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Abstract Lamins form a dense meshwork at the inner surface of the inner nuclear membrane (INM), where they interact with other nuclear envelope proteins such as emerin. Emerin is an integral membrane protein that is part of the LEM (LAP2/emerin/MAN1) domain family, and mutations in either emerin or lamin A/C can result in Emery-Dreifuss muscular dystrophy (EDMD) and other striated muscle diseases. Emerin is retained at the INM through direct interaction with lamin A/C, and emerin’s proper subcellular localization is critical for its ability to influence the mechanical properties of the nucleus and participate in various signaling processes. Nonetheless, the requirements for interaction between emerin and lamin A/C at the INM remain incompletely understood. Here, we report that two distinct regions of lamin A/C are each sufficient to properly localize emerin to the INM and prevent emerin’s lateral diffusion within the INM. In addition to a previously described region of the lamin A/C tail domain able to bind emerin, we identify a novel emerin-interacting domain comprising the linker between the rod and Ig-like fold domains of lamin A/C. We further demonstrate that stably anchoring emerin to the INM requires assembly of A-type lamins into a filamentous network. Collectively, our findings suggest a revised model for emerin retention at the INM, which predicts that two independent lamin A/C domains are required to retain emerin at the nuclear envelope, thereby illuminating how diverse mutations in lamin A/C result in EDMD. 

ABSTRACT The ability of cells to sense and respond to mechanical signals is essential for many biological processes that form the basis of cell identity, tissue development and maintenance. This process, known as mechanotransduction, involves crucial feedback between mechanical force and biochemical signals, including epigenomic modifications that establish transcriptional programs. These programs, in turn, reinforce the mechanical properties of the cell and its ability to withstand mechanical perturbation. The nucleus has long been hypothesized to play a key role in mechanotransduction due to its direct exposure to forces transmitted through the cytoskeleton, its role in receiving cytoplasmic signals and its central function in gene regulation. However, parsing out the specific contributions of the nucleus from those of the cell surface and cytoplasm in mechanotransduction remains a substantial challenge. In this Review, we examine the latest evidence on how the nucleus regulates mechanotransduction, both via the nuclear envelope (NE) and through epigenetic and transcriptional machinery elements within the nuclear interior. We also explore the role of nuclear mechanotransduction in establishing a mechanical memory, characterized by a mechanical, epigenetic and transcriptomic cell state that persists after mechanical stimuli cease. Finally, we discuss current challenges in the field of nuclear mechanotransduction and present technological advances that are poised to overcome them. 

Abstract Lamins are nuclear intermediate filament proteins with diverse functions, ranging from organizing chromatin and regulating gene expression to providing structural support to the nucleus. Mammalian cells express two types of lamins, A-type and B-type, which, despite their similar structure and biochemical properties, exhibit distinct differences in expression, interaction partners, and function. One major difference is that A-type lamins have a significantly larger effect on the mechanical properties of the nucleus, which are crucial for protecting the nucleus from cytoskeletal forces, enabling cell migration through confined spaces, and contributing to cellular mechanotransduction. The molecular mechanism underlying this difference has remained unresolved. Here, we applied custom-developed biophysical and proteomic assays to lamin-deficient cell lines engineered to express specific full-length lamin proteins, lamin truncations, or chimeras combining domains from A- and B-type lamins, to systematically determine their contributions to nuclear mechanics. We found that although all expressed lamins contribute to the biophysical properties of the nuclear interior and confer some mechanical stability to the nuclear envelope, which is sufficient to protect the nuclear envelope from small cell-intrinsic forces and ensure proper positioning of nuclear pores, A-type lamins endow cells with a unique ability to resist large forces on the nucleus. Surprisingly, this effect was conferred through the A-type lamin rod domain, rather than the head or tail domains, which diverge more substantially between A- and B-type lamins and play important roles in lamin network formation. Collectively, our work provides an improved understanding of the distinct functions of different lamins in mammalian cells and may also explain why mutations in the A-type lamin rod domain cause more severe muscle defects in mouse models than other mutations. 

Filamin A interacting protein 1-like (FILIP1L) is a multifunctional protein that plays a role in cancer progression, apoptosis, and angiogenesis. However, FILIP1L remains a relatively underexplored protein, and many of FILIP1L’s key mechanisms and interacting partners are still unknown. Using immunofluorescence staining for endogenous FILIP1L and high-resolution confocal microscopy, we show that FILIP1L colocalizes with the mitochondrial marker, TOM20, suggesting a novel connection between FILIP1L and mitochondria. Further exploring the relationship between FILIP1L and mitochondria might provide a deeper understanding of the emerging function of FILIP1L in health and disease. 

ABSTRACT Lamins are intermediate filament proteins that contribute to numerous cellular functions, including nuclear morphology and mechanical stability. The N-terminal head domain of lamin is crucial for higher order filament assembly and function, yet the effects of commonly used N-terminal tags on lamin function remain largely unexplored. Here, we systematically studied the effect of two differently sized tags on lamin A (LaA) function in a mammalian cell model engineered to allow for precise control of expression of tagged lamin proteins. Untagged, FLAG-tagged and GFP-tagged LaA completely rescued nuclear shape defects when expressed at similar levels in lamin A/C-deficient (Lmna–/–) MEFs, and all LaA constructs prevented increased nuclear envelope ruptures in these cells. N-terminal tags, however, altered the nuclear localization of LaA and impaired the ability of LaA to restore nuclear deformability and to recruit emerin to the nuclear membrane in Lmna–/– MEFs. Our finding that tags impede some LaA functions but not others might explain the partial loss of function phenotypes when tagged lamins are expressed in model organisms and should caution researchers using tagged lamins to study the nucleus. 

ABSTRACT As cells migrate through biological tissues, they must frequently squeeze through micron-sized constrictions in the form of interstitial pores between extracellular matrix fibers and/or other cells. Although it is now well recognized that such confined migration is limited by the nucleus, which is the largest and stiffest organelle, it remains incompletely understood how cells apply sufficient force to move their nucleus through small constrictions. Here, we report a mechanism by which contraction of the cell rear cortex pushes the nucleus forward to mediate nuclear transit through constrictions. Laser ablation of the rear cortex reveals that pushing forces behind the nucleus are the result of increased intracellular pressure in the rear compartment of the cell. The pushing forces behind the nucleus depend on accumulation of actomyosin in the rear cortex and require Rho kinase (ROCK) activity. Collectively, our results suggest a mechanism by which cells generate elevated intracellular pressure in the posterior compartment to facilitate nuclear transit through three-dimensional (3D) constrictions. This mechanism might supplement or even substitute for other mechanisms supporting nuclear transit, ensuring robust cell migrations in confined 3D environments. 

Abstract Lamins A and C, encoded by theLMNAgene, are nuclear intermediate filaments that provide structural support to the nucleus and contribute to chromatin organization and transcriptional regulation.LMNAmutations cause muscular dystrophies, dilated cardiomyopathy, and other diseases. The mechanisms by which manyLMNAmutations result in muscle-specific diseases have remained elusive, presenting a major hurdle in the development of effective treatments. Previous studies using striated muscle laminopathy mouse models found that cytoskeletal forces acting on mechanically fragileLmna-mutant nuclei led to transient nuclear envelope rupture, extensive DNA damage, and activation of DNA damage response (DDR) pathways in skeletal muscle cells in vitro and in vivo. Furthermore, hearts ofLmnamutant mice have elevated activation of the tumor suppressor protein p53, a central regulator of DDR signaling. We hypothesized that elevated p53 activation could present a pathogenic mechanism in striated muscle laminopathies, and that eliminating p53 activation could improve muscle function and survival in laminopathy mouse models. Supporting a pathogenic function of p53 activation in muscle, stabilization of p53 was sufficient to reduce contractility and viability in wild-type muscle cells in vitro. Using three laminopathy models, we found that increased p53 activity inLmna-mutant muscle cells primarily resulted from mechanically induced damage to the myonuclei, and not from altered transcriptional regulation due to loss of lamin A/C expression. However, global deletion of p53 in a severe muscle laminopathy model did not reduce the disease phenotype or increase survival, indicating that additional drivers of disease must contribute to the disease pathogenesis. 

Lamins are nuclear intermediate filament proteins that are ubiquitously found in metazoan cells, where they contribute to nuclear morphology, stability, and gene expression. Lamin-like sequences have recently been identified in distantly related eukaryotes, but it remains unclear whether these proteins share conserved functions with the lamins found in metazoans. Here, we investigate conserved features between metazoan and amoebozoan lamins using a genetic complementation system to express the Dictyostelium discoideum lamin-like protein NE81 in mammalian cells lacking either specific lamins or all endogenous lamins. We report that NE81 localizes to the nucleus in cells lacking Lamin A/C, and that NE81 expression improves nuclear circularity, reduces nuclear deformability, and prevents nuclear envelope rupture in these cells. However, NE81 did not completely rescue loss of Lamin A/C, and was unable to restore normal distribution of metazoan lamin interactors, such as emerin and nuclear pore complexes, which are frequently displaced in Lamin A/C deficient cells. Collectively, our results indicate that the ability of lamins to modulate the morphology and mechanical properties of nuclei may have been a feature present in the common ancestor of Dictyostelium and animals, whereas other, more specialized interactions may have evolved more recently in metazoan lineages.