During the first period of this grant we have focused on understanding how mitochondria, the powerhouses of any cell, can adapt their ability to produce energy to the activity state of neurons. When neurons communicate to each other, there are several changes occurring in a neuron that facilitate their capacity to send information. A key molecular process consists in the entry of calcium into the neuron that is transmitting information. We discovered that part of that calcium enters directly into mitochondria, which acts as a signal that activates their capacity to produce energy for the active neuron. As such, mitochondria produce energy proportionally to what was consumed before, making the process of sending information energetically sustainable. However, while we understand now that calcium entry in mitochondria is necessary for producing energy, we hypothesized that after performing its activating action, calcium should leave mitochondria in a timely manner. We discovered a protein called LETM1, whose dysfunction is heavily linked to epilepsy in humans, which drives that mitochondrial calcium exit after a neuron has ended its activity. Experimentally removing this protein using genetic tools drives an increased amount of ATP in neurons as a consequence of poorly coordinated exit of calcium from mitochondria, staying there longer times. Given that LETM1 dysfunction is associated to epilepsy, we believe such increase in ATP may imbalance neuronal networks in a way that generates disease. We are currently investigating how these conditions may modulate higher endbrain functions, such as memory formation, and how they may affect the ability of neurons to communicate to each other.
On the other hand, our project has progressed in better understanding how neurons can preserve their energy levels independently of mitochondria, allowing them to transmit information and preserve brain function. ATP, the main molecular fuel of neuronal communication, can be obtained by two main processes: glycolysis (using glucose) or mitochondrial oxidative phosphorylation (which uses oxygen and other substrates). Our main results for this first period of the grant have been focused on oxidative phosphorylation, which is a process driven by mitochondria. However, to better understand the molecular mechanisms facilitating glucose use by neurons, we have developed novel molecular technologies that allow seeing how many glucose transporters are in synapses, the regions of the neurons that communicate information. The surface of a synapse is decorated with a large variety of molecules that facilitate neuronal communication. One of these molecules is called GLUT4, which is a protein that has the ability of transporting glucose inside the neuron. Intuitively, the higher the number of these transporters, the better the ability of a synapse to capture surrounding glucose to facilitate its function. However, understanding quantitatively how many proteins of a particular type are present in the surface of a synapse, and whether this number can change dynamically depending on the needs, remains poorly understood. We have developed novel technologies that allow isolating all proteins present in the synaptic surface, both at rest or during neurotransmission, allowing to understand how surface proteins adapt to the activity of the neuron. This now includes GLUT4, but it is a novel technology that expands what could be studied before and can provide a holistic picture of how the surface landscape of synapses changes when neurons talk to each other.
