SUMO is a small protein that is becoming recognized as an important regulator of neuronal function. It exits as four homologues, of which 3 (SUMO1-3) are expressed in brain tissue. SUMO-2 and SUMO-3 differ only by 3 N-terminal amino acids and are functionally indistinguishable. Together, they share ~50% homology with SUMO-1, which has been shown to strongly interact with the GluK2 and GluK3 subunits (Martin et al 2007; Wilkinson et al 2008). SUMOylation of GluK2 occurs following kainate -induced internalisation, while de-SUMOylation prevents such internalisation, showing that SUMOylation is required for kainate-induced GluK2 internalisation (Martin et al 2007) . SUMO-1 also causes a run-down in KAR-mediated EPSCs in the hippocampus, while de-SUMOylation causes a run-up, suggesting that SUMOylation is an important regulator of synaptic transmission. In addition, SUMO-1 has also been shown to regulate glutamate release (Feligioni et al 2009), with SUMOylation increasing kainate-mediated release and de-SUMOylation decresing release. Interestingly, SUMOylation had the opposite effect on KCl-induced glutamate release. Similar results were seen for either kainate- or KCl-induced Ca2+ influx into synaptosomes. Thus SUMOylation can affect excitatory synaptic transmission at either pre-synaptic or post-synaptic sites.
Conjugation of both SUMO-1 and SUMO2/3 have been shown to increase following focal ischaemia (Cimarosti et al 2008), a model for stroke. This was accompanied by a decrease in the expression of AMPA and KAR receptors in the regions both within the infarct and in adjacent areas. Interestingly, while both SUMO-1 and SUMO-2/3 conjugation is apparent in the infarct, only SUMO-2/3 conjugation was seen outside the region. Thus increased SUMOylation and reduced AMPAR/KAR expression may be a physiological response to ischaemia, and thus a tagret for pharmacological intervention. Indeed, SUMOylation may be involved in a range of neurological disorders (Anderson et al 2009).
PICK1 is a protein that was first identified as interacting with PKCalpha via a PDZ domain (protein-protein interaction motifs). It has been shown to interact with a number of glutamate receptors - GluA2 and GluA3 AMPA receptor subunits (Dev et al 1999, Xia et al 1999), the GluK1 and GluK2 subuits of the kainate receptor (Hirbec et al 2003) and the mGlu7 receptor (Dev et al 2000) via their extreme C-terminal PDZ binding motifs.
In contrast to AMPA receptors, where PICK1 directed phosphorylation of the GluA2 subunit releases AMPARs from their interaction with GRIP so making them available for surface expression, the effects of PICK1 on kainate receptors is to enhance the interaction with GRIP so stabilising these receptors at the cell surface. Thus, disruption of PICK1-PDZ interactions in the hippocampus results in an increase in the AMPAR-mediated EPSC and a decrease in the KAR-mediated EPSC (Hirbec et al 2003). A similar decrease in KAR-mediated EPSC following PICK1-PDZ interactions is seen in the perirhinal cortex (Park et al 2006).
PICK1 has reently been shown to bind to the C-termius of the GluK5 subunit, alongside a weak interaction of this subunit with SNAP25, a protein intimately involved with internalisation processes (Selak et al 2009).
GRIP is a multi-PDZ domain proteins that binds to GluK2 subunit and two splice variants of the GluK1 subunit (GluK12a and GluK12b) of the kainate receptor (Hirbec et al 2003). While there is little literature to detail the PDZ domains to which these subunits bind, a construct containing PDZ domains 4-7 interacts with GluK12b and GluK12c, GluK2 and GluK3a. Thus it is likely that they use the same domains as AMPA receptors, PDZ 4 and 5 (Dong et al 1997).
The functions of ABP/GRIP appear to be many and varied. Given the multiple PDZ domain structure and the ability to interact with a wide range of different proteins, an obvious primary function would appear to be as a scaffolding and targetting protein, taking receptors and other proteins to their final destinations within synapses and anchoring them there. In contrast to the effects on GluA2, the binding of GluK1 and 2 subunits to GRIP is enhanced by PKC-mediated phosphorylation of Ser880/886 in GluK12b (and equivalent resudues in GluK2) (Hirbec et al 2003), thus stabilising the GRIP/KAR interaction. This appears to take place at the plasma membrane as blocking the GluK12b or GluK2 interactions with GRIP results in a decrease in the KAR-mediated EPSC recorded from hippocampal neurons. Given the opposite effects of phosphorylation and localisation between KAR and AMPAR interactions with GRIP, it is likely that there are two populations of GRIP found at intracellular and membrane locations. In this regard, it is interesting the some splice variants of ABP can be palmitoylated, and so be anchored in the plasma membrane (DeSouza et al 2002).