Identification of Fibroblast Growth Factor Receptor 3 (FGFR3) as a Protein Receptor for Botulinum Neurotoxin Serotype A (BoNT/A)

Abstract

Botulinum neurotoxin serotype A (BoNT/A) causes transient muscle paralysis by entering motor nerve terminals (MNTs) where it cleaves the SNARE protein Synaptosomal-associated protein 25 (SNAP25206) to yield SNAP25197. Cleavage of SNAP25 results in blockage of synaptic vesicle fusion and inhibition of the release of acetylcholine. The specific uptake of BoNT/A into pre-synaptic nerve terminals is a tightly controlled multistep process, involving a combination of high and low affinity receptors. Interestingly, the C-terminal binding domain region of BoNT/A, HC/A, is homologous to fibroblast growth factors (FGFs), making it a possible ligand for Fibroblast Growth Factor Receptors (FGFRs). Here we present data supporting the identification of Fibroblast Growth Factor Receptor 3 (FGFR3) as a high affinity receptor for BoNT/A in neuronal cells. HC/A binds with high affinity to the two extra-cellular loops of FGFR3 and acts similar to an agonist ligand for FGFR3, resulting in phosphorylation of the receptor. Native ligands for FGFR3; FGF1, FGF2, and FGF9 compete for binding to FGFR3 and block BoNT/A cellular uptake. These findings show that FGFR3 plays a pivotal role in the specific uptake of BoNT/A across the cell membrane being part of a larger receptor complex involving ganglioside- and protein-protein interactions.

Uptake and transport of clostridial neurotoxins

“…In agreement, several research groups have demonstrated that BoNT/A is retrogradely transported in different neuronal types, including hippocampal, tectal, and motor neurons [113,114], and undergoes transcytosis in the visual system [115,116]. Additionally, FGFR3 was identified as a new protein receptor for BoNT/A in resting conditions [117]. Since FGRF3 may undergo receptor-mediated endocytosis followed by long-range transport upon stimulation with its ligand, it might be able to mediate the uptake, sorting, and transport of BoNT/A along its trafficking route.…”

Characterization of Botulinum Neurotoxin A Subtypes 1 Through 5 by Investigation of Activities in Mice, in Neuronal Cell Cultures, and In Vitro

“…While it has previously been reported and is widely accepted that SV2 is the primary protein receptor for BoNT/A1, with SV2C having the highest affinity (42), FGFR3 has recently been reported to be a higher-affinity receptor for BoNT/A1 than SV2 (45). Thus, future studies are required to determine whether the differences in A3 and A5 activity in human cells are due to differential SV2 receptor expression in the different NCB models or to other factors such as other protein receptors, ganglioside binding, glycosylation, or neuronal activity.…”

The C-Terminal Heavy-Chain Domain of Botulinum Neurotoxin A Is Not the Only Site That Binds Neurons, as the N-Terminal Heavy-Chain Domain Also Plays a Very Active Role in Toxin-Cell Binding and Interactions

“…Besides, it is not easy to predict the structural and conformational changes induced both in the molecule and at the membrane surface (at various stages), making it difficult to assert any strong assumptions regarding the role of various domains and the receptors involved. This has been proved repeatedly where new receptors and factors influencing toxin internalization are identified (59)(60)(61)(62) with an increased understanding of the toxinhost interaction. It is really necessary to confirm the data obtained from such studies in vivo before making any strong interpretations.…”