Rapid growth and fusion of protocells in surface-adhered membrane networks

Background 

A protocell is defined as a self-organized, endogenously ordered and spherical collection of lipids, proposed as a stepping-stone towards understanding the origin of life [1].  Researchers have aimed to generate protocells from a bottom-up approach by determining the minimum requirements for creating one. This resulted in the basic primitive protocell model made up of two components namely a) a membrane component and b) encapsulated genetic material, usually RNA or DNA.

The membrane component is essential, since bilayer membranes that surround all types of cells as we know them today are thought to have originated from spontaneous self- assembly of amphiphilic molecules. Novel ways to induce the spontaneous formation of lipid vesicles from lipid reservoirs known as nucleation have been identified. Also, the presence of solid particle surfaces consisting of natural minerals or synthetic materials has been shown to enhance this spontaneous nucleation of vesicles from fatty acids [2]. Furthermore, the increase in vesicle membrane size or membrane growth to mimic cell growth could occur by vesicle–vesicle fusion initiated by surface functionalization, or due to various sources of external stimuli such as fusogenic peptides, synthetic lipids, polyethylene glycol, ions, an electric field pulse, and light irradiation [3].

The second characteristic of protocells is thought to be the compartmentalization of nucleic acids. Compartmentalization is key to enabling Darwinian evolution of organisms by inhibiting the random mixing of genetic polymers such as DNA which eventually results in the coupling of genotype and phenotype [4]. Upon successful generation of vesicles, the encapsulation of genetic materials such as RNA or DNA is a key step towards adding complexity to protocells. However, to be truly considered living these protocells need to undergo cycles of growth and division and replicate endogenous genetic material along the process [4].

Approach

In this study, the authors propose a primitive growth mechanism controlled by elevated surrounding temperature. Depositing a multilamellar lipid reservoir on a SiO₂ surface leads to the formation of a double bilayer on the surface. The distal bilayer of the two stacked bilayers ruptures due to continuous tensile stress giving rise to a network of nanotubes on the proximal bilayer [5]. Regions of the nanotubes swell over time to form unilamellar vesicles, as has been previously reported.

Key findings

• The authors use an Infrared-B laser (λ=1470 nm) to achieve a mild temperature increase in the vicinity of the membrane. The laser radiation is not focused, but affects an elliptical surface area which gives rise to a thermal This leads to the instant formation and growth of strictly unilamellar vesicles exclusively from the lipid nanotubes.
• They observe elevated temperature-induced protocell. This is concluded based on a negative correlation between the total number of vesicles and the average diameter of the protocells. They observe fusion events on vesicles predominantly present on the same nanotube. However, they go on to identify the fusion of vesicles located on different nanotubes in later stages.
• To identify the mechanism of fusion they use a mathematical model to reproduce the above-mentioned fusion The initial mechanism of vesicle fusion occurring on vesicles located on the same nanotube is supported by their simulations. On the other hand, the second mechanism of fusion results in the stabilization of a pore (a gap between the top of the two vesicles and the neck of the two vesicles), which makes it less probable in their simulation. Therefore, the authors conclude that the fusion process should predominantly start at the interconnected base of vesicles being the nanotube.

Why this work is important? 

• The physicochemical mechanisms driving the nucleation, growth, and division of protocells are among the key questions in understanding the origin of life. Evidence supporting the hypothesis of solid surfaces playing a role in the formation of lipid nanotube networks that give rise to the formation of protocells is convincing. Fragments of these nanotubes swell into giant, strictly unilamellar vesicular compartments over a few This relatively slow process is entirely self-driven and only requires a lipid reservoir as a source, a solid surface, and surrounding aqueous media. With temperature elevation, this process is enhanced and achieved on the scale of a few mins which is impressive.
• The temperature range of the Lost City hydrothermal vents is 40-90 °C, surprisingly similar to the experimental conditions used in this work that promotes vesicle formation, growth, and fusion [6]. Fusion is described here to occur predominantly between vesicles on interconnected nanotubes. Redistribution of RNA between the vesicles is also demonstrated. This all proves that their previously described novel primitive protocell formation mechanism [5] could be enhanced by elevated temperatures similar to the protocell studies carried out on environments such as hydrothermal vents and warm

Questions for the authors

• To be truly considered as ‘living’, the protocells need to be able to undergo cycles of growth and division. Do the authors intend to investigate whether these vesicles undergo division upon attaining a maximum diameter? If so what drives division in this case?
• In your model, it would be interesting to look at the fusion of independent vesicles located on different nanotubes. Could you investigate if a pore formation is limiting fusion in this case too.
• Lastly, as a future perspective, in order to add a metabolic characteristic to these protocells would you study the ability of these protocells to redistribute self-replicating genetic material?

References

1. Chen, I. A. & Walde, P. From self-assembled vesicles to protocells. Cold Spring Harb. Perspect. Biol. 2, (2010)
2. Hanczyc, M. M., Fujikawa, S. M. & Szostak, J. W. Experimental Models of Primitive Cellular Compartments: Encapsulation, Growth, and Science (80-.). 302, 618– 622 (2003).
3. Xu, B. Y., Xu, J. & Yomo, T. A protocell with fusion and division. Biochemical Society Transactions 47 1909–1919 (2019).
4. Szostak, W., Bartel, D. P. & Luisi, P. L. Synthesizing life. Nature vol. 409 387–390 (2001).
5. Köksal, E. S. et al. Nanotube-Mediated Path to Protocell Formation. ACS Nano 13(6) 388405 (2019).
6. Martin, , Baross, J., Kelley, D. & Russell, M. J. Hydrothermal vents and the origin of life. Nature Reviews Microbiology vol. 6 805–814 (2008).

Tags: membrane, primitive, protocell, protocells, vesicle

doi: https://doi.org/10.1242/prelights.20022

Read preprint (No Ratings Yet)