Going Where siRNA Has Never Gone Before
Small interfering RNA (siRNA) has the potential to treat cancer, along with a number of other conditions. But getting it to the cancerous tissue is a challenge: although tissue-permeable nanocarriers for siRNA have reached depths of up to 20–40 μm, metastatic cancer often spreads to depths of 300 μm from blood vessels. So researchers are working to develop ways to penetrate deeper tissues.
Kou Okuro, Takuzo Aida, and co-workers have developed a nanocarrier that can deliver siRNA to unprecedented depths (DOI:
10.1021/jacs.8b12501). The researchers made the smallest siRNA-containing reactive nanocaplet yet reported by adding I
2 to a preincubated mixture of siRNA and
AzGu, an azide (N
3)-appended macromonomer bearing multiple guanidium ion units, causing oxidative polymerization of
AzGu that attaches it to the siRNA strand. The researchers then attached a bioadhesive dendron and a benzophenone derivative through a click reaction, which in turn allowed them to attach transferin, a protein that induces transcytosis. Using luciferase as a target gene, the researchers show significant knockdown in spheroids of hepatocarcinoma cells at depths of nearly 70 μm. This strategy could allow siRNA to reach tissues that have previously been inaccessible and potentially even cross the blood–brain barrier, since brain endothelial cells have high levels of transferrin receptors.
Follow the Gold: Examining Au Nanocrystals and the Chemistry of Electron Microscopy
Electron microscopy has revolutionized our view of the world at small scales, but it has its limitations. Chief among them is that electron beams scatter off air, requiring samples to be in high vacuum to obtain clear images. Recently, liquid cell electron microscopy has provided a way to overcome this limitation, by using an atomically thin substance such as graphene to create a microscopic liquid cell in an otherwise vacuum environment. This approach comes with its own challenges, though: the very electron beam used for imaging interacts with the contents of the liquid cell to give rise to unintended chemical reactions, potentially contaminating the processes being studied. Worse, little is known about these unintended reactions, making them difficult to account for.
Now Paul Alivisatos and co-workers have probed the chemistry of a liquid cell under an electron beam through the controlled etching of gold nanocrystals (DOI:
10.1021/jacs.9b00082). They placed gold nanocrystals of known size and shape into a graphene-encapsulated liquid cell containing FeCl
3. The oxidative products the electron beam creates in the aqueous solution etch the gold nanocrystals, and the researchers followd the evolution of the gold nanocrystals as they shrink. They examined 150 etching trajectories to provide data on how the process proceeded on average. This work provides both data and a template for chemists to account for the electron beam-driven chemistry in the liquid cell, expanding the reach of this valuable technique.
Graphene Membrane Freezes Biomolecules for (Electron) Camera
A glass slide can trap a tissue or cellular sample for viewing in an optical microscope, but trapping molecules, which are smaller, requires freezing the sample in water ice long enough for an electron microscope to capture an image. But ice does not always trap the molecules in the right way. Instead, the air–water interface may attract the molecules, creating a layer of molecules denatured by the air and which may only show one side to the camera, like vamping fashion models. Researchers have tried using several materials, such as DNA scaffolds and lipid monolayers, to prepare samples for cryo-electron microscopy (cryo-EM), but most require labor-intensive optimization.
Now Nan Liu and co-workers have demonstrated that a graphene membrane studded with 20S proteasome ligands can bind histidine-tagged proteins and complexes for cryo-EM (DOI:
10.1021/jacs.8b13038). The ligands, which the team fixed to the graphene using a previously reported technique of their own, solve the problem of migrating and mis-oriented target biomolecules. The researchers report low background noise and near-atomic-resolution imaging of the test biomolecule. Because the grid captures the target biomolecules, this method might allow for simultaneous biomolecule purification and cryo-EM preparation, simplifying the process and saving time.
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