Human Cell Atlas
Posted on by Lawrence Tabak, D.D.S., Ph.D.
There are 37 trillion or so cells in our bodies that work together to give us life. But it may surprise you that we still haven’t put a good number on how many distinct cell types there are within those trillions of cells.
That’s why in 2016, a team of researchers from around the globe launched a historic project called the Human Cell Atlas (HCA) consortium to identify and define the hundreds of presumed distinct cell types in our bodies. Knowing where each cell type resides in the body, and which genes each one turns on or off to create its own unique molecular identity, will revolutionize our studies of human biology and medicine across the board.
Since its launch, the HCA has progressed rapidly. In fact, it has already reached an important milestone with the recent publication in the journal Science of four studies that, together, comprise the first multi-tissue drafts of the human cell atlas. This draft, based on analyses of millions of cells, defines more than 500 different cell types in more than 30 human tissues. A second draft, with even finer definition, is already in the works.
Making the HCA possible are recent technological advances in RNA sequencing. RNA sequencing is a topic that’s been mentioned frequently on this blog in a range of research areas, from neuroscience to skin rashes. Researchers use it to detect and analyze all the messenger RNA (mRNA) molecules in a biological sample, in this case individual human cells from a wide range of tissues, organs, and individuals who voluntarily donated their tissues.
By quantifying these RNA messages, researchers can capture the thousands of genes that any given cell actively expresses at any one time. These precise gene expression profiles can be used to catalogue cells from throughout the body and understand the important similarities and differences among them.
In one of the published studies, funded in part by the NIH, a team co-led by Aviv Regev, a founding co-chair of the consortium at the Broad Institute of MIT and Harvard, Cambridge, MA, established a framework for multi-tissue human cell atlases . (Regev is now on leave from the Broad Institute and MIT and has recently moved to Genentech Research and Early Development, South San Francisco, CA.)
Among its many advances, Regev’s team optimized single-cell RNA sequencing for use on cell nuclei isolated from frozen tissue. This technological advance paved the way for single-cell analyses of the vast numbers of samples that are stored in research collections and freezers all around the world.
Using their new pipeline, Regev and team built an atlas including more than 200,000 single-cell RNA sequence profiles from eight tissue types collected from 16 individuals. These samples were archived earlier by NIH’s Genotype-Tissue Expression (GTEx) project. The team’s data revealed unexpected differences among cell types but surprising similarities, too.
For example, they found that genetic profiles seen in muscle cells were also present in connective tissue cells in the lungs. Using novel machine learning approaches to help make sense of their data, they’ve linked the cells in their atlases with thousands of genetic diseases and traits to identify cell types and genetic profiles that may contribute to a wide range of human conditions.
By cross-referencing 6,000 genes previously implicated in causing specific genetic disorders with their single-cell genetic profiles, they identified new cell types that may play unexpected roles. For instance, they found some non-muscle cells that may play a role in muscular dystrophy, a group of conditions in which muscles progressively weaken. More research will be needed to make sense of these fascinating, but vital, discoveries.
The team also compared genes that are more active in specific cell types to genes with previously identified links to more complex conditions. Again, their data surprised them. They identified new cell types that may play a role in conditions such as heart disease and inflammatory bowel disease.
Two of the other papers, one of which was funded in part by NIH, explored the immune system, especially the similarities and differences among immune cells that reside in specific tissues, such as scavenging macrophages [2,3] This is a critical area of study. Most of our understanding of the immune system comes from immune cells that circulate in the bloodstream, not these resident macrophages and other immune cells.
These immune cell atlases, which are still first drafts, already provide an invaluable resource toward designing new treatments to bolster immune responses, such as vaccines and anti-cancer treatments. They also may have implications for understanding what goes wrong in various autoimmune conditions.
Scientists have been working for more than 150 years to characterize the trillions of cells in our bodies. Thanks to this timely effort and its advances in describing and cataloguing cell types, we now have a much better foundation for understanding these fundamental units of the human body.
But the latest data are just the tip of the iceberg, with vast flows of biological information from throughout the human body surely to be released in the years ahead. And while consortium members continue making history, their hard work to date is freely available to the scientific community to explore critical biological questions with far-reaching implications for human health and disease.
 Single-nucleus cross-tissue molecular reference maps toward understanding disease gene function. Eraslan G, Drokhlyansky E, Anand S, Fiskin E, Subramanian A, Segrè AV, Aguet F, Rozenblatt-Rosen O, Ardlie KG, Regev A, et al. Science. 2022 May 13;376(6594):eabl4290.
 Cross-tissue immune cell analysis reveals tissue-specific features in humans. Domínguez Conde C, Xu C, Jarvis LB, Rainbow DB, Farber DL, Saeb-Parsy K, Jones JL,Teichmann SA, et al. Science. 2022 May 13;376(6594):eabl5197.
 Mapping the developing human immune system across organs. Suo C, Dann E, Goh I, Jardine L, Marioni JC, Clatworthy MR, Haniffa M, Teichmann SA, et al. Science. 2022 May 12:eabo0510.
Ribonucleic acid (RNA) (National Human Genome Research Institute/NIH)
Studying Cells (National Institute of General Medical Sciences/NIH)
Regev Lab (Broad Institute of MIT and Harvard, Cambridge, MA)
NIH Support: Common Fund; National Cancer Institute; National Human Genome Research Institute; National Heart, Lung, and Blood Institute; National Institute on Drug Abuse; National Institute of Mental Health; National Institute on Aging; National Institute of Allergy and Infectious Diseases; National Institute of Neurological Disorders and Stroke; National Eye Institute
Posted on by Dr. Francis Collins
A Happy New Year to one and all! While many of us were busy wrapping presents, the journal Science announced its much-anticipated scientific breakthroughs of 2018. In case you missed the announcement , it was another banner year for the biomedical sciences.
The 2018 Breakthrough of the Year went to biomedical science and its ability to track the development of life—one cell at a time—in a variety of model organisms. This newfound ability opens opportunities to understand the biological basis of life more systematically than ever before. Among Science’s “runner-up” breakthroughs, more than half had strong ties to the biomedical sciences and NIH-supported research.
Sound intriguing? Let’s take a closer look at some of the amazing science conducted in 2018, starting with Science’s Breakthrough of the Year.
Development Cell by Cell: For millennia, biologists have wondered how a single cell develops into a complete multicellular organism, such as a frog or a mouse. But solving that mystery was almost impossible without the needed tools to study development systematically, one cell at a time. That’s finally started to change within the last decade. I’ve highlighted the emergence of some of these powerful tools on my blog and the interesting ways that they were being applied to study development.
Over the past few years, all of this technological progress has come to a head. Researchers, many of them NIH-supported, used sophisticated cell labeling techniques, nucleic acid sequencing, and computational strategies to isolate thousands of cells from developing organisms, sequence their genetic material, and determine their location within that developing organism.
In 2018 alone, groundbreaking single-cell analysis papers were published that sequentially tracked the 20-plus cell types that arise from a fertilized zebrafish egg, the early formation of organs in a frog, and even the creation of a new limb in the Axolotl salamander. This is just the start of amazing discoveries that will help to inform us of the steps, or sometimes missteps, within human development—and suggest the best ways to prevent the missteps. In fact, efforts are now underway to gain this detailed information in people, cell by cell, including the international Human Cell Atlas and the NIH-supported Human BioMolecular Atlas Program.
An RNA Drug Enters the Clinic: Twenty years ago, researchers Andrew Fire and Craig Mello showed that certain small, noncoding RNA molecules can selectively block genes in our cells from turning “on” through a process called RNA interference (RNAi). This work, for the which these NIH grantees received the 2006 Nobel Prize in Physiology or Medicine, soon sparked a wave of commercial interest in various noncoding RNA molecules for their potential to silence the expression of a disease-causing gene.
After much hard work, the first gene-silencing RNA drug finally came to market in 2018. It’s called Onpattro™ (patisiran), and the drug uses RNAi to treat the peripheral nerve disease that can afflict adults with a rare disease called hereditary transthyretin-mediated amyloidosis. This hard-won success may spark further development of this novel class of biopharmaceuticals to treat a variety of conditions, from cancer to cardiovascular disorders, with potentially greater precision.
Rapid Chemical Structure Determination: Last October, two research teams released papers almost simultaneously that described an incredibly fast new imaging technique to determine the structure of smaller organic chemical compounds, or “small molecules“ at atomic resolution. Small molecules are essential components of molecular biology, pharmacology, and drug development. In fact, most of our current medicines are small molecules.
News of these papers had many researchers buzzing, and I highlighted one of them on my blog. It described a technique called microcrystal electron diffraction, or MicroED. It enabled these NIH-supported researchers to take a powder form of small molecules (progesterone was one example) and generate high-resolution data on their chemical structures in less than a half-hour! The ease and speed of MicroED could revolutionize not only how researchers study various disease processes, but aid in pinpointing which of the vast number of small molecules can become successful therapeutics.
How Cells Marshal Their Contents: About a decade ago, researchers discovered that many proteins in our cells, especially when stressed, condense into circumscribed aqueous droplets. This so-called phase separation allows proteins to gather in higher concentrations and promote reactions with other proteins. The NIH soon began supporting several research teams in their groundbreaking efforts to explore the effects of phase separation on cell biology.
Over the past few years, work on phase separation has taken off. The research suggests that this phenomenon is critical in compartmentalizing chemical reactions within the cell without the need of partitioning membranes. In 2018 alone, several major papers were published, and the progress already has some suggesting that phase separation is not only a basic organizing principle of the cell, it’s one of the major recent breakthroughs in biology.
Forensic Genealogy Comes of Age: Last April, police in Sacramento, CA announced that they had arrested a suspect in the decades-long hunt for the notorious Golden State Killer. As exciting as the news was, doubly interesting was how they caught the accused killer. The police had the Golden Gate Killer’s DNA, but they couldn’t determine his identity, that is, until they got a hit on a DNA profile uploaded by one of his relatives to a public genealogy database.
Though forensic genealogy falls a little outside of our mission, NIH has helped to advance the gathering of family histories and using DNA to study genealogy. In fact, my blog featured NIH-supported work that succeeded in crowdsourcing 600 years of human history.
The researchers, using the online profiles of 86 million genealogy hobbyists with their permission, assembled more than 5 million family trees. The largest totaled more than 13 million people! By merging each tree from the crowd-sourced and public data, they were able to go back about 11 generations—to the 15th century and the days of Christopher Columbus. Though they may not have caught an accused killer, these large datasets provided some novel insights into our family structures, genes, and longevity.
An Ancient Human Hybrid: Every year, researchers excavate thousands of bone fragments from the remote Denisova Cave in Siberia. One such find would later be called Denisova 11, or “Denny” for short.
Oh, what a fascinating genomic tale Denny’s sliver of bone had to tell. Denny was at least 13 years old and lived in Siberia roughly 90,000 years ago. A few years ago, an international research team found that DNA from the mitochondria in Denny’s cells came from a Neanderthal, an extinct human relative.
In 2018, Denny’s family tree got even more interesting. The team published new data showing that Denny was female and, more importantly, she was a first generation mix of a Neanderthal mother and a father who belonged to another extinct human relative called the Denisovans. The Denisovans, by the way, are the first human relatives characterized almost completely on the basis of genomics. They diverged from Neanderthals about 390,000 years ago. Until about 40,000 years ago, the two occupied the Eurasian continent—Neanderthals to the west, and Denisovans to the east.
Denny’s unique genealogy makes her the first direct descendant ever discovered of two different groups of early humans. While NIH didn’t directly support this research, the sequencing of the Neanderthal genome provided an essential resource.
As exciting as these breakthroughs are, they only scratch the surface of ongoing progress in biomedical research. Every field of science is generating compelling breakthroughs filled with hope and the promise to improve the lives of millions of Americans. So let’s get started with 2019 and finish out this decade with more truly amazing science!
 “2018 Breakthrough of the Year,” Science, 21 December 2018.
NIH Support: These breakthroughs represent the culmination of years of research involving many investigators and the support of multiple NIH institutes.