Stimulus secretion coupling in pancreatic beta-cells

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Impaired insulin secretion is a key step in the pathogenesis of type 2 diabetes, along with inefficient use of insulin by target tissues. The two main components of secretion are calcium entry into pancreatic beta cells and triggering of insulin granule exocytosis by that calcium. We have modeled both components and their contributions to diabetes. We have focused mainly on the mechanisms of calcium oscillations over a range of periods, from seconds to minutes. The slower class of oscillations (5 - 10 minute period) is the main driver of pulsatile insulin concentration in the circulation, which has been shown to be optimal for the response of insulin-sensitive tissues, especially the liver. Details of the historical development of the model can be found in our reports from previous years and in a current review article (ref. #1). We call the resulting model the integrated oscillator model (IOM) to indicate that the oscillations result from the partnership of an electrical oscillator (EO) and a metabolic oscillator (MO). The electrical oscillator (EO) is based on negative feedback of calcium onto ion channels, directly onto calcium-activated potassium (K(Ca)) channels and indirectly onto ATP-dependent potassium (K(ATP)) channels because calcium reduces the ATP/ADP ratio. The metabolic oscillator (MO) is governed by positive feedback of fructose 1,6 bisphosphate (FBP) on the enzyme in glycolysis that produces it, phosphofructokinase (PFK). The MO communicates with the EO via the K(ATP) channels, which transduce the metabolic state of the cell (ATP/ADP ratio) into electrical depolarization. K(ATP) channels are of clinical significance as they are a target of insulin-stimulating drugs, such as the sulfonylureas tolbutamide and glyburide, the first class of oral medications developed for the treatment of Type 2 Diabetes. Severe gain-of-function mutations of K(ATP) are a major cause of neo-natal diabetes mellitus, whereas moderate gain-of-function mutations have been linked in genome-wide association studies (GWAS) to the milder but more common adult-onset form of diabetes, type 2 diabetes. Conversely, loss-of-function mutations of K(ATP) are a major cause of familial hyperinsulinism, a hereditary disease found in children in which beta cells are persistently electrically active and secrete insulin in the face of normal or low glucose, causing life-threatening hypoglycemia. Another major cause of hyperinsulinism is excessive activity of the enzyme glucokinase (GK), which also plays a key role in the DOM. Notwithstanding the central role of PFK, the model predicted that oscillations in calcium and ATP could occur in the absence of the most important isoform of PFK, PFKm, because other isoforms could substitute for it. This was described in last year's report, and a paper has now appeared (ref. #2). Because increases in cytosolic calcium activate pumps that consume ATP to bring calcium back down, any oscillation in calcium will be reflected in a oscillations in ATP/ADP. However, our model shows that when ATP/ADP is the cause, not just a consequence, of calcium oscillations, then the ATP/ADP ratio remains constant as glucose is increased within the range that supports oscillations. This theoretical result runs counter to the natural expectation that increases in glucose increase ATP production. However, this neglects the fact that, when ATP, acting through KATP channels, is the driver of the calcium oscillations, then consumption of ATP by calcium pumps increases in tandem. This novel theoretical prediction is straightforward to test, and experiments in the Satin laboratory have confirmed that the ATP/ADP ratio is nearly invariant within the oscillatory regime, as detailed in ref. #3. This supports our hypothesis that ATP/ADP is the primary active driver of the calcium oscillations. We are therefore confident that metabolic oscillations are central in the generation of calcium oscillations and of pulsatile insulin secretion. However, we have come to appreciate that metabolic oscillations come in two flavors. They can arise through the autonomous activity of a glycolytic oscillator, which we call Active Metabolic Oscillations (AMOs), or by passively responding to consumption of ATP due to the need to pump calcium out of the cells, which we call Passive Metabolic Oscillations (PMOs). At the higher glucose concentrations (ca. 200 mg/dl) usually studied in vitro, PMOs predominate, but physiologically glucose mainly fluctuates in a lower range from a basal level of less than 100 mg/dl to post-prandial level of about 120 mg/dl. Large amplitude calcium oscillations, which are obligatory in PMO mode do not occur in basal glucose, yet pulsatile insulin secretion, albeit of small amplitude, still occurs. This led us to propose a hypothesis that the basal insulin pulses are driven by AMOs. Not only is this hypothesis novel, but the need to explain the basal oscillations had not been previously recognized in the field. Furthermore, as glucose increases after meals, AMOs smoothly give way to PMOs. In ref. #4 we made the analogy to a hybrid gas-electric car, in which an electric motor active at low speeds smoothly hands over control to a gasoline engine at higher speeds. This concept was featured on the cover of that issue of the journal. Another area of contention in islet biology is how the hundreds of beta cells in each pancreatic islet coordinate their activity to produce coherent oscillation. For many years, islets were viewed as democratic, with all the cells contributing and no central pacemaker needed to conduct the orchestra. Recently, however, an alternative hypothesis has been proposed that a small subset of beta cells, called "hub cells" because they are highly coupled to the rest of the beta cells, are required to coherent activity. The existence of such hub cells is well established, but we and others have questioned the suggestion that coordinated oscillations cannot occur in their absence. For one thing, this would render islets very vulnerable to loss of a handful of cells, which seems like a poor design principle. The intriguing and challenging hub hypothesis has led to a number of modeling studies by others to investigate whether islets could or do work in this oligarchical manner. We have summarized the results of those studies in a review paper(ref. #5) and concluded that most of the experimental observations can be accommodated but that the existence of obligatory pacemakers is unlikely.