Long-Term GABA Administration Induces Alpha Cell-Mediated Beta-like Cell Neogenesis.

The recent discovery that genetically modified α cells can regenerate and convert into ß-like cells in vivo holds great promise for diabetes research. However, to eventually translate these findings to human, it is crucial to discover compounds with similar activities. Herein, we report the identification of GABA as an inducer of α-to-ß-like cell conversion in vivo. This conversion induces α cell replacement mechanisms through the mobilization of duct-lining precursor cells that adopt an α cell identity prior to being converted into ß-like cells, solely upon sustained GABA exposure. Importantly, these neo-generated ß-like cells are functional and can repeatedly reverse chemically induced diabetes in vivo. Similarly, the treatment of transplanted human islets with GABA results in a loss of α cells and a concomitant increase in ß-like cell counts, suggestive of α-to-ß-like cell conversion processes also in humans. This newly discovered GABA-induced α cell-mediated ß-like cell neogenesis could therefore represent an unprecedented hope toward improved therapies for diabetes.

Pax4 acts as a key player in pancreas development and plasticity.

The embryonic development of the pancreas is orchestrated by a complex and coordinated transcription factor network. Neurogenin3 (Neurog3) initiates the endocrine program by activating the expression of additional transcription factors driving survival, proliferation, maturation and lineage allocation of endocrine precursors. Among the direct targets of Neurog3, Pax4 appears as one of the key regulators of ß-cell specification. Indeed, mice lacking Pax4 die a few days postpartum, as they develop severe hyperglycemia due to the absence of mature pancreatic ß-cells. Pax4 also directly regulates the expression of Arx, a gene that plays a crucial role in α-cell specification. Comparative analysis of Pax4 and Arx mutants, as well as Arx/Pax4 double mutants, showed that islet subtype destiny is mainly directed by cross-repression of the Pax4 and Arx factors. Importantly, the ectopic expression of Pax4 in α-cells was found sufficient to induce their neogenesis and conversion into ß-like cells, not only during development but also in adult rodents. Therefore, differentiated endocrine α-cells can be considered as a putative source for insulin-producing ß-like cells. These findings have clearly widened our understanding regarding pancreatic development, but they also open new research avenues in the context of diabetes research.

The inactivation of Arx in pancreatic α-cells triggers their neogenesis and conversion into functional ß-like cells.

Recently, it was demonstrated that pancreatic new-born glucagon-producing cells can regenerate and convert into insulin-producing ß-like cells through the ectopic expression of a single gene, Pax4. Here, combining conditional loss-of-function and lineage tracing approaches, we show that the selective inhibition of the Arx gene in α-cells is sufficient to promote the conversion of adult α-cells into ß-like cells at any age. Interestingly, this conversion induces the continuous mobilization of duct-lining precursor cells to adopt an endocrine cell fate, the glucagon(+) cells thereby generated being subsequently converted into ß-like cells upon Arx inhibition. Of interest, through the generation and analysis of Arx and Pax4 conditional double-mutants, we provide evidence that Pax4 is dispensable for these regeneration processes, indicating that Arx represents the main trigger of α-cell-mediated ß-like cell neogenesis. Importantly, the loss of Arx in α-cells is sufficient to regenerate a functional ß-cell mass and thereby reverse diabetes following toxin-induced ß-cell depletion. Our data therefore suggest that strategies aiming at inhibiting the expression of Arx, or its molecular targets/co-factors, may pave new avenues for the treatment of diabetes.

[Reprogramming pancreatic cells to ß cells]. / Reprogrammation des cellules pancréatiques en cellules ß

Type 1 diabetes (T1DM) is a common metabolic disorder affecting an ever-increasing number of patients worldwide. T1DM is caused by the selective destruction of pancreatic insulin-producing ß-cells by the immune system. Such loss results in chronic hyperglycemia and could induce a number of cardio-vascular complications. Despite the classical insulin-based therapy, compared to healthy people, patients with T1DM display a shortened life expectancy due to the treatment's inability to strictly regulate glycemic levels. An alternative therapy involves pancreatic islet transplantation but the shortage of donors and the required immuno-suppressive treatments limit the widespread use of this approach. Therefore, the search of new approaches to generate functional ß-cells is of growing interest. In this review, we describe several novel strategies aiming at the conversion of diverse pancreatic cells into ß-cells, such as acinar, ductal, and endocrine cells. Clearly, such promising results could open new research avenues in the context of type 1 diabetes research.

Conversion of Gastrointestinal Somatostatin-Expressing D Cells Into Insulin-Producing Beta-Like Cells Upon Pax4 Misexpression.

Type 1 diabetes results from the autoimmune-mediated loss of insulin-producing beta-cells. Accordingly, important research efforts aim at regenerating these lost beta-cells by converting pre-existing endogenous cells. Following up on previous results demonstrating the conversion of pancreatic somatostatin delta-cells into beta-like cells upon Pax4 misexpression and acknowledging that somatostatin-expressing cells are highly represented in the gastrointestinal tract, one could wonder whether this Pax4-mediated conversion could also occur in the GI tract. We made use of transgenic mice misexpressing Pax4 in somatostatin cells (SSTCrePOE) to evaluate a putative Pax4-mediated D-to-beta-like cell conversion. Additionally, we implemented an ex vivo approach based on mice-derived gut organoids to assess the functionality of these neo-generated beta-like cells. Our results outlined the presence of insulin+ cells expressing several beta-cell markers in gastrointestinal tissues of SSTCrePOE animals. Further, using lineage tracing, we established that these cells arose from D cells. Lastly, functional tests on mice-derived gut organoids established the ability of neo-generated beta-like cells to release insulin upon stimulation. From this study, we conclude that the misexpression of Pax4 in D cells appears sufficient to convert these into functional beta-like cells, thus opening new research avenues in the context of diabetes research.

Neurog3 misexpression unravels mouse pancreatic ductal cell plasticity.

In the context of type 1 diabetes research and the development of insulin-producing ß-cell replacement strategies, whether pancreatic ductal cells retain their developmental capability to adopt an endocrine cell identity remains debated, most likely due to the diversity of models employed to induce pancreatic regeneration. In this work, rather than injuring the pancreas, we developed a mouse model allowing the inducible misexpression of the proendocrine gene Neurog3 in ductal cells in vivo. These animals developed a progressive islet hypertrophy attributed to a proportional increase in all endocrine cell populations. Lineage tracing experiments indicated a continuous neo-generation of endocrine cells exhibiting a ductal ontogeny. Interestingly, the resulting supplementary ß-like cells were found to be functional. Based on these findings, we suggest that ductal cells could represent a renewable source of new ß-like cells and that strategies aiming at controlling the expression of Neurog3, or of its molecular targets/co-factors, may pave new avenues for the improved treatments of diabetes.

ß-Cell Replacement Strategies: The Increasing Need for a "ß-Cell Dogma".

Type 1 diabetes is an auto-immune disease resulting in the loss of pancreatic ß-cells and, consequently, in chronic hyperglycemia. Insulin supplementation allows diabetic patients to control their glycaemia quite efficiently, but treated patients still display an overall shortened life expectancy and an altered quality of life as compared to their healthy counterparts. In this context and due to the ever increasing number of diabetics, establishing alternative therapies has become a crucial research goal. Most current efforts therefore aim at generating fully functional insulin-secreting ß-like cells using multiple approaches. In this review, we screened the literature published since 2011 and inventoried the selected markers used to characterize insulin-secreting cells generated by in vitro differentiation of stem/precursor cells or by means of in vivo transdifferentiation. By listing these features, we noted important discrepancies when comparing the different approaches for the initial characterization of insulin-producing cells as true ß-cells. Considering the recent advances achieved in this field of research, the necessity to establish strict guidelines has become a subject of crucial importance, especially should one contemplate the next step, which is the transplantation of in vitro or ex vivo generated insulin-secreting cells in type 1 diabetic patients.

GABA signaling stimulates α-cell-mediated ß-like cell neogenesis.

Diabetes is a chronic and progressing disease, the number of patients increasing exponentially, especially in industrialized countries. Regenerating lost insulin-producing cells would represent a promising therapeutic alternative for most diabetic patients. To this end, using the mouse as a model, we reported that GABA, a food supplement, could induce insulin-producing beta-like cell neogenesis offering an attractive and innovative approach for diabetes therapeutics.

Implication of REDD1 in the activation of inflammatory pathways.

In response to endotoxemia, the organism triggers an inflammatory response, and the visceral adipose tissue represents a major source of proinflammatory cytokines. The regulation of inflammation response in the adipose tissue is thus of crucial importance. We demonstrated that Regulated in development and DNA damage response-1 (REDD1) is involved in inflammation. REDD1 expression was increased in response to lipopolysaccharide (LPS) in bone marrow derived macrophages (BMDM) and in epidydimal adipose tissue. Loss of REDD1 protected the development of inflammation, since the expression of proinflammatory cytokines (TNFα, IL-6, IL-1ß) was decreased in adipose tissue of REDD1-/- mice injected with LPS compared to wild-type mice. This decrease was associated with an inhibition of the activation of p38MAPK, JNK, NF-κB and NLRP3 inflammasome leading to a reduction of IL-1ß secretion in response to LPS and ATP in REDD1-/- BMDM. Although REDD1 is an inhibitor of mTORC1, loss of REDD1 decreased inflammation independently of mTORC1 activation but more likely through oxidative stress regulation. Absence of REDD1 decreases ROS associated with a dysregulation of Nox-1 and GPx3 expression. Absence of REDD1 in macrophages decreases the development of insulin resistance in adipocyte-macrophage coculture. Altogether, REDD1 appears to be a key player in the control of inflammation.

Ectopic expression of Pax4 in pancreatic δ cells results in ß-like cell neogenesis.

The recent demonstration that pancreatic α cells can be continuously regenerated and converted into ß-like cells upon ectopic expression of Pax4 opened new avenues of research in the endocrine cell differentiation and diabetes fields. To determine whether such plasticity was also shared by δ cells, we generated and characterized transgenic animals that express Pax4 specifically in somatostatin-expressing cells. We demonstrate that the ectopic expression of Pax4 in δ cells is sufficient to induce their conversion into functional ß-like cells. Importantly, this conversion induces compensatory mechanisms involving the reactivation of endocrine developmental processes that result in dramatic ß-like cell hyperplasia. Importantly, these ß-like cells are functional and can partly reverse the consequences of chemically induced diabetes.

From pancreas morphogenesis to ß-cell regeneration.

Type 1 diabetes is a metabolic disease resulting in the selective loss of pancreatic insulin-producing ß-cells and affecting millions of people worldwide. The side effects of diabetes are varied and include cardiovascular, neuropathologic, and kidney diseases. Despite the most recent advances in diabetes care, patients suffering from type 1 diabetes still display a shortened life expectancy compared to their healthy counterparts. In an effort to improve ß-cell-replacement therapies, numerous approaches are currently being pursued, most of these aiming at finding ways to differentiate stem/progenitor cells into ß-like cells by mimicking embryonic development. Unfortunately, these efforts have hitherto not allowed the generation of fully functional ß-cells. This chapter summarizes recent findings, allowing a better insight into the molecular mechanisms underlying the genesis of ß-cells during the course of pancreatic morphogenesis. Furthermore, a focus is made on new research avenues concerning the conversion of pre-existing pancreatic cells into ß-like cells, such approaches holding great promise for the development of type 1 diabetes therapies.

From pancreatic islet formation to beta-cell regeneration.

Diabetes mellitus represents a major healthcare burden and, due to the increasing prevalence of type I diabetes and the complications arising from current treatments, other alternative therapies must be found. Type I diabetes arises as a result of a cell-mediated autoimmune destruction of insulin producing pancreatic ß-cells. Thus, a cell replacement therapy would be appropriate, using either in vitro or in vivo cell differentiation/reprogramming from different cell sources. Increasing our understanding of the molecular mechanisms controlling endocrine cell specification during pancreas morphogenesis and gaining further insight into the complex transcriptional network and signaling pathways governing ß-cell development should facilitate efforts to achieve this ultimate goal, that is to regenerate insulin-producing ß-cells. This review will therefore describe briefly the genetic program underlying mouse pancreas development and present new insights regarding ß-cell regeneration.

Adult duct-lining cells can reprogram into ß-like cells able to counter repeated cycles of toxin-induced diabetes.

It was recently demonstrated that embryonic glucagon-producing cells in the pancreas can regenerate and convert into insulin-producing ß-like cells through the constitutive/ectopic expression of the Pax4 gene. However, whether α cells in adult mice display the same plasticity is unknown. Similarly, the mechanisms underlying such reprogramming remain unclear. We now demonstrate that the misexpression of Pax4 in glucagon(+) cells age-independently induces their conversion into ß-like cells and their glucagon shortage-mediated replacement, resulting in islet hypertrophy and in an unexpected islet neogenesis. Combining several lineage-tracing approaches, we show that, upon Pax4-mediated α-to-ß-like cell conversion, pancreatic duct-lining precursor cells are continuously mobilized, re-express the developmental gene Ngn3, and successively adopt a glucagon(+) and a ß-like cell identity through a mechanism involving the reawakening of the epithelial-to-mesenchymal transition. Importantly, these processes can repeatedly regenerate the whole ß cell mass and thereby reverse several rounds of toxin-induced diabetes, providing perspectives to design therapeutic regenerative strategies.