We thank Christina Pyrgaki, Tao Tong, and Alison North from the Bioimaging Resource Center at the Rockefeller University for assistance and support. We also thank Xiphias Ge Zhu for movie editing. This work was supported by the Human Frontier Science Program Organization (PC).

Animal care and experimentation were performed according to procedures approved by the Institutional Animal Care and Use Committee at the Rockefeller University.
1. Tissue Preparation
<ol>
	<li>Perform standard intracardiac perfusion with ~20 mL of 1x phosphate buffered saline (PBS) at 4 &#176;C until the blood is completely removed from the tissue.</li>
	<li>Switch the perfusate to ~20 mL of fixative solution (4% paraformaldehyde (PFA) in 1x PBS) at 4 &#176;C until the neck and tail have significantly stiffened. 
	CAUTION: PFA is toxic. Avoid contact with skin, eyes, and mucous membrane. Solutions should be made inside a fume hood.</li>
	<li>Dissect the fat pads of interest carefully to remove the entire fat pad without damaging it. Use blunt-ended forceps to avoid pinching or squeezing the tissue. Avoid contaminating the dissected tissue with any fur.</li>
	<li>Post-fix the tissue in 4% PFA in 1x PBS overnight at 4 &#176;C. For each fat pad, post-fix with ~10 mL of fixation solution in a 15 mL conical tube.</li>
	<li>Wash the tissue 3 times (1 h each) with 1x PBS at room temperature (RT).</li>
	<li>Store the tissue for short-term (up to 2 weeks) in 1x PBS at 4 &#176;C and protected from light. For long-term storage (e.g., 1-2 years), switch the buffer to 1x PBS with 0.05% sodium azide (as a preservative). 
	CAUTION: Sodium azide is highly toxic when ingested orally or absorbed through the skin. Concentrated sodium azide solutions (at 5% or greater) should be handled under a fume hood.</li>
</ol>
2. Delipidation and Permeabilization
Timing: 1-2 days
<ol>
	<li>Prepare 20%, 40%, 60%, and 80% methanol gradient with B1n buffer (Table 1) (v/v), e.g., for 20% methanol/B1n buffer, mix 20 mL of methanol and 80 mL of B1n buffer. Store all buffers at 4 &#176;C. Glycine crystals will precipitate from 60% and 80% methanol/B1n buffers due to saturation. Avoid removing the crystals; use the liquid solution for the following washes. 
	CAUTION: Methanol is volatile, irritant, and flammable. Avoid skin or eye contact.</li>
	<li>Gently remove the hair from the sample under a dissection microscope. Any hair, lint, or debris attached to the sample will cause shadows during imaging. Transfer the cleaned sample into a new tube.</li>
	<li>Perform all following steps on ice or at 4 &#176;C unless otherwise indicated. For small samples (e.g., posterior subcutaneous/perigonadal fat pads from young lean mice), use 2 mL microcentrifuge tubes with 1.6 mL of the solution. For large samples or samples with high lipid content (e.g., fat pads from obese mice), use 5 mL tubes with 4 mL of the solution.
	<ol>
		<li>Place the tubes containing the samples horizontally on an orbital shaker set at ~ 100 rpm. Make sure the samples can move freely inside the tube.</li>
	</ol>
	</li>
	<li>Dehydrate the sample in a graded series of 20%, 40%, 60%, 80%, and 100% methanol/B1n buffer for the indicated time (Table 2). Minimize the carry-over solution.</li>
	<li>Delipidate the sample with 100% dichloromethane (DCM) (3 times), each with the incubation time indicated in Table 2. The sample should sink to the bottom of the tube at the end of the second and third DCM washes. If not, extend the incubation time to ensure full delipidation (e.g., extend from 30 min to 1-2 h). 
	CAUTION: DCM should be handled inside a fume hood. Use glass containers for storage and microcentrifuge tubes that are made from polypropylene for incubations. The microcentrifuge tubes should not be used for long-term storage of DCM. Petri dishes and serological pipettes that are made from polystyrene are not compatible with DCM. DCM is highly volatile. Transfer the sample into fresh solution quickly to prevent desiccation.</li>
	<li>Wash twice with 100% methanol for the indicated time (Table 2).</li>
	<li>Optional: If the sample is not well-perfused, the color of hemoglobin from the remaining red blood cells might cause strong autofluorescence. Bleach with 5% H2O2 in methanol (1 volume of 30% H2O2 to 5 volumes of methanol) overnight at 4 &#176;C.</li>
	<li>Rehydrate the sample in a reversed methanol/B1n buffer series: 80%, 60%, 40%, 20% methanol/B1n, and 100% B1n buffer (2 times) for the indicated time (Table 2).</li>
	<li>Wash the sample with PTxwH buffer (Table 1) for 2 h at RT.</li>
	<li>Store the delipidated sample in PTxwH buffer at 4&#176; C (up to 1 year) or proceed immediately to the immunostaining step.</li>
</ol>
3. Whole Mount Immunostaining
Timing: 8-10 days
Note: All of the following steps should be carried out at RT unless noted otherwise, with shaking and protection from light. For small samples, use 2 mL microcentrifuge tubes with 1.6 mL of solution. For large samples, use 5 mL tubes with 4 mL of solution. It is recommended to first validate the antibodies on small pieces of the tissue or methanol-treated tissue sections.
<ol>
	<li>Dilute the primary antibody in PTxwH buffer to the recommended concentration. Centrifuge the diluted antibody solution at ~20,000 x g for 10 min to prevent introducing antibody precipitates or aggregations.</li>
	<li>Incubate the delipidated sample with the primary antibody solution for the indicated time (Table 2).</li>
	<li>Wash the sample with PTxwH buffer in a series of incubation steps: 5 min, 10 min, 15 min, 20 min, 1 h, 2 h, 4 h, and overnight.</li>
	<li>Dilute the secondary antibody in PTxwH buffer to the recommended concentration. Centrifuge the diluted antibody solution at ~20,000 x g for 10 min to prevent introducing antibody precipitates or aggregations. 
	Note: To avoid high background caused by tissue autofluorescence, secondary antibodies conjugated with fluorophores that emit light in the red and far-red regions are recommended. Depending on the number of laser lines available on a microscope, up to 3-4 markers can be imaged simultaneously in one sample.</li>
	<li>Incubate the sample with the secondary antibody solution for the indicated time (Table 2).</li>
	<li>Wash sample with PTxwH buffer in a series of incubation steps: 5 min, 10 min, 15 min, 20 min, 1 h, 2 h, 4 h, and overnight.</li>
	<li>Optional: For tissue types that are fragile, fix the stained tissue with 4% PFA in 1x PBS overnight at 4 &#176;C to help preserve tissue morphology. 
	Note: This step may increase imaging background. It is not necessary to perform this step for adipose tissue.</li>
	<li>Wash the sample with 1x PBS in a series of incubation steps: 5 min, 10 min, and 30 min.</li>
	<li>Gently remove lint from the sample under a dissection microscope.</li>
</ol>
4. Tissue Clearing
Timing: 1-2 days
<ol>
	<li>Optional: Embed the sample in agarose to facilitate sample mounting for light-sheet microscopy. 
	Note: This step is strongly recommended for adipose tissue to stabilize its shape during imaging.
	<ol>
		<li>Prepare the embedding solution with 1% agarose in 1x PBS (w/v). Cool the embedding solution to ~ 40 &#176;C to avoid exposing the sample to excessive heat.</li>
		<li>Place the cleaned sample into a mold (e.g., Petri dish or weighing boat) and arrange it into the desired position. Avoid any liquid carryover. Gently pour the embedding solution over the sample. Avoid any air bubbles.</li>
		<li>Let the agarose fully solidify at RT. Cut out a block containing the sample. 
		Note: All of the following steps including overnight incubations should be carried out at RT, with shaking and protection from light. For small samples, use 2 mL microcentrifuge tubes with 1.6 mL of solution. For large samples, use 5 mL tubes with 4 mL of solution.</li>
	</ol>
	</li>
	<li>Dehydrate the sample in methanol gradient with H2O: 25%, 50%, 75%, and 100% (3 times) for the indicated time (Table 2). 
	Note: Sample can be left overnight at the 100% methanol step.</li>
	<li>Incubate the sample in 100% DCM for 1 h with shaking (3 times). Sample should sink to the bottom of the tube at the end of each DCM wash. If not, extend the incubation time to ensure full removal of methanol. Sample can be left overnight at the 100% DCM step.</li>
	<li>Incubate the sample in dibenzyl ether (DBE) overnight with mild shaking to achieve refractive index matching. 
	Note: Sample will eventually become transparent in visible light. 
	CAUTION: DBE is hazardous. Avoid any contact with skin. Handle it inside a fume hood with double-layered gloves. Discard the gloves as soon as it is contaminated with DBE. Use glass containers for storage and microcentrifuge tubes (polypropylene) for incubations. The microcentrifuge tubes should not be used for long-term storage of DBE. Petri dishes and serological pipettes that are made from polystyrene are not compatible with DBE. Clean any spill with 100% ethanol.</li>
	<li>Incubate the sample with fresh DBE with mild shaking for 2 h. Store the sample at RT in the dark or proceed directly to imaging. 
	Note: Samples are recommended to be imaged within a month. Although certain fluorophores are more stable than others, long-term storage of the samples at this stage is not recommended.</li>
</ol>
5. Microscopy
<ol>
	<li>Imaging with a light-sheet microscope that is compatible with DBE. 
	Note: Only objectives that are approved for organic solvent-based imaging and matched with the refractive index of DBE can be used as dipping objectives in DBE-immersed imaging.
	<ol>
		<li>Mount the agarose-embedded sample onto a holder that can prevent movement during imaging. Immerse the sample into a DBE-filled chamber.</li>
		<li>Collect a z-stack covering the whole sample (e.g., 1-2X magnification) to obtain whole-tissue distribution/structure of the marker of interest.</li>
		<li>Switch to an objective with higher magnification (e.g., 4-12X magnification) to zoom in on the regions of interest to image detailed structures. 
		Note: Collagen is a major contributor to the extracellular matrix of adipose tissue, which surrounds all fat cells13. Collagen has a typical emission spectrum ranging around 400 nm to 550 nm14. Imaging the sample with the 488 laser line (green) and collecting the emitted light at a wavelength range that lies within the emission spectrum of collagen (e.g., 525-550 nm) will provide the tissue autofluorescence signal, which was previously shown to delineate fat cell contour and overall tissue architecture12.</li>
	</ol>
	</li>
	<li>Imaging with an inverted confocal or a two-photon microscope.
	<ol>
		<li>Place the agarose-embedded sample into a chamber slide with a glass bottom without touching any plastic parts (or other DBE-compatible imaging chamber that can seal). Make sure DBE does not leak out.</li>
		<li>Choose the objectives with longer working distances to achieve deeper penetration. 
		Note: Imaging should be done with an inverted confocal or two-photon microscope through the glass bottom of the chamber slide. Avoid direct contact between the sample and dipping objectives that are not suggested for DBE-based imaging.</li>
	</ol>
	</li>
</ol>

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E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tracking+adipogenesis+during+white+adipose+tissue+development,+expansion+and+regeneration.">Tracking adipogenesis during white adipose tissue development, expansion and regeneration.</a> Nature Medicine. 19 (10), 1338-1344 (2013).</li><li>Bartness, T. J., Liu, Y., Shrestha, Y. B., Ryu, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neural+innervation+of+white+adipose+tissue+and+the+control+of+lipolysis.">Neural innervation of white adipose tissue and the control of lipolysis.</a> Frontiers in Neuroendocrinology. 35 (4), 473-493 (2014).</li><li>Morrison, S. F., Madden, C. J., Tupone, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Central+Neural+Regulation+of+Brown+Adipose+Tissue+Thermogenesis+and+Energy+Expenditure.">Central Neural Regulation of Brown Adipose Tissue Thermogenesis and Energy Expenditure.</a> Cell Metabolism. 19 (5), 741-756 (2014).</li><li>Xue, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hypoxia-Independent+Angiogenesis+in+Adipose+Tissues+during+Cold+Acclimation.">Hypoxia-Independent Angiogenesis in Adipose Tissues during Cold Acclimation.</a> Cell Metabolism. 9 (1), 99-109 (2009).</li><li>Shimizu, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vascular+rarefaction+mediates+whitening+of+brown+fat+in+obesity.">Vascular rarefaction mediates whitening of brown fat in obesity.</a> The Journal of Clinical Investigation. 124 (5), 2099-2112 (2014).</li><li>Abe, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+reconstruction+of+brain+section+images+for+creating+axonal+projection+maps+in+marmosets.">3D reconstruction of brain section images for creating axonal projection maps in marmosets.</a> Journal of Neuroscience Methods. 286, 102-113 (2017).</li><li>Richardson, D. S., Lichtman, J. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Clarifying+Tissue+Clearing.">Clarifying Tissue Clearing.</a> Cell. 162 (2), 246-257 (2015).</li><li>Renier, N., Wu, Z., Simon, D. J., Yang, J., Ariel, P., Tessier-Lavigne, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=iDISCO:+A+Simple,+Rapid+Method+to+Immunolabel+Large+Tissue+Samples+for+Volume+Imaging.">iDISCO: A Simple, Rapid Method to Immunolabel Large Tissue Samples for Volume Imaging.</a> Cell. 159 (4), 896-910 (2014).</li><li>Renier, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mapping+of+Brain+Activity+by+Automated+Volume+Analysis+of+Immediate+Early+Genes.">Mapping of Brain Activity by Automated Volume Analysis of Immediate Early Genes.</a> Cell. 165 (7), 1789-1802 (2016).</li><li>Chi, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-Dimensional+Adipose+Tissue+Imaging+Reveals+Regional+Variation+in+Beige+Fat+Biogenesis+and+PRDM16-Dependent+Sympathetic+Neurite+Density.">Three-Dimensional Adipose Tissue Imaging Reveals Regional Variation in Beige Fat Biogenesis and PRDM16-Dependent Sympathetic Neurite Density.</a> Cell Metabolism. 27 (1), 226-236 (2018).</li><li>Khan, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metabolic+Dysregulation+and+Adipose+Tissue+Fibrosis:+Role+of+Collagen+VI.">Metabolic Dysregulation and Adipose Tissue Fibrosis: Role of Collagen VI.</a> Molecular and Cellular Biology. 29 (6), 1575-1591 (2009).</li><li>Croce, A. 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The authors have nothing to disclose.

Adipo-Clear is a straightforward and robust method for clearing adipose tissue, which can be easily performed in a regular lab setup. In comparison to other solvent-based clearing methods such as iDISCO/iDISCO+10,11,12, Adipo-Clear is particularly optimized for clearing adipose tissue and other tissue with high fat content. The delipidation step completely removes lipids from adipose, and therefore facilitates immunolabeling thr...

Until recently, adipose tissue was conceived of as an amorphous collection of fat cells. Over the past few decades, our understanding has grown more sophisticated, with fat now recognized to be a complex organ containing different types of adipocytes, as well as adipocyte precursors, immune cells, fibroblasts, the vasculature, and nerve projections. Interactions among these adipose-resident cells have pronounced effects on adipose tissue and organismal physiology and pathophysiology1. Although emerging studies have unraveled important molecular mechanisms underlying certain interactions, a more comprehensive understanding requires reliable structural profiling of the entire tissue in three dimensions (3D).
Our current knowledge of adipose tissue morphology is largely based on histological analysis of thin sections (5 &#956;m) with relatively high-magnification imaging (more than 10X)2,3. However, this approach has several significant limitations. First, intricate filamentous structures such as sympathetic nerves and the vasculature, which are known to play important roles in adipose function4,5,6,7, are difficult to evaluate through thin sections. Second, due to its seemingly amorphous shape and the lack of representative structural units to focus on, it is difficult to appreciate adipose tissue structures based only on section staining. Third, adipose tissue has a very high lipid content, creating challenges in obtaining consistent serial sections that are suitable for 3D anatomical reconstruction, a conventional method used to study whole brain morphology8. Given these factors, there is a great need for a whole-mount approach that can provide 3D visualization of an entire adipose depot while still achieving cellular resolution.
3D volumetric imaging of an entire organ is challenging due to the obscuring effects of light scatter. A major source of light scatter in biological tissues comes from lipid-aqueous interfaces. Although the efforts to eliminate scatter by removing lipids have been ongoing for over a century, there have been a large number of recent innovations9. One such newly developed tissue-clearing method is immunolabeling-enabled 3D imaging of solvent-cleared organs (iDISCO/iDISCO+)10,11. However, adipose tissue presents a particular challenge given its high level of lipids, and therefore, additional modifications to the iDISCO/iDISCO+ protocol are required to fully extract the lipids while protecting the tissue from collapsing. The modified protocol we have developed, now called Adipo-Clear, employs methanol/dichloromethane-based delipidation of adipose tissue to achieve optimal transparency suitable for high-resolution volumetric imaging12. Because the delipidation step largely quenches endogenously expressed fluorescent proteins such as GFP and RFP, visualization of such proteins must be achieved by immunolabeling. Overall, this simple and robust protocol can be applied to study tissue-level organization of adipose-resident cells, lineage tracing of adipocyte progenitor cells, and adipose morphogenesis during development.

<table>
	<tbody>
		<tr>
			<td>1x phosphate buffered saline</td>
			<td>Corning</td>
			<td>21-040-CV</td>
			<td></td>
		</tr>
		<tr>
			<td>Paraformaldehyde</td>
			<td>Sigma Aldrich</td>
			<td>P6148-1KG</td>
			<td></td>
		</tr>
		<tr>
			<td>Methanol</td>
			<td>Fisher Scientific</td>
			<td>A412SK-4</td>
			<td></td>
		</tr>
		<tr>
			<td>Triton X-100</td>
			<td>Sigma Aldrich</td>
			<td>X100-500ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Tween 20</td>
			<td>Sigma Aldrich</td>
			<td>P2287-500ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Heparin</td>
			<td>Sigma Aldrich</td>
			<td>H3393-100KU</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichloromethane</td>
			<td>Sigma Aldrich</td>
			<td>270991</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen peroxide 30%</td>
			<td>Fisher Scientific</td>
			<td>325-100</td>
			<td></td>
		</tr>
		<tr>
			<td>Benzyl ether</td>
			<td>Sigma Aldrich</td>
			<td>108014</td>
			<td></td>
		</tr>
		<tr>
			<td>Agarose</td>
			<td>Invitrogen</td>
			<td>16500500</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium azide</td>
			<td>Sigma Aldrich</td>
			<td>71289-5G</td>
			<td></td>
		</tr>
		<tr>
			<td>Glycine</td>
			<td>Fisher Scientific</td>
			<td>BP381-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Rabbit polyclonal anti-Tyrosine Hydroxylase</td>
			<td>Millipore</td>
			<td>AB152</td>
			<td>1:200 dilution</td>
		</tr>
		<tr>
			<td>Goat polyclonal anti-CD31/PECAM-1</td>
			<td>R&#38;D Systems</td>
			<td>AF3628</td>
			<td>Final concentration of 2 &#181;g/mL</td>
		</tr>
		<tr>
			<td>Rat monoclonal anti-CD68, Clone FA-11</td>
			<td>Bio-Rad</td>
			<td>MCA1957</td>
			<td>Final concentration of 2 &#181;g/mL</td>
		</tr>
		<tr>
			<td>Donkey anti-rabbit IgG (H+L) Alexa Fluor 647</td>
			<td>Jackson ImmunoResearch</td>
			<td>711-605-152</td>
			<td>Final concentration of 5-10 &#181;g/mL</td>
		</tr>
		<tr>
			<td>Donkey anti-goat IgG (H+L) Alexa Fluor 568</td>
			<td>Invitrogen</td>
			<td>A11077</td>
			<td>Final concentration of 5-10 &#181;g/mL</td>
		</tr>
		<tr>
			<td>Donkey anti-rat IgG (H+L) Alexa Fluor 647</td>
			<td>Jackson ImmunoResearch</td>
			<td>712-605-153</td>
			<td>Final concentration of 5-10 &#181;g/mL</td>
		</tr>
		<tr>
			<td>Imaging chamber</td>
			<td>ibidi</td>
			<td>80287</td>
			<td></td>
		</tr>
		<tr>
			<td>Light sheet microscope</td>
			<td>LaVision BioTec</td>
			<td>Ultramicroscope II</td>
			<td></td>
		</tr>
		<tr>
			<td>Imaging software</td>
			<td>LaVision BioTec</td>
			<td>Imspector software</td>
			<td></td>
		</tr>
		<tr>
			<td>Microscopy visualization software</td>
			<td>Bitplane</td>
			<td>Imaris</td>
			<td></td>
		</tr>
	</tbody>
</table>

adipo-clear: a tissue clearing method for three-dimensional imaging of adipose tissue

Adipose tissue plays a central role in energy homeostasis and thermoregulation. It is composed of different types of adipocytes, as well as adipocyte precursors, immune cells, fibroblasts, blood vessels, and nerve projections. Although the molecular control of cell type specification and how these cells interact have been increasingly delineated, a more comprehensive understanding of these adipose-resident cells can be achieved by visualizing their distribution and architecture throughout the whole tissue. Existing immunohistochemistry and immunofluorescence approaches to analyze adipose histology rely on thin paraffin-embedded sections. However, thin sections capture only a small portion of tissue; as a result, the conclusions can be biased by what portion of tissue is analyzed. We have therefore developed an adipose tissue clearing technique, Adipo-Clear, to permit comprehensive three-dimensional visualization of molecular and cellular patterns in whole adipose tissues. Adipo-Clear was adapted from iDISCO/iDISCO+, with specific modifications made to completely remove the lipid stored in the tissue while preserving native tissue morphology. In combination with light-sheet fluorescence microscopy, we demonstrate here the use of the Adipo-Clear method to obtain high-resolution volumetric images&#160;of an entire adipose tissue.

Adipo-Clear prepared whole fat pads can be imaged in 3D to analyze how tissue morphology and cellular interactions are affected in the lean and obese states. This method can be easily applied to analyze general adipose structure by collecting the tissue autofluorescence signal in the green channel. We have previously shown that the autofluorescence signal in adipose overlays favorably with perilipin staining, a commonly used marker to outline mature adipocytes12. F...

Watch this Scientific Journal Video about Adipo-Clear: A Tissue Clearing Method for Three-Dimensional Imaging of Adipose Tissue at JoVE.com

Adipo-Clear: A Tissue Clearing Method for Three-Dimensional Imaging of Adipose Tissue

Due to the high lipid content, adipose tissue has been challenging to visualize using traditional histological methods. Adipo-Clear is a tissue clearing technique that allows robust labeling and high-resolution volumetric fluorescent imaging of adipose tissue. Here, we describe the methods for sample preparation, pretreatment, staining, clearing, and mounting for imaging.

Adipose tissue plays a central role in energy homeostasis and thermoregulation. It is composed of different types of adipocytes, as well as adipocyte ...

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JoVE Journal

Biology

26.9K Views.  The Rockefeller University. Due to the high lipid content, adipose tissue has been challenging to visualize using traditional histological methods. Adipo-Clear is a tissue clearing technique that allows robust labeling and high-resolution volumetric fluorescent imaging of adipose tissue. Here, we describe the methods for sample preparation, pretreatment, staining, clearing, and mounting for imaging.

Video: Adipo-Clear: A Tissue Clearing Method for Three-Dimensional Imaging of Adipose Tissue

Actin Co-Sedimentation Assay; for the Analysis of Protein Binding to F-Actin

The actin cytoskeleton within the cell is a network of actin filaments that allows the movement of cells and cellular processes, and that generates tension and helps maintains cellular shape.  Although the actin cytoskeleton is a rigid structure, it is a dynamic structure that is constantly remodeling.  A number of proteins can bind to the actin cytoskeleton.  The binding of a particular protein to F-actin is often desired to support cell biological observations or to further understand dynamic processes due to remodeling of the actin cytoskeleton. The actin co-sedimentation assay is an in vitro assay routinely used to analyze the binding of specific proteins or protein domains with F-actin.  The basic principles of the assay involve an incubation of the protein of interest (full length or domain of) with F-actin, ultracentrifugation step to pellet F-actin and analysis of the protein co-sedimenting with F-actin.  Actin co-sedimentation assays can be designed accordingly to measure actin binding affinities and in competition assays.

Proteins bind to filamentous actin (F-actin) through distinct actin binding modules. In this video we demonstrate the procedure of actin co-sedimentation, which is an in vitro assay routinely used to analyze proteins or specific domains that bind F-actin.

The actin cytoskeleton within the cell is a network of actin filaments that allows the movement of cells and cellular processes, and that generates ...

Hi-C: A Method to Study the Three-dimensional Architecture of Genomes.

The three-dimensional folding of chromosomes compartmentalizes the genome and and can bring distant functional elements, such as promoters and enhancers, into close spatial proximity 2-6. Deciphering the relationship between chromosome organization and genome activity will aid in understanding genomic processes, like transcription and replication. However, little is known about how chromosomes fold. Microscopy is unable to distinguish large numbers of loci simultaneously or at high resolution. To date, the detection of chromosomal interactions using chromosome conformation capture (3C) and its subsequent adaptations required the choice of a set of target loci, making genome-wide studies impossible 7-10.
We developed Hi-C, an extension of 3C that is capable of identifying long range interactions in an unbiased, genome-wide fashion. In Hi-C, cells are fixed with formaldehyde, causing interacting loci to be bound to one another by means of covalent DNA-protein cross-links. When the DNA is subsequently fragmented with a restriction enzyme, these loci remain linked. A biotinylated residue is incorporated as the 5' overhangs are filled in. Next, blunt-end ligation is performed under dilute conditions that favor ligation events between cross-linked DNA fragments. This results in a genome-wide library of ligation products, corresponding to pairs of fragments that were originally in close proximity to each other in the nucleus. Each ligation product is marked with biotin at the site of the junction. The library is sheared, and the junctions are pulled-down with streptavidin beads. The purified junctions can subsequently be analyzed using a high-throughput sequencer, resulting in a catalog of interacting fragments.
Direct analysis of the resulting contact matrix reveals numerous features of genomic organization, such as the presence of chromosome territories and the preferential association of small gene-rich chromosomes. Correlation analysis can be applied to the contact matrix, demonstrating that the human genome is segregated into two compartments: a less densely packed compartment containing open, accessible, and active chromatin and a more dense compartment containing closed, inaccessible, and inactive chromatin regions. Finally, ensemble analysis of the contact matrix, coupled with theoretical derivations and computational simulations, revealed that at the megabase scale Hi-C reveals features consistent with a fractal globule conformation.

The Hi-C method allows unbiased, genome-wide identification of chromatin interactions (1). Hi-C couples proximity ligation and massively parallel sequencing. The resulting data can be used to study genomic architecture at multiple scales: initial results identified features such as chromosome territories, segregation of open and closed chromatin, and chromatin structure at the megabase scale.

The three-dimensional folding of chromosomes compartmentalizes the genome and and can bring distant functional elements, such as promoters and enhancers, ...

A System for Culturing Iris Pigment Epithelial Cells to Study Lens Regeneration in Newt

Salamanders like newt and axolotl possess the ability to regenerate many of its lost body parts such as limbs, the tail with spinal cord, eye, brain, heart, the jaw 1. Specifically, newts are unique for its lens regeneration capability. Upon lens removal, IPE cells of the dorsal iris transdifferentiate to lens cells and eventually form a new lens in about a month 2,3. This property of regeneration is never exhibited by the ventral iris cells. The regeneration potential of the iris cells can be studied by making transplants of the in vitro cultured IPE cells. For the culture, the dorsal and ventral iris cells are first isolated from the eye and cultured separately for a time period of 2 weeks (Figure 1). These cultured cells are reaggregated and implanted back to the newt eye. Past studies have shown that the dorsal reaggregate maintains its lens forming capacity whereas the ventral aggregate does not form a lens, recapitulating, thus the in vivo process (Figure 2) 4,5. This system of determining regeneration potential of dorsal and ventral iris cells is very useful in studying the role of genes and proteins involved in lens regeneration.

In newt, the lens regenerates always from the dorsal iris by transdifferentiation of the iris pigmented epithelial cells (IPEs). Here we describe a procedure to culture dorsal and ventral newt IPE cells and their implantation to the newt eye. The implanted cells are then studied by tissue sectioning and immunohistochemistry.

Salamanders like newt and axolotl possess the ability to regenerate many of its lost body parts such as limbs, the tail with spinal cord, eye, brain, ...

Quantitative Comparison of cis-Regulatory Element (CRE) Activities in Transgenic Drosophila melanogaster

Gene expression patterns are specified by cis-regulatory element (CRE) sequences, which are also called enhancers or cis-regulatory modules. A typical CRE possesses an arrangement of binding sites for several transcription factor proteins that confer a regulatory logic specifying when, where, and at what level the regulated gene(s) is expressed. The full set of CREs within an animal genome encodes the organism&prime;s program for development1, and empirical as well as theoretical studies indicate that mutations in CREs played a prominent role in morphological evolution2-4. Moreover, human genome wide association studies indicate that genetic variation in CREs contribute substantially to phenotypic variation5,6. Thus, understanding regulatory logic and how mutations affect such logic is a central goal of genetics. 
Reporter transgenes provide a powerful method to study the in vivo function of CREs. Here a known or suspected CRE sequence is coupled to heterologous promoter and coding sequences for a reporter gene encoding an easily observable protein product. When a reporter transgene is inserted into a host organism, the CRE&prime;s activity becomes visible in the form of the encoded reporter protein. P-element mediated transgenesis in the fruit fly species Drosophila (D.) melanogaster7 has been used for decades to introduce reporter transgenes into this model organism, though the genomic placement of transgenes is random. Hence, reporter gene activity is strongly influenced by the local chromatin and gene environment, limiting CRE comparisons to being qualitative. In recent years, the phiC31 based integration system was adapted for use in D. melanogaster to insert transgenes into specific genome landing sites8-10. This capability has made the quantitative measurement of gene and, relevant here, CRE activity11-13 feasible. The production of transgenic fruit flies can be outsourced, including phiC31-based integration, eliminating the need to purchase expensive equipment and/or have proficiency at specialized transgene injection protocols. 
Here, we present a general protocol to quantitatively evaluate a CRE&prime;s activity, and show how this approach can be used to measure the effects of an introduced mutation on a CRE&prime;s activity and to compare the activities of orthologous CREs. Although the examples given are for a CRE active during fruit fly metamorphosis, the approach can be applied to other developmental stages, fruit fly species, or model organisms. Ultimately, a more widespread use of this approach to study CREs should advance an understanding of regulatory logic and how logic can vary and evolve.

Quantitative Comparison of cis-Regulatory Element CRE Activities in Transgenic Drosophila melanogaster

Phenotypic variation for traits can result from mutations in cis-regulatory element (CRE) sequences that control gene expression patterns. Methods derived for use in Drosophila melanogaster can quantitatively compare the levels of spatial and temporal patterns of gene expression mediated by modified or naturally occurring CRE variants.

Gene expression patterns are specified by cis-regulatory element (CRE) sequences, which are also called enhancers or cis-regulatory modules. A typical CRE ...

FRET Imaging in Three-dimensional Hydrogels

Imaging of F&#246;rster resonance energy transfer (FRET)&#160;is a powerful tool for examining cell biology in real-time. Studies utilizing FRET commonly employ two-dimensional (2D) culture, which does not mimic the three-dimensional (3D) cellular microenvironment. A method to perform quenched emission FRET imaging using conventional widefield epifluorescence microscopy of cells within a 3D hydrogel environment is presented. Here an analysis method for ratiometric FRET probes that yields linear ratios over the probe activation range is described. Measurement of intracellular cyclic adenosine monophosphate (cAMP) levels is demonstrated in chondrocytes under forskolin stimulation using a probe for EPAC1 activation (ICUE1) and the ability to detect differences in cAMP signaling dependent on hydrogel material type, herein a photocrosslinking hydrogel (PC-gel, polyethylene glycol dimethacrylate) and a thermoresponsive hydrogel (TR-gel). Compared with 2D FRET methods, this method requires little additional work. Laboratories already utilizing FRET imaging in 2D can easily adopt this method to perform cellular studies in a 3D microenvironment. It can further be applied to high throughput drug screening in engineered 3D microtissues. Additionally, it is compatible with other forms of FRET imaging, such as anisotropy measurement and fluorescence lifetime imaging (FLIM), and with advanced microscopy platforms using confocal, pulsed, or modulated illumination.

F&#246;rster resonance energy transfer (FRET) imaging is a powerful tool for real-time cell biology studies. Here a method for FRET imaging cells in physiologic three-dimensional (3D) hydrogel microenvironments using conventional epifluorescence microscopy is presented. An analysis for ratiometric FRET probes that yields linear ratios over the activation range is described.

Imaging of F&#246;rster resonance energy transfer (FRET)&#160;is a powerful tool for examining cell biology in real-time. Studies utilizing FRET commonly ...

ACT-PRESTO: Biological Tissue Clearing and Immunolabeling Methods for Volume Imaging

The identification and exploration of the detailed organization of organs or of the whole body at the cellular level are fundamental challenges in biology. Transitional methods require a substantial amount of time and effort to obtain a 3D image and including sectioning the intact tissue, immunolabeling, and imaging serially-sectioned tissue, which produces a loss of information at each step of the process. In recently developed approaches for high-resolution imaging within intact tissue, molecular characterization has been restricted to the labeling of proteins. However, currently available protocols for organ clearing require a considerably long process time, making it difficult to implement tissue clearing techniques in the lab. We recently established a rapid and highly-reproducible protocol termed ACT-PRESTO (active clarity technique&#8211;pressure related efficient and stable transfer of macromolecules into organs), which allows for tissue clearance within several hours. Moreover, ACT-PRESTO enables rapid immunolabeling with conventional methods and accelerates antibody penetration into the deep layer of densely-formed, thick specimens by applying pressure or convection flow. We describe how to prepare tissues, how to clear by lipid removal using electrophoresis, and how to immuno-stain by a pressure-assisted delivery. The rapidity and consistency of the protocol will expedite the performance of 3D histological research and volume-based diagnoses.

ACT-PRESTO (active clarity technique&#8211;pressure related efficient and stable transfer of macromolecules into organs) enables rapid, efficient, but inexpensive tissue clearing and rapid antibody penetration into cleared tissues by passive diffusion or pressure-assisted delivery. Using this method, a wide range of tissues can be cleared, immunolabeled by multiple antibodies, and volume-imaged.

The identification and exploration of the detailed organization of organs or of the whole body at the cellular level are fundamental challenges in ...

Visualization of 3D White Adipose Tissue Structure Using Whole-mount Staining

Adipose tissue is an important metabolic organ with high plasticity and is responsive to environmental stimuli and nutrient status. As such, various techniques have been developed to study the morphology and biology of adipose tissue. However, conventional visualization methods are limited to studying the tissue in 2D sections, failing to capture the 3D architecture of the whole organ. Here we present whole-mount staining, an immunohistochemistry method that preserves intact adipose tissue morphology with minimal processing steps. Hence, the structures of adipocytes and other cellular components are maintained without distortion, achieving the most representative 3D visualization of the tissue. In addition, whole-mount staining can be combined with lineage tracing methods to determine cell fate decisions. However, this technique has some limitations to providing accurate information regarding deeper parts of adipose tissue. To overcome this limitation, whole-mount staining can be further combined with tissue clearing techniques to remove the opaqueness of tissue and allow for complete visualization of entire adipose tissue anatomy using light-sheet fluorescent microscopy. Therefore, a higher resolution and more accurate representation of adipose tissue structures can be captured with the combination of these techniques.

The focus of the present study is to demonstrate the whole-mount immunostaining and visualization technique as an ideal method for 3D imaging of adipose tissue architecture and cellular component.

Adipose tissue is an important metabolic organ with high plasticity and is responsive to environmental stimuli and nutrient status. As such, various ...

Human Adipose Tissue Micro-fragmentation for Cell Phenotyping and Secretome Characterization

In the past decade, adipose tissue transplants have been widely used in plastic surgery and orthopaedics to enhance tissue repletion and/or regeneration. Accordingly, techniques for harvesting and processing human adipose tissue have evolved in order to quickly and efficiently obtain large amounts of tissue. Among these, the closed system technology represents an innovative and easy-to-use system to harvest, process, and re-inject refined fat tissue in a short time and in the same intervention (intra-operatively). Adipose tissue is collected by liposuction, washed, emulsified, rinsed and minced mechanically into cell clusters of 0.3 to 0.8 mm. Autologous transplantation of mechanically fragmented adipose tissue has shown remarkable efficacy in different therapeutic indications such as aesthetic medicine and surgery, orthopedic and general surgery. Characterization of micro-fragmented adipose tissue revealed the presence of intact small vessels within the adipocyte clusters; hence, the perivascular niche is left unperturbed. These clusters are enriched in perivascular cells (i.e., mesenchymal stem cell (MSC) ancestors) and in vitro analysis showed an increased release of growth factors and cytokines involved in tissue repair and regeneration, compared to enzymatically derived MSCs. This suggests that the superior therapeutic potential of microfragmented adipose tissue is explained by a higher frequency of presumptive MSCs and enhanced secretory activity. Whether these added pericytes directly contribute to higher growth factor and cytokine production is not known. This clinically approved procedure allows the transplantation of presumptive MSCs without the need for expansion and/or enzymatic treatment, thus bypassing the requirements of GMP guidelines, and reducing the costs for cell-based therapies.

Here, we present human adipose tissue enzyme-free micro-fragmentation using a closed system device. This new method allows the obtainment of sub-millimeter clusters of adipose tissue suitable for in vivo transplantation, in vitro culture, and further cell isolation and characterization.

In the past decade, adipose tissue transplants have been widely used in plastic surgery and orthopaedics to enhance tissue repletion and/or regeneration. ...

Single-Cell Resolution Three-Dimensional Imaging of Intact Organoids

Organoid technology, in vitro 3D culturing of miniature tissue, has opened a new experimental window for cellular processes that govern organ development and function as well as disease. Fluorescence microscopy has played a major role in characterizing their cellular composition in detail and demonstrating their similarity to the tissue they originate from. In this article, we present a comprehensive protocol for high-resolution 3D imaging of whole organoids upon immunofluorescent labeling. This method is widely applicable for imaging of organoids differing in origin, size and shape. Thus far we have applied the method to airway, colon, kidney, and liver organoids derived from healthy human tissue, as well as human breast tumor organoids and mouse mammary gland organoids. We use an optical clearing agent, FUnGI, which enables the acquisition of whole 3D organoids with the opportunity for single-cell quantification of markers. This three-day protocol from organoid harvesting to image analysis is optimized for 3D imaging using confocal microscopy.

The entire 3D structure and cellular content of organoids, as well as their phenotypic resemblance to the original tissue can be captured using the single-cell resolution 3D imaging protocol described here. This protocol can be applied to a wide range of organoids varying in origin, size and shape.

Organoid technology, in vitro 3D culturing of miniature tissue, has opened a new experimental window for cellular processes that govern organ development ...

Y-27632 Enriches the Yield of Human Melanocytes from Adult Skin Tissues

The isolation and culture of primary melanocytes from skin tissues is very important for biological research and has been widely used for clinical applications. Isolating primary melanocytes from skin tissues by the conventional method usually takes about 3 to 4 weeks to passage sufficiently. More importantly, the tissues used are usually newborn foreskins and it is still a challenge to efficiently isolate primary melanocytes from adult tissues. We recently developed a new isolation method for melanocytes that adds Y-27632, a Rho kinase inhibitor, to the initial culture medium for 48 h. Compared with the conventional protocol, this new method dramatically increases the yield of melanocytes and shortens the time required to isolate melanocytes from foreskin tissues. We now describe this new method in more detail using adult epidermis to efficiently culture primary melanocytes. Importantly, we show that melanocytes obtained from adult tissues prepared by this new method can function normally. This new protocol will significantly benefit studies of pigmentation defects and melanomas using primary melanocytes prepared from easily accessed adult skin tissues.

This paper reports that the addition of Y-27632 to TIVA medium can significantly increase the yield of melanocytes from adult skin tissues.