Functional restoration of diseased solid organs is a broad goal motivating intensive effort in biomedical research. Replacement or regeneration of pancreatic islets of Langerhans, endocrine organs that secrete insulin and glucagon, has emerged as a paradigm for organ restoration in recent years. Deficiency of insulin-producing islet β-cells underlies the pathogenesis of diabetes mellitus, a disease with devastating autoimmune (type 1) and pandemic (type 2) forms. Islet replacement in diabetes is ultimately limited, however, by our inadequate understanding of mechanisms controlling islet formation and growth. Thus, islet replacement is a specific challenge to the consensus that knowledge about solid organ development and expansion can be used to restore organ function in human diseases.
To meet this challenge, my group has created new approaches to produce, expand, and regenerate islets. We discovered Drosophila endocrine cells, including insulin-producing cells, that are functional orthologs of mammalian islet cells, and we generated genetic screens to identify evolutionarily conserved programs that control the development, expansion, and reprogramming of islet cells. We identified new FACS (fluorescence-activated cell sorting)-based methods to purify specific classes of cells that generate the pancreas and islets, including native pancreatic progenitor cells, providing a powerful platform to accelerate use of pancreatic and embryonic stem cells for islet studies and replacement. We elucidated new molecular pathways that control proliferation of β-cells in physiological settings or islet tumors. We envision that modulation of these pathways will be useful for stimulating expansion of functional islets for diabetes, and for treating neuroendocrine cancer.
Genetic Dissection of Islet Cell Development and Function in DrosophilaRegulation of growth and metabolism by insulin and glucagon-like peptides is a conserved feature in metazoans. Despite this ancient regulatory heritage and a century of knowledge that islet hormones are crucial to human health, our understanding of the genetic basis for pancreatic islet cell differentiation, expansion, and function is surprisingly meager. With our Stanford colleagues, we discovered endocrine cells that secrete insulin and glucagon-like peptides in Drosophila melanogaster, an organism superbly suited for discovering genes governing tissue and organ development. Our studies showed remarkable physiological parallels between these Drosophila endocrine cells and pancreatic islet β- and α-cells, including the mechanisms governing glucose sensing and hormone secretion. These parallels motivated us to screen mutants to identify conserved regulators of islet cell development and function. To the extent that Drosophila cells that secrete insulin and glucagon-like peptides represent ancestral islet cells, our work has also begun to define genetic pathways for reprogramming cells toward an islet cell fate, another strategy for islet regeneration or replacement.
Purifying Pancreatic Progenitor/Stem Cells
Purification of stem and progenitor cells from an organ is a powerful way to achieve organ replacement and to investigate the basis for organ development. In a celebrated paradigm, successful purification of hematopoietic progenitors and stem cells from bone marrow ultimately produced cell-based therapies for a diverse range of human diseases and transformed modern medical practice. We use cell purification, genetics, imaging, and genomics tools to investigate the basis of pancreas islet development and growth. For example, we have successfully used FACS to isolate subsets of multipotent, self-renewing progenitors in the pancreatic islet lineage from mice and humans. Our group is using this resource to investigate the developmental biology of pancreatic stem and progenitor cells. Pancreatic progenitor cells provide a natural source for generating new islets. Since mature islets derive from islet progenitor cells, our emerging FACS strategies also provide methods for isolating and evaluating differentiating human cells in the islet lineage produced from nonpancreatic sources, regardless of origin. Thus, we expect our distinct approaches should accelerate use of sources like embryonic or induced pluripotent stem cells to generate functional islet cells.
Decoding the Basis of Physiological and Pathological β-Cell Proliferation
Once viewed as postmitotic and incapable of significant proliferation, mature β-cells in the adult pancreas are now recognized to have a significant capacity to replicate, and thereby maintain, β-cell mass. For this reason, expansion of islets in culture or in the pancreas may become a therapeutic option for diabetes. However, prior attempts to expand cultured islets with mitogens have been bedeviled by the loss of key β-cell features, like insulin expression, that accompanies proliferation. Thus, it remains elusive how adult β-cells "remember" their differentiated fate while proliferating. To decode the mechanisms controlling β-cell proliferation, we seek to identify genetic and epigenetic pathways that govern expression of hallmark β-cell factors and cell cycle regulators.
For example, our group has studied a rare human cancer syndrome to gain fundamental insights about mechanisms controlling pathological and physiological β-cell growth. We have investigated the basis of islet tumor formation in type 1 multiple endocrine neoplasia (MEN1), a familial cancer syndrome I have managed as an oncologist, which results from mutation of the MEN1 gene. A key observation in MEN1 motivating our work is that β-cells in islet tumors often remain "functional," leading to elevated circulating insulin levels and symptoms from hypoglycemia. Thus, proliferating β-cells in MEN1 patients often maintain their defining functions. The MEN1 gene encodes a protein called menin, and our studies have unveiled a novel epigenetic mechanism of tumor suppression by menin. In studies of Men1-deficient mice with features of MEN1 syndrome, we showed that menin promotes histone methylation and expression of genes encoding the cyclin-dependent kinase inhibitors p27Kip1, p18INK4C, and other cell cycle regulators in islet β-cells. Thus, menin-dependent histone modifications control islet β-cell proliferation.
In addition to menin roles in tumor suppression, our recent work shows for the first time that menin also regulates histone modifications and β-cell proliferation during physiological growth. Common states such as pregnancy stimulate adaptive β-cell expansion, and we have discovered that menin levels in β-cells are strikingly attenuated in pregnancy and other physiological states. Prevention of menin attenuation in pregnant mice impairs β-cell histone modification and adaptive islet growth, producing gestational diabetes in a unique genetic model. Thus, excess menin function in human islets could underlie common subsets of type 2 diabetes.
Summary
Our efforts in the past several years have created opportunities for harnessing knowledge about the molecular and cellular basis of pancreatic development and growth to restore pancreas islet function and to treat endocrine cancers. Our work with Drosophila, mice, human islet organogenesis and diseases, cell purification, and chromatin regulation has revealed mechanisms underlying islet development, adaptations, and disease pathogenesis. Coupled with our clinical collaborations, our discoveries will provide tools and expertise that may lead to production of islet regeneration therapies for type 1 diabetes, improved treatments and tests to mitigate or prevent type 2 diabetes, and new therapeutic strategies for neuroendocrine cancers.
