Extracellular and Intracellular Regulation of G Protein-Coupled Receptor Signaling Public Deposited

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  • March 19, 2019
  • Rittiner, Joseph
    • Affiliation: Eshelman School of Pharmacy, Division of Chemical Biology and Medicinal Chemistry
  • Signaling through G protein-coupled receptors (GPCRs) is an essential part of cellular communication, and nearly half of modern pharmaceuticals target GPCRs in some way. GPCRs are also subject to extensive biological regulation, which is incompletely understood. Here, I examined both extracellular metabolic control of GPCR signaling, and intracellular feedback mechanisms which regulate downstream signal transduction and receptor desensitization. I first found that the nucleotide adenosine 5'-monophosphate (AMP) is an agonist of the A1 adenosine receptor. Previously, AMP was thought to signal exclusively via hydrolysis to adenosine, and no AMP receptor was known to exist. Using a novel real-time assay of adenosine receptor activity, I showed that AMP directly activates A1R independent of hydrolysis to adenosine, but that activation of the adenosine A2B receptor required hydrolysis to adenosine. I also identified a histidine residue in the A1R binding pocket critical for receptor activation by AMP, but not adenosine. These results suggest that some of the A1R-mediated physiological effects attributed to adenosine may in fact be directly caused by AMP. Furthermore, I found that extracellular loop 2 partially determines A1R sensitivity to AMP, and that AMP stimulation elicits differential activation of signaling cascades downstream of A1R. In subsequent work, I found that the novel lipid kinase diacylglycerol kinase eta (DGKh) positively modulates signaling downstream of muscarinic and purinergic GPCRs. This effect required DGKh catalytic activity, but only a minimal level of DGKh expression. DGKh expression also suppressed extracellular signal-related kinase (ERK) phosphorylation downstream of protein kinase C (PKC) and both activation and depletion of PKC abolished the DGKh effect on GPCR signaling, indicating that DGKh enhances GPCR signaling by suppressing PKC activation, likely reducing receptor desensitization. Enhanced expression of DGKh is linked to bipolar disorder (BPD), suggesting that increased DGKh activity contributes to the dysregulation of GPCR and PKC signaling in BPD and that DGKh inhibitors may have therapeutic potential for the treatment of BPD. Together, my work expands the current understanding of adenosine receptor signaling, implies an enhanced role for multiple endogenous ligands and functional selectivity in physiological GPCR signaling, and highlights GPCR-modulating enzymes as targets for drug discovery.
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  • In Copyright
  • Zylka, Mark J.
  • Doctor of Philosophy
Graduation year
  • 2013

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