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Janice R. Naegele


Neuroscience and Behavior




Recent studies suggest that synaptic integration of transplanted GABAergic progenitors derived from the medial ganglionic eminence (MGE) into the dentate gyrus of mice with temporal lobe epilepsy (TLE) may lead to seizure suppression (Henderson, Gupta et al., under revision). However, the molecular mechanisms for synaptic integration of MGE-derived GABAergic interneurons into the adult hippocampal circuit are not known. Prior work suggests that neuroligin 2, a cell adhesion molecule that is located specifically to the inhibitory postsynaptic sites, together with its presynaptic partner neurexin, regulates the formation and stabilization of inhibitory synapses in the brain and in vitro.

To examine inhibitory synapse formation, I developed an in vitro model system in which MGE-derived GABAergic interneurons were plated onto relatively mature hippocampal cells. I showed that MGE-derived GABAergic progenitors matured and differentiated into major functional subtypes of hippocampal GABAergic interneurons, including those that co-express somatostatin, neuropeptide Y and calbindin. Furthermore, these interneurons formed GABAergic synapses onto the hippocampal neurons, as indicated by the expression of the inhibitory cell adhesion molecule neuroligin 2 and scaffolding protein gephyrin. I further characterized the role of gephyrin in dendritic and perisomatic synapse formation. By 14DIV, almost all of the gephyrin clusters were in close contact with VGAT-Venus+ GABAergic presynaptic terminals, indicating that the gephyrin clusters became more localized to the synapses as the neurons matured. Additionally, at 14DIV, almost all of the VGAT-Venus+ GABAergic terminals were in close contact with the gephyrin clusters on the soma, but only 50% of the presynaptic terminals on the dendrites were associated with gephyrin. These results suggested that perisomatic and dendritic synapse formation may involve different molecular mechanisms or a different time course. The present study also showed that the tri-culture contained dentate GCs. In TLE, these cells undergo neuronal and synaptic plasticity. Therefore, GABAergic synaptogenesis onto the GCs is a striking feature of this new in vitro model. The triculture system described here will allow close examination of GABAergic synapse formation and application of genetic tools, such as adeno-associated virus-mediated gene delivery, to manipulate and investigate specific GABAergic synaptic proteins and their roles in inhibitory GABAergic synaptogenesis.



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