One of the greatest hopes in the medical research community is that tissues will one day be grown and implanted into the human body to replace aging or defective tissues and organs.  There has been much research into how to best assemble cells into complex tissues, which feature many cell types.

Past efforts have generally focused on growing cells in special molds and then grouping the molded microtissues together into a larger tissue.  Now researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory have developed a novel approach to build self-assembling microtissues by coating cells in complementary DNA. 

The new approach is a bottom-up method, which should allow for greater tissue complexity.  Carolyn Bertozzi, principal investigator in the research and the director of the DOE’s Molecular Foundry nanoscience research facility at Berkeley Lab, states, "Our method allows the assembly of multicellular structures from the ‘bottom up'.  In other words, we can control the neighbors of each individual cell in a mixed population. By this method, it may be possible to assemble tissues with more sophisticated properties."

Normally, DNA is found only in the chromatin material inside the cell’s nucleus (as well as small amounts being found inside mitochondria).  In order to get DNA on the outer surface of the cell, the researchers first genetically engineered the cells to have special artificial sugars on their surface.  They then created pieces of DNA 20 bases long which bind to these sugars and cover the cells.

Once the cells had their complementary DNA markers attached, they would group together forming microtissues.  The assembly is dependent on concentration and DNA complexity.  If the concentration of cells with the complementary strands is equal, then the tissue is heterogeneous.  If one is in a smaller concentration, the more numerous type groups around the less numerous type.

Scientists also found that simple repeating complementary sequences, such as the pair (CACACA…, TGTGTG…) cause the cells to bind faster than more complex sequences.  Finally, researchers found they could control how fast the cells bind by controlling how many sugars are produced for the cells' surfaces, and thus how many DNA molecules are attached to the cells' surfaces.

More methods of directed assembly should also be possible says Ms. Bertozzi.  She states, "For example, it might be possible to cluster DNA strands on specific cellular structures. Thus, distribution of DNA on the cell surface might be yet another parameter we can exploit to guide cell-cell interactions."

The team used their methods to create a two-cell tissue out of hematopoietic progenitor cells (a kind of stem cell for blood cells). These depend on the presence of the growth factor interleukin-3 by combining them in microtissues with CHO cells (Chinese hamster ovary cells) that were engineered to secrete interleukin-3.  When separated, the two cell types would not grow; but together, they created a microtissue.

More complex designs should also be possible.  Says Ms. Bertozzi, "Since DNA has essentially an unlimited capacity for information storage, there is no theoretical limit on the number of different cell types we can assemble in a structure.  In practice, I think structures with three or four cell types are quite feasible. Such structures would be relevant to many biological organs."

The scientists hope to tweak the method as it could allow for microtissues to be assembled in bulk reactors like chemical engineers create organic chemicals.  The key challenge remaining is that many cells lack the pathways needed to synthesize the sugars to attach the DNA.  Thus, the researchers are looking at alternatives to this approach.

The new work is reported in the March 2 early edition of the journal PNAS.