The discovery of the myofibroblast has opened new perspectives for the comprehension of the biological mechanisms involved in wound healing and fibrotic diseases. In recent years, many advances have been made in understanding important aspects of myofibroblast basic biological characteristics. This review summarizes such advances in several fields, such as the following: i) force production by the myofibroblast and mechanisms of connective tissue remodeling; ii) factors controlling the expression of α-smooth muscle actin, the most used marker of myofibroblastic phenotype and, more important, involved in force generation by the myofibroblast; and iii) factors affecting genesis of the myofibroblast and its differentiation from precursor cells, in particular epigenetic factors, such as DNA methylation, microRNAs, and histone modification. We also review the origin and the specific features of the myofibroblast in diverse fibrotic lesions, such as systemic sclerosis; kidney, liver, and lung fibrosis; and the stromal reaction to certain epithelial tumors. Finally, we summarize the emerging strategies for influencing myofibroblast behavior in vitro and in vivo, with the ultimate goal of an effective therapeutic approach for myofibroblast-dependent diseases.

Recent developments in myofibroblast biology: paradigms for connective tissue remodeling.

A unique feature of systemic sclerosis (SSc) that distinguishes it from other fibrotic disorders is that autoimmunity and vasculopathy characteristically precede fibrosis. Moreover, fibrosis in SSc is not restricted to a single organ, but rather affects many organs and accounts for much of the morbidity and mortality associated with this disease. Although immunomodulatory drugs have been used extensively in the treatment of SSc, no therapy to date has been able to reverse or slow the progression of tissue fibrosis or substantially modify the natural progression of the disease. In this Review, we highlight recent studies that shed light on the cellular and molecular mechanisms underlying the fibrotic process in SSc and that identify cellular processes and intra- and extracellular proteins as potential novel targets for therapy in this prototypic multisystemic fibrotic disease.

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Systemic sclerosis: a prototypic multisystem fibrotic disorder.

The DNA methyltransferase Dnmt1 is responsible for cytosine methylation in mammals and has a role in gene silencing. DNA methylation represses genes partly by recruitment of the methyl-CpG-binding protein MeCP2, which in turn recruits a histone deacetylase activity. Here we show that Dnmt1 is itself associated with histone deacetylase activity in vivo. Consistent with this association, we find that one of the known histone deacetylases, HDAC1, has the ability to bind Dnmt1 and can purify methyltransferase activity from nuclear extracts. We have identified a transcriptional repression domain in Dnmt1 that functions, at least partly, by recruiting histone deacetylase activity and shows homology to the repressor domain of the trithorax-related protein HRX (also known as MLL and ALL-1). Our data show a more direct connection between DNA methylation and histone deacetylation than was previously considered. We suggest that the process of DNA methylation, mediated by Dnmt1, may depend on or generate an altered chromatin state via histone deacetylase activity.

DNA methyltransferase Dnmt1 associates with histone deacetylase activity.

THE mixed-lineage leukaemia gene (MLL/HRX/ALL-1) is disrupted by chromosomal translocation in human acute leukaemias that often display mixed lymphoid–myeloid phenotypes and present in infancy1–4. MLL possesses a highly conserved SET domain also found in Drosophila trithorax (trx) and Polycomb group (Pc-G) genes, which are known to regulate homeotic genes (HOM-C) in a positive or negative fashion, respectively5. Mll was targeted in mice by homologous recombination in embryonic stem (ES) cells to assess its role in pattern development. Mll heterozygous (+/−) mice had retarded growth, displayed haematopoietic abnormalities, and demonstrated bidirectional homeotic transformations of the axial skeleton as well as sternal malformations. Mll deficiency (−/−) was embryonic lethal. Anterior boundaries of Hoxa-7 and Hoxc-9 expression were shifted posteriorly in Mll +/− embryos, but their expression was abolished in Mll −/− embryos. Thus Mll is required for proper segment identity in mammals, displays haplo-insufficiency, and positively regulates Hox gene expression.

Altered Hox expression and segmental identity in Mll -mutant mice

Recombinant Human DNA (Cytosine-5) Methyltransferase I. EXPRESSION, PURIFICATION, AND COMPARISON OF DE NOVO AND MAINTENANCE METHYLATION

Mice carrying the Tight skin (Tsk) mutation have thickened skin and visceral fibrosis resulting from an accumulation of extracellular matrix molecules. These and other connective tissue abnormalities have made Tskl + mice models for scleroderma, hereditary emphysema, and myocardial hypertrophy. Previously we localized Tsk to mouse chromosome 2 in a region syntenic with human chromosome 15. The microfibrillar glycoprotein gene, fibrillin 1 (FBN1), on human chromosome 15q, provided a candidate for the Tsk mutation. We now demonstrate that the Tsk chromosome harbors a 30- to 40-kb genomic duplication within the Fbn1 gene that results in a larger than normal in-frame Fbn1 transcript. These findings provide hypotheses to explain some of the phenotypic characteristics of Tskl + mice and the lethality of Tsk/Tsk embryos.

/pdf/a-tandem-duplication-within-the-fibrillin-1-gene-is-43rvyqw54o.pdf

A tandem duplication within the fibrillin 1 gene is associated with the mouse tight skin mutation.

A series of translocation break points found in a subset of human acute leukemias have one of the breaks on human chromosome 11q23. This region has recently been cloned and a large gene, ALL-1, with homology to the Drosophila trithorax gene has been identified. This paper describes the cloning, sequencing, and mapping of the mouse homolog of ALL-1. We have found a motif present in All-1 that shows homology to the zinc-binding domain of DNA (cytosine-5) methyltransferases (EC 2.1.1.63). Sequence analysis of the murine All-1 gene has identified distinct regions of homology with the human ALL-1 gene; these highly conserved domains may define regions of functional significance in mammals. In addition, we have identified alternatively spliced forms of All-1 within one of the zinc-finger domains, suggesting that there may be different targets and/or functions for All-1 proteins. Finally, we report that All-1 resides in the proximal portion of mouse chromosome 9 and is a candidate for a mutation that results in skeletal transformations during embryonic development.

https://www.pnas.org/content/pnas/90/13/6350.full.pdf

Qing Ma

Papers

A tandem duplication within the fibrillin 1 gene is associated with the mouse tight skin mutation.

Analysis of the murine All-1 gene reveals conserved domains with human ALL-1 and identifies a motif shared with DNA methyltransferases