DNase I hypersensitive sites and transcriptional activation of the lamin A/C gene

Normal and aberrant splicing of LMNA

The LMNA gene gives rise to at least three isoforms (lamin A, C, lamin AΔ10) as a result of normal alternative splicing, regulated by cis- and trans-acting regulatory factors, as well as the 5' and 3' untranslated regions of the gene. The two main isoforms, lamin A and C, are constitutive components of the fibrous nuclear lamina and have diverse physiological roles, ranging from mechanical nuclear membrane maintenance to gene regulation. The clinical spectrum of diseases (called 'laminopathies') caused by LMNA mutations is broad, including at least eight well-characterised phenotypes, some of which are confined to the skeletal muscles or skin, while others are multisystemic. This review discusses the different alternatively spliced isoforms of LMNA and the regulation of LMNA splicing, as well as the subgroup of mutations that affect splicing of LMNA pre-mRNA, and also seeks to bridge the mis-splicing of LMNA at transcript level and the resulting clinical phenotypes. Finally, we discuss the manipulation of LMNA splicing by splice-switching antisense oligonucleotides and its therapeutic potential for the treatment of some laminopathies.

Nuclear lamins in the brain - new insights into function and regulation

The nuclear lamina is an intermediate filament meshwork composed largely of four nuclear lamins - lamins A and C (A-type lamins) and lamins B1 and B2 (B-type lamins). Located immediately adjacent to the inner nuclear membrane, the nuclear lamina provides a structural scaffolding for the cell nucleus. It also interacts with both nuclear membrane proteins and the chromatin and is thought to participate in many important functions within the cell nucleus. Defects in A-type lamins cause cardiomyopathy, muscular dystrophy, peripheral neuropathy, lipodystrophy, and progeroid disorders. In contrast, the only bona fide link between the B-type lamins and human disease is a rare demyelinating disease of the central nervous system - adult-onset autosomal-dominant leukoencephalopathy, caused by a duplication of the gene for lamin B1. However, this leukoencephalopathy is not the only association between the brain and B-type nuclear lamins. Studies of conventional and tissue-specific knockout mice have demonstrated that B-type lamins play essential roles in neuronal migration in the developing brain and in neuronal survival. The importance of A-type lamin expression in the brain is unclear, but it is intriguing that the adult brain preferentially expresses lamin C rather than lamin A, very likely due to microRNA-mediated removal of prelamin A transcripts. Here, we review recent studies on nuclear lamins, focusing on the function and regulation of the nuclear lamins in the central nervous system.

Cell-type-specific interactions at regulatory motifs in the first intron of the lamin A gene

Lamins A, C and C2 are alternatively spliced products of the LMNA gene; lamins A and C are expressed in differentiated somatic cells, whereas lamin C2 is expressed in germ cells. We have analyzed a segment of the first intron of the LMNA gene for cell-type-specific regulatory elements. We identified a 420-bp fragment that increased promoter activity in lamin A-expressing cells but repressed activity in undifferentiated cells. DNase I footprinting and electrophoretic mobility shift assays revealed two binding motifs, footprinted region A (FPRA) and FPRB. The hepatocyte nuclear factor-3beta was bound to FPRA only in somatic cell extracts and this motif had an inhibitory effect on promoter activity. The retinoic X receptor beta, RXRbeta, bound near FPRB with extracts from lamin A- or C2-expressing cells, and this site enhanced promoter activity. We have, thus, identified two novel binding sites for transcription factors in a region likely to function as an important regulatory element for the cell-type-specific transcription of A-type lamins.

Chromatin folding and gene expression: new tools to reveal the spatial organization of genes

An important aim in biology is to understand how gene expression is regulated in the context of chromatin. Much progress has been made towards cracking the 'histone code', which describes the composition and organization of chromatin at high resolution. At the lower resolution provided by microscopy, nuclear compartmentalization has been linked to the control of gene expression and silencing. I will review two new techniques able to reveal the three-dimensional organization of individual loci, providing a view of the folding of the chromatin fibre at an intermediate level of resolution. Carter and colleagues and Tolhuis and colleagues have used the new techniques to demonstrate direct physical contact between the locus control region (LCR) and expressed genes in the active murine beta-globin locus. The techniques will allow us to assess the role of locus organization when transcription is directed by distant regulatory elements. The new techniques (and their foreseeable descendants) will permit investigation of many genomic activities involving physical contact between separate regions of any genome. As such, they provide us with a new level of resolution at which to investigate the functional significance of chromatin organization as patterns of gene expression are initiated and modulated during development.

DNase I-hypersensitive sites enhance alpha1(I) collagen gene expression in hepatic stellate cells

Liver fibrosis is characterized by a dramatic increase in the expression of type I collagen. Several deoxyribonuclease (DNase) I-hypersensitive sites (HS) have been located in the distal 5'-flanking region of the alpha1(I) collagen gene that are specific to collagen-producing cells. To assess the role of the DNase I-HS in regulating alpha1(I) collagen gene expression in hepatic stellate cells (HSCs), 3 transgenic mouse lines expressing collagen-alpha1(I) reporter genes were used (Krempen et al. Gene Expr 1999;8:151-163). The pCol9GFP transgene contains the collagen gene promoter (-3122 to +111) linked to the green fluorescent protein (GFP) reporter gene. The pCol9GFP-HS4,5 transgene contains HS4,5 and pColGFP-HS8,9 contains HS8,9 positioned upstream of the collagen promoter in pCol9GFP. HSCs isolated from transgenic mice containing pCol9GFPHS4,5 and pColGFP-HS8,9 showed earlier and higher GFP expression patterns than HSCs isolated from pCol9GFP mice. HSCs from pCol9GFP-HS4,5 showed the highest levels of GFP expression and culture-induced expression correlated with induction of the endogenous alpha1(I) collagen gene. After CCl(4) administration, pCol9GFP-HS4,5 mice showed increased GFP expression compared with pCol9GFP mice in both whole liver extracts and isolated HSCs. Several sites for DNA-protein interactions in both HS4 and HS5 were identified that included a binding site for activator protein 1. In conclusion, DNase I-HS4,5 enhance expression of the alpha1(I) collagen gene promoter in HSCs both in vitro and in vivo after a fibrogenic stimulus. The collagen-GFP transgenic mice provide a convenient and reliable model system to investigate the molecular mechanisms controlling increased collagen expression during fibrosis.

SP3 and AP-1 mediate transcriptional activation of the lamin A proximal promoter

Lamin A is a major component of the nuclear lamina that is expressed in various types of differentiated cells. We have analysed previously the putative promoter sequences of the gene and shown that the rat lamin A proximal promoter contains two essential motifs, a GC box that can bind to Sp1 and Sp3, and an AP-1 motif that can bind to c-Jun and c-Fos. In this study we have investigated the role of Sp1 and Sp3 in transactivation of the promoter. Functional analysis of the promoter in Drosophila SL2 cells has demonstrated that it is inactive in the absence of Sp proteins. Activation by expression of Sp3 is more pronounced than that by Sp1 although both proteins can bind to the GC box in vitro; activation clearly depends on an intact GC box as deduced from mutant analysis. Promoter activity in SL2 cells also requires an intact AP-1 motif, which can bind to endogenous Drosophila Jun and Fos proteins. Furthermore, overexpression of c-Jun and c-Fos results in fourfold activation of the promoter in PCC-4 embryonal carcinoma cells. Our demonstration that activation of the lamin A proximal promoter is mediated by Sp3 and AP-1 transcription factors affords a basis for further studies on the regulation of this important gene during development and disease.