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Koromina, Maria (2019) Investigating the contribution of kainate receptors to neurodevelopmental disorders. PhD thesis, University of Nottingham.

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Abstract

Kainate receptors (KARs) are ionotropic glutamate receptors involved in presynaptic and postsynaptic neuroplasticity mechanisms. They form functional ion channels by tetrameric combinations of five different subunits (GRIK1-GRIK5, GluK1-GluK5) and they are modulated by auxiliary proteins Neto1 and Neto2. In this study, it was hypothesized that common and rare loss of function and pathological coding alleles within KAR subunit and NETO genes contribute to risk for developing neuropsychiatric disorders.

One of the aims of this study was to screen for common and rare coding GRIK and NETO variation within individuals with neurodevelopmental diseases and control populations. The coding regions of kainate receptor subunit and NETO genes were analysed using sequencing data from 1745 individuals with severe neuropsychiatric disorders, 741 individuals with ASD or intellectual disability (ID), 2095 population controls and 128 family member with schizophrenia belonging to a mega pedigree. The present findings suggest an excess of singleton and rare loss of function (LoF) and missense variants within the SCZ cases compared to controls (p = 1.8 x 10-10), as well as a significant enrichment of LoF, missense and regulatory variants with neuropsychiatric phenotypes (first discovery phase p = 1.6 x 10-11; second discovery phase p = 1.3 x 10-25) and with autism spectrum disorder (p = 6.9 x 10-18). Single allele associations for 9 coding variants were significantly replicated (p < 5 x 10-8) using ExAC cohort data (N > 45,000).

The relationship between cognitive performance and a deletion allele within GluK4, which is reported as protective against risk for bipolar disorder, was also investigated within 1,642 individuals from the TwinsUK study. Individuals with the GluK4 protective deletion allele performed significantly better in Spatial Working Memory compared to insertion homozygotes when adjusted for a clinical diagnosis. GluK4 deletion carriers who had a mental health problem (predominantly depression) showed better performance in visuo-spatial ability and mental processing speed compared to individuals with mental health problems homozygous for the insertion.

Another aim of this study was to investigate and characterise the pharmacological and electrophysiological properties of wild-type and mutated KARs. First, the effect of the human Neto proteins was assessed on GluK2 and GluK2/GluK4 receptors. It was found that h.Neto1-S (human Neto1 short isoform) increased glutamate sensitivity of GluK2 receptors by 4-fold, whilst it decreased glutamate sensitivity of GluK2/GluK4 receptors by 26-fold. Moreover, h.Neto1-S slowed the desensitisation rate of GluK2 receptors by 2.5-fold. The full Neto2 isoform (h.Neto2) decreased by 150-fold glutamate sensitivity of GluK2/GluK4 receptors, whilst having a less clear effect on the agonist sensitivity and the decay kinetics of GluK2 channels. In addition, the functional effect of three rare damaging missense variants ((GluK2(K525E), GluK4(Y555N) and GluK4(L825W)) located within functional domains of GluKs was assessed by performing voltage clamp assays on Xenopus oocytes expressing mutated KAR subtypes. These mutations affected significantly the agonist sensitivity by decreasing glutamate sensitivity and increasing kainate sensitivity. Moreover, these damaging mutations led to a significant decrease of the desensitisation rate (~5-fold) of these channels following either glutamate or kainate application.

Taken together, these novel discoveries define aspects of the GRIK and NETO genetic contribution to mental illness, provide a comprehensive pharmacological characterization of two different KAR subtypes and demonstrate how rare functional mutations may alter the KAR channel activity. Overall, this research provides a better understanding of the link between genetic risk, biological processes and potential therapeutic avenues for brain disorders.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Knight, Helen Miranda Mellor, Ian Richard
Keywords:	Kainate receptors, Neurodevelopmental disorders, Rare variants, Loss of function variants, Burden analysis, Cognitive impairment, GRIK4, Agonist sensitivity, Decay kinetics
Subjects:	Q Science > QP Physiology
Faculties/Schools:	UK Campuses > Faculty of Medicine and Health Sciences > School of Life Sciences
Item ID:	56257
Depositing User:	Koromina, Maria
Date Deposited:	25 Jul 2019 08:33
Last Modified:	19 Jul 2021 04:30
URI:	https://eprints.nottingham.ac.uk/id/eprint/56257

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