Ullman Lab Research

Cell Division – Nuclear Assembly and Function

When a higher eukaryotic cell divides, nuclei undergo dramatic remodeling (Prunuske and Ullman, 2006, LaJoie and Ullman, 2017; see figure below, where the pore protein POM121 is green).

Series of images showing nuclei remodeling

Disassembly of nuclear envelope involves dispersal of the nuclear membrane as well as solubilization of nuclear pore complexes, which break into small subunits. We are particularly interested in the process by which the nuclear pore and the nuclear envelope are coordinately reassembled during cell division. In collaboration with the Frost lab at UCSF, we have found that the nuclear membrane protein LEM2 coalesces around microtubules that remain connected to chromosomes after chromosome segregation. LEM2 recruits and works cooperatively with the ESCRT factor CHMP7 to seal the nuclear envelope (Gu, LaJoie, Allen et al., 2017, von Appen, LaJoie, Johnson et al., 2020). This montage from a live imaging experiment shows the organization and rapid time frame of these events.

Series of images showing how LEM2 works with ESCRT factor CHMP7 to seal the nuclear envelope

We are also interested in how processes at the reforming nucleus are connected to cell cycle regulation and how morphological changes in nuclear structure affect nuclear function in the context of cancer (Chow et al., 2012a) and diseases such as progeria. We have new projects starting up, in collaboration with Niwa lab at the University of California, San Diego, to understand the interconnections between nuclear envelope and endoplasmic reticulum function.

Cell Division – Cytokinetic Abscission

One striking phenotype that arises when levels of Nup153 are depleted in mammalian cells is a delay in cytokinesis (Mackay et al., 2009, Mackay et al., 2010b, Mackay and Ullman, 2011). We track this phenotype both with live cell imaging (as further described in a video protocol, Mackay et al, 2010a) and by monitoring midbodies (see figure), which are structures formed between the two daughter cells that organize the final separation of the cells, or abscission. This unexpected finding led us to an interest in how the role of Nup153 is tied into the timing of midbody resolution, and we found that impairing Nup153 function prevents the Aurora B mediated abscission checkpoint from being satisfied (Mackay et al., 2010b). We are now interested in elucidating the molecular link that connects defects at the nuclear pore complexto Aurora B.

Additionally, we are working downstream to understand how the process of abscission is regulated. Abscission, like NE closure, relies on ESCRT factors, in this case to remodel membranes in the thin intercellular bridge to facilitate abscission. We have found that certain ESCRT factors localize to cytoplasmic granules when the abscission checkpoint is active and that this corresponds to a delay in their recruitment to the site of abscission (Williams et al., 2020). We are studying how these granules–or Abscission Checkpoint Bodies (see figure)–form and the full scope of their regulatory role. Like other checkpoints, the abscission checkpoint is a quality control mechanism that works to prevent cell dysfunction that otherwise can contribute to cancerous cell growth.