Description:

The Simpson Querrey Institute for Epigenetics presents:

Abigail Buchwalter, PhD

Assistant Professor, Cardiovascular Research Institute
University of California at San Francisco

"Dissecting the Influence of the Nuclear Lamina on Heterochromatin Function and Nuclear Integrity"

Abstract:

The nuclear lamina is a filamentous protein meshwork that underlies the inner nuclear membrane of almost all eukaryotic nuclei. This structure scaffolds heterochromatin at the nuclear periphery and enhances nuclear rigidity. Mechanistic dissection of these key functions has been limited by functional redundancies between lamin isoforms and lamin-associated nuclear membrane proteins. 

Here, we remove three nuclear lamins and lamin B receptor (LBR) in mouse embryonic stem cells (mESCs) and show that heterochromatin marked by the H3K9me2 modification detaches from the nuclear periphery. Mutant mESCs sustain naïve pluripotency and maintain H3K9me2 across the genome but cannot repress H3K9me2-marked genes or transposons. Further, mutant cells fail to differentiate into epiblast-like cells (EpiLCs), a transition that requires the expansion of H3K9me2 across the genome.  Mutant EpiLCs can silence naïve pluripotency genes and activate epiblast-stage genes. However, H3K9me2 cannot repress markers of alternative fates, including primitive endoderm. We conclude that the lamins and LBR control the spatial position, dynamic remodeling, and repressive capacity of H3K9me2-marked heterochromatin to shape cell fate decisions.

While an intact lamina is thought to maintain nuclear integrity, we were surprised to find that lamin-null mESCs do not undergo nuclear rupture. However, displacement of heterochromatin from the nuclear periphery by removal of both the lamins and LBR induces frequent nuclear ruptures, indicating that the peripheral heterochromatin layer contributes to nuclear robustness. Surprisingly, stem cells retain pluripotency and viability in spite of frequent nuclear rupture. We have identified separation-of-function mutants of LBR that can rescue differentiation capacity but not nuclear rupture. In ongoing experiments, we are using this system to explore the sensing and repair of nuclear rupture in pluripotency.