But here’s the good news. They discovered the obstacle by removing it: by reducing the levels of two specific proteins within the cell, they avoided the barrier altogether. Once the proteins were limited, researchers witnessed considerably enhanced generation of mature cells from blood precursor cells.
“From a fundamental perspective, this is very important because this mechanism counteracts the development of precursor cells into red blood cells, thereby establishing a balance between developed cells and the progenitor population,” said Emery Bresnick, professor of cell and regenerative biology and director of the UW-Madison Blood Research Program at the UW School of Medicine and Public Health (SMPH). Bresnick was the senior author of the paper, published August, 2014 by the American Society of Hematology’s scientific journal, Blood.
“In the context of translation, if you want to maximize the output of end-stage red blood cells, which we’re not able to do at this time, our study provides a rational approach involving lowering the levels of these subunits,” he said.
The proteins of interest are subunits of the exosome complex, an RNA garbage disposal composed of almost a dozen proteins that form an elongated donut-like structure. Composed of long chains of nucleotides, RNA provides instructions to the cell, builds proteins for specific functions, and helps turn genes on and off. Yet, just like having too many cooks in the kitchen, too much RNA can be problematic. Enter the exosome complex.
“The exosome complex selectively degrades both coding and non-coding RNAs to control levels of specific RNAs in the cell. It acts as a post-transcriptional regulator of gene expression, because the cell needs to degrade RNA once it’s no longer needed,” said Skye McIver, postdoctoral researcher at the SMPH and one of three co-first authors of the paper. Andrew DeVilbiss, UW graduate student in the cell and molecular pathology program, and Yoona Kang, prior graduate student in Cell and Molecular Biology whose thesis research laid the foundation for this line of inquiry, were also co-first authors.
A potential target for drug therapies, the exosome complex could provide a way for doctors to adjust the blood development system to produce red blood cells, like in the instance of EPO-resistant anemias, when the development process is rendered inefficient or resistant to known therapeutics. This would be good news for many, whether they’re suffering from anemias or working to stock blood banks.
The primary obstacle now in converting stem cells into red blood cells, whether those stem cells are in your body or in a research lab, involves late stage maturation.
“The problem isn’t simply getting erythroid precursors produced by the bucket, but understanding how these cells systematically lose their nuclei and organelles to become a red blood cell, the final product,” said Bresnick. “This is the bottleneck, even in the stem cell world of embryonic and induced pluripotent stem cells. We know little about how the cell orchestrates the intricate processes that constitute late-stage maturation.”
At the end of red blood cell development, the precursor cell must eject its own genetic material in a process called enucleation. Discarding the nucleus helps make the cell more flexible for traveling through tiny blood vessels and makes way for more hemoglobin, thereby allowing the cell to carry more oxygen. But while it’s clear why enucleation is important, the definitive process of how the cell does it has gone unanswered, largely because of the system’s complexity.
Besides ejecting the nucleus, the cell must be cleared of other organelles like the endoplasmic reticulum and mitochondria. This process, known as autophagy, is linked to a pair of transcription factors that control gene expression important in red blood cell development and many other functions. Because they knew GATA1 and Foxo3 actually promote autophagy in the cell, the Bresnick lab asked, ‘do proteins these transcription factors repress play an important role in cell maturation?’ They identified exosc8 and exosc9, two units of the exosome that ultimately established this development barrier.
The lab will continue studying the exosome, because many RNAs in the cell are not degraded by the exosome. Determining exactly how the exosome decides what RNA to dispose of may provide an even better understanding of the newly discovered barricade.
“One goal we have is to establish the specific RNA targets the exosome is regulating that are responsible for the blockade,” said Bresnick. “In doing so, we might even uncover targets that are easier to manipulate than the exosome itself.”
University of Wisconsin School of Medicine and Public Health