Even cells need to take out the trash in order to maintain a healthy clean environment. And scientists are now uncovering the harmful effects when cells instead begin to hoard their garbage.
Aging, on the cellular level is—at its core—the increasing inability for cells to repair themselves over time. As cells begin to break down faster than they can be repaired, the risk of age-related diseases escalates. Cancer, heart disease and neurological conditions such as Alzheimer’s disease are some of aging’s most deadly effects.
As a result, scientists have long searched for ways to give our cells a little help and improve our quality of life as we age. For example, recent research has pointed to a connection between fasting (restricting calories) and a longer lifespan, though the molecular mechanisms behind this connection remain somewhat cryptic.
In two studies published this month in the Proceedings of the National Academy of Sciences and eLife, Gottschling and colleagues discover that a particular long-lasting protein builds up over time in certain cell types, causing the buildup of a protein hoard that damages the cell beyond repair.
Clearing out the Cobwebs
Some cells, such as those that make up the skin or that reside in the gut, are continually replenished by a stockpile of adult stem cells. But other cells, such as those found in the eye and brain, last for years, decades and—in some cases—our entire lifetimes.
Within and surrounding these long-lived cells are similarly long-lived proteins which help the cell perform essential functions. For example, the lens of the human eye, which helps focus light, is made up of these proteins that arise during embryonic development and last for a lifetime.
“Shortly after you’re born, that’s it, you get no more of that protein and it lives with you the rest of your life,” explained Gottschling.
As a result, if those proteins degrade and die, new ones don’t replace them—the result is the age-related disease called cataracts.
But scientists weren’t exactly sure of the relationship between these dying proteins and the onset of conditions such as cataracts, and other disease related to aging. Did these conditions occur because the proteins were dying? Or rather because the proteins were building up to toxic levels?
So Gottschling and his team set up a series of experiments to find out.
They developed a laboratory model by using yeast cells. Interestingly, yeast cells share several key properties with human stem cells, and are often the focus of early-stage research into basic, fundamental concepts of biology.
Like stem cells, yeast cells grow and divide asymmetrically. In other words, a ‘mother’ cell will produce many ‘daughter’ cells, but will itself remain intact. In general, yeast mother cells produce up to 35 daughter cells before dying—which usually takes just a few days.
Here, the research team used a special labeling technique that marked individual proteins that exist within and surrounding these mother cells. These microscopic tracking devices then told researchers how these proteins behaved over the entire lifespan of the mother cell as it aged.
The team found a total of 135 long-lived proteins within the mother cell. But what really surprised them was what they found upon closer examination: all but 21 of these 135 proteins appeared to have no function. They appeared to be trash.
“No one’s ever seen proteins like this before [in aging],” said Nathanial Thayer, a graduate student in the Gottschling Lab and lead author of one of the studies.
Added Gottschling, “With the number of different fragments [in the mother cell], we think they’re going to cause trouble. As the daughter yeast cells grow and split off, somehow mom retains all these protein bits.”
This startling discovery opened up an entirely new set of questions, explained Gottschling.
“It’s not clear whether the mother’s trash keeper function is a selfless act designed to give her daughters the best start possible, or if she’s hanging on to them for another reason.”
Hungry, Hoarding Mother Cells
So Gottschling and his team took a closer look at one of these proteins, known as Pma1.
Recent work by the Gottschling Lab found that cells lose their acidity over time, which itself leads to the deterioration of the cells’ primary energy source. The team hypothesized that Pma1 was somehow intricately tied to corresponding levels of pH (high pH levels indicate an acidic environment, while lower pH levels signify a more basic environment).
In the second study published in eLife, led by Postdoctoral Fellow Dr. Kiersten Henderson, the team made several intriguing discoveries about the role of Pma1.
First, they uncovered a key difference between mother and daughter cells: daughter cells are born with no Pma1. As a result, they are far more acidic than their mothers. But when they ramped up Pma1 in the mother cells, the acidity levels in subsequent generations of daughter cells changed accordingly.
“When we boosted levels of the protein, daughter cells were born with Pma1 and became more basic (they had a lower pH), just like their mothers.”
Further examination uncovered the true relationship between Pma1 and these cells. At its most fundamental, Pma1 helps the mother cells eat.
“Pma1 plays a key role in cellular feeding,” said Gottschling. “The protein sits on the surface of cells and helps them take in nutrients from their environment.”
Pma1 gives the mother cell the ability to gorge herself. The more access to food she has, the easier it is for her to produce more daughter cells. By hoarding Pma1, the mother cell can churn out more offspring. Unfortunately, she is also signing her own death certificate—she’s creating a more basic environment that, in the end, proves toxic and contributes to her death.
The hoarding, it turns out, may not all be due to the mother cells’ failure to ‘take out the trash.’ Instead, she wants to keep eating and producing daughters—and hoarding Pma1 allows her to do just that.
“There’s this whole trade off of being able to divide quickly and the negative side is that the individual, the mother, does not get to live as long.”
Together, the results from these two studies provide a huge boost for researchers like Gottschling who are trying to unravel the molecular mysteries of aging. But the process is incredibly intricate, and there will likely be no one simple solution to improving quality of life as we get older.
“The whole issue of aging is so complex that we’re still laying the groundwork of possibilities of how things can go awry,” said Gottschling. “And so we’re still learning what is going on. We’re defining the aging process.”