Shape Deformation, Budding and Division of Giant Vesicles and Artificial Cells: A Review

Abstract

The understanding of the shape-change dynamics leading to the budding and division of artificial cells has gained much attention in the past few decades due to an increased interest in designing stimuli-responsive synthetic systems and minimal models of biological self-reproduction. In this respect, membranes and their composition play a fundamental role in many aspects related to the stability of the vesicles: permeability, elasticity, rigidity, tunability and response to external changes. In this review, we summarise recent experimental and theoretical work dealing with shape deformation and division of (giant) vesicles made of phospholipids and/or fatty acids membranes. Following a classic approach, we divide the strategies used to destabilise the membranes into two different types, physical (osmotic stress, temperature and light) and chemical (addition of amphiphiles, the addition of reactive molecules and pH changes) even though they often act in synergy when leading to a complete division process. Finally, we review the most important theoretical methods employed to describe the equilibrium shapes of giant vesicles and how they provide ways to explain and control the morphological changes leading from one equilibrium structure to another.

Keywords: ADE theory; artificial cells; budding; division; giant vesicles; protocells; systems chemistry.

Figure 1

(a) Schematic representations of cone-, cylinder- and inverse-cone-shaped lipids. Reproduced from [35]. (b) Self-reproduction cycle for binary vesicles composed of DLPE/DPPC = 2/8. The green and red arrows show the budding of the second daughter vesicle and the granddaughter vesicle, respectively. Scale bar 5 $\mathsf{\mu }$m. Reproduced from [35]. (c) Lipid bilayer in liquid-ordered phase (Lo) (composed of SM and cholesterol) and liquid-disordered phase Ld (composed of DOPC). Reprinted with permission from Ref. [66]. 2018, Royal Society of Chemistry. (d) Budding (left, T = 30 ${}^{\circ }$C) and complete budding (right, T = 35 ${}^{\circ }$C) of phase-separated vesicles made of sphingomyelin, DOPC and cholesterol. Scale bar 5 $\mathsf{\mu }$m. Reprinted with permission from Ref. [65]. 2003, Springer Nature.

Figure 2

Light-triggered division of GUVs. Panel I: (a) An example of an azobenzene-containing phosphatidylcholine (azoPC) that can be isomerised between its cis- and trans-configurations; (b) budding transition of an azoPC vesicle following the illumination with either 365 or 460 nm light. The space-time plot demonstrates the reversibility of the process. After 33 s, vesicle fission is initiated by intense white light illumination. Adapted with permission from Ref. [67]. 2017, American Chemical Society. Panel II: (a) In the first step, GUVs deform after the addition of a higher osmolarity sucrose solution. In the second step, illumination leads to local lipid peroxidation of the outer membrane leaflet in the presence of the photosensitiser Ce6; (b) mechanism of Ce6-mediated lipid peroxidation. Illumination at the 405 nm wavelength triggers the generation of reactive oxygen species (ROS) in close proximity to the lipid tails. The ROS causes the peroxidation of the lipids in the outer leaflet and hence an asymmetric area increase. Reprinted with permission from Ref. [73]. 2021, American Chemical Society.

Figure 3

General scheme of the system AOT and PANI-ES. The polymerisation of aniline occurs on the surface of AOT vesicles. In symmetric membranes, the addition of AOT micelles leads to the deformation and growth of the vesicles; in asymmetric membranees, growth and division into two daughter vesicles are observed. Reproduced from [88].

Figure 4

(a) Amplification of DNA within a GUV. Vesicles contain PCR reagents and a fluorescent probe: template DNA, primers, fluorescent tag SYBR Green I, deoxynucleoside triphosphates, DNA polymerase and Mg${}^{2+}$. Reprinted with permission from Ref. [96]. 2011, Springer Nature. (b) Chemical structures of membrane molecule V, amphiphile catalyst C, membrane precursor V* and electrolyte E. Reproduced from [97]. (c) Adhesion and fusion between a target GUV and a conveyer GUV. The surface charge of the target GUV changes to cationic due to the protonation of the POPC as well as the increase in the cationic membrane lipid V from its precursor. These two types of GUVs fuse, and the transport of dNTP from the conveyer GV to the target GUV proceeds. Reproduced from [97]. (d) Production of cationic membrane lipid V from its precursor V* after the transport mechanism depicted in (c). Reproduced from [97].

Figure 5

(a) Self−division of a GUV triggered by the urea–urease reaction. Transformation of a GUV from a spherical shape through prolate and pear shapes into two daughter vesicles. (b) Change of the fluorescence intensity of pyranine vs. time inside the GUVs. (c) FRAP (fluorescence recovery after photobleaching) experiment using fluorescein sodium salt as a fluorescent probe. One of the two daughter vesicles (indicated by a yellow arrow) was irradiated with a laser pulse after the division process. The lack of fluorescence recovery is proof of the effective separation between the two daughter vesicles. The time between consecutive snapshots is 30 s. (a,b) reproduced from [102], (c) reproduced from the Supplementary Information of [102].

Figure 6

(Panel I) v-$\mathrm{\Delta }a$ phase diagram of the strict-bilayer-coupling model showing different classes of shapes; (a–e) are tridimensional representative shapes of the five classes reported in the phase diagram. Reprinted with permission from Ref. [106]. 2009, Wiley. (Panel II) Self-division of a GUV. (a) Shape transformation of pH-sensitive GUVs governed by the urea–urease enzymatic reaction; (b) numerically simulated equilibrium shapes of GUVs using the Surface Evolver software; reproduced from [104]. (Panel III) Morphological transitions of nanovesicles using coarse-grained molecular dynamics simulations; (a) time series of budded nanovesicle; (b) division of nanovesicle by fission of membrane neck. Reprinted with permission from Ref. [110]. 2021, American Chemical Society.

Figure 7

Graphical summary of the most important physical and chemical stimuli for inducing budding and self-division of GVs, as reviewed in this paper, with the corresponding principal references.