Type 1 or juvenile diabetes is a genetically complex disease caused by an autoimmune destruction of insulin-producing beta (β) cells. Thus, the challenge finding new treatments or a cure can be divided into two problems: blocking or reversing the autoimmune attack and providing new β cells. We have focused on the latter, pursuing three complementary approaches aimed at making new β cells for diabetics.
Directed Differentiation of Stem Cells
We analyze the genes and cells that form the pancreas and use that information to direct differentiation of multipotent stem cells toward the β-cell fate. Genetic marking in mice has allowed us to map the lineage of progenitor cells that give rise to the exocrine, endocrine, and ductal components of the pancreas. In parallel, using biological and chemical screens, we investigate the regulatory genes that specify pancreatic cell fates. These studies have identified a set of transcription factors and intercellular signaling molecules (growth factors) that are responsible for the stepwise differentiation of normal pancreatic development. This genetic and cellular knowledge of pancreatic developmental biology guides our approach to the directed differentiation of stem cells. Using both embryonic and induced pluripotent stem cells (ES and iPS, respectively, from mice and humans), we aim to create new β cells, using a stepwise differentiation protocol wherein specific signals are used to tell the cells which fate to adopt.
Direct Reprogramming of One Differentiated Cell into Another Cell Type
In an alternative approach, we have explored the possibility of manipulating cell fates by turning one differentiated cell into another kind of differentiated cell. In this case, one converts or "transdifferentiates" one cell into another without reversion to a stem cell state. In our first success, we have managed to turn fully differentiated exocrine cells into functional β cells by the ectopic expression of just three transcription factors. This direct reprogramming sidesteps some of the problems associated with directed differentiation from stem cells and may be applied to a variety of cells in regenerative medicine. We are exploring the rules or constraints that govern conversion of one cell type into another.
Replication of β Cells
Our third approach focuses on β-cell replication. In adult animals, new pancreatic β cells are not formed by the differentiation of a precursor or stem cell but by the simple process of self-duplication from pre-existing β cells. We are therefore interested in understanding the signals that stimulate β-cell replication. The relative roles of physiological need (glucose levels), stress from a disease state (obesity and/or diabetes), and overall controls on organ size all come to bear on the issue of setting and maintaining a particular number of β cells and islets. To find ways to manipulate β-cell mass, we aim to identify the importance of these internal and external signals. We have recently reported on betatrophin, a hormone that induced β-cell replication, and now aim to understand its mechanism of action.
We use a wide variety of techniques, including functional genomics, chemical screening, tissue explants and grafting for analyzing inductive signals, and developmental genetics for direct assays of gene function. Most of our work is done with human cells, using mice as a guide.
Should we be successful in directing the differentiation of human cells into functional β cells, or find signals that cause β cell replication and regeneration in vivo, we will extend our findings to clinical applications for the treatment of diabetes.
Some of these projects were also supported by grants from the National Institutes of Health, the Beta Cell Biology Consortium, the Juvenile Diabetes Research Foundation, and the Harvard Stem Cell Institute.
As of October 31, 2013