Expression and synthesis of high mobility group chromosomal proteins in different rat skeletal cell lines during myogenesis

Functional Diversity of Non-Histone Chromosomal Protein HmgB1

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.

HuR and miR-1192 regulate myogenesis by modulating the translation of HMGB1 mRNA

Upon muscle injury, the high mobility group box 1 (HMGB1) protein is upregulated and secreted to initiate reparative responses. Here we show that HMGB1 controls myogenesis both in vitro and in vivo during development and after adult muscle injury. HMGB1 expression in muscle cells is regulated at the translational level: the miRNA miR-1192 inhibits HMGB1 translation and the RNA-binding protein HuR promotes it. HuR binds to a cis-element, HuR binding sites (HuRBS), located in the 3'UTR of the HMGB1 transcript, and at the same time miR-1192 is recruited to an adjacent seed element. The binding of HuR to the HuRBS prevents the recruitment of Argonaute 2 (Ago2), overriding miR-1192-mediated translation inhibition. Depleting HuR reduces myoblast fusion and silencing miR-1192 re-establishes the fusion potential of HuR-depleted cells. We propose that HuR promotes the commitment of myoblasts to myogenesis by enhancing the translation of HMGB1 and suppressing the translation inhibition mediated by miR-1192.

HMGA1 levels influence mitochondrial function and mitochondrial DNA repair efficiency

HMGA chromatin proteins, a family of gene regulatory factors found at only low concentrations in normal cells, are almost universally overexpressed in cancer cells. HMGA proteins are located in the nuclei of normal cells except during the late S/G(2) phases of the cell cycle, when HMGA1, one of the members of the family, reversibly migrates to the mitochondria, where it binds to mitochondrial DNA (mtDNA). In many cancer cells, this controlled shuttling is lost and HMGA1 is found in mitochondria throughout the cell cycle. To investigate the effects of HMGA1 on mitochondria, we employed a genetically engineered line of human MCF-7 cells in which the levels of transgenic HMGA1 protein could be reversibly controlled. "Turn-ON" and "turn-OFF" time course experiments were performed with these cells to either increase or decrease intracellular HMGA1 levels, and various mitochondrial changes were monitored. Results demonstrated that changes in both mtDNA levels and mitochondrial mass inversely paralleled changes in HMGA1 concentrations, strongly implicating HMGA1 in the regulation of these parameters. Additionally, the level of cellular reactive oxygen species (ROS) increased and the efficiency of repair of oxidatively damaged mtDNA decreased as consequences of elevated HMGA1 expression. Increased ROS levels and reduced repair efficiency in HMGA1-overexpressing cells likely contribute to the increased occurrence of mutations in mtDNA frequently observed in cancer cells.

The inflammatory response to skeletal muscle injury: illuminating complexities

Injury of skeletal muscle, and especially mechanically induced damage such as contusion injury, frequently occurs in contact sports, as well as in accidental contact sports, such as hockey and squash. The large variations with regard to injury severity and affected muscle group, as well as non-specificity of reported symptoms, complicate research aimed at finding suitable treatments. Therefore, in order to increase the chances of finding a successful treatment, it is important to understand the underlying mechanisms inherent to this type of skeletal muscle injury and the cellular processes involved in muscle healing following a contusion injury. Arguably the most important of these processes is inflammation since it is a consistent and lasting response. The inflammatory response is dependent on two factors, namely the extent of actual physical damage and the degree of muscle vascularization at the time of injury. However, long-term anti-inflammatory treatment is not necessarily effective in promoting healing, as indicated by various studies on NSAID treatment. Because of the factors named earlier, human studies on the inflammatory response to contusion injury are limited, but several experimental animal models have been designed to study muscle damage and regeneration. The early recovery phase is characterized by the overlapping processes of inflammation and occurrence of secondary damage. Although neutrophil infiltration has been named as a contributor to the latter, no clear evidence exists to support this claim. Macrophages, although forming part of the inflammatory response, have been shown to have a role in recovery, rather than in exacerbating secondary damage. Several probable roles for this cell type in the second phase of recovery, involving resolution processes, have been identified and include the following: (i) phagocytosis to remove cellular debris; (ii) switching from a pro- to anti-inflammatory phenotype in regenerating muscle; (iii) preventing muscle cells from undergoing apoptosis; (iv) releasing factors to promote muscle precursor cell activation and growth; and (v) secretion of cytokines and growth factors to facilitate vascular and muscle fibre repair. These many different roles suggest that a single treatment with one specific target cell population (e.g. neutrophils, macrophages or satellite cells) may not be equally effective in all phases of the post-injury response. To find the optimal targeted, but time-course-dependent, treatments requires substantial further investigations. However, the techniques currently used to induce mechanical injury vary considerably in terms of invasiveness, tools used to induce injury, muscle group selected for injury and contractile status of the muscle, all of which have an influence on the immune and/or cytokine responses. This makes interpretation of the complex responses more difficult. After our review of the literature, we propose that a standardized non-invasive contusion injury is the ideal model for investigations into the immune responses to mechanical skeletal muscle injury. Despite its suitability as a model, the currently available literature with respect to the inflammatory response to injury using contusion models is largely inadequate. Therefore, it may be premature to investigate highly targeted therapies, which may ultimately prove more effective in decreasing athlete recovery time than current therapies that are either not phase-specific, or not administered in a phase-specific fashion.

HMGN1 modulates estrogen-mediated transcriptional activation through interactions with specific DNA-binding transcription factors

HMGN1, an abundant nucleosomal binding protein, can affect both the chromatin higher order structure and the modification of nucleosomal histones, but it alters the expression of only a subset of genes. We investigated specific gene targeting by HMGN1 in the context of estrogen induction of gene expression. Knockdown and overexpression experiments indicated that HMGN1 limits the induction of several estrogen-regulated genes, including TFF1 and FOS, which are induced by estrogen through entirely distinct mechanisms. HMGN1 specifically interacts with estrogen receptor alpha (ER alpha), both in vitro and in vivo. At the TFF1 promoter, estrogen increases HMGN1 association through recruitment by the ER alpha. HMGN1 S20E/S24E, although deficient in binding nucleosomal DNA, still interacts with ER alpha and, strikingly, still represses estrogen-driven activation of the TFF1 gene. On the FOS promoter, which lacks the ER alpha binding sites, constitutively bound serum response factor (SRF) mediates estrogen stimulation. HMGN1 also interacts specifically with SRF, but HMGN1 S20E/S24E does not. Consistent with the protein interactions, only wild-type HMGN1 significantly inhibits the estrogen-driven activation of the FOS gene. Mechanistically, the inhibition of estrogen induction of several ER alpha-associated genes, including TFF1, by HMGN1 correlates with decreased levels of acetylation of Lys9 on histone H3. Together, these findings indicate that HMGN1 regulates the expression of particular genes via specific protein-protein interactions with transcription factors at target gene regulatory regions.

A role for chromosomal protein HMGN1 in corneal maturation

Corneal differentiation and maturation are associated with major changes in the expression levels of numerous genes, including those coding for the chromatin-binding high-mobility group (HMG) proteins. Here we report that HMGN1, a nucleosome-binding protein that alters the structure and activity of chromatin, affects the development of the corneal epithelium in mice. The corneal epithelium of Hmgn1(-/-) mice is thin, has a reduced number of cells, is poorly stratified, is depleted of suprabasal wing cells, and its most superficial cell layer blisters. In mature Hmgn1(-/-)mice, the basal cells retain the ovoid shape of immature cells, and rest directly on the basal membrane which is disorganized. Gene expression was modified in Hmgn1(-/-) corneas: glutathione-S-transferase (GST)alpha 4 and GST omega 1, epithelial layer-specific markers, were selectively reduced while E-cadherin and alpha-, beta-, and gamma-catenin, components of adherens junctions, were increased. Immunofluorescence analysis reveals a complete co-localization of HMGN1 and p 63 in small clusters of basal corneal epithelial cells of wild-type mice, and an absence of p 63 expressing cells in the central region of the Hmgn1(-/-) cornea. We suggest that interaction of HMGN1 with chromatin modulates the fidelity of gene expression and affects corneal development and maturation.

HMGN1 is dispensable for myogenesis and adipogenesis

Expression of key regulatory and tissue specific proteins necessary for myogenesis and adipogenesis are dependent on functional SWI/SNF enzymes that hydrolyze ATP to remodel chromatin and allow factors access to chromatinized DNA. Functional chromatin structural changes also can be facilitated by the high mobility group-N1 (HMGN1) protein. HMGN1 is a chromatin architectural protein that specifically interacts with nucleosomes and has been shown to facilitate the reversal of repressive chromatin structure, thereby making it more conducive for transcription. To determine if HMGN1 functions in myogenesis or adipogensis, two SWI/SNF-dependent processes, we used RNA interference to created stable cell lines with reduced HMGN1 protein levels and differentiated them along the myogenic and adipogenic pathways. We show that neither myogenesis nor adipogenesis was affected by reduced HMGN1 protein levels. We further demonstrate that HMGN1 levels naturally decrease as a function of contact-mediated cell cycle arrest, thereby explaining the lack of requirement for HMGN1 in these cellular differentiation processes.

Down-regulation of nucleosomal binding protein HMGN1 expression during embryogenesis modulates Sox9 expression in chondrocytes

We find that during embryogenesis the expression of HMGN1, a nuclear protein that binds to nucleosomes and reduces the compaction of the chromatin fiber, is progressively down-regulated throughout the entire embryo, except in committed but continuously renewing cell types, such as the basal layer of the epithelium. In the developing limb bud, the expression of HMGN1 is complementary to Sox9, a master regulator of the chondrocyte lineage. In limb bud micromass cultures, which faithfully mimic in vivo chondrogenic differentiation, loss of HMGN1 accelerates differentiation. Expression of wild-type HMGN1, but not of a mutant HMGN1 that does not bind to chromatin, in Hmgn1-/- micromass cultures inhibits Sox9 expression and retards differentiation. Chromatin immunoprecipitation analysis reveals that HMGN1 binds to Sox9 chromatin in cells that are poised to express Sox9. Loss of HMGN1 elevates the amount of HMGN2 bound to Sox9, suggesting functional redundancy among these proteins. These findings suggest a role for HMGN1 in chromatin remodeling during embryogenesis and in the activation of Sox9 during chondrogenesis.

Developmental role of HMGN proteins in Xenopus laevis

HMGN proteins are architectural chromatin proteins that reduce the compaction of the chromatin fiber, facilitate access to nucleosomes and modulate replication and transcription processes. Here we demonstrate that in Xenopus laevis, the expression and cellular location of the HMGN proteins are developmentally regulated and that their misexpression leads to gross developmental defects in post-blastula embryos. HMGN transcripts and proteins are present throughout oogenesis; however, the proteins stored in the cytoplasm are not associated with lampbrush chromosomes, and are rapidly degraded when oocytes mature into eggs. During embryogenesis, HMGN expression is first detected in blastula stages and progresses to a tissue-specific expression reaching relative high levels in the mesodermal and neuroectodermal regions of tadpoles. Only after midblastula transition (MBT), alterations in the HMGN levels by either microinjection of recombinant proteins or by morpholino-antisense oligo treatments produced embryos with imperfectly closed blastopore, distorted body axis and showed abnormal head structures. Analyses of animal cap explants indicated that HMGN proteins are involved in the regulation of mesoderm specific genes. In addition, HMGN misexpression caused altered expression of specific genes at MBT rather than global changes of transcription rates. Our results demonstrate that proper embryonic development of Xenopus laevis requires precisely regulated levels of HMGN proteins and suggest that these nucleosomal binding proteins modulate the expression of specific genes.

Regulated expression and subcellular localization of HMGB1, a chromatin protein with a cytokine function

High mobility group box protein 1 (HMGB1) has been considered as a ubiquitous nuclear protein with an architectural function, but even early reports have described its presence outside of the nucleus. Today, we have only started to understand the extranuclear and extracellular functions of HMGB1: we know that it participates in developmental and differentiation processes, triggers and modulates many of the inflammatory cascades in the body, and may even be involved in the metastatic invasion programme of cancer cells. Given such diverse roles, it is important to know which cells express HMGB1, where, and how much. The present review deals with the expression pattern of HMGB1 and provides evidence that, far from being housekeeping, the HMGB1 gene is tightly regulated. This can have implications for therapeutic intervention on inflammatory diseases as well as cancer.

Morphine treatment affects the regulation of high mobility group I-type chromosomal phosphoproteins in C6 glioma cells

High mobility group (HMG) I-type chromosomal phosphoproteins HMG I/Y and HMG I-C were investigated following morphine treatment of C6BU-1 glioma. Cells were labelled with [32P]-orthophosphoric acid. Electrophoretic profiles and autoradiograms of the control cells revealed the presence of HMG I and HMG I-C proteins. HMG Y was not detected. Northern blot analysis showed a single HMG I/Y transcript. Treatment with morphine lowered the [32P]-incorporation in HMG I and HMGI-C proteins and the level of the HMG I/Y transcript. However, it did not change the protein ratios on the Coomassie stained gels. These results suggest that morphine may trigger independent reaction pathways affecting either transcription regulation and/or postsynthetic phosphorylation of the preexisting HMG I-type proteins. In addition, opposing changes in the postsynthetic phosphorylation of HMG 14 and histones H1AB were also noticed.

Gene expression screening in Xenopus identifies molecular pathways, predicts gene function and provides a global view of embryonic patterning

In a large-scale gene expression screen 1765 randomly picked cDNAs were analyzed by whole-mount in situ hybridization in Xenopus embryos. Two hundred and seventy three unique, differentially expressed genes were identified, 204 of which are novel in Xenopus. Partial DNA sequences and expression patterns were documented and assembled into a database, 'AXelDB'. Approximately 30% of cDNAs analyzed represent differentially expressed genes and about 5% show highly regionalized expression. Novel marker genes and potential developmental regulators were found. Differential expression of mitochondrial genes was observed. Marker genes were used to study regionalization of the entire gastrula as well as the tail forming region and the epidermis of the tailbud embryo. Four 'synexpression' groups representing genes with shared, complex expression pattern that predict molecular pathways involved in patterning and differentiation were identified. According to their probable functional significance these groups are designated as Delta1, Bmp4, ER-import and Chromatin group. Within synexpression groups, a likely function of genes without sequence similarity can be predicted. The results indicate that synexpression groups have strong prognostic value. A cluster analysis was made by comparing gene expression patterns to derive a novel parameter, 'tissue relatedness'. In conclusion, this study describes a semi-functional approach to investigate genes expressed during early development and provides global insight into embryonic patterning.

HMG-X, a Xenopus gene encoding an HMG1 homolog, is abundantly expressed in the developing nervous system

We used a PCR-based subtraction cloning procedure with Concanavalin A-treated and untreated animal caps from stage 9 Xenopus embryos to search for genes the expression of which is induced during neurogenesis. One of these genes was found to encode a homolog of mammalian HMG 1 and 2, hence named HMG-X. HMG-X mRNA was maternally transmitted, up-regulated in neuroectoderm-derived tissues throughout early development, and eventually down-regulated in all adult tissues examined except ovary. Our data suggest that we have identified a gene for a member of the HMG1/2 family that could have an important role in neurogenesis.

Stability of the maize chromosomal high-mobility-group proteins, HMGa and HMGb, in vivo

Chromosomal non-histone high-mobility-group (HMG) proteins represent essential components of eukaryotic chromatin and have also been isolated from a variety of plants. In maize, studies on structure and function of the two larger of the four major HMG proteins have recently been performed and are now extended by analysis of their in vivo stability using pulse-chase experiments in a cell suspension culture. The half-life of the analyzed HMGa and HMGb proteins was found to be 65 h or more than 78 h, respectively.

Molecular cloning, expression analysis, and chromosomal localization of mouse Hmg1-containing sequences

We isolated clones encoding the mouse high-mobility-group (Hmg) chromatin protein, Hmg1, from a 7.5-day mouse embryo cDNA library. The translated amino acid sequence encodes a protein of 24,890 daltons and is identical to previously characterized mouse, rat, and hamster Hmg1. However, comparison of the two mouse Hmg1 cDNA sequences revealed nine sequence alterations. This observation, together with the finding of a complex pattern of hybridizing bands in genomic Southern analysis, suggests that mouse Hmg1 is encoded by a multigene family. The expression of Hmg1 was examined by Northern analysis of RNA isolated from the early mouse embryo and revealed a predominant 1.5-kb transcript in conjunction with low levels of a 2.5-kb transcript. Further analysis of mouse embryos by in situ hybridization showed that Hmg1 transcripts are expressed in high abundance during early mouse embryogenesis. As development progresses, Hmg1 transcript abundance is modulated in a spatially restricted and developmentally regulated manner. Chromosomal localization with recombinant inbred strains revealed that Hmg1-related sequences are widely dispersed in the mouse genome. Here we also report the mapping of six Hmg1 loci to mouse Chromosomes (Chrs) 10, 13, 16, and 17.

Stimulatory effect of the maize HMGa protein on reporter gene expression in maize protoplasts

The high mobility group (HMG) proteins represent a class of chromosomal non-histone proteins with an assumed influence on transcription. In this context, the effect of the maize HMGa protein on reporter gene expression was examined. Transient co-transformation experiments in maize protoplasts with plasmid constructs directing the synthesis of the maize HMGa protein and with a luciferase reporter plasmid demonstrated a stimulatory effect of the HMGa protein on the reporter gene expression. Additional experiments with HMGa deletion constructs indicated that the HMG-Box DNA-binding motif is important for the observed effect, while the acidic carboxy-terminal domain of the HMGa protein appears to be dispensable.

Differential expression of the chromosomal high mobility group proteins 14 and 17 during the onset of differentiation in mammalian osteoblasts and promyelocytic leukemia cells

The expression of chromosomal proteins HMG 14 and HMG 17 during proliferation and differentiation into the osteoblast and monocyte phenotypes was studied. Cellular levels of HMG 14 and HMG 17 mRNA were assayed in primary cultures of calvarial-derived rat osteoblasts under conditions that: (1) support complete expression of the mature osteocytic phenotype and development of a bone tissue-like organization; and (2) where development of osteocytic phenotypic properties are both delayed and reduced in extent of expression. HMG 14 and HMG 17 are preferentially expressed in proliferating osteoblasts and decline to basal levels post-proliferatively at the onset of extracellular matrix mineralization. In contrast, under conditions that are not conducive to extracellular matrix mineralization, HMG 14 is maximally expressed following the downregulation of proliferation. Consistent with previous reports by Bustin and co-workers [Crippa et al., 1990], HMG 14 and HMG 17 are expressed in proliferating HL-60 promyelocytic leukemia cells and downregulated post-proliferatively following phorbol ester-induced monocytic differentiation. However, differentiation into the monocyte phenotype is accompanied by reinitiation of HMG 17 gene expression. The results indicate that the levels of HMG 14 and HMG 17 mRNA are selectively down-regulated during differentiation.