Characterization of Neuromuscular Communication in the C. elegans Pharynx During Feeding

2018-02-08T00:00:00Z (GMT) by Alena Kozlova
Communication between motor neurons and their target muscles is crucial for proper muscle contraction and function. Neurotransmitter acetylcholine mediates this communication. It is released from the motor neuron and binds receptors embedded in the muscle cell membrane. Acetylcholine stimulates two very different types of contractions in adjacent muscle cells in the C. elegans pharynx, called pumping and peristalsis. We were interested to understand what signaling mechanisms that stimulate peristalsis. To examine muscle contractions and Ca2+ transients during peristalsis in wild-type animals and various mutants affecting acetylcholine signaling we used time-lapse imaging. We found that while mutants affecting the eat-2 nicotinic acetylcholine receptor exhibited reduced pumping, they also exhibit hyperstimulated peristalses. This hyperstimulation depends on crosstalk with a muscarinic acetylcholine receptor GAR-3, and we suggest crosstalk results from acetylcholine spillover from synapses that normally stimulate pumping to receptors in adjacent cells. The crosstalk between EAT-2 and GAR-3 underlies feeding defects in a eat-2 mutant and contributes to the dietary restriction and extended adult lifespan observed in these animals. The NK-2 family homeobox transcription factor CEH-28 is expressed exclusively in the M4 motor neuron. Mutants affecting ceh-28 exhibit “stuffed pharynx” phenotype, loss of M4 polarity and defects in synapse morphology. Instead of having synapses only in the posterior isthmus, M4 forms abnormally sized and spaced synapses throughout the entire isthmus in ceh-28 mutants. We were interested to understand why these mutants develop “stuffed pharynx” phenotype. We examined M4 activity and found that in wild-type animals Ca2+ transients were larger in the posterior part of the M4 where it forms synapses with isthmus muscles. In contrast, in ceh-28 mutants Ca2+ transients were low and uniform along the entire length of M4. We also looked at Ca2+ transients in the pharyngeal isthmus muscles and found that ceh-28 mutants have lower Ca2+ signals than wild-type animals. This indicates that function of M4 and the isthmus muscles is impaired in ceh-28 mutants. We suggest that in ceh-28 mutants abnormal morphology and synaptic organization of M4 results in the reduced M4 activity and reduced function of the isthmus muscles, which eventually lead to the “stuffed pharynx” phenotype.