin proteins (NHP), plus excipients.The associated proteins are thought to stabilize the neurotoxin [21].*Proteins complexes are not present in IncobotulinumtoxinA, which is a formulation of botulinum toxin serotype A [22].

Figure 3

Array of proteins associated with the presynaptic vesicle membrane [32]. The figure illustrates a suggested complex array of proteins that are anchored to the membrane of the vesicle that is exposed to the presynaptic terminal during the process of vesicle recycling.

Figure 4

Neuromuscular Transmission: The role of the SNARE complex in acetylcholine release. The action potential travels along the presynaptic neuron and leads to intracellular calcium influx; this induces the adherence of vesicles charged with acetylcholine (Ach) to the presynaptic terminal membrane, where the SNARE protein complex participates in the vesicle exocytosis and release of Acetylcholine (Ach) to the neuromuscular junction. Ach molecules activate the postsynaptic nAChR (N1 Subtype), and open the central ion channel allowing a net inward flux of sodium whichleads to muscle contraction.

Figure 5

Botulinum neurotoxin blocking Ach release from the neuromuscular junction. The heavy chain (Hc) of neurotoxin binds to the presynaptic array of receptors exposed during vesicle recycling. The carboxyl terminal of Hc binds to specific gangliosides and proteins associated with the vesicle membrane (Syt and SV2 isoforms). Lateral movements of the exposed membrane increase the contact of BoNT with more proteins of the presynaptic receptor complex, and the molecule is endocytosed. The intravesicle acidification dissociates the Lc from Hc, and then the amino region of the heavy chain participates in the translocation of Lc into the cytoplasm to cleave specific proteins from the SNAREv and SNAREt complex according the neurotoxin subtype. BoNT/A, C, E target SNAP-25, BoNT/C also cleave syntaxin and BoNT/B, D, F, G target VAMP. The final result is blocking of the Ach release, preventing postsynaptic receptors activation.

Figure 6

Representation of neutralizing and non-neutralizing antibodies against BoNT/A, and BoNT/B [106]. Neutralizing antibodies against BoNT/A <1> and B <2> affect neurotoxin heavy chain carboxyl terminal (Hcc), preventing its binding to presynaptic cells. Antibodies lacking neutralizing effects against BoNT/A <4> and BoNT/B <3> do not affect neurotoxin binding. According to somestudies, cross antibodies can recognize BoNT/A and BoNT/B <5,6>, but they do not affect neurotoxin binding [106]. A recent study reported neutralizing cross-reacting antibodies between neurotoxin serotypes <7> [107].