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| The broad goal of our research is to understand how eukaryotic cells establish their distinct morphology and organization in order to carry out their specialized functions. We are working on a variety of topics ranging from cell polarity, asymmetric cell division, polycystic kidney disease to adaptive evolution, as it is our view that the fundamental principles in biology can be best learned if one could take a broad approach to its problems. Below is a summary of the research projects in our lab: |
I. Regulatory circuits that control cell polarization
The ability of cells to break symmetry and establish a robust polar axis does not rely on any pre-existing external asymmetry. Environmental cues or cues provided from a cell ’s history harness this intrinsic ability to establish polarity in physiologically required orientations. We have used yeast as a model organism to explore the intrinsic mechanisms that can drive cell polarization. Our previous work suggested that two different positive feedback loops are capable of driving symmetry breaking, with one involving actin cytoskeleton-dependent transport and the other involving the GTPase cycle of Cdc42. Recently we have used quantitative imaging and mathematical modeling to investigate the general principles that underlie the dynamic maintenance of cell polarity. We are presently extending the quantitative analysis to the entire set of proteins that define the polar cortical domain in order to understand the dynamic organization of various functional modules at the site of polarized growth.
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II. Control of actin dynamics during cell polarization and motility
The actin cytoskeleton plays major roles in physiological processes such as cell polarization and cell motility. These functions are intimately related to actin ’s dynamic properties. Our work has been focused on the nucleation step of actin filament assembly because this is the rate limiting step in actin polymerization and is an important target for regulation. Through genetic analysis and in vitro actin polymerization assays, we identified several highly conserved protein factors, such as the Arp2/3 complex and a WASP family protein, that are required for assembly of cortical actin filaments. Our current work focuses on understanding the structural basis for the activity of these actin assembly factors using cryo electron microscopy and fluorescence spectroscopy, and the in vivo function and regulation of these proteins using yeast and mouse models. |
III. Using fluorescence correlation spectroscopy (FCS) and FLIM-based FRET measurements to understand protein dynamics and interactions in live cells
Achieving a quantitative understanding of cellular systems requires temporally and spatially resolved characterization of dynamic molecular interactions in live cell settings. We have been exploring the use of FCS and fluorescence cross correlation spectroscopy (FCCS) to study cytosolic and nuclear biochemistry of mobile molecules tagged with autofluorescent proteins. We have recently applied this analysis to the MAP kinase signaling cascade and obtained useful parameters for quantitative modeling of this highly conserved signaling cascade. We are also exploring the use of fluorescence lifetime measurement (FLIM)-based FRET analysis to decipher the in vivo protein interactions in compact and complex cytoskeletal structures involved in cell division. |
IV. Adaptive evolution of the cell division system
A hallmark of biological systems is their remarkable ability to adapt to external or internal perturbations through rapid generation of heritable phenotypic variations. It has been hypothesized that the evolvability of cellular systems is embedded within the complex design features of the underlying molecular networks, which in turn are built through evolutionary processes. We are using yeast and cultured mammalian cells as models to understand whether and how the cell division system is capable of rapid adaptive evolution, how the evolved pathways are linked to the pre-existing molecular network and how cells generate rapid genomic and transcriptome changes to drive the evolutionary process. |
V. The design principles in the mitotic exit control network
Successful cell division is marked not only by the physical separation of progeny cells but more importantly by the correct inheritance of genetic materials. Pioneering work in yeast has shown that mitosis and cytokinesis during cell division are coordinated through an intricate signaling network that controls the exit from mitosis. This network has a great deal of complexity and functional redundancy which underlie the robustness and adaptability that have been observed with this important cell cycle control. We have begun to use computational approaches to study the network design principles of the mitotic exit control. Our goal is to use interweaving modeling and experimentation to understand the timing and spatial sensor functions of this network and to explore the adaptive response of the network when individual components are inhibited. |
VI. Asymmetric cell division during mouse oocyte maturation
Mouse oocytes undergo polarization during meiosis II, during which the centrally located spindle moves to a subcortical region where a cortical actin cap that contains myosin-II assembles. This asymmetric placement of the spindle and formation of the actin-based contractile structure are critical for extrusion of the polar body. Our recent work using a reconstituted system showed that the cortical actomyosin structure can be induced by microinjected DNA-coated beads through a pathway of chromatin-directed cortical myosin-II assembly that involves the MAP kinase cascade and the small GTPase Ran. Ongoing experiments are being carried out to elucidate the biochemical details of this unique pathway of regulation of the cortical cytoskeleton. |
VII. The cellular basis of autosomal dominant polycystic kidney disease (ADPKD)
Cells that constitute mammalian epithelial and endothelial tissues have the ability to detect and adjust to significant environmental stress while carrying out their specialized functions. Loss of such ability could result in altered cell polarity, morphology and proliferation that ultimately lead to diseases. In polarized renal epithelial cells, the apical surface is marked by a single cilium that is thought to act as a mechanical sensor mediated through cilia-associated proteins such as polycystin 1 and 2. Mutations in these proteins result in autosomal dominant polycystic kidney disease (ADPKD), the most common human genetic disease. We are using a combination of network modeling, mouse genetics, and microaray analysis to understand how loss of polycystin function gives rise to ADPKD. |
