Diminished insulin secretory response to glucose but normal insulin and glucagon secretory responses to arginine in a family with maternally inherited diabetes and deafness caused by mitochondrial tRNA(LEU(UUR)) gene mutation

Endocrine Manifestations and New Developments in Mitochondrial Disease

Mitochondrial diseases are a group of common inherited diseases causing disruption of oxidative phosphorylation. Some patients with mitochondrial disease have endocrine manifestations, with diabetes mellitus being predominant but also include hypogonadism, hypoadrenalism, and hypoparathyroidism. There have been major developments in mitochondrial disease over the past decade that have major implications for all patients. The collection of large cohorts of patients has better defined the phenotype of mitochondrial diseases and the majority of patients with endocrine abnormalities have involvement of several other systems. This means that patients with mitochondrial disease and endocrine manifestations need specialist follow-up because some of the other manifestations, such as stroke-like episodes and cardiomyopathy, are potentially life threatening. Also, the development and follow-up of large cohorts of patients means that there are clinical guidelines for the management of patients with mitochondrial disease. There is also considerable research activity to identify novel therapies for the treatment of mitochondrial disease. The revolution in genetics, with the introduction of next-generation sequencing, has made genetic testing more available and establishing a precise genetic diagnosis is important because it will affect the risk for involvement for different organ systems. Establishing a genetic diagnosis is also crucial because important reproductive options have been developed that will prevent the transmission of mitochondrial disease because of mitochondrial DNA variants to the next generation.

Acute and long-term disruption of glycometabolic control after SARS-CoV-2 infection

Patients with coronavirus disease 2019 (COVID-19) are reported to have a greater prevalence of hyperglycaemia. Cytokine release as a consequence of severe acute respiratory syndrome coronavirus 2 infection may precipitate the onset of metabolic alterations by affecting glucose homeostasis. Here we describe abnormalities in glycometabolic control, insulin resistance and beta cell function in patients with COVID-19 without any pre-existing history or diagnosis of diabetes, and document glycaemic abnormalities in recovered patients 2 months after onset of disease. In a cohort of 551 patients hospitalized for COVID-19 in Italy, we found that 46% of patients were hyperglycaemic, whereas 27% were normoglycaemic. Using clinical assays and continuous glucose monitoring in a subset of patients, we detected altered glycometabolic control, with insulin resistance and an abnormal cytokine profile, even in normoglycaemic patients. Glycaemic abnormalities can be detected for at least 2 months in patients who recovered from COVID-19. Our data demonstrate that COVID-19 is associated with aberrant glycometabolic control, which can persist even after recovery, suggesting that further investigation of metabolic abnormalities in the context of long COVID is warranted.

Mitochondrial Bioenergetics and Dynamics in Secretion Processes

Secretion is an energy consuming process that plays a relevant role in cell communication and adaptation to the environment. Among others, endocrine cells producing hormones, immune cells producing cytokines or antibodies, neurons releasing neurotransmitters at synapsis, and more recently acknowledged, senescent cells synthesizing and secreting multiple cytokines, growth factors and proteases, require energy to successfully accomplish the different stages of the secretion process. Calcium ions (Ca²⁺) act as second messengers regulating secretion in many of these cases. In this setting, mitochondria appear as key players providing ATP by oxidative phosphorylation, buffering Ca²⁺ concentrations and acting as structural platforms. These tasks also require the concerted actions of the mitochondrial dynamics machinery. These proteins mediate mitochondrial fusion and fission, and are also required for transport and tethering of mitochondria to cellular organelles where the different steps of the secretion process take place. Herein we present a brief overview of mitochondrial energy metabolism, mitochondrial dynamics, and the different steps of the secretion processes, along with evidence of the interaction between these pathways. We also analyze the role of mitochondria in secretion by different cell types in physiological and pathological settings.

Glucose metabolism derangements in adults with the MELAS m.3243A>G mutation

The m.3243A>G mutation in the mitochondrial gene MT-TL1 leads to a wide clinical spectrum ranging from asymptomatic carriers to MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) at the severe end. Diabetes mellitus (DM) occurs in mitochondrial diseases, with the m.3243A>G mutation being the most common mutation associated with mitochondrial DM. The pathogenesis of mitochondrial DM remains largely unknown, with previous studies suggesting that impaired insulin secretion is the major factor. In this study we used stable isotope infusion techniques to assess glucose metabolism in vivo and under physiological conditions in 5 diabetic and 11 non-diabetic adults with the m.3243A>G mutation and 10 healthy adult controls. Our results revealed increased glucose production due to increased gluconeogenesis in both diabetic and non-diabetic subjects with the m.3243A>G mutation. In addition, diabetic subjects demonstrated insulin resistance and relative insulin deficiency, resulting in an inability to increase glucose oxidation which can explain the development of DM in these subjects. Non-diabetic subjects showed normal insulin sensitivity; and therefore, they were able to increase their glucose oxidation rate. The ability to increase glucose utilization can act as a compensatory mechanism that explains why these subjects do not have DM despite the higher rate of glucose production. These results suggest that increased gluconeogenesis is not enough to cause DM and the occurrence of combined insulin resistance and relative insulin deficiency are needed to develop DM in individuals with the m.3243A>G mutation. Therefore, multiple defects in insulin and glucose metabolism are required for DM to occur in individuals with mitochondrial diseases. The results of this study uncover previously undocumented alterations in glucose metabolism in individuals with the m.3243A>G mutation that contribute significantly to our understanding of the pathogenesis of mitochondrial DM and can have significant implications for its management.

Glycaemic instability correlates with a hyperglucagonaemic response in patients with type 1 diabetes without residual beta-cell function

We investigated the association between arginine-stimulated glucagon secretion (AUCIRG) and several parameters of glycaemic variability in 12 patients with type 1 diabetes without residual beta-cell function. AUCIRG positively correlated with the SD and mean amplitude of glycaemic excursions, thus glucagon might contribute to glycaemic instability, independent of endogenous insulin.

Mitochondrial dysfunction in pancreatic β cells

In pancreatic β cells, mitochondria play a central role in coupling glucose metabolism to insulin exocytosis, thereby ensuring strict control of glucose-stimulated insulin secretion. Defects in mitochondrial function impair this metabolic coupling, and ultimately promote apoptosis and β cell death. Various factors have been identified that may contribute to mitochondrial dysfunction. In this review we address the emerging concept of complex links between these factors. We also discuss the role of the mitochondrial genome and mutations associated with diabetes, the effect of oxidative stress and reactive oxygen species, the sensitivity of mitochondria to lipotoxicity, and the adaptive dynamics of mitochondrial morphology. Better comprehension of the molecular mechanisms contributing to mitochondrial dysfunction will help drive the development of effective therapeutic approaches.

Remission of diabetes mellitus in cats cannot be predicted by the arginine stimulation test

BACKGROUND

Cats with diabetes mellitus frequently achieve clinical remission, suggesting residual β-cell function. Responsiveness of β-cells to arginine persists the longest during diabetes progression, making the intravenous arginine stimulation test (IVAST) a useful tool to assess residual insulin and glucagon secretion.

HYPOTHESIS

Diabetic cats with and without remission will have different arginine-induced insulin or glucagon response.

ANIMALS

Seventeen cats with diabetes, 7 healthy cats.

METHODS

Blood samples collected on admission and during subsequent IVAST. Glucose, insulin, and glucagon were measured. Response to IVAST was assessed by calculating the insulin and glucagon area under the curve (AUC) and the AUC glucagon-to-insulin ratio. Diabetic cats were treated with insulin and were followed for 18 weeks. Remission was defined as normoglycemia and disappearance of clinical signs of diabetes for ≥4 weeks, without requiring insulin.

RESULTS

Seven diabetic cats (41%) achieved remission. On admission, blood glucose concentration was significantly lower in cats with remission (median, 389 mg/dL; range, 342-536 mg/dL) than in those without remission (median, 506 mg/dL; range, 266-738 mg/dL). After IVAST, diabetic cats with remission had higher AUC glucagon-to-insulin ratios (median, 61; range, 34-852) than did cats without remission (median, 26; range, 20-498); glucose, insulin, and glucagon AUCs were not different. Diabetic cats had lower insulin AUC than did healthy cats but comparable glucagon AUC.

CONCLUSIONS AND CLINICAL IMPORTANCE

Diabetic cats with and without remission have similar arginine-stimulated insulin secretion on admission. Although cats with remission had lower blood glucose concentrations and higher AUC glucagon-to-insulin ratios, large overlap between groups prevents use of these parameters in clinical practice.

Role of mitochondria in beta-cell function and dysfunction

Pancreatic beta-cells are poised to sense glucose and other nutrient secretagogues to regulate insulin exocytosis, thereby maintaining glucose homeostasis. This process requires translation of metabolic substrates into intracellular messengers recognized by the exocytotic machinery. Central to this metabolism-secretion coupling, mitochondria integrate and generate metabolic signals, thereby connecting glucose recognition to insulin exocytosis. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic calcium, to the stimulation of insulin release. This review describes the mitochondrion-dependent pathways of regulated insulin secretion. Mitochondrial defects, such as mutations and reactive oxygen species production, are discussed in the context of beta-cell failure that may participate to the etiology of diabetes.

Genetic influences on the insulin response of the beta cell to different secretagogues

AIMS/HYPOTHESIS

The aim of the present study was to estimate the heritability of the beta cell insulin response to glucose and to glucose combined with glucagon-like peptide-1 (GLP-1) or with GLP-1 plus arginine.

METHODS

This was a twin-family study that included 54 families from the Netherlands Twin Register. The participants were healthy twin pairs and their siblings of the same sex, aged 20 to 50 years. Insulin response of the beta cell was assessed by a modified hyperglycaemic clamp with additional GLP-1 and arginine. Insulin sensitivity index (ISI) was assessed by the euglycaemic-hyperinsulinaemic clamp. Multivariate structural equation modelling was used to obtain heritabilities and the genetic factors underlying individual differences in BMI, ISI and secretory responses of the beta cell.

RESULTS

The heritability of insulin levels in response to glucose was 52% and 77% for the first and second phase, respectively, 53% in response to glucose + GLP-1 and 80% in response to an additional arginine bolus. Insulin responses to the administration of glucose, glucose + GLP-1 and glucose + GLP-1 + arginine were highly correlated (0.62< r <0.79). Heritability of BMI and ISI was 74% and 60% respectively. The genetic factors that influenced BMI and ISI explained about half of the heritability of insulin levels in response to the three secretagogues. The other half was due to genetic factors specific to the beta cell.

CONCLUSIONS/INTERPRETATION

In healthy adults, genetic factors explain most of the individual differences in the secretory capacity of the beta cell. These genetic influences are partly independent from the genes that influence BMI and ISI.

High prevalence of impaired glucose homeostasis and myopathy in asymptomatic and oligosymptomatic 3243A>G mitochondrial DNA mutation-positive subjects

INTRODUCTION

The point mutation of 3243A>G mtDNA is the most frequent cause of mitochondrial diabetes, often presenting as the syndrome maternally inherited diabetes and deafness (MIDD). The mutation may also cause myopathy, ataxia, strokes, ophthalmoplegia, epilepsy, and cardiomyopathy in various combinations. Consequently, it is difficult to predict the "phenotypic risk profile" of 3243A>G mutation-positive subjects. The 3243A>G mutation coexists in cells with wild-type mtDNA, a phenomenon called heteroplasmy. The marked variability in mutation loads in different tissues is the main explanation for the different phenotypes associated with this mutation.

AIM

The aim of the study was to screen asymptomatic and oligosymptomatic 3243A>G mtDNA carriers for diabetes and myopathy.

METHODS

The study is a case-control study. Nineteen adult 3243A>G carriers presumed to be normoglycemic and matched healthy controls were subjected to an oral glucose tolerance test. Twenty-six adult 3243A>G carriers with unknown myopathy status and 17 healthy controls had a maximal cycle test and a muscle biopsy performed. The mutation loads were quantified in blood and muscle biopsies and correlated to the clinical manifestations of the mutation.

RESULTS

In the presumed normoglycemic 3243A>G-positive subjects, one subject had overt diabetes, and 10 subjects had impaired glucose tolerance. Sixteen of the 26 subjects with unknown oxidative capacity fulfilled criteria for myopathy. The mutation load in blood and muscle correlated with the age for diagnosis of impaired glucose homeostasis and hearing impairment (rho = -0.71 to -0.78; P < 0.0001).

CONCLUSION

The findings suggest that 3243A>G mutation carriers should be screened for diabetes and myopathy.

m.3243A>G mutation in mitochondrial DNA leads to decreased insulin sensitivity in skeletal muscle and to progressive beta-cell dysfunction

OBJECTIVE

To study insulin sensitivity and perfusion in skeletal muscle together with the beta-cell function in subjects with the m.3243A>G mutation in mitochondrial DNA, the most common cause of mitochondrial diabetes.

RESEARCH DESIGN AND METHODS

We measured skeletal muscle glucose uptake and perfusion using positron emission tomography and 2-[18F]fluoro-2-deoxyglucose and [15O]H2O during euglycemic hyperinsulinemia in 15 patients with m.3243A>G. These patients included five subjects with no diabetes as defined by the oral glucose tolerance test (OGTT) (group 1), three with GHb <6.1% and newly found diabetes by OGTT (group 2), and seven with a previously diagnosed diabetes (group 3). Control subjects consisted of 13 healthy individuals who were similar to the carriers of m.3243A>G with respect to age and physical activity. Beta-cell function was assessed using the OGTT and subsequent mathematical modeling.

RESULTS

Skeletal muscle glucose uptake was significantly lower in groups 1, 2, and 3 than in the control subjects. The glucose sensitivity of beta-cells in group 1 patients was similar to that of the control subjects, whereas in group 2 and 3 patients, the glucose sensitivity was significantly lower. The insulin secretion parameters correlated strongly with the proportion of m.3243A>G mutation in muscle.

CONCLUSIONS

Our findings show that subjects with m.3243A>G are insulin resistant in skeletal muscle even when beta-cell function is not markedly impaired or glucose control compromised. We suggest that both the skeletal muscle insulin sensitivity and the beta-cell function are affected before the onset of the mitochondrial diabetes caused by the m.3243A>G mutation.

Mitochondrial damages and the regulation of insulin secretion

Pancreatic beta-cells are able to respond to nutrients, principally glucose, as the primary stimulus for insulin exocytosis. This unique feature requires translation of metabolic substrates into intracellular messengers recognized by the exocytotic machinery. Central to this signal transduction mechanism, mitochondria integrate and generate metabolic signals, thereby coupling glucose recognition with insulin secretion. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic Ca2+, in the stimulation of insulin exocytosis. Mitochondrial defects, such as mutations and ROS (reactive oxygen species) production, might be associated with beta-cell failure in the course of diabetes. mtDNA (mitochondrial DNA) mutation A3243G is associated with MIDD (mitochondrial inherited diabetes and deafness). A common hypothesis to explain the link between the genotype and the phenotype is that the mutation might impair mitochondrial metabolism expressly required for beta-cell functions, although this assumption lacks direct demonstration. mtDNA-deficient cellular models are glucose-unresponsive and are defective in mitochondrial function. Recently, we used clonal cytosolic hybrid cells (namely cybrids) harbouring mitochondria derived from MIDD patients. Compared with control mtDNA from the same patient, the A3243G mutation markedly modified metabolic pathways. Moreover, cybrid cells carrying patient-derived mutant mtDNA exhibited deranged cell Ca2+ handling and elevated ROS under metabolic stress. In animal models, transgenic mice lacking expression of the mitochondrial genome specifically in beta-cells are diabetic and their islets are incable of releasing insulin in response to glucose. These various models demonstrate the fragility of nutrient-stimulated insulin secretion, caused primarily by defective mitochondrial function.

Beta-cell function in individuals carrying the mitochondrial tRNA leu (UUR) mutation

OBJECTIVES

To assess the beta-cell function in individuals with mitochondrial DNA A3243G mutation with normal glucose tolerance (NGT) or diabetes mellitus (DM). Furthermore, in diabetic individuals, we evaluated the effect of coenzyme Q10 supplementation on insulin secretory response.

METHODS

Eight mutation-positive individuals with NGT (n = 4) or DM (n = 4) were studied. beta-Cell function was evaluated by C-peptide levels before and after a mixed liquid meal (Sustacal) challenge and by first-phase insulin response.

RESULTS

Fasting and Sustacal-stimulated C-peptide levels were significantly lower in diabetic patients than that in controls (area under the curve: 104.1 +/- 75.7 vs 520.8 +/- 173.8, P = 0.001), whereas in individuals with NGT, this response was preserved (area under the curve: 537.8 +/- 74.3 vs 520.8 +/- 179.8, P = 0.87). The duration of diabetes was negatively correlated with fasting C-peptide levels (r = -0.961, P = 0.038). Among the 3 patients with residual insulin secretion, the short-term treatment with coenzyme Q10 (3 months) improved C-peptide levels in 2 of them. The first-phase insulin response was diminished in 2 individuals with NGT, the oldest ones.

CONCLUSIONS

We showed an impaired insulin secretory capacity in individuals carrying the A3243G mutation, this possibly being the primary defect contributing to the development of DM. In addition, our data suggest that this could be a functional defect.

Diabetes-associated mitochondrial DNA mutation A3243G impairs cellular metabolic pathways necessary for beta cell function

AIMS/HYPOTHESIS

Mitochondrial DNA (mtDNA) mutations cause several diseases, including mitochondrial inherited diabetes and deafness (MIDD), typically associated with the mtDNA A3243G point mutation on tRNALeu gene. The common hypothesis to explain the link between the genotype and the phenotype is that the mutation might impair mitochondrial metabolism expressly required for beta cell functions. However, this assumption has not yet been tested.

METHODS

We used clonal osteosarcoma cytosolic hybrid cells (namely cybrids) harbouring mitochondria derived from MIDD patients and containing either exclusively wild-type or mutated (A3243G) mtDNA. According to the importance of mitochondrial metabolism in beta cells, we studied the impact of the mutation on key parameters by comparing stimulation of these cybrids by the main insulin secretagogue glucose and the mitochondrial substrate pyruvate.

RESULTS

Compared with control mtDNA from the same patient, the A3243G mutation markedly modified metabolic pathways leading to a high glycolytic rate (2.8-fold increase), increased lactate production (2.5-fold), and reduced glucose oxidation (-83%). We also observed impaired NADH responses (-56%), negligible mitochondrial membrane potential, and reduced, only transient ATP generation. Moreover, cybrid cells carrying patient-derived mutant mtDNA exhibited deranged cell calcium handling with increased cytosolic loads (1.4-fold higher), and elevated reactive oxygen species (2.6-fold increase) under glucose deprivation.

CONCLUSIONS/INTERPRETATION

The present study demonstrates that the mtDNA A3243G mutation impairs crucial metabolic events required for proper cell functions, such as coupling of glucose recognition to insulin secretion.

Effect of donor age on function of isolated human islets

This study intended to evaluate the impact of donor age on the function of isolated islets. Analysis of human islets from cadaveric donors (age 16-70 years) was performed using glucose-stimulated insulin release (GSIR) (n = 93), islet ATP content (n = 27), diabetic nude mouse bioassay (n = 72), and the insulin secretory function after single-donor clinical islet allotransplantation (n = 7). The GSIR index was significantly higher in younger donors (age < or =40 years) than in older donors and negatively correlated with the donor age (r = -0.535). Islet ATP was higher in younger donors (115.7 +/- 17.7 vs. 75.7 +/- 6.6 pmol/microg DNA). The diabetes reversal rate of mice with 2,000 IE was significantly higher in younger donors (96 vs. 68%). C-peptide increment to glucose during intravenous glucose tolerance test at days 90-120 after clinical transplantation showed negative correlation with donor age (r = -0.872) and positive correlation with the islet mass (r = 0.832). On the other hand, acute insulin response to arginine only showed correlation with the islet mass and not with donor age. These results show that insulin secretory response to glucose deteriorates with increasing age and that it may be related to changes in ATP generation in beta-cells.

In beta-cells, mitochondria integrate and generate metabolic signals controlling insulin secretion

Pancreatic beta-cells are unique neuroendocrine cells displaying the peculiar feature of responding to nutrients, principally glucose, as primary stimulus. This requires translation of a metabolic substrate into intracellular messengers recognized by the exocytotic machinery. Central to this signal transduction mechanism, mitochondria integrate and generate metabolic signals, thereby coupling glucose recognition to insulin secretion. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic calcium, to the stimulation of insulin exocytosis. This review describes the mitochondrion-dependent pathways of regulated insulin secretion. In particular, importance of cataplerotic and anaplerotic processes is discussed, with special attention to the mitochondrial enzyme glutamate dehydrogenase. Mitochondrial defects, such as mutations and reactive oxygen species production, are presented in the context of beta-cell failure in the course of type 2 diabetes.

Current literature in diabetes

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of diabetes/metabolism. Each bibliography is divided into 17 sections: 1 Books, Reviews & Symposia; 2 General; 3 Genetics; 4 Epidemiology; 5 Immunology; 6 Prediction; 7 Prevention; 8 INTERVENTION: a&rpar General; b&rpar Pharmacology; 9 Pathology: a&rpar General; b&rpar Cardiovascular; c&rpar Neurological; d&rpar Renal; 10 Endocrinology & Metabolism; 11 Nutrition; 12 Animal Studies; 13 Techniques. Within each section, articles are listed in alphabetical order with respect to author (9 Weeks journals - Search completed at 1st Aug 2001)