Development of red genetically encoded biosensor for visualization of intracellular glucose dynamics

RIBOsensor for FRET-based, real-time ribose measurements in live cells

d-Ribose is a building block of many essential biomolecules, including all nucleic acids, and its supplementation can enhance energy production, particularly under stress conditions such as ischemia and heart failure. The distribution, biosynthesis, and regulation of ribose in mammalian systems remain poorly understood. To explore intracellular ribose dynamics, we developed a genetically encoded fluorescence resonance energy transfer (FRET) sensor using ribose binding protein (RBP) and enhanced cyan and yellow fluorescent proteins (FPs). The RIBOsensor, which positions one FP near the active site of RBP, achieves the necessary sensitivity for cellular imaging by increasing the FRET signal upon ribose binding, compared to traditional N- and C-terminal FP orientations. This sensor rapidly, reversibly, and selectively detects labile ribose in live cells-enabling longitudinal studies-and can be employed for intracellular ribose quantitation, which provides a valuable tool for investigating ribose transport and metabolism in normal and disease states.

Glucose modulates IRF6 transcription factor dimerization to enable epidermal differentiation

Non-energetic roles for glucose are largely unclear, as is the interplay between transcription factors (TFs) and ubiquitous biomolecules. Metabolomic analyses uncovered elevation of intracellular glucose during differentiation of diverse cell types. Human and mouse tissue engineered with glucose sensors detected a glucose gradient that peaked in the outermost differentiated layers of the epidermis. Free glucose accumulation was essential for epidermal differentiation and required the SGLT1 glucose transporter. Glucose affinity chromatography uncovered glucose binding to diverse regulatory proteins, including the IRF6 TF. Direct glucose binding enabled IRF6 dimerization, DNA binding, genomic localization, and induction of IRF6 target genes, including essential pro-differentiation TFs GRHL1, GRHL3, HOPX, and PRDM1. These data identify a role for glucose as a gradient morphogen that modulates protein multimerization in cellular differentiation.

Spatial diversity of in vivo tissue immunity

The immune system exhibits spatial diversity in in vivo tissues. Immune cells are strategically distributed within tissues to maintain the organ integrity. Advanced technologies such as intravital imaging and spatial transcriptomics have revealed the spatial heterogeneity of immune cell distribution and function within organs such as the liver, kidney, intestine, and lung. In addition, these technologies visualize nutrient and oxygen environments across tissues. Recent spatial analyses have suggested that a functional immune niche is determined by interactions between immune and non-immune cells in an appropriate nutrient and oxygen environment. Understanding the spatial communication between immune cells, environment, and surrounding non-immune cells is crucial for developing strategies to control immune responses and effectively manage inflammatory diseases.

Molecular Spies in Action: Genetically Encoded Fluorescent Biosensors Light up Cellular Signals

Cellular function is controlled through intricate networks of signals, which lead to the myriad pathways governing cell fate. Fluorescent biosensors have enabled the study of these signaling pathways in living systems across temporal and spatial scales. Over the years there has been an explosion in the number of fluorescent biosensors, as they have become available for numerous targets, utilized across spectral space, and suited for various imaging techniques. To guide users through this extensive biosensor landscape, we discuss critical aspects of fluorescent proteins for consideration in biosensor development, smart tagging strategies, and the historical and recent biosensors of various types, grouped by target, and with a focus on the design and recent applications of these sensors in living systems.

A bright red fluorescent genetically encoded sensor for lactate imaging

Lactate plays a crucial role in energy metabolism and greatly impacts protein activities, exerting diverse physiological and pathological effects. Therefore, convenient lactate assays for tracking spatiotemporal dynamics in living cells are desirable. In this paper, we engineered and optimized a red fluorescent protein sensor for l-lactate named FiLa-Red. This indicator exhibited a maximal fluorescence change of 730 % and an apparent dissociation constant (Kd) of approximately 460 μM. By utilizing FiLa-Red and other sensors, we monitored energy metabolism in a multiplex manner by simultaneously tracking lactate and NAD+/NADH abundance in the cytoplasm, nucleus, and mitochondria. The FiLa-Red sensor is expected to be a useful tool for performing metabolic analysis in vitro, in living cells and in vivo.

Development of fluorescence lifetime biosensors for ATP, cAMP, citrate, and glucose using the mTurquoise2-based platform

Single-fluorescent protein (FP)-based FLIM (fluorescence lifetime imaging microscopy) biosensors can visualize intracellular processes quantitatively. They require a single wavelength for detection, which facilitates multi-color imaging. However, their development has been limited by the absence of a general design framework and complex screening processes. In this study, we engineered FLIM biosensors for ATP (adenosine triphosphate), cAMP (cyclic adenosine monophosphate), citrate, and glucose by inserting each sensing domain into mTurquoise2 (mTQ2) between Tyr-145 and Phe-146 using peptide linkers. Fluorescence intensity-based screening yielded FLIM biosensors with a 0.5 to 1.0 ns dynamic range upon analyte binding, demonstrating that the mTQ2(1-145)-GT-X-EF-mTQ2(146-238) backbone is a versatile platform for FLIM biosensors, allowing for simple intensity-based screening while providing dual-functional biosensors for both FLIM and intensity-based imaging. As a proof of concept, we monitored cAMP and Ca2+ dynamics simultaneously in living cells by dual-color imaging. Our results complement recent studies, establishing mTQ2 as a valuable framework for developing FLIM biosensors.

Elucidating the spatiotemporal dynamics of glucose metabolism with genetically encoded fluorescent biosensors

Glucose metabolism has been well understood for many years, but some intriguing questions remain regarding the subcellular distribution, transport, and functions of glycolytic metabolites. To address these issues, a living cell metabolic monitoring technology with high spatiotemporal resolution is needed. Genetically encoded fluorescent sensors can achieve specific, sensitive, and spatiotemporally resolved metabolic monitoring in living cells and in vivo, and dozens of glucose metabolite sensors have been developed recently. Here, we highlight the importance of tracking specific intermediate metabolites of glycolysis and glycolytic flux measurements, monitoring the spatiotemporal dynamics, and quantifying metabolite abundance. We then describe the working principles of fluorescent protein sensors and summarize the existing biosensors and their application in understanding glucose metabolism. Finally, we analyze the remaining challenges in developing high-quality biosensors and the huge potential of biosensor-based metabolic monitoring at multiple spatiotemporal scales.

Imaging immunometabolism in situ in live animals

Immunometabolism is a rapidly developing field that holds great promise for diagnostic and therapeutic benefits to human diseases. The field has emerged based on seminal findings from in vitro and ex vivo studies that established the fundamental role of metabolism in immune cell effector functions. Currently, the field is acknowledging the necessity of investigating cellular metabolism within the natural context of biological processes. Examining cells in their native microenvironment is essential not only to reveal cell-intrinsic mechanisms but also to understand how cross-talk between neighboring cells regulates metabolism at the tissue level in a local niche. This necessity is driving innovation and advancement in multiple imaging-based technologies to enable analysis of dynamic intracellular metabolism at the single-cell level, with spatial and temporal resolution. In this review, we tally the currently available imaging-based technologies and explore the emerging methods of Raman and autofluorescence lifetime imaging microscopy, which hold significant potential and offer broad applications in the field of immunometabolism.

Distinct lactate metabolism between hepatocytes and myotubes revealed by live cell imaging with genetically encoded indicators

The process of glycolysis breaks down glycogen stored in muscles, producing lactate through pyruvate to generate energy. Excess lactate is then released into the bloodstream. When lactate reaches the liver, it is converted to glucose, which muscles utilize as a substrate to generate ATP. Although the biochemical study of lactate metabolism in hepatocytes and skeletal muscle cells has been extensive, the spatial and temporal dynamics of this metabolism in live cells are still unknown. We observed the dynamics of metabolism-related molecules in primary cultured hepatocytes and a skeletal muscle cell line upon lactate overload. Our observations revealed an increase in cytoplasmic pyruvate concentration in hepatocytes, which led to glucose release. Skeletal muscle cells exhibited elevated levels of lactate and pyruvate levels in both the cytoplasm and mitochondrial matrix. However, mitochondrial ATP levels remained unaffected, indicating that the increased lactate can be converted to pyruvate but is unlikely to be utilized for ATP production. The findings suggest that excess lactate in skeletal muscle cells is taken up into mitochondria with little contribution to ATP production. Meanwhile, lactate released into the bloodstream can be converted to glucose in hepatocytes for subsequent utilization in skeletal muscle cells.

Mitochondrial ATP concentration decreases immediately after glucose administration to glucose-deprived hepatocytes

Hepatocytes can switch their metabolic processes in response to nutrient availability. However, the dynamics of metabolites (such as lactate, pyruvate, and ATP) in hepatocytes during the metabolic switch remain unknown. In this study, we visualized metabolite dynamics in primary cultured hepatocytes during recovery from glucose-deprivation. We observed a decrease in the mitochondrial ATP concentration when glucose was administered to hepatocytes under glucose-deprivation conditions. In contrast, there was slight change in the cytoplasmic ATP concentration. A decrease in mitochondrial ATP concentration was associated with increased protein synthesis rather than glycogen synthesis, activation of urea cycle, and production of reactive oxygen species. These results suggest that mitochondrial ATP is important in switching metabolic processes in the hepatocytes.

Antagonistic regulation of salt and sugar chemotaxis plasticity by a single chemosensory neuron in Caenorhabditis elegans

The nematode Caenorhabditis elegans memorizes various external chemicals, such as ions and odorants, during feeding. Here we find that C. elegans is attracted to the monosaccharides glucose and fructose after exposure to these monosaccharides in the presence of food; however, it avoids them without conditioning. The attraction to glucose requires a gustatory neuron called ASEL. ASEL activity increases when glucose concentration decreases. Optogenetic ASEL stimulation promotes forward movements; however, after glucose conditioning, it promotes turning, suggesting that after glucose conditioning, the behavioral output of ASEL activation switches toward glucose. We previously reported that chemotaxis toward sodium ion (Na+), which is sensed by ASEL, increases after Na+ conditioning in the presence of food. Interestingly, glucose conditioning decreases Na+ chemotaxis, and conversely, Na+ conditioning decreases glucose chemotaxis, suggesting the reciprocal inhibition of learned chemotaxis to distinct chemicals. The activation of PKC-1, an nPKC ε/η ortholog, in ASEL promotes glucose chemotaxis and decreases Na+ chemotaxis after glucose conditioning. Furthermore, genetic screening identified ENSA-1, an ortholog of the protein phosphatase inhibitor ARPP-16/19, which functions in parallel with PKC-1 in glucose-induced chemotactic learning toward distinct chemicals. These findings suggest that kinase-phosphatase signaling regulates the balance between learned behaviors based on glucose conditioning in ASEL, which might contribute to migration toward chemical compositions where the animals were previously fed.

Mid-infrared photoacoustic spectroscopy based on ultrasound detection for blood component analysis

For the non-invasive measurement of biological tissue, a piezoelectric photoacoustic spectroscopy (PZT-PAS) system that detects a single frequency of ultrasound induced by the irradiation of pulse-modulated mid-infrared laser light was developed. PA spectra of the optical phantom and biological samples were obtained, and the relationship between the PA signal intensity and optical absorbance in the fingerprint region (930-1,200 cm^-1) was analyzed to estimate the optical absorbance. The resonance vibration of the induced ultrasound was utilized to further increase the signal strength for biological tissue measurement. Consequently, PA spectrum reflecting the absorption of components in biological tissues was obtained.

Modern Microscopic Approaches to Astrocytes

Microscopy started as the histological analysis based on intrinsic optical properties of tissues such as the refractive index and light absorption, and is expanding to include the visualization of organelles by chemical staining, localization of molecules by immunostaining, physiological measurements such as Ca²⁺ imaging, functional manipulation by optogenetics, and comprehensive analysis of chemical composition by Raman spectra. The microscope is one of the most important tools in neuroscience, which aims to reveal the complex intercellular communications underlying brain function and pathology. Many aspects of astrocytes, including the structures of their fine processes and physiological activities in concert with neurons and blood vessels, were revealed in the course of innovations in modern microscopy. The evolution of modern microscopy is a consequence of breakthroughs in spatiotemporal resolutions and expansions in molecular and physiological targets due to the progress in optics and information technology, as well as the inventions of probes using organic chemistry and molecular biology. This review overviews the modern microscopic approach to astrocytes.

Vitamin C supplementation for diabetes management: A comprehensive narrative review

Growing evidence suggests that vitamin C supplementation may be an effective adjunct therapy in the management of people with diabetes. This paper critically reviews the current evidence on effects of vitamin C supplementation and its potential mechanisms in diabetes management. Evidence from meta-analyses of randomized controlled trials (RCTs) show favourable effects of vitamin C on glycaemic control and blood pressure that may be clinically meaningful, and mixed effects on blood lipids and endothelial function. However, evidence is mostly of low evidence certainty. Emerging evidence is promising for effects of vitamin C supplementation on some diabetes complications, particularly diabetic foot ulcers. However, there is a notable lack of robust and well-designed studies exploring effects of vitamin C as a single compound supplement on diabetes prevention and patient-important outcomes (i.e. prevention and amelioration of diabetes complications). RCTs are also required to investigate potential preventative or ameliorative effects of vitamin C on gestational diabetes outcomes. Oral vitamin C doses of 500-1000 mg per day are potentially effective, safe, and affordable for many individuals with diabetes. However, personalisation of supplementation regimens that consider factors such as vitamin C status, disease status, current glycaemic control, vitamin C intake, redox status, and genotype is important to optimize vitamin C's therapeutic effects safely. Finally, given a high prevalence of vitamin C deficiency in patients with complications, it is recommended that plasma vitamin C concentration be measured and monitored in the clinic setting.

Glucose as a Protein-Condensing Cellular Solute

The present study reports a surprising protein-condensing effect of glucose, prompted by our accidental observation during chemical library screening under a high-glucose condition. We noticed "glucosing-out" of certain compounds, in which physiological concentrations of glucose induced compound aggregation. Adapting the "glucosing-out" concept to proteins, our proteomic analysis identified three cellular proteins (calmodulin, rho guanine nucleotide exchange factor 40, and polyubiquitin-C) that displayed robust glucose-dependent precipitation. One of these proteins, calmodulin, formed glucose-dependent condensates that control cellular glycogenolysis in hepatic cells. Our findings suggest that glucose is a heretofore underappreciated driver of protein phase separation that may have profound effects on cellular homeostasis.

Visualizing physiological parameters in cells and tissues using genetically encoded indicators for metabolites

The study of metabolism is undergoing a renaissance. Since the year 2002, over 50 genetically-encoded fluorescent indicators (GEFIs) have been introduced, capable of monitoring metabolites with high spatial/temporal resolution using fluorescence microscopy. Indicators are fusion proteins that change their fluorescence upon binding a specific metabolite. There are indicators for sugars, monocarboxylates, Krebs cycle intermediates, amino acids, cofactors, and energy nucleotides. They permit monitoring relative levels, concentrations, and fluxes in living systems. At a minimum they report relative levels and, in some cases, absolute concentrations may be obtained by performing ad hoc calibration protocols. Proper data collection, processing, and interpretation are critical to take full advantage of these new tools. This review offers a survey of the metabolic indicators that have been validated in mammalian systems. Minimally invasive, these indicators have been instrumental for the purposes of confirmation, rebuttal and discovery. We envision that this powerful technology will foster metabolic physiology.