Imaging bioluminescent indicators shows Ca2+ and ATP permeability thresholds in live cells attacked by complement

Targeting the tissue-complosome for curbing inflammatory disease

Hyperactivated local tissue is a cardinal feature of immune-mediated inflammatory diseases of various organs such as the joints, the gut, the skin, or the lungs. Tissue-resident structural and stromal cells, which get primed during repeated or long-lasting bouts of inflammation form the basis of this sensitization of the tissue. During priming, cells change their metabolism to make them fit for the heightened energy demands that occur during persistent inflammation. Epigenetic changes and, curiously, an activation of intracellularly expressed parts of the complement system drive this metabolic invigoration and enable tissue-resident cells and infiltrating immune cells to employ an arsenal of inflammatory functions, including activation of inflammasomes. Here we provide a current overview on complement activation and inflammatory transformation in tissue-occupying cells, focusing on fibroblasts during arthritis, and illustrate ways how therapeutics directed at complement C3 could potentially target the complosome to unprime cells in the tissue and induce long-lasting abatement of inflammation.

Complement's favourite organelle-Mitochondria?

The complement system, well known for its central role in innate immunity, is currently emerging as an unexpected, cell‐autonomous, orchestrator of normal cell physiology. Specifically, an intracellularly active complement system—the complosome—controls key pathways of normal cell metabolism during immune cell homeostasis and effector function. So far, we know little about the exact structure and localization of intracellular complement components within and among cells. A common scheme, however, is that they operate in crosstalk with other intracellular immune sensors, such as inflammasomes, and that they impact on the activity of key subcellular compartments. Among cell compartments, mitochondria appear to have built a particularly early and strong relationship with the complosome and extracellularly active complement—not surprising in view of the strong impact of the complosome on metabolism. In this review, we will hence summarize the current knowledge about the close complosome–mitochondria relationship and also discuss key questions surrounding this novel research area.

Complement C5b-9 and Cancer: Mechanisms of Cell Damage, Cancer Counteractions, and Approaches for Intervention

The interactions of cancer cells with components of the complement system are highly complex, leading to an outcome that is either favorable or detrimental to cancer cells. Currently, we perceive only the "tip of the iceberg" of these interactions. In this review, we focus on the complement terminal C5b-9 complex, known also as the complement membrane attack complex (MAC) and discuss the complexity of its interaction with cancer cells, starting with a discussion of its proposed mode of action in mediating cell death, and continuing with a portrayal of the strategies of evasion exhibited by cancer cells, and closing with a proposal of treatment approaches targeted at evasion strategies. Upon intense complement activation and membrane insertion of sufficient C5b-9 complexes, the afflicted cells undergo regulated necrotic cell death with characteristic damage to intracellular organelles, including mitochondria, and perforation of the plasma membrane. Several pro-lytic factors have been proposed, including elevated intracellular calcium ion concentrations and activated JNK, Bid, RIPK1, RIPK3, and MLKL; however, further research is required to fully characterize the effective cell death signals activated by the C5b-9 complexes. Cancer cells over-express a multitude of protective measures which either block complement activation, thus reducing the number of membrane-inserted C5b-9 complexes, or facilitate the elimination of C5b-9 from the cell surface. Concomitantly, cancer cells activate several protective pathways that counteract the death signals. Blockage of complement activation is mediated by the complement membrane regulatory proteins CD46, CD55, and CD59 and by soluble complement regulators, by proteases that cleave complement proteins and by protein kinases, like CK2, which phosphorylate complement proteins. C5b-9 elimination and inhibition of cell death signals are mediated by caveolin and dynamin, by Hsp70 and Hsp90, by the mitochondrial stress protein mortalin, and by the protein kinases PKC and ERK. It is conceivable that various cancers and cancers at different stages of development will utilize distinct patterns of these and other MAC resistance strategies. In order to enhance the impact of antibody-based therapy on cancer, novel precise reagents that block the most effective protective strategies will have to be designed and applied as adjuvants to the therapeutic antibodies.

Real-time analysis of the detailed sequence of cellular events in mAb-mediated complement-dependent cytotoxicity of B-cell lines and of chronic lymphocytic leukemia B-cells

Complement-dependent cytotoxicity is an important mechanism of action of certain mAbs used in cancer immunotherapy, including ofatumumab and rituximab. However, the detailed sequence of cellular changes that occur in nucleated cells attacked by mAb and complement has not been delineated. Recently developed CD20 mAbs, engineered to form hexamers on binding to cells, react with B-cells in serum, chelate C1q, and then activate complement and promote cell killing considerably more effectively than their wild-type precursors. We used these engineered mAbs as a model to investigate the sequence of events that occur when mAbs bind to B-cell lines and to primary cells from patients with chronic lymphocytic leukemia and then activate complement. Based on four-color confocal microscopy real-time movies and high resolution digital imaging, we find that after CD20 mAb binding and C1q uptake, C3b deposits on cells, followed by Ca(2+) influx, revealed by bright green signals generated on cells labeled with FLUO-4, a Ca(2+) indicator. The bright FLUO-4/Ca(2+) signal fades, replaced by punctate green signals in mitochondria, indicating Ca(2+) localization. This step leads to mitochondrial poisoning followed by cell death. The entire sequence is completed in <2 min for hexamerization-enhanced CD20 mAb-mediated killing. To our knowledge this is the first time the entire process has been characterized in detail in real time. By identifying multiple discrete steps in the cytotoxic pathway for nucleated cells our findings may inform future development and more effective application of complement-fixing mAbs to cancer treatment.

Strategy of mutual compensation of green and red mutants of firefly luciferase identifies a mutation of the highly conservative residue E457 with a strong red shift of bioluminescence

Bioluminescence spectra of firefly luciferases demonstrate highly pH-sensitive spectra changing the color from green to red light when pH is lowered from alkaline to acidic. This reflects a change of ratio of the green and red emitters in the bimodal spectra of bioluminescence. We show that the mutations strongly stabilizing green (Y35N) or red (H433Y) emission compensate each other leading to the WT color of firefly luciferase. We further used this compensating ability of Y35N to search for strong red-shifting mutations in the C-domain of firefly luciferase by random mutagenesis. The discovered mutation E457K substantially increased the contribution of the red emitter and caused a 12 nm red shift of the green emitter as well. E457 is highly conservative not only in beetle luciferases but also in a whole ANL superfamily of adenylating enzymes and forms a conservative structural hydrogen bond with V471. Our results suggest that the removal of this hydrogen bond only mildly affects luciferase properties and that most of the effect of E457K is caused by the introduction of positive charge. E457 forms a salt bridge with R534 in most ANL enzymes including pH-insensitive luciferases which is absent in pH-sensitive firefly luciferases. The mutant A534R shows that this salt bridge is not important for pH-sensitivity but considerably improves in vivo thermostability. Although E457 is located far from the oxyluciferin-binding site, the properties of the mutant E457K suggest that it affects color by influencing the AMP binding.

Prolonged SRC kinase activation, a mechanism to turn transient, sublytic complement activation into a sustained pathological condition in retinal pigment epithelium cells

Age-related macular degeneration (AMD) is a slowly progressing multifactorial disease involving genetic abnormalities and environmental insults. Genetic studies have demonstrated that polymorphisms in different complement proteins increase the risk for developing AMD. Previously, we have shown that in retinal pigment epithelium (RPE) monolayers, exposure to oxidative stress reduced complement inhibition on the cell surface, with the resulting increase in complement activation leading to vascular endothelial growth factor (VEGF) release and VEGF-receptor-2-mediated disruption of the monolayer barrier function. Complement activation was found to be sublytic and transient and require the assembly of the membrane attack complex (MAC). Here, we asked how this transient, sublytic complement activation could trigger long-term pathological changes in RPE cells. The initial activation of the L-type voltage-gated calcium channels was followed by calcium influx and activation of several kinases. While Erk/Ras activation was found to be transient, Src kinase phosphorylation was sustained. We have shown previously that Src kinase controls VEGF release from RPE cells by altering the activity of the L-type channel. We propose that the prolonged Src kinase activation, and its resulting effects on membrane depolarization and calcium influx, leads to sustained VEGF secretion. In addition, the previously shown effect of the autocrine positive feedback loop in RPE cells, involving VEGF-induced VEGF production and secretion via VEGFR-2 receptors, will augment and prolong the effects of sublytic complement activation. In summary, identification of the links between oxidative stress, chronic, low-grade activation of the complement system, and elevated VEGF expression and secretion might offer opportunities to selectively inhibit pathological VEGF release only.

Serum complement enhances the responses of genotoxin- and oxidative stress-sensitive Escherichia coli bioreporters

Bacterial bioreporters are limited in their abilities to detect large polar molecules due to their membrane selectivity. In this study, the activity of serum complement was used to bypass this undesired selectivity. Initially, the serum complement activity was assessed using the responses of a bacterial bioreporter harboring a recA::luxCDABE transcriptional fusion when exposed to the chemotherapy drug, mitomycin C (MMC). Using 50 °C-treated serum, the limit of detection for this bacterial sensor was lowered by nearly 450-fold, from 31 μg/L to 0.07 μg/L MMC. Real-time quantitative PCR demonstrated that serum-treated cultures responded more strongly to 100 μg/L MMC, with 3.1-fold higher recA expression levels. Subsequent experiments with other bioreporter strains also found enhanced sensitivities and responses. Finally, combining each of the above findings, tests were performed to demonstrate the potential application of the recA::luxCDABE bioreporter within a lab-on-a-CD platform as a point-of-care diagnostic to measure chemotherapeutic drug concentrations within blood.

Sublytic membrane-attack-complex (MAC) activation alters regulated rather than constitutive vascular endothelial growth factor (VEGF) secretion in retinal pigment epithelium monolayers

Uncontrolled activation of the alternative complement pathway and secretion of vascular endothelial growth factor (VEGF) are thought to be associated with age-related macular degeneration (AMD). Previously, we have shown that in RPE monolayers, oxidative-stress reduced complement inhibition on the cell surface. The resulting increased level of sublytic complement activation resulted in VEGF release, which disrupted the barrier facility of these cells as determined by transepithelial resistance (TER) measurements. Induced rather than basal VEGF release in RPE is thought to be controlled by different mechanisms, including voltage-dependent calcium channel (VDCC) activation and mitogen-activated protein kinases. Here we examined the potential intracellular links between sublytic complement activation and VEGF release in RPE cells challenged with H(2)O(2) and complement-sufficient normal human serum (NHS). Disruption of barrier function by H(2)O(2) + NHS rapidly increased Ras expression and Erk and Src phosphorylation, but had no effect on P38 phosphorylation. Either treatment alone had little effect. TER reduction could be attenuated by inhibiting Ras, Erk and Src activation, or blocking VDCC or VEGF-R2 activation, but not by inhibiting P38. Combinatorial analysis of inhibitor effects demonstrated that sublytic complement activation triggers VEGF secretion via two pathways, Src and Ras-Erk, with the latter being amplified by VEGF-R2 activation, but has no effect on constitutive VEGF secretion mediated via P38. Finally, effects on TER were directly correlated with release of VEGF; and sublytic MAC activation decreased levels of zfp36, a negative modulator of VEGF transcription, resulting in increased VEGF expression. Taken together, identifying how sublytic MAC induces VEGF expression and secretion might offer opportunities to selectively inhibit pathological VEGF release only.

Rational design of novel red-shifted BRET pairs: Platforms for real-time single-chain protease biosensors

Bioluminescence resonance energy transfer (BRET) systems to date have been dominated by use of blue-green Renilla luciferase (Rluc) as the energy donor. Although effective in many cases, the expense and unfavorable biochemical attributes of the substrate (phenylcoelenterazine) limit utility of Rluc-based BRET systems. Herein we report a series of novel BRET pairs based on luciferases that utilize D-luciferin, resulting in red-shifted photonic outputs, favorable biochemical attributes, and increased efficacy. We developed a modified Förster equation to predict optimal BRET luciferase donor-fluorophore pairs and identified tdTomato as the optimal red fluorophore acceptor for click beetle green luciferase (CBG). A prototypical single-chain protease biosensor, capable of reporting on executioner caspase activity in live cells and in real-time, was generated by inserting a DEVD linker between CBG and tdTomato and validated in vitro with recombinant caspases and in cellulo with apoptosis-sensitive and -resistant cell lines. High signal-to-noise ratios ( approximately 33) and Z' factors (0.85) were observed in live cell longitudinal studies, sufficient for high-throughput screening. Thus, we illustrate a general methodology for the rational design of new BRET systems and provide a novel single-chain BRET protease biosensor that is long lived, red-shifted, and utilizes D-luciferin.

Illuminating cellular physiology: recent developments

Bioluminescent methods are gaining more and more attention among scientists due to their sensitivity, selectivity and simplicity; coupled with the fact that the bioluminescence can be monitored both in vitro and in vivo. Since the discovery of bioluminescence in the 19th century, enzymes involved in the bioluminescent process have been isolated and cloned. The bioluminescent reactions in several different organisms have also been fully characterized and used as reporters in a wide variety of biochemical assays. From the 1990s it became clear that bioluminescence can be detected and quantified directly from inside a living cell. This gave rise to numerous possibilities for the in vivo monitoring of intracellular processes non-invasively using bioluminescent molecules as reporters. This review describes recent developments in the area of bioluminescent imaging for cell biology. Newly developed imaging methods allow transcriptional/translational regulation, signal transduction, protein-protein interaction, oncogenic transformation, cell and protein trafficking, and target drug action to be monitored in vivo in real-time with high temporal and spatial resolution; thus providing researchers with priceless information on cellular functions. Advantages and limitations of these novel bioluminescent methods are discussed and possible future developments identified.

Activation of kainate receptors sensitizes oligodendrocytes to complement attack

Glutamate excitotoxicity and complement attack have both been implicated separately in the generation of tissue damage in multiple sclerosis and in its animal model, experimental autoimmune encephalomyelitis. Here, we investigated whether glutamate receptor activation sensitizes oligodendrocytes to complement attack. We found that a brief incubation with glutamate followed by exposure to complement was lethal to oligodendrocytes in vitro and in freshly isolated optic nerves. Complement toxicity was induced by activation of kainate but not of AMPA receptors and was abolished by removing calcium from the medium during glutamate priming. Dose-response studies showed that sensitization to complement attack is induced by two distinct kainate receptor populations displaying high and low affinities for glutamate. Oligodendrocyte death by complement required the formation of the membrane attack complex, which in turn increased membrane conductance and induced calcium overload and mitochondrial depolarization as well as a rise in the level of reactive oxygen species. Treatment with the antioxidant Trolox and inhibition of poly(ADP-ribose) polymerase-1, but not of caspases, protected oligodendrocytes against damage induced by complement. These findings indicate that glutamate sensitization of oligodendrocytes to complement attack may contribute to white matter damage in acute and chronic neurological disorders.

Advances in in vivo bioluminescence imaging of gene expression

To advance our understanding of biological processes as they occur in living animals, imaging strategies have been developed and refined that reveal cellular and molecular features of biology and disease in real time. One rapid and accessible technology for in vivo analysis employs internal biological sources of light emitted from luminescent enzymes, luciferases, to label genes and cells. Combining this reporter system with the new generation of charge coupled device (CCD) cameras that detect the light transmitted through the animal's tissues has opened the door to sensitive in vivo measurements of mammalian gene expression in living animals. Here, we review the development and application of this imaging strategy, in vivo bioluminescence imaging (BLI), together with in vivo fluorescence imaging methods, which has enabled the real-time study of immune cell trafficking, of various genetic regulatory elements in transgenic mice, and of in vivo gene transfer. BLI has been combined with fluorescence methods that together offer access to in vivo measurements that were not previously available. Such studies will greatly facilitate the functional analysis of a wide range of genes for their roles in health and disease.

Imaging of light emission from the expression of luciferases in living cells and organisms: a review

Luciferases are enzymes that emit light in the presence of oxygen and a substrate (luciferin) and which have been used for real-time, low-light imaging of gene expression in cell cultures, individual cells, whole organisms, and transgenic organisms. Such luciferin-luciferase systems include, among others, the bacterial lux genes of terrestrial Photorhabdus luminescens and marine Vibrio harveyi bacteria, as well as eukaryotic luciferase luc and ruc genes from firefly species (Photinus) and the sea pansy (Renilla reniformis), respectively. In various vectors and in fusion constructs with other gene products such as green fluorescence protein (GFP; from the jellyfish Aequorea), luciferases have served as reporters in a number of promoter search and targeted gene expression experiments over the last two decades. Luciferase imaging has also been used to trace bacterial and viral infection in vivo and to visualize the proliferation of tumour cells in animal models.

Measurement of proteases using chemiluminescence-resonance-energy-transfer chimaeras between green fluorescent protein and aequorin

Homogeneous assays, without a separation step, are essential for measuring chemical events in live cells and for drug discovery screens, and are desirable for making measurements in cell extracts or clinical samples. Here we demonstrate the principle of chemiluminescence resonance energy transfer (CRET) as a homogeneous assay system, using two proteases as models, one extracellular (alpha-thrombin) and the other intracellular (caspase-3). Chimaeras were engineered with aequorin as the chemiluminescent energy donor and green fluorescent protein (GFP) or enhanced GFP as the energy acceptors, with a protease linker (6 or 18 amino acid residues) recognition site between the donor and acceptor. Flash chemiluminescent spectra (20--60 s) showed that the spectra of chimaeras matched GFP, being similar to that of luminous jellyfish, justifying their designation as 'Rainbow' proteins. Addition of the protease shifted the emission spectrum to that of aequorin in a time- and dose-dependent manner. Separation of the proteolysed fragments showed that the ratio of green to blue light matched the extent of proteolysis. The caspase-3 Rainbow protein was able to provide information on the specificity of caspases in vitro and in vivo. It was also able to monitor caspase-3 activation in cells provoked into apoptosis by staurosporine (1 or 2 microM). CRET can also monitor GFP fluor formation. The signal-to-noise ratio of our Rainbow proteins is superior to that of fluorescence resonance energy transfer, providing a potential platform for measuring agents that interact with the reactive site between the donor and acceptor.

A chimeric reporter gene allowing for clone selection and high-throughput screening of reporter cell lines expressing G-protein-coupled receptors

Efficient screening of ligands interacting with G-protein-coupled receptors is central for modern drug development. Here, we describe an optimized reporter vector primarily intended for use in reporter cell lines expressing such receptors. The construct consists of a synthetic enhancer containing 9x TRE (12-O-tetradecanoylphorbol-13-acetate-responsive elements) fused to a minimal CMV (cytomegalovirus) promoter. Activation of the promoter construct leads to the expression of a chimeric reporter protein based on the genes for enhanced green fluorescent protein and Photinus luciferase. The chimeric protein allows for both clonal selection by fluorescence, which facilitates the selection of optimal reporter cell lines and high-throughput screening by luminescens. In designing the vector, increasing numbers of TRE motifs were tested in front of two different minimal promoters. The reporter gene was more strongly inducible with increasing numbers of TRE motifs. The constructs were tested in two cell lines, CHO and HeLa. The latter regulated reporter gene activity stronger in response to PMA (phorbol 12-myristate 13-acetate) stimulation and were used to construct HF1 reporter cell lines. Model experiments were carried out on these reporter cells transfected with the human BLTR, human CCR5, or the rat alpha(1b) receptor. After maximal agonist stimulation reporter gene activity was increased 200-, 15-, and 50-fold, respectively.