Analysis of nerve terminal dysfunction in autoimmune neuropathy

In studies on the pathophysiology of the autoimmune neuropathy, Miller Fisher syndrome (MFS), monoclonal antibodies to the disialosyl epitopes on GQ1b, GT1a and GD3 gangliosides have been produced. Antibodies to these complex gangliosides are thought to be crucial in the pathogenesis of MFS. These antibodies have recently been shown to produce complement dependent, glial and/or neuronal injury at mouse neuromuscular junctions (NMJs). Three antibodies (EG1, LB1 and R24) were identified as producing selective terminal Schwann cell (TSC) injury whilst sparing neuronal membranes at NMJs in BALB/c and C57BL/6 mouse strains in a dose dependent manner. These changes occur in the absence of observable acute physiological or morphological changes to the nerve terminal, suggesting that TSC injury or loss has no major short-term influence on synapse function. Having compared the suitability of two common ex vivo muscle preparations for use in characterisation studies, TSC selective antibodies were compared, and EG1 was identified as most suitable for further investigations on the role of the TSC in mammalian NMJ function and as a possible disease target in MFS. The effect of this antibody when combined with normal human serum as a source of complement in ex vivo hemidiaphragm preparations was independent of complement regulators DAF1 and CD59a. Cell specific promoter sequences have been used to produce a mouse line that expresses green-fluorescent protein (GFP) in TSCs, and cyan-fluorescent protein (CFP) in axons (CK mouse). Live imaging techniques were used to study the acute and chronic effects of EG1 mAb mediated, complement dependent glial injury in this system. It is shown that TSC injury is characterised by loss of GFP staining, occurring within 20 minutes of complement exposure. Repopulation of the NMJs with GFP-positive cell bodies is first evident at day 2. The origin of these returning Schwann cells is discussed, and three possible sources are considered – the last myelinating Schwann cell, non-myelinating Schwann cells lying more proximally in the peripheral nervous system, and muscle stem cells. At day 7, the number of GFP-positive cell bodies seen at the NMJ is higher (7-12 per NMJ) than prior to antibody exposure (3-5 per NMJ). This process of enhanced repopulation is not dependent on an intact axon as it is retained following axotomy. At 3 months, minor remodelling of the NMJ is seen, and is more pronounced at one year. Repeat antibody exposure within 48 hours does not injure returning processes, or delay repopulation. Instead, extra-junctional TSC processes are formed, with associated axon sprouts in the absence of gross terminal axon injury. Anti-GQ1b containing serum from a MFS patient is shown to induce murine TSC death in a similar manner to murine monoclonal antibodies described previously. This suggests that TSCs are a potential new disease target in human disease if the ganglioside profile of mouse and human TSCs is equivalent. Ganglioside distribution and antibody binding are examined on a series of human muscles. These studies demonstrate for the first time that components of the human NMJ are potentially susceptible to anti-ganglioside antibody mediated injury, by virtue of their ganglioside profile. This study suggests that TSCs may be a previously unrecognised site of immune-mediated nerve injury. It also describes a new technique for observing chronic TSC injury and recovery in an in vivo mouse model system, which could be used for human disease modelling