Physiology: Pancreatic β-cell heterogeneity revisited

Development, regeneration, and physiological expansion of functional β-cells: Cellular sources and regulators

Pancreatic regeneration is a complex process observed in both normal and pathological conditions. The aim of this review is to provide a comprehensive understanding of the emergence of a functionally active population of insulin-secreting β-cells in the adult pancreas. The renewal of β-cells is governed by a multifaceted interaction between cellular sources of genetic and epigenetic factors. Understanding the development and heterogeneity of β-cell populations is crucial for functional β-cell regeneration. The functional mass of pancreatic β-cells increases in situations such as pregnancy and obesity. However, the specific markers of mature β-cell populations and postnatal pancreatic progenitors capable of increasing self-reproduction in these conditions remain to be elucidated. The capacity to regenerate the β-cell population through various pathways, including the proliferation of pre-existing β-cells, β-cell neogenesis, differentiation of β-cells from a population of progenitor cells, and transdifferentiation of non-β-cells into β-cells, reveals crucial molecular mechanisms for identifying cellular sources and inducers of functional cell renewal. This provides an opportunity to identify specific cellular sources and mechanisms of regeneration, which could have clinical applications in treating various pathologies, including in vitro cell-based technologies, and deepen our understanding of regeneration in different physiological conditions.

Identification of genomic regions and genes associated with subclinical ketosis in periparturient dairy cows

Subclinical ketosis (SCK) is a prevalent metabolic disorder that occurs during the transition to lactation period. It is defined as a high blood concentration of ketone bodies (beta-hydroxybutyric acid f ≥ 1.2 mmol/L) within the first few weeks of lactation, and often presents without clinical signs. SCK is mainly caused by negative energy balance (NEB). The objective of this study is to identify single nucleotide polymorphisms (SNPs) associated with SCK using genome-wide association studies (GWAS), and to predict the biological functions of proximal genes using gene-set enrichment analysis (GSEA). Blood samples were collected from 112 Holstein cows between 5 and 18 days postpartum to determine the incidence of SCK. Genomic DNA extracted from both SCK and healthy cows was examined using the Illumina Bovine SNP50K BeadChip for genotyping. GWAS revealed 194 putative SNPs and 163 genes associated with those SNPs. Additionally, GSEA showed that the genes retrieved by Database for Annotation, Visualization, and Integrated Discovery (DAVID) belonged to calcium signaling, starch and sucrose, immune network, and metabolic pathways. Furthermore, the proximal genes were found to be related to germ cell and early embryo development. In summary, this study proposes several feasible SNPs and genes associated with SCK through GWAS and GSEA. These candidates can be utilized in selective breeding programs to reduce the genetic risk for SCK and subfertility in high-performance dairy cows.

Endocrine islet β-cell subtypes with differential function are derived from biochemically distinct embryonic endocrine islet progenitors that are regulated by maternal nutrients

Endocrine islet b cells comprise heterogenous cell subsets. Yet when/how these subsets are produced and how stable they are remain unknown. Addressing these questions is important for preventing/curing diabetes, because lower numbers of b cells with better secretory function is a high risk of this disease. Using combinatorial cell lineage tracing, scRNA-seq, and DNA methylation analysis, we show here that embryonic islet progenitors with distinct gene expression and DNA methylation produce b-cell subtypes of different function and viability in adult mice. The subtype with better function is enriched for genes involved in vesicular production/trafficking, stress response, and Ca2+-secretion coupling, which further correspond to differential DNA methylation in putative enhancers of these genes. Maternal overnutrition, a major diabetes risk factor, reduces the proportion of endocrine progenitors of the b-cell subtype with better-function via deregulating DNA methyl transferase 3a. Intriguingly, the gene signature that defines mouse b-cell subtypes can reliably divide human cells into two sub-populations while the proportion of b cells with better-function is reduced in diabetic donors. The implication of these results is that modulating DNA methylation in islet progenitors using maternal food supplements can be explored to improve b-cell function in the prevention and therapy of diabetes.

The postnatal pancreatic microenvironment guides β cell maturation through BMP4 production

Glucose homeostasis depends on regulated insulin secretion from pancreatic β cells, which acquire their mature phenotype postnatally. The functional maturation of β cells is regulated by a combination of cell-autonomous and exogenous factors; the identity of the latter is mostly unknown. Here, we identify BMP4 as a critical component through which the pancreatic microenvironment regulates β cell function. By combining transgenic mouse models and human iPSCs, we show that BMP4 promotes the expression of core β cell genes and is required for proper insulin production and secretion. We identified pericytes as the primary pancreatic source of BMP4, which start producing this ligand midway through the postnatal period, at the age β cells mature. Overall, our findings show that the islet niche directly promotes β cell functional maturation through the timely production of BMP4. Our study highlights the need to recapitulate the physiological postnatal islet niche for generating fully functional stem-cell-derived β cells for cell replacement therapy for diabetes.

The neuropeptide allatostatin C from clock-associated DN1p neurons generates the circadian rhythm for oogenesis

Metazoan species optimize the timing of reproduction to maximize fitness. To understand how biological clocks direct reproduction, we investigated the neural substrates that produce oogenesis rhythms in the genetically amenable model organism Drosophila melanogaster. The neuropeptide allatostatin C (AstC) is an insect counterpart of the vertebrate neuropeptide somatostatin, which suppresses gonadotropin production. A subset of the brain circadian pacemaker neurons produces AstC. We have uncovered that these clock-associated AstC neurons generate the circadian oogenesis rhythm via brain insulin-producing cells and the insect gonadotropin juvenile hormone. Identification of a conserved neuropeptide pathway that links female reproduction and the biological clock offers insight into the molecular mechanisms that direct reproductive timing.

The link between the biological clock and reproduction is evident in most metazoans. The fruit fly Drosophila melanogaster, a key model organism in the field of chronobiology because of its well-defined networks of molecular clock genes and pacemaker neurons in the brain, shows a pronounced diurnal rhythmicity in oogenesis. Still, it is unclear how the circadian clock generates this reproductive rhythm. A subset of the group of neurons designated “posterior dorsal neuron 1” (DN1p), which are among the ∼150 pacemaker neurons in the fly brain, produces the neuropeptide allatostatin C (AstC-DN1p). Here, we report that six pairs of AstC-DN1p send inhibitory inputs to the brain insulin-producing cells, which express two AstC receptors, star1 and AICR2. Consistent with the roles of insulin/insulin-like signaling in oogenesis, activation of AstC-DN1p suppresses oogenesis through the insulin-producing cells. We show evidence that AstC-DN1p activity plays a role in generating an oogenesis rhythm by regulating juvenile hormone and vitellogenesis indirectly via insulin/insulin-like signaling. AstC is orthologous to the vertebrate neuropeptide somatostatin (SST). Like AstC, SST inhibits gonadotrophin secretion indirectly through gonadotropin-releasing hormone neurons in the hypothalamus. The functional and structural conservation linking the AstC and SST systems suggest an ancient origin for the neural substrates that generate reproductive rhythms.

Beta cell dysfunction in diabetes: the islet microenvironment as an unusual suspect

Cells in different tissues, including endocrine cells in the pancreas, live in complex microenvironments that are rich in cellular and acellular components. Intricate interactions with their microenvironment dictate most cellular properties, such as their function, structure and size, and maintain tissue homeostasis. Pancreatic islets are populated by endocrine, vascular and immune cells that are immersed in the extracellular matrix. While the intrinsic properties of beta cells have been vastly investigated, our understanding of their interactions with their surroundings has only recently begun to unveil. Here, we review current research on the interplay between the islet cellular and acellular components, and the role these components play in beta cell physiology and pathophysiology. Although beta cell failure is a key pathomechanism in diabetes, its causes are far from being fully elucidated. We, thus, propose deleterious alterations of the islet niche as potential underlying mechanisms contributing to beta cell failure. In sum, this review emphasises that the function of the pancreatic islet depends on all of its components. Graphical abstract.

Time Course of Normalization of Functional β-Cell Capacity in the Diabetes Remission Clinical Trial After Weight Loss in Type 2 Diabetes

OBJECTIVE

To assess functional β-cell capacity in type 2 diabetes during 2 years of remission induced by dietary weight loss.

RESEARCH DESIGN AND METHODS

A Stepped Insulin Secretion Test with Arginine was used to quantify functional β-cell capacity by hyperglycemia and arginine stimulation. Thirty-nine of 57 participants initially achieved remission (HbA_1c <6.5% [<48 mmol/mol] and fasting plasma glucose <7 mmol/L on no antidiabetic drug therapy) with a 16.4 ± 7.7 kg weight loss and were followed up with supportive advice on avoidance of weight regain. At 2 years, 20 participants remained in remission in the study. A nondiabetic control (NDC) group, matched for age, sex, and weight after weight loss with the intervention group, was studied once.

RESULTS

During remission, median (interquartile range) maximal rate of insulin secretion increased from 581 (480-811) pmol/min/m² at baseline to 736 (542-998) pmol/min/m² at 5 months, 942 (565-1,240) pmol/min/m² at 12 months (P = 0.028 from baseline), and 936 (635-1,435) pmol/min/m² at 24 months (P = 0.023 from baseline; n = 20 of 39 of those initially in remission). This was comparable to the NDC group (1,016 [857-1,507] pmol/min/m²) by 12 (P = 0.064) and 24 (P = 0.244) months. Median first-phase insulin response increased from baseline to 5 months (42 [4-67] to 107 [59-163] pmol/min/m²; P < 0.0001) and then remained stable at 12 and 24 months (110 [59-201] and 125 [65-166] pmol/min/m², respectively; P < 0.0001 vs. baseline) but lower than that of the NDC group (250 [226-429] pmol/min/m²; P < 0.0001).

CONCLUSIONS

A gradual increase in assessed functional β-cell capacity occurred after weight loss, becoming similar to that of NDC group participants by 12 months. This result was unchanged at 2 years with continuing remission of type 2 diabetes.

Newer perspective on the coupling between glucose-mediated signaling and β-cell functionality

Insulin secretion by the pancreatic β-cells is elicited in response to elevated extracellular glucose concentration. In addition to triggering insulin secretion, glucose-induced signal regulates β-cell proliferation and survival. However, the molecular mechanism underlying the effects of glucose on the β-cell functionality still remains unclear. Glucokinase, a hexokinase isozyme that catalyzes the phosphorylation of glucose, acts as the glucose sensor in the β-cells. To investigate the mechanisms of glucose signaling in the regulation of β-cell functions, we analyzed the role of glucokinase in insulin secretion, β-cell proliferation and β-cell apoptosis, using β-cell-specific glucokinase-haploinsufficient (Gck+/-) mice and allosteric glucokinase activators (GKAs). Glucokinase-mediated glucose metabolism (1) suppresses endoplasmic reticulum (ER) stress-induced β-cell apoptosis via inducing insulin receptor substrate-2 (IRS-2) expression and expression of ER stress-related molecules, (2) promotes adaptive β-cell proliferation through activation of the Forkhead Box M1 (FoxM1)/polo-like kinase-1 (PLK1)/centromere protein-A (CENP-A) pathway, (3) induces islet inflammation by promoting interaction of islet-derived S100 calcium-binding protein A8 (S100A8) with macrophages, (4) induces the expression of Fibulin-5 (Fbln5), an extracellular matrix protein to regulate β-cell functions, and (5) activates other unknown pathways. Glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase 4 (DPP-4) inhibitors have been found to possibly compensate for dysregulation of glucose metabolism in the β-cells. This review provides an update and overview of the recent advances in the study of β-cell pathophysiology and some therapeutic possibilities focusing on glucose-/glucokinase-mediated signaling.

β-Cell Maturation and Identity in Health and Disease

The exponential increase of patients with diabetes mellitus urges for novel therapeutic strategies to reduce the socioeconomic burden of this disease. The loss or dysfunction of insulin-producing β-cells, in patients with type 1 and type 2 diabetes respectively, put these cells at the center of the disease initiation and progression. Therefore, major efforts have been taken to restore the β-cell mass by cell-replacement or regeneration approaches. Implementing novel therapies requires deciphering the developmental mechanisms that generate β-cells and determine the acquisition of their physiological phenotype. In this review, we summarize the current understanding of the mechanisms that coordinate the postnatal maturation of β-cells and define their functional identity. Furthermore, we discuss different routes by which β-cells lose their features and functionality in type 1 and 2 diabetic conditions. We then focus on potential mechanisms to restore the functionality of those β-cell populations that have lost their functional phenotype. Finally, we discuss the recent progress and remaining challenges facing the generation of functional mature β-cells from stem cells for cell-replacement therapy for diabetes treatment.

A glucose-sensing neuron pair regulates insulin and glucagon in Drosophila

Although glucose-sensing neurons were identified more than 50 years ago, the physiological role of glucose sensing in metazoans remains unclear. Here we identify a pair of glucose-sensing neurons with bifurcated axons in the brain of Drosophila. One axon branch projects to insulin-producing cells to trigger the release of Drosophila insulin-like peptide 2 (dilp2) and the other extends to adipokinetic hormone (AKH)-producing cells to inhibit secretion of AKH, the fly analogue of glucagon. These axonal branches undergo synaptic remodelling in response to changes in their internal energy status. Silencing of these glucose-sensing neurons largely disabled the response of insulin-producing cells to glucose and dilp2 secretion, disinhibited AKH secretion in corpora cardiaca and caused hyperglycaemia, a hallmark feature of diabetes mellitus. We propose that these glucose-sensing neurons maintain glucose homeostasis by promoting the secretion of dilp2 and suppressing the release of AKH when haemolymph glucose levels are high.

Repercussions of low fructose-drinking water in male rats

Fructose consumption has increased worldwide, and it has been associated with the development of metabolic diseases such as insulin resistance (IR) and steatosis. The aim was to evaluate if lower fructose concentrations may cause pancreatic structural abnormalities, leading to a glucose intolerance without steatosis in male rats. Young male rats orally received 7% fructose solution for 12 weeks. Body weight, food, water, and energy intake were measured. An oral glucose tolerance test (OGTT) was performed. After final experimental period, all rats were anaesthetized and killed. Blood samples were collected for biochemical analyses and organs (liver and pancreas) were processed for morphological analyses. Fructose consumption was not associated with lipid accumulation in liver. However, fructose administration was associated with an increased area under curve from OGTT and an increased percentage of insulin-positive cells, high beta cell mass and reduced pancreatic islet area. Fructose supplementation (7%) did not cause steatosis, but it led to abnormal morphology and function of pancreatic islet cells, contributing for glucose intolerance development. Our findings demonstrate that even low fructose concentrations may cause deleterious effects in animals.

Electrophysiology of the pancreatic islet β-cell sweet taste receptor TIR3

Over recent years, the presence of the sweet taste receptor TIR3 in rodent and human insulin-producing pancreatic islet β-cells was documented. The activation of this receptor by sweet-tasting sucralose mimics several biochemical and functional effects of D-glucose in the β-cells. The present study extends this analogy to the bioelectrical response of β-cells. In this respect, sucralose was inefficient in the absence of D-glucose, but induced on occasion electrical activity in mouse β-cells exposed to low non-stimulatory concentrations of the hexose and potentiated, in a concentration-related manner, the response to stimulatory concentrations of D-glucose. These data indicate that sucralose, acting as an agonist of the TIR3 receptor, exerts an excitatory effect upon pancreatic β-cell bioelectrical activity.

Individual islet respirometry reveals functional diversity within the islet population of mice and human donors

OBJECTIVE

Islets from the same pancreas show remarkable variability in glucose sensitivity. While mitochondrial respiration is essential for glucose-stimulated insulin secretion, little is known regarding heterogeneity in mitochondrial function at the individual islet level. This is due in part to a lack of high-throughput and non-invasive methods for detecting single islet function.

METHODS

We have developed a novel non-invasive, high-throughput methodology capable of assessing mitochondrial respiration in large-sized individual islets using the XF96 analyzer (Agilent Technologies).

RESULTS

By increasing measurement sensitivity, we have reduced the minimal size of mouse and human islets needed to assess mitochondrial respiration to single large islets of >35,000 μm2 area (∼210 μm diameter). In addition, we have measured heterogeneous glucose-stimulated mitochondrial respiration among individual human and mouse islets from the same pancreas, allowing population analyses of islet mitochondrial function for the first time.

CONCLUSIONS

We have developed a novel methodology capable of analyzing mitochondrial function in large-sized individual islets. By highlighting islet functional heterogeneity, we hope this methodology can significantly advance islet research.

Heterogeneity in the Beta-Cell Population: a Guided Search Into Its Significance in Pancreas and in Implants

PURPOSE OF REVIEW

Intercellular differences in function have since long been noticed in the pancreatic beta-cell population. Heterogeneity in cellular glucose responsiveness is considered of physiological and pathological relevance. The present review updates evidence for the physiologic significance of beta-cell heterogeneity in the pancreas. It also briefly discusses what this role would imply for beta-cell implants in diabetes.

RECENT FINDINGS

Over the past 3 years, functionally different beta cells have been related to mechanisms that may underlie their heterogeneity in the pancreas, such as the stage in their life cycle and the degree of their clustering to islets with varying vascularization. Markers were identified for detecting these subpopulations in tissues. The existence of a functional heterogeneity in the pancreatic beta-cell population is further supported. Views on its origin and methods for its analysis in pancreas and implants will help guide the search into its significance in beta-cell biology, pathology, and therapy.

Pancreatic Beta Cell Identity in Humans and the Role of Type 2 Diabetes

Pancreatic beta cells uniquely synthetize, store, and release insulin. Specific molecular, functional as well as ultrastructural traits characterize their insulin secretion properties and survival phentoype. In this review we focus on human islet/beta cells, and describe the changes that occur in type 2 diabetes and could play roles in the disease as well as represent possible targets for therapeutical interventions. These include transcription factors, molecules involved in glucose metabolism and insulin granule handling. Quantitative and qualitative insulin release patterns and their changes in type 2 diabetes are also associated with ultrastructural features involving the insulin granules, the mitochondria, and the endoplasmic reticulum.

Interrogating islets in health and disease with single-cell technologies

BACKGROUND

Blood glucose levels are tightly controlled by the coordinated actions of hormone-producing endocrine cells that reside in pancreatic islets. Islet cell malfunction underlies diabetes development and progression. Due to the cellular heterogeneity within islets, it has been challenging to uncover how specific islet cells contribute to glucose homeostasis and diabetes pathogenesis. Recent advances in single-cell technologies and computational methods have opened up new avenues to resolve islet heterogeneity and study islet cell states in health and disease.

SCOPE OF REVIEW

In the past year, a multitude of studies have been published that used single-cell approaches to interrogate the transcriptome and proteome of the different islet cell types. Here, we summarize the conclusions of these studies, as well as discuss the technologies used and the challenges faced with computational analysis of single-cell data from islet studies.

MAJOR CONCLUSIONS

By analyzing single islet cells from rodents and humans at different ages and disease states, the studies reviewed here have provided new insight into endocrine cell function and facilitated a high resolution molecular characterization of poorly understood processes, including regeneration, maturation, and diabetes pathogenesis. Gene expression programs and pathways identified in these studies pave the way for the discovery of new targets and approaches to prevent, monitor, and treat diabetes.