Cholinergic augmentation of insulin release requires ankyrin-B

Metabolic and Molecular Amplification of Insulin Secretion

The pancreatic β cells are at the hub of myriad signals to regulate the secretion of an adequate amount of insulin needed to re-establish postprandial euglycemia. The β cell possesses sophisticated metabolic enzymes and a variety of extracellular receptors and channels that amplify insulin secretion in response to autocrine, paracrine, and neurohormonal signals. Considerable research has been undertaken to decipher the mechanisms regulating insulin secretion. While the triggering pathway induced by glucose is needed to initiate the exocytosis process, multiple other stimuli modulate the insulin secretion response. This chapter will discuss the recent advances in understanding the role of the diverse glucose- and fatty acid-metabolic coupling factors in amplifying insulin secretion. It will also highlight the intracellular events linking the extracellular receptors and channels to insulin secretion amplification. Understanding these mechanisms provides new insights into learning more about the etiology of β-cell failure and paves the way for developing new therapeutic strategies for type 2 diabetes.

A cell-autonomous mechanism regulates BCAA catabolism in white adipocytes and systemic metabolic balance

Elevated plasma branched-chain amino acids (BCAAs) are strongly associated with obesity, insulin resistance (IR), and diabetes in humans and rodent models. However, the mechanisms of BCAA dysregulation and its systemic, organ, and cell-specific implications in the development of obesity and IR are not well understood. To gain mechanistic insight into the causes and effects of plasma BCAA elevations, we leveraged mouse models with high circulating BCAA levels prior to the onset of obesity and IR. Young mice lacking ankyrin-B in white adipose tissue (WAT) or bearing an ankyrin-B variant that causes age-driven metabolic syndrome exhibit downregulation of BCAA catabolism selectively in WAT and excess plasma BCAAs. Using cellular assays, we demonstrated that ankyrin-B promotes the surface localization of the amino acid transporter Asct2 in white adipocytes, and its deficit impairs BCAA uptake. Excess BCAA supplementation worsened glucose tolerance and insulin sensitivity across genotypes. In contrast, BCAA overconsumption only increased adiposity in control mice, implicating WAT utilization of BCAAs in their obesogenic effects. These results shed light into the mechanistic underpinnings of metabolic syndrome caused by ankyrin-B deficits and provide new evidence of the relevance of WAT in the regulation of systemic BCAA levels, adiposity, and glucose homeostasis.

A developmental pathway for epithelial-to-motoneuron transformation in C. elegans

Motoneurons and motoneuron-like pancreatic β cells arise from radial glia and ductal cells, respectively, both tube-lining progenitors that share molecular regulators. To uncover programs underlying motoneuron formation, we studied a similar, cell-division-independent transformation of the C. elegans tube-lining Y cell into the PDA motoneuron. We find that lin-12/Notch acts through ngn-1/Ngn and its regulator hlh-16/Olig to control transformation timing. lin-12 loss blocks transformation, while lin-12(gf) promotes precocious PDA formation. Early basal expression of ngn-1/Ngn and hlh-16/Olig depends on sem-4/Sall and egl-5/Hox. Later, coincident with Y cell morphological changes, ngn-1/Ngn expression is upregulated in a sem-4/Sall and egl-5/Hox-dependent but hlh-16/Olig-independent manner. Subsequently, Y cell retrograde extension forms an anchored process priming PDA axon extension. Extension requires ngn-1-dependent expression of the cytoskeleton organizers UNC-119, UNC-44/ANK, and UNC-33/CRMP, which also activate PDA terminal-gene expression. Our findings uncover cell-division-independent regulatory events leading to motoneuron generation, suggesting a conserved pathway for epithelial-to-motoneuron/motoneuron-like cell differentiation.

Rashid et al. report on a conserved epithelial-to-motoneuron transformation pathway in C. elegans requiring ngn-1/Ngn and hlh-16/Olig. lin-12/Notch regulates transformation timing through these genes, while ngn-1/Ngn and hlh-16/Olig expression levels are regulated by sem-4/Sall and egl-5/Hox. Unexpectedly, the cytoskeleton organizers UNC-119, UNC-44, and UNC-33, which are ngn-1/Ngn targets, promote motoneuron terminal identity.

Mechanisms underlying the role of ankyrin-B in cardiac and neurological health and disease

The ANK2 gene encodes for ankyrin-B (ANKB), one of 3 members of the ankyrin family of proteins, whose name is derived from the Greek word for anchor. ANKB was originally identified in the brain (B denotes “brain”) but has become most widely known for its role in cardiomyocytes as a scaffolding protein for ion channels and transporters, as well as an interacting protein for structural and signaling proteins. Certain loss-of-function ANK2 variants are associated with a primarily cardiac-presenting autosomal-dominant condition with incomplete penetrance and variable expressivity characterized by a predisposition to supraventricular and ventricular arrhythmias, arrhythmogenic cardiomyopathy, congenital and adult-onset structural heart disease, and sudden death. Another independent group of ANK2 variants are associated with increased risk for distinct neurological phenotypes, including epilepsy and autism spectrum disorders. The mechanisms underlying ANKB's roles in cells in health and disease are not fully understood; however, several clues from a range of molecular and cell biological studies have emerged. Notably, ANKB exhibits several isoforms that have different cell-type–, tissue–, and developmental stage– expression profiles. Given the conservation within ankyrins across evolution, model organism studies have enabled the discovery of several ankyrin roles that could shed important light on ANKB protein-protein interactions in heart and brain cells related to the regulation of cellular polarity, organization, calcium homeostasis, and glucose and fat metabolism. Along with this accumulation of evidence suggesting a diversity of important ANKB cellular functions, there is an on-going debate on the role of ANKB in disease. We currently have limited understanding of how these cellular functions link to disease risk. To this end, this review will examine evidence for the cellular roles of ANKB and the potential contribution of ANKB functional variants to disease risk and presentation. This contribution will highlight the impact of ANKB dysfunction on cardiac and neuronal cells and the significance of understanding the role of ANKB variants in disease.

The Hyperglycemia Stranglehold Stifles Cutaneous Epithelial‒Mesenchymal Plasticity and Functional Wound Closure

Iterative cycles of epithelial‒mesenchymal transition (EMT) and mesenchymal-to-epithelial transition (MET) are responsible for epithelial plasticity necessary to achieve functional wound closure. Restoration of the barrier function of the repaired skin is a hallmark of functional wound closure. Both EMT and MET are subject to control by glycemic status. A new article by Tan et al (2020) supports the notion that hyperglycemia blunts epithelial plasticity.

Cargo hold and delivery: Ankyrins, spectrins, and their functional patterning of neurons

The highly polarized, typically very long, and nonmitotic nature of neurons present them with unique challenges in the maintenance of their homeostasis. This architectural complexity serves a rich and tightly controlled set of functions that enables their fast communication with neighboring cells and endows them with exquisite plasticity. The submembrane neuronal cytoskeleton occupies a pivotal position in orchestrating the structural patterning that determines local and long-range subcellular specialization, membrane dynamics, and a wide range of signaling events. At its center is the partnership between ankyrins and spectrins, which self-assemble with both remarkable long-range regularity and micro- and nanoscale specificity to precisely position and stabilize cell adhesion molecules, membrane transporters, ion channels, and other cytoskeletal proteins. To accomplish these generally conserved, but often functionally divergent and spatially diverse, roles these partners use a combinatorial program of a couple of dozens interacting family members, whose code is not fully unraveled. In a departure from their scaffolding roles, ankyrins and spectrins also enable the delivery of material to the plasma membrane by facilitating intracellular transport. Thus, it is unsurprising that deficits in ankyrins and spectrins underlie several neurodevelopmental, neurodegenerative, and psychiatric disorders. Here, I summarize key aspects of the biology of spectrins and ankyrins in the mammalian neuron and provide a snapshot of the latest advances in decoding their roles in the nervous system.

Blocking Ca2+ Channel β3 Subunit Reverses Diabetes

Voltage-gated Ca²⁺ channels (Ca_v) are essential for pancreatic beta cell function as they mediate Ca²⁺ influx, which leads to insulin exocytosis. The β3 subunit of Ca_v (Ca_vβ₃) has been suggested to regulate cytosolic Ca²⁺ ([Ca²⁺]_i) oscillation frequency and insulin secretion under physiological conditions, but its role in diabetes is unclear. Here, we report that islets from diabetic mice show Ca_vβ₃ overexpression, altered [Ca²⁺]_i dynamics, and impaired insulin secretion upon glucose stimulation. Consequently, in high-fat diet (HFD)-induced diabetes, Ca_vβ₃-deficient (Ca_vβ₃^−/−) mice showed improved islet function and enhanced glucose tolerance. Normalization of Ca_vβ₃ expression in ob/ob islets by an antisense oligonucleotide rescued the altered [Ca²⁺]_i dynamics and impaired insulin secretion. Importantly, transplantation of Ca_vβ₃^−/− islets into the anterior chamber of the eye improved glucose tolerance in HFD-fed mice. Ca_vβ₃ overexpression in human islets also impaired insulin secretion. We thus suggest that Ca_vβ₃ may serve as a druggable target for diabetes treatment.

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Pancreatic islets from diabetic mice have increased level of Ca_vβ₃

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Overexpression of Ca_vβ₃ in islets alters Ca²⁺ dynamics and impairs insulin secretion

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Deficiency of Ca_vβ₃ prevents islet dysfunction and glucose intolerance in diabetes

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Blocking Ca_vβ₃ improves islet function and glucose tolerance after onset of diabetes

Cell-autonomous adiposity through increased cell surface GLUT4 due to ankyrin-B deficiency

Obesity typically is linked to caloric imbalance as a result of overnutrition. Here we propose a cell-autonomous mechanism for adiposity as a result of persistent cell surface glucose transporter type 4 (GLUT4) in adipocytes resulting from impaired function of ankyrin-B (AnkB) in coupling GLUT4 to clathrin-mediated endocytosis. Adipose tissue-specific AnkB-KO mice develop obesity and progressive pancreatic islet dysfunction with age or high-fat diet (HFD). AnkB-deficient adipocytes exhibit increased lipid accumulation associated with increased glucose uptake and impaired endocytosis of GLUT4. AnkB binds directly to GLUT4 and clathrin and promotes their association in adipocytes. AnkB variants that fail to restore normal lipid accumulation and GLUT4 localization in adipocytes are present in 1.3% of European Americans and 8.4% of African Americans, and are candidates to contribute to obesity susceptibility in humans.

The evolving role of ankyrin-B in cardiovascular disease

Over the past decade, ankyrin-B has been identified as a prominent player in cardiac physiology. Ankyrin-B has a multitude of functions, with roles in expression, localization, and regulation of proteins critical for cardiac excitability, cytoskeletal integrity, and signaling. Furthermore, human ANK2 variants that result in ankyrin-B loss of function are associated with "ankyrin-B syndrome," a complex cardiac phenotype that may include bradycardia and heart rate variability, conduction block, atrial fibrillation, QT interval prolongation, and potentially fatal catecholaminergic polymorphic ventricular tachycardia. However, our understanding of the molecular mechanisms underlying ankyrin-B function at baseline and in disease is still not fully developed owing to the complexity of ankyrin-B gene regulation, number of ankyrin-B-associated molecules, multiple roles of ankyrin-B in the heart and other organs that modulate cardiac function, and a host of unexpected clinical phenotypes. In this review, we summarize known roles of ankyrin-B in the heart and the impact of ankyrin-B dysfunction in animal models and in human disease as well as highlight important new findings illustrating the complexity of ankyrin-B signaling.

A Novel Mechanism for Human Cardiac Ankyrin-B Syndrome due to Reciprocal Chromosomal Translocation

BACKGROUND

Cardiac rhythm abnormalities are a leading cause of morbidity and mortality in developed countries. Loss-of-function variants in the ANK2 gene can cause a variety of cardiac rhythm abnormalities including sinus node dysfunction, atrial fibrillation and ventricular arrhythmias (called the "ankyrin-B syndrome"). ANK2 encodes ankyrin-B, a molecule critical for the membrane targeting of key cardiac ion channels, transporters, and signalling proteins.

METHODS AND RESULTS

Here, we describe a family with a reciprocal chromosomal translocation between chromosomes 4q25 and 9q26 that transects the ANK2 gene on chromosome 4 resulting in loss-of-function of ankyrin-B. Select family members with ankyrin-B haploinsufficiency due to the translocation displayed clinical features of ankyrin-B syndrome. Furthermore, evaluation of primary lymphoblasts from a carrier of the translocation showed altered levels of ankyrin-B as well as a reduced expression of downstream ankyrin-binding partners.

CONCLUSIONS

Thus, our data conclude that, similar to previously described ANK2 loss-of-function "point mutations", large chromosomal translocations resulting in ANK2 haploinsufficiency are sufficient to cause the human cardiac ankyrin-B syndrome. The unexpected ascertainment of ANK2 dysfunction via the discovery of a chromosomal translocation in this family, the determination of the familial phenotype, as well as the complexities in formulating screening and treatment strategies are discussed.

Dysfunction of the β2-spectrin-based pathway in human heart failure

β2-Spectrin is critical for integrating membrane and cytoskeletal domains in excitable and nonexcitable cells. The role of β2-spectrin for vertebrate function is illustrated by dysfunction of β2-spectrin-based pathways in disease. Recently, defects in β2-spectrin association with protein partner ankyrin-B were identified in congenital forms of human arrhythmia. However, the role of β2-spectrin in common forms of acquired heart failure and arrhythmia is unknown. We report that β2-spectrin protein levels are significantly altered in human cardiovascular disease as well as in large and small animal cardiovascular disease models. Specifically, β2-spectrin levels were decreased in atrial samples of patients with atrial fibrillation compared with tissue from patients in sinus rhythm. Furthermore, compared with left ventricular samples from nonfailing hearts, β2-spectrin levels were significantly decreased in left ventricle of ischemic- and nonischemic heart failure patients. Left ventricle samples of canine and murine heart failure models confirm reduced β2-spectrin protein levels. Mechanistically, we identify that β2-spectrin levels are tightly regulated by posttranslational mechanisms, namely Ca(2+)- and calpain-dependent proteases. Furthermore, consistent with this data, we observed Ca(2+)- and calpain-dependent loss of β2-spectrin downstream effector proteins, including ankyrin-B in heart. In summary, our findings illustrate that β2-spectrin and downstream molecules are regulated in multiple forms of cardiovascular disease via Ca(2+)- and calpain-dependent proteolysis.

ARA290 Improves Insulin Release and Glucose Tolerance in Type 2 Diabetic Goto-Kakizaki Rats

Effects of ARA290 on glucose homeostasis were studied in type 2 diabetic Goto-Kakizaki (GK) rats. In GK rats receiving ARA290 daily for up to 4 wks, plasma glucose concentrations were lower after 3 and 4 wks, and hemoglobin A1c (Hb A1c) was reduced by ~20% without changes in whole body and hepatic insulin sensitivity. Glucose-stimulated insulin secretion was increased in islets from ARA290-treated rats. Additionally, in response to glucose, carbachol and KCl, islet cytoplasmic free Ca²⁺ concentrations, [Ca²⁺]_i, were higher and the frequency of [Ca²⁺]_i oscillations enhanced compared with placebo. ARA290 also improved stimulus-secretion coupling for glucose in GK rat islets, as shown by an improved glucose oxidation rate, ATP production and acutely enhanced glucose-stimulated insulin secretion. ARA290 also exerted an effect distal to the ATP-sensitive potassium (K_ATP) channel on the insulin exocytotic pathway, since the insulin response was improved following islet depolarization by KCl when K_ATP channels were kept open by diazoxide. Finally, inhibition of protein kinase A completely abolished effects of ARA290 on insulin secretion. In conclusion, ARA290 improved glucose tolerance without affecting hematocrit in diabetic GK rats. This effect appears to be due to improved γ-cell glucose metabolism and [Ca²⁺]_i handling, and thereby enhanced glucose-induced insulin release.

Ankyrin-B metabolic syndrome combines age-dependent adiposity with pancreatic β cell insufficiency

Rare functional variants of ankyrin-B have been implicated in human disease, including hereditary cardiac arrhythmia and type 2 diabetes (T2D). Here, we developed murine models to evaluate the metabolic consequences of these alterations in vivo. Specifically, we generated knockin mice that express either the human ankyrin-B variant R1788W, which is present in 0.3% of North Americans of mixed European descent and is associated with T2D, or L1622I, which is present in 7.5% of African Americans. Young AnkbR1788W/R1788W mice displayed primary pancreatic β cell insufficiency that was characterized by reduced insulin secretion in response to muscarinic agonists, combined with increased peripheral glucose uptake and concomitantly increased plasma membrane localization of glucose transporter 4 (GLUT4) in skeletal muscle and adipocytes. In contrast, older AnkbR1788W/R1788W and AnkbL1622I/L1622I mice developed increased adiposity, a phenotype that was reproduced in cultured adipocytes, and insulin resistance. GLUT4 trafficking was altered in animals expressing mutant forms of ankyrin-B, and we propose that increased cell surface expression of GLUT4 in skeletal muscle and fatty tissue of AnkbR1788W/R1788W mice leads to the observed age-dependent adiposity. Together, our data suggest that ankyrin-B deficiency results in a metabolic syndrome that combines primary pancreatic β cell insufficiency with peripheral insulin resistance and is directly relevant to the nearly one million North Americans bearing the R1788W ankyrin-B variant.

Integration of modeling with experimental and clinical findings synthesizes and refines the central role of inositol 1,4,5-trisphosphate receptor 1 in spinocerebellar ataxia

A suite of models was developed to study the role of inositol 1,4,5-trisphosphate receptor 1 (IP3R1) in spinocerebellar ataxias (SCAs). Several SCAs are linked to reduced abundance of IP3R1 or to supranormal sensitivity of the receptor to activation by its ligand inositol 1,4,5-trisphosphate (IP3). Detailed multidimensional models have been created to simulate biochemical calcium signaling and membrane electrophysiology in cerebellar Purkinje neurons. In these models, IP3R1-mediated calcium release is allowed to interact with ion channel response on the cell membrane. Experimental findings in mice and clinical observations in humans provide data input for the models. The SCA modeling suite helps interpret experimental results and provides suggestions to guide experiments. The models predict IP3R1 supersensitivity in SCA1 and compensatory mechanisms in SCA1, SCA2, and SCA3. Simulations explain the impact of calcium buffer proteins. Results show that IP3R1-mediated calcium release activates voltage-gated calcium-activated potassium channels in the plasma membrane. The SCA modeling suite unifies observations from experiments in a number of SCAs. The cadre of simulations demonstrates the central role of IP3R1.

Cortical cholinergic dysregulation as a long-term consequence of neonatal hypoglycemia

Neonatal hypoglycemia limits the glucose supply to cells, affecting the function of brain due to its high energy demand. This can cause long-term consequences in brain function, leading to memory and cognitive deficits. The present study evaluated the cholinergic functional regulation in cerebral cortex of one month old rats exposed to neonatal hypoglycemia to understand the long-term effects of early life stress. Receptor binding and gene expression studies were done in the cerebral cortex to analyze the changes in total muscarinicreceptors, muscarinic M1, M2, M3 receptors, and the enzymes involved in acetylcholine metabolism, cholineacetyl transferase and acetylcholine esterase. Neonatal hypoglycemia decreased total muscarinic receptors (p < 0.001) with reduced muscarinic M1, M2, and M3 receptor genes (p < 0.001) in one month old rats. The reduction in acetylcholine metabolism is indicated by the downregulated cholineacetyl transferase, upregulated acetylcholine esterase, and decreased vesicular acetylcholine transporter expression. These alterations in cholinergic function in one month old rat brain indicates the longterm consequences of neonatal hypoglycemia in cortical function, which can contribute to the onset of many disease conditions in later stages of life.

βIV-Spectrin and CaMKII facilitate Kir6.2 regulation in pancreatic beta cells

Identified over a dozen years ago in the brain and pancreatic islet, βIV-spectrin is critical for the local organization of protein complexes throughout the nervous system. βIV-Spectrin targets ion channels and adapter proteins to axon initial segments and nodes of Ranvier in neurons, and βIV-spectrin dysfunction underlies ataxia and early death in mice. Despite advances in βIV-spectrin research in the nervous system, its role in pancreatic islet biology is unknown. Here, we report that βIV-spectrin serves as a multifunctional structural and signaling platform in the pancreatic islet. We report that βIV-spectrin directly associates with and targets the calcium/calmodulin-dependent protein kinase II (CaMKII) in pancreatic islets. In parallel, βIV-spectrin targets ankyrin-B and the ATP-sensitive potassium channel. Consistent with these findings, βIV-spectrin mutant mice lacking CaMKII- or ankyrin-binding motifs display selective loss of expression and targeting of key protein components, including CaMKIIδ. βIV-Spectrin-targeted CaMKII directly phosphorylates the inwardly-rectifying potassium channel, Kir6.2 (alpha subunit of KATP channel complex), and we identify the specific residue, Kir6.2 T224, responsible for CaMKII-dependent regulation of KATP channel function. CaMKII-dependent phosphorylation alters channel regulation resulting in KATP channel inhibition, a cellular phenotype consistent with aberrant insulin regulation. Finally, we demonstrate aberrant KATP channel phosphorylation in βIV-spectrin mutant mice. In summary, our findings establish a broader role for βIV-spectrin in regulation of cell membrane excitability in the pancreatic islet, define the pathway for CaMKII local control in pancreatic beta cells, and identify the mechanism for CaMKII-dependent regulation of KATP channels.

A single divergent exon inhibits ankyrin-B association with the plasma membrane

Vertebrate ankyrin-B and ankyrin-G exhibit divergent subcellular localization and function despite their high sequence and structural similarity and common origin from a single ancestral gene at the onset of chordate evolution. Previous studies of ankyrin family diversity have focused on the C-terminal regulatory domain. Here, we identify an ankyrin-B-specific linker peptide connecting the ankyrin repeat domain to the ZU5₂-UPA module that inhibits binding of ankyrin-B to membrane protein partners E-cadherin and neurofascin 186 and prevents association of ankyrin-B with epithelial lateral membranes as well as neuronal plasma membranes. The residues of the ankyrin-B linker required for autoinhibition are encoded by a small exon that is highly divergent between ankyrin family members but conserved in the ankyrin-B lineage. We show that the ankyrin-B linker suppresses activity of the ANK repeat domain through an intramolecular interaction, likely with a groove on the surface of the ANK repeat solenoid, thereby regulating the affinities between ankyrin-B and its binding partners. These results provide a simple evolutionary explanation for how ankyrin-B and ankyrin-G have acquired striking differences in their plasma membrane association while maintaining overall high levels of sequence similarity.

New horizons in cellular regulation by inositol polyphosphates: insights from the pancreatic β-cell

Studies of inositol polyphosphates in the pancreatic β-cell have led to an exciting synergism between new discoveries regarding their cellular roles and new insights into β-cell function. Because the loss or malfunction of the β-cell is central to diabetes, these studies open the possibility of new pharmacological interventions in a disease that has reached epidemic proportions worldwide. Using the β-cell as our prime but not exclusive example, we examine the inositol polyphosphates in three main groups: 1) inositol 1,4,5-trisphosphate and its influence on Ca(2+) signaling, specifically in a cell in which cytoplasmic-free Ca(2+) concentration is principally increased by plasma membrane standing voltage-gated Ca(2+) channels; 2) higher inositol polyphosphates including a novel second messenger inositol 3,4,5,6-tetrakisphosphate and a regulatory role for inositol hexakisphosphate in β-cell Ca(2+) homeostasis and exo- and endocytosis; and 3) inositol pyrophosphates and their role in β-cell exocytosis, together with the exciting possibility of being novel targets for therapy in diabetes. We conclude with some of the new perspectives that are likely to become apparent in the next few years.

The pancreatic islet as a signaling hub

Over the last two decades we have focused on beta cell signal transduction, bringing many new insights, especially in the context of insulin signal transduction, the role of inositol polyphosphates and the regulation of cytoplasmic free Ca(2+) concentration. However, there has been a growing awareness that the beta cell, which is mandatory for insulin secretion, has a unique context within the micro-organ of the pancreatic Islet of Langerhans. In this environment the beta cell both mediates and receives paracrine regulation, critical for the control of blood glucose homeostasis. Failure of an appropriate beta cell function leads to the development of diabetes mellitus. In our quest to understand the molecular events maintaining beta cell function we have faced two key challenges. Firstly, whilst there are many similarities between signal transduction in pancreatic islets between the much used rodent models and humans there are some notable differences. Critical distinctions between rodent and primate can be made in the structure of the islet, including the arrangement of the islet cells, the innervation pattern and the microcirculation. This means that important signaling interactions between islets cells, mediated through for example insulin, glucagon, GABA, glutamate and ATP, will have a unique human framework. The second challenge was to be able to take the discoveries we have made using in vitro systems and examine them in an in vivo context. Advances in in vivo imaging achieved by utilizing the anterior chamber of the eye as a transplantation site for pancreatic islets make it possible for non-invasive, longitudinal studies at single cell resolution in real time of islet cell physiology and pathology. Thus it is becoming possible to study the insulin secreting pancreatic beta cell within the framework of the unique micro-organ, the Islet of Langerhans, for the first time in a physiological context, i.e. when being innervated and connected to the blood supply.

Noninvasive in vivo model demonstrating the effects of autonomic innervation on pancreatic islet function

The autonomic nervous system is thought to modulate blood glucose homeostasis by regulating endocrine cell activity in the pancreatic islets of Langerhans. The role of islet innervation, however, has remained elusive because the direct effects of autonomic nervous input on islet cell physiology cannot be studied in the pancreas. Here, we used an in vivo model to study the role of islet nervous input in glucose homeostasis. We transplanted islets into the anterior chamber of the eye and found that islet grafts became densely innervated by the rich parasympathetic and sympathetic nervous supply of the iris. Parasympathetic innervation was imaged intravitally by using transgenic mice expressing GFP in cholinergic axons. To manipulate selectively the islet nervous input, we increased the ambient illumination to increase the parasympathetic input to the islet grafts via the pupillary light reflex. This reduced fasting glycemia and improved glucose tolerance. These effects could be blocked by topical application of the muscarinic antagonist atropine to the eye, indicating that local cholinergic innervation had a direct effect on islet function in vivo. By using this approach, we found that parasympathetic innervation influences islet function in C57BL/6 mice but not in 129X1 mice, which reflected differences in innervation densities and may explain major strain differences in glucose homeostasis. This study directly demonstrates that autonomic axons innervating the islet modulate glucose homeostasis.

The pancreatic beta cell as a paradigm for advances in inositide research

In a previous review for Advances in Enzyme Research (Berggren and Barker, 2008) we outlined the history of our involvement in discovering important roles for inositides in the insulin secreting pancreatic beta cell. In this current appraisal we bring the work up to date and project how we believe this field will continue to develop in the future. Recently, we have seen an important synergism between the growth in our understanding of inositide function and our knowledge of beta cell stimulus-secretion coupling in both physiological and pathophysiological contexts. Important advances have been made in three areas. 1. The classic regulation of cytoplasmic free Ca(2+) concentration [Ca(2+)](i) by Inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and its receptor, 2. A novel role of the inositol pyrophosphates, especially 5-diphosphoinositol pentakisphosphate (5-PP-InsP(5)), in exocytosis, and 3. The unique signaling roles of PI3K pathways instituted by the engagement of the insulin receptor in an autocrine, positive feed-back loop. We examine each of these in turn and close with an assessment of the likely future directions the research will take.

A likelihood ratio-based Mann-Whitney approach finds novel replicable joint gene action for type 2 diabetes

The potential importance of the joint action of genes, whether modeled with or without a statistical interaction term, has long been recognized. However, identifying such action has been a great challenge, especially when millions of genetic markers are involved. We propose a likelihood ratio-based Mann-Whitney test to search for joint gene action either among candidate genes or genome-wide. It extends the traditional univariate Mann-Whitney test to assess the joint association of genotypes at multiple loci with disease, allowing for high-order statistical interactions. Because only one overall significance test is conducted for the entire analysis, it avoids the issue of multiple testing. Moreover, the approach adopts a computationally efficient algorithm, making a genome-wide search feasible in a reasonable amount of time on a high performance personal computer. We evaluated the approach using both theoretical and real data. By applying the approach to 40 type 2 diabetes (T2D) susceptibility single-nucleotide polymorphisms (SNPs), we identified a four-locus model strongly associated with T2D in the Wellcome Trust (WT) study (permutation P-value < 0.001), and replicated the same finding in the Nurses' Health Study/Health Professionals Follow-Up Study (NHS/HPFS) (P-value = 3.03×10-11). We also conducted a genome-wide search on 385,598 SNPs in the WT study. The analysis took approximately 55 hr on a personal computer, identifying the same first two loci, but overall a different set of four SNPs, jointly associated with T2D (P-value = 1.29×10-5). The nominal significance of this same association reached 4.01×10-6 in the NHS/HPFS.

Ankyrin-B protein in heart failure: identification of a new component of metazoan cardioprotection

Ankyrins (ankyrin-R, -B, and -G) are adapter proteins linked with defects in metazoan physiology. Ankyrin-B (encoded by ANK2) loss-of-function mutations are directly associated with human cardiovascular phenotypes including sinus node disease, atrial fibrillation, ventricular tachycardia, and sudden cardiac death. Despite the link between ankyrin-B dysfunction and monogenic disease, there are no data linking ankyrin-B regulation with common forms of human heart failure. Here, we report that ankyrin-B levels are altered in both ischemic and non-ischemic human heart failure. Mechanistically, we demonstrate that cardiac ankyrin-B levels are tightly regulated downstream of reactive oxygen species, intracellular calcium, and the calcium-dependent protease calpain, all hallmarks of human myocardial injury and heart failure. Surprisingly, β(II)-spectrin, previously thought to mediate ankyrin-dependent modulation in the nervous system and heart, is not coordinately regulated with ankyrin-B or its downstream partners. Finally, our data implicate ankyrin-B expression as required for vertebrate myocardial protection as hearts deficient in ankyrin-B show increased cardiac damage and impaired function relative to wild-type mouse hearts following ischemia reperfusion. In summary, our findings provide the data of ankyrin-B regulation in human heart failure, provide insight into candidate pathways for ankyrin-B regulation in acquired human cardiovascular disease, and surprisingly, implicate ankyrin-B as a molecular component for cardioprotection following ischemia.

Vann Bennett: how ankyrin holds it all together. Interview by Caitlin Sedwick

Since discovering ankyrin, Bennett has worked to understand its many functions at cell membranes.

Genome-wide association study of type 2 diabetes in a sample from Mexico City and a meta-analysis of a Mexican-American sample from Starr County, Texas

AIMS/HYPOTHESIS

We report a genome-wide association study of type 2 diabetes in an admixed sample from Mexico City and describe the results of a meta-analysis of this study and another genome-wide scan in a Mexican-American sample from Starr County, TX, USA. The top signals observed in this meta-analysis were followed up in the Diabetes Genetics Replication and Meta-analysis Consortium (DIAGRAM) and DIAGRAM+ datasets.

METHODS

We analysed 967 cases and 343 normoglycaemic controls. The samples were genotyped with the Affymetrix Genome-wide Human SNP array 5.0. Associations of genotyped and imputed markers with type 2 diabetes were tested using a missing data likelihood score test. A fixed-effects meta-analysis including 1,804 cases and 780 normoglycaemic controls was carried out by weighting the effect estimates by their inverse variances.

RESULTS

In the meta-analysis of the two Hispanic studies, markers showing suggestive associations (p < 10(-5)) were identified in two known diabetes genes, HNF1A and KCNQ1, as well as in several additional regions. Meta-analysis of the two Hispanic studies and the recent DIAGRAM+ dataset identified genome-wide significant signals (p < 5 × 10(-8)) within or near the genes HNF1A and CDKN2A/CDKN2B, as well as suggestive associations in three additional regions, IGF2BP2, KCNQ1 and the previously unreported C14orf70.

CONCLUSIONS/INTERPRETATION

We observed numerous regions with suggestive associations with type 2 diabetes. Some of these signals correspond to regions described in previous studies. However, many of these regions could not be replicated in the DIAGRAM datasets. It is critical to carry out additional studies in Hispanic and American Indian populations, which have a high prevalence of type 2 diabetes.

Inositol 1,4,5-trisphosphate receptor 1 mutation perturbs glucose homeostasis and enhances susceptibility to diet-induced diabetes

The inositol 1,4,5-trisphosphate receptors (IP3Rs) as ligand-gated Ca(2)(+) channels are key modulators of cellular processes. Despite advances in understanding their critical role in regulating neuronal function and cell death, how this family of proteins impact cell metabolism is just emerging. Unexpectedly, a transgenic mouse line (D2D) exhibited progressive glucose intolerance as a result of transgene insertion. Inverse PCR was used to identify the gene disruption in the D2D mice. This led to the discovery that Itpr1 is among the ten loci disrupted in chromosome 6. Itpr1 encodes for IP3R1, the most abundant IP3R isoform in mouse brain and also highly expressed in pancreatic β-cells. To study IP3R1 function in glucose metabolism, we used the Itpr1 heterozygous mutant mice, opt/+. Glucose homeostasis in male mice cohorts was examined by multiple approaches of metabolic phenotyping. Under regular diet, the opt/+ mice developed glucose intolerance but no insulin resistance. Decrease in second-phase glucose-stimulated blood insulin level was observed in opt/+ mice, accompanied by reduced β-cell mass and insulin content. Strikingly, when fed with high-fat diet, the opt/+ mice were more susceptible to the development of hyperglycemia, glucose intolerance, and insulin resistance. Collectively, our studies identify the gene Itpr1 being interrupted in the D2D mice and uncover a novel role of IP3R1 in regulation of in vivo glucose homeostasis and development of diet-induced diabetes.

Coordinating electrical activity of the heart: ankyrin polypeptides in human cardiac disease

INTRODUCTION

Over the past ten years, ankyrin polypeptides have emerged as players in cardiac excitation-contraction coupling. Once thought to solely play a structural role, loss-of-function variants of genes encoding ankyrin polypeptides have highlighted how this protein mediates subcellular localization of various electrical components of the excitation-contraction coupling machinery. Evidence has revealed how disruption of this localization is the primary cause of various cardiomyopathies, ranging from long-QT syndrome 4, to sinus node disease, to more common forms of arrhythmias.

AREAS COVERED

The roles of ankyrin polypeptides in excitation-contraction coupling in the heart and the development of ankyrin-specific cardiomyopathies. How ankyrin polypeptides may be involved in structural and electrical remodeling of the heart, post-myocardial infarct. How ankyrin interactions with membrane-bound ion channels may regulate these channels' response to stimuli. New data, which offers the potential for unique therapies, for not only combating heart disease, but also for wider applications to various disease states.

EXPERT OPINION

The ankyrin family of adapter proteins is emerging as an intimate player in cardiac excitation-contraction coupling. Until recently, these proteins have gone largely unappreciated for their importance in proper cardiac function. New insights into how these proteins function within the heart are offering potentially new avenues for therapies against cardiomyopathy.

The role of M(3)-muscarinic receptor signaling in insulin secretion

Recently, M(3)-muscarinic receptor (M3R) has been identified as the bona fide receptor responsible for the cholinergic regulation of glucose-induced insulin release. The molecular mechanisms of such regulation have also begun to be unravelled. These include the conventional G protein-dependent pathways involving calcium mobilization and activation of protein kinase C. In addition, recent studies also provided evidence for G protein-independent pathways in the regulation of insulin secretion by M3R. These include phosphorylation/arrestin-dependent activation of protein kinase D1, Src family kinase-dependent activation of the sodium channel NALCN and the involvement of regulator of G protein signaling (RGS)-4. Time has now come to extend these studies which were done mainly in rodents to human and explore the potential for targeting such pathways at different levels for the treatment of diseases with impaired insulin secretion such as type II diabetes.

Short telomeres compromise β-cell signaling and survival

The genetic factors that underlie the increasing incidence of diabetes with age are poorly understood. We examined whether telomere length, which is inherited and known to shorten with age, plays a role in the age-dependent increased incidence of diabetes. We show that in mice with short telomeres, insulin secretion is impaired and leads to glucose intolerance despite the presence of an intact β-cell mass. In ex vivo studies, short telomeres induced cell-autonomous defects in β-cells including reduced mitochondrial membrane hyperpolarization and Ca²⁺ influx which limited insulin release. To examine the mechanism, we looked for evidence of apoptosis but found no baseline increase in β-cells with short telomeres. However, there was evidence of all the hallmarks of senescence including slower proliferation of β-cells and accumulation of p16^INK4a. Specifically, we identified gene expression changes in pathways which are essential for Ca²⁺-mediated exocytosis. We also show that telomere length is additive to the damaging effect of endoplasmic reticulum stress which occurs in the late stages of type 2 diabetes. This additive effect manifests as more severe hyperglycemia in Akita mice with short telomeres which had a profound loss of β-cell mass and increased β-cell apoptosis. Our data indicate that short telomeres can affect β-cell metabolism even in the presence of intact β-cell number, thus identifying a novel mechanism of telomere-mediated disease. They implicate telomere length as a determinant of β-cell function and diabetes pathogenesis.

Novel insights into the function of β-cell M3 muscarinic acetylcholine receptors: therapeutic implications

Impaired function of pancreatic β-cells is one of the hallmarks of type 2 diabetes. β-cell function is regulated by the activity of many hormones and neurotransmitters, which bind to specific cell surface receptors. The M(3) muscarinic acetylcholine receptor (M3R) belongs to the superfamily of G protein-coupled receptors and, following ligand dependent activation, selectively activates G proteins of the G(q/11) family. Recent studies with M3R mutant mice strongly suggest that β-cell M3Rs play a central role in promoting insulin release and maintaining correct glucose homeostasis. In this review, we highlight recent studies indicating that β-cell M3Rs and components of downstream signaling pathways might represent promising new targets for the treatment of type 2 diabetes.

Science Signaling Podcast: 16 March 2010

Reduction of ankyrin-B activity impairs maximal insulin release and is a risk factor for diabetes.