Joshua Levitz - US grants

Project Summary In many biological systems G protein-coupled receptors (GPCRs) provide a crucial molecular link between the dynamics of the extracellular environment and the associated intracellular signaling response. In the nervous system, GPCRs serve as detectors of precise patterns of neurotransmitter release and are able to, in turn, modulate neuronal excitability and synaptic transmission. Of particular importance are the class C metabotropic glutamate (mGluR) and GABA receptors (GABABR), which respond to the major excitatory and inhibitory neurotransmitters, respectively, and serve as drug targets for neurological and psychiatric disorders. Unfortunately, our understanding of their underlying molecular mechanisms of signaling remain limited due to a lack of methods for the direct measurement and manipulation of their activity with high specificity and spatial and temporal precision. Furthermore, the biophysical activation mechanism of class C GPCRs is particularly challenging to decipher because unlike class A GPCRs, such as rhodopsin or ß-adrenergic receptors, they contain large, extracellular ligand binding domains (LBDs) that multimerize and couple, via a poorly understood mechanism, to a transmembrane domain (TMD). Our recent work has established new optical methods for directly measuring mGluR assembly and conformational dynamics at the single molecule level and has also produced an optogenetic method to manipulate receptors with subtype selectivity and high spatiotemporal precision using photoswitchable tethered ligands. These breakthroughs have advanced our understanding of how mGluRs dimerize and the initial molecular motions that lead to cooperative receptor activation, but many fundamental questions remain. In research area 1 we will dissect the activation mechanism of mGluRs and GABABRs in a quantitative, interdisciplinary way using optical approaches, including single molecule Forster resonance energy transfer (FRET) to measure conformational dynamics, in conjunction with functional reporters and detailed structural analysis. The long-term goal is to understand, biophysically, how allosteric inter-domain and inter-subunit coupling interactions permit orthosteric and allosteric ligand binding to produce G protein activation. This work will give major insight into the fundamental activation processes of a large class of membrane receptors and should provide a deeper understanding of their molecular pharmacology. In research area 2 we will improve and harness the power of optical sensors of activation and optogenetic control of receptors to probe the kinetics of different mGluR subtypes at the level of activation, signaling, and desensitization and to dissect their spatiotemporal signaling profiles at hippocampal synapses. In the long term we plan to use this information to probe the mechanism of induction of long-term depression by pre-synaptic, post-synaptic, and glial mGluR populations. This work will provide a dynamic picture of mGluR signaling that has been missing from the field and will strengthen our molecular understanding of the role of these receptors in synaptic modulation in health and disease.

PROJECT SUMMARY In the nervous system, G protein-coupled receptors (GPCRs) sense neuromodulators to initiate intracellular signaling cascades that control a plethora of brain functions. Of particular importance are the opioid receptors which contribute to pain sensation, reward processing and mood regulation. Consistent with these roles, opioid receptors serve as major drug targets for a variety of disorders. Agonists of the mu-opioid receptor (MOR) are common analgesics and also have potential as antidepressants. However, a challenge with opioid- based treatment is the propensity for addiction, tolerance and dangerous side effects. Unfortunately, limitations in the precision of pharmacological approaches have hampered our ability to dissect the molecular, cellular and circuit level mechanisms of MOR-mediated disease treatment and develop improved therapeutic strategies. We previously established methodologies for optical control of GPCRs using photopharmacology, which we will adapt for the MOR in the R61 phase. In aim 1, we will develop a range of new photoswitchable compounds for the MOR using a combination of structure-based prediction, chemical synthesis and functional analysis. In aim 2, we will adapt these compounds to make photoswitchable orthogonal remotely-tethered ligands (PORTLs) which covalently attach to target receptors with a labeling tag, such as SNAP, CLIP or Halo. To enable targeting of native receptors, we will further extend our system to develop nanobody-photoswitch conjugates (NPCs) which bind to native receptors and deliver a PORTL for reversible optical control. NPCs can either be genetically- encoded to permit cell type-targeting or can be purified in vitro and directly applied to the sample in a gene-free approach. Together, this will provide a new, widely-applicable toolset for MORs while also developing an engineering framework for extension of this approach across different receptor types. In the R33 phase, we will harness our toolset in vivo to probe the basis of MOR-mediated antidepressant effects in mice (aim 3). We hypothesize that activation of MORs localized on interneurons of the medial prefrontal cortex (mPFC) induce disinhibition that leads to normalization of dysfunctional prefrontal circuits. PORTLs and NPCs will enable targeting to the cell types and brain regions of interest with sufficient spatiotemporal precision to probe the relationship between receptor activation and behavioral modulation. We will use behavioral assays of relevance to depression, as well as measures of reward and dependence with the expectation that targeted photo-activation can produce antidepressant effects with minimal side effects. To gain further mechanistic insight, we will perform slice electrophysiology and in vivo 2-photon calcium imaging to measure the effects of MOR activation on mPFC activity. This work will validate our toolset in vivo and provide a key step toward understanding the mechanism of MOR modulation for depression treatment.