Some viruses and bacteria use a syringe-like system to infect host cells. Scientists have found a way to repurpose this system to deliver therapeutic proteins to human cells. This breakthrough could help to overcome a major obstacle in fulfilling the promise of CRISPR technology for gene editing and provide a new way to deliver protein-based drugs.
“It’s astonishing,” said Feng Jiang, a microbiologist at the Chinese Academy of Medical Sciences Institute of Pathogen Biology in Beijing. “It is a huge breakthrough.” The research, co-authored by microbiologist Feng Zhang at the Broad Institute of the Massachusetts Institute of Technology (MIT) and Harvard, was published in the journal Nature.
Removing a CRISPR Bottleneck
Zhang was an early pioneer of CRISPR-Cas9 genome editing, which uses the Cas9 enzyme to precisely edit DNA. The technology has significant potential for gene editing and treating a range of diseases. However, “One of the major bottlenecks for gene editing is delivery,” reported Zhang. Researchers have been investigating methods to get the reagents required for the process, including the Cas9 enzyme and the RNA segment that guides it, into cells.
Because cells in the liver, eye, or blood can be reached with existing technology, most gene editing clinical trials have focused on diseases in those organs. “The reason we don’t see brain or kidney diseases getting tackled is because we don’t have good delivery systems,” Zhang remarked.
Harnessing a Bacterial Hired Gun
The model for the delivery device that the researchers used is found in some bacteria. It consists of molecular spikes used to punch holes in host cells’ membranes. The bacteria use the holes to deliver proteins into the cell. Jiang and his colleagues have reported on their work engineering this system in the bioluminescent bacterium Photorhabdus asymbiotica, which lives in nematodes in the guts of insects. In the nematode, the bacterium uses the syringe to deliver its toxin into the insects’ host cells. After the toxin kills the insect, the nematode eats the remains. “The bacterium can be viewed as a hired gun to kill this insect,” said Joseph Kreitz, a molecular biologist at the MIT and co-author of the research.
Although the molecular syringe is designed to work in insect cells, Jiang and his colleagues showed that they could engineer the system to deliver chosen proteins from mammals, plants, and fungi into human cells, demonstrating how it could potentially be used in medical applications.
Applying AI
The researchers used an artificial intelligence program called AlphaFold, which predicts protein structures, to help them modify parts of the syringe known as the tail fibers. These components play an important role in guiding the injector toward the target cell. By applying the AI program, the team was able to modify the syringe so that it could recognize mice and human cells. “That was the moment when it all came together,” said Kreitz.
Potential Medical Uses
Having successfully engineered the syringes, the researchers showed that they could carry various proteins, including Cas9 and cancer-killing toxins, and deliver them into human cells cultivated in the lab, and into the brains of mice. This is especially significant since the Cas9 protein is about five times larger than the proteins the syringes usually carry.
Although the system did not support delivery of the mRNA guide needed for CRISPR-Cas9 genome editing, the researchers are working on this. With further investigation, they say the technique could someday overcome current barriers in gene editing and be used to deliver other protein-based drugs. “It’s still early for this approach, but I think it’s really important to explore [the system’s ability] to be able to treat many different types of diseases that affect human health,” said Zhang.
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