Researchers have discovered a new role for biogenic monoamines such as dopamine and serotonin in the reproductive development of planarian flatworms, suggesting that these neurotransmitters serve as important signaling molecules beyond brain function in several species.

The functions of biogenic monoamines such as dopamine and serotonin in the brain as regulators of mood, learning and memory, stress responses, and the body’s fight-or-flight response are well known.

But these neurotransmitters were present in nature long before the brain emerged on the evolutionary tree. They are also common in plants, bacteria, and single-celled organisms, but their functions are much less understood.

Scientists at the Morgridge Research Institute have added a new challenge to monoamines. They play an important role in the reproductive organs of planarian flatworms and appear to be critical for the development of both female and male germ cells (cells that produce eggs and sperm).

In this issue of the magazine Proceedings of the National Academy of Sciences (PNAS) on June 25, 2024, the regenerative biology team at Morgridge showed that such transmitters are not just signals from the planarian brain. They are also highly localized within somatic gonadal cells that regulate germ cell development.

Discovery and research methods

“We are excited about this because it reveals a new paradigm for signaling from the niche cell to the germ cell,” says researcher Melanie Issigonis, lead author of the study.

This surprising discovery began in a separate study published in 2022 by former graduate student Umair Khan and Morgridge researcher Phil Newmark, also an investigator at the Howard Hughes Medical Institute. They set out to characterize the transcriptomes of planarian ovaries and testes (which are hermaphroditic) and came up with a long list of genes with enriched expression in the ovary. One of the most popular hits was aromatic L-amino acid decarboxylase (AADC), an evolutionarily conserved enzyme important for the production of monoamines.

They assumed, with surprise, that the samples were contaminated by surrounding brain tissue, but later research confirmed this finding. Khan then suppressed the gene’s expression etc.To examine the role of the reproductive system in development.

“When you knocked out this enzyme in sexually reproducing planarians, the phenotype was stunning,” Issigonis says. “The ovaries were gone. In fact, the entire female reproductive system was completely eliminated.”

Issigonis says the opposite happens in the testicles. If you cut a normal testicle like a watermelon, there are stem cells around the perimeter like skin, but they are produced in very small quantities. “When Umair was shot etc.“The testicles were completely filled with germline stem cells,” he says. “There was no sperm production; essentially, the testicles were filled with germ cell tumors.”

Formation and action of monoamines

Issigonis’ subsequent research sought to answer two questions: What monoamine does AADC produce, and where does it come from? They sought an answer by sequencing the single-cell RNA of somatic cells in the “niche” that surrounds and supports germ cells.

They found enriched expression in somatic niche cells etc. and another gene, nrp’s which encodes a nonribosomal peptide synthetase (NRPS). It was impressive because, unlike aadc , nrp’s It is expressed only in the reproductive system, not in neurons.

Then when they destroyed nrp’s They revealed the same phenotype as observed upon suppression in sexual planarians etc. : complete disruption of the female system and a sharp accumulation of male reproductive cells.

This was a key clue that AADC and NRPS work together. Further mass spectrometric analysis by colleagues Rui Chen and Jim Collins of UT Southwestern provided evidence that the two enzymes produce β-alanyltryptamines (BATTs), which consist of β-alanine conjugated to the monoamine tryptamine. The Collins lab discovered that in schistosomes (parasitic cousins ​​of planarians), males produce BATT to trigger egg production in females.

The complex was named the “BATT Signal” after the bat-shaped skylight used to alert Batman in the comic book series. Planarians also have a clear flashing signal. The team found that BATT is most prevalent when planarians reach sexual maturity and have mature reproductive organs.

“We thought, wow, sexual animals make a lot of BATT,” he says. “And when we actually dropped both of them nrp’s or etc. “BATT disappeared. This showed us that we were on the right track.”

This work and research in schistosomes by the Collins lab have revealed that β-alanyl-monoamine conjugates can act as important signals. Since NRPS enzymes are present throughout the animal kingdom, this suggests that the new monoamine conjugates may also serve as signaling molecules in other animals. The next steps are to understand how these new signals work.