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The construction of biomimetic cellular structures is a quickly growing area in scientific research. This rapid growth comes from the variety of purposes of cell mimics and from the potential applications that can be offered. It provides the opportunity to understand and investigate the biochemical concepts of the cell and cellular life which gives possible insights into the origin of life on Earth. Another purpose of constructing and designing cell mimics with protocellular structures is the development of new applications by introducing new functionalities that not exist in biological cells. This research area provides a wide spectrum of potential applications in many areas such as biotechnology, pharmaceuticals, drug delivery, cell engineering, bio-factories, biosensors, and bioremediation. The complex nature of biological cells makes it difficult to study and understand all aspects of their structure and function. However, the built artificial cell structures can be easier to control and more robust system than biological cells. These cell-like structures makes it easier to investigate and study the biochemical interactions in the natural cells.
Moreover, they gave a chance to explore new potential applications rather than biological cells. In order to make artificial cells, the essential requirements for cellular life should be taken in consideration. These requirements are a combination of three key components that cooperate and link to each other to enable the biological cells to display its functions. The first component is a semi-permeable membrane that acts as a container to protect the cellular constituents from the external environment and permits selective exchange of materials and energy. The second is the presence of bio-macromolecules such as DNA and RNA that act as genetic information carriers which give the cell the ability to control the dynamics and makes it capable of Darwinian evolution.
The third component is the presence of many metabolic pathways that provide the required energy to cells. Metabolism is fundamental to maintain and construct cellular components which is essential for cell growth and, eventually, helps it to split into two similar copies (self-reproduction). Based on their substantial characteristics, the artificial cell structures are divided into two general categories; typical and non-typical as outlined in Figure 1. The typical artificial cells are imitators of biological cell containing cell-like structures and showing the key characteristics of natural cells. They should be able to exhibit one or more functions of the actual biological cell such as self-reproduction, and/or metabolism. There are two construction approaches to make typical artificial cells; top-down approach and bottom- up approach. The top- down approach relies on reducing the complexity of a living biological cell (or organisms such as viruses or bacteria). In this approach, the genomes are either stripped down to the lowest required number of genes and substances that needed to retain the cellular life, or replaced completely by synthetic genes.
For example, Venter et al designed a genome sequence and synthesize it chemically. Then, they transplanted it into a recipient cells. The new synthetic cell was controlled only by the transplanted synthetic genome and it was able to show cell proprieties such as the capability to self-replicate. On the other hand, the bottom-up approach relies on the assembly of non-living components that can be either biological or non-biological components. As a result of the assembly, the complexity of the system will be increased and resulting artificial cell will exhibit some properties that exist in the biological living cell.
For example, Lentini el al described the construction of artificial cell engineered with lipid vesicles containing chemical-sensing genetic device. The engineered artificial cell enabled E. coli cells to sense and translate chemical message which E. coli on its own cannot detect it. The non-typical artificial cell structures, so-called ‘protocells’, are synthesized or modified materials that only exhibit and mimic some of cells properties and functions. They imitate the shape, morphology, surface characteristics, or some particular functions of the biological cell. Based on their nature, the protocells are classified into two categories, cell membrane mimetic structures such as vesicles made from liposomes, polymersomes, proteinosomes, and colloidal particles, in addition to, membrane-free structures such as hydrogels, aqueous two-phase systems (ATPS), and coacervates. The phenomenon where macromolecules associate with each other resulting liquid- liquid macrophase separation is called coacervation. Due to the coacervation, there will be two phases; a dense, polymer-rich called “coacervate phase” which characterized under microscope as droplets and a dilute polymer-poor phase called “equilibrium phase” as outlined. The formed droplets can be evolved in size from nano, micro, even to meso- droplets. Depending on the nature of macromolecules coacervation is divided into two categories: simple and complex coacervation. Simple coacervation takes place when only a single polymer associated with coacervation process. This polymer normally contains multiple functions or charges that involved in coacervates formation such as polyampholytes and protein.
On the other hand, complex coacervation takes place when two polymers (polyelectrolytes) with opposite charges interact with each other in solution or colloid. In general, the driving force of complex coacervation is the electrostatic interaction between the polyelectrolytes. However, complex coacervation can be promoted via non-covalent interaction such as hydrophobic interactions in Elastin-like proteins and cation–π interaction. Apparently, complex coacervates exhibit similarities to the intercellular phase separation. Pak et al suggested that the intracellular phase separation may be a result of a complex coacervation mechanism. They showed that complex coacervation promotes intracellular phase separation for a disordered cellular protein, Nephrin intracellular domain (NICD). A negatively charged NICD (basic) was co-assembled with positively charged NICD (acidic) to produce microdroplets. The driving forces of this phase separation were not only the electrostatic interactions but also aromatic and hydrophobic interactions come from the side chains located across the protein. The capability of phase separation is not the only remarkable propriety of coacervates. They are able to show some life-like properties.
In addition to their spontaneous formation, they exhibit a simple form of metabolism by selective absorption of some organic molecules from the surrounding medium. The coacervates life-like properties made them receive considerable attention in synthetic cell applications. However, their application in mimic cell research is still limited because they are relatively instable due to their membrane-free nature. To increase the stability of the coacervate microdroplets, Mason et al presented and designed a new protocell model by making biodegradable complex coacervate microdroplets under physiological conditions. The microdroplets were made by spontaneous co-assembly of negatively and positively charged biopolymers of amylose derivatives. Subsequently, a membrane was formed on the microdroplets surface by self-assembly of a synthetic triblock copolymer. The structure of the copolymer is shown in Figure 5 and it consists of three components; hydrophilic, hydrophobic, and poly anionic blocks.
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