Botulinum neurotoxins: Future innovations

Abstract

Botulinum neurotoxins (BoNTs) are multi-domain proteins whose potent and selective actions on nerve endings have led to innovations in both basic and clinical science. The various BoNT domains are responsible for binding to gangliosides and proteins associated with nerve cell membranes, internalization into the cell, and cleavage of one or more SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) proteins necessary for vesicle docking and fusion. Novel modifications to BoNT molecules, such as the creation of chimeras, helped identify the protein domains responsible for various aspects of BoNT action, such as localized effects. Other molecular modifications have been introduced in attempts to increase the specificity of BoNTs for autonomic or sensory neurons, with the ultimate goal of optimizing therapeutic selectivity. This research, in turn, has led to the development of BoNT-based proteins that can target non-SNARE substrates such as phosphatase and tensin homolog (PTEN). Still others are developing different BoNT serotypes, subtypes, or variants that are longer- or shorter-acting or have faster onset for various clinical purposes. New formulations of BoNTs that provide convenience for both patients and physicians are under investigation. Novel clinical uses are being evaluated for onabotulinumtoxinA, including in the prevention of post-operative atrial fibrillation. All these innovations capitalize on the unique properties of BoNTs, which continue to intrigue scientists and clinicians across numerous fields of study.

Figure 1.

Ribbon diagram of botulinum neurotoxin A, subtype 1 (BoNT/A1), showing the organization of individual domains. The C-terminal portion of the heavy chain (H_CC; yellow) binds to nerve terminals, its N-terminal portion (H_CN; gold) contributes to internalization and the H_N domain (blue) translocates the light chain (LC; red) into the cytosol; the L_C is a zinc-dependent metalloprotease (Zn2+ shown in teal) that cleaves specific SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins that mediate vesicle fusion with membranes. A peptide belt (green) surrounds the LC, and a disulfide bond (magenta) links the LC to HN domain. Image of 3BTA (Lacy DB, Tepp W, Cohen AC, DasGupta BR, Stevens RC. Crystal structure of botulinum neurotoxin type A and implications for toxicity. Nat Struct Biol. 1998;5(10):898-902) created with Discovery Studio 2017 R2 (BIOVIA, Dassault Systèmes).

Figure 2.

Cleavage sites of BoNT serotypes on SNAP-25, VAMP-2, and syntaxin. (adapted from von Berg L, Stern D, Pauly D, Mahrhold S, Weisemann J, Jentsch L, Hansbauer EM, Müller C, Avondet MA, Rummel A, Dorner MB, Dorner BG. Functional detection of botulinum neurotoxin serotypes A to F by monoclonal neoepitope-specific antibodies and suspension array technology. Sci Rep. 2019 Apr 2;9(1):553; licensed under CC by 4.0 https://creativecommons.org/licenses/by/4.0/).

Figure 3.

Prof. J. Oliver Dolly in his laboratory, Sept. 2019.

Figure 4.

Drs. Amy Brideau-Andersen and Mitchell Brin, March 2022.

Figure 5.

Reported Actions of BoNT/A on Skin and Skin Cells. BoNT/A has been explored in mild facial lines,^[72] postoperative scarring,^[73] excessive sweating,^[74] rosacea,^[75,76] skin sebum production,^[–79] biomechanical properties of skin,^[80,81] chronic pruritis,^[82] Hailey-Hailey disease,^[83] plantar keratoderma,^[84] keloids,^[85] and psoriasis.^[–88]* In dermal fibroblasts, BoNT/A inhibits cell senescence and promotes collagen production.^[89,90] In myofibroblasts, BoNT/A inhibits cell proliferation, contraction, and collagen production.^[70,91] *This is not a comprehensive list; the reader is referred to several recent reviews.^[–94]