Fragment C of tetanus toxin: new insights into its neuronal signaling pathway

Abstract

When Clostridium tetani was discovered and identified as a Gram-positive anaerobic bacterium of the genus Clostridium, the possibility of turning its toxin into a valuable biological carrier to ameliorate neurodegenerative processes was inconceivable. However, the non-toxic carboxy-terminal fragment of the tetanus toxin heavy chain (fragment C) can be retrogradely transported to the central nervous system; therefore, fragment C has been used as a valuable biological carrier of neurotrophic factors to ameliorate neurodegenerative processes. More recently, the neuroprotective properties of fragment C have also been described in vitro and in vivo, involving the activation of Akt kinase and extracellular signal-regulated kinase (ERK) signaling cascades through neurotrophin tyrosine kinase (Trk) receptors. Although the precise mechanism of the molecular internalization of fragment C in neuronal cells remains unknown, fragment C could be internalized and translocated into the neuronal cytosol through a clathrin-mediated pathway dependent on proteins, such as dynamin and AP-2. In this review, the origins, molecular properties and possible signaling pathways of fragment C are reviewed to understand the biochemical characteristics of its intracellular and synaptic transport.

Keywords: Trk receptors; clathrin-mediated pathway; dynamin; fragment C; neurotrophin; tetanus toxin.

Figure 1

Diagram of the tetanus toxin molecule. The targeting and the translocation domains are located in the heavy-chain (HC), whereas the catalytic domain is located in the light-chain (LC) of the molecule. Its proteolytic activity is Zn²⁺-dependent, and heavy-metal chelators generate inactive apo-neurotoxins. The position of the cleavage of the tetanus-toxin molecule by papain is indicated. The digestion yields two fragments; one of them, fragment C, is approximately 50 kDa [29].

Figure 2

Proposed internalization pathway of tetanus toxin. The ganglioside-recognition domain in the C-terminal region of HC allows the toxin to be internalized into the neuron. The light chain of the toxin (LC) cleaves the soluble NSF attachment receptor (SNARE) complex, inhibiting neurotransmitter release.

Figure 3

A schematic representation of the structures of gangliosides GT1b, GM1 and GD1b. The strongest and most specific ganglioside association with fragment C of tetanus toxin is with GT1b, since the targeting domain of the toxin contains two binding sites that can accommodate NeuAc residues.

Figure 4

(a) Image from the RCSB PD of PDB ID 1A8D [47]; (b) Illustration of the carbohydrate-binding sites for lactose (LAC), galactose (GAL), sialic acid (SIA) and N-acetyl-galactosamine (NGA) in tetanus toxin. These binding sites are localized in the beta-trefoil domain of fragment C (residues 865–1315). Adapted from [40].

Figure 5

(a) Image from the RCSB PDB of PDB ID 1FV3 regarding Site-1 of fragment C [51]; (b) Image from the RCSB PDB of PDB ID 1YYN regarding Site-2 of fragment C [52].

Figure 6

Proposed pathway for the hybrid protein β-Gal-fragment C. Once the hybrid protein is injected intramuscularly, it is found in large uncoated vesicles and then transported retrogradely to the endoplasmic reticulum (ER). Hypothetical pathways are indicated with dashed arrows. Adapted from [63].

Figure 7

Proposed signaling pathway for fragment C. Based on the results obtained by Aguilera and co-workers, fragment C can induce cerebellar granular cell survival under stress conditions, activating signaling pathways associated with Trk receptors that include the activation of PLC, the Ras/MAPK pathway and the PI3 pathway, leading to the survival of the cell [88,89]. Another possible retrograde pathway of fragment C of tetanus toxin is shared by p75^NTR, TrkB and BDNF, which is strongly dependent on the activities of the small GTPases Rab5 and Rab7 [56].