A single-channel method for evaluation of very magnitudes of Ca2+ ion fluxes through epsilon4/zeta1 N-methyl-D-aspartate receptor channels in bilayer lipid membranes

Transmembrane Signaling with Lipid-Bilayer Assemblies as a Platform for Channel-Based Biosensing

Artificial and natural lipid membranes that elicit transmembrane signaling is are useful as a platform for channel-based biosensing. In this account we summarize our research on the design of transmembrane signaling associated with lipid bilayer membranes containing nanopore-forming compounds. Channel-forming compounds, such as receptor ion-channels, channel-forming peptides and synthetic channels, are embedded in planar and spherical bilayer lipid membranes to develop highly sensitive and selective biosensing methods for a variety of analytes. The membrane-bound receptor approach is useful for introducing receptor sites on both planar and spherical bilayer lipid membranes. Natural receptors in biomembranes are also used for designing of biosensing methods.

Lobe specific Ca2+-calmodulin nano-domain in neuronal spines: a single molecule level analysis

Calmodulin (CaM) is a ubiquitous Ca²⁺ buffer and second messenger that affects cellular function as diverse as cardiac excitability, synaptic plasticity, and gene transcription. In CA1 pyramidal neurons, CaM regulates two opposing Ca²⁺-dependent processes that underlie memory formation: long-term potentiation (LTP) and long-term depression (LTD). Induction of LTP and LTD require activation of Ca²⁺-CaM-dependent enzymes: Ca²⁺/CaM-dependent kinase II (CaMKII) and calcineurin, respectively. Yet, it remains unclear as to how Ca²⁺ and CaM produce these two opposing effects, LTP and LTD. CaM binds 4 Ca²⁺ ions: two in its N-terminal lobe and two in its C-terminal lobe. Experimental studies have shown that the N- and C-terminal lobes of CaM have different binding kinetics toward Ca²⁺ and its downstream targets. This may suggest that each lobe of CaM differentially responds to Ca²⁺ signal patterns. Here, we use a novel event-driven particle-based Monte Carlo simulation and statistical point pattern analysis to explore the spatial and temporal dynamics of lobe-specific Ca²⁺-CaM interaction at the single molecule level. We show that the N-lobe of CaM, but not the C-lobe, exhibits a nano-scale domain of activation that is highly sensitive to the location of Ca²⁺ channels, and to the microscopic injection rate of Ca²⁺ ions. We also demonstrate that Ca²⁺ saturation takes place via two different pathways depending on the Ca²⁺ injection rate, one dominated by the N-terminal lobe, and the other one by the C-terminal lobe. Taken together, these results suggest that the two lobes of CaM function as distinct Ca²⁺ sensors that can differentially transduce Ca²⁺ influx to downstream targets. We discuss a possible role of the N-terminal lobe-specific Ca²⁺-CaM nano-domain in CaMKII activation required for the induction of synaptic plasticity.

Calmodulin is a versatile Ca²⁺ signal mediator and a buffer in a wide variety of body organs including the heart and brain. In the brain, calmodulin regulates intracellular molecular processes that change the strength of connectivity between neurons, thus contributing to various brain functions including memory formation. The exact molecular mechanism as to how calmodulin regulates these processes is not yet known. Interestingly, in other excitable tissues, including the heart, each of two lobes of calmodulin responds differentially toward Ca²⁺ influx and toward its target molecules (e.g., ion channels). This way, calmodulin precisely controls the Ca²⁺ dynamics of the cell. We wish to test if a similar mechanism may be operational in neurons so that two lobes of calmodulin interact differentially with Ca²⁺ ions to activate different downstream molecules that control the strength of connections between neurons. We constructed a detailed simulation of calmodulin that allows us to keep track of its interactions with Ca²⁺ ions and target proteins at the single molecule level. The simulation predicts that two lobes of calmodulin respond differentially to Ca²⁺ influx both in space and in time. This work opens a door to future experimental testing of the lobe-specific control of neural function by calmodulin.