(49b) Competitive Tuning: Competition’s Role in Setting the Frequency-Dependence of Calmodulin Target Proteins Important in Learning and Memory

More than 10 million people suffer from learning and memory disorders. Ranging from Alzheimerâ??s and Huntingtonâ??s disease to autism and schizophrenia, these disorders are caused by disruptions in protein signaling networks that control the strength of neuronal connections. Normally, dynamic changes in the strength of synaptic connections (called synaptic plasticity) are brought about through the exquisite coordination of neurotransmitter release, protein synthesis, protein localization, and cytoskeletal reorganization. The timing, magnitude, and location of these processes are determined by protein binding events within protein signaling networks and, in particular, enzyme activation initiated by Ca2+-flux through NMDA receptors (NMDARs) in the synapse. Therefore, elucidation of the mechanisms that regulate NMDAR-dependent long-term plasticity depends on a complete understanding of the endogenous tuning mechanisms that pair precise patterns of enzyme activation to certain calcium signals.

The calcium sensor calmodulin (CaM) acts as a common activator of the networks responsible for both the strengthening and weakening of synaptic connections. This is possible, in part, because different calmodulin target proteins are â??tunedâ?? to different calcium signals by their unique binding dynamics. Such systems are traditionally studied in vitro or in vivo, but economic and practical limitations have created a demand for computational models capable of predicting molecular phenomena. These predictions can then be used to guide focused experimental studies.

Previous computational studies have demonstrated the role of binding dynamics, feedback loops, and spatial effects in regulating enzyme activation during synaptic calcium signaling, but competition may serve as an additional tuning mechanism. The concentration of CaM binding sites in the cell far exceeds that of CaM itself, and in vitro studies have demonstrated competitive inhibition among neuronal CaM target proteins. But, despite its implicit presence in many computational models, competition among CaM-dependent proteins has not been explicitly investigated as a regulator of enzymatic activation during synaptic plasticity.

In the present study, a mathematical model of eight experimentally-characterized postsynaptic CaM targets is used to investigate competitionâ??s potential role as a regulator of synaptic plasticity. Specifically, a pair of mathematical models is developed. A â??competitiveâ?? model simulates CaM binding to calcium and eight CaM binding targetsâ?? adenylyl cyclase type I (AC1), the adenylyl cyclase type VIII N-terminus (AC8-Nt), the adenylyl cyclase type VIII C_2b domain (AC8-Ct), calcineurin (CaN), Ca²⁺/CaM-dependent protein kinase II (CaMKII), myosin light chain kinase (MLCK), neurogranin (Ng), and nitric oxide synthase (NOS). An â??isolatedâ?? model simulates CaM binding to calcium and just one target of interest (TOI) from the eight present in the competitive model. Because our model is devoid of complicating feedback loops and spatial effects, the differences in the CaM-binding of each protein in the competitive and isolated models are the pure competitive effects of this system. Indeed, we find that competition for CaM serves as an additional tuning mechanism; the presence of competitors both shifts and sharpens the Ca²⁺ frequency-dependence of target proteins. Notably, simulated competition is necessary to recreate the in vivo frequency-dependence of several notable CaM target proteins. We conclude that competitive tuning is an important dynamical process underlying learning and memory. In addition, we find that our results explain seemingly contradictory experimental studies wherein genetic knockout of neurogranin leads to increases in CaMKII activation, but has been shown to both decrease and increase synaptic strength. This and future work in characterizing the critical components of Ca²⁺/CaM protein signaling may lead to the identification of therapeutics that can treat neurological disorders by modulating target protein binding through competition.