Worm Breeder's Gazette 11(4): 106

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.

The Roles of AVL and DVB in Defecation

Steve McIntire, Joshua Kaplan, Erik Jorgensen and Bob Horvitz

A series of three muscle contractions occurs in C.  elegans during 
defecation (Jim Thomas, Genetics 124:855-872, 1990): (1) the posterior 
body muscle contraction (pboc) forces the gut contents forward, (2) 
the anterior body muscle contraction (aboc) forces the gut contents 
backward, and (3) the expulsion muscle contraction (exp) forces the 
gut contents through the anus.  This sequence is repeated every 50 
Two GABAergic neurons, AVL and DVB, are jointly required for exps.  
As a first step toward identifying the neurons involved in defecation, 
we have shown that unc-25 animals, which lack immunoreactivity to the 
neurotransmitter GABA, are constipated due to a reduction of exps.  
Killing two GABAergic neurons, AVL and DVB, nearly eliminates exps 
producing a constipated phenotype very similar to that of unc-25.  
Killing either DVB or AVL alone decreases the frequency of exps, but 
does not lead to constipation.
In addition, AVL is also required for abocs.  Laser ablation of AVL 
results in animals with only very rare abocs.  Thus, this neuron 
controls two of the muscle groups that contract during defecation.  
This result is more intriguing because unc-25 animals, which lack 
detectable GABA staining, have normal abocs.  Based on these results, 
we propose that AVL stimulates abocs via a second neurotransmitter.  
Interestingly, Jim Thomas has identified six genes (denoted aex genes) 
that when mutated cause constipation due to defects in both abocs and 
exps.  Perhaps these genes encode molecules required for the 
development or function of AVL and DVB, or of cells that mediate the 
effects of AVL and DVB.
Using the known connectivities of AVL and DVB, we are currently 
expanding our laser experiments to identify other neurons that control 
defecation movements.
Existing mutations that eliminate the function of AVL or DVB have 
been identified using laser surgery.  The above experiments suggest a 
strategy for identifying mutations that affect the function of either 
AVL or DVB.  Mutants lacking AVL should become constipated after 
killing DVB, while mutants lacking DVB should become constipated after 
killing AVL.  We tested these predictions using mutants we had 
previously shown to lack AVL (unc-73) or DVB (sem-4).  In 70% of unc-
73(e936) animals the AVL axonal process, which is normally found in 
the ventral nerve cord, is found in the dorsal cord.  As a consequence,
the average frequency of exps in unc-73 animals is reduced to 50%; 
however, unc-73 animals are not constipated.  In 70% of unc-73 animals,
killing DVB reduces the frequency of exps to less than 10% and 
results in severely constipated animals.  In 50% of sem-4(n1378) 
animals DVB is missing as assayed by GABA expression (M.  Basson, 
personal communication).  sem-4 animals miss half of their expulsions, 
but are not constipated.  In 50% of sem-4 animals, killing AVL reduces 
expulsions to less than 10% and causes the animals to become 
constipated.  We have begun to screen through known mutants using the 
laser to identify those lacking AVL or DVB function.  So far we have 
found that unc-2, lack AVL 
We are conducting genetic screens for new mutations that eliminate 
the function of AVL and DVB.  Among mutations that make unc-73 or sem-
4 animals constipated should be mutations that specifically disrupt 
DVB and AVL function, respectively.  In addition to these synthetic-
constipated mutants, such screens should also generate mutants that 
are defective in other steps of defecation, similar to those first 
described by Jim Thomas.  We have screened 7,000 unc-73 genomes and 
are presently characterizing 100 constipated strains.  Similarly, we 
have screened 10,000 sem-4 genomes and are characterizing 34 
constipated strains.