For centuries, some of the world’s leading scholars worked endlessly with one objective, to turn ordinary metals into gold. Now, scientists of the 21st century are trying their hand at another type of alchemy, turning one cell type into another. This process, known as ‘transdifferentiation’ or ‘direct reprogramming’ can create a potentially limitless source of some of the body’s most poorly proliferative cells. But where does transdifferentiation stand in a world where induced pluripotent stem cells (iPSCs) are stealing all the headlines?
The Origins of Transdifferentiation
When Helen Blau was experimenting with some neat biochemical wizardry to fuse two different cell types together in the 1980s, little did she know that this simple experiment would form the basis of our understanding about the control of cellular identity in years to come. The purpose was to create one mega cell or ‘heterokaryon’ with two nuclei and a shared cytoplasm and examine how the gene expression program of the occupant nuclei would influence each other in this rather unconventional flat share. Blau fused human amniocytes with mouse myocytes to easily identify the gene products from each nucleus. Remarkably, the resultant heterokaryon started to express human muscle-specific genes.
Even then, reprogramming itself was nothing new. In the 1960’s John Gurdon had shown in frogs that nuclei from differentiated cells could be reprogrammed by the specialized environment of the egg. But this required going right back to the very beginning of development.
Waddington pictured the process of differentiation as a marble, representing a pluripotent cell, at the top of a hill. During development, the marble rolls down the hill taking one of several routes through many valleys, before finally stopping at the bottom as a terminally differentiated cell. Gurdon’s technique provided a long and treacherous mission back up the summit of pluripotency. Experiments with heterokaryons showed that there was a shortcut.
Magical MyoD Promotes Conversion to Skeletal Muscle Cells
Not long after came the identification of the gene responsible for conversion of cells to myoblasts, the transcription factor MyoD, in experiments by Harold Weintraub’s lab. The forced expression of this gene alone led to the conversion of a variety of cell types to skeletal muscle, and thus direct reprogramming was born.
What works for some, however, does not work for all, and Weintraub’s team soon identified ‘refractory’ cell types, that despite coaxing, stubbornly refused to switch fates. Weintraub postulated that transitions between cell types with a similar developmental origin would be easiest because they would require shorter journeys across the developmental landscape. The ease with which blood cells could be persuaded to switch lineages in experiments the following decade seemed to confirm this.
Expanding the Repertoire of Transdifferentiation
Then came a quantum leap in 2010: Vierbuchen turned fibroblasts into neurons and in doing so transversed the barrier of embryonic germ layer, from mesoderm to ectoderm. A wide variety of cell types have now been made from fibroblasts, which are chosen as a starting point because they are easy to obtain and culture. These include hepatocytes (endoderm), cardiomyocytes (mesoderm), macrophages (mesoderm), smooth muscle cells, and recently pancreatic islet cells (endoderm).
All of these cell types can be generated by the forced expression of a cocktail of transcription factors, by tinkering with miRNA, or a combination of both. This has raised the possibility that just about any cell type can be created in a dish. All you need are the right ingredients.
Direct Reprogramming vs. Induced Pluripotent Stem Cells
The ultimate aim of these attempts at cellular alchemy is to create new functioning tissue for cell replacement therapy. So far, no trial in humans has used transdifferentiated cells, and iPSCs, which made it into the clinic for the first time last year, have certainly had a head start. Nonetheless, these transcriptionally-rewired cells have several potential advantages over iPSCs. For starters, direct reprogramming can occur without cell division so there is less risk of mutation. This also makes the process a lot faster and reduces the risk of teratoma formation.
Manipulating Cell Fate in vivo
Perhaps the biggest advantage that direct programming offers over iPSCs is the ability to make reprogrammed cells directly inside a living organism. One group recently tried to create iPSCs in vivo using a doc-inducible system to turn on the expression of classical Yamanaka reprogramming factors in mouse tissues. The mice developed crippling teratomas in multiple organs and were completely wiped out; yet, several studies have reported safe and efficient transdifferentiation in vivo.
Zhou and colleagues injected a cocktail of reprogramming factors directly into the pancreas of mice and converted exocrine cells into insulin-producing β-cells with an efficiency of 20%. Using a similar strategy, other studies have converted cardiac fibroblasts into cardiomyocytes and astrocytes into neurons in vivo. One (of the many) remaining problems is the successful integration of these cells into resident tissues. In Zhou’s case, most of the induced β-cells remained scattered and did not contribute to existing islets, which probably explained their limited capacity to restore glucose homeostasis. Nonetheless, these experiments provide proof-of-concept that cells resident within living organisms can be instructed to stop what they are doing and undergo a radical makeover.
The alchemists never succeeded in making gold. But in just a few decades, scientists have managed to convert the humble fibroblast into functioning heart and nerve cells, the 21st century equivalent of turning water into wine. We will be watching closely to see what miraculous transformations the next few years have in store.