Brain signals help reproductive cells survive heat stress, new worm study suggests

A recent study published in Current Biology provides evidence that the brain sends molecular signals to reproductive cells to help them withstand high temperatures. Scientists found that certain small genetic molecules in the nervous system of worms communicate with sperm and eggs to modify their ability to survive heat stress. These findings suggest that an animal’s sensory perception of its environment plays a role in protecting its fertility, independent of direct physical damage from the heat.

Rising global temperatures threaten reproduction across many species, making it important to understand how organisms adapt to environmental stress. The reproductive system relies heavily on small RNA molecules, which are tiny pieces of genetic code that control gene activity without producing proteins. These molecules act like biological switches, helping to buffer developmental processes against outside changes and maintain reproductive health.

Past research indicated that surrounding support tissues might pass environmental signals to reproductive cells. The authors of the current paper aimed to test whether the nervous system, which directly senses the outside world, could regulate the reproductive system’s resilience to heat via these small RNA pathways. They used the microscopic roundworm *Caenorhabditis elegans* as a model organism to track these molecular messages.

The researchers first examined worms missing the *rde-4* gene. This gene is required to produce certain small RNA molecules involved in gene regulation. The scientists raised these mutant worms at a standard temperature of 20 degrees Celsius and an elevated temperature of 25 degrees Celsius.

At 20 degrees Celsius, the mutant worms remained fertile. At 25 degrees Celsius, 80.6 percent of the eggs laid by the mutant worms (out of a sample of 500) were unfertilized, compared to a base rate of just 0.06 percent (out of 236) in normal worms. The scientists determined this defect was linked to sperm failure and massive chromosomal abnormalities.

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To see if the nervous system was responsible for this failure, the authors inserted a working copy of the *rde-4* gene exclusively into the neurons of the mutant worms. This genetic addition partially restored the production of small RNA molecules targeted at sperm development.

Expressing the gene only in the neurons also partially rescued the worms’ fertility under heat stress. The percentage of unfertilized eggs dropped from 89.5 percent in the fully mutant worms to 50.6 percent in those with the restored neuronal gene. To confirm this was an authentic brain-to-reproductive-cell signal, they controlled for stray gene expression in the reproductive tract and tested other tissues like muscle, which did not improve fertility.

The scientists also performed a mosaic analysis. They engineered worms to have normal brains but a mutated reproductive system. These mosaic worms showed milder fertility defects at 25 degrees Celsius compared to fully mutant worms, producing 74.0 percent unfertilized eggs versus 97.1 percent. This provides evidence that neuronal signaling alone can bolster the heat tolerance of sperm.

Next, the authors tested whether specific sensory functions affected this protective process. They genetically disabled various sensory receptors in the worms, including those dedicated to sensing heat, taste, and oxygen. Altering the heat and taste receptors produced no changes in fertility.

Disabling the *gcy-35* gene, which helps specific neurons sense oxygen, partially restored fertility in the worms lacking the *rde-4* gene. The rate of unfertilized eggs fell from 95.7 percent to 69.8 percent. Removing the *tax-2* gene, which is broadly required for sensory responses, also reduced the rate of unfertilized eggs to 38.0 percent from a baseline of 83.7 percent in the mutants.

Placing the worms in artificially low oxygen environments of 1 percent and 7 percent oxygen also improved their reproductive success at high temperatures. The authors suggest the worms’ nervous systems associate low oxygen levels with dense bacterial growth. This oxygen-depleted state signals abundant food and a safe environment for breeding, prompting the nervous system to relax its inhibition of reproduction.

The scientists then compared standard laboratory worms to wild worm strains to see how domestication affects this pathway. The standard lab worms carry specific genetic variations in genes linked to oxygen sensing and behavior, adapting them to artificial petri dish environments.

Reverting these specific genes to their wild, ancestral versions improved the heat tolerance of the worms’ reproductive systems. When testing worms with ancestral versions of the *npr-1* and *glb-5* genes, the percentage of unfertilized eggs dropped from 98.4 percent to 92.6 percent. This suggests that the sensory pathways regulating fertility are highly adaptable and tend to shift based on the organism’s natural habitat.

Finally, the scientists examined the reproductive cells for signs of DNA damage under a microscope. They used a specialized fluorescent marker to look specifically for double-strand breaks in the genetic code.

At the elevated temperature of 25 degrees Celsius, worms missing the *rde-4* gene showed a widespread increase in DNA damage across their reproductive cells. This damage was observed even in the stem cells that constantly divide to produce new sperm and eggs.

Restoring the gene in the neurons, or disabling the oxygen-sensing neural circuits, reduced the amount of DNA damage. These microscopic observations suggest that neuronal signaling helps maintain the structural integrity of the reproductive genome during heat stress. The perception of environmental safety appears to trigger protective mechanisms at the cellular level.

It is incorrect to conclude that sensory perception entirely overrides the physical impacts of extreme heat. The neural signals act to modify and buffer the reproductive system, rather than granting complete immunity to severe environmental temperatures. The fertility of the worms was only partially rescued in these experiments, indicating that heat still exerts a direct, negative physical toll on the cells.

Additionally, the exact physical pathway transporting the signal from the nervous system to the reproductive organs remains unidentified. The researchers ruled out one common transport protein, but the specific biological mechanism carrying these genetic molecules across tissue boundaries is still unknown. Future research could explore whether different transport proteins or cellular channels facilitate this communication. Additional studies could also investigate whether similar neural communication networks operate in more complex animals.

The study, “Neuronal RNAi and oxygen-sensing circuit shape germline resilience to heat stress,” was authored by Chee Kiang Ewe, Hanna Achache, Hanna Schön, Leonid Kontorovich, Guy Teichman, Shir Weiss, Anna Mogilevskaya, Myriam Valenski, Sarit Anava, Rutwik Bardapurkar, Hila Gingold, Rachel Posner, Olga Antonova, Mario de Bono, Yonatan B. Tzur, and Oded Rechavi.