Behavior: C. elegans does a spit take

Eating can turn into spitting because individual parts of a muscle cell are able to contract in different ways.
  1. Michael Hendricks  Is a corresponding author
  1. Department of Biology, McGill University, Canada

Like all animals, the tiny worm Caenorhabditis elegans dislikes bad-tasting food – a probably common occurrence in the compost where it lives (Frézal and Félix, 2015). In fact, recent data showed that just like you and I, the worms can spit out foul-tasting chemicals such as reactive oxygen species (Bhatla and Horvitz, 2015b; Bhatla et al., 2015a). To explain how these types of behaviors can emerge, scientists often focus on correlations between brain activity, sensory inputs and behavioral outputs. However, while neural activity understandably comes to the fore, attempts at mechanistic explanations will always fall short if they do not include another class of excitable cells that are essential for behavior: muscles. Now, in eLife, Robert Horvitz and colleagues from Massachusetts Institute of Technology (MIT) and University of California, Berkeley – including Steven Sando as first author – report on the impressive complexity in muscle coordination required for worms to spit out their food (Sando et al., 2021).

The feeding organ of C. elegans contains a pump that ingests and grinds bacteria before passing them on to the gut. Like the nervous system in the guts of mammals, this ‘pharynx’ is somewhat a fiefdom of its own. Formed of 20 neurons and 20 muscle cells isolated from the rest of the nervous system (both physically and in terms of neuronal connections), the organ regulates food intake autonomously (Avery and Shtonda, 2003; White et al., 1986). In particular, two structures in the pharynx ensure that the worms can eat properly: the metastomal filter stops large particles from entering while the pharyngeal valve acts as a one-way check and keeps food moving in the right direction (Fang-Yen et al., 2009). So how can such a dedicated pump suddenly reverse direction?

By analyzing high-speed videos, Sando et al. noticed that when the worms are spitting, the rate of pumping increases in the pharynx. This seems counterintuitive: if food tasted unpleasant, you probably would not start gorging on it even faster. However, the metastomal filter and pharyngeal valve are held open during this increase, allowing the contents of the pharynx to be rapidly flushed back into the environment.

To examine how the valve stayed open during spitting, the team then focused on a set of muscles known as pm3s. These three muscle cells contract and relax rhythmically to help the pharynx pump food, and to allow the pharyngeal valve to open and close. However, during spitting, pm3s play two simultaneous roles: the anterior portions of the cells stay contracted to keep the valve open, while their posterior sections rapidly contract and relax to drive food out of the pharynx.

To confirm that these changes came from pm3s themselves – and not from forces impinging on the muscle or the valve – Sando et al. had a closer look at muscle activation during pumping and spitting. To do so, they expressed a calcium-sensitive fluorescent protein in pharyngeal muscles, as the concentration of calcium ions increases inside a contracting cell. This revealed that in spitting animals, sustained calcium signals were localized around the pharyngeal valve. This result is consistent with the anterior portion of pm3s (and only this portion) contracting to hold the valve open. But how is this complex activity state of pm3 regulated?

A pharyngeal neuron call M1 is essential for spitting – killing this cell with a laser stops the spitting response in worms. Based on the cells that M1 connects to and further experiments, Sando et al. suggest that this neuron integrates multiple signals that correspond to noxious tastes. The signaling output of the M1 neuron varies in strength according to these inputs: weak activation leads to opening of the pharyngeal valve, and only strong activation results in the valve opening and increased pumping necessary to eject food. In turn, various degrees of spitting behavior could emerge from these different inputs thanks to local contraction of cellular portions of the pm3 muscles.

Sando et al. stopped short of exploring the cellular mechanisms that allow local contraction of pm3s. In other systems, like mammalian smooth muscle, contractility patterns are determined by the spatial and temporal dynamics of calcium ions. These patterns arise from a complex interplay between various sources of ions and the channels or regulatory proteins that compartmentalize and shape calcium dynamics inside a cell. A similar mechanism could be happening here, with various levels of M1 activation targeting different sources of – or regulatory pathways for – intracellular calcium ions in pm3s.

Taken together, the results from Sando et al. highlight that muscles are not just passive conduits for neural commands: instead, they can exhibit dynamics that arise from the interplay between neural signals and their own, varying physiological properties. The functional insights of this study, along with the power of C. elegans genetics, offers an opportunity to study complex muscle dynamics and their neural regulation in a compact and accessible system.

References

    1. White JG
    2. Southgate E
    3. Thomson JN
    4. Brenner S
    (1986) The structure of the nervous system of the nematode Caenorhabditis elegans
    Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 314:1–340.
    https://doi.org/10.1098/rstb.1986.0056

Article and author information

Author details

  1. Michael Hendricks

    Michael Hendricks is in the Department of Biology, McGill University, Montreal, Canada

    For correspondence
    michael.hendricks@mcgill.ca
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-3408-3858

Publication history

  1. Version of Record published: August 3, 2021 (version 1)

Copyright

© 2021, Hendricks

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 624
    Page views
  • 42
    Downloads
  • 0
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Michael Hendricks
(2021)
Behavior: C. elegans does a spit take
eLife 10:e71813.
https://doi.org/10.7554/eLife.71813

Further reading

    1. Biochemistry and Chemical Biology
    2. Neuroscience
    Jinli Geng, Yingjun Tang ... Xiaodong Liu
    Research Article Updated

    Dynamic Ca2+ signals reflect acute changes in membrane excitability, and also mediate signaling cascades in chronic processes. In both cases, chronic Ca2+ imaging is often desired, but challenged by the cytotoxicity intrinsic to calmodulin (CaM)-based GCaMP, a series of genetically-encoded Ca2+ indicators that have been widely applied. Here, we demonstrate the performance of GCaMP-X in chronic Ca2+ imaging of cortical neurons, where GCaMP-X by design is to eliminate the unwanted interactions between the conventional GCaMP and endogenous (apo)CaM-binding proteins. By expressing in adult mice at high levels over an extended time frame, GCaMP-X showed less damage and improved performance in two-photon imaging of sensory (whisker-deflection) responses or spontaneous Ca2+ fluctuations, in comparison with GCaMP. Chronic Ca2+ imaging of one month or longer was conducted for cultured cortical neurons expressing GCaMP-X, unveiling that spontaneous/local Ca2+ transients progressively developed into autonomous/global Ca2+ oscillations. Along with the morphological indices of neurite length and soma size, the major metrics of oscillatory Ca2+, including rate, amplitude and synchrony were also examined. Dysregulations of both neuritogenesis and Ca2+ oscillations became discernible around 2–3 weeks after virus injection or drug induction to express GCaMP in newborn or mature neurons, which were exacerbated by stronger or prolonged expression of GCaMP. In contrast, neurons expressing GCaMP-X were significantly less damaged or perturbed, altogether highlighting the unique importance of oscillatory Ca2+ to neural development and neuronal health. In summary, GCaMP-X provides a viable solution for Ca2+ imaging applications involving long-time and/or high-level expression of Ca2+ probes.

    1. Neuroscience
    Maria Cecilia Martinez, Camila Lidia Zold ... Mariano Andrés Belluscio
    Research Article

    The automatic initiation of actions can be highly functional. But occasionally these actions cannot be withheld and are released at inappropriate times, impulsively. Striatal activity has been shown to participate in the timing of action sequence initiation and it has been linked to impulsivity. Using a self-initiated task, we trained adult male rats to withhold a rewarded action sequence until a waiting time interval has elapsed. By analyzing neuronal activity we show that the striatal response preceding the initiation of the learned sequence is strongly modulated by the time subjects wait before eliciting the sequence. Interestingly, the modulation is steeper in adolescent rats, which show a strong prevalence of impulsive responses compared to adults. We hypothesize this anticipatory striatal activity reflects the animals’ subjective reward expectation, based on the elapsed waiting time, while the steeper waiting modulation in adolescence reflects age-related differences in temporal discounting, internal urgency states, or explore–exploit balance.