On Thursday April 1, the Department of Biology hosted Niels Ringstad, professor in the Department of Cell Biology at the New York University Grossman School of Medicine. Ringstad delivered a talk titled “Modulation of Behavior by Host-Microbe Interactions” for the department’s seminar series, highlighting recent findings from his lab about the powerful effects of microbes on the behavior of the model organism Caenorhabditis elegans (C. elegans).
The nematode C. elegans serves as an ideal organism for studying how effects on the animal nervous system can alter behavior, as the simple nervous system of C. elegans functions remarkably similar to our own. Instead of the billions of neurons that humans have, C. elegans has just 302, which have been mapped in close detail. This allows researchers like Ringstad to utilize C. elegans as a powerful tool to identify the neural circuits and molecular mechanisms behind animal behavior.
Ringstad began his presentation with a picture of a common household occurrence: a moldy strawberry. While the immune system helps protect the body from infection if the moldy strawberry is consumed, Ringstad argued that the nervous system also plays a major — if not even more important — role by preventing the consumption of the strawberry in the first place.
Your visual system can tell you that the strawberry is moldy, your olfactory system can detect that the strawberry has gone bad and your gustatory and tactile senses can also tell that the strawberry is lacking its usually sweet aroma.
Ringstad further emphasized that these layers of defense can coordinate our everyday interactions with microbes. Bacteria can influence human neurochemistry, and Ringstad noted that the same host-microbe interactions are present in C. elegans, which must constantly decide which bacteria to eat and which to avoid.
“There’s... nothing more important to a worm than whether or not they eat bacteria that are good for them,” Ringstad said.
To study this, his team designed behavioral experiments where worms were given a choice between consuming harmless Escherichia coli and pathogenic Enterococcus faecalis. The result? Worms rapidly migrated toward the nutritive E. coli and avoided the toxic E. faecalis. Furthermore, Ringstad found that mutating key genes involved in chemosensory transduction prevented the worms from finding and accumulating on sources of nutrient bacteria.
Ringstad’s group then began identifying the mechanisms underlying this behavior.
“[Worms] have these little bumps and turrets and grooves on their face that house sensilla; they have [both] mechanosensory [and] chemosensory sensilla,” Ringstad described.
Due to the C. elegans’ complex sensory systems, Ringstad’s group focused on studying a subset of chemosensory sensilla that can function as either taste or odor receptors to elicit a microbe-specific response. They identified three specific neurons that preferentially responded to E. faecalis stimuli and five neurons that responded to E. coli. Interestingly, one sensory neuron, known as AWC, was only activated when E. coli was removed from, but not when worms were presented with the bacterium.
“This neuron does not respond to stimulus presentation, but to stimulus removal,” Ringstad explained. It’s a so-called “off” neuron.”
His group also demonstrated that the signals from E. coli-responsive neurons converged onto interneurons, like the AIB interneuron, that were directly responsible for driving chemotaxis behavior by activating neurons to orient the worms towards patches of E. coli.
Ringstad’s lab then went on to determine what specific chemicals in bacteria the worms were detecting. By comparing the metabolomic profiles of E. coli and E. faecalis, they identified a group of pungent molecules called polyamines — which are produced by bacterial metabolism and often associated with decaying matter.
The lab confirmed that odorants could activate certain sensory neurons at relatively high nanomolar concentrations, but they were surprised to discover that polyamines given to worms at concentrations as low as a one trillionth of a milligram could elicit ultrasensitive responses from the AWC interneuron. Further experiments also confirmed that C. elegans largely relied on sensing polyamine metabolites to detect nutritive bacteria.
Ringstad then shifted to a discussion of his lab’s work on the mechanosensory neurons that worms use to detect bacterial biofilms. These sensory neurons were of particular interest to his group because they released dopamine when activated and visibly affected behavior.
“If you transfer [worms] onto a bacterial lawn, immediately after transferring, they're freaked out,” Ringstad described. “But as soon as they feel the texture of the lawn, they start to slow down... [there’s kind of] anxiolytic or pacifying effects of food sensing on worm behavior.”
Using RNA-sequencing analysis of dopaminergic neurons, Ringstad’s group identified a potassium channel called TWK-2 required for regulating food-response behavior. Knocking out the TWK-2 gene led to an unexpected discovery; in worms lacking both TWK-2 and another sensory ion channel (TRP-4), which was thought to be the primary mechanoreceptor for tactile food sensing, an alternate mechanoreceptor mechanism involving three subunits of a sodium-calcium ion channel, known as the Deg/ENaC channel, activated the slowing behavior in response to food.
The lab’s follow-up work testing mechanical stimuli responses revealed that TRP and the trio of Deg/ENaC channels work together to achieve a precise, linear encoding of force.
“Accurate encoding of this mechanosensory stimulus actually requires collaboration between these different channels, and they’re all [coexpressed] all in the same neuron,” Ringstad concluded.
Finally, Ringstad posed a question exploring the converse to his previous discussions of how worms detect and respond to bacteria: Could bacteria manipulate the brains of predatory worms to defend themselves?
“That’s not totally crazy,” Ringstad said. “One of the most potent anti-nematode compounds out there is a compound called Avermectin, which comes from bacteria. Bacteria have already developed defense systems against nematodes.”
To this end, Ringstad’s group screened microbe-derived small molecules and studied their effects on egg-laying behavior in worms, which serves as a simple system to study the signaling of serotonin, a crucial neurotransmitter, in vivo. Among other compounds, they identified a microbial metabolite called 2,5-diketopiperazine gliotoxin, which was previously unknown to interact with serotonin signaling.
Furthemore, gliotoxin exerted similar effects to previously known serotonin signaling-related agonists at just 20 micromolar concentrations and also acted via a novel mechanism to act on serotonin signaling different from serotonin-reuptake inhibitors.
Ringstad also noted that he used the Caenorhabditis Natural Diversity Resource (CaeNDR), developed by Professor Erik Andersen at Hopkins, to identify C. elegans strains from across the world with different resistance and sensitivity to gliotoxin.
Ringstad concluded by discussing his lab’s continued screening of thousands of microbial extracts with potentially similar effects to gliotoxin, which could translate into novel compounds for psychopharmacology. He reasoned that further exploring the complex interactions between microbes and animals could have important implications for human health.
“We think it’s very interesting that these chemoreceptor mechanisms are so sensitive to microbial metabolites,” Ringstad explained. “We’re wondering if other animals like us have similar mechanisms... because we are landlords to lots of bacteria. They live within us and... actually control our neurochemistry.”
