Imaging the cAMP Signaling Microdomain of the Primary Cilium Using Targeted FRET-Based Biosensors.

Optical approaches have revolutionized our view of second messenger signaling in organelles, allowing precise time-resolved assessment of soluble signaling molecules in situ. Among the most challenging of subcellular signaling microdomains to assay is the primary cilium. A petite but visually arresting organelle, the primary cilium extends from the cell surface of most non-dividing cells. Recently, the concept of the primary cilium as an independent cAMP signaling organelle has attracted substantial interest. The cilium sequesters a very specific subset of ciliary cAMP-linked GPCRs in its membrane (e.g., 5-HT6, D1R, MCR4, FFAR4, TGR5), as well as other key components of the cAMP signaling machinery that include adenylyl cyclases, GNAS, phosphodiesterases, PKA holoenzyme, and biologically important PKA targets. Here we provide a practical guide to assessing ciliary cAMP signals in live cells using targeted genetically encoded FRET biosensors. Key experimental difficulties include gathering sufficient signal from such a small, photon-limited volume, and the susceptibility of cilia to movement artifacts. Other challenges are associated with the fidelity of sensor targeting and the difficulties in distinguishing between cAMP signals produced exclusively within the cilium vs. those that emanate from the cell body. Here we describe ratio imaging approaches used in our lab for time-resolved visualization of ciliary cAMP in cultured renal cells. These methods can be readily adapted to other cell types and microscopy platforms according to the needs of the user.

Cyclic AMP does double duty to support parallel signaling in primary cilium and cytosol.

The primary cilium maintains all of the necessary machinery to generate and interpret cAMP signals within its tiny volume, leading to the supposition that ciliary cAMP provides unique biological instructions separate from those derived from the rest of the cell body. A new paper by Truong et al. has used optogenetic and chemogenetic tricks to selectively manipulate cAMP signaling within the primary cilium. Their data show that ciliary but not cytosolic message preferentially regulates transcriptional activity via the hedgehog pathway leading to actions on zebrafish development. Computer modeling provides a rational explanation as to how the geometry of this organelle enables it to tune out cAMP signals from the cell body in order to pick up messages generated in the cilium.

Anti-EGFR VHH-armed death receptor ligand-engineered allogeneic stem cells have therapeutic efficacy in diverse brain metastatic breast cancers.

Basal-like breast cancer (BLBC) shows brain metastatic (BM) capability and overexpresses EGFR and death-receptors 4/5 (DR4/5); however, the anatomical location of BM prohibits efficient drug-delivery to these targetable markers. In this study, we developed BLBC-BM mouse models featuring different patterns of BMs and explored the versatility of estem cell (SC)-mediated bi-functional EGFR and DR4/5-targeted treatment in these models. Most BLBC lines demonstrated a high sensitivity to EGFR and DR4/5 bi-targeting therapeutic protein, EVDRL [anti-EGFR VHH (EV) fused to DR ligand (DRL)]. Functional analyses using inhibitors and CRISPR-Cas9 knockouts revealed that the EV domain facilitated in augmenting DR4/5-DRL binding and enhancing DRL-induced apoptosis. EVDRL secreting stem cells alleviated tumor-burden and significantly increased survival in mouse models of residual-tumor after macrometastasis resection, perivascular niche micrometastasis, and leptomeningeal metastasis. This study reports mechanism based simultaneous targeting of EGFR and DR4/5 in BLBC and defines a new treatment paradigm for treatment of BM.

cAMPing out with the keepers of the gate: Adenylyl cyclases get cozy with Orai.

Calcium and cyclic AMP form the cornerstones of two ancient signaling systems represented in nearly every kingdom of life. Not surprisingly, these old and ubiquitous messenger molecules have co-evolved multiple means to regulate one another. Zhang et al. describe a new twist on this theme related to the intimate union between the calcium-activated adenylyl cyclase, AC8, and the store-operated Ca2+ channel, Orai1.

Polycystin 2 regulates mitochondrial Ca2+ signaling, bioenergetics, and dynamics through mitofusin 2.

Mitochondria and the endoplasmic reticulum (ER) have an intimate functional relationship due to tethering proteins that bring their membranes in close (~30 nm) apposition. One function of this interorganellar junction is to increase the efficiency of Ca2+ transfer into mitochondria, thus stimulating mitochondrial respiration. Here, we showed that the ER cation-permeant channel polycystin 2 (PC2) functions to reduce mitochondria-ER contacts. In cell culture models, PC2 knockdown led to a 50% increase in mitofusin 2 (MFN2) expression, an outer mitochondrial membrane GTPase. Live-cell super-resolution and electron microscopy analyses revealed enhanced MFN2-dependent tethering between the ER and mitochondria in PC2 knockdown cells. PC2 knockdown also led to increased ER-mediated mitochondrial Ca2+ signaling, bioenergetic activation, and mitochondrial density. Mutation or deletion of the gene encoding for PC2 results in autosomal dominant polycystic kidney disease (ADPKD), a condition characterized by numerous fluid-filled cysts. In cell culture models and mice with kidney-specific PC2 knockout, knockdown of MFN2 rescued defective mitochondrial Ca2+ transfer and diminished cell proliferation in kidney cysts. Consistent with these results, cyst-lining epithelial cells from human ADPKD kidneys had a twofold increase in mitochondria and MFN2 expression. Our data suggest that PC2 normally serves to limit key mitochondrial proteins at the ER-mitochondrial interface and acts as a checkpoint for mitochondrial biogenesis and bioenergetics. Loss of this regulation may contribute to the increased oxidative metabolism and aberrant cell proliferation typical of kidney cysts in ADPKD.

Direct visualization of cAMP signaling in primary cilia reveals up-regulation of ciliary GPCR activity following Hedgehog activation.

The primary cilium permits compartmentalization of specific signaling pathways, including elements of the Hedgehog (Hh) pathway. Hh transcriptional activity is thought to be negatively regulated by constitutively high ciliary cAMP maintained by the Gα(s)-coupled GPCR, GPR161. However, cilia also sequester many other Gα(s)-coupled GPCRs with unknown potential to regulate Hh. Here we used biosensors optimized for ciliary cAMP and strategies to isolate signals in the cilium from the cell body and neighboring cells. We found that ciliary cAMP was not elevated relative to cellular cAMP, inconsistent with constitutive cAMP production. Gα(s)-coupled GPCRs (e.g., the 5-HT6 serotonin and D1R dopamine receptor) had reduced ability to generate cAMP upon trafficking to the ciliary membrane. However, activation of the Hh pathway restored or amplified GPCR function to permit cAMP elevation selectively in the cilium. Hh therefore enables its own local GPCR-dependent cAMP regulatory circuit. Considering that GPCRs comprise much of the druggable genome, these data suggest alternative strategies to modify Hh signaling.

Interrogating cyclic AMP signaling using optical approaches.

Optical reporters for cAMP represent a fundamental advancement in our ability to investigate the dynamics of cAMP signaling. These fluorescent sensors can measure changes in cAMP in single cells or in microdomains within cells as opposed to whole populations of cells required for other methods of measuring cAMP. The first optical cAMP reporters were FRET-based sensors utilizing dissociation of purified regulatory and catalytic subunits of PKA, introduced by Roger Tsien in the early 1990s. The utility of these sensors was vastly improved by creating genetically encoded versions that could be introduced into cells with transfection, the first of which was published in the year 2000. Subsequently, improved sensors have been developed using different cAMP binding platforms, optimized fluorescent proteins, and targeting motifs that localize to specific microdomains. The most common sensors in use today are FRET-based sensors designed around an Epac backbone. These rely on the significant conformational changes in Epac when it binds cAMP, altering the signal between FRET pairs flanking Epac. Several other strategies for optically interrogating cAMP have been developed, including fluorescent translocation reporters, dimerization-dependent FP based biosensors, BRET (bioluminescence resonance energy transfer)-based sensors, non-FRET single wavelength reporters, and sensors based on bacterial cAMP-binding domains. Other newly described mammalian cAMP-binding proteins such as Popdc and CRIS may someday be exploited in sensor design. With the proliferation of engineered fluorescent proteins and the abundance of cAMP binding targets in nature, the field of optical reporters for cAMP should continue to see rapid refinement in the coming years.

Recent advances in understanding the extracellular calcium-sensing receptor.

The extracellular calcium-sensing receptor (CaR), a ubiquitous class C G-protein-coupled receptor (GPCR), is responsible for the control of calcium homeostasis in body fluids. It integrates information about external Ca 2+ and a surfeit of other endogenous ligands into multiple intracellular signals, but how is this achieved? This review will focus on some of the exciting concepts in CaR signaling and pharmacology that have emerged in the last few years.

"Store-operated" cAMP signaling contributes to Ca2+-activated Cl- secretion in T84 colonic cells.

Apical cAMP-dependent CFTR Cl(-) channels are essential for efficient vectorial movement of ions and fluid into the lumen of the colon. It is well known that Ca(2+)-mobilizing agonists also stimulate colonic anion secretion. However, CFTR is apparently not activated directly by Ca(2+), and the existence of apical Ca(2+)-dependent Cl(-) channels in the native colonic epithelium is controversial, leaving the identity of the Ca(2+)-activated component unresolved. We recently showed that decreasing free Ca(2+) concentration ([Ca(2+)]) within the endoplasmic reticulum (ER) lumen elicits a rise in intracellular cAMP. This process, which we termed "store-operated cAMP signaling" (SOcAMPS), requires the luminal ER Ca(2+) sensor STIM1 and does not depend on changes in cytosolic Ca(2+). Here we assessed the degree to which SOcAMPS participates in Ca(2+)-activated Cl(-) transport as measured by transepithelial short-circuit current (Isc) in polarized T84 monolayers in parallel with imaging of cAMP and PKA activity using fluorescence resonance energy transfer (FRET)-based reporters in single cells. In Ca(2+)-free conditions, the Ca(2+)-releasing agonist carbachol and Ca(2+) ionophore increased Isc, cAMP, and PKA activity. These responses persisted in cells loaded with the Ca(2+) chelator BAPTA-AM. The effect on Isc was enhanced in the presence of the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), inhibited by the CFTR inhibitor CFTRinh-172 and the PKA inhibitor H-89, and unaffected by Ba(2+) or flufenamic acid. We propose that a discrete component of the "Ca(2+)-dependent" secretory activity in the colon derives from cAMP generated through SOcAMPS. This alternative mode of cAMP production could contribute to the actions of diverse xenobiotic agents that disrupt ER Ca(2+) homeostasis, leading to diarrhea.

The inner and outer compartments of mitochondria are sites of distinct cAMP/PKA signaling dynamics.

Cyclic AMP (cAMP)-dependent phosphorylation has been reported to exert biological effects in both the mitochondrial matrix and outer mitochondrial membrane (OMM). However, the kinetics, targets, and effectors of the cAMP cascade in these organellar domains remain largely undefined. Here we used sensitive FRET-based sensors to monitor cAMP and protein kinase A (PKA) activity in different mitochondrial compartments in real time. We found that cytosolic cAMP did not enter the matrix, except during mitochondrial permeability transition. Bicarbonate treatment (expected to activate matrix-bound soluble adenylyl cyclase) increased intramitochondrial cAMP, but along with membrane-permeant cAMP analogues, failed to induce measureable matrix PKA activity. In contrast, the OMM proved to be a domain of exceptionally persistent cAMP-dependent PKA activity. Although cAMP signaling events measured on the OMM mirrored those of the cytosol, PKA phosphorylation at the OMM endured longer as a consequence of diminished control by local phosphatases. Our findings demonstrate that mitochondria host segregated cAMP cascades with distinct functional and kinetic signatures.

An aryleneethynylene fluorophore for cell membrane staining.

The use of an amphiphilic aryleneethynylene fluorophore as a plasma membrane marker in fixed and living mammalian cells and liposome model systems is demonstrated. We show here that the optical properties of the novel dye are almost independent on pH, in the range 5.0-8.0. Spectroscopic characterization performed on unilamellar liposomes ascertained that the fluorescence intensity of the aryleneethynylene fluorophore greatly increases after incorporation in lipidic membranes. Experiments performed on different mammalian cells demonstrated that the novel membrane marker exhibits fast staining and a good photostability that make it a suitable tool for live cell imaging. Importantly, the aryleneethynylene fluorophore was also shown to be a fast and reliable blue membrane marker in classical multicolor immunofluorescence experiments. This study adds new important findings to the recent exploitation of the wide class of aryleneethynylene molecules as luminescent markers for biological investigations.

Termination and activation of store-operated cyclic AMP production.

Diverse pathophysiological processes (e.g. obesity, lifespan determination, addiction and male fertility) have been linked to the expression of specific isoforms of the adenylyl cyclases (AC1-AC10), the enzymes that generate cyclic AMP (cAMP). Our laboratory recently discovered a new mode of cAMP production, prominent in certain cell types, that is stimulated by any manoeuvre causing reduction of free [Ca(2+) ] within the lumen of the endoplasmic reticulum (ER) calcium store. Activation of this 'store-operated' pathway requires the ER Ca(2+) sensor, STIM1, but the identity of the enzymes responsible for cAMP production and how this process is regulated is unknown. Here, we used sensitive FRET-based sensors for cAMP in single cells combined with silencing and overexpression approaches to show that store-operated cAMP production occurred preferentially via the isoform AC3 in NCM460 colonic epithelial cells. Ca(2+) entry via the plasma membrane Ca(2+) channel, Orai1, suppressed cAMP production, independent of store refilling. These findings are an important first step towards defining the functional significance and to identify the protein composition of this novel Ca(2+) /cAMP crosstalk system.

Critical role for calcium mobilization in activation of the NLRP3 inflammasome.

The NLRP3 (nucleotide-binding domain, leucine-rich-repeat-containing family, pyrin domain-containing 3) inflammasome mediates production of inflammatory mediators, such as IL-1ß and IL-18, and as such is implicated in a variety of inflammatory processes, including infection, sepsis, autoinflammatory diseases, and metabolic diseases. The proximal steps in NLRP3 inflammasome activation are not well understood. Here we elucidate a critical role for Ca(2+) mobilization in activation of the NLRP3 inflammasome by multiple stimuli. We demonstrate that blocking Ca(2+) mobilization inhibits assembly and activation of the NLRP3 inflammasome complex, and that during ATP stimulation Ca(2+) signaling is pivotal in promoting mitochondrial damage. C/EPB homologous protein, a transcription factor that can modulate Ca(2+) release from the endoplasmic reticulum, amplifies NLRP3 inflammasome activation, thus linking endoplasmic reticulum stress to activation of the NLRP3 inflammasome. Our findings support a model for NLRP3 inflammasome activation by Ca(2+)-mediated mitochondrial damage.

Pseudomonas aeruginosa Homoserine lactone activates store-operated cAMP and cystic fibrosis transmembrane regulator-dependent Cl- secretion by human airway epithelia.

The ubiquitous bacterium Pseudomonas aeruginosa frequently causes hospital-acquired infections. P. aeruginosa also infects the lungs of cystic fibrosis (CF) patients and secretes N-(3-oxo-dodecanoyl)-S-homoserine lactone (3O-C12) to regulate bacterial gene expression critical for P. aeruginosa persistence. In addition to its effects as a quorum-sensing gene regulator in P. aeruginosa, 3O-C12 elicits cross-kingdom effects on host cell signaling leading to both pro- or anti-inflammatory effects. We find that in addition to these slow effects mediated through changes in gene expression, 3O-C12 also rapidly increases Cl(-) and fluid secretion in the cystic fibrosis transmembrane regulator (CFTR)-expressing airway epithelia. 3O-C12 does not stimulate Cl(-) secretion in CF cells, suggesting that lactone activates the CFTR. 3O-C12 also appears to directly activate the inositol trisphosphate receptor and release Ca(2+) from the endoplasmic reticulum (ER), lowering [Ca(2+)] in the ER and thereby activating the Ca(2+)-sensitive ER signaling protein STIM1. 3O-C12 increases cytosolic [Ca(2+)] and, strikingly, also cytosolic [cAMP], the known activator of CFTR. Activation of Cl(-) current by 3O-C12 was inhibited by a cAMP antagonist and increased by a phosphodiesterase inhibitor. Finally, a Ca(2+) buffer that lowers [Ca(2+)] in the ER similar to the effect of 3O-C12 also increased cAMP and I(Cl). The results suggest that 3O-C12 stimulates CFTR-dependent Cl(-) and fluid secretion in airway epithelial cells by activating the inositol trisphosphate receptor, thus lowering [Ca(2+)] in the ER and activating STIM1 and store-operated cAMP production. In CF airways, where CFTR is absent, the adaptive ability to rapidly flush the bacteria away is compromised because the lactone cannot affect Cl(-) and fluid secretion.

The {omega}-3 fatty acid eicosapentaenoic acid elicits cAMP generation in colonic epithelial cells via a "store-operated" mechanism.

Eicosapentaenoic acid (EPA) is an omega-3 polyunsaturated fatty acid abundant in fish oil that exerts a wide spectrum of documented beneficial health effects in humans. Because dietary interventions are relatively inexpensive and are widely assumed to be safe, they have broad public appeal. Their endorsement can potentially have a major impact on human health, but hard mechanistic evidence that specifies how these derivatives work at the cellular level is limited. EPA (50 microM) caused a small elevation of cytoplasmic Ca(2+) concentration ([Ca(2+)]) in intact NCM460 human colonic epithelial cells as measured by fura 2 and a profound drop of [Ca(2+)] within the endoplasmic reticulum (ER) of permeabilized cells as monitored by compartmentalized mag-fura 2. Total internal reflection fluorescence microscopy showed that this loss of ER store [Ca(2+)] led to translocation of the ER-resident transmembrane Ca(2+) sensor STIM1. Using sensitive FRET-based sensors for cAMP in single cells, we further found that EPA caused a substantial increase in cellular cAMP concentration, a large fraction of which was dependent on the drop in ER [Ca(2+)], but independent of cytosolic Ca(2+). An additional component of the EPA-induced cAMP signal was sensitive to the phosphodiesterase inhibitor isobutyl methylxanthine. We conclude that EPA slowly releases ER Ca(2+) stores, resulting in the generation of cAMP. The elevated cAMP is apparently independent of classical G protein-coupled receptor activation and is likely the consequence of a newly described "store-operated" cAMP signaling pathway that is mediated by STIM1.

"cAMP sponge": a buffer for cyclic adenosine 3', 5'-monophosphate.

BACKGROUND: While intracellular buffers are widely used to study calcium signaling, no such tool exists for the other major second messenger, cyclic AMP (cAMP). METHODS/PRINCIPAL FINDINGS: Here we describe a genetically encoded buffer for cAMP based on the high-affinity cAMP-binding carboxy-terminus of the regulatory subunit RIbeta of protein kinase A (PKA). Addition of targeting sequences permitted localization of this fragment to the extra-nuclear compartment, while tagging with mCherry allowed quantification of its expression at the single cell level. This construct (named "cAMP sponge") was shown to selectively bind cAMP in vitro. Its expression significantly suppressed agonist-induced cAMP signals and the downstream activation of PKA within the cytosol as measured by FRET-based sensors in single living cells. Point mutations in the cAMP-binding domains of the construct rendered the chimera unable to bind cAMP in vitro or in situ. Cyclic AMP sponge was fruitfully applied to examine feedback regulation of gap junction-mediated transfer of cAMP in epithelial cell couplets. CONCLUSIONS: This newest member of the cAMP toolbox has the potential to reveal unique biological functions of cAMP, including insight into the functional significance of compartmentalized signaling events.