Previous publications from our group [Gil, Chaib, Pelliccioni and Aguilera (2000) FEBS Lett. 481, 177–182; Gil, Chaib, Blasi and Aguilera (2001) Biochem. J. 356, 97–103] have reported the activation, in rat brain synaptosomes, of several phosphoproteins, such as neurotrophin tyrosine kinase (Trk) A receptor, phospholipase Cγ-1, protein kinase C (PKC) isoforms and extracellular-signal-regulated kinases 1 and 2 (ERK-1/2). In the present study, we examined, by means of phospho-specific antibodies, the activation of the signalling cascades involving neurotrophin Trk receptor, Akt kinase and ERK pathway, in cultured cortical neurons from foetal rat brain, by tetanus toxin (TeTx) as well as by the C-terminal part of its heavy chain (HC-TeTx). TeTx and HC-TeTx induce fast and transient phosphorylation of Trk receptor at Tyr674 and Tyr675, but not at Tyr490, although the potency of TeTx in this action was higher when compared with HC-TeTx action. Moreover, HC-TeTx and TeTx also induced phosphorylation of Akt (at Ser473 and Thr308) and of ERK-1/2 (Thr202/Tyr204), in a time- and concentration-dependent manner. The detection of TeTx- and HC-TeTx-induced phosphorylation at Ser9 of glycogen synthase kinase 3β confirms Akt activation. In the extended analysis of the ERK pathway, phosphorylation of the Raf, mitogen-activated protein kinase kinase (MEK)-1/2 and p90Rsk kinases and phosphorylation of the transcription factor cAMP-response-element-binding protein were detected. The use of tyrphostin AG879, an inhibitor of Trk receptors, demonstrates their necessary participation in the HC-TeTx-induced activation of Akt and ERK pathways, as well as in the phosphorylation of phospholipase Cγ-1. Furthermore, both pathways are totally dependent on phosphatidylinositol 3-kinase action, and they are independent of PKC action, as assessed using wortmannin and Ro-31-8220 as inhibitors. The activation of PKC isoforms was determined by their translocation from the cytosolic compartment to the membranous compartment, showing a clear HC-TeTx-induced translocation of PKC-α and -β, but not of PKC-ε.

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.