Entirely artificial signal transduction with adrenaline

Synthetic Artificial Apoptosis-Inducing Receptor for On-Demand Deactivation of Engineered Cells

The design of a fully synthetic, chemical "apoptosis-inducing receptor" (AIR) molecule is reported that is anchored into the lipid bilayer of cells, is activated by the incoming biological input, and responds with the release of a secondary messenger-a highly potent toxin for cell killing. The AIR molecule has four elements, namely, an exofacial trigger group, a bilayer anchor, a toxin as a secondary messenger, and a self-immolative scaffold as a mechanism for signal transduction. Receptor installation into cells is established via a robust protocol with minimal cell handling. The synthetic receptor remains dormant in the engineered cells, but is effectively triggered externally by the addition of an activating biomolecule (enzyme) or in a mixed cell population through interaction with the surrounding cells. In 3D cell culture (spheroids), receptor activation is accessible for at least 5 days, which compares favorably with other state of the art receptor designs.

Controlled dimerization of artificial membrane receptors for transmembrane signal transduction

In biology, membrane-spanning proteins are responsible for the transmission of chemical signals across membranes, including the signal recognition-mediated conformational change of transmembrane receptors at the cell surface, and a trigger of an intracellular phosphorylation cascade. The ability to reproduce such biological processes in artificial systems has potential applications in smart sensing, drug delivery, and synthetic biology. Here, an artificial transmembrane receptors signaling system was designed and constructed based on modular DNA scaffolds. The artificial transmembrane receptors in this system are composed of three functional modules: signal recognition, lipophilic transmembrane linker, and signal output modules. Adenosine triphosphate (ATP) served as an external signal input to trigger the dimerization of two artificial receptors on membranes through a proximity effect. This effect induced the formation of a G-quadruplex, which served as a peroxidase-like enzyme to facilitate a signal output measured by either fluorescence or absorbance in the lipid bilayer vesicles. The broader utility of this modular method was further demonstrated using a lysozyme-binding aptamer instead of an ATP-binding aptamer. Therefore, this work provides a modular and generalizable method for the design of artificial transmembrane receptors. The flexibility of this synthetic methodology will allow researchers to incorporate different functional modules while retaining the same molecular framework for signal transduction.

An artificial transmbrane signal transducer was developed through the chemical input-mediated dimerization of artificial DNA transmembrane receptors and the subsequent activation of a cascade of events inside the vesicles.

Artificial Signal Transduction

Communication between and inside cells as well as their response to external stimuli relies on elaborated systems of signal transduction. They all require a directional transmission across membranes, often realized by primary messenger docking onto external receptor units and subsequent internalization of the signal in form of a released second messenger. This in turn starts a cascade of events which ultimately control all functions of the living cell. Although signal transduction is a fundamental biological process realized by supramolecular recognition and multiplication events with small molecules, chemists have just begun to invent artificial models which allow to study the underlying rules, and one day perhaps to rescue damaged transduction systems in nature. This review summarizes the exciting pioneering efforts of chemists to create simple models for the basic principles of signal transduction across a membrane. It starts with first attempts to establish molecular recognition events on liposomes with embedded receptor amphiphiles and moves on to simple transmembrane signaling across lipid bilayers. More elaborated systems step by step incorporate more elements of cell signaling, such as primary and secondary messenger or a useful cellular response such as cargo release.

Supramolecular Chemistry in the Biomembrane

The combination of supramolecular functional systems with biomolecular chemistry has been a fruitful exercise for decades, leading to a greater understanding of biomolecules and to a great variety of applications, for example, in drug delivery and sensing. Within these developments, the phospholipid bilayer membrane, surrounding live cells, with all its functions has also intrigued supramolecular chemists. Herein, recent efforts from the supramolecular chemistry community to mimic natural functions of lipid membranes, such as sensing, molecular recognition, membrane fusion, signal transduction, and gated transport, are reviewed.

Artificial signal transduction with primary and secondary messengers

The complete, entirely artificial, signal-transduction process was realized with a pair of tailored transmembrane units that were equipped with receptor- and reactive sites at both amphiphilic ends. Thus, docking of the primary messenger, transmission of the signal, and release of the secondary messenger could all be imitated in a single experimental setup. The system imitates the signaling principle of receptor tyrosine kinases and employs bisphosphonate head-groups for oligoamine-recognition and a pair of thiol nucleophiles and pyridine disulfide tail-groups for intravesicle S(N)2 displacement. This system operates in a unidirectional fashion, does not suffer from intervesicle competition, and is highly sensitive towards the lipid composition of the membrane and the nature of the primary messenger.

Transmission of binding information across lipid bilayers

A synthetic transmembrane receptor that is capable of transmitting binding information across a lipid bilayer membrane is reported. The binding event is based on aggregation of the receptor triggered by copper(II) complexation to ethylenediamine functionalities. By labelling the receptor with fluorescent dansyl groups, the copper(II) binding event could be monitored by measuring the extent of fluorescence quenching. Comparing the receptor with a control receptor lacking the transmembrane linkage revealed that the transmembrane receptor binds copper(II) ions more tightly than the non-spanning control receptor at low copper(II) concentrations. Since the intrinsic binding to copper(II) is the same for both receptors, this effect was attributed to synergy between the connected interior and exterior binding sides of the transmembrane receptor. Thus, this is the first reported artificial signalling event in which binding of a messenger on one side of the membrane leads to a cooperative binding event on the opposite side of the membrane, resembling biological signalling systems and helping us to get a better understanding of the requirements for more effective artificial signalling systems.