Yulong Li - US grants

Neuromodulators are essential signaling molecules that regulate many neural processes, including cognition, mood, memory, and sleep, through their influence on brain circuits. Monitoring the release and distribution of neuromodulators in behaving animals is critical for understanding the diverse functions of these molecules. A major impediment to developing this understanding is the lack of tools that can monitor these compounds at the temporal, spatial and concentration scales relevant to these brain processes. Filling this technological gap is one of the most pressing needs in neuroscience research. Our proposal directly bridges this gap by developing a platform of new tools for chronic, non- invasive monitoring of neuromodulators at millisecond, subcellular, and nanomolar resolution. Genetically-encoded fluorescent indicators for calcium and glutamate have transformed investigation of dynamic brain processes in the major model systems, including worms, flies, rodents, and increasingly primates. Building on our prior experience in developing these tools, we now propose to build a new suite of GPCR-activation-based (GRAB) genetically-encoded fluorescent indicators for neuromodulators. Our preliminary data shows we can generate GRABs with >500% fluorescence change and nanomolar affinity in mammalian cells. We propose to further develop and validate these prototypes in cultured neurons, flies, rodent brain slices, anesthetized and behaving mice to maximize their utility. In Aim 1, we will develop GRAB indicators for acetylcholine, serotonin, and norepinephrine by iteratively screening libraries that systematically vary in insertion site, linkers, cpGFP sequence, and FP-GPCR protein surface interface. The dimensions of optimization will be dF/F, membrane surface expression, affinity, and non-disruption of endogenous signaling. Our targeted performance levels are >10x dF/F, nanomolar range affinity and <10 millisecond on-rates in vitro. In Aim 2, performance of top candidate GRAB indicators from the in vitro screen will be validated following long-term expression in drosophila olfactory system, in brain slice, in anesthetized and behaving mouse cortex. Feedback from these experiments will guide iterative optimization in Aim 1. Successful completion of our Aims will yield a suite of powerful molecular constructs, cell-type specific viral tools and technical approaches that will be broadly disseminated to the neuroscience community. The GRAB indicators can be easily integrated with existing mouse models of human mental disorders. Since these probes for neuromodulators are well-suited for a wide range of preparations, and a large number of investigators, they will have a multiplicative impact on our understanding of neural circuit function and dysfunction when combined with other advances supported by the BRAIN Initiative.

SUMMARY Animal behaviors are orchestrated by the sophisticated nervous system, which is dynamically regulated by neuromodulators including lipids and neuropeptides. Endocannabinoids (eCBs) are neurolipids exist broadly in the brain and regulate learning and memory, addiction, pain sensation, and food intake. Among neuropeptides, cholecystokinin (CCK) is involved in nutrient sensing, food intake, and sleep regulation, and oxytocin (OXT) and vasopressin (AVP) play important roles in various aspects of social behaviors. However, how and when lipid and neuropeptide transmission occur in the brain are largely unclear. Existing methods (e.g. microdialysis) that measures brain chemical content suffer from low temporal and spatial resolution. Additionally, since neurolipid and neuropeptide releases often require repetitive neuronal firing and can occur at both axonal and dendritic sites, activity of the neuromodulator- releasing neurons cannot reliably predict where and when neurolipids and neuropeptides are released. Here we propose to develop a set of new tools for long-term monitoring of neurolipids and neuropeptides. Our strategy taps into their natural receptors, human G protein-coupled receptors (GPCRs), which are coupled to GFP. In the presence of neurolipids or neuropeptides, these GPCR Activation-Based (GRAB) sensors transform ligand binding-induced conformational changes into rapid fluorescent signals. We aim to develop and optimize neurolipid and neuropeptide GRAB sensors with >500% fluorescence change (dF/F) and 10- nanomolar affinity in vitro and validate these novel tools in brain slices ex vivo and mouse behavioral paradigms in vivo. In Aim 1, we will develop GRAB sensors for endocannabinoids, CCK, vasopressin, and OXT by systematically varying key sites involved in ligand binding, conformational change, etc. In Aim 2, we will validate the performance of these sensors in brain slice following long-term expression using viral tools. In Aim 3, we will use three different imaging methods (fiber photometry, epifluorescence and 2-photon imaging coupled with GRIN lens) in different behavioral paradigms to test in vivo performance of the novel GRAB sensors in mice. Feedback from experiments in Aims 2-3 will guide iterative optimization in Aim 1. Successful completion of our proposal will yield a suite of powerful tools and technical approaches, which will greatly facilitate studies of neurolipids and neuropeptides under both physiological and pathological conditions, helping reveal disease mechanisms, providing therapeutic guidance, and eventually benefiting human health.

Neuromodulators regulate addiction, attention, cognition, mood, memory, motivation, sleep, and more through their influence on brain circuits. Classic tools for measuring neuromodulation in the brain have poor spatial and temporal resolution. This has hampered the discovery of the diverse and complex functions neuromodulation plays during behavior. Over the past few years, new indicators for imaging neuromodulator dynamics have begun to dismantle these barriers. However, all existing neuromodulator indicators have significant limitations. The goal of this proposal is to optimize our GPCR-activation-based (GRAB) genetically-encoded fluorescent indicators of four major neuromodulators: dopamine (DA), acetylcholine (ACh), norepinephrine (NE), and serotonin (5-HT). We will make their responses bigger and more specific, create red versions for multiplexed imaging, and make them easier for end-users to successfully deploy in vivo. In Aim 1, we will optimize GRAB indicators for DA, ACh, NE, and 5-HT by iteratively screening libraries via high-content confocal imaging and FACS. We will vary insertion site, linkers, cpGFP, FP-GPCR protein surface interface, and thermostabilizing GPCR residues on a range of chimeric GCPR sensor backbones. Library generation will be prioritized by computational prediction of function from GPCR structures. The dimensions of optimization will be brightness, dF/F0, ligand selectivity, affinity, and non-disruption of endogenous signals. Top hits will be validated following long-term expression in mammalian brain slice and behaving mice. Our targeted performance levels are: 1000x ligand selectivity across all neuromodulators (3rd gen), >5x SNR improvement over 2nd generation indicators in vitro and in vivo (3rd gen), and reliable single-trial subcellular resolution of graded responses with in vivo 2-photon imaging of cortex during behavior for all neuromodulators (4th gen). In Aim 2, we will use the same approach as Aim 1 to develop and validate in vivo 1st and 2nd generation red GRABs for the same neuromodulators to enable simultaneous imaging of multiple signals. Our targeted performance levels for second generation, spectrally orthogonal red GRABs are 10x dF/F in vitro, >50% dF/F in vivo responses. We will also engineer out any photoactivation of red GRAB fluorescence, demonstrate multiplexed imaging and optogenetic stimulation with zero opsin excitation crosstalk from imaging light. In Aim 3, we will optimize GRAB packaging and distribution for maximum end-user ease of use. We will quantify the best FPs for in vivo coexpression with GRABs, engineer viral-genetic strategies for robust, brain- wide GRAB expression from systemic AAV injection, and make cre-reporter mouse lines for the best green GRAB of each neuromodulator. Optimized plasmids, AAVs, and mice will be broadly disseminated. Successful completion of our Aims will yield an optimized suite of powerful molecular tools packaged for maximum utility and ease of use. Since these probes are well-suited for a large number of investigators, they will have a multiplicative impact on our understanding of neural circuit function and dysfunction.