Researchers from the Georgia Institute of Technology and EmoryUniversity found that chromatin compaction is required for properembryonic stem cell differentiation to occur. Chromatin, which iscomposed of histone proteins and DNA, packages DNA into a smallervolume so that it fits inside a cell. A study published on May 10, 2012 in the journal PLoS Genetics found that embryonic stem cells lacking several histone H1subtypes and exhibiting reduced chromatin compaction suffered fromimpaired differentiation under multiple scenarios and demonstratedinefficiency in silencing genes that must be suppressed to inducedifferentiation. “While researchers have observed that embryonic stem cellsexhibit a relaxed, open chromatin structure and differentiatedcells exhibit a compact chromatin structure, our study is the firstto show that this compaction is not a mere consequence of thedifferentiation process but is instead a necessity fordifferentiation to proceed normally,” said Yuhong Fan, anassistant professor in the Georgia Tech School of Biology. Commercial LED Displays
Fan and Todd McDevitt, an associate professor in the Wallace H.Coulter Department of Biomedical Engineering at Georgia Tech andEmory University, led the study with assistance from Georgia Techgraduate students Yunzhe Zhang and Kaixiang Cao, researchtechnician Marissa Cooke, and postdoctoral fellow Shiraj Panjwani. The work was supported by the National Institutes of Health’sNational Institute of General Medical Sciences (NIGMS), theNational Science Foundation, a Georgia Cancer CoalitionDistinguished Scholar Award, and a Johnson & Johnson/GeorgiaTech Healthcare Innovation Award. To investigate the impact of linker histones and chromatin foldingon stem cell differentiation, the researchers used embryonic stemcells that lacked three subtypes of linker histone H1 — H1c, H1dand H1e — which is the structural protein that facilitates thefolding of chromatin into a higher-order structure. They found thatthe expression levels of these H1 subtypes increased duringembryonic stem cell differentiation, and embryonic stem cellslacking these H1s resisted spontaneous differentiation for aprolonged time, showed impairment during embryoid bodydifferentiation and were unsuccessful in forming a high-qualitynetwork of neural cells. Indoor Advertising LED Display
“This study has uncovered a new, regulatory function forhistone H1, a protein known mostly for its role as a structuralcomponent of chromosomes,” said Anthony Carter, who overseesepigenetics grants at NIGMS. “By showing that H1 plays a partin controlling genes that direct embryonic stem celldifferentiation, the study expands our understanding of H1’sfunction and offers valuable new insights into the cellularprocesses that induce stem cells to change into specific celltypes.” During spontaneous differentiation, the majority of the H1triple-knockout embryonic stem cells studied by the researchersretained a tightly packed colony structure typical ofundifferentiated cells and expressed high levels of Oct4 for aprolonged time. Oct4 is a pluripotency gene that maintains anembryonic stem cell’s ability to self-renew and must be suppressedto induce differentiation. “H1 depletion impaired the suppression of the Oct4 and Nanogpluripotency genes, suggesting a novel mechanistic link by which H1and chromatin compaction may mediate pluripotent stem celldifferentiation by contributing to the epigenetic silencing ofpluripotency genes,” explained Fan. “While a significantreduction in H1 levels does not interfere with embryonic stem cellself-renewal, it appears to impair differentiation.” The researchers also used a rotary suspension culture methoddeveloped by McDevitt to produce with high efficiency homogonous 3Dclumps of embryonic stem cells called embryoid bodies. China Commercial LED Displays
Embryoidbodies typically contain cell types from all three germ layers –the ectoderm, mesoderm and endoderm — that give rise to thevarious types of tissues and structures in the body. However, themajority of the H1 triple-knockout embryoid bodies formed in rotarysuspension culture lacked differentiated structures and displayedgene expression signatures characteristic of undifferentiated stemcells. “H1 triple-knockout embryoid bodies displayed a reduced levelof activation of many developmental genes and markers in rotaryculture, suggesting that differentiation to all three germ layerswas affected.” noted McDevitt. The embryoid bodies also lacked the epigentic changes at thepluripotency genes necessary for differentiation, according to Fan. “When we added one of the deleted H1 subtypes to the embryoidbodies, Oct4 was suppressed normally and embryoid bodydifferentiation continued,” explained Fan.
“Theepigenetic regulation of Oct4 expression by H1 was also evident inmouse embryos.” In another experiment, the researchers provided an environment thatwould encourage embryonic stem cells to differentiate into neuralcells. However, the H1 triple-knockout cells were defective informing neuronal and glial cells and a neural network, which isessential for nervous system development. Only 10 percent of the H1triple-knockout embryoid bodies formed neurites and they producedon average eight neurites each. In contrast, half of the normalembryoid bodies produced, on average, 18 neurites. In future work, the researchers plan to investigate whethercontrolling H1 histone levels can be used to influence thereprogramming of adult cells to obtain induced pluripotent stemcells, which are capable of differentiating into tissues in a waysimilar to embryonic stem cells.