Stimulated Raman Scattering Imaging Sheds New Light on Lipid Droplet Biology

METTL3 mediates atheroprone flow-induced glycolysis in endothelial cells

Atheroprone flow-increased glycolysis in vascular endothelial cells (ECs) is pivotal in EC dysfunction and the initiation of atherosclerosis. Methyltransferase 3 (METTL3) is a major m6A methyltransferase for RNA N6-mehtyladenosine (m6A) modifications to regulate epitranscriptome and cellular functions. With the atheroprone flow upregulating METTL3 and m6A RNA hypermethylation, we investigate the role of METTL3 in atheroprone flow-induced glycolysis in ECs in vitro and in vivo. Compared to pulsatile shear stress (PS, atheroprotective flow), oscillatory shear stress (OS, atheroprone flow) increases METTL3 expression to enhance the m6A modifications of mRNAs encoding HK1, PFKFB3, and GCKR, which are rate-limiting enzymes of glycolysis. These augmented m6A modifications increase the expressions of HK1 and PFKFB3 while decreasing GCKR, resulting in elevated EC glycolysis, as revealed by seahorse analysis. Moreover, a stimulated Raman scattering (SRS) imaging study demonstrates the elevation of glucose incorporation into de novo synthesized lipids in ECs under atheroprone flow in vitro and in vivo. Empagliflozin, a sodium-glucose cotransporter-2 inhibitor (SGLT2i) drug, represses METTL3 expression, thereby mitigating OS-induced glycolysis in ECs. These data suggest mechanisms by which METTL3 links EC mechanotransduction with metabolic reprogramming under atherogenic conditions.

Enzyme-Instructed Self-Assembly Reprograms Fatty Acid Metabolism for Cancer Therapeutics

Enzyme-instructed self-assembly (EISA) is actively explored as a promising therapeutic approach for cancer treatment. However, the metabolic response of cancer cells to EISA remains under-studied. Here, by stimulated Raman scattering (SRS) imaging in C─H, fingerprint, and silent windows, it is found that the formation of peptide assemblies within and around cancer cells significantly enhances both lipids catabolism and fatty acids (FAs) uptake. It is further found that the increased uptake of FAs aids the resistance of cancer cells under EISA treatment, likely to cope with the stress induced by the peptide assemblies. Combining EISA with FAs uptake inhibition leads to enhanced cancer suppression compared to EISA alone, while additional FAs supplementation rescue cancer cells from EISA treatment, both in vitro and in 3D-culture spheroid models. These findings shed new light on the impact of EISA on the metabolic activities of cancer cells and suggest a new approach for improved cancer therapy.

Stimulated Raman Scattering Microscopy Facilitates the Discovery of Diacylglycerol O-Acyltransferase 2 as a Target to Enhance Iodine Uptake in Papillary Thyroid Carcinoma

Radioactive 131I therapy is a primary treatment for papillary thyroid carcinoma (PTC), with approximately 30% of patients developing iodine-refractory disease. There is an urgent clinical need to improve iodine uptake in PTC. Previous research suggested a connection between triglyceride (TG) accumulation and decreased iodine uptake in benign thyroid cells. Notably, TG accumulation has been found to be a marker of aggressive human PTC. Therefore, it is crucial to elucidate whether TG accumulation affects iodine uptake in PTC, which may lead to a new way for enhancement of iodine uptake. Here, by combining stimulated Raman scattering (SRS) microscopy and deuterated Raman tags, we first quantitatively analyzed the level of TG and its source in the K1 cell with low iodine uptake and the TPC-1 cell with high iodine uptake. It was found that K1 cells had significantly greater TG accumulation than TPC-1 cells, primarily due to an increased exogenous uptake of fatty acids. Further RNA-seq transcriptome experiments revealed that the underlying mechanism could be upregulation of lipid biosynthesis, uptake, and transport-related genes, along with down-regulation of fatty acid β-oxidation and lipolysis-related genes in K1 cells. Among the upregulated lipid biosynthesis genes, diacylglycerol O-acyltransferase 2 (DGAT2) is of great importance as the rate-limiting enzyme in TG biosynthesis. Notably, the inhibition of DGAT2 led to a significant increase in the expression of iodine uptake-related proteins, namely, sodium iodide symporter (NIS) and thyroglobulin (Tg), in K1 cells. Further Gene Ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses unraveled that DGAT2 inhibition could enhance thyroid hormone synthesis, for which iodine is an essential raw material, by alleviating endoplasmic reticulum stress and upregulating the pathways related to protein glycosylation and transmembrane transport. In summary, our study has shown that SRS microscopy facilitates the discovery of DGAT2 as a potential target to enhance iodine uptake in PTC, which holds promise for improving treatment outcome of iodine-refractory PTC.

Cancer Cell Line Classification Using Raman Spectroscopy of Cancer-Derived Exosomes and Machine Learning

Liquid biopsies are an emerging, noninvasive tool for cancer diagnostics, utilizing biological fluids for molecular profiling. Nevertheless, the current methods often lack the sensitivity and specificity necessary for early detection and real-time monitoring. This work explores an advanced approach to improving liquid biopsy techniques through machine learning analysis of the Raman spectra measured to classify distinct exosome solutions by their cancer origin. This was accomplished by conducting principal component analysis (PCA) of the Raman spectra of exosomes from three cancer cell lines (COLO205, A375, and LNCaP) to extract chemically significant features. This reduced set of features was then utilized to train a linear discriminant analysis (LDA) classifier to predict the source of the exosomes. Furthermore, we investigated differences in the lipid composition in these exosomes by their spectra. This spectral similarity analysis revealed differences in lipid profiles between the different cancer cell lines as well as identified the predominant lipids across all exosomes. Our PCA-LDA framework achieved 93.3% overall accuracy and F1 scores of 98.2%, 91.1%, and 91.0% for COLO205, A375, and LNCaP, respectively. Our results from spectral similarity analysis were also shown to support previous findings of lipid dynamics due to cancer pathology and pertaining to exosome function and structure. These findings underscore the benefits of enhancing Raman spectroscopy analysis with machine learning, laying the groundwork for the development of early noninvasive cancer diagnostics and personalized treatment strategies. This work potentially establishes the foundation for refining the classification model and optimizing exosome extraction and detection from clinical samples for clinical translation.

Raman analysis of lipids in cells: Current applications and future prospects

Lipids play an important role in the regulation of cell life processes. Although there are various lipid detection methods, Raman spectroscopy, a non-invasive technique, provides the detailed chemical composition of lipid profiles without a complex sample preparation procedure and possesses greater potential in basic biology, clinical diagnosis and disease therapy. In this review, we summarized the characteristics and advantages of Raman-based techniques and their primary contribution to illustrating cellular lipid metabolism.

Label-Free Quantification of Apoptosis and Necrosis Using Stimulated Raman Scattering Microscopy

Recombinant proteins are critical for modern therapeutics and diagnostics, with Chinese hamster ovary (CHO) cells serving as the primary production platform. However, environmental and chemical stressors in bioreactors often trigger cell death, particularly apoptosis, posing a significant challenge to recombinant protein manufacturing. Rapid, label-free methods to monitor cell death are essential for ensuring better production quality. Stimulated Raman scattering (SRS) microscopy offers a powerful, label-free approach to measure lipid and protein compositions in live cells. We demonstrate that SRS microscopy enables rapid and reagent-free analysis of apoptotic and necrotic transitions. Our results show that apoptotic cells exhibit higher protein concentrations, while necrotic cells show an opposite trend. To enhance analysis, we developed a quantitative single-cell analysis pipeline that extracts chemotypic and phenotypic signatures of apoptosis and necrosis, enabling the identification of subpopulations with varied responses to stressors or treatments. Furthermore, the cell death analysis was successfully generalized to other stressors and cell types. This study highlights SRS microscopy as a robust and non-invasive tool for rapid monitoring of live cell apoptotic and necrotic transitions. Our method and findings hold potential for improving quality control in CHO cell-based biopharmaceutical production and for evaluating cell death in diverse biological contexts.

Modulating lipid droplet dynamics in neurodegeneration: an emerging area of molecular pharmacology

Neurodegenerative diseases (NDDs) are characterised by the progressive loss of neurons in the central nervous system (CNS), resulting in memory impairment, cognition abnormalities, and motor dysfunctions. The common pathological features include altered energy metabolism, neuroinflammation, loss of neurons, aberrant protein aggregation, and synaptic dysfunction. Lipids, fundamental components of cell membranes play a critical role in energy storage and cell signaling. The brain, comprising approximately 60% lipid content by dry weight, underscores the significance of lipid dynamics in maintaining CNS integrity. Variations in lipid distribution across brain regions further highlight their specialised functions. Dysregulation of lipid metabolism, encompassing synthesis, transport, and utilization, has been implicated in the pathogenesis of neurodegenerative diseases. Lipid droplets (LDs), key intermediates of lipid metabolism, accumulate in neurons, microglia, and astrocytes, particularly in aging brains. The deposition of these LDs disrupts cellular homeostasis and links the dynamics of LDs to pathology of disease. Therefore, this review explores the pivotal role of lipid metabolism and LDs in NDDs, providing insights into their contributions to neuronal dysfunction and potential therapeutic implications.

Synthetic Lipid Biology

Cells contain thousands of different lipids. Their rapid and redundant metabolism, dynamic movement, and many interactions with other biomolecules have justly earned lipids a reputation as a vexing class of molecules to understand. Further, as the cell's hydrophobic metabolites, lipids assemble into supramolecular structures─most commonly bilayers, or membranes─from which they carry out myriad biological functions. Motivated by this daunting complexity, researchers across disciplines are bringing order to the seeming chaos of biological lipids and membranes. Here, we formalize these efforts as "synthetic lipid biology". Inspired by the idea, central to synthetic biology, that our abilities to understand and build biological systems are intimately connected, we organize studies and approaches across numerous fields to create, manipulate, and analyze lipids and biomembranes. These include construction of lipids and membranes from scratch using chemical and chemoenzymatic synthesis, editing of pre-existing membranes using optogenetics and protein engineering, detection of lipid metabolism and transport using bioorthogonal chemistry, and probing of lipid-protein interactions and membrane biophysical properties. What emerges is a portrait of an incipient field where chemists, biologists, physicists, and engineers work together in proximity─like lipids themselves─to build a clearer description of the properties, behaviors, and functions of lipids and membranes.

Artificial Intelligence-Assisted Stimulated Raman Histology: New Frontiers in Vibrational Tissue Imaging

Frozen section biopsy, introduced in the early 1900s, still remains the gold standard methodology for rapid histologic evaluations. Although a valuable tool, it is labor-, time-, and cost-intensive. Other challenges include visual and diagnostic variability, which may complicate interpretation and potentially compromise the quality of clinical decisions. Raman spectroscopy, with its high specificity and non-invasive nature, can be an effective tool for dependable and quick histopathology. The most promising modality in this context is stimulated Raman histology (SRH), a label-free, non-linear optical process which generates conventional H&E-like images in short time frames. SRH overcomes limitations of conventional Raman scattering by leveraging the qualities of stimulated Raman scattering (SRS), wherein the energy gets transferred from a high-power pump beam to a probe beam, resulting in high-energy, high-intensity scattering. SRH's high resolution and non-requirement of preprocessing steps make it particularly suitable when it comes to intrasurgical histology. Combining SRH with artificial intelligence (AI) can lead to greater precision and less reliance on manual interpretation, potentially easing the burden of the overburdened global histopathology workforce. We review the recent applications and advances in SRH and how it is tapping into AI to evolve as a revolutionary tool for rapid histologic analysis.

Role of lipid droplets in neurodegenerative diseases: From pathogenesis to therapeutics

Neurodegenerative diseases (NDDs) are a series of disorders characterized by the progressive loss of specific neurons, leading to cognitive and locomotor impairment. NDDs affect millions of patients worldwide but lack effective treatments. Dysregulation of lipids, particularly the accumulation of lipid droplets (LDs), is strongly implicated in the pathogenesis of NDDs. How LDs contribute to the occurrence and development of NDDs, and their potential as therapeutic targets remain to be addressed. In present review, we first introduce the processes of LDs formation, transportation and degradation. We then highlight how the accumulation of LDs contributes to the pathogenesis of NDDs in a cell type-specific manner. Moreover, we discuss currently available methods for detecting LDs and elaborate on LDs-based therapeutic strategies for NDDs. Lastly, we identify gaps that need to be filled to better leverage LD-based theranostics in NDDs and other diseases. We hope this review could shed light on the role of LDs in NDDs and facilitate the development of novel therapeutic strategies for NDDs.

Mid-infrared chemical imaging of living cells enabled by plasmonic metasurfaces

Mid-Infrared (MIR) chemical imaging provides rich chemical information of biological samples in a label-free and non-destructive manner. Yet, its adoption to live-cell analysis is limited by the strong attenuation of MIR light in water, often necessitating cell culture geometries that are incompatible with the prolonged viability of cells and with standard high-throughput workflow. Here, we introduce a new approach to MIR microscopy, where cells are imaged through their localized near-field interaction with a plasmonic metasurface. Chemical contrast of distinct molecular groups provided sub-cellular resolution images of the proteins, lipids, and nucleic acids in the cells that were collected using an inverted MIR microscope. Time-lapse imaging of living cells demonstrated that their behaviors, including motility, viability, and substrate adhesion, can be monitored over extended periods of time using low-power MIR light. The presented approach provides a method for the non-perturbative MIR imaging of living cells, which is well-suited for integration with modern high-throughput screening technologies for the label-free, high-content chemical imaging of living cells.

The Duality of Raman Scattering

ConspectusFirst predicted more than 100 years ago, Raman scattering is a cornerstone of photonics, spectroscopy, and imaging. The conventional framework of understanding Raman scattering was built on Raman cross section σRaman. Carrying a dimension of area, σRaman characterizes the interaction strength between light and molecules during inelastic scattering. The numerical values of σRaman turn out to be many orders of magnitude smaller in comparison to the linear absorption cross sections σAbsorption of similar molecular systems. Such an enormous gap has been the reason for researchers to believe the extremely feeble Raman scattering ever since its discovery. However, this prevailing picture is conceptually problematic or at least incomplete due to the fact that Raman scattering and linear absorption belong to different orders of light-matter interaction.In this Account, we will summarize an alternate way to think about Raman scattering, which we term stimulated response formulation. To capture the third-order interaction nature of Raman scattering, we introduced stimulated Raman cross section, σSRS, defined as the intrinsic molecular property in response to the external photon fluxes. Foremost, experimental measurement of σSRS turns out to be not weak at all or even larger when fairly compared with electronic counterparts of the same order. The analytical expression for σSRS derived from quantum electrodynamics also supports the measurement and proves that σSRS is intrinsically strong. Hence, σRaman and σSRS can be extremely small and large, respectively, for the same molecule at the same time. Our subsequent theoretical studies show that stimulated response formulation can unify spontaneous emission, stimulated emission, spontaneous Raman, and stimulated Raman via eq 10, in a coherent and symmetric way. In particular, an Einstein-coefficient-like equation, eq 12a, was derived, showing that σRaman can be explicitly expressed as σSRS multiplied by an effective photon flux arising from zero-point fluctuation of the vacuum. The feeble vacuum fluctuation hence explains how σSRS can be intrinsically strong while, at the same time, σRaman ends up being many orders of magnitude smaller when both compared to the electronic counterparts. These two sides of the same coin prompted us to propose "the duality of Raman scattering" (Table 1). Finally, this formulation naturally leads to a quantitative treatment of stimulated Raman scattering (SRS) microscopy, providing an intuitive, molecule-centric explanation as to how SRS microscopy can outperform regular Raman microscopy. Hence, as unveiled by the new formulation, a duality of Raman scattering has emerged, with implications for both fundamental science and practical technology.

Stimulated Raman Scattering Microscopy Reveals Aberrant Triglyceride Accumulation in Lymphatic Metastasis of Papillary Thyroid Carcinoma

Lipid metabolic alterations are known to play a crucial role in cancer metastasis. As a key hub in lipid metabolism, intracellular neutral lipid accumulation in lipid droplets (LDs) has become a signature of aggressive human cancers. Nevertheless, it remains unclear whether lipid accumulation displays distinctive features in metastatic lesions compared to the primary ones. Here, we integrated multicolor stimulated Raman scattering (SRS) imaging with confocal Raman spectroscopy on the same platform to quantitatively analyze the amount and composition of LDs in intact human thyroid tissues in situ without any processing or labeling. Inspiringly, we found aberrant accumulation of triglycerides (TGs) in lymphatic metastases but not in normal thyroid, primary papillary thyroid carcinoma (PTC), or normal lymph node. In addition, the unsaturation degree of unsaturated TGs was significantly higher in the lymphatic metastases from patients diagnosed with late-stage (T3/T4) PTC compared to those of patients diagnosed with early-stage (T1/T2) PTC. Furthermore, both public sequencing data analysis and our RNA-seq transcriptomic experiment showed significantly higher expression of alcohol dehydrogenase-1B (ADH1B), which is critical to lipid uptake and transport, in lymphatic metastases relative to the primary ones. In summary, these findings unravel the lipid accumulation as a novel marker and therapeutic target for PTC lymphatic metastasis that has a poor response to the regular radioactive iodine therapy.