The video outlines a detailed protocol for preparing sapphire discs with HeLa cells for imaging and analysis of protein distribution. It involves several steps, including washing, incubation with reducing and gold precursor solutions, polymerization in epoxy resin, and freeze-thaw techniques, leading to the visualization of proteins in cellular structures.

gold nanoparticle synthesis for mammalian cells using sapphire discs

Direct Synthesis of Gold Nanoparticles for Protein Localization Analysis

In the postgenomic era, the development of green fluorescent protein (GFP) as a single-molecule reporter for light microscopy has revolutionized the field of modern life science research1,2. For decades, electron microscopy (EM) has been a powerful tool for intuitively observing the cellular ultrastructure with nanoscale resolution3; however, the precise identification and localization of protein molecules remain challenging.
The most commonly used EM labeling technique is the immunoelectron microscopy (IEM) labeling technique, which is based on the antigen-antibody reaction. However, although many techniques have been developed in the field of IEM labeling, including pre-embedding IEM and post-embedding IEM (on resin sections or hydrated cryosections), it still suffers from low labeling efficiency (&#60;10%)4,5, which is related to the sample preparation and antibody quality. To overcome these limitations, developing genetically encoded tags has great application potential.
Two main types of EM tags have been thoroughly explored in recent years. One type is the DAB staining method, which utilizes tags such as APEX2 to oxidize 3,3&#39;-diaminobenzidine (DAB) to osmiophilic polymers for EM visualization6,7,8,9,10,11,12. It enables the labeling of high-abundance proteins in subcellular regions but is not suitable for single-molecule counting. The other type utilizes metal-binding proteins, such as ferritin13 and metallothionein (MT)14,15,16,17,18,19,20,21,22,23,24,25,26, to generate electron-dense metal deposits in situ for EM visualization. Only the latter has real potential for single-molecule visualization and counting. The molecular size of ferritin is too large (~450 kD) for it to be used as a promising tag, whereas the small size (~5 kD) of MT and its ability to bind various ions through its 20 cysteines have attracted great attention. Several labs have tried to label purified MT-fusion proteins or MT-expressing cells by incubating directly with Au+. These attempts have initially proved that MT tags can bind gold ions to form high-contrast signals, but none have really achieved the effective identification of individual proteins in cells, and they are not widely applicable14,15,16,17,18,19,20,21,22,23.
The ANSM-based AuNP synthesis technique, which involves synthesizing 2-6 nm-sized AuNPs directly on cysteine-rich tags (e.g., MT, the MT variants MTn and MT&#945;, AFP) as electron-dense labels for EM visualization, is the first reliable and applicable approach for protein labeling and single molecule detection in cells24,25,26. It allows for the specific synthesis of AuNPs on isolated tag fusion proteins and has achieved an unprecedented labeling efficiency in nonfixed or chemically fixed prokaryotic (E. coli) and eukaryotic (S. pombe) cells. However, implementing the same protocol in more advanced systems such as mammalian cells or even tissues involves additional challenges, such as the more complex intracellular redox homeostasis and more fragile cellular structure.
This study presents a clonable EM labeling technology, which combines the novel ANSM-based AuNP synthesis technique for labeling genetically encoded cysteine-rich tags (MT) with the HPF/FSF-rehydration-HPF/FSF sample preparation method, enables the unambiguous single-molecule identification of tagged proteins in the ER membrane, ER lumen, and mitochondrial matrix in HeLa cells. The current method combines the characteristics of high labeling efficiency, a high signal-to-noise ratio, single-molecule labeling, and strong universality, and this method has broad application prospects in life science research.

Author Spotlight: Direct Synthesis of EM-Visible Gold Nanoparticles in Cells for Protein Localization Analysis with Well-Preserved Ultrastructure  

Direct Synthesis of EM-Visible Gold Nanoparticles in Cells for Protein Localization Analysis with Well-Preserved Ultrastructure

Protein detection and localization are essential in biological research. Our protocol describes a novel EM-labeling technology using genetically-encoded tags which allows single-molecule visualization in situ with well-preserved ultrastructure. It is crucial to avoid over-fixation.The labeling method is based on the activity of cysteines in the tags, which are highly sensitive to aldehyde cross-linking. Our protocol avoids use of aldehyde fixatives and preserves the tag activity and ultrastructure. at the same time.EM labeling has always been challenging, suffering from poor morphology or inefficient labeling. Our protocols synthesizes in situ nanoparticles directly on individual proteins to provide clear and precise localization of the target proteins, with a high signal-to-noise ratio, high efficiency, and specificity. We have so far achieved excellent labeling in isolated proteins, E.coli cells, yeast cells, and HeLa cells.We will explore and optimize the method for application to tissue samples. Combining the prevailing cryo-FIB and cryo-EM technologies will allow us to address important biological questions.

Watch this Scientific Journal Video about Gold Nanoparticle Synthesis for Mammalian Cells Using Sapphire Discs at JoVE.com

Gold Nanoparticle Synthesis for Mammalian Cells Using Sapphire Discs  

Take a pre-prepared HPF and FSF treated sapphire disc containing stable HeLa cells, wash it with PBS-A buffer for five minutes, and repeat this process three times. Transfer the sapphire discs to a 35-millimeter culture dish containing one milliliter of PBS-A buffer at room temperature. Replace the PBS-A buffer in the culture dish with one milliliter of reducing solution and incubate for one hour at room temperature.Prepare the gold precursor in one milliliter of reducing solution and immediately mix by vortexing. Add 80 microliters of 500-millimolar D-Penicillamine in double distilled water to the gold precursor solution. And immediately vortex.Replace the solution in the cultured dish with one milliliter of gold precursor solution and incubate for two hours at four degrees Celsius. Add 20 to 100 microliters of the freshly prepared sodium borohydride solution to the gold precursor solution, immediately shake to mix well, and incubate for five minutes at room temperature. Replace the solution with two milliliters of PBS-A buffer to stop the reaction.Then prepare the epoxy resin mixture according to the manufacturer's instructions, transfer the sapphire discs to flat bottom embedding capsules, and infiltrate with resin for at least 30 minutes at room temperature. Polymerize the specimen at 60 degrees Celsius in an oven for at least 18 hours. Prepare ultra-thin sections of the polymerized specimen by trimming the specimen blocks carefully using a razor to expose the sapphire discs.Dip the block tips with sapphire discs into liquid nitrogen for several seconds and thaw room temperature. Repeat the freeze-thaw action several times to detach the discs from the blocks. The EGFP-MTn-KDEL protein appeared as two to five-nanometer sized gold nanoparticles exclusively distributed in the peripheral ER lumen and in the perinuclear space of the nuclear envelope.The well-preserved ultra structure enabled the single molecule identification of tagged proteins and facilitated the analysis of organelle interactions, such as ER mitochondria interactions. Nanoparticles of the Ost4-EGFP-MTn protein delineated the ER membrane in the outer membrane of the nuclear envelope. Likewise, the Mito-acGFP-MTn expressing cells exhibited specific labeling in the mitochondrial matrix.

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Research

JoVE Journal

Biology

73 Görüntüleme.  National Institute of Biological Sciences, Beijing. Stabil HeLa hücreleri içeren önceden hazırlanmış bir HPF ve FSF ile muamele edilmiş safir disk alın, PBS-A tamponu ile beş dakika yıkayın ve bu işlemi üç kez tekrarlayın. Safir diskleri oda sıcaklığında bir mililitre PBS-A tamponu içeren 35 milimetrelik bir kültür kabına aktarın. Kültür kabındaki PBS-A tamponunu bir mililitre indirgeyici çözelti ile değiştirin ve oda sıcaklığında bir saat inkübe edin.Altın öncülünü bir mililitre indirgeyici çö...