by Francisco Galdos
Two blocks from my apartment in Bogotá is a small bakery that specializes in making a typical Colombian bread known as a roscón. Roscones are made with a sweet bread that is filled with a caramel like spread known as arequipe and is baked in the shape of a large donut. Fresh out of the oven, the roscón is sprinkled with sugar that melts perfectly at the edges of the delicacy. If eaten with a fresh cup of Colombian coffee, the sugar from the roscón complements the smoky taste of the coffee to create an unforgettable experience, and yet, this is all so easy. I go to the bakery, sit with my coffee, my roscón, and simply eat. I don’t really have to think too much about the process of eating. In fact, no one really has to think about what goes on when we eat; our digestive system simply seems to take care of the whole thing. The sugars from the roscón are broken down in the mouth by salivary enzymes, and further broken down in the intestines, and a spew of glucose enters the blood stream to circulate throughout the body. As the levels of glucose rise, a cell in the pancreas known as the beta cell detects the rising levels and secretes insulin, allowing my cells to uptake the glucose in my blood that came from my roscón. The balance is perfect. I can eat ten roscones in one sitting, and still my beta cells work to keep my blood glucose within an ideal range. I simply go on with my life letting my cells do all of the work.
Imagine for a day, if I had to do the work of my beta cell. As soon as I eat the roscón, glucose begins to be absorbed in my blood, and if the glucose goes too high above the ideal range, anything from blindness to severe nerve damage would eventually take hold. What is the solution? I take insulin. But wait! I gave myself too much—now my cells have eaten up too much of my blood glucose, and I’m about to go into a coma! You give me a banana, and here we are again—my glucose is too high. Somehow, the beta cell is able to take care of this tedious job, and yet, in 2010, more than 25.8 million Americans had to play the role of their beta cells because of the loss or dysfunction of these cells due to a disease known as diabetes.
There are two types of diabetes: type 1 and type 2. In type 1, patients’ immune systems attack their own beta cells, leaving them with fewer beta cells and a lifelong dependence on insulin shots. In type 2, patients’ bodies become resistant to insulin, and the beta cell often overworks itself to the point that it begins to malfunction and die, again leaving patients dependent on insulin. Since Charles Best and Frederich Banting discovered insulin in 1921, scientists have long tried to find treatments and cures for diabetes. Just last month, scientists published efforts to create an artificial pancreas that could effectively replace the beta cell. Even with such refined technology, however, the pancreatic beta cell still does a better job than our glucose-sensing computers and insulin pumps. Physicians have attempted to transplant beta cells from cadavers into patients with diabetes, and although temporarily successful, they still only see limited success in getting patients to be insulin-independent. Stem cell biologists have studied the development of beta cells and have attempted to generate them in vitro for transplantation into patients. Many names were given to the cells that came out of these efforts: beta-like cells, polyhormonal cells, fetal-like beta cells, and the list goes on (1). From all of these attempts to differentiate pluripotent stem cells into beta cells, many cells did not end up secreting insulin according to the amount of glucose they were exposed to, and some produced hormones that were not normally made in beta cells. Essential factors were missing from the process, and the final step in making mature beta cells like those found in healthy patients was missing.
On October 9, 2014, after more than a decade of work that aimed to generate fully functional beta cells, a team led by Douglas Melton at the Harvard Stem Cell Institute was able to generate beta cells from pluripotent stem cells. When transplanted into mice with diabetes, these beta cells were able to better regulate their blood glucose levels, providing what could be a possible cure for type 1 diabetes (2). How was Melton’s group able to achieve such a remarkable success? A good place to start is an overview of where beta cells come from. During embryonic development, the cells of the zygote—the product of the fertilized egg—begin to divide and eventually make a structure known as a blastocyst, which has a group of cells clustered together to form the inner cell mass. This blastocyst implants in the uterine wall, and, as development continues, the cells of the inner cell mass continue to divide. Eventually, gastrulation occurs, where the cells begin to take up their positions and differentiate into three layers called the “germ layers.” Each of these germ layers creates specific parts of the body, with the endoderm specifically making tissues of the gut, as well as our cell of interest—the beta cell (1). During each step in the development of a beta cell, specific genes are expressed that are responsible for specifying the cell’s function and the next step in the cell’s developmental path. In the case of the beta cell, the pluripotent cell of the early embryo develops into definitive endoderm. Cells from neighboring tissues and cells send signals to instruct the definitive endoderm to differentiate into the next step in the developmental pathway to becoming a pancreatic cell (1). Signals from a structure called the notochord allow for definitive endoderm to differentiate into pancreatic endoderm, which begins to turn on key pancreatic genes necessary for the function of pancreatic cells. At this point, cells called “pancreatic progenitors” are made, which are capable of making all of the cells that make up the pancreatic islets of Langerhans—home of the beta cell and the neighboring cells that are important for maintaining glucose regulation and metabolism. Prior to Melton’s discovery, scientists were able to make these progenitors and were even to able to make insulin-producing cells; however, these cells were not able to secrete insulin according to the amount of glucose they were given—a key function of a beta cell—and they often expressed genes that are not normally expressed in beta cells.
Scientists have used small molecules and proteins in vitro in order to drive the generation of these various intermediate steps in the development of beta cells. These small molecules or proteins often interact with other proteins in a cell that may activate or inhibit certain signaling pathways, eventually leading to changes in gene expression. As a result of these changes, the cell will differentiate and change its function, making it challenging to figure out the right combination of factors necessary to obtain the right cell. The right cocktail to get from the pancreatic progenitor to the fully functional beta cell was missing from previous studies, and after testing over 150 combinations of over 70 compounds, Melton’s group found 11 factors that allowed them to get to the final step of making a functional beta cell (2). More importantly, when they compared their derived cell to cells taken from the pancreata of human cadavers and to the polyhormonal cells that were previously derived, the stem cell-derived beta cells responded comparably to the cells from the human pancreata. This indicated that they were secreting insulin according to how they would in beta cells in the human body. Gene expression analysis also found that the derived beta cells had similar patterns to that of the primary cells from human cadavers. As Melton discusses in his paper, work remains to be done to understand the molecular biology of how the factors that they identified are allowing for the progenitors to differentiate into beta cells. Importantly, if pancreatic progenitors are transplanted into a mouse over a period of six months, they are able to differentiate and mature into fully functional beta cells, but the process still remains unknown. Melton’s group is now beginning trials on non-human primates. If these trials are effective in controlling glucose levels in primates, it may well be possible to see the beginning of human clinical trials that may lead to a treatment and perhaps an effective cure for diabetes. With this exciting finding, the field can now begin to dissect how a beta cell is able to regulate glucose with such fine-tuned precision, and scientists may be able to find a solution to one of the most prevalent healthcare problems in the world.
Francisco Galdos ’15 is a Human Developmental and Regenerative Biology concentrator in Quincy House.
- Pagliuca, F. & Melton, D. How to make a functional β-cell. Development (Cambridge, England) 140, 2472-2483, doi:10.1242/dev.093187 (2013).
- Pagliuca, F. W. et al. Generation of Functional Human Pancreatic β Cells In Vitro. Cell 159, 428-439, doi:10.1016/j.cell.2014.09.040 (2014).