Lineage tracing reveals the dynamic contribution of Hes1+ cells to the developing and adult pancreas

Impact of Sox9 dosage and Hes1-mediated Notch signaling in controlling the plasticity of adult pancreatic duct cells in mice

In the adult pancreas, there has been a long-standing dispute as to whether stem/precursor populations that retain plasticity to differentiate into endocrine or acinar cell types exist in ducts. We previously reported that adult Sox9-expressing duct cells are sufficiently plastic to supply new acinar cells in Sox9-IRES-CreERT2 knock-in mice. In the present study, using Sox9-IRES-CreERT2 knock-in mice as a model, we aimed to analyze how plasticity is controlled in adult ducts. Adult duct cells in these mice express less Sox9 than do wild-type mice but Hes1 equally. Acinar cell differentiation was accelerated by Hes1 inactivation, but suppressed by NICD induction in adult Sox9-expressing cells. Quantitative analyses showed that Sox9 expression increased with the induction of NICD but did not change with Hes1 inactivation, suggesting that Notch regulates Hes1 and Sox9 in parallel. Taken together, these findings suggest that Hes1-mediated Notch activity determines the plasticity of adult pancreatic duct cells and that there may exist a dosage requirement of Sox9 for keeping the duct cell identity in the adult pancreas. In contrast to the extended capability of acinar cell differentiation by Hes1 inactivation, we obtained no evidence of islet neogenesis from Hes1-depleted duct cells in physiological or PDL-induced injured conditions.

Growth Factor Independence-1 (Gfi1) Is Required for Pancreatic Acinar Unit Formation and Centroacinar Cell Differentiation

BACKGROUND & AIMS

The genetic specification of the compartmentalized pancreatic acinar/centroacinar unit is poorly understood. Growth factor independence-1 (Gfi1) is a zinc finger transcriptional repressor that regulates hematopoietic stem cell maintenance, pre-T-cell differentiation, formation of granulocytes, inner ear hair cells, and the development of secretory cell types in the intestine. As GFI1/Gfi1 is expressed in human and rodent pancreas, we characterized the potential function of Gfi1 in mouse pancreatic development.

METHODS

Gfi1 knockout mice were analyzed at histological and molecular levels, including qRT-PCR, in situ hybridization, immunohistochemistry, and electron microscopy.

RESULTS

Loss of Gfi1 impacted formation and structure of the pancreatic acinar/centroacinar unit. Histologic and ultrastructural analysis of Gfi1-null pancreas revealed specific defects at the level of pancreatic acinar cells as well as the centroacinar cells (CACs) in Gfi1^-/- mice when compared with wild-type littermates. Pancreatic endocrine differentiation, islet architecture, and function were unaffected. Organ domain patterning and the formation of ductal cells occurred normally during the murine secondary transition (E13.5-E14.5) in the Gfi1^-/- pancreas. However, at later gestational time points (E18.5), expression of cellular markers for CACs was substantially reduced in Gfi1^-/- mice, corroborated by electron microscopy imaging of the acinar/centroacinar unit. The reduction in CACs was correlated with an exocrine organ defect. Postnatally, Gfi1 deficiency resulted in severe pancreatic acinar dysplasia, including loss of granulation, autolytic vacuolation, and a proliferative and apoptotic response.

CONCLUSIONS

Gfi1 plays an important role in regulating the development of pancreatic CACs and the function of pancreatic acinar cells.

Hes3 is expressed in the adult pancreatic islet and regulates gene expression, cell growth, and insulin release

The transcription factor Hes3 is a component of a signaling pathway that supports the growth of neural stem cells with profound consequences in neurodegenerative disease models. Here we explored whether Hes3 also regulates pancreatic islet cells. We showed that Hes3 is expressed in human and rodent pancreatic islets. In mouse islets it co-localizes with alpha and beta cell markers. We employed the mouse insulinoma cell line MIN6 to perform in vitro characterization and functional studies in conditions known to modulate Hes3 based upon our previous work using neural stem cell cultures. In these conditions, cells showed elevated Hes3 expression and nuclear localization, grew efficiently, and showed higher evoked insulin release responses, compared with serum-containing conditions. They also exhibited higher expression of the transcription factor Pdx1 and insulin. Furthermore, they were responsive to pharmacological treatments with the GLP-1 analog Exendin-4, which increased nuclear Hes3 localization. We employed a transfection approach to address specific functions of Hes3. Hes3 RNA interference opposed cell growth and affected gene expression as revealed by DNA microarrays. Western blotting and PCR approaches specifically showed that Hes3 RNA interference opposes the expression of Pdx1 and insulin. Hes3 overexpression (using a Hes3-GFP fusion construct) confirmed a role of Hes3 in regulating Pdx1 expression. Hes3 RNA interference reduced evoked insulin release. Mice lacking Hes3 exhibited increased islet damage by streptozotocin. These data suggest roles of Hes3 in pancreatic islet function.

Plasticity and dedifferentiation within the pancreas: development, homeostasis, and disease

Cellular identity is established by genetic, epigenetic, and environmental factors that regulate organogenesis and tissue homeostasis. Although some flexibility in fate potential is beneficial to overall organ health, dramatic changes in cellular identity can have disastrous consequences. Emerging data within the field of pancreas biology are revising current beliefs about how cellular identity is shaped by developmental and environmental cues under homeostasis and stress conditions. Here, we discuss the changes occurring in cellular states upon fate modulation and address how our understanding of the nature of this fluidity is shaping therapeutic approaches to pancreatic disorders such as diabetes and cancer.

ADAM10 regulates Notch function in intestinal stem cells of mice

BACKGROUND & AIMS

A disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) is a cell surface sheddase that regulates physiologic processes, including Notch signaling. ADAM10 is expressed in all intestinal epithelial cell types, but the requirement for ADAM10 signaling in crypt homeostasis is not well defined.

METHODS

We analyzed intestinal tissues from mice with constitutive (Vil-Cre;Adam10(f/f) mice) and conditional (Vil-CreER;Adam10(f/f) and Leucine-rich repeat-containing GPCR5 [Lgr5]-CreER;Adam10(f/f) mice) deletion of ADAM10. We performed cell lineage-tracing experiments in mice that expressed a gain-of-function allele of Notch in the intestine (Rosa26(NICD)), or mice with intestine-specific disruption of Notch (Rosa26(DN-MAML)), to examine the effects of ADAM10 deletion on cell fate specification and intestinal stem cell maintenance.

RESULTS

Loss of ADAM10 from developing and adult intestine caused lethality associated with altered intestinal morphology, reduced progenitor cell proliferation, and increased secretory cell differentiation. ADAM10 deletion led to the replacement of intestinal cell progenitors with 2 distinct, post-mitotic, secretory cell lineages: intermediate-like (Paneth/goblet) and enteroendocrine cells. Based on analysis of Rosa26(NICD) and Rosa26(DN-MAML) mice, we determined that ADAM10 controls these cell fate decisions by regulating Notch signaling. Cell lineage-tracing experiments showed that ADAM10 is required for survival of Lgr5(+) crypt-based columnar cells. Our findings indicate that Notch-activated stem cells have a competitive advantage for occupation of the stem cell niche.

CONCLUSIONS

ADAM10 acts in a cell autonomous manner within the intestinal crypt compartment to regulate Notch signaling. This process is required for progenitor cell lineage specification and crypt-based columnar cell maintenance.

Gastrin induces ductal cell dedifferentiation and β-cell neogenesis after 90% pancreatectomy

Induction of β-cell mass regeneration is a potentially curative treatment for diabetes. We have recently found that long-term gastrin treatment results in improved metabolic control and β-cell mass expansion in 95% pancreatectomised (Px) rats. In this study, we investigated the underlying mechanisms of gastrin-induced β-cell mass expansion after Px. After 90%-Px, rats were treated with gastrin (Px+G) or vehicle (Px+V), pancreatic remnants were harvested on days 1, 3, 5, 7, and 14 and used for gene expression, protein immunolocalisation and morphometric analyses. Gastrin- and vehicle-treated Px rats showed similar blood glucose levels throughout the study. Initially, after Px, focal areas of regeneration, showing mesenchymal cells surrounding ductal structures that expressed the cholecystokinin B receptor, were identified. These focal areas of regeneration were similar in size and cell composition in the Px+G and Px+V groups. However, in the Px+G group, the ductal structures showed lower levels of keratin 20 and β-catenin (indicative of duct dedifferentiation) and higher levels of expression of neurogenin 3 and NKX6-1 (indicative of endocrine progenitor phenotype), as compared with Px+V rats. In Px+G rats, β-cell mass and the number of scattered β-cells were significantly increased compared with Px+V rats, whereas β-cell replication and apoptosis were similar in the two groups. These results indicate that gastrin treatment-enhanced dedifferentiation and reprogramming of regenerative ductal cells in Px rats, increased β-cell neogenesis and fostered β-cell mass expansion.

The nuclear hormone receptor family member NR5A2 controls aspects of multipotent progenitor cell formation and acinar differentiation during pancreatic organogenesis

The orphan nuclear receptor NR5A2 is necessary for the stem-like properties of the epiblast of the pre-gastrulation embryo and for cellular and physiological homeostasis of endoderm-derived organs postnatally. Using conditional gene inactivation, we show that Nr5a2 also plays crucial regulatory roles during organogenesis. During the formation of the pancreas, Nr5a2 is necessary for the expansion of the nascent pancreatic epithelium, for the subsequent formation of the multipotent progenitor cell (MPC) population that gives rise to pre-acinar cells and bipotent cells with ductal and islet endocrine potential, and for the formation and differentiation of acinar cells. At birth, the NR5A2-deficient pancreas has defects in all three epithelial tissues: a partial loss of endocrine cells, a disrupted ductal tree and a >90% deficit of acini. The acinar defects are due to a combination of fewer MPCs, deficient allocation of those MPCs to pre-acinar fate, disruption of acinar morphogenesis and incomplete acinar cell differentiation. NR5A2 controls these developmental processes directly as well as through regulatory interactions with other pancreatic transcriptional regulators, including PTF1A, MYC, GATA4, FOXA2, RBPJL and MIST1 (BHLHA15). In particular, Nr5a2 and Ptf1a establish mutually reinforcing regulatory interactions and collaborate to control developmentally regulated pancreatic genes by binding to shared transcriptional regulatory regions. At the final stage of acinar cell development, the absence of NR5A2 affects the expression of Ptf1a and its acinar specific partner Rbpjl, so that the few acinar cells that form do not complete differentiation. Nr5a2 controls several temporally distinct stages of pancreatic development that involve regulatory mechanisms relevant to pancreatic oncogenesis and the maintenance of the exocrine phenotype.

Proliferating pancreatic beta-cells upregulate ALDH

High levels of aldehyde dehydrogenase (ALDH) activity have been regarded as a specific feature of progenitor cells and stem cells. Hence, as an indicator of ALDH activity, aldefluor fluorescence has been widely used for the identification and isolation of stem and progenitor cells. ALDH activity was recently detected in embryonic mouse pancreas, and specifically and exclusively in adult centroacinar and terminal duct cells, suggesting that these duct cells may harbor cells of endocrine and exocrine differentiation potential in the adult pancreas. Here, we report the presence of aldefluor+ beta-cells in a beta-cell proliferation model, partial pancreatectomy. The aldefluor+ beta-cells are essentially all positive for Ki-67 and expressed high levels of cell-cycle activators such as CyclinD1, CyclinD2, and CDK4, suggesting that they are mitotic cells. Our data thus reveal a potential change in ALDH activity of proliferating beta-cells, which provides a novel method for the isolation and analysis of proliferating beta-cells. Moreover, our data also suggest that aldefluor lineage-tracing is not a proper method for analyzing progenitor or stem activity in the adult pancreas.

Endothelial cells contribute to generation of adult ventricular myocytes during cardiac homeostasis

Cardiac tissue undergoes renewal with low rates. Although resident stem cell populations have been identified to support cardiomyocyte turnover, the source of the cardiac stem cells and their niche remain elusive. Using Cre/Lox-based cell lineage tracing strategies, we discovered that labeling of endothelial cells in the adult heart yields progeny that have cardiac stem cell characteristics and express Gata4 and Sca1. Endothelial-derived cardiac progenitor cells were localized in the arterial coronary walls with quiescent and proliferative cells in the media and adventitia layers, respectively. Within the myocardium, we identified labeled cardiomyocytes organized in clusters of single-cell origin. Pulse-chase experiments showed that generation of individual clusters was rapid but confined to specific regions of the heart, primarily in the right anterior and left posterior ventricular walls and the junctions between the two ventricles. Our data demonstrate that endothelial cells are an intrinsic component of the cardiac renewal process.

Transcriptional control of mammalian pancreas organogenesis

The field of pancreas development has markedly expanded over the last decade, significantly advancing our understanding of the molecular mechanisms that control pancreas organogenesis. This growth has been fueled, in part, by the need to generate new therapeutic approaches for the treatment of diabetes. The creation of sophisticated genetic tools in mice has been instrumental in this progress. Genetic manipulation involving activation or inactivation of genes within specific cell types has allowed the identification of many transcription factors (TFs) that play critical roles in the organogenesis of the pancreas. Interestingly, many of these TFs act at multiple stages of pancreatic development, and adult organ function or repair. Interaction with other TFs, extrinsic signals, and epigenetic regulation are among the mechanisms by which TFs may play context-dependent roles during pancreas organogenesis. Many of the pancreatic TFs directly regulate each other and their own expression. These combinatorial interactions generate very specific gene regulatory networks that can define the different cell lineages and types in the developing pancreas. Here, we review recent progress made in understanding the role of pancreatic TFs in mouse pancreas formation. We also summarize our current knowledge of human pancreas development and discuss developmental pancreatic TFs that have been associated with human pancreatic diseases.

Sox9: a master regulator of the pancreatic program

Over the last decade, it has been discovered that the transcription factor Sox9 plays several critical roles in governing the development of the embryonic pancreas and the homeostasis of the mature organ. While analysis of pancreata from patients affected by the Sox9 haploinsufficiency syndrome campomelic dysplasia initially alluded to a functional role of Sox9 in pancreatic morphogenesis, transgenic mouse models have been instrumental in mechanistically dissecting such roles. Although initially defined as a marker and maintenance factor for pancreatic progenitors, Sox9 is now considered to fulfill additional indispensable functions during pancreogenesis and in the postnatal organ through its interactions with other transcription factors and signaling pathways such as Fgf and Notch. In addition to maintaining both multipotent and bipotent pancreatic progenitors, Sox9 is also required for initiating endocrine differentiation and maintaining pancreatic ductal identity, and it has recently been unveiled as a key player in the initiation of pancreatic cancer. These functions of Sox9 are discussed in this article, with special emphasis on the knowledge gained from various loss-of-function and lineage tracing mouse models. Also, current controversies regarding Sox9 function in healthy and injured adult pancreas and unanswered questions and avenues of future study are discussed.

ALDH expression characterizes G1-phase proliferating beta cells during pregnancy

High levels of aldehyde dehydrogenase (ALDH) activity have been detected in various progenitor and stem cells. Thus, Aldefluor fluorescence, which represents precisely the ALDH activity, has been widely used for the identification, evaluation, and isolation of stem and progenitor cells. Recently, ALDH activity was detected in embryonic and adult mouse pancreas, specifically in adult centroacinar and terminal duct cells supposed to harbor endocrine and exocrine progenitor cells in the adult pancreas. Nevertheless, ALDH activity and aldeflour fluorescence have not been examined in beta cells. Here, we report a dynamic increase in the number of aldeflour+ beta cells during pregnancy. Interestingly, nearly all these aldeflour+ beta cells are positive for Ki-67, suggesting that they are in an active cell cycle (G1, S and M phases). To determine precisely at which phase beta cells activate ALDH activity and thus become aldeflour+, we co-stained insulin with additional proliferation markers, phosphohistone3 (PHH3, a marker for M-phase proliferating cells) and Bromodeoxyuridine (BrdU, a marker for S-phase proliferating cells). Our data show little aldeflour+ beta cells that were positive for either PHH3, or BrdU, suggesting that beta cells activate ALDH and become Aldefluor+ when they enter G1-phase of active cell cycle, but may downregulate ALDH when they leave G1-phase and enter S phase. Our data thus reveal a potential change in ALDH activity of proliferating beta cells during pregnancy, which provides a novel method for isolation and analysis of proliferating beta cells. Moreover, our data also suggest that caution needs to be taken on interpretation of Aldefluor lineage-tracing data in pancreas.

M2 macrophages promote beta-cell proliferation by up-regulation of SMAD7

Determination of signaling pathways that regulate beta-cell replication is critical for beta-cell therapy. Here, we show that blocking pancreatic macrophage infiltration after pancreatic duct ligation (PDL) completely inhibits beta-cell proliferation. The TGFβ superfamily signaling inhibitor SMAD7 was significantly up-regulated in beta cells after PDL. Beta cells failed to proliferate in response to PDL in beta-cell-specific SMAD7 mutant mice. Forced expression of SMAD7 in beta cells by itself was sufficient to promote beta-cell proliferation in vivo. M2, rather than M1 macrophages, seem to be the inducers of SMAD7-mediated beta-cell proliferation. M2 macrophages not only release TGFβ1 to directly induce up-regulation of SMAD7 in beta cells but also release EGF to activate EGF receptor signaling that inhibits TGFβ1-activated SMAD2 nuclear translocation, resulting in TGFβ signaling inhibition. SMAD7 promotes beta-cell proliferation by increasing CyclinD1 and CyclinD2, and by inducing nuclear exclusion of p27. Our study thus reveals a molecular pathway to potentially increase beta-cell mass through enhanced SMAD7 activity induced by extracellular stimuli.

Increase of beta cell mass by beta cell replication, but not neogenesis, in the maternal pancreas in mice

Strategies for increasing functional beta cell mass are effective for diabetes therapy. Although controversy remains on the existence of facultative beta cell progenitors in the adult pancreas, most evidence does not support such a possibility. One of the greatest physiological increases in beta cells has been detected in the maternal pancreas during pregnancy, following neonatal period and in the setting of insulin resistance. However, no systematical analysis on the beta cell growth in this period has been ever performed. Here we analyzed beta cell replication by quantifying BrdU incorporated beta cells at different time points in the pregnant mice. Similarly, we evaluated the possible involvement of beta cell neogenesis (differentiation from progenitor cells) by analyzing expression of Neurog3, a key determinant of pancreatic endocrine cell neogenesis during embryogenesis, in the exocrine pancreas. We found a dynamic increase in beta cell replication, but failed to detect beta cell neogenesis, demonstrating that beta cell growth in the maternal pancreas during pregnancy is predominantly attributable to beta cell replication, rather than beta cell neogenesis.

Transient cytokine treatment induces acinar cell reprogramming and regenerates functional beta cell mass in diabetic mice

Reprogramming of pancreatic exocrine cells into cells resembling beta cells may provide a strategy for treating diabetes. Here we show that transient administration of epidermal growth factor and ciliary neurotrophic factor to adult mice with chronic hyperglycemia efficiently stimulates the conversion of terminally differentiated acinar cells to beta-like cells. Newly generated beta-like cells are epigenetically reprogrammed, functional and glucose responsive, and they reinstate normal glycemic control for up to 248 d. The regenerative process depends on Stat3 signaling and requires a threshold number of Neurogenin 3 (Ngn3)-expressing acinar cells. In contrast to previous work demonstrating in vivo conversion of acinar cells to beta-like cells by viral delivery of exogenous transcription factors, our approach achieves acinar-to-beta-cell reprogramming through transient cytokine exposure rather than genetic modification.

Keratin 79 identifies a novel population of migratory epithelial cells that initiates hair canal morphogenesis and regeneration

The formation of epithelial tubes underlies the development of diverse organs. In the skin, hair follicles resemble tube-like structures with lumens that are generated through poorly understood cellular rearrangements. Here, we show that creation of the hair follicle lumen is mediated by early outward movement of keratinocytes from within the cores of developing hair buds. These migratory keratinocytes express keratin 79 (K79) and stream out of the hair germ and into the epidermis prior to lumen formation in the embryo. Remarkably, this process is recapitulated during hair regeneration in the adult mouse, when K79(+) cells migrate out of the reactivated secondary hair germ prior to formation of a new hair canal. During homeostasis, K79(+) cells line the hair follicle infundibulum, a domain we show to be multilayered, biochemically distinct and maintained by Lrig1(+) stem cell-derived progeny. Upward movement of these cells sustains the infundibulum, while perturbation of this domain during acne progression is often accompanied by loss of K79. Our findings uncover previously unappreciated long-distance cell movements throughout the life cycle of the hair follicle, and suggest a novel mechanism by which the follicle generates its hollow core through outward cell migration.

Gene expression profiles in human and mouse primary cells provide new insights into the differential actions of vitamin D3 metabolites

1α,25-Dihydroxyvitamin D₃ (1α,25(OH)₂D₃) had earlier been regarded as the only active hormone. The newly identified actions of 25-hydroxyvitamin D₃ (25(OH)D₃) and 24R,25-dihydroxyvitamin D₃ (24R,25(OH)₂D₃) broadened the vitamin D₃ endocrine system, however, the current data are fragmented and a systematic understanding is lacking. Here we performed the first systematic study of global gene expression to clarify their similarities and differences. Three metabolites at physiologically comparable levels were utilized to treat human and mouse fibroblasts prior to DNA microarray analyses. Human primary prostate stromal P29SN cells (hP29SN), which convert 25(OH)D₃ into 1α,25(OH)₂D₃ by 1α-hydroxylase (encoded by the gene CYP27B1), displayed regulation of 164, 171, and 175 genes by treatment with 1α,25(OH)₂D₃, 25(OH)D₃, and 24R,25(OH)₂D₃, respectively. Mouse primary Cyp27b1 knockout fibroblasts (mCyp27b1^−/−), which lack 1α-hydroxylation, displayed regulation of 619, 469, and 66 genes using the same respective treatments. The number of shared genes regulated by two metabolites is much lower in hP29SN than in mCyp27b1^−/−. By using DAVID Functional Annotation Bioinformatics Microarray Analysis tools and Ingenuity Pathways Analysis, we identified the agonistic regulation of calcium homeostasis and bone remodeling between 1α,25(OH)₂D₃ and 25(OH)D₃ and unique non-classical actions of each metabolite in physiological and pathological processes, including cell cycle, keratinocyte differentiation, amyotrophic lateral sclerosis signaling, gene transcription, immunomodulation, epigenetics, cell differentiation, and membrane protein expression. In conclusion, there are three distinct vitamin D₃ hormones with clearly different biological activities. This study presents a new conceptual insight into the vitamin D₃ endocrine system, which may guide the strategic use of vitamin D₃ in disease prevention and treatment.

The use of stem cells for pancreatic regeneration in diabetes mellitus

The endocrine pancreas represents an interesting arena for regenerative medicine and cell therapeutics. One of the major pancreatic diseases, diabetes mellitus is a metabolic disorder caused by having an insufficient number of insulin-producing β cells. Replenishment of β cells by cell transplantation can restore normal metabolic control. The shortage in donor pancreata has meant that the demand for transplantable β cells has outstripped the supply, which could be met by using alternative sources of stem cells. This situation has opened up new areas of research, such as cellular reprogramming and in vivo β-cell regeneration. Pluripotent stem cells seem to be the best option for clinical applications of β-cell regeneration in the near future, as these cells have been demonstrated to represent an unlimited source of functional β cells. Although compelling evidence shows that the adult pancreas retains regenerative capacity, it remains unclear whether this organ contains stem cells. Alternatively, specialized cell types within or outside the pancreas retain plasticity in proliferation and differentiation. Cellular reprogramming or transdifferentiation of exocrine cells or other types of endocrine cells in the pancreas could provide a long-term solution.

Significance of Notch1-signaling pathway in human pancreatic development and carcinogenesis

In animal studies, Notch1-signaling pathway plays an important role in the pancreatic embryogenesis by promoting pancreatic progenitor cells self-renewal and exocrine linage development. The persistent activation of Notch pathway could arrest the organ development and keep cells at an undifferentiated stage. Studies have shown that Notch1-signaling pathway is upregulated in invasive pancreatic ductal adenocarcinoma (PDAC). Here we examined the expression pattern of Notch1 and Hes1 in human fetal pancreatic tissues to elucidate the role of Notch1 in human pancreatic embryonic development. We also compared Notch1 expression in tissues from PDAC, chronic pancreatitis and pancreatic intraepithelial neoplasm. Our data show that Notch1/Hes1-signaling pathway is activated during early pancreatic embryogenesis and reaches the highest at birth. After pancreas is fully developed, Notch1/Hes1 pathway is inactivated even though Notch1 protein cell-surface expression is upregulated. We also showed that the expression of both Notch1 and Hes1 are present in 50% (33/66) of PDACs, but not in pancreatic intraepithelial neoplasms. These findings indicate that Notch1 activation is only apparent in late stage of pancreatic carcinogenesis, suggesting that treatment with Notch-signaling inhibitors including γ-secretase should be selectively used for PDACs with confirmed Notch1-signaling activation.

The plastic pancreas

Pancreas homeostasis is based on replication of differentiated cells in order to maintain proper organ size and function under changing physiological demand. Recent studies suggest that acinar cells, the most abundant cell type in the pancreas, are facultative progenitors capable of reverting to embryonic-like multipotent progenitor cells under injury conditions associated with inflammation. In parallel, it is becoming apparent that within the endocrine pancreas, hormone-producing cells can lose or switch their identity under metabolic stress or in response to single gene mutations. This new view of pancreas dynamics suggests interesting links between pancreas regeneration and pathologies including diabetes and pancreatic cancer.

Neurogenin3 activation is not sufficient to direct duct-to-beta cell transdifferentiation in the adult pancreas

It remains controversial whether adult pancreatic ducts harbor facultative beta cell progenitors. Because neurogenin3 (Ngn3) is a key determinant of pancreatic endocrine cell neogenesis during embryogenesis, many studies have also relied upon Ngn3 expression as evidence of beta cell neogenesis in adults. Recently, however, Ngn3 as a marker of adult beta cell neogenesis has been called into question by reports of Ngn3 expression in fully-developed beta cells. Nevertheless, direct evidence as to whether Ngn3 activation in adult pancreatic duct cells may lead to duct-to-beta cell transdifferentiation is lacking. Here we studied two models of Ngn3 activation in adult pancreatic duct cells (low-dose alloxan treatment and pancreatic duct ligation) and lineage-traced Ngn3-activated duct cells by labeling them through intraductal infusion with a cell-tagging dye, CFDA-SE No dye-labeled beta cells were found during the follow-up in either model, suggesting that activation of Ngn3 in duct cells is not sufficient to direct their transdifferentiation into beta cells. Therefore, Ngn3 activation in duct cells is not a signature for adult beta cell neogenesis.

Pancreas organogenesis: from lineage determination to morphogenesis

The pancreas is an essential organ for proper nutrient metabolism and has both endocrine and exocrine function. In the past two decades, knowledge of how the pancreas develops during embryogenesis has significantly increased, largely from developmental studies in model organisms. Specifically, the molecular basis of pancreatic lineage decisions and cell differentiation is well studied. Still not well understood are the mechanisms governing three-dimensional morphogenesis of the organ. Strategies to derive transplantable β-cells in vitro for diabetes treatment have benefited from the accumulated knowledge of pancreas development. In this review, we provide an overview of the current understanding of pancreatic lineage determination and organogenesis, and we examine future implications of these findings for treatment of diabetes mellitus through cell replacement.

Lhx2 balances progenitor maintenance with neurogenic output and promotes competence state progression in the developing retina

The LIM-Homeodomain transcription factor Lhx2 is an essential organizer of early eye development and is subsequently expressed in retinal progenitor cells (RPCs). To determine its requirement in RPCs, we performed a temporal series of conditional inactivations in mice with the early RPC driver Pax6 α-Cre and the tamoxifen-inducible Hes1(CreERT2) driver. Deletion of Lhx2 caused a significant reduction of the progenitor population and a corresponding increase in neurogenesis. Precursor fate choice correlated with the time of inactivation; early and late inactivation led to the overproduction of retinal ganglion cells (RGCs) and rod photoreceptors, respectively. In each case, however, the overproduction was selective, occurring at the expense of other cell types and indicating a role for Lhx2 in generating cell type diversity. RPCs that persisted in the absence of Lhx2 continued to generate RGC precursors beyond their normal production window, suggesting that Lhx2 facilitates a transition in competence state. These results identify Lhx2 as a key regulator of RPC properties that contribute to the ordered production of multiple cell types during retinal tissue formation.

Metabolic regulation of cellular plasticity in the pancreas

Obese individuals exhibit an increase in pancreatic β cell mass; conversely, scarce nutrition during pregnancy has been linked to β cell insufficiency in the offspring [reviewed in 1, 2]. These phenomena are thought to be mediated mainly through effects on β cell proliferation, given that a nutrient-sensitive β cell progenitor population in the pancreas has not been identified. Here, we employed the fluorescent ubiquitination-based cell-cycle indicator system to investigate β cell replication in real time and found that high nutrient concentrations induce rapid β cell proliferation. Importantly, we found that high nutrient concentrations also stimulate β cell differentiation from progenitors in the intrapancreatic duct (IPD). Furthermore, using a new zebrafish line where β cells are constitutively ablated, we show that β cell loss and high nutrient intake synergistically activate these progenitors. At the cellular level, this activation process causes ductal cell reorganization as it stimulates their proliferation and differentiation. Notably, we link the nutrient-dependent activation of these progenitors to a downregulation of Notch signaling specifically within the IPD. Furthermore, we show that the nutrient sensor mechanistic target of rapamycin (mTOR) is required for endocrine differentiation from the IPD under physiological conditions as well as in the diabetic state. Thus, this study reveals critical insights into how cells modulate their plasticity in response to metabolic cues and identifies nutrient-sensitive progenitors in the mature pancreas.

β-Cells are not generated in pancreatic duct ligation-induced injury in adult mice

The existence of adult β-cell progenitors remains the most controversial developmental biology topic in diabetes research. It has been reported that β-cell progenitors can be activated by ductal ligation-induced injury of adult mouse pancreas and apparently act in a cell-autonomous manner to double the functional β-cell mass within a week by differentiation and proliferation. Here, we demonstrate that pancreatic duct ligation (PDL) does not activate progenitors to contribute to β-cell mass expansion. Rather, PDL stimulates massive pancreatic injury, which alters pancreatic composition and thus complicates accurate measurement of β-cell content via traditional morphometry methodologies that superficially sample the pancreas. To overcome this potential bias, we quantified β-cells from the entire pancreas and observed that β-cell mass and insulin content are totally unchanged by PDL-induced injury. Lineage-tracing studies using sequential administration of thymidine analogs, rat insulin 2 promoter-driven cre-lox, and low-frequency ubiquitous cre-lox reveal that PDL does not convert progenitors to the β-cell lineage. Thus, we conclude that β-cells are not generated in injured adult mouse pancreas.

Sox9 and programming of liver and pancreatic progenitors

Recent advances in developmental biology have greatly expanded our understanding of progenitor cell programming and the fundamental roles that Sox9 plays in liver and pancreas organogenesis. In the last 2 years, several studies have dissected the behavior of the Sox9+ duct cells in adult organs, but conflicting results have left unanswered the long-standing question of whether physiologically functioning progenitors exist in adult liver and pancreas. On the other hand, increasing evidence suggests that duct cells function as progenitors in the tissue restoration process after injury, during which embryonic programs are sometimes reactivated. This article discusses the role of Sox9 in programming liver and pancreatic progenitors as well as controversies in the field.

No evidence for β cell neogenesis in murine adult pancreas

Whether facultative β cell progenitors exist in the adult pancreas is a major unsolved question. To date, lineage-tracing studies have provided conflicting results. To track β cell neogenesis in vivo, we generated transgenic mice that transiently coexpress mTomato and GFP in a time-sensitive, nonconditional Cre-mediated manner, so that insulin-producing cells express GFP under control of the insulin promoter, while all other cells express mTomato (INSCremTmG mice). Newly differentiated β cells were detected by flow cytometry and fluorescence microscopy, taking advantage of their transient coexpression of GFP and mTomato fluorescent proteins. We found that β cell neogenesis predominantly occurs during embryogenesis, decreases dramatically shortly after birth, and is completely absent in adults across various models of β cell loss, β cell growth and regeneration, and inflammation. Moreover, we demonstrated upregulation of neurogenin 3 (NGN3) in both proliferating ducts and preexisting β cells in the ligated pancreatic tail after pancreatic ductal ligation. These results are consistent with some recent reports, but argue against the widely held belief that NGN3 marks cells undergoing endocrine neogenesis in the pancreas. Our data suggest that β cell neogenesis in the adult pancreas occurs rarely, if ever, under either normal or pathological conditions.

TGFβ receptor signaling is essential for inflammation-induced but not β-cell workload-induced β-cell proliferation

Protection and restoration of a functional β-cell mass are fundamental strategies for prevention and treatment of diabetes. Consequently, knowledge of signals that determine the functional β-cell mass is of immense clinical relevance. Transforming growth factor β (TGFβ) superfamily signaling pathways play a critical role in development and tissue specification. Nevertheless, the role of these pathways in adult β-cell homeostasis is not well defined. Here, we ablated TGFβ receptor I and II genes in mice undergoing two surgical β-cell replication models (partial pancreatectomy or partial duct ligation), representing two triggers for β-cell proliferation, increased β-cell workload and local inflammation, respectively. Our data suggest that TGFβ receptor signaling is necessary for baseline β-cell proliferation. By either provision of excess glucose or treatment with exogenous insulin, we further demonstrated that inflammation and increased β-cell workload are both stimulants for β-cell proliferation but are TGFβ receptor signaling dependent and independent, respectively. Collectively, by using a pancreas-specific TGFβ receptor-deleted mouse model, we have identified two distinct pathways that regulate adult β-cell proliferation. Our study thus provides important information for understanding β-cell proliferation during normal growth and in pancreatic diseases.

Neurogenin 3+ cells contribute to β-cell neogenesis and proliferation in injured adult mouse pancreas

We previously showed that injury by partial duct ligation (PDL) in adult mouse pancreas activates Neurogenin 3 (Ngn3)⁺ progenitor cells that can differentiate to β cells ex vivo. Here we evaluate the role of Ngn3⁺ cells in β cell expansion in situ. PDL not only induced doubling of the β cell volume but also increased the total number of islets. β cells proliferated without extended delay (the so-called ‘refractory' period), their proliferation potential was highest in small islets, and 86% of the β cell expansion was attributable to proliferation of pre-existing β cells. At sufficiently high Ngn3 expression level, upto 14% of all β cells and 40% of small islet β cells derived from non-β cells. Moreover, β cell proliferation was blunted by a selective ablation of Ngn3⁺ cells but not by conditional knockout of Ngn3 in pre-existing β cells supporting a key role for Ngn3⁺ insulin⁻ cells in β cell proliferation and expansion. We conclude that Ngn3⁺ cell-dependent proliferation of pre-existing and newly-formed β cells as well as reprogramming of non-β cells contribute to in vivo β cell expansion in the injured pancreas of adult mice.

Spatiotemporal patterns of multipotentiality in Ptf1a-expressing cells during pancreas organogenesis and injury-induced facultative restoration

Pancreatic multipotent progenitor cells (MPCs) produce acinar, endocrine and duct cells during organogenesis, but their existence and location in the mature organ remain contentious. We used inducible lineage-tracing from the MPC-instructive gene Ptf1a to define systematically in mice the switch of Ptf1a(+) MPCs to unipotent proacinar competence during the secondary transition, their rapid decline during organogenesis, and absence from the mature organ. Between E11.5 and E15.5, we describe tip epithelium heterogeneity, suggesting that putative Ptf1a(+)Sox9(+)Hnf1β(+) MPCs are intermingled with Ptf1a(HI)Sox9(LO) proacinar progenitors. In the adult, pancreatic duct ligation (PDL) caused facultative reactivation of multipotency factors (Sox9 and Hnf1β) in Ptf1a(+) acini, which undergo rapid reprogramming to duct cells and longer-term reprogramming to endocrine cells, including insulin(+) β-cells that are mature by the criteria of producing Pdx1(HI), Nkx6.1(+) and MafA(+). These Ptf1a lineage-derived endocrine/β-cells are likely formed via Ck19(+)/Hnf1β(+)/Sox9(+) ductal and Ngn3(+) endocrine progenitor intermediates. Acinar to endocrine/β-cell transdifferentiation was enhanced by combining PDL with pharmacological elimination of pre-existing β-cells. Thus, we show that acinar cells, without exogenously introduced factors, can regain aspects of embryonic multipotentiality under injury, and convert into mature β-cells.

Wnt signaling regulates postembryonic hypothalamic progenitor differentiation

Previous studies have raised the possibility that Wnt signaling may regulate both neural progenitor maintenance and neuronal differentiation within a single population. Here we investigate the role of Wnt/β-catenin activity in the zebrafish hypothalamus and find that the pathway is first required for the proliferation of unspecified hypothalamic progenitors in the embryo. At later stages, including adulthood, sequential activation and inhibition of Wnt activity is required for the differentiation of neural progenitors and negatively regulates radial glia differentiation. The presence of Wnt activity is conserved in hypothalamic progenitors of the adult mouse, where it plays a conserved role in inhibiting the differentiation of radial glia. This study establishes the vertebrate hypothalamus as a model for Wnt-regulated postembryonic neural progenitor differentiation and defines specific roles for Wnt signaling in neurogenesis.

Development and regeneration in the endocrine pancreas

The pancreas is composed of two compartments that deliver digestive enzymes and endocrine hormones to control the blood sugar level. The endocrine pancreas consists of functional units organized into cell clusters called islets of Langerhans where insulin-producing cells are found in the core and surrounded by glucagon-, somatostatin-, pancreatic polypeptide-, and ghrelin-producing cells. Diabetes is a devastating disease provoked by the depletion or malfunction of insulin-producing beta-cells in the endocrine pancreas. The side effects of diabetes are multiple, including cardiovascular, neuropathological, and kidney diseases. The analyses of transgenic and knockout mice gave major insights into the molecular mechanisms controlling endocrine pancreas genesis. Moreover, the study of animal models of pancreas injury revealed that the pancreas has the propensity to undergo regeneration and opened new avenues to develop novel therapeutic approaches for the treatment of diabetes. Thus, beside self-replication of preexisting insulin-producing cells, several potential cell sources in the adult pancreas were suggested to contribute to beta-cell regeneration, including acinar, intraislet, and duct epithelia. However, regeneration in the adult endocrine pancreas is still under controversial debate.

Notch1 is not required for acinar-to-ductal metaplasia in a model of Kras-induced pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma is believed to arise from precursor lesions termed pancreatic intraepithelial neoplasia (PanIN). Mouse models have demonstrated that targeted expression of activated K-ras to mature acinar cells in the pancreas induces the spontaneous development of PanIN lesions; implying acinar-to-ductal metaplasia (ADM) is a key event in this process. Recent studies suggest Notch signaling is a key regulator of ADM. To assess if Notch1 is required for K-ras driven ADM we employed both an in vivo mouse model and in vitro explant culture system, in which an oncogenic allele of K-ras is activated and Notch1 is deleted simultaneously in acinar cells. Our results demonstrate that oncogenic K-ras is sufficient to drive ADM both in vitro and in vivo but that loss of Notch1 has a minimal effect on this process. Interestingly, while loss of Notch1 in vivo does not affect the severity of PanIN lesions observed, the overall numbers of lesions were greater in mice with deleted Notch1. This suggests Notch1 deletion renders acinar cells more susceptible to formation of K-ras-induced PanINs.

ALDH1B1 is a potential stem/progenitor marker for multiple pancreas progenitor pools

Aldehyde dehydrogenase (ALDH) genes are increasingly associated with stem/progenitor cell status but their role in the maintenance of pluripotency remains uncertain. In a screen conducted for downstream Ngn3 target genes using ES derived pancreas progenitors we identified Aldh1b1, encoding a mitochondrial enzyme, as one of the genes strongly up regulated in response to Ngn3 expression. We found both by in situ hybridization and immunofluorescence using a specific antibody that ALDH1B1 is exclusively expressed in the emerging pancreatic buds of the early embryo (9.5 dpc) in a Pdx1 dependent manner. Around the time of secondary transition, ALDH1B1 expression was restricted in the tip tripotent progenitors of the branching epithelium and in a subset of the trunk epithelium. Expression in the latter was Ngn3 dependent. Subsequently, ALDH1B1 expression persisted only in the tip cells that become restricted to the exocrine lineage and declined rapidly as these cells mature. In the adult pancreas we identified rare ALDH1B1(+) cells that become abundant following pancreas injury in either the caerulein or streptozotocin paradigms. Blocking ALDH catalytic activity in pancreas embryonic explants resulted in reduced size of the explants and accelerated differentiation suggesting for the first time that ALDH activity may be necessary in the developing pancreas for the maintenance and expansion of progenitor pools.

Hippo signaling regulates pancreas development through inactivation of Yap

The mammalian pancreas is required for normal metabolism, with defects in this vital organ commonly observed in cancer and diabetes. Development must therefore be tightly controlled in order to produce a pancreas of correct size, cell type composition, and physiologic function. Through negative regulation of Yap-dependent proliferation, the Hippo kinase cascade is a critical regulator of organ growth. To investigate the role of Hippo signaling in pancreas biology, we deleted Hippo pathway components in the developing mouse pancreas. Unexpectedly, the pancreas from Hippo-deficient offspring was reduced in size, with defects evident throughout the organ. Increases in the dephosphorylated nuclear form of Yap are apparent throughout the exocrine compartment and correlate with increases in levels of cell proliferation. However, the mutant exocrine tissue displays extensive disorganization leading to pancreatitis-like autodigestion. Interestingly, our results suggest that Hippo signaling does not directly regulate the pancreas endocrine compartment as Yap expression is lost following endocrine specification through a Hippo-independent mechanism. Altogether, our results demonstrate that Hippo signaling plays a crucial role in pancreas development and provide novel routes to a better understanding of pathological conditions that affect this organ.

Notch signaling differentially regulates the cell fate of early endocrine precursor cells and their maturing descendants in the mouse pancreas and intestine

Notch signaling inhibits differentiation of endocrine cells in the pancreas and intestine. In a number of cases, the observed inhibition occurred with Notch activation in multipotential cells, prior to the initiation of endocrine differentiation. It has not been established how direct activation of Notch in endocrine precursor cells affects their subsequent cell fate. Using conditional activation of Notch in cells expressing Neurogenin3 or NeuroD1, we examined the effects of Notch in both organs, on cell fate of early endocrine precursors and maturing endocrine-restricted cells, respectively. Notch did not preclude the differentiation of a limited number of endocrine cells in either organ when activated in Ngn3(+) precursor cells. In addition, in the pancreas most Ngn3(+) cells adopted a duct but not acinar cell fate; whereas in intestinal Ngn3(+) cells, Notch favored enterocyte and goblet cell fates, while selecting against endocrine and Paneth cell differentiation. A small fraction of NeuroD1(+) cells in the pancreas retain plasticity to respond to Notch, giving rise to intraislet ductules as well as cells with no detectable pancreatic lineage markers that appear to have limited ultrastructural features of both endocrine and duct cells. These results suggest that Notch directly regulates cell fate decisions in multipotential early endocrine precursor cells. Some maturing endocrine-restricted NeuroD1(+) cells in the pancreas switch to the duct lineage in response to Notch, indicating previously unappreciated plasticity at such a late stage of endocrine differentiation.

Bmi1 is required for regeneration of the exocrine pancreas in mice

BACKGROUND & AIMS

Bmi1 is a member of the Polycomb protein family and represses transcription by modifying chromatin organization at specific promoters. Bmi1 is implicated in the control of stem cell self-renewal and has been shown to regulate cell proliferation, tissue homeostasis, and differentiation. Bmi1 is present in a subpopulation of self-renewing pancreatic acinar cells and is expressed in response to pancreatic damage. We investigated the role of Bmi1 in regeneration of exocrine pancreas.

METHODS

Acute pancreatitis was induced in Bmi1(-/-) mice with cerulein; pancreatic cell regeneration, differentiation, and apoptosis were assessed. Cultured Bmi1(-/-) and wild-type primary acini were analyzed in vitro to determine acinar-specific consequences of Bmi1 deletion. To investigate cell autonomous versus non-cell autonomous roles for Bmi1 in vivo, pancreatitis was induced in Bmi1(-/-) mice reconstituted with a wild-type hematopoietic system.

RESULTS

Bmi1 expression was up-regulated in the exocrine pancreas during regeneration after cerulein-induced pancreatitis. Exocrine regeneration was impaired following administration of cerulein to Bmi1(-/-) mice. Pancreata of Bmi1(-/-) mice were hypoplastic, and the exocrine pancreas was replaced with ductal metaplasia that had increased apoptosis and decreased cell proliferation compared with that of wild-type mice. Expression of Cdkn2a and p53-dependent apoptotic genes was markedly up-regulated in Bmi1(-/-) pancreas compared with wild-type mice after injury. Furthermore, after transplantation of bone marrow from wild-type to Bmi1(-/-) mice, the chimeric mice had intermediate levels of pancreatic hypoplasia and significant but incomplete rescue of impaired exocrine regeneration after cerulein injury.

CONCLUSIONS

Bmi1 contributes to regeneration of the exocrine pancreas after cerulein-induced injury through cell autonomous mechanisms, in part by regulating Cdkn2a expression, and non-cell autonomous mechanisms.

Beta cell count instead of beta cell mass to assess and localize growth in beta cell population following pancreatic duct ligation in mice

Background

Pancreatic-tail duct ligation (PDL) in adult rodents has been reported to induce beta cell generation and increase beta cell mass but increases in beta cell number have not been demonstrated. This study examines whether PDL increases beta cell number and whether this is caused by neogenesis of small clusters and/or their growth to larger aggregates.

Methodology

Total beta cell number and its distribution over small (<50 µm), medium, large (>100 µm) clusters was determined in pancreatic tails of 10-week-old mice, 2 weeks after PDL or sham.

Principal findings

PDL increased total beta cell mass but not total beta cell number. It induced neogenesis of small beta cell clusters (2.2-fold higher number) which contained a higher percent proliferating beta cells (1.9% Ki67+cells) than sham tails (<0.2%); their higher beta cell number represented <5% of total beta cell number and was associated with a similar increase in alpha cell number. It is unknown whether the regenerative process is causally related to the inflammatory infiltration in PDL-tails. Human pancreases with inflammatory infiltration also exhibited activation of proliferation in small beta cell clusters.

Conclusions/significance

The PDL model illustrates the advantage of direct beta cell counts over beta cell mass measurements when assessing and localizing beta cell regeneration in the pancreas. It demonstrates the ability of the adult mouse pancreas for neogenesis of small beta cell clusters with activated beta cell proliferation. Further studies should investigate conditions under which neoformed small beta cell clusters grow to larger aggregates and hence to higher total beta cell numbers.

Progress in understanding the relationship of microRNAs with pancreas development and stem cell differentiation

MicroRNAs (miRNAs), a class of non-coding small RNAs involved in post-transcriptional gene regulation, play important roles in embryonic development, cell fate determination, and growth regulation. It has been shown that miRNAs contribute to the embryonic pancreas development by regulating several important transcriptional factors. Stem cells can be induced to differentiate into insulin-producing cells in vitro by mimicking the development of pancreatic β cells in vivo. MiRNAs may have regulatory roles in the maintenance and differentiation of stem cells. Therefore, elucidation of the mechanisms by which miRNAs regulate pancreas development and stem cell differentiation will provide novel insights into the development of therapeutic approach for diabetes.

Exocrine ontogenies: on the development of pancreatic acinar, ductal and centroacinar cells

This review summarizes our current understanding of exocrine pancreas development, including the formation of acinar, ductal and centroacinar cells. We discuss the transcription factors associated with various stages of exocrine differentiation, from multipotent progenitor cells to fully differentiated acinar and ductal cells. Within the branching epithelial tree of the embryonic pancreas, this involves the progressive restriction of multipotent pancreatic progenitor cells to either a central "trunk" domain giving rise to the islet and ductal lineages, or a peripheral "tip" domain giving rise to acinar cells. This review also discusses the soluble morphogens and other signaling pathways that influence these events. Finally, we examine centroacinar cells as an enigmatic pancreatic cell type whose lineage remains uncertain, and whose possible progenitor capacities continue to be explored.

A Notch-dependent molecular circuitry initiates pancreatic endocrine and ductal cell differentiation

In the pancreas, Notch signaling is thought to prevent cell differentiation, thereby maintaining progenitors in an undifferentiated state. Here, we show that Notch renders progenitors competent to differentiate into ductal and endocrine cells by inducing activators of cell differentiation. Notch signaling promotes the expression of Sox9, which cell-autonomously activates the pro-endocrine gene Ngn3. However, at high Notch activity endocrine differentiation is blocked, as Notch also induces expression of the Ngn3 repressor Hes1. At the transition from high to intermediate Notch activity, only Sox9, but not Hes1, is maintained, thus de-repressing Ngn3 and initiating endocrine differentiation. In the absence of Sox9 activity, endocrine and ductal cells fail to differentiate, resulting in polycystic ducts devoid of primary cilia. Although Sox9 is required for Ngn3 induction, endocrine differentiation necessitates subsequent Sox9 downregulation and evasion from Notch activity via cell-autonomous repression of Sox9 by Ngn3. If high Notch levels are maintained, endocrine progenitors retain Sox9 and undergo ductal fate conversion. Taken together, our findings establish a novel role for Notch in initiating both ductal and endocrine development and reveal that Notch does not function in an on-off mode, but that a gradient of Notch activity produces distinct cellular states during pancreas development.

Conditional gene expression in the mouse inner ear using Cre-loxP

In recent years, there has been significant progress in the use of Cre-loxP technology for conditional gene expression in the inner ear. Here, we introduce the basic concepts of this powerful technology, emphasizing the differences between Cre and CreER. We describe the creation and Cre expression pattern of each Cre and CreER mouse line that has been reported to have expression in auditory and vestibular organs. We compare the Cre expression patterns between Atoh1-CreER(TM) and Atoh1-CreER(T2) and report a new line, Fgfr3-iCreER(T2), which displays inducible Cre activity in cochlear supporting cells. We also explain how results can vary when transgenic vs. knock-in Cre/CreER alleles are used to alter gene expression. We discuss practical issues that arise when using the Cre-loxP system, such as the use of proper controls, Cre efficiency, reporter expression efficiency, and Cre leakiness. Finally, we introduce other methods for conditional gene expression, including Flp recombinase and the tetracycline-inducible system, which can be combined with Cre-loxP mouse models to investigate conditional expression of more than one gene.

Different levels of Notch signaling regulate quiescence, renewal and differentiation in pancreatic endocrine progenitors

Genetic studies have implicated Notch signaling in the maintenance of pancreatic progenitors. However, how Notch signaling regulates the quiescent, proliferative or differentiation behaviors of pancreatic progenitors at the single-cell level remains unclear. Here, using single-cell genetic analyses and a new transgenic system that allows dynamic assessment of Notch signaling, we address how discrete levels of Notch signaling regulate the behavior of endocrine progenitors in the zebrafish intrapancreatic duct. We find that these progenitors experience different levels of Notch signaling, which in turn regulate distinct cellular outcomes. High levels of Notch signaling induce quiescence, whereas lower levels promote progenitor amplification. The sustained downregulation of Notch signaling triggers a multistep process that includes cell cycle entry and progenitor amplification prior to endocrine differentiation. Importantly, progenitor amplification and differentiation can be uncoupled by modulating the duration and/or extent of Notch signaling downregulation, indicating that these processes are triggered by distinct levels of Notch signaling. These data show that different levels of Notch signaling drive distinct behaviors in a progenitor population.

Mind bomb 1 is required for pancreatic β-cell formation

During early pancreatic development, Notch signaling represses differentiation of endocrine cells and promotes proliferation of Nkx6-1(+)Ptf1a(+) multipotent progenitor cells (MPCs). Later, antagonistic interactions between Nkx6 transcription factors and Ptf1a function to segregate MPCs into distal Nkx6-1(-)Ptf1a(+) acinar progenitors and proximal Nkx6-1(+)Ptf1a(-) duct and β-cell progenitors. Distal cells are initially multipotent, but evolve into unipotent, acinar cell progenitors. Conversely, proximal cells are bipotent and give rise to duct cells and late-born endocrine cells, including the insulin producing β-cells. However, signals that regulate proximodistal (P-D) patterning and thus formation of β-cell progenitors are unknown. Here we show that Mind bomb 1 (Mib1) is required for correct P-D patterning of the developing pancreas and β-cell formation. We found that endoderm-specific inactivation of Mib1 caused a loss of Nkx6-1(+)Ptf1a(-) and Hnf1β(+) cells and a corresponding loss of Neurog3(+) endocrine progenitors and β-cells. An accompanying increase in Nkx6-1(-)Ptf1a(+) and amylase(+) cells, occupying the proximal domain, suggests that proximal cells adopt a distal fate in the absence of Mib1 activity. Impeding Notch-mediated transcriptional activation by conditional expression of dominant negative Mastermind-like 1 (Maml1) resulted in a similarly distorted P-D patterning and suppressed β-cell formation, as did conditional inactivation of the Notch target gene Hes1. Our results reveal iterative use of Notch in pancreatic development to ensure correct P-D patterning and adequate β-cell formation.

Notch-mediated patterning and cell fate allocation of pancreatic progenitor cells

Early pancreatic morphogenesis is characterized by the transformation of an uncommitted pool of pancreatic progenitor cells into a branched pancreatic epithelium that consists of 'tip' and 'trunk' domains. These domains have distinct molecular signatures and differentiate into distinct pancreatic cell lineages. Cells at the branched tips of the epithelium develop into acinar cells, whereas cells in the trunk subcompartment differentiate into endocrine and duct cells. Recent genetic analyses have highlighted the role of key transcriptional regulators in the specification of these subcompartments. Here, we analyzed in mice the role of Notch signaling in the patterning of multipotent pancreatic progenitor cells through mosaic overexpression of a Notch signaling antagonist, dominant-negative mastermind-like 1, resulting in a mixture of wild-type and Notch-suppressed pancreatic progenitor cells. We find that attenuation of Notch signaling has pronounced patterning effects on multipotent pancreatic progenitor cells prior to terminal differentiation. Relative to the wild-type cells, the Notch-suppressed cells lose trunk marker genes and gain expression of tip marker genes. The Notch-suppressed cells subsequently differentiate into acinar cells, whereas duct and endocrine populations are formed predominantly from the wild-type cells. Mechanistically, these observations could be explained by a requirement of Notch for the expression of the trunk determination gene Nkx6.1. This was supported by the finding of direct binding of RBP-jκ to the Nkx6.1 proximal promoter.

Tamoxifen-Induced Cre-loxP Recombination Is Prolonged in Pancreatic Islets of Adult Mice

Tamoxifen (Tm)-inducible Cre recombinases are widely used to perform gene inactivation and lineage tracing studies in mice. Although the efficiency of inducible Cre-loxP recombination can be easily evaluated with reporter strains, the precise length of time that Tm induces nuclear translocation of CreER^Tm and subsequent recombination of a target allele is not well defined, and difficult to assess. To better understand the timeline of Tm activity in vivo, we developed a bioassay in which pancreatic islets with a Tm-inducible reporter (from Pdx1^PB-CreER^Tm;R26R^lacZ mice) were transplanted beneath the renal capsule of adult mice previously treated with three doses of 1 mg Tm, 8 mg Tm, or corn oil vehicle. Surprisingly, recombination in islet grafts, as assessed by expression of the β-galactosidase (β-gal) reporter, was observed days or weeks after Tm treatment, in a dose-dependent manner. Substantial recombination occurred in islet grafts long after administration of 3×8 mg Tm: in grafts transplanted 48 hours after the last Tm injection, 77.9±0.4% of β-cells were β-gal+; in β-cells placed after 1 week, 46.2±5.0% were β-gal+; after 2 weeks, 26.3±7.0% were β-gal+; and after 4 weeks, 1.9±0.9% were β-gal+. Islet grafts from mice given 3×1 mg Tm showed lower, but notable, recombination 48 hours (4.9±1.7%) and 1 week (4.5±1.9%) after Tm administration. These results show that Tm doses commonly used to induce Cre-loxP recombination may continue to label significant numbers of cells for weeks after Tm treatment, possibly confounding the interpretation of time-sensitive studies using Tm-dependent models. Therefore, investigators developing experimental approaches using Tm-inducible systems should consider both maximal recombination efficiency and the length of time that Tm-induced Cre-loxP recombination occurs.