Modular platform for low-light microscopy

Human Adipose-Derived Stem Cells Reduce Cellular Damage after Experimental Spinal Cord Injury in Rats

Traumatic spinal cord injury (SCI) is a devastating condition without an effective therapy. Cellular therapies are among the promising treatment strategies. Adult stem cells, such as mesenchymal stem cells, are often used clinical research for their immunomodulatory and regenerative potential. This study aimed to evaluate the effect of human adipose tissue-derived stem cells (ADSC) infusion through the cauda equina in rats with SCI. The human ADSC from bariatric surgery was isolated, expanded, and characterized. Wistar rats were subjected to blunt SCI and were divided into four groups. Two experimental groups (EG): EG1 received one ADSC infusion after SCI, and EG2 received two infusions, the first one after SCI and the second infusion seven days after the injury. Control groups (CG1 and CG2) received infusion with a culture medium. In vivo, cell tracking was performed 48 h and seven days after ADSC infusion. The animals were followed up for 40 days after SCI, and immunohistochemical quantification of myelin, neurons, and astrocytes was performed. Cellular tracking showed cell migration towards the injury site. ADSC infusion significantly reduced neuronal loss, although it did not prevent the myelin loss or enhance the area occupied by astrocytes compared to the control group. The results were similar when comparing one or two cell infusions. The injection of ADSC distal to the injured area was shown to be a safe and effective method for cellular administration in spinal cord injury.

Incorporation and Assembly of a Light-Emitting Enzymatic Reaction into Model Protein Condensates

Eukaryotic cells partition enzymes and other cellular components into distinct subcellular compartments to generate specialized biochemical niches. A subclass of these compartments form in the absence of lipid membranes, via liquid-liquid phase separation of proteins to form biomolecular condensates or "membraneless organelles" such as nucleoli, stress granules, and P-bodies. Because of their propensity to form compartments from simple starting materials, membraneless organelles are an attractive target for engineering new functionalities in both living cells and protocells. In this work, we demonstrate incorporation of a novel enzymatic activity in protein coacervates with the light-generating enzyme, NanoLuc, to produce bioluminescence. Using condensates comprised of the disordered RGG domain of Caenorhabditis elegans LAF-1, we functionalized condensates with enzymatic activity in vitro and show that enzyme localization to coacervates enhances assembly and activity of split enzymes. To build condensates that function as light-emitting reactors, we designed a NanoLuc enzyme flanked by RGG domains. The resulting condensates concentrated NanoLuc by 10-fold over bulk solution and displayed significantly increased reaction rates. We further show that condensate viscosity impacts light emission due to diffusion-limited behavior. Because our model condensates have low viscosities, we predict NanoLuc diffusion-limited behavior in most other condensates and thus propose the condensate-Nanoluc system as a potential strategy for high-throughput screening of condensate targeting drugs. By splitting the NanoLuc enzyme into its constituent components, we demonstrate that NanoLuc activity can be reconstituted via co-condensation. In addition, we demonstrate control of the spatial localization of the enzyme within condensates by targettng NanoLuc to the surface of in vitro condensates. Collectively, this work demonstrates that membraneless organelles can be endowed with localized enzymatic activity and that this activity can be spatially and temporally controlled via biochemical reconstitution and design of protein surfactants.

Microfluidics-Coupled Radioluminescence Microscopy for In Vitro Radiotracer Kinetic Studies

Integrated bioassay systems that combine microfluidics and radiation detectors can deliver medical radiopharmaceuticals to live cells with precise timing, while minimizing radiation dose and sample volume. However, the spatial resolution of many radiation imaging systems is limited to bulk cell populations. Here, we demonstrate microfluidics-coupled radioluminescence microscopy (μF-RLM), a new integrated system that can image radiotracer uptake in live adherent cells growing inside microincubators with spatial resolution better than 30 μm. Our method enables on-chip radionuclide imaging by incorporating an inorganic scintillator plate (CdWO4) into a microfluidic chip. We apply this approach to investigate the factors that influence the dynamic uptake of [18F]fluorodeoxyglucose (FDG) by cancer cells. In the first experiment, we measured the effect of flow on FDG uptake of cells and found that a continuous flow of the radiotracer led to fourfold higher uptake than static incubation, suggesting that convective replenishment enhances molecular radiotracer transport into cells. In the second set of experiments, we applied pharmacokinetic modeling to show that lactic acidosis inhibits FDG uptake by cancer cells in vitro and that this decrease is primarily due to downregulation of FDG transport into the cells. The other two rate constants, which represent FDG export and FDG metabolism, were relatively unaffected by lactic acidosis. Lactic acidosis is common in solid tumors because of the dysregulated metabolism and inefficient vasculature. In conclusion, μF-RLM is a simple and practical approach for integrating high-resolution radionuclide imaging within standard microfluidics devices, thus potentially opening venues for investigating the efficacy of radiopharmaceuticals in in vitro cancer models.

Towards systems tissue engineering: Elucidating the dynamics, spatial coordination, and individual cells driving emergent behaviors

Biomaterial systems have enabled the in vitro production of complex, emergent tissue behaviors that were not possible with conventional two-dimensional culture systems, allowing for analysis of both normal development and disease processes. We propose that the path towards developing the design parameters for biomaterial systems lies with identifying the molecular drivers of emergent behavior through leveraging technological advances in systems biology, including single cell omics, genetic engineering, and high content imaging. This growing research opportunity at the intersection of the fields of tissue engineering and systems biology - systems tissue engineering - can uniquely interrogate the mechanisms by which complex tissue behaviors emerge with the potential to capture the contribution of i) dynamic regulation of tissue development and dysregulation, ii) single cell heterogeneity and the function of rare cell types, and iii) the spatial distribution and structure of individual cells and cell types within a tissue. By leveraging advances in both biological and materials data science, systems tissue engineering can facilitate the identification of biomaterial design parameters that will accelerate basic science discovery and translation.

Single-cell radioluminescence microscopy with two-fold higher sensitivity using dual scintillator configuration

Radioluminescence microscopy (RLM) is an imaging technique that allows quantitative analysis of clinical radiolabeled drugs and probes in single cells. However, the modality suffers from slow data acquisition (15–30 minutes), thus critically affecting experiments with short-lived radioactive drugs. To overcome this issue, we suggest an approach that significantly accelerates data collection. Instead of using a single scintillator to image the decay of radioactive molecules, we sandwiched the radiolabeled cells between two scintillators. As proof of concept, we imaged cells labeled with [¹⁸F]FDG, a radioactive glucose popularly used in oncology to image tumors. Results show that the double scintillator configuration increases the microscope sensitivity by two-fold, thus reducing the image acquisition time by half to achieve the same result as the single scintillator approach. The experimental results were also compared with Geant4 Monte Carlo simulation to confirm the two-fold increase in sensitivity with only minor degradation in spatial resolution. Overall, these findings suggest that the double scintillator configuration can be used to perform time-sensitive studies such as cell pharmacokinetics or cell uptake of short-lived radiotracers.

Thermochemiluminescent peroxide crystals

Chemiluminescence, a process of transduction of energy stored within chemical bonds of ground-state reactants into light via high-energy excited intermediates, is known in solution, but has remained undetected in macroscopic crystalline solids. By detecting thermally induced chemiluminescence from centimeter-size crystals of an organic peroxide here we demonstrate direct transduction of heat into light by thermochemiluminescence of bulk crystals. Heating of crystals of lophine hydroperoxide to ~115 °C results in detectable emission of blue-green light with maximum at 530 nm with low chemiluminescent quantum yield [(2.1 ± 0.1) × 10^‒7 E mol^‒1]. Spectral comparison of the thermochemiluminescence in the solid state and in solution revealed that the solid-state thermochemiluminescence of lophine peroxide is due to emission from deprotonated lophine. With selected 1,2-dioxetane, endoperoxide and aroyl peroxide we also establish that the thermochemiluminescence is common for crystalline peroxides, with the color of the emitted light varying from blue to green to red.

Chemiluminescence is known in solution, but has remained undetected in macroscopic crystalline solids so far. Here the authors demonstrate direct transduction of heat into light by thermochemiluminescence in a centimeter-size lophine hydroperoxide crystal.

Single-Cell Imaging Using Radioluminescence Microscopy Reveals Unexpected Binding Target for [18F]HFB

PURPOSE

Cell-based therapies are showing great promise for a variety of diseases, but remain hindered by the limited information available regarding the biological fate, migration routes and differentiation patterns of infused cells in trials. Previous studies have demonstrated the feasibility of using positron emission tomography (PET) to track single cells utilising an approach known as positron emission particle tracking (PEPT). The radiolabel hexadecyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]HFB) was identified as a promising candidate for PEPT, due to its efficient and long-lasting labelling capabilities. The purpose of this work was to characterise the labelling efficiency of [¹⁸F]HFB in vitro at the single-cell level prior to in vivo studies.

PROCEDURES

The binding efficiency of [¹⁸F]HFB to MDA-MB-231 and Jurkat cells was verified in vitro using bulk gamma counting. The measurements were subsequently repeated in single cells using a new method known as radioluminescence microscopy (RLM) and binding of the radiolabel to the single cells was correlated with various fluorescent dyes.

RESULTS

Similar to previous reports, bulk cell labelling was significantly higher with [¹⁸F]HFB (18.75 ± 2.47 dpm/cell, n = 6) than 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) (7.59 ± 0.73 dpm/cell, n = 7; p ≤ 0.01). However, single-cell imaging using RLM revealed that [¹⁸F]HFB accumulation in live cells (8.35 ± 1.48 cpm/cell, n = 9) was not significantly higher than background levels (4.83 ± 0.52 cpm/cell, n = 12; p > 0.05) and was 1.7-fold lower than [¹⁸F]FDG uptake in the same cell line (14.09 ± 1.90 cpm/cell, n = 13; p < 0.01). Instead, [¹⁸F]HFB was found to bind significantly to fragmented membranes associated with dead cell nuclei, suggesting an alternative binding target for [¹⁸F]HFB.

CONCLUSION

This study demonstrates that bulk analysis alone does not always accurately portray the labelling efficiency, therefore highlighting the need for more routine screening of radiolabels using RLM to identify heterogeneity at the single-cell level.

In silico optimization of radioluminescence microscopy

Radioluminescence microscopy (RLM) is a high-resolution method for imaging radionuclide uptake in live cells within a fluorescence microscopy environment. Although RLM currently provides sufficient spatial resolution and sensitivity for cell imaging, it has not been systematically optimized. This study seeks to optimize the parameters of the system by computational simulation using a combination of numerical models for the system's various components: Monte-Carlo simulation for radiation transport, 3D optical point-spread function for the microscope, and stochastic photosensor model for the electron multiplying charge coupled device (EMCCD) camera. The relationship between key parameters and performance metrics relevant to image quality is examined. Results show that Lu2 O3 :Eu yields the best performance among 5 different scintillator materials, and a thickness: 8 μm can best balance spatial resolution and sensitivity. For this configuration, a spatial resolution of ~20 μm and sensitivity of 40% can be achieved for all 3 magnifications investigated, provided that the user adjusts pixel binning and electron multiplying (EM) gain accordingly. Hence the primary consideration for selecting the magnification should be the desired field of view and magnification for concurrent optical microscopy studies. In conclusion, this study estimates the optimal imaging performance achievable with RLM and promotes further development for more robust imaging of cellular processes using radiotracers.

Toward a Droplet-Based Single-Cell Radiometric Assay

Radiotracers are widely used to track molecular processes, both in vitro and in vivo, with high sensitivity and specificity. However, most radionuclide detection methods have spatial resolution inadequate for single-cell analysis. A few existing methods can extract single-cell information from radioactive decays, but the stochastic nature of the process precludes high-throughput measurement (and sorting) of single cells. In this work, we introduce a new concept for translating radioactive decays occurring stochastically within radiolabeled single-cells into an integrated, long-lasting fluorescence signal. Single cells are encapsulated in radiofluorogenic droplets containing molecular probes sensitive to byproducts of ionizing radiation (primarily reactive oxygen species, or ROS). Different probes were examined in bulk solutions, and dihydrorhodamine 123 (DHRh 123) was selected as the lead candidate due to its sensitivity and reproducibility. Fluorescence intensity of DHRh 123 in bulk increased at a rate of 54% per Gy of X-ray radiation and 15% per MBq/ml of 2-deoxy-2-[¹⁸F]-fluoro-d-glucose ([¹⁸F]FDG). Fluorescence imaging of microfluidic droplets showed the same linear response, but droplets were less sensitive overall than the bulk ROS sensor (detection limit of 3 Gy per droplet). Finally, droplets encapsulating radiolabeled cancer cells allowed, for the first time, the detection of [¹⁸F]FDG radiotracer uptake in single cells through fluorescence activation. With further improvements, we expect this technology to enable quantitative measurement and selective sorting of single cells based on the uptake of radiolabeled small molecules.

Modular low-light microscope for imaging cellular bioluminescence and radioluminescence

Low-light microscopy methods are receiving increased attention as new applications have emerged. One such application is to allow longitudinal imaging of light-sensitive cells with no phototoxicity and no photobleaching of fluorescent biomarkers. Another application is for imaging signals that are inherently dim and undetectable using standard microscopy techniques, such as bioluminescence, chemiluminescence or radioluminescence. In this protocol, we provide instructions on how to build a modular low-light microscope (1-4 d) by coupling two microscope objective lenses, back to back from each other, using standard optomechanical components. We also provide directions on how to image dim signals such as those of radioluminescence (1-1.5 h), bioluminescence (∼30 min) and low-excitation fluorescence (∼15 min). In particular, radioluminescence microscopy is explained in detail, as it is a newly developed technique that enables the study of small-molecule transport (e.g., radiolabeled drugs, metabolic precursors and nuclear medicine contrast agents) by single cells without perturbing endogenous biochemical processes. In this imaging technique, a scintillator crystal (e.g., CdWO₄) is placed in close proximity to the radiolabeled cells, where it converts the radioactive decays into optical flashes detectable using a sensitive camera. Using the image reconstruction toolkit provided in this protocol, the flashes can be reconstructed to yield high-resolution images of the radiotracer distribution. With appropriate timing, the three aforementioned imaging modalities may be performed together on a population of live cells, allowing the user to perform parallel functional studies of cell heterogeneity at the single-cell level.

Performance evaluation of 18 F radioluminescence microscopy using computational simulation

PURPOSE

Radioluminescence microscopy can visualize the distribution of beta-emitting radiotracers in live single cells with high resolution. Here, we perform a computational simulation of ¹⁸ F positron imaging using this modality to better understand how radioluminescence signals are formed and to assist in optimizing the experimental setup and image processing.

METHODS

First, the transport of charged particles through the cell and scintillator and the resulting scintillation is modeled using the GEANT4 Monte-Carlo simulation. Then, the propagation of the scintillation light through the microscope is modeled by a convolution with a depth-dependent point-spread function, which models the microscope response. Finally, the physical measurement of the scintillation light using an electron-multiplying charge-coupled device (EMCCD) camera is modeled using a stochastic numerical photosensor model, which accounts for various sources of noise. The simulated output of the EMCCD camera is further processed using our ORBIT image reconstruction methodology to evaluate the endpoint images.

RESULTS

The EMCCD camera model was validated against experimentally acquired images and the simulated noise, as measured by the standard deviation of a blank image, was found to be accurate within 2% of the actual detection. Furthermore, point source simulations found that a reconstructed spatial resolution of 18.5 μm can be achieved near the scintillator. As the source is moved away from the scintillator, spatial resolution degrades at a rate of 3.5 μm per μm distance. These results agree well with the experimentally measured spatial resolution of 30-40 μm (live cells). The simulation also shows that the system sensitivity is 26.5%, which is also consistent with our previous experiments. Finally, an image of a simulated sparse set of single cells is visually similar to the measured cell image.

CONCLUSIONS

Our simulation methodology agrees with experimental measurements taken with radioluminescence microscopy. This in silico approach can be used to guide further instrumentation developments and to provide a framework for improving image reconstruction.