GABA-B . GABBR2 . Amyloid Precursor Protein . APP . Rett Syndrome
Epileptic Encephalopathy

Molecular Neurobiology Synaptic Plasticity
 

From the GABA-B receptor proteome to brain functions
and therapeutic concepts

My laboratory studies how the molecular composition of GABA-B receptors
(GBRs), the G protein-coupled receptors for the neurotransmitter GABA, influences
neuronal activity. Because GBRs are implicated in the pathophysiology of neurological
and psychiatric disorders, we also aim at targeting molecularly defined
GBR signaling complexes for therapy. In collaboration with Prof. B. Fakler (University
of Freiburg, Germany), we identifed 30 GBR-associated proteins (Pin & Bettler,
Nature 540, 2016; Bettler & Fakler, Curr. Opin. Neurobiol. 45, 2017). We have
mapped the interactions of several of these proteins with each other and with the
GBR subunits GB1 and GB2 (Fig. 1). We found that GBR components associate
in a modular fashion into a variety of functionally distinct multi-protein complexes.
We analyzed several GBR-associated proteins for their effects on receptor signaling,
neuronal excitability, brain network activity and behavior. Auxiliary KCTD proteins,
for example, regulate the kinetics of GBR-induced currents, explaining kinetic
discrepancies between currents observed in different neurons (Fritzius et al., J.
Neurosci. 37, 2017). KCTD proteins influence both strength and frequency of thalamic
spindle oscillations, showing that kinetic effects of the KCTDs on GBR signaling
regulate network activity (Ulrich et al., Neuropharmacol. 136, 2018). Accordingly,
lack of KCTD16 in mice also influences behavioral responses (Cathomas et
al., Behav. Brain Res. 317, 2017).
Amyloid precursor protein (APP), adherens junction-associated protein 1 (AJAP1)
and PILRα-associated neural protein (PIANP) form three mutually exclusive GBR
complexes by binding to the N-terminal sushi domain of GB1a (Schwenk et al., Nat.
Neurosci. 19, 2016). Because this sushi domain mediates axonal localization, we
tested whether axonal trafficking of GBRs is impaired in APP-/-, AJAP1-/- or PIANP-/-
mice (Dinamarca et al., Nat. Commun. 10, 2019). Selectively APP-/- mice
exhibited a decrease in axonal GBRs (Fig. 2A) and a consequent deficit in GBRmediated
inhibition of neurotransmitter release. Trafficking of APP/GBR complexes
in axons was visualized using time-lapse imaging (Fig. 2B-D). APP associates
with JIP3 and CSTN3 proteins (Fig. 1) of the axonal trafficking machinery. Complex
formation with GBRs stabilizes APP at the cell surface and reduces proteolysis of
APP to Aβ, a component of senile plaques in Alzheimer’s disease. These findings
establish a link between APP/GBR complex formation, axonal trafficking of GBRs
and Aβ production.
The SHRM4 protein, which is genetically associated with intellectual disability and
epilepsy, controls GBR cell surface expression. Knockdown of Shrm4 in rodents
impairs GBR activity, induces anxiety-like behaviors and increases susceptibility to
seizures (Zapata et al., Nat. Commun. 8, 2017). Collaborative work further showed
that GBRs shape the auditory map (Vickers et al., Neuron 99, 2018), evoke distinct
responses in astrocytes (Mariotti et al., Nat. Commun. 9, 2018) and regulate cocaine-
induced behaviors (Edwards et al., Nat. Neurosci 20, 2017).

Drug Discovery
Clinical use of GBR drugs is limited to agonists and the treatment of narcolepsy,
spasticity and alcohol use disorders. One reason for the limited use of GBR agonists
is that global GBR activation results in unwanted side effects. The recognition
that GBRs form a variety of multi-protein complexes provides opportunities for targeting
cell-type specific functions and individual signaling pathways (Rosenbaum
et al., Nat Rev Drug Discov (2020). https://doi.org/10.1038/s41573-020-0086-4.
Targeting molecularly defined receptor complexes should reduce side effects and
enable new therapeutic applications. In collaboration with the medicinal chemistry
group of K. Strømgaard (Pharmaceutical University, Copenhagen), we developed peptide-based inhibitors of the GBR/KCTD interaction (Sereikaite et al., J. Med.
Chem. 62, 2019). These inhibitors are expected to exhibit anxiolytic properties
by preventing GBR desensitization. We are currently also developing peptides interfering
with defined presynaptic GBR complexes to facilitate glutamate release,
particularly for the treatment of cognitive dysfunctions.

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