How Brain Stem Cells Could Stay Forever Young

As we age, so do the cells that make up our bodies. To keep us spry as we get older, our bodies rely on adult stem cells to replace the cells in our tissues and organs. Adult stem cells can only generate cell types specific to the organ or tissue that they live in. For instance, heart stem cells can only help regenerate or repair the heart, same for brain stem cells and the brain, etc.

While adult stem cells have built-in mechanisms to help them avoid the aging process for as long as possible, they can only delay the inevitable for so long. So as the function of our bodies decline, so does adult stem cell function and with it our ability to regenerate damaged tissue.

But now a new study has found out what happens to cause the aging of adult stem cells and points at ways to avoid it and keep these stem cells “forever young.”

Brain stem cells stay youthful

A group from Zurich, Switzerland studied how brain stem cells stay young as the brain ages. In a study published in Science on Friday, they found that young brain stem cells divide in a way that routes damaged proteins and aging-related factors away from the daughter stem cells and into their non-stem cell progeny, thus keeping brain stem cells healthy and youthful.

stemcelldivision

Brain stem cells divide asymmetrically into a daughter stem cell and a non-stem daughter cell that can differentiate into other brain cells (Image adapted from Berika et al., 2014).

The Zurich group took a closer look at brain stem cells in adult mouse brains and found that they divide asymmetrically. This means that instead of equally dividing its cellular components between two daughter cells, the mother cell instead herds all of the damaged proteins and aging factors into the non-stem daughter cell, leaving the new stem cell unexposed to cell damage. In this way, the new stem cell is protected and is able to maintain its regenerative capacity.

A barrier against aging?

Brain stem cells are able to preferentially shuttle damaged proteins into their non-stem cell progeny by a diffusion barrier called the endoplasmic reticulum (ER). The ER is a membrane structure in cells that has a number of important functions including deciding what factors or proteins end up in which cells.

The authors observed that during the division of brain stem cells, the ER forms a barrier between the non-stem and stem cell progeny that keeps the damaged proteins and aging factors in the non-stem daughter cell. This ER barrier remains intact during the division of young brain stem cells, however, they weren’t sure this was the case with older brain stem cells.

The scientists watched older brain stem cells to see if this anti-aging barrier was able to hold up with advancing age. They observed that this barrier actually weakens with age and allows aging factors to go with the stem cell progeny. This happens because an important cell structure called the nuclear lamina, which regulates cell division, doesn’t function properly in the old stem cells. When they disrupted the lamina structure in young brain stem cells, as expected, the anti-aging barrier couldn’t properly block the transfer of aging-factors into the new daughter stem cells.

youngvsold

Young brain stem cells on the left divide asymmetrically and have a barrier that keeps age-related damage in the non-stem daughter cell (red). This barrier weakens in older brain stem cells and aging factors are transferred to the stem cell progeny. (Moore et al., 2015)

 

Thus it seems that brain stem cells maintain their youth by generating diffusible barriers that block the transfer of damaged proteins and aging factors into their stem cell progeny during cell division. The strength of this barrier weakens with age, and when this happens, aging factors are more evenly divided between the non-stem and stem cell progeny, potentially causing stem cell damage and reducing their regenerative function.

Anti-aging implications

The authors note at the end of their report that further studies should be done to determine whether this anti-aging mechanism is unique to brain stem cells or if it occurs in other adult stem cells or cancer cells which display stem cell like properties. If similar anti-aging barriers exist, then targeting the age-related breakdown of this barrier could be a potential strategy to keep adult stem cells forever young and humans feeling and acting younger for a little longer.


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New Regenerative Liver Cells Identified

It’s common knowledge that your liver is a champion when it comes to regeneration. It’s actually one of the few internal organs in the human body that can robustly regenerate itself after injury. Other organs such as the heart and lungs do not have the same regenerative response and instead generate scar tissue to protect the injured area. Liver regeneration is very important to human health as the liver conducts many fundamental processes such as making proteins, breaking down toxic substances, and making new chemicals required to digest your food.

The human liver.

The human liver

Over the years, scientists have suggested multiple theories for why the liver has this amazing regenerative capacity. What’s known for sure is that mature hepatocytes (the main cell type in the liver) will respond to injury by dividing and proliferating to make more hepatocytes. In this way, the liver can regrow up to 70% of itself within a matter of a few weeks. Pretty amazing right?

So what is the source of these regenerative hepatocytes? It was originally thought that adult liver stem cells (called oval cells) were the source, but this theory has been disproved in the past few years. The answer to this million-dollar question, however, likely comes from a study published last week in the journal Cell.

Hybrid hepatocytes (shown in green) divide and regenerate the liver in response to injury. (Image source: Font-Burgada et al., 2015)

Hybrid hepatocytes (green) divide and regenerate the liver in response to injury. (Image source: Font-Burgada et al., 2015)

A group at UCSD led by Dr. Michael Karin reported a new population of liver cells called “hybrid hepatocytes”. These cells were discovered in an area of the healthy liver called the portal triad. Using mouse models, the CIRM-funded group found that hybrid hepatocytes respond to chemical-induced injury by massively dividing to replace damaged or lost liver tissue. When they took a closer look at these newly-identified cells, they found that hybrid hepatocytes were very similar to normal hepatocytes but differed slightly with respect to the types of liver genes that they expressed.

A common concern associated with regenerative tissue and cells is the development of cancer. Actively dividing cells in the liver can acquire genetic mutations that can cause hepatocellular carcinoma, a common form of liver cancer.

What makes this group’s discovery so exciting is that they found evidence that hybrid hepatocytes do not cause cancer in mice. They showed this by transplanting a population of hybrid hepatocytes into multiple mouse models of liver cancer. When they dissected the liver tumors from these mice, none of the transplanted hybrid cells were present. They concluded that hybrid hepatocytes are robust and efficient at regenerating the liver in response to injury, and that they are a safe and non-cancer causing source of regenerating liver cells.

Currently, liver transplantation is the only therapy for end-stage liver diseases (often caused by cirrhosis or hepatitis) and aggressive forms of liver cancer. Patients receiving liver transplants from donors have a good chance of survival, however donated livers are in short supply, and patients who actually get liver transplants have to take immunosuppressant drugs for the rest of their lives. Stem cell-derived liver tissue, either from embryonic or induced pluripotent stem cells (iPSC), has been proposed as an alternative source of transplantable liver tissue. However, safety of iPSC-derived tissue for clinical applications is still being addressed due to the potential risk of tumor formation caused by iPSCs that haven’t fully matured.

This study gives hope to the future of cell-based therapies for liver disease and avoids the current hurdles associated with iPSC-based therapy. In a press release from UCSD, Dr. Karin succinctly summarized the implications of their findings.

“Hybrid hepatocytes represent not only the most effective way to repair a diseased liver, but also the safest way to prevent fatal liver failure by cell transplantation.”

This exciting and potentially game-changing research was supported by CIRM funding. The first author, Dr. Joan Font-Burgada, was a CIRM postdoctoral scholar from 2012-2014. He reached out to CIRM regarding his publication and provided the following feedback:

CIRM Postdoctoral Fellow Jean Font-Burgada

CIRM postdoctoral scholar Joan Font-Burgada

“I’m excited to let you know that work CIRM funded through the training program will be published in Cell. I would like to express my most sincere gratitude for the opportunity I was given. I am convinced that without CIRM support, I could not have finished my project. Not only the training was excellent but the resources I was offered allowed me to work with enough independence to explore new avenues in my project that finally ended up in this publication.”

 

We at CIRM are always thrilled and proud to hear about these success stories. More importantly, we value feedback from our grantees on how our funding and training has supported their science and helped them achieve their goals. Our mission is to develop stem cell therapies for patients with unmet medical needs, and studies such as this one are an encouraging sign that we are making progress towards to achieving this goal.


Related links:

UCSD Press Release

CIRM Spotlight on Liver Disease Research

CIRM Spotlight on Living with Liver Disease