Deficiencies in lamin B1 and lamin B2 cause neurodevelopmental defects and distinct nuclear shape abnormalities in neurons

Ancient Eukaryotic Origin and Evolutionary Plasticity of Nuclear Lamina

The emergence of the nucleus was a major event of eukaryogenesis. How the nuclear envelope (NE) arose and acquired functions governing chromatin organization and epigenetic control has direct bearing on origins of developmental/stage-specific expression programs. The configuration of the NE and the associated lamina in the last eukaryotic common ancestor (LECA) is of major significance and can provide insight into activities within the LECA nucleus. Subsequent lamina evolution, alterations, and adaptations inform on the variation and selection of distinct mechanisms that subtend gene expression in distinct taxa. Understanding lamina evolution has been difficult due to the diversity and limited taxonomic distributions of the three currently known highly distinct nuclear lamina. We rigorously searched available sequence data for an expanded view of the distribution of known lamina and lamina-associated proteins. While the lamina proteins of plants and trypanosomes are indeed taxonomically restricted, homologs of metazoan lamins and key lamin-binding proteins have significantly broader distributions, and a lamin gene tree supports vertical evolution from the LECA. Two protist lamins from highly divergent taxa target the nucleus in mammalian cells and polymerize into filamentous structures, suggesting functional conservation of distant lamin homologs. Significantly, a high level of divergence of lamin homologs within certain eukaryotic groups and the apparent absence of lamins and/or the presence of seemingly different lamina proteins in many eukaryotes suggests great evolutionary plasticity in structures at the NE, and hence mechanisms of chromatin tethering and epigenetic gene control.

Lamin B1 and lamin B2 are long-lived proteins with distinct functions in retinal development

Lamin B1 and lamin B2 are essential building blocks of the nuclear lamina, a filamentous meshwork lining the nucleoplasmic side of the inner nuclear membrane. Deficiencies in lamin B1 and lamin B2 impair neurodevelopment, but distinct functions for the two proteins in the development and homeostasis of the CNS have been elusive. Here we show that embryonic depletion of lamin B1 in retinal progenitors and postmitotic neurons affects nuclear integrity, leads to the collapse of the laminB2 meshwork, impairs neuronal survival, and markedly reduces the cellularity of adult retinas. In stark contrast, a deficiency of lamin B2 in the embryonic retina has no obvious effect on lamin B1 localization or nuclear integrity in embryonic retinas, suggesting that lamin B1, but not lamin B2, is strictly required for nucleokinesis during embryonic neurogenesis. However, the absence of lamin B2 prevents proper lamination of adult retinal neurons, impairs synaptogenesis, and reduces cone photoreceptor survival. We also show that lamin B1 and lamin B2 are extremely long-lived proteins in rod and cone photoreceptors. OF interest, a complete absence of both proteins during postnatal life has little or no effect on the survival and function of cone photoreceptors.

ARL3 regulates trafficking of prenylated phototransduction proteins to the rod outer segment

The small GTPase, ADP-ribosylation factor-like 3 (ARL3), has been proposed to participate in the transport of proteins in photoreceptor cells. Moreover, it has been implicated in the pathogenesis associated with X-linked retinitis pigmentosa (XLRP) resulting from mutations in the ARL3 GTPase activating protein, retinitis pigmentosa 2 (RP2). To determine the importance of ARL3 in rod photoreceptor cells, we generated transgenic mice expressing a dominant active form of ARL3 (ARL3-Q71L) under a rod-specific promoter. ARL3-Q71L animals exhibited extensive rod cell death after post-natal day 30 (PN30) and degeneration was complete by PN70. Prior to the onset of cell death, rod photoresponse was significantly reduced along with a robust decrease in rod phosphodiesterase 6 (PDE6) and G-protein receptor kinase-1 (GRK1) levels. Furthermore, assembled phosphodiesterase-6 (PDE6) subunits, rod transducin and G-protein receptor kinase-1 (GRK1) accumulated on large punctate structures within the inner segment in ARL3-Q71L retina. Defective trafficking of prenylated proteins is likely due to sequestration of prenyl binding protein δ (PrBPδ) by ARL3-Q71L as we demonstrate a specific interaction between these proteins in the retina. Unexpectedly, our studies also revealed a novel role for ARL3 in the migration of photoreceptor nuclei. In conclusion, this study identifies ARL3 as a key player in prenylated protein trafficking in rod photoreceptor cells and establishes the potential role for ARL3 dysregulation in the pathogenesis of RP2-related forms of XLRP.

Alzheimer's disease: An acquired neurodegenerative laminopathy

The nucleus is typically depicted as a sphere encircled by a smooth surface of nuclear envelope. For most cell types, this depiction is accurate. In other cell types and in some pathological conditions, however, the smooth nuclear exterior is interrupted by tubular invaginations of the nuclear envelope, often referred to as a “nucleoplasmic reticulum,” into the deep nuclear interior. We have recently reported a significant expansion of the nucleoplasmic reticulum in postmortem human Alzheimer's disease brain tissue. We found that dysfunction of the nucleoskeleton, a lamin-rich meshwork that coats the inner nuclear membrane and associated invaginations, is causal for Alzheimer's disease-related neurodegeneration in vivo. Additionally, we demonstrated that proper function of the nucleoskeleton is required for survival of adult neurons and maintaining genomic architecture. Here, we elaborate on the significance of these findings in regard to pathological states and physiological aging, and discuss cellular causes and consequences of nuclear envelope invagination.

Role of Intermediate Filaments in Vesicular Traffic

Intermediate filaments are an important component of the cellular cytoskeleton. The first established role attributed to intermediate filaments was the mechanical support to cells. However, it is now clear that intermediate filaments have many different roles affecting a variety of other biological functions, such as the organization of microtubules and microfilaments, the regulation of nuclear structure and activity, the control of cell cycle and the regulation of signal transduction pathways. Furthermore, a number of intermediate filament proteins have been involved in the acquisition of tumorigenic properties. Over the last years, a strong involvement of intermediate filament proteins in the regulation of several aspects of intracellular trafficking has strongly emerged. Here, we review the functions of intermediate filaments proteins focusing mainly on the recent knowledge gained from the discovery that intermediate filaments associate with key proteins of the vesicular membrane transport machinery. In particular, we analyze the current understanding of the contribution of intermediate filaments to the endocytic pathway.

Impaired mechanical response of an EDMD mutation leads to motility phenotypes that are repaired by loss of prenylation

There are roughly 14 distinct heritable autosomal dominant diseases associated with mutations in lamins A/C, including Emery-Dreifuss muscular dystrophy (EDMD). The mechanical model proposes that the lamin mutations change the mechanical properties of muscle nuclei, leading to cell death and tissue deterioration. Here, we developed an experimental protocol that analyzes the effect of disease-linked lamin mutations on the response of nuclei to mechanical strain in living Caenorhabditis elegans We found that the EDMD mutation L535P disrupts the nuclear mechanical response specifically in muscle nuclei. Inhibiting lamin prenylation rescued the mechanical response of the EDMD nuclei, reversed the muscle phenotypes and led to normal motility. The LINC complex and emerin were also required to regulate the mechanical response of C. elegans nuclei. This study provides evidence to support the mechanical model and offers a potential future therapeutic approach towards curing EDMD.

Lamin in inflammation and aging

Aging is characterized by a progressive loss of tissue function and an increased susceptibility to injury and disease. Many age-associated pathologies manifest an inflammatory component, and this has led to the speculation that aging is at least in part caused by some form of inflammation. However, whether or not inflammation is truly a cause of aging, or is a consequence of the aging process is unknown. Recent work using Drosophila has uncovered a mechanism where the progressive loss of lamin-B in the fat body upon aging triggers systemic inflammation. This inflammatory response perturbs the local immune response of the neighboring gut tissue and leads to hyperplasia. Here, we will discuss the literature connecting lamins to aging and inflammation.

Recent advances in understanding nuclear size and shape

Size and shape are important aspects of nuclear structure. While normal cells maintain nuclear size within a defined range, altered nuclear size and shape are associated with a variety of diseases. It is unknown if altered nuclear morphology contributes to pathology, and answering this question requires a better understanding of the mechanisms that control nuclear size and shape. In this review, we discuss recent advances in our understanding of the mechanisms that regulate nuclear morphology, focusing on nucleocytoplasmic transport, nuclear lamins, the endoplasmic reticulum, the cell cycle, and potential links between nuclear size and size regulation of other organelles. We then discuss the functional significance of nuclear morphology in the context of early embryonic development. Looking toward the future, we review new experimental approaches that promise to provide new insights into mechanisms of nuclear size control, in particular microfluidic-based technologies, and discuss how altered nuclear morphology might impact chromatin organization and physiology of diseased cells.

Non-farnesylated B-type lamin can tether chromatin inside the nucleus and its chromatin interaction requires the Ig-fold region

Lamins are thought to direct heterochromatin to the nuclear lamina (NL); however, this function of lamin has not been clearly demonstrated in vivo. To address this, we analyzed polytene chromosome morphology when artificial lamin variants were expressed in Drosophila endoreplicating cells. We found that the CaaX-motif-deleted B-type lamin Dm₀, but not A-type lamin C, was able to form a nuclear envelope-independent layer that was closely associated with chromatin. Other nuclear envelope proteins were not detected in this "ectopic lamina," and the associated chromatin showed a repressive histone modification maker but not a permissive histone modification marker nor RNA polymerase II proteins. Furthermore, deletion of the C-terminal lamin-Ig-fold domain prevents chromatin association with this ectopic lamina. Thus, non-farnesylated B-type lamin Dm₀ can form an ectopic lamina and induce changes to chromatin structure and status inside the interphase nucleus.

Lamin B1 protein is required for dendrite development in primary mouse cortical neurons

Lamin B1, a key component of the nuclear lamina, plays an important role in brain development and function. A duplication of the human lamin B1 (LMNB1) gene has been linked to adult-onset autosomal dominant leukodystrophy, and mouse and human loss-of-function mutations in lamin B1 are susceptibility factors for neural tube defects. In the mouse, experimental ablation of endogenous lamin B1 (Lmnb1) severely impairs embryonic corticogenesis. Here we report that in primary mouse cortical neurons, LMNB1 overexpression reduces axonal outgrowth, whereas deficiency of endogenous Lmnb1 results in aberrant dendritic development. In the absence of Lmnb1, both the length and complexity of dendrites are reduced, and their growth is unresponsive to KCl stimulation. This defective dendritic outgrowth stems from impaired ERK signaling. In Lmnb1-null neurons, ERK is correctly phosphorylated, but phospho-ERK fails to translocate to the nucleus, possibly due to delocalization of nuclear pore complexes (NPCs) at the nuclear envelope. Taken together, these data highlight a previously unrecognized role of lamin B1 in dendrite development of mouse cortical neurons through regulation of nuclear shuttling of specific signaling molecules and NPC distribution.

Lamin Dysfunction Mediates Neurodegeneration in Tauopathies

The filamentous meshwork formed by the lamin nucleoskeleton provides a scaffold for the anchoring of highly condensed heterochromatic DNA to the nuclear envelope, thereby establishing the three-dimensional architecture of the genome [1]. Insight into the importance of lamins to cellular viability can be gleaned from laminopathies, severe disorders caused by mutations in genes encoding lamins. A cellular consequence of lamin dysfunction in laminopathies is relaxation of heterochromatic DNA [1]. Similarly, we have recently reported the widespread relaxation of heterochromatin in tauopathies [1]: age-related progressive neurodegenerative disorders, including Alzheimer's disease, that are pathologically characterized by aggregates of phosphorylated tau protein in the brain [2, 3]. Here we demonstrate that acquired lamin misregulation though aberrant cytoskeletal-nucleoskeletal coupling promotes relaxation of heterochromatin and neuronal death in an in vivo model of neurodegenerative tauopathy. Genetic manipulation of lamin function significantly modifies neurodegeneration in vivo, demonstrating that lamin pathology plays a causal role in tau-mediated neurotoxicity. We show that lamin dysfunction is conserved in human tauopathy, as super-resolution microscopy reveals a significantly disrupted nuclear lamina in postmortem tissue from human Alzheimer's disease brain. Our study provides strong evidence that tauopathies are neurodegenerative laminopathies and identifies a new pathway mediating neuronal death in currently untreatable human neurodegenerative disorders, including Alzheimer's disease.

Mutation of the nuclear lamin gene LMNB2 in progressive myoclonus epilepsy with early ataxia

We studied a consanguineous Palestinian Arab family segregating an autosomal recessive progressive myoclonus epilepsy (PME) with early ataxia. PME is a rare, often fatal syndrome, initially responsive to antiepileptic drugs which over time becomes refractory and can be associated with cognitive decline. Linkage analysis was performed and the disease locus narrowed to chromosome 19p13.3. Fourteen candidate genes were screened by conventional Sanger sequencing and in one, LMNB2, a novel homozygous missense mutation was identified that segregated with the PME in the family. Whole exome sequencing excluded other likely pathogenic coding variants in the linked interval. The p.His157Tyr mutation is located in an evolutionarily highly conserved region of the alpha-helical rod of the lamin B2 protein. In vitro assembly analysis of mutant lamin B2 protein revealed a distinct defect in the assembly of the highly ordered fibrous arrays typically formed by wild-type lamin B2. Our data suggests that disruption of the organisation of the nuclear lamina in neurons, perhaps through abnormal neuronal migration, causes the epilepsy and early ataxia syndrome and extends the aetiology of PMEs to include dysfunction in nuclear lamin proteins.

Nuclear envelope: positioning nuclei and organizing synapses

The nuclear envelope plays an essential role in nuclear positioning within cells and tissues. This review highlights advances in understanding the mechanisms of nuclear positioning during skeletal muscle and central nervous system development. New findings, particularly about A-type lamins and Nesprin1, may link nuclear envelope integrity to synaptic integrity. Thus synaptic defects, rather than nuclear mispositioning, may underlie human pathologies associated with mutations of nuclear envelope proteins.

Regulation of homologous recombinational repair by lamin B1 in radiation-induced DNA damage

DNA double-strand breaks (DSBs) are the major lethal lesion induced by ionizing radiation (IR). RAD51-dependent homologous recombination (HR) is one of the most important pathways in DSB repair and genome integrity maintenance. However, the mechanism of HR regulation by RAD51 remains unclear. To understand the mechanism of RAD51-dependent HR, we searched for interacting partners of RAD51 by a proteomics analysis and identified lamin B1 in human cells. Lamins are nuclear lamina proteins that play important roles in the structural organization of the nucleus and the regulation of chromosome functions. Immunoblotting analyses revealed that siRNA-mediated lamin B1 depletion repressed the DNA damage-dependent increase of RAD51 after IR. The repression was abolished by the proteasome inhibitor MG132, suggesting that lamin B1 stabilizes RAD51 by preventing proteasome-mediated degradation in cells with IR-induced DNA damage. We also showed that lamin B1 depletion repressed RAD51 focus formation and decreased the survival rates after IR. On the basis of these results, we propose that lamin B1 promotes DSB repair and cell survival by maintaining the RAD51 protein levels for HR upon DSB induction after IR.

Messenger RNA processing is altered in autosomal dominant leukodystrophy

Adult-onset autosomal dominant leukodystrophy (ADLD) is a slowly progressive neurological disorder characterized by autonomic dysfunction, followed by cerebellar and pyramidal features. ADLD is caused by duplication of the lamin B1 gene (LMNB1), which leads to its increased expression. The molecular pathways involved in the disease are still poorly understood. Hence, we analyzed global gene expression in fibroblasts and whole blood of LMNB1 duplication carriers and used Gene Set Enrichment Analysis to explore their gene signatures. We found that LMNB1 duplication is associated with dysregulation of genes involved in the immune system, neuronal and skeletal development. Genes with an altered transcriptional profile clustered in specific genomic regions. Among the dysregulated genes, we further studied the role of RAVER2, which we found to be overexpressed at mRNA and protein level. RAVER2 encodes a putative trans regulator of the splicing repressor polypyrimidine tract binding protein (PTB) and is likely implicated in alternative splicing regulation. Functional studies demonstrated an abnormal splicing pattern of several PTB-target genes and of the myelin protein gene PLP1, previously demonstrated to be involved in ADLD. Mutant mice with different lamin B1 expression levels confirmed that Raver2 expression is dependent on lamin B1 in neural tissue and determines an altered splicing pattern of PTB-target genes and Plp1. Overall our results demonstrate that deregulation of lamin B1 expression induces modified splicing of several genes, likely driven by raver-2 overexpression, and suggest that an alteration of mRNA processing could be a pathogenic mechanism in ADLD.

Sun1 deficiency leads to cerebellar ataxia in mice

Migration and organization of the nucleus are essential for the proliferation and differentiation of cells, including neurons. However, the relationship between the positioning of the nucleus and cellular morphogenesis remains poorly understood. Inherited recessive cerebellar ataxia has been attributed to mutations in SYNE1, a component of the linker of nucleoskeleton and cytoskeleton (LINC) complex. Regardless, Syne1-mutant mice present with normal cerebellar development. The Sad1-Unc-84 homology (SUN)-domain proteins are located at the inner nuclear membrane and recruit Syne proteins through the KASH domain to the outer nuclear membrane. Here, we report an unrecognized contribution of Sun1 and Sun2 to the postnatal development of murine cerebellum. Mice depleted of Sun1 showed a marked reduction in the cerebellar volume, and this phenotype is exacerbated with additional loss of a Sun2 allele. Consistent with these histological changes, Sun1(-/-) and Sun1(-/-)Sun2(+/-) mice exhibited defective motor coordination. Results of immunohistochemical analyses suggested that Sun1 is highly expressed in Purkinje cells and recruits Syne2 to the periphery of the nucleus. Approximately 33% of Purkinje cells in Sun1(-/-) mice and 66% of Purkinje cells in Sun1(-/-)Sun2(+/-) mice were absent from the surface of the internal granule layer (IGL), whereas the proliferation and migration of granule neurons were unaffected. Furthermore, the Sun1(-/-)Sun2(+/-) Purkinje cells exhibited retarded primary dendrite specification, reduced dendritic complexity and aberrant patterning of synapses. Our findings reveal a cell-type-specific role for Sun1 and Sun2 in nucleokinesis during cerebellar development, and we propose the use of Sun-deficient mice as a model for studying cerebellar ataxia that is associated with mutation of human SYNE genes or loss of Purkinje cells.

Lamin-B in systemic inflammation, tissue homeostasis, and aging

Gradual loss of tissue function (or homeostasis) is a natural process of aging and is believed to cause many age-associated diseases. In human epidemiology studies, the low-grade and chronic systemic inflammation in elderly has been correlated with the development of aging related pathologies. Although it is suspected that tissue decline is related to systemic inflammation, the cause and consequence of these aging phenomena are poorly understood. By studying the Drosophila fat body and gut, we have uncovered a mechanism by which lamin-B loss in the fat body upon aging induces age-associated systemic inflammation. This chronic inflammation results in the repression of gut local immune response, which in turn leads to the over-proliferation and mis-differentiation of the intestinal stem cells, thereby resulting in gut hyperplasia. Here we discuss the implications and remaining questions in light of our published findings and new observations.

Lamins at the crossroads of mechanosignaling

The intermediate filament proteins, A- and B-type lamins, form the nuclear lamina scaffold adjacent to the inner nuclear membrane. Lamins also contribute to chromatin regulation and various signaling pathways affecting gene expression. In this review, Osmanagic-Myers et al. focus on the role of nuclear lamins in mechanosensing and also discuss how disease-linked lamin mutants may impair the response of cells to mechanical stimuli and influence the properties of the extracellular matrix.

The intermediate filament proteins, A- and B-type lamins, form the nuclear lamina scaffold adjacent to the inner nuclear membrane. B-type lamins confer elasticity, while A-type lamins lend viscosity and stiffness to nuclei. Lamins also contribute to chromatin regulation and various signaling pathways affecting gene expression. The mechanical roles of lamins and their functions in gene regulation are often viewed as independent activities, but recent findings suggest a highly cross-linked and interdependent regulation of these different functions, particularly in mechanosignaling. In this newly emerging concept, lamins act as a “mechanostat” that senses forces from outside and responds to tension by reinforcing the cytoskeleton and the extracellular matrix. A-type lamins, emerin, and the linker of the nucleoskeleton and cytoskeleton (LINC) complex directly transmit forces from the extracellular matrix into the nucleus. These mechanical forces lead to changes in the molecular structure, modification, and assembly state of A-type lamins. This in turn activates a tension-induced “inside-out signaling” through which the nucleus feeds back to the cytoskeleton and the extracellular matrix to balance outside and inside forces. These functions regulate differentiation and may be impaired in lamin-linked diseases, leading to cellular phenotypes, particularly in mechanical load-bearing tissues.

Accessorizing and anchoring the LINC complex for multifunctionality

The linker of nucleoskeleton and cytoskeleton (LINC) complex, composed of outer and inner nuclear membrane Klarsicht, ANC-1, and Syne homology (KASH) and Sad1 and UNC-84 (SUN) proteins, respectively, connects the nucleus to cytoskeletal filaments and performs diverse functions including nuclear positioning, mechanotransduction, and meiotic chromosome movements. Recent studies have shed light on the source of this diversity by identifying factors associated with the complex that endow specific functions as well as those that differentially anchor the complex within the nucleus. Additional diversity may be provided by accessory factors that reorganize the complex into higher-ordered arrays. As core components of the LINC complex are associated with several diseases, understanding the role of accessory and anchoring proteins could provide insights into pathogenic mechanisms.

Role of lamin b1 in chromatin instability

Nuclear lamins play important roles in the organization and structure of the nucleus; however, the specific mechanisms linking lamin structure to nuclear functions are poorly defined. We demonstrate that reducing nuclear lamin B1 expression by short hairpin RNA-mediated silencing in cancer cell lines to approximately 50% of normal levels causes a delay in the cell cycle and accumulation of cells in early S phase. The S phase delay appears to be due to the stalling and collapse of replication forks. The double-strand DNA breaks resulting from replication fork collapse were inefficiently repaired, causing persistent DNA damage signaling and the assembly of extensive repair foci on chromatin. The expression of multiple factors involved in DNA replication and repair by both nonhomologous end joining and homologous repair is misregulated when lamin B1 levels are reduced. We further demonstrate that lamin B1 interacts directly with the promoters of some genes associated with DNA damage response and repair, including BRCA1 and RAD51. Taken together, the results suggest that the maintenance of lamin B1 levels is required for DNA replication and repair through regulation of the expression of key factors involved in these essential nuclear functions.

Lamins: nuclear intermediate filament proteins with fundamental functions in nuclear mechanics and genome regulation

Lamins are intermediate filament proteins that form a scaffold, termed nuclear lamina, at the nuclear periphery. A small fraction of lamins also localize throughout the nucleoplasm. Lamins bind to a growing number of nuclear protein complexes and are implicated in both nuclear and cytoskeletal organization, mechanical stability, chromatin organization, gene regulation, genome stability, differentiation, and tissue-specific functions. The lamin-based complexes and their specific functions also provide insights into possible disease mechanisms for human laminopathies, ranging from muscular dystrophy to accelerated aging, as observed in Hutchinson-Gilford progeria and atypical Werner syndromes.

Mice that express farnesylated versions of prelamin A in neurons develop achalasia

Neurons in the brain produce lamin C but almost no lamin A, a consequence of the removal of prelamin A transcripts by miR-9, a brain-specific microRNA. We have proposed that miR-9-mediated regulation of prelamin A in the brain could explain the absence of primary neurological disease in Hutchinson-Gilford progeria syndrome, a genetic disease caused by the synthesis of an internally truncated form of farnesyl-prelamin A (progerin). This explanation makes sense, but it is not entirely satisfying because it is unclear whether progerin-even if were expressed in neurons-would be capable of eliciting neuropathology. To address that issue, we created a new Lmna knock-in allele, Lmna(HG-C), which produces progerin transcripts lacking an miR-9 binding site. Mice harboring the Lmna(HG-C) allele produced progerin in neurons, but they had no pathology in the central nervous system. However, these mice invariably developed esophageal achalasia, and the enteric neurons and nerve fibers in gastrointestinal tract were markedly abnormal. The same disorder, achalasia, was observed in genetically modified mice that express full-length farnesyl-prelamin A in neurons (Zmpste24-deficient mice carrying two copies of a Lmna knock-in allele yielding full-length prelamin A transcripts lacking a miR-9 binding site). Our findings indicate that progerin and full-length farnesyl-prelamin A are toxic to neurons of the enteric nervous system.

Stress sensitivity and mechanotransduction during heart development

Early in embryogenesis, the heart begins its rhythmic contractions as a tube that helps perfuse the nascent vasculature, but the embryonic heart soon changes shape and mechanical properties, like many other developing organs. A key question in the field is whether stresses in development impact the underlying gene circuits and, if so, how? Here, we attempt to address this question as we review the mechanical maturation of heart - and, to a limited extent, lung and blood - with a focus on a few key abundant structural proteins whose expression dynamics have been suggested to be directly sensitive to mechanical stress. In heart maturation, proliferating fibroblasts deposit increasing amounts of collagenous matrix in parallel with cardiomyocytes expressing more sarcomeric proteins that increase the contractile stress and strength of the tissue, which in turn pumps more blood at higher stress throughout the developing vasculature. Feedback of beating cardiomyocytes on the expression of matrix by fibroblasts seems a reasonable model, with both synthesis and turnover of matrix and contractile elements achieving a suitable balance. Based on emerging evidence for coiled-coil biopolymers that are tension-stabilized against degradation, a minimal network model of a dynamic cell-matrix interaction is proposed. This same concept is extended to nuclear mechanics as regulated by stress on the nuclear structural proteins called lamins, which are examined in part because of the prominence of mutations in these coiled-coil proteins in diseases of the heart, amongst other organs/tissues. Variations in lamin levels during development and across adult tissues are to some extent known and appear to correlate with extracellular matrix mechanics, which we illustrate across heart, lung, and blood development. The formal perspective here on the mechanochemistry of tissue development and homeostasis could provide a useful framework for 'big data' quantitative biology, particularly of stress-sensitive differentiation, maturation, and disease processes.

An absence of nuclear lamins in keratinocytes leads to ichthyosis, defective epidermal barrier function, and intrusion of nuclear membranes and endoplasmic reticulum into the nuclear chromatin

B-type lamins (lamins B1 and B2) have been considered to be essential for many crucial functions in the cell nucleus (e.g., DNA replication and mitotic spindle formation). However, this view has been challenged by the observation that an absence of both B-type lamins in keratinocytes had no effect on cell proliferation or the development of skin and hair. The latter findings raised the possibility that the functions of B-type lamins are subserved by lamins A and C. To explore that idea, we created mice lacking all nuclear lamins in keratinocytes. Those mice developed ichthyosis and a skin barrier defect, which led to death from dehydration within a few days after birth. Microscopy of nuclear-lamin-deficient skin revealed hyperkeratosis and a disordered stratum corneum with an accumulation of neutral lipid droplets; however, BrdU incorporation into keratinocytes was normal. Skin grafting experiments confirmed the stratum corneum abnormalities and normal BrdU uptake. Interestingly, the absence of nuclear lamins in keratinocytes resulted in an interspersion of nuclear/endoplasmic reticulum membranes with the chromatin. Thus, a key function of the nuclear lamina is to serve as a "fence" and prevent the incursion of cytoplasmic organelles into the nuclear chromatin.

The nuclear envelope: an intriguing focal point for neurogenetic disease

Mutations in genes encoding nuclear envelope proteins cause a wide range of inherited diseases, many of which are neurological. We review the genetic causes and what little is known about pathogenesis of these nuclear envelopathies that primarily affect striated muscle, peripheral nerve and the central nervous system. We conclude by providing examples of experimental therapeutic approaches to these rare but important neuromuscular diseases.

Actin and microtubules play distinct roles in governing the anisotropic deformation of cell nuclei in response to substrate strain

Physical forces arising in the cellular microenvironment have been hypothesized to play a major role in governing cell function. Moreover, it is thought that gene regulation may be sensitive to nuclear deformations taking place in response to extracellular forces over short and long timescales. Although nuclear responses to mechanical stimuli over long timescales are relatively well studied, the short-term responses are poorly understood. Therefore, to characterize the short-term instantaneous deformation of the nucleus in a mechanically dynamic environment, we exposed MDCK epithelial monolayers to varying mechanical strain fields. The results reveal that nuclei deform anisotropically in response to substrate strain, specifically, the minor nuclear axis is significantly more deformable than the major axis. We show that upon microtubule depolymerization, nuclear deformation anisotropy completely disappears. Moreover, the removal of actin causes a significant increase in nuclear deformation along the minor axis and a corresponding increase in mechanical anisotropy. The results demonstrate that the nucleus deforms in a manner that is very much dependent on the direction of strain and the characteristics of the strain field. Actin and microtubules also appear to play distinct roles in controlling the anisotropic deformation of the nucleus in response to mechanical forces that arise in the cellular microenvironment.

Nuclear envelope protein MAN1 regulates clock through BMAL1

Circadian clocks serve as internal pacemakers that influence many basic homeostatic processes; consequently, the expression and function of their components are tightly regulated by intricate networks of feedback loops that fine-tune circadian processes. Our knowledge of these components and pathways is far from exhaustive. In recent decades, the nuclear envelope has emerged as a global gene regulatory machine, although its role in circadian regulation has not been explored. We report that transcription of the core clock component BMAL1 is positively modulated by the inner nuclear membrane protein MAN1, which directly binds the BMAL1 promoter and enhances its transcription. Our results establish a novel connection between the nuclear periphery and circadian rhythmicity, therefore bridging two global regulatory systems that modulate all aspects of bodily functions.

The Caenorhabditis elegans SUN protein UNC-84 interacts with lamin to transfer forces from the cytoplasm to the nucleoskeleton during nuclear migration

The nucleoplasmic domain of the Caenorhabditis elegans SUN protein UNC-84 interacts with lamin. If this interaction is disrupted, a partial failure in nuclear migration occurs.

Nuclear migration is a critical component of many cellular and developmental processes. The nuclear envelope forms a barrier between the cytoplasm, where mechanical forces are generated, and the nucleoskeleton. The LINC complex consists of KASH proteins in the outer nuclear membrane and SUN proteins in the inner nuclear membrane that bridge the nuclear envelope. How forces are transferred from the LINC complex to the nucleoskeleton is poorly understood. The Caenorhabditis elegans lamin, LMN-1, is required for nuclear migration and interacts with the nucleoplasmic domain of the SUN protein UNC-84. This interaction is weakened by the unc-84(P91S) missense mutation. These mutant nuclei have an intermediate nuclear migration defect—live imaging of nuclei or LMN-1::GFP shows that many nuclei migrate normally, others initiate migration before subsequently failing, and others fail to begin migration. At least one other component of the nucleoskeleton, the NET5/Samp1/Ima1 homologue SAMP-1, plays a role in nuclear migration. We propose a nut-and-bolt model to explain how forces are dissipated across the nuclear envelope during nuclear migration. In this model, SUN/KASH bridges serve as bolts through the nuclear envelope, and nucleoskeleton components LMN-1 and SAMP-1 act as both nuts and washers on the inside of the nucleus.

Do lamin B1 and lamin B2 have redundant functions?

Lamins B1 and B2 have a high degree of sequence similarity and are widely expressed from the earliest stages of development. Studies of Lmnb1 and Lmnb2 knockout mice revealed that both of the B-type lamins are crucial for neuronal migration in the developing brain. These observations naturally posed the question of whether the two B-type lamins might play redundant functions in the development of the brain. To explore that issue, Lee and coworkers generated "reciprocal knock-in mice" (knock-in mice that produce lamin B1 from the Lmnb2 locus and knock-in mice that produce lamin B2 from the Lmnb1 locus). Both lines of knock-in mice manifested neurodevelopmental abnormalities similar to those in conventional knockout mice, indicating that lamins B1 and B2 have unique functions and that increased production of one B-type lamin cannot compensate for the loss of the other.

The nuclear lamina is mechano-responsive to ECM elasticity in mature tissue

How cells respond to physical cues in order to meet and withstand the physical demands of their immediate surroundings has been of great interest for many years, with current research efforts focused on mechanisms that transduce signals into gene expression. Pathways that mechano-regulate the entry of transcription factors into the cell nucleus are emerging, and our most recent studies show that the mechanical properties of the nucleus itself are actively controlled in response to the elasticity of the extracellular matrix (ECM) in both mature and developing tissue. In this Commentary, we review the mechano-responsive properties of nuclei as determined by the intermediate filament lamin proteins that line the inside of the nuclear envelope and that also impact upon transcription factor entry and broader epigenetic mechanisms. We summarize the signaling pathways that regulate lamin levels and cell-fate decisions in response to a combination of ECM mechanics and molecular cues. We will also discuss recent work that highlights the importance of nuclear mechanics in niche anchorage and cell motility during development, hematopoietic differentiation and cancer metastasis, as well as emphasizing a role for nuclear mechanics in protecting chromatin from stress-induced damage.

Nuclear lamins and neurobiology

Much of the work on nuclear lamins during the past 15 years has focused on mutations in LMNA (the gene for prelamin A and lamin C) that cause particular muscular dystrophy, cardiomyopathy, partial lipodystrophy, and progeroid syndromes. These disorders, often called "laminopathies," mainly affect mesenchymal tissues (e.g., striated muscle, bone, and fibrous tissue). Recently, however, a series of papers have identified important roles for nuclear lamins in the central nervous system. Studies of knockout mice uncovered a key role for B-type lamins (lamins B1 and B2) in neuronal migration in the developing brain. Also, duplications of LMNB1 (the gene for lamin B1) have been shown to cause autosome-dominant leukodystrophy. Finally, recent studies have uncovered a peculiar pattern of nuclear lamin expression in the brain. Lamin C transcripts are present at high levels in the brain, but prelamin A expression levels are very low-due to regulation of prelamin A transcripts by microRNA 9. This form of prelamin A regulation likely explains why "prelamin A diseases" such as Hutchinson-Gilford progeria syndrome spare the central nervous system. In this review, we summarize recent progress in elucidating links between nuclear lamins and neurobiology.

Reciprocal knock-in mice to investigate the functional redundancy of lamin B1 and lamin B2

Lamins B1 and B2 (B-type lamins) have very similar sequences and are expressed ubiquitously. In addition, both Lmnb1- and Lmnb2-deficient mice die soon after birth with neuronal layering abnormalities in the cerebral cortex, a consequence of defective neuronal migration. The similarities in amino acid sequences, expression patterns, and knockout phenotypes raise the question of whether the two proteins have redundant functions. To investigate this topic, we generated "reciprocal knock-in mice"-mice that make lamin B2 from the Lmnb1 locus (Lmnb1(B2/B2)) and mice that make lamin B1 from the Lmnb2 locus (Lmnb2(B1/B1)). Lmnb1(B2/B2) mice produced increased amounts of lamin B2 but no lamin B1; they died soon after birth with neuronal layering abnormalities in the cerebral cortex. However, the defects in Lmnb1(B2/B2) mice were less severe than those in Lmnb1-knockout mice, indicating that increased amounts of lamin B2 partially ameliorate the abnormalities associated with lamin B1 deficiency. Similarly, increased amounts of lamin B1 in Lmnb2(B1/B1) mice did not prevent the neurodevelopmental defects elicited by lamin B2 deficiency. We conclude that lamins B1 and B2 have unique roles in the developing brain and that increased production of one B-type lamin does not fully complement loss of the other.

Concentration-dependent lamin assembly and its roles in the localization of other nuclear proteins

The nuclear lamina (NL) consists of lamin polymers and proteins that bind to the polymers. Disruption of NL proteins such as lamin and emerin leads to developmental defects and human diseases. However, the expression of multiple lamins, including lamin-A/C, lamin-B1, and lamin-B2, in mammals has made it difficult to study the assembly and function of the NL. Consequently, it has been unclear whether different lamins depend on one another for proper NL assembly and which NL functions are shared by all lamins or are specific to one lamin. Using mouse cells deleted of all or different combinations of lamins, we demonstrate that the assembly of each lamin into the NL depends primarily on the lamin concentration present in the nucleus. When expressed at sufficiently high levels, each lamin alone can assemble into an evenly organized NL, which is in turn sufficient to ensure the even distribution of the nuclear pore complexes. By contrast, only lamin-A can ensure the localization of emerin within the NL. Thus, when investigating the role of the NL in development and disease, it is critical to determine the protein levels of relevant lamins and the intricate shared or specific lamin functions in the tissue of interest.

Nuclear lamin stiffness is a barrier to 3D migration, but softness can limit survival

Lamins impede 3D migration but also promote survival against migration-induced stresses.

Cell migration through solid tissue often involves large contortions of the nucleus, but biological significance is largely unclear. The nucleoskeletal protein lamin-A varies both within and between cell types and was shown here to contribute to cell sorting and survival in migration through constraining micropores. Lamin-A proved rate-limiting in 3D migration of diverse human cells that ranged from glioma and adenocarcinoma lines to primary mesenchymal stem cells (MSCs). Stoichiometry of A- to B-type lamins established an activation barrier, with high lamin-A:B producing extruded nuclear shapes after migration. Because the juxtaposed A and B polymer assemblies respectively conferred viscous and elastic stiffness to the nucleus, subpopulations with different A:B levels sorted in 3D migration. However, net migration was also biphasic in lamin-A, as wild-type lamin-A levels protected against stress-induced death, whereas deep knockdown caused broad defects in stress resistance. In vivo xenografts proved consistent with A:B-based cell sorting, and intermediate A:B-enhanced tumor growth. Lamins thus impede 3D migration but also promote survival against migration-induced stresses.

Regulation of Myelination in the Central Nervous System by Nuclear Lamin B1 and Non-coding RNAs

Adult-onset autosomal dominant leukodystrophy (ADLD) is a progressive and fatal hereditary demyelination disorder characterized initially by autonomic dysfunction and loss of myelin in the central nervous system (CNS). Majority of ADLD is caused by a genomic duplication of the nuclear lamin B1 gene (LMNB1) encoding lamin B1 protein, resulting in increased gene dosage in brain tissue. In vitro, excessive lamin B1 at the cellular level reduces transcription of myelin genes, leading to premature arrest of oligodendrocyte differentiation. Murine models of ADLD overexpressing LMNB1 exhibited age-dependent motor deficits and myelin defects, which are associated with reduced occupancy of the Yin Yang 1 transcription factor at the promoter region of the proteolipid protein gene. Lamin B1 overexpression mediates oligodendrocyte cell-autonomous neuropathology in ADLD and suggests lamin B1 as an important regulator of myelin formation and maintenance during aging. Identification of microRNA-23 (miR-23) as a negative regulator of lamin B1 can ameliorate the consequences of excessive lamin B1 at the cellular level. miR-23a-overexpressing mice display enhanced oligodendrocyte differentiation and myelin synthesis. miR-23a targets include a protein coding transcript PTEN (phosphatase and tensin homolog on chromosome 10), and a long noncoding RNA (2700046G09Rik), indicating a unique role for miR-23a in the coordination of proteins and noncoding RNAs in generating and maintaining healthy myelin. Here, we provide a concise review of the current literature on clinical presentations of ADLD and how lamin B1 affects myelination and other developmental processes. Moreover, we address the emerging role of non-coding RNAs (ncRNAs) in modulating gene networks, specifically investigating miR-23 as a potential target for the treatment of ADLD and other demyelinating disorders.

Sizing and shaping the nucleus: mechanisms and significance

The size and shape of the nucleus are tightly regulated, indicating the physiological significance of proper nuclear morphology, yet the mechanisms and functions of nuclear size and shape regulation remain poorly understood. Correlations between altered nuclear morphology and certain disease states have long been observed, most notably many cancers are diagnosed and staged based on graded increases in nuclear size. Here we review recent studies investigating the mechanisms regulating nuclear size and shape, how mitotic events influence nuclear morphology, and the role of nuclear size and shape in subnuclear chromatin organization and cancer progression.

The nuclear lamina regulates germline stem cell niche organization via modulation of EGFR signaling

Stem cell niche interactions have been studied extensively with regard to cell polarity and extracellular signaling. Less is known about the way in which signals and polarity cues integrate with intracellular structures to ensure appropriate niche organization and function. Here, we report that nuclear lamins function in the cyst stem cells (CySCs) of Drosophila testes to control the interaction of CySCs with the hub. This interaction is important for regulation of CySC differentiation and organization of the niche that supports the germline stem cells (GSCs). Lamin promotes nuclear retention of phosphorylated ERK in the CySC lineage by regulating the distribution of specific nucleoporins within the nuclear pores. Lamin-regulated nuclear epidermal growth factor (EGF) receptor signaling in the CySC lineage is essential for proliferation and differentiation of the GSCs and the transient amplifying germ cells. Thus, we have uncovered a role for the nuclear lamina in the integration of EGF signaling to regulate stem cell niche function.

Nuclear assembly as a target for anti-cancer therapies

Current anti-cancer therapies have a great deal of undesirable side effects; therefore, there is a need to develop efficient and cancer cell-specific new drugs without strong dose-limiting side effects. In my opinion, mechanisms of nuclear assembly and organization represent a novel platform for drug targets, which might fulfill these criteria. The nuclear stiffness and organization of some cancer types are often compromised, making them more vulnerable for further targeting the mechanisms of nuclear integrity than their normal counterparts. Here I will discuss the nuclear organization of normal cells and cancer cells, the molecular mechanisms that govern nuclear assembly with emphasis on those that, in my view, might be considered as targets for future anti-cancer therapies.

Significance of higher-order chromatin architecture for neuronal function and dysfunction

Recent studies in neurons indicate that the large-scale chromatin architectural framework, including chromosome territories or lamina-associated chromatin, undergoes dynamic changes that represent an emergent level of regulation of neuronal gene-expression. This phenomenon has been implicated in neuronal differentiation, long-term potentiation, seizures, and disorders of neural plasticity such as Rett syndrome and epilepsy.

Nuclear envelope in nuclear positioning and cell migration

Hauling and anchoring the nucleus within immobile or motile cells, tissues, and/or syncytia represents a major challenge. In the past 15 years, Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes) have emerged as evolutionary-conserved molecular devices that span the nuclear envelope and provide interacting interfaces for cytoskeletal networks and molecular motors to the nuclear envelope. Here, we review the molecular composition of LINC complexes and focus on how their genetic alteration in vivo has provided a wealth of information related to the relevance of nuclear positioning during tissue development and homeostasis with a special emphasis on the central nervous system. As it may be relevant for metastasis in a range of cancers, the involvement of LINC complexes in migration of nonneuronal cells via its interaction with the perinuclear actin cap will also be developed.

Functional architecture of the cell's nucleus in development, aging, and disease

In eukaryotes, the function of the cell's nucleus has primarily been considered to be the repository for the organism's genome. However, this rather simplistic view is undergoing a major shift, as it is increasingly apparent that the nucleus has functions extending beyond being a mere genome container. Recent findings have revealed that the structural composition of the nucleus changes during development and that many of these components exhibit cell- and tissue-specific differences. Increasing evidence is pointing to the nucleus being integral to the function of the interphase cytoskeleton, with changes to nuclear structural proteins having ramifications affecting cytoskeletal organization and the cell's interactions with the extracellular environment. Many of these functions originate at the nuclear periphery, comprising the nuclear envelope (NE) and underlying lamina. Together, they may act as a "hub" in integrating cellular functions including chromatin organization, transcriptional regulation, mechanosignaling, cytoskeletal organization, and signaling pathways. Interest in such an integral role has been largely stimulated by the discovery that many diseases and anomalies are caused by defects in proteins of the NE/lamina, the nuclear envelopathies, many of which, though rare, are providing insights into their more common variants that are some of the major issues of the twenty-first century public health. Here, we review the contributions that mouse mutants have made to our current understanding of the NE/lamina, their respective roles in disease and the use of mice in developing potential therapies for treating the diseases.

Broken nuclei--lamins, nuclear mechanics, and disease

Mutations in lamins, which are ubiquitous nuclear intermediate filaments, lead to a variety of disorders including muscular dystrophy and dilated cardiomyopathy. Lamins provide nuclear stability, help connect the nucleus to the cytoskeleton, and can modulate chromatin organization and gene expression. Nonetheless, the diverse functions of lamins remain incompletely understood. We focus here on the role of lamins on nuclear mechanics and their involvement in human diseases. Recent findings suggest that lamin mutations can decrease nuclear stability, increase nuclear fragility, and disturb mechanotransduction signaling, possibly explaining the muscle-specific defects in many laminopathies. At the same time, altered lamin expression has been reported in many cancers, where the resulting increased nuclear deformability could enhance the ability of cells to transit tight interstitial spaces, thereby promoting metastasis.

New Lmna knock-in mice provide a molecular mechanism for the 'segmental aging' in Hutchinson-Gilford progeria syndrome

Lamins A and C (products of the LMNA gene) are found in roughly equal amounts in peripheral tissues, but the brain produces mainly lamin C and little lamin A. In HeLa cells and fibroblasts, the expression of prelamin A (the precursor to lamin A) can be reduced by miR-9, but the relevance of those cell culture studies to lamin A regulation in the brain was unclear. To address this issue, we created two new Lmna knock-in alleles, one (Lmna(PLAO-5NT)) with a 5-bp mutation in a predicted miR-9 binding site in prelamin A's 3' UTR, and a second (Lmna(PLAO-UTR)) in which prelamin A's 3' UTR was replaced with lamin C's 3' UTR. Neither allele had significant effects on lamin A levels in peripheral tissues; however, both substantially increased prelamin A transcript levels and lamin A protein levels in the cerebral cortex and the cerebellum. The increase in lamin A expression in the brain was more pronounced with the Lmna(PLAO-UTR) allele than with the Lmna(PLAO-5NT) allele. With both alleles, the increased expression of prelamin A transcripts and lamin A protein was greater in the cerebral cortex than in the cerebellum. Our studies demonstrate the in vivo importance of prelamin A's 3' UTR and its miR-9 binding site in regulating lamin A expression in the brain. The reduced expression of prelamin A in the brain likely explains why children with Hutchinson-Gilford progeria syndrome (a progeroid syndrome caused by a mutant form of prelamin A) are spared from neurodegenerative disease.

Generation and characterization of a conditional deletion allele for Lmna in mice

Extensive efforts have been devoted to study A-type lamins because mutations in their gene, LMNA in humans, are associated with a number of diseases. The mouse germline mutations in the A-type lamins (encoded by Lmna) exhibit postnatal lethality at either 4-8 postnatal (P) weeks or P16-18 days, depending on the deletion alleles. These mice exhibit defects in several tissues including hearts and skeletal muscles. Despite numerous studies, how the germline mutation of Lmna, which is expressed in many postnatal tissues, affects only selected tissues remains poorly understood. Addressing the tissue specific functions of Lmna requires the generation and careful characterization of conditional Lmna null alleles. Here we report the creation of a conditional Lmna knockout allele in mice by introducing loxP sites flanking the second exon of Lmna. The Lmna(flox/flox) mice are phenotypically normal and fertile. We show that Lmna homozygous mutants (Lmna(Δ/Δ)) generated by germline Cre expression display postnatal lethality at P16-18 days with defects similar to a recently reported germline Lmna knockout mouse that exhibits the earliest lethality compared to other germline knockout alleles. This conditional knockout mouse strain should serve as an important genetic tool to study the tissue specific roles of Lmna, which would contribute toward the understanding of various human diseases associated with A-type lamins.

Regulation of nucleotide excision repair by nuclear lamin b1

The nuclear lamins play important roles in the structural organization and function of the metazoan cell nucleus. Recent studies on B-type lamins identified a requirement for lamin B1 (LB1) in the regulation of cell proliferation in normal diploid cells. In order to further investigate the function of LB1 in proliferation, we disrupted its normal expression in U-2 OS human osteosarcoma and other tumor cell lines. Silencing LB1 expression induced G1 cell cycle arrest without significant apoptosis. The arrested cells are unable to mount a timely and effective response to DNA damage induced by UV irradiation. Several proteins involved in the detection and repair of UV damage by the nucleotide excision repair (NER) pathway are down-regulated in LB1 silenced cells including DDB1, CSB and PCNA. We propose that LB1 regulates the DNA damage response to UV irradiation by modulating the expression of specific genes and activating persistent DNA damage signaling. Our findings are relevant to understanding the relationship between the loss of LB1 expression, DNA damage signaling, and replicative senescence.

FGF signalling regulates chromatin organisation during neural differentiation via mechanisms that can be uncoupled from transcription

Changes in higher order chromatin organisation have been linked to transcriptional regulation; however, little is known about how such organisation alters during embryonic development or how it is regulated by extrinsic signals. Here we analyse changes in chromatin organisation as neural differentiation progresses, exploiting the clear spatial separation of the temporal events of differentiation along the elongating body axis of the mouse embryo. Combining fluorescence in situ hybridisation with super-resolution structured illumination microscopy, we show that chromatin around key differentiation gene loci Pax6 and Irx3 undergoes both decompaction and displacement towards the nuclear centre coincident with transcriptional onset. Conversely, down-regulation of Fgf8 as neural differentiation commences correlates with a more peripheral nuclear position of this locus. During normal neural differentiation, fibroblast growth factor (FGF) signalling is repressed by retinoic acid, and this vitamin A derivative is further required for transcription of neural genes. We show here that exposure to retinoic acid or inhibition of FGF signalling promotes precocious decompaction and central nuclear positioning of differentiation gene loci. Using the Raldh2 mutant as a model for retinoid deficiency, we further find that such changes in higher order chromatin organisation are dependent on retinoid signalling. In this retinoid deficient condition, FGF signalling persists ectopically in the elongating body, and importantly, we find that inhibiting FGF receptor (FGFR) signalling in Raldh2−/− embryos does not rescue differentiation gene transcription, but does elicit both chromatin decompaction and nuclear position change. These findings demonstrate that regulation of higher order chromatin organisation during differentiation in the embryo can be uncoupled from the machinery that promotes transcription and, for the first time, identify FGF as an extrinsic signal that can direct chromatin compaction and nuclear organisation of gene loci.

Changes in the position of genes within the nucleus and in their local organisation frequently correlate with whether or not genes are turned on. However, little is known about how such nuclear organisation is controlled and whether this can be separated from the mechanisms that promote transcription. We show here that central nuclear position and chromatin de-compaction correlate with onset of expression at key neural differentiation gene loci in the mouse embryo. Conversely, the locus of a gene that is down-regulated as neural differentiation commences exhibits a shift towards the nuclear periphery as this takes place. Importantly, we show that signalling through the fibroblast growth factor (FGF) pathway regulates changes at this level of nuclear organisation. FGF represses differentiation gene transcription and keeps differentiation gene loci compact and at the nuclear periphery. By blocking FGF signalling in a retinoid deficient embryo in which differentiation genes are not expressed, we further show that control of nuclear organisation by FGF is not just a consequence of gene transcription. These findings are the first to demonstrate that such higher order nuclear organisation is regulated in the developing embryo, that this takes place downstream of FGF signaling, and can be uncoupled from the machinery of gene transcription.

The contrasting roles of lamin B1 in cellular aging and human disease

The nuclear lamina underlies the inner nuclear membrane and consists of a proteinaceous meshwork of intermediate filaments: the A- and B-type lamins. Mutations in LMNA (encoding lamin A and C) give rise to a variety of human diseases including muscular dystrophies, cardiomyopathies and the premature aging syndrome progeria (HGPS). Duplication of the LMNB1 locus, leading to elevated levels of lamin B1, causes adult-onset autosomal dominant leukodystrophy (ADLD), a rare genetic disease that leads to demyelination in the central nervous system (CNS). Conversely, reduced levels of lamin B1 have been observed in HGPS patient derived fibroblasts, as well as fibroblasts and keratinocytes undergoing replicative senescence, suggesting that the regulation of lamin B1 is important for cellular physiology and disease. However, the causal relationship between low levels of lamin B1 and cellular senescence and its relevance in vivo remain unclear. How do elevated levels of lamin B1 cause disease and why is the CNS particularly susceptible to lamin B1 fluctuations? Here we summarize recent findings as to how perturbations of lamin B1 affect cellular physiology and discuss the implications this has on senescence, HGPS and ADLD.

Is LMNB1 a susceptibility gene for neural tube defects in humans?

BACKGROUND

Lamins are intermediate filament proteins that form a major component of the nuclear lamina, a protein complex at the surface of the inner nuclear membrane. Numerous clinically diverse conditions, termed laminopathies, have been found to result from mutation of LMNA. In contrast, coding or loss of function mutations of LMNB1, encoding lamin B1, have not been identified in human disease. In mice, polymorphism in Lmnb1 has been shown to modify risk of neural tube defects (NTDs), malformations of the central nervous system that result from incomplete closure of the neural folds.

METHODS

Mutation analysis by DNA sequencing was performed on all exons of LMNB1 in 239 samples from patients with NTDs from the United Kingdom, Sweden, and United States. Possible functional effects of missense variants were analyzed by bioinformatics prediction and fluorescence in photobleaching.

RESULTS

In NTD patients, we identified two unique missense variants that were predicted to disrupt protein structure/function and represent putative contributory mutations. Fluorescence loss in photobleaching analysis showed that the A436T variant compromised stability of lamin B1 interaction within the lamina.

CONCLUSION

The genetic basis of human NTDs appears highly heterogenous with possible involvement of multiple predisposing genes. We hypothesize that rare variants of LMNB1 may contribute to susceptibility to NTDs.

Farnesylation of lamin B1 is important for retention of nuclear chromatin during neuronal migration

The role of protein farnesylation in lamin A biogenesis and the pathogenesis of progeria has been studied in considerable detail, but the importance of farnesylation for the B-type lamins, lamin B1 and lamin B2, has received little attention. Lamins B1 and B2 are expressed in nearly every cell type from the earliest stages of development, and they have been implicated in a variety of functions within the cell nucleus. To assess the importance of protein farnesylation for B-type lamins, we created knock-in mice expressing nonfarnesylated versions of lamin B1 and lamin B2. Mice expressing nonfarnesylated lamin B2 developed normally and were free of disease. In contrast, mice expressing nonfarnesylated lamin B1 died soon after birth, with severe neurodevelopmental defects and striking nuclear abnormalities in neurons. The nuclear lamina in migrating neurons was pulled away from the chromatin so that the chromatin was left "naked" (free from the nuclear lamina). Thus, farnesylation of lamin B1--but not lamin B2--is crucial for brain development and for retaining chromatin within the bounds of the nuclear lamina during neuronal migration.

Nuclear positioning

The nucleus is the largest organelle and is commonly depicted in the center of the cell. Yet during cell division, migration, and differentiation, it frequently moves to an asymmetric position aligned with cell function. We consider the toolbox of proteins that move and anchor the nucleus within the cell and how forces generated by the cytoskeleton are coupled to the nucleus to move it. The significance of proper nuclear positioning is underscored by numerous diseases resulting from genetic alterations in the toolbox proteins. Finally, we discuss how nuclear position may influence cellular organization and signaling pathways.