X-ray tomography exposes cell nuclei

Editorial

Rebecca Pool

Wednesday, November 23, 2016 - 21:00
Image: Skeletonized structure of DNA material, heterochromatin, in a mouse’s nerve cell. [Berkeley Lab/UCSF]
 
Using X-ray imaging tools, researchers from Berkeley Lab have mapped the reorganization of genetic material that takes place when a stem cell matures into a nerve cell.
 
Detailed 3D reconstructions of mouse olfactory cells reveal an unexpected connectivity in chromatin within the cell nucleus, providing a new understanding of the cell’s evolving architecture.
 
These renderings show a tightly packed form of DNA called heterochromatin as it exists in a mouse cell’s nucleus at different stages of cell development: a multipotent stem cell (left), a neuronal progenitor (middle), and a mature nerve cell (right). [Berkeley Lab, UCSF]
 
To study the cells, researchers turned to soft Xray tomography at Berkeley Lab’s Advanced Light Source (ALS) to capture images of frozen olfactory nerve cells at different stages of maturity during cell differentiation.
 
Cells at each stage were imaged from dozens of different angles using X-rays with each set of 2D images being used to calculate a 3D reconstruction of a cell detailing the changing chromatin formations in the nuclei.
 
In this way, researchers were also able to measure the dense packing in a form of chromatin called heterochromatin, learning about the importance of a specific protein in controlling the compaction of heterochromatin and its confinement to the nucleus.
 
“It’s a new way of looking at the nucleus where we don’t have to chemically treat the cell,” said Carolyn Larabell, director of the National Center for X-ray Tomography, a joint program of Berkeley Lab and UC San Francisco (UCSF).
 
“Being able to directly image and quantify changes in the nucleus is enormously important and has been on cell biologists’ wish list for many years,” she adds.
 
This computer rendering shows the skeletonized structure of heterochromatin (red represents a thin region while white represents a thick region), a tightly packed form of DNA, surrounding another form of DNA-carrying material known as euchromatin (dark blue represents a thin region and yellow represent the thickest) in a mouse’s mature nerve cell. [Berkeley Lab/UCSF]
 
According to Larabell, chromatin is notoriously sensitive to chemical stains and other chemical additives often used in biological imaging.
 
“Until now, it has only been possible to image the nucleus indirectly by staining it, in which case the researcher has to take a leap of faith that the stain was evenly distributed,” she adds.
 
However, these results shed light on how patterning and reorganization of chromatin in a nucleus relate to a cell’s specialised function as specific genes are activated or silenced.
 
“We’re trying to understand how the reorganization of chromatin affects gene expression,” highlights Larabell. “No one’s been able to study this at the human level yet.”
 
With the proven success of the imaging technique, Larabell said it’s possible to perform statistical analyses based on large collections of cell nuclei images sorted by different stages of development.
 
Coupled with other types of imaging techniques, researchers hope to isolate individual gene-selection processes in upcoming work.
 
Research is published in Cell Reports.
 
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