Regulation of calcium-permeable AMPA receptors by auxiliary subunits in cerebellar neurons

AMPA receptors (AMPARs) mediate the majority of fast excitatory synaptic transmission in the central nervous system. They exist as homo- or hetero-tetrameric assemblies of GluA1-4 subunits. Subunit composition is a crucial determinant of AMPAR biophysical and pharmacological properties. Notably, inclusion of the GluA2 subunit renders AMPARs impermeable to calcium ions as result of editing at the ‘Q/R site’. Despite the predominant expression of calcium-impermeable AMPARs (CI-AMPARs), calcium-permeable AMPARs (CP-AMPARs) play key roles in multiple physiological aspects of transmission and the influx of calcium ions through CP-AMPARs contributes to neuronal death in several neurological disorders. Given the critical roles played by CP-AMPARs in normal synaptic function and neurodegenerative processes, it is essential to understand their regulation. Neuronal AMPARs exist as complexes with accessory proteins such as transmembrane AMPAR regulatory proteins (TARPs), cornichons (CNIHs), cysteine-knot AMPA receptor-modulating proteins (CKAMPs/shisas) and GSG1L. These ancillary proteins not only govern receptor delivery to the cell membrane but also actively shape their biophysical and pharmacological properties. This thesis describes an investigation into subunit-specific AMPAR trafficking by TARPs in cerebellar granule cells (GCs). In the epileptic and ataxic stargazer mouse, a mutation in the stargazin (γ 2) gene and consequent loss of functional γ 2 protein results in a complete absence of AMPAR-mediated synaptic transmission at cerebellar mossy fibre (MF) to GC synapses. This led to the identification of γ 2 as the prototypical TARP. I used, siRNA constructs to manipulate the AMPAR subunit and TARP content in stargazer neurons to study the interplay between the type I TARP γ 2 and the less well understood type II TARP γ 7 in the synaptic targeting of CI- and CP-AMPARs. The last chapter of my thesis presents an investigation into the expression and function of AMPARs in a mouse model of juvenile Batten disease, a condition in which changes in cerebellar CP-AMPAR had been proposed by others. Overall, my results establish an important role of γ 7 in CP-AMPAR regulation in GCs and demonstrate a presynaptic rather than postsynaptic change, in the Batten disease model.