THE LINK BETWEEN PDS AND MITOCHONDRIA IN EPILEPTOGENESIS

Epilepsy in adults is dominated by acquired forms of the disease, e.g. pathologies developing after traumatic or ischemic brain injury or hemorrhagic bleeding. Since there is a time window between the precipitating insult and the first occurrence of unprovoked recurrent seizures, there is room for therapeutic intervention. With an ever increasing age of the population and a prognosed increase in stroke-incidence in the next decades, identifying therapeutic strategies to prevent epileptogenesis is of outmost importance. One promising idea on how to find a way to prevent epilepsies is related to mitochondria. Disturbances of mitochondrial functions have been linked to epileptogenesis, and this may be due to processes ranging from ATP deficiency to cell death (i.e. neuronal loss). Enhanced production of reactive oxygen species may also play a crucial role. Interestingly, one of the major excitability-regulating family of ion channels, L-type voltage-gated calcium channels (LTCCS), has been repeatedly demonstrated to be potentiated by redox modulation. This is intriguing because we have recently identified potentiation of LTCCs as a crucial factor in the formation of a potential epileptogenic electrical phenomenon, the paroxysmal depolarization shift (PDS). The PDS represents the cellular correlate of interictal spikes (IIS). The postulate of PDS being epileptogenic stems primarily from the observation that IIS emerge prior to seizures in animal models of acquired epilepsy and from the close resemblance of PDS to giant depolarizing potentials (GDPs), electrical events that have been recognized as key elements in cellular remodelling during neuronal development. Hence it is possible that PDS are induced by mitochondrial malfunction, e.g. augmented ROS production. However, since PDS involve enhanced activity of LTCCs, relentless Ca2+ influx that arises in the course of PDS may overburden mitochondria, causing further mitochondrial disturbances. It is an obvious task in that respect to identify the starting point of a potentially vicious cycle consisting of ROS-induced potentiation of LTCC resulting in PDS formation and PDS-induced Ca2+ overload of mitochondria, which is known to lead to generation of ROS. The group of the applicant has focused on the role of LTCCs in normal and abnormal discharge activities in recent years using primarily electrophysiological approaches. The main collaborator has gained expertise in imaging of mitochondrial dynamics and has been involved in implementation of excellent tools to experimentally interfere with mitochondrial functions. Hence, this proposal applies for funding that enables a combination of these approaches to make an important contribution in the study of a link between PDS and mitochondria as a target mechanism for the identification of anti-epileptogenic therapeutic strategies.

Mitochondria represent the main site of the generation of ATP, an energy-carrying molecule of cells, and have therefore be termed as cellular "powerstation". Besides ATP production, mitochondria perform several other vital functions. It is hardly surprising therefore, that functional deficits of mitochondria have been linked to various diseases. One such disease is epilepsy, which can be due to genetic causes or may be acquired during life. In the latter case, insults to the brain trigger a so-called latency period, which remains without symptoms, but during which epileptogenic processes are thought to take place that ultimately lead to spontaneously occurring seizures. Mitochondrial dysfunction is thought to play an essential role in these processes. One additional characteristic phenomenon of the latency period is the occurrence of paroxysmal depolarization shifts (PDS), which typically arise synchronously in large number of cells in our near to the region of neuronal damage, and which therefore can be detected as brief (< 1 s) electrographic spikes. It has been hypothesized that PDS are also involved in the process of epileptogenesis. However, it has not been addressed so far, whether there is a link between the occurrence of PDS and mitochondrial dysfunction. This is where the current project set in. The main result of our work is that PDS come with a pronounced influx of Ca2+ ions through a certain type of ion channels in the neuronal membrane, which is known in the field as Cav1.3 channels of the L-type calcium channel family. We were able to show that the Cav1.3 channel-mediated Ca2+ influx is involved in the stimulation of mitochondrial ATP production. However, in a situation of exaggerated levels of Ca2+ influx, as it is in the case of PDS, mitochondria stop generating ATP, but instead consume available ATP in an attempt to prevent mitochondrial damage. The responsible mechanism was identified by us as a reversal of the operating mode of the mitochondrial enzyme that normally produces ATP, the ATP synthase, which then acts as an ATP-driven "protective device". Our results have several implications. First, they provide evidence that PDS may not represent primarily a pathological phenomenon, but that they may on the contrary serve to induce cyto- (and in particular mito-) protective processes. And secondly, our finding of a central regulatory function of L-type calcium channels in this process questions a dogma in the field of neurodegenerative diseases, that L-type calcium channels should be inhibited to reduce neuronal damage under unfavorable conditions. This raises several questions that need to be addressed in future research. However, caution should be exercised when aiming to employ L-type calcium channel inhibition therapeutically: it should be taken into account that an L-type channel-mediated neuroprotective mechanism might be equally eliminated.