Making Pancreatic Beta Cells from Embryonic Stem Cells


Type 1 diabetes results from an inability to produce sufficient quantities of the hormone insulin. Without insulin, the body does not receive the signal to take up sugar from the blood, and the result is high blood sugar levels, which are damaging to tissues, and a general wasting of tissues because they cannot take up enough sugar to feed them.

Pancreatic beta cells

The cells in the pancreas that produce insulin are the beta cells, and animal studies have shown that transplantation of new beta cells into diabetic animals can reverse and even in some cases cure the diabetic animals. Therefore researchers have tried to make beta cells from pluripotent stem cells in order to make a source of beta cells for transplantation.

Unfortunately, beta cell production in the laboratory has been fraught with problems. The cells produced by differentiation of embryonic stem cells do not have the characteristics of mature beta cells and they produce little insulin and are not glucose responsive (D’Amour, et al., (2006) Nat Biotechnol 24, 1392-1401).

A different strategy, however, works much better. Instead of differentiating stem cells into beta cells, differentiate them into those cells that will form beta cells and other types of pancreatic cells in the embryo – immature endocrine cell precursors – and then transplant those into the pancreas of diabetic mice. In this case, the endocrine cell precursors differentiate in the bodies of the mice into pancreatic beta cells that greatly resemble normal beta cells.

Why don’t embryonic stem cells for beta cells in culture? This question was pursued by a collaboration between research team led by Maike Sander at UC San Diego and a company called ViaCyte, Inc.

When it comes to endocrine precursors transplanted into mice, Dr. Sander noted that, “We found that the endocrine cells retrieved from transplanted mice are remarkably similar to primary human endocrine cells.” He continued, “This shows that hESCs (human embryonic stem cells) can differentiate into endocrine cells that are almost indistinguishable from their primary human counterparts.”

Well, ESCs can make perfectly fine beta cells in the mouse body, but not in culture. What’s up with that?

Sander and her colleagues examined the gene expression patterns of embryonic stem cells as they were differentiated and compared them with the gene expression patterns in those cells that were transplanted into mice and allowed to differentiate inside the body of the mouse.

What Sander and her team found was astounding. As cells progress through their developmental program, particular genes are brought on-line and expressed, and then turned off as the cells passed through each stage of endocrine cell differentiation. The cellular machinery that shuts off genes after they have been activated consists of a family of proteins that remodel chromatin known as the Polycomb group (PcG). PcG-mediated repression of genes silenced those genes that were only turned on temporarily once they were no longer required.

Two major Polycomb repressive complexes (PRCs) have been described. The PRC2 complex contains the histone methyltransferase enhancer of zeste homologue 2 (EZH2), which together with embryonic ectoderm development (EED) and suppressor of zeste 12 homolog (SUZ12) catalyses the trimethylation of histone H3 at lysine K27 (H3K27me3). The EZH2 SET domain confers this activity. Multiple forms of the PRC1 complex exist and these contain combinations of at least four PC proteins (CBX2, CBX4, CBX7 and CBX8), six PSC proteins (BMI1, MEL18, MBLR, NSPC1, RNF159 and RNF3), two RING proteins (RNF1 and RNF2), three PH proteins (HPH1, HPH2 and HPH3) and two SCML proteins (SCML1 1 and SCML2). Some results have suggested that PRC1 complexes are recruited by the affinity of chromodomains in chromobox (Cbx) proteins to the H3K27me3 mark. PRC1 recruitment results in the RNF1 and RNF2-mediated ubiquitylation of histone H2A on lysine 119, which is thought to be important for transcriptional repression. PC, Polycomb; PSC, Posterior sex combs ; SCML, Sex combs on midleg .
Two major Polycomb repressive complexes (PRCs) have been described. The PRC2 complex contains the histone methyltransferase enhancer of zeste homologue 2 (EZH2), which together with embryonic ectoderm development (EED) and suppressor of zeste 12 homolog (SUZ12) catalyses the trimethylation of histone H3 at lysine K27 (H3K27me3). The EZH2 SET domain confers this activity. Multiple forms of the PRC1 complex exist and these contain combinations of at least four PC proteins (CBX2, CBX4, CBX7 and CBX8), six PSC proteins (BMI1, MEL18, MBLR, NSPC1, RNF159 and RNF3), two RING proteins (RNF1 and RNF2), three PH proteins (HPH1, HPH2 and HPH3) and two SCML proteins (SCML1 1 and SCML2). Some results have suggested that PRC1 complexes are recruited by the affinity of chromodomains in chromobox (Cbx) proteins to the H3K27me3 mark. PRC1 recruitment results in the RNF1 and RNF2-mediated ubiquitylation of histone H2A on lysine 119, which is thought to be important for transcriptional repression. PC, Polycomb; PSC, Posterior sex combs ; SCML, Sex combs on midleg .

In the transplanted endocrine precursors, Sanders and his team noted an orderly progression of genes that were turned on and then turned off once as needed. However, in the embryonic stem cells that were differentiated into beta cells in culture, they discovered that these cells failed to express the majority of genes critical for endocrine cell function. The main reason for this appeared to be that the PcG-mediated repression of genes was not fully eliminated when particular genes had to be expressed at specific developmental stages. Thus these cultured cells failed to fully eliminate PcG-mediated repression on endocrine-specific genes, which contributes to the abnormality of the culture-derived beta cells.

Sander commented: “This information will help devise protocols to generate functional insulin-producing beta cells in vitro. This will be important not only for cell therapies, but also for identifying disease mechanisms that underlie the pathogenesis of diabetes.”