Generation of human islet cell type-specific identity genesets.

Generation of surrogate cells with stable functional identities is crucial for developing cell-based therapies. Efforts to produce insulin-secreting replacement cells to treat diabetes require reliable tools to assess islet cellular identity. Here, we conduct a thorough single-cell transcriptomics meta-analysis to identify robustly expressed markers used to build genesets describing the identity of human α-, ß-, γ- and δ-cells. These genesets define islet cellular identities better than previously published genesets. We show their efficacy to outline cell identity changes and unravel some of their underlying genetic mechanisms, whether during embryonic pancreas development or in experimental setups aiming at developing glucose-responsive insulin-secreting cells, such as pluripotent stem-cell differentiation or in adult islet cell reprogramming protocols. These islet cell type-specific genesets represent valuable tools that accurately benchmark gain and loss in islet cell identity traits.

Adult pancreatic islet endocrine cells emerge as fetal hormone-expressing cells.

The precise developmental dynamics of the pancreatic islet endocrine cell types, and their interrelation, are unknown. Some authors claim the persistence of islet cell differentiation from precursor cells after birth ("neogenesis"). Here, using four conditional cell lineage tracing ("pulse-and-chase") murine models, we describe the natural history of pancreatic islet cells, once they express a hormone gene, until late in life. Concerning the contribution of early-appearing embryonic hormone-expressing cells to the formation of islets, we report that adult islet cells emerge from embryonic hormone-expressing cells arising at different time points during development, without any evidence of postnatal neogenesis. We observe specific patterns of hormone gene activation and switching during islet morphogenesis, revealing that, within each cell type, cells have heterogeneous developmental trajectories. This likely applies to most maturating cells in the body, and explains the observed phenotypic variability within differentiated cell types. Such knowledge should help devising novel regenerative therapies.

Mouse models characterize GNAO1 encephalopathy as a neurodevelopmental disorder leading to motor anomalies: from a severe G203R to a milder C215Y mutation.

GNAO1 encephalopathy characterized by a wide spectrum of neurological deficiencies in pediatric patients originates from de novo heterozygous mutations in the gene encoding Gαo, the major neuronal G protein. Efficient treatments and even the proper understanding of the underlying etiology are currently lacking for this dominant disease. Adequate animal models of GNAO1 encephalopathy are urgently needed. Here we describe establishment and characterization of mouse models of the disease based on two point mutations in GNAO1 with different clinical manifestations. One of them is G203R leading to the early-onset epileptic seizures, motor dysfunction, developmental delay and intellectual disability. The other is C215Y producing much milder clinical outcomes, mostly-late-onset hyperkinetic movement disorder. The resultant mouse models show distinct phenotypes: severe neonatal lethality in GNAO1[G203R]/ + mice vs. normal vitality in GNAO1[C215Y]/ + . The latter model further revealed strong hyperactivity and hyperlocomotion in a panel of behavioral assays, without signs of epilepsy, recapitulating the patients' manifestations. Importantly, despite these differences the two models similarly revealed prenatal brain developmental anomalies, such as enlarged lateral ventricles and decreased numbers of neuronal precursor cells in the cortex. Thus, our work unveils GNAO1 encephalopathy as to a large extent neurodevelopmental malady. We expect that this understanding and the tools we established will be instrumental for future therapeutic developments.

Cell-specific alterations in Pitx1 regulatory landscape activation caused by the loss of a single enhancer.

Developmental genes are frequently controlled by multiple enhancers sharing similar specificities. As a result, deletions of such regulatory elements have often failed to reveal their full function. Here, we use the Pitx1 testbed locus to characterize in detail the regulatory and cellular identity alterations following the deletion of one of its enhancers (Pen). By combining single cell transcriptomics and an in-embryo cell tracing approach, we observe an increased fraction of Pitx1 non/low-expressing cells and a decreased fraction of Pitx1 high-expressing cells. We find that the over-representation of Pitx1 non/low-expressing cells originates from a failure of the Pitx1 locus to coordinate enhancer activities and 3D chromatin changes. This locus mis-activation induces a localized heterochrony and a concurrent loss of irregular connective tissue, eventually leading to a clubfoot phenotype. This data suggests that, in some cases, redundant enhancers may be used to locally enforce a robust activation of their host regulatory landscapes.

Pancreatic Ppy-expressing γ-cells display mixed phenotypic traits and the adaptive plasticity to engage insulin production.

The cellular identity of pancreatic polypeptide (Ppy)-expressing γ-cells, one of the rarest pancreatic islet cell-type, remains elusive. Within islets, glucagon and somatostatin, released respectively from α- and δ-cells, modulate the secretion of insulin by ß-cells. Dysregulation of insulin production raises blood glucose levels, leading to diabetes onset. Here, we present the genetic signature of human and mouse γ-cells. Using different approaches, we identified a set of genes and pathways defining their functional identity. We found that the γ-cell population is heterogeneous, with subsets of cells producing another hormone in addition to Ppy. These bihormonal cells share identity markers typical of the other islet cell-types. In mice, Ppy gene inactivation or conditional γ-cell ablation did not alter glycemia nor body weight. Interestingly, upon ß-cell injury induction, γ-cells exhibited gene expression changes and some of them engaged insulin production, like α- and δ-cells. In conclusion, we provide a comprehensive characterization of γ-cells and highlight their plasticity and therapeutic potential.

A Map of Human Type 1 Diabetes Progression by Imaging Mass Cytometry.

Type 1 diabetes (T1D) results from the autoimmune destruction of insulin-producing ß cells. A comprehensive picture of the changes during T1D development is lacking due to limited sample availability, inability to sample longitudinally, and the paucity of technologies enabling comprehensive tissue profiling. Here, we analyzed 1,581 islets from 12 human donors, including eight with T1D, using imaging mass cytometry (IMC). IMC enabled simultaneous measurement of 35 biomarkers with single-cell and spatial resolution. We performed pseudotime analysis of islets through T1D progression from snapshot data to reconstruct the evolution of ß cell loss and insulitis. Our analyses revealed that ß cell destruction is preceded by a ß cell marker loss and by recruitment of cytotoxic and helper T cells. The approaches described herein demonstrate the value of IMC for improving our understanding of T1D pathogenesis, and our data lay the foundation for hypothesis generation and follow-on experiments.

Diabetes relief in mice by glucose-sensing insulin-secreting human α-cells.

Cell-identity switches, in which terminally differentiated cells are converted into different cell types when stressed, represent a widespread regenerative strategy in animals, yet they are poorly documented in mammals. In mice, some glucagon-producing pancreatic α-cells and somatostatin-producing δ-cells become insulin-expressing cells after the ablation of insulin-secreting ß-cells, thus promoting diabetes recovery. Whether human islets also display this plasticity, especially in diabetic conditions, remains unknown. Here we show that islet non-ß-cells, namely α-cells and pancreatic polypeptide (PPY)-producing γ-cells, obtained from deceased non-diabetic or diabetic human donors, can be lineage-traced and reprogrammed by the transcription factors PDX1 and MAFA to produce and secrete insulin in response to glucose. When transplanted into diabetic mice, converted human α-cells reverse diabetes and continue to produce insulin even after six months. Notably, insulin-producing α-cells maintain expression of α-cell markers, as seen by deep transcriptomic and proteomic characterization. These observations provide conceptual evidence and a molecular framework for a mechanistic understanding of in situ cell plasticity as a treatment for diabetes and other degenerative diseases.

Pancreatic islet-autonomous insulin and smoothened-mediated signalling modulate identity changes of glucagon+ α-cells.

The mechanisms that restrict regeneration and maintain cell identity following injury are poorly characterized in higher vertebrates. Following ß-cell loss, 1-2% of the glucagon-producing α-cells spontaneously engage in insulin production in mice. Here we explore the mechanisms inhibiting α-cell plasticity. We show that adaptive α-cell identity changes are constrained by intra-islet insulin- and Smoothened-mediated signalling, among others. The combination of ß-cell loss or insulin-signalling inhibition, with Smoothened inactivation in α- or δ-cells, stimulates insulin production in more α-cells. These findings suggest that the removal of constitutive 'brake signals' is crucial to neutralize the refractoriness to adaptive cell-fate changes. It appears that the maintenance of cell identity is an active process mediated by repressive signals, which are released by neighbouring cells and curb an intrinsic trend of differentiated cells to change.

Pancreatic α Cell-Derived Glucagon-Related Peptides Are Required for ß Cell Adaptation and Glucose Homeostasis.

Pancreatic α cells may process proglucagon not only to glucagon but also to glucagon-like peptide-1 (GLP-1). However, the biological relevance of paracrine GLP-1 for ß cell function remains unclear. We studied effects of locally derived insulin secretagogues on ß cell function and glucose homeostasis using mice with α cell ablation and with α cell-specific GLP-1 deficiency. Normally, intestinal GLP-1 compensates for the lack of α cell-derived GLP-1. However, upon aging and metabolic stress, glucose tolerance is impaired. This was partly rescued with the DPP-4 inhibitor sitagliptin, but not with glucagon administration. In isolated islets from these mice, glucose-stimulated insulin secretion was heavily impaired and exogenous GLP-1 or glucagon rescued insulin secretion. These data highlight the importance of α cell-derived GLP-1 for glucose homeostasis during metabolic stress and may impact on the clinical use of systemic GLP-1 agonists versus stabilizing local α cell-derived GLP-1 by DPP-4 inhibitors in type 2 diabetes.

Converting Adult Pancreatic Islet α Cells into ß Cells by Targeting Both Dnmt1 and Arx.

Insulin-producing pancreatic ß cells in mice can slowly regenerate from glucagon-producing α cells in settings like ß cell loss, but the basis of this conversion is unknown. Moreover, it remains unclear if this intra-islet cell conversion is relevant to diseases like type 1 diabetes (T1D). We show that the α cell regulators Aristaless-related homeobox (Arx) and DNA methyltransferase 1 (Dnmt1) maintain α cell identity in mice. Within 3 months of Dnmt1 and Arx loss, lineage tracing and single-cell RNA sequencing revealed extensive α cell conversion into progeny resembling native ß cells. Physiological studies demonstrated that converted α cells acquire hallmark ß cell electrophysiology and show glucose-stimulated insulin secretion. In T1D patients, subsets of glucagon-expressing cells show loss of DNMT1 and ARX and produce insulin and other ß cell factors, suggesting that DNMT1 and ARX maintain α cell identity in humans. Our work reveals pathways regulated by Arx and Dnmt1 that are sufficient for achieving targeted generation of ß cells from adult pancreatic α cells.

Dnmt1 activity is dispensable in δ-cells but is essential for α-cell homeostasis.

In addition to ß-cells, pancreatic islets contain α- and δ-cells, which respectively produce glucagon and somatostatin. The reprogramming of these two endocrine cell types into insulin producers, as observed after a massive ß-cell ablation in mice, may help restoring a functional ß-cell mass in type 1 diabetes. Yet, the spontaneous α-to-ß and δ-to-ß conversion processes are relatively inefficient in adult animals and the underlying epigenetic mechanisms remain unclear. Several studies indicate that the conserved chromatin modifiers DNA methyltransferase 1 (Dnmt1) and Enhancer of zeste homolog 2 (Ezh2) are important for pancreas development and restrict islet cell plasticity. Here, to investigate the role of these two enzymes in α- and δ-cell development and fate maintenance, we genetically inactivated them in each of these two cell types. We found that loss of Dnmt1 does not enhance the conversion of α- or δ-cells toward a ß-like fate. In addition, while Dnmt1 was dispensable for the development of these two cell types, we noticed a gradual loss of α-, but not δ-cells in adult mice. Finally, we found that Ezh2 inactivation does not enhance α-cell plasticity, and, contrary to what is observed in ß-cells, does not impair α-cell proliferation. Our results indicate that both Dnmt1 and Ezh2 play distinct roles in the different islet cell types.

Blockade of glucagon signaling prevents or reverses diabetes onset only if residual ß-cells persist.

Glucagon secretion dysregulation in diabetes fosters hyperglycemia. Recent studies report that mice lacking glucagon receptor (Gcgr(-/-)) do not develop diabetes following streptozotocin (STZ)-mediated ablation of insulin-producing ß-cells. Here, we show that diabetes prevention in STZ-treated Gcgr(-/-) animals requires remnant insulin action originating from spared residual ß-cells: these mice indeed became hyperglycemic after insulin receptor blockade. Accordingly, Gcgr(-/-) mice developed hyperglycemia after induction of a more complete, diphtheria toxin (DT)-induced ß-cell loss, a situation of near-absolute insulin deficiency similar to type 1 diabetes. In addition, glucagon deficiency did not impair the natural capacity of α-cells to reprogram into insulin production after extreme ß-cell loss. α-to-ß-cell conversion was improved in Gcgr(-/-) mice as a consequence of α-cell hyperplasia. Collectively, these results indicate that glucagon antagonism could i) be a useful adjuvant therapy in diabetes only when residual insulin action persists, and ii) help devising future ß-cell regeneration therapies relying upon α-cell reprogramming.

A Variant of GJD2, Encoding for Connexin 36, Alters the Function of Insulin Producing ß-Cells.

Signalling through gap junctions contributes to control insulin secretion and, thus, blood glucose levels. Gap junctions of the insulin-producing ß-cells are made of connexin 36 (Cx36), which is encoded by the GJD2 gene. Cx36-null mice feature alterations mimicking those observed in type 2 diabetes (T2D). GJD2 is also expressed in neurons, which share a number of common features with pancreatic ß-cells. Given that a synonymous exonic single nucleotide polymorphism of human Cx36 (SNP rs3743123) associates with altered function of central neurons in a subset of epileptic patients, we investigated whether this SNP also caused alterations of ß-cell function. Transfection of rs3743123 cDNA in connexin-lacking HeLa cells resulted in altered formation of gap junction plaques and cell coupling, as compared to those induced by wild type (WT) GJD2 cDNA. Transgenic mice expressing the very same cDNAs under an insulin promoter revealed that SNP rs3743123 expression consistently lead to a post-natal reduction of islet Cx36 levels and ß-cell survival, resulting in hyperglycemia in selected lines. These changes were not observed in sex- and age-matched controls expressing WT hCx36. The variant GJD2 only marginally associated to heterogeneous populations of diabetic patients. The data document that a silent polymorphism of GJD2 is associated with altered ß-cell function, presumably contributing to T2D pathogenesis.

Connective tissue growth factor modulates adult ß-cell maturity and proliferation to promote ß-cell regeneration in mice.

Stimulation of endogenous ß-cell expansion could facilitate regeneration in patients with diabetes. In mice, connective tissue growth factor (CTGF) is expressed in embryonic ß-cells and in adult ß-cells during periods of expansion. We discovered that in embryos CTGF is necessary for ß-cell proliferation, and increased CTGF in ß-cells promotes proliferation of immature (MafA(-)) insulin-positive cells. CTGF overexpression, under nonstimulatory conditions, does not increase adult ß-cell proliferation. In this study, we tested the ability of CTGF to promote ß-cell proliferation and regeneration after partial ß-cell destruction. ß-Cell mass reaches 50% recovery after 4 weeks of CTGF treatment, primarily via increased ß-cell proliferation, which is enhanced as early as 2 days of treatment. CTGF treatment increases the number of immature ß-cells but promotes proliferation of both mature and immature ß-cells. A shortened ß-cell replication refractory period is also observed. CTGF treatment upregulates positive cell-cycle regulators and factors involved in ß-cell proliferation, including hepatocyte growth factor, serotonin synthesis, and integrin ß1. Ex vivo treatment of whole islets with recombinant human CTGF induces ß-cell replication and gene expression changes consistent with those observed in vivo, demonstrating that CTGF acts directly on islets to promote ß-cell replication. Thus, CTGF can induce replication of adult mouse ß-cells given a permissive microenvironment.

Targeting GLP-1 receptors for repeated magnetic resonance imaging differentiates graded losses of pancreatic beta cells in mice.

AIMS/HYPOTHESIS: Non-invasive imaging of beta cells is a much-needed development but is one that faces significant biological and technological hurdles. A relevant imaging method should at least allow for an evaluation over time of the mass of beta cells under physiological and pathological conditions, and for an assessment of novel therapies. We, therefore, investigated the ability of a new MRI probe to repeatedly measure the loss of beta cells in a rodent model. METHODS: We developed an innovative nanoparticle probe that targets the glucagon-like peptide 1 receptor, and can be used for both fluorescence imaging and MRI. Using fluorescence, we characterised the specificity and biodistribution of the probe. Using 1.5 T MRI, we longitudinally imaged the changes in insulin content in male and female mice of the RIP-DTr strain, which mimic the changes expected in type 1 and type 2 diabetes, respectively. RESULTS: We showed that this probe selectively labelled beta cells in situ, imaged in vivo native pancreatic islets and evaluated their loss after diphtheria toxin administration, in a model of graded beta cell deletion. Thus, using clinical MRI, the probe quantitatively differentiates, in the same mouse strain, between female animals featuring a 50% loss of beta cells and the males featuring an almost complete loss of beta cells. CONCLUSIONS/INTERPRETATION: The approach addresses several of the hurdles that have so far limited the non-invasive imaging of beta cells, including the potential to repeatedly monitor the very same animals using clinically available equipment, and to differentiate graded losses of beta cells.

Diabetes recovery by age-dependent conversion of pancreatic δ-cells into insulin producers.

Total or near-total loss of insulin-producing ß-cells occurs in type 1 diabetes. Restoration of insulin production in type 1 diabetes is thus a major medical challenge. We previously observed in mice in which ß-cells are completely ablated that the pancreas reconstitutes new insulin-producing cells in the absence of autoimmunity. The process involves the contribution of islet non-ß-cells; specifically, glucagon-producing α-cells begin producing insulin by a process of reprogramming (transdifferentiation) without proliferation. Here we show the influence of age on ß-cell reconstitution from heterologous islet cells after near-total ß-cell loss in mice. We found that senescence does not alter α-cell plasticity: α-cells can reprogram to produce insulin from puberty through to adulthood, and also in aged individuals, even a long time after ß-cell loss. In contrast, before puberty there is no detectable α-cell conversion, although ß-cell reconstitution after injury is more efficient, always leading to diabetes recovery. This process occurs through a newly discovered mechanism: the spontaneous en masse reprogramming of somatostatin-producing δ-cells. The juveniles display 'somatostatin-to-insulin' δ-cell conversion, involving dedifferentiation, proliferation and re-expression of islet developmental regulators. This juvenile adaptability relies, at least in part, upon the combined action of FoxO1 and downstream effectors. Restoration of insulin producing-cells from non-ß-cell origins is thus enabled throughout life via δ- or α-cell spontaneous reprogramming. A landscape with multiple intra-islet cell interconversion events is emerging, offering new perspectives for therapy.

Leptin engages a hypothalamic neurocircuitry to permit survival in the absence of insulin.

The dogma that life without insulin is incompatible has recently been challenged by results showing the viability of insulin-deficient rodents undergoing leptin monotherapy. Yet, the mechanisms underlying these actions of leptin are unknown. Here, the metabolic outcomes of intracerebroventricular (i.c.v.) administration of leptin in mice devoid of insulin and lacking or re-expressing leptin receptors (LEPRs) only in selected neuronal groups were assessed. Our results demonstrate that concomitant re-expression of LEPRs only in hypothalamic γ-aminobutyric acid (GABA) and pro-opiomelanocortin (POMC) neurons is sufficient to fully mediate the lifesaving and antidiabetic actions of leptin in insulin deficiency. Our analyses indicate that enhanced glucose uptake by brown adipose tissue and soleus muscle, as well as improved hepatic metabolism, underlies these effects of leptin. Collectively, our data elucidate a hypothalamic-dependent pathway enabling life without insulin and hence pave the way for developing better treatments for diseases of insulin deficiency.

Loss of prohibitin induces mitochondrial damages altering ß-cell function and survival and is responsible for gradual diabetes development.

Prohibitins are highly conserved proteins mainly implicated in the maintenance of mitochondrial function and architecture. Their dysfunctions are associated with aging, cancer, obesity, and inflammation. However, their possible role in pancreatic ß-cells remains unknown. The current study documents the expression of prohibitins in human and rodent islets and their key role for ß-cell function and survival. Ablation of Phb2 in mouse ß-cells sequentially resulted in impairment of mitochondrial function and insulin secretion, loss of ß-cells, progressive alteration of glucose homeostasis, and, ultimately, severe diabetes. Remarkably, these events progressed over a 3-week period of time after weaning. Defective insulin supply in ß-Phb2(-/-) mice was contributed by both ß-cell dysfunction and apoptosis, temporarily compensated by increased ß-cell proliferation. At the molecular level, we observed that deletion of Phb2 caused mitochondrial abnormalities, including reduction of mitochondrial DNA copy number and respiratory chain complex IV levels, altered mitochondrial activity, cleavage of L-optic atrophy 1, and mitochondrial fragmentation. Overall, our data demonstrate that Phb2 is essential for metabolic activation of mitochondria and, as a consequence, for function and survival of ß-cells.