Calcium Signaling in Astrocytes: Modeling Fura-2AM Measurements

Abstract

Event Abstract Back to Event Calcium Signaling in Astrocytes: Modeling Fura-2AM Measurements Eeva Toivari1, Tiina Manninen1*, Amit K. Nahata2, Tuula O. Jalonen3 and Marja-Leena Linne1 1 Tampere University of Technology, Finland 2 Kidney Care Center at DCI, United States 3 School of Medicine, St. Georges University, Department of Physiology and Neuroscience, Grenada Purpose: In addition to wet lab and clinical studies, mathematical models can be used to better understand and verify the hypothesis concerning complex molecular level interactions, including the role of intracellular calcium stores such as endoplasmic reticulum, in cellular functions. Ionic equilibrium in astrocytes can be disturbed by transmitters, for example by serotonin and Alzheimer's disease related amyloid plaques forming amyloid-ß peptides (Aß). We studied the effects of small amounts of Aß fragments on the neurotransmitter-induced calcium signals in astrocytes by doing Fura-2-acetoxymethyl ester (Fura-2AM) measurements and computer modeling based on calcium-induced kinetic reactions. Method: Fura-2AM measurements were done to study the effects of transmitters, such as serotonin and glutamate, on the cytosolic concentration of calcium in astrocytes separately and together with neurotoxic Aß25-35. Fura-2AM is a membrane penetrating derivative of the radiometric calcium indicator Fura-2 used in biochemistry to measure intracellular calcium concentrations by fluorescence. Aß25-35 is a synthetic derivative of the longer peptide Aß42 which predominates in the neuritic plaques. A computational model, introduced by Di Garbo et al. (2007), was adjusted to mimic the calcium signals measured with the calcium imaging technique. The model includes kinetic reaction equations, for example, the effect of both metabotropic and ionotropic receptors, the flux of calcium from/to extracellular matrix, the pumping of calcium from cytosol to the endoplasmic reticulum and the leak back to cytosol, and the release of calcium from the endoplasmic reticulum via inositol 1,4,5-trisphosphate receptors. Results: Though serotonin and glutamate have earlier been shown to induce calcium release in astrocytes (Jalonen et al. 1997, Kimelberg et al. 1997), it is shown in this study how serotonin and Aß25-35, when added together, clearly increase the amplitudes of the calcium signal. Thus, even small amounts of neurotoxic Aß25-35 can have a severe effect on the intracellular ionic equilibrium. In the modeling part of this study, different experimental setups were computationally simulated using different values of the model parameters and/or different inputs. The model presented by Di Garbo et al. (2007) was found to sufficiently explain the results measured by us using the Fura-2AM technique. For example, phenomena related to unfilled intracellular calcium stores were observed both by calcium imaging and computer simulations. Thus, model simulations support the hypothesis about the importance of intracellular calcium stores, such as endoplasmic reticulum, to calcium signaling in astrocytes. Conclusions: We conclude that the experimental results on astrocytic calcium oscillations can be reproduced with the computational model originally proposed by Di Garbo et al. (2007). Modeling the mechanisms of intracellular calcium oscillations in astrocytes is important, as astrocytes have an essential role in regulating the central nervous system microenvironment.