Tau Tubulin Kinase TTBK2 Sensitivity of Glutamate Receptor GluK2

Abstract

Background/Aims: Inherited, autosomal dominant spinocerebellar ataxia type 11 (SCA11) is caused by loss of function mutations of TTBK2 (tau tubulin kinase 2). Mutations observed in patients with SCA11 include truncated TTBK2(450). The present study explored the possibility that TTBK2 influences the function of the glutamate receptor GluK2. Methods: GluK2 was expressed in Xenopus oocytes without and with additional expression of wild type TTBK2, the truncated mutant TTBK2(450), or the kinase dead mutants TTBK2(KD) and TTBK2(450/KD). GluK2 current was determined by dual electrode voltage clamp and GluK2 protein abundance in the cell membrane utilizing confocal microscopy. Results: Glutamate exposure of GluK2 expressing oocytes generated a current, which was significantly lower in oocytes expressing GluK2 together with TTBK2 wt or TTBK2(KD) than in oocytes expressing GluK2 alone or together with either TTBK2(450) or TTBK2(450/KD). According to confocal microscopy of EGFP-tagged GluK2, TTBK2 wt decreased the GluK2 protein abundance in the cell membrane. Overexpression of an inactive RAB5(N133I) mutant but not RAB5wt could reverse the TTBK2 effect on GluK2 suggesting that RAB5 function is required for the effect. Conclusions: TTBK2 down-regulates GluK2 activity by decreasing the receptor protein abundance in the cell membrane via RAB5-dependent endocytosis, an effect that may protect against neuroexcitotoxicity.

Spinocerebellar ataxia type 11 (SCA11): An update

“…( 4) Regulation of membrane transporters and receptors: TTBK2 increases γ-aminobutyric acid (GABA) intracellular transport by retarding the clearance of betaine/GABA transporter 1 (BGT1) from the cell membrane, thus increasing GABA-induced currents (Almilaji et al, 2013). Conversely, TTBK2 may restrict glutamate influx through endocytosis of the small GTPase RAB5-mediated glutamate kainate receptor subunit 2 (GluK2) (Nieding et al, 2016). TTBK2 also regulates sodium/glucose cotransporter 1 (SGLT1) and synaptic vesicle protein 2A (SV2A).…”

Spinocerebellar ataxia type 11 (SCA11): TTBK2 variants, functions and associated disease mechanisms

“…Since then, several cellular functions have been attributed to TTBK2, but few have been described in detail. TTBK2 potential functions include: (1) ciliogenesis [ 44 ]; (2) regulation of microtubule dynamics [ 40 ]; (3) modulation of membrane transporters and receptors [ 45 – 47 ]; (4) phosphorylation of the transactive response DNA-binding protein 43 kDa (TDP-43) [ 48 ]; (5) mitosis [ 49 , 50 ]; (6) cancer progression [ 51 ]; (7) and maintenance of the connectivity and viability of Purkinje cells [ 52 ]. A summarised description of TTBK2 targets in these cellular processes can be found in Fig.…”

“…Other neuronal targets of TTBK2 include the synaptic vesicle protein 2A (SV2A) [ 67 ], the betain/GABA transporter 1 (BGT1) [ 45 ] and the glutamate (kainate) receptor subunit 2 (GluK2) [ 47 ].…”

“…TTBK2 seems to have opposite effects on BGT1 and GluK2 activities in Xenopus oocytes models. Co-expression of TTBK2 with BGT1 enhanced GABA induced currents [ 45 ] while co-expression with GluK2 decreased glutamate-induced currents [ 47 ]. Upregulation of BGT1 and downregulation of GluK2 by TTBK2 is mediated, at least in part, by respectively increasing or decreasing BGT1 and GluK2 protein abundances at the cell membrane (Fig.…”

“…Upregulation of BGT1 and downregulation of GluK2 by TTBK2 is mediated, at least in part, by respectively increasing or decreasing BGT1 and GluK2 protein abundances at the cell membrane (Fig. 2 b and c) [ 45 , 47 ]. Moreover, it is possible that TTBK2 stimulates Gluk2 internalization through small G-protein (GTPase) Rab5-dependent endocytosis, ultimately mediating neuroexcitotoxicity protection [ 47 ].…”