Poly(I:C) promotes TNFa/TNFR1-dependent oligodendrocyte death in mixed glial cultures

Steelman, Andrew J; Li, Jianrong

To date several studies have demonstrated reduced preOL viability following LPS stimulation of microglia/ preOL co-cultures [28] or mixed glial cultures containing astrocytes [17, 24, 25, 41] . Mechanistically, it has been shown that NO-derived peroxynitrite is required for mediating toxicity to preOLs in microglia/preOL co-cultures [28] and may indeed participate in the initial reduction of preOL viability (after 24 h) in mixed cultures [41] . However, 48 h of LPS stimulation in mixed glial cultures results in a loss of preOL viability that is attributable to TNFα [24, 25, 41] . As with other studies, the current set of experiments implicates aberrant TNFα production in the demise of oligodendrocytes [17, 24, 41, 42] , but the exact role of TNFα in the pathogenesis of demyelination in vivo are not yet fully understood. For instance, stimulation of mixed glial cultures with LPS results in TNFα production, which is toxic to oligodendrocytes in vitro [17, 24, 43] . TNFα has also been shown to be directly toxic to oligodendrocytes in vitro [44] . Moreover, in transgenic mice in which astrocytes express transmembrane TNFα demonstrate oligodendrocyte death and develop demyelination [42] . In agreement with this observation, we have shown that both astrocytes and oligodendroglial TNFR1 are required for TNFα-mediated oligodendrocyte toxicity in vitro [25] . Furthermore, direct cell-cell contact between astrocytes and oligodendrocytes mediates the TNFα toxicity. Therefore, accumulating data suggest that microglia as well as astrocytes are required for TLR agonist induced preOL toxicity. It is important to note that the mixed glial cultures used in this study did not contain neurons. As functional neurons constitutively express several immunoregulatory molecules such as CD200 [45] and CX3CL1 [46] that are capable of modulating microglia activation and cytokine production, it is possible that stimulation of mixed neuronal cultures with poly(I:C) might mitigate the toxic effect on preOL. Thus, future experiments are needed to examine the effect of neurons on microglia inflammatory responses and preOL viability. However, several lines of evidence indicate that TLR ligation can contribute to tissue destruction within the CNS. For instance, stereotaxic injection of zymosan, a TLR2 ligand, into either the corpus callosum [20] or spinal cord [21] causes microglia activation and demyelination. Likewise, stereotaxic injection of LPS into the spinal cord induces demyelination [19] . Moreover, poly(I:C) injection into the substantia nigra pars compacta was shown to trigger microglia activation and neurodegeneration at high doses or susceptibility to neurodegeneration at subtoxic levels [18] . While these studies suggest that acute microglia activation subsequent to TLR ligation is detrimental to the surrounding tissues, certainly not all microglial responses result in neurodegeneration. Some can in fact promote tissue repair [4] . For instance, intravitreal injection of zymosan is associated with an increase in TGFβ1 and IL-1β production and induces myelination of retina [47] ; and LPS treatment concurrent to detergent ethidium bromide-induced demyelination promotes repair, possibly through the recruitment of oligodendrocyte progenitors to the lesion [48] .