Role of histone H1 in neural differentiation of embryonic stem cells
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Linker histone H1 is a key structural protein facilitating the formation of higher order chromatin structures and regulates specific gene expression. In mammals, there exist 11 closely related H1 variants. Previous studies show that H1 depletion by 50% impairs specific gene regulation and differentiation of embryonic stem cells (ESCs). However, the mechanisms by which H1 and its variants regulate ESC differentiation remain elusive. Here, we demonstrate a dosage effect of H1 variants in mouse ESCs through severe H1 depletion and mutation analysis. We establish ultra-low H1 ESCs by sequential depletion of six somatic H1 variants. These cells exhibit normal ESC morphology and self-renewal. During neural differentiation, the total H1 level gradually increases, and H1 depletion reveals a dosage effect in neurite formation, induction of neural lineage-specific genes, and silencing of pluripotency-associated genes such as Oct4 and Nanog. In addition, severe H1 depletion causes reduced cell proliferation and cellular senescence in neural lineages. Significantly, Oct4 knockdown effectively restores neural differentiation and partially rescues the reduction in cell proliferation and cellular senescence. These results suggest that H1 is crucial for neural differentiation of ESCs and its regulation in the process acts in a dosage dependent, rather than a variant specific, manner. Another part of this thesis centers on analysis of H1 mutations frequently occurred in follicular lymphoma or transformed follicular lymphoma. These mutations in H1 are clustered in the globular and C-terminal domains directly involved in chromatin binding. By comparing the properties of wild-type human H1c (hH1c) and mutant hHcS102F expressed in H1c/H1d/H1e triple knockout mouse ESCs, we find that S102F mutation dramatically impairs the association of hH1c with chromatin. These results suggest that the identified H1 mutations in follicular lymphoma most likely result in a loss-of-function phenotype by reducing the binding affinity of H1 for chromatin, thus compromising chromatin compaction and the regulation of specific genes.