Chang Lab

Cell density

LIFE IN A CROWDED ENVIRONMENT

Although biochemical reactions have been largely studied in the context of purified proteins in dilute solutions, the molecular environment inside living cells is completely different. The cytoplasm is highly crowded environment, chock full of components including ribosomes, large protein or protein-RNA complexes, filamentous cytoskeletal elements, membranous and non-membranous organelles of diverse sizes as well as metabolites, ions and water. This crowded nature of the cytoplasm may hinder movement of large complexes while allowing for movement of small proteins or molecules. Macromolecular crowding may also contribute to the mechanical properties of the cell and organization of components, for instance through formation of phase separations. The density and crowding of the cytoplasm is not a constant, but varies during the cell cycle and development, as well as in aging and disease states. As we are interested in how cellular processes perform in the context of living cells, we see crowding and other cytoplasmic properties as critical but largely unappreciated parameters that can have a large impact on the biology of living cells. We are working to address questions such as:

• How do physical properties of the cytoplasm and nucleoplasm affect cellular processes, such as cytoskeletal dynamics, endocytosis or growth? How does osmotic pressure generated by macromolecules affect intracellular organization?

• How is cytoplasmic density regulated, for instance at different stages of the cell cycle, or in response to certain perturbations or physiological states? What are the molecules that control or contribute cytoplasmic density?

Our recent stories on density in fission yeast:

Supergrowth. When fission yeast cells are inhibited from growing (in volume), intracellular density and the concentration of many proteins all increase. Surprisingly, when this inhibition was lifted, these cells exhibit “supergrowth” — growing about twice as fast as normal. This supergrowth behavior suggests that some aspect of cell density dictates how fast cells grow, which may be one mechanism for density homeostasis.

Regulation of cell density in the cell cycle. Using a new method of quantitative phase imaging, we show that intracellular density varies during the fission yeast cell cycle and also exhibits spatial gradients within the cell.

The cytoplasm dampens microtubule dynamics. Using osmotic shifts, we discovered that cytoplasmic concentration tunes the rate of microtubule growth and shrinkage. At high concentrations of cytoplasm, microtubules freeze. We find that these effects of the cytoplasm are through viscosity rather than crowding effects. These findings begin to inform on how the cytoplasm affects biochemical reactions within cells.

Regulation of nuclear size by osmotic pressure. It is still mysterious how the size of the nucleus scales with size of the cell. We propose that macromolecules within the nucleus and cytoplasm generate colloid osmotic pressure that specifies the nuclear size.