UCSF

AMPA-type glutamate receptors are known to play a critical role in both basal synaptic transmission and in acute forms of plasticity, such as LTP or LTD, but less is known about their role in neuronal homeostasis. A model for bidirectional synaptic scaling is emerging with the GluA2 AMPA receptor subunit playing a central role. Through my dissertation work, I found that GluA2, but not GluA1, is necessary for synaptic scaling-up, and I showed that GluA2 is also sufficient for mediating this phenomenon. Additionally, I discovered that the membrane-proximal C-terminal domain of GluA2 is required for scaling-up following chronic pharmacological silencing of network activity. Precisely how the GluA2 membrane-proximal C-terminal domain mediates synaptic insertion of AMPA receptors following chronic silencing remains to be elucidated. However, in probing the molecular mechanisms of synaptic homeostasis, we rely on tools and manipulations such as network-wide activity blockade that create highly artificial environments, and few studies have relied on the silencing of individual cells. I therefore employed 4 distinct strategies to chronically hyperpolarize or silence neurons to assess the intrinsic ability of a single neuron to engage synaptic scaling programs. I first hyperpolarized neurons with the inwardly-rectifying potassium channel, Kir2.1, after which I attempted to hyperpolarize neurons with more temporal control by expressing the inhibitory DREADD, hM4Di, and activating GIRK channels via CNO-mediated activation of the DREADD. I then employed a CRISPR/Cas9 strategy to eliminate all neuronal voltage dependent sodium channels. In the final series of experiments, I set out to achieve tighter temporal control over the abolishment of sodium current. I therefore rescued with the skeletal sodium channel, Nav1.4, on this Nav-null background. Despite employing these four unique strategies to hyperpolarize or silence individual neurons, I observed no evidence for any cell-intrinsic scaling mechanisms. In fact, contrary to our expectations, it seemed that a more Hebbian, non-homeostatic, program overrode synaptic scaling.