Herein, we demonstrate an optimized BODIPY 493/503 fluorescence-based protocol for lipid droplet characterization in liver tissue. Through the use of orthogonal projections and 3D reconstructions, the fluorophore allows for successful discrimination between microvesicular and macrovesicular steatosis and may represent a complementary approach to the classical histological protocols for hepatic steatosis assessment.
Lipid droplets (LDs) are specialized organelles that mediate lipid storage and play a very important role in suppressing lipotoxicity and preventing dysfunction caused by free fatty acids (FAs). The liver, given its critical role in the body’s fat metabolism, is persistently threatened by the intracellular accumulation of LDs in the form of both microvesicular and macrovesicular hepatic steatosis. The histologic characterization of LDs is typically based on lipid-soluble diazo dyes, such as Oil Red O (ORO) staining, but a number of disadvantages consistently hamper the use of this analysis with liver specimens. More recently, lipophilic fluorophores 493/503 have become popular for visualizing and locating LDs due to their rapid uptake and accumulation into the neutral lipid droplet core. Even though most applications are well-described in cell cultures, there is less evidence demonstrating the reliable use of lipophilic fluorophore probes as an LD imaging tool in tissue samples. Herein, we propose an optimized boron dipyrromethene (BODIPY) 493/503-based protocol for the evaluation of LDs in liver specimens from an animal model of high-fat diet (HFD)-induced hepatic steatosis. This protocol covers liver sample preparation, tissue sectioning, BODIPY 493/503 staining, image acquisition, and data analysis. We demonstrate an increased number, intensity, area ratio, and diameter of hepatic LDs upon HFD feeding. Using orthogonal projections and 3D reconstructions, it was possible to observe the full content of neutral lipids in the LD core, which appeared as nearly spherical droplets. Moreover, with the fluorophore BODIPY 493/503, we were able to distinguish microvesicles (1 µm < d ≤ 3 µm), intermediate vesicles (3 µm < d ≤ 9 µm), and macrovesicles (d > 9 µm), allowing the successful discrimination of microvesicular and macrovesicular steatosis. Overall, this BODIPY 493/503 fluorescence-based protocol is a reliable and simple tool for hepatic LD characterization and may represent a complementary approach to the classical histological protocols.
Lipid droplets (LDs), classically viewed as energy depots, are specialized cellular organelles that mediate lipid storage, and they comprise a hydrophobic neutral lipid core, which mainly contains cholesterol esters and triglycerides (TGs), encapsulated by a phospholipid monolayer1,2,3.
LD biogenesis occurs in the endoplasmic reticulum (ER), starting with the synthesis of triacylglycerol (TAG) and sterol esters. Neutral lipids are diffused between the leaflets of the ER bilayer at low concentrations but coalesce into oil lenses that grow and bud into nearly spherical droplets from ER membrane when their intracellular concentration increases4. Subsequently, proteins from the ER bilayer and cytosol, particularly the perilipin (PLIN) protein family, translocate to the surfaces of the LDs to facilitate budding5,6,7,8,9.
Through new fatty acid synthesis and LD fusion or coalescence, LDs grow into different sizes. Accordingly, the size and number of LDs differ considerably across different cell types. Small droplets (300-800 nm diameter), which are known as initial LDs (iLDs), can be formed by nearly all cells4. Later in LD formation, most cells are able to convert some iLDs into larger ones-expanding LDs (eLDs >1 µm in diameter). Yet, just specific cell types, such as adipocytes and hepatocytes, have the capacity to form giant or supersized LDs (up to tens of microns in diameter)4,10.
LDs play a very important role in the regulation of cellular lipid metabolism, suppressing lipotoxicity, and preventing ER stress, mitochondrial dysfunction, and, ultimately, cell death caused by free fatty acids (FAs)11,12,13,14. Furthermore, LDs have also been implicated in the regulation of gene expression, viral replication protein sequestration, and membrane trafficking and signaling15,16,17. Therefore, the misregulation of LD biogenesis is a hallmark of chronic diseases associated with metabolic syndrome, obesity, type 2 diabetes mellitus (T2DM), and/or arteriosclerosis, to name just a few18,19,20.
The liver, as a metabolic hub, is mostly responsible for lipid metabolism by storing and processing lipids, and, therefore, it is constantly threatened by lipotoxicity21. Hepatic steatosis (HS) is a common feature of a series of progressive liver diseases and is characterized by excessive intracellular lipid accumulation in the form of cytosolic LDs that, ultimately, may lead to liver metabolic dysfunction, inflammation, and advanced forms of nonalcoholic fatty liver disease22,23,24,25. HS occurs when the rate of fatty acid oxidation and export as triglycerides within very low-density lipoproteins (VLDLs) is lower than the rate of hepatic fatty acid uptake from the plasma and de novo fatty acid synthesis26. The hepatic accumulation of lipids often occurs in two forms-microvesicular and macrovesicular steatosis-and these display distinct cytoarchitectonic characteristics27. Typically, microvesicular steatosis is characterized by the presence of small LDs dispersed throughout the hepatocyte with the nucleus placed centrally, whereas macrovesicular steatosis is characterized by the presence of a single large LD that occupies the greater part of the hepatocyte, pushing the nucleus to the periphery28,29. Notably, these two types of steatosis are often found together, and it remains unclear how these two LD patterns influence disease pathogenesis, as evidence is still inconsistent31,32,33,34. Yet, such type of analysis is often employed as a "reference standard" in preclinical and clinical studies to understand the dynamic behavior of LDs and characterize hepatic steatosis29,34,35,36.
Liver biopsies, the gold standard for diagnosing and grading HS, are routinely assessed by histological hematoxylin and eosin (H&E) analysis, where lipid droplets are evaluated as unstained vacuoles in H&E-stained liver sections37. While acceptable for macrovesicular steatosis evaluation, this type of staining generally narrows the assessment of microvesicular steatosis38. Lipid-soluble diazo dyes, such as Oil Red O (ORO), are classically combined with brightfield microscopy to analyze intracellular lipid stores, but these still have a number of disadvantages: (i) the usage of ethanol or isopropanol in the staining process, which often causes the disruption of the native LDs and occasional fusion despite the cells being fixed39; (ii) the time-consuming nature, as ORO solution requires fresh powder dissolving and filtering due to the limited shelf life, thus contributing to less consistent results; (iii) and the fact that ORO stains more than just lipid droplets and often overestimates hepatic steatosis38.
Consequently, cell-permeable lipophilic fluorophores, such as Nile Red, have been used in either live or fixed samples to overcome some of the aforementioned limitations. However, the non-specific nature of cellular lipid organelle labeling repeatedly narrows LD assessments40. Moreover, the spectral properties of Nile Red vary according to the polarity of the environment, which can often lead to spectral shifts41.
The lipophilic fluorescent probe 1,3,5,7,8-pentamethyl-4-bora-3a,4adiaza-s-indacene (excitation wavelength: 480 nm; emission maximum: 515 nm; BODIPY 493/503) exhibits hydrophobicity characteristics that allow its rapid uptake by intracellular LDs, accumulates in the lipid droplet core, and, subsequently, emits bright green fluorescence12. Unlike Nile Red, BODIPY 493/503 is insensitive to the environment polarity and has been shown to be more selective, as it displays high brightness for LD imaging. In order to stain neutral LDs, this dye can be used in live or fixed cells and successfully coupled with other staining and/or labeling methods42. Another advantage of the dye is that it requires little effort to place into a solution and is stable, thus eliminating the need to freshly prepare it for each experiment42. Even though the BODIPY 493/503 probe has been successfully employed to visualize the localization and dynamics of LDs in cell cultures, some reports have also demonstrated the reliable use of this dye as an LD imaging tool in tissues including the human vastus lateralis muscle43, the rat soleus muscle42, and the mouse intestine44.
Herein, we propose an optimized BODIPY 493/503-based protocol as an alternative analytical approach for the evaluation of the LD number, area, and diameter in liver specimens from an animal model of hepatic steatosis. This procedure covers liver sample preparation, tissue sectioning, staining conditions, image acquisition, and data analysis.
All animal procedures performed in this study were approved by the Coimbra Institute for Clinical and Biomedical Research (iCBR) Animal Welfare Body (ORBEA, #9/2018) and complied with the Animal Care National and European Directives and with the ARRIVE guidelines.
1. Experimental design
2. Liver sample preparation
3. Frozen tissue sectioning
4. BODIPY staining
5. Lipid quantification
6. Image acquisition
7. Analysis of images
8. Statistical analysis
The successful execution of this technique should result in clear lipid droplet staining for the simultaneous characterization of the LD morphology (shape and lipid core density based on the 3D reconstruction) along with their spatial distribution, number per total area, and average size (assessed with the pipeline above described, Figure 1).
Figure 1: Sample image processing using CellProfiler. (A) Original image of LDs stained with BODIPY 493/503 (green) and nuclei stained with DAPI (blue) in the CTL group. (B) The differentiated LDs (magenta) of the CTL group were overlaid using the OverlayOutlines module. (C) Original image of LDs stained with BODIPY 493/503 (green) and nuclei stained with DAPI (blue) in the HFD group. (D) The differentiated LDs (magenta) of the HFD group were overlaid using the OverlayOutlines module. Images were taken at 20x magnification using a laser point scanning confocal microscope. Scale bars = 50 µm. Abbreviations: CTL = control group; HFD = high-fat diet group. Please click here to view a larger version of this figure.
Serum TGs, total cholesterol, LDL-c, HDL-c, as well as the hepatic TG contents were evaluated (Table 1). HFD-fed animals presented a pronounced dyslipidemic profile, characterized by an accentuated accumulation of hepatic TGs (345% when compared to CTL, p < 0.001), along with a subtle increase in circulating TG levels (129% when compared to CTL, p > 0.05).
Parameter | CTL | HFD | |
Serum | |||
Total Cholesterol (mg/dL) | 56.67 ± 9.35 | 80.40 ± 7.45 | |
LDL (mg/dL) | 7.66 ± 0.84 | 7.40 ± 1.03 | |
HDL (mg/dL) | 17.83 ± 2.93 | 28.40 ± 2.40 * | |
Triglycerides (mg/dL) | 154.8 ± 40.17 | 201.2 ± 38.12 | |
Liver | |||
Triglycerides (mg/g) | 15.29 ± 1.31 | 52.83 ± 6.73 *** |
Table 1: Serum TGs, total cholesterol, LDL-c, HDL-c, and the hepatic TG contents. Data are expressed as mean ± SEM (n = 5-6 per group). *** p < 0.001 versus CTL. Statistical analysis was performed using a Student's unpaired t-test. Abbreviations: CTL = control group; HFD = high-fat diet group.
Figure 2 shows representative wide-area images (tile scans with a 10x objective) of liver sections in which LDs are stained with BODIPY 493/503 (green) and nuclei are stained with DAPI (blue). Individual LDs of various sizes were successfully visualized with BODIPY 493/503 staining, which showed a widespread distribution pattern in the HFD-fed animals.
Figure 2: Representative images of LD accumulation in hepatic tissue. LDs stained with BODIPY 493/503 (green), and nuclei stained with DAPI (blue). Images were taken at 10x magnification using a laser point scanning confocal microscope. Scale bars = 200 µm. Abbreviations: CTL = control group; HFD = high-fat diet group. Please click here to view a larger version of this figure.
Figure 3A shows representative orthogonal projections with a 20x objective and 3D images with a 40x objective of hepatic LDs. Single-plane images were processed and analyzed by CellProfiler (version 4.2.5) to assess the fluorescence intensity, number, area, and diameter of the LCs (Figure 3B–E). It was possible to confirm that the HFD-fed animals displayed an increased number of hepatic LDs (151% vs. CTL, Figure 3B), and this was corroborated by the augmented BODIPY 493/503 fluorescence intensity (182% vs. CTL, p < 0.001, Figure 3C). Moreover, the area ratio of LDs nearly tripled (360% vs. CTL, p < 0.0001, Figure 3D) as they presented larger diameters (182% vs. CTL, Figure 3E) in the HFD-fed animals. To assess the size distribution, the LDs were categorized into three groups according to the diameter ranges: 1 µm < d ≤ 3 µm, 3 µm < d ≤ 9 µm, and d > 9 µm (Figure 3F). The animals fed the HFD showed a greater than 20% increase in the number of supersized, macrovesicular hepatic LDs (d > 9 µm, p < 0.0001) along with a nearly three-fold reduction in the number of microvesicles smaller than 3 µm in diameter (1 µm < d ≤ 3 µm, p < 0.0001).
Figure 3: Orthogonal/3D views of LDs and data analysis. (A) Representative orthogonal projections and 3D rendered images of hepatic lipid droplets (green, BODIPY 493/503) and nuclei (blue, DAPI) in the liver of mice fed a normal or HFD diet. Images were taken at 20x (left) and 40x (right) magnification using a laser point scanning confocal microscope. Scale bars = 20 µm. (B) Number of hepatic LDs. (C) Fluorescence intensity of the hepatic LDs. (D) Area ratio of the hepatic LDs. (E) Lipid droplet diameters. (F) Fractional ratios of groups of hepatic lipid droplets with different diameters: 1 µm < d ≤ 3 µm, 3 µm < d ≤ 9 µm, and d > 9 µm. Data are presented as mean ± SEM. n = 5-6 mice/group; * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001. Statistical analysis was performed using a Student's unpaired t-test. Abbreviations: CTL = control group; HFD = high-fat diet group. Please click here to view a larger version of this figure.
This BODIPY 493/503 fluorescence-based protocol for LD assessment aimed to develop a new imaging approach for the evaluation of hepatic steatosis. Given the strong correlation between obesity and fatty liver disease, the Western-style high-fat diet was used to establish an animal model of hepatic steatosis26. A robust increase in hepatic TG contents was confirmed by a quantitative triglycerides colorimetric assay kit, which suggested a heightened hepatic lipidosis scenario in the HFD-fed animals. Subsequently, the degree of LD accumulation was visualized by the fluorescent probe BODIPY 493/503 under low magnification. As expected, the BODIPY 493/503 staining revealed a widespread distribution of vesicular structures across the hepatic tissue in the HFD group. Resorting to orthogonal projections and 3D reconstructions, it was possible to observe that the LD core presented a full content of neutral lipids, which appeared as nearly spherical droplets. Moreover, the robust increase in the LD area per total area was clearly evident (360% vs. CTL group), and this area was most likely filled with neutral TGs given their quantitative score in the HFD group (52.83 mg/g ± 6.73 mg/g; 346% vs. CTL group). To further characterize the LD dynamics upon HFD feeding, the micro or macrovesicular nature of hepatic steatosis was analyzed. Using the LD diameter ranges previously described in literature4,46,47, it was possible to discriminate a significant drop in the microvesicular LD count (1 µm < d ≤ 3 µm), which paralleled a proportional increase in LD macrovesicles (d > 9 µm). Several studies have reported that LDs in hepatocytes can form supersized LDs (up to tens of microns in diameter)4,46,47, which is in line with the present results.
In recent years, classical lipid dyes have been gradually substituted with a new array of fluorescent lipophilic probes, such as BODIPY, given their neutral and planar structure stabilized by a difluoroboron complex48; these probes have been shown to be very effective at tagging LDs to study their morphology, dynamics, and interaction with other organelles in living cells and some fixed tissues49,50. Within the fluorescent-stained lipid droplets protocol presented herein for hepatic steatosis assessment, there are some critical steps for the success of the technique that mostly rely on tissue preparation and image acquisition. Since the BODIPY signal can be bleached by UV light51, LD imaging acquisition must occur before imaging other fluorescent dyes that are excited by UV light, such as the commonly known DNA fluorescent dyes. Furthermore, it is highly recommended to protect the microscope slides from light exposure in order to prevent BODIPY fluorescence quenching before imaging. Even though the 3D reconstruction of LDs is very useful for the study of their morphology, it is extremely time-consuming, since 3D-reconstructed images are composed of several independent 2D images. Therefore, researchers should consider avoiding this step when the primary goal of the experiment is solely the assessment of micro and macrovesicular hepatic steatosis. To avoid bias, two independent observers blinded to the treatment group should carry out the data acquisition and analysis. The semi-automated image processing software (Cell Profiler pipeline) further contributes to a semi-blind quantification and overcomes the increased processing time consistently observed in manual analysis.
This technique may be extended to broader applications in combination with immunohistochemistry staining of other tissues and species, as long as the optimal BODIPY concentrations and image acquisition settings have been determined to maximize the signal/background ratio. Such experimental settings may allow the simultaneous detection of other antigens provided that the second fluorophore used does not overlap with the BODIPY excitation/emission spectra.
Overall, the optimized BODIPY 493/503 fluorescence-based protocol presented herein is a reliable and simple tool for LD characterization and may represent a complementary approach to the classical histological protocols often employed to confirm and grade hepatic steatosis.
The authors have nothing to disclose.
This research was funded by National and European Funds via the Portuguese Science and Technology Foundation (FCT), European Regional Development Fund (FEDER), and Programa Operacional Factores de Competitividade (COMPETE): 2020.09481.BD, UIDP/04539/2020 (CIBB), and POCI-01-0145-FEDER-007440. The authors would like to thank the support of iLAB – Microscopy and Bioimaging Lab, a facility of the Faculty of Medicine of the University of Coimbra and a member of the national infrastructure PPBI-Portuguese Platform of BioImaging (POCI-01-0145-FEDER-022122), as well as support from FSE CENTRO-04-3559-FSE-000142.
1.6 mm I.D. silicone tubing, I.V mini drip set | Fisher Scientific | ||
4,4-difluoro-1,3,5,7,8-pentametil-4-bora-3a,4a-diaza-s-indaceno (BODIPY 493/503) | Sigma-Aldrich, Lyon, France | D3922 | |
4',6-diamidino-2-phenylindole (DAPI) | Molecular Probes Inc, Invitrogen, Eugene, OR | D1306 | |
70% ethanol | Honeywell | 10191455 | |
Adobe Illustrator CC | Adobe Inc. | Used to design the figures | |
Automatic analyzer Hitachi 717 | Roche Diagnostics Inc., Mannheim, Germany | 8177-30-0010 | |
Barrier pen (Liquid blocker super pap pen) | Daido Sangyo Co., Ltd, Japon | _ | |
Blade | Leica | 221052145 | Used in the cryostat |
Cell Profiler version 4.2.5 | https://cellprofiler.org/releases/ | Used to analyse the acquired images | |
Coverslips | Menzel-Glaser, Germany | _ | |
Cryomolds | Tissue-Tek | _ | |
Cryostat (including specimen disc and heat extractor) CM3050 S | Leica Biosystems | _ | |
Dimethyl Sulfoxide (DMSO) | Sigma-Aldrich, Lyon, France | D-8418 | Used to dissolve Bodipy for the 5 mg/mL stock solution. CAUTION: Toxic and flammable. Vapors may cause irritation. Manipulate in a fume hood. Avoid direct contact with skin. Wear rubber gloves, protective eye goggles. |
Dry ice container (styrofoam cooler) | Novolab | A26742 | |
Dumont forceps | Fine Science Tools, Germany | 11295-10 | |
Glass Petri dish (H 25 mm, ø 150 mm) |
Thermo Scientific | 150318 | Used to weigh the liver after dissection |
Glycergel | DAKO Omnis | S303023 | |
GraphPad Prism software, version 9.3.1 | GraphPad Software, Inc., La Jolla, CA, USA | ||
High-fat diet | Envigo, Barcelona, Spain | MD.08811 | |
Ketamine (Nimatek 100 mg/mL) | Dechra | 791/01/14DFVPT | Used at a final concentration of 75 mg/kg |
Laser scanning confocal microscope (QUASAR detection unit; ) | Carl Zeiss, germany | LSM 710 Axio Observer Z1 microscope | |
Medetomidine (Sedator 1 mg/mL) | Dechra | 1838 ESP / 020/01/07RFVPT | Used at a final concentration of 1 mg/kg |
Needle | BD microlance | 300635 | |
No 15 Sterile carbon steel scalpel Blade |
Swann-Morton | 205 | |
Objectives 10x (Plan-Neofluar 10x/0.3), 20x (Plan-Apochromat 20x/0.8) and 40x (Plan-Neofluar 40x/1.30 Oil) | Carl Zeiss, Germany | ||
Paint brushes | Van Bleiswijck Amazon B07W7KJQ2X | Used to handle cryosections | |
Peristaltic pump (Minipuls 3) | Gilson | 1004170 | |
Phosphate-buffered saline (PBS, pH ~ 7.4) | Sigma-Aldrich, Lyon, France | P3813 | |
Scalpel handle, 125 mm (5"), No. 3 | Swann-Morton | 0208 | |
Slide staining system StainTray | Simport Scientific | M920 | |
Standard diet | Mucedola | 4RF21 | |
Superfrost Plus microscope slides | Menzel-Glaser, Germany | J1800AMNZ | |
Tissue-Tek OCT mounting media | VWR CHEMICALS | 361603E | |
Triglycerides colorimetric assay kit | Cayman Chemical | 10010303 | |
Ultrasonic bath | Bandelin Sonorex | TK 52 | |
Vannas spring scissors – 3 mm cutting edge |
Fine Science Tools, Germany | 15000-00 | |
ZEN Black software | Zeiss |