MAPK activation by acetylcholine in C. elegans pharynx

초록

The importance of acetylcholine neurotransmission is highlighted in Alzheimer's disease, where massive loss of cholinergic neurons causes memory loss. One of the suggested mechanisms of acetylcholine transmission is activating ERK-1/2, which is essential in learning and memory.In this study we dissect a muscarinic receptor pathway to MAPK in the C. elegans pharynx, showing that MPK-1, the C. elegans ortholog of ERK-2, is one of the muscarinic receptor targets, and we define the physiological role of the pathway. 30 min treatment of whole worms with 1mM arecoline, a muscarinic agonist, induces MPK-1 activation. Loss-of-function mutations of ras or nPKC (TPA-1) reduce MPK-1 activation, as does co-treatment with a MEK inhibitor or with a muscarinic antagonist atropine. These results suggest that arecoline activates MPK-1 through muscarinic receptors, PKC, ras and mek. To study the physiological role of the pathway, we use arecoline or aldicarb to increase acetylcholine signaling. A G protein beta subunit mutant gpb-2 is hypersensitive to arecoline because of excessive Gq signaling and misregulation of pharyngeal pumping: in the presence of 5mM arecoline they arrest in the first larval stage and die. MPK-1 activation in the gpb-2 mutant is higher than in the wild type and introduction of a reduced function mutation of mpk-1 can rescue arecoline hypersensitivity of the gpb-2 mutant, suggesting that MPK-1 is involved in Gq signaling. Aldicarb prevents pharyngeal pumping in wild-type worms. Both a ras mutant and a TPA-1 mutant, which show reduced MPK-1 activation by arecoline, also show resistance to aldicarb; the pharynx continues to pump in its presence. This could mean that downregulation of Gq-coupled muscarinic signaling enables worms to survive excessive acetylcholine signaling. In other words, muscarinic signaling coupled to Gq could activate the MAPK pathway under physiological conditions to regulate pharyngeal pumping.