Achieving Cross Time-Domain Multiplexed Signal Cascade and Cancer Exosomes Identification by Bridging Long Lifetime Phosphor to NIR-II Lanthanide Energy Transfer

Designing lanthanide luminescence lifetime sensors in the second near-infrared (NIR-II) window holds great potentials for physiological studies. However, the single lifetime signal is confined to one or two orders of magnitude of signal variation, which limits the sensitivity of lifetime probes. In this study, a lifetime cascade system, i.e., ZGO:Mn, Eu-DNA-1/TCPP-PEI70K @Yb-AptEpCAM , with a variety of signals (τm , τn , τµ , τm /τn and τm /τµ ) is constructed for exosome identification using time-domain multiplexing. The sensitized ligand TCPP acts as both target-modulated switch and a bridge for connecting long lifetime ZGO:Mn, Eu-DNA-1 emitter to lanthanide Yb3+ . This drives successive dual-path energy transfer and forms two D(donor) -A(acceptor) pairs. The lifetime variation is dominantly modulated by arranging TCPP as energy intermediate relay to covert milliseconds to nanoseconds to microseconds. It enables a broad lifetime range of six orders of magnitude. The presence of exosome specifically recognizes aptamers on TCPP-PEI70K @Yb-AptEpCAM to impede D-A pairs and reverse multiplexed response signals of the lifetime cascade system. The ratio lifetime signals τm /τn and τm /τµ achieve prominent exosome quantification and exosome type differentiation attributed to signal amplification. The cascade system relying on lifetime criteria can realize precise quantization and provide an effective strategy for subsequent physiological study.

Fluorescence lifetime image microscopy prediction with convolutional neural networks for cell detection and classification in tissues

Convolutional neural networks (CNNs) and other deep-learning models have proven to be transformative tools for the automated analysis of microscopy images, particularly in the domain of cellular and tissue imaging. These computer-vision models have primarily been applied with traditional microscopy imaging modalities (e.g. brightfield and fluorescence), likely due to the availability of large datasets in these regimes. However, more advanced microscopy imaging techniques could, potentially, allow for improved model performance in various computational histopathology tasks. In this work, we demonstrate that CNNs can achieve high accuracy in cell detection and classification without large amounts of data when applied to histology images acquired by fluorescence lifetime imaging microscopy (FLIM). This accuracy is higher than what would be achieved with regular single or dual-channel fluorescence images under the same settings, particularly for CNNs pretrained on publicly available fluorescent cell or general image datasets. Additionally, generated FLIM images could be predicted from just the fluorescence image data by using a dense U-Net CNN model trained on a subset of ground-truth FLIM images. These U-Net CNN generated FLIM images demonstrated high similarity to ground truth and improved accuracy in cell detection and classification over fluorescence alone when used as input to a variety of commonly used CNNs. This improved accuracy was maintained even when the FLIM images were generated by a U-Net CNN trained on only a few example FLIM images.

Raman Imaging and Fluorescence Lifetime Imaging Microscopy for Diagnosis of Cancer State and Metabolic Monitoring

Hurdles for effective tumor therapy are delayed detection and limited effectiveness of systemic drug therapies by patient-specific multidrug resistance. Non-invasive bioimaging tools such as fluorescence lifetime imaging microscopy (FLIM) and Raman-microspectroscopy have evolved over the last decade, providing the potential to be translated into clinics for early-stage disease detection, in vitro drug screening, and drug efficacy studies in personalized medicine. Accessing tissue- and cell-specific spectral signatures, Raman microspectroscopy has emerged as a diagnostic tool to identify precancerous lesions, cancer stages, or cell malignancy. In vivo Raman measurements have been enabled by recent technological advances in Raman endoscopy and signal-enhancing setups such as coherent anti-stokes Raman spectroscopy or surface-enhanced Raman spectroscopy. FLIM enables in situ investigations of metabolic processes such as glycolysis, oxidative stress, or mitochondrial activity by using the autofluorescence of co-enzymes NADH and FAD, which are associated with intrinsic proteins as a direct measure of tumor metabolism, cell death stages and drug efficacy. The combination of non-invasive and molecular-sensitive in situ techniques and advanced 3D tumor models such as patient-derived organoids or microtumors allows the recapitulation of tumor physiology and metabolism in vitro and facilitates the screening for patient-individualized drug treatment options.

AIEgen-Based Lifetime-Probes for Precise Furin Quantification and Identification of Cell Subtypes

Biochemical sensing probes based on aggregation-induced-emission luminogens (AIEgens) are widely used in biological imaging and therapy, chemical sensing, and material sciences. However, it is still a great challenge to quantify the targets through fluorescence intensity of AIEgen probes due to their undesirable aggregations. Here, a PyTPA-ZGO probe with three lifetime signals for precise quantification of furin is constructed: the lifetime signal 1 and signal 2 comes from AIEgen PyTPA-P (τ_Pn ) and inorganic nanoparticles Zn₂ GeO₄ :Mn²⁺ -NH₂ (τ_Zn ), respectively, while the lifetime signal 3 is marked as the composite dual-lifetime signal (CDLS_n , C D L S n = τ Z n τ P n ). In contrast, the fluorescence intensity signal of PyTPA-P shows defectively quantitative performance. Furthermore, it is found that the CDLS_n exhibits higher significant differences than the two other lifetime signals (τ_Pn and τ_Zn ) thanks to its wide range between the maximum and minimum signal values and small standard deviation. Therefore, CDLS_n is further used to accurately identify cell subtypes based on the specific concentration of furin in each subtype. The lifetime criterion can realize precise quantification, and it should be a promising direction of AIEgen-based quantitative analysis in the future.

Enhanced Exciton Quantum Coherence in Single CsPbBr3 Perovskite Quantum Dots using Femtosecond Two-Photon Near-Field Scanning Optical Microscopy

Cesium-halide perovskite quantum dots (QDs) have gained tremendous interest as quantum emitters in quantum information processing applications due to their optical and photophysical properties. However, engineering excitonic states in quantum dots requires a deep knowledge of the coherent dynamics of their excitons at a single-particle level. Here, we use femtosecond time-resolved two-photon near-field scanning optical microscopy (NSOM) to reveal coherences involving a single cesium lead bromide perovskite QD (CsPbBr₃) at room temperature. We show that, compared to other nonperovskite nanoparticles, the electronic coherence on a single perovskite QD has a relatively long lifetime of ca. 150 fs, whereas CdSe QDs have exciton coherence times shorter than 75 fs at room temperature. One possible explanation for the longer coherence time observed for the CsPbBr₃ perovskite system is related to the exciton fine structure of these perovskite QDs compared to other nanoparticles. These perovskite QDs exhibit interesting optical properties that differ from those of the traditional QDs including bright triplet exciton states. In fact, due to the small amplitude of the energy gap fluctuations of dipole-allowed triplet states in perovskite QDs, the coherent superposition could be preserved for longer times. Furthermore, single-particle excitation approach implemented in this work allows us to remove effects of heterogeneity that are usually present in ensemble averaging experiments at room temperature. The realization of quantum-mechanical phase-coherence of a charge carrier that can operate at room temperature is an issue of great importance for the potential application of coherent electronic phenomena in electronic and optoelectronic devices. These interesting findings provide further evidence of the great potential of these perovskite QDs as candidates for quantum computing and information processing applications.

Frequency-doubled femtosecond Er-doped fiber laser for two-photon excited fluorescence imaging

A femtosecond frequency-doubled erbium-doped fiber laser with an adjustable pulse repetition rate is developed and applied in two-photon excited fluorescence microscopy. The all-fiber laser system provides the fundamental pulse at 1560 nm wavelength with 22 fs duration for the second harmonic generation, resulting in 1.35 nJ, 60 fs pulses at 780 nm. The repetition rate is adjusted by a pulse picker unit built-in within the amplifier chain, directly providing transform-limited pulses for any chosen repetition rate between 1 and 12 MHz. We employed the laser source to drive a scanning two-photon excited fluorescence microscope for ex vivo rat skin and other samples' imaging at various pulse repetition rates. Due to compactness, ease of operation, and suitable pulse characteristics, the laser source can be considered as an attractive alternative for Ti:Sapphire laser in biomedical imaging.

Increased intracellular Cl- concentration improves airway epithelial migration by activating the RhoA/ROCK Pathway

In the airway, Cl- is the most abundant anion and is critically involved in transepithelial transport. The correlation of the abnormal expression and activation of chloride channels (CLCs), such as cystic fibrosis transmembrane conductance regulators (CFTRs), anoctamin-1, and CLC-2, with cell migration capability suggests a relationship between defective Cl- transport and epithelial wound repair. However, whether a correlation exists between intracellular Cl- and airway wound repair capability has not been explored thus far, and the underlying mechanisms involved in this relationship are not fully defined. Methods: In this work, the alteration of intracellular chloride concentration ([Cl-]i) was measured by using a chloride-sensitive fluorescent probe (N-[ethoxycarbonylmethyl]-6-methoxyquinolium bromide). Results: We found that clamping with high [Cl-]i and 1 h of treatment with the CLC inhibitor CFTR blocker CFTRinh-172 and chloride intracellular channel inhibitor IAA94 increased intracellular Cl- concentration ([Cl-]i) in airway epithelial cells. This effect improved epithelial cell migration. In addition, increased [Cl-]i in cells promoted F-actin reorganization, decreased cell stiffness, and improved RhoA activation and LIMK1/2 phosphorylation. Treatment with the ROCK inhibitor of Y-27632 and ROCK1 siRNA significantly attenuated the effects of increased [Cl-]i on LIMK1/2 activation and cell migration. In addition, intracellular Ca2+ concentration was unaffected by [Cl-]i clamping buffers and CFTRinh-172 and IAA94. Conclusion: Taken together, these results suggested that Cl- accumulation in airway epithelial cells could activate the RhoA/ROCK/LIMK cascade to induce F-actin reorganization, down-regulate cell stiffness, and improve epithelial migration.

Fluorescence lifetime imaging microscopy: fundamentals and advances in instrumentation, analysis, and applications

SIGNIFICANCE

Fluorescence lifetime imaging microscopy (FLIM) is a powerful technique to distinguish the unique molecular environment of fluorophores. FLIM measures the time a fluorophore remains in an excited state before emitting a photon, and detects molecular variations of fluorophores that are not apparent with spectral techniques alone. FLIM is sensitive to multiple biomedical processes including disease progression and drug efficacy.

AIM

We provide an overview of FLIM principles, instrumentation, and analysis while highlighting the latest developments and biological applications.

APPROACH

This review covers FLIM principles and theory, including advantages over intensity-based fluorescence measurements. Fundamentals of FLIM instrumentation in time- and frequency-domains are summarized, along with recent developments. Image segmentation and analysis strategies that quantify spatial and molecular features of cellular heterogeneity are reviewed. Finally, representative applications are provided including high-resolution FLIM of cell- and organelle-level molecular changes, use of exogenous and endogenous fluorophores, and imaging protein-protein interactions with Förster resonance energy transfer (FRET). Advantages and limitations of FLIM are also discussed.

CONCLUSIONS

FLIM is advantageous for probing molecular environments of fluorophores to inform on fluorophore behavior that cannot be elucidated with intensity measurements alone. Development of FLIM technologies, analysis, and applications will further advance biological research and clinical assessments.

Real-Time Imaging and Simultaneous Quantification of Mitochondrial H2O2 and ATP in Neurons with a Single Two-Photon Fluorescence-Lifetime-Based Probe

Mitochondrial oxidative stress and energy metabolism are vital biological events and are involved in various physiological and pathological processes such as apoptosis and necrosis. However, it remains unclear how the dynamic patterns of mitochondrial hydrogen peroxide (H₂O₂) and adenosine-5'-triphosphate (ATP) change in these events and, more importantly, how they affect each other. Herein, we developed a single two-photon fluorescence-lifetime-based probe (TFP), which offered real-time imaging and the simultaneous determination of mitochondrial H₂O₂ and ATP changes in two well-separated fluorescence channels without spectral crosstalk. The fluorescence lifetime of TFP exhibited good responses and selectivity in the detection ranges of 0.4-10 μM H₂O₂ and 0.5-15 mM ATP, taking advantage of accuracy and the quantitative ability of fluorescence lifetime imaging. Using this useful probe, we studied the relationship between H₂O₂ and ATP in mitochondria and visualized the dynamic level changes of mitochondrial H₂O₂ and ATP induced by the superoxide anion (O₂^•-). It was discovered that O₂^•- stimulation in a short period of time (8 min) temporarily changes the levels of H₂O₂ and ATP in mitochondria, and neurons were capable of recovering to the initial state in a short time. However, increasing time of up to 50 min of O₂^•- stimulation led to permanent oxidative damage and an energy deficiency. Meanwhile, it was first found that the exogenous stimulation of O₂^•- and H₂O₂ had different impacts on the levels of mitochondrial H₂O₂ and ATP, in which O₂^•- demonstrated more severe and negative consequences. As a matter of fact, this work not only has provided a general molecular design methodology for multiple species imaging but also has revealed oxidative-stress-induced intracellular functions related to H₂O₂ and ATP in mitochondria based on this developed TFP probe.

Local tissue manipulation via a force- and pressure-controlled AFM micropipette for analysis of cellular processes

Local manipulation of complex tissues at the single-cell level is challenging and requires excellent sealing between the specimen and the micromanipulation device. Here, biological applications for a recently developed loading technique for a force- and pressure-controlled fluidic force microscope micropipette are described. This technique allows for the exact positioning and precise spatiotemporal control of liquid delivery. The feasibility of a local loading technique for tissue applications was investigated using two fluorescent dyes, with which local loading behaviour could be optically visualised. Thus, homogeneous intracellular distribution of CellTracker Red and accumulation of SYTO 9 Green within nuclei was realised in single cells of a tissue preparation. Subsequently, physiological micromanipulation experiments were performed. Salivary gland tissue was pre-incubated with the Ca2+-sensitive dye OGB-1. An intracellular Ca2+ rise was then initiated at the single-cell level by applying dopamine via micropipette. When pre-incubating tissue with the nitric oxide (NO)-sensitive dye DAF-FM, NO release and intercellular NO diffusion was observed after local application of the NO donor SNP. Finally, local micromanipulation of a well-defined area along irregularly shaped cell surfaces of complex biosystems was shown for the first time for the fluidic force microscope micropipette. Thus, this technique is a promising tool for the investigation of the spatiotemporal effects of locally applied substances in complex tissues.

Elevated Cytosolic Cl- Concentrations in Dendritic Knobs of Mouse Vomeronasal Sensory Neurons

In rodents, the vomeronasal system controls social and sexual behavior. However, several mechanistic aspects of sensory signaling in the vomeronasal organ remain unclear. Here, we investigate the biophysical basis of a recently proposed vomeronasal signal transduction component-a Ca(2+)-activated Cl(-) current. As the physiological role of such a current is a direct function of the Cl(-) equilibrium potential, we determined the intracellular Cl(-) concentration in dendritic knobs of vomeronasal neurons. Quantitative fluorescence lifetime imaging of a Cl(-)-sensitive dye at the apical surface of the intact vomeronasal neuroepithelium revealed increased cytosolic Cl(-) levels in dendritic knobs, a substantially lower Cl(-) concentration in vomeronasal sustentacular cells, and an apparent Cl(-) gradient in vomeronasal neurons along their dendritic apicobasal axis. Together, our data provide a biophysical basis for sensory signal amplification in vomeronasal neuron microvilli by opening Ca(2+)-activated Cl(-) channels.

Sperm metabolism is altered during storage by female insects: evidence from two-photon autofluorescence lifetime measurements in bedbugs

We explore the possibility of characterizing sperm cells without the need to stain them using spectral and fluorescence lifetime analyses after multi-photon excitation in an insect model. The autofluorescence emission spectrum of sperm of the common bedbug, Cimex lectularius, was consistent with the presence of flavins and NAD(P)H. The mean fluorescence lifetimes showed smaller variation in sperm extracted from the male (tau m, τm = 1.54-1.84 ns) than in that extracted from the female sperm storage organ (tau m, τm = 1.26-2.00 ns). The fluorescence lifetime histograms revealed four peaks. These peaks (0.18, 0.92, 2.50 and 3.80 ns) suggest the presence of NAD(P)H and flavins and show that sperm metabolism can be characterized using fluorescence lifetime imaging. The difference in fluorescence lifetime variation between the sexes is consistent with the notion that female animals alter the metabolism of sperm cells during storage. It is not consistent, however, with the idea that sperm metabolism represents a sexually selected character that provides females with information about the male genotype.

Sensing of intracellular environments by fluorescence lifetime imaging of exogenous fluorophores

Fluorescence lifetime imaging (FLIM) has been recognized as a powerful microscopy technique to examine environments in living systems. The fluorescence lifetime does not depend on the photobleaching and optical conditions, which allows us to obtain quantitative information on intracellular environments by analyzing the fluorescence lifetime. A variety of exogenous fluorophores have been applied in FLIM measurements to examine cellular processes. Information on the correlation between the fluorescence lifetime and the physiological parameters is essential to elucidate the cellular environments from the fluorescence lifetime measurements of exogenous fluorophores. In this review, exogenous fluorophores used for lifetime-based sensing are summarized, with the expectation that it becomes a basis for selecting the fluorophore used to investigate the intracellular environment with FLIM. Experimental results of the intracellular sensing of pH, metal ions, oxygen, viscosity, and other physiological parameters on the basis of the FLIM measurements are described along with a brief explanation of the mechanism of the change in the fluorescence lifetime.

Intracellular chloride concentration of the mouse vomeronasal neuron

Background

The vomeronasal organ (VNO) is specialized in detecting pheromone and heterospecific cues in the environment. Recent studies demonstrate the involvement of multiple ion channels in VNO signal transduction, including the calcium-activated chloride channels (CACCs). Opening of CACCs appears to result in activation of VNO neuron through outflow of Cl⁻ ions. However, the intracellular Cl⁻ concentration remains undetermined.

Results

We used the chloride ion quenching dye, MQAE, to measure the intracellular Cl⁻ concentration of VNO neuron in live VNO slices. The resting Cl⁻ concentration in the VNO neurons is measured at 84.73 mM. Urine activation of the VNO neurons causes a drop in Cl⁻ concentration, consistent with the notion of an efflux of Cl⁻ to depolarize the cells. Similar observation is made for VNO neurons from mice with deletion of the transient receptor potential canonical channel 2 (TRPC2), which have a resting Cl⁻ concentrations at 81 mM.

Conclusions

The VNO neurons rest at high intracellular Cl⁻ concentration, which can lead to depolarization of the cell when chloride channels open. These results also provide additional support of TRPC2-independent pathway of VNO activation.

Molecular probes for fluorescence lifetime imaging

Visualization of biological processes and pathologic conditions at the cellular and tissue levels largely relies on the use of fluorescence intensity signals from fluorophores or their bioconjugates. To overcome the concentration dependency of intensity measurements, evaluate subtle molecular interactions, and determine biochemical status of intracellular or extracellular microenvironments, fluorescence lifetime (FLT) imaging has emerged as a reliable imaging method complementary to intensity measurements. Driven by a wide variety of dyes exhibiting stable or environment-responsive FLTs, information multiplexing can be readily accomplished without the need for ratiometric spectral imaging. With knowledge of the fluorescent states of the molecules, it is entirely possible to predict the functional status of biomolecules or microevironment of cells. Whereas the use of FLT spectroscopy and microscopy in biological studies is now well-established, in vivo imaging of biological processes based on FLT imaging techniques is still evolving. This review summarizes recent advances in the application of the FLT of molecular probes for imaging cells and small animal models of human diseases. It also highlights some challenges that continue to limit the full realization of the potential of using FLT molecular probes to address diverse biological problems and outlines areas of potential high impact in the future.

Photoluminescence imaging of Zn2+in living systems

Advances in PL imaging techniques, such as confocal microscopy, two photon microscopy, lifetime and optical imaging techniques, have made remarkable contributions in Zn²⁺tracking.

ANG-2 for quantitative Na(+) determination in living cells by time-resolved fluorescence microscopy

Sodium ions (Na(+)) play an important role in a plethora of cellular processes, which are complex and partly still unexplored. For the investigation of these processes and quantification of intracellular Na(+) concentrations ([Na(+)]i), two-photon coupled fluorescence lifetime imaging microscopy (2P-FLIM) was performed in the salivary glands of the cockroach Periplaneta americana. For this, the novel Na(+)-sensitive fluorescent dye Asante NaTRIUM Green-2 (ANG-2) was evaluated, both in vitro and in situ. In this context, absorption coefficients, fluorescence quantum yields and 2P action cross-sections were determined for the first time. ANG-2 was 2P-excitable over a broad spectral range and displayed fluorescence in the visible spectral range. Although the fluorescence decay behaviour of ANG-2 was triexponential in vitro, its analysis indicates a Na(+)-sensitivity appropriate for recordings in living cells. The Na(+)-sensitivity was reduced in situ, but the biexponential fluorescence decay behaviour could be successfully analysed in terms of quantitative [Na(+)]i recordings. Thus, physiological 2P-FLIM measurements revealed a dopamine-induced [Na(+)]i rise in cockroach salivary gland cells, which was dependent on a Na(+)-K(+)-2Cl(-) cotransporter (NKCC) activity. It was concluded that ANG-2 is a promising new sodium indicator applicable for diverse biological systems.

Asante Calcium Green and Asante Calcium Red--novel calcium indicators for two-photon fluorescence lifetime imaging

For a comprehensive understanding of cellular processes and potential dysfunctions therein, an analysis of the ubiquitous intracellular second messenger calcium is of particular interest. This study examined the suitability of the novel Ca²⁺-sensitive fluorescent dyes Asante Calcium Red (ACR) and Asante Calcium Green (ACG) for two-photon (2P)-excited time-resolved fluorescence measurements. Both dyes displayed sufficient 2P fluorescence excitation in a range of 720–900 nm. In vitro, ACR and ACG exhibited a biexponential fluorescence decay behavior and the two decay time components in the ns-range could be attributed to the Ca²⁺-free and Ca²⁺-bound dye species. The amplitude-weighted average fluorescence decay time changed in a Ca²⁺-dependent way, unraveling in vitro dissociation constants K_D of 114 nM and 15 nM for ACR and ACG, respectively. In the presence of bovine serum albumin, the absorption and steady-state fluorescence behavior of ACR was altered and its biexponential fluorescence decay showed about 5-times longer decay time components indicating dye-protein interactions. Since no ester derivative of ACG was commercially available, only ACR was evaluated for 2P-excited fluorescence lifetime imaging microscopy (2P-FLIM) in living cells of American cockroach salivary glands. In living cells, ACR also exhibited a biexponential fluorescence decay with clearly resolvable short (0.56 ns) and long (2.44 ns) decay time components attributable to the Ca²⁺-free and Ca²⁺-bound ACR species. From the amplitude-weighted average fluorescence decay times, an in situ K_D of 180 nM was determined. Thus, quantitative [Ca²⁺]_i recordings were realized, unraveling a reversible dopamine-induced [Ca²⁺]_i elevation from 21 nM to 590 nM in salivary duct cells. It was concluded that ACR is a promising new Ca²⁺ indicator dye for 2P-FLIM recordings applicable in diverse biological systems.

Two-photon chloride imaging using MQAE in vitro and in vivo

This protocol describes a technique for high-resolution chloride imaging of living cells using a quinoline-based chloride (Cl(-)) indicator dye, MQAE (N-[6-methoxyquinolyl] acetoethyl ester). Bath-applied to acute brain slices, MQAE provides high-quality labeling of neuronal cells and their processes. In living anesthetized mice, cortical cells are labeled using the multicell bolus loading procedure. In combination with two-photon microscopy, this procedure enables in vivo visualization of cell bodies of neurons and astrocytes as well as some astrocytic processes and allows one to monitor changes in the intracellular chloride concentration in dozens of individual cells.

Long working distance fluorescence lifetime imaging with stimulated emission and electronic time delay

In this work, long working distance fluorescence lifetime imaging is realized with stimulated emission in combination with electronic time delay control. Spatial coherence, as a result of stimulated emission, supports unattenuated fluorescence detection at extended distance, using low NA optics. An electronic time delayed trigger provides an advantageous way in adjusting the pulse separation and probing the fluorescence lifetime in the nanosecond ranges. The fluorescence lifetime of selected fluorophores is accurately determined through the pump-probe configuration. The characteristics and applications in fluorescence lifetime measurement of stimulated emission are investigated and summarized succinctly here.

Long-distance fluorescence lifetime imaging using stimulated emission

Long-distance stimulated emission imaging has recently been demonstrated as a novel approach for the characterization and imaging of samples containing fluorescent moieties. We present an extension of this methodology through a pump-probe setup for fluorescence lifetime determination and imaging. We measure fluorescence lifetimes of Rhodamine 6G at different solutions and indocyanine green using long-distance fluorescence lifetime imaging.

Activation of the bumetanide-sensitive Na+,K+,2Cl- cotransporter (NKCC2) is facilitated by Tamm-Horsfall protein in a chloride-sensitive manner

Active transport of NaCl across thick ascending limb (TAL) epithelium is accomplished by Na(+),K(+),2Cl(-) cotransporter (NKCC2). The activity of NKCC2 is determined by vasopressin (AVP) or intracellular chloride concentration and includes its amino-terminal phosphorylation. Co-expressed Tamm-Horsfall protein (THP) has been proposed to interact with NKCC2. We hypothesized that THP modulates NKCC2 activity in TAL. THP-deficient mice (THP(-/-)) showed an increased abundance of intracellular NKCC2 located in subapical vesicles (+47% compared with wild type (WT) mice), whereas base-line phosphorylation of NKCC2 was significantly decreased (-49% compared with WT mice), suggesting reduced activity of the transporter in the absence of THP. Cultured TAL cells with low endogenous THP levels and low base-line phosphorylation of NKCC2 displayed sharp increases in NKCC2 phosphorylation (+38%) along with a significant change of intracellular chloride concentration upon transfection with THP. In NKCC2-expressing frog oocytes, co-injection with THP cRNA significantly enhanced the activation of NKCC2 under low chloride hypotonic stress (+112% versus +235%). Short term (30 min) stimulation of the vasopressin V2 receptor pathway by V2 receptor agonist (deamino-cis-D-Arg vasopressin) resulted in enhanced NKCC2 phosphorylation in WT mice and cultured TAL cells transfected with THP, whereas in the absence of THP, NKCC2 phosphorylation upon deamino-cis-D-Arg vasopressin was blunted in both systems. Attenuated effects of furosemide along with functional and structural adaptation of the distal convoluted tubule in THP(-/-) mice supported the notion that NaCl reabsorption was impaired in TAL lacking THP. In summary, these results are compatible with a permissive role for THP in the modulation of NKCC2-dependent TAL salt reabsorptive function.

Two-photon microscopy and fluorescence lifetime imaging reveal stimulus-induced intracellular Na+ and Cl− changes in cockroach salivary acinar cells

The intracellular ion homeostasis in cockroach salivary acinar cells during salivation is not satisfactorily understood. This is mainly due to technical problems regarding strong tissue autofluorescence and ineffective ion concentration quantification. For minimizing these problems, we describe the successful application of two-photon (2P) microscopy partly in combination with fluorescence lifetime imaging microscopy (FLIM) to record intracellular Na+ and Cl− concentrations ([Na+]i, [Cl−]i) in cockroach salivary acinar cells. Quantitative 2P-FLIM Cl− measurements with the dye N-(ethoxycarbonylmethyl)-6-methoxy-quinolinium bromide indicate that the resting [Cl−]i is 1.6 times above the Cl− electrochemical equilibrium but is not influenced by pharmacological inhibition of the Na+-K+-2Cl− cotransporter (NKCC) and anion exchanger using bumetanide and 4,4′-diisothiocyanatodihydrostilbene-2,2′-disulfonic acid disodium salt. In contrast, rapid Cl− reuptake after extracellular Cl− removal is almost totally NKCC mediated both in the absence and presence of dopamine. However, in physiological saline [Cl−]i does not change during dopamine stimulation although dopamine stimulates fluid secretion in these glands. On the other hand, dopamine causes a decrease in the sodium-binding benzofuran isophthalate tetra-ammonium salt (SBFI) fluorescence and an increase in the Sodium Green fluorescence after 2P excitation. This opposite behavior of both dyes suggests a dopamine-induced [Na+]i rise in the acinar cells, which is supported by the determined 2P-action cross sections of SBFI. The [Na+]i rise is Cl− dependent and inhibited by bumetanide. The Ca2+-ionophore ionomycin also causes a bumetanide-sensitive [Na+]i rise. We propose that a Ca2+-mediated NKCC activity in acinar peripheral cells attributable to dopamine stimulation serves for basolateral Na+ uptake during saliva secretion and that the concomitantly transported Cl− is recycled back to the bath.

Imaging carious dental tissues with multiphoton fluorescence lifetime imaging microscopy

In this study, multiphoton excitation was utilized to image normal and carious dental tissues noninvasively. Unique structures in dental tissues were identified using the available multimodality (second harmonic, autofluorescence, and fluorescence lifetime analysis) without labeling. The collagen in dentin exhibits a strong second harmonic response. Both dentin and enamel emit strong autofluorescence that reveals in detail morphological features (such as dentinal tubules and enamel rods) and, despite their very similar spectral profiles, can be differentiated by lifetime analysis. Specifically, the carious dental tissue exhibits a greatly reduced autofluorescence lifetime, which result is consistent with the degree of demineralization, determined by micro-computed tomography. Our findings suggest that two-photon excited fluorescence lifetime imaging may be a promising tool for diagnosing and monitoring dental caries.

Genetically encoded optical sensors for monitoring of intracellular chloride and chloride-selective channel activity

This review briefly discusses the main approaches for monitoring chloride (Cl⁻), the most abundant physiological anion. Noninvasive monitoring of intracellular Cl⁻ ([Cl⁻]i) is a challenging task owing to two main difficulties: (i) the low transmembrane ratio for Cl⁻, approximately 10:1; and (ii) the small driving force for Cl⁻, as the Cl⁻ reversal potential (E_Cl) is usually close to the resting potential of the cells. Thus, for reliable monitoring of intracellular Cl⁻, one has to use highly sensitive probes. From several methods for intracellular Cl⁻ analysis, genetically encoded chloride indicators represent the most promising tools. Recent achievements in the development of genetically encoded chloride probes are based on the fact that yellow fluorescent protein (YFP) exhibits Cl⁻-sensitivity. YFP-based probes have been successfully used for quantitative analysis of Cl⁻ transport in different cells and for high-throughput screening of modulators of Cl⁻-selective channels. Development of a ratiometric genetically encoded probe, Clomeleon, has provided a tool for noninvasive estimation of intracellular Cl⁻ concentrations. While the sensitivity of this protein to Cl⁻ is low (EC₅₀ about 160 mM), it has been successfully used for monitoring intracellular Cl⁻ in different cell types. Recently a CFP–YFP-based probe with a relatively high sensitivity to Cl⁻ (EC₅₀ about 30 mM) has been developed. This construct, termed Cl-Sensor, allows ratiometric monitoring using the fluorescence excitation ratio. Of particular interest are genetically encoded probes for monitoring of ion channel distribution and activity. A new molecular probe has been constructed by introducing into the cytoplasmic domain of the Cl⁻-selective glycine receptor (GlyR) channel the CFP–YFP-based Cl-Sensor. This construct, termed BioSensor-GlyR, has been successfully expressed in cell lines. The new genetically encoded chloride probes offer means of screening pharmacological agents, analysis of Cl⁻ homeostasis and functions of Cl⁻-selective channels under different physiological and pathological conditions.