A scalable, clinic-friendly recipe for converting skin cells to muscle cells

Way back in 1987, about two decades before Shinya Yamanaka would go on to identify four proteins that can reprogram skin cells into induced pluripotent stem cells (iPSCs), Harold Weintraub’s lab identified the first “master control” protein, MyoD, which can directly convert a skin cell into a muscle cell. Though MyoD opened up new approaches for teasing out the molecular mechanisms of a cell’s identity, it did not produce therapeutic paths for replacing muscle damaged by disease and injury.

That’s because MyoD-generated muscle cells are not amenable to a clinical setting. For a cell therapy to be viable, you need to manufacture large amounts of your product to treat many people. But these MyoD cells do not grow well enough to be effective to serve as a cell replacement therapy. Generating iPSC-derived muscle cells provides the potential of overcoming this limitation but the capacity of the embryonic stem cell-like iPSC for unlimited growth carries a risk of forming tumors after the transplanting iPSC-derived cell therapies into the muscle.

169572_web

This image shows iMPCs stained for markers of muscle stem, progenitor and differentiated cells. iMPCs recapitulate muscle differentiation in a dish. Credit: Ori Bar-Nur and Mattia Gerli

A recent study in Stem Cell Reports, by Konrad Hochedlinger’s lab at Massachusetts General Hospital and the Harvard Stem Cell Institute, may provide a work around. The team came up with a recipe that calls for the temporary activation of MyoD in mouse skin cells, along with the addition of three molecules that boost cell reprogramming. The result? Cells they dubbed induced myogenic progenitor cells, or iMPCs, that can make self-sustaining copies of themselves and can be scaled up for manufacturing purposes. On top of that, they show that these iMPCs carry the hallmarks of muscle stem cells and generate muscle fibers when transplanted into mice with leg injuries without signs of tumor formation.

A lot of work still remains to be done, like confirming that these iMPCs truly have the same characteristics as muscle stem cells. But if everything pans out, the potential applications for people suffering from various muscle disorders and injuries is very exciting, as co-first author Mattia FM Gerli, PhD points out in a press release:

in7czFjH_400x400

Mattia FM Gerli, PhD

“Patient-specific iMPCs could be used for personalized medicine by treating patients with their own genetically matched cells. If disease-causing mutations are known, as is the case in many muscular dystrophies, one could in principle repair the mutation in iMPCs prior to transplantation of the corrected cells back into the patient.”

Taking Steps Toward Personalized Heart Transplants

Over five million Americans have heart failure (HF), a condition in which the heart muscles become too weak to pump an adequate amount of blood, oxygen and nutrients to the body’s other organs. People with heart failure suffer from shortness of breath, chronic weakness and a fifty percent chance of dying within the first five years of diagnosis.

SystolicDiastolic_Heartfailure.5518685646fab

Heart failure weakens the heart’s ability to pump blood to the body.
Image credit: Fran Milner, http://www.franimation.com

Heart transplants: swapping one disease for another
The only true cure for heart failure is an organ transplant but donor organs are in limited supply. And those lucky enough to receive a transplant need to take life long immunosuppressive drugs to fight off organ rejection, which often leads to other serious health problems like risk of infection, high blood pressure, diabetes and kidney failure.

It’s not a pretty picture for patients and with a cost to the nation of $32 billion annually, heart failure affects us all.

In a recent Circulation Research journal article, a team at the Massachusetts General Hospital (MGH) Center for Regenerative Medicine (CRM) reported on an incremental yet important step toward an alternative approach to heart transplants: growing bioengineered hearts with a patient’s own stem cells.

Bioengineered hearts: been there, done that
Bioengineered hearts may sound far fetched but eight years have already passed since researchers showed it’s possible with animal models. In that 2008 study, scientists at the University of Minnesota soaked rat hearts in detergents that cleaned away all of the heart cells, leaving behind a scaffold of connective tissue that is secreted by the cells. They then “re-seeded” the scaffold with rat progenitor heart muscle cells. Incredibly, after 4 days in a bioreactor, the tissue began contracting. After 8 days, using electrical stimulation, the hearts could pump with measurable strength.

heartscaffold

Custom built bioreactor with a partially recellularized human whole-heart cardiac scaffold. Image credit: Bernhard Jank, MD, Ott Lab, CRM MGH

Harald Ott, the first author of that proof of concept study, now leads the MGH team in the current study. With an eye toward bringing this method to the clinic, Ott aimed to reproduce the rat studies using human cells and hearts. Specific consent was obtained to recover seventy-three human hearts donated after brain death or cardiac death and were determined to be unusable for clinical transplant.

Just as in the previous study, heart scaffolds were created by clearing away the cells from these organs. Half a billion human heart muscle cells, or cardiomyocytes, that had been grown from human induced pluripotent stem cells (iPS) were then injected into the scaffolds. At this early stage of research, the team did not attempt to reseed the entire heart but specifically focused on areas around the left ventricle – the heart chamber responsible for pumping oxygenated blood out to the rest of the body.

In order to recellularize the scaffold under conditions that resemble the contractions that actually occur in a human heart inside the body, pressure was rhythmically applied using a balloon inside the ventricle – watch it in action by downloading this video.

Screen Shot 2016-03-15 at 9.22.17 AM

Decellularized heart scaffold is injected with iPS heart muscle cells and incubated in a bioreactor to grow new heart muscle (Circ Res. 2016;118:56-72).

After incubating the heart for 14 days in a bioreactor, the team confirmed the cardiomyocytes had formed into functioning heart muscle, or myocardial, tissue that contracted upon electrical stimulation (watch the beating heart by downloading this video).

Looking ahead to a “grow yourself a new heart” future
These results set a course for a future in which scientists could grow a new, personalized heart for people with heart failure. Since the bioengineered heart would be built using iPS cells derived from the patient’s own skin or blood sample, this technique would likely get around the problems of organ rejection and the need for immunosuppressive drugs. Additional analysis in the current study also confirmed that the donor heart scaffold itself, which is void of cells, probably will not pose tissue rejection problems either.

The other key problem, a limited supply of donor hearts, was also addressed. Heart scaffolds donated after cardiac death performed just as well as those donated after brain death which would make this technique available to more patients waiting anxiously for a transplant.

The MGH team would be the first to tell you this “grow yourself a new heart” scenario is still off on the horizon. But as first author, Jacques Guyette, stated in a news release, there are milestones to reach along the way:

“Regenerating a whole heart is most certainly a long-term goal that is several years away, so we are currently working on engineering a functional myocardial [heart muscle tissue] patch that could replace cardiac tissue damaged due a heart attack or heart failure.”

Related Links:

Perfecting the use of stem cells as drug delivery mules shows promise in brain tumors

Stem cells loaded with cancer-killing herpes virus (red) attacking a brain tumor cell (green). Courtesy HSCI

The innate tendency of stem cells to seek out inflammation—combined with the fact that our bodies see tumors as inflammation—has led many teams to try to harness stem cells as delivery vehicles for cancer therapies. CIRM funds a team at City of Hope in Duarte, California that aims to treat brain tumors with stem cells loaded with an agent that can be turned into a form of chemotherapy.

Now, a team at the Harvard Stem Cell Institute and Massachusetts General Hospital have used stem cells to revive a therapy, once considered highly promising, that failed in early clinical trials.

A number of viruses have the ability to kill tumors. In particular, some viruses naturally kill rapidly dividing cells like those found in tumors. But as so often happens, early success in mice did not carry over to the first trials in humans. Researchers reasoned that our body’s immune system cleared out the virus before it could do its deadly deed with the cancer cells.

In this study the Harvard team decided to shield the virus inside stem cells. They then encased the stem cells in a gel that they had previously shown could enhance the ability of the stem cells to remain alive after transplantation.

In a mouse model of glioblastoma, the most common form of brain cancer, the combination extended the life of the mice after the cells were placed at the site where the tumors were surgically debulked, a procedure performed on most human patients with the cancer.

The herpes virus they used had been tagged with an imaging protein that allowed the team to verify that the stem cells lived long enough for the virus to replicate and kill the residual tumor left behind after surgery. It is this residual cancer than makes this brain tumor almost uniformly fatal.

Because a few cancer cells are resistant to the herpes virus, the team gave their killer a second weapon, using genetic engineering techniques to help the virus kill cancer cells. The lead researcher, Khalid Shah, discussed the combination therapy in an article in Genetic Engineering & Biotechnology News:

“Our approach can overcome problems associated with current clinical procedures. The work will have direct implications for designing clinical trials using oncolytic viruses, not only for brain tumors, but for other solid tumors.”

Shah predicted the process would enter clinical trials in two to three years. They published the current research in the Journal of the National Cancer Institute.

Don Gibbons