Saturday, 1 June 2013

#chemclub Reviews: Protocells

Welcome to the first in a new series: #chemclub reviews. Every month there'll be a short article about an area of chemistry, the idea being to allow readers who are only vaguely familiar with a topic to learn some more as quickly and painlessly as possible. I hope that these will be a useful supplement to your regular reading!

The key goal is to provide you with context, or the 'big picture' of a topic, to help you more fully appreciate new research in the area. The articles will assume basic chemistry knowledge, or at least the ability to use Wikipedia.

Naturally, I won't be writing all of these: my own knowledge is pretty limited. A few friends have kindly offered to write about their own areas of interest or expertise. If you'd like to get involved, drop me an email!

First up is a topic close to my own area of research: self-reproducing protocells.

This area is important for two main reasons: it’s a crucial part of building synthetic protocells, and it’s potentially relevant to the origins of life. In either case, we want to model simple organisms. Any simple organism that is bounded by a membrane needs some way to grow and divide. In modern biology there’s a complex apparatus by which the cell produces more membrane, but in synthetic life it’s much simpler if the membrane can copy itself directly.

Conceptual scheme of a protocell. Reproduced from Nature 2001409, 387-390.
I'm going to focus on the use of chemical reactions to drive cell growth and division. That’s only one aspect of protocell research; I chose it simply because it’s what I know! This approach was pioneered by Pier Luigi Luisi in the early 90s using the simplest model of a cell you can imagine: a micelle. Over 10 years or so Luisi and co-workers developed a number of self-reproducing micelles based on several chemical reactions, such as ester hydrolysis and the oxidation of alcohols to carboxylic acids. He quickly shifted focus to self-reproducing vesicles, which tended to be more complex, but are better models of cells.

The mechanism of Luisi’s systems is pretty simple. Micelles and vesicles are made of surfactants and are capable of solubilising organic molecules in water. For this reason, any reaction between an organic molecule and a water-soluble molecule can be catalysed by micelles or vesicles. If the product of a reaction like this is itself a surfactant, and hence can form more micelles or vesicles, then you have the basis for self-reproducing micelles or vesicles.

To sum it up in a tweet: a molecule aggregates into a micelle or vesicle, which catalyses the formation of more of that molecule, allowing for the formation of more micelles or vesicles.

The actual dynamic behaviour of these models is quite complicated and depends on many factors. Sometimes vesicles gradually grow into bizarre, extended shapes and only divide when prompted to environmentally; other times they divide spontaneously into two without any prompting. I won’t go into these effects here as they’re very involved; suffice to say, the ideal picture of a spherical protocell cleanly dividing in two is not generally realistic.

How, then, do we go from these simple models of a self-reproducing cell membrane to a full-fledged protocell? This question was tackled head-on in a 2001 paper in Nature by Luisi, with Jack Szostak and David Bartel. Szostak is famous for his work on telomeres, for which he received the Nobel prize in 2009. These days he does a great deal of work on protocells and the origins of life; he is a major proponent of the “RNA world” hypothesis and has done some brilliant work to support it.

The 2001 paper proposes a way forward to a living protocell: a self-reproducing vesicle contains a self-replicating genetic polymer, such as the RNA world’s proposed ribozyme. If the replication of the gene and the membrane can be coupled, Szostak and Luisi suggest that it may be capable of evolution and hence life. Whether you agree or not, a system that did this would at least be a convincing model of life.

This coupling between the replication of genetic material and a protocell membrane has been on the agenda since long before 2001 – it was a feature of the ‘chemoton’ model proposed by G├ínti in the 70s, for example. Achieving this experimentally has been, and remains, a major challenge. Very early on, Luisi encapsulated a self-replicating oligonucleotide in simple protocells, and enzymatic nucleotide replication in vesicles has been reported by others. However, the most impressive effort was reported by Sugawara in 2011 in Nature Chemistry.

This paper is pretty complicated, so bear with me! Sugawara’s vesicles are made up of a cationic surfactant. This is produced by hydrolysis of a precursor; this hydrolysis is catalysed by a molecule which is embedded in the membrane. So the precursor molecule is outside the protocell, and when it encounters the membrane it is hydrolysed into a surfactant. The surfactant associates with the vesicle and hence the vesicle can grow.

Here’s where it gets clever: inside the protocell is DNA, and the enzymes needed to replicate it by the polymerase chain reaction. As DNA is negatively charged, and the membrane is positively charged (due to the cationic surfactant), the two associate. When the DNA associates with the membrane, it causes it to deform – which induces division!

Core-and-shell reproduction. Reproduced from Nature Chemistry 2011, 3, 775-781.
This is perhaps the simplest coupling between the replication of a genetic polymer and the membrane you can imagine: when a cell has enough DNA, it divides. This means that cells in which DNA is produced more quickly will divide more quickly; if a mutation arose which allowed the DNA to replicate faster, it would be selected for. The key limitation is that the catalyst for membrane formation is not reproduced. It’s not quite the coupling proposed by Szostak, Bartel and Luisi, but it’s a step in the right direction and an impressive piece of work.

I've only just scratched the surface of the world of protocells here. There are many examples of static protocells carrying out functions other than replication. To give but one example, Stephen Mann has reported silica-based protocells which allow for pH control enzymatic reactions inside the cells.

Hopefully this quick review has highlighted some key themes of protocell research:
  • Moving away from enzyme-based reactions to chemical reactions, especially prebiotically-plausible reactions (such as peptide catalysis).
  • Coupling different functions of protocells, especially replication of genetic molecules with division of the protocell.
  • Modelling primitive natural selection and related processes such as mutation and competition.
All systems to date suffer from being overly simple (containing only one component, for example), from incomplete replication (one or more essential components are not replicated, leading to death by dilution), or from lack of coupling (all components are replicated, but there is no control, preventing selection or evolution). The ultimate goal would be a system which can reproduce all of its components, and in which these reactions are coupled. We’d be hard pressed to call such a system inanimate!

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