Polycomb Complex in Embryonic Development and stem cells
DNA in eukaryotic cells is hierarchically organized into chromatin which is a complex of DNA and proteins. This complex forms chromosomes within the nucleus of a eukaryotic cell. The proteins that generally shape the chromatin are histone proteins and the core histones are H2A, H2B, H3 & H4. The fundamental unit of chromatin is a nucleosome which comprises of 147 base pairs of DNA wrapped around an octamer of histone proteins in 1.8 helical turns. Histone H1 binds the linker DNA between the two adjacent nucleosomes causing further compaction of chromatin into higher order chromatin structures, often referred to as solenoids. The dynamics of the chromatin plays a crucial role in gene expression (production of proteins) which regulates almost all biological processes in the cell. This dynamics is regulated by many factors such as, histone variants, histone modifications and the binding of non-histone proteins to DNA. Histone variants are non-core histone proteins that associate with chromatin only to certain parts of chromatin or during certain stages of cell cycle that regulate gene expression. The non- histone proteins that bind to chromatin and regulate gene expression are RNA polymerase, transcription factors, mediators etc. Post-translation modifications (PTMs) are covalently linked chemical groups that are added to proteins after the information coded in RNA is decoded into proteins, hence the term post-translational. Post-translational modifications on chromatin are of several types such as phosphorylations, acetylation and methylation. These dimensional structure of histones revealed that histones tails protrude outward from the nucleosome core. These histone tails undergo post-translational modifications that influence large number of biological processes such as DNA replication, Transcription, DNA repair and Chromosome compaction. Histones PTMs generate cking sites or modulate the affinity of nuclear proteins for chromatin. This recognition of PTMS is number of domains present in certain proteins that are themselves part of large protein ting chromatin remodeling and transcription. Two groups of Protein complexes called trithorax and PCG exemplify how histone PTMs are involved in transcription regulation Both these complexes were identified 50 year ago for their role in activation and repression of HOX gene respectively during early embryonic development. ThxGeatalyses the deposition of trimethyl gr amino acid lysine of histone H3 (H3K4me3) and PeG is responsible for trimethylation of lysine histone H3 (H3K27ME3). Both these methylation share opposing effects on transcription act and repression respectively. Both TrxG and PeG are important for normal development of an e However, due to space restrictions this topic will only focus on Polycomb group of proteins
Recruitment of PRC1 and PRC2 onto chromatin Recruitment of PRC2 to chromatin has received a considerable attention, several leading scientists have proposed pathways for how PRC2 is recruited to chromatin. One of the widely accepted hypothesis is that PRC2 is recruited to CpG islands on the chromatin. Three different mechanism have been proposed for PRC2 recruitment on CpG island: 1. Proteins that associate with PRC2 only under specific cell conditions. 2. Sequence specific transcription factors. 3. Non-coding (RNAs that do not code for
The recruitment of PRC1 depends on the activity of PRC2. The H3K27me3 mark acts as docking site for binding of PRC1. The CBX chromo domain binds to the chromatin containing this methylation mark. In addition, non-canonical PRC1 associates with other proteins and is targeted to specific sites onto chromatin directly without the need of a methylation mark.
Taken together, several modes of recruitment PcG proteins have been proposed as outlined above, yet the Exact roles of these interaction is not fully understood.
Polycomb function during embryogenesis:
The role of polycomb during embryogenesis was studied by the analysis of mice for deletion (knockout) of various polycomb components. Such studies have revealed key functions of the various polycomb components during embryonic development usually exhibiting gastrulation defects.
Specifically, knockout of PRC2 components, Suz12, Ezh2 and Eed leads to the death of the embryo.
drawing early post-implantation stage. However the knockout of Jarid2 leads to defects in neuronal tube formation. Knockout of the catalytic subunit of PRC1, RING1B, results in embryonic lethality and in trast Ring 1A knockout mice are viable.
polycomb functions in embryonic stem cells:
It has thus become increasingly clear that histone modifications (epigenetic regulation) play a critical role in determining embryonic stem cells (ESCs) fates. Elucidating the mechanistic pathways by which the PcG proteins act and interact to fine-tune the regulation of cell fate specific genes is highly important for understanding embryonic development. Since these regulatory pathways are often impeded in cancer, this knowledge could offer us the possibility to directly target mis- regulated pathways with specific drugs.
ESCs are derived from the inner cell mass of the pre-implantation embryo at the stage of blastocyst. A hallmark of ESCs is their pluripotency, which bestows them with the ability to give rise to all cell types of an embryo. When reintroduced into host blastocysts, ESCs are capable of generating into all cell types, including germ cells. In ESCs, pluripotency is maintained by the prevention of differentiation and the promotion are cultured under the proper conditions. Differentiation of ESCs in vitro recapitulates the same hierarchical steps of embryo development that occurs in vivo. During development, lineage-specific transcription factors are regulated by epigenetic events that impose either a permissive or a repressive environment over cell fate. of proliferation. Thus, ESCs can self-renew continuously in vitro in test tube) if they are Found.
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