Lamins and Disease

Abnormal development of the cerebral cortex and cerebellum in the setting of lamin B2 deficiency

Nuclear lamins are components of the nuclear lamina, a structural scaffolding for the cell nucleus. Defects in lamins A and C cause an array of human diseases, including muscular dystrophy, lipodystrophy, and progeria, but no diseases have been linked to the loss of lamins B1 or B2. To explore the functional relevance of lamin B2, we generated lamin B2-deficient mice and found that they have severe brain abnormalities resembling lissencephaly, with abnormal layering of neurons in the cerebral cortex and cerebellum. This neuronal layering abnormality is due to defective neuronal migration, a process that is dependent on the organized movement of the nucleus within the cell. These studies establish an essential function for lamin B2 in neuronal migration and brain development.

Hierarchies, multiple energy barriers, and robustness govern the fracture mechanics of alpha-helical and beta-sheet protein domains

The fundamental fracture mechanisms of biological protein materials remain largely unknown, in part, because of a lack of understanding of how individual protein building blocks respond to mechanical load. For instance, it remains controversial whether the free energy landscape of the unfolding behavior of proteins consists of multiple, discrete transition states or the location of the transition state changes continuously with the pulling velocity. This lack in understanding has thus far prevented us from developing predictive strength models of protein materials. Here, we report direct atomistic simulation that over four orders of magnitude in time scales of the unfolding behavior of alpha-helical (AH) and beta-sheet (BS) domains, the key building blocks of hair, hoof, and wool as well as spider silk, amyloids, and titin. We find that two discrete transition states corresponding to two fracture mechanisms exist. Whereas the unfolding mechanism at fast pulling rates is sequential rupture of individual hydrogen bonds (HBs), unfolding at slow pulling rates proceeds by simultaneous rupture of several HBs. We derive the hierarchical Bell model, a theory that explicitly considers the hierarchical architecture of proteins, providing a rigorous structure-property relationship. We exemplify our model in a study of AHs, and show that 3-4 parallel HBs per turn are favorable in light of the protein's mechanical and thermodynamical stability, in agreement with experimental findings that AHs feature 3.6 HBs per turn. Our results provide evidence that the molecular structure of AHs maximizes its robustness at minimal use of building materials.

Age-related changes of nuclear architecture in Caenorhabditis elegans

Mutations in lamins cause premature aging syndromes in humans, including the Hutchinson-Gilford Progeria Syndrome (HGPS) and Atypical Werner Syndrome. It has been shown that HGPS cells in culture undergo age-dependent progressive changes in nuclear architecture. However, it is unknown whether similar changes in nuclear architecture occur during the normal aging process. We have observed that major changes of nuclear architecture accompany Caenorhabditis elegans aging. We found that the nuclear architecture in most nonneuronal cell types undergoes progressive and stochastic age-dependent alterations, such as changes of nuclear shape and loss of peripheral heterochromatin. Furthermore, we show that the rate of these alterations is influenced by the insulin/IGF-1 like signaling pathway and that reducing the level of lamin and lamin-associated LEM domain proteins leads to shortening of lifespan. Our work not only provides evidence for changes of nuclear architecture during the normal aging process of a multicellular organism, but also suggests that HGPS is likely a result of acceleration of the normal aging process. Because the nucleus is vital for many cellular functions, our studies raise the possibility that the nucleus is a prominent focal point for regulating aging.

Lamin B1 is required for mouse development and nuclear integrity

Lamins are key structural components of the nuclear lamina, an intermediate filament meshwork that lies beneath the inner nuclear membrane. Lamins play a role in nuclear architecture, DNA replication, and gene expression. Mutations affecting A-type lamins have been associated with a variety of human diseases, including muscular dystrophy, cardiomyopathy, lipodystrophy, and progeria, but mutations in B-type lamins have never been identified in humans or in experimental animals. To investigate the in vivo function of lamin B1, the major B-type lamin, we generated mice with an insertional mutation in Lmnb1. The mutation resulted in the synthesis of a mutant lamin B1 protein lacking several key functional domains, including a portion of the rod domain, the nuclear localization signal, and the CAAX motif (the carboxyl-terminal signal for farnesylation). Homozygous Lmnb1 mutant mice survived embryonic development but died at birth with defects in lung and bone. Fibroblasts from mutant embryos grew under standard cell-culture conditions but displayed grossly misshapen nuclei, impaired differentiation, increased polyploidy, and premature senescence. Thus, the lamin B1 mutant mice provide evidence for a broad and nonredundant function of lamin B1 in mammalian development. These mutant mice and cell lines derived from them will be useful models for studying the role of the nuclear lamina in various cellular processes.

MAN1 and emerin have overlapping function(s) essential for chromosome segregation and cell division in Caenorhabditis elegans

Emerin and MAN1 are LEM domain-containing integral membrane proteins of the vertebrate nuclear envelope. The function of MAN1 is unknown, whereas emerin is known to interact with nuclear lamins, barrier-to-autointegration factor (BAF), nesprin-1 alpha, and a transcription repressor. Mutations in emerin cause X-linked recessive Emery-Dreifuss muscular dystrophy. Emerin and MAN1 homologs are both conserved in Caenorhabditis elegans, but loss of Ce-emerin has no detectable phenotype. We therefore used C. elegans to test the hypothesis that Ce-MAN1 overlaps functionally with Ce-emerin. Supporting this model, Ce-MAN1 interacted directly with Ce-lamin and Ce-BAF in vitro and required Ce-lamin for its nuclear envelope localization. Interestingly, RNA interference-mediated removal of approximately 90% of Ce-MAN1 was lethal to approximately 15% of embryos. However, in the absence of Ce-emerin, approximately 90% reduction of Ce-MAN1 was lethal to all embryos by the 100-cell stage, with a phenotype involving repeated cycles of anaphase chromosome bridging and cytokinesis ["cell untimely torn" (cut) phenotype]. Immunostaining showed that the anaphase-bridged chromatin specifically retained a mitosis-specific phosphohistone H3 epitope and failed to recruit detectable Ce-lamin or Ce-BAF. These findings show that LEM domain proteins are essential for cell division and that Ce-emerin and Ce-MAN1 share at least one and possibly multiple overlapping functions, which may be relevant to Emery-Dreifuss muscular dystrophy.