Directed differentiation of human pluripotent stem cells into functional insulin-producing beta-like cells holds great promise for cell replacement therapy for patients suffering from diabetes. This approach also offers the unique opportunity to study otherwise inaccessible aspects of human beta cell development and function in vitro. Here, we show that current pancreatic progenitor differentiation protocols promote precocious endocrine commitment, ultimately resulting in the generation of non-functional polyhormonal cells. Omission of commonly used BMP inhibitors during pancreatic specification prevents precocious endocrine formation while treatment with retinoic acid followed by combined EGF/KGF efficiently generates both PDX1 + and subsequent PDX1 + /NKX6.1 + pancreatic progenitor populations, respectively. Precise temporal activation of endocrine differentiation in PDX1 + /NKX6.1 + progenitors produces glucose-responsive beta-like cells in vitro that exhibit key features of bona fide human beta cells, remain functional after short-term transplantation, and reduce blood glucose levels in diabetic mice. Thus, our simplified and scalable system accurately recapitulates key steps of human pancreas development and provides a fast and reproducible supply of functional human beta-like cells.

Human induced pluripotent stem (iPS) cells are remarkably similar to embryonic stem (ES) cells, but recent reports suggest that there may be important differences between them. We performed a systematic comparison of human iPS cells generated from hepatocytes (representative of endoderm), skin fibroblasts (mesoderm) and melanocytes (ectoderm). All low passage iPS cells analyzed retain a transcriptional memory of the original cells. The persistent expression of somatic genes can be partially explained by incomplete promoter DNA methylation. This epigenetic mechanism underlies a robust form of memory that can be found in iPS cells generated by multiple laboratories using different methods, including RNA transfection. Incompletely silenced genes tend to be isolated from other genes that are repressed during reprogramming, indicating that recruitment of the silencing machinery may be inefficient at isolated genes. Knockdown of the incompletely reprogrammed gene C9orf64 reduces the efficiency of human iPS cell generation, suggesting that somatic memory genes may be functionally relevant during reprogramming.

The regenerative abilities of freshwater planarians are based on neoblasts, stem cells maintained throughout the animal's life. We show that a member of the Bruno-like family of RNA binding proteins is critical for regulating neoblasts in the planarian Schmidtea mediterranea. Smed-bruno-like (bruli) mRNA and protein are expressed in neoblasts and the central nervous system. Following bruli RNAi, which eliminates detectable Bruli protein, planarians initiate the proliferative response to amputation and form small blastemas but then undergo tissue regression and lysis. We characterize the neoblast population by using antibodies recognizing SMEDWI-1 and Histone H4 (monomethyl-K20) and cell-cycle markers to label subsets of neoblasts and their progeny. bruli knockdown results in a dramatic reduction/elimination of neoblasts. Our analyses indicate that neoblasts lacking Bruli can respond to wound stimuli and generate progeny that can form blastemas and differentiate; yet, they are unable to self-renew. These results suggest that Bruli is required for stem cell maintenance.

Germ cells are required for the successful propagation of sexually reproducing species. Understanding the mechanisms by which these cells are specified and how their totipotency is established and maintained has important biomedical and evolutionary implications. Freshwater planarians serve as fascinating models for studying these questions. They can regenerate germ cells from fragments of adult tissues that lack reproductive structures, suggesting that inductive signaling is involved in planarian germ cell specification. To study the development and regeneration of planarian germ cells, we have functionally characterized an ortholog of nanos, a gene required for germ cell development in diverse organisms, from Schmidtea mediterranea. In the hermaphroditic strain of this species, Smed-nanos mRNA is detected in developing, regenerating, and mature ovaries and testes. However, it is not detected in the vast majority of newly hatched planarians or in small tissue fragments that will ultimately regenerate germ cells, consistent with an epigenetic origin of germ cells. We show that Smed-nanos RNA interference (RNAi) results in failure to develop, regenerate, or maintain gonads in sexual planarians. Unexpectedly, Smed-nanos mRNA is also detected in presumptive testes primordia of asexual individuals that reproduce strictly by fission. These presumptive germ cells are lost after Smed-nanos RNAi, suggesting that asexual planarians specify germ cells, but their differentiation is blocked downstream of Smed-nanos function. Our results reveal a conserved function of nanos in germ cell development in planarians and suggest that these animals will serve as useful models for dissecting the molecular basis of epigenetic germ cell specification.epigenesis ͉ germ cell specification ͉ RNA interference ͉ Schmidtea mediterranea ͉ Platyhelminthes

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Several families of evolutionarily conserved axon guidance cues orchestrate the precise wiring of the nervous system during embryonic development. The remarkable plasticity of freshwater planarians provides the opportunity to study these molecules in the context of neural regeneration and maintenance. Here we characterize a homologue of the Slit family of guidance cues from the planarian Schmidtea mediterranea. Smed-slit is expressed along the planarian midline, in both dorsal and ventral domains. RNA interference (RNAi) targeting Smed-slit results in the collapse of many newly regenerated tissues at the midline; these include the cephalic ganglia, ventral nerve cords, photoreceptors, and the posterior digestive system. Surprisingly, Smed-slit RNAi knockdown animals also develop morphologically distinguishable, ectopic neural structures near the midline in uninjured regions of intact and regenerating planarians. These results suggest that Smed-slit acts not only as a repulsive cue required for proper midline formation during regeneration but that it may also act to regulate the behavior of neural precursors at the midline in intact planarians.

The number of patients worldwide suffering from the chronic disease diabetes mellitus is growing at an alarming rate. Insulin-secreting beta-cells in the islet of Langerhans are damaged to different extents in diabetic patients, either through an autoimmune reaction present in type 1 diabetic patients or through inherent changes within beta-cells that affect their function in patients suffering from type 2 diabetes. Cell replacement strategies via islet transplantation offer potential therapeutic options for diabetic patients. However, the discrepancy between the limited number of donor islets and the high number of patients who could benefit from such a treatment reflects the dire need for renewable sources of high-quality beta-cells. Human embryonic stem cells (hESCs) are capable of self-renewal and can differentiate into components of all three germ layers, including all pancreatic lineages. The ability to differentiate hESCs into beta-cells highlights a promising strategy to meet the shortage of beta-cells. Here, we review the different approaches that have been used to direct differentiation of hESCs into pancreatic and beta-cells. We will focus on recent progress in the understanding of signaling pathways and transcription factors during embryonic pancreas development and how this knowledge has helped to improve the methodology for high-efficiency beta-cell differentiation in vitro.

SummaryEmbryonic stem cells (ESCs) have adopted an accelerated cell-cycle program with shortened gap phases and precocious expression of cell-cycle regulatory proteins, including cyclins and cyclin-dependent kinases (CDKs). We examined the effect of CDK inhibition on the pathways regulating proliferation and survival of ESCs. We found that inhibiting cyclin-dependent kinase 1 (CDK1) leads to activation of the DNA damage response, nuclear p53 stabilization, activation of a subset of p53 target genes including NOXA, and negative regulation of the anti-apoptotic protein MCL1 in human and mouse ESCs, but not differentiated cells. We demonstrate that MCL1 is highly expressed in ESCs and loss of MCL1 leads to ESC death. Finally, we show that clinically relevant CDK1 inhibitors prevent formation of ESC-derived tumors and induce necrosis in established ESC-derived tumors. Our data demonstrate that ES cells are uniquely sensitive to CDK1 inhibition via a p53/NOXA/MCL1 pathway.