Models of primitive cellular life: polymerases and templates in liposomes

Modelling energy-harvesting processes in primitive cells: Proton transport across bilayers driven by the oxidation of sulfite

A frequently debated topic related to the origin of life centers around the question of how complex forms of life on today's Earth may have evolved over time from simpler predecessors. For example, the question of how proton concentration gradients across cellular membranes developed in ancestral protocells remains unanswered. This process, which is indispensable for the generation of chemical energy in modern organisms, is driven by energy derived from redox processes in the respiratory chain. Since it is highly unlikely that the complex machinery of the respiratory chain was available on early Earth, we provide an example of how proton gradients can be established in less complex systems. Utilizing liposomes as models of primitive cells, we were able to generate proton gradients of about two pH units across the liposome bilayers using redox reactions as the driving force. Electrons were transferred from sodium sulfite present on the outside of the liposomes to ferricyanide, which was trapped on the inside. A lipid-soluble phenazine derivative served as a shuttle that transferred both electrons and protons across the lipid bilayer. Because sulfite would have been an abundant reduced solute available to the earliest cells, we propose that it may have been a primary source of redox energy for primitive chemiosmotic energy transduction.

ATP mediated stimuli responsive supramolecular assembly

Adenosine triphosphate has been employed as a biomolecular building block to fabricate pH and enzyme responsive compartmentalized supramolecular assemblies sequestering silver nanoparticles (AgNPs) and doxorubicin in the core and increase the therapeutic efficacy. Detailed investigations reveal that meticulous design can integrate chemical enrichment, stimuli responsiveness and targeted delivery within compartmentalized models.

Windowless detection geometry for sum frequency scattering spectroscopy in the C-D and amide I regions

Understanding the structure and chemistry of nanoscopic surfaces is an important challenge for biointerface sciences. Sum frequency scattering (SFS) spectroscopy can specifically probe the surfaces of nanoparticles, vesicles, liposomes, and other materials relevant to biomaterial research, and, as a vibrational spectroscopy method, it can provide molecular level information about the surface chemistry. SFS is particularly promising to probe the structure of proteins, and other biological molecules, at nanoparticle surfaces. Here, amide I spectra can provide information about protein folding and orientation, while spectra in the C-D and C-H stretching regions allow experiments to determine the mode of interaction between particle surfaces and proteins. Methods used currently employ a closed liquid cell or cuvette, which works extremely well for C-H and phosphate regions but is often impeded in the amide I and C-D regions by a strong background signal that originates from the window material of the sample cells. Here, we discuss a windowless geometry for collecting background-free and high-fidelity SFS spectra in the amide I and C-D regions. We demonstrate the improvement in spectra quality by comparing SFS spectra of unextruded, multilamellar vesicles in a sample cuvette with those recorded using the windowless geometry. The sample geometry we propose will enable new experiments using SFS as a probe for protein-particle interactions.

Designing Artificial Cells towards a New Generation of Biosensors

The combination of biological and synthetic materials has great potential to generate new types of biosensors. Toward this goal, recent advances in artificial cell development have demonstrated the capacity to detect a variety of analytes and environmental changes by encapsulating genetically encoded sensors within bilayer membranes, expanding the contexts within which biologically based sensing can operate. This chassis not only acts as a container for cell-free sensors, but can also play an active role in artificial cell sensing by serving as an additional gate mediating the transfer of environmental information. Here, we focus on recent progress toward stimuli-responsive artificial cells and discuss strategies for membrane functionalization in order to expand cell-free biosensing capabilities and applications.

Impact of Particle Size and Polydispersity Index on the Clinical Applications of Lipidic Nanocarrier Systems

Lipid-based drug delivery systems, or lipidic carriers, are being extensively employed to enhance the bioavailability of poorly-soluble drugs. They have the ability to incorporate both lipophilic and hydrophilic molecules and protecting them against degradation in vitro and in vivo. There is a number of physical attributes of lipid-based nanocarriers that determine their safety, stability, efficacy, as well as their in vitro and in vivo behaviour. These include average particle size/diameter and the polydispersity index (PDI), which is an indication of their quality with respect to the size distribution. The suitability of nanocarrier formulations for a particular route of drug administration depends on their average diameter, PDI and size stability, among other parameters. Controlling and validating these parameters are of key importance for the effective clinical applications of nanocarrier formulations. This review highlights the significance of size and PDI in the successful design, formulation and development of nanosystems for pharmaceutical, nutraceutical and other applications. Liposomes, nanoliposomes, vesicular phospholipid gels, solid lipid nanoparticles, transfersomes and tocosomes are presented as frequently-used lipidic drug carriers. The advantages and limitations of a range of available analytical techniques used to characterize lipidic nanocarrier formulations are also covered.

Recent Progress in Micro/Nanoreactors toward the Creation of Artificial Organelles

Artificial organelles created from a bottom up approach are a new type of engineered materials, which are not designed to be living but, instead, to mimic some specific functions inside cells. By doing so, artificial organelles are expected to become a powerful tool in biomedicine. They can act as nanoreactors to convert a prodrug into a drug inside the cells or as carriers encapsulating therapeutic enzymes to replace malfunctioning organelles in pathological conditions. For the design of artificial organelles, several requirements need to be fulfilled: a compartmentalized structure that can encapsulate the synthetic machinery to perform an enzymatic function, as well as a means to allow for communication between the interior of the artificial organelle and the external environment, so that substrates and products can diffuse in and out the carrier allowing for continuous enzymatic reactions. The most recent and exciting advances in architectures that fulfill the aforementioned requirements are featured in this review. Artificial organelles are classified depending on their constituting materials, being lipid and polymer-based systems the most prominent ones. Finally, special emphasis will be put on the intracellular response of these newly emerging systems.

Encapsulation of Nucleic Acids into Giant Unilamellar Vesicles by Freeze-Thaw: a Way Protocells May Form

Protocells are believed to consist of a lipid membrane and encapsulated nucleic acid. As the lipid membrane is impermeable to macromolecules like nucleic acids, the processes by which nucleic acids become encapsulated inside lipid membrane compartments are still unknown. In this paper, a freeze-thaw method was modified and applied to giant unilamellar vesicles (GUVs) and deoxyribonucleic acid (DNA) in mixed solution resulting in the efficient encapsulation of 6.4 kb plasmid DNA and similar length linear DNA into GUVs. The mechanism of encapsulation was followed by observing the effect of freeze-thaw temperatures on GUV morphological change, DNA encapsulation and ice crystal formation, and analyzing their correlation. Following ice crystal formation, the shape of spherical GUVs was altered and membrane integrity was damaged and this was found to be a necessary condition for encapsulation. Heating alone had no effects on DNA encapsulation, but was helpful for restoring the spherical shape and membrane integrity of GUVs damaged during freezing. These results suggested that freeze-thaw could promote the encapsulation of DNA into GUVs by a mechanism: the vesicle membrane was breached by ice crystal formation during freezing, DNA entered into damaged GUVs through these membrane gaps and was encapsulated after the membrane was resealed during the thawing process. The process described herein therefore describes a simple way for the encapsulation of nucleic acids and potentially other macromolecules into lipid vesicles, a process by which early protocells might have formed.

Flexible Proteins at the Origin of Life

Almost all modern proteins possess well-defined, relatively rigid scaffolds that provide structural preorganization for desired functions. Such scaffolds require the sufficient length of a polypeptide chain and extensive evolutionary optimization. How ancestral proteins attained functionality, even though they were most likely markedly smaller than their contemporary descendants, remains a major, unresolved question in the origin of life. On the basis of evidence from experiments and computer simulations, we argue that at least some of the earliest water-soluble and membrane proteins were markedly more flexible than their modern counterparts. As an example, we consider a small, evolved in vitro ligase, based on a novel architecture that may be the archetype of primordial enzymes. The protein does not contain a hydrophobic core or conventional elements of the secondary structure characteristic of modern water-soluble proteins, but instead is built of a flexible, catalytic loop supported by a small hydrophilic core containing zinc atoms. It appears that disorder in the polypeptide chain imparts robustness to mutations in the protein core. Simple ion channels, likely the earliest membrane protein assemblies, could also be quite flexible, but still retain their functionality, again in contrast to their modern descendants. This is demonstrated in the example of antiamoebin, which can serve as a useful model of small peptides forming ancestral ion channels. Common features of the earliest, functional protein architectures discussed here include not only their flexibility, but also a low level of evolutionary optimization and heterogeneity in amino acid composition and, possibly, the type of peptide bonds in the protein backbone.

Generating minimal living systems from non-living materials and increasing their evolutionary abilities

We review lessons learned about evolutionary transitions from a bottom-up construction of minimal life. We use a particular systemic protocell design process as a starting point for exploring two fundamental questions: (i) how may minimal living systems emerge from non-living materials? and (ii) how may minimal living systems support increasingly more evolutionary richness? Under (i), we present what has been accomplished so far and discuss the remaining open challenges and their possible solutions. Under (ii), we present a design principle we have used successfully both for our computational and experimental protocellular investigations, and we conjecture how this design principle can be extended for enhancing the evolutionary potential for a wide range of systems.This article is part of the themed issue 'The major synthetic evolutionary transitions'.

Sustainable proliferation of liposomes compatible with inner RNA replication

Although challenging, the construction of a life-like compartment via a bottom-up approach can increase our understanding of life and protocells. The sustainable replication of genome information and the proliferation of phospholipid vesicles are requisites for reconstituting cell growth. However, although the replication of DNA or RNA has been developed in phospholipid vesicles, the sustainable proliferation of phospholipid vesicles has remained difficult to achieve. Here, we demonstrate the sustainable proliferation of liposomes that replicate RNA within them. Nutrients for RNA replication and membranes for liposome proliferation were combined by using a modified freeze-thaw technique. These liposomes showed fusion and fission compatible with RNA replication and distribution to daughter liposomes. The RNAs in daughter liposomes were repeatedly used as templates in the next RNA replication and were distributed to granddaughter liposomes. Liposome proliferation was achieved by 10 cycles of iterative culture operation. Therefore, we propose the use of culturable liposomes as an advanced protocell model with the implication that the concurrent supplement of both the membrane material and the nutrients of inner reactions might have enabled protocells to grow sustainably.

The progene hypothesis: the nucleoprotein world and how life began

In this article, I review the results of studies on the origin of life distinct from the popular RNA world hypothesis. The alternate scenario postulates the origin of the first bimolecular genetic system (a polynucleotide gene and a polypeptide processive polymerase) with simultaneous replication and translation and includes the following key features: 1. The bimolecular genetic system emerges not from mononucleotides and monoamino acids, but from progenes, namely, trinucleotides aminoacylated on 3'-end by a non-random amino acid (NpNpNp ~ pX ~ Aa, where N--deoxyribo- or ribonucleoside, p--phosphate, X--a bifunctional agent, for example ribose, Aa--amino acid, ~ macroerge bond). Progenes are used as substrates for simultaneous synthesis of a polynucleotide and a polypeptide. Growth of the system is controlled by the growing polypeptide, and the bimolecular genetic system emerges as an extremely rare event. The first living being (virus-like organism protoviroid, Protoviroidum primum) arises and reproduces in prebiotic liposome-like structures using progenes. A population of protoviroids possessing the genetic system evolves in accordance with the Darwinian principle. Early evolution from protoviroid world to protocell world is shortly described. 2. The progene forming mechanism (NpNp + Np ~ pX ~ Aa) makes it possible to explain the emergence of the prebiotic physicochemical group genetic code, as well as the selection of organic compounds for the future genetic system from the racemic environment. 3. The protoviroid is reproduced on a progene basis via replicative transcription-translation (RTT, the first molecular genetic process) that is similar to its modern counterparts. Nothing is required for the emergence and reproduction of the protoviroid except for progenes and conditions for their formation. 4. The general scheme of early evolution is as follows: prebiotic world → protoviroid (nucleoprotein) world → protocell (DNA-RNA-protein) world → LUCA (Last Universal Common Ancestor) → modern cell world. This scheme exclude the existence of an independent RNA world as predecessor of the cellular world.

Controlled Transcription of Exogenous mRNA in Platelets Using Protocells

Transcribing exogenous RNA in eukaryotic cells requires delivering DNA to their nuclei and changing their genome. Nuclear delivery is often inefficient, limiting the potential scope of gene therapy and synthetic biology. These challenges may be overcome by techniques that allow for extranucleate transcription within eukaryotic cells. Protocells have been developed that enable transcription inside of liposomes; however, it has not yet been demonstrated whether this technology can be extended for use within eukaryotic cells. Here we show RNA-synthesizing nanoliposomes allow transcription of exogenous RNA inside anucleate cells. To accomplish this, components of transcription were encapsulated into liposomes and delivered to platelets. These liposomes were capable of light-induced transcription in platelets, providing proof-of-concept that protocell technology can be adapted for use within mammalian cells.

Progress toward synthetic cells

The complexity of even the simplest known life forms makes efforts to synthesize living cells from inanimate components seem like a daunting task. However, recent progress toward the creation of synthetic cells, ranging from simple protocells to artificial cells approaching the complexity of bacteria, suggests that the synthesis of life is now a realistic goal. Protocell research, fueled by advances in the biophysics of primitive membranes and the chemistry of nucleic acid replication, is providing new insights into the origin of cellular life. Parallel efforts to construct more complex artificial cells, incorporating translational machinery and protein enzymes, are providing information about the requirements for protein-based life. We discuss recent advances and remaining challenges in the synthesis of artificial cells, the possibility of creating new forms of life distinct from existing biology, and the promise of this research for gaining a deeper understanding of the nature of living systems.

Piecing Together Cell-like Systems

Several laboratories are pursuing the synthesis of cellular systems from different directions, including those that begin with simple chemicals to those that exploit existing cells. The methods that begin with nonliving components tend to focus on mimicking specific features of life, such as genomic replication, protein synthesis, sensory systems, and compartment formation, growth, and division. Conversely, the more prevalent synthetic biology approaches begin with something that is already alive and seek to impart new behavior on existing cells. Here we discuss advances in building cell-like systems that mimic key features of life with defined components.

Effects of compartment size on the kinetics of intracompartmental multimeric protein synthesis

The cell contents are encapsulated within a compartment, the volume of which is a fundamental physical parameter that may affect intracompartmental reactions. However, there have been few studies to elucidate whether and how volume changes alone can affect the reaction kinetics. It is difficult to address these questions in vivo, because forced cell volume changes, e.g., by osmotic inflation/deflation, globally alters the internal state. Here, we prepared artificial cell-like compartments with different volumes but with identical constituents, which is not possible with living cells, and synthesized two tetrameric enzymes, β-glucuronidase (GUS) and β-galactosidase (GAL), by cell-free protein synthesis. Tetrameric GUS but not GAL was synthesized more quickly in smaller compartments. The difference between the two was dependent on the rate-limiting step and the reaction order. The observed acceleration mechanism would be applicable to living cells as multimeric protein synthesis in a microcompartment is ubiquitous in vivo.

Cell-free protein synthesis inside giant unilamellar vesicles analyzed by flow cytometry

Lipid vesicles have been used as model cell systems, in which an in-vitro transcription-translation system (IVTT) is encapsulated to carry out intravesicular protein synthesis. Despite a large number of previous studies, a quantitative understanding of how protein synthesis inside the vesicles is affected by the lipid membrane remains elusive. This is mainly because of the heterogeneity in structural properties of the lipid vesicles used in the experiments. We investigated the effects of the phospholipid membrane on green fluorescent protein (GFP) synthesis occurring inside cell-sized giant unilamellar vesicles (GUV), which have a defined quantity of lipids relative to the reaction volume. We first developed a method to distinguish GUV from multilamellar vesicles using flow cytometry (FCM). Using this method, we investigated the time course of GFP synthesis using one of the IVTT, the PURE system, and found that phospholipid in the form of GUV has little effect on GFP synthesis based on three lines of investigation. (1) GFP synthesis inside the GUV was not dependent on the size of GUV (2) or on the fraction of cholesterol or anionic phospholipid constituting the GUV, and (3) GFP synthesis proceeded similarly in GUV and in the test tube. The present results suggest that GUV provides an ideal reaction environment that does not affect the internal biochemical reaction. On the other hand, we also found that internal GFP synthesis is strongly dependent on the chemical composition of the outer solution.

Synthetic cells and organelles: compartmentalization strategies

The recent development of RNA replicating protocells and capsules that enclose complex biosynthetic cascade reactions are encouraging signs that we are gradually getting better at mastering the complexity of biological systems. The road to truly cellular compartments is still very long, but concrete progress is being made. Compartmentalization is a crucial natural methodology to enable control over biological processes occurring within the living cell. In fact, compartmentalization has been considered by some theories to be instrumental in the creation of life. With the advancement of chemical biology, artificial compartments that can mimic the cell as a whole, or that can be regarded as cell organelles, have recently received much attention. The membrane between the inner and outer environment of the compartment has to meet specific requirements, such as semi-permeability, to allow communication and molecular transport over the border. The membrane can either be built from natural constituents or from synthetic polymers, introducing robustness to the capsule.

Achievements and open questions in the self-reproduction of vesicles and synthetic minimal cells

Supramolecular chemistry was enriched, about twenty years ago, by the discovery of the self-reproduction of micelles and vesicles. The dynamic aspects and complexity of these systems makes them good models for biological compartments. For example, the self-reproduction of vesicles suggests that the growth in size and number of a vesicle population resembles the pattern of living cells in several aspects, but it take place solely due to physical forces. Several reports demonstrate that reverse micelles, micelles, sub-micrometric and giant vesicles can self-reproduce, generating new particles at the expenses of a suitable precursor. Recently, similar studies are in progress on more complex vesicle-based systems, namely semi-synthetic minimal cells. These are artificial cell-like compartments that are built by filling liposomes with the minimal number of biomolecules, such as DNA, ribosomes, enzymes, etc., in order to construct a living cell in the laboratory. This approach aims to investigate the minimal requirements for molecular systems in order to display some living properties, while it finds relevance in origins of life studies and in synthetic (constructive) biology.

Constructing partial models of cells

Understanding the origin of life requires knowledge not only of the origin of biological molecules such as amino acids, nucleotides and their polymers, but also the manner in which those molecules are integrated into the organized systems that characterize cellular life. In this article, we introduce a constructive approach to understand how biological molecules can be arranged to achieve a higher-order biological function: replication of genetic information.

Membrane transport in primitive cells

Although model protocellular membranes consisting of monoacyl lipids are similar to membranes composed of contemporary diacyl lipids, they differ in at least one important aspect. Model protocellular membranes allow for the passage of polar solutes and thus can potentially support cell-to functions without the aid of transport machinery. The ability to transport polar molecules likely stems from increased lipid dynamics. Selectively permeable vesicle membranes composed of monoacyl lipids allow for many lifelike processes to emerge from a remarkably small set of molecules.

The emergence of ribozymes synthesizing membrane components in RNA-based protocells

A significant problem of the origin of life is the emergence of cellular self-replication. In the context of the "RNA world", a crucial concern is how the RNA-based protocells could achieve the ability to produce their own membrane. Here we show, with the aid of a computer simulation, that for these protocells, there would be "immediately" a selection pressure for the emergence of a ribozyme synthesizing membrane components. The ribozyme would promote the enlargement of cellular space and favor the incoming (by permeation) of RNA's precursors, thus benefit the replication of inner RNA, including itself. Via growth and division, protocells containing the ribozyme would achieve superiority and spread in the system, and meanwhile the ribozyme would spread in the system. The present work is inspiring because it suggests that the transition from molecular self-replication to cellular self-replication might have occurred naturally (and necessarily) in the origin of life, leading to the emergence of Darwinian evolution at the cellular level.

Chemical evolution of amphiphiles: glycerol monoacyl derivatives stabilize plausible prebiotic membranes

The self-assembly of simple amphiphiles like fatty acids into cell-like membranous structures suggests that such structures were available on prebiotic Earth to support the origin of cellular life. However, the composition of primitive membranes remains unclear because the physical properties of the aqueous environment in which they assembled are relatively unconstrained in terms of temperature, pH, and ionic concentrations. It seems likely that early membranes were composed of mixtures of various amphiphiles in an aqueous medium warmed by geothermal activity prevalent in the Archean era. To better understand the properties of mixed bilayers formed by binary mixtures of single-chain amphiphiles under these conditions, we conducted stability experiments, using membranes composed of various fatty acids having hydrocarbon chain length between 8 and 18 carbons, in mixtures with their glycerol monoacyl amphiphile derivatives (GMAs). The parameters investigated were critical vesicle concentration (CVC), encapsulation, and temperature-dependent stability. We found that hydrocarbon chain length and the presence of GMAs were major factors related to membrane stability. As chain length increased, GMA additions decreased the CVC of the mixtures 4- to 9-fold. Encapsulation ability also increased significantly as a function of chain length, which reduced permeation of small marker molecules. However, long exposures to temperatures in excess of 60 degrees C resulted in a total release of encapsulated solutes and extensive mixing of the membrane components between vesicles. We conclude that GMAs can significantly increase the stability of mixed amphiphile membranes, but further studies are required to establish model membranes that are stable at elevated temperatures.

Peptide membranes in chemical evolution

Simple surfactants achieve remarkable long-range order in aqueous environments. This organizing potential is seen most dramatically in biological membranes where phospholipid assemblies both define cell boundaries and provide a ubiquitous structural scaffold for controlling cellular chemistry. Here we consider simple peptides that also spontaneously assemble into exceptionally ordered scaffolds, and review early data suggesting that these structures maintain the functional diversity of proteins. We argue that such scaffolds can achieve the required molecular order and catalytic agility for the emergence of chemical evolution.

Permeation of membranes by ribose and its diastereomers

It was recently found that ribose permeates membranes an order of magnitude faster than its diastereomers arabinose and xylose (Sacerdote, M. G.; Szostak, J. W. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 6004). On this basis it was hypothesized that differences in membrane permeability to aldopentoses provide a mechanism for preferential delivery of ribose to primitive cells for subsequent selective incorporation into nucleotides and their polymers. However, the origins of these unusually large differences have not been well understood. We address this issue in molecular dynamics simulations combined with free energy calculations. It is found that the free energy of transferring ribose from water to the bilayer is lower by 1.5-2 kcal/mol than the barrier for transferring the other two aldopentoses. The calculated and measured permeability coefficients are in excellent agreement. The sugar structures that permeate the membrane are beta-pyranoses, with a possible contribution of the alpha-anomer for arabinose. The furanoid form of ribose is not substantially involved in permeation, even though it is non-negligibly populated in aqueous solution. The differences in free energy of transfer between ribose and arabinose or xylose are attributed, at least in part, to stronger highly cooperative, intramolecular interactions between consecutive exocyclic hydroxyl groups, which are stable in nonpolar media but rare in water. Water/hexadecane partition coefficients of the sugars obtained from separate molecular dynamics simulations correlate with the calculated permeability coefficients, in qualitative agreement with the Overton rule. The relevance of our calculations to understanding the origins of life is discussed.

Polymer hydrogel capsules: en route toward synthetic cellular systems

Engineered synthetic cellular systems are expected to become a powerful biomedical platform for the development of next-generation therapeutic carrier vehicles. In this mini-review, we discuss the potential of polymer capsules derived by the layer-by-layer assembly as a platform system for the construction of artificial cells and organelles. We outline the characteristics of polymer capsules that make them unique for these applications, and we describe several successful examples of microencapsulated catalysis, including biologically relevant enzymatic reactions. We also provide examples of subcompartmentalized polymer capsules, which represent a major step toward the creation of synthetic cells.

Sugar synthesis in a protocellular model leads to a cell signalling response in bacteria

The design of systems with life-like properties from simple chemical components may offer insights into biological processes, with the ultimate goal of creating an artificial chemical cell that would be considered to be alive. Most efforts to create artificial cells have concentrated on systems based on complex natural molecules such as DNA and RNA. Here we have constructed a lipid-bound protometabolism that synthesizes complex carbohydrates from simple feedstocks, which are capable of engaging the natural quorum sensing mechanism of the marine bacterium Vibrio harveyi and stimulating a proportional bioluminescent response. This encapsulated system may represent the first step towards the realization of a cellular 'mimic' and a starting point for 'bottom-up' designs of other chemical cells, which could perhaps display complex behaviours such as communication with natural cells.

Self-assembly and function of primitive cell membranes

We describe possible pathways for separating amphiphilic molecules from organic material on the early earth to form membrane-bound structures required for the start of cellular life. We review properties of the first membranes and their function as permeability barriers. Finally, we discuss the emergence of protein-mediated ion transport across membranes, which facilitated many other cellular functions.

Model protocells from single-chain lipids

Significant progress has been made in the construction of laboratory models of protocells. Most frequently the developed vesicle systems utilize single-chain lipids rather than the double-chain lipids typically found in biological membranes. Although single-chain lipids yield less robust vesicles, their dynamic characteristics are highly exploitable for protocellular functions. Herein the advantages of using single-chain lipids in the construction of protocells are discussed.

Compositional inheritance: comparison of self-assembly and catalysis

Genetic inheritance in modern cells is due to template-directed replication of nucleic acids. However, the difficulty of prebiotic synthesis of long information-carrying polymers like RNA raises the question of whether some other form of heredity is possible without polymers. As an alternative, the lipid world theory has been proposed, which considers non-covalent assemblies of lipids, such as micelles and vesicles. Assemblies store information in the form of a non-random molecular composition, and this information is passed on when the assemblies divide, i.e. the assemblies show compositional inheritance. Here, we vary several important assumptions of previous lipid world models and show that compositional inheritance is relevant more generally than the context in which it was originally proposed. Our models assume that interaction occurs between nearest neighbour molecules only, and account for spatial segregation of molecules of different types within the assembly. We also draw a distinction between a self-assembly model, in which the composition is determined by mutually favourable interaction energies between the molecules, and a catalytic model, in which the composition is determined by mutually favourable catalysis. We show that compositional inheritance occurs in both models, although the self-assembly case seems more relevant if the molecules are simple lipids. In the case where the assemblies are composed of just two types of molecules, there is a strong analogy with the classic two-allele Moran model from population genetics. This highlights the parallel between compositional inheritance and genetic inheritance.

Template-directed synthesis of a genetic polymer in a model protocell

Contemporary phospholipid-based cell membranes are formidable barriers to the uptake of polar and charged molecules ranging from metal ions to complex nutrients. Modern cells therefore require sophisticated protein channels and pumps to mediate the exchange of molecules with their environment. The strong barrier function of membranes has made it difficult to understand the origin of cellular life and has been thought to preclude a heterotrophic lifestyle for primitive cells. Although nucleotides can cross dimyristoyl phosphatidylcholine membranes through defects formed at the gel-to-liquid transition temperature, phospholipid membranes lack the dynamic properties required for membrane growth. Fatty acids and their corresponding alcohols and glycerol monoesters are attractive candidates for the components of protocell membranes because they are simple amphiphiles that form bilayer membrane vesicles that retain encapsulated oligonucleotides and are capable of growth and division. Here we show that such membranes allow the passage of charged molecules such as nucleotides, so that activated nucleotides added to the outside of a model protocell spontaneously cross the membrane and take part in efficient template copying in the protocell interior. The permeability properties of prebiotically plausible membranes suggest that primitive protocells could have acquired complex nutrients from their environment in the absence of any macromolecular transport machinery; that is, they could have been obligate heterotrophs.

Question 9: prospects for the construction of artificial cells or protocells

The construction of artificial cells or protocells that are a simplified version of contemporary cells will have implications for both the understanding of the origins of cellular Life and the design of "cell-like" chemical factories. In this short communication, we discuss the progress and remaining issues related to the construction of protocells from metabolic products. We further outline the de novo design of a simple chemical system that mimics the functional properties of a living cell without being composed of molecules of biological origin, thereby addressing issues related to Life's origins.