Random mutations turn RNA droplets into cell-like structures that move and store cargo

A small number of random genetic mutations cause simple RNA droplets to transform into cell-like structures. In this process, they form cavities, can move and are able to take up cargo.

Research led by Dr. Kerstin Göpfrich at the Center for Molecular Biology of Heidelberg University (ZMBH), in cooperation with experts from the German Cancer Research Center and the University of Augsburg, has demonstrated this using nanostructures based on ribonucleic acid. Just a few molecular changes are enough to trigger these complex processes in very simple synthetic cells—a principle that the researchers also hope to harness for medical applications.

Building a minimal cell

How life could have emerged from nonliving matter is one of the oldest unanswered questions in science. For this to happen, a minimal cell must at least retain its genetic material while also being able to change, as Göpfrich explains.

Creating such a minimal living cell from scratch using nonliving components is one of the main goals of synthetic biology—the field in which Göpfrich and her research group at the ZMBH work. The molecular biologist uses RNA droplets in her research. These are liquid-like, membraneless nanostructures composed of synthetically produced ribonucleic acid (RNA) that spontaneously self-assemble.

The paper is published in the journal Nature Chemical Engineering.

Mutations create stable cavities

These RNA droplets can encapsulate the genetic template from which they are transcribed. "That establishes a basic prerequisite for a system that can evolve," says William Verstraeten, a doctoral candidate in Göpfrich's research group. The scientists were surprised to find that deliberately modifying the DNA template did not cause the droplets to dissolve, as expected.

"Instead, they formed large cavities known as vacuoles, which remained stable for days or even weeks," says Göpfrich. According to her, a mutation rate of just around 3% was sufficient to produce these complex changes in the primitive synthetic cells.

The team led by Dr. Michael Platten at the German Cancer Research Center used a specialized sequencing technology to determine which mutations were responsible for the structural changes inside the droplets. This made it possible to create a "model mutant" and reliably reproduce the effect.

"The vacuoles form when more and more new RNA enters the already viscous interior of the droplets during transcription. The rapid accumulation of mass can cause local instabilities, from which the cavities form," explains Dr. Christoph Weber from the Institute of Physics at the University of Augsburg.

The experiments conducted by the scientists confirmed that vacuole formation requires RNA droplets to have a radius of more than 20 micrometers and contain more than approximately 2% genetically modified material.

Movement, storage and potential immune functions

Further investigations showed that the cavities redistribute mass inside the droplets, producing tiny movements on the micrometer scale. According to the researchers, mutated RNA droplets traveled significantly farther than nonmutated ones and collided with one another more frequently; as a result, they fused and grew.

The vacuoles also act as internal storage compartments. "They take up small molecules and can store this cargo inside them," says Göpfrich.

"Mutagenesis therefore provides us with a new architecture and new functions that would be very difficult to design and build from scratch," emphasizes the molecular biologist. As part of Heidelberg University's SynthImmune Cluster of Excellence, the researchers aim to harness this process of mutation and selection to generate molecular functions from minimal synthetic components.

"In the future, we want to use this tool to develop synthetic immune effectors capable of evolving specific immune functions on their own."