Intracellular Electrophysiology: An electrochemical atlas of organelles

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PROJECT SUMMARY The long-term objective of this proposal is to build an electrochemical atlas of organelles to guide the rational manipulation of inter-organelle contacts in the context of neurodegenerative diseases. A new mode of intracellular communication is emerging at the level of organelles, whereby the membranes of two juxtaposed organelles are physically connected, via protein-protein interactions on their cytoplasmic faces, referred to as inter-organelle contacts. Ions and small molecules are actively transferred from one organelle to the other across these contacts, traversing two sealed membranes. Inter-organelle contacts are vital to cell function, tissue homeostasis and physiology because they regulate processes ranging from lipid metabolism to apoptosis. However, we still do not know what signals initiate contact formation or what switches on chemical transport across contacts, nor can we discriminate between functional and dysfunctional contacts. Hence, although we know of specific mutations in proteins that disrupt contact, leading to diverse neurodegenerative diseases, we still do not know how to restore these contacts and treat those diseases. I posit that the electrochemical states of organelles, alone and in contact, will inform which pathways and molecules should be targeted to rectify aberrant contacts in disease states. My rationale is that, if we abstract out the molecular details, inter-organelle contacts resemble neuronal synapses. Even in synapses, ions flow on cue across two sealed, abutting membranes. Just as ion-transport mechanisms across neuronal membranes were revealed by Hodgkin and Huxley’s electrochemical model, an analogous model of organelle membranes will reveal ion flow mechanisms across inter-organelle contacts and which specific flows are impacted in disease. I propose to build an electrochemical atlas of organelles as a universal reference to study contacts in health and disease. This atlas will be a compendium of equations comprising electrochemical models of major organelles, alone and in contact. By enabling us to discriminate normal and aberrant contacts, I envisage the atlas will reveal common pathways across diseases that can be targeted to restore contacts with impacted ion flows. The inability to assay ions or voltage in organelles has prevented the development of electrochemical models of their membranes. Over the last decade, my lab developed a chemical platform to quantify ions and voltage in organelles. By integrating electrophysiology to this platform, I propose to now map out the electrochemical characteristics of organelle membranes in isolation and in contact, and make an electrochemical atlas of organelles. We will apply the atlas to elucidate how Ca2+ flow across aberrant contacts between the endoplasmic reticulum and the lysosome can be rectified to restore lysosomal Ca2+ in parkinsonism. Dysregulated lysosomal Ca2+ is a common factor across many neurodegenerative diseases and the value of the electrochemical atlas is its pioneering ability to reveal common pathways that can be targeted for treatment in a disease cross-cutting manner.