Detection of differentially regulated subsarcolemmal calcium signals activated by vasoactive agonists in rat pulmonary artery smooth muscle cells

Modulation of Anionic Lipid Bilayers by Specific Interplay of Protons and Calcium Ions

Biomembranes, important building blocks of living organisms, are often exposed to large local fluctuations of pH and ionic strength. To capture changes in the membrane organization under such harsh conditions, we investigated the mobility and hydration of zwitterionic and anionic lipid bilayers upon elevated H₃O⁺ and Ca²⁺ content by the time-dependent fluorescence shift (TDFS) technique. While the zwitterionic bilayers remain inert to lower pH and increased calcium concentrations, anionic membranes are responsive. Specifically, both bilayers enriched in phosphatidylserine (PS) and phosphatidylglycerol (PG) become dehydrated and rigidified at pH 4.0 compared to at pH 7.0. However, their reaction to the gradual Ca²⁺ increase in the acidic environment differs. While the PG bilayers exhibit strong rehydration and mild loosening of the carbonyl region, restoring membrane properties to those observed at pH 7.0, the PS bilayers remain dehydrated with minor bilayer stiffening. Molecular dynamics (MD) simulations support the strong binding of H₃O⁺ to both PS and PG. Compared to PS, PG exhibits a weaker binding of Ca²⁺ also at a low pH.

Long-term high-altitude hypoxia influences pulmonary arterial L-type calcium channel-mediated Ca2+ signals and contraction in fetal and adult sheep

Long-term hypoxia (LTH) has a profound effect on pulmonary arterial vasoconstriction in the fetus and adult. Dysregulation in Ca²⁺ signaling is important during the development of LTH-induced pulmonary hypertension. In the present study, we tested the hypothesis that L-type Ca²⁺ channels (Ca_L), which are voltage dependent and found in smooth, skeletal, and cardiac muscle, are important in the adaptation of pulmonary arterial contractions in postnatal maturation and in response to LTH. Pulmonary arteries were isolated from fetal or adult sheep maintained at low or high altitude (3,801 m) for >100 days. The effects were measured using an L-type Ca²⁺ channel opener FPL 64176 (FPL) in the presence or absence of an inhibitor, Nifedipine (NIF) on arterial contractions, intracellular Ca²⁺ oscillations, and ryanodine receptor-driven Ca²⁺ sparks. FPL induced pulmonary arterial contractions in all groups were sensitive to NIF. However, when compared with 125 mM K⁺, FPL contractions were greater in fetuses than in adults. FPL reduced Ca²⁺ oscillations in myocytes of adult but not fetal arteries, independently of altitude. The FPL effects on Ca²⁺ oscillations were reversed by NIF in myocytes of hypoxic but not normoxic adults. FPL failed to enhance Ca²⁺ spark frequency and had little impact on spatiotemporal firing characteristics. These data suggest that Ca_L-dependent contractions are largely uncoupled from intracellular Ca²⁺ oscillations and the development of Ca²⁺ sparks. This raises questions regarding the coupling of pulmonary arterial contractility to membrane depolarization, attendant Ca_L facilitation, and the related associations with the activation of Ca²⁺ oscillations and Ca²⁺ sparks.

Nanojunctions of the Sarcoplasmic Reticulum Deliver Site- and Function-Specific Calcium Signaling in Vascular Smooth Muscles

Vasoactive agents may induce myocyte contraction, dilation, and the switch from a contractile to a migratory-proliferative phenotype(s), which requires changes in gene expression. These processes are directed, in part, by Ca²⁺ signals, but how different Ca²⁺ signals are generated to select each function is enigmatic. We have previously proposed that the strategic positioning of Ca²⁺ pumps and release channels at membrane-membrane junctions of the sarcoplasmic reticulum (SR) demarcates cytoplasmic nanodomains, within which site- and function-specific Ca²⁺ signals arise. This chapter will describe how nanojunctions of the SR may: (1) define cytoplasmic nanospaces about the plasma membrane, mitochondria, contractile myofilaments, lysosomes, and the nucleus; (2) provide for functional segregation by restricting passive diffusion and by coordinating active ion transfer within a given nanospace via resident Ca²⁺ pumps and release channels; (3) select for contraction, relaxation, and/or changes in gene expression; and (4) facilitate the switch in myocyte phenotype through junctional reorganization. This should serve to highlight the need for further exploration of cellular nanojunctions and the mechanisms by which they operate, that will undoubtedly open up new therapeutic horizons.

The complex nature of calcium cation interactions with phospholipid bilayers

Understanding interactions of calcium with lipid membranes at the molecular level is of great importance in light of their involvement in calcium signaling, association of proteins with cellular membranes, and membrane fusion. We quantify these interactions in detail by employing a combination of spectroscopic methods with atomistic molecular dynamics simulations. Namely, time-resolved fluorescent spectroscopy of lipid vesicles and vibrational sum frequency spectroscopy of lipid monolayers are used to characterize local binding sites of calcium in zwitterionic and anionic model lipid assemblies, while dynamic light scattering and zeta potential measurements are employed for macroscopic characterization of lipid vesicles in calcium-containing environments. To gain additional atomic-level information, the experiments are complemented by molecular simulations that utilize an accurate force field for calcium ions with scaled charges effectively accounting for electronic polarization effects. We demonstrate that lipid membranes have substantial calcium-binding capacity, with several types of binding sites present. Significantly, the binding mode depends on calcium concentration with important implications for calcium buffering, synaptic plasticity, and protein-membrane association.

From contraction to gene expression: nanojunctions of the sarco/endoplasmic reticulum deliver site- and function-specific calcium signals

Calcium signals determine, for example, smooth muscle contraction and changes in gene expression. How calcium signals select for these processes is enigmatic. We build on the "panjunctional sarcoplasmic reticulum" hypothesis, describing our view that different calcium pumps and release channels, with different kinetics and affinities for calcium, are strategically positioned within nanojunctions of the SR and help demarcate their respective cytoplasmic nanodomains. SERCA2b and RyR1 are preferentially targeted to the sarcoplasmic reticulum (SR) proximal to the plasma membrane (PM), i.e., to the superficial buffer barrier formed by PM-SR nanojunctions, and support vasodilation. In marked contrast, SERCA2a may be entirely restricted to the deep, perinuclear SR and may supply calcium to this sub-compartment in support of vasoconstriction. RyR3 is also preferentially targeted to the perinuclear SR, where its clusters associate with lysosome-SR nanojunctions. The distribution of RyR2 is more widespread and extends from this region to the wider cell. Therefore, perinuclear RyR3s most likely support the initiation of global calcium waves at L-SR junctions, which subsequently propagate by calcium-induced calcium release via RyR2 in order to elicit contraction. Data also suggest that unique SERCA and RyR are preferentially targeted to invaginations of the nuclear membrane. Site- and function-specific calcium signals may thus arise to modulate stimulus-response coupling and transcriptional cascades.

Permeabilization activated reduction in fluorescence: A novel method to measure kinetics of protein interactions with intracellular structures

Understanding kinetic information is fundamental in understanding biological function. Advanced imaging technologies have fostered the development of kinetic analyses in cells. We have developed Permeabilization Activated Reduction in Fluorescence (PARF) analysis for determination of apparent t1/2 and immobile fraction, describing the dissociation of a protein of interest from intracellular structures. To create conditions where dissociation events are observable, cells expressing a fluorescently-tagged protein are permeabilized with digitonin, diluting the unbound protein into the extracellular media. As the media volume is much larger than the cytosolic volume, the concentration of the unbound pool decreases drastically, shifting the system out of equilibrium, favoring dissociation events. Loss of bound protein is observed as loss of fluorescence from intracellular structures and can be fit to an exponential decay. We compared PARF dissociation kinetics with previously published equilibrium kinetics as determined by FRAP. PARF dissociation rates agreed with the equilibrium-based FRAP analysis predictions of the magnitude of those rates. When used to investigate binding kinetics of a panel of cytoskeletal proteins, PARF analysis revealed that filament stabilization resulted in slower fluorescence loss. Additionally, commonly used "general" F-actin labels display differences in kinetic properties, suggesting that not all fluorescently-tagged actin labels interact with the actin network in the same way. We also observed differential dissociation kinetics for GFP-VASP depending on which cellular structure was being labeled. These results demonstrate that PARF analysis of non-equilibrium systems reveals kinetic information without the infrastructure investment required for other quantitative approaches such as FRAP, photoactivation, or in vitro reconstitution assays. © 2016 Wiley Periodicals, Inc.