Abstracts

Rationale: DLX proteins are transcription factors important for the normal development of GABAergic interneurons. DLX1 mutant mice lose a subset of cortical and hippocampal somatostatin-, NPY-, VIP-, and calretinin-positive interneurons die at ~1 month of age. Selective interneuron loss is accompanied by a decrease in inhibitory synaptic transmission onto cortical and hippocampal pyramidal neurons, and late-onset of epilepsy (Cobos et al 2005). Excitatory synaptic transmission onto surviving interneurons also undergoes changes following interneuron death (Jones et al 2011). Spontaneous seizures become apparent at a low frequency in some mice as young as 2 months and in most mice by 6 months (Cobos et al 2005). To further investigate hippocampal circuit changes triggered by selective interneuron loss, and test a cell therapy approach to reverse these deficits a series of electrophysiological, anatomical and transplantation experiments were performed. Methods: Voltage- and current clamp recordings in acute hippocampal slices. Embryonic dissection and postnatal transplantation of interneuron progenitors isolated from the medial ganglionic eminence (MGE) of GFP+ donor embryos. Immunohistochemistry for GAD67, NeuN, parvalbumin, somatostatin, NPY and calretinin. Results: We report homeostatic compensation in excitatory connections between pyramidal neurons of DLX1-/- mice following interneuron loss. Specifically, decreased glutamatergic synaptic connections between hippocampal CA3 and CA1 pyramidal neurons (assessed in voltage-clamp recordings of miniature and evoked EPSCs), and decreased excitability of CA1 pyramidal neurons (assessed in current-clamp recordings of input resistance and rheobase). Next, we generated new interneurons by transplanting MGE progenitors into hippocampi of P2 DLX1-/- and wild-type mice. At 30 days after transplantation, MGE cell grafts generated GABAergic interneurons that functionally integrated and restored inhibitory synaptic transmission in DLX1-/- mice to WT levels. MGE-derived interneurons stained positively for interneuron-specific antibodies, as described (Alvarez-Dolado et al. 2006). Interestingly, DLX1-/- mice receiving MGE transplants exhibited normal levels of excitatory synaptic activity between pyramidal neurons, and normal pyramidal neuron excitability. Conclusions: Our results suggest that changes in inhibitory synaptic activity, or interneuron number, elicit homeostatic compensation in the excitatory circuitry of the hippocampus in vivo. Furthermore, circuit deficits caused by DLX1 deficiency and subsequent interneuron loss can be corrected by MGE progenitor cell transplant.