Intracellular Spermine Is a Key Player in GSG1L's Regulation of Calcium-Permeable AMPAR Channel Conductance and Recovery from Desensitization.

Calcium-permeable AMPA-type glutamate receptors (CP-AMPARs) contribute to excitatory synaptic transmission and play pivotal roles in normal and detrimental forms of plasticity. Most AMPARs are associated with auxiliary subunits. Transmembrane AMPAR regulatory proteins (TARPs), the prototypical auxiliary subunits, enhance current through CP-AMPAR channels by increasing single-channel conductance and relieving the characteristic voltage-dependent block by endogenous intracellular polyamines, such as spermine. In contrast, the atypical auxiliary subunit GSG1L negatively regulates CP-AMPARs, suppressing current flow by promoting polyamine block and reducing channel conductance. Here, we investigated the role of polyamines in these opposing effects. We show that, in the absence of auxiliary subunits, intracellular spermine decreases CP-AMPAR single-channel conductance. This effect is prevented by the presence of TARPs but enhanced by the presence of GSG1L. Unexpectedly, intracellular spermine is necessary for both GSG1L's attenuation of CP-AMPAR channel conductance and its characteristic slowing of recovery from desensitization. These various effects are determined by specific residues within the channel's selectivity filter and within GSG1L's C-tail. Together, our findings reveal that intracellular polyamines play an essential role in GSG1L's unique ability to negatively regulate many of the key properties of CP-AMPARs.