A specific three-dimensional multicellular framework is often criticial for proper cellular function. The ability to design such a framework would be useful for many purposes, but has proven elusive.
One useful purpose would be for organ repair. Another useful purpose would be the study of diseases whose progress is impacted by multicellular topology (e.g., cancer).
Zev Gartner and Carolyn Bertozzi (University of California and Lawrence Berkeley National Laboratory, Berkeley) have constructed three-dimensional assemblies of living cells. They have used their approach to initiate short-distance cell-cell chemical communication.
Developing the strategy.
The scientists needed to figure out a way to get their cells to assemble in controlled three-dimensional structures. They achieved this by first affixing DNA molecules to the surface of one batch of cells.
They then affixed different DNA molecules (which chemically bind to the other DNA molecules) to the surface of the other batch of cells. When they mixed the two populations of cells together, in greater than a 1:50 ratio, they achieved clusters of cells.
An important point is that the more predominant cells were affixed to the surface of the less predominant cells. This is a controlled three-dimensional architecture.
Controlling cell-cell assembly.
So far the scientists have shown that they could assemble cells into three-dimensional architectures. However, they desired more control over assembly: a rational means to control cell-cell assembly, the capacity to purify the resulting assemblies, and the capacity to disassemble the cell-cell contacts.
The scientists found that decreasing the concentration of cells or DNA, or using more complex DNA molecules that experienced more difficulty in forming chemical bonds, decreased the rate of cell-cell assembly. This told the scientists that it should be straightforward to assemble cells into predictable three-dimensional architectures, using one or more of these three variables.
The scientists were able to purify desired three-dimensional assemblies from undesired structures, such as individual cells, using fluorescence-activated cell sorting. This technique can use size, fluorescence properties, and many other cellular charactertistics to rapidly sort cells (as fast as thousands of cells per second).
Cell-cell contacts, through the DNA linkages, were easily disassembled by increasing the temperature, or adding an enzyme that chemically degrades the DNA linkages. The first disassembly method is presumably reversible as long as the cells are entrapped in a matrix (such as a gel), while the second disassembly method is irreversible.
Fabricating a chemical communication network.
The scientists' ultimate goal was the generation of a three-dimensional cellular architecture that exhibits a function. In this way, their assemblies would primitively mimic living cell-cell assemblies.
They achieved this by assembling Chinese hamster ovary cells with blood stem cells. The scientists genetically engineered the Chinese hamster ovary cells to produce interleukin molecules.
Interleukin molecules stimulate blood stem cells to mature into full-fledged blood cells, and the blood stem cells cannot survive without the interleukin molecules. Thus, one of the cells in the assembly stimulates the other cells in the assembly.
After fabrication of their cell-cell assemblies, the scientists embedded them in a sugar gel, and added molecules to the gel that chemically inactivate interleukin molecules. This prevented long-range transport of the interleukin molecules through the gel, enabling the scientists to confirm the presence of short-range cell-cell communication.
After 16 hours, the cell-cell assemblies grew, due to the transfer of interleukin molecules from the Chinese hamster ovary cells to the blood stem cells. Thus, a chemical communication network was fabricated.
Advantages of this development.
There are several very attractive features of this method of cell-cell assembly. The process can be performed under conditions typically used for cell culturing.
Genetically modified cells are not required. The assemblies can be transferred after fabrication to any environment, such as a gel or some other scaffold, for further manipulation or study.
DNA is a useful assembly tool. This is because DNA molecules can be designed to direct cell-cell assembly without instigating an immune response.
These scientists have designed a method to rationally assemble cells into predictable three-dimensional structures. It should be very useful towards fabricating artificial organs and combating diseases, such as cancer, which are dependent on the three-dimensional arrangement of cells.
for more information:
Gartner, Z. J.; Bertozzi, C. R.
Programmed assembly of 3-dimensional microtissues with defined
cellular connectivity.
Proc. Natl. Acad. Sci. USA 2009, 106, 4606-4610.