Polarized THG microscopy identifies compositionally different lipid droplets in mammalian cells

Nuclear envelope-associated lipid droplets are enriched in cholesteryl esters and increase during inflammatory signaling

Cholesteryl esters (CEs) and triacylglycerols (TAGs) are stored in lipid droplets (LDs), but their compartmentalisation is not well understood. Here, we established a hyperspectral stimulated Raman scattering microscopy system to identify and quantitatively assess CEs and TAGs in individual LDs of human cells. We found that nuclear envelope-associated lipid droplets (NE-LDs) were enriched in cholesteryl esters compared to lipid droplets in the cytoplasm. Correlative light-volume-electron microscopy revealed that NE-LDs projected towards the cytoplasm and associated with type II nuclear envelope (NE) invaginations. The nuclear envelope localization of sterol O-acyltransferase 1 (SOAT1) contributed to NE-LD generation, as trapping of SOAT1 to the NE further increased their number. Upon stimulation by the pro-inflammatory cytokine TNFα, the number of NE-LDs moderately increased. Moreover, TNFα-induced NF-κB nuclear translocation was fine-tuned by SOAT1: increased SOAT1 activity and NE-LDs associated with faster NF-κB translocation, whereas reduced SOAT1 activity and NE-LDs associated with slower NF-κB translocation. Our findings suggest that the NE is enriched in CEs and that cholesterol esterification can modulate nuclear translocation.

Exploring liquid-liquid phase separation in vitro and in vivo using multimodal nonlinear optical imaging

Liquid-liquid phase separation leads to the formation of liquid droplets (LqDs) such as P granules in Caenorhabditis elegans (C. elegans). In this study, we demonstrate the label-free visualization of LqDs using multimodal nonlinear optical imaging both in vitro and in vivo. In vitro measurements with polymerized adenine [poly(A)], we found significantly higher poly(A) concentrations in LqDs compared to surrounding solutions, with the limit of detection (LoD) of 32 mg/mL. In vivo measurements, we performed label-free imaging of C. elegans. Despite efforts to detect P granules within P lineage cells in both wild-type C. elegans and green fluorescent protein (GFP)-tagged strains, no clear RNA-specific signals were observed. This indicates that the RNA concentration in P granules is lower than anticipated and falls below our in vitro LoD. These results underscore the challenges of label-free RNA detection in P granules.

Probing compositional engineering effects on lead-free perovskite-inspired nanocrystal thin films using correlative nonlinear optical microscopy

We introduce the use of correlative third-harmonic generation and multiphoton-induced luminescence microscopy to investigate the impact of manganese (Mn) doping on bismuth (Bi)-based perovskite-inspired nanocrystal thin films. The technique was found to be extremely sensitive to the microscopic features of the perovskite film and its structural compositions, allowing the unambiguous detection of compositionally different emitters in the perovskite film and manipulation of their nonlinear optical responses. Our work unveils a new way to investigate, manipulate, and exploit perovskite-inspired functional materials for nonlinear optical conversion at the nanoscale.

Lipid content evaluation of Drosophila tumour associated haemocytes through Third Harmonic Generation measurements

Non-linear microscopy is a powerful imaging tool to examine structural properties and subcellular processes of various biological samples. The competence of Third Harmonic Generation (THG) includes the label free imaging with diffraction-limited resolution and three-dimensional visualization with negligible phototoxicity effects. In this study, THG records and quantifies the lipid content of Drosophila haemocytes, upon encountering normal or tumorigenic neural cells, in correlation with their shape or their state. We show that the lipid accumulations of adult haemocytes are similar before and after encountering normal cells. In contrast, adult haemocytes prior to their interaction with cancer cells have a low lipid index, which increases while they are actively engaged in phagocytosis only to decrease again when haemocytes become exhausted. This dynamic change in the lipid accrual of haemocytes upon encountering tumour cells could potentially be a useful tool to assess the phagocytic capacity or activation state of tumour-associated haemocytes.

Third harmonic imaging contrast from tubular structures in the presence of index discontinuity

Accurate interpretation of third harmonic generation (THG) microscopy images in terms of sample optical properties and microstructure is generally hampered by the presence of excitation field distortions resulting from sample heterogeneity. Numerical methods that account for these artifacts need to be established. In this work, we experimentally and numerically analyze the THG contrast obtained from stretched hollow glass pipettes embedded in different liquids. We also characterize the nonlinear optical properties of 2,2[Formula: see text]-thiodiethanol (TDE), a water-soluble index-matching medium. We find that index discontinuity not only changes the level and modulation amplitude of polarization-resolved THG signals, but can even change the polarization direction producing maximum THG near interfaces. We then show that a finite-difference time-domain (FDTD) modeling strategy can accurately account for contrast observed in optically heterogeneous samples, whereas reference Fourier-based numerical approaches are accurate only in the absence of index mismatch. This work opens perspectives for interpreting THG microscopy images of tubular objects and other geometries.

Cancer metabolic features allow discrimination of tumor from white blood cells by label-free multimodal optical imaging

Circulating tumor cells (CTCs) are tumor cells that have penetrated the circulatory system preserving tumor properties and heterogeneity. Detection and characterization of CTCs has high potential clinical values and many technologies have been developed for CTC identification. These approaches remain challenged by the extraordinary rarity of CTCs and the difficulty of efficiently distinguishing cancer from the much larger number of white blood cells in the bloodstream. Consequently, there is still a need for efficient and rapid methods to capture the broad spectrum of tumor cells circulating in the blood. Herein, we exploit the peculiarities of cancer metabolism for discriminating cancer from WBCs. Using deuterated glucose and Raman microscopy we show that a) the known ability of cancer cells to take up glucose at greatly increased rates compared to non-cancer cells results in the lipid generation and accumulation into lipid droplets and, b) by contrast, leukocytes do not appear to generate visible LDs. The difference in LD abundance is such that it provides a reliable parameter for distinguishing cancer from blood cells. For LD sensitive detections in a cell at rates suitable for screening purposes, we test a polarization-sensitive digital holographic imaging (PSDHI) technique that detects the birefringent properties of the LDs. By using polarization-sensitive digital holographic imaging, cancer cells (prostate cancer, PC3 and hepatocarcinoma cells, HepG2) can be rapidly discriminated from leukocytes with reliability close to 100%. The combined Raman and PSDHI microscopy platform lays the foundations for the future development of a new label-free, simple and universally applicable cancer cells' isolation method.

Label-free third harmonic generation imaging and quantification of lipid droplets in live filamentous fungi

We report the utilization of Third-Harmonic Generation microscopy for label-free live cell imaging of lipid droplets in the hypha of filamentous fungus Phycomyces blakesleeanus. THG microscopy images showed bright spherical features dispersed throughout the hypha cytoplasm in control conditions and a transient increase in the number of bright features after complete nitrogen starvation. Colocalization analysis of THG and lipid-counterstained images disclosed that the cytoplasmic particles were lipid droplets. Particle Size Analysis and Image Correlation Spectroscopy were used to quantify the number density and size of lipid droplets. The two analysis methods both revealed an increase from 16 × 10^-3 to 23 × 10^-3 lipid droplets/µm² after nitrogen starvation and a decrease in the average size of the droplets (range: 0.5-0.8 µm diameter). In conclusion, THG imaging, followed by PSA and ICS, can be reliably used for filamentous fungi for the in vivo quantification of lipid droplets without the need for labeling and/or fixation. In addition, it has been demonstrated that ICS is suitable for THG microscopy.

Recent Advancements in Optical Harmonic Generation Microscopy: Applications and Perspectives

Second harmonic generation (SHG) and third harmonic generation (THG) microscopies have emerged as powerful imaging modalities to examine structural properties of a wide range of biological tissues. Although SHG and THG arise from very different contrast mechanisms, the two are complimentary and can often be collected simultaneously using a modified multiphoton microscope. In this review, we discuss the needed instrumentation for these modalities as well as the underlying theoretical principles of SHG and THG in tissue and describe how these can be leveraged to extract unique structural information. We provide an overview of recent advances showing how SHG microscopy has been used to evaluate collagen alterations in the extracellular matrix and how this has been used to advance our knowledge of cancers, fibroses, and the cornea, as well as in tissue engineering applications. Specific examples using polarization-resolved approaches and machine learning algorithms are highlighted. Similarly, we review how THG has enabled developmental biology and skin cancer studies due to its sensitivity to changes in refractive index, which are ubiquitous in all cell and tissue assemblies. Lastly, we offer perspectives and outlooks on future directions of SHG and THG microscopies and present unresolved questions, especially in terms of overall miniaturization and the development of microendoscopy instrumentation.

Universal phase behaviors of intracellular lipid droplets

Lipid droplets are cytoplasmic microscale organelles involved in energy homeostasis and handling of cellular lipids and proteins. The core structure is mainly composed of two kinds of neutral lipids, triglycerides and cholesteryl esters, which are coated by a phospholipid monolayer and proteins. Despite the liquid crystalline nature of cholesteryl esters, the connection between the lipid composition and physical states is poorly understood. Here, we present a universal intracellular phase diagram of lipid droplets, semiquantitatively consistent with the in vitro phase diagram, and reveal that cholesterol esters cause the liquid-liquid crystal phase transition under near-physiological conditions. We moreover combine in vivo and in vitro studies, together with the theory of confined liquid crystals, to suggest that the radial molecular alignments in the liquid crystallized lipid droplets are caused by an anchoring force at the droplet surface. Our findings on the phase transition of lipid droplets and resulting molecular organization contribute to a better understanding of their biological functions and diseases.

Foam Cells: One Size Doesn't Fit All

Chronic inflammation in many infectious and metabolic diseases, and some cancers, is accompanied by the presence of foam cells. These cells form when the intracellular lipid content of macrophages exceeds their capacity to maintain lipid homeostasis. Concurrently, critical macrophage immune functions are diminished. Current paradigms of foam cell formation derive from studies of atherosclerosis. However, recent studies indicate that the mechanisms of foam cell biogenesis during tuberculosis differ from those operating during atherogenesis. Here, we review how foam cell formation and function vary with disease context. Since foam cells are therapeutic targets in atherosclerosis, further research on the disease-specific mechanisms of foam cell biogenesis and function is needed to explore the therapeutic consequences of targeting these cells in other diseases.

Surface third-harmonic generation at a two-photon-polymerized micro-interferometer for real-time on-chip refractive index monitoring

A micro-interferometer based on surface third-harmonic generation (THG) at two-photon-polymerized SU-8 cuboids for real-time monitoring of the refractive index changes of target fluids, which can be easily integrated into microfluidic photonic systems, is demonstrated. The third-harmonic (TH) interferogram is selectively generated only from the target volume by a simple vertical pumping, thereby eliminating the needs for complicated coupling and alignments. The dependence of the generated TH to the input pump polarization state is thoroughly investigated. The THG efficiency by linearly polarized excitation is found to be 2.6 × 10^-7, which is the most efficient at the SU-8-air interface and independent of the input polarization direction. The THG efficiency from the SU-8-air interface is 12.17 times higher than that from the glass-air interface and 4.93 times higher than that from the SU-8-glass interface. Real-time monitoring of argon gas pressure is demonstrated using the micro- interferometer. The surface TH from two-photon-polymerized 3D structures offers novel design flexibility to the nonlinear optical light sources for microfluidic and microelectronic devices.

Dynamic Remodeling of the Host Cell Membrane by Virulent Mycobacterial Sulfoglycolipid-1

Lipids dictate membrane properties to modulate lateral membrane organization, lipid/protein diffusion and lipid-protein interactions, thereby underpinning proper functioning of cells. Mycobacterium tuberculosis harnesses the power of its atypical cell wall lipids to impact immune surveillance machinery centered at the host cell membrane. However, the role of specific virulent lipids in altering host cellular functions by modulating membrane organization and the associated signaling response are still pertinent unresolved questions. Here, combining membrane biophysics and cell biology, we elucidate how virulent Mtb sulfoglycolipids hijack the host cell membrane, affecting its order, fluidity, and stiffness along with manipulating the linked cytoskeleton. The functional outcome of this perturbation was assayed by monitoring membrane-associated autophagy signaling. These actions form a part of the overall response to commandeer host membrane-associated immune processes during infection. The findings on the mechanism of action of Mtb lipids on host cell membrane structure and downstream signaling will deepen the collective understanding of their functional aspects in membrane-dictated bacterial survival, pathogenesis and drug resistance and reveal suitable membrane driven-therapeutic intervention points and diagnostic tools.

Detection of the T cell activation state using nonlinear optical microscopy

The ability to monitor the activation state of T-cells during immunotherapy is of great importance. Although specific activation markers do exist, their abundance and complicated regulation cannot definitely define the activation state of the cells. Previous studies have shown that Third Harmonic Generation (THG) imaging could distinguish between activated versus resting microglia and healthy versus cancerous cells, mainly based on their lipid-body profiles. In the present study, mitogen or antigen-stimulated T-cells were subjected to THG imaging microscopy. Qualitative and quantitative analysis showed statistically significant increase of THG mean area and intensity in activated versus resting T-cells. The connection of THG imaging to chemical information was achieved using Raman spectroscopy, which showed significant differences between the activation processes and controls, correlating of THG signal area with cholesterol and lipid compounds, but not with triglycerides. The obtained results suggested a potential employment of nonlinear microscopy in evaluating of T-cell activation, which is expected to be largely appreciated in the clinical practice.

Aster Proteins Facilitate Nonvesicular Plasma Membrane to ER Cholesterol Transport in Mammalian Cells

The mechanisms underlying sterol transport in mammalian cells are poorly understood. In particular, how cholesterol internalized from HDL is made available to the cell for storage or modification is unknown. Here, we describe three ER-resident proteins (Aster-A, -B, -C) that bind cholesterol and facilitate its removal from the plasma membrane. The crystal structure of the central domain of Aster-A broadly resembles the sterol-binding fold of mammalian StARD proteins, but sequence differences in the Aster pocket result in a distinct mode of ligand binding. The Aster N-terminal GRAM domain binds phosphatidylserine and mediates Aster recruitment to plasma membrane-ER contact sites in response to cholesterol accumulation in the plasma membrane. Mice lacking Aster-B are deficient in adrenal cholesterol ester storage and steroidogenesis because of an inability to transport cholesterol from SR-BI to the ER. These findings identify a nonvesicular pathway for plasma membrane to ER sterol trafficking in mammals.

Storage lipid studies in tuberculosis reveal that foam cell biogenesis is disease-specific

Foam cells are lipid-laden macrophages that contribute to the inflammation and tissue damage associated with many chronic inflammatory disorders. Although foam cell biogenesis has been extensively studied in atherosclerosis, how these cells form during a chronic infectious disease such as tuberculosis is unknown. Here we report that, unlike the cholesterol-laden cells of atherosclerosis, foam cells in tuberculous lung lesions accumulate triglycerides. Consequently, the biogenesis of foam cells varies with the underlying disease. In vitro mechanistic studies showed that triglyceride accumulation in human macrophages infected with Mycobacterium tuberculosis is mediated by TNF receptor signaling through downstream activation of the caspase cascade and the mammalian target of rapamycin complex 1 (mTORC1). These features are distinct from the known biogenesis of atherogenic foam cells and establish a new paradigm for non-atherogenic foam cell formation. Moreover, they reveal novel targets for disease-specific pharmacological interventions against maladaptive macrophage responses.

Label-free imaging of atherosclerotic plaques using third-harmonic generation microscopy

Multiphoton microscopy using laser sources in the mid-infrared range (MIR, 1,300 nm and 1,700 nm) was used to image atherosclerotic plaques from murine and human samples. Third harmonic generation (THG) from atherosclerotic plaques revealed morphological details of cellular and extracellular lipid deposits. Simultaneous nonlinear optical signals from the same laser source, including second harmonic generation and endogenous fluorescence, resulted in label-free images of various layers within the diseased vessel wall. The THG signal adds an endogenous contrast mechanism with a practical degree of specificity for atherosclerotic plaques that complements current nonlinear optical methods for the investigation of cardiovascular disease. Our use of whole-mount tissue and backward scattered epi-detection suggests THG could potentially be used in the future as a clinical tool.

Lipid droplet functions beyond energy storage

Lipid droplets are cytoplasmic organelles that store neutral lipids and are critically important for energy metabolism. Their function in energy storage is firmly established and increasingly well characterized. However, emerging evidence indicates that lipid droplets also play important and diverse roles in the cellular handling of lipids and proteins that may not be directly related to energy homeostasis. Lipid handling roles of droplets include the storage of hydrophobic vitamin and signaling precursors, and the management of endoplasmic reticulum and oxidative stress. Roles of lipid droplets in protein handling encompass functions in the maturation, storage, and turnover of cellular and viral polypeptides. Other potential roles of lipid droplets may be connected with their intracellular motility and, in some cases, their nuclear localization. This diversity highlights that lipid droplets are very adaptable organelles, performing different functions in different biological contexts. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Role for formin-like 1-dependent acto-myosin assembly in lipid droplet dynamics and lipid storage

Lipid droplets (LDs) are cellular organelles specialized in triacylglycerol (TG) storage undergoing homotypic clustering and fusion. In non-adipocytic cells with numerous LDs this is balanced by poorly understood droplet dissociation mechanisms. We identify non-muscle myosin IIa (NMIIa/MYH-9) and formin-like 1 (FMNL1) in the LD proteome. NMIIa and actin filaments concentrate around LDs, and form transient foci between dissociating LDs. NMIIa depletion results in decreased LD dissociations, enlarged LDs, decreased hydrolysis and increased storage of TGs. FMNL1 is required for actin assembly on LDs in vitro and for NMIIa recruitment to LDs in cells. We propose a novel acto-myosin structure regulating lipid storage: FMNL1-dependent assembly of myosin II-functionalized actin filaments on LDs facilitates their dissociation, thereby affecting LD surface-to-volume ratio and enzyme accessibility to TGs. In neutrophilic leucocytes from MYH9-related disease patients NMIIa inclusions are accompanied by increased lipid storage in droplets, suggesting that NMIIa dysfunction may contribute to lipid imbalance in man.

miR-29a Promotes Lipid Droplet and Triglyceride Formation in HCV Infection by Inducing Expression of SREBP-1c and CAV1

Aims: To examine the regulation of SREBP-1c and CAV1 by microRNA-29a (miR-29a) in cells infected with hepatitis C virus (HCV) in an attempt to control HCV-induced non-alcoholic fatty liver disease. Methods: In order to examine the manipulation of SREBP-1c and CAV1 by miR-29a, oleic acid (OA)-treated JFH-I-infected Huh-7 cells were used. OA was added 24 h post-transfection and gene expression was investigated by qRT-PCR at 48 h post treatment. The functional impact of the observed alteration in SREBP-1c and CAV1 expression was analyzed by examining lipid droplet (LD) and triglyceride (TG) content at 72 h post-OA treatment using light microscopy and spectrophotometry, respectively. Viral load was quantified by qRT-PCR at 72 h post-transfection. Results: OA treatment induced the expression of miR-29a and SREBP-1c, as compared to untreated cells. Forced miR-29a expression led to a significant up-regulation of SREBP-1c as well as CAV1 compared to mock untransfected cells. Ectopic expression of miR-29a resulted in a marked increase in LDs and their respective TGs, while miR-29a antagomirs decreased both the LD and TG content compared to mock untransfected cells. Moreover, forcing the expression of miR-29a in JFH-1 HCV-infected Huh-7 cells resulted in 53% reduction in viral titers compared to mock untransfected Huh-7 cells. Conclusion: Inducing miR-29a expression significantly induces SREBP-1c and CAV1 expression, thereby increasing LDs as well as their respective TGs. Nonetheless, forcing the expression of miR-29a resulted in reduction of HCV RNA levels in Huh-7 cells.

D38-cholesterol as a Raman active probe for imaging intracellular cholesterol storage

We generated a highly deuterated cholesterol analog (D38-cholesterol) and demonstrated its use for selective vibrational imaging of cholesterol storage in mammalian cells. D38-cholesterol produces detectable signals in stimulated Raman scattering (SRS) imaging, is rapidly taken up by cells, and is efficiently metabolized by acyl-CoA cholesterol acyltransferase to form cholesteryl esters. Using hyperspectral SRS imaging of D38-cholesterol, we visualized cholesterol storage in lipid droplets. We found that some lipid droplets accumulated preferentially unesterified D38-cholesterol, whereas others stored D38-cholesteryl esters. In steroidogenic cells, D38-cholesteryl esters and triacylglycerols were partitioned into distinct sets of lipid droplets. Thus, hyperspectral SRS imaging of D38-cholesterol demonstrates a heterogeneous incorporation of neutral lipid species, i.e., free cholesterol, cholesteryl esters, and triacylglycerols, between individual lipid droplets in a cell.

Third-harmonic generation imaging of three-dimensional microstructures fabricated by photopolymerization

We demonstrate the capability of polarized third-harmonic generation (THG) for high contrast imaging of three-dimensional microstructures fabricated by photopolymerization. Using circular polarization of fundamental light, background-free optically-sectioned THG images were obtained from laser-written photopolymerized microstructures. The technique has great potential for simple and noninvasive characterization of photopolymerized devices, which typically show poor contrast in conventional light microscopy.

Quantitative analysis of light scattering in polarization-resolved nonlinear microscopy

Polarization resolved nonlinear microscopy (PRNM) is a powerful technique to gain microscopic structural information in biological media. However, deep imaging in a variety of biological specimens is hindered by light scattering phenomena, which not only degrades the image quality but also affects the polarization state purity. In order to quantify this phenomenon and give a framework for polarization resolved microscopy in thick scattering tissues, we develop a characterization methodology based on four wave mixing (FWM) process. More specifically, we take advantage of two unique features of FWM, meaning its ability to produce an intrinsic in-depth local coherent source and its capacity to quantify the presence of light depolarization in isotropic regions inside a sample. By exploring diverse experimental layouts in phantoms with different scattering properties, we study systematically the influence of scattering on the nonlinear excitation and emission processes. The results show that depolarization mechanisms for the nonlinearly generated photons are highly dependent on the scattering center size, the geometry used (epi/forward) and, most importantly, on the thickness of the sample. We show that the use of an un-analyzed detection makes the polarization-dependence read-out highly robust to scattering effects, even in regimes where imaging might be degraded. The effects are illustrated in polarization resolved imaging of myelin lipid organization in mouse spinal cords.