Abstracts

Rationale: Dysfunction and/or loss of GABA interneurons can lead to synaptic excitation and inhibition imbalance, hyperexcitability, seizures, and epilepsy. Transplantation of exogenous inhibitory neurons could suppress hyperactivity by supplementing synaptic inhibition in the host brain. Progenitor cells from the medial ganglionic eminence (MGE) retain the ability to migrate broadly within the host brain, differentiate into specified interneuron subtypes, namely parvalbumin- (PV) and somatostatin-positive (Sst), and form pre- and post-synaptic interfaces with native neurons (Wichterle et al 1999; Alvarez-Dolado et al 2006). MGE transplants effectively decreased seizure phenotypes, reduced behavioral comorbidities of epilepsy, and reversed circuit plasticity induced by loss of inhibition (Baraban et al 2009; Hunt et al 2013; Howard et al 2014). To better understand how specific cell types might provide therapeutic relief for epilepsy, the mechanisms by which transplanted MGE cells integrate within host circuitry must still be defined. Here we will determine whether transplanted interneurons form subtype-appropriate connections with pyramidal cells.Methods: We performed MGE transplants using donor cells from embryonic day 13 mice. For dual whole cell experiments, G42 mice (expressing GAD67-GFP driven by a PV promotor; Chattopdhyaya et al 2004) served as donors. For optogenetics, donor Ai32 X GAD2-Cre mice express a channelrhodopsin-2/EYFP fusion protein in GABAergic neurons. Transplants were made into the cortices of wild type mice on postnatal day 2. We made dual whole cell electrophysiological recordings from GFP+ PV interneurons and pyramidal neurons in acute slices from transplanted or control (non-transplanted, age-matched G42) mice 35 days after transplant. We used a combination of current and voltage clamp to alternately evoke spikes and record inhibitory or excitatory postsynaptic currents (I- or EPSCs) in neuron pairs to test for reciprocal synaptic connectivity. We measured connection probability, synaptic failures and paired pulse ratio to compare release probability, and PSC kinetics, statistically comparing transplanted interneuron/native pyramidal and native interneuron/native pyramidal pairs. For optogenetics experiments, amplitude and kinetics of light-evoked IPSCs in pyramidal neurons were used to compare increases in aggregate inhibitory tone provided by transplantation.Results: As expected, MGE-derived neurons migrated widely from the injection site and incorporated in the host hippocampus as functional fast-spiking (PV) and regular-spiking non-pyramidal (Sst) interneurons; these properties were similar to native interneurons. Transplanted interneurons received excitatory input from the host circuit in a manner similar to native interneuron populations. MGE-derived interneurons made synaptic connections within the host circuit.Conclusions: These experiments clarify how transplanted interneuron subtypes integrate into host circuitry and begin to provide mechanistic insights into how cell transplantation controls hyperexcitability in epileptic neural circuits. Supported by NINDS R01-NS071785.