Since coming to the NIH, your focus shifted from cell cycle to nuclear morphology…
The lab mascot (a toy yeast cell) really needed a coffee break, Cohen-Fix explains. “I take science seriously, but I don't take myself too seriously.”
When I first came to the United States with my husband and two kids, I expected it to be a temporary move. I planned to return to Israel and become a researcher, but, after living here for a few years, we found that we were very comfortable here. My husband and I decided to look for jobs in the States instead, and, of all the places I interviewed, NIH was one of my favorites.
One of the great things about working at the NIH is that NIH researchers don't write grants, so in a way we have a lot more freedom to pursue risky projects than most academic researchers. That's why, about six years after I got to the NIH, I felt able to start shifting my lab from cell cycle to nuclear architecture. I did that because there are so many talented people working on cell cycle, and I felt that my time and resources would be better spent focusing on something less studied.
Now nobody in my lab studies the cell cycle. Except, of course, it turns out that we actually are still studying the cell cycle; we just didn't know it. [Laughs] There's no escaping the cell cycle.
Several years ago I started wondering why the nucleus looks the way it does. Yeast don't have a nuclear skeleton that we know of, so what is it that determines nuclear shape? Why is the nucleus round? We decided to look for mutants whose nuclei weren't round.
In the literature we found a yeast mutant called spo7Δ
. Absence of the Spo7 protein results in over-proliferation of the ER membrane, which is contiguous with the nuclear membrane. For us, the interesting thing about spo7
mutants was that, although their DNA looks normal by DAPI staining, they have a flare-like nuclear extension. We showed that these flares are always found at the nuclear envelope adjacent to the nucleolus, which in wild-type yeast cells forms a crescent shape up against the edge of the nucleus. This says that not all areas of the nuclear envelope are equal; for reasons we still don't understand, the extra membrane created by spo7
mutants accumulates over the nucleolus, causing the nucleolus to change shape, but nowhere else around the nucleus.
“Not all areas of the nuclear envelope are equal.”
Flares appear at the nucleoli (green), but not the DNA-containing region (blue), of nuclei in spo7Δ mutant yeast cells.
What we've found most recently—and this isn't published yet—is that these flares also form in yeast cells that are delayed in mitosis. We think that what's happening is that, when the mitotic checkpoint turns on, it blocks chromosome segregation but not membrane synthesis. Because yeast nuclear membranes remain intact during cell division, the cells end up with all this extra membrane, which they stick over the nucleolus. Why that happens, and how, is something we're working on right now.
We've also looked in C. elegans, where defects in a process analogous to the yeast Spo7 pathway result in impaired nuclear envelope disassembly and reassembly. We think this is due to expansion of the ER membrane that causes a “traffic jam” that prevents the nuclear envelope from being properly absorbed into the ER at mitosis. This has led us to consider additional questions—for example, in higher eukaryotes, when the nuclear membrane reforms after mitosis, how does it “know” to make one round nucleus instead of lots of little micro-nuclei around individual chromosomes?
But ultimately one of the most fascinating questions that we're trying to figure out is what determines the nuclear-to-cell volume ratio. It turns out that probably in many systems, but definitely in yeast, there's a constant ratio between nuclear volume and cell volume, and nobody knows how that is determined. No mutants have been found where this is completely abolished—except in cancer cells—so is this ratio important for maintaining normal cell division, or is it simply disrupted as a consequence of transformation?