Location: HSB G-328, 10:30am (unless otherwise noted)
March 06, 2014
Matthew Good, Ph.D., University California - Berkeley
Host: Stan Froehner
Cells exist in a wide variety of shapes and sizes – from round eggs millimeters in diameter to highly elongated neurons with sub-micron extensions. Despite dramatic physical differences, the function of these cells depends on the activities of intracellular structures and organelles, requiring that these internal processes adapt to a wide range of cell geometries. The mitotic spindle, a dynamic microtubule polymer-based structure required for chromosome segregation, provides an important example of this flexibility. For example, during Xenopus early embryo development, cell volume reduces nearly a million-fold due to division in the absence of growth, and spindle adjusts to the spatial dimensions of a cell. However, an open question was whether cell size directly controls spindle size or whether a gene expression program tied to development orchestrates the scaling phenomenon. Due to the difficulty of modulating cell size in embryos I developed a cell-like system with tunable geometry to decipher the mechanism of intracellular size-regulation. By combining droplet microfluidics and cell-free cytoplasmic extracts, I was able to generate spindle and nucleus structures inside compartments whose diameter can be tuned from microns to millimeters, recapitulating the spindle size-scaling trend observed during Xenopus embryogenesis. By precisely modulating droplet diameter and aspect ratio, I discovered that spindle growth is intrinsically tied to compartment volume, but not mechanical feedback from boundaries, suggesting that limiting amounts of cytoplasmic material restrict maximum spindle size. Using this technology it is possible to test whether additional organelles respond to changes in cell size or shape and whether protein signaling networks can act as sensors of cell geometry. The significance of this research lies in the intimate connection between cell size and embryo developmental transitions, and the observation that cell and organelle size are often misregulated in disease.