Brunner syndrome associated MAOA dysfunction in human dopaminergic neurons results in NMDAR hyperfunction and increased network activity

BackgroundMonoamine neurotransmitter abundance affects motor control, emotion, and cognitive function and is regulated by monoamine oxidases. Amongst these, monoamine oxidase A (MAOA) catalyzes the degradation of dopamine, norepinephrine, and serotonin into their inactive metabolites. Loss-of-function mutations in the X-linked MAOA gene cause Brunner syndrome, which is characterized by various forms of impulsivity, maladaptive externalizing behavior, and mild intellectual disability. Impaired MAOA activity in individuals with Brunner syndrome results in bioamine aberration, but it is currently unknown how this affects neuronal function.MethodsWe generated human induced pluripotent stem cell (hiPSC)-derived dopaminergic (DA) neurons from three individuals with Brunner syndrome carrying different mutations, and used CRISPR/Cas9 mediated homologous recombination to rescue MAOA function. We used these lines to characterize morphological and functional properties of DA neuronal cultures at the single cell and neuronal network level in vitro.ResultsBrunner syndrome DA neurons showed reduced synaptic density but hyperactive network activity. Intrinsic functional properties and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated synaptic transmission were not affected by MAOA dysfunction. Instead, we show that the neuronal network hyperactivity is mediated by upregulation of the GRIN2A and GRIN2B subunits of the N-methyl-D-aspartate receptor (NMDAR), and rescue of MAOA results in normalization of NMDAR function as well as restoration of network activity.ConclusionsOur data suggest that MAOA dysfunction in Brunner syndrome increases activity of dopaminergic neurons through upregulation of NMDAR function, which may contribute to Brunner syndrome associated phenotypes.

Brunner Syndrome associated MAOA dysfunction in human induced dopaminergic neurons results in dysregulated NMDAR expression and increased network activity

Monoamine oxidase A (MAOA) is an enzyme that catalyzes the degradation of dopamine, noradrenaline, and serotonin. Regulation of monoamine neurotransmitter abundance through MAOA activity strongly affects motor control, emotion, and cognitive function. Mutations in MAOA cause Brunner Syndrome, which is characterized by impulsive aggressive behavior and mild intellectual disability (ID). The impaired MAOA activity in Brunner Syndrome patients results in bioamine aberration, but it is currently unknown how this affects neuronal function. MAOA is highly expressed in serotonergic and dopaminergic neurons, and dysfunction of both neurotransmission systems is associated with aggressive behavior in mice and humans. Research has so far mainly focused on the serotonergic system. Here, we generated human induced pluripotent stem cell-derived induced dopaminergic neurons (iDANs) from individuals with known MAOA mutations, to investigate MAOA-dependent effects on dopamine neuronal function in the context of Brunner Syndrome. We assessed iDAN lines from three patients and combined data from morphological analysis, gene expression, single-cell electrophysiology, and network analysis using micro-electrode arrays (MEAs). We observed mutation-dependent functional effects as well as overlapping changes in iDAN morphology. The most striking effect was a clear increase in N-methyl-D-aspartate (NMDA) receptor mRNA expression in all patient lines. A marked increase was also seen in coordinated network activity (network bursts) on the MEA in all patient lines, while single-cell intrinsic properties and spontaneous excitatory postsynaptic currents activity appeared normal. Together, our data indicate that dysfunction of MAOA leads to increased coordinated network activity in iDANs, possibly caused by increased synaptic NMDA receptor expression.