Recent trends in synthetic biology have brought the ambition of constructing life in the laboratory — albeit limited for the moment to single cells — into the realm of possibility. This ambition arises not only from the Faustian dream of the scientist creating life, but also from a rational observation: even the simplest modern unicellular organisms are so extremely complex (a few thousand genes and tens of thousands of chemicals inside a tiny container, and all organized in, for example, regulatory networks). This elicits the question: is this complexity really necessary for (cellular) life, or can we, in the laboratory, construct something much simpler that has the characteristic of life, yet consists of a very limited number of components? Since the late 1980s1 this notion of a 'minimal cell' has been widely described in the literature2,3, and several groups around the world are currently engaged in this fascinating research4,5,6.

Critical to these endeavours is the concept of a 'minimal genome', which poses a second question: what is the minimal number of genes that permits (most of) the basic functions of the living cell? Such 'synthetic' cells represent one of the most ambitious goals in synthetic biology. They are relevant for investigating the self-organizing abilities and emergent properties of chemical systems — for example, in origin-of-life studies and for the realization of chemical autopoietic systems7 that continuously self-replicate — and can also have biotechnological applications.

Writing in Nature Chemistry, Tadashi Sugawara and co-workers now describe how DNA replication inside lipid compartments can induce those lipid compartments to grow and ultimately divide8 — this behaviour resembles, to a certain extent, the more complex cellular self-reproduction. The researchers had shown9 in previous studies that under certain conditions giant vesicles (tens of micrometres in size) were capable of growth and division. An amphiphilic catalyst was incorporated into the membrane of the vesicle that was able to convert lipidic precursor molecules into membrane lipids by cleavage of a terminal moiety. When precursor molecules were added to a vesicle-containing solution, they were converted into membrane lipids and incorporated into the vesicles' membranes. This allowed the vesicles to first grow, and ultimately divide, if sufficient precursor molecules were provided. Conveniently, the large size of giant vesicles allows direct observation, in this case by fluorescence microscopy, of what is happening in the sample.

The present study also shows the growth and the division of giant vesicles, but now it is enriched by the replication of DNA inside the vesicle (Fig. 1). DNA encapsulated within the large cavities of the vesicles is first amplified by the polymerase chain reaction (PCR) — catalysed by a polymerase enzyme that makes copies of DNA sequences. The PCR reaction occurs inside the giant vesicles, simulating cellular DNA replication. As in the previous studies mentioned above, the addition of cationic membrane precursors leads to the formation of cationic lipids that are in turn incorporated in the membrane. Now, however, the researchers have prepared a membrane that also contains a zwitterionic lipid, so that the central cavity can contain an aqueous solution. An anionic lipid is also added to the membrane to enhance the vesicle's stability to changes in temperature and ionic strength. DNA (which is polyanionic) interacts strongly with the newly formed cationic membrane lipids. Increased DNA concentration due to replication therefore prompts faster incorporation of new lipids into the vesicle membrane and speeds up vesicle growth and division.

Figure 1: Schematic representation of an approach towards the synthetic construction of cell-like systems.
figure 1

Separated molecular components are assembled into spatially and functionally organized vesicles — microscopic compartments with sizes varying from about 0.1 to 100 micrometres — with a shell formed from phospholipids or fatty acids. DNA and a polymerase enzyme (among other components) are encapsulated inside 10-micrometre-sized lipid vesicles, whose membrane also contained an amphiphilic catalyst. An internal DNA amplification synthesis step first produces new DNA molecules. A lipid precursor is then added externally, and converted into a membrane molecule by the catalyst. This allows membrane growth, and ultimately division, to give 'daughter' vesicles.

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This synergism is the result of a series of complex (and unfortunately not yet completely elucidated) chemical interactions at the membrane boundary between freshly produced DNA, the membrane lipids, the amphiphilic catalyst and the incoming cationic membrane precursor. Sugawara and co-workers explain the behaviour observed in terms of local accumulation of cationic membrane lipids around the DNA-rich inner-membrane areas. The complexity of the mechanism is rooted in the interplay between physical and chemical phenomena occurring at the vesicle membrane. Importantly, the researchers demonstrated that the efficiency of vesicle growth and division was dependent on the amount of DNA produced within vesicles, which was in turn controlled by varying the number of PCR cycles. By analogy, this orchestrated dynamics can be thought of as effectively modelling one of the intrinsic characteristics of prokaryotic cells — namely, chromosomal replication coupled with cell division.

The current study provides a proof-of-principle that a complex dynamic system, involving molecular transformations and supramolecular structures, can be artificially constructed by exploiting chemical reactions and self-organizing patterns. At this stage, there are two limitations to these exciting achievements. The first is the fact that the two main events (DNA replication and vesicle growth) occur one after the other rather than synchronously — although they can be considered to be functionally coupled because the first strongly affects the second. The second limitation concerns the very specific chemistry needed to observe the above-mentioned behaviour; that is, the cationic membrane precursor used in this study was designed and prepared especially for this particular system. Despite these two points, the construction of these dynamic vesicles certainly extends the repertoire of complex biochemical reactions reconstituted inside lipid vesicles2 in a significant manner, and as such paves the way to experiments with more complex systems.

There remain open questions before minimal synthetic (or semi-synthetic) cells can be created in the laboratory. The most important is the requirement of a full 'core and shell' reproduction2, where all of the components of a synthetic cell can be produced in a replicating cycle7. In the current study, for example, the DNA and membrane cationic lipids are produced in situ, whereas the two catalysts (the polymerase enzyme and the catalyst for surfactant synthesis) are not. This means that even if the observed dynamics proceed for several cycles (generations), it would not ultimately lead to a population of active compartments, because the catalysts themselves are not replicated. Newly formed vesicles that lack polymerase, for example, will not amplify their internal DNA. Ribozyme-based systems have been proposed as candidates for achieving core-and-shell reproduction, especially for modelling primitive cells3. A ribozyme is an RNA molecule that has catalytic activity, just like an enzyme. If a ribozyme could catalyse the synthesis of lipids, and its self-replication could be induced by another ribozyme, the resulting system could self-reproduce without the need for additional molecules. However this does not seem feasible yet. Another possibility, perhaps more promising, involves the intra-vesicle biosynthesis of proteins (enzymes), so that they could in turn catalyse DNA replication, RNA and ribosome synthesis, and lipid synthesis, thus reconstituting a minimal biochemical network for self-maintenance.

All this is very challenging, and places research on the bottom-up construction of synthetic cells at the cutting-edge of a multidisciplinary field that embraces synthetic biology, the origin of life, systems chemistry and the concepts of self-organization and emergence. The abrupt rise of interest for constructing minimal cells in the laboratory might be due to a diffuse sense of confidence that its achievement is indeed an experimentally accessible target.