Identification of the site at which phospholipase A2 neurotoxins localize to produce their neuromuscular blocking effects.

Experiments were conducted on mouse hemidiaphragm preparations using five phospholipase A2 neurotoxins of differing chain structures and antigenicities [notexin (one chain); crotoxin (two chains not covalently bound), beta-bungarotoxin (two chains covalently bound); taipoxin (three chains), and textilotoxin (five chains; one copy each of three chains and two copies of a fourth chain)]. Three clostridial neurotoxins (botulinum neurotoxin types A and B, and tetanus toxin) were used in comparison experiments. Phospholipase A2 neurotoxins produced concentration-dependent blockade of neuromuscular transmission. There was no obvious relationship between chain structure and potency, but there was an indication of a relationship between chain structure and binding. The binding of notexin was substantially reversible, the binding of crotoxin was slightly reversible, and the binding of beta-bungarotoxin, taipoxin and textilotoxin was poorly reversible. Experiments with neutralizing antibodies indicated that phospholipase A2 neurotoxins became associated with binding sites on or near the cell surface. This binding did not produce neuromuscular blockade. When exposed to physiological temperatures and nerve stimulation, bound toxin disappeared from accessibility to neutralizing antibody. This finding suggests that there was some form of molecular rearrangement. The two most likely possibilities are: (1) there was a change in the conformation of the toxin molecule, or (2) there was a change in the relationship between the toxin and the membrane. The molecular rearrangement step did not produce neuromuscular blockade. At a later time there was onset of paralysis; the amount of time necessary for onset of blockade was a function of toxin concentration. Phospholipase A2 neurotoxins were not antagonized by drugs that inhibit receptor-mediated endocytosis. In addition, phospholipase A2 neurotoxins did not display the pH-induced conformational changes that are typical of other endocytosed proteins, such as clostridial neurotoxins. However, phospholipase A2 neurotoxins were antagonized by strontium, and this antagonism was expressed against toxins that were free in solution and toxins that were bound to the cell surface. Limited antagonism was expressed after toxins had undergone molecular rearrangement, and no antagonism was expressed after toxin-induced neuromuscular blockade. The cumulative data suggest that phospholipase A2 neurotoxins are not internalized to produce their poisoning effects. These toxins appear to act on the plasma membrane, and this is the site at which they initiate the events that culminate in neuromuscular blockade.