Quantitative analysis of inositol lipids and inositol phosphates in synaptosomes and microvessels by column chromatography: comparison of the mass analysis and the radiolabelling methods

Insertion of circularly permuted cyan fluorescent protein into the ligand-binding domain of inositol 1,4,5-trisphosphate receptor for enhanced FRET upon binding of fluorescent ligand

Binding of fluorescent ligand (FL) to the cyan fluorescent protein (CFP)-coupled ligand-binding domain of the inositol 1,4,5-trisphosphate (IP₃) receptor (CFP-LBP) produces fluorescence (Förster) resonance energy transfer (FRET). A competitive fluorescent ligand assay (CFLA), using the FRET signal from competition between FLs and IP₃, can measure IP₃ concentration. The FRET signal should be enhanced by attaching a FRET donor to an appropriate position. Herein, we inserted five different circularly permuted CFPs in the loop between the second and third α-helices to generate membrane-targeted fluorescent ligand-binding proteins (LBPs). Two such proteins, LBP-cpC157 and LBP-cpC173, localized at the plasma membrane, displayed FRET upon binding the high-affinity ligand fluorescent adenophostin A (F-ADA), and exhibited a decreased fluorescence emission ratio (480 nm / 535 nm) by 1.6- to 1.8-fold that of CFP-LBP. In addition, binding of a fluorescent low-affinity ligand (F-LL) also reduced the fluorescence ratio in a concentration-dependent manner, with EC₅₀ values for LBP-cpC157 and LBP-cpC173 of 34.7 nM and 27.6 nM, respectively. These values are comparable to that with CFP-LBP (29.2 nM), indicating that insertion of cpC157 and cpC173 did not disrupt LBP structure and function. The effect of 100 nM F-LL on the decrease in fluorescence ratio was reversed upon addition of IP₃, indicating binding competition between F-LL and IP₃. We also constructed cytoplasmic fluorescent proteins cyLBP-cpC157 and cyLBP-cpC173, and bound them to DYK beads for imaging analyses. Application of F-ADA decreased the fluorescence ratio of the beads from the periphery to the center over 3 - 5 min. Application of F-LL also decreased the fluorescence ratio of cyLBP-cpC157 and cyLBP-cpC173 by 20-25%, and subsequent addition of IP₃ recovered the fluorescence ratio in a concentration-dependent manner. The EC₅₀ value and Hill coefficient obtained by curve fitting against the IP₃-dependent recovery of fluorescence ratio can be used to estimate the IP₃ concentration.

Competitive Fluorescent Ligand Assay for Inositol 1,4,5-Trisphosphate

We present a novel method, termed competitive fluorescent ligand assay for inositol 1,4,5-trisphosphate (CFLA-IP₃), to measure inositol 1,4,5-trisphosphate (IP₃). This method is based on fluorescence resonance energy transfer (FRET) between two fluorescent molecules, a fluorescent IP₃-binding protein and its fluorescent ligand. Binding of these fluorescent molecules generates a FRET signal, and the IP₃-dependent decrease in the FRET signal due to displacement of the fluorescent ligand is detected by fluorescence microscopy.

[Advances in methods for analyzing IP3 signaling and understanding of coupled Ca2+ and IP3 oscillations]

Inositol 1,4,5-trisphosphate (IP₃) is an important intracellular messenger produced by phospholipase C via the activation of G-protein-coupled receptor- or receptor-tyrosine-kinase-mediated pathways, and is involved in numerous responses to hormones, neurotransmitters, and growth factors through the releases of Ca²⁺ from intracellular stores via IP₃ receptors. IP₃-mediated Ca²⁺ signals often exhibit complex spatial and temporal organizations, such as Ca²⁺ oscillations. Recently, new methods have become available to measure IP₃ concentration ([IP₃]) using AlphaScreen technology, fluorescence polarization, and competitive ligand binding assay (CFLA). These methods are useful for the high throughput screening in drug discovery. Calcium ions generate versatile intracellular signals such as Ca²⁺ oscillations and waves. Fluorescent sensors molecules to monitor changes in [IP₃] in single living cells are crucial to study the mechanism for the spatially and temporally regulated Ca²⁺ signals. In particular, FRET-based IP₃ sensors are useful for the quantitative monitoring intracellular [IP₃], and allowed to uncovered the oscillatory IP₃ dynamics in association with Ca²⁺ oscillations. A mathematical model of coupled Ca²⁺ and IP₃ oscillations predicts that Ca²⁺ oscillations are the result of modulation of the IP₃ receptor by intracellular Ca²⁺, and that the period is modulated by the accompanying IP₃ oscillations. These model predictions have also been confirmed experimentally. At present, however, usefulness of FRET-based IP₃ sensors are limited by their relatively small change in fluorescence. Development of novel IP₃ sensors with improve dynamic range would be important for understanding the regulatory mechanism of Ca²⁺ signaling and for in vivo IP₃ imaging.

A Miniaturized Column Chromatography Method for Measuring Receptor-Mediated Inositol Phosphate Accumulation

Inositol phosphates (IPs), such as 1,4,5-inositol-trisphosphate (IP3), comprise a ubiquitous intracellular signaling cascade initiated in response to G protein-coupled receptor-mediated activation of phospholipase C. Classical methods for measuring intracellular accumulation of these molecules include time-consuming high-performance liquid chromatography (HPLC) separation or large-volume, gravity-fed anion-exchange column chromatography. More recent approaches, such as radio-receptor and AlphaScreen™ assays, offer higher throughput. However, these techniques rely on measurement of IP3itself, rather than its accumulation with other downstream IPs, and often suffer from poor signal-to-noise ratios due to the transient nature of IP3. The authors have developed a miniaturized, anion-exchange chromatography method for measuring inositol phosphate accumulation in cells that takes advantage of signal amplification achieved through measuring IP3and downstream IPs. This assay uses centrifugation of 96-well-formatted anion-exchange mini-columns for the isolation of radiolabeled inositol phosphates from cell extracts, followed by low-background dry-scintillation counting. This improved assay method measures receptor-mediated IP accumulation with signal-to-noise and pharmacological values comparable to the classical large-volume, column-based methods. Assay validation data for recombinant muscarinic receptor 1, galanin receptor 2, and rat astrocyte metabotropic glutamate receptor 5 are presented. This miniaturized protocol reduces reagent usage and assay time as compared to large-column methods and is compatible with standard 96-well scintillation counters.

Nonradioactive analysis of phosphatidylinositides and other anionic phospholipids by anion-exchange high-performance liquid chromatography with suppressed conductivity detection

Phosphatidylinositol 4,5-biphosphate (PIP(2)) modulates the function of numerous ion transporters and channels, as well as cell signaling and cytoskeletal proteins. To study PIP(2) levels of cells without radiolabeling, we have developed a new method to quantify anionic phospholipid species. Phospholipids are extracted and deacylated to glycero-head groups, which are then separated by anion-exchange HPLC and detected by suppressed conductivity measurements. The major anionic head groups can be quantified in single runs with practical detection limits of about 100 pmol, and the D3 isoforms of phosphatidylinositol phosphate (PIP) and PIP(2) are detected as shoulder peaks. In HeLa, Hek 293 and COS cells, as well as intact heart, PIP(2) amounts to 0.5 to 1.5% of total anionic phospholipid (10 to 30 micromol/liter cell water or 0.15 to 0.45 nmol/mg protein). In cell cultures, overexpression of Type I PIP5-kinase specifically increases PIP(2), whereas overexpression of Type II PI4-kinase can increase both PIP and PIP(2). Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) and the D3 isomers of PIP(2) are detected after treatment of cells with pervanadate; in yeast, overexpression of a phosphatidylinositol 3-kinase (VPS34) specifically increases phosphatidylinositol 3-phosphate (PI3P). Using isolated cardiac membranes, lipid kinase and lipid phosphatase activities can be monitored with the same methods. Upon addition of ATP, PIP increases while PIP(2) remains low; exogenous PIP(2) is rapidly degraded to PIP and phosphatidylinositol (PI). In summary, the HPLC methods described here can be used to probe multiple aspects of phosphatidylinositide (Ptide) metabolism without radiolabeling.

Quantitative chromatographic analysis of inositol phospholipids and related compounds

The metabolism of phospholipids and the mobilization of second messengers such as inositol-1,4,5-trisphosphate, 1,2-diacylglycerol (DAG) and arachidonic acid (AA) from phospholipids is commonly studied by radiolabelling phospholipids with [3H]myo-inositol or [32P]ATP and measuring the incorporation of radioactivity in different phospholipids or their hydrolysis products. However, for the radiolabelling method to accurately reflect changes in the compound's mass, it is essential that the tissue is labelled to isotopic equilibrium which is difficult to achieve. To circumvent the disadvantages of the radiolabelling method, several analytical procedures have been developed for the mass analysis of phospholipids and inositolphosphates (IPs). Quantitation of the mass or the radiolabelling of phospholipids is a complex multi-step procedure that involves quantitative isolation of phospholipids, fractionation of individual phospholipids and either determination of radioactivity in each component or the measurement of their mass. Phospholipids, DAG and AA are extracted from tissue sample with organic solvents such as chloroform-methanol (2:1) containing HCl or formic acid. The extract is separated by TLC, cartridge-column chromatography or HPLC on a reversed-phase column. Phospholipids are quantitated by measuring inorganic phosphate, absorption at 200 nm or mass spectrometry. Inositol phosphates are extracted with perchloric acid or trichloroacetic acid and separated by ion-exchange cartridge-column or HPLC with an ion-exchange column. IPs are quantitated by measuring inorganic phosphate or by using enzymatic reaction, metal-dye coupling, NMR or mass spectrometry.

Age-dependent neurotoxicity in rats chronically exposed to low level lead ingestion: phospholipid metabolism in synaptosomes and microvessels

The uptake of [3H]Ch and [3H]MI by synaptosomes or microvessels, the concentration of membrane phospholipids, and the incorporation of [3H]Ch or [3H]MI into the respective phospholipids in synaptosomes or microvessels, were studied in samples obtained from the brain of control rats and rats exposed to a low-level lead ingestion starting prenatally, neonatally or at an adult age. The Vmax values for the uptake of [3H]Ch by control-neonatal and control-adult samples were significantly different. However, there was no significant difference in the Vmax values for the uptake of [3H]MI by control-neonatal and control-adult samples. The same was true for the Km values for the uptake of [3H]Ch or [3H]MI. Chronic exposure of embryonic and neonatal rats to a low-level lead ingestion inhibited the rate of uptake of [3H]Ch and [3H]Mi by the brain synaptosomes or microvessels, reduced the concentrations of Ch and MI phospholipids in membranes of these tissues, and did not effect the incorporation of [3H]Ch and [3H]MI into the respective membrane phospholipids. In adult rats, these changes were not observed following chronic exposure. These observations suggest that Ch and MI transport mechanisms in the brain of embryonic and neonatal rats are sensitive to chronic low-level lead ingestion but Ch and MI transport mechanisms in the brain of adult rats are not. A lead-induced decrease in the availability of Ch and MI in the brain may be responsible for the observed decrease in the concentrations of phospholipids.