The complex, 3D micro-anatomy of the human liver. (Image source: WikiMedia Commons)

One of the Holy Grails of stem cell research is growing body parts to replace those damaged by disease or injury. Enormous strides have been made in a key first step: mastering recipes for maturing stem cells into various specialized cell types. But a lawn of, say, liver cells in a petri dish is not a functioning liver. Organs have complex, three-dimensional structures with intricate communication between multiple cell types.

Scientists are actively devising methods to overcome this challenge. For instance, cultivating cells onto biological scaffolds help mold the cells into the shape of a particular organ or tissue. And retooled 3D printers using “bio ink” can seed layers of different cells onto these scaffolds to create specified structures.

This week, a UCSF team added an ingenious new tool to this tissue engineering tool kit.  As reported on Monday in Nature Methods, the lab of Zev Gartner took advantage of DNA’s Velcro-like chemistry to build layers of different cell types in a specified pattern.

DNA – it’s not just for genetics anymore

A DNA fragment is made of two complimentary strands that bind together with high specificity. (Image source: Visionlearning)

DNA is a molecule made of two thin strands. Each strand is specifically attracted to the other based on a unique sequence of genetic information. So if two strands of a short DNA fragment are peeled apart, they will only rejoin to each other and not some other fragment with a different sequence.  While DNA usually resides in the nucleus of a cell, the team worked out a method to temporarily attach copies of a strand of DNA on the outside of, let’s call it, “cell A”. The opposite strand of that DNA fragment was attached to “cell B”. When mixed together the two cells became attached to each other via the matching DNA sequences. Other cells with different DNA fragments floated on by.

The screen shot below from a really neat time-lapse video, which accompanies the research publication, shows how a rudimentary 3D cell structure could be built with a series of different cell-DNA fragment combinations. In this case, the team first attached DNA fragments onto a petri dish in a specific pattern. At the thirty-second mark in the video, you can see that cells with matching DNA fragments have attached to the DNA on the dish.

This video demonstrates the assembly of 3D cell structures with the help of DNA “Velcro” (image source: Todhunter et al. Nature Methods 2015 Aug 31st)

The new technique, dubbed DNA programmed assembly of cells (DPAC), opens up a lot possibilities according to Gartner in a UCSF press release:

 “We can take any cell type we want and program just where it goes. We can precisely control who’s talking to whom and who’s touching whom at the earliest stages. The cells then follow these initially programmed spatial cues to interact, move around, and develop into tissues over time.”

The Quest still continues with possible victories along the way

 Of course, this advance is still a far cry from the quest for whole organs derived from stem cells. The cell assemblies using DPAC can only be grown up to about 100 microns, the thickness of a human hair. Beyond that size, the innermost cells get starved of oxygen and nutrients. Gartner says that obstacle is a current focus in the lab:

“We’re working on building functional blood vessels into these tissues. We can get the right cells in the right positions but haven’t figured out how to perfuse them with blood or a substitute efficiently yet.”

In the meantime, building these small 3D “organoids” from stem cells certainly could be put to good use as a means to test drug toxicity on human tissue or as a way to study human disease.

Related Links:

• Video: CIRM Tissue Engineering and Stem Cell Research Workshop