Increased NR2B-dependent early network oscillations in a human stem cell-derived model for autism

The term autism spectrum disorder (ASD) comprises a range of neurodevelopmental disorders that are characterized by impaired social interaction, repetitive behavior and deficits in speech and nonverbal communication. ASD susceptibility has a strong genetic component. Additionally, a number of environmental risk factors have been suggested to contribute to ASD pathogenesis. Valproic acid (VPA) is an antiepileptic drug with histone deacetylase (HDAC) inhibition activity. VPA intake during pregnancy is associated with a significantly increased risk of the unborn child to develop neurodevelopmental disorders including ASD.
In rodent models it has been shown that in-utero exposure to VPA results in an autism-related behavioral phenotype in the offspring. On the neuronal level, alterations in excitatory and inhibitory synaptic transmission as well as enhanced NMDA receptor-mediated long-term potentiation (LTP) have been proposed to contribute to these phenotypes.
However, the effect of VPA on developing human neuronal networks is largely unknown. Here, we use human embryonic stem cell (hESC)-derived neuronal cultures as an in-vitro model of early cortical development.
We show that hESC-derived neurons co-cultured with primary mouse astrocytes acquire passive and active membrane properties of mature neurons. They can be differentiated into synaptically connected neuronal networks containing glutamatergic and GABAergic neurons. After approximately two months of differentiation the cultured human neuronal network develops a spontaneous rhythmic synaptic activity resembling NMDA receptor-driven cortical early network oscillations (cENOs) reported in the early postnatal rodent brain.
Chronic VPA exposure strongly increased onset and frequency of spontaneous postsynaptic currents and human ENOs in-vitro due to NMDA receptor overexpression. Furthermore, the density of glutamatergic synapses was increased, as well as synaptic NMDA receptor to AMPA receptor conductance density, indicating that enhanced NMDA receptor activation facilitates the recruitment of glutamatergic neurons to generate increased network activity.
By contrast, the density of GABAergic synapses and frequency of miniature GABAergic currents were unchanged.
Finally, enhanced frequency of human ENOs could be rectified by partial block of NR2B NMDA receptors using a low concentration of Ifenprodil (1 μM). This indicates that NR2B NMDA receptors might constitute a promising target for treatment of hyperexcitability and hyperconnectivity in the framework of ASD.
Taken together, we show that embryonic VPA exposure generates hyperexcitability in human neuronal networks by NR2B NMDA receptor hyperactivation leading to enhanced frequency of ENOs, which can be rescued by partial block of NR2B NMDA receptors.