Stem cell stories that caught our eye: building an embryo and reviving old blood stem cells

Building an embryo in the lab from stem cells
The human body has been studied for centuries yet little is known about the first 14 days of human development when the fertilized embryo implants into the mother’s uterus and begins to divide and grow. Being able to precisely examine this critical time window may help researchers better understand why 75% of conceptions never implant and why 30% of pregnancies end in miscarriage.

This lack of knowledge is due in part to a lack of embryos to study. Researchers rely on embryos donated by couples who’ve gone through in vitro fertilization to get pregnant and have left over embryos that are otherwise discarded. Using mouse stem cells, a research team from Cambridge University reports today in Nature that they’ve generated a cellular structure that has the hallmarks of a fertilized embryo.

embryo

Stem cell-modeled mouse embryo (left) Mouse embryo (right); The red part is embryonic and the blue extra-embryonic.
Credit: Sarah Harrison and Gaelle Recher, Zernicka-Goetz Lab, University of Cambridge

This technique has been tried before without success. The breakthrough here was in the types of cells used. Rather that only relying on embryonic stems cells (ESCs), this study also included extra-embryonic trophoblast stem cells (TSCs), the cell type that goes on to form the placenta.

When grown on a 3D scaffold made from biological materials, the two cell types self-organized themselves into a pattern that closely resembles the early development of a true embryo. In a press release that was picked up by many media outlets, senior author Zernicka-Goetz spoke about the importance of including both TSCs and ESCs:

“We knew that interactions between the different types of stem cell are important for development, but the striking thing that our new work illustrates is that this is a real partnership – these cells truly guide each other. Without this partnership, the correct development of shape and form and the timely activity of key biological mechanisms doesn’t take place properly.”

The researchers think that lab-made embryos from mouse or human stem cells have little chance of developing into a fetus because other cell types critical for continued growth are not included. And there’s much to be learned by focusing on these very early events:

“We are very optimistic that this will allow us to study key events of this critical stage of human development without actually having to work on embryos.  Knowing how development normally occurs will allow us to understand why it so often goes wrong,” says Zernicka-Goetz.

Reviving old blood stem cells, part 1: repair the garbage collectors
One of the reasons that our bodies begin to deteriorate in old age is a weakening, dysfunctional immune system that increases the risk for serious infection, blood cancers and chronic inflammatory diseases like atherosclerosis (hardening of the arteries). Reporting this week in Nature, a UCSF research team presents evidence that a breakdown in our cell’s natural garbage collecting system may be partially to blame.

The team focused on a process called autophagy (literally meaning self “auto”-eating “phagy”) that keeps cells functioning properly by degrading faulty proteins and cellular structures. In particular, they examined autophagy in blood-forming stem cells, which give rise to all the cell types of the immune system. They found that autophagy was not working in 70 percent of blood stem cells from old mice. And these cells had all the hallmarks of an old cell. And the other 30 percent? In those cells, autophagy was fully functional and they looked like blood stem cells found in young mice.

The team went on to show that in blood stem cells, autophagy had an additional role that until now had not been observed: it helped slow the activity of the stem cells back to its default state by gobbling up excess mitochondria, the structures that produces a cell’s energy needs. Without this quieting of the stem cell, the over-active mitochondria led to chemical modification of the cell’s DNA that disrupted the blood stem cells’ ability to give rise to a proper balance of immune cells. In fact, young mice with genetic modifications that block autophagy generated blood stem cells with these old age-related characteristics.

But the researchers were also able to restore autophagy in blood stem cells collected from old mice by adding various drugs. Team lead Emmanuelle Passegué is optimistic this result could be translated into a therapeutic approach:

“This discovery might provide an interesting therapeutic angle to use in re-activating autophagy in all of the old HSCs, to slow the aging of the blood system and to improve engraftment during bone marrow or HSC transplantation,” Passegué said in a university press release.

Reviving old blood stem cells, part 2: fix the aging neighborhood
Another study this week focused on age-related disruptions in the function of blood stem cells but in this case an aging neighborhood is to blame. Blood stem cells form and hang out in areas of the bone marrow called niches. Researchers at the Cincinnati Children’s Hospital Medical Center and the University of Ulm in Germany reported this week in EMBO that the age of the niche affects blood stem cell function.

bonemarrow

Microscopy of bone marrow. Red staining indicates osteopotin, blue staining indicates cell nuclei. Credit: University of Ulm

 

When blood stem cells from two-year old mice were transplanted into the bone marrow of eight-week old mice, the older stem cells took on characteristics of young stem cells including an enhance ability to form all the different cell types of the immune system. In trying to understand what was going on, the researchers focused on a bone marrow cell called an osteoblast which gives rise to bone. Osteoblasts produce osteopontin, a protein that plays an important role in the structure of the bone marrow. The team showed that as the bone marrow ages, osteopontin levels go down. And this reduction had effects on the health of blood stem cells. But, as team lead Hartmut Geiger mentions in a press release, this impact could be reversed which points to a potential new therapeutic strategy for age-related disease:

“We show that the place where HSCs form in the bone marrow loses osteopontin upon aging, but if you give back the missing protein to the blood-forming cells they suddenly rejuvenate and act younger. Our study points to exciting novel ways to have a better immune system and possibly less blood cancer upon aging by therapeutically targeting the place where blood stem cells form.”

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