Regulation of mitochondrial ATP synthesis by neuronal Cav1

Mitochondria are fundamental cell components because they enable effective energy production. They are also involved in many important biochemical pathways and provide essential cellular signaling molecules and metabolites. Therefore, mitochondria represent crucial organelles for most cell types, including nerve cells. In addition, mitochondria also harbor key elements of programmed cell death (apoptosis), which puts them center stage in neurodegenerative mechanisms. Energy production and metabolism of mitochondria are regulated by intracellular and/or intramitochondrial Ca2+, which also plays a central role in the induction of apoptosis. Neurodegenerative mitochondrial effects have been widely attributed to Ca2+-influx via glutamate receptors of the so called NMDA-type. Other routes of Ca2+ rises in neurons, such as the one provided by voltage-gated calcium channels of the L-type (LTCC), were considered less harmful. Nevertheless, it has been proposed that LTCC-mediated Ca2+-influx may exert detrimental effects under certain conditions. Indeed, elevated levels of LTCC activity were suggested to contribute to the pathogenesis of Parkinsons disease and to play a precipitating role in epileptogenesis. However, in contrast to the proposed role in neuropathology, evidence was also presented that LTCC-mediated Ca2+-influx exerts beneficial effects and promotes neuronal survival. It remained unclear whether mitochondria were involved in such LTCC-dependent neuroprotective mechanisms, too. One of our recent findings may help to shed light on this issue: w e were able to show that LTCC- mediated Ca2+-influx affects mitochondrial function in opposite manners, which depends on the size of the Ca2+-elevation. LTCC-mediated Ca2+-influx that is brought about by stimulation of neuronal activity was found to promote mitochondrial ATP production. However, under conditions of extensive LTCC availability, mitochondria turned into ATP consumers rather than producing this energy carrier. Hence, LTCC-mediated Ca2+-influx may stand at the crossroads of neuroprotective and neurodegenerative mitochondrial processes. Therefore, the mechanistic details underlying the regulatory link between LTCC and mitochondria shall be investigated in future studies proposed in this grant proposal. The overall aim of this project will be to contribute to molecular and functional identification of interactions between LTCC and mitochondria that lead to promotion of ATP production. The results of this project can be envisaged to enable development of new therapeutic strategies for the treatment of neuronal diseases.

Mitochondria are fundamental cell components because they enable effective energy production. They are also involved in many important biochemical pathways and provide essential cellular signaling molecules and metabolites. Therefore, mitochondria represent crucial organelles for most cell types, including nerve cells. Many neurological disorders are characterized by bioenergetic dysfunction. Therefore, understanding how mitochondrial function can potentially go wrong is of utmost importance. The results of this project shed light on both of these aspects. According to our work, calcium entry through a special group of calcium channels called L-type calcium channels (LTCC) is on one hand beneficial in stimulating mitochondrial bioenergetics, however under pathological setting, such as those seen during epileptiform activity, LTCCs provide calcium influx that is high enough to switch mitochondrial oxidative phosphorylation off. We could show that LTCC-mediated stimulation of mitochondrial bioenergetics could in principle occur at three different calcium-sensitive points. Interestingly, if one of these systems fails, another one takes over and ensures that sufficient energy is supplied to meet the prevailing requirement. In this way neurons gain substantial metabolic flexibility. Most importantly our experiments enabled us to identify glycerol-3-phosphate shuttle system (G3PS) as an essential back-up system. The existence of this biochemical transport mechanism in neurons has long remained questionable. However, we could show that the G3PS is also vitally important in neurons and its activity increases only when necessary. In addition to uncovering the details of how calcium stimulates mitochondrial bioenergetics, we also shed light on the pathway that transmits these signals from distantly-located synapses to neuronal bodies. Neuronal endoplasmic reticulum and a subtype of calcium release channels termed Ip3 receptors act as main players in distributing these LTCC-mediated calcium signals to the mitochondria located in neuronal somata. In order to extrapolate from the physiological LTCC-mitochondria interaction to pathological settings, we focused our attention on a particular type of epileptiform activity called paroxysmal depolarization shifts. We could show that L-type calcium channels provide essential calcium entry pathway which causes aberrant changes to the mitochondrial metabolism which in a longer run may lead to the development of epilepsy. The data of this project provide a description how neurons meet their energy requirements under various intensities of neuronal firing. The link between LTCCs and mitochondria should therefore be considered when LTCC inhibitors are being proposed as a potential treatment for neurologic disorders. Inhibition of this communication needs to be done in a careful way, because it may impinge on the potentially important signaling pathway.