Structure and function of the hippocampal CA2 region in temporal lobe epilepsy

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Human temporal lobe epilepsy (TLE) is characterized by refractory epileptic seizures mainly of hippocampal origin together with structural changes in the hippocampus. The latter include the loss of principal cells and interneurons in parts of the Ammons horn (CA3, CA1) and hilus, reactive gliosis and the dispersion of the granule cell layer. In contrast, the neurons of the CA2 region are mostly resistant to the pathological changes induced by TLE. Despite this conspicuous feature the role of the CA2 region in TLE has hardly been investigated up to now due to a lack of suitable methods to precisely distinguish CA2 principal cells from the neighboring regions CA3 and CA1. Recently, novel methods (e.g., viral tracing, CA2-specific antibodies and transgenic mouse lines) have been developed allowing for targeted investigation of CA2 and yield new insights into its structure, connectivity and function in the healthy brain. We are planning to use these methods, both, in a well-established mouse model for TLE and in human hippocampal tissue obtained from epilepsy surgery to investigate the role of CA2 with respect to its synaptic integration in the epileptic hippocampal network. Our project aims at determining a critical role that the preserved CA2 region might play in seizure onset and propagation since CA3 and CA1 are functionally highly impaired due to neuronal loss in these regions.We will therefore use the intrahippocampal kainate model in mice and human hippocampal tissue from epilepsy surgery, which we obtain directly from the Dept. of Neurosurgery. Using histological methods (immunocytochemistry, in situ hybridization) we will quantify the degree of preservation of the CA2 region in the animal model and patient tissue and correlate it with clinical parameters (e.g., frequency of seizures, duration of epilepsy, age at epilepsy onset). We will characterize the mossy fiber projection in the CA2 region with confocal and electron microscopy using a transgenic mouse line that expresses green-fluorescent protein in dentate granule cells and their axons. Specific viral tracers will allow us to detect the synaptic in- and outputs of the CA2 region and to characterize its integration in to the pathological hippocampal network. Furthermore, we will perform electrophysiological in vivo recordings of local field potentials and single cell activity to investigate the functional role of CA2 in epileptic activity. In addition, we will perform patch-clamp recordings in acute slices from human hippocampal tissue and epileptic mice to measure the physiological properties of CA2 pyramidal cells and possible alterations in epilepsy. The broad approach of our project, which combines the easy accessibility and flexibility of the animal model with the clinical condition of human TLE promises new insights into the role of CA2, which might be seminal for new therapeutic approaches.