Worm Breeder's Gazette 15(4): 24 (October 1, 1998)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
|1||Department of Molecular Biology and Oncology, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, TX 75235-9148|
When placed on an agar plate containing the insecticide aldicarb, worms hypercontract, stop moving and eating, arrest growth, and die. Aldicarb is an inhibitor of the enzyme acetylcholinesterase. When this enzyme is off, the concentration of the neurotransmitter acetylcholine increases to a level that is toxic to the worm. What are the target receptors for acetylcholine that lead to the toxic effect of aldicarb?
Of 21 genes shown by Miller et al (PNAS 93:12593) to confer sensitivity to the toxic effect of aldicarb, most function presynaptically, in the release of acetylcholine. Only ric-3 was proposed to function in the reception of acetylcholine postsynaptically. The paucity of clear postsynaptic aldicarb-resistant mutants lead us to hypothesize that there is more than one target for acetylcholine that mediates its toxic effect, at least one in the body muscles and at least one in pharyngeal muscles, the two major muscle groups in the worm that are affected by aldicarb.
Acetylcholine acts on two types of receptors, nicotinic receptors, which are ligand-gated ion channels, and muscarinic receptors, which are G-protein coupled receptors. The main nicotinic response in body muscles is mediated by the nicotinic acetylcholine receptor subunits UNC-29, UNC-38, and LEV-1 (Fleming et al, J. Neurosci. 17:5843). Although mutations in these genes alone do not confer aldicarb-resistance, they can contribute to resistance in a sensitized genetic background (Yook and Jorgensen WM97). The main nicotinic response in pharyngeal muscles is mediated by the gene product of eat-18 (Raizen et al, Genetics 141: 1365). There are no known mutations in a worm muscarinic receptor but the behavioral response to muscarinic agonists can be attenuated by the muscarinic antagonist atropine (LA, unpublished observations).
We tested whether combinations of an unc-29 mutation, an eat-18 mutation, and the drug atropine can confer resistance to aldicarb. We made the following observations:
Our interpretation of these results is that the acetylcholine targets that mediate the toxic effect of aldicarb include three receptors: a pharyngeal nicotinic receptor, a body muscle nicotinic receptor, and a muscarinic receptor. If only one of these targets is eliminated, there is no resistance to aldicarb. When two targets are eliminated, there is partial aldicarb resistance. The problem with this simple redundancy model is that when all three targets are eliminated, there is no greater aldicarb-resistance (and in fact, there is decreased resistance). An explanation for the effect of atropine on the double mutant is that when all three types of cholinergic receptors are blocked, the worm is so sick that it dies of reasons unrelated to the effect of aldicarb. It is known that in a complete absence of acetylcholine stimulation of its various receptors (in a cha-1 or unc-17 null mutant, where no acetylcholine is released), the worm arrests as a larvae. Note that both in eat-18 and in the unc-29 eat-18 double, atropine causes a growth retardation in the absence of aldicarb.
Lastly, how do we explain the hypersensitivity of eat-18 to aldicarb? In the absence of pharyngeal muscle nicotinic reception, perhaps there is an upregulation of a muscarinic pathway. Thus, less acetylcholine needs to build up to exert a toxic effect.