Behind the paper: Uncovering the twin origins of human cortical astrocytes

September 7, 2022

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Dr. Denise Allen and Dr. Tomasz Nowakowski on the College of California, San Francisco not too long ago revealed an article in Science the place they revealed a twin origin for astrocytes within the human cortex. Utilizing a mixture of destiny mapping and single cell evaluation, they revealed that the 2 stem cell niches within the creating cortex give rise to spatially, morphologically, and molecularly distinct populations of astrocytes. The Node requested them to present us a behind the scenes have a look at how the story got here collectively:

How did you get began on this undertaking?

TJN: For a really very long time, we’ve got been within the query of why the brains of primates and people are a lot bigger and sophisticated than the brains of mice, which we examine incessantly within the laboratory. Variations in mind dimension may be discovered very early on throughout improvement, and due to this fact it was believable to hypothesize that variations in the way in which radial glia, which act as neural stem cells, develop may contribute to those variations. In our prior work, we discovered that animals with giant brains, similar to primates or people, include a better range of radial glia subtypes in comparison with mice. Specifically, we discovered that based mostly on gene expression profiles, radial glia might be divided into truncated radial glia and outer radial glia, that are positioned in two anatomically distinct niches of the creating cortex.

DA: Throughout my undergraduate neuroscience courses I used to be all the time struck by the deep data we’ve got concerning the improvement of neurons within the cerebral cortex, however astrocytes and different glia gave the impression to be so typically ignored. Throughout my rotation within the Nowakowski lab, I grew to become fascinated with Tom’s preliminary information that recommended distinct subtypes of radial glia may give rise to distinct astrocyte populations. I used to be actually excited by the truth that giant mind mammals, together with primates and people, appear to have a special repertoire of radial glia in comparison with rodents, in addition to rather more complicated astrocytes. So the chance to check the distinctive options of human improvement with a concentrate on astrocytes was a dream come true.

What was already recognized concerning the developmental trajectories of radial glia within the creating mind previous to your work?

Quite a lot of work has been carried out probing the differentiation of outer radial glia (also called basal radial glia). Quite a few papers have proven that they offer rise to neurons, oligodendrocytes and supposedly the vast majority of astrocytes, however the function of truncated radial glia has not been studied in nice element. Earlier research have recommended that as a result of few mitotic cells may be discovered within the ventricular zone stem cell area of interest throughout midgestation in primates and people, that the truncated radial glia that reside on this zone are unlikely to function a serious supply of latest cells. We determined to problem this assumption by labeling progenitors within the ventricular zone and figuring out the fates of the ensuing cells. To our shock, we discovered that neurons, oligodendrocytes, and astrocytes proceed to be produced by ventricular zone progenitors.

Are you able to summarize your findings? What was the important thing experiment?

The important thing experiment concerned labeling progenitor cells that occupy anatomically distinct niches that truncated and outer radial glia cells, and tracing the fates of cells that they produce. We discovered that whereas each populations broadly produce related cell varieties (neurons, astrocytes and oligodendrocytes), they produce very distinct subtypes of astrocytes. In a collection of very hanging outcomes, we discovered that truncated radial glia give rise to astrocytes that migrate to the cortical plate, whereas outer radial glia give rise to astrocytes that don’t migrate, and as a substitute differentiate regionally within the outer subventricular zone.

We frequently converse concerning the range of neurons, however classical research have additionally proven that astrocytes may be remarkably numerous, even in early improvement. I needed to additional discover the variety of the astrocyte subtypes we recognized, however it’s difficult to attach these classical descriptions of astrocyte subtypes to modern-day descriptions similar to these derived from single cell sequencing. To resolve this drawback, I took benefit of a way known as Patch-seq which gave us the power to aspirate the contents of a cell that was beforehand outlined based mostly on its morphology and place, after which performing sequencing of that mobile contents to find out a molecular identification. This evaluation was key for bridging our mobile definitions based mostly on morphology and developmental cell lineage, and linking them to molecular markers. This allowed me to carry the story in full circle.

When doing the analysis, did you may have any explicit end result or eureka second that has caught with you?

The very first experiment I carried out that concerned labeling these two totally different stem cell niches resulted in a distribution of cell varieties that might not have been extra totally different. Many comparisons in developmental biology relaxation upon small variations between circumstances. To see such a stark distinction within the distribution of cells–particularly of glia–was an thrilling second that outlined the course of the undertaking very early on in my PhD.

One other shocking discovering was once we began carefully evaluating classical drawings by Ramon y Cajal and Retzius and to pictures of our astrocyte subtypes. Remarkably, we discovered that our “dense bulbous” astrocytes have been clearly depicted in these early information, however these cells have hardly ever been talked about in trendy literature. This realization gave us lots of confidence that the cells we have been observing have been an actual phenomenon and never an experimental artifact. These cells had simply been mendacity in wait, ready for somebody to place the highlight on them.

And what concerning the flipside? Any frustrations or despair?

The Universe actually conspired towards us once we have been attempting to complete experiments for the revision of the paper. I arrange the final three revision experiments in late December, once we instantly discovered that it was time to maneuver our lab to a brand new constructing at UCSF. We got here up with an elaborate system to maintain the cultures going whereas we moved and so they appeared to have survived, till somebody instantly seen that the incubator had failed and the alarm hadn’t gone off. What adopted was two months of situation after situation attempting to repeat these final two experiments, however we lastly bought there ultimately!

The place will this story take the lab?

This work has impressed a number of new tasks within the lab. We’re excited to look at if related findings may be replicated in different fashions of mind improvement similar to cerebral organoids, what these distinctive subpopulations appear like within the grownup mind, and what function they may play in illness states. I’m additionally hoping this work can even entice extra trainees curious about glial improvement to the lab!

What’s subsequent for you/the lab after this paper? Tell us in case you are persevering with this analysis, or beginning/searching for a brand new place.

Denise has graduated and is at present interviewing for computational biologist roles in biotech. She is trying ahead to delving into the “large information” aspect of biology, and dealing in the direction of making a major affect on sufferers’ lives.