Skip to main content

Acute oxygen sensing and oxygen tolerance in C. elegans

Periodic Reporting for period 2 - OXYGEN SENSING (Acute oxygen sensing and oxygen tolerance in C. elegans)

Reporting period: 2020-07-01 to 2021-12-31

Animals adapt certain physiological and behavioural patterns appropriate for their living environment to ensure health and survival. Variations in the surrounding environment are always accompanied with physiological and behavioural adaptations such as migration and hibernation, in which oxygen (O2) plays an important role. Animals get acclimated to live at certain ambient O2 concentrations of sea level, high altitude, underground or aquatic habitats. Certain animals are extremely tolerant to low O2 (hypoxia) exposure. In contrast, the human brain is particularly sensitive to hypoxia. Interruption of the O2 supply for a few minutes leads to irreversible neurological damage. Animals have evolved sophisticated mechanisms to cope with both acute and chronic low oxygen availability in order to survive. However, notwithstanding intensive research, the precise molecular mechanisms of acute and prolonged O2 adaptations remain poorly understood.

Here we are studying acute O2 sensing and low O2 tolerance in round worm Caenorhabditis elegans, a widely used genetic model organism in which several molecules critical for O2 sensing were initially discovered. This animal displays extreme tolerance to O2 deprivation. It can survive several days without any O2 supply, but it is mechanistically unclear how C. elegans manages to survive so long without O2. We are conducting an analysis to identify the components important for survival during O2 deprivation, and apply our findings to humans. C. elegans not only displays extreme tolerance to O2 deprivation but also responds acutely to rapid O2 variations. It prefers O2 levels close to 7%. A rapid change of O2 tension from 7% induces dramatic increases of its locomotory speed. The robust behavioural response to the variation of O2 tensions offers us the opportunity to perform mutagenesis screens for mutants that are unable to respond to the changes in O2 levels. We have identified a collection of molecules required for acute O2 sensation, which are now thoroughly investigated in the context of a well-constructed neural circuit coordinating behaviour.

Our objective is to discover conserved molecular and neural circuit principles of acute O2 sensing and to gain deep insight into the mechanism mediating tolerance to low O2 levels. We aim to gain a better understanding of O2 sensation by O2 sensing organs in mammals such as carotid body, and inform advances in therapy for the neurological disorders such as cerebral hemorrhage and ischemia.
Acute O2 sensing is highly plastic and is responsive to the environmental changes, chemicals and pathogens. Many cellular pathways have been implicated in acute responses to the rapid changes of O2 tension. We are investigating the pro-inflammatory cytokine interleukin 17 (IL-17), which acts as a neuromodulator participating in the signal transduction of acute O2 responses in C. elegans. By combining high-throughput genetic screen, biochemical analysis and RNA sequencing, we discovered several key components in IL-17 signaling and used these molecules as the entry point to dissect the molecular and neural circuit details of acute O2 sensation. The identification of these molecules also provoked us to investigate the potential involvement of IL-17 in the other cellular processes. We found that IL-17 has a broad impact of worm physiology.

Cyclic nucleotide has long been implicated in acute sensation of O2 in C. elegans. Our current studies re-defined the contribution of cyclic nucleotide in acute O2 sensation, elucidated how the intracellular cyclic nucleotide levels affect the acute responses to rapid changes of O2 tension, and established a connection between G protein signaling and Cyclic nucleotide in the modulation of acute sensation of O2. We also revealed an intriguing role of cGMP in supporting animals’ survival under limited O2 supply, and identified the cyclic nucleotide channels that are involved in this process. In addition, a large-scale genetic screen is currently in process to identify the regulators of anoxia tolerance in C. elegans, and several interesting candidates have already been obtained.
The robust locomotory responses of C. elegans to the variations of O2 tension and its extreme tolerance to O2 deprivation allow us to perform high throughput forward genetic screens with deep coverage, which is unlikely to be accomplished in other systems. We are in a unique position to systematically dissect acute O2 sensing and anoxia tolerance without prior knowledge, without bias and without ethical issues of animal research. The forward genetic screen turns out to be effective in the identification of the relevant molecules essential for acute O2 sensing. It leads to the discovery of several novel and unexpected components in IL-17 signaling. These discoveries reveal how IL-17 transmits signals to the transcription factors, alters gene expression and modulates acute sensation of O2 and the other cellular processes. We are currently translating our findings into mammalian systems, and testing new drugs that target these novel components in order to find the potential candidates for the treatment of IL-17 related diseases.

Even though a simple but elegant genome-wide screen for mutants with altered anoxia tolerance is still at its beginning stage, we have already identified a conserved molecule essential for animals' prolonged survival without O2 supply. It demonstrates that our approach is feasible in isolating relevant factors, which have the protective effects to the cells during O2 deprivation. We are aiming to identify more such molecules in the screen, elucidate how C. elegans reprogram its gene expression to survive extreme low O2 exposure, and apply these discoveries in the mammalian system. This part is highly relevant to Ischemia/reperfusion related disorders, which are the most common causes of debilitating diseases and death in western countries. Effective approaches are still lacking in the prevention of catastrophic consequences caused by ischemia/reperfusion. Our research with the little worm C. elegans may generate a big impact, leading to the the therapeutic innovation for the treatment of hypoxia sequelae.