System Maintenance Notice

Please note that maintenance work is scheduled between 4 AM and 5 AM - CET. During this time, you may experience reduced performance. Thanks for your patience.

Publications

Journal Article Ahead of print

PubMed: 40994051 DOI: 10.1093/brain/awaf356

Normalization of network activity in an epilepsy model with a constitutively active GABBR2 variant.

Brain

Authors: Michal Stawarski, Daniel Ulrich, Sebastian Reinartz, Jochen Schwenk, Li-Yuan Chen, Diego Fernandez-Fernandez, Bartosz Adam Frycz, Mehdi Tafti, Yoon Jeon, Ho Lee, Je Kyung Seong, Murim Choi, Robert Lütjens, Martin Gassmann, Bernd Fakler, Tania Rinaldi Barkat, Bernhard Bettler

Affiliations

  • Department of Biomedicine, Pharmazentrum, University of Basel, CH-4056 Basel, Switzerland.
  • Institute of Physiology II, University of Freiburg, 79104 Freiburg, Germany.
  • Department of Biomedical Sciences, Faculty of Biology and Medicine, University of Lausanne, CH-1005 Lausanne, Switzerland.
  • Department of Cancer Biomedical Science, Graduate School of Cancer Science and Policy, National Cancer Center, Gyeonggi 10408, Republic of Korea.
  • Korea Mouse Phenotyping Center, College of Veterinary Medicine, Seoul National University, Seoul 08826, Republic of Korea.
  • Department of Biomedical Sciences, Seoul National University College of Medicine, Seoul 03080, Republic of Korea.
  • Addex Therapeutics, CH-1202 Geneva, Switzerland.
  • Signalling Research Centres BIOSS and CIBSS, University of Freiburg, 79104 Freiburg, Germany.

Abstract

The neurotransmitter GABA activates G protein-coupled GABAB receptors (GBRs) that mediate neuronal inhibition in the brain. These receptors function as obligate heterodimers, consisting of the GB1 and GB2 subunits, with GB1 binding GABA and GB2 interacting with the G protein. The monoallelic variants p.A567T, p.S695I, and p.I705N in the GABBR2 gene, which encodes the GB2 subunit, have been associated with epileptic encephalopathy and Rett-like disorders. The clinical phenotypes overlap with those seen in individuals with monoallelic loss-of-function variants in GABBR1, the gene encoding the GB1 subunit. To investigate the effects of these GABBR2 variants on GBR function, we expressed the variants in heterologous cells and evaluated their pharmacological profiles using a luciferase reporter assay. Furthermore, we introduced the epileptic encephalopathy-associated p.I705N variant into the mouse Gabbr2 gene to examine its impact on neuronal and network activity. These mice were analyzed using proteomic approaches, in combination with in vitro and in vivo electrophysiological techniques. Finally, we evaluated whether the observed network alterations could be reversed pharmacologically. In heterologous cells, all variants displayed strong constitutive activity, reaching 50-100% of the maximal GABA-induced activity of wild-type receptors. This gain-of-function effect was evident regardless of whether the variants were expressed as individual subunits or as heterodimeric receptors. EEG recordings from Gabbr2I704N/+ mice revealed abnormal high-amplitude synchronization in the δ frequency band, without overt seizures. Electrophysiological recordings from brain slices confirmed an increase in constitutive activity in both pre- and postsynaptic GBRs, but also revealed a significant reduction in receptor responsiveness to agonists. Proteomic analysis of brain tissue further revealed a downregulation of both GB1 and GB2 subunits, along with several G protein signaling components. This downregulation likely serves as an adaptive response to the heightened constitutive activity, reducing not only the activity of the variant receptors but also the signaling of wild-type receptors. In vivo recordings from the auditory cortex of awake Gabbr2I704N/+ mice revealed reduced spontaneous neuronal activity and a slower decline in neuronal activity following auditory stimuli. Treatment with a positive allosteric modulator of GBRs normalized spontaneous network activity and the termination of neuronal activity after sensory stimulation in these mice. In conclusion, our findings indicate that the clinical phenotypes associated with constitutively active GABBR2 variants are driven by an adaptive downregulation of GBRs and their key signaling components. Therefore, in monoallelic individuals, positive allosteric modulators that enhance wild-type receptor activity may provide a promising therapeutic strategy.

MeSH

Cells, Genes, Epilepsy, Autism Spectrum Disorder, Brain, Phenotype, Neurodevelopmental Disorders, Brain Diseases, Therapeutics, Down-Regulation, Neurology, Intellectual Disability