Cowan Group Research
Symmetry breaking during cell polarization
Cell polarity allows for spatial specialization within a cell, such as directed transport, migration, or growth. Cell polarity also controls asymmetric cell division, a dominant mechanism for dictating cell fate changes during development. The diverse roles of this fundamental organizational concept mean that understanding cell polarity is essential to understanding both normal development and diseases. We are investigating how cell polarity is established.
- Figure 1 (Click to view legend)
We are using one-cell C. elegans embryos as a model system to determine how cell polarity is established. One-cell C. elegans embryos polarize in response to a signal from the sperm-provided centrosomes. Centrosomes induce a local structural reorganization of the acto-myosin network: whereas most of the cortex undergoes stochastic contractions, the area nearest the centrosomes becomes non-contractile. This local change in cortex activity marks the functional symmetry-breaking event that allows polarization. Once cortical symmetry has been broken, mutual antagonism between antagonistic polarity proteins - the anterior and posterior PA R proteins - facilitates self-organizing polarization. PA R polarity controls cell fate determinant segregation and asymmetric cell division.
Using a combination of forward and reverse genetics, in vivo biochemistry, high-resolution time-lapse microscopy, ultrastructural reconstruction, automated quantitative analysis, and mechanical manipulations, we are investigating the following questions:
How do centrosomes communicate with the cortex?
After the sperm centrosomes are delivered to the egg during fertilization, they wander randomly in the cytoplasm for approximately thirty minutes. A dense network of cytoplasmic microtubules prevents centrosomes from moving too far away from the cortex. Upon a cell cycle signal, centrosomes are activated and signal to the cortex to change acto-myosin contractility. Centrosomes can initiate polarity from any position within the embryo, but the efficiency of polarization increases when centrosomes are close to the cortex. We are investigating the mechanisms and functions of centrosome positioning to determine how accurate information is supplied to the cortex during symmetry breaking. In a genetic screen for centrosomal molecules that may mediate signaling from centrosomes to the cortex, we identified the Aurora family kinase AIR-1. Embryos depleted of AIR-1 often fail to break cortical symmetry in response to the centrosomes and instead undergo spontaneous polarization. AIR-1 depleted embryos often have multiple polarity axes, leading to mis-segregation of cell fate determinants during cell division. AIR-1 is required for centrosome growth but this function is distinct from AIR-1’s roles in polarity establishment. We are trying to understand how AIR-1 both positively and negatively regulates cortical symmetry breaking to ensure that a single polarity axis is formed.
What regulates PA R polarity in response to cortical symmetry breaking?
- Figure 2 (Click to view legend)
After the initial symmetry-breaking event in the cortex, mutually exclusive PA R protein domains drive the establishment of a stable cell polarity axis. The balance between the amounts of anterior and posterior PA R domain components appears essential for normal polarization. Controlling PA R protein amounts - both absolute and at the cortex - is an important regulatory point. Total PA R protein levels appear to depend on processing of relevant mRNAs, while the proportion of cortical PA R proteins appears to be influenced by intracellular trafficking. We are looking at the molecular mechanisms by which these pathways control cortical PA R protein localization.
How does cortical polarity control cytoplasmic asymmetry?
- Figure 3 (Click to view legend)
The establishment of polarity at the cortex provides spatial information to polarize the entire cell, ultimately allowing asymmetric changes in gene expression and cell fate. The cytoplasmic fate determinant PIE-1 is restricted to the posterior half of one-cell embryos and thus is inherited only by cells in the germline lineage. PIE-1 forms a concentration gradient in response to two distinct activities that change the apparent diffusion of PIE-1 in the cytoplasm: in the anterior, MEX-5 increases PIE-1 mobility, and in the posterior, MEX-1 decreases PIE-1 mobility. MEX-1 and MEX-5 in turn affect each other. We are using mathematical models and biochemistry to understand the parameters that are important for PIE-1 mobility.