Protocell Design

Chem Soc Rev Dynamic Article Links Cite this: Chem. Soc. Rev., 2012, 41, 79–85 www.rsc.org/csr TUTORIAL REVIEW Designs for life: protocell models in the laboratory Alicja J. Dzieciol and Stephen Mann* Received 9th August 2011 DOI: 10.1039/c1cs15211d Compartmentalization of primitive biochemical reactions within membrane-bound water micro- droplets is considered an essential step in the origin of life. In the absence of complex biochemical machinery, the hypothetical precursors to the first biological cells (protocells) would be dependent on the self-organization of their components and physicochemical conditions of the environment to attain a basic level of autonomy and evolutionary viability. Many researchers consider the self-organization of lipid and fatty acid molecules into bilayer vesicles as a simple form of membrane-based compartmentalization that can be developed for the experimental design and construction of plausible protocell models. In this tutorial review, we highlight some of the recent advances and issues concerning the construction of simple cell-like systems in the laboratory. Overcoming many of the current scientific challenges should lead to new types of chemical bio-reactors and artificial cell-like entities, and bring new insights concerning the possible pathways responsible for the origin of life. 1. Introduction theories fail to explain the origin of cellularity—how did the first cells emerged in a world devoid of biological evolution? Along with the theory of evolution, the cell theory of life is the Solving this long-standing mystery is of deep significance as 1 most important generalisation in biology. The latter theory even small advances in our understanding of the origin of life states that biological cells are the basic components and will help to bridge the conspicuous disconnection between building blocks of all known living organisms, and that the non-living and living forms of matter, as well as contribute to primary unit of life is represented by the operation of a single the development of a unifying theory for the emergence of cell subjected to Darwinian evolution. However, these two key biology within a physical universe. The foundation of modern research on the origin of life can be traced back to Oparin who emphasized the concept of Centre for Organized Matter Chemistry, School of Chemistry, University of Bristol, Bristol BS8 1TS, UK. molecular evolution as a chemical progenitor of biological E-mail: [email protected] evolution.2 Oparin proposed that life developed from simple Alicja J. Dzieciol was born in Stephen Mann is Professor of 1989 and comes from Lower Chemistry, Director of the Silesia, Poland. She is studying Centre for Organized Matter for a MSci in Chemistry at the Chemistry, and Principal University of Bristol, UK. of the Bristol Centre for Currently in her final year, Functional Nanomaterials at she is pursuing a project the University of Bristol, UK. focusing on fatty acid vesicles His research interests are as protocell models under the focused on the chemical guidance of Professor Stephen synthesis, characterization Mann. In her spare time, she and emergence of complex enjoys long-distance cycling. forms of organized matter, including models of protocell assembly. Prof. Mann was Alicja J. Dzieciol Stephen Mann elected as a Fellow of the Royal Society, UK, in 2003. In 2011, he was awarded the Royal Society of Chemistry de Gennes Medal, and was a recipient of the Chemical Society of France, French-British Prize. This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 79–85 79 non-living molecules through a spontaneous and gradual build within the confines of the cell membrane8 only the core criteria up of molecular complexity. A commonly accepted hypothesis of life are of immediate interest for the design and construction is that the physicochemical conditions on the early Earth of protocell models (Fig. 1). All living cells share the following favoured chemical reactions that produced simple organic characteristics:9 compounds from inorganic precursors, and that these water- - a semi-permeable membrane that encloses the cell consti- soluble organic molecules underwent subsequent reactions to tuents and acts as a selective barrier between the cell interior generate an expanding library of molecular structures with and the external environment, regulating the flow of materials increasing complexity and new properties.3 The hypothesis in and out of the cell (compartmentalization). considers three of these properties—small-molecule catalysis, - genetic information carried in the form of double-stranded molecular self-generation, and amphiphilicity—as critical for molecules of DNA which is inherited by daughter cells during the emergence of a self-contained chemical system that respec- cell division. tively would provide the basis for the development of coupled - template polymerisation to copy hereditary information reaction cycles (primitive metabolism), self-replication (copying (replication). of informational molecules), and compartmentalization - transcription of the genetic information stored in DNA (self-assembly of enclosed membranes). How this could be into RNA, and translation of RNA into proteins (the ‘‘central achieved, at least in principle, is the subject of much current dogma’’). research in which plausible chemical and physical pathways - protein-based catalysts that participate in myriad chemical are modelled both experimentally and theoretically. This has transformations for self-maintainance and regeneration given rise to various scenarios in which proponents such as (metabolism), as well as in transcription, translation and DNA ribonucleic acid (RNA world),4 peptides/proteins (metabolic replication. This feedback loop between DNA and proteins is world)5 and lipid/fatty acids (compartmentalization-first the basis of the self-reproducing capacity of living cells. hypothesis)6 vie for precedence. - a steady state (homeostasis) based on a non-equilibrium At the same time, it has been increasingly recognised that system that requires a continuous influx of energy from the the integration and collective operation of primitive processes surroundings to sustain life, grow and divide. of replication, metabolism and compartmentalization, rather than their individual pre-eminence, could represent a critical step in the emergence of life.7 This view sees life as a systems property that is maintained under non-equilibrium conditions by flows of energy and matter from the surrounding environment. As a consequence, the system functions to increase its own viability through various selection pressures that ultimately lead to a Darwinian evolutionary capacity. Signifi- cantly, this notion of a systems-based criticality has focused recent attention on the design and construction of plausible models of autonomous chemical systems—a research field that might be called protobiology. Central to this activity is the concept of the protocell, which has been defined in various ways, ranging from a plausible representation of a hypothetical precursor to the first biological cell, through to the description of a synthetic cell-like entity that contains non-biologically relevant components. For most practical purposes, laboratory investigations aim to construct protocell Fig. 1 Systems representation of cellular life showing core criteria of immediate interest for the design and construction of protocell models involving simple membrane-bound cell-like structures models.7 Two primary mechanistic features must emerge for life; exhibiting at least some of the key characteristics of modern (i) a systems interface with the environment based on the cell cells. Examples of this approach are highlighted in this tutorial membrane and embedded protein-based sensors, and (ii) a systems review with particular attention placed on the use of lipid or network for internalized self-processing via processes of metabolism, fatty acid vesicles. Such studies should help to elucidate the replication etc. Together, these processes constitute a dissipative non- types of molecules and physical conditions compatible with equilibrium system that operates through nanoscale miniaturization the design and construction of protocells, and provide clues and is maintained by continuous active exchange between the intra- about the possible pre-biotic pathways responsible for the cellular milieu and surrounding environment. The self-referential formation of cellular progenitors on the early Earth. nature of life is manifest in the steady state of materials and energy fluxes (homeostasis), which necessitates that the hierarchical networks of internal self-processing must be capable of passively or actively 2. Key features of living cells for protocell assimilating novel environmental inputs into pre-existing processes modelling without undermining viability. In tension with this implicit conserva- tive nature, is the enduring ability of the cell to adapt to novel We begin by considering some of the key features of living disturbances by permanent transformations in the systems interface cells. As even the simplest known living cells—such as the and internal processing networks to maintain and enhance viability. parasitic bacterium Mycoplasma genitalium—consist of a This is not realized at the level of the individual cell but by selection highly complex network of hundreds of genes and proteins pressures on cell populations (Darwinian evolution). 80 Chem. Soc. Rev., 2012, 41, 79–85 This journal

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