Thermosensation: How a human-infective nematode finds its host

Abstract

The parasitic nematode Strongyloides stercoralis locates human hosts via thermal cues through unknown neural mechanisms. A new study finds the heat-sensing neuron AFD mediates attraction to human body heat. Interestingly, this neuron also mediates thermotaxis in the nematode C. elegans.

Parasitic helminths impose severe health and economic burdens worldwide, infecting an estimated one billion people^1,2. Available therapeutics only target active infections with variable efficacy, highlighting the need to develop preventative measures^3,4.

Strongyloides stercoralis is a parasitic nematode that infects humans by penetrating the skin. It seeks its host upon reaching the developmentally arrested third-larval (iL3) stage, which is a stress-induced survival state akin to the C. elegans dauer stage^5,6. Though host detection relies on both thermal and chemical cues, thermal drive is prioritized above chemosensory input^2,3,6. S. stercoralis is attracted to temperatures slightly above human core body temperature (40°C)^2,3,7. The distant nematode cousin, C. elegans, also senses temperature, but its preferred temperature is significantly lower (15 to 25°C)^8,9. Importantly, C. elegans readily avoids heating above 30°C, a temperature that conversely attracts the human-seeking S. stercoralis^2,9. Although much is known about the mechanisms underlying C. elegans thermosensation, it remains largely unclear how S. stercoralis senses temperature to engage in host-seeking behavior. This is in part due to the paucity of genetic tools in S. stercoralis. The popular model organism C. elegans, however, is a useful model for comparative studies to shed light into this knowledge gap, due to the robust genetic techniques available.

In this issue, Bryant and colleagues brilliantly employ genetic approaches to investigate the neural mechanisms underlying S. stercoralis thermotaxis, presenting the first detailed mechanistic study of thermosensation in a non-Caenorhabditis nematode¹⁰. Previously, these authors demonstrated that S. stercoralis thermotaxis exhibits striking parallels to C. elegans, including robust thermotaxis behavior dependent on the cyclic GMP-gated (CNG) cation channel subunit TAX-4^2,3. In their new work, they now uncover the underlying neural mechanisms by identifying the primary thermosensory neuron mediating thermotaxis in S. stercoralis infective larvae.

C. elegans thermotaxis is primarily mediated by the amphid sensory neuron AFD (referred to here as Ce-AFD), which has a unique finger-like dendritic process extending to the nose tip^8,9. Laser ablation studies identified Ce-AFD as being essential for thermotaxis¹¹. Calcium imaging experiments found that Ce-AFD is a primary heat-sensitive neuron that shows increases in calcium upon warming^12-14 (Figure 1A). Electrophysiology recordings further corroborated this^13,15. The molecular mechanisms of Ce-AFD thermosensing depend on three receptor guanylate cyclases, GCY-8, GCY-18, and GCY-23, that are expressed exclusively in Ce-AFD and directly sense temperature to synthesize cGMP^16,17 (Figure 1A). This enables cGMP to open the CNG channel TAX-4/TAX-2, resulting in neuronal activation¹² (Figure 1A).

Figure 1. Heat-sensing mechanisms in C. elegans versus S. stercoralis.

Heat-sensing mechanisms in S. stercoralis show striking parallels to those found in C. elegans, but exhibit a biphasic calcium response, suggesting a more complex mechanism. In both panels, upper red trace represents neuronal calcium level, while lower black trace represents corresponding temperature change. Both traces plotted across time. A) Top: Warming ramps above cultivation temperature induce excitatory responses in Ce-AFD. Bottom: The Ce-AFD signal transduction cascade mediating thermotaxis in C. elegans. Upon warming above ambient temperatures, guanylate cyclases GCY-8, −18, −23 are activated to catalyze the production of cGMP from GTP. cGMP opens cyclic-nucleotide gated (CNG) channels to allow cation influx, resulting in neuronal excitation.(B) Top: Warming ramps above cultivation temperature induce a biphasic response in Ss-AFD, characterized by a transient inhibitory response followed by a linear relaxation upon approaching host body temperature. Bottom: Proposed Ss-AFD signal transduction cascade mediating neuronal inhibition. Decreases in both calcium and cGMP are observed upon warming above ambient temperatures. Phophodiesterase potentially mediates this initial transient inhibitory phase in Ss-AFD. Models made using Biorender.

In search of the primary sensory neuron mediating host-seeking thermotaxis in S. stercoralis, the authors first conducted a phylogenetic analysis, and found that three receptor guanylate cyclases contained distant homology with C. elegans GCY-23. All three S. stercoralis receptor guanylate cyclases are expressed specifically in the TAX-4-expressing head neuron ALD, whose elaborate multi-layered ‘lamellae’ draw parallels to Ce-AFD. These similarities in both molecular profiles and morphology led the authors to name these three S. stercoralis receptor guanylate cyclases as GCY-23.1, GCY-23.2, and GCY-23.3, and henceforth refer to ALD as S. stercoralis AFD neuron (Ss-AFD) (Figure 1B). This also suggests that Ss-AFD may use conserved cGMP signaling, which is required for C. elegans thermotaxis.

Importantly, chemogenetic silencing of Ss-AFD neurons yielded a severe defect in thermotaxis, indicating that Ss-AFD is required for heat-seeking in S. stercoralis. Of note, the authors report here and in previous work that S. stercoralis heat-seeking preference is context-dependent, based on both cultivation temperature and starting temperature when placed on a thermal gradient. Although placement at warm temperatures above standard cultivation temperature (23°C) results in the heat-seeking response, S. stercoralis exhibits bimodal preference when placed at 23°C and becomes cryophilic when placed below this. The authors speculate that this valence-switch may function as a dispersal mechanism in the presence of low temperature to increase the probability of locating a host². Both Ss-AFD and TAX-4 are required for negative and positive thermotaxis in S. stercoralis, strongly suggesting that Ss-AFD functions as the primary sensory neuron mediating thermotaxis. By comparison, C. elegans also exhibits context-dependent thermal preference, and valence control is mediated at the synapses between AFD and the downstream interneuron AIY via differential release of neurotransmitters¹⁸.

Next, the authors sought to determine whether Ss-AFD shares similar functions to Ce-AFD with respect to neuronal activity. As discussed above, Ce-AFD functions as a primary heat-sensitive neuron and exhibits an increase in calcium level upon warming (Figure 1A). Unexpectedly, warming ramps above cultivation temperature induced a biphasic thermosensory calcium response in Ss-AFD, consisting of a transient inhibition period followed by a secondary linear increase in calcium above baseline levels (Figure 1B). This biphasic response is in striking contrast to the singular excitatory response observed in Ce-AFD upon warming (Figure 1A). Warming-triggered inhibition of Ss-AFD in the initial phase was further verified using the cGMP sensor FlincG3, demonstrating that warming above ambient temperatures reduces intracellular cGMP levels. Taken together, both calcium and cGMP imaging data support the idea that warming inhibits rather than excites Ss-AFD, indicating that Ss-AFD’s role in thermotaxis is more complex than Ce-AFD.

How does warming inhibit Ss-AFD? To address this question, the authors asked whether Ss-AFD-specific receptor guanylate cyclases share similar functions to their C. elegans counterpart by ectopically expressing them in the C. elegans TAX-4-expressing sensory neuron ASER, which is only known to sense cold but not heat¹⁹. Intriguingly, ectopic expression of Ss-AFD receptor guanylate cyclases (GCY-23.1, GCY-23.2, or GCY-23.3) individually conferred heat-sensitivity to the C. elegans ASER neuron. This suggests that Ss-AFD receptor guanylate cyclases function similarly to their C. elegans homolog GCY-23 and are activated by warming to synthesize cGMP, which would open the CNG channel TAX-4/TAX-2 to excite Ss-AFD neurons in S. stercoralis. As the authors observed inhibition rather than excitation of Ss-AFD by warming, additional mechanisms must exist in S. stercoralis to counteract warming-induced activation of receptor guanylate cyclases, leading to a decrease in cGMP level upon warming. Warming above cultivation temperature is known to induce excitation of Ce-AFD to trigger heat avoidance (negative thermotaxis) in C. elegans¹³. As such, warming-induced inhibition of Ss-AFD would be consistent with its role in driving positive thermotaxis such that this parasitic nematode is attracted to the human host.

In summary, Bryant et al. present an interesting study revealing neural mechanisms of heat sensation in the human parasitic nematode S. stercoralis. Their work also sheds light on the underlying molecular mechanisms. The authors discover that heat-sensing mechanisms in S. stercoralis share striking resemblance to those required for C. elegans thermotaxis, such as dependence on an amphid sensory neuron with elaborate dendritic structures that expresses thermosensitive receptor guanylate cyclases. However, unlike Ce-AFD, which is activated by warming (Figure 1A), an increase above ambient temperature gives rise to a biphasic response in Ss-AFD — transient inhibition followed by linear relaxation upon approaching host body temperatures (Figure 1B). These findings indicate that GCYs are unlikely to function as the primary drivers of the initial inhibitory warming response in Ss-AFD, as both calcium and cGMP levels are reduced. This raises many interesting questions. For example, what is the molecular identity of this unknown temperature sensor that mediates the initial inhibitory phase of the warming response in Ss-AFD? The authors speculate that phosphodiesterase represents a strong candidate, as it functions to cleave cGMP; this would reduce cytosolic cGMP levels and thereby induce cellular inhibition via closing of CNG channels (Figure 1B). Similar mechanisms are observed in vertebrate phototransduction: light activates opsin and G protein to stimulate phosphodiesterase, resulting in cGMP hydrolysis and cellular hyperpolarization via closure of CNG channels in both rod and cones²⁰. It will be interesting to determine whether warming-induced Ss-AFD inhibition is phosphodiesterase-dependent, and if so, whether phosphodiesterase functions directly as a thermosensor, or indirectly in the process by acting downstream of an unknown thermosensor. It also remains unknown as to what mediates the secondary excitatory phase of the Ss-AFD warming response. The S. stercoralis GCY-23 homologs GCY-23.1, GCY-23.2, and GCY-23.3, are likely candidates, as the authors confirmed that they confer heat sensitivity to otherwise heat-insensitive cells when ectopically expressed in C. elegans. Further, what is the relationship between the sensors and pathways that mediate each phase of the warming-induced responses in Ss-AFD and how do they functionally interact to produce the complex calcium responses of Ss-AFD and behavioral responses of the animal? Finally, how does S. stercoralis execute the valence switch from thermal attraction to repulsion and how this is shaped by prior experience? In conclusion, this study elegantly uncovers mechanisms of heat seeking in parasitic nematodes, providing insights that may unveil potential avenues for the development of preventative measures against parasitic nematode infection.