The legend of the fountain of youth has captivated people for centuries, with the promise that its rejuvenating waters can turn someone young again. Nature has its own version of this myth, as rejuvenation is a part of every organism’s life cycle. Think about it like this: babies are always born young, regardless of the age of their parents. Nature’s rejuvenation program is the process of meiosis, which is responsible for the creation of gametes, or sex cells, like eggs and sperm. Previous research establishes that cells accumulate damage as they age, which limits their lifespan. Excitingly, cells that undergo meiosis experience full lifespan resetting and the clearance of any age-associated damage. Exactly how this rejuvenation event occurs remains mysterious and is a subject of research in the joint lab of UC Berkeley Professors Gloria Brar and Elçin Ünal.
Recent findings from the lab, published this week in Nature Communications, describe a breakthrough in the way gametogenic rejuvenation is studied and the key molecular players involved. It begins with a machine the lab has nicknamed “Buddy.”
Life is a movie
Buddy is a high-tech microfluidic microscope named after the budding yeast (S. cerevisiae) that the lab uses as a model organism to study aging and gametogenic rejuvenation. Many age-associated changes that happen in yeast are also observed in human cells. However, instead of looking for white hairs or wrinkles, yeast aging can be measured by the number of times the cell replicates. Before Buddy, traditional methods of studying gametogenic rejuvenation were low-throughput and labor intensive. Old and young yeast cells would be induced to undergo meiosis to produce a collection of four gametes. To measure the lifespan of an individual gamete, researchers would use a microscope to count every time the cell forms a new daughter cell after budding. They would then remove it with a tiny needle, so as not to confuse the new cells with the original gamete. This process continues until the gamete eventually dies, and then the cell’s lifespan is tallied up. Old cells have a shorter lifespan compared to young cells. However, gametes that come from old cells have the same lifespan as gametes that come from young cells, indicating that lifespan resetting and rejuvenation has occurred.
“I conducted these experiments during my postdoc more than twelve years ago,” Ünal reflects on the old method, a process that involved separating daughter cells every 90 to 120 minutes.“I actually had a microscope in the lab and a microscope at home, so I could work around the clock,” she adds. A typical experiment would take anywhere from 3 to 5 days, depending on how willing Ünal was to stay awake for long periods of time. Buddy obviates the need for all-nighters by recording a video, or “movie,” of the gamete budding. Silvan Spiri, a postdoctoral researcher in the Brar-Ünal lab and lead author of the study, helped establish this lifespan-measurer in the lab.
He used a few genetic tricks to prepare individual gametes for analysis: He introduced mutations to decrease the cells’ stickiness, which prevents clogging the microfluidic device inside Buddy, and promotes efficient separation between the gamete and newly formed daughter cells. Spiri also used a genetic toolkit to switch the gamete’s mating response off, to ensure that the cells analyzed were truly gametes and not the fusion of two mated cells. Individual gametes are isolated and deposited into small traps on a chip, which can accommodate imaging of thousands of cells. A camera in the microscope captures days-long movies of the gametes budding. Every time a gamete buds, the daughter cell is washed away by media that’s flowed in through microfluidic tubing, while the gamete remains in the trap. These movies are then analyzed to count the gamete’s replicative lifespan.
Getting Buddy to work was a collaborative effort. From integrating the microfluidic workflow to optimizing gamete imaging, Spiri brainstormed solutions with other lab members and put them to the test. “Science is a very interconnected process,” says Spiri. “It was like a puzzle set where I put the pieces together from the work of other scientists.”
Calling the junkyard
Using a microscope like Buddy, researchers can more easily probe questions about rejuvenation since data collection is easier, more reproducible, and higher throughput. This method has enabled Spiri to get a head start in determining the key players responsible for rejuvenation. To decipher the rejuvenation program, one interesting pathway Spiri explored was autophagy, which is a mechanism cells can use to degrade certain components. Over time, different cellular components, including organized structures like organelles, fall apart. If part of an organelle is too damaged, it’s risky for a cell to keep it, so they can mark it for degradation by autophagy. “There are specific proteins that can signal to the autophagy machinery: we have a damaged part, please remove this,” Spiri explains. “It’s like if you call the junkyard to take it away for you.”
Spiri identified the first two factors that are necessary to complete gametogenic rejuvenation. These factors are autophagy receptors responsible for signaling the degradation of the endoplasmic reticulum (ER), an organelle that is important for protein folding and trafficking. When these two factors are missing, gametes from old cells are not completely rejuvenated and have a shorter lifespan. Why? “If you remove these two receptors, you can imagine that the ER has no more phone to call the junkyard,” Spiri hypothesizes. “We can imagine that the cell accumulates ER parts that are not functioning correctly, and this can lead to a crisis.”
Nature’s fountain of youth
Spiri’s work has big implications for the aging field. The accumulation of cellular damage over time is thought to contribute to age-associated diseases like neurodegeneration and cancer. “The natural rejuvenation event that accompanies the production of gametes shows us that age-associated cellular damage can be reversed,” Brar says. “This discovery uncovers the first specific factors that drive this reversal.”
The two factors identified using the Buddy platform are just the beginning. Spiri is investigating other candidates involved in proteostasis, which is the balance of protein synthesis, folding, and degradation that occurs in the cells. These candidates could be part of a mechanism responsible for resetting the cell’s proteome to a young state, so the next generation can start fresh. “I’m interested in figuring out these mechanisms and deploying them in different aging models to see if what we learn from the gametogenic rejuvenation program can reset proteostasis in aging-related diseases,” Spiri says. Taking advantage of these existing clean-up mechanisms has the potential for making a therapeutic difference. With these findings, scientists may be one step closer to the trailhead that leads to the fountain of youth.
Maia Reyes is a postdoctoral researcher in the Brar lab. She received her PhD in Molecular and Cell Biology from UC Berkeley.