Scar forming cells -- fibroblasts -- are chemically tricked to morph into cardiomyocytes -- heart cells

Researchers are describing that the human body is much more strange and pliable than previously thought -- and that's a good thing.  A shining example is the work of Professor Deepak Srivastava, MD, a professor at the Univ. of California - San Francisco (UCSF) and its affiliate Gladstone Institute labs.  Dr. Srivastava first discovered that a cocktail of three genes injected into mice could provoke fibroblast cells to become cardiomyocytes -- the cells that pump blood in a beating heart.  Now his team has managed to use a slightly more advanced formulation to get human fibroblasts to do the same thing.

I. Cardiofibroblasts Offer a Ready Made Stock of Cells For Heart Repair in Mice

While the holy grail of tissue engineering has long thought to have been to develop fully pluripotent, sustainable stem cell colonies and then trigger them to differentiate into desired cells of any kind, that has proven tricky in practice.  Complex tissue parameters -- pressure, local nutrients, and signals from surrounding cells -- have all been found to influence what target a stem cell decides to become, making what seemed an easy route far harder.  So some researchers like Dr. Srivastava have turned in part to look into ways to trick local cell types near a damaged tissue into becoming the type of cells in the healthy version of that tissue via gene therapy.  

Repairing a (literally) broken heart is one attractive target: every year nearly 600,000 Americans die of heart disease.  Heart disease is the number one killer in the U.S., according to the latest statistics from the U.S. Centers for Disease Control and Prevention.  It accounts for nearly 600,000 deaths in the U.S. a year, or just under 1 in 4 deaths.

Heart disease
Heart disease kills nearly one in four Americans in the long run. [Image Source: Soracc Photo]

Dr. Srivastava's team considered fibroblasts a particularly attractive target.  Recent papers such as Camelliti, et al. '04 and Turner, et al. '09 reveal that fibroblasts in heart tissue -- cardiofibroblasts -- are found in greater quantities (50-60% of the total cell population) than heart muscle cells (30-40%) -- also known as cadiomyocytes -- despite the cardiomyocytes dominating 75 percent of the cell volume.

In a healthy individual these special helper cells are like the gardeners of the regal estate that is the human heart.  They maintain the collagen and other biopolymers found in the extracellular matrix, keep the heart elastic and receptive to electrical signals from the "pacemaker" -- the sinoatrial (SA) node.  They also play a key role in a a damaged heart, depositing scar tissue.  While this may save the heart from infection and internal bleeding, it also thickens the heart and prevents it from fully healing, over time leading to death.

But what if you could trick some of the abundant population of myofibroblasts to become myocardiocytes when heart damage (e.g. a heart attack) occurs, repopulating the damaged region with health tissue, rather than scar tissue.

In 2012 Dr. Srivastava proved [abstract] that approach had legs, locating a trio of genes he dubbed GMT -- GATA4, MEF2C, and TBX5.  When injected into cardiofibroblasts in mice, GMT triggered them to become cardiomyocytes.  But would the approach work with humans?

Mice tests
The fibroblast idea worked on mice in a 2012 study. [Image Source: Science Museum]

That question was put to the test in the UCSF researcher's latest study.  The team isolated cardiofibroblasts from three different sources -- fetal heart cells, embryonic stem cells, and neonatal skin cells -- and injected them with GMT.  So what happened?

II. Transforming Human Cells Took a Little More Work

Ji-dong Fu, a PhD staff scientist who worked on the human tests describes the frustrating initial results, remarking, "When we injected GMT into each of the three types of human fibroblasts, nothing happened -- they never transformed -- so we went back to the drawing board to look for additional genes that would help initiate the transformation.  We narrowed our search to just 16 potential genes, which we then screened alongside GMT, in the hopes that we could find the right combination."

With 16 new genes on tap, they began to test different combinations and slowly eliminate the non-essential genes.  They found that at a minimum a five-gene mix -- GMT plus ESRRG and MESP1 -- was necessary.  But a seven-gene mix -- GMT, ESRRG, MESP1, MYOCD, and ZFPM2 -- the resulting cells were much better differentiated, being virtually indistinguishable from native cardiomyocytes.

Cardiac Myocytes
Using a modified cocktail human cardiofibroblasts were tricked into becoming cardiomyocytes.
[Image Source: UCSF/Stem Cell]

The team also tested different cell signalling chemicals to see their effects on the transformation rate.  They found that by exposing the cells to TGF-β -- a small intercellular protein that promotes differentiation -- they could speed up the process.

Professor Fu comments, "While almost all the cells in our study exhibited at least a partial transformation, about 20% of them were capable of transmitting electrical signals -- a key feature of beating heart cells.  Clearly, there are some yet-to-be-determined barriers preventing a more complete transformation for many of the cells. For example, success rates might be improved by transforming the fibroblasts within living hearts rather than in a dish -- something we also observed during our initial experiments in mice."

Dr. Srivastava enthuses on the success in making the gene therapy work in reprogramming human cells, remarking, "Fibroblasts make up about 50% of all cells in the heart and therefore represent a vast pool of cells that could one day be harnessed and reprogrammed to create new muscle.  Our findings here serve as proof of concept that human fibroblasts can be reprogrammed successfully into beating heart cells."

"With more than five million heart attack survivors in the United States, who have hearts that are no longer able to beat at full capacity, our findings -- along with recently published findings from our colleagues -- come at a critical time.  We've now laid a solid foundation for developing a way to reverse the damage -- something previously thought impossible -- and changing the way that doctors may treat heart attacks in the future."
Fibroblasts to myocytes
The next step is to test the technology in vivo on live pigs. [Image Source: UCSF/Stem Cell]

Professor Fu (the first author) and Dr. Srivastava (the senior author) -- along with eight other undergraduate, graduate, doctoral, and postdoctoral researchers from UCSF and the Gladstone Institutes authored a paper, which is published in the peer-reviewed tissue engineering journal Stem Cell Reports.

The next step will be to test the five and seven component blends on the pigs, which are closer on a cellular level to humans than rats.  Eventually the researchers hope to reformulate their drugs into "small, drug-like molecules", which would offer safe and efficient delivery of the transformative therapy to cardiac tissue of heart patients.

Sources: Gladstone Insitutes, Stem Cell Reports [Press Release]

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