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Clostridial neurotoxins potently and specifically inhibit neurotransmitter release in defined cell types. Here we report that a catalytically active derivative (termed LH N /A) of the type A neurotoxin from Clostridium botulinum has been coupled to a lectin obtained from Erythrina cristagalli to form a novel conjugate. This conjugate exhibits an in vitro selectivity for nociceptive afferents compared with the anatomically adjacent spinal neurons, as assessed using in vitro primary neuronal culture systems to measure inhibition of release of neurotransmitters. Chemical conjugates prepared between E. cristagalli lectin and either natively sourced LH N /A or recombinant LH N /A purified from Escherichia coli are assessed, and equivalence of the recombinant material are demonstrated. Furthermore, the dependence of inhibition of neurotransmitter release on the cleavage of SNAP-25 is demonstrated through the use of an endopeptidase-deficient LH N /A conjugate variant. The duration of action of inhibition of neurotransmitter released by the conjugate in vitro is assessed and is comparable with that observed with Clostridium botulinum neurotoxin. Finally, in vivo electrophysiology shows that these in vitro actions have biological relevance in that sensory transmission from nociceptive afferents through the spinal cord is significantly attenuated. These data demonstrate that the potent endopeptidase activity of clostridial neurotoxins can be selectively retargeted to cells of interest and that inhibition of release of neurotransmitters from a neuronal population of therapeutic relevance to the treatment of pain can be achieved. The clostridial neurotoxin (CNT)1 family includes tetanus toxin (TeNT), produced by Clostridium tetani, and the seven antigenically distinct botulinum neurotoxins produced from strains of Clostridium botulinum (BoNTs). These proteins are responsible for the conditions of tetanus and botulism, respectively, that develop as a direct result of inhibition of Ca 2ϩ -dependent neurotransmitter release, a mechanism of action common to all the CNTs. In the case of BoNTs, intoxication of the neuromuscular junction is thought to occur in at least three phases: an initial binding phase, an internalization phase, and finally a neurotransmitter blockade phase (1).All CNTs have a similar structure and consist of a heavy chain (ϳ100 kDa) covalently joined to a light chain (ϳ50 kDa) by a single disulfide bond. Proteolytic cleavage of the heavy chain of C. botulinum neurotoxin type A (BoNT/A) generates two fragments of ϳ50 kDa each. The C-terminal domain (H C ) is required for target cell binding, with the N-terminal domain (H N ) being proposed to be involved in intracellular membrane translocation (2). Under conditions in which the disulfide bond between the light and heavy chains is maintained, trypsin cleavage results in a 100-kDa species termed LH N /A (light chain plus N-terminal heavy chain domain) representing a catalytically active, non-cell binding, non-toxic derivative of BoNT/A. In addition to obtainin...

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Botulinum neurotoxins (BoNTs) cause flaccid paralysis by inhibiting neurotransmission at cholinergic nerve terminals. Each BoNT consists of three domains that are essential for toxicity: the binding domain, the translocation domain, and the catalytic light-chain domain. BoNT modular architecture is associated with a multistep mechanism that culminates in the intracellular proteolysis of SNARE (soluble N-ethylmaleimide-sensitive-fusion-protein attachment protein receptor) proteins, which prevents synaptic vesicle exocytosis. As the most toxic proteins known, BoNTs have been extensively studied and are used as pharmaceutical agents to treat an increasing variety of disorders. This review summarizes the level of sophistication reached in BoNT engineering and highlights the diversity of approaches taken to utilize the modularity of the toxin. Improved efficiency and applicability have been achieved by direct mutagenesis and interserotype domain rearrangement. The scope of BoNT activity has been extended to nonneuronal cells and offers the basis for novel biomolecules in the treatment of secretion disorders.

Ricin A-chain (RTA) is an N-glycosidase which removes a specific adenine residue from the large rRNA of eukaryotic ribosomes. As a consequence, the ribosome is inactivated and protein synthesis is inhibited leading to cell death. This report describes the effects on enzyme activity of specific mutations of the conserved active site Glu177. The activity of mutant proteins was initially screened using an in vitro translation system. It was found that mutagenesis of Glu177 to Lys led to an apparent total inactivation of the enzyme, Glu177 to Ala had a small effect on activity, whereas the conservative Glu177 to Asp mutation had a significant effect. The properties of Glu177 to Asp were investigated more closely. Mutant protein was purified from an Escherichia coli expression system and kinetic analysis of the depurination activity assessed using salt-washed yeast ribosomes. It was shown that the Km of the mutant protein was unchanged when compared to data of wild type RTA; however, the kcat was significantly decreased (49-fold compared to wild type RTA). This suggests that Glu177 plays a predominant role in the rate-limiting step of the enzymatic mechanism and not in substrate binding. These data are discussed in relation to other reports of ricin Glu177 substitutions.

Clostridial neurotoxins potently and specifically inhibit neurotransmitter release in defined cell types by a mechanism that involves cleavage of specific components of the vesicle docking/fusion complex, the SNARE complex. A derivative of the type A neurotoxin from Clostridium botulinum (termed LH N /A) that retains catalytic activity can be prepared by proteolysis. The LH N /A, however, lacks the putative native binding domain (H C ) of the neurotoxin and is thus unable to bind to neurons and effect inhibition of neurotransmitter release. Here we report the chemical conjugation of LH N /A to an alternative cell-binding ligand, wheat germ agglutinin (WGA). When applied to a variety of cell lines, including those that are ordinarily resistant to the effects of neurotoxin, WGA-LH N /A conjugate potently inhibits secretory responses in those cells. Inhibition of release is demonstrated to be ligand mediated and dose dependent and to occur via a mechanism involving endopeptidase-dependent cleavage of the natural botulinum neurotoxin type A substrate. These data confirm that the function of the H C domain of C. botulinum neurotoxin type A is limited to binding to cell surface moieties. The data also demonstrate that the endopeptidase and translocation functions of the neurotoxin are effective in a range of cell types, including those of nonneuronal origin. These observations lead to the conclusion that a clostridial endopeptidase conjugate that can be used to investigate SNARE-mediated processes in a variety of cells has been successfully generated.

Clostridial neurotoxins potently and specifically inhibit neurotransmitter release in defined cell types. Previously reported data have demonstrated that the catalytically active LH(N) endopeptidase fragment of botulinum neurotoxin type A (termed LH(N)/A) can be retargeted to a range of cell types in vitro to lead to inhibition of secretion of a range of transmitters. Here, we report the synthesis of endopeptidase conjugates with in vitro selectivity for nociceptive afferents compared to spinal neurons. Chemical conjugates prepared between Erythrina cristagalli lectin and LH(N)/A are assessed in vitro and in in vivo models of pain. Chemical conjugates prepared between E. cristagalli lectin and either natively sourced LH(N)/A, or recombinant LH(N)/A purified from Escherichia coli are assessed, and equivalence of the recombinant material is demonstrated. The duration of action of inhibition of neurotransmitter release by the conjugate in vitro is also assessed and is comparable to that observed with Clostridium botulinum neurotoxin. Selectivity of targeting and therapeutic potential have been confirmed by in vivo electrophysiology studies. Furthermore, the analgesic properties of the conjugate have been assessed in in vivo models of pain and extended duration effects observed. These data provide proof of principle for the concept of retargeted clostridial endopeptidases as novel analgesics.

The ability to chemically couple proteins to LH(N)-fragments of clostridial neurotoxins and create novel molecules with selectivity for cells other than the natural target cell of the native neurotoxin is well established. Such molecules are able to inhibit exocytosis in the target cell and have the potential to be therapeutically beneficial where secretion from a particular cell plays a causative role in a disease or medical condition. To date, these molecules have been produced by chemical coupling of the LH(N)-fragment and the targeting ligand. This is, however, not a suitable basis for producing pharmaceutical agents as the products are ill defined, difficult to control and heterogeneous. Also, the molecules described to date have targeted neuroendocrine cells that are susceptible to native neurotoxins, and therefore the benefit of creating a molecule with a novel targeting domain has been limited. In this paper, the production of a fully recombinant fusion protein from a recombinant gene encoding both the LH(N)-domain of a clostridial neurotoxin and a specific targeting domain is described, together with the ability of such recombinant fusion proteins to inhibit secretion from non-neuronal target cells. Specifically, a novel protein consisting of the LH(N)-domains of botulinum neurotoxin type C and epidermal growth factor (EGF) that is able to inhibit secretion of mucus from epithelial cells is reported. Such a molecule has the potential to prevent mucus hypersecretion in asthma and chronic obstructive pulmonary disease.