A human IgM signals axon outgrowth: coupling lipid raft to microtubules.

Mouse and human IgMs support neurite extension from primary cerebellar granule neurons. In this study using primary hippocampal and cortical neurons, we demonstrate that a recombinant human IgM, rHIgM12, promotes axon outgrowth by coupling membrane domains (lipid rafts) to microtubules. rHIgM12 binds to the surface of neuron and induces clustering of cholesterol and ganglioside GM1. After cell binding and membrane fractionation, rHIgM12 gets segregated into two pools, one associated with lipid raft fractions and the other with the detergent-insoluble cytoskeleton-containing pellet. Membrane-bound rHIgM12 co-localized with microtubules and co-immuno precipitated with ß3-tubulin. rHIgM12-membrane interaction also enhanced the tyrosination of α-tubulin indicating a stabilization of new neurites. When presented as a substrate, rHIgM12 induced axon outgrowth from primary neurons. We now demonstrate that a recombinant human mAb can induce signals in neurons that regulate membrane lipids and microtubule dynamics required for axon extension. We propose that the pentameric structure of the IgM is critical to cross-link membrane lipids and proteins resulting in signaling cascades.

Remyelination induced by a DNA aptamer in a mouse model of multiple sclerosis.

Multiple sclerosis (MS) is a debilitating inflammatory disease of the central nervous system (CNS) characterized by local destruction of the insulating myelin surrounding neuronal axons. With more than 200 million MS patients worldwide, the absence of treatments that prevent progression or induce repair poses a major challenge. Anti-inflammatory therapies have met with limited success only in preventing relapses. Previous screening of human serum samples revealed natural IgM antibodies that bind oligodendrocytes and promote both cell signaling and remyelination of CNS lesions in an MS model involving chronic infection of susceptible mice by Theiler's encephalomyelitis virus and in the lysolecithin model of focal demyelination. This intriguing result raises the possibility that molecules with binding specificity for oligodendrocytes or myelin components may promote therapeutic remyelination in MS. Because of the size and complexity of IgM antibodies, it is of interest to identify smaller myelin-specific molecules with the ability to promote remyelination in vivo. Here we show that a 40-nucleotide single-stranded DNA aptamer selected for affinity to murine myelin shows this property. This aptamer binds multiple myelin components in vitro. Peritoneal injection of this aptamer results in distribution to CNS tissues and promotes remyelination of CNS lesions in mice infected by Theiler's virus. Interestingly, the selected DNA aptamer contains guanosine-rich sequences predicted to induce folding involving guanosine quartet structures. Relative to monoclonal antibodies, DNA aptamers are small, stable, and non-immunogenic, suggesting new possibilities for MS treatment.

Cellular mechanisms of central nervous system repair by natural autoreactive monoclonal antibodies.

Natural autoreactive monoclonal IgM antibodies have demonstrated potential as therapeutic agents for central nervous system (CNS) disease. These antibodies bind surface antigens on specific CNS cells, activating intracellular repair-promoting signals. IgM antibodies that bind to surface antigens on oligodendrocytes enhanced remyelination in animal models of multiple sclerosis. IgM antibodies that bind to neurons stimulate neurite outgrowth and prevent neuron apoptosis. The neuron-binding IgM antibodies may have utility in CNS axon- or neuron-damaging diseases, such as amyotrophic lateral sclerosis, stroke, spinal cord injury, or secondary progressive multiple sclerosis. Recombinant remyelination-promoting IgM antibodies have been generated for formal toxicology studies and, after Food and Drug Administration approval, a phase 1 clinical trial. Natural autoreactive monoclonal antibodies directed against CNS cells represent novel therapeutic molecules to induce repair of the nervous system.