Schwille and her team are expert membrane-wranglers. The Mins, the researchers found, would pop on and off the membranes and make them wave and swirl 1.
But when they added the Mins to 3D spheres of lipids, the structures burst like soap bubbles, says Schwille. Her group and others have overcome this problem using microfluidic techniques to construct cell-sized membrane containers, or liposomes, that can tolerate multiple insertions of proteins — either into the membranes themselves or into the interior. Cell-sized liposomes created on a microfluidic chip. Centrifugal force pulls the droplets through layers of dense lipids that encapsulate them along the way.
They come out at the other end as liposomes measuring 10—20 micrometres across — about the size of an average plant or animal cell. On the chip, two channels containing lipid molecules converge on a water-filled channel and spit out cell-sized liposomes that can hold various biological molecules, either stuck through the membrane or free-floating inside the container 3. His group has experimented with pressurizing, deforming and reshaping the liposomes to take on non-spherical shapes that mimic cells better. Microfluidic devices give researchers more control to move, sort and manipulate liposomes using micro-channels that operate almost like circuits.
This year, the Dekker lab designed a chip that could mechanically split a liposome in two by pushing it up against a sharp point 4. Examples include the force it takes to divide a cell, and what types of physical manipulation the liposomes can tolerate. Along the same lines, his team has also played around with the shape of living Escherichia coli cells — making them wider or square by growing them in nanofabricated silicone chambers.
In this way, team members can see how cell shape affects the division machinery, and assess how the Min proteins work in cells of different size and shape 5. Almost anything life-like requires cellular energy, usually in the form of ATP. To do this, his team took advantage of new microfluidic techniques. First, they stabilized GUVs by placing them inside water-in-oil droplets surrounded by a viscous shell of polymers.
They loaded these membranes with an enzyme called ATP synthase, which acts as a kind of molecular waterwheel, creating ATP energy from precursor molecules as protons flow through the membrane. Spatz explains that researchers could cycle the GUVs around the microchannel again for another protein injection, to sequentially add components. For instance, the next step could be to add a component that will automatically set up the proton gradient for the system.
Another Max Planck synthetic-biology group led by biochemist Tobias Erb has been chipping away at other approaches to constructing cellular metabolic pathways. Erb, a group leader at the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany, takes a blank-slate approach to synthesizing cellular metabolic pathways. His group sketched out a system design that could convert CO 2 into malate, a key metabolite produced during photosynthesis. The team predicted that the pathway would be even more efficient than photosynthesis. Next, Erb and his team searched databases for enzymes that might perform each of the reactions.
For a few, they needed to tweak existing enzymes into designer ones. In the end, they found 17 enzymes from 9 different organisms, including E. The reaction, perhaps unsurprisingly, was inefficient and slow 7.
Expertise. Insights. Illumination.
By grinding up spinach in a blender, and adding its photosynthesis machinery to their enzyme system in the test tube, the biologists can drive the production of ATP and the conversion of CO 2 to malate — solely by shining ultraviolet light on it. The researchers add circles of DNA called plasmids that they have designed to perform a particular function, along with all the machinery needed to make proteins from DNA.
For instance, her group has made liposome bioreactors that can sense an antibiotic in their environment through membrane pores and can generate a bioluminescent signal in response 8.
By fusing simple bioreactors together sequentially, the team can construct more-complex genetic circuits. But the systems start to break down as they expand to include ten or so components. This is a major challenge for the field, Adamala says. For much simpler synthetic cells, biologists must find other ways to impose that control.
This could be through external gatekeeping, in which the experimenter decides which liposomes get mixed together and when. It might also be accomplished through chemical tags that regulate which liposomes can fuse together, or through a time-release system. Another key to making a cell is getting the software right. For living systems, this is done by genes — from hundreds for some microbes, to tens of thousands for humans. How many genes a synthetic cell will need to run itself is a matter of healthy debate. Schwille and others would like to keep it in the neighbourhood of a few dozen.
Others, such as Adamala, think that synthetic cells need — genes. Some have chosen to start with something living. Synthetic biologist John Glass and his colleagues at the J.
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Craig Venter Institute JCVI in La Jolla, California, took one of the smallest-known microbial genomes on the planet, that of the bacterium Mycoplasma mycoides , and systematically disrupted its genes to identify the essential ones. Once they had that information, they chemically stitched together a minimal genome in the laboratory. This synthesized genome contained genes — about half of what was in the original organism — and it was transplanted into a related bacterial species, Mycoplasma capricolum 9. Glass thinks that it will be hard to decrease that number much more: take any gene away, and it either kills the cells or slows their growth to near zero, he says.
In that case, both the software and the hardware of the cell would be synthetic from the start. If it could grow and divide, that would be a tremendous step. But many argue that to truly represent a living system, it would also have to evolve and adapt to its environment. This is the goal with the most unpredictable results and also the biggest challenges, says Schwille. And they have seen a handful of gene-sequence changes popping up.
Erb says that working out how to add evolution to synthetic cells is the only way to make them interesting. That little bit of messiness in biological systems is what allows them to improve their performance. Synthetic cells could lead to insights about how life might look on other planets. Releasing such an organism into the human body or the environment would be risky, but a top-down engineered organism with unknown and unpredictable behaviours might be even riskier.
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