Artificial Cells: Synthetic Compartments with Life-like Functionality and Adaptivity

Abstract

Cells are highly advanced microreactors that form the basis of all life. Their fascinating complexity has inspired scientists to create analogs from synthetic and natural components using a bottom-up approach. The ultimate goal here is to assemble a fully man-made cell that displays functionality and adaptivity as advanced as that found in nature, which will not only provide insight into the fundamental processes in natural cells but also pave the way for new applications of such artificial cells. In this Account, we highlight our recent work and that of others on the construction of artificial cells. First, we will introduce the key features that characterize a living system; next, we will discuss how these have been imitated in artificial cells. First, compartmentalization is crucial to separate the inner chemical milieu from the external environment. Current state-of-the-art artificial cells comprise subcompartments to mimic the hierarchical architecture of eukaryotic cells and tissue. Furthermore, synthetic gene circuits have been used to encode genetic information that creates complex behavior like pulses or feedback. Additionally, artificial cells have to reproduce to maintain a population. Controlled growth and fission of synthetic compartments have been demonstrated, but the extensive regulation of cell division in nature is still unmatched. Here, we also point out important challenges the field needs to overcome to realize its full potential. As artificial cells integrate increasing orders of functionality, maintaining a supporting metabolism that can regenerate key metabolites becomes crucial. Furthermore, life does not operate in isolation. Natural cells constantly sense their environment, exchange (chemical) signals, and can move toward a chemoattractant. Here, we specifically explore recent efforts to reproduce such adaptivity in artificial cells. For instance, synthetic compartments have been produced that can recruit proteins to the membrane upon an external stimulus or modulate their membrane composition and permeability to control their interaction with the environment. A next step would be the communication of artificial cells with either bacteria or another artificial cell. Indeed, examples of such primitive chemical signaling are presented. Finally, motility is important for many organisms and has, therefore, also been pursued in synthetic systems. Synthetic compartments that were designed to move in a directed, controlled manner have been assembled, and directed movement toward a chemical attractant is among one of the most life-like directions currently under research. Although the bottom-up construction of an artificial cell that can be truly considered "alive" is still an ambitious goal, the recent work discussed in this Account shows that this is an active field with contributions from diverse disciplines like materials chemistry and biochemistry. Notably, research during the past decade has already provided valuable insights into complex synthetic systems with life-like properties. In the future, artificial cells are thought to contribute to an increased understanding of processes in natural cells and provide opportunities to create smart, autonomous, cell-like materials.

Figure 1

Prominent classes of multicompartmentalized vesicles. Their design is often inspired by the architecture of a eukaryotic cell (middle). Adapted with permission from refs (−10). Copyrights 2009 and 2014 Wiley, 2013 American Association for the Advancement of Science, and 2010 American Chemical Society.

Figure 2

SGCs are examples of the complex behavior that arises when combining genetic elements. (A) A serial transcriptional activation cascade that produces deGFP. Each σ factor activates its successor by interacting with its promoter, as indicated by solid arrows. (B) This circuit generates a pulse in deGFP production due to two competing expression cascades. Addition of σ⁷⁰ induces deGFP production by the stimulatory (lower) circuit, but the inhibitory (upper) circuit is triggered simultaneously and causes a delayed suppression. (C) An inducible transcriptional repression unit that can switch outputs. In the presence of IPTG, deCFP is produced; replacement by ATc represses deCFP production and stimulates deGFP expression. Adapted with permission from refs ( and 18). Copyright 2012 and 2016 American Chemical Society.

Figure 3

Self-reproduction of vesicles coupled to internal DNA amplification. A polymerase chain reaction (PCR) in the vesicle’s lumen amplifies the encapsulated DNA, and a catalyst in the membrane generates new membrane components from supplemented precursors. Importantly, the DNA accelerates membrane formation and induces budding and fission of the vesicle. Adapted with permission from refs ( and 29). Copyright 2011 Macmillan Publishers Ltd.

Figure 4

Reversible assembly of a His-tagged protein on the membrane of a GUV. The assembly is driven by the catalytic activity of ADH, which changes the pH of the vesicle’s lumen. (A) Schematic of the reversible assembly. (B) Alternating addition of two substrates (indicated by arrows) drives membrane assembly and disassembly of the His-tagged protein. (C) Fluorescence microscopy images of GUVs corresponding to the time points in panel B. Adapted with permission from ref (35). Copyright 2015 Wiley.

Figure 5

Oscillating buckling patterns observed for two colloidosomes in response to temperature variations and an internal oscillating reaction. Reproduced with permission from ref (39). Copyright 2016 Wiley.

Figure 6

Colloidosome signaling. (A) A glucose oxidase (GOx)-filled colloidosome produces hydrogen peroxide, which induces polymerization of the NIPAM shell of a secondary colloidosome. Consequently, the permeability of the PNIPAM-colloidosome becomes thermoresponsive, influencing the kinetics of an internal reaction. (B) Fluorescence microcopy image of red fluorescent GOx-colloidosomes that induced the polymerization of a green fluorescent shell around a PNIPAM-colloidosome. (C) Kinetics of an enzymatic reaction inside the PNIPAM-colloidosomes before (black) and after (red) polymerization of the NIPAM shell. Adapted with permission from ref (43). Copyright 2016 Wiley.

Figure 7

Glucose-fueled propulsion of enzyme-loaded stomatocytes. (A) The nanomotor’s cavity is loaded with enzymes during self-assembly. Fuel can diffuse in, but the enzymes remain trapped, producing thrust. (B) The nanomotors’ speed varies with glucose concentration. Adapted with permission from ref (48). Copyright 2016 American Chemical Society.