Early embryonic developmental roles of Huntington and H1 linker histones

Abstract

Embryogenesis is a crucial developmental continuum that is responsible for the determination, patterning, and lineage specification and maturation of all individual cell types and profiles of organogenesis representing the overall body plan. The Huntingtin protein (Htt) and H1 linker histones are two structurally and functionally distinct classes of proteins, which have only recently been shown to be essential for early development; however their precise developmental roles have not been adequately explored. In this study, we have utilized established embryonic stem cell (ESC) differentiation paradigms that recapitulate the multistage program of early embryonic development, with Htt knock-out (Htt KO) ESCs, mutant Htt knock-in ESCs, and H1 linker histone triple knock-out ESC line (H1 tKO) ESCs, to explore their respective developmental roles during early embryogenesis. In the third and fourth chapter of this thesis, we demonstrate that Htt plays important functions in 1) the specification and survival of three cardinal germ layer cell types, 2) neural induction and early neurogenesis, and 3) the specification and maturation of tissue-specific lineages and neural subtypes. On the other hand, the presence of the mutant Htt causes differential impairments in these seminal processes and these cumulative developmental deficits are associated with significant alterations in Notch, Hest, Hes5 and STAT3 signaling pathways. In the fifth chapter, we demonstrate that the H1 linker histone proteins appear to be dispensable for the maintenance of undifferentiated ESCs; however, cell survival of germ layers during EB formation is significantly impaired. Furthermore, we demonstrate that there are severe disruption in the specification of neural stem cells and the elaboration and maturation of multiple neural cell lineages. Additionally, the absence of these variants disrupts the developmental programs of cardiomyocyte, hepatic, and pancreatic differentiation; the latter also exhibits ectopic expression of neural and cardiomyocyte genes with precocious neurogenesis and cardiomyogenesis. Interestingly, the targeted deletion of only two H1 linker histone variants results in similar disruptions to the specification and maturation of neural lineages. Our cumulative experimental findings demonstrate essential developmental roles of both Htt and H1 linker histone proteins in the entire early programs of embryogenesis, whereas mHtt differentially impairs these seminal developmental processes.