G protein co-signaling and challenges for translational research

Abstract

The Gq-linked G protein coupled receptors (GPCRs) and their signaling pathways are important clinical targets for the dementia of Alzheimer’s disease and cognitive decline with aging. Gq stimulates phospholipase C-β1 (PLC-β1) activity, increasing levels of inositol-1, 4, 5-trisphosphate (IP3) and diacylglycerol, to initiate mobilization of intracellular Ca2+ and activation of protein kinase C, respectively. While high concentrations of ligand typically evoke large sustained increases in cytosolic Ca2+ levels, it has long been appreciated that the dynamics of the Ca2+ increase are more complex and consistent with multiple levels of regulation. Physiologically relevant concentrations of Gq-ligands evoke rhythmic fluctuations or an oscillation in the level of cytosolic Ca2+. Downstream targets are tuned to respond to the frequency of the Ca2+ oscillations which in turn, reflect the oscillations in IP3 levels. Oscillatory behavior depends on the assembly of self-organizing interactions. The components that contribute to and regulate the Ca2+ oscillator have been unclear, precluding transfer of this fundamental knowledge from bench to bedside. Many GPCRs that signal with Gq also co-signal with G12. G protein co-signaling could therefore regulate the Ca2+ oscillator. This letter explores the potential relationship between Ca2+ oscillations, G protein co-signaling and cellular response in the context of our recent observations. We found that Gq efficacy is synergistic with phosphatidic acid, (PA), a signaling mediator generated downstream of activated G12 and RhoA. Regulation by PA depends on interaction with the unique PLC-β1 PA binding region. G protein co-signaling is therefore a mechanism for GPCRs to collectively assemble self-organizing interactions that regulate the Ca2+ oscillator.

Purinoreceptors and ectonucleotidases control ATP-induced calcium waveforms and calcium-dependent responses in microglia: Roles of P2 receptors and CD39 in ATP-stimulated microglia

“…P 2 Y receptors are GPCRs, of which P 2 Y 1, P 2 Y 2, P 2 Y 4, P 2 Y 6, and P 2 Y 11 promote IP 3 -dependent ER Ca 2+ release via activating G αq proteins ( Kettenmann et al, 2011 ). This pathway includes PKC-dependent negative-feedback inhibition of PLC β ( Litosch, 2013 ), which gives rise to stable IP 3 oscillations and periodic intracellular Ca 2+ release ( Skupin et al, 2008 ). Negative feedback inhibition is a property of classical biochemical oscillators, for which the enzyme reaction rates determine the periodicity and decay of products like IP 3 ( Tyson, 2002 ), Although these oscillations may be stochastic, deterministic representations ( Sneyd et al, 1995 ; Tyson, 2002 ) amenable to ordinary differential equation modeling are commonly used, given their ability to approximate the amplitude and peak-to-peak intervals of experimentally-measured Ca 2+ release events ( Cao et al, 2014 ).…”

“…We model this as a deterministic process, for simplicity, although the Ca 2+ spiking that results resembles that which is observed experimentally ( Skupin et al, 2008 ). As the cytosolic Ca 2+ rises following IP 3 receptor opening, DAG and Ca 2+ promote PKC-dependent inhibition of PLC- β ( Litosch, 2013 ). We also include the activity of ectonucleoside triphosphate diphosphohydrolase-1, also known as CD39.…”

Regulating G protein activity by lipase-independent functions of phospholipase C

“…Levels of PA also increase through the RhoA-stimulated phospholipase D activity (PLD) [12,31]. Intracellular targets for PA-regulation include PLC [66] and accessory proteins which modulate the G protein GTPase cycle [113].…”

“…Receptor tyrosine kinases (RTK) increase the lipase activity of PLC-ε and PLC-γ. PA stimulates PLC-β, PLC-δ, PLC-γ and PLC-ε lipase activity [66].…”

Regulation of phospholipase C-β1GTPase-activating protein (GAP) function and relationship to Gqefficacy

“…The answer to this challenging question is important to basic research and drug discovery. G q is a key regulator of most human behavior but its potential as a high‐value therapeutic target remains to be fully realized . G q receives and processes information from the ligand‐activated GPCR to implement changes in signaling networks as is necessary to achieve appropriate response.…”