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  • Recent results suggest that TARPs are not

    2024-04-16

    Recent results suggest that TARPs are not the only auxiliary AMPAR subunits. Using a proteomic strategy (affinity purification of native AMPAR complexes followed by mass-spectrometric analysis), cornichon-related proteins (CNIHs) have been recently identified as components of the AMPAR protein microcomplex in the DLPC receptor (Schwenk et al., 2009). Like TARPs, CNIHs enhance surface expression of AMPARs. Also like TARPs, they prolong both deactivation and desensitization of AMPARs, without any noticeable effect on the time course of recovery from desensitization (Schwenk et al., 2009). Using a similar proteomic approach, the recent work by von Engelhardt et al. (2010) published in Science identified another new protein associated with AMPAR subunits, the cystine-knot AMPAR modulating protein 44 (CKAMP44). Both the structure and the function of this new protein are remarkable. In contrast to TARPs and CNIHs, CKAMP44 has a single putative transmembrane segment. Rather uniquely, it contains several cysteine residues presumably forming a cystine knot, similar to peptide toxins. Like TARPs, but unlike CNIHs, CKAMP44 contains a PDZ ligand motif. When coexpressed with AMPAR subunits, CKAMP44 has several unique effects. First, it only minimally alters AMPAR surface expression. Second, it affects the deactivation and desensitization time constant in a unique way, prolonging deactivation while accelerating desensitization. This is remarkable, because most modulators of AMPAR gating, including TARPs and CNIHs, consistently prolong both deactivation and desensitization (Partin et al., 1996). Finally, it slows the recovery of AMPARs from desensitization. Again, this is different from TARPs, which accelerate recovery (Morimoto-Tomita et al., 2009) and CNIHs, which have no effect (Schwenk et al., 2009). Thus, the results show that the effects of CKAMP44 are in many ways opposite to those of TARPs and cornichons. This suggests the possibility of a reciprocal regulation of AMPAR gating in the brain. Which types of neurons express CKAMP44 and in which subcellular domains is CKAMP44 located? Within the hippocampus, the strongest expression is observed in dentate gyrus granule cells. This selective pattern is different from the expression pattern of both TARPs (TARP γ-2, for example, being highly enriched in cerebellar granule cells) and CNIHs (which are expressed throughout the brain). Furthermore, FLAG-tagging suggests that CKAMP44 is concentrated at synapses. This subcellular distribution is similar to that of TARPs, but different from that of CNIHs, which appear to be also located in extrasynaptic plasma membrane areas (Schwenk et al., 2009). This suggests the possibility that TARPs and CKAMP44 may primarily regulate the properties of postsynaptic receptors, whereas CNIHs could also modify extrasynaptic receptors, making them more responsive to glutamate spillover. What is the functional significance of CKAMP44 for excitatory synaptic transmission? Using CKAMP44 knockout and overexpression, von Engelhardt et al. (2010) show that CKAMP44 prolongs the decay time course of the EPSC, at least in the presence of cyclothiazide. Furthermore, CKAMP44 affects the short-term dynamics of excitatory synaptic transmission at the medial and lateral perforant path synapses on hippocampal granule cells, shifting the paired-pulse ratio toward depression. Thus, postsynaptic factors, i.e., AMPAR desensitization, contribute to paired-pulse depression at these synapses. How the contribution of desensitization is related to synaptic structure, especially the spacing of presynaptic terminals, remains to be determined (Trussell et al., 1993). In summary, accumulating evidence suggests that multiple auxiliary subunits regulate the properties of native AMPARs in a complex antagonistic way (Figure 1). Thus, the fine-tuning of gating kinetics by auxiliary subunits, which is well established for voltage-gated channels, also applies to AMPARs, the main types of receptors mediating glutamatergic synaptic transmission in the brain. Differential expression of auxiliary subunits may contribute to the large range of gating kinetics observed for native AMPARs and EPSCs at glutamatergic synapses. Whether additional auxiliary subunits beyond TARPs, CNIHs, and CKAMP44 contribute to this regulation remains to be explored.