Research

My research focuses on how cellular protein networks establish spatial organization within and among cells, and how this organization drives cell fate decisions in development. What are the key features of protein networks that drive long range spatial patterning?

I currently work on cell polarity and the pathways that allow cells to differentiate top and bottom, front from back. This relatively simple concept lies at the heart of many diverse and essential developmental processes in biology, from bacteria to humans. Yet, it often involves complex signaling networks.

My current focus is the highly conserved PAR polarity network, which plays an essential role throughout metazoan development. It regulates asymmetric and cell fate inheritance during cell division, the establishment of body plans, axon-dendrite specification, cell migration, and the establishment of epithelial tissue architecture. Defects in the PAR network are associated with numerous developmental defects and cancer.

My goal is to identify the physical principles that underly pattern generation by the PAR network and how these principles allow for a system that drives polarity in a wide range of cells that differ dramatically in size, shape, and function. As a model system, I work on early embryonic development of the nematode worm Caenorhabditis elegans. Here, PAR polarity plays a central role in orchestrating in a series of asymmetric cell divisions that are critical for generating the major cell lineages of the adult animal.

PAR polarity first appears in the one-cell embryo, as two groups of antagonistic PAR proteins are segregated into complementary domains along the long axis of the embryo (right). Polarization of PAR proteins is triggered in response to a large scale reorganization of a highly contractile, membrane-associated actomyosin meshwork (below), which drives asymmetric transport of PAR proteins within the cell.

Genetic approaches have revealed most of the key players and many key interactions. However, bridging the nanometer scale of protein interactions to the formation of cell scale patterns remains elusive.

Bacteria are now known to exhibit a highly complex cellular organization involving multiple types of cytoskeletal elements and membrane compartmentalization. This organization is particularly apparent during the process of cell division.

Just as in eukaryotes, a variety of highly coordinated processes must occur to ensure a successful cell division. Bacteria possess machinery to ensure the proper segregation of the DNA, including structures that appear to form a rudimentary mitotic apparatus and DNA translocating proteins. Bacteria also possess mechanisms to specify the site of division, which in Escherichia coli involves a reaction-diffusion driven oscillation of a cell division inhibitor.

In order to divide, E. coli assembles a ring-like structure of division proteins known as the “divisome.” The divisome is responsible for inducing the architectural changes required to split the cell in two and includes the tubulin homologue, FtsZ, which is the primary structural component of the ring. While in Jon Beckwith’s group, I was particularly interested in how division proteins are recruited and assembled into this structure. During my Ph.D, we developed several techniques to tackle this problem, including an imaging based approach to identify functional interactions in vivo and a morphology based screen for mutations that affect the division process [2-4]. These assays revealed unexpected complexity in the assembly process and suggested that the apparent linear assembly pathway of the divisome is due to its being composed of a hierarchical set of protein complexes.

After leaving the lab, I continued to be involved in several projects. First, we applied techniques I developed during my Ph.D. to identify functional interactions in vivo in order to address evolutionary conservation of the divisome assembly process between E. coli and the gram-positive bacterium, Bacillus subtilis [5]. Second, we identified a unique role for FtsN among the essential divisome components in being required for a fast-lysis phenotype in response to β-lactams, suggesting a potential regulatory role for FtsN in initiating the process of cell division [6].

FtsL localizes to midcell in E. coli [1].

A characteristic “wrinkled” colony due to a mutation in cell division gene FtsI [2].

Placement of the cell division site in the bacterium E. coli is a good example of a system that relies on positional information provided by overlapping signaling pathways: the self-organizing oscillatory pattern-forming Min system specifies the cell middle, while a DNA sensing pathway prevents division from occurring on top of and thereby damaging the bacterial chromosomes. Here division has been blocked, but nascent cell division sites (yellow) can still be seen forming at regular intervals between the segregated DNA (blue). [3]

Once formed, these so-called ‘PAR domains’ define the anterior-posterior (A-P) axis of the animal and provide for the spatial regulation of numerous downstream pathways that drive division asymmetry, from the biased microtubule pulling forces that displace the mitotic spindle from the cell center to the unequal partitioning of cytoplasmic cell fate determinants into the daughter cells. This general pattern is then repeated in a series of polarized cell divisions that ultimately restrict germ cell fate into two stem-cell like precursors.

A reaction-diffusion scheme for PAR polarity based on reciprocal antagonistic feedback between membrane-associated PAR species. [2]

Asymmetric cell divisions segregate germ cell fate in the

C. elegans P lineage.

To answer these questions, I am taking an integrative approach, combining genetics, pharmacologic, and quantitative imaging-based approaches, along with the formulation and testing of quantitative models for polarity establishment. We have begun to define the mobility of PAR proteins in cells and how these mobilities are controlled to allow enrichment of PAR proteins within domains [1], and to test theoretical models for actin-dependent polarization in the one cell embryo [2].

•How do dynamic protein interactions yield a stable boundary between PAR domains?
•What governs the size and position of domains?
•How is domain size measured and controlled?
•How is pattern formation coupled to changes in the underlying cytoskeletal meshwork?

By analyzing polarization from the molecular to systems-level scale, my work seeks to identify the core features that drive and regulate pattern formation by the PAR protein network, and ultimately determine the consequences of these features for cell polarity in different cells and tissues during development.

[1] Goehring et al. (2011) Journal of Cell Biology. [2] Goehring et al. (2011) Science.

[1] Ghigo et al. (1999) Mol. Micro. [2] Goehring et al. (2007a) J. Bact. [3] Goehring et al. (2006) Mol. Micro.[4] Goehring et al. (2006) Genes Dev. [5] Robichon et al. (2008) J. Bact. [6] Chung et al. (2009) PNAS.