mTOR regulates longevity through a bile-acid like hormonal mechanism and DHS- 26/DHRS1

This study reveals that mTOR regulates organismal longevity in *C. elegans* through a conserved neuroendocrine mechanism where its downregulation increases bile acid-like dafachronic acid production, which in turn activates the nuclear receptor DAF-12 and the dehydrogenase DHS-26/DHRS1 to extend lifespan.

The Gist

Imagine your body as a bustling city with a central command center called mTOR. This command center is like a busy traffic controller that decides when the city should be in "growth mode" (building new roads, expanding neighborhoods) and when it should be in "maintenance mode" (saving energy, fixing old pipes). Scientists have long known that if you tell this traffic controller to slow down, the city (the organism) tends to last much longer. But until now, no one knew exactly how the command center sends out the order to slow down the whole city.

This paper reveals that the mTOR command center uses a special messenger system to talk to the rest of the body, kind of like a hormonal postman.

Here is how the story unfolds, step-by-step:

1. The "Bile Acid" Postman
The researchers discovered that when mTOR slows down, it triggers the production of a specific chemical messenger called Dafachronic Acid (DA). Think of DA as a "bile acid-like" letter. In the tiny worm they studied (C. elegans), this letter is crucial. If the worm can't make this letter, or if the receiver can't read it, the "longevity" message never gets delivered, and the worm doesn't live longer.

2. The Receiver (The Lock and Key)
Every letter needs a receiver. In this case, the receiver is a protein called DAF-12. You can think of DAF-12 as a special lock on the cell's door. The DA letter is the key. When the key fits into the lock, it opens the door to a set of instructions that tell the organism to live longer. Interestingly, this lock (DAF-12) is very similar to a lock found in humans (called FXR), suggesting this system is a fundamental part of how animals work.

3. The Missing Piece: The DHS-26/DHRS1 Machine
The scientists then asked: "What happens after the key turns the lock?" They found a specific worker machine called DHS-26 (in worms) or DHRS1 (in mice).

• Think of DHS-26 as a specialized factory worker that only shows up when the "longevity" signal is active.
• This worker is essential. Without DHS-26, the message stops, and the worm doesn't live longer.
• This worker is stationed in a very specific part of the worm's "brain" (canal-associated neurons), which suggests the brain is the control tower sending these signals out to the rest of the body.

4. The Universal Connection
The most exciting part of the discovery is that this isn't just a worm thing. The researchers found that mice have the same worker (DHRS1), and it responds to the same signals (mTOR and the nuclear receptor). This means the "mTOR → Hormone Letter → Worker Machine" chain is an ancient, shared system across different animals, from worms to mice.

In Summary
The paper explains that mTOR doesn't just tell cells to grow or stop growing; it acts like a system-wide manager. When it decides to extend life, it sends out a chemical "letter" (the bile acid-like hormone). This letter unlocks a specific door (DAF-12), which then activates a critical worker (DHS-26/DHRS1) in the brain. This worker then helps the whole organism slow down and live longer. It's a neat, coordinated relay race where the brain, the hormones, and the cells all work together to manage the clock of life.

Technical Summary

Technical Summary: mTOR Regulates Longevity Through a Bile-Acid Like Hormonal Mechanism and DHS-26/DHRS1

Problem Statement
The mTOR signaling pathway is a fundamental regulator of cellular metabolism and growth, with its downregulation consistently shown to extend lifespan across diverse taxa. In the nematode Caenorhabditis elegans, it is established that mTOR exerts cell non-autonomous control over organismal longevity. However, the specific molecular mechanisms mediating this systemic regulation remain elusive. A critical gap exists in understanding how mTOR signaling translates into organism-wide lifespan extension, particularly regarding the involvement of hormonal signaling and downstream effector genes.

Methodology
To address these gaps, the authors employed a multi-faceted approach combining genetic manipulation, functional genomics, and comparative analysis:

• Genetic Manipulation: The study utilized C. elegans strains with deletions in raga-1 (the ortholog of mammalian RRAGA), a key regulator of TORC1, to observe effects on lifespan and hormone production.
• Hormonal and Genetic Dependency Analysis: The researchers assessed the dependence of lifespan extension on dafachronic acid (DA) biosynthetic genes and the DA-cognate nuclear hormone receptor DAF-12 (a homolog of the mammalian farnesoid X receptor, FXR).
• Functional Genomic Screens: Systematic screens were conducted to identify downstream regulatory targets of the mTOR-steroid axis.
• Comparative Analysis: The expression and regulation of the identified target were examined in murine models to determine evolutionary conservation, specifically looking at the interaction between mTOR signaling and FXR in mice.

Key Contributions and Results
The study identifies a novel neuroendocrine mechanism linking mTOR signaling to longevity:

1. mTOR-DA Axis: Deletion of raga-1 was found to enhance the production of dafachronic acid (DA), a bile acid-like hormone. This lifespan extension was strictly dependent on DA biosynthetic genes and the nuclear receptor DAF-12, establishing a direct link between mTOR inhibition and steroid hormone signaling.
2. Identification of DHS-26/DHRS1: Through functional genomic screens, the authors identified DHS-26 (and its human/murine ortholog DHRS1) as a previously uncharacterized short-chain dehydrogenase. This enzyme acts as a critical downstream effector of the mTOR-steroid axis, essential for mediating organismal longevity.
3. Neuroendocrine Localization: DHS-26 expression is prominent in the canal-associated neurons (CANs) of C. elegans. Since CANs are essential for growth and development, this localization suggests a neuroendocrine mechanism where the nervous system regulates systemic longevity via hormone availability.
4. Evolutionary Conservation: The study demonstrates that murine DHRS1 is similarly regulated by mTOR signaling and the nuclear receptor FXR. This indicates that the mTOR-DHS-26/DHRS1 axis is an evolutionarily conserved pathway across metazoans.

Significance and Claims
The paper claims that these findings elucidate a systemic mechanism by which mTOR signaling impacts metazoan longevity. Specifically, the authors posit that mTOR regulates lifespan by controlling the availability of bile acid-like hormones and the subsequent signal transduction through nuclear receptors. The identification of DHS-26/DHRS1 as a conserved effector provides a molecular bridge between metabolic signaling (mTOR) and hormonal regulation (DA/FXR), offering a concrete explanation for the cell non-autonomous effects of mTOR on aging. The study underscores the importance of neuroendocrine regulation in the aging process and highlights a conserved pathway potentially relevant to mammalian biology.