Chemical Communication in Artificial Cells: Basic Concepts, Design and Challenges

Abstract

In the past decade, the focus of bottom-up synthetic biology has shifted from the design of complex artificial cell architectures to the design of interactions between artificial cells mediated by physical and chemical cues. Engineering communication between artificial cells is crucial for the realization of coordinated dynamic behaviours in artificial cell populations, which would have implications for biotechnology, advanced colloidal materials and regenerative medicine. In this review, we focus our discussion on molecular communication between artificial cells. We cover basic concepts such as the importance of compartmentalization, the metabolic machinery driving signaling across cell boundaries and the different modes of communication used. The various studies in artificial cell signaling have been classified based on the distance between sender and receiver cells, just like in biology into autocrine, juxtacrine, paracrine and endocrine signaling. Emerging tools available for the design of dynamic and adaptive signaling are highlighted and some recent advances of signaling-enabled collective behaviours, such as quorum sensing, travelling pulses and predator-prey behaviour, are also discussed.

Keywords: artificial cells; bottom-up synthetic biology; chemical signaling; collective behaviours; dynamic colloidal systems; systems chemistry.

FIGURE 1

Basic concepts of cell chemical signaling. (A) The signaling chain include a sender and a receiver cell, a signal (generated and encoded in the sender cell; and transduced within the receiver cell) and a propagation medium. (B) Chemical signaling can be characterized by the distance between sender and receiver cells (L) compared to the typical size of the cells (d). (C) Different modes of signaling are possible, from simple linear or cascade modes to more sophisticated interactive of networked modes. (D) Cell signaling is dynamic and adaptive: for instance, living cells utilize switches to regulate signal production (concentration, frequency, etc.), and advanced reconfigurable systems to regulate signal responses, such as receptor internalization.

FIGURE 2

Distance based classification of signaling. (A) Self-signaling (autocrine signaling) within artificial cells can be mediated by simple enzyme cascades and enzyme cross-talk to self-modulate chemical behaviour while morphological changes can also be elicited via cytoskeleton formation and division into second-generation cells. (B) Juxtacrine signaling occurs between sender and receiver cells that are in direct contact and its regulation depends on the type of membranes separating them. In the case of sender and receiver cells having continuous membranes (lipid bilayers), the insertion of protein/peptide pores into the membrane allows regulation of communication between themselves and with the environment while also avoiding dilution of signaling molecules due to limited pore channels. Sender and receiver cells having discontinuous membranes exhibit fast-unchanneled exchange of signaling molecules between themselves and the environment. (C) Paracrine signaling occurs between sender and receiver cells which are separated by distances larger than their size. The signaling molecules get diluted and may be degraded before reaching the receiver cell, where they can trigger a chemical response via cascade signaling or undergo amplification for increasing the signaling distances. Signaling molecules can also trigger morphological responses, for e.g., via activating membrane-immobilized chain growth agents leading to polymer brush growth on the membrane of the receiver cell allowing modulation of permeability.

FIGURE 3

Dynamic control over artificial cell signaling. (A) Various strategies can be implemented to regulate signal production or release that can be schematically split in two categories: switchable reactions, such as sequence-specific ssDNA binding and release, switchable gene expression (e.g., using riboswitches), or activation of enzyme reactions, and controllable permeabilization, via gene-directed in situ synthesis of protein pores, membrane fluidization and formation of defects in continuous lipid- or polymer-based membranes, or pore gating in discontinuous membranes. (B) Reversible modulation of artificial cell-cell communication could also be achieved by dynamically changing the distance between sender and receiver cell, e.g. using physical manipulation or chemical binding strategies, or by exploiting the motility of artificial cells. (C) Signal reception and transduction within receiver cells can last be regulated by switchable reception, e.g., using stimuli-responsive pores or strategies to enhanced signal transduction across membranes such as membrane-addressed coacervates, and programmable transduction pathways, e.g. using specific addressing into sub-compartments.

FIGURE 4

Collective behaviours enabled by cell signaling. (A) Artificial cells capable of population density-dependent behavior, i.e. quorum sensing, contain both sender and receiver DNA templates so that they can both send and receive protein signals (blue) to trigger the expression of a green fluorescent protein at high population densities, where the loss of self-generated signaling molecules by dilution is countered by the receipt of signals secreted by neighbouring cells (indicated by strong blue hues around clustered cells). (B) Travelling pulse behaviour in artificial cells can be realized by propagation of signals from a single sender cell through an array of receiver cells, where the signaling molecules are capable of diffusing across the membranes between the cells and are coupled to a feedforward genelet circuit. (C) Two types of predator-prey behaviour are observed depending on the distance between predator and prey: contact predation, when they are in direct contact allowing immediate degradation (killing) and trafficking of contents of prey cell, and non-contact predation, when they are separated by small distances ( $L\ge d$ ) leading to slow degradation of prey cell.