Contributions of different kainate receptor subunits to the properties of recombinant homomeric and heteromeric receptors

Abstract

The tetrameric kainate receptors can be assembled from a combination of five different subunit subtypes. While GluK1-3 subunits can form homomeric receptors, GluK4 and GluK5 require a heteromeric partner to assemble, traffic to the membrane surface, and produce a functional channel. Previous studies have shown that incorporation of a GluK4 or GluK5 subunit changes both receptor pharmacology and channel kinetics. We directly compared the functional characteristics of recombinant receptors containing either GluK4 or GluK5 in combination with the GluK1 or GluK2 subunit. In addition, we took advantage of mutations within the agonist binding sites of GluK1, GluK2, or GluK5 to isolate the response of the wild-type partner within the heteromeric receptor. Our results suggest that GluK1 and GluK2 differ primarily in their pharmacological properties, but that GluK4 and GluK5 have distinct functional characteristics. In particular, while binding of agonist to only the GluK5 subunit appears to activate the channel to a non-desensitizing state, binding to GluK4 does produce some desensitization. This suggests that GluK4 and GluK5 differ fundamentally in their contribution to receptor desensitization. In addition, mutation of the agonist binding site of GluK5 results in a heteromeric receptor with a glutamate sensitivity similar to homomeric GluK1 or GluK2 receptors, but which requires higher agonist concentrations to produce desensitization. This suggests that onset of desensitization in heteromeric receptors is determined more by the number of subunits bound to agonist than by the identity of those subunits. The distinct, concentration-dependent properties observed with heteromeric receptors in response to glutamate or kainate are consistent with a model in which either subunit can activate the channel, but in which occupancy of both subunits within a dimer is needed to allow desensitization of GluK2/K5 receptors.

The auxiliary subunits Neto1 and Neto2 have distinct, subunit-dependent effects at recombinant GluK1- and GluK2-containing kainate receptors

“…The GluK1 kainate receptor subunits are able to form functional homomeric channels that respond to glutamate with rapidly desensitizing currents (Sommer et al, 1992; Swanson & Heinemann, 1998; Fisher & Fisher, 2014). We examined the impact of Neto1 and Neto2 on the characteristics of the whole-cell responses of GluK1 homomers (Figure 1).…”

“…These studies show that the effects of the two Neto subunits are both distinct from one another and dependent upon the identity of the pore-forming subunits. Recombinant kainate receptors containing either GluK1 or GluK2 subunits have very similar functional characteristics (Fisher & Fisher, 2014). However, with the addition of Neto1 or Neto2 subunits these receptors behave quite differently from one another.…”

“…In the absence of Neto subunits, GluK1 and GluK2 each form homomeric receptors with similar functional properties, characterized by relatively low sensitivity to glutamate and rapid and complete onset of desensitization with slow recovery from desensitization (Sommer et al, 1992; Heckmann et al,. 1996; Paternain et al, 1998; Fisher & Fisher, 2014). …”

Amino-terminal domains of kainate receptors determine the differential dependence on Neto auxiliary subunits for trafficking

“…4 A and E), indicating that the mossy fiber synapses can accommodate additional GluK2/5 receptors. In contrast, expression of GluK1 with GluK5, which can form heteromeric receptors (28,29), had no effect on the slow EPSCs (Fig. 4 B and E).…”

A kainate receptor subunit promotes the recycling of the neuron-specific K+-Cl− co-transporter KCC2 in hippocampal neurons

“…GluK2 normally exists in heteromeric combinations with other GluK subunits (15) and associates with Neto2 (16 -18). To test whether GluK2 and Neto2 together can further regulate total KCC2 abundance in vitro, we co-expressed KCC2 in combinations with GluK subunits and Neto2.…”