Ca(2+) signaling in arterioles and small arteries of conscious, restrained, optical biosensor mice.

UNLABELLED: Two-photon fluorescence microscopy and conscious, restrained optical biosensor mice were used to study smooth muscle Ca(2+) signaling in ear arterioles. Conscious mice were used in order to preserve normal mean arterial blood pressure (MAP) and sympathetic nerve activity (SNA). ExMLCK mice, which express a genetically-encoded smooth muscle-specific FRET-based Ca(2+) indicator, were equipped with blood pressure telemetry and immobilized for imaging. MAP was 101 ± 4 mmHg in conscious restrained mice, similar to the freely mobile state (107 ± 3 mmHg). Oscillatory vasomotion or irregular contractions were observed in most arterioles (71%), with the greatest oscillatory frequency observed at 0.25 s(-1). In a typical arteriole with an average diameter of ~35 µm, oscillatory vasomotion of a 5-6 µm magnitude was accompanied by nearly uniform [Ca(2+)] oscillations from ~0.1 to 0.5 µM, with maximum [Ca(2+)] occurring immediately before the rapid decrease in diameter. Very rapid, spatially uniform "Ca(2+) flashes" were also observed but not asynchronous propagating Ca(2+) waves. In contrast, vasomotion and dynamic Ca(2+) signals were rarely observed in ear arterioles of anesthetized exMLCK biosensor mice. Hexamethonium (30 µg/g BW, i.p.) caused a fall in MAP to 74 ± 4 mmHg, arteriolar vasodilation, and abolition of vasomotion and synchronous Ca(2+) transients. SUMMARY: MAP and heart rate (HR) were normal during high-resolution Ca(2+) imaging of conscious, restrained mice. SNA induced continuous vasomotion and irregular vasoconstrictions via spatially uniform Ca(2+) signaling within the arterial wall. FRET-based biosensor mice and two-photon imaging provided the first measurements of [Ca(2+)] in vascular smooth muscle cells in arterioles of conscious animals.

A method for noninvasive longitudinal measurements of [Ca2+] in arterioles of hypertensive optical biosensor mice.

We used two-photon (2-p) Förster resonance energy transfer (FRET) microscopy to provide serial, noninvasive measurements of [Ca(2+)] in arterioles of living "biosensor" mice. These express a genetically encoded Ca(2+) indicator (GECI), either FRET-based exMLCK or intensity-based GCaMP2. The FRET ratios, Rmin and Rmax, required for in vivo Ca(2+) calibration of exMLCK were obtained in isolated arteries. For in vivo experiments, mice were anesthetized (1.5% isoflurane), and arterioles within a depilated ear were visualized through the intact skin (i.e., noninvasively), by 2-p excitation of exMLCK (at 820 nm) or GCaMP2 (at 920 nm). Spontaneous or agonist-evoked [Ca(2+)] transients in arteriolar smooth muscle cells were imaged (at 2 Hz) with both exMLCK and GCaMP2. To examine changes in arteriolar [Ca(2+)] that might accompany hypertension, five exMLCK mice were implanted with telemetric blood pressure transducers and osmotic minipumps containing ANG II (350 ng·kg(-1)·min(-1)) and fed a high (6%)-salt diet for 9 days. [Ca(2+)] was measured every other day in five smooth muscle cells of two to three arterioles in each animal. Prior to ANG II/salt, [Ca(2+)] was 246 ± 42 nM. [Ca(2+)] increased transiently to 599 nM on day 2 after beginning ANG II/salt, then remained elevated at 331 ± 42 nM for 4 more days, before returning to 265 ± 47 nM 6 days after removal of ANG II/salt. In summary, two-photon excitation of exMLCK and GCaMP2 provides a method for noninvasive, longitudinal quantification of [Ca(2+)] dynamics and vascular structure in individual arterioles of a particular animal over an extended period of time, a capability that should enhance future studies of hypertension and vascular function.

The 1.6 Å resolution structure of a FRET-optimized Cerulean fluorescent protein.

Genetically encoded cyan fluorescent proteins (CFPs) bearing a tryptophan-derived chromophore are commonly used as energy-donor probes in Förster resonance energy transfer (FRET) experiments useful in live cell-imaging applications. In recent years, significant effort has been expended on eliminating the structural and excited-state heterogeneity of these proteins, which has been linked to undesirable photophysical properties. Recently, mCerulean3, a descendant of enhanced CFP, was introduced as an optimized FRET donor protein with a superior quantum yield of 0.87. Here, the 1.6 Šresolution X-ray structure of mCerulean3 is reported. The chromophore is shown to adopt a planar trans configuration at low pH values, indicating that the acid-induced isomerization of Cerulean has been eliminated. ß-Strand 7 appears to be well ordered in a single conformation, indicating a loss of conformational heterogeneity in the vicinity of the chromophore. Although the side chains of Ile146 and Leu167 appear to exist in two rotamer states, they are found to be well packed against the indole group of the chromophore. The Ser65 reversion mutation allows improved side-chain packing of Leu220. A structural comparison with mTurquoise2 is presented and additional engineering strategies are discussed.

Association with nitric oxide synthase on insulin secretory granules regulates glucokinase protein levels.

Glucokinase (GCK) association with insulin-secretory granules is controlled by interaction with nitric oxide synthase (NOS) and is reversed by GCK S-nitrosylation. Nonetheless, the function of GCK sequestration on secretory granules is unknown. Here we report that the S-nitrosylation blocking V367M mutation prevents GCK accumulation on secretory granules by inhibiting association with NOS. Expression of this mutant is reduced compared with a second S-nitrosylation blocking GCK mutant (C371S) that accumulates to secretory granules and is expressed at levels greater than wild type. Even so, the rate of degradation for wild type and mutant GCK proteins were not significantly different from one another, and neither mutation disrupted the ability of GCK to be ubiquitinated. Furthermore, gene silencing of NOS reduced endogenous GCK content but did not affect ß-actin content. Treatment of GCK(C371S) expressing cells with short interfering RNA specific for NOS also blocked accumulation of this protein to secretory granules and reduced expression levels to that of GCK(V367M). Conversely, cotransfection of catalytically inactive NOS increased GCK-mCherry levels. Expression of GCK(C371S) in ßTC3 cells enhanced glucose metabolism compared with untransfected cells and cells expressing wild type GCK, even though this mutant has slightly reduced enzymatic activity in vitro. Finally, molecular dynamics simulations revealed that V367M induces conformational changes in GCK that are similar to S-nitrosylated GCK, thereby suggesting a mechanism for V367M-inhibition of NOS association. Our findings suggest that sequestration of GCK on secretory granules regulates cellular GCK protein content, and thus cellular GCK activity, by acting as a storage pool for GCK proteins.

Transformation of postingestive glucose responses after deletion of sweet taste receptor subunits or gastric bypass surgery.

The glucose-dependent secretion of the insulinotropic hormone glucagon-like peptide-1 (GLP-1) is a critical step in the regulation of glucose homeostasis. Two molecular mechanisms have separately been suggested as the primary mediator of intestinal glucose-stimulated GLP-1 secretion (GSGS): one is a metabotropic mechanism requiring the sweet taste receptor type 2 (T1R2) + type 3 (T1R3) while the second is a metabolic mechanism requiring ATP-sensitive K(+) (K(ATP)) channels. By quantifying sugar-stimulated hormone secretion in receptor knockout mice and in rats receiving Roux-en-Y gastric bypass (RYGB), we found that both of these mechanisms contribute to GSGS; however, the mechanisms exhibit different selectivity, regulation, and localization. T1R3(-/-) mice showed impaired glucose and insulin homeostasis during an oral glucose challenge as well as slowed insulin granule exocytosis from isolated pancreatic islets. Glucose, fructose, and sucralose evoked GLP-1 secretion from T1R3(+/+), but not T1R3(-/-), ileum explants; this secretion was not mimicked by the K(ATP) channel blocker glibenclamide. T1R2(-/-) mice showed normal glycemic control and partial small intestine GSGS, suggesting that T1R3 can mediate GSGS without T1R2. Robust GSGS that was K(ATP) channel-dependent and glucose-specific emerged in the large intestine of T1R3(-/-) mice and RYGB rats in association with elevated fecal carbohydrate throughout the distal gut. Our results demonstrate that the small and large intestines utilize distinct mechanisms for GSGS and suggest novel large intestine targets that could mimic the improved glycemic control seen after RYGB.

An IFN-γ-stimulated ATF6-C/EBP-ß-signaling pathway critical for the expression of Death Associated Protein Kinase 1 and induction of autophagy.

The IFN family of cytokines operates a frontline defense against pathogens and neoplastic cells in vivo by controlling the expression of several genes. The death-associated protein kinase 1 (DAPK1), an IFN-γ-induced enzyme, controls cell cycle, apoptosis, autophagy, and tumor metastasis, and its expression is frequently down-regulated in a number of human tumors. Although the biochemical action of DAPK1 is well understood, mechanisms that regulate its expression are unclear. Previously, we have shown that transcription factor C/EBP-ß is required for the basal and IFN-γ-induced expression of DAPK1. Here, we show that ATF6, an ER stress-induced transcription factor, interacts with C/EBP-ß in an IFN-stimulated manner and is obligatory for Dapk1 expression. IFN-stimulated proteolytic processing of ATF6 and ERK1/2-mediated phosphorylation of C/EBP-ß are necessary for these interactions. More importantly, IFN-γ failed to activate autophagic response in cells lacking either ATF6 or C/EBP-ß. Consistent with these observations, the Atf6(-/-) mice were highly susceptible to lethal bacterial infections compared with the wild-type mice. These studies not only unravel an IFN signaling pathway that controls cell growth and antibacterial defense, but also expand the role of ATF6 beyond ER stress.

Naturally occurring glucokinase mutations are associated with defects in posttranslational S-nitrosylation.

Posttranslational activation of glucokinase (GCK) through S-nitrosylation has been recently observed in the insulin-secreting pancreatic beta-cell; however, the function of this molecular mechanism in regulating the physiology of insulin secretion is not well understood. To more fully understand the function of posttranslational regulation of GCK, we examined two naturally occurring GCK mutations that map to residues proximal to the S-nitrosylated cysteine and cause mild fasting hyperglycemia (maturity-onset diabetes of the young; subtype glucokinase). The kinetics of recombinantly generated GCK-R369P and GCK-V367M were assessed in vitro. The GCK-R369P protein has greatly reduced catalytic activity (relative activity index 0.05 vs. 1.00 for wild type), whereas the GCK-V367M has near normal kinetics (relative activity index 1.26 vs. 1.00 for wild type). Quantitative imaging and biochemical assays were used to assess the effect of these mutants on the metabolic response to glucose, GCK activation, and S-nitrosylation of GCK in betaTC3 insulinoma cells. Expression of either mutant in betaTC3 cells did not affect the metabolic response to 5 mM glucose. However, expression of either mutant blocked the effects of insulin on glucose-stimulated nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate reduction, suggesting defects in posttranslational regulation of GCK. Each of these mutations blocked GCK activation, and prevented posttranslational cysteine S-nitrosylation. Our findings link defects in hormone-regulated GCK S-nitrosylation to hyperglycemia and support a role for posttranslational regulation of GCK S-nitrosylation as a vital regulatory mechanism for glucose-stimulated insulin secretion.

A technique for simultaneous measurement of Ca2+, FRET fluorescence and force in intact mouse small arteries.

FRET (Forster resonance energy transfer)-based biosensor molecules are powerful tools to reveal specific molecular interactions in cells. Typically however, they are used in cultured cells that (inevitably) express different genes than their counterparts in intact organisms. In such cells it may be impossible to administer physiological stimuli and measure physiological outputs. Here, through the use of transgenic mice that express a FRET-based myosin light chain kinase (MLCK) biosensor molecule, we report a technique for dynamically observing activation and regulation of MLCK within the smooth muscle cells of intact, functioning small arteries, together with measurement of arterial force production and intracellular [Ca(2+)].

FRET by fluorescence polarization microscopy.

The widespread success in using genetically encoded fluorescent proteins (FPs) to track protein motion in living cells has led to extensive interest in measuring Förster resonance energy transfer (FRET) between two FPs of different colors. FRET occurs over distances less than 10-nm and can thus be used to detect protein-protein interactions and changes in protein conformation. However, FP-FRET measurements are complicated by the spectral properties of FPs. Consequently, extensive correction or photo-destructive approaches have been used to detect the presence of FRET. Since these methods limit the temporal and spatial resolution of FRET measurements, they are not well suited for many live-cell imaging applications. Here, we describe an alternative approach to detect FP-FRET by measuring fluorescence anisotropies (AFRET). Since FPs are large in size, excitation of FPs with polarized light results in highly polarized emission. In this case, FRET to a second FP that lies outside the photoselection plane will depolarize the fluorescence. This method provides high contrast and unambiguous indication of FRET using a simple image collection strategy that can be easily adapted to any modality including widefield and laser scanning approaches. In this chapter, we will discuss the theory behind AFRET imaging, calculation of FP anisotropies using fluorescent microscopes, and configuration of microscopes for AFRET experiments.

X-ray structure of Cerulean GFP: a tryptophan-based chromophore useful for fluorescence lifetime imaging.

The crystal structure of the cyan-fluorescent Cerulean green fluorescent protein (GFP), a variant of enhanced cyan fluorescent protein (ECFP), has been determined to 2.0 A. Cerulean bears an internal fluorophore composed of an indole moiety derived from Y66W, conjugated to the GFP-like imidazolinone ring via a methylene bridge. Cerulean undergoes highly efficient fluorescence resonance energy transfer (FRET) to yellow acceptor molecules and exhibits significantly reduced excited-state heterogeneity. This feature was rationally engineered in ECFP by substituting His148 with an aspartic acid [Rizzo et al. (2004) Nat. Biotechnol. 22, 445], rendering Cerulean useful for fluorescence lifetime imaging microscopy (FLIM). The X-ray structure is consistent with a single conformation of the chromophore and surrounding residues and may therefore provide a structural rationale for the previously described monoexponential fluorescence decay. Unexpectedly, the carboxyl group of H148D is found in a buried position, directly contacting the indole nitrogen of the chromophore via a bifurcated hydrogen bond. Compared to the similarly constructed ECFP chromophore, the indole group of Cerulean is rotated around the methylene bridge to adopt a cis-coplanar conformation with respect to the imidazolinone ring, resulting in a close edge-to-edge contact of the two ring systems. The double-humped absorbance spectrum persists in single-crystal absorbance measurements, casting doubt on the idea that ground state conformational heterogeneity forms the basis of the two overlapping transitions. At low pH, a blue shift in absorbance of 10-15 nm suggests a pH-induced structural transition that proceeds with a time constant of 47 (+/-2) min and is reversible. Possible interpretations in terms of chromophore isomerization are presented.

Optimization of pairings and detection conditions for measurement of FRET between cyan and yellow fluorescent proteins.

Detection of Förster resonance energy transfer (FRET) between cyan and yellow fluorescent proteins is a key method for quantifying dynamic processes inside living cells. To compare the different cyan and yellow fluorescent proteins, FRET efficiencies were measured for a set of the possible donor:acceptor pairs. FRET between monomeric Cerulean and Venus is more efficient than the ECFP:EYFP pair and has a 10% greater Förster distance. We also compared several live cell microscopy methods for measuring FRET. The greatest contrast for changes in intramolecular FRET is obtained using a combination of ratiometric and spectral imaging. However, this method is not appropriate for establishing the presence of FRET without extra controls. Accurate FRET efficiencies are obtained by fluorescence lifetime imaging microscopy, but these measurements are difficult to collect and analyze. Acceptor photobleaching is a common and simple method for measuring FRET efficiencies. However, when applied to cyan to yellow fluorescent protein FRET, this method becomes prone to an artifact that leads to overestimation of FRET efficiency and false positive signals. FRET was also detected by measuring the acceptor fluorescence anisotropy. Although difficult to quantify, this method is exceptional for screening purposes, because it provides high contrast for discriminating FRET.

Regulation of two insulin granule populations within the reserve pool by distinct calcium sources.

Insulin granule trafficking is a key step of glucose-stimulated insulin secretion from pancreatic beta cells. Using quantitative live cell imaging, we examined insulin granule movements within the reserve pool upon secretory stimulation in betaTC3 cells. For this study, we developed a custom image analysis program that permitted automatic tracking of the individual motions of over 20,000 granules. This analysis of a large sample size enabled us to study micro-populations of granules that were not quantifiable in previous studies. While over 90% of the granules depend on Ca2+ efflux from the endoplasmic reticulum for their mobilization, a small and fast-moving population of granules responds to extracellular Ca2+ influx after depolarization of the plasma membrane. We show that this differential regulation of the two granule populations is consistent with localized Ca2+ signals, and that the cytoskeletal network is involved in both types of granule movement. The fast-moving granules are correlated temporally and spatially to the replacement of the secreted insulin granules, which supports the hypothesis that these granules are responsible for replenishing the readily releasable pool. Our study provides a model by which glucose and other secretory stimuli can regulate the readily releasable pool through the same mechanisms that regulate insulin secretion.

The role of phosphatidic acid in the regulation of the Ras/MEK/Erk signaling cascade.

Phosphatidic acid (PA) is an important second messenger produced by the activation of numerous cell surface receptors. Recent data have suggested that PA regulates multiple cellular processes. This review addresses primarily the role of PA in the regulation of the Erk1/2 cascade pathway. A model for the regulation of Erk1/2 phosphorylation by cell surface receptors is presented. According to this model, agonists stimulate the binding of GTP to Ras and the activation of phospholipase D to generate phosphatidic acid. PA promotes the binding of cRaf-1 kinase to the membrane, where it interacts with Ras.GTP and other regulatory components of the pathway. Ras-Raf complexes remain bound to the surface of endosomes, where scaffolding complexes involving Ras, cRaf-1, MEK and Erk are formed. Complete activation and coupling of the cascade requires endocytosis, a process that is also modulated by PA.