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Biological Chemistry

Nanoparticles Regulate Gene Expression

Molecular Biology: Synthetic transcription factors built on gold nanoparticles can be designed to turn on particular genes, offering a potential tool for tissue engineers

by Katherine Bourzac
September 3, 2014

Nucleus Seekers
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Credit: ACS Nano
Artificial transcription factors made from gold nanoparticles (green) can slip into the nucleus of a human cell (red) because of a peptide on their surfaces. Both scale bars are 2 µm.
20140903lnp2-Figure4b.jpg
Credit: ACS Nano
Artificial transcription factors made from gold nanoparticles (green) can slip into the nucleus of a human cell (red) because of a peptide on their surfaces. Both scale bars are 2 µm.

Researchers have decorated gold nanoparticles with peptides and polyamides to impart them with all of the natural functions of gene-regulating transcription factors (ACS Nano 2014, DOI: 10.1021/nn501589f). The team hopes to use these artificial transcription factors to program stem cells to create specific tissues or to revert cells back to earlier developmental states.

Transcription factors are master regulators of gene expression, and certain ones play a particularly important role during development, helping to influence the fate of a given stem cell in an embryo. For researchers looking to program stem cells to grow new tissue in the lab, controlling gene expression is key. To do so, some have tried to make synthetic molecules that can perform some of the functions of natural transcription factors.

Transcription factors have three main functions, says KiBum Lee, a chemical biologist at Rutgers University, Piscataway. One domain acts as a kind of subcellular address, assuring that the protein gets delivered to the nucleus, the site of gene transcription. Another domain binds to a specific sequence of DNA on a chromosome that is near the gene to be turned on. Finally, a third part recruits the cellular machinery that transcribes the gene’s DNA into messenger RNA.

Synthetic versions of all three parts exist separately, and researchers previously have used them singly or in pairs to help regulate gene expression. But Lee and his colleagues are the first to bring them all together in a single nanoparticle.

Their nanoparticles, which they call NanoScripts, start with a 9-nm-wide gold core. The researchers then coat the gold particles with mercaptoundeconic acid, which allows them to decorate the surface with any kind of functional molecule using standard amide coupling. In this study, the team coupled two different peptides, one that allows the particles to enter the nucleus and another that recruits RNA polymerase and other proteins involved in transcription. They also coupled a polyamide designed to bind to a DNA sequence corresponding to the genes they planned to activate in their tests.

To test the NanoScripts, Lee’s group engineered a human cell line to contain an alkaline phosphatase gene to serve as a reporter. They could measure the level of expression of this gene through a chemiluminescent assay. Cells treated with NanoScripts expressed the gene at 15-fold higher levels than cells treated with naked gold nanoparticles. NanoScript particles lacking one of the three components produced less than fourfold increases in the reporter gene expression, showing the need for all three molecules in turning on the gene. And cells treated with the DNA-binding polyamide and polymerase-recruiting peptide alone, unanchored to the nanoparticle, showed just a 2.2-fold increase in expression.

Aseem Z. Ansari, a biochemist at the University of Wisconsin, Madison, who also develops synthetic transcription factors, says the study clearly demonstrates that the NanoScripts can regulate gene expression in cells. Down the road, he notes, the team will have to study what happens to the gold nanoparticles after the NanoScripts do their job, to make sure no toxic effects arise.

Lee’s group is now developing NanoScript particles that can regulate gene expression to nudge stem cells toward muscle or nerve cells, as well as to induce adult cells to return to a stem-cell-like state.

Synthetic Transcription Factor
20140903lnp2-Figure1b.jpg
Credit: ACS Nano
New nanoparticles called NanoScripts (right) mimic the main functions of natural transcription factors (left). One peptide on the particles (red cone) serves as a nuclear localization signal (NLS). A second peptide (green cone) plays the part of an activation domain (AD) that recruits the cell’s transcription machinery to a chromosome. A polyamide (pink) acts as a DNA-binding domain (DBD) for a target gene. And the gold nanoparticle (yellow), similar to the linking domain (LD) of a natural protein, holds all of the functional components together.
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