Designing β Cells
Authors: Matthias Hebrok
Journal: Cell Metabolism (2017)

Diabetes is a growing epidemic, and many patients depend on insulin injections to control the disease and minimize long-term complications. In a recent manuscript published in Science, Xie et al., 2016Xie M. Ye H. Wang H. Charpin-El Hamri G. Lormeau C. Saxena P. Stelling J. Fussenegger M. Science. 2016; 354: 1296-1301Crossref PubMed Scopus (138) Google Scholar generate insulin-producing cells from a somatic embryonic kidney cell line through minimal genetic modification capable of regulating glucose levels in diabetic mice. Diabetes is a growing epidemic, and many patients depend on insulin injections to control the disease and minimize long-term complications. In a recent manuscript published in Science, Xie et al., 2016Xie M. Ye H. Wang H. Charpin-El Hamri G. Lormeau C. Saxena P. Stelling J. Fussenegger M. Science. 2016; 354: 1296-1301Crossref PubMed Scopus (138) Google Scholar generate insulin-producing cells from a somatic embryonic kidney cell line through minimal genetic modification capable of regulating glucose levels in diabetic mice. Recent surveys indicate that the number of people suffering from diabetes worldwide is greater than 415 million (http://www.idf.org/about-diabetes/facts-figures). Even more concerning is the fact that many more (greater than 85 million people in the U.S. alone, according to the Centers for Disease Control and Prevention) are afflicted by pre-diabetes, a condition defined by impaired blood sugar control that often precedes full-blown diabetes. While the vast majority of these patients are suffering from type 2 diabetes (T2D), often accompanied by increased insulin resistance due to higher-than-normal body mass, type 1 diabetes (T1D), caused by an autoimmune response to insulin-producing β cells, is on the rise as well. Thus, either a functional defect (T2D) or the destruction (T1D) of β cells is the underlying cause of the development of the most prominent forms of diabetes. Considering that loss of β cell function/number and, by extension, the loss or reduction of the sugar-lowering hormone insulin appears to be the main culprit for the development of diabetes, multiple avenues have been explored in the recent past to produce functional insulin-producing cells for cell therapy. Transplantation of pancreatic islets harboring β cells isolated from cadaveric donors has shown great promise (Moassesfar et al., 2016Moassesfar S. Masharani U. Frassetto L.A. Szot G.L. Tavakol M. Stock P.G. Posselt A.M. Am. J. Transplant. 2016; 16: 518-526Crossref PubMed Scopus (52) Google Scholar), but unfortunately the demand by far outstrips the supply. Human stem cell-derived β cells are another promising alternative (Pagliuca et al., 2014Pagliuca F.W. Millman J.R. Gürtler M. Segel M. Van Dervort A. Ryu J.H. Peterson Q.P. Greiner D. Melton D.A. Cell. 2014; 159: 428-439Abstract Full Text Full Text PDF PubMed Scopus (1316) Google Scholar, Rezania et al., 2014Rezania A. Bruin J.E. Arora P. Rubin A. Batushansky I. Asadi A. O'Dwyer S. Quiskamp N. Mojibian M. Albrecht T. et al.Nat. Biotechnol. 2014; 32: 1121-1133Crossref PubMed Scopus (971) Google Scholar, Russ et al., 2015Russ H.A. Parent A.V. Ringler J.J. Hennings T.G. Nair G.G. Shveygert M. Guo T. Puri S. Haataja L. Cirulli V. et al.EMBO J. 2015; 34: 1759-1772Crossref PubMed Scopus (381) Google Scholar), but whether such cells can restore normoglycemia in diabetic patients has not been demonstrated as of yet. A recent manuscript by Xie and colleagues published in Science suggests an exciting alternative approach by demonstrating that synthetically programmed somatic cells unrelated to insulin-producing cells can fulfill essential functions of specialized β cells (Xie et al., 2016Xie M. Ye H. Wang H. Charpin-El Hamri G. Lormeau C. Saxena P. Stelling J. Fussenegger M. Science. 2016; 354: 1296-1301Crossref PubMed Scopus (138) Google Scholar). β cells are highly evolved entities that fulfill critical and highly regulated functions: release insulin when the ambient blood glucose levels increase above a certain threshold (∼90–100 mg/dL in humans) and, perhaps more importantly, cease secretion as soon as sugar levels have returned to the baseline. To accomplish these feats, β cells have evolved to accurately measure sugar levels and generate an astounding amount of insulin that is concentrated and then stored in specialized secretion vesicles, and have developed a sophisticated secretory pathway that enables efficient and rapid release of the hormone upon increases in endogenous Ca2+ levels (Figure 1). It is widely believed that optimal glucose control requires all of these components. Xie and colleagues now demonstrate that an established non-β cell line, human embryonic kidney cells (HEK293), can be minimally modified to enable glucose sensing and insulin secretion. Ectopic expression of a calcium channel appears to be sufficient to confer glucose sensitivity to HEK293 cells. Parallel expression of insulin under control of elements of the calcium-responsive NFAT promoter results in glucose-responsive secretion of the hormone. Additional modifications include the ectopic expression of both a short version of the glucagon-like peptide (GLP-1), a known insulin secretagogue, and its receptor (GLP1R). The results are interesting and provocative, at least on the surface questioning the notion that only fully matured β cells possessing all regulatory and secretory elements could provide glycemic control. However, while impressive, the designer β-like cells are not capable of fully restoring normoglycemia in diabetic animals. Fasting glucose levels remain clearly above the accepted threshold when HEK-β cells are transplanted into a T2D mouse model. Similarly, glucose levels at fasting and upon challenge are higher in HEK-β cell-treated mice whose endogenous β cells were destroyed through injection with alloxan, a β cell toxin. These functional deficiencies indicate that at least some regulatory elements present in mature β cells are not recapitulated in the designer HEK-β cells. The findings raise other important questions. For example, the authors use alloxan-treated mice as a model for T1D. While the compound eliminates rodent β cells, the depletion occurs in the absence of an autoimmune disease, a critical element of human T1D that is present in NOD mice (Anderson and Bluestone, 2005Anderson M.S. Bluestone J.A. Annu. Rev. Immunol. 2005; 23: 447-485Crossref PubMed Scopus (840) Google Scholar). Testing the activity of designer β cells in a true T1D model might result in important insights into which components of a β cell induce the autoimmune response. In other words, it is well documented that T1D patients have auto-antibodies against β cell proteins, including insulin. Does this mean that any cell expressing the key constituents of a β cell constitutes a target for a misaligned immune system, or would designer β cells lacking many such properties be immune protected? Defining the minimal composition of a designer β cell that remains invisible to an immune system primed to destroy endogenous β cells would be of great promise. On the flip side, using these designer cells to clearly identify the antigens (either individually or combinatorially) that launch an immune response could also aid in identifying the triggers that initiate T1D. Furthermore, while the current HEK-β cells display remarkable features with regards to regulating blood glucose levels, perfect control found in endogenous β cells is not achieved. β cells depend on an exquisite glucose-sensing machinery to ensure appropriate insulin release, in large part mediated by the expression of glucokinase (GK; Matschinsky, 2009Matschinsky F.M. Nat. Rev. Drug Discov. 2009; 8: 399-416Crossref PubMed Scopus (333) Google Scholar), not other hexokinases likely present in HEK cells (of note, ectopic expression of GK in HEK cells proved toxic at higher glucose concentrations). In addition, storage of large numbers of properly processed insulin permits rapid response and prolonged activity. Designer β cells might have to be further optimized to achieve glucose control equivalent to their endogenous counterparts. Efforts to direct differentiation of stem and progenitor cells to functional β cells have accelerated over the recent past, and therapies based on stem cell-derived products are being developed at an increasing rate. While it can be debated whether designer cells starting with differentiated cells of a non-β cell nature will be as efficient as those that have been generated in a process resembling embryonic organ formation, the properties with regard to linking glucose sensing to insulin production/secretion in the synthetic cells are already remarkable. Considering that the diabetes epidemic has not shown signs of abating, there remains a clear need for β cell replacement strategies. Bioengineering approaches combining synthetic biology with directed cell differentiation to generate true equivalents of functional β cells might dramatically change current treatment options and, consequently, the lives of patients suffering from this disease. M.H. serves on the SAB for Semma Therapeutics.