The Nuclear Envelope in Development, Disease and Ageing

Basic Sciences

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Abstract

The vertebrate nuclear lamina is a protein meshwork associated with the nuclear face of the inner nuclear membrane (INM). It provides anchoring sites for chromatin domains, and is an important determinant of interphase nuclear architecture, DNA replication and chromatin organization. The major components of the lamina are the intermediate filament-like proteins, the nuclear lamins, The lamins are grouped into 2 classes, A-type and B-type. The B-type lamins are encoded by 2 genes and are constitutively expressed whereas the A-type lamins are spliced variants from a single gene (Lmna) and their expression is developmentally regulated. They are not expressed in early embryos or adult stem cell and their expression correlates with the terminal differentiation of various lineages. We derived mice that did not express the A-type lamins. Development of the Lamin null mice was overtly normal, but by 4-5 weeks they developed a severe form of muscular dystrophy, had abnormal hearts and were dead by 8 weeks. In humans different mutations in the Lamin A gene are responsible for at least 8 inherited diseases. These include 2 forms of muscular dystrophy, dilated cardiomyopathy, 2 types of Familial Partial Lipodystrohy, one of which also affects skeletal development, a peripheral neuropathy and most recently, the premature ageing condition called Hutchinson Gilford Progeria. We have derived mice with mutations in the Lmna gene that correspond to most of these diseases. We have produced mouse lines that develop muscular dystrophy, dilated cardiomyopathy, and a progeric phenotype. The mice with progeria are providing novel insights into fundamental aspects of aging and how the structure of the nucleus developmentally regulates cell proliferation. In addition, we have engineered a mutant mouse line harboring yet another isoform of Lamin A/C protein, known as N195K point mutation and associated in humans with cases of dilated cardiomyopathy (DCM). Although the homozygous N195K/N195K mice appeared to live longer and developed no visible muscular wasting of fat loss compared to the initially derived Lamin A/C null animals, these animals ultimately succumb by the age of 4 months due to acute heart failure preceded by cardiac aberrant rhythm similar to the clinical findings in human DCM patients. We were also able to derive a mouse line allowing for a conditional elimination of Lamin A/C gene via a conventional loxP/Cre system. These animals provide an alternative of studying the nuclear lamina functioning in specific subsets of tissues and/or in an inducible manner by crossing with a spectrum of Cre-expressing transgenic lines thus bypassing the early lethality observed in Lamin A/C null mice. We exploited this mouse strain in the lab to study the effect of Lamin A/C loss on the hair cycle (via crossing with K14-Cre transgenic line) and on the maturation of immune system cellular components in the bone niche environment harboring the hematopoietic stem cells (via mating with Col2a-Cre transgenic mice). In summary, the analysis of the laminopathies is providing novel information into how the structure of the nucleus is important to its function. This is particularly relevant, as in addition to the diseases associated with the lamins (the laminopathies), at least 2 other disease have been linked to mutations in proteins associated with the nuclear envelope. Furthermore, a recent proteomics analysis of the components of the nuclear envelope suggested that an additional 14 diseases might also be linked to altered nuclear envelope proteins. In addition to these diseases we are also looking at what effects Lamin deficiency has on a cells ability to replicate its DNA, chromosome segregation/location and chromatin organization/gene regulation and whether lamin loss may affect tumor development.