This work was supported by funds from the Research Grants Council Hong Kong, CUHK direct grant scheme, United College endowment fund, and the TUYF Charitable Trust. The figures in this work were adapted from our previous publication, &#34;ARF6-Rac1 signaling-mediated neurite outgrowth is potentiated by the neuronal adaptor FE65 through orchestrating ARF6 and ELMO1&#34; published in the FASEB Journal in October 2020.

1. Mammalian cell culture and transfection
<ol>
	<li>Plate 2 x 106 cells in a 100 mm culture dish. Use four dishes for each transfection. 
	NOTE: The number of cells required may vary for different cell lines. Optimization may be necessary before proceeding to the isolation step.</li>
	<li>The next day, transfect the cells with Lipofectamine according to the manufacturer&#39;s instructions.</li>
</ol>
2. Cell harvest
<ol>
	<li>Discard the culture medium 48 h post-transfection.</li>
	<li>Wash the cells with ice-cold PBS (10 mM sodium phosphates, 2.68 mM potassium chloride, 140 mM sodium chloride) twice.</li>
	<li>Add 1 mL of ice-cold PBS+ (PBS supplemented with 0.5x protease inhibitor cocktail and 0.5x phosphatase inhibitor cocktail) to each dish.</li>
	<li>Collect the cells with a cell scraper and transfer the cell suspension to a 15 mL centrifuge tube.</li>
	<li>Pellet the cells by centrifugation using a swing bucket rotor at 400 x g for 5 min.</li>
	<li>Discard the supernatant and resuspend the cell pellet gently in 5 mL of homogenization buffer (HB; 250 mM sucrose, 3 mM imidazole at pH 7.4, 1 mM EDTA supplemented with 0.03 mM cycloheximide, 1x protease inhibitor cocktail, and 1x phosphatase inhibitor cocktail).</li>
	<li>Collect the cells by centrifugation at 1,300 x g for 10 min.</li>
	<li>Resuspend the cell pellet in 1 mL of HB.</li>
	<li>Homogenize the cells with a Dounce homogenizer for 15-20 strokes. 
	NOTE: Other homogenization methods, for example, passing the sample through a syringe, could be used. The homogenization efficiency could be revealed by observing the homogenate under a phase-contrast microscope.</li>
	<li>Transfer the homogenate to a 2 mL centrifugation tube. 
	NOTE: Harvest 50 &#181;L of homogenate with 12.5 &#181;L of 5x sample buffer and label it as total lysate.</li>
	<li>Add 0.7 mL of HB to the homogenate.</li>
	<li>Spin the diluted homogenate at 2,000 x g for 10 min at 4 &#176;C. 
	NOTE: The pellet contains nuclei and unbroken cells.</li>
	<li>Collect 1.5 mL of the supernatant and repeat step 2.12 once.</li>
	<li>Collect 1.4 mL of the supernatant and label it as post-nuclear supernatant (PNS).</li>
</ol>
3. Density gradient column preparation
<ol>
	<li>Transfer 1.2 mL of PNS to an ultracentrifuge tube.</li>
	<li>Add 1 mL of 62% sucrose solution (2.351 M sucrose, 3 mM imidazole at pH 7.4) to the sample and mix well by gentle pipetting. 
	NOTE: The resultant solution is a 40.6% sucrose solution.</li>
	<li>Add 3.3 mL of 35% sucrose solution (1.177 M sucrose, 3 mM imidazole at pH 7.4) carefully on top of the sample.</li>
	<li>Add 2.2 mL of 25% sucrose solution (0.806 M sucrose, 3 mM imidazole at pH 7.4) carefully on top of the 35% sucrose solution. 
	NOTE: The refractive index of the 62%, 40.6%, 35%, and 25% sucrose solutions at room temperature are 1.44, 1.40, 1.39, and 1.37, respectively. The refractive indexes of the sucrose solutions&#160;could be checked with a refractometer to ensure the precision and consistency of the experiment.</li>
	<li>Fill up the ultracentrifugation tube with HB. 
	&#8203;NOTE: Temporarily store the prepared density gradient column at 4 &#176;C.</li>
</ol>
4. Fractionation and recovery of recycling endosome-containing fraction
<ol>
	<li>Centrifuge the column at 210,000 x g for 3 h at 4 &#176;C.</li>
	<li>Collect 12 fractions (1 mL each) carefully, starting from the top of the gradient. 
	NOTE: The recycling endosomes should be found at the interface between 35% and 25% sucrose solutions. The collected fractions can be snap-frozen in liquid nitrogen and stored at -80 &#176;C.</li>
	<li>Dilute all the fractions with 1 mL of dilution buffer (3 mM imidazole at pH 7.4, 1 mM EDTA).</li>
	<li>Centrifuge the diluted sample at 100,000 x g for 1 h at 4 &#176;C.</li>
	<li>Aspirate the supernatant and add 50 &#181;L of 1x sample buffer to harvest the fractions.</li>
	<li>Analyze the protein contents in the fractions by western blotting.</li>
</ol>

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The authors declare that they have no conflicts of interest with the contents of this article.

The above protocol outlines the procedures for isolating recycling endosomes from cultured cells by ultracentrifugation. The reliability of this method has been demonstrated by the latest publication22, proving that recycling endosomes are successfully isolated from other organelles (Figure 1), such as the Golgi apparatus and mitochondria. Some critical steps need to be paid attention to for obtaining a good separation result. While preparing the sucrose solutions, it...

Endosomal trafficking is an essential physiological process that implicates various biological events1, for example, the transportation of signaling receptors, ion channels, and adhesion molecules. Proteins localized at the plasma membrane are internalized by endocytosis2. The internalized proteins are then sorted by the early endosome3. Some of the proteins are targeted to lysosomes for degradation4. However, a significant amount of proteins are recycled back to the cell surface by fast recycling and slow recycling processes. In fast recycling, proteins leave the early endosomes and directly return to the plasma membrane. Conversely, in slow recycling, proteins are first sorted to the endocytic recycling compartment and then transported back to the plasma membrane. Various cargo proteins, for example, clathrin, retromer complex, retriever complex and Wiskott-Aldrich syndrome protein, and SCAR Homologue (WASH) complex, participate in such membrane recycling processes4,5,6,7,8,9. The balance of the endocytosis and recycling event is crucial for cell survival and contributes to various cellular events10, for instance, cell adhesion, cell migration, cell polarity, and signal transduction.
ARF6, a small GTPase, is a reported regulator of endocytic trafficking7,11,12. Of interest, various research groups have illustrated the importance of ARF6 in endocytic recycling13,14,15,16,17. The study aims to investigate the relationship between ARF6-mediated neurite outgrowth and endocytic recycling. The previous report suggests that the activation of ARF6 is upstream to Rac1 activity through acting on ELMO1-dedicator of cytokinesis 1 (DOCK180) complex18. However, how ARF6 triggers ELMO1-DOCK180 mediated Rac1 signaling remains unclear. Density gradient ultracentrifugation was employed to investigate the role of ARF6-mediated endocytic recycling in such process. By using that, the recycling endosome-containing fraction was obtained from cell lysates19. The fraction was subjected to Western blotting for protein content analysis. The immunoblot results revealed that under the presence of FE65, a brain-enriched adaptor protein, active ARF6 substantially increased the level of ELMO1 in the recycling endosome-containing fraction. The following protocol includes the procedures for (1) transfecting mammalian cells; (2) preparing the samples and density gradient columns; and (3) obtaining the recycling endosome-containing fraction.

<table><tbody><tr><td>1 mL, Open-Top Thickwall Polypropylene Tube, 11 x 34 mm</td><td>Beckman Coulter</td><td>347287</td><td></td></tr><tr><td>100 mm tissue culture dish</td><td>SPL</td><td>20100</td><td></td></tr><tr><td>13.2 mL, Certified Free Open-Top Thinwall Ultra-Clear Tube, 14 x 89 mm</td><td>Beckman Coulter</td><td>C14277</td><td></td></tr><tr><td>5x Sample Buffer</td><td>GenScript</td><td>MB01015</td><td></td></tr><tr><td>cOmplete, EDTA-free Protease Inhibitor Cocktail</td><td>Roche</td><td>11873580001</td><td></td></tr><tr><td>COX IV (3E11) Rabbit mAb</td><td>Cell Signaling Technology</td><td>4850S</td><td>Rabbit monoclonal antibody for detecting COX IV.</td></tr><tr><td>Cycloheximide</td><td>Sigma-Aldrich</td><td>C1988</td><td></td></tr><tr><td>Dounce Tissue Grinder, 7 mL</td><td>DWK Life Sciences</td><td>357542</td><td></td></tr><tr><td>Dulbecco&#039;s Modified Eagle Medium (DMEM) with low glucose</td><td>HyClone</td><td>SH30021.01</td><td></td></tr><tr><td>ELMO1 antibody (B-7)</td><td>Santa Cruz Biotechnology</td><td>SC-271519</td><td>Mouse monoclonal antibody for detecting ELMO1.</td></tr><tr><td>EndoFree Plasmid Maxi Kit</td><td>QIAGEN</td><td>12362</td><td></td></tr><tr><td>FE65 antibody (E-20)</td><td>Santa Cruz Biotechnology</td><td>SC-19751</td><td>Goat polyclonal antibody for detecting FE65.</td></tr><tr><td>Fetal Bovine Serum, Research Grade</td><td>HyClone</td><td>SV30160.03</td><td></td></tr><tr><td>GAPDH Monoclonal Antibody (6C5)</td><td>Ambion</td><td>AM4300</td><td>Mouse monoclonal antibody for detecting GAPDH.</td></tr><tr><td>ImageLab Software</td><td>Bio-Rad</td><td></td><td>Measurement of band intensity</td></tr><tr><td>Imidazole</td><td>Sigma-Aldrich</td><td>I2399</td><td></td></tr><tr><td>Lipofectamine 2000 Transfection Reagent</td><td>Invitrogen</td><td>11668019</td><td></td></tr><tr><td>Monoclonal Anti-&amp;beta;-COP antibody</td><td>Sigma</td><td>G6160</td><td>Mouse monoclonal antibody for detecting &amp;beta;-COP.</td></tr><tr><td>Myc-tag (9B11) mouse mAb</td><td>Cell Signaling Technology</td><td>2276S</td><td>Mouse monoclonal antibody for detecting myc tagged proteins.</td></tr><tr><td>OmniPur EDTA, Disodium Salt, Dihydrate</td><td>Calbiochem</td><td>4010-OP</td><td></td></tr><tr><td>Optima L-100 XP</td><td>Beckman Coulter</td><td>392050</td><td></td></tr><tr><td>Optima MAX-TL</td><td>Beckman Coulter</td><td>A95761</td><td></td></tr><tr><td>Opti-MEM I Reduced Serum Media</td><td>Gibco</td><td>31985070</td><td></td></tr><tr><td>PBS Tablets</td><td>Gibco</td><td>18912014</td><td></td></tr><tr><td>PhosSTOP</td><td>Roche</td><td>4906845001</td><td></td></tr><tr><td>RAB11A-Specific Polyclonal antibody</td><td>Proteintech</td><td>20229-1-AP</td><td>Rabbit polyclonal antibody for detecting Rab11.</td></tr><tr><td>Sucrose</td><td>Affymetrix</td><td>AAJ21931A4</td><td></td></tr><tr><td>SW 41 Ti Swinging-Bucket Rotor</td><td>Beckman Coulter</td><td>331362</td><td></td></tr><tr><td>TLA-120.2 Fixed-Angle Rotor</td><td>Beckman Coulter</td><td>362046</td><td></td></tr><tr><td>Trypsin-EDTA (0.05%), phenol red</td><td>Gibco</td><td>25300062</td><td></td></tr></tbody></table>

density gradient ultracentrifugation for investigating endocytic recycling in mammalian cells

Endosomal trafficking is an essential cellular process that regulates a broad range of biological events. Proteins are internalized from the plasma membrane and then transported to the early endosomes. The internalized proteins could be transited to the lysosome for degradation or recycled back to the plasma membrane. A robust endocytic recycling pathway is required to balance the removal of membrane materials from endocytosis. Various proteins are reported to regulate the pathway, including ADP-ribosylation factor 6 (ARF6). Density gradient ultracentrifugation is a classical method for cell fractionation. After the centrifugation, organelles are sedimented at their isopycnic surface. The fractions are collected and used for other downstream applications. Described here is a protocol to obtain a recycling endosome-containing fraction from transfected mammalian cells using density gradient ultracentrifugation. The isolated fractions were subjected to standard Western blotting for analyzing their protein contents. By employing this method, we identified that the plasma membrane targeting of engulfment and cell motility 1 (ELMO1), a Ras-related C3 botulinum toxin substrate 1 (Rac1) guanine nucleotide exchange factor, is through ARF6-mediated endocytic recycling.

After fractionating the untransfected HEK293 cells by density gradient ultracentrifugation, 12 fractions were collected starting from the top of the gradient. The harvested fractions were diluted with the dilution buffer in a 1:1 ratio and subjected to a second round of centrifugation. The samples were then subjected to western blotting for analyzing their protein contents. As shown in Figure 1, the recycling endosome marker Rab11 is detected in fraction 720. Other su...

Watch this Scientific Journal Video about Density Gradient Ultracentrifugation for Investigating Endocytic Recycling in Mammalian Cells at JoVE.com

Density Gradient Ultracentrifugation for Investigating Endocytic Recycling in Mammalian Cells

This paper aims to present a protocol for preparing recycling endosomes from mammalian cells using sucrose density gradient ultracentrifugation.

Endosomal trafficking is an essential cellular process that regulates a broad range of biological events. Proteins are internalized from the plasma ...

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Research

JoVE Journal

Biochemistry

4.0K Views.  The Chinese University of Hong Kong. This paper aims to present a protocol for preparing recycling endosomes from mammalian cells using sucrose density gradient ultracentrifugation.

Video: Density Gradient Ultracentrifugation for Investigating Endocytic Recycling in Mammalian Cells

Extraction of Extracellular Vesicles from Whole Tissue

Circulating and interstitial small membrane-bound extracellular vesicles (EVs) represent promising targets for the development of novel diagnostic or prognostic biomarker assays, and likely serve as important players in the progression of a vast spectrum of diseases. Current research is focused on the characterization of vesicles secreted from multiple cell and tissue types in order to better understand the role of EVs in the pathogenesis of conditions including neurodegeneration, inflammation, and cancer. However, globally consistent and reproducible techniques to isolate and purify vesicles remain in progress. Moreover, methods for extraction of EVs from solid tissue ex vivo are scarcely described. Here, we provide a detailed protocol for extracting small EVs of interest from whole fresh or frozen tissues, including brain and tumor specimens, for further characterization. We demonstrate the adaptability of this method for multiple downstream analyses, including electron microscopy and immunophenotypic characterization of vesicles, as well as quantitative mass spectrometry of EV proteins.

Here, we provide a detailed protocol to isolate small extracellular vesicles (EVs) from whole tissues, including brain and tumor specimens. This method offers a reproducible technique to extract EVs from solid tissue for further downstream analyses.

Circulating and interstitial small membrane-bound extracellular vesicles (EVs) represent promising targets for the development of novel diagnostic or ...

Cancer Research

Cell Fractionation of U937 Cells in the Absence of High-speed Centrifugation

In this protocol we detail a method to obtain subcellular fractions of U937 cells without the use of ultracentrifugation or indiscriminate detergents. This method utilizes hypotonic buffers, digitonin, mechanical lysis and differential centrifugation to isolate the cytoplasm, mitochondria and plasma membrane. The process can be scaled to accommodate the needs of researchers, is inexpensive and straightforward. This method will allow researchers to determine protein localization in cells without specialized centrifuges and without the use of commercial kits, both of which can be prohibitively expensive. We have successfully used this method to separate cytosolic, plasma membrane and mitochondrial proteins in the human monocyte cell line U937.

Here, we present a protocol to isolate the plasma membrane, cytoplasm and mitochondria of U937 cells without the use of high-speed centrifugation. This technique can be used to purify subcellular fractions for subsequent examination of protein localization via immunoblotting.

In this protocol we detail a method to obtain subcellular fractions of U937 cells without the use of ultracentrifugation or indiscriminate detergents. ...

Cell Fractionation of U937 Cells by Isopycnic Density Gradient Purification

This protocol describes a method to obtain subcellular protein fractions from mammalian cells using a combination of detergents, mechanical lysis, and isopycnic density gradient centrifugation. The major advantage of this procedure is that it does not rely on the sole use of solubilizing detergents to obtain subcellular fractions. This makes it possible to separate the plasma membrane from other membrane-bound organelles of the cell. This procedure will facilitate the determination of protein localization in cells with a reproducible, scalable, and selective method. This method has been successfully used to isolate cytosolic, nuclear, mitochondrial, and plasma membrane proteins from the human monocyte cell line, U937. Although optimized for this cell line, this procedure may serve as a suitable starting point for the subcellular fractionation of other cell lines. Potential pitfalls of the procedure and how to avoid them are discussed as are alterations that may need to be considered for other cell lines.

This fractionation protocol will allow researchers to isolate cytoplasmic, nuclear, mitochondrial, and membrane proteins from mammalian cells. The latter two subcellular fractions are further purified via isopycnic density gradient.

This protocol describes a method to obtain subcellular protein fractions from mammalian cells using a combination of detergents, mechanical lysis, and ...

Eukaryotic Polyribosome Profile Analysis

Protein synthesis is a complex cellular process that is regulated at many levels. For example, global translation can be inhibited at the initiation phase or the elongation phase by a variety of cellular stresses such as amino acid starvation or growth factor withdrawal. Alternatively, translation of individual mRNAs can be regulated by mRNA localization or the presence of cognate microRNAs. Studies of protein synthesis frequently utilize polyribosome analysis to shed light on the mechanisms of translation regulation or defects in protein synthesis. In this assay, mRNA/ribosome complexes are isolated from eukaryotic cells.  A sucrose density gradient separates mRNAs bound to multiple ribosomes known as polyribosomes from mRNAs bound to a single ribosome or monosome. Fractionation of the gradients allows isolation and quantification of the different ribosomal populations and their associated mRNAs or proteins. Differences in the ratio of polyribosomes to monosomes under defined conditions can be indicative of defects in either translation initiation or elongation/termination. Examination of the mRNAs present in the polyribosome fractions can reveal whether the cohort of individual mRNAs being translated changes with experimental conditions. In addition, ribosome assembly can be monitored by analysis of the small and large ribosomal subunit peaks which are also separated by the gradient. In this video, we present a method for the preparation of crude ribosomal extracts from yeast cells, separation of the extract by sucrose gradient and interpretation of the results.  This procedure is readily adaptable to mammalian cells.

This article describes a protocol for the extraction of translating ribosomes from eukaryotic cells. Once extracted, ribosomes are separated into monosomes and polyribosomes by sucrose gradient fractionation to allow different ribosomal populations to be analyzed. As such, this method is the gold standard for examining the regulation of translation.

Protein synthesis is a complex cellular process that is regulated at many levels. For example, global translation can be inhibited at the initiation phase ...

Biology

Isolation of Mitochondrial Contact Sites

An Improved Method to Isolate Mitochondrial Contact Sites

Mitochondria are present in virtually all eukaryotic cells and perform essential functions that go far beyond energy production, for instance, the synthesis of iron-sulfur clusters, lipids, or proteins, Ca2+ buffering, and the induction of apoptosis. Likewise, mitochondrial dysfunction results in severe human diseases such as cancer, diabetes, and neurodegeneration. In order to perform these functions, mitochondria have to communicate with the rest of the cell across their envelope, which consists of two membranes. Therefore, these two membranes have to interact constantly. Proteinaceous contact sites between the mitochondrial inner and outer membranes are essential in this respect. So far, several contact sites have been identified. In the method described here, Saccharomyces cerevisiae mitochondria are used to isolate contact sites and, thus, identify candidates that qualify for contact site proteins. We used this method to identify the mitochondrial contact site and cristae organizing system (MICOS) complex, one of the major contact site-forming complexes in the mitochondrial inner membrane, which is conserved from yeast to humans. Recently, we further improved this method to identify a novel contact site consisting of Cqd1 and the Por1-Om14 complex.

Author Spotlight: Unveiling Mitochondrial Contact Sites and Architectural Insights 

Mitochondrial contact sites are protein complexes that interact with mitochondrial inner and outer membrane proteins. These sites are essential for the communication between the mitochondrial membranes and, thus, between the cytosol and the mitochondrial matrix. Here, we describe a method to identify candidates qualifying for this specific class of proteins.

Mitochondria are present in virtually all eukaryotic cells and perform essential functions that go far beyond energy production, for instance, the ...

Enrichment of Native and Recombinant Extracellular Vesicles of Mycobacteria

Most bacteria, including mycobacteria, generate extracellular vesicles (EVs). Since bacterial EVs (bEVs) contain a subset of cellular components, including metabolites, lipids, proteins, and nucleic acids, several groups have evaluated either the native or recombinant versions of bEVs for their protective potency as subunit vaccine candidates. Unlike native EVs, recombinant EVs are molecularly engineered to contain one or more immunogens of interest. Over the last decade, different groups have explored diverse approaches for generating recombinant bEVs. However, here, we report the design, construction, and enrichment of recombinant mycobacterial EVs (mEVs) in mycobacteria. Towards that, we use Mycobacterium smegmatis (Msm), an avirulent soil mycobacterium as the model system. We first describe the generation and enrichment of native EVs of Msm. Then, we describe the design and construction of recombinant mEVs that contain either mCherry, a red fluorescent reporter protein, or EsxA (Esat-6), a prominent immunogen of Mycobacterium tuberculosis. We achieve this by separately fusing mCherry and EsxA N-termini with the C-terminus of a small Msm protein Cfp-29. Cfp-29 is one of the few abundantly present proteins of MsmEVs. The protocol to generate and enrich recombinant mEVs from Msm remains identical to the generation and enrichment of native EVs of Msm.

This protocol details the enrichment of native mycobacterial extracellular vesicles (mEVs) from axenic cultures of Mycobacterium smegmatis (Msm) and how mCherry (a red fluorescent reporter)-containing recombinant MsmEVs can be designed and enriched. Lastly, it verifies the novel approach with the enrichment of MsmEVs containing the EsxA protein of Mycobacterium tuberculosis.

Most bacteria, including mycobacteria, generate extracellular vesicles (EVs). Since bacterial EVs (bEVs) contain a subset of cellular components, ...

Immunology and Infection

Assessment of Selective mRNA Translation in Mammalian Cells by Polysome Profiling

Regulation of protein synthesis represents a key control point in cellular response to stress. In particular, discreet RNA regulatory elements were shown to allow to selective translation of specific mRNAs, which typically encode for proteins required for a particular stress response. Identification of these mRNAs, as well as the characterization of regulatory mechanisms responsible for selective translation has been at the forefront of molecular biology for some time. Polysome profiling is a cornerstone method in these studies. The goal of polysome profiling is to capture mRNA translation by immobilizing actively translating ribosomes on different transcripts and separate the resulting polyribosomes by ultracentrifugation on a sucrose gradient, thus allowing for a distinction between highly translated transcripts and poorly translated ones. These can then be further characterized by traditional biochemical and molecular biology methods. Importantly, combining polysome profiling with high throughput genomic approaches allows for a large scale analysis of translational regulation.

The ability of cells to adapt to stress is crucial for their survival. Regulation of mRNA translation is one such adaptation strategy, providing for rapid regulation of the proteome. Here, we provide a standardized polysome profiling protocol to identify specific mRNAs that are selectively translated under stress conditions.

Regulation of protein synthesis represents a key control point in cellular response to stress. In particular, discreet RNA regulatory elements were shown ...

Enhanced Bacterial Extracellular Vesicles Isolation for Microbiota Multi-Omics

Isolation and Purification of Bacterial Extracellular Vesicles from Human Feces Using Density Gradient Centrifugation

Bacterial extracellular vesicles (BEVs) are nanovesicles derived from bacteria that play an active role in bacteria-bacteria and bacteria-host communication, transferring bioactive molecules such as proteins, lipids, and nucleic acids inherited from the parent bacteria. BEVs derived from the gut microbiota have effects within the gastrointestinal tract and can reach distant organs, resulting in significant implications for physiology and pathology. Theoretical investigations that explore the types, quantities, and roles of BEVs derived from human feces are crucial for understanding the secretion and function of BEVs from the gut microbiota. These investigations also necessitate an improvement in the current strategy for isolating and purifying BEVs.
This study optimized the isolation and purification process of BEVs by establishing two density gradient centrifugation (DGC) modes: Top-down and Bottom-up. The enriched distribution of BEVs was determined in fractions 6 to 8 (F6-F8). The effectiveness of the approach was evaluated based on particle morphology, size, concentration, and protein content. The particle and protein recovery rates were calculated, and the presence of specific markers was analyzed to compare the recovery and purity of the two DGC modes. The results indicated that the Top-down centrifugation mode had lower contamination levels and achieved a recovery rate and purity similar to that of the Bottom-up mode. A centrifugation time of 7 h was sufficient to achieve a fecal BEV concentration of 108/mg.
Apart from feces, this method could be applied to other body fluid types with proper modification according to the differences in components and viscosity. In conclusion, this detailed and reliable protocol would facilitate the standardized isolation and purification of BEVs and thus, lay a foundation for subsequent multi-omics analysis and functional experiments.

Author Spotlight: Accelerating Research on Bacterial Extracellular Vesicles Separation and Heterogeneity 

This study describes a method to isolate and purify bacterial extracellular vesicles (BEVs) enriched from human feces via density gradient centrifugation (DGC), identifies the physical characteristics of BEVs from morphology, particle size, and concentration, and discusses the potential applications of the DGC approach in clinical and scientific research.

Bacterial extracellular vesicles (BEVs) are nanovesicles derived from bacteria that play an active role in bacteria-bacteria and bacteria-host ...

Density Gradient Ultracentrifugation

Density gradient ultracentrifugation is a common technique used to isolate and purify biomolecules and cell structures. This technique exploits the fact that, in suspension, particles that are more dense than the solvent will sediment, while those that are less dense will float. A high-speed ultracentrifuge is used to accelerate this process in order to separate biomolecules within a density gradient, which can be established by layering liquids of decreasing density in a centrifuge tube.
The video will cover the principles of density gradient ultracentrifugation, including a procedure that demonstrates sample preparation, creation of a sucrose gradient, ultracentrifugation, and collection of fractionated analytes. The applications section discusses isolation of multi-protein complexes, isolation of nucleic acid complexes, and separation using cesium chloride density gradients.
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Density gradient ultracentrifugation is a common approach to isolate and purify cell structures for biochemical experiments. The technique uses a high-speed, or ultra, centrifuge to nondestructively separate cellular components in a density gradient. This video describes the principles of density gradient ultracentrifugation, provides a general procedure using a sucrose gradient, and discusses some applications.
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Let&#39;s start by examining the principles of ultracentrifuges and density gradients. A suspension contains particles in a liquid solvent. Because of gravity, particles denser than the solvent sediment out while those less dense than the solvent float. The greater the difference in density between the particle and the solvent, the faster the separation.
An ultracentrifuge contains a unit called a rotor, which rotates at highly controlled speeds, simulating a strong gravitational field. Within this field, the differences in density between particles and the solvent are magnified.
The strength of the field depends on the speed of rotation. Even a small rotor at a relatively low rotational speed can create a force thousands of times stronger than the earth&#39;s gravitational field.
If a tube contains several liquids of different densities, centrifugation will keep them in separate layers in order of density, with the densest liquid closest to the base. Such a layering of multiple liquids is called a &#34;density gradient.&#34; There are two types. In step gradients, liquids of decreasing density are carefully layered on top one another. In continuous gradients, the liquids are mixed in varying proportions, so the density decreases smoothly from the base upwards.
Cellular organelles can be separated using a step gradient, through &#34;isopycnic density-gradient centrifugation.&#34; This is the simplest and most common centrifugation procedure.
This procedure is used to separate the cellular structures. The more dense the organelle, the further it descends-with mitochondria at the top and nucleic acids towards the bottom.
Now that you know the principles behind the technique, let&#39;s see it in the lab.
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Before the procedure is started, the manufacturer&#39;s speed and density ratings should be noted, and the ultracentrifuge checked for corrosion. This procedure uses a swinging-bucket rotor.
First, the cellular material is prepared by homogenizing the cells, which nondestructively releases their organelles. The homogenate may be fractionated through preliminary low-speed centrifugation, to remove low-density components. Next, the sucrose solutions are prepared.
Sucrose is added in increasing amounts so each solution is more concentrated, and therefore denser, than the preceding one. The exact densities of the solutions will depend on the components to be separated, which vary between organisms. The solutions should have densities between those of the components to be separated, with the last solution denser than the densest component of the analyte. Techniques for separating components denser than sucrose, like nucleic acids, are described in the applications.
The sucrose gradient is now created in a clean centrifuge tube. A pipette is used to draw up the most concentrated sucrose solution. With the tube held upright, the pipette tip is placed high against the wall, and the liquid dispensed steadily down. It&#39;s important that the working area is kept free of vibrations and other disturbances.
After replacing the tip, the remaining solutions are added in order of decreasing density. They are dispensed carefully to form distinct layers and avoid mixing. Finally, about half a milliliter of the cellular sample is added atop the gradient, and the tube is weighed. This is used to balance the weight distribution, the next step of the process.
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Centrifugation should begin as soon as possible. The tube is placed in the rotor, which is then balanced by placing blank solutions of equal weight in opposing slots. The rotor is placed in the ultracentrifuge and the system sealed. The temperature and rotation speed and time are set. Typical values are 4 &#176;C with a force of over 100,000 x g for 16 h.
After centrifugation, the tube is withdrawn from the rotor, taking care to keep it upright and undisturbed. The different cellular components have fractionated into discrete bands between the solution layers. The fractions can be collected with a syringe. Alternately, the bottom of the tube can be punctured with a fine, sterilized needle and the outflow collected in sterile tubes. The cellular components have now been isolated. They can be stored at -80 &#176;C.
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<div class="chapter_text" id="script_4">
Now that we&#39;ve seen the basic procedure, let&#39;s look at some applications.
A typical application is the isolation of multi-protein complexes in plant cells. In this example, complexes responsible for cyclic electron flow are being isolated from the thylakoid, the site of the light reaction in photosynthesis. This procedure uses discrete solutions of 14 to 45% sucrose. Centrifugation occurs over 100,000 x g for 14 h at 4 &#176;C.
Because nucleic acids are denser than sucrose, isopycnic centrifugation cannot separate them from organelles nondestructively.
A different technique, known as &#34;rate-zonal centrifugation&#34; is used. It separates organelles based on their sedimentation rates, which depend not only on their densities, but also on their conformations. A continuous gradient is used to separate the components based on this property.
The procedural steps are similar to those for isopycnic cases. In this example, RNA-ribosome complexes are isolated using a continuous gradient of 5% to 20%, centrifuged at 230,000 x g. Centrifugation is interrupted after a few hours to prevent co-precipitation.
Nucleic acid strands can be separated from each other on the basis of density.
This is because strands rich in guanine and cytosine are denser than those rich in adenine and thiamine. In this case, the gradient cannot be made of sucrose, because sucrose is less dense than nucleic acids. Instead, cesium chloride gradients, typically from 1.65 to 1.75 g/mL are used, as they have sufficient density and a low viscosity.
Here we see plankton DNA being purified using a continuous cesium chloride gradient. Centrifugation occurs at over 1,000,000 x g for 18 h under vacuum.
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<div class="chapter_text" id="script_5">
You&#39;ve just watched JoVE&#39;s video on ultracentrifugation with a sucrose density gradient. You should now understand how a density gradient works, how to construct a step gradient, and how to load and operate an ultracentrifuge. Thanks for watching!
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Density Gradient Ultracentrifugation: Principle and Applications

Density gradient ultracentrifugation is a common technique used to isolate and purify biomolecules and cell structures. This technique exploits the fact ...

Education

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Chemistry

Gradient Centrifugation Based Purification of Nuclei: A Technique to Obtain Ultrapure Fraction of Intact Nuclei Using Density Gradient Centrifugation

Source: Narayanan, A. et al. <a href="https://www.jove.com/v/61542/nuclei-isolation-from-fresh-frozen-brain-tumors-for-single-nucleus">Nuclei Isolation from Fresh Frozen Brain Tumors for Single-Nucleus RNA-seq and ATAC-seq.</a> J. Vis. Exp. (2020)
In this video, we describe a simple and efficient protocol using gradient centrifugation to isolate single nuclei. Once isolated, these nuclei can be used for sequencing and genome analysis.

Oczyszczanie jąder oparte na wirowaniu gradientowym: technika otrzymywania ultraczystej frakcji nienaruszonych jąder za pomocą wirowania w gradicynacji gęstości

Source: Narayanan, A. et al. Nuclei Isolation from Fresh Frozen Brain Tumors for Single-Nucleus RNA-seq and ATAC-seq. J. Vis. Exp. (2020)
In this video, ...