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Cell fusion is an important cellular process in which several uninuclear cells (cells with a single nucleus) combine to form a multinuclear cell, known as a syncytium. Cell fusion occurs during differentiation of muscle, bone and trophoblast cells, during embryogenesis, and during morphogenesis. Cell fusion is a necessary event in the maturation of cells so that they maintain their specific functions throughout growth.
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History
In 1847 Theodore Schwann expanded upon the theory that all living organisms are composed of cells when he added to it that discrete cells are the basis of life. Schwann observed that in certain cells the walls and cavities of the cells coalesce together. It was this observation that provided the first hint that cells fuse. It was not until 1960 that cell biologists deliberately fused cells for the first time. To fuse the cells, biologists combined isolated mouse cells, with the same kind of tissue, and induced fusion of their outer membrane using the Sendai virus (a respiratory virus in mice). Each of the fused hybrid cells contained a single nucleus with chromosomes from both fusion partners. Synkaryon became the name of this type of cell combined with a nucleus. In the late 1960s biologists successfully fused cells of different types and from different species. The hybrid products of these fusions, heterokaryon, were hybrids that maintained two or more separate nuclei. This work was headed by Henry Harris at the University of Oxford and Nils Ringertz from Sweden's Karolinska Institute. These two men are responsible for reviving the interest of cell fusion. The hybrid cells interested biologists in the area of how different kinds of cytoplasm affect different kinds of nuclei. The work conducted by Henry and Nils showed that proteins from one gene fusion affect gene expression in the other partner's nucleus, and vice versa. These hybrid cells that were created were considered forced exceptions to normal cellular integrity and it was not until 2002 that the possibility of cell fusion between cells of different types may have a real function in mammals.
Two Types of Cell Fusion
There are two different types of cell fusion that can occur. These two types include homotypic and heterotypic cell fusion.
Homotypic cell fusion occurs between cells of the same type. An example of this would be osteoclasts or myofibers being fusing together with their respective type of cells. Whenever the two nuclei merge a synkaryon is produced. Cell fusion normally occurs with nuclear fusion, but in the absence of nuclear fusion, the cell would be described as a binucleated heterokaryon. A heterokaryon is the melding of two or more cells into one and it may reproduce itself for several generations. If two of the same type of cells fuse, but their nuclei do not fuse, then the resulting cell is called a syncytium.
Heterotypic cell fusion occurs between cells of different types, making it the exact opposite of homotypic cell fusion. The result of this fusion is also a synkaryon produced by the merging of the nuclei, and a binucleated heterokaryon in the absence of nuclear fusion. An example of this would be BMDCs being fused with parenchymatous organs.
Three Methods for Fusing Cells
There are three methods that cell biologists use to fuse cells. These three ways include electrical cell fusion, polyethylene glycol cell fusion, and sendai virus induced cell fusion.
Electrical cell fusion, is an essential step in some of the most innovative methods in modern biology. This method begins when two cells are brought into contact by dielectrophoresis. Dielectrophoresis uses a high frequency alternating current, unlike electrophoresis in which a direct current is applied. Once the cells are brought together, a pulsed voltage is applied. The pulse voltage causes the cell membrane to permeate and subsequent combining of the membranes and the cells then fuse. After this, alternative voltage is applied for a brief period of time to stabilize the process. The result of this is that the cytoplasm has mixed together and the cell membrane has completely fused. All that remains separate is the nuclei, which will fuse at a later time within the cell, making the result a heterokaryon cell.
Polyethylene glycol cell fusion is the simplest, but most toxic, way to fuse cells. In this type of cell fusion polyethylene glycol, PEG, acts as a dehydrating agent and fuses not only plasma membranes but also intracellular membranes. This leads to cell fusion since PEG induces cell agglutination and cell-to-cell contact. Though this type of cell fusion is the most widely used, it still has downfalls. Oftentimes PEG can cause uncontrollable fusion of multiple cells, leading to the appearance of giant polykaryons. Also, standard PEG cell fusion is poorly reproducible and different types of cells have various fusion susceptibilities. This type of cell fusion is widely used for the production of somatic cell hybrids and for nuclear transfer in mammalian cloning.
Sendai virus induced cell fusion occurs in four different temperature stages. During the first stage,which lasts no longer than 10 minutes, viral adsorption takes place and the adsorbed virus can be inhibited by viral antibodies. The second stage, which is 20 minutes, is pH dependent and an addition of viral antiserum can still inhibit ultimate fusion. In the third, antibody-refractory stage, viral envelope constituents remain detectable on the surface of cells. During the fourth stage cell fusion becomes evident and HA neuraminidase and fusion factor begin to disappear. The first and second stages are the only two that are pH dependent.
Cell Fusion for Human Therapy
Alternative forms of restoring organ function and replacing damaged cells are needed with donor organs and tissue for transplantation being so scarce. It is because of the scarcity that biologists have begun considering the potential for therapeutic cell fusion. Biologists have been discussing implications of the observation that cell fusion can occur with restorative effects following tissue damage or cell transplantation. Though using cell fusion for this is being talked about and worked on, there are still many challenges those who wish to implement cell fusion as a therapeutic tool face. These challenges include choosing the best cells to use for the reparative fusion, determining the best way to introduce the chosen cells into the desired tissue, discovering methods to increase the incidence in cell fusion, and ensuring that the resulting fusion products will function properly. If these challenges can be overcome then cell fusion may have therapeutic potential.