Clostridial neurotoxins: from toxins to therapeutic tools?

Tetanus toxin and botulinum toxins are powerful neurotoxins which block neurotransmitter release through an unknown mechanism my means of their light chains. The heavy chains provide the machinery for neuroselective binding, internalization, retrograde intraaxonal transport, and translocation of the L-chains into the cytosole. We have cloned and sequenced the structural genes of tetanus toxin and of five serologically distinct botulinum toxins to identify structurally and functionally conserved subdomains. The minimum essential domains of the L-chains of tetanus and botulinum toxin type A were identified by combined in vitro transcription and microinjection of L-chain specific mRNA into identified presynaptic neurons of Aplysia californica. In addition, a nontoxic mutant of tetanus was generated by replacing histidine(237) by a proline residue. The development of nontoxic neuroselective transporter molecules carrying various marker enzymes is discussed.