Chromatin Remodeling and Vascular Development
For the lab’s first decade, we focused on the role of two key ATP-dependent chromatin-remodeling complexes in vascular development: SWI/SNF and NuRD. We first reported that these complexes transcriptionally and antagonistically regulate yolk sac vascular Wnt signaling at midgestation (Griffin and Curtis et al, 2011; Curtis and Griffin, 2012). Further analysis of SWI/SNF mutants at later stages of development revealed that they had defective venous specification due to misregulated expression of the venous transcription factor COUP-TFII (Davis et al, 2013) and defective capillary integrity due to misregulated expression of the serum response factor cofactors MRTFA/B (Menendez and Ong et al, 2017). These studies are important because they were among the first to reveal the contribution of chromatin-remodeling complexes to vascular development, and they established a model for how we could glean interesting genetic and molecular information from vascular phenotypes in chromatin-remodeling complex mutants. Notably, our approach has been tractable and productive because we can frequently attribute the specific phenotypes generated in our mutants to a single misregulated gene or signaling pathway. Therefore, this unbiased approach is revealing genes for which tight transcriptional regulation or timing is essential during vascular development. See below for descriptions of how additional SWI/SNF and NuRD vascular mutants have opened up new areas of research for the lab.
Central research approach. Because chromatin-remodeling complexes modulate expression of specific target genes, we exploit these complexes to identify critical genes for vascular development and maintenance. Specifically, we genetically delete the central ATPase of these complexes (which is critical for their function) in murine vascular cells. (1) We then look for a resulting vascular phenotype, which indicates critical target genes were misregulated in our mutants. (2) Finally, we use candidate or unbiased approaches to identify the misregulated gene(s) underlying the phenotype. This tractable approach combines strengths of forward and reverse genetics and has revealed novel aspects of vascular biology to us.
Plasmin-mediated vascular fragility
While analyzing vascular development in NuRD chromatin-remodeling complex mutants, we discovered that these mutants develop sudden and reproducible vascular rupture at midgestation due to transcriptional misregulation of genes that activate the extracellular matrix protease plasmin. Genetic reduction of plasmin activators rescued vascular matrix proteolysis and rupture in our mutants (Ingram et al, 2011). We subsequently discovered that NuRD continues to regulate plasmin activation and vascular integrity later in embryonic development, with particularly important consequences for hepatic matrix degradation and sinusoidal vascular rupture (Crosswhite et al, 2016). This work is important because it reveals a novel, chromatin-based mechanism for regulating plasmin activation. It also highlights the detrimental effects of excessive plasmin on embryonic blood vessels and thereby challenges the dogma that plasmin-mediated matrix degradation is predominantly beneficial during development because it promotes sprouting angiogenesis. We recently built upon this embryonic work to show that elevation of plasmin likewise causes matrix degradation and hepatic sinusoidal bleeding in adult mice after acetaminophen overdose (Gao et al, 2018). Altogether, our work is generating new information about plasmin regulation and is identifying contexts in which therapeutic plasmin inhibition could protect vascular integrity.
Our ongoing analysis of embryos with mutated chromatin-remodeling enzymes has piqued our interest in the programmed necrosis (necroptosis) cell death pathway. Our research indicates that this form of cell death causes embryonic vascular fragility and rupture. We are leveraging NIH funding (R01HL134778) to study causes and effects of endothelial cell necroptosis in development and disease. Stay tuned as the lab publishes its first papers in this exciting new field.