Coincidence detection at the level of phospholipase C activation mediated by the m4 muscarinic acetylcholine receptor.
BACKGROUND
One of the principal mechanisms by which G-protein-coupled receptors evoke cellular responses is through the activation of phospholipase C (PLC) and the subsequent release of Ca2+ from intracellular stores. Receptors that couple to pertussis toxin (PTX)-insensitive G proteins typically evoke large increases in PLC activity and intracellular Ca2+ release. In contrast, receptors that use only PTX-sensitive G proteins usually generate weak PLC-dependent responses, but efficiently regulate a second effector enzyme, adenylyl cyclase. For example, in many cell types, agonist binding by the m4 muscarinic acetylcholine receptor (m4 receptor) results in a strong inhibition of adenylyl cyclase and very little stimulation of PLC activity or release of intracellular Ca2+. We have investigated whether the weak, PTX-sensitive stimulation of PLC activity by the m4 receptor can play a significant role in the generation of cellular responses.


RESULTS
We report here that PTX-sensitive Ca2+ release mediated by the m4 receptor in transfected Chinese hamster ovary cells is greatly enhanced when endogenous purinergic receptors simultaneously activate a PTX-insensitive signaling pathway. Furthermore, m4-receptor-induced transcription of the c-fos gene (a Ca(2+)-sensitive response) is similarly potentiated when purinergic receptors are coactivated. These enhanced m4-receptor-dependent Ca2+ responses do not require an influx of external Ca2+, and occur in the absence of detectable purinergic-receptor-stimulated Ca2+ release; they apparently require the activation of both PTX-sensitive and PTX-insensitive G-protein pathways. Measurements of phosphoinositide hydrolysis indicate that the enhancement of m4-receptor-mediated Ca2+ signaling by purinergic receptors is due to a synergistic increase in agonist-stimulated PLC activity.


CONCLUSIONS
These studies demonstrate that the potency of m4-receptor-mediated PLC signaling is highly dependent upon the presence or absence of other PLC-activating agonists. The ability of the m4 receptor to evoke a strong, but conditional, activation of PLC may allow this type of receptor to participate in a coincidence-detection system that amplifies simultaneous PLC-activating signals through a mechanism involving crosstalk between PTX-sensitive and PTX-insensitive G-protein pathways.
Carroll
R C
RC
Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
Morielli
A D
AD
Peralta
E G
EG
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism; D006224:Cricetinae; D004789:Enzyme Activation; D019204:GTP-Binding Proteins / Q000378:metabolism; D016762:Genes, fos; D011976:Receptors, Muscarinic / Q000378:metabolism*; D011983:Receptors, Purinergic / Q000378:metabolism; D015398:Signal Transduction; D014158:Transcription, Genetic; D010738:Type C Phospholipases / Q000378:metabolism*
D011976:Receptors, Muscarinic; D011983:Receptors, Purinergic; D000255:Adenosine Triphosphate; D010738:Type C Phospholipases; D019204:GTP-Binding Proteins; D002118:Calcium