Lipid vesicles as possible intermediates in the origin of life

A relationship between membrane properties forms the basis of a selectivity mechanism for vesicle self-reproduction

Self-reproduction and the ability to regulate their composition are two essential properties of terrestrial biotic systems. The identification of non-living systems that possess these properties can therefore contribute not only to our understanding of their functioning but also hint at possible prebiotic processes that led to the emergence of life. Growing lipid vesicles have been previously established as having the capacity to self-reproduce. Here it is demonstrated that vesicle self-reproduction can occur only at selected values of vesicle properties. We treat as an example a simple vesicle with membrane elastic properties defined by a membrane bending modulus kappa and spontaneous curvature C0, whose volume variation depends on the membrane hydraulic permeability Lp and whose membrane area doubles in time Td. Vesicle self-reproduction is described as a process in which a growing vesicle first transforms its shape from a sphere into a budded shape of two spheres connected by a narrow neck, and then splits into two spherical daughter vesicles. We show that budded vesicle shapes can be reached only under the condition that Td Lpkappa C0(4)> or =1.85. Thus, in a growing vesicle population containing vesicles of different composition, only the vesicles for which this condition is fulfilled can increase their number in a self-reproducing manner. The obtained results also suggest that at times much longer than Td the number of vesicles with their properties near the "edge" in the system parameter space defined by the minimum value of the product Td Lpkappa C0(4), will greatly exceed the number of any other vesicles.

Gene Expression within Cell-Sized Lipid Vesicles

Functional protein synthesis was observed in cell-sized lipid vesicles following encapsulation of a gene-expression system. Expression of rsGFP (red-shifted green fluorescent protein) within individual vesicles was observed by fluorescence microscopy. Interestingly, at the early stage of the reaction, the expression efficiency inside the vesicle was remarkably higher than that in the solution outside. The synthesized rsGFP in individual vesicles is safe from attack by proteinase K added to the external aqueous solution. Studies on cell-sized vesicles expressing protein should contribute to a fundamental understanding of certain aspects of living systems and will be useful for practical applications, such as the construction of microreactors.

A novel system of self-reproducing giant vesicles

Novel self-reproducing giant vesicles, consisting of a vesicular amphiphile with an imine group in its hydrophobic chain, were constructed. This vesicular amphiphile, the product of a dehydrocondensation reaction between amphiphilic aldehyde and a lipophilic aniline derivative, could be prepared within the giant vesicles. When a protected form of the aldehyde precursor was added to a suspension of giant vesicles containing the lipophilic aniline precursor and a catalyst, dehydrocondensation between the two precursors took place inside the vesicles and produced the same amphiphile as the one which constitutes the original vesicle. The newly formed amphiphiles self-assembled in the inner water pool to form small vesicles, which were eventually extruded through the outer layer of the original vesicle to the bulk water. Accordingly, this kinetic system can be designated as a self-reproducing system of giant vesicles.

Early assembly of cellular life

Popular hypotheses that attempt to explain the origin of prebiotic molecules and cellular life capable of growth and division are not always agreed upon. In this manuscript, information on early bacterial life on Earth is examined using information from several disciplines. For example, knowledge can be integrated from physics, thermodynamics, planetary sciences, geology, biogeochemistry, lipid chemistry, primordial cell structures, cell and molecular biology, microbiology, metabolism and genetics. The origin of life also required a combination of elements, compounds and environmental physical-chemical conditions that allowed cells to assemble in less than a billion years. This may have been widespread in the subsurface of the early Earth located at microscopic physical domains.

Enhancement of apparent substrate selectivity of proteinase K encapsulated in liposomes through a cholate-induced alteration of the bilayer permeability

Proteinase K-containing liposomes with highly selective membrane permeability properties were prepared. The selectivity obtained was with respect to the two substrate molecules added to the external aqueous phase of the liposomes: acetyl-L-Ala-Ala-Ala-p-nitroanilide (Ac-AAA-pNA) and succinyl-L-Ala-Ala-Ala-p-nitroanilide (Suc-AAA-pNA). The liposome-forming lipid used was POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and modulation of the membrane permeability was achieved using the detergent cholate. Proteinase K-containing mixed liposomes (PKCL) were prepared by adding cholate to preformed proteinase K-containing POPC liposomes (PKL) at a defined effective cholate/POPC molar ratio in the liposomal bilayer membrane R(e). Proteinase K was kept inside PKCL with a negligible amount of leakage into the bulk aqueous phase at R(e) < or = 0.30. At higher R(e), leakage of proteinase K was pronounced, even under conditions where POPC/cholate mixed liposomes seemed to be still intact (0.30 < R(e) < or = 0.39). At R(e) < or = 0.30, the reactivity of proteinase K in the PKCL measured with the externally added substrate Ac-AAA-pNA increased with increasing R(e), while the reactivity measured with Suc-AAA-pNA remained low, regardless of the R(e) value. This showed that externally added Ac-AAA-pNA molecules permeated the liposomal membrane more easily than Suc-AAA-pNA by modulating the membrane with cholate. Consequently, Ac-AAA-pNA was hydrolyzed in PKCL with considerably higher apparent substrate selectivity in comparison with the cases of proteinase K in PKL and free proteinase K (without liposomal encapsulation). The results obtained clearly demonstrate that the prepared PKCL can be utilized as a kind of nano-scaled bioreactor system which can take up a particular target substrate with high apparent substrate selectively from the external phase of the liposomes. Inside the liposomes, the target substrate is then converted into the corresponding products.

Membrane self-assembly processes: steps toward the first cellular life

This review addresses the question of the origin of life, with emphasis on plausible boundary structures that may have initially provided cellular compartmentation. Some form of compartmentation is a necessary prerequisite for maintaining the integrity of interdependent molecular systems that are associated with metabolism, and for permitting variations required for speciation. The fact that lipid-bilayer membranes define boundaries of all contemporary living cells suggests that protocellular compartments were likely to have required similar, self-assembled boundaries. Amphiphiles such as short-chain fatty acids, which were presumably available on the early Earth, can self-assemble into stable vesicles that encapsulate hydrophilic solutes with catalytic activity. Their suspensions in aqueous media have therefore been used to investigate nutrient uptake across simple membranes and encapsulated catalyzed reactions, both of which would be essential processes in protocellular life forms.

Shape behavior of lipid vesicles as the basis of some cellular processes

The basic principles that govern the shape behavior of phospholipid vesicle shapes are discussed. The important membrane parameters of the system are defined by presenting the expressions for the relevant contributions to the system's mechanical energy. In the description of the rather unique shape behavior of lipid vesicles, the emphasis is on providing a qualitative understanding of the dependence of vesicle shape on the parameters of the system. The vesicle shape behavior is then related to biologically important phenomena. Some examples are given of how the results of the shape behavior of lipid vesicles can be applied to the analysis of cellular systems. Red blood cell shape and shape transformations, vesicle fission and fusion processes, and the phenomenon of cellular polarity are considered. It is reasoned that the current biological processes that involve changes of membrane conformation may have their origin in the general shape behavior of closed lamellar membranes.

Influence of ionic inorganic solutes on self-assembly and polymerization processes related to early forms of life: implications for a prebiotic aqueous medium

A commonly accepted view is that life began in a marine environment, which would imply the presence of inorganic ions such as Na+, Cl-, Mg2+, Ca2+, and Fe2+. We have investigated two processes relevant to the origin of life--membrane self-assembly and RNA polymerization--and established that both are adversely affected by ionic solute concentrations much lower than those of contemporary oceans. In particular, monocarboxylic acid vesicles, which are plausible models of primitive membrane systems, are completely disrupted by low concentrations of divalent cations, such as magnesium and calcium, and by high sodium chloride concentrations as well. Similarly, a nonenzymatic, nontemplated polymerization of activated RNA monomers in ice/eutectic phases (in a solution of low initial ionic strength) yields oligomers with > 80% of the original monomers incorporated, but polymerization in initially higher ionic strength aqueous solutions is markedly inhibited. These observations suggest that cellular life may not have begun in a marine environment because the abundance of ionic inorganic solutes would have significantly inhibited the chemical and physical processes that lead to self-assembly of more complex molecular systems.

Morphology: an ambiguous indicator of biogenicity

This paper deals with the difficulty of decoding the origins of natural structures through the study of their morphological features. We focus on the case of primitive life detection, where it is clear that the principles of comparative anatomy cannot be applied. A range of inorganic processes are described that result in morphologies emulating biological shapes, with particular emphasis on geochemically plausible processes. In particular, the formation of inorganic biomorphs in alkaline silica-rich environments are described in detail.

The molecular roots of compositional inheritance

Non-covalent compositional assemblies, made of monomeric mutually catalytic molecules, constitute an alternative to alphabet-based informational biopolymers as a mechanism of primordial inheritance. Such assemblies appear implicitly in many "Metabolism First" origin of life scenarios, and more explicitly in the Graded Autocatalysis Replication Domain (GARD) model [Segréet al. (2000). Proc. Natl Acad. Sci. U.S.A.97, 4112-4117]. In the present work, we provide a detailed analysis of the quantitative molecular roots of such behavior. It is demonstrated that the fidelity of reproduction provided by a newly defined heritability measure eta(*)(s), strongly depends on the values of molecular recognition parameters and on assembly size. We find that if the catalytic rate acceleration coefficients are distributed normally, transfer of compositional information becomes impossible, due to frequent "compositional error catastrophes". In contrast, if the catalytic acceleration rates obey a lognormal distribution, as actually predicted by a statistical formalism for molecular repertoires, high reproduction fidelity is obtained. There is also a clear dependence on assembly size N, whereby maximal eta is seen in a narrow range around N approximately 3.5 N(G)/lambda, where N(G)is the size of the primordial molecular repertoire and lambda is a molecular interaction statistical parameter. Such relationships help define the physicochemical conditions that could underlie the early steps in pre-biotic evolution.

Composing life

Textbooks often assert that life began with specialized complex molecules, such as RNA, that are capable of making their own copies. This scenario has serious difficulties, but an alternative has remained elusive. Recent research and computer simulations have suggested that the first steps toward life may not have involved biopolymers. Rather, non-covalent protocellular assemblies, generated by catalyzed recruitment of diverse amphiphilic and hydrophobic compounds, could have constituted the first systems capable of information storage, inheritance and selection. A complex chain of evolutionary events, yet to be deciphered, could then have led to the common ancestors of today's free-living cells, and to the appearance of DNA, RNA and protein enzymes.