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

Identification of tissue-specific regulatory region in the zebrafish lamin A promoter

Lamins are major structural proteins present in the nuclei of metazoan cells and contribute significantly to nuclear organization and function. The expression of different types of lamins is developmentally regulated and lamin A is detectable in most differentiated tissues. Although the proximal promoter of the mammalian lamin A gene has been characterized, the tissue-specific regulatory elements of the gene have not been identified. In this study, we have cloned and functionally characterized a 2.99 kb segment upstream of exon 1 in the zebrafish lamin A gene. This fragment was able to drive GFP expression in several tissues of the developing embryo at 14-72 h post fertilization in stable transgenic lines. Deletion fragments of the 2.99 kb promoter were analyzed by microinjection into zebrafish embryos in transient assays as well as by luciferase reporter assays in cultured cells. A minimal promoter segment of 1.24 kb conferred tissue-specific expression of GFP in the zebrafish embryo as well as in a myoblast cell line. An 86 bp fragment of this 1.24 kb segment was able to activate a heterologous promoter in myoblasts. Mutational analysis revealed the importance of muscle-specific regulatory motifs in the promoter. Our results have important implications for understanding the tissue-specific regulation and functions of the lamin A 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.

Different prelamin A forms accumulate in human fibroblasts: a study in experimental models and progeria

Lamin A is a component of the nuclear lamina mutated in a group of human inherited disorders known as laminopathies. Among laminopathies, progeroid syndromes and lipodystrophies feature accumulation of prelamin A, the precursor protein which, in normal cells, undergoes a multi-step processing to yield mature lamin A. It is of utmost importance to characterize the prelamin A form accumulated in each laminopathy, since existing evidence shows that drugs acting on protein processing can improve some pathological aspects. We report that two antibodies raised against differently modified prelamin A peptides show a clear specificity to full-length prelamin A or carboxymethylated farnesylated prelamin A, respectively. Using these antibodies, we demonstrated that inhibition of the prelamin A endoprotease ZMPSTE24 mostly elicits accumulation of full-length prelamin A in its farnesylated form, while loss of the prelamin A cleavage site causes accumulation of carboxymethylated prelamin A in progeria cells. These results suggest a major role of ZMPSTE24 in the first prelamin A cleavage step.

Adult stem cell maintenance and tissue regeneration in the ageing context: the role for A-type lamins as intrinsic modulators of ageing in adult stem cells and their niches

Adult stem cells have been identified in most mammalian tissues of the adult body and are known to support the continuous repair and regeneration of tissues. A generalized decline in tissue regenerative responses associated with age is believed to result from a depletion and/or a loss of function of adult stem cells, which itself may be a driving cause of many age-related disease pathologies. Here we review the striking similarities between tissue phenotypes seen in many degenerative conditions associated with old age and those reported in age-related nuclear envelope disorders caused by mutations in the LMNA gene. The concept is beginning to emerge that nuclear filament proteins, A-type lamins, may act as signalling receptors in the nucleus required for receiving and/or transducing upstream cytosolic signals in a number of pathways central to adult stem cell maintenance as well as adaptive responses to stress. We propose that during ageing and in diseases caused by lamin A mutations, dysfunction of the A-type lamin stress-resistant signalling network in adult stem cells, their progenitors and/or stem cell niches leads to a loss of protection against growth-related stress. This in turn triggers an inappropriate activation or a complete failure of self-renewal pathways with the consequent initiation of stress-induced senescence. As such, A-type lamins should be regarded as intrinsic modulators of ageing within adult stem cells and their niches that are essential for survival to old age.

Nuclear lamins: major factors in the structural organization and function of the nucleus and chromatin

Over the past few years it has become evident that the intermediate filament proteins, the types A and B nuclear lamins, not only provide a structural framework for the nucleus, but are also essential for many aspects of normal nuclear function. Insights into lamin-related functions have been derived from studies of the remarkably large number of disease-causing mutations in the human lamin A gene. This review provides an up-to-date overview of the functions of nuclear lamins, emphasizing their roles in epigenetics, chromatin organization, DNA replication, transcription, and DNA repair. In addition, we discuss recent evidence supporting the importance of lamins in viral infections.

Generation of transgenic rats expressing green fluorescent protein in S-100beta-producing pituitary folliculo-stellate cells and brain astrocytes

Folliculo-stellate (FS) cells are known to act as sustentacular cells or scavenger cells in the anterior lobe. However, the precise function and origin of FS cells are still under discussion. Like brain astrocytes, FS cells contain S-100beta protein, and FS cells can be detected immunocytochemically using antibodies for S-100beta protein after fixation; however, living FS cells can not be detected. The generation of transgenic rats expressing green fluorescent protein (GFP) under the control of S-100beta protein gene promoter may allow the detection of living FS cells, which may be an excellent tool for the study of FS cells. With the aim of generation of transgenic rats, we analyzed the promoter activity of the S-100beta gene and found that intron 1 is important for cell-specific expression of the S-100beta gene. Therefore, we obtained a DNA construct containing GFP gene under a part of the S-100 promoter with intron 1. We transfected the construct into rat embryos and succeeded in generating transgenic rats. The transgenic rats expressed GFP in FS cells specifically in the anterior lobe. GFP is also expressed in other known S-100beta-expressing cells, i.e. brain astrocytes, adipocytes, and chondrocytes. We believe that the newly generated transgenic rats will provide a new approach for the study of FS cells and other S-100beta protein-producing cells.

An essential GT motif in the lamin A promoter mediates activation by CREB-binding protein

Lamin A is an important component of nuclear architecture in mammalian cells. Mutations in the human lamin A gene lead to highly degenerative disorders that affect specific tissues. In studies directed towards understanding the mode of regulation of the lamin A promoter, we have identified an essential GT motif at -55 position by reporter gene assays and mutational analysis. Binding of this sequence to Sp transcription factors has been observed in electrophoretic mobility shift assays and by chromatin immunoprecipitation studies. Further functional analysis by co-expression of recombinant proteins and ChIP assays has shown an important regulatory role for CREB-binding protein in promoter activation, which is mediated by the GT motif.