New yeast assembly technique yields living materials
IImagine a dressing that can seal a wound almost instantly. Or a filter to clean up toxic spills that might detect and adapt to its environment. These are just some of the applications that may be possible for materials constructed from living cells.
Artificial living materials (ELMs) can theoretically take on the properties of tissue, which means they can grow and spread. Previously, scientists have successfully engineered cells to fit together in moldable materials, but it has been difficult to precisely control and shape how they fit together without chemical modifications that could damage the cells.
While the scientists were able to fashion ELMs made of bacterial cells by sculpting biofilm-building proteins, directing eukaryotic cells to where they’re supposed to go has been more difficult. In a study published in Scientists progress on November 4, scientists led the genetically modified baker’s yeast (Saccharomyces cerevisiae) to be assembled in ELM. Using microscopic “tweezers,” they were able to precisely control the shape and size of the resulting ELM without chemical modifications.
“It’s really difficult to introduce biological functions into materials,” says Sara Molinari, a synthetic biologist at Rice University who was not involved in the study. Nonetheless, it was a goal worth pursuing because “yeasts are better for certain applications,” says Molinari.
To try to glue eukaryotic cells together, the authors used what are called very high affinity protein-protein (IPP) interactions among four synthetic proteins. previously derived from bacteria. These interactions, as their name suggests, cause the proteins to squeeze together extremely tightly. Proteins come in pairs that form strong PPI bonds between them, like a lock and key: SpyTag and SpyCatcher, and Im7 and CL7. The researchers also took advantage of yeast’s natural tendency to form colonies via weak interactions.
Diagram of assembly of baker’s yeast (Saccharomyces cerevisiae) cells in living multicellular material. The AGA1 and AGA2 proteins allow the display of target proteins, such as SpyTag, SpyCatcher, CL7 and Im7, on the cell surface. AGA2 is naturally expressed in yeast, while AGA1 is tethered to the target protein and covalently binds AGA2. Intercellular IPPs assemble individual cells into networks.
The team cloned genes encoding SpyTag, SpyCatcher, Im7 and CL7 into yeasts, which then started expressing these proteins on their extracellular membranes. Then, the researchers used optical tweezers– a non-invasive technique that uses lasers to manipulate living cells – to bring together individual yeast cells containing complementary PPI-forming proteins and to separate other cells. These tweezers allowed researchers to measure the strength of interactions between cells while monitoring and assessing the nature of living cell assembly at the microscopic level.
“One thing I’ve never seen before is using optical tweezers to trap individual cells. I thought that was really cool,” says Molinari.
After an ultra-strong PPI forms between two yeast cells, the cells continue to divide, forming more ultra-strong bonds with their daughter cells.
This technique could be used to produce self-propagating ELMs that have useful functions, such as extracting uranium from seawater, which could be used as a renewable source of nuclear energy, according to the paper. The researchers engineered the production of a uranium-sequestering protein in yeast and found that the material continued to grow and produce more protein. “There is a huge reserve of uranium in the oceans”, writes Fei Sunchemist and biological engineer at the Hong Kong University of Science and Technology, in an email to The scientist. “Self-growing ELMs, with their abilities to produce efficient uranium-binding ligands, can provide cost-effective solutions to challenges faced by the chemical separation and energy industries.” Sun led the study with colleagues Richard Lakerveld, a chemical engineer, and Jinqing Huang, a biophysicist chemist.
The researchers also successfully cloned a sticky, water-resistant molecule derived from the sea blue mussel (Mytilus edulis) in a separate yeast batch that also contains SpyTag and SpyCatcher. These cells effectively stuck to a variety of things, including skin and glass. “The resulting ELMs have proven to be extremely effective bioglues” that could be used for wound healing, Sun says.
“Engineering Living Materials . . . [have] the promise of revolutionizing the field of materials,” and new applications such as those described in this study are “encouraging,” says Molinari. However, she points out that ELMs assembled using this method are always less than 20 microns in diameter and that more research likely needs to be done before yeast can be made into ELMs on a large scale.
“Overall, the ability to functionalize single cells with genetic engineering and precisely assemble them into structured materials with microfluidic and optical tweezers provides a rich platform for new classes of advanced materials,” says Sun. “It’s unprecedented and transformative.”