Our results provide a mechanistic framework to help us understand the selective vulnerability of striatal and cortical neurons in HD. We show here that synaptic activity controls expression of the chaperonin TRiC, which in turn modulates inclusion formation and toxicity of mtHtt. Moreover, the electrical properties of these neurons coupled with the predominant effect of the small guanine nucleotide-binding protein Rhes and the CREB—PGC-1α pathway in this cell type29,37
renders them particularly sensitive to the toxic effects of mtHtt. Specifically, excessive extrasynaptic activity increases the relative Rhes level and, in conjunction with mtHtt, decreases CREB—PGC-1α activity to promote neuronal cell death. Coupled with a decrease in synaptic transmission, which also contributes to a decrease in CREB activity in the presence of mtHtt, neurons become increasingly vulnerable to injury and death (). We speculate that these findings imply that therapies for HD that minimize excessive extrasynaptic activity while maintaining or enhancing normal synaptic activity will yield considerable benefit. Notably, it has been contentious whether mtHtt inclusions are cytotoxic. The presence of inclusions may reflect either successful sequestration of toxic soluble oligomers of mtHtt and thus contribute to neuroprotection, or insufficient sequestration of toxic soluble mtHtt, which can no longer be accommodated by the inclusion formation process, and is thus toxic. Here, we report that synaptic activity-driven inclusions are neuroprotective, supporting the former premise.
Figure 6 Schematic model showing the role of physiological synaptic vs. excessive extrasynaptic NMDAR activity in the neurodegeneration of Huntington’s disease. Physiological synaptic NMDAR activity promotes neuroprotection, in part by facilitating non-toxic (more ...)
Nonetheless, we also demonstrate that cultured neurons containing mtHtt inclusions become increasingly vulnerable to excitotoxic insults, consistent with the notion that over time, accumulation of excitotoxic challenges may eventually contribute to the death of mtHtt-expressing neurons in HD6
. We show not only that mtHtt increases the vulnerability of neurons to relatively low concentrations of exogenous glutamate, but also that suppressing spontaneous excitatory synaptic activity can mimic this phenomenon via the excitotoxic effect of endogenous glutamate apparently acting on extrasynaptic receptors.
Our data using primary neurons manifesting synaptic activity may also explain why others had reported direct mtHtt toxicity in the absence of excitotoxic insult in yeast, various cell lines, and synaptically-immature neurons. That is, the presence of normal synaptic activity in our mature neuronal cultures leads to the formation of inclusions, thereby avoiding toxic oligomers of mtHtt, whereas these other cell types lack synaptic activity and therefore are exposed to soluble mtHtt oligomers.
Our findings further suggest that the balance between synaptic and extrasynaptic NMDAR activity may be critical in determining neuronal cell survival in HD. We show that low concentrations of the NMDAR antagonist, memantine, afford the advantage of restoring excitatory balance by maintaining physiological synaptic activity while blocking excessive extrasynaptic NMDAR stimulation9,10
. Inhibiting excessive extrasynaptic NMDAR activity decreased Rhes levels, thus preserving mtHtt inclusions and lessening mtHtt cytotoxicity29
. To the contrary, blocking synaptic activity with high-dose memantine decreased inclusion formation and increased soluble mtHtt. Soluble mtHtt interferes with the neuroprotective CREB—PGC-1α pathway, thus accelerating neuronal cell death. Indeed, prolonged administration of high-dose memantine may contribute to toxicity for several reasons, including interference with other neuroprotective pathways that are dependent on synaptic activity, such as the phosphatidylinositol 3 kinase (PI3K)-Akt cascade38,39
. Thus, we posit that restoring excitatory balance, which is disrupted by mtHtt protein, can affect protein misfolding and protect neurons in HD, as also suggested in a small, open-label human clinical trial of memantine in HD patients40
. This novel concept of balancing synaptic and extrasynaptic neuronal NMDAR activity may also lead to strategies to combat cell injury and death associated with other neurodegenerative disorders.