Paper #17 - Caenorhabditis elegans exhibit a coupling between the defecation motor program and directed locomotion

TitleCaenorhabditis elegans exhibit a coupling between the defecation motor program and directed locomotion

Year: 2015

Summary: This will be my second post I think that actually has nothing to do with neurons.

When worms defecate, they apparently have a very characteristic body pattern consisting first of posterior contraction (both dorsal and ventral), followed by anterior contraction (again, both dorsal and ventral) followed by expulsion. Subdividing this process into these three steps (called pBoc, aBoc and Exp) goes back to at least (Thomas, Genetics, 1990). In this paper, they refer to this as the defecation motor program (DMP).

This process occurs via a calcium wave that starts in the posterior intestinal cells and propagates via innexins to the anterior intestinal cells. I have some questions about this* but I'll hold off on them for now.  The calcium influx into these intestinal cells induces pumping protons from the intestinal cells into the pseucoelomic space and acidifying it. Body wall muscles then respond to this acidication via proton-gated ion channels (PBO-5/6) and contract.

In this paper, they make measurements of worms over the L4 to young adult transition, recording video for 10 hours at 10 fps.
This enables them to make the striking observation that seconds prior to defecation, worms often start going backwards, with a peak probability of backwards motion at around 8 seconds prior the first body contraction (pBoc). After the pBoc but before the aBoc (that little 4.5 second window), almost no worms go forward, and about half go backwards (compared to ~10-15% before the pBoc). After the aBoc, no more than 1-2% of the worms go backwards for the first 10 seconds or so.  They call this pattern of activity the 'DMP associated motor program', or DAMP.

I'm a little sad they don't discuss worm- to-worm, or trial-to-trial covariance between aspects of this motor program, since it's clearly not super deterministic. Additionally, all their plots show the fraction of worms moving 'forwards', 'backwards' or not moving. Rather than discretizing the data and setting some velocity cutoffs (worse, this is after a 2-second smoothing too, reducing the temporal resolution significantly for a 5-second ish event), why not use the actual velocity information?

Next they investigate the coupling between the locomotion pattern (again, they call this DAMP) and the defecation motor program (again, DMP). The Thomas paper mentioned above apparently showed that gentle touch could reset the defecation cycle phase, so they asked whether resetting the defecation cycle reset the DAMP cycle. It did. Regardless of tapping on the plate or touching the worms, each cycle of defecation had the same propensity for backwards motion between the body contractions.

Interestingly in the case of extended vibration (15 s of 1kHz vibration from a piezo), the worms upregulate their overall probability of forward motion over the next ~3-4 minutes. This has the consequence that a smaller fraction of worms go backwards during the interBoc duration.

To look at how defecation calcium waves affect the locomotion program, they looked at two mutants that break the defecation motor program: egl-8 and inx-16. egl-8 mutants (egl-8 is a phospholipase C) don't do pBocs for some unexplained reason, and inx-16 don't do aBocs because they lack innexins (here, confusingly called pannexins) that allow propagation of a calcium wave.

In the egl-8 mutants without posterior body contractions, the worms still typically start moving in reverse before defecation. However, sometimes the worms would initiate a

In inx-16 mutants that are missing innexins and therefore sometimes don't to aBocs, backward motion begins after the pBoc only in the case when subsequent aBocs occur.

Questions I still have:

  • * How is this temporally coordinated? Waves are generally smooth things, yet the body contractions occur as two very discrete events, separated by a not-insignificant amount of time (4-6s!)
  • Does forward/backward motion not cease during defecation-related body contractions? How do you score forward/backwards motion during this process?
  • Do neurons really not participate in this process? How do you prove that?
  • What sets the timing of defecation? What variable dictates the phase of the oscillation?
    • Presumably food modulates the timing of defecation?