The immediate objective of this proposal is to build an accurate dynamic model of activation and autophosphorylation of the signaling protein Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) during influx of Ca2+ into the postsynaptic spine through NMDA receptors. The work will proceed in three stages. First, investigators will validate a model of activation of individual monomeric catalytic subunits by Ca2+ and CaM and refine its kinetic parameters by comparing the model to experiments. The deterministic model is implemented in Mathematica;the output of the model will be tested against bench assays of the enzymatic activity of monomeric subunits of CaMKII under a wide range of concentrations of the subunits, CaM, and Ca2+. The concentrations will mimic both in vivo and in vitro conditions. In the second stage, investigators will construct a model of activation of the dodecameric holoenzyme of CaMKII, based on the model validated in the first stage. They will model cooperative activation of subunit dimers within the holoenzyme, and three different paths of autophosphorylation of its subunits. The models will be constructed in the program MCell, which supports spatially correct stochastic models of protein interactions and enzymatic activation, within biologically realistic geometries. This model will employ a new rule-based algorithm to specify the locations and behavior of subunits in holoenzymes. It will be constructed in a well-mixed volume to enable testing by comparison to bench experiments with holoenzymes under a wide range of concentrations of subunits, CaM, and Ca2+. The comparisons will be used to optimize four new parameters in the holoenzyme model, and to choose the most accurate model for progression of autophosphorylation within the holoenzyme. In the third stage, investigators will introduce optimized models of the CaMKII holoenzyme into a larger MCell model of Ca2+ influx into spines through NMDA receptors and its removal by pumps and exchangers. Simulations in MCell with this model will be used to test hypotheses about parameters governing activation of CaMKII in spines. The intellectual merit of the proposal lies in its utility in the study of mechanisms of learning in the central nervous system. The regulatory machinery in a spine controls synaptic strength by regulating activity-dependent changes such as LTP and LTD. We know much about the regulatory enzymes in a spine and we have hypotheses about enzymatic networks that regulate the cellular processes controlling synaptic plasticity, including insertion and removal of glutamate receptors and changes in the shape of the spine actin cytoskeleton. However, at the present stage of analysis, qualitative studies with mutant animals, or over-expression and knock-down of particular enzymes are the dominant paradigm in the field and they are not adequate to bring our knowledge to the next level, which is to establish the timing of the action of each of these players, and the precise conditions and position in the regulatory network at which each one becomes important. To reach that level of understanding, we need better quantitative models and methods. CaMKII is one of the the initial enzymes activated by Ca2+ coming through NMDA receptors during induction of LTP. A well-validated quantitative model of its activation in the powerful MCell program will provide a starting point and an example for the construction of dynamic models of successive steps in spine regulatory pathways. The broader impacts include the educational goal of fostering introduction of computational techniques into cellular neurobiological research. A female postdoctoral fellow will be trained in experimental techniques to test computational models, and in the use of MCell. Undergraduate students (including minority students) will be involved in the work through summer research programs at Caltech and Salk. All models will be made available to the community for download. The models of CaMKII holoenzymes will be a first example of simulation in MCell of interactions within a cytosolic multiprotein complex. The syntax for doing this will be published, and taught in the regular workshops on MCell sponsored by NSF. The proposal has medical significance. Deficiencies in spine signaling pathways that use CaMKII are associated with working memory deficits similar to those that underlie schizophrenia and related thought disorders. A quantitative understanding of the factors governing activation of CaMKII during synaptic activity, and its role in controlling synaptic plasticity will facilitate development of clinically useful pharmacological agents that target specific aspects of synaptic dysfunction with fewer undesirable side effects.
The quantitative model we develop will lay the foundation for increasingly complex molecular models of regulation of synaptic plasticity and mechanisms that underlie reward. Such models will facilitate development of clinically useful pharmacological interventions to target specific aspects of synaptic dysfunction in drug addiction, and in co-morbid mental illnesses, with fewer undesirable side effects.
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