CKAMP59: an AMPA or a GABA receptor auxiliary subunit?

• Applicant: Professor Dr. Jakob von Engelhardt
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496461701

Control of expression and function of ionotropic receptors is essential for information processing, learning and memory. A few years ago, we identified proteins of the CKAMP family (CKAMP39, CKAMP44, CKAMP52, and CKAMP59), which as auxiliary subunits of AMPA receptors influence their transport to the cell surface, synaptic localization and function. Little is known about the function of CKAMP59 and the three so far published studies show contradictory results. Thus, a recently published study identifies CKAMP59 as a GABAA receptor interacting protein and casts doubts on its role as an auxiliary subunit of AMPA receptors. The conflicting findings may be explained by the use of different CAKMP59 splice variants and a mutant variant that lacks the AMPA receptor interaction site. Alterations of excitatory and inhibitory synapse function in CA1 neurons of CKAMP59 knockout mice indeed suggest that CKAMP59 influences AMPA and GABAA receptors. Our preliminary show that CKAMP59 reduces AMPA receptor expression in distal synapses under basal condition but is required for the activity dependent upregulation of synaptic AMPA receptor number during LTP. However, global CKAMP59 knockout mice were used in our experiments and those of the published studies. Thus, possible confounders such as neurodevelopmental changes or homeostatic dysregulation with indirect influences on the function of excitatory or inhibitory synapse function complicate the interpretation of the findings. Undebated is that CKAMP59 plays an essential role in hippocampal synaptic function and behavior (absence of LTP in CA1 and reduced fear memory in CKAMP59 knockout mice). However, the underlying mechanisms of the function of CKAMP59 are not resolved.We propose to investigate with electrophysiological, immunohistochemical and anatomical methods the expression of CKAMP59 splice variants and their influence on the subcellular localization and function of AMPA and GABAA receptors. We will perform experiments with HEK293 cells and, importantly, with conditional CKAMP59 knockout mice and with re-expression of CKAMP59 splice variants in knockout mice. The mechanisms of the influence of CKAMP59 on synaptic plasticity (LTP and LTD) will be investigated by electrophysiological means in acute brain slices and in primary hippocampal cell cultures with quantifications of the activity-dependent localization, insertion into the cell membrane and diffusion of AMPA receptors. The planned study should answer whether CKAMP59 is a bona fide subunit of native AMPA and/or GABAA receptors and which role CKAMP59 splice variants play for synaptic function in the hippocampus.

• DFG Programme: Research Grants