Synapses represent key transduction machines that convert incoming electrical information in the form of action potentials into a secreted chemical message which in turn is converted back into a postsynaptic electrical response. The orchestration of these events is thought to underlie critical mechanisms of learning and memory, and dysfunction of synaptic communication is suspected to be central in a number of diseased states of brain function. Synapses however are located at significant distances from cell bodies, and the highly-localized cell biological machinery must rely on local sources of ATP for function. In addition to the ion pumps that are present on both pre and postsynaptic membranes that work to restore ionic balance following activity both compartments contain high concentrations of proteins that must consume ATP in their role carrying out signal transduction, as well as key membrane trafficking events delivering and retrieving proteins and lipids to and from the plasma membrane. Because of these high energy needs, a large fraction of nerve terminals and postsynaptic dendrites are endowed with local mitochondria. Little is known however about intracellular ATP concentrations at these sites, how these concentrations are impacted by synaptic activity, how metabolic needs are coupled to activity, how the presence of local mitochondria impacts local ATP levels, or the extent to which local synaptic ATP generation relies on glycolysis versus oxidative phosphorylation. A local direct reporter of ATP levels is required to access this information. Given that mitochondrial dysfunction has been implicated in a number of neurodegenerative diseases, the ability to directly measure ATP concentration dynamics at individual nerve terminals will be valuable for examining ATP metabolism in these diseased states as well. Here we propose to develop imaging methodology to help fill this information gap. Our approach will be to develop a combined chemo-luminescence and fluorescence approach in the form of a genetically encoded and synaptically targeted ATP indicator that will provide calibrated, dynamic, intracellular synaptic ATP concentration profiles in living nerve terminals.
Information flow in the brain is mediated by transduction of electrical information into chemical information and back again at chemical synapses. The functioning of the human brain relies on the careful orchestration of delivering neurotransmitter-laden vesicles to sites at nerve terminals where they can be used to deliver this chemical message on demand. Many known genetic mutations in diseases such as Parkinson's disease, migraine headache and schizophrenia are linked to proteins that control synapse function. Our work is aimed at understanding the machinery at a molecular level to better ensure the success of future therapies for these types of neuronal diseases. Here we are proposing to develop technologies that will allow us to examine how synapses regulate their energy supply, which is thought to be a critical area of malfunction in certain neurological diseases.
|Rangaraju, Vidhya; Calloway, Nathaniel; Ryan, Timothy A (2014) Activity-driven local ATP synthesis is required for synaptic function. Cell 156:825-35|