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One specialization of vertebrate presynaptic neuronal membranes is their multifold enrichment in complex gangliosides, suggesting that these sialoglycolipids may play a major functional role in synaptic transmission. We tested this hypothesis directly by studying neuromuscular synapses of mice lacking complex gangliosides attributable to deletion of the gene coding for beta1,4 GalNAc-transferase (GM2/GD2 synthase), which catalyzes an early step in ganglioside synthesis. Our studies show that complex gangliosides are surprisingly redundant for regulated neurotransmitter release under normal physiological conditions. In contrast, we show that they are membrane receptors for both the paralytic botulinum neurotoxin type-A and human neuropathy-associated anti-ganglioside autoantibodies that arise through molecular mimicry with microbial structures. These data prove the critical importance of complex gangliosides in mediating pathophysiological events at the neuromuscular synapse.

Miller Fisher syndrome-associated anti-GQ1b ganglioside antibodies produce an acute complement-dependent neuroexocytic effect at the mouse neuromuscular junction (NMJ) that closely resembles the effect of alpha-latrotoxin (LTx). This pathophysiological effect is accompanied by morphological disruption of the nerve terminal involving the loss of major cytoskeletal components, including neurofilament. Both LTx and the membrane attack complex of complement form membrane pores that allow free ionic movement and we have previously hypothesized that Ca2+ ingress and the subsequent activation of Ca2+-dependent proteases, calpains, may lead to substrate degradation resulting in structural disorganization of the terminal. Here, we treated mouse NMJs in hemidiaphragm preparations with anti-GQ1b antibodies and complement, or with LTx in the presence and absence of extracellular Ca2+, and studied possible neuroprotective effects of the calpain inhibitors calpeptin and calpain inhibitor V. Both Ca2+ depletion and calpain inhibition protected the cytoskeleton from degradation, as assessed by immunohistological and ultrastructural analysis. Calpain inhibitors may therefore be useful therapeutically in limiting nerve terminal and axonal injury in autoimmune peripheral neuropathy and in human latrodectism.

No abstract

Ganglioside binding properties were calculated from half-maximal binding studies and graded as follows: 0,-; <10 2 , +; >10 2 <10 3 , ++; >10 3 , +++. LPS reactivity as assessed by ELISA and TLC overlay is scored as present (+) or absent (-). See Methods.