Loss of A-type lamin expression compromises nuclear envelope integrity leading to muscular dystrophy

Cardiac and skeletal muscle manifestations in the G608G mouse model of Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a rare premature aging disorder resulting from de novo mutations in the lamin A gene. Children with HGPS typically pass away in their teenage years due to cardiovascular diseases such as atherosclerosis, myocardial infarction, heart failure, and stroke. In this study, we characterized the G608G HGPS mouse model and explored cardiac and skeletal muscle function, along with senescence-associated phenotypes in fibroblasts. Homozygous G608G HGPS mice exhibited cardiac dysfunction, including decreased cardiac output and stroke volume, and impaired left ventricle relaxation. Additionally, skeletal muscle exhibited decreased isometric tetanic torque, muscle atrophy, and increased fibrosis. HGPS fibroblasts showed nuclear abnormalities, decreased proliferation, and increased expression of senescence markers. These findings provide insights into the pathophysiology of the G608G HGPS mouse model and inform potential therapeutic strategies for HGPS.

YEATS domain-containing protein GAS41 regulates nuclear shape by working in concert with BRD2 and the mediator complex in colorectal cancer

The maintenance of nuclear shape is essential for cellular homeostasis and disruptions in this process have been linked to various pathological conditions, including cancer, laminopathies, and aging. Despite the significance of nuclear shape, the precise molecular mechanisms controlling it are not fully understood. In this study, we have identified the YEATS domain-containing protein 4 (GAS41) as a previously unidentified factor involved in regulating nuclear morphology. Genetic ablation of GAS41 in colorectal cancer cells resulted in significant abnormalities in nuclear shape and inhibited cancer cell proliferation both in vitro and in vivo. Restoration experiments revealed that wild-type GAS41, but not a YEATS domain mutant devoid of histone H3 lysine 27 acetylation or crotonylation (H3K27ac/cr) binding, rescued the aberrant nuclear phenotypes in GAS41-deficient cells, highlighting the importance of GAS41's binding to H3K27ac/cr in nuclear shape regulation. Further experiments showed that GAS41 interacts with H3K27ac/cr to regulate the expression of key nuclear shape regulators, including LMNB1, LMNB2, SYNE4, and LEMD2. Mechanistically, GAS41 recruited BRD2 and the Mediator complex to gene loci of these regulators, promoting their transcriptional activation. Disruption of GAS41-H3K27ac/cr binding caused BRD2, MED14 and MED23 to dissociate from gene loci, leading to nuclear shape abnormalities. Overall, our findings demonstrate that GAS41 collaborates with BRD2 and the Mediator complex to control the expression of crucial nuclear shape regulators.

Activity-dependent synthesis of Emerin gates neuronal plasticity by regulating proteostasis

Neurons dynamically regulate their proteome in response to sensory input, a key process underlying experience-dependent plasticity. We characterized the visual experience-dependent nascent proteome within a brief, defined time window after stimulation using an optimized metabolic labeling approach. Visual experience induced cell type-specific and age-dependent alterations in the nascent proteome, including proteostasis-related processes. We identified Emerin as the top activity-induced candidate plasticity protein and demonstrated that its rapid activity-induced synthesis is transcription-independent. In contrast to its nuclear localization and function in myocytes, activity-induced neuronal Emerin is abundant in the endoplasmic reticulum and broadly inhibits protein synthesis, including translation regulators and synaptic proteins. Downregulating Emerin shifted the dendritic spine population from predominantly mushroom morphology to filopodia and decreased network connectivity. In mice, decreased Emerin reduced visual response magnitude and impaired visual information processing. Our findings support an experience-dependent feed-forward role for Emerin in temporally gating neuronal plasticity by negatively regulating translation.

Multi-level transcriptomic analysis of LMNA -related dilated cardiomyopathy identifies disease-driving processes

LMNA- related dilated cardiomyopathy ( LMNA -DCM) is one of the most severe forms of DCM. The incomplete understanding of the molecular disease mechanisms results in lacking treatment options, leading to high mortality amongst patients. Here, using an inducible, cardiomyocyte-specific lamin A/C depletion mouse model, we conducted a comprehensive transcriptomic study, combining both bulk and single nucleus RNA sequencing, and spanning LMNA -DCM disease progression, to identify potential disease drivers. Our refined analysis pipeline identified 496 genes already misregulated early in disease. The expression of these genes was largely driven by disease specific cardiomyocyte sub-populations and involved biological processes mediating cellular response to DNA damage, cytosolic pattern recognition, and innate immunity. Indeed, DNA damage in LMNA -DCM hearts was significantly increased early in disease and correlated with reduced cardiomyocyte lamin A levels. Activation of cytosolic pattern recognition in cardiomyocytes was independent of cGAS, which is rarely expressed in cardiomyocytes, but likely occurred downstream of other pattern recognition sensors such as IFI16. Altered gene expression in cardiac fibroblasts and immune cell infiltration further contributed to tissue-wide changes in gene expression. Our transcriptomic analysis further predicted significant alterations in cell-cell communication between cardiomyocytes, fibroblasts, and immune cells, mediated through early changes in the extracellular matrix (ECM) in the LMNA -DCM hearts. Taken together, our work suggests a model in which nuclear damage in cardiomyocytes leads to activation of DNA damage responses, cytosolic pattern recognition pathway, and other signaling pathways that activate inflammation, immune cell recruitment, and transcriptional changes in cardiac fibroblasts, which collectively drive LMNA -DCM pathogenesis.

Eliminating elevated p53 signaling fails to rescue skeletal muscle defects or extend survival in lamin A/C-deficient mice

Lamins A and C, encoded by the LMNA gene, are nuclear intermediate filaments that provide structural support to the nucleus and contribute to chromatin organization and transcriptional regulation. LMNA mutations cause muscular dystrophies, dilated cardiomyopathy, and other diseases. The mechanisms by which many LMNA mutations 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 fragile Lmna-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 of Lmna mutant 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 in Lmna-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.

Perinuclear damage from nuclear envelope deterioration elicits stress responses that contribute to LMNA cardiomyopathy

Mutations in the LMNA gene encoding lamins A/C cause an array of tissue-selective diseases, with the heart being the most commonly affected organ. Despite progress in understanding the perturbations emanating from LMNA mutations, an integrative understanding of the pathogenesis underlying cardiac dysfunction remains elusive. Using a novel conditional deletion model capable of translatome profiling, we observed that cardiomyocyte-specific Lmna deletion in adult mice led to rapid cardiomyopathy with pathological remodeling. Before cardiac dysfunction, Lmna-deleted cardiomyocytes displayed nuclear abnormalities, Golgi dilation/fragmentation, and CREB3-mediated stress activation. Translatome profiling identified MED25 activation, a transcriptional cofactor that regulates Golgi stress. Autophagy is disrupted in the hearts of these mice, which can be recapitulated by disrupting the Golgi. Systemic administration of modulators of autophagy or ER stress significantly delayed cardiac dysfunction and prolonged survival. These studies support a hypothesis wherein stress responses emanating from the perinuclear space contribute to the LMNA cardiomyopathy development.

Proteomic characterization of human LMNA-related congenital muscular dystrophy muscle cells

LMNA-related congenital muscular dystrophy (L-CMD) is caused by mutations in the LMNA gene, encoding lamin A/C. To further understand the molecular mechanisms of L-CMD, proteomic profiling using DIA mass spectrometry was conducted on immortalized myoblasts and myotubes from controls and L-CMD donors each harbouring a different LMNA mutation (R249W, del.32 K and L380S). Compared to controls, 124 and 228 differentially abundant proteins were detected in L-CMD myoblasts and myotubes, respectively, and were associated with enriched canonical pathways including synaptogenesis and necroptosis in myoblasts, and Huntington's disease and insulin secretion in myotubes. Abnormal nuclear morphology and reduced lamin A/C and emerin abundance was evident in all L-CMD cell lines compared to controls, while nucleoplasmic aggregation of lamin A/C was restricted to del.32 K cells, and mislocalization of emerin was restricted to R249W cells. Abnormal nuclear morphology indicates loss of nuclear lamina integrity as a common feature of L-CMD, likely rendering muscle cells vulnerable to mechanically induced stress, while differences between L-CMD cell lines in emerin and lamin A localization suggests that some molecular alterations in L-CMD are mutation specific. Nonetheless, identifying common proteomic alterations and molecular pathways across all three L-CMD lines has highlighted potential targets for the development of non-mutation specific therapies.

DNA double-stranded breaks, a hallmark of aging, defined at the nucleotide resolution, are increased and associated with transcription in the cardiac myocytes in LMNA-cardiomyopathy

AIMS

An intrinsic feature of gene transcription is the formation of DNA superhelices near the transcription bubble, which are resolved upon induction of transient double-stranded breaks (DSBs) by topoisomerases. Unrepaired DSBs are pathogenic as they lead to cell cycle arrest, senescence, inflammation, and organ dysfunction. We posit that DSBs would be more prevalent at the genomic sites that are associated with gene expression. The objectives were to identify and characterize genome-wide DSBs at the nucleotide resolution and determine the association of DSBs with transcription in cardiac myocytes.

METHODS AND RESULTS

We identified the genome-wide DSBs in ∼1 million cardiac myocytes per heart in three wild-type and three myocyte-specific LMNA-deficient (Myh6-Cre:LmnaF/F) mice by END-Sequencing. The prevalence of DSBs was 0.8% and 2.2% in the wild-type and Myh6-Cre:LmnaF/F myocytes, respectively. The END-Seq signals were enriched for 8 and 6764 DSBs in the wild-type and Myh6-Cre:LmnaF/F myocytes, respectively (q < 0.05). The DSBs were preferentially localized to the gene regions, transcription initiation sites, cardiac transcription factor motifs, and the G quadruplex forming structures. Because LMNA regulates transcription through the lamin-associated domains (LADs), we defined the LADs in cardiac myocytes by a Cleavage Under Targets & Release Using Nuclease (CUT&RUN) assay (N = 5). On average there were 818 LADs per myocyte. Constitutive LADs (cLADs), defined as LADs that were shared by at least three genomes (N = 2572), comprised about a third of the mouse cardiac myocyte genomes. Transcript levels of the protein-coding genes located at the cLADs (N = 3975) were ∼16-fold lower than those at the non-LAD regions (N = ∼17 778). The prevalence of DSBs was higher in the non-LAD as compared to the cLAD regions. Likewise, DSBs were more common in the loss-of-LAD regions, defined as the genomic regions in the Myh6-Cre:LmnaF/F that were juxtaposed to the LAD regions in the wild-type myocytes.

CONCLUSION

To our knowledge, this is the first identification of the DSBs, at the nucleotide resolution in the cardiovascular system. The prevalence of DSBs was higher in the genomic regions associated with transcription. Because transcription is pervasive, DSBs are expected to be common and pathogenic in various states and aging.

Nesprin proteins: bridging nuclear envelope dynamics to muscular dysfunction

This review presents a comprehensive exploration of the pivotal role played by the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, with a particular focus on Nesprin proteins, in cellular mechanics and the pathogenesis of muscular diseases. Distinguishing itself from prior works, the analysis delves deeply into the intricate interplay of the LINC complex, emphasizing its indispensable contribution to maintaining cellular structural integrity, especially in mechanically sensitive tissues such as cardiac and striated muscles. Additionally, the significant association between mutations in Nesprin proteins and the onset of Dilated Cardiomyopathy (DCM) and Emery-Dreifuss Muscular Dystrophy (EDMD) is highlighted, underscoring their pivotal role in disease pathogenesis. Through a comprehensive examination of DCM and EDMD cases, the review elucidates the disruptions in the LINC complex, nuclear morphology alterations, and muscular developmental disorders, thus emphasizing the essential function of an intact LINC complex in preserving muscle physiological functions. Moreover, the review provides novel insights into the implications of Nesprin mutations for cellular dynamics in the pathogenesis of muscular diseases, particularly in maintaining cardiac structural and functional integrity. Furthermore, advanced therapeutic strategies, including rectifying Nesprin gene mutations, controlling Nesprin protein expression, enhancing LINC complex functionality, and augmenting cardiac muscle cell function are proposed. By shedding light on the intricate molecular mechanisms underlying nuclear-cytoskeletal interactions, the review lays the groundwork for future research and therapeutic interventions aimed at addressing genetic muscle disorders.

mTOR Inhibition Prolongs Survival and Has Beneficial Effects on Heart Function After Onset of Lamin A/C Gene Mutation Cardiomyopathy in Mice

BACKGROUND

Mutations in LMNA encoding nuclear envelope proteins lamin A/C cause dilated cardiomyopathy. Activation of the AKT/mTOR (RAC-α serine/threonine-protein kinase/mammalian target of rapamycin) pathway is implicated as a potential pathophysiologic mechanism. The aim of this study was to assess whether pharmacological inhibition of mTOR signaling has beneficial effects on heart function and prolongs survival in a mouse model of the disease, after onset of heart failure.

METHODS

We treated male LmnaH222P/H222P mice, after the onset of heart failure, with placebo or either of 2 orally bioavailable mTOR inhibitors: everolimus or NV-20494, a rapamycin analog highly selective against mTORC1. We examined left ventricular remodeling, and the cell biological, biochemical, and histopathologic features of cardiomyopathy, potential drug toxicity, and survival.

RESULTS

Everolimus treatment (n=17) significantly reduced left ventricular dilatation and increased contractility on echocardiography, with a 7% (P=0.018) reduction in left ventricular end-diastolic diameter and a 39% (P=0.0159) increase fractional shortening compared with placebo (n=17) after 6 weeks of treatment. NV-20494 treatment (n=15) yielded similar but more modest and nonsignificant changes. Neither drug prevented the development of cardiac fibrosis. Drug treatment reactivated suppressed autophagy and inhibited mTORC1 signaling in the heart, although everolimus was more potent. With regards to drug toxicity, everolimus alone led to a modest degree of glucose intolerance during glucose challenge. Everolimus (n=20) and NV-20494 (n=20) significantly prolonged median survival in LmnaH222P/H222P mice, by 9% (P=0.0348) and 11% (P=0.0206), respectively, compared with placebo (n=20).

CONCLUSIONS

These results suggest that mTOR inhibitors may be beneficial in patients with cardiomyopathy caused by LMNA mutations and that further study is warranted.

The structure and function of lamin A/C: Special focus on cardiomyopathy and therapeutic interventions

Lamins are inner nuclear membrane proteins that belong to the intermediate filament family. Lamin A/C lie adjacent to the heterochromatin structure in polymer form, providing skeletal to the nucleus. Based on the localization, lamin A/C provides nuclear stability and cytoskeleton to the nucleus and modulates chromatin organization and gene expression. Besides being the structural protein making the inner nuclear membrane in polymer form, lamin A/C functions as a signalling molecule involved in gene expression as an enhancer inside the nucleus. Lamin A/C regulates various cellular pathways like autophagy and energy balance in the cytoplasm. Its expression is highly variable in differentiated tissues, higher in hard tissues like bone and muscle cells, and lower in soft tissues like the liver and brain. In muscle cells, including the heart, lamin A/C must be expressed in a balanced state. Lamin A/C mutation is linked with various diseases, such as muscular dystrophy, lipodystrophy, and cardiomyopathies. It has been observed that a good number of mutations in the LMNA gene impact cardiac activity and its function. Although several works have been published, there are still several unexplored areas left regarding the lamin A/C function and structure in the cardiovascular system and its pathological state. In this review, we focus on the structural organization, expression pattern, and function of lamin A/C, its interacting partners, and the pathophysiology associated with mutations in the lamin A/C gene, with special emphasis on cardiovascular diseases. With the recent finding on lamin A/C, we have summarized the possible therapeutic interventions to treat cardiovascular symptoms and reverse the molecular changes.

A Hierarchical Mechanotransduction System: From Macro to Micro

Mechanotransduction is a strictly regulated process whereby mechanical stimuli, including mechanical forces and properties, are sensed and translated into biochemical signals. Increasing data demonstrate that mechanotransduction is crucial for regulating macroscopic and microscopic dynamics and functionalities. However, the actions and mechanisms of mechanotransduction across multiple hierarchies, from molecules, subcellular structures, cells, tissues/organs, to the whole-body level, have not been yet comprehensively documented. Herein, the biological roles and operational mechanisms of mechanotransduction from macro to micro are revisited, with a focus on the orchestrations across diverse hierarchies. The implications, applications, and challenges of mechanotransduction in human diseases are also summarized and discussed. Together, this knowledge from a hierarchical perspective has the potential to refresh insights into mechanotransduction regulation and disease pathogenesis and therapy, and ultimately revolutionize the prevention, diagnosis, and treatment of human diseases.

ABHD7-mediated depalmitoylation of lamin A promotes myoblast differentiation

LMNA gene mutation can cause muscular dystrophy, and post-translational modification plays a critical role in regulating its function. Here, we identify that lamin A is palmitoylated at cysteine 522, 588, and 591 residues, which are reversely catalyzed by palmitoyltransferase zinc finger DHHC-type palmitoyltransferase 5 (ZDHHC5) and depalmitoylase α/β hydrolase domain 7 (ABHD7). Furthermore, the metabolite lactate promotes palmitoylation of lamin A by inhibiting the interaction between it and ABHD7. Interestingly, low-level palmitoylation of lamin A promotes, whereas high-level palmitoylation of lamin A inhibits, murine myoblast differentiation. Together, these observations suggest that ABHD7-mediated depalmitoylation of lamin A controls myoblast differentiation.

Sex-specific preservation of neuromuscular function and metabolism following systemic transplantation of multipotent adult stem cells in a murine model of progeria

Onset and rates of sarcopenia, a disease characterized by a loss of muscle mass and function with age, vary greatly between sexes. Currently, no clinical interventions successfully arrest age-related muscle impairments since the decline is frequently multifactorial. Previously, we found that systemic transplantation of our unique adult multipotent muscle-derived stem/progenitor cells (MDSPCs) isolated from young mice-but not old-extends the health-span in DNA damage mouse models of progeria, a disease of accelerated aging. Additionally, induced neovascularization in the muscles and brain-where no transplanted cells were detected-strongly suggests a systemic therapeutic mechanism, possibly activated through circulating secreted factors. Herein, we used ZMPSTE24-deficient mice, a lamin A defect progeria model, to investigate the ability of young MDSPCs to preserve neuromuscular tissue structure and function. We show that progeroid ZMPST24-deficient mice faithfully exhibit sarcopenia and age-related metabolic dysfunction. However, systemic transplantation of young MDSPCs into ZMPSTE24-deficient progeroid mice sustained healthy function and histopathology of muscular tissues throughout their 6-month life span in a sex-specific manner. Indeed, female-but not male-mice systemically transplanted with young MDSPCs demonstrated significant preservation of muscle endurance, muscle fiber size, mitochondrial respirometry, and neuromuscular junction morphometrics. These novel findings strongly suggest that young MDSPCs modulate the systemic environment of aged animals by secreted rejuvenating factors to maintain a healthy homeostasis in a sex-specific manner and that the female muscle microenvironment remains responsive to exogenous regenerative cues in older age. This work highlights the age- and sex-related differences in neuromuscular tissue degeneration and the future prospect of preserving health in older adults with systemic regenerative treatments.

Molecular Pathways and Animal Models of Cardiomyopathies

Cardiomyopathies are a heterogeneous group of disorders of the heart muscle that ultimately result in congestive heart failure. Rapid progress in genetics, molecular and cellular biology with breakthrough innovative genetic-engineering techniques, such as next-generation sequencing and multiomics platforms, stem cell reprogramming, as well as novel groundbreaking gene-editing systems over the past 25 years has greatly improved the understanding of pathogenic signaling pathways in inherited cardiomyopathies. This chapter will focus on intracellular and intercellular molecular signaling pathways that are activated by a genetic insult in cardiomyocytes to maintain tissue and organ level regulation and resultant cardiac remodeling in certain forms of cardiomyopathies. In addition, animal models of different clinical forms of human cardiomyopathies with their summaries of triggered key molecules and signaling pathways will be described.

Bulky glycocalyx shields cancer cells from invasion-associated stresses

The glycocalyx-that forms a protective barrier around cells-has been implicated in cancer cell proliferation, survival, and metastasis. However, its role in maintaining the integrity of DNA/nucleus during migration through dense matrices remains unexplored. In this study, we address this question by first documenting heterogeneity in glycocalyx expression in highly invasive MDA-MB-231 breast cancer cells, and establishing a negative correlation between cell size and glycocalyx levels. Next, we set-up transwell migration through 3 µm pores, to isolate two distinct sub-populations and to show that the early migrating cell sub-population possesses a bulkier glycocalyx and undergoes less DNA damage and nuclear rupture, assessed using γH2AX foci formation and nuclear/cytoplasmic distribution of Ku70/80. Interestingly, enzymatic removal of glycocalyx led to disintegration of the nuclear membrane indicated by increased cytoplasmic localisation of Ku70/80, increased nuclear blebbing and reduction in nuclear area. Together, these results illustrate an inverse association between bulkiness of the glycocalyx and nuclear stresses, and highlights the mechanical role of the glycocalyx in shielding migration associated stresses.

Heterologous expression of Dictyostelium discoideum NE81 in mouse embryo fibroblasts reveals conserved mechanoprotective roles of lamins

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.

Lamin A/C mediates microglial activation by modulating cell proliferation and immune response

Lamin A/C is involved in macrophage activation and premature aging, also known as progeria. As the resident macrophage in brain, overactivation of microglia causes brain inflammation, promoting aging and brain disease. In this study, we investigated the role of Lamin A/C in microglial activation and its impact on progeria using Lmna-/- mice, primary microglia, Lmna knockout (Lmna-KO) and Lmna-knockdown (Lmna-KD) BV2 cell lines. We found that the microglial activation signatures, including cell proliferation, morphology changes, and proinflammatory cytokine secretion (IL-1β, IL-6, and TNF-α), were significantly suppressed in all Lamin A/C-deficient models when stimulated with LPS. TMT-based quantitative proteomic and bioinformatic analysis were further applied to explore the mechanism of Lamin A/C-regulated microglia activation from the proteome level. The results revealed that immune response and phagocytosis were impaired in Lmna-/- microglia. Stat1 was identified as the hub protein in the mechanism by which Lamin A/C regulates microglial activation. Additionally, DNA replication, chromatin organization, and mRNA processing were also altered by Lamin A/C, with Ki67 fulfilling the main hub function. Lamin A/C is a mechanosensitive protein and, the immune- and proliferation-related biological processes are also regulated by mechanotransduction. We speculate that Lamin A/C-mediated mechanotransduction is required for microglial activation. Our study proposes a novel mechanism for microglial activation mediated by Lamin A/C.

Lamins as structural nuclear elements through evolution

Lamins are nuclear intermediate filament proteins with important, well-established roles in humans and other vertebrates. Lamins interact with DNA and numerous proteins at the nuclear envelope to determine the mechanical properties of the nucleus, coordinate chromatin organization, and modulate gene expression. Many of these functions are conserved in the lamin homologs found in basal metazoan organisms, including Drosophila and Caenorhabditis elegans. Lamin homologs have also been recently identified in non-metazoans, like the amoeba Dictyostelium discoideum, yet how these proteins compare functionally to the metazoan isoforms is only beginning to emerge. A better understanding of these distantly related lamins is not only valuable for a more complete picture of eukaryotic evolution, but may also provide new insights into the function of vertebrate lamins.

Effect of morphological change on the maturation of human induced pluripotent stem cell-derived cardiac tissue in rotating flow culture

Introduction

Understanding the critical factors for the maturation of human induced pluripotent stem cell (hiPSC)-derived cardiac tissue is important for further development of culture techniques. Rotating flow culture, where the tissues float in the culture medium by balancing its gravitational settling and the medium flow generated in rotating disk-shaped culture vessels, is one of culture systems used for tissue engineering. It has previously been demonstrated that rotating flow culture leads to the formation of matured cardiac tissue with higher levels of function and structure than the other culture systems. However, the detailed mechanisms underlying the maturation of cardiac tissue remain unclear. This study investigated the maturation process of hiPSC-derived cardiac tissue in rotating flow culture with a focus on morphological changes in the tissue, which is a trigger for maturation.

Methods

The cardiac tissue, which consisted of cardiomyocytes derived from hiPSCs, was cultured on the 3D scaffold of poly (lactic-co-glycolic) acid (PLGA)-aligned nanofibers, in rotating flow culture for 5 days. During the culture, the time profile of projected area of tissue and formation of maturation marker proteins (β-myosin heavy chain and Connexin-43), tissue structure, and formation of nuclear lamina proteins (Lamin A/C) were compared with that in static suspension culture.

Results

The ratio of the projected area of tissue significantly decreased from Day 0 to Day 3 due to tissue shrinkage. In contrast, Western blot analysis revealed that maturation protein markers of cardiomyocytes significantly increased after Day 3. In addition, in rotating flow culture, flat-shaped nuclei and fiber-like cytoskeletal structures were distributed in the surface region of tissue where medium flow was continuously applied. Moreover, Lamin A/C, which are generally formed in differentiated cells owing to mechanical force across the cytoskeleton and critically affect the maturation of cardiomyocytes, were significantly formed in the tissue of rotating flow culture.

Conclusions

In this study, we found that spatial heterogeneity of tissue structure and tissue shrinkage occurred in rotating flow culture, which was not observed in static suspension culture. Moreover, from the quantitative analysis, it was also suggested that tissue shrinkage in rotating flow culture contributed its following tissue maturation. These findings showed one of the important characteristics of rotating flow culture which was not revealed in previous studies.

Revisiting the truncated lamin A produced by a commonly used strain of Lmna knockout mice

The Lmna knockout mouse (Lmna-/-) created by Sullivan and coworkers in 1999 has been widely used to examine lamin A/C function. The knockout allele contains a deletion of Lmna intron 7-exon 11 sequences and was reported to be a null allele. Later, Jahn and coworkers discovered that the mutant allele produces a 54-kDa truncated lamin A and identified, by RT-PCR, a Lmna cDNA containing exon 1-7 + exon 12 sequences. Because exon 12 encodes prelamin A's CaaX motif, the mutant lamin A is assumed to be farnesylated. In the current study, we found that the truncated lamin A in Lmna-/- mouse embryonic fibroblasts (MEFs) was predominantly nucleoplasmic rather than at the nuclear rim, leading us to hypothesize that it was not farnesylated. Our study revealed that the most abundant Lmna transcripts in Lmna-/- MEFs contain exon 1-7 but not exon 12 sequences. Exon 1-7 + exon 12 transcripts were detectable by PCR but in trace amounts. We suspect that these findings explain the nucleoplasmic distribution of the truncated lamin A in Lmna-/- MEFs, and subsequent cell transduction experiments support this suspicion. A truncated lamin A containing exon 1-7 sequence was nucleoplasmic, whereas a lamin A containing exon 1-7 + exon 12 sequences was located along the nuclear rim. Our study explains the nucleoplasmic targeting of truncated lamin A in Lmna-/- MEFs and adds to our understanding of a commonly used strain of Lmna-/- mice.

Prelamin A and ZMPSTE24 in premature and physiological aging

As human longevity increases, understanding the molecular mechanisms that drive aging becomes ever more critical to promote health and prevent age-related disorders. Premature aging disorders or progeroid syndromes can provide critical insights into aspects of physiological aging. A major cause of progeroid syndromes which result from mutations in the genes LMNA and ZMPSTE24 is disruption of the final posttranslational processing step in the production of the nuclear scaffold protein lamin A. LMNA encodes the lamin A precursor, prelamin A and ZMPSTE24 encodes the prelamin A processing enzyme, the zinc metalloprotease ZMPSTE24. Progeroid syndromes resulting from mutations in these genes include the clinically related disorders Hutchinson-Gilford progeria syndrome (HGPS), mandibuloacral dysplasia-type B, and restrictive dermopathy. These diseases have features that overlap with one another and with some aspects of physiological aging, including bone defects resembling osteoporosis and atherosclerosis (the latter primarily in HGPS). The progeroid syndromes have ignited keen interest in the relationship between defective prelamin A processing and its accumulation in normal physiological aging. In this review, we examine the hypothesis that diminished processing of prelamin A by ZMPSTE24 is a driver of physiological aging. We review features a new mouse (LmnaL648R/L648R) that produces solely unprocessed prelamin A and provides an ideal model for examining the effects of its accumulation during aging. We also discuss existing data on the accumulation of prelamin A or its variants in human physiological aging, which call out for further validation and more rigorous experimental approaches to determine if prelamin A contributes to normal aging.

Chromatin-modifying enzymes as modulators of nuclear size during lineage differentiation

The mechanism of nuclear size determination and alteration during normal lineage development and cancer pathologies which is not fully understood. As recently reported, chromatin modification can change nuclear morphology. Therefore, we screened a range of pharmacological chemical compounds that impact the activity of chromatin-modifying enzymes, in order to get a clue of the specific types of chromatin-modifying enzymes that remarkably effect nuclear size and shape. We found that interrupted activity of chromatin-modifying enzymes is associated with nuclear shape abnormalities. Furthermore, the activity of chromatin-modifying enzymes perturbs cell fate determination in cellular maintenance and lineage commitment. Our results indicated that chromatin-modifying enzyme regulates cell fate decision during lineage differentiation and is associate with nuclear size alteration.

Heterologous expression of Dictyostelium discoideum NE81 in mouse embryo fibroblasts reveals conserved mechanoprotective roles of lamins

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 if 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.

Systematic in vivo candidate evaluation uncovers therapeutic targets for LMNA dilated cardiomyopathy and risk of Lamin A toxicity

BACKGROUND

Dilated cardiomyopathy (DCM) is a severe, non-ischemic heart disease which ultimately results in heart failure (HF). Decades of research on DCM have revealed diverse aetiologies. Among them, familial DCM is the major form of DCM, with pathogenic variants in LMNA being the second most common form of autosomal dominant DCM. LMNA DCM is a multifactorial and complex disease with no specific treatment thus far. Many studies have demonstrated that perturbing candidates related to various dysregulated pathways ameliorate LMNA DCM. However, it is unknown whether these candidates could serve as potential therapeutic targets especially in long term efficacy.

METHODS

We evaluated 14 potential candidates including Lmna gene products (Lamin A and Lamin C), key signaling pathways (Tgfβ/Smad, mTor and Fgf/Mapk), calcium handling, proliferation regulators and modifiers of LINC complex function in a cardiac specific Lmna DCM model. Positive candidates for improved cardiac function were further assessed by survival analysis. Suppressive roles and mechanisms of these candidates in ameliorating Lmna DCM were dissected by comparing marker gene expression, Tgfβ signaling pathway activation, fibrosis, inflammation, proliferation and DNA damage. Furthermore, transcriptome profiling compared the differences between Lamin A and Lamin C treatment.

RESULTS

Cardiac function was restored by several positive candidates (Smad3, Yy1, Bmp7, Ctgf, aYAP1, Sun1, Lamin A, and Lamin C), which significantly correlated with suppression of HF/fibrosis marker expression and cardiac fibrosis in Lmna DCM. Lamin C or Sun1 shRNA administration achieved consistent, prolonged survival which highly correlated with reduced heart inflammation and DNA damage. Importantly, Lamin A treatment improved but could not reproduce long term survival, and Lamin A administration to healthy hearts itself induced DCM. Mechanistically, we identified this lapse as caused by a dose-dependent toxicity of Lamin A, which was independent from its maturation.

CONCLUSIONS

In vivo candidate evaluation revealed that supplementation of Lamin C or knockdown of Sun1 significantly suppressed Lmna DCM and achieve prolonged survival. Conversely, Lamin A supplementation did not rescue long term survival and may impart detrimental cardiotoxicity risk. This study highlights a potential of advancing Lamin C and Sun1 as therapeutic targets for the treatment of LMNA DCM.

A CRISPR-based rapid DNA repositioning strategy and the early intranuclear life of HSV-1

The relative positions of viral DNA genomes to the host intranuclear environment play critical roles in determining virus fate. Recent advances in the application of chromosome conformation capture-based sequencing analysis (3 C technologies) have revealed valuable aspects of the spatiotemporal interplay of viral genomes with host chromosomes. However, to elucidate the causal relationship between the subnuclear localization of viral genomes and the pathogenic outcome of an infection, manipulative tools are needed. Rapid repositioning of viral DNAs to specific subnuclear compartments amid infection is a powerful approach to synchronize and interrogate this dynamically changing process in space and time. Herein, we report an inducible CRISPR-based two-component platform that relocates extrachromosomal DNA pieces (5 kb to 170 kb) to the nuclear periphery in minutes (CRISPR-nuPin). Based on this strategy, investigations of herpes simplex virus 1 (HSV-1), a prototypical member of the human herpesvirus family, revealed unprecedently reported insights into the early intranuclear life of the pathogen: (I) Viral genomes tethered to the nuclear periphery upon entry, compared with those freely infecting the nucleus, were wrapped around histones with increased suppressive modifications and subjected to stronger transcriptional silencing and prominent growth inhibition. (II) Relocating HSV-1 genomes at 1 hr post infection significantly promoted the transcription of viral genes, termed an 'Escaping' effect. (III) Early accumulation of ICP0 was a sufficient but not necessary condition for 'Escaping'. (IV) Subnuclear localization was only critical during early infection. Importantly, the CRISPR-nuPin tactic, in principle, is applicable to many other DNA viruses.

Nuclear damage in LMNA mutant iPSC-derived cardiomyocytes is associated with impaired lamin localization to the nuclear envelope

The LMNA gene encodes the nuclear envelope proteins Lamins A and C, which comprise a major part of the nuclear lamina, provide mechanical support to the nucleus, and participate in diverse intracellular signaling. LMNA mutations give rise to a collection of diseases called laminopathies, including dilated cardiomyopathy (LMNA-DCM) and muscular dystrophies. Although nuclear deformities are a hallmark of LMNA-DCM, the role of nuclear abnormalities in the pathogenesis of LMNA-DCM remains incompletely understood. Using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from LMNA mutant patients and healthy controls, we show that LMNA mutant iPSC-CM nuclei have altered shape or increased size compared to healthy control iPSC-CM nuclei. The LMNA mutation exhibiting the most severe nuclear deformities, R249Q, additionally caused reduced nuclear stiffness and increased nuclear fragility. Importantly, for all cell lines, the degree of nuclear abnormalities corresponded to the degree of Lamin A/C and Lamin B1 mislocalization from the nuclear envelope. The mislocalization was likely due to altered assembly of Lamin A/C. Collectively, these results point to the importance of correct lamin assembly at the nuclear envelope in providing mechanical stability to the nucleus and suggest that defects in nuclear lamina organization may contribute to the nuclear and cellular dysfunction in LMNA-DCM.

Caenorhabditis elegans models for striated muscle disorders caused by missense variants of human LMNA

Striated muscle laminopathies caused by missense mutations in the nuclear lamin gene LMNA are characterized by cardiac dysfunction and often skeletal muscle defects. Attempts to predict which LMNA variants are pathogenic and to understand their physiological effects lag behind variant discovery. We created Caenorhabditis elegans models for striated muscle laminopathies by introducing pathogenic human LMNA variants and variants of unknown significance at conserved residues within the lmn-1 gene. Severe missense variants reduced fertility and/or motility in C. elegans. Nuclear morphology defects were evident in the hypodermal nuclei of many lamin variant strains, indicating a loss of nuclear envelope integrity. Phenotypic severity varied within the two classes of missense mutations involved in striated muscle disease, but overall, variants associated with both skeletal and cardiac muscle defects in humans lead to more severe phenotypes in our model than variants predicted to disrupt cardiac function alone. We also identified a separation of function allele, lmn-1(R204W), that exhibited normal viability and swimming behavior but had a severe nuclear migration defect. Thus, we established C. elegans avatars for striated muscle laminopathies and identified LMNA variants that offer insight into lamin mechanisms during normal development.

Nanotube patterning reduces macrophage inflammatory response via nuclear mechanotransduction

The inflammatory immune environment surrounding titanium bone implants determines the formation of osseointegration, and nanopatterning on implant surfaces modulates the immune microenvironment in the implant region. Among many related mechanisms, the mechanism by which nanopatterning controls macrophage inflammatory response still needs to be elucidated. In this paper, we found that inhibition of the nuclear envelope protein lamin A/C by titania nanotubes (TNTs) reduced the macrophage inflammatory response. Knockdown of lamin A/C reduced macrophage inflammatory marker expression, while overexpression of lamin A/C significantly elevated inflammatory marker expression. We further found that suppression of lamin A/C by TNTs limited actin polymerization, thereby reducing the nuclear translocation of the actin-dependent transcriptional cofactor MRTF-A, which subsequently reduced the inflammatory response. In addition, emerin, which is a key link between lamin A/C and actin, was delocalized from the nucleus in response to mechanical stimulation by TNTs, resulting in reduced actin organization. Under inflammatory conditions, TNTs exerted favourable osteoimmunomodulatory effects on the osteogenic differentiation of mouse bone marrow-derived stem cells (mBMSCs) in vitro and osseointegration in vivo. This study shows and confirms for the first time that lamin A/C-mediated nuclear mechanotransduction controls macrophage inflammatory response, and this study provides a theoretical basis for the future design of immunomodulatory nanomorphologies on the surface of metallic bone implants.

Lamin A/C Ablation Restricted to Vascular Smooth Muscle Cells, Cardiomyocytes, and Cardiac Fibroblasts Causes Cardiac and Vascular Dysfunction

Mutations in the LMNA gene (encoding lamin A/C proteins) cause several human cardiac diseases, including dilated cardiomyopathies (LMNA-DCM). The main clinical risks in LMNA-DCM patients are sudden cardiac death and progressive left ventricular ejection fraction deterioration, and therefore most human and animal studies have sought to define the mechanisms through which LMNA mutations provoke cardiac alterations, with a particular focus on cardiomyocytes. To investigate if LMNA mutations also cause vascular alterations that might contribute to the etiopathogenesis of LMNA-DCM, we generated and characterized Lmna^flox/floxSM22αCre mice, which constitutively lack lamin A/C in vascular smooth muscle cells (VSMCs), cardiac fibroblasts, and cardiomyocytes. Like mice with whole body or cardiomyocyte-specific lamin A/C ablation, Lmna^flox/floxSM22αCre mice recapitulated the main hallmarks of human LMNA-DCM, including ventricular systolic dysfunction, cardiac conduction defects, cardiac fibrosis, and premature death. These alterations were associated with elevated expression of total and phosphorylated (active) Smad3 and cleaved (active) caspase 3 in the heart. Lmna^flox/floxSM22αCre mice also exhibited perivascular fibrosis in the coronary arteries and a switch of aortic VSMCs from the 'contractile' to the 'synthetic' phenotype. Ex vivo wire myography in isolated aortic rings revealed impaired maximum contraction capacity and an altered response to vasoconstrictor and vasodilator agents in Lmna^flox/floxSM22αCre mice. To our knowledge, our results provide the first evidence of phenotypic alterations in VSMCs that might contribute significantly to the pathophysiology of some forms of LMNA-DCM. Future work addressing the mechanisms underlying vascular defects in LMNA-DCM may open new therapeutic avenues for these diseases.

AKT2-mediated nuclear deformation leads to genome instability during epithelial-mesenchymal transition

Nuclear deformation has been observed in some cancer cells for decades, but its underlying mechanism and biological significance remain elusive. To address these questions, we employed human lung cancer A549 cell line as a model in context with transforming growth factor β (TGFβ)-induced epithelial-mesenchymal transition. Here, we report that nuclear deformation induced by TGFβ is concomitant with increased phosphorylation of lamin A at Ser390, defective nuclear lamina and genome instability. AKT2 and Smad3 serve as the downstream effectors for TGFβ to induce nuclear deformation. AKT2 directly phosphorylates lamin A at Ser390, whereas Smad3 is required for AKT2 activation upon TGFβ stimulation. Expression of the lamin A mutant with a substitution of Ser390 to Ala or suppression of AKT2 or Smad3 prevents nuclear deformation and genome instability induced by TGFβ. These findings reveal a molecular mechanism for TGFβ-induced nuclear deformation and establish a role of nuclear deformation in genome instability during epithelial-mesenchymal transition.

Current Trends in Anti-Aging Strategies

The process of aging manifests from a highly interconnected network of biological cascades resulting in the degradation and breakdown of every living organism over time. This natural development increases risk for numerous diseases and can be debilitating. Academic and industrial investigators have long sought to impede, or potentially reverse, aging in the hopes of alleviating clinical burden, restoring functionality, and promoting longevity. Despite widespread investigation, identifying impactful therapeutics has been hindered by narrow experimental validation and the lack of rigorous study design. In this review, we explore the current understanding of the biological mechanisms of aging and how this understanding both informs and limits interpreting data from experimental models based on these mechanisms. We also discuss select therapeutic strategies that have yielded promising data in these model systems with potential clinical translation. Lastly, we propose a unifying approach needed to rigorously vet current and future therapeutics and guide evaluation toward efficacious therapies.

Perinuclear damage from nuclear envelope deterioration elicits stress responses that contribute to LMNA cardiomyopathy

Mutations in the LMNA gene encoding nuclear lamins A/C cause a diverse array of tissue-selective diseases, with the heart being the most commonly affected organ. Despite progress in understanding the molecular perturbations emanating from LMNA mutations, an integrative understanding of the pathogenesis leading to cardiac dysfunction remains elusive. Using a novel cell-type specific Lmna deletion mouse model capable of translatome profiling, we found that cardiomyocyte-specific Lmna deletion in adult mice led to rapid cardiomyopathy with pathological remodeling. Prior to the onset of cardiac dysfunction, lamin A/C-depleted cardiomyocytes displayed nuclear envelope deterioration, golgi dilation/fragmentation, and CREB3-mediated golgi stress activation. Translatome profiling identified upregulation of Med25, a transcriptional co-factor that can selectively dampen UPR axes. Autophagy is disrupted in the hearts of these mice, which can be recapitulated by disrupting the golgi or inducing nuclear damage by increased matrix stiffness. Systemic administration of pharmacological modulators of autophagy or ER stress significantly improved the cardiac function. These studies support a hypothesis wherein stress responses emanating from the perinuclear space contribute to the development of LMNA cardiomyopathy.

Teaser

Interplay of stress responses underlying the development of LMNA cardiomyopathy.

Overview of cellular homeostasis-associated nuclear envelope lamins and associated input signals

With the discovery of the role of the nuclear envelope protein lamin in human genetic diseases, further diverse roles of lamins have been elucidated. The roles of lamins have been addressed in cellular homeostasis including gene regulation, cell cycle, cellular senescence, adipogenesis, bone remodeling as well as modulation of cancer biology. Features of laminopathies line with oxidative stress-associated cellular senescence, differentiation, and longevity and share with downstream of aging-oxidative stress. Thus, in this review, we highlighted various roles of lamin as key molecule of nuclear maintenance, specially lamin-A/C, and mutated LMNA gene clearly reveal aging-related genetic phenotypes, such as enhanced differentiation, adipogenesis, and osteoporosis. The modulatory roles of lamin-A/C in stem cell differentiation, skin, cardiac regulation, and oncology have also been elucidated. In addition to recent advances in laminopathies, we highlighted for the first kinase-dependent nuclear lamin biology and recently developed modulatory mechanisms or effector signals of lamin regulation. Advanced knowledge of the lamin-A/C proteins as diverse signaling modulators might be biological key to unlocking the complex signaling of aging-related human diseases and homeostasis in cellular process.

Lateral thinking in syndromic congenital cardiovascular disease

Syndromic birth defects are rare diseases that can present with seemingly pleiotropic comorbidities. Prime examples are rare congenital heart and cardiovascular anomalies that can be accompanied by forelimb defects, kidney disorders and more. Whether such multi-organ defects share a developmental link remains a key question with relevance to the diagnosis, therapeutic intervention and long-term care of affected patients. The heart, endothelial and blood lineages develop together from the lateral plate mesoderm (LPM), which also harbors the progenitor cells for limb connective tissue, kidneys, mesothelia and smooth muscle. This developmental plasticity of the LPM, which founds on multi-lineage progenitor cells and shared transcription factor expression across different descendant lineages, has the potential to explain the seemingly disparate syndromic defects in rare congenital diseases. Combining patient genome-sequencing data with model organism studies has already provided a wealth of insights into complex LPM-associated birth defects, such as heart-hand syndromes. Here, we summarize developmental and known disease-causing mechanisms in early LPM patterning, address how defects in these processes drive multi-organ comorbidities, and outline how several cardiovascular and hematopoietic birth defects with complex comorbidities may be LPM-associated diseases. We also discuss strategies to integrate patient sequencing, data-aggregating resources and model organism studies to mechanistically decode congenital defects, including potentially LPM-associated orphan diseases. Eventually, linking complex congenital phenotypes to a common LPM origin provides a framework to discover developmental mechanisms and to anticipate comorbidities in congenital diseases affecting the cardiovascular system and beyond.

InterLINCing Chromatin Organization and Mechanobiology in Laminopathies

PURPOSE OF REVIEW

In this review, we explore the chromatin-related consequences of laminopathy-linked mutations through the lens of mechanotransduction.

RECENT FINDINGS

Multiple studies have highlighted the role of the nuclear lamina in maintaining the integrity of the nucleus. The lamina also has a critical role in 3D genome organization. Mutations in lamina proteins associated with various laminopathies result in the loss of organization of DNA at the nuclear periphery. However, it remains unclear if or how these two aspects of lamin function are connected. Recent data suggests that unlinking the cytoskeleton from the nuclear lamina may be beneficial to slow progress of deleterious phenotypes observed in laminopathies. In this review, we highlight emerging data that suggest interlinked chromatin- and mechanical biology-related pathways are interconnected in the pathogenesis of laminopathies.

Bioengineering a miniaturized in vitro 3D myotube contraction monitoring chip to model muscular dystrophies

Quantification of skeletal muscle functional contraction is essential to assess the outcomes of therapeutic procedures for neuromuscular disorders. Muscle three-dimensional "Organ-on-chip" models usually require a substantial amount of biological material, which rarely can be obtained from patient biopsies. Here, we developed a miniaturized 3D myotube culture chip with contraction monitoring capacity at the single cell level. Optimized micropatterned substrate design enabled to obtain high culture yields in tightly controlled microenvironments, with myotubes derived from primary human myoblasts displaying spontaneous contractions. Analysis of nuclear morphology confirmed similar myonuclei structure between obtained myotubes and in vivo myofibers, as compared to 2D monolayers. LMNA-related Congenital Muscular Dystrophy (L-CMD) was modeled with successful development of diseased 3D myotubes displaying reduced contraction. The miniaturized myotube technology can thus be used to study contraction characteristics and evaluate how diseases affect muscle organization and force generation. Importantly, it requires significantly fewer starting materials than current systems, which should substantially improve drug screening capability.

SARS-CoV-2, HIV, and HPV: Convergent evolution of selective regulation of cGAS-STING signaling

Recognizing aberrant cytoplasmic double-stranded DNA and stimulating innate immunity is essential for the host's defense against viruses and tumors. Cyclic GMP-AMP (cGAMP) synthase (cGAS) is a cytosolic DNA sensor that synthesizes the second messenger 2'3'-cGAMP and subsequently activates stimulator of interferon genes (STING)-mediated activation of TANK-binding kinase 1 (TBK1)/interferon regulatory factor 3 (IRF3) and the production of type I interferon (IFN-I). Both the cGAS-STING-mediated IFN-I antiviral defense and the countermeasures developed by diverse viruses have been extensively studied. However, recent studies have revealed a convergent evolutionary feature of severe acute respiratory syndrome coronavirus 2 and human immunodeficiency virus (HIV) viral proteins in terms of the selective regulation of cGAS-STING-mediated nuclear factor-κB (NF-κB) signaling without any effect on cGAS-STING-mediated TBK1/IRF3 activation and IFN production. The potential beneficial effect of this cGAS-STING-mediated, NF-κB-dependent antiviral effect, and the possible detrimental effect of IFN-I in the pathogenesis of coronavirus disease 2019 and HIV infection deserve more attention and future investigation.

Genotype-Phenotype Correlations in Human Diseases Caused by Mutations of LINC Complex-Associated Genes: A Systematic Review and Meta-Summary

Mutations in genes encoding proteins associated with the linker of nucleoskeleton and cytoskeleton (LINC) complex within the nuclear envelope cause different diseases with varying phenotypes including skeletal muscle, cardiac, metabolic, or nervous system pathologies. There is some understanding of the structure of LINC complex-associated proteins and how they interact, but it is unclear how mutations in genes encoding them can cause the same disease, and different diseases with different phenotypes. Here, published mutations in LINC complex-associated proteins were systematically reviewed and analyzed to ascertain whether patterns exist between the genetic sequence variants and clinical phenotypes. This revealed LMNA is the only LINC complex-associated gene in which mutations commonly cause distinct conditions, and there are no clear genotype-phenotype correlations. Clusters of LMNA variants causing striated muscle disease are located in exons 1 and 6, and metabolic disease-associated LMNA variants are frequently found in the tail of lamin A/C. Additionally, exon 6 of the emerin gene, EMD, may be a mutation "hot-spot", and diseases related to SYNE1, encoding nesprin-1, are most often caused by nonsense type mutations. These results provide insight into the diverse roles of LINC-complex proteins in human disease and provide direction for future gene-targeted therapy development.

Lamin A precursor localizes to the Z-disc of sarcomeres in the heart and is dynamically regulated in muscle cell differentiation

The lamin A precursor, prelamin A, requires extensive processing to yield mature lamin A and effect its primary function as a structural filament of the nucleoskeleton. When processing is perturbed, nuclear accumulation of prelamin A is toxic and causes laminopathic diseases such as Hutchinson-Gilford progeria syndrome and cardiomyopathy. However, the physiological role of prelamin A is largely unknown and we sought to identify novel insights about this. Using rodent heart tissue, primary cells and the C2C12 model of myofibrillogenesis, we investigated the expression and localization patterns of prelamin A in heart and skeletal muscle cells. We found that endogenous prelamin A was detectable in mouse heart localized to the sarcomere in both adult mouse heart and isolated neonatal rat cardiomyocytes. We investigated the regulation of prelamin A in C2C12 myofibrillogenesis and found it was dynamically regulated and organized into striations upon myofibril formation, colocalizing with the Z-disc protein α-actinin. These data provide evidence that prelamin A is a component of the sarcomere, underpinning a physiological purpose for unprocessed prelamin A. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Blockage of lamin-A/C loss diminishes the pro-inflammatory macrophage response

Mutations and defects in nuclear lamins can cause major pathologies, including inflammation and inflammatory diseases. Yet, the underlying molecular mechanisms are not known. We now report that the pro-inflammatory activation of macrophages, as induced by LPS or pathogenic E. coli, reduces Lamin-A/C levels thereby augmenting pro-inflammatory gene expression and cytokine secretion. We show that the activation of bone-marrow-derived macrophages (BMDMs) causes the phosphorylation and degradation of Lamin-A/C, as mediated by CDK1 and Caspase-6, respectively, necessary for upregulating IFN-β expression. Enhanced IFN-β expression subsequently increases pro-inflammatory gene expression via the IFN-β-STAT axis. Pro-inflammatory gene expression was also amplified in the complete absence of Lamin-A/C. Alternatively, pharmacological inhibition of either Lamin-A/C phosphorylation or degradation significantly downregulated pro-inflammatory gene expression, as did the targeting of IFN-β-STAT pathway members, i.e. phospho-STAT1 and phospho-STAT3. As Lamin-A/C is a previously unappreciated regulator of the pro-inflammatory macrophage response, our findings suggest novel opportunities to treat inflammatory diseases.

Lamin A/C impairments cause mitochondrial dysfunction by attenuating PGC1α and the NAMPT-NAD+ pathway

Mutations in the lamin A/C gene (LMNA) cause laminopathies such as the premature aging Hutchinson Gilford progeria syndrome (HGPS) and altered lamin A/C levels are found in diverse malignancies. The underlying lamin-associated mechanisms remain poorly understood. Here we report that lamin A/C-null mouse embryo fibroblasts (Lmna-/- MEFs) and human progerin-expressing HGPS fibroblasts both display reduced NAD+ levels, unstable mitochondrial DNA and attenuated bioenergetics. This mitochondrial dysfunction is associated with reduced chromatin recruitment (Lmna-/- MEFs) or low levels (HGPS) of PGC1α, the key transcription factor for mitochondrial homeostasis. Lmna-/- MEFs showed reduced expression of the NAD+-biosynthesis enzyme NAMPT and attenuated activity of the NAD+-dependent deacetylase SIRT1. We find high PARylation in lamin A/C-aberrant cells, further decreasing the NAD+ pool and consistent with impaired DNA base excision repair in both cell models, a condition that fuels DNA damage-induced PARylation under oxidative stress. Further, ATAC-sequencing revealed a substantially altered chromatin landscape in Lmna-/- MEFs, including aberrantly reduced accessibility at the Nampt gene promoter. Thus, we identified a new role of lamin A/C as a key modulator of mitochondrial function through impairments of PGC1α and the NAMPT-NAD+ pathway, with broader implications for the aging process.

The LEM-ESCRT toolkit: Repair and maintenance of the nucleus

The eukaryotic genome is enclosed in a nuclear envelope that protects it from potentially damaging cellular activities and physically segregates transcription and translation.Transport across the NE is highly regulated and occurs primarily via the macromolecular nuclear pore complexes.Loss of nuclear compartmentalization due to defects in NPC function and NE integrity are tied to neurological and ageing disorders like Alzheimer’s, viral pathogenesis, immune disorders, and cancer progression.Recent work implicates inner-nuclear membrane proteins of the conserved LEM domain family and the ESCRT machinery in NE reformation during cell division and NE repair upon rupture in migrating cancer cells, and generating seals over defective NPCs. In this review, we discuss the recent in-roads made into defining the molecular mechanisms and biochemical networks engaged by LEM and many other integral inner nuclear membrane proteins to preserve the nuclear barrier.

Low lamin A levels enhance confined cell migration and metastatic capacity in breast cancer

Aberrations in nuclear size and shape are commonly used to identify cancerous tissue. However, it remains unclear whether the disturbed nuclear structure directly contributes to the cancer pathology or is merely a consequence of other events occurring during tumorigenesis. Here, we show that highly invasive and proliferative breast cancer cells frequently exhibit Akt-driven lower expression of the nuclear envelope proteins lamin A/C, leading to increased nuclear deformability that permits enhanced cell migration through confined environments that mimic interstitial spaces encountered during metastasis. Importantly, increasing lamin A/C expression in highly invasive breast cancer cells reflected gene expression changes characteristic of human breast tumors with higher LMNA expression, and specifically affected pathways related to cell-ECM interactions, cell metabolism, and PI3K/Akt signaling. Further supporting an important role of lamins in breast cancer metastasis, analysis of lamin levels in human breast tumors revealed a significant association between lower lamin A levels, Akt signaling, and decreased disease-free survival. These findings suggest that downregulation of lamin A/C in breast cancer cells may influence both cellular physical properties and biochemical signaling to promote metastatic progression.

Nesprin-1 LINC complexes recruit microtubule cytoskeleton proteins and drive pathology in Lmna-mutant striated muscle

Mutations in LMNA, the gene encoding A-type lamins, cause laminopathies-diseases of striated muscle and other tissues. The aetiology of laminopathies has been attributed to perturbation of chromatin organization or structural weakening of the nuclear envelope (NE) such that the nucleus becomes more prone to mechanical damage. The latter model requires a conduit for force transmission to the nucleus. NE-associated Linker of Nucleoskeleton and Cytoskeleton (LINC) complexes are one such pathway. Using clustered regularly interspaced short palindromic repeats to disrupt the Nesprin-1 KASH (Klarsicht, ANC-1, Syne Homology) domain, we identified this LINC complex protein as the predominant NE anchor for microtubule cytoskeleton components, including nucleation activities and motor complexes, in mouse cardiomyocytes. Loss of Nesprin-1 LINC complexes resulted in loss of microtubule cytoskeleton proteins at the nucleus and changes in nuclear morphology and positioning in striated muscle cells, but with no overt physiological defects. Disrupting the KASH domain of Nesprin-1 suppresses Lmna-linked cardiac pathology, likely by reducing microtubule cytoskeleton activities at the nucleus. Nesprin-1 LINC complexes thus represent a potential therapeutic target for striated muscle laminopathies.

Nuclear F-actin and Lamin A antagonistically modulate nuclear shape

Nuclear shape influences cell migration, gene expression and cell cycle progression, and is altered in disease states like laminopathies and cancer. What factors and forces determine nuclear shape? We find that nuclei assembled in Xenopus egg extracts in the presence of dynamic F-actin exhibit a striking bilobed nuclear morphology with distinct membrane compositions in the two lobes and accumulation of F-actin at the inner nuclear envelope. The addition of Lamin A (encoded by lmna), which is absent from Xenopus eggs, results in rounder nuclei, suggesting that opposing nuclear F-actin and Lamin A forces contribute to the regulation of nuclear shape. Nuclear F-actin also promotes altered nuclear shape in Lamin A-knockdown HeLa cells and, in both systems, abnormal nuclear shape is driven by formins and not Arp2/3 or myosin. Although the underlying mechanisms might differ in Xenopus and HeLa cells, we propose that nuclear F-actin filaments nucleated by formins impart outward forces that lead to altered nuclear morphology unless Lamin A is present. Targeting nuclear actin dynamics might represent a novel approach to rescuing disease-associated defects in nuclear shape.

Glucocorticoids enhance chemotherapy-driven stress granule assembly and impair granule dynamics leading to cell death

Stress granules (SGs) can assemble in cancer cells upon chemotoxic stress. Glucocorticoids function during stress responses and are administered with chemotherapies. The roles of glucocorticoids in SG assembly and disassembly pathways are unknown. We examined whether combining glucocorticoids such as cortisone with chemotherapies from the vinca alkaloid family, which dismantle the microtubule network, affects SG assembly and disassembly pathways and influences cell viability in cancer cells and human-derived organoids. Cortisone augmented SG formation when combined with vinorelbine (VRB). Live-cell imaging showed that cortisone increased SG assembly rates but reduced SG clearance rates after stress, by increasing protein residence times within the SGs. Mechanistically, VRB and cortisone signaled through the integrated stress response mediated by eIF2α (also known as EIF2S1), yet induced different kinases, with cortisone activating the GCN2 kinase (also known as EIF2AK4). Cortisone increased VRB-induced cell death and reduced the population of cells trapped in mitotic catastrophe. These effects were mediated by the core SG proteins G3BP1 and G3BP2. In conclusion, glucocorticoids induce SG assembly and cell death when administered with chemotherapies, suggesting that combining glucocorticoids with chemotherapies can enhance cancer cell chemosensitivity.

Summary: Combining cortisone with the chemotherapy vinorelbine enhances the assembly of stress granules that are less likely to be cleared from the cells, augmenting vinorelbine-induced cell death.

LINCing Senescence and Nuclear Envelope Changes

The nuclear envelope (NE) has emerged as a nexus for cellular organization, signaling, and survival. Beyond its role as a barrier to separate the nucleoplasm from the cytoplasm, the NE's role in supporting and maintaining a myriad of other functions has made it a target of study in many cellular processes, including senescence. The nucleus undergoes dramatic changes in senescence, many of which are driven by changes in the NE. Indeed, Lamin B1, a key NE protein that is consistently downregulated in senescence, has become a marker for senescence. Other NE proteins have also been shown to play a role in senescence, including LINC (linker of nucleoskeleton and cytoskeleton) complex proteins. LINC complexes span the NE, forming physical connections between the cytoplasm to the nucleoplasm. In this way, they integrate nuclear and cytoplasmic mechanical signals and are essential not only for a variety of cellular functions but are needed for cell survival. However, LINC complex proteins have been shown to have a myriad of functions in addition to forming a LINC complex, often existing as nucleoplasmic or cytoplasmic soluble proteins in a variety of isoforms. Some of these proteins have now been shown to play important roles in DNA repair, cell signaling, and nuclear shape regulation, all of which are important in senescence. This review will focus on some of these roles and highlight the importance of LINC complex proteins in senescence.

Rare and potential pathogenic mutations of LMNA and LAMA4 associated with familial arrhythmogenic right ventricular cardiomyopathy/dysplasia with right ventricular heart failure, cerebral thromboembolism and hereditary electrocardiogram abnormality

Background

Arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) is associated with ventricular arrhythmia, heart failure (HF), and sudden death. Thromboembolism is also an important and serious complication of ARVC/D. However, the etiology of ARVC/D and thromboembolism and their association with genetic mutations are unclear.

Methods

Genomic DNA samples of peripheral blood were conducted for whole-exome sequencing (WES) and Sanger sequencing in the ARVC/D family. Then, we performed bioinformatics analysis for genes susceptible to cardiomyopathies and arrhythmias. Further, we analyzed how the potential pathogenic mutations were affecting the hydrophobicity and phosphorylation of amino acids and their joint pathogenicity by ProtScale, NetPhos and ORVAL algorisms.

Results

We discovered a Chinese Han family of ARVC/D with right ventricular HF (RVHF), cerebral thromboembolism, arrhythmias (atrial fibrillation, atrial standstill, multifocal ventricular premature, complete right bundle block and third-degree atrioventricular block) and sudden death. Based on the WES data, the variants of LMNA p.A242V, LAMA4 p.A225P and RYR2 p.T858M are highly conserved and predicated as “deleterious” by SIFT and MetaSVM algorithms. Their CADD predicting scores are 33, 27.4 and 25.8, respectively. These variants increase the hydrophobicity of their corresponding amino acid residues and their nearby sequences by 0.378, 0.266 and 0.289, respectively. The LAMA4 and RYR2 variants lead to changes in protein phosphorylation at or near their corresponding amino acid sites. There were high risks of joint pathogenicity for cardiomyopathy among these three variants. Cosegregation analysis indicated that LMNA p.A242V might be an important risk factor for ARVC/D, electrocardiogram abnormality and cerebral thromboembolism, while LAMA4 p.A225P may be a pathogenic etiology of ARVC/D and hereditary electrocardiogram abnormality.

Conclusions

The LMNA p.A242V may participate in the pathogenesis of familial ARVC/D with RVHF and cerebral thromboembolism, while LAMA4 p.A225P may be associated with ARVC/D and hereditary electrocardiogram abnormality.