High-Mobility Group (HMG) chromosomal proteins are the most numerous nuclear non-histone proteins. HMGB domain proteins are the most abundant and well-studied HMG proteins. They are involved in variety of biological processes. HMGB1 and HMGB2 were the first members of HMGB-family to be discovered and are found in all studied eukaryotes. Despite the high degree of homology, HMGB1 and HMGB2 proteins differ from each other both in structure and functions. In contrast to HMGB2, there is a large pool of works devoted to the HMGB1 protein whose structure–function properties have been described in detail in our previous review in 2020. In this review, we attempted to bring together diverse data about the structure and functions of the HMGB2 protein. The review also describes post-translational modifications of the HMGB2 protein and its role in the development of a number of diseases. Particular attention is paid to its interaction with various targets, including DNA and protein partners. The influence of the level of HMGB2 expression on various processes associated with cell differentiation and aging and its ability to mediate the differentiation of embryonic and adult stem cells are also discussed.

The functioning of DNA in the cell nucleus is ensured by a multitude of proteins, whose interactions with DNA as well as with other proteins lead to the formation of a complicated, organized, and quite dynamic system known as chromatin. This review is devoted to the description of properties and structure of the progenitors of the most abundant non-histone protein of the HMGB family—the HmgB1 protein. The proteins of the HMGB family are also known as “architectural factors” of chromatin, which play an important role in gene expression, transcription, DNA replication, and repair. However, as soon as HmgB1 goes outside the nucleus, it acquires completely different functions, post-translational modifications, and change of its redox state. Despite a lot of evidence of the functional activity of HmgB1, there are still many issues to be solved related to the mechanisms of the influence of HmgB1 on the development and treatment of different diseases—from oncological and cardiovascular diseases to pathologies during pregnancy and childbirth. Here, we describe molecular structure of the HmgB1 protein and discuss general mechanisms of its interactions with other proteins and DNA in cell.

Non-histone nuclear proteins HMGB1 and HMGB2 (High Mobility Group) are involved in many biological processes, such as replication, transcription, and repair. The HMGB1 and HMGB2 proteins consist of a short N-terminal region, two DNA-binding domains, A and B, and a C-terminal sequence of glutamic and aspartic acids. In this work, the structural organization of calf thymus HMGB1 and HMGB2 proteins and their complexes with DNA were studied using UV circular dichroism (CD) spectroscopy. Post-translational modifications (PTM) of HMGB1 and HMGB2 proteins were determined with MALDI mass spectrometry. We have shown that despite the similar primary structures of the HMGB1 and HMGB2 proteins, their post-translational modifications (PTMs) demonstrate quite different patterns. The HMGB1 PTMs are located predominantly in the DNA-binding A-domain and linker region connecting the A and B domains. On the contrary, HMGB2 PTMs are found mostly in the B-domain and within the linker region. It was also shown that, despite the high degree of homology between HMGB1 and HMGB2, the secondary structure of these proteins is also slightly different. We believe that the revealed structural properties might determine the difference in the functioning of the HMGB1 and HMGB2 as well as their protein partners.

Linker histone H1 is one of the main chromatin proteins which plays an important role in organizing eukaryotic DNA into a compact structure. There is data indicating that cell type-specific post-translational modifications of H1 modulate chromatin activity. Here, we compared histone H1 variants from NIH/3T3, mouse embryonic fibroblasts (MEFs), and mouse embryonic stem (ES) cells using matrix-assisted laser desorption/ ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FT-ICR-MS). We found significant differences in the nature and positions of the post-translational modifications (PTMs) of H1.3-H1.5 variants in ES cells compared to differentiated cells. For instance, methylation of K75 in the H1.2-1.4 variants; methylation of K108, K148, K151, K152 K154, K155, K160, K161, K179, and K185 in H1.1, as well as of K168 in H1.2; phosphorylation of S129, T146, T149, S159, S163, and S180 in H1.1, T180 in H1.2, and T155 in H1.3 were identified exclusively in ES cells. The H1.0 and H1.2 variants in ES cells were characterized by an enhanced acetylation and overall reduced expression levels. Most of the acetylation sites of the H1.0 and H1.2 variants from ES cells were located within their C-terminal tails known to be involved in the stabilization of the condensed chromatin. These data may be used for further studies aimed at analyzing the functional role played by the revealed histone H1 PTMs in the self-renewal and differentiation of pluripotent stem cells.

Interaction of HMGB1 nonhistone chromosomal protein with DNA was studied using circular dichroism spectroscopy and thermal denaturation of DNA. Melting DNA in the complex was shown to be a biphasic process. The characteristic melting temperatures of unbound DNA and the DNA bound to HMGB1 in 0.25 mM EDTA solutions were found to beTmI=44.0±0.5°C andTmII=62.0±1°C, respectively. It was shown that the binding of the HMGB1 molecule affects the melting of the DNA region approximately 30 b.p. long.

Introduction: Embryo implantation is characterized by the process of naïve-to-primed pluripotency transition in epiblast cells, involving an anabolic boost, mitochondrial remodeling, and increased proliferation. Yet, the molecular mechanisms underlying these extensive changes remain poorly understood. High mobility group box 1 (HMGB1) is a non-histone, redox-sensitive chromatin protein involved in diverse cellular processes, however its role in pluripotency control has not been fully characterized. Methods: To determine the function of HMGB1 in mouse embryonic stem cells (ESCs), Hmgb1 -knockout (KO) ESCs were generated using CRISPR/Cas9 system. KO ESCs were analyzed for cell proliferation, cell cycle progression, and apoptosis, as well as for levels of active mitochondria, mitochondrial membrane potential, and reactive oxygen species (ROS) using fluorescent-based reagents and flow cytometry. Pluripotency was assessed by analyzing the expression of pluripotency markers with immunocytochemistry, Western blotting, qRT-PCR, as well as by teratoma formation assay. Naïve-to-primed pluripotency transition was investigated by in vitro culture. Molecular analysis was performed with RNA sequencing, bioinformatics, qRT-PCR, and Western blotting. The ultrastructure of mitochondria was examined using transmission electron microscopy. Results: We first successfully generated HMGB1 KO in mouse ESCs and showed that HMGB1 function is dispensable for both cell viability and pluripotency maintenance, while it is required for the cell proliferation boost during the naïve-to-primed pluripotency transition. Molecular and transcriptomic analysis identified the involvement of HMGB1 in the regulation of energy metabolism processes by regulating mitochondrial structure and function, as well as ROS homeostasis. Loss of HMGB1 function in mouse ESCs results in altered mitochondrial structure and excessive ROS production. HMGB1-dependent elevated ROS levels negatively regulate cell proliferation during the transition from naïve to primed pluripotency in vitro . Conclusion: While HMGB1 deficiency does not impair self-renewal in the naïve state, it causes a marked reduction in proliferation as cells advance to primed pluripotency. Our findings thus identify HMGB1 as a key regulator of mitochondrial integrity and ROS homeostasis during the naïve-to-primed pluripotency transition.