Introduction

Living organisms are all composed of cells. Multicellular organisms are composed of several kinds of specialized cells, spatially organized and interacting with each other in complex ways; unicellular organisms, on the other hand, are composed ofjust one cell that must accomplish all the necessary functions for its maintenance and reproduction. The cellular nature ofall forms of life and the evidence that no life exists at hierarchical levels below the cell, can be seen as the most striking property of living organisms.

When we look to an isolated cell, e.g., a bacterium, the most obvious fact lies in front of our eyes: the living can be appreciated as a single entity which is well differentiated from its environment. In other words, the cell has a defined boundary that separates the inside from the outside.

And looking at this interaction between these two regions, we are also able to capture the blue print of cellular life—on the basis ofa phenomenological approach. First ofall, cells are alive, but "life" cannot be found in any ofits individual components—rather, "life" is a collective property that stems from the self-organization of the cellular components and processes—it is a distributed, emergent property. From the historical point of view, several authors recognized that the very turning point of the transition from nonlife to life is the formation of a cell or, in more general term, of a compartment. Harold J. Morowitz, in his book Beginning of Cellular Life,1 firmly advocates this idea, showing that the universal nature of living compartments is irreducible, since a simpler form of life cannot be found; moreover, he emphasizes the universality of cellular architecture, containing a plasma membrane that separates the cell from its environment.

Before going on with the issue of "compartmentation", a short note on the current theories on the origins of life is called for. The main assumption held by scientists studying origins of life is that life originates from inanimate matter through a spontaneous and gradual increase of molecular complexity, from simple molecules to bio-monomers (as amino acids, sugars, aromatic bases, lipids), to macromolecules and thereby to activity (the so-called Oparin-Haldane hypothesis). Within this framework, two kinds of view have dominated the scientific debate in the last century, namely (i) the metabolism-first approach and (ii) the gene-first approach. Although it is often difficult to sharply categorize researchers in one or the other school of thought, authors such as Alexander I. Oparin, Sidney Fox, Freeman Dyson, Stuart Kauffman, Harold J. Morowitz and Günter Wächtershäuser can be broadly defined as supporters of the first view. Other authors such as John B. S.

Haldane, Leonard Troland, Manfred Eigen, Leslie Orgel, Francis Crick, Carl Woese, Walter Gilbert and Gerard Joyce can be rather considered as adhering to the gene-first hypothesis. An historical discussion of the alternation among different theories, their derivations and evolution has been presented by Iris Fry in her recent book on the origin of life.6

Without going into details, this dichotomy ultimately derives from the co-occurrence—in modern cells—of two kind of biopolymers: proteins (enzymes) and nucleic acids (DNA, RNAs), that produce each other in a cyclic manner. Different theories try to explain which of the two was the first agent, the initiator of the cycle.

The "compartment" approach can be considered as being independent of the above-mentioned cases, with the favorite theme of compartmentalized reaction networks. It does not directly resolve the metabolism/genes dilemma, but emphasizes instead the role of compartments in the origin of life. Some of the authors mentioned above have also integrated in their views in one form or another the notion of compartment, such as Dyson or Morowitz. The approach is generally based on enclosed reaction (also genetic) networks in a self-organized bounded system that determines a series ofemergent properties such as selective permeability, establishment of electrochemical gradients, sustenance ofout-of-equilibrium processes and above all the emergence of cellular individuality.

A basic model for compartments is provided by lipid vesicles (liposomes). Liposomes as well as other kind of vesicles form spontaneously when certain amphiphilic molecules are dispersed in water, (Fig. 12.1). The lipid molecules self-assemble to form a bilayer semi-permeable membrane that encloses an inner aqueous phase and presents a hydrophobic barrier between the environment and the vesicle. What is important from the point of view of the origin of life is the biogenesis of liposome monomers (i.e., the membrane-forming molecules) under prebiotic conditions, as will be discussed in the following paragraphs.

This compartment approach is embodied in the lines ofresearch carried out by David Deamer, Doron Lancet, Yoiko Nakatani, the late Guy Ourisson, Tetsuya Yomo and our group. Over the last 30 years, a considerable number ofinvestigations have been devoted to the study of vesicles composed of phospholipids and other important surfactants such as fatty acids, giving rise to several remarkable findings regarding vesicle properties and reactivity.

In this chapter, we shall illustrate some of the basic concepts underlying cellular and precellular evolution. Toward this aim, we shall discuss first what may be defined as the blue print ofcellular life;

Corresponding author: Pier Luigi Luisi—Biology Department, University of Rome "RomaTre" Viale Guglielmo Marconi, 446; 00146 Rome, Italy. Email: [email protected]

Figure 12.1. The spontaneous self-assembly of membrane-forming surfactants into a vesicle, with an inner water pool that can host water-soluble molecules. Any hydrophobic molecules present will be positioned in the membrane; if ionic surfactants are involved in vesicle formation, the ionic and/or polar solutes with complementary changes will be adsorbed on the surface. Notice that the formation of vesicles yields a three-phase microheterogeneous system (inside/boundary/outside), making possible the establishment of chemical gradients, segregation of macromolecules, selective permeability and above all the emergence of cell-like individuality. Double-chain surfactants as well as some classes of single-chain surfactants can form vesicles under a variety of conditions. In addition to bilayers, mono-layer membranes can be formed by so-called "bola" amphiphiles. Passive entrapment of solutes within vesicles is a process that does not require additional energy; and the self-assembly of surfactants proceeds spontaneously as well (AG < 0).

Figure 12.1. The spontaneous self-assembly of membrane-forming surfactants into a vesicle, with an inner water pool that can host water-soluble molecules. Any hydrophobic molecules present will be positioned in the membrane; if ionic surfactants are involved in vesicle formation, the ionic and/or polar solutes with complementary changes will be adsorbed on the surface. Notice that the formation of vesicles yields a three-phase microheterogeneous system (inside/boundary/outside), making possible the establishment of chemical gradients, segregation of macromolecules, selective permeability and above all the emergence of cell-like individuality. Double-chain surfactants as well as some classes of single-chain surfactants can form vesicles under a variety of conditions. In addition to bilayers, mono-layer membranes can be formed by so-called "bola" amphiphiles. Passive entrapment of solutes within vesicles is a process that does not require additional energy; and the self-assembly of surfactants proceeds spontaneously as well (AG < 0).

the most powerful theory in this regard is the theory of autopoiesis, which will next be briefly sketched. After that we will describe some of the most basic features of liposomes as models for early cells and give a short review on biochemical reactions occurring in liposomes as a model system for simple cells. We shall also examine a striking reactive pathway ofvesicles, namely that ofvesicle self-reproduction and the possible integration of metabolism-first and/or gene-first approaches within the framework of the compartment approach. Finally, we shall provide an introduction to the notion and research of"minimal cells"—tangible objects synthesized in the laboratory— that might display minimal cellular functions. This paves the way to an understanding ofthe structure and functions ofthe early cell and its evolution to the complex cells of to-day.

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