Quantitative Properties and Receptor Reserve of the IP(3) and Calcium Branch of G(q)-coupled Receptor Signaling

J Gen Physiol. 2013 May;141(5):521-35. doi: 10.1085/jgp.201210886.

Abstract

Gq-coupled plasma membrane receptors activate phospholipase C (PLC), which hydrolyzes membrane phosphatidylinositol 4,5-bisphosphate (PIP2) into the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). This leads to calcium release, protein kinase C (PKC) activation, and sometimes PIP2 depletion. To understand mechanisms governing these diverging signals and to determine which of these signals is responsible for the inhibition of KCNQ2/3 (KV7.2/7.3) potassium channels, we monitored levels of PIP2, IP3, and calcium in single living cells. DAG and PKC are monitored in our companion paper (Falkenburger et al. 2013. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.201210887). The results extend our previous kinetic model of Gq-coupled receptor signaling to IP3 and calcium. We find that activation of low-abundance endogenous P2Y2 receptors by a saturating concentration of uridine 5'-triphosphate (UTP; 100 µM) leads to calcium release but not to PIP2 depletion. Activation of overexpressed M1 muscarinic receptors by 10 µM Oxo-M leads to a similar calcium release but also depletes PIP2. KCNQ2/3 channels are inhibited by Oxo-M (by 85%), but not by UTP (<1%). These differences can be attributed purely to differences in receptor abundance. Full amplitude calcium responses can be elicited even after PIP2 was partially depleted by overexpressed inducible phosphatidylinositol 5-phosphatases, suggesting that very low amounts of IP3 suffice to elicit a full calcium release. Hence, weak PLC activation can elicit robust calcium signals without net PIP2 depletion or KCNQ2/3 channel inhibition.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Calcium / metabolism*
  • Cell Membrane / metabolism
  • Cell Membrane / physiology
  • Cells, Cultured
  • Diacylglycerol Kinase / metabolism
  • GTP-Binding Protein alpha Subunits, Gq-G11 / metabolism*
  • Humans
  • Inositol 1,4,5-Trisphosphate / metabolism*
  • KCNQ2 Potassium Channel / metabolism
  • KCNQ3 Potassium Channel / metabolism
  • Phosphoinositide Phospholipase C / metabolism
  • Potassium Channels / metabolism*
  • Protein Kinase C / metabolism
  • Receptor, Muscarinic M1 / metabolism
  • Receptors, Cell Surface / metabolism*
  • Receptors, Purinergic P2Y2 / metabolism*
  • Signal Transduction
  • Type C Phospholipases / metabolism

Substances

  • KCNQ2 Potassium Channel
  • KCNQ2 protein, human
  • KCNQ3 Potassium Channel
  • KCNQ3 protein, human
  • Potassium Channels
  • Receptor, Muscarinic M1
  • Receptors, Cell Surface
  • Receptors, Purinergic P2Y2
  • Inositol 1,4,5-Trisphosphate
  • Diacylglycerol Kinase
  • Protein Kinase C
  • Type C Phospholipases
  • Phosphoinositide Phospholipase C
  • GTP-Binding Protein alpha Subunits, Gq-G11
  • Calcium