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 that are functional orthologs of mammalian insulin-producing islet β-cells and glucagon-producing islet α-cells. We generated genetic screens to identify evolutionarily conserved programs that control the development, expansion, and functional maturation of islet cells. To purify and interrogate specific classes of cells that generate the pancreas and islets, we developed FACS (fluorescence-activated cell sorting)-based and high-throughput gene expression-profiling methods. This permitted isolation and investigations of native pancreatic progenitor cells, which provide a powerful platform to accelerate use of embryonic stem cells for islet studies and replacement. We elucidated new molecular pathways that control the function and proliferation of β-cells in physiological or pathological settings. We envision that modulation of these pathways will be useful for stimulating expansion of functional islets for diabetes, for treating neuroendocrine cancer, and for investigating pathogenesis of pancreatic adenocarcinoma.
Genetic Dissection of Islet Cell Development and Function in Drosophila
Regulation 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. We discovered the metabolic functions of 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 investigate the lineage and molecular mechanisms regulating development of these cells, with a goal to identify conserved regulators of islet cell development and function. Our current work is also focused on using fly genetics and physiological studies of insulin-producing cells to investigate the function of genes implicated in human type 2 diabetes.
Purifying Progenitor Cells for Pancreas Development and Reconstructing Cancer Pathogenesis
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 pancreatic progenitor cells. Our group is using this resource to investigate the developmental biology of native pancreatic stem and progenitor cells. These efforts have revealed new ways to purify, culture, and develop subsets of human fetal and adult pancreatic cells amenable to genetic and cell biology investigations. We are using insights from these approaches to discover the transcription factor networks controlling endocrine differentiation, to profile the epigenetic landscape of endocrine cells during aging, and to understand how genetic and epigenetic modulation can be used to control conversion between endocrine cell types.
Our approaches have also provided opportunities to investigate human pancreas development and have suggested new ways to reconstruct early genetic steps of human pancreas cancer development. These distinct approaches should advance progress in the development of functional replacement islet cells from native pancreatic stem cells or pluripotent stem cell lines and investigations of early stages in pathogenesis of human pancreatic adenocarcinoma.
Decoding the Basis of β-Cell Proliferation and Function
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 proliferation accompanied by the loss of key β-cell features, like insulin expression. Thus, it remains elusive how adult β-cells "remember" their differentiated fate and function while proliferating. To decode the mechanisms controlling β-cell proliferation, we seek to identify native genetic and epigenetic pathways that govern expression of hallmark β-cell factors and cell cycle regulators.
For example, our group has studied replicative senescence in aging human and mouse β-cells to gain insights about mechanisms controlling pathological and physiological β-cell growth. Recently we identified conserved pathways linking growth factor signaling and epigenetic regulators like Enhancer of Zeste Homologue 2 (EZH2) to age-dependent expression of the β-cell cycle inhibitor p16INK4a. We are now investigating how modulation of this EZH2-INK4a pathway may be useful for expanding functional β-cell mass in mouse and human islets.
In collaboration with Stanford colleagues, including Gerald Crabtree (HHMI, Stanford University), we are also investigating the function of calcineurin/Nuclear Factor of Activated T-cells (NFAT) signaling in controlling β-cell function and growth. Studies by our group and others indicate this is a pathway central to β-cell biology in human islets and diabetes. In mouse and human islets, this pathway likely links activity-dependent calcium transients (evoked by glucose and other insulin secretagogues in β-cells) to changes in β-cell transcriptional regulation, physiological maturation, and growth control. We are using genetic and pharmacological approaches to assess how activation of this pathway may be useful for generating or expanding functional β-cells in diabetes, and how pathway inhibitors may attenuate pathological growth or function in neuroendocrine cancers.
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 model pancreatic cancer genetics. Our work withDrosophila, mice, and human tissues has revealed conserved mechanisms underlying islet development, functional maturation, physiological adaptation, proliferation, 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 diagnostic or therapeutic strategies for pancreatic endocrine and exocrine cancers.
Grants from the National Institutes of Health and JDRF provided partial support for these projects.
As of January 23, 2013