Background During inflammatory demyelination, TNF receptor 1 (TNFR1) mediates detrimental proinflammatory effects of soluble TNF (solTNF), whereas TNFR2 mediates beneficial effects of transmembrane TNF (tmTNF) through oligodendroglia, microglia, and possibly other cell types. This model supports the use of selective inhibitors of solTNF/TNFR1 as anti-inflammatory drugs for central nervous system (CNS) diseases. A potential obstacle is the neuroprotective effect of solTNF pretreatment described in cultured neurons, but the relevance in vivo is unknown. Methods To address this question, we generated mice with neuron-specific depletion of TNFR1, TNFR2, or inhibitor of NF-κB kinase subunit β (IKKβ), a main downstream mediator of TNFR signaling, and applied experimental models of inflammatory demyelination and acute and preconditioning glutamate excitotoxicity. We also investigated the molecular and cellular requirements of solTNF neuroprotection by generating astrocyte-neuron co-cultures with different combinations of wild-type (WT) and TNF and TNFR knockout cells and measuring N-methyl-d-aspartate (NMDA) excitotoxicity in vitro. Results Neither neuronal TNFR1 nor TNFR2 protected mice during inflammatory demyelination. In fact, both neuronal TNFR1 and neuronal IKKβ promoted microglial responses and tissue injury, and TNFR1 was further required for oligodendrocyte loss and axonal damage in cuprizone-induced demyelination. In contrast, neuronal TNFR2 increased preconditioning protection in a kainic acid (KA) excitotoxicity model in mice and limited hippocampal neuron death. The protective effects of neuronal TNFR2 observed in vivo were further investigated in vitro. As previously described, pretreatment of astrocyte-neuron co-cultures with solTNF (and therefore TNFR1) protected them against NMDA excitotoxicity. However, protection was dependent on astrocyte, not neuronal TNFR1, on astrocyte tmTNF-neuronal TNFR2 interactions, and was reproduced by a TNFR2 agonist. Conclusions These results demonstrate that neuronal TNF receptors perform fundamentally different roles in CNS pathology in vivo, with neuronal TNFR1 and IKKβ promoting microglial inflammation and neurotoxicity in demyelination, and neuronal TNFR2 mediating neuroprotection in excitotoxicity. They also reveal that previously described neuroprotective effects of solTNF against glutamate excitotoxicity in vitro are indirect and mediated via astrocyte tmTNF-neuron TNFR2 interactions. These results consolidate the concept that selective inhibition of solTNF/TNFR1 with maintenance of TNFR2 function would have combined anti-inflammatory and neuroprotective properties required for safe treatment of CNS diseases.

BACKGROUNDDuring inflammatory demyelination TNF receptor 1 (TNFR1) mediates detrimental proinflammatory effects of soluble TNF, whereas TNFR2 mediates beneficial effects of transmembrane TNF through oligodendrocytes, microglia, and possibly other cell types. This model supports use of selective inhibitors of soluble TNF/TNFR1 as antinflammatory drugs for CNS disease. A potential obstacle is the neuroprotective effect of soluble TNF pretreatment described in cultured neurons, but the in vivo relevance is unknown. METHODSTo address this question we generated mice with neuron-specific depletion of TNFR1, TNFR2 or IKKβ and applied experimental models of inflammatory demyelination and acute and preconditioning glutamate excitotoxicity. We also investigated the molecular and cellular requirements of soluble TNF (and therefore TNFR1) neuroprotection by generating astrocyte-neuron co-cultures with different combinations of wildtype and TNF and TNF receptor knockout cells and measuring NMDA excitotoxicity in vitro.RESULTSNeither neuronal TNFR1 nor TNFR2 protected mice during inflammatory demyelination. In fact, both neuronal TNFR1 and neuronal IKKβ promoted microglial responses and tissue injury, and TNFR1 was further required for oligodendrocyte loss and axonal damage in cuprizone demyelination. In contrast, neuronal TNFR2 increased preconditioning protection in a kainic acid excitotoxicity model in mice, and limited hippocampal neuron death. The neuroprotective effects of neuronal TNFR2 observed in vivo were further investigated in vitro. Here as expected, pretreatment of astrocyte-neuron co-cultures with soluble TNF protected them against NMDA excitotoxicity. However, protection was dependent on astrocyte, not neuronal TNFR1, on astrocyte transmembrane TNF-neuronal TNFR2 interactions, and was reproduced by a TNFR2 agonist. CONCLUSIONSThese results demonstrate that neuronal TNF receptors perform fundamentally different roles in CNS pathology in vivo, with neuronal TNFR1 and IKKβ promoting microglial inflammation and neurotoxicity in demyelination, and neuronal TNFR2 mediating neuroprotection in excitotoxicity. They also reveal that previously-described neuroprotective effects of soluble TNF (and therefore TNFR1) against glutamate excitotoxicity in vitro are indirect, and mediated by astrocyte transmembrane TNF-neuron TNFR2 interactions. These results consolidate the concept that selective inhibition of soluble TNF/TNFR1 with maintenance of TNFR2 function would have anti-inflammatory and neuroprotective properties required for the safe treatment of CNS disease.