Imaging bioluminescence by detecting localized haemodynamic contrast from photosensitized vasculature

Bioluminescent probes are widely used to monitor biomedically relevant processes and cellular targets in living animals. However, the absorption and scattering of visible light by tissue drastically limit the depth and resolution of the detection of luminescence. Here we show that bioluminescent sources can be detected with magnetic resonance imaging by leveraging the light-mediated activation of vascular cells expressing a photosensitive bacterial enzyme that causes the conversion of bioluminescent emission into local changes in haemodynamic contrast. In the brains of rats with photosensitized vasculature, we used magnetic resonance imaging to volumetrically map bioluminescent xenografts and cell populations virally transduced to express luciferase. Detecting bioluminescence-induced haemodynamic signals from photosensitized vasculature will extend the applications of bioluminescent probes.

Light-regulated gene expression in Bacteria: Fundamentals, advances, and perspectives

Numerous photoreceptors and genetic circuits emerged over the past two decades and now enable the light-dependent i.e., optogenetic, regulation of gene expression in bacteria. Prompted by light cues in the near-ultraviolet to near-infrared region of the electromagnetic spectrum, gene expression can be up- or downregulated stringently, reversibly, non-invasively, and with precision in space and time. Here, we survey the underlying principles, available options, and prominent examples of optogenetically regulated gene expression in bacteria. While transcription initiation and elongation remain most important for optogenetic intervention, other processes e.g., translation and downstream events, were also rendered light-dependent. The optogenetic control of bacterial expression predominantly employs but three fundamental strategies: light-sensitive two-component systems, oligomerization reactions, and second-messenger signaling. Certain optogenetic circuits moved beyond the proof-of-principle and stood the test of practice. They enable unprecedented applications in three major areas. First, light-dependent expression underpins novel concepts and strategies for enhanced yields in microbial production processes. Second, light-responsive bacteria can be optogenetically stimulated while residing within the bodies of animals, thus prompting the secretion of compounds that grant health benefits to the animal host. Third, optogenetics allows the generation of precisely structured, novel biomaterials. These applications jointly testify to the maturity of the optogenetic approach and serve as blueprints bound to inspire and template innovative use cases of light-regulated gene expression in bacteria. Researchers pursuing these lines can choose from an ever-growing, versatile, and efficient toolkit of optogenetic circuits.

Hemodynamic molecular imaging of tumor-associated enzyme activity in the living brain

Molecular imaging could have great utility for detecting, classifying, and guiding treatment of brain disorders, but existing probes offer limited capability for assessing relevant physiological parameters. Here, we describe a potent approach for noninvasive mapping of cancer-associated enzyme activity using a molecular sensor that acts on the vasculature, providing a diagnostic readout via local changes in hemodynamic image contrast. The sensor is targeted at the fibroblast activation protein (FAP), an extracellular dipeptidase and clinically relevant biomarker of brain tumor biology. Optimal FAP sensor variants were identified by screening a series of prototypes for responsiveness in a cell-based bioassay. The best variant was then applied for quantitative neuroimaging of FAP activity in rats, where it reveals nanomolar-scale FAP expression by xenografted cells. The activated probe also induces robust hemodynamic contrast in nonhuman primate brain. This work thus demonstrates a potentially translatable strategy for ultrasensitive functional imaging of molecular targets in neuromedicine.

Target-responsive vasoactive probes for ultrasensitive molecular imaging

The ability to monitor molecules volumetrically throughout the body could provide valuable biomarkers for studies of healthy function and disease, but noninvasive detection of molecular targets in living subjects often suffers from poor sensitivity or selectivity. Here we describe a family of potent imaging probes that can be activated by molecules of interest in deep tissue, providing a basis for mapping nanomolar-scale analytes without the radiation or heavy metal content associated with traditional molecular imaging agents. The probes are reversibly caged vasodilators that induce responses detectable by hemodynamic imaging; they are constructed by combining vasoactive peptides with synthetic chemical appendages and protein blocking domains. We use this architecture to create ultrasensitive biotin-responsive imaging agents, which we apply for wide-field mapping of targets in rat brains using functional magnetic resonance imaging. We also adapt the sensor design for detecting the neurotransmitter dopamine, illustrating versatility of this approach for addressing biologically important molecules.

Neurotransmitter-Responsive Nanosensors for T2-Weighted Magnetic Resonance Imaging

Neurotransmitter-sensitive contrast agents for magnetic resonance imaging (MRI) have recently been used for mapping signaling dynamics in live animal brains, but paramagnetic sensors for T₁-weighted MRI are usually effective only at micromolar concentrations that themselves perturb neurochemistry. Here we present an alternative molecular architecture for detecting neurotransmitters, using superparamagnetic iron oxide nanoparticles conjugated to tethered neurotransmitter analogs and engineered neurotransmitter binding proteins. Interactions between the nanoparticle conjugates result in clustering that is reversibly disrupted in the presence of neurotransmitter analytes, thus altering T₂-weighted MRI signals. We demonstrate this principle using tethered dopamine and serotonin analogs, together with proteins selected for their ability to competitively bind either the analogs or the neurotransmitters themselves. Corresponding sensors for dopamine and serotonin exhibit target-selective relaxivity changes of up to 20%, while also operating below endogenous neurotransmitter concentrations. Semisynthetic magnetic particle sensors thus represent a promising path for minimally perturbative studies of neurochemical analytes.

Characterization and engineering of photoactivated adenylyl cyclases

Cyclic nucleoside monophosphates (cNMP) serve as universal second messengers in signal transduction across prokaryotes and eukaryotes. As signaling often relies on transiently formed microdomains of elevated second messenger concentration, means to precisely perturb the spatiotemporal dynamics of cNMPs are uniquely poised for the interrogation of the underlying physiological processes. Optogenetics appears particularly suited as it affords light-dependent, accurate control in time and space of diverse cellular processes. Several sensory photoreceptors function as photoactivated adenylyl cyclases (PAC) and hence serve as light-regulated actuators for the control of intracellular levels of 3′, 5′-cyclic adenosine monophosphate. To characterize PACs and to refine their properties, we devised a test bed for the facile analysis of these photoreceptors. Cyclase activity is monitored in bacterial cells via expression of a fluorescent reporter, and programmable illumination allows the rapid exploration of multiple lighting regimes. We thus probed two PACs responding to blue and red light, respectively, and observed significant dark activity for both. We next engineered derivatives of the red-light-sensitive PAC with altered responses to light, with one variant, denoted DdPAC, showing enhanced response to light. These PAC variants stand to enrich the optogenetic toolkit and thus facilitate the detailed analysis of cNMP metabolism and signaling.

Primer-Aided Truncation for the Creation of Hybrid Proteins

Proteins frequently display modular architecture with several domains and segments connected by linkers. Proper protein functionality hinges on finely orchestrated interactions among these constituent elements. The underlying modularity lends itself to the engineering of hybrid proteins via modular rewiring; novel properties can thus be obtained, provided the linkers connecting the individual elements are conducive to productive interactions. As a corollary, the process of protein engineering often encompasses the generation and screening of multiple linker variants. To aid these steps, we devised the PATCHY method (primer-aided truncation for the creation of hybrid proteins) to readily generate hybrid gene libraries of predefined composition. We applied PATCHY to the mechanistic characterization of hybrid receptors that possess blue-light-regulated histidine kinase activity. Comprehensive sampling of linker composition revealed that catalytic activity and response to light are primarily functions of linker length. Variants with linkers of 7n residues mostly have light-repressed activity but those with 7n + 1 residues mostly have inverted, light-induced activity. We further probed linker length in the context of single residue exchanges that also lead to an inversion of the signal response. As in the original context, activity is only observed for certain periodic linker lengths. Taken together, these results provide mechanistic insight into signaling strategies employed by sensory photoreceptors and sensor histidine kinases. PATCHY represents an adequate and facile method to efficiently generate and probe hybrid gene libraries and to thereby identify key determinants for proper function.

Library-Aided Probing of Linker Determinants in Hybrid Photoreceptors

Signaling proteins comprise interaction and effector modules connected by linkers. Throughout evolution, these recurring modules have multiply been recombined to produce the present-day plethora of signaling proteins. Likewise, modular recombination lends itself to the engineering of hybrid signal receptors, whose functionality hinges on linker topology, sequence, and length. Often, numerous linkers must be assessed to obtain functional receptors. To expedite linker optimization, we devised the PATCHY strategy (primer-aided truncation for the creation of hybrid proteins) for the facile construction of hybrid gene libraries with defined linker distributions. Empowered by PATCHY, we engineered photoreceptors whose signal response was governed by linker length: whereas blue-light-repressed variants possessed linkers of 7n or 7n+5 residues, variants with 7n+1 residues were blue-light-activated. Related natural receptors predominantly displayed linker lengths of 7n and 7n+5 residues but rarely of 7n+1 residues. PATCHY efficiently explores linker sequence space to yield functional hybrid proteins including variants transcending the natural repertoire of signaling proteins.

Biophysical, mutational, and functional investigation of the chromophore-binding pocket of light-oxygen-voltage photoreceptors

As light-regulated actuators, sensory photoreceptors underpin optogenetics and numerous applications in synthetic biology. Protein engineering has been applied to fine-tune the properties of photoreceptors and to generate novel actuators. For the blue-light-sensitive light-oxygen-voltage (LOV) photoreceptors, mutations near the flavin chromophore modulate response kinetics and the effective light responsiveness. To probe for potential, inadvertent effects on receptor activity, we introduced these mutations into the engineered LOV photoreceptor YF1 and determined their impact on light regulation. While several mutations severely impaired the dynamic range of the receptor (e.g., I39V, R63K, and N94A), residue substitutions in a second group were benign with little effect on regulation (e.g., V28T, N37C, and L82I). Electron paramagnetic resonance and absorption spectroscopy identified correlated effects for certain of the latter mutations on chromophore environment and response kinetics in YF1 and the LOV2 domain from Avena sativa phototropin 1. Carefully chosen mutations provide a powerful means to adjust the light-response function of photoreceptors as demanded for diverse applications.

Application of the combination of isotope ratio monitoring with isotope dilution mass spectrometry to the determination of glucose in serum

Isotope ratio monitoring combined with n((13)C)/n((12)C) isotope dilution mass spectrometry (IRM/IDMS) provides results of low uncertainty of the order of 0.1% if it is applied to the analysis of simple mixtures as found in organic chemistry, even if only low (13)C spike additives to the sample are used. If the method is applied to the analysis of systems that require large-scale sample preparation prior to the measurement, such as the determination of glucose in serum, the results obtained exhibit a higher uncertainty that is comparable to that of the conventional gas chromatography/isotope dilution mass spectrometry (GC/IDMS) method. The reason for this observation is that the small contribution that the IRM/IDMS method makes to the uncertainty budget of the result is superimposed on a large contribution due to the sample preparation. It appears therefore that the IRM/IDMS method has no advantage over the conventional GC/IDMS method. However, if a series of measurements is carried out, and if a suitable experimental design is chosen, the IRM/IDMS method can provide valuable additional information. The influence of sample preparation on each individual result can be quantified as its deviation from the average value of all results of the series. From these data conclusions can be drawn for an improvement in sample preparation.

Memory effects in combining isotope ratio monitoring with isotope dilution mass spectrometry

Combining isotope ratio monitoring with isotope dilution techniques provides very accurate results in the quantitative analysis of volatile organic chemical compounds by gas chromatography/mass spectrometry (GC/MS). However, this method requires that spikes highly enriched in (13)C be used. This may lead to memory effects which will be investigated in more detail. They occur when the component of the mixture to be investigated exhibits an isotope ratio which is different from that of the component eluted earlier from the column during the chromatographic separation process. A residue of this component, which is shown in the gas chromatogram as tailing, falsifies the result of the isotope ratio measurement. This also leads to false amount-of-substance measurement results. Memory effects can be avoided by using spikes of low (13)C content, by adjusting the composition of the reference solution to that of the sample, or by ensuring effective sample preparation, thus separating disturbing mixture components prior to the measurement. Copyright 1999 John Wiley & Sons, Ltd.

Identification of trimethyllead in urine by high-performance liquid chromatography with column switching and chemical reaction detection and by liquid chromatography-mass spectrometry

Incorporated tetraalkyllead compounds are metabolized in the liver and the highly toxic trialkyllead species are excreted via the urine. The procedure for the determination of these metabolites in urine consists of solid-phase enrichment, reversed-phase pre-column high-performance liquid chromatography (HPLC) and chemical reaction detection. As urine is a very complex matrix, it must be questioned whether the retention time alone is a sufficient criterion for the identification of the analytes. For the trimethyllead ion the validity of the results was examined by selectivity checks of the chemical reaction detector, by the application of different stationary and mobile phases in single and dual pre-column HPLC systems and by the use of thermospray LC-mass spectrometry as an independent method. The results demonstrated that the recommended method is accurate for the determination of trimethyllead in urine samples.

Dissociation of antiplatelet effects from myocardial cytoprotective activity during acute myocardial ischemia in cats by a new carbacyclin derivative (ZK 36 375)

We studied the potential therapeutic value of the chemically stable carbacyclin analogue ZK 36 375 during acute myocardial ischemia and compared the cardiovascular and anti- and disaggregatory effects of the compound in vitro and ex vivo. In anesthetized cats the left anterior descending coronary artery was ligated, and 30 min later an intravenous infusion of ZK 36 375 (3.6 micrograms/kg X min) or vehicle was initiated and continued for 4.5 h. ZK 36 375 reduced the ST-segment elevation at 2-5 h (p less than 0.01) when compared to vehicle-treated cats. ZK 36 375 significantly inhibited both the loss of creatine phosphokinase--specific activity and the decrease in the percentage of bound cathepsin D in the infarcted area of the myocardium (p less than 0.05). ZK 36 375 did not reverse ischemia-induced formation of platelet aggregates in vivo and was found ex vivo to be two to three orders of magnitude less active in preventing platelet aggregation, redispersing platelet aggregates, and relaxing bovine coronary arteries than prostacyclin (PGI2) or its (5E) stereoisomer ZK 36 374. It is concluded that ZK 36 375 has a significant cardioprotective activity in acute myocardial ischemia of the cat that can be dissociated from antiplatelet effects in vivo.

Beneficial effects of a new carbacyclin derivative, ZK 36 374, in acute myocardial ischemia

The potential therapeutic value of the chemically stable carbacyclin analog ZK 36 374 was studied in acute myocardial ischemia (MI). In anesthetized cats, the left anterior descending coronary artery was ligated and 30 min later an i.v. infusion of ZK 36 374 (0.18 microgram/kg X min) on vehicle was initiated and continued for 4.5 hr. ZK 36 374 reduced the ST-segment elevation at 2 to 5 hr (P less than .01) when compared to vehicle-treated MI cats. ZK 36 374 completely prevented the loss of CK specific activities and the decrease in percentage of bound cathepsin D in the infarcted area of the myocardium (P less than .01), but had no influences on any of these parameters in shamoperated animals. In addition, ZK 36 374 reversed the MI-induced decrease in circulating platelet count toward the preinfarction levels, probably by dispersion of circulating platelet aggregates. ZK 36 374 prevented the ischemia-induced loss of myocardial catecholamines from adrenergic nerve terminals. ZK 36 374, at 0.18 microgram/kg X min, exerted a maximum antiplatelet effect, whereas a significant decrease in arterial blood pressure was seen at 1.79 microgram/kg X min (-30-40%). This indicates a considerable dissociation between antiplatelet and blood pressure-lowering activities of ZK 36 374 in this model. The data demonstrate a significant protective effect of ZK 36 374 in acute MI that might be associated with its platelet-stabilizing, antiadrenergic and myocardial cytoprotective activities.

Prostacyclin prevents ischemia-induced increase of lactate and cyclic AMP in ischemic myocardium

The effect of prostacyclin (PGI2, 0.5 nmol . kg-1 . min-1,i.v.) on myocardial metabolism was studied in cats subjected to 5 h of myocardial ischemia (MI) and compared to vehicle-treated MI cats. MI was followed by a 52% decrease in ATP and a concomitant increase (2-3 fold) in lactate and lactate/pyruvate ratio in the severely ischemic area. PGI2 prevented this increase in lactate with an unchanged ATP and lactate/pyruvate ratio. Moreover, PGI2 abolished the ischemia-induced in myocardial cAMP. It is concluded, the PGI2 exerts its beneficial actions on ischemic myocardium partly via cAMP-linked mechanisms.