Many people probably think of mice as unwanted household pests. But over more than a century, mice have proven to be incredibly valuable in medical research. One of many examples is how studies in mice are now helping researchers understand how mammalian genomes work, including the human genome. Scientists have spent decades inactivating, or “knocking out,” individual genes in laboratory mice to learn which tissues or organs are affected when a specific gene is out of order, providing valuable clues about its function.
More than a decade ago, NIH initiated a project called KOMP—the Knockout Mouse Project . The goal was to use homologous recombination (exchange of similar or identical DNA) in embryonic stem cells from a standard mouse strain to knock out all of the mouse protein-coding genes. That work has led to wide availability of such cell lines to investigators with interest in specific genes, saving time and money. But it’s one thing to have a cell line with the gene knocked out, it’s even more interesting (and challenging) to determine the phenotype, or observable characteristics, of each knockout. To speed up that process in a scientifically rigorous and systematic manner, NIH and other research funding agencies teamed to launch an international research consortium to turn those embryonic stem cells into mice, and ultimately to catalogue the functions of the roughly 20,000 genes that mice and humans share. The consortium has just released an analysis of the phenotypes of the first 1,751 new lines of unique “knockout mice” with much more to come in the months ahead. This initial work confirms that about a third of all protein-coding genes are essential for live birth, helping to fill in a major gap in our understanding of the genome.
Caption: From stem cells to bone. Human bone cell progenitors, derived from stem cells, were injected under the skin of mice and formed mineralized structures containing cartilage (1-2) and bone (3). Credit: Loh KM and Chen A et al., 2016
To help people suffering from a wide array of injuries and degenerative diseases, scientists and bioengineers have long dreamed of creating new joints and organs using human stem cells. A major hurdle on the path to achieving this dream has been finding ways to steer stem cells into differentiating into all of the various types of cells needed to build these replacement parts in a fast, efficient manner.
Now, an NIH-funded team of researchers has reported important progress on this front. The researchers have identified for the first time the precise biochemical signals needed to spur human embryonic stem cells to produce 12 key types of cells, and to do so rapidly. With these biochemical “recipes” in hand, researchers say they should be able to generate pure populations of replacement cells in a matter of days, rather than the weeks or even months it currently takes. In fact, they have already demonstrated that their high-efficiency approach can be used to produce potentially therapeutic amounts of human bone, cartilage, and heart tissue within a very short time frame.
If you’re curious what innovations are coming out of the Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative, take a look at this video shot via a microscope. What at first glance looks like water dripping through pipes is actually a cool new technology for swiftly and efficiently analyzing the gene activity of thousands of individual cells. You might have to watch this video several times and use the pause button to catch all of the steps, but it provides a quick overview of how the Drop-seq microfluidic device works.
First, a nanoliter-sized droplet of lysis buffer containing a bead with a barcoded sequencing primer on its surface slides downward through the straight channel at the top of the screen. At the same time, fluid containing individual cells flows through the curved channels on either side of the bead-bearing channel—you can catch a fleeting glimpse of a tiny cell in the left-hand channel about 5 seconds into the video. The two streams (barcoded-bead primers and cells) feed into a single channel where they mix, pass through an oil flow, and get pinched off into oily drops. Most are empty, but some contain a bead or a cell—and a few contain both. At the point where the channel takes a hard left, these drops travel over a series of bumps that cause the cell to rupture and release its messenger RNA—an indicator of what genes are active in the cell. The mRNAs are captured by the primer on the bead from which, after the drops are broken, they can be transcribed, amplified, and sequenced to produce STAMPS (single-cell transcriptomes attached to microparticles). Because each bead contains its own unique barcode that enables swift identification of the transcriptomes of individual cells, this process can be done simultaneously on thousands of cells in a single reaction.