Artificial cells: prospects for biotechnology

Protein synthesis in liposomes with a minimal set of enzymes

In a significant step towards the construction of the semi-synthetic minimal cell, a protein expression system with a minimal set of pure and specific enzymes is required. A novel cell-free transcription and translation system named PURESYSTEM (PS), consisting of a specified set of 36 enzymes and ribosomes, has been entrapped in POPC liposomes for protein synthesis. The PS has been used to transcribe and translate an Enhanced Green Fluorescent Protein (EGFP) gene from plasmid DNA. The synthesis is confirmed by the EGFP fluorescence emitting liposomes on fluorometric analysis and on confocal microscopy analysis. Furthermore the PS encapsulated into POPC liposomes can drive the expression of the plsB and plsC genes encoding for the sn-glycerol-3-phosphate acyltransferase (GPAT) and 1-acyl-sn-glycerol-3-phosphate acyltransferase (LPAAT) involved in the first step of the "salvage pathway" for synthesis of POPC. The expression of GPAT and LPAAT in liposomes would in principle allow the production of the cell boundary from within.

Structural analyses of a hypothetical minimal metabolism

By integrating data from comparative genomics and large-scale deletion studies, we previously proposed a minimal gene set comprising 206 protein-coding genes. To evaluate the consistency of the metabolism encoded by such a minimal genome, we have carried out a series of computational analyses. Firstly, the topology of the minimal metabolism was compared with that of the reconstructed networks from natural bacterial genomes. Secondly, the robustness of the metabolic network was evaluated by simulated mutagenesis and, finally, the stoichiometric consistency was assessed by automatically deriving the steady-state solutions from the reaction set. The results indicated that the proposed minimal metabolism presents stoichiometric consistency and that it is organized as a complex power-law network with topological parameters falling within the expected range for a natural metabolism of its size. The robustness analyses revealed that most random mutations do not alter the topology of the network significantly, but do cause significant damage by preventing the synthesis of several compounds or compromising the stoichiometric consistency of the metabolism. The implications that these results have on the origins of metabolic complexity and the theoretical design of an artificial minimal cell are discussed.

Question 7: modelling minimal 'lipid-peptide' cells

This contribution is aimed to give support to 'bottom-up' approaches to the minimal or early cell research project. Even if, from this perspective, the most simple living cell still seems very far away, the analysis of less complex, infrabiological cellular systems (some of which could be relatively soon implemented in the lab) probably holds the key, or one of the fundamental keys, to the problem of origins of life. On these lines, we propose a simulation model to study the transition from proto-metabolic 'lipid' cells to 'lipid-peptide' cells, as a critical step in which self-reproducing vesicles could develop into more functionalized supramolecular systems.

Question 8: bridging the gap between in silico and in vitro approaches to minimal cells

In this short paper we argue for the relevance and value of theoretical models in the field of origins of life, but also claim that both theoreticians and experimentalists should make an effort to come together and interact more closely to obtain more fruitful and significant results. As an example, we present our own modeling approach to protocell dynamics, including some simulation results, to show that it is possible to develop computational tools that start bridging that traditional gap between theory and experiments.

Dynamic microcompartmentalization of giant unilamellar vesicles by sol-gel transition and temperature induced shrinking/swelling of poly(N-isopropyl acrylamide)

Giant unilamellar vesicles (GUVs) were microinjected with aqueous solutions of poly(-isopropyl acrylamide) (PNIPAAm). Temperature-dependent sol-gel phase transitions of the solutions, followed by shrinking and swelling of the resulting hydrogel, were studied in the presence of a variety of co-solutes within the GUV. Reversible formation of a dense, spherical hydrogel structure (compartment) was observed in all cases with defined shrinking/swelling behaviour at temperatures above the lower critical solution temperatures (LCSTs). Nanotube-mediated merging of two vesicles with thus formed compartments resulted in a single GUV with two internalized hydrogel structures. As an application example, we demonstrate how fluorescent nanoparticles can be immobilized in such gel structures.

Chemical Aspects of Synthetic Biology

Synthetic biology as a broad and novel field has also a chemical branch: whereas synthetic biology generally has to do with bioengineering of new forms of life (generally bacteria) which do not exist in nature, 'chemical synthetic biology' is concerned with the synthesis of chemical structures such as proteins, nucleic acids, vesicular forms, and other which do not exist in nature. Three examples of this 'chemical synthetic biology' approach are given in this article. The first example deals with the synthesis of proteins that do not exist in nature, and dubbed as 'the never born proteins' (NBPs). This research is related to the question why and how the protein structures existing in our world have been selected out, with the underlying question whether they have something very particular from the structural or thermodynamic point of view (for example, the folding). The NBPs are produced in the laboratory by the modern molecular biology technique, the phage display, so as to produce a very large library of proteins having no homology with known proteins. The second example of chemical synthetic biology has also to do with the laboratory synthesis of proteins, but, this time, adopting a prebiotic synthetic procedure, the fragment condensation of short peptides, where short means that they have a length that can be obtained by prebiotic methods; for example, from the condensation of N-carboxy anhydrides. The scheme is illustrated and discussed, being based on the fragment condensation catalyzed by peptides endowed with proteolitic activity. Selection during chain growth is determined by solubility under the contingent environmental conditions, i.e., the peptides which result insoluble are eliminated from further growth. The scheme is tested preliminarily with a synthetic chemical fragment-condensation method and brings to the synthesis of a 44-residues-long protein, which has no homology with known proteins, and which has a stable tertiary folding. Finally, the third example, dubbed as 'the minimal cell project'. Here, the aim is to synthesize a cell model having the minimal and sufficient number of components to be defined as living. For this purpose, liposomes are used as shell membranes, and attempts are made to introduce in the interior a minimal genome. Several groups all around the world are active in this field, and significant results have been obtained, which are reviewed in this article. For example, protein expression has been obtained inside liposomes, generally with the green fluorescent protein, GFP. Our last attempts are with a minimal genome consisting of 37 enzymes, a set which is able to express proteins using the ribosomal machinery. These minimal cells are not yet capable of self-reproduction, and this and other shortcomings within the project are critically reviewed.

Life cycle of a minimal protocell--a dissipative particle dynamics study

Cross-reactions and other systematic difficulties generated by the coupling of functional chemical subsystems pose the largest challenge for assembling a viable protocell in the laboratory. Our current work seeks to identify and clarify such key issues as we represent and analyze in simulation a full implementation of a minimal protocell. Using a 3D dissipative particle dynamics simulation method, we are able to address the coupled diffusion, self-assembly, and chemical reaction processes required to model a full life cycle of a protocell composed of coupled genetic, metabolic, and container subsystems. Utilizing this minimal structural and functional representation of the constituent molecules, their interactions, and their reactions, we identify and explore the nature of the many linked processes for the full protocellular system. Obviously the simplicity of this simulation method combined with the inherent system complexity prevents us from expecting quantitative simulation predictions from these investigations. However, we report important findings on systemic processes, some previously predicted and some newly discovered, as we couple the protocellular self-assembly processes and chemical reactions.

Quantification of structural properties of cell-sized individual liposomes by flow cytometry

We describe a new high-throughput method of quantifying the structural properties of individual cell-sized liposomes. An internal aqueous solution of liposomes was labeled with a green fluorescent marker and the membrane with a red marker. The double-labeled liposomes were analyzed using flow cytometry, and the internal aqueous volume and lipid membrane volume of each liposome were measured. The experimental results indicate that both the internal aqueous and lipid membrane volumes positively correlate with the intensity of forward-scatter (FS) and side-scatter (SS) signals in a logarithmic scale. In addition, liposomes in 18 small areas gated by log(FS) and log(SS) were sorted by fluorescence-activated cell sorting (FACS), and observed by optical microscopy. Structural characteristics observed in the microscopy images of heterogeneous liposomes correlated with FACS data. Because this method does not employ any particular assumption about the shape and structure of liposomes, flow cytometry is a powerful tool for estimating the internal and membrane volumes of individual cell-sized liposomes with heterogeneous shapes and structures.

Towards synthesis of a minimal cell

Construction of a chemical system capable of replication and evolution, fed only by small molecule nutrients, is now conceivable. This could be achieved by stepwise integration of decades of work on the reconstitution of DNA, RNA and protein syntheses from pure components. Such a minimal cell project would initially define the components sufficient for each subsystem, allow detailed kinetic analyses and lead to improved in vitro methods for synthesis of biopolymers, therapeutics and biosensors. Completion would yield a functionally and structurally understood self-replicating biosystem. Safety concerns for synthetic life will be alleviated by extreme dependence on elaborate laboratory reagents and conditions for viability. Our proposed minimal genome is 113 kbp long and contains 151 genes. We detail building blocks already in place and major hurdles to overcome for completion.

Experimentally tracing the key steps in the origin of life: The aromatic world

Life is generally believed to emerge on Earth, to be at least functionally similar to life as we know it today, and to be much simpler than modern life. Although minimal life is notoriously difficult to define, a molecular system can be considered alive if it turns resources into building blocks, replicates, and evolves. Primitive life may have consisted of a compartmentalized genetic system coupled with an energy-harvesting mechanism. How prebiotic building blocks self-assemble and transform themselves into a minimal living system can be broken into two questions: (1) How can prebiotic building blocks form containers, metabolic networks, and informational polymers? (2) How can these three components cooperatively organize to form a protocell that satisfies the minimal requirements for a living system? The functional integration of these components is a difficult puzzle that requires cooperation among all the aspects of protocell assembly: starting material, reaction mechanisms, thermodynamics, and the integration of the inheritance, metabolism, and container functionalities. Protocells may have been self-assembled from components different from those used in modern biochemistry. We propose that assemblies based on aromatic hydrocarbons may have been the most abundant flexible and stable organic materials on the primitive Earth and discuss their possible integration into a minimal life form. In this paper we attempt to combine current knowledge of the composition of prebiotic organic material of extraterrestrial and terrestrial origin, and put these in the context of possible prebiotic scenarios. We also describe laboratory experiments that might help clarify the transition from nonliving to living matter using aromatic material. This paper presents an interdisciplinary approach to interface state of the art knowledge in astrochemistry, prebiotic chemistry, and artificial life research.

Phenotypic diversity and chaos in a minimal cell model

Gánti's chemoton model (Gánti, T., 2002. On the early evolution of biological periodicity. Cell. Biol. Int. 26, 729) is considered as an iconic example of a minimal protocell including three key subsystems: membrane, metabolism and information. The three subsystems are connected through stoichiometrical coupling which ensures the existence of a replication cycle for the chemoton. Our detailed exploration of a version of this model indicates that it displays a wide range of complex dynamics, from regularity to chaos. Here, we report the presence of a very rich set of dynamical patterns potentially displayed by a protocell as described by this implementation of a chemoton-like model. The implications for early cellular evolution and synthesis of artificial cells are discussed.

Toward the Emergence of Nanoneurosurgery: Part III—Nanomedicine: Targeted Nanotherapy, Nanosurgery, and Progress Toward the Realization of Nanoneurosurgery

The notion of nanotechnology has evolved since its inception as a fantastic conceptual idea to its current position as a mainstream research initiative with broad applications among all divisions of science. In the first part of this series, we reviewed the structures and principles that comprise the main body of knowledge of nanoscience and nanotechnology. In the second part, we discussed applications of nanotechnology to the emerging field of nanomedicine, with specific attention on medical diagnostics and imaging. This article further explores the applications of nanotechnology to nanomedicine. Specific attention is given to developments in therapeutic modalities, including advanced drug delivery systems and targeted nanotherapy, which will form the basis for the treatment arm of mature nanomedicine. A variety of modalities are discussed, including polymeric nanoparticles, micelles, liposomes, dendrimers, fullerenes, hydrogels, nanoshells, and smart surfaces. Applications of nanotechnology to nanosurgery and nanoneurosurgery are presented. Femtosecond laser systems, nanoneedles, and nanotweezers are presented as technologies that are operational at the nanoscale level and have the potential to revolutionize the practice of neurosurgery in a profound and momentous way.

Approaches to semi-synthetic minimal cells: a review

Following is a synthetic review on the minimal living cell, defined as an artificial or a semi-artificial cell having the minimal and sufficient number of components to be considered alive. We describe concepts and experiments based on these constructions, and we point out that an operational definition of minimal cell does not define a single species, but rather a broad family of interrelated cell-like structures. The relevance of these researches, considering that the minimal cell should also correspond to the early simple cell in the origin of life and early evolution, is also explained. In addition, we present detailed data in relation to minimal genome, with observations cited by several authors who agree on setting the theoretical full-fledged minimal genome to a figure between 200 and 300 genes. However, further theoretical assumptions may significantly reduce this number (i.e. by eliminating ribosomal proteins and by limiting DNA and RNA polymerases to only a few, less specific molecular species). Generally, the experimental approach to minimal cells consists in utilizing liposomes as cell models and in filling them with genes/enzymes corresponding to minimal cellular functions. To date, a few research groups have successfully induced the expression of single proteins, such as the green fluorescence protein, inside liposomes. Here, different approaches are described and compared. Present constructs are still rather far from the minimal cell, and experimental as well as theoretical difficulties opposing further reduction of complexity are discussed. While most of these minimal cell constructions may represent relatively poor imitations of a modern full-fledged cell, further studies will begin precisely from these constructs. In conclusion, we give a brief outline of the next possible steps on the road map to the minimal cell.

Toward an artificial cell based on gene expression in vesicles

We present a new experimental approach to build an artificial cell using the translation machinery of a cell-free expression system as the hardware and a DNA synthetic genome as the software. This approach, inspired by the self-replicating automata of von Neumann, uses cytoplasmic extracts, encapsulated in phospholipid vesicles, to assemble custom-made genetic circuits to develop the functions of a minimal cell. Although this approach can find applications, especially in biotechnology, the primary goal is to understand how a DNA algorithm can be designed to build an operating system that has some of the properties of life. We provide insights on this cell-free approach as well as new results to transform step by step a long-lived vesicle bioreactor into an artificial cell. We show how the green fluorescent protein can be anchored to the membrane and we give indications of a possible insertion mechanism of integral membrane proteins. With vesicles composed of different phospholipids, the fusion protein alpha-hemolysin-eGFP can be expressed to reveal patterns on the membrane. The specific degradation complex ClpXP from E. coli is introduced to create a sink for the synthesized proteins. Perspectives and subsequent limitations of this approach are discussed.

The place of metabolism in the origin of life

Metabolism and replication are generally considered essential features of biological life. Workers in the field of the origin of life are mostly split into two groups, depending on which of these two functions is postulated to have occurred first. Because of difficulties encountered by the replication-first (or genetics-first) approach, some workers have postulated that a highly developed metabolism must have originated before replication and the formation of a genetic apparatus. However, as supporters of a replication-first approach have pointed out, and as is discussed in this article, the alternative metabolism-first approach has fundamental problems that have not been sufficiently addressed.

How was membrane permeability produced in an RNA world?

Darwinian evolution in an RNA World required that catalysts be encapsulated in membranes since this would allow superior catalysts to benefit from the products of their own reactions. However, typical membranes are relatively impermeable to polar and complex molecules and, thus, even primitive cells had to have RNA-based mechanisms for the uptake of external nutrients and the excretion of waste products. Nucleic acids form weak non-specific contacts with the surface of the lipid membrane in the presence of divalent cations, and strongly binding species can be obtained in the course of SELEX experiments. The only currently suggested mechanism for the production of permeability is through formation of supramolecular RNA complexes capable of destabilizing and transiently opening lipid membranes by action from one side, but neither natural nor selected examples of RNA channels or transmembrane shuttles are known so far. The necessity to evolve proteins could be strongly driven by the need to build defined hydrophobic structures that when integrated into membranes could provide selective permeability.

Interaction between complementary liposomes: a process leading to multicompartment systems formation

Interaction of complementary liposomes induces a series of processes, involving reorganization of their membrane lipids, which lead to the formation of large aggregates. In several cases these aggregates exhibit multicompartment structures and only primitively mimic, in some aspects at least, the multicompartmental features of cells. Similar multicompartment structures were repeatedly obtained following the interaction of a diversity of complementary liposomal pairs. Thus, a working hypothesis is proposed, according to which, molecular recognition of liposomes induces the formation of multicompartment structures.

Expression of a cascading genetic network within liposomes

Liposomes have long been used as possible compartments for artificial cells, and it has been shown that liposomes can sustain various types of biochemical reactions. To elevate the degree of molecular complexity of the system in liposomes, we have constructed a two-stage genetic network encapsulated in liposomes. This two-stage genetic network was constructed with the plasmid pTH, in which the protein product of the first stage (T7 RNA polymerase) is required to drive the protein synthesis of the second stage (GFP). We show that the two-stage genetic network constructed in a cell-free expression system is functional within liposomes.

A vesicle bioreactor as a step toward an artificial cell assembly

An Escherichia coli cell-free expression system is encapsulated in a phospholipid vesicle to build a cell-like bioreactor. Large unilamellar vesicles containing extracts are produced in an oil-extract emulsion. To form a bilayer the vesicles are transferred into a feeding solution that contains ribonucleotides and amino acids. Transcription-translation of plasmid genes is isolated in the vesicles. Whereas in bulk solution expression of enhanced GFP stops after 2 h, inside the vesicle permeability of the membrane to the feeding solution prolongs the expression for up to 5 h. To solve the energy and material limitations and increase the capacity of the reactor, the alpha-hemolysin pore protein from Staphylococcus aureus is expressed inside the vesicle to create a selective permeability for nutrients. The reactor can then sustain expression for up to 4 days with a protein production of 30 muM after 4 days. Oxygen diffusion and osmotic pressure are critical parameters to maintain expression and avoid vesicle burst.

Determination of the core of a minimal bacterial gene set

The availability of a large number of complete genome sequences raises the question of how many genes are essential for cellular life. Trying to reconstruct the core of the protein-coding gene set for a hypothetical minimal bacterial cell, we have performed a computational comparative analysis of eight bacterial genomes. Six of the analyzed genomes are very small due to a dramatic genome size reduction process, while the other two, corresponding to free-living relatives, are larger. The available data from several systematic experimental approaches to define all the essential genes in some completely sequenced bacterial genomes were also considered, and a reconstruction of a minimal metabolic machinery necessary to sustain life was carried out. The proposed minimal genome contains 206 protein-coding genes with all the genetic information necessary for self-maintenance and reproduction in the presence of a full complement of essential nutrients and in the absence of environmental stress. The main features of such a minimal gene set, as well as the metabolic functions that must be present in the hypothetical minimal cell, are discussed.

Basic autonomy as a fundamental step in the synthesis of life

In the search for the primary roots of autonomy (a pivotal concept in Varela's comprehensive understanding of living beings), the theory of autopoiesis provided an explicit criterion to define minimal life in universal terms, and was taken as a guideline in the research program for the artificial synthesis of biological systems. Acknowledging the invaluable contribution of the autopoietic school to present biological thinking, we offer an alternative way of conceiving the most basic forms of autonomy. We give a bottom-up account of the origins of "self-production" (or self-construction, as we propose to call it), pointing out which are the minimal material and energetic requirements for the constitution of basic autonomous systems. This account is, indeed, committed to the project of developing a general theory of biology, but well grounded in the universal laws of physics and chemistry. We consider that the autopoietic theory was formulated in highly abstract terms and, in order to advance in the implementation of minimal autonomous systems (and, at the same time, make major progress in exploring the origins of life), a more specific characterization of minimal autonomous systems is required. Such a characterization will not be drawn from a review of the autopoietic criteria and terminology (à la Fleischaker) but demands a whole reformulation of the question: a proper naturalization of the concept of autonomy. Finally, we also discuss why basic autonomy, according to our account, is necessary but not sufficient for life, in contrast with Varela's idea that autopoiesis was a necessary and sufficient condition for it.

Origin of sex revisited

Why did sex ever arise in the first place? Why it does not disappear in view of the greater efficiency of asexuals? These are clearly two different questions, and we suggest here that the solution for the origin of sex does not necessarily come from theoretical considerations based on currently existing genetic systems. Thus, while we agree with a number of authors in that the emergence of sex (understood as the exchange of genetic material between genomes) is deeply rooted in the origin of life and happened during the very early stages in the transition from individual genes ('replicators') to bacteria-like cells ('reproducers'), we challenge the idea that recombinational repair was the major selective force for the emergence of sex. Taking the stochastic corrector model as a starting point, we provide arguments that question the putative costs of redundancy in primitive protocells. In addition, if genes that cause intragenomic conflict (i.e., parasites) are taken into account, it is certainly wrong to suggest that cellular fusion would be beneficial at the population level (although this strong claim needs some qualifications). However, when a continuous input of deleterious mutations that impair the fitness of the protocell as a whole is considered in the model (in the realistic range in which stable mutant distributions of quasi-species within compartments are established), there are circumstances when sex could be beneficial as a side effect of the dynamic equilibrium between cellular fusion-mutation-selection. The scenario we have explored numerically is fully consistent with the idea that the universal ancestor was not a discrete entity but an ensemble of proto-organisms that exchanged much genetic information.

Nanotechnology: convergence with modern biology and medicine

The worldwide emergence of nanoscale science and engineering was marked by the announcement of the National Nanotechnology Initiative (NNI) in January 2000. Recent research on biosystems at the nanoscale has created one of the most dynamic science and technology domains at the confluence of physical sciences, molecular engineering, biology, biotechnology and medicine. This domain includes better understanding of living and thinking systems, revolutionary biotechnology processes, the synthesis of new drugs and their targeted delivery, regenerative medicine, neuromorphic engineering and developing a sustainable environment. Nanobiosystems research is a priority in many countries and its relevance within nanotechnology is expected to increase in the future.

Autopoiesis: a review and a reappraisal

The aim of the paper is to review critically the notion of autopoiesis as presented by Maturana and Varela. In particular, recognizing that there are difficulties in obtaining a complete and clear picture from the primary literature, an effort is made to present a coherent view-also based on many years of personal contact with Francisco Varela. The paper begins with a few historical notes to highlight the cultural background from which the notion of autopoiesis arose. The basic principles of autopoiesis as a theory of cellular life are then described, emphasizing also what autopoiesis is not: not an abstract theory, not a concept of artificial life, not a theory about the origin of life-but rather a pragmatic blueprint of life based on cellular life. It shown how this view leads to a conceptually clear definition of minimal life and to a logical link with related notions, such as self-organization, emergence, biological autonomy, auto-referentiality, and interactions with the environment. The perturbations brought about by the environment are seen as changes selected and triggered by the inner organization of the living. These selective coupling interactions impart meaning to the minimal life and are thus defined by Maturana and Varela with the arguable term of "cognition". This particular view on the mutual interactions between living organism and environment leads these authors to the notion of "enaction", and to the surprising view that autopoiesis and cognition are two complementary, and in a way equivalent, aspects of life. It is then shown how cognition, so defined, permits us to build a bridge between biology and cognitive science. Autopoiesis also allows one to conceive chemical models of minimal cellular life that can be implemented experimentally. The corresponding work on "chemical autopoiesis" is then reviewed. The surprising impact of autopoiesis in the social sciences ("social autopoiesis") is also briefly discussed. This review also comments on why the theory of autopoiesis had, and still has, a difficult time being accepted into the mainstream of life-science research. Finally, it is pointed out that the new interest in system biology and complexity theories may lead to a reappraisal of autopoiesis and related notions, as outlined also by other authors, such as Tibor Ganti and Stuart Kauffmann.

Bridging nonliving and living matter

Assembling non-biological materials (geomaterials) into a proto-organism constitutes a bridge between nonliving and living matter. In this article we present a simple step-by-step route to assemble a proto-organism. Many pictures have been proposed to describe this transition within the origins-of-life and artificial life communities, and more recently alternative pictures have been emerging from advances in nanoscience and biotechnology. The proposed proto-organism lends itself to both traditions and defines a new picture based on a simple idea: Given a set of required functionalities, minimize the physicochemical structures that support these functionalities, and make sure that all structures self-assemble and mutually enhance each other's existence. The result is the first concrete, rational design of a simple physicochemical system that integrates the key functionalities in a thermodynamically favorable manner as a lipid aggregate integrates proto-genes and a proto-metabolism. Under external pumping of free energy, the metabolic processes produce the required building blocks, and only specific gene sequences enhance the metabolic kinetics sufficiently for the whole system to survive. We propose an experimental implementation of the proto-organism with a discussion of our experimental results, together with relevant results produced by other experimental groups, and we specify what is still missing experimentally. Identifying the missing steps is just as important as providing the road map for the transition. We derive the kinetic and thermodynamic conditions of each of the proto-organism subsystems together with relevant theoretical and computational results about these subsystems. We present and discuss detailed 3D simulations of the lipid aggregation processes. From the reaction kinetics we derive analytical aggregate size distributions, and derive key properties of the metabolic efficiency and stability. Thermodynamics and kinetics of the ligation directed self-replication of the proto-genes is discussed, and we summarize the full life cycle of the proto-organism by comparing size, replication time, and energy with the biomass efficiency of contemporary unicells. Finally, we also compare our proto-organism picture with existing origins-of-life and protocell pictures. By assembling one possible bridge between nonliving and living matter we hope to provide a piece in the ancient puzzle about who we are and where we come from.

Toward the engineering of minimal living cells

The article focuses on the notion of a synthetic or semi-synthetic minimal cell, defined as a system that has the minimal and sufficient structural conditions for cellular life. It is emphasized that two complementary approaches are in principle possible, defined as "bottom-up" and "top-down" approaches. The first one aims at the construction of a minimal cell starting from scratch, and it is argued that a very serious bottle-neck to this pathway lies in the origination of specific macro-molecular sequences, as in nature those were constructed most likely by a particular contingent set of conditions. The top-down approaches utilize extant genes and enzymes, and the work in this case is based on the incorporation of the minimal and sufficient amount of such macromolecules into liposomes, as models for the shell of biological cells. The first phase of this ambitious project foresees the study of conditions under which complex molecular biology reactions takes place in the compartments of liposomes. Examples of these reactions are provided, for example, the production of RNA throughout Q-beta replicase in a self-reproducing vesicle system; or PC Reaction in phospholipid vesicles; or even the incorporation of ribosomes in liposomes, with the production of polypeptide chains. The use of giant vesicles is also illustrated. These systems, due to their large size, offer the advantage that by way of special micro-injection techniques, all sort of biochemical agents can be directly introduced in the compartment; and that the reaction can be followed by optical microscopy. In the final part of the article, the outlook of increasing the complexity of these liposome systems so as to arrive at first semi-synthetic cells is discussed.

The first cell membranes

Organic compounds are synthesized in the interstellar medium and can be delivered to planetary surfaces such as the early Earth, where they mix with endogenous species. Some of these compounds are amphiphilic, having polar and nonpolar groups on the same molecule. Amphiphilic compounds spontaneously self-assemble into more complex structures such as bimolecular layers, which in turn form closed membranous vesicles. The first forms of cellular life required self-assembled membranes that were likely to have been produced from amphiphilic compounds on the prebiotic Earth. Laboratory simulations show that such vesicles readily encapsulate functional macromolecules, including nucleic acids and polymerases. The goal of future investigations will be to fabricate artificial cells as models of the origin of life.