Cell division is the great domestic drama of a cell's life. In sickness and in health, cell division by mitosis is the complicated yet critical process by which a mother cell divides into two daughter cells. But first the mother cell has to pack up her cellular household contents, disassembling and dividing up everything for her soon-to-be-formed daughters. How cells manage division has been exhaustively studied for well over a century and yet basic mysteries remain. We know that some organelles such as the endoplasmic reticulum (ER) are pulled apart before division but keep their tubular membrane structure intact. Others like the Golgi, the organelle that sorts out proteins for delivery inside or outside the cell, go to pieces after the prophase of mitosis through what's called a "choreographed disassembly process."
But where does the Golgi go once it is in pieces? Until now, microscopes and other imaging systems could only look so closely before the disassembled Golgi parts blurred into ultra-tiny "puncta" and an unresolvable haze. Now thanks to new 3-D super-resolution imaging methods, Dylan Burnette and Prabuddha Sengupta in Jennifer Lippincott-Schwartz's lab at the Eunice Shriver National Institute of Child Health and Human Development (NICHD) at NIH in Bethesda, MD, have a clearer picture and a clear answer.
The researchers started with two plausible theories: In the ER-linked hypothesis, the Golgi puncta and enzyme haze are closely held by the ER; in the non-ER-linked model, the puncta and haze float about on their own, waiting for cytokinesis when the two daughter cells separate and the Golgi body reappears as stacks of membrane-bound "cisternae," ready to sort proteins from the reassembled ER. Powered by their new imaging technologies, which gave them far greater resolution than was ever possible before, the researchers saw clear support of the ER-linked model—the enzyme haze sticking close to ER markers with the puncta clustering near ER exits.
For a second line of proof, the NICHD researchers followed up with a pharmacological-based trapping assay that showed Golgi enzymes being held tightly by the ER during mitosis. Which means, say the researchers, that Golgi enzymes redistribute into the ER during mitosis and that they must follow an ER export pathway to reform the Golgi at the end of mitosis. Their work not only resolves a basic cellular question but shows what new solutions await as these new technologies give us keener vision and sharper tools.