Chromatin Modification in iPS Cells

Chromatin is composed of nucleosomes—structures consisting of DNA wound around histone proteins. Condensed chromatin suppresses gene expression, while relaxed chromatin allows transcription factors to access DNA and activate genes. During somatic to iPS cell reprogramming, scientists remodel chromatin from its compact form to a more accessible state, enabling the gene expression necessary for pluripotency.

Methylation strengthens histone-DNA interactions and suppresses gene transcription, while acetylation weakens these interactions and loosens chromatin to make DNA accessible. Histone demethylases remove methyl groups to activate pluripotency genes, whereas histone acetyltransferases add acetyl groups to open chromatin. These opposing modifications allow scientists to control gene expression during cell reprogramming.

Histone variants like H2AZ and H3.3 replace major histone proteins and alter chromatin structure by changing the amount of DNA wound around them. H2AZ increases DNA accessibility to transcription factors and often carries increased acetylation, enhancing the conversion of somatic cells to induced pluripotent stem cells by promoting pluripotency gene expression.

Inhibitors of histone deacetylases and histone methyltransferases increase reprogramming efficiency by preventing chromatin compaction. Similarly, DNA methylase inhibitors loosen chromatin and allow expression of genes essential for pluripotency. These inhibitors work by blocking enzymes that normally suppress gene expression, making chromatin more accessible for transcription factor binding.

During chromatin remodeling, entire nucleosomes physically relocate along the DNA, altering which genes are accessible to transcription factors. This repositioning, combined with histone modifications, transforms condensed chromatin into a relaxed form. Nucleosome relocation is catalyzed by chromatin remodeling complexes that scientists add to somatic cells to enhance pluripotent stem cell conversion.

DNA methylation causes chromatin compaction, similar to histone methylation, making DNA inaccessible to transcription factors. During reprogramming, scientists add DNA methylase inhibitors to loosen chromatin and enable expression of genes essential for pluripotency. Reducing DNA methylation is therefore a key strategy to facilitate induced pluripotent stem cell generation.

Scientists add histone demethylases and acetyltransferases to loosen chromatin and increase DNA accessibility. They also use inhibitors of histone deacetylases, histone methyltransferases, and DNA methylases to prevent chromatin compaction. Additionally, histone variants and chromatin remodeling complexes are introduced to alter gene expression patterns and promote the conversion of somatic cells to pluripotent stem cells.