In Situ Synthesis of Phospholipid Membranes

Diacylation of Peptides Enables the Construction of Functional Vesicles for Drug-Carrying Liposomes

Membrane-forming phospholipids are generated in cells by enzymatic diacylation of non-amphiphilic polar head groups. Analogous non-enzymatic processes may have been relevant at the origin of life and could have practical utility in membrane synthesis. However, aqueous head group diacylation is challenging in the absence of enzymes. The use of charged peptides instead of canonical phospholipid head groups offers advantages with respect to ease of acylation and chemical diversity. Here we demonstrate that native chemical ligation (NCL) enables in situ synthesis of diacylated lipopeptides (D-ALPs), which spontaneously self-assemble into micron-sized vesicles resembling cellular membranes. Diacylation occurs between non-amphiphilic peptides possessing an N-terminal cysteine, and acyl thioesters. Peptide head groups endow unique membrane functions, which is demonstrated by incorporation of an arginine-glycine-aspartic acid (RGD) motif, resulting in vesicle targeting to αvβ3 integrin-overexpressing cancer cells. The biocompatibility and functional group programmability of D-ALPs supports their broad utility as membrane mimetics.

Protocells by spontaneous reaction of cysteine with short-chain thioesters

All known forms of life are composed of cells, whose boundaries are defined by lipid membranes that separate and protect cell contents from the environment. It is unknown how the earliest forms of life were compartmentalized. Several models have suggested a role for single-chain lipids such as fatty acids, but the membranes formed are often unstable, particularly when made from shorter alkyl chains (≤C8) that were probably more prevalent on prebiotic Earth. Here we show that the amino acid cysteine can spontaneously react with two short-chain (C8) thioesters to form diacyl lipids, generating protocell-like membrane vesicles. The three-component reaction takes place rapidly in water using low concentrations of reactants. Silica can catalyse the formation of protocells through a simple electrostatic mechanism. Several simple aminothiols react to form diacyl lipids, including short peptides. The protocells formed are compatible with functional ribozymes, suggesting that coupling of multiple short-chain precursors may have provided membrane building blocks during the early evolution of cells.

Rapid Formation of Non-canonical Phospholipid Membranes by Chemoselective Amide-Forming Ligations with Hydroxylamines

There has been increasing interest in methods to generate synthetic lipid membranes as key constituents of artificial cells or to develop new tools for remodeling membranes in living cells. However, the biosynthesis of phospholipids involves elaborate enzymatic pathways that are challenging to reconstitute in vitro. An alternative approach is to use chemical reactions to non-enzymatically generate natural or non-canonical phospholipids de novo. Previous reports have shown that synthetic lipid membranes can be formed in situ using various ligation chemistries, but these methods lack biocompatibility and/or suffer from slow kinetics at physiological pH. Thus, it would be valuable to develop chemoselective strategies for synthesizing phospholipids from water-soluble precursors that are compatible with synthetic or living cells Here, we demonstrate that amide-forming ligations between lipid precursors bearing hydroxylamines and α-ketoacids (KAs) or potassium acyltrifluoroborates (KATs) can be used to prepare non-canonical phospholipids at physiological pH conditions. The generated amide-linked phospholipids spontaneously self-assemble into cell-like micron-sized vesicles similar to natural phospholipid membranes. We show that lipid synthesis using KAT ligation proceeds extremely rapidly, and the high selectivity and biocompatibility of the approach facilitates the in situ synthesis of phospholipids and associated membranes in living cells.

Biomimetic construction of phospholipid membranes by direct aminolysis ligations

Construction of artificial cells requires the development of straightforward methods for mimicking natural phospholipid membrane formation. Here we describe the use of direct aminolysis ligations to spontaneously generate biomimetic phospholipid membranes from water-soluble starting materials. Additionally, we explore the suitability of such biomimetic approaches for driving the in situ formation of native phospholipid membranes. Our studies suggest that non-enzymatic ligation reactions could have been important for the synthesis of phospholipid-like membranes during the origin of life, and might be harnessed as simplified methods to enable the generation of lipid compartments in artificial cells.

In situ synthesis of artificial lipids

Lipids constitute one of the most enigmatic family of biological molecules. Although the importance of lipids as basic units of compartmental structure and energy storage is well-acknowledged, deciphering the biosynthesis and precise roles of specific lipid species has been challenging. To better understand the structure and function of these biomolecules, there is a burgeoning interest in developing strategies to produce noncanonical lipids in a controlled manner. This review covers recent advances in the area of in situ generation of synthetic lipids. Specifically, we report several approaches that constitute a powerful toolbox for achieving noncanonical lipid synthesis. We describe how these methodologies enable the direct construction of synthetic lipids, helping to address fundamental questions related to the cell biology of lipid biosynthesis, trafficking, and signaling. We envision that highlighting the current advances in artificial lipid synthesis will pave the way for broader interest into this emerging class of biomimetic molecules.

Rapid and Sequential Dual Oxime Ligation Enables De Novo Formation of Functional Synthetic Membranes from Water-Soluble Precursors

Cell membranes define the boundaries of life and primarily consist of phospholipids. Living organisms assemble phospholipids by enzymatically coupling two hydrophobic tails to a soluble polar head group. Previous studies have taken advantage of micellar assembly to couple single-chain precursors, forming non-canonical phospholipids. However, biomimetic nonenzymatic coupling of two alkyl tails to a polar head-group remains challenging, likely due to the sluggish reaction kinetics of the initial coupling step. Here we demonstrate rapid de novo formation of biomimetic liposomes in water using dual oxime bond formation between two alkyl chains and a phosphocholine head group. Membranes can be generated from non-amphiphilic, water-soluble precursors at physiological conditions using micromolar concentrations of precursors. We demonstrate that functional membrane proteins can be reconstituted into synthetic oxime liposomes from bacterial extracts in the absence of detergent-like molecules.

Synthesis of lipid membranes for artificial cells

A major goal of synthetic biology is to understand the transition between non-living matter and life. The bottom-up development of an artificial cell would provide a minimal system with which to study the border between chemistry and biology. So far, a fully synthetic cell has remained elusive, but chemists are progressing towards this goal by reconstructing cellular subsystems. Cell boundaries, likely in the form of lipid membranes, were necessary for the emergence of life. In addition to providing a protective barrier between cellular cargo and the external environment, lipid compartments maintain homeostasis with other subsystems to regulate cellular processes. In this Review, we examine different chemical approaches to making cell-mimetic compartments. Synthetic strategies to drive membrane formation and function, including bioorthogonal ligations, dissipative self-assembly and reconstitution of biochemical pathways, are discussed. Chemical strategies aim to recreate the interactions between lipid membranes, the external environment and internal biomolecules, and will clarify our understanding of life at the interface of chemistry and biology.

A protocell with fusion and division

A protocell is a synthetic form of cellular life that is constructed from phospholipid vesicles and used to understand the emergence of life from a nonliving chemical network. To be considered 'living', a protocell should be capable of self-proliferation, which includes successive growth and division processes. The growth of protocells can be achieved via vesicle fusion approaches. In this review, we provide a brief overview of recent research on the formation of a protocell, fusion and division processes of the protocell, and encapsulation of a defined chemical network such as the genetic material. We also provide some perspectives on the challenges and future developments of synthetic protocell research.

Protobiotic Systems Chemistry Analyzed by Molecular Dynamics

Systems chemistry has been a key component of origin of life research, invoking models of life's inception based on evolving molecular networks. One such model is the graded autocatalysis replication domain (GARD) formalism embodied in a lipid world scenario, which offers rigorous computer simulation based on defined chemical kinetics equations. GARD suggests that the first pre-RNA life-like entities could have been homeostatically-growing assemblies of amphiphiles, undergoing compositional replication and mutations, as well as rudimentary selection and evolution. Recent progress in molecular dynamics has provided an experimental tool to study complex biological phenomena such as protein folding, ligand-receptor interactions, and micellar formation, growth, and fission. The detailed molecular definition of GARD and its inter-molecular catalytic interactions make it highly compatible with molecular dynamics analyses. We present a roadmap for simulating GARD's kinetic and thermodynamic behavior using various molecular dynamics methodologies. We review different approaches for testing the validity of the GARD model by following micellar accretion and fission events and examining compositional changes over time. Near-future computational advances could provide empirical delineation for further system complexification, from simple compositional non-covalent assemblies towards more life-like protocellular entities with covalent chemistry that underlies metabolism and genetic encoding.

Biomimetic Generation and Remodeling of Phospholipid Membranes by Dynamic Imine Chemistry

Biomimetic liposomes have a wide array of applications in several areas, ranging from medicinal chemistry to synthetic biology. Due to their biocompatibility and biological relevance, there is particular interest in the formation of synthetic phospholipid vesicles and the development of methods to tune their properties in a controlled manner. However, while true biological membranes are capable of responding to environmental stimuli by enzymatically remodeling their composition, synthetic liposomes are typically static once formed. Herein we report the chemoselective reaction of the natural amine-containing lysosphingomyelin with a series of long-chain aldehydes to form imines. This transformation results in the formation of phospholipid liposomes that are in dynamic equilibrium with the aldehyde-amine form. The reversibility of the imine linkage is exploited in the synthesis of vesicles that are capable of responding to external stimuli such as temperature or the addition of small molecules.

In Situ Lipid Membrane Formation Triggered by Intramolecular Photoinduced Electron Transfer

A major goal of synthetic biology is the development of rational methodologies to construct self-assembling non-natural membranes, which could enable the efficient fabrication of artificial cellular systems from purely synthetic components. However, spatiotemporal control of artificial membrane formation remains both challenging and limited in scope. Here, we describe a new methodology to promote biomimetic phospholipid membrane formation by the photochemical activation of a catalyst-sensitizer dyad via an intramolecular photoinduced electron-transfer process. Our results offer future opportunities to exert spatiotemporal control over artificial cellular constructs.