Clinical trials are underway to test whether rapamycin, a drug that has served as an immune suppressor for decades, could also treat cancer and neurodegeneration. Scientists are also interested in exploring its anti-aging properties.
Rapamycin gets its name from Rapa Nui, the native term for Easter Island. In the 1960s, scientists went to the island in search of new antimicrobials. They found that the island's soil harbors bacteria "a compound with remarkable antifungal, immunosuppressive, and antitumor properties."
For many years, scientists have believed that rapamycin exerts most of its effect by blocking the appropriately-named mechanistic target of rapamycin (mTOR). However, they also suspected that the drug might work through more than just this cell signaling pathway.
Now, by uncovering a second cell target for rapamycin, a recent study offers valuable insights into the drug's potential as a neuroprotective, anti-aging agent.
The second target is a protein called transient receptor potential mucolipin 1 (TRPML1). Targeting TRPML1 appears to spur a recycling process that stops cells clogging up with waste material and faulty proteins.
Accumulation of faulty proteins in cells is a characteristic of aging. It is also a hallmark of Alzheimer's, Parkinson's, and other neurodegenerative diseases.
The study is the work of researchers at the University of Michigan in Ann Arbor and the Zhejiang University of Technology in China. They report their findings in a recent paper.
The principal study investigator is Haoxing Xu, who supervises a laboratory in the Department of Molecular, Cellular, and Developmental Biology, at the University of Michigan.
"The identification of a new target of rapamycin offers an insight in developing the next generation of rapamycin, which will have a more specific effect on neurodegenerative disease," says co-lead study author Wei Chen, who works in Xu's laboratory.
Rapamycin and autophagy
Since the discovery of rapamycin, its various uses as an immune suppressor have extended from preventing immune rejection of organ transplants to the coating of stents that prop open coronary arteries.
The Food and Drug Administration (FDA) have also approved several rapamycin derivatives, or "rapalogs," for clinical trials to evaluate their effectiveness in targeting cancer cells and treating neurodegenerative diseases. In addition, studies in mammals, flies, and other organisms have shown that rapamycin can lengthen lifespan.
When rapamycin blocks mTOR, it halts cell growth. That is why drug developers are interested in its potential as an anticancer agent because uncontrolled growth of cells is a primary feature of cancer.
However, blocking mTOR also sets autophagy in motion. Autophagy is another cell process that clears away and recycles damaged cell components and proteins that have the wrong shape and do not work correctly.
Autophagy depends on cell recycling compartments called lysosomes to break down the waste materials into molecular building blocks that the cell can use again.
"The main function of the lysosome is to maintain the healthy state of the cell because it degrades the harmful stuff within the cell," explains co-lead study author Xiaoli Zhang, who also works in Xu's laboratory.
"During stress conditions," she adds, "autophagy can lead to [...] cell survival by degrading dysfunctional components and providing the building blocks of cells, such as amino acids and lipids."
TRPML1 and lysosomes
TRPML1 is a protein that sits on the surface of lysosomes and acts as a channel for calcium ions. It conveys signals that control the function of lysosomes.
The team used a "lysosome patch clamp" to investigate the role of TRPML1. This highly sophisticated technique allows researchers to observe the channel's operation. The team used cultures of mammalian and human cells in their study.
Using the patch clamp, the team could show that rapamycin was able to open the TRPML1 channel in cells' lysosomes independently of mTOR. It did not matter whether mTOR was active or inactive; the effect was the same.
The researchers also found that rapamycin could not trigger autophagy in cells that lacked TRPML1. This showed that rapamycin needed TRPML1 to enhance autophagy.
The authors conclude that "identification of TRPML1 as an additional [rapamycin] target, independent of mTOR, may lead to a better mechanistic understanding of [rapamycin's] effects on cellular clearance."
"We think lysosomal TRPML1 may contribute significantly to the neuroprotective and anti-aging effects of rapamycin," says Chen.
"Without this channel, you get neurodegeneration. If you stimulate the channel, it's anti-neurodegeneration."