Release of the cytokine TNFα comprises an alternative mechanism by which glia might modulate LTP. Cultured astrocytes constitutively release TNFα, which induce the addition of AMPA receptors to neuronal membranes, thereby enhancing excitatory synaptic transmission (Beattie et al., 2002; Stellwagen and Malenka, 2006). To assess the involvement of TNFα in the strengthening of excitatory transmission in the human glial chimeric mice, we first confirmed that TNFα increased both AMPA receptor current (Fig. 5A) and AMPA GluR1 immunolabeling in hippocampal slices (Fig. 5B). In contrast, TNFα did not affect expression of the NMDA receptor NR1 subunit in the same slices (Fig. 5B). On that basis, we next asked whether chimeric mice expressed human TNFα. Using qPCR we found that human-specific sequence encoding TNFα was indeed highly expressed in the chimeras, yet undetectable in unengrafted mice (Fig. S4A). Immunolabeling confirmed that the human glial chimeras exhibited significant increases in both TNFα and GluR1, but not of NR1 (Fig. 5C). We thus asked whether the inhibition of TNFα production might suppress excitatory hippocampal transmission in chimeric mice, and if so, whether TNFα inhibition might abrogate the effects of human glial chimerization on LTP.
