Technique borrowed from nature, and honed using artificial intelligence, could spur the development of better drug-delivery systems.
The bacterium Photorhabdus asymbiotica uses molecular spikes to pierce a hole in the membranes of host cells.
Credit: F. Zhang et al./Nature
Researchers have hijacked a molecular ‘syringe’ that some viruses and bacteria use to infect their hosts, and put it to work delivering potentially therapeutic proteins into human cells grown in the laboratory.
“It’s astonishing,” says Feng Jiang, a microbiologist at the Chinese Academy of Medical Sciences Institute of Pathogen Biology in Beijing. “It is a huge breakthrough.”
The technique, published in Nature on 29 March1, could offer a new way to administer protein-based drugs, but will need more testing before it can be used in people. With further optimization, the approach might also be useful for delivering the components needed for CRISPR–Cas9 genome editing.
Difficult delivery
The medical applications of CRISPR are currently limited by the challenges of getting the reagents — the DNA-cutting Cas9 enzyme and a short piece of RNA that guides Cas9 to a specific region in the genome — into cells.
“One of the major bottlenecks for gene editing is delivery,” says study co-author Feng Zhang, a molecular biologist at the Broad Institute of MIT and Harvard in Cambridge, Massachusetts, and an early pioneer of the CRISPR–Cas9 technique. Limited options have restricted most clinical trials to editing genomes in liver, eye or blood cells, because those cells can be reached using the current delivery methods, he says. “The reason we don’t see brain or kidney diseases getting tackled is because we don’t have good delivery systems.”
While Zhang and his collaborators searched for ways of transporting proteins into human cells, microbiologists were learning more about an unusual group of bacteria that use molecular spikes to pierce a hole in the membranes of host cells. The bacteria then transport proteins through the perforation and into the cell, exploiting the host’s physiology in their favour.
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Last year, Jiang and his colleagues reported that they could manipulate this syringe-like system in the bioluminescent bacterium Photorhabdus asymbiotica, loading proteins of their choosing from mammals, plants and fungi into the syringe2. Normally, the bacterium lives inside nematodes and uses its syringe to transport a toxin into the cells of insects infected by the nematode. The toxin kills the insect, and the nematode eats the remains. “The bacterium can be viewed as a hired gun to kill this insect,” says co-author Joseph Kreitz, a molecular biologist at the Massachusetts Institute of Technology in Cambridge.
In Zhang’s lab, Kreitz and his collaborators were working on ways to engineer the P. asymbiotica molecular syringe so that it would recognize human cells. They focused on a region of the syringe called the tail fibre, which normally binds to a protein found on insect cells. Using the artificial-intelligence program AlphaFold, which predicts protein structures, the team designed ways to modify the tail fibre so that it would recognize mouse and human cells instead. “Once we had the image, it was very easy to modify it for our uses,” says Kreitz. “That was the moment when it all came together.”
They then loaded the syringes with various proteins, including Cas9 and toxins that could be used to kill cancer cells, and delivered them into human cells grown in the lab, and into the brains of mice.
Flexible system
The system was unable to transport the mRNA guide needed for CRISPR–Cas9 genome editing, but the team is developing ways to do this, says Kreitz. The fact that the system was able to ferry Cas9 into cells speaks to the technique’s flexibility, he adds, given that the Cas9 protein is about five times larger than the syringes’ usual cargo.
The syringe story is reminiscent of the way that researchers such as Zhang developed CRISPR–Cas9 — a system that many microorganisms rely on in nature to defend against viruses and other pathogens — for use as a genome-editing technique, says Asaf Levy, a computational microbiologist at the Hebrew University of Jerusalem. Similar to the early days of CRISPR–Cas9 research, the bacterial syringes are studied by only a handful of labs, and their roles in microbial ecology are only beginning to be understood.
Yet they could have a transformational effect on medicine, says Levy. “The evolution of this thing is quite amazing,” he says. “The fact that you can engineer both the payload and the specificity is ultracool.”
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