Label-Free Imaging of Lipid Droplets in Prostate Cells Using Stimulated Raman Scattering Microscopy and Multivariate Analysis

Seeing is Believing: Developing Multimodal Metabolic Insights at the Molecular Level

This outlook explores how two different molecular imaging approaches might be combined to gain insight into dynamic, subcellular metabolic processes. Specifically, we discuss how matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) and stimulated Raman scattering (SRS) microscopy, which have significantly pushed the boundaries of imaging metabolic and metabolomic analyses in their own right, could be combined to create comprehensive molecular images. We first briefly summarize the recent advances for each technique. We then explore how one might overcome the inherent limitations of each individual method, by envisioning orthogonal and interchangeable workflows. Additionally, we delve into the potential benefits of adopting a complementary approach that combines both MSI and SRS spectro-microscopy for informing on specific chemical structures through functional-group-specific targets. Ultimately, by integrating the strengths of both imaging modalities, researchers can achieve a more comprehensive understanding of biological and chemical systems, enabling precise metabolic investigations. This synergistic approach holds substantial promise to expand our toolkit for studying metabolites in complex environments.

Label-Free In Vivo Monitoring of Lipid Droplets for Real-Time Assessment of Adipose Activator-Induced Tumor Suppression

To enhance our comprehension of the fundamental mechanisms driving tumor metabolism and metastasis, it is essential to dynamically monitor intratumoral lipid droplet (LD) and collagen processes in vivo. Traditional LD analysis in tumors predominantly relies on observations of in vitro cells or tissue slices, which unfortunately hinder real-time insights into the dynamic behavior of LDs during in vivo tumor progression. In this study, we developed a dual-modality imaging technique that combines coherent anti-Stokes Raman scattering (CARS) and second-harmonic generation (SHG) microscopy for in vivo monitoring of tumor LDs and collagen alterations, assisted by a murine breast cancer 4T1 cell-based dorsal skinfold window. Specifically, we accomplished real-time observations and quantitative analysis of the LD size, density, and collagen alignment within living tumors through CARS/SHG imaging. Additionally, our findings demonstrate that real-time LD monitoring provides a valuable means of assessing the efficacy of anticancer drugs in vivo. We evaluated the impact of adipose activators on lipid metabolism, oxidative stress, and tumor suppression by monitoring changes in LD size and density. Overall, this study highlights the potential of dual-modality CARS/SHG microscopy as a sensitive and flexible tool for antitumor therapeutic strategies.

Spectral fingerprinting of cellular lipid droplets using stimulated Raman scattering microscopy and chemometric analysis

Hyperspectral stimulated Raman scattering (SRS) microscopy is a powerful method for direct visualisation and compositional analysis of cellular lipid droplets. Here we report the application of spectral phasor analysis as a convenient method for the segmentation of lipid droplets using the hyperspectral SRS spectrum in the high wavenumber and fingerprint region of the spectrum. Spectral phasor analysis was shown to discriminate six fatty acids based on vibrational spectroscopic features in solution. The methodology was then applied to studying fatty acid metabolism and storage in a mammalian cancer cell model and during drug-induced steatosis in a hepatocellular carcinoma cell model. The accumulation of fatty acids into cellular lipid droplets was shown to vary as a function of the degree of unsaturation, whilst in a model of drug-induced steatosis, the detection of increased saturated fatty acid esters was observed. Taking advantage of the fingerprint and high wavenumber regions of the SRS spectrum has yielded a greater insight into lipid droplet composition in a cellular context. This approach will find application in the label-free profiling of intracellular lipids in complex disease models.

Discrimination of lipid composition and cellular localization in human liver tissues by stimulated Raman scattering microscopy

Significance

The molecular mechanisms driving the progression from nonalcoholic fatty liver (NAFL) to fibrosing steatohepatitis (NASH) are insufficiently understood. Techniques enabling the characterization of different lipid species with both chemical and spatial information can provide valuable insights into their contributions to the disease progression.

Aim

We extend the utility of stimulated Raman scattering (SRS) microscopy to characterize and quantify lipid species in liver tissue sections from patients with NAFL and NASH.

Approach

We applied a dual-band hyperspectral SRS microscopy system for imaging tissue sections in both the C-H stretching and fingerprint regions. The same sections were imaged with polarization microscopy for detecting birefringent liquid crystals in the tissues.

Results

Our imaging and analysis pipeline provides accurate classification and quantification of free cholesterol, saturated cholesteryl esters (CEs), unsaturated CE, and triglycerides in liver tissue sections. The subcellular resolution enables investigations of the heterogeneous distribution of saturated CE, which has been under-examined in previous studies. We also discovered that the birefringent crystals, previously found to be associated with NASH development, are predominantly composed of saturated CE.

Conclusions

Our method allows for a detailed characterization of lipid composition in human liver tissues and enables further investigation into the potential mechanism of NASH progression.

Raman microscopy reveals how cell inflammation activates glucose and lipid metabolism

Metabolism of endothelial cells (ECs) depends on the availability of the energy substrates. Since the endothelium is the first line of defence against inflammation in the cardiovascular system and its dysfunction can lead to the development of cardiovascular diseases, it is important to understand how glucose metabolism changes during inflammation. In this work, glucose uptake was studied in human microvascular endothelial cells (HMEC-1) in high glucose (HG), and additionally in an inflammatory state, using Raman imaging. HG state was induced by incubation of ECs with a deuterated glucose analogue, while the EC inflammation was caused by TNF-α pre-treatment. Spontaneous and stimulated Raman scattering spectroscopy provided comprehensive information on biochemical changes, including lipids and the extent of unsaturation induced by excess glucose in ECs., induced by excess glucose in ECs. In this work, we indicated spectroscopic markers of metabolic changes in ECs as a strong increase in the ratio of the intensity of lipids / (proteins + lipids) bands and an increase in the level of lipid unsaturation and mitochondrial changes. Inflamed ECs treated with HG, revealed enhanced glucose uptake, and intensified lipid production i.a. of unsaturated lipids. Additionally, increased cytochrome c signal in the mitochondrial region indicated higher mitochondrial activity and biogenesis. Raman spectroscopy is a powerful method for determining the metabolic markers of ED which will better inform understanding of disease onset, development, and treatment.

Rapid Intracellular Detection and Analysis of Lipid Droplets' Morpho-Chemical Composition by Phase-Guided Raman Sampling

Intracellular lipid droplets (LDs) are dynamic, complex organelles involved in nearly all aspects of cellular metabolism. In situ characterization methods are primarily limited to fluorescence imaging, which yields limited chemical information, or Raman spectroscopy, which provides excellent chemical profiling but very low throughput. Here, we propose a new paradigm where locations of both large and small droplets are obtained automatically from high-resolution phase images and fed into a galvomirror-controlled Raman sampling arm to obtain the full spectrum of each LD efficiently. Using this phase-guided Raman sampling, we can characterize hundreds of LDs within a single cell in minutes and easily acquire more than 40,000 high-quality spectra. The data set revealed strong, cell line-dependent, cell-dependent, and individual droplet-dependent composition changes to various culture conditions. In particular, we revealed a strong competitive relationship between mono- and polyunsaturated fatty acids, where supplementation with one led to a relative decrease in the other.

Investigating ionizing radiation-induced changes in breast cancer cells using stimulated Raman scattering microscopy

Significance

Altered lipid metabolism of cancer cells has been implicated in increased radiation resistance. A better understanding of this phenomenon may lead to improved radiation treatment planning. Stimulated Raman scattering (SRS) microscopy enables label-free and quantitative imaging of cellular lipids but has never been applied in this domain.

Aim

We sought to investigate the radiobiological response in human breast cancer MCF7 cells using SRS microscopy, focusing on how radiation affects lipid droplet (LD) distribution and cellular morphology.

Approach

MCF7 breast cancer cells were exposed to either 0 or 30 Gy (X-ray) ionizing radiation and imaged using a spectrally focused SRS microscope every 24 hrs over a 72-hr time period. Images were analyzed to quantify changes in LD area per cell, lipid and protein content per cell, and cellular morphology. Cell viability and confluency were measured using a live cell imaging system while radiation-induced lipid peroxidation was assessed using BODIPY C11 staining and flow cytometry.

Results

The LD area per cell and total lipid and protein intensities per cell were found to increase significantly for irradiated cells compared to control cells from 48 to 72 hrs post irradiation. Increased cell size, vacuole formation, and multinucleation were observed as well. No significant cell death was observed due to irradiation, but lipid peroxidation was found to be greater in the irradiated cells than control cells at 72 hrs.

Conclusions

This pilot study demonstrates the potential of SRS imaging for investigating ionizing radiation-induced changes in cancer cells without the use of fluorescent labels.

Analytical Techniques for Single-Cell Biochemical Assays of Lipids

Lipids are essential cellular components forming membranes, serving as energy reserves, and acting as chemical messengers. Dysfunction in lipid metabolism and signaling is associated with a wide range of diseases including cancer and autoimmunity. Heterogeneity in cell behavior including lipid signaling is increasingly recognized as a driver of disease and drug resistance. This diversity in cellular responses as well as the roles of lipids in health and disease drive the need to quantify lipids within single cells. Single-cell lipid assays are challenging due to the small size of cells (∼1 pL) and the large numbers of lipid species present at concentrations spanning orders of magnitude. A growing number of methodologies enable assay of large numbers of lipid analytes, perform high-resolution spatial measurements, or permit highly sensitive lipid assays in single cells. Covered in this review are mass spectrometry, Raman imaging, and fluorescence-based assays including microscopy and microseparations.

Label-Free Cytometric Evaluation of Mitosis via Stimulated Raman Scattering Microscopy and Spectral Phasor Analysis

Hyperspectral stimulated Raman scattering (SRS) microscopy is a robust imaging tool for the analysis of biological systems. Here, we present a unique perspective, a label-free spatiotemporal map of mitosis, by integrating hyperspectral SRS microscopy with advanced chemometrics to assess the intrinsic biomolecular properties of an essential process of mammalian life. The application of spectral phasor analysis to multiwavelength SRS images in the high-wavenumber (HWN) region of the Raman spectrum enabled the segmentation of subcellular organelles based on innate SRS spectra. Traditional imaging of DNA is primarily reliant on using fluorescent probes or stains which can affect the biophysical properties of the cell. Here, we demonstrate the label-free visualization of nuclear dynamics during mitosis coupled with an evaluation of its spectral profile in a rapid and reproducible manner. These results provide a snapshot of the cell division cycle and chemical variability between intracellular compartments in single-cell models, which is central to understanding the molecular foundations of these fundamental biological processes. The evaluation of HWN images by phasor analysis also facilitated the differentiation between cells in separate phases of the cell cycle based solely on their nuclear SRS spectral signal, which offers an interesting label-free approach in combination with flow cytometry. Therefore, this study demonstrates that SRS microscopy combined with spectral phasor analysis is a valuable method for detailed optical fingerprinting at the subcellular level.

Raman spectroscopy for profiling physical and chemical properties of atmospheric aerosol particles: A review

Atmosphere aerosols have significant impact on human health and the environment. Aerosol particles have a number of characteristics that influence their health and environmental effects, including their size, shape, and chemical composition. A great deal of difficulty is associated with quantifying and identifying atmospheric aerosols because these parameters are highly variable on a spatial and temporal scale. An important component of understanding aerosol fate is Raman Spectroscopy (RS), which is capable of resolving chemical compositions of individual particles. This review presented strategic techniques, especially RS methods for characterizing atmospheric aerosols. The nature and properties of atmospheric aerosols and their influence on environment and human health were briefly described. Analytical methodologies that offer insight into the chemistry and multidimensional properties of aerosols were discussed. In addition, perspectives for practical applications of atmospheric aerosols using RS are featured.

A Flexible Chamber for Time-Lapse Live-Cell Imaging with Stimulated Raman Scattering Microscopy

Stimulated Raman scattering (SRS) microscopy is a label-free chemical imaging technology. Live-cell imaging with SRS has been demonstrated for many biological and biomedical applications. However, long-term time-lapse SRS imaging of live cells has not been widely adopted. SRS microscopy often uses a high numerical aperture (NA) water-immersion objective and a high NA oil-immersion condenser to achieve high-resolution imaging. In this case, the gap between the objective and the condenser is only a few millimeters. Therefore, most commercial stage-top environmental chambers cannot be used for SRS imaging because of their large thickness with a rigid glass cover. This paper describes the design and fabrication of a flexible chamber that can be used for time-lapse live-cell imaging with transmitted SRS signal detection on an upright microscope frame. The flexibility of the chamber is achieved by using a soft material - a thin natural rubber film. The new enclosure and chamber design can be easily added to an existing SRS imaging setup. The testing and preliminary results demonstrate that the flexible chamber system enables stable, long-term, time-lapse SRS imaging of live cells, which can be used for various bioimaging applications in the future.