ARTIFICIAL GOLGI: Sample molecules in an artificial golgi apparatus are moved around by applying voltages at electrodes and held in place with the help of magnets. Along the way, they are exposed to enzymes that can modify them chemically in an assembly-line fashion. Image: courtesy Robert Linhardt et al.
More and more synthetic versions of key parts of the human cell, including chromosomes, have been developed by scientists in the past decade or so. Now researchers are aiming even higher, developing the first working artificial prototype of an "organ" of a human cell—the Golgi apparatus, which helps modify biomolecules and package them for delivery around the cell.
The Golgi apparatus is an organelle, akin to a miniature organ in a cell, made up of a network of sacs piled together like a stack of pancakes. The role it plays in chemically modifying proteins is crucial for their stability and function, and it also helps manufacture complex sugars. However, the Golgi apparatus remains one of the most poorly understood organelles in the body.
"The sacs are fluid and constantly change shape, so it's difficult to get a handle on," explains researcher Robert Linhardt, a chemist at Rensselaer Polytechnic Institute. "And while we know the general direction of the flow of vesicles between stacks, we don't really know what cargoes they're carrying."
To better dissect how the Golgi apparatus works, Linhardt and his colleagues tried creating a synthetic version of it, designing a square-millimeter-sized lab-on-a-chip to mimic the assembly line of enzymes within the Golgi apparatus that modify a biomolecule. The sample molecules are attached to magnetic particles, suspended in a watery droplet 300-billionths of a liter in size and placed on the chip. When the desired location on the chip for those molecules is electrically charged, it becomes more attractive to water, causing the droplet to flow there. A larger magnet can then be kept under that spot to keep the magnetic particles attached to the biomolecules in place. In this way, the drop can be moved through chambers loaded with an assembly line of enzymes, as well as sugars and other raw materials the enzymes might attach to the sample.
In experiments with an inactive precursor of heparin, a widely used blood thinner, the scientists found their device could quickly and efficiently modify the anticoagulant to make it functional, findings they are scheduled to detail in the August 12 issue of the Journal of the American Chemical Society. The researchers suggest that an artificial Golgi could lead to a faster, safer method of producing heparin than current techniques, which employ animal tissue.
This device is apparently the first instance of an artificial organelle, says biochemist and glycobiologist Jerry Turnbull of the University of Liverpool in England, who did not participate in this study. Although the proof-of-concept system Linhardt demonstrated is simple, employing just one enzyme, it "clearly has potential to be greatly diversified as a tool," he adds.
Scientists have experimented with building cells up piece by piece for decades, including the creation of simple artificial cells in the form of bubbles made of synthetic cell membranes, to better understand how life on Earth might have began. Then, in 1997, scientists devised the first artificial human chromosome. And earlier this year, molecular technologist George Church of Harvard University and his colleagues reported a breakthrough in developing artificial ribosomes, bodies inside each cell that make proteins based on instructions from DNA. Although the first artificial ribosome was created in 1968, this recent work assembled ribosomes in the kind of environment where protein synthesis normally occurs, unlike before. Such ribosomes could be inexpensive cell-independent alternatives to current industrial processes that manufacture proteins using cells, Church says.
Now that Linhardt and his colleagues have devised a primitive artificial Golgi, in the future they plan on creating a synthetic endoplasmic reticulum (ER) as well, the organelle into which ribosomes are studded, where protein synthesis and folding take place.
"We'd even like to integrate an artificial Golgi and ER together," Linhardt says. "We're basically taking pieces of a cell and making them on electronic chips, and hopefully moving on to even more complex systems. Of course, organelles are pretty complicated, with the mitochondria and chloroplasts almost having the complexity of a bacterial cell, so there's a lot of work to go."
In the near term, the artificial Golgi could help scientists understand how the actual organelle modifies biomolecules—for instance, how it glycosylates or adds sugars to proteins. The sugars on a protein can affect its structure and function, how the immune system reacts to it and how long it remains in the bloodstream.
"Over half of all proteins are glycosylated, so it's clear that it's an important process, but we just don't have good ways of studying it in the cell," Linhardt says. "This platform gives us an artificial system to test it with, see what's important."