Flora Lamson Hewlett Chair and Professor of Cell and Developmental BiologyLab Homepage: http://mcb.berkeley.edu/labs/heald/
My laboratory’s ongoing research explores two fundamental areas of biology. The first is cell division, in particular the assembly and function of the mitotic spindle, which in all eukaryotes performs the universal and essential task of accurately distributing the duplicated genome to two daughter cells. Our second major project is biological size control, which so far we have been studying at the level of intracellular organelles, but in the longer term will span multiple levels, from cells to organisms. The common themes to our research are the use of Xenopus, which provides unique and powerful systems for in vitro and in vivo studies, and extensive collaboration with experts in structural biology, biophysics, bioengineering and proteomics.
Cell Division. Mitosis is arguably the most dramatic event in the life of a cell. Chromosomes condense, organelles vesiculate, and the microtubule cytoskeleton rearranges into a bipolar spindle that attaches to chromosomes and segregates a complete set to each daughter cell. Although the morphological changes that occur during mitosis were first observed over a century ago, we still do not understand how these dynamic events are orchestrated. We develop assays in Xenopus egg extracts to elucidate mitotic events and identify novel players. The extract system is ideal because it provides the complexity of total cytoplasm, yet is open, allowing control of cell cycle progression, specific inactivation of individual components, high-resolution observation, and biochemical and biophysical approaches. We take advantage of this system to address fundamental questions: 1. How do chromosomes condense during mitosis? 2. How do biochemical signals from chromosomes contribute to spindle assembly? 3. How does the spindle assemble and adapt to different cellular contexts, and 4. What is the contribution of cytoplasmic membranes and how are they reorganized and distributed? These projects inform our ultimate objective: 5. Can we reconstitute the spindle from purified components?
Figure 1. Time-lapse images of spindles assembled in Xenopus egg extract around a bead coated with RCC1, the guanine nucleotide exchange factor for Ran (green, upper panel) or a bead coated with chromatin (blue, lower panel). This experiment shows that a single protein (RCC1) can induce spindle assembly, but the bead oscillates within the spindle.
Size Control. Size varies widely in biology at many levels: the animal, the cells that make up the animal, and the contents of the cells, but we don’t understand how scaling occurs so that everything fits and functions properly. Correct organelle size is crucial for cell function, architecture, and division, but the control systems that a cell uses to regulate the size of its organelles are virtually unknown. Macromolecular cellular assemblies like the ribosome or nuclear pore self assemble from a fixed number of subunits and do not vary in size. In contrast, many complex organelles adjust in size to suit a particular cellular environment. Loss of certain tumor suppressors, or overexpression of oncogenes such as myc can result in an increase in cell and nuclear size. Although size-related cell morphology changes are characteristic of cancer pathology and used diagnostically, the precise nature of these changes and their underlying molecular mechanisms are poorly understood.
Xenopus frogs offer two physiological contexts in which we can investigate organelle size control. First, we can compare Xenopus laevis to the smaller, related species Xenopus tropicalis, which lays smaller eggs and has proportionally smaller cells throughout development. Second, we can compare different stages of Xenopus laevis embryogenesis, as the ~1 millimeter diameter egg rapidly cleaves to form smaller blastomeres. A unique aspect of our approach has been to prepare cytoplasmic extracts from eggs and embryos that recapitulate organelle scaling in vitro, which we can use to identify molecular differences that underlie size changes. Together with the laboratory of Dan Fletcher, we are building synthetic cells of defined sizes by encapsulating Xenopus cytoplasm inside droplets and unilamellar vesicles.
Figure 2. In vitro scaling systems. A. Xenopus laevis (left) and Xenopus tropicalis (right) egg extracts generate different sized nuclei and mitotic spindles in vitro. (B) Extracts can be prepared from embryos at the 4 cell (left) or 4000 cell (right) embryos that form different-sized spindles in vitro. (C) Spindle assembly reactions can be performed inside different-sized droplets of cytoplasm in oil.
Wilbur J, Heald R. (2013) Mitotic spindle scaling during Xenopus development by kif2a and importin α. eLife 2013;2:e00290.
Whitehead E, Heald R, Wilbur J (2013) N-terminal phosphorylation of p60 katanin directly regulates microtubule severing. J Mol Biol 425, 214-221. (PMC3540178)
Xiao B, Freedman BS, Miller KE, Heald R, Marko JF (2012) Histone H1 compacts DNA under force and during chromatin assembly. Mol Biol Cell 23, 4864-71. (PMC3521692)
Patel K, Nogales E, Heald R. (2012) Multiple domains of human CLASP contribute to microtubule dynamics and organization in vitro and in Xenopus egg extracts. Cytoskeleton 69, 155-165. (PMC3315288)
Halpin D, Kalab P, Wang J, Weis K, Heald R. (2011) Spindle assembly around RCC1 coated beads in Xenopus egg extracts. PLoS Biol 9, e1001225. (PMC3246454)
Kieserman EK, Heald R. (2011) Mitotic chromosome size scaling in Xenopus. Cell Cycle 10, 3863-70. (PMC3266116)
Loughlin R, Wilbur JD, McNally FJ, Nedelec F, Heald R. (2011) Katanin contributes to interspecies spindle length scaling in Xenopus. Cell 147, 1937- 1407. (PMC3240848)
Soderholm JF, Bird SL, Kalab P, Sampathkumar Y, Hasegawa K, Uehara-Bingen M, Weis K, Heald R. (2011) Importazole, a small molecule inhibitor of the transport receptor importin-β. ACS Chem Biol 6, 700-8. (PMC3137676)
Loughlin R, Heald R, Nedelec F. (2010) A computational model predicts Xenopus meiotic spindle organization. J Cell Biol 191, 1239-1249. (PMC3010074)
Levy DL, Heald R. (2010) Nuclear size is regulated by importin alpha and NTF2 in Xenopus. Cell 143, 288-298. (PMC2966892)
Photo credit: Mark Hanson at Mark Joseph Studios.
Last Updated 2013-02-25