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yeast

Provocative prions may protect yeast cells from stress

Prions have a notorious reputation. They cause neurodegenerative disease, namely mad cow/Creutzfeld-Jakob disease. And the way these protein particles propagate – getting other proteins to join the pile – can seem insidious.

Yet prion formation could represent a protective response to stress, research from Emory University School of Medicine and Georgia Tech suggests.

A yeast protein called Lsb2, which can trigger prion formation by other proteins, actually forms a “metastable” prion itself in response to elevated temperatures, the scientists report.

The results were published this week in Cell Reports.

Higher temperatures cause proteins to unfold; this is a major stress for yeast cells as well as animal cells, and triggers a “heat shock” response. Prion formation could be an attempt by cells to impose order upon an otherwise chaotic jumble of misfolded proteins, the scientists propose.

A glowing red clump can be detected in yeast cells containing a Lsb2 prion (left), because Lsb2 is hooked up to a red fluorescent protein. In other cells lacking prion activity (right), the Lsb2 fusion protein is diffuse.

“What we found suggests that Lsb2 could be the regulator of a broader prion-forming response to stress,” says Keith Wilkinson, PhD, professor of biochemistry at Emory University School of Medicine.

The scientists call the Lsb2 prion metastable because it is maintained in a fraction of cells after they return to normal conditions but is lost in other cells. Lsb2 is a short-lived, unstable protein, and mutations that keep it around longer increase the stability of the prions.

The Cell Reports paper was the result of collaboration between Wilkinson, Emory colleague Tatiana Chernova, PhD, assistant professor of biochemistry, and the laboratory of Yury Chernoff, PhD in Georgia Tech’s School of Biological Sciences.

“It’s fascinating that stress treatment may trigger a cascade of prion-like changes, and that the molecular memory of that stress can persist for a number of cell generations in a prion-like form,” Chernoff says.”Our further work is going to check if other proteins can respond to environmental stresses in a manner similar to Lsb2.” Read more

Posted on by Quinn Eastman in Uncategorized Leave a comment

When cells fix DNA the wrong way

Cells sometimes “fix” DNA the wrong way, creating an extra mutation, Emory scientists have revealed.

Biologist Gray Crouse, PhD, and radiation oncologist Yoke Wah Kow, PhD, recently published a paper in Proceedings of the National Academy of Sciences that shows how mismatch repair can introduce mutations in nondividing cells. Their paper was recognized by the National Institute of Environmental Health Sciences as an extramural paper of the month. The first author is lead research specialist Gina Rodriguez.

In DNA, a mismatch is when the bases on the two DNA strands do not conform to Watson-Crick rules, such as G with T or A with C. Mismatches can be introduced into DNA through copying errors as well as some kinds of DNA damage.

If the cell “fixes” the wrong side, that will introduce a mutation (see diagram). So how does the cell know which side of the mismatch needs to be repaired? Usually mismatch repair is tied to DNA replication. Replication enzymes appear to somehow mark the recently copied strand as being the one to replace — exactly how cells accomplish this is an active area of research.

In some situations, mismatch repair could introduce mutations into DNA.

Overall, mismatch repair is a good thing, from the point of view of preventing cancer. Inherited deficiencies in mismatch repair enzymes lead to an accumulation of mutations and an increased risk of colon cancer and other types of cancer.

But many of the cells in our bodies, such as muscle cells and neurons, have stopped dividing more or less permanently (in contrast with the colon). That means they no longer need to replicate their DNA. Other cells, such as resting white blood cells, have stopped dividing temporarily. Mutations in nondividing cells may have implications for aging and cancer formation in some tissues.

Through clever experimental design, Crouse’s team was able to isolate examples of when mismatch repair occurred in the absence of DNA replication.

As the NIEHS Newsletter notes:

“The researchers introduced specific mispairs into the DNA of yeast cells in a way that let them observe the very rare event of non-strand dependent DNA repair. They found that mispairs, not repaired during replication, sometimes underwent mismatch repair later when the cells were no longer dividing. This repair was not strand dependent and sometimes introduced mutations into the DNA sequence that allowed cells to resume growth. In one case, they observed such mutations arising in cells that had been in a non-dividing state for several days.”

Although the Emory team’s research was performed on yeast, the mechanisms of mismatch repair are highly conserved in mammalian cells. Their results could also shed light on a process that takes place in the immune system called somatic hypermutation, in which mutations fine-tune antibody genes to make the most potent antibodies.

Posted on by Quinn Eastman in Cancer 2 Comments