aims conference, 2012
DESCRIPTION
The Dual Oscillator Model for Pancreatic Islets Richard Bertram Department of Mathematics and Programs in Neuroscience and Molecular Biophysics Florida State University, Tallahassee, FL. AIMS Conference, 2012. Collaborators. Arthur Sherman (NIH) Leslie Satin (U. Michigan) - PowerPoint PPT PresentationTRANSCRIPT
The Dual Oscillator Model for Pancreatic Islets
Richard Bertram
Department of Mathematicsand
Programs in Neuroscience and Molecular Biophysics Florida State University, Tallahassee, FL.
AIMS Conference, 2012
Collaborators
Arthur Sherman (NIH)Leslie Satin (U. Michigan)Matthew Merrins (U. Michigan)
Funding: NSF-DMS0917664
What is an Islet of Langerhans?Cluster of electrically coupled hormone-secreting cells,located throughout the pancreas. The human pancreas has about 1 million islets.
Courtesy of Rohit Kulkarni
Immunostained for glucagon (green) and insulin (red)
Insulin Secretion is Pulsatile
Peripheral insulin measurements in the blood of humansexhibits oscillations, suggesting that insulin is secretedin a pulsatile manner.
Porksen et al.,AJP, 273:E908,1997
measured
deconvolved
Central Question:
What is the mechanism for oscillations ininsulin secretion from pancreatic β-cells?
Islets are Electrically Excitable
Islets are like nerve cells in that they produce electricalimpulses. During an upstroke of an impulse Ca2+ entersthe cells, causing insulin to be released.
insulin
calcium
Gilon et al.,JBC, 268:22265,1993
Fast Bursting Oscillations
Simultaneous fast Ca2+ and voltage measurements from a mouseislet in 11.1 mM glucose. From Zhang et al., Biophys. J., 84:2852, 2003
Slow Bursting Oscillations
Slow oscillations of Ca2+ and voltage from an islet...
Zhang et al., Biophys. J., 84:2852, 2003Smith et al., FEBS Lett., 261-187, 1990
…have period similar to slowinsulin oscillations measuredfrom a mouse in vivo (Nunemaker et al., Diabetes,54:3517, 2005)
A BA B
Compound Bursting Oscillations
Henquin et al., Eur. J. Physiol., 393:322, 1982
Bursting oscillations superimposed on a slow wave of activity
The Dual Oscillator Model
Central Hypothesis
Fast, slow, and compound oscillations can all be produced by a mechanism with two coupledoscillators.
Calcium oscillator controls fast bursting
Ca2+ K(Ca) channels
Metabolic oscillator controls slower oscillation
ATP K(ATP) channels
Metabolic Oscillations Can Occur Through Oscillations in Glycolysis
Dano et al., Nature, 402:320-322, 1999
Glycolyticoscillationsin yeast
Modeling Approach
• Cells within an islet are electrically coupled, leading to synchronization within an islet. Our modeltherefore describes a single cell within the synchronizedcell population.
• A β-cell is very small and spherical (diameter ≈10 microns). Modeled as a single compartment;equations depend on time only.
Electrical Component of the DOM
Voltage equation reflects Kirchoff’s current law
Second equation describes dynamics of the K+ activationvariable n. This depends on the voltage.
Electrical/Calcium Components of the DOM
)(/))((
/)( )()(
ckIfcnVnn
CIIIIV
cCa
n
mATPKCaKKCa
Ca2+ enters the cell through L-type Ca2+ channels. The freecytosolic Ca2+ activates K(Ca) channels. Thus, there is mutualfeedback between the electrical and Ca2+ components.
ER is the Endoplasmic Reticulum
Fast Oscillations with the DOMWhen glycolysis is non-oscillatory, the DOM produces fast burstingoscillations, due to the electrical/calcium components of the model.
The ER acts as a slow Ca2+ filter, setting the period ofbursting through its interaction with the cytosol.
Bertram andSherman, BMB,66:1313, 2004
Glycolytic Component of the DOM
Key feature: The product F1,6BP feedsback positively onto the allostericenzyme PFK1 (phosphofructokinase 1).Leads to oscillations due to substratedepletion.
substrate
product
Glycolytic Oscillations Produced if Glucokinase Rate is in the Right Range
(A) Intermediate JGK
(B) High JGK
Glycolytic oscillations in muscleextracts (Tornheim, Diabetes,46:1375, 1997)
Solid-F6P, Dashed-FBP
Bertram et al.,BJ, 87:3074, 2004
Mitochondrial Component
Includes equations for mitochondrial NADH concentration, innermembrane potential, Ca2+ concentration, and ADP/ATP concentrations.
Final 3-compartment model:
Bertram et al.,BJ, 92:1544, 2007
ATP acts on K(ATP) channels to affect V
The Three Types of Activity can be Reproduced by the Model
No glycolytic oscillations
With glycolyticoscillations
With glycolyticoscillations
Bertram et al., Am. J. Physiol., 293:E890, 2007
Experiment
Model
Model
A Test for Glycolytic Oscillations
Fructose-2,6-bisphosphate(F2,6BP) is a high-affinity activator of PFK1
Idea: Modify the activity of the key enzyme PFK1 andsee if islet oscillations behave as predicted by the model.
Competes with F1,6BP for binding to PFK1, thusreducing the positivefeedback
Model Prediction
F2,6BP should make metabolic oscillations smaller and faster by changing the F1,6BP nullcline
Merrins et al.,PLoS One, 2012
F6P
F1,6BP
Experimental Test of the Prediction
Use an adenovirus to overexpressthe Bifunctional Enzyme 2 (BIF2)into islets. Then use moleculartagging to degrade either thekinase or phosphatase action of theenzyme.
Kinase makes F2,6BP from F6P
Phosphatase converts F2,6BP backto F6P
Prediction Confirmed
With kinase theoscillations incalcium arefaster and smaller
Merrins et al.,PLoS One, 2012
Prediction Confirmed
With phosphataseoscillationsare slower and larger
Merrins et al.,PLoS One, 2012
Next Step
Use a FRET sensor to directly measure oscillationsin the concentration of a molecule that is in theglycolytic pathway and downstream of PFK1. Dothese oscillations exist?
Thank You!