Nuclear mechanotransduction: response of the lamina to extracellular stress with implications in aging

Nucleocytoplasmic transport senses mechanical forces independently of cell density in cell monolayers

Cells sense and respond to mechanical forces through mechanotransduction, which regulates processes in health and disease. In single adhesive cells, mechanotransduction involves the transmission of force from the extracellular matrix to the cell nucleus, where it affects nucleocytoplasmic transport (NCT) and the subsequent nuclear localization of transcriptional regulators, such as YAP (also known as YAP1). However, if and how NCT is mechanosensitive in multicellular systems is unclear. Here, we characterize and use a fluorescent sensor of nucleocytoplasmic transport (Sencyt) and demonstrate that NCT responds to mechanical forces but not cell density in cell monolayers. Using monolayers of both epithelial and mesenchymal phenotype, we show that NCT is altered in response both to osmotic shocks and to the inhibition of cell contractility. Furthermore, NCT correlates with the degree of nuclear deformation measured through nuclear solidity, a shape parameter related to nuclear envelope tension. In contrast, YAP is sensitive to cell density, showing that the YAP response to cell-cell contacts is not via a mere mechanical effect of NCT. Our results demonstrate the generality of the mechanical regulation of NCT.

The nexus of nuclear envelope dynamics, circular economy and cancer cell pathophysiology

The nuclear envelope (NE) is a critical component in maintaining the function and structure of the eukaryotic nucleus. The NE and lamina are disassembled during each cell cycle to enable an open mitosis. Nuclear architecture construction and deconstruction is a prime example of a circular economy, as it fulfills a highly efficient recycling program bound to continuous assessment of the quality and functionality of the building blocks. Alterations in the nuclear dynamics and lamina structure have emerged as important contributors to both oncogenic transformation and cancer progression. However, the knowledge of the NE breakdown and reassembly is still limited to a fraction of participating proteins and complexes. As cancer cells contain highly diverse nuclei in terms of DNA content, but also in terms of nuclear number, size, and shape, it is of great interest to understand the intricate relationship between these nuclear features in cancer cell pathophysiology. In this review, we provide insights into how those NE dynamics are regulated, and how lamina destabilization processes may alter the NE circular economy. Moreover, we expand the knowledge of the lamina-associated domain region by using strategic algorithms, including Artificial Intelligence, to infer protein associations, assess their function and location, and predict cancer-type specificity with implications for the future of cancer diagnosis, prognosis and treatment. Using this approach we identified NUP98 and MECP2 as potential proteins that exhibit upregulation in Acute Myeloid Leukemia (LAML) patients with implications for early diagnosis.

Low-intensity pulsed ultrasound reduces alveolar bone resorption during orthodontic treatment via Lamin A/C-Yes-associated protein axis in stem cells

BACKGROUND

The bone remodeling during orthodontic treatment for malocclusion often requires a long duration of around two to three years, which also may lead to some complications such as alveolar bone resorption or tooth root resorption. Low-intensity pulsed ultrasound (LIPUS), a noninvasive physical therapy, has been shown to promote bone fracture healing. It is also reported that LIPUS could reduce the duration of orthodontic treatment; however, how LIPUS regulates the bone metabolism during the orthodontic treatment process is still unclear.

AIM

To investigate the effects of LIPUS on bone remodeling in an orthodontic tooth movement (OTM) model and explore the underlying mechanisms.

METHODS

A rat model of OTM was established, and alveolar bone remodeling and tooth movement rate were evaluated via micro-computed tomography and staining of tissue sections. In vitro, human bone marrow mesenchymal stem cells (hBMSCs) were isolated to detect their osteogenic differentiation potential under compression and LIPUS stimulation by quantitative reverse transcription-polymerase chain reaction, Western blot, alkaline phosphatase (ALP) staining, and Alizarin red staining. The expression of Yes-associated protein (YAP1), the actin cytoskeleton, and the Lamin A/C nucleoskeleton were detected with or without YAP1 small interfering RNA (siRNA) application via immunofluorescence.

RESULTS

The force treatment inhibited the osteogenic differentiation potential of hBMSCs; moreover, the expression of osteogenesis markers, such as type 1 collagen (COL1), runt-related transcription factor 2, ALP, and osteocalcin (OCN), decreased. LIPUS could rescue the osteogenic differentiation of hBMSCs with increased expression of osteogenic marker inhibited by force. Mechanically, the expression of LaminA/C, F-actin, and YAP1 was downregulated after force treatment, which could be rescued by LIPUS. Moreover, the osteogenic differentiation of hBMSCs increased by LIPUS could be attenuated by YAP siRNA treatment. Consistently, LIPUS increased alveolar bone density and decreased vertical bone absorption in vivo. The decreased expression of COL1, OCN, and YAP1 on the compression side of the alveolar bone was partially rescued by LIPUS.

CONCLUSION

LIPUS can accelerate tooth movement and reduce alveolar bone resorption by modulating the cytoskeleton-Lamin A/C-YAP axis, which may be a promising strategy to reduce the orthodontic treatment process.

The upregulation of lamin A/C as a compensatory mechanism during tight junction disruption in renal tubular cells mediated by calcium oxalate crystals

Calcium oxalate monohydrate (COM), the most important crystal causing kidney stone disease, upregulates lamin A/C but downregulates zonula occludens-1 (ZO-1) in renal tubular cells. While roles for F-actin and α-tubulin and their association with ZO-1 are known to regulate COM-mediated tight junction (TJ) disruption, roles of lamin A/C and its interplay with ZO-1 in COM kidney stone model remain unclear and are thus the objectives of this study. Lamin A/C was knocked down in MDCK cells by silencing RNA specific for LMNA (siLMNA). Both wild-type (WT) and siLMNA cells were treated with COM for 48-h compared with the untreated (control) cells. Western blotting and immunofluorescence staining revealed upregulated lamin A/C and downregulated ZO-1 in the COM-treated WT cells. siLMNA successfully reduced lamin A/C expression in both control and COM-treated cells. Nonetheless, siLMNA did not reverse the effect of COM on the decreases in ZO-1 and transepithelial resistance, but further reduced their levels in both control and COM-treated cells. Protein-protein interaction analysis demonstrated that two cytoskeletal proteins (actin and tubulin) served as the linkers to connect lamin A/C with ZO-1 and occludin (both of which are the TJ proteins). Altogether, these data implicate that lamin A/C and ZO-1 are indirectly associated to control TJ function, and ZO-1 expression is regulated by lamin A/C. Moreover, COM-induced upregulation of lamin A/C most likely serves as a compensatory mechanism to cope with the downregulation of ZO-1 during COM-mediated TJ disruption.

A single stiffened nucleus alters cell dynamics and coherence in a monolayer

Force transmission throughout a monolayer is the result of complex interactions between cells. Monolayer adaptation to force imbalances such as singular stiffened cells provides insight into the initiation of disease and fibrosis. Here, NRK-52E cells transfected with ∆50LA, which significantly stiffens the nucleus. These stiffened cells were sparsely placed in a monolayer of normal NRK-52E cells. Through morphometric analysis and temporal tracking, the impact of the singular stiffened cells shows a pivotal role in mechanoresponse of the monolayer. A method for a detailed analysis of the spatial aspect and temporal progression of the nuclear boundary was developed and used to achieve a full description of the phenotype and dynamics of the monolayers under study. Our findings reveal that cells are highly sensitive to the presence of mechanically impaired neighbors, leading to generalized loss of coordination in collective cell migration, but without seemingly affecting the potential for nuclear lamina fluctuations of neighboring cells. Reduced translocation in neighboring cells appears to be compensated by an increase in nuclear rotation and dynamic variation of shape, suggesting a "frustration" of cells and maintenance of motor activity. Interestingly, some characteristics of the behavior of these cells appear to be dependent on the distance to a ∆50LA cell, pointing to compensatory behavior in response to force transmission imbalances in a monolayer. These insights may suggest the long-range impacts of single cell defects related to tissue dysfunction.

Crucial Role of Lamin A/C in the Migration and Differentiation of MSCs in Bone

Lamin A/C, intermediate filament proteins from the nuclear lamina encoded by the LMNA gene, play a central role in mediating the mechanosignaling of cytoskeletal forces into nucleus. In fact, this mechanotransduction process is essential to ensure the proper functioning of other tasks also mediated by lamin A/C: the structural support of the nucleus and the regulation of gene expression. In this way, lamin A/C is fundamental for the migration and differentiation of mesenchymal stem cells (MSCs), the progenitors of osteoblasts, thus affecting bone homeostasis. Bone formation is a complex process regulated by chemical and mechanical cues, coming from the surrounding extracellular matrix. MSCs respond to signals modulating the expression levels of lamin A/C, and therefore, adapting their nuclear shape and stiffness. To promote cell migration, MSCs need soft nuclei with low lamin A content. Conversely, during osteogenic differentiation, lamin A/C levels are known to be increased. Several LMNA mutations present a negative impact in the migration and osteogenesis of MSCs, affecting bone tissue homeostasis and leading to pathological conditions. This review aims to describe these concepts by discussing the latest state-of-the-art in this exciting area, focusing on the relationship between lamin A/C in MSCs' function and bone tissue from both, health and pathological points of view.

Lamin A/C Mechanotransduction in Laminopathies

Mechanotransduction translates forces into biological responses and regulates cell functionalities. It is implicated in several diseases, including laminopathies which are pathologies associated with mutations in lamins and lamin-associated proteins. These pathologies affect muscle, adipose, bone, nerve, and skin cells and range from muscular dystrophies to accelerated aging. Although the exact mechanisms governing laminopathies and gene expression are still not clear, a strong correlation has been found between cell functionality and nuclear behavior. New theories base on the direct effect of external force on the genome, which is indeed sensitive to the force transduced by the nuclear lamina. Nuclear lamina performs two essential functions in mechanotransduction pathway modulating the nuclear stiffness and governing the chromatin remodeling. Indeed, A-type lamin mutation and deregulation has been found to affect the nuclear response, altering several downstream cellular processes such as mitosis, chromatin organization, DNA replication-transcription, and nuclear structural integrity. In this review, we summarize the recent findings on the molecular composition and architecture of the nuclear lamina, its role in healthy cells and disease regulation. We focus on A-type lamins since this protein family is the most involved in mechanotransduction and laminopathies.

Evaluation of musculoskeletal phenotype of the G608G progeria mouse model with lonafarnib, pravastatin, and zoledronic acid as treatment groups

Hutchinson-Gilford progeria syndrome (HGPS) is a uniformly fatal condition that is especially prevalent in skin, cardiovascular, and musculoskeletal systems. A wide gap exists between our knowledge of the disease and a promising treatment or cure. The aim of this study was to first characterize the musculoskeletal phenotype of the homozygous G608G BAC-transgenic progeria mouse model, and to determine the phenotype changes of HGPS mice after a five-arm preclinical trial of different treatment combinations with lonafarnib, pravastatin, and zoledronic acid. Microcomputed tomography and CT-based rigidity analyses were performed to assess cortical and trabecular bone structure, density, and rigidity. Bones were loaded to failure with three-point bending to assess strength. Contrast-enhanced µCT imaging of mouse femurs was performed to measure glycosaminoglycan content, thickness, and volume of the femoral head articular cartilage. Advanced glycation end products were assessed with a fluorometric assay. The changes demonstrated in the cortical bone structure, rigidity, stiffness, and modulus of the HGPS G608G mouse model may increase the risk for bending and deformation, which could result in the skeletal dysplasia characteristic of HGPS. Cartilage abnormalities seen in this HGPS model resemble changes observed in the age-matched WT controls, including early loss of glycosaminoglycans, and decreased cartilage thickness and volume. Such changes might mimic prevalent degenerative joint diseases in the elderly. Lonafarnib monotherapy did not improve bone or cartilage parameters, but treatment combinations with pravastatin and zoledronic acid significantly improved bone structure and mechanical properties and cartilage structural parameters, which ameliorate the musculoskeletal phenotype of the disease.

Effect of mechanical loading and substrate elasticity on the osteogenic and adipogenic differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are highly sensitive to biomechanics of their extracellular environment. Generally, a higher elasticity of culture substrates can drive cells into the osteogenic lineage, whereas low substrate elasticity results in adipogenesis. Applied mechanical loading by cyclic strain is another major variable influencing cell fate. Yet, little is known about the simultaneous effect of both cues. Therefore, the present study investigated the relative importance of both cues on differentiation. MSCs were cultured in an osteogenic and also an adipogenic environment on soft polyacrylamide (PAAm; E = 23 ± 0.3 kPa), stiff PAAm (111 ± 2 kPa), and polydimethylsiloxane (PDMS; E = 1,5 ± 0.07 MPa) either unstrained or with 8% cyclic strain at 1 Hz. Without strain, the relative expression of the early osteogenic marker alkaline phosphatase (ALP) was significantly higher (78%) on PDMS than on both PAAm. With 8% cyclic strain, ALP expression increased for all groups in comparison with unstrained controls. The highest increase was observed for the soft PAAm by 36%. Moreover, relative oil red O (ORO) expression-indicating adipogenesis-was the highest for unstrained soft PAAm. On the other hand, the percentage of ORO positive cells significantly decreased by 57% and 69% for soft and stiff PAAm when strained. In conclusion, biomaterial elasticity and mechanical loading can act simultaneously on cell differentiation. Substrate elasticity is an important factor, regulating the differentiation, but cyclic strain can drive MSCs towards the osteogenesis even on the softest substrate. As such, the osteogenic effect of mechanical loading can overrule the adipogenic effect of soft substrates, thereby acting as an inhibitor.

Mechanosensing at the Nuclear Envelope by Nuclear Pore Complex Stretch Activation and Its Effect in Physiology and Pathology

Cell fate is correlated to mechanotransduction, in which forces transmitted by the cytoskeleton filaments alter the nuclear shape, affecting transcription factor import/export, cells transcription activity and chromatin distribution. There is in fact evidence that stem cells cultured in 3D environments mimicking the native niche are able to maintain their stemness or modulate their cellular function. However, the molecular and biophysical mechanisms underlying cellular mechanosensing are still largely unclear. The propagation of mechanical stimuli via a direct pathway from cell membrane integrins to SUN proteins residing in the nuclear envelop has been demonstrated, but we suggest that the cells’ fate is mainly affected by the force distribution at the nuclear envelope level, where the SUN protein transmits the stimuli via its mechanical connection to several cell structures such as chromatin, lamina and the nuclear pore complex (NPC). In this review, we analyze the NPC structure and organization, which have not as yet been fully investigated, and its plausible involvement in cell fate. NPC is a multiprotein complex that spans the nuclear envelope, and is involved in several key cellular processes such as bidirectional nucleocytoplasmic exchange, cell cycle regulation, kinetochore organization, and regulation of gene expression. As several connections between the NPC and the nuclear envelope, chromatin and other transmembrane proteins have been identified, it is reasonable to suppose that nuclear deformations can alter the NPC structure. We provide evidence that the transmission of mechanical forces may significantly affects the basket conformation via the Nup153-SUN1 connection, both altering the passage of molecules through it and influencing the state of chromatin packing. Finally, we review the known correlations between a pathological NPC structure and diseases such as cancer, autoimmune disease, aging and laminopathies.

The role of lamin A/C in mesenchymal stem cell differentiation

Lamin A/C is the major architectural protein of cell nucleus in charge of the nuclear mechanosensing. By integrating extracellular mechanical and biochemical signals, lamin A/C regulates multiple intracellular events including mesenchymal stem cell (MSC) fate determination. Herein, we review the recent findings about the effects and mechanisms of lamin A/C in governing MSC lineage commitment, with a special focus on osteogenesis and adipogenesis. Better understanding of MSC differentiation regulated by lamin A/C could provide insights into pathogenesis of age-related osteoporosis.

Shear stress induced nuclear shrinkage through activation of Piezo1 channels in epithelial cells

The cell nucleus responds to mechanical cues with changes in size, morphology, and motility. Previous work showed that external forces couple to nuclei through the cytoskeleton network, but we show here that changes in nuclear shape can be driven solely by calcium levels. Fluid shear stress applied to MDCK cells caused the nuclei to shrink through a Ca2+ dependent signaling pathway. Inhibiting mechanosensitive Piezo1 channels with GsMTx4 prevented nuclear shrinkage. Piezo1 knockdown also significantly reduced the nuclear shrinkage. Activation of Piezo1 with the agonist Yoda1 caused similar nucleus shrinkage without shear stress. These results demonstrate that Piezo1 channel is a key element for transmitting shear force input to nuclei. To ascertain the relative contributions of Ca2+ to cytoskeleton perturbation, we examined the F-actin reorganization under shear stress and static conditions, and showed that reorganization of the cytoskeleton is not necessary for nuclear shrinkage. These results emphasize the role of the mechanosensitive channels as primary transducers in force transmission to the nucleus.

Endothelial progerin expression causes cardiovascular pathology through an impaired mechanoresponse

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disorder characterized by accelerated cardiovascular disease with extensive fibrosis. It is caused by a mutation in LMNA leading to expression of truncated prelamin A (progerin) in the nucleus. To investigate the contribution of the endothelium to cardiovascular HGPS pathology, we generated an endothelium-specific HGPS mouse model with selective endothelial progerin expression. Transgenic mice develop interstitial myocardial and perivascular fibrosis and left ventricular hypertrophy associated with diastolic dysfunction and premature death. Endothelial cells show impaired shear stress response and reduced levels of endothelial nitric oxide synthase (eNOS) and NO. On the molecular level, progerin impairs nucleocytoskeletal coupling in endothelial cells through changes in mechanoresponsive components at the nuclear envelope, increased F-actin/G-actin ratios, and deregulation of mechanoresponsive myocardin-related transcription factor-A (MRTFA). MRTFA binds to the Nos3 promoter and reduces eNOS expression, thereby mediating a profibrotic paracrine response in fibroblasts. MRTFA inhibition rescues eNOS levels and ameliorates the profibrotic effect of endothelial cells in vitro. Although this murine model lacks the key anatomical feature of vascular smooth muscle cell loss seen in HGPS patients, our data show that progerin-induced impairment of mechanosignaling in endothelial cells contributes to excessive fibrosis and cardiovascular disease in HGPS patients.

Mechanisms of vascular aging: What can we learn from Hutchinson-Gilford progeria syndrome?

Aging is the main risk factor for cardiovascular disease (CVD). The increased prevalence of CVD is partly due to the global increase in life expectancy. In this context, it is essential to identify the mechanisms by which aging induces CVD, with the ultimate aim of reducing its incidence. Both atherosclerosis and heart failure significantly contribute to age-associated CVD morbidity and mortality. Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disorder caused by the synthesis of progerin, which is noted for accelerated aging and CVD. This mutant form of prelamin A induces generalised atherosclerosis, vascular calcification, and cardiac electrophysiological abnormalities, leading to premature aging and death, mainly due to myocardial infarction and stroke. This review discusses the main vascular structural and functional abnormalities during physiological and premature aging, as well as the mechanisms involved in the exacerbated CVD and accelerated aging induced by the accumulation of progerin and prelamin A. Both proteins are expressed in non-HGPS individuals, and physiological aging shares many features of progeria. Research into HGPS could therefore shed light on novel mechanisms involved in the physiological aging of the cardiovascular system.

Nucleoskeletal stiffness regulates stem cell migration and differentiation through lamin A/C

Stem cell-based tissue engineering provides a prospective strategy to bone tissue repair. Bone tissue repair begins at the recruitment and directional movement of stem cells, and ultimately achieved on the directional differentiation of stem cells. The migration and differentiation of stem cells are regulated by nucleoskeletal stiffness. Mechanical properties of lamin A/C contribute to the nucleoskeletal stiffness and consequently to the regulation of cell migration and differentiation. Nuclear lamin A/C determines cell migration through the regulation of nucleoskeletal stiffness and rigidity and involve in nuclear-cytoskeletal coupling. Moreover, lamin A/C is the essential core module regulating stem cell differentiation. The cells with higher migration ability tend to have enhanced differentiation potential, while the optimum amount of lamin A/C in migration and differentiation of MSCs is in conflict. This contrary phenomenon may be the result of mechanical microenvironment modulation.

Expression of Nuclear Lamin Proteins in Endothelial Cells is Sensitive to Cell Passage and Fluid Shear Stress

Introduction

Vascular cells are regulated by continuous hemodynamic forces in vivo, and mechanical forces such as shear stress are proposed to involve in the progression of cardiovascular diseases such as atherosclerosis. Lamin A/C makes up the nuclear lamina, which structurally supports the nucleus while also functionally participates in chromatin organization and gene transcription. Diseases caused by lamin or other nuclear proteins are called laminopathies. One example, Hutchinson Gilford Progeria Syndrome (HGPS) where young patients show signs of accelerated aging, is caused by de novo mutations on the lamin A/C gene. Vasculature of HGPS patients shares many similarities with people of advanced age, suggesting a role for lamin in vascular aging.

Methods

In this study, we examined how arterial shear stress affects lamin A/C expression in bovine aortic endothelial cells at different population doubling levels (PDL). We also used fluorescence image analysis to examine nuclear shape changes with shear stress and PDL.

Results

Our results suggest that laminar shear stress downregulated lamin A/C expression in low PDL cells, but the effect was reversed in high PDL cells. Nuclear shape changes were more prominent after shear stress in low PDL cells. Moreover, lamin A/C accumulated more at the nuclear periphery after exposure to shear stress.

Conclusions

Overall, our results indicate that both shear stress and cell passage can have an impact on lamin expressions at transcriptional and translational levels, as we continue to understand the effect of shear stress on endothelial lamina as part of the vascular aging process.

Mechano-adaptation of the stem cell nucleus

Exogenous mechanical forces are transmitted through the cell and to the nucleus, initiating mechanotransductive signaling cascades with profound effects on cellular function and stem cell fate. A growing body of evidence has shown that the force sensing and force-responsive elements of the nucleus adapt to these mechanotransductive events, tuning their response to future mechanical input. The mechanisms underlying this “mechano-adaptation” are only just beginning to be elucidated, and it remains poorly understood how these components act and adapt in tandem to drive stem cell differentiation. Here, we review the evidence on how the stem cell nucleus responds and adapts to physical forces, and provide a perspective on how this mechano-adaptation may function to drive and enforce stem cell differentiation.

Aging in the Cardiovascular System: Lessons from Hutchinson-Gilford Progeria Syndrome

Aging, the main risk factor for cardiovascular disease (CVD), is becoming progressively more prevalent in our societies. A better understanding of how aging promotes CVD is therefore urgently needed to develop new strategies to reduce disease burden. Atherosclerosis and heart failure contribute significantly to age-associated CVD-related morbimortality. CVD and aging are both accelerated in patients suffering from Hutchinson-Gilford progeria syndrome (HGPS), a rare genetic disorder caused by the prelamin A mutant progerin. Progerin causes extensive atherosclerosis and cardiac electrophysiological alterations that invariably lead to premature aging and death. This review summarizes the main structural and functional alterations to the cardiovascular system during physiological and premature aging and discusses the mechanisms underlying exaggerated CVD and aging induced by prelamin A and progerin. Because both proteins are expressed in normally aging non-HGPS individuals, and most hallmarks of normal aging occur in progeria, research on HGPS can identify mechanisms underlying physiological aging.

Cell Mechanosensors and the Possibilities of Using Magnetic Nanoparticles to Study Them and to Modify Cell Fate

The use of magnetic nanoparticles (MNPs) is a promising technique for future advances in biomedical applications. This idea is supported by the availability of MNPs that can target specific cell components, the variety of shapes of MNPs and the possibility of finely controlling the applied magnetic forces. To examine this opportunity, here we review the current developments in the use of MNPs to mechanically stimulate cells and, specifically, the cell mechanotransduction systems. We analyze the cell components that may act as mechanosensors and their effect on cell fate and we focus on the promising possibilities of controlling stem-cell differentiation, inducing cancer-cell death and treating nervous-system diseases.

Dynamic regulation of nuclear architecture and mechanics-a rheostatic role for the nucleus in tailoring cellular mechanosensitivity

Nuclear architecture, a function of both chromatin and nucleoskeleton structure, is known to change with stem cell differentiation and differs between various somatic cell types. These changes in nuclear architecture are associated with the regulation of gene expression and genome function in a cell-type specific manner. Biophysical stimuli are known effectors of differentiation and also elicit stimuli-specific changes in nuclear architecture. This occurs via the process of mechanotransduction whereby extracellular mechanical forces activate several well characterized signaling cascades of cytoplasmic origin, and potentially some recently elucidated signaling cascades originating in the nucleus. Recent work has demonstrated changes in nuclear mechanics both with pluripotency state in embryonic stem cells, and with differentiation progression in adult mesenchymal stem cells. This review explores the interplay between cytoplasmic and nuclear mechanosensitivity, highlighting a role for the nucleus as a rheostat in tuning the cellular mechano-response.

Nuclear Mechanics and Stem Cell Differentiation

Stem cells are characterized by their self-renewal and multi-lineage differentiation potential. Stem cell differentiation is a prerequisite for the application of stem cells in regenerative medicine and clinical therapy. In addition to chemical stimulation, mechanical cues play a significant role in regulating stem cell differentiation. The integrity of mechanical sensors is necessary for the ability of cells to respond to mechanical signals. The nucleus, the largest and stiffest cellular organelle, interacts with the cytoskeleton as a key mediator of cell mechanics. Nuclear mechanics are involved in the complicated interactions of lamins, chromatin and nucleoskeleton-related proteins. Thus, stem cell differentiation is intimately associated with nuclear mechanics due to its indispensable role in mechanotransduction and mechanical response. This paper reviews several main contributions of nuclear mechanics, highlights the hallmarks of the nuclear mechanics of stem cells, and provides insight into the relationship between nuclear mechanics and stem cell differentiation, which may guide clinical applications in the future.

Under Pressure: Mechanical Stress Management in the Nucleus

Cells are constantly adjusting to the mechanical properties of their surroundings, operating a complex mechanochemical feedback, which hinges on mechanotransduction mechanisms. Whereas adhesion structures have been shown to play a central role in mechanotransduction, it now emerges that the nucleus may act as a mechanosensitive structure. Here, we review recent advances demonstrating that mechanical stress emanating from the cytoskeleton can activate pathways in the nucleus which eventually impact both its structure and the transcriptional machinery.

Molecular insights into the premature aging disease progeria

Hutchinson-Gilford progeria syndrome (HGPS) is an extremely rare premature aging disease presenting many features resembling the normal aging process. HGPS patients die before the age of 20 years due to cardiovascular problems and heart failure. HGPS is linked to mutations in the LMNA gene encoding the intermediate filament protein lamin A. Lamin A is a major component of the nuclear lamina, a scaffold structure at the nuclear envelope that defines mechanochemical properties of the nucleus and is involved in chromatin organization and epigenetic regulation. Lamin A is also present in the nuclear interior where it fulfills lamina-independent functions in cell signaling and gene regulation. The most common LMNA mutation linked to HGPS leads to mis-splicing of the LMNA mRNA and produces a mutant lamin A protein called progerin that tightly associates with the inner nuclear membrane and affects the dynamic properties of lamins. Progerin expression impairs many important cellular processes providing insight into potential disease mechanisms. These include changes in mechanosignaling, altered chromatin organization and impaired genome stability, and changes in signaling pathways, leading to impaired regulation of adult stem cells, defective extracellular matrix production and premature cell senescence. In this review, we discuss these pathways and their potential contribution to the disease pathologies as well as therapeutic approaches used in preclinical and clinical tests.

Cell Mechanosensitivity is Enabled by the LINC Nuclear Complex

Mechanoresponses in mesenchymal stem cells (MSCs) guide both differentiation and function. In this review, we focus on advances in0 our understanding of how the cytoplasmic cytoskeleton, nuclear envelope and nucleoskeleton, which are connected via LINC (Linker of Nucleoskeleton and Cytoskeleton) complexes, are emerging as an integrated dynamic signaling platform to regulate MSC mechanobiology. This dynamic interconnectivity affects mechanical signaling and transfer of signals into the nucleus. In this way, nuclear and LINC-mediated cytoskeletal connectivity play a critical role in maintaining mechanical signaling that affects MSC fate by serving as both mechanosensory and mechanoresponsive structures. We review disease and age related compromises of LINC complexes and nucleoskeleton that contribute to the etiology of musculoskeletal diseases. Finally we invite the idea that acquired dysfunctions of LINC might be a contributing factor to conditions such as aging, microgravity and osteoporosis and discuss potential mechanical strategies to modulate LINC connectivity to combat these conditions.

Nuclear mechanotransduction: forcing the nucleus to respond

Cell phenotype and fate are driven by the mechanical properties of their surrounding environment. Changes in matrix rigidity or application of force have been shown to impact profoundly cell behavior and phenotype, demonstrating that the molecular mechanisms which "sense" and transduce these signals into biochemical pathways are central in cell biology. In this commentary, we discuss recent evidence showing that mechanotransduction mechanisms occur in the nucleus, allowing dynamic regulation of the nucleoskeleton in response to mechanical stress. We will review this nucleoskeletal response and its impact on both nuclear structure and function.

Hutchinson-Gilford progeria syndrome as a model for vascular aging

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disorder caused by a de novo genetic mutation that leads to the accumulation of a splicing isoform of lamin A termed progerin. Progerin expression alters the organization of the nuclear lamina and chromatin. The life expectancy of HGPS patients is severely reduced due to critical cardiovascular defects. Progerin also accumulates in an age-dependent manner in the vascular cells of adults that do not carry genetic mutations associated with HGPS. The molecular mechanisms that lead to vascular dysfunction in HGPS may therefore also play a role in vascular aging. The vascular phenotypic and molecular changes observed in HGPS are strikingly similar to those seen with age, including increased senescence, altered mechanotransduction and stem cell exhaustion. This article discusses the similarities and differences between age-dependent and HGPS-related vascular aging to highlight the relevance of HGPS as a model for vascular aging. Induced pluripotent stem cells derived from HGPS patients are suggested as an attractive model to study vascular aging in order to develop novel approaches to treat cardiovascular disease.

Nuclear stiffening inhibits migration of invasive melanoma cells

During metastasis, melanoma cells must be sufficiently deformable to squeeze through extracellular barriers with small pore sizes. We visualize and quantify deformability of single cells using micropipette aspiration and examine the migration potential of a population of melanoma cells using a flow migration apparatus. We artificially stiffen the nucleus with recombinant overexpression of Δ50 lamin A, which is found in patients with Hutchison Gilford progeria syndrome and in aged individuals. Melanoma cells, both WM35 and Lu1205, both show reduced nuclear deformability and reduced cell invasion with the expression of Δ50 lamin A. These studies suggest that cellular aging including expression of Δ50 lamin A and nuclear stiffening may reduce the potential for metastatic cancer migration. Thus, the pathway of cancer metastasis may be kept in check by mechanical factors in addition to known chemical pathway regulation.

Nanomechanics of vascular endothelium

The mechanical characteristics of endothelial cells reveal four distinct compartments, namely glycocalyx, cell cortex, cytoplasm and nucleus. There is accumulating evidence that endothelial nanomechanics of these individual compartments control vascular physiology. Depending on protein composition, filament formation and interaction with cross-linker proteins, these four compartments determine endothelial stiffness. Structural organization and mechanical properties directly influence physiological processes such as endothelial barrier function, nitric oxide release and gene expression. This review will focus on endothelial nanomechanics and its impact on vascular function.

Isolated nuclei adapt to force and reveal a mechanotransduction pathway in the nucleus

Mechanical forces influence many aspects of cell behaviour. Forces are detected and transduced into biochemical signals by force-bearing molecular elements located at the cell surface, in adhesion complexes or in cytoskeletal structures. The nucleus is physically connected to the cell surface through the cytoskeleton and the linker of nucleoskeleton and cytoskeleton (LINC) complex, allowing rapid mechanical stress transmission from adhesions to the nucleus. Although it has been demonstrated that nuclei experience force, the direct effect of force on the nucleus is not known. Here we show that isolated nuclei are able to respond to force by adjusting their stiffness to resist the applied tension. Using magnetic tweezers, we found that applying force on nesprin-1 triggers nuclear stiffening that does not involve chromatin or nuclear actin, but requires an intact nuclear lamina and emerin, a protein of the inner nuclear membrane. Emerin becomes tyrosine phosphorylated in response to force and mediates the nuclear mechanical response to tension. Our results demonstrate that mechanotransduction is not restricted to cell surface receptors and adhesions but can occur in the nucleus.

Nuclear forces and cell mechanosensing

Cells respond to mechanical signals, but the subcellular mechanisms are not well understood. The nucleus has recently emerged as an important mechanosensory organelle in the cell, as it is intimately connected to the cytoskeleton. Mechanical forces applied to cells that act on membrane-embedded receptors are transmitted through the cytoskeleton to the nuclear surface. Interfering with linkers of the nucleus to the cytoskeleton causes defects in cell mechanosensing and cell function. In this chapter, we discuss recent work in this area, highlighting the role that the nuclear linkages with the cytoskeleton play in cellular mechanotransduction.

Mechanics of the nucleus

The nucleus is the distinguishing feature of eukaryotic cells. Until recently, it was often considered simply as a unique compartment containing the genetic information of the cell and associated machinery, without much attention to its structure and mechanical properties. This article provides compelling examples that illustrate how specific nuclear structures are associated with important cellular functions, and how defects in nuclear mechanics can cause a multitude of human diseases. During differentiation, embryonic stem cells modify their nuclear envelope composition and chromatin structure, resulting in stiffer nuclei that reflect decreased transcriptional plasticity. In contrast, neutrophils have evolved characteristic lobulated nuclei that increase their physical plasticity, enabling passage through narrow tissue spaces in their response to inflammation. Research on diverse cell types further demonstrates how induced nuclear deformations during cellular compression or stretch can modulate cellular function. Pathological examples of disturbed nuclear mechanics include the many diseases caused by mutations in the nuclear envelope proteins lamin A/C and associated proteins, as well as cancer cells that are often characterized by abnormal nuclear morphology. In this article, we will focus on determining the functional relationship between nuclear mechanics and cellular (dys-)function, describing the molecular changes associated with physiological and pathological examples, the resulting defects in nuclear mechanics, and the effects on cellular function. New insights into the close relationship between nuclear mechanics and cellular organization and function will yield a better understanding of normal biology and will offer new clues into therapeutic approaches to the various diseases associated with defective nuclear mechanics.

LMNA-associated cardiocutaneous progeria: an inherited autosomal dominant premature aging syndrome with late onset

Hutchinson-Gilford Progeria Syndrome (HGPS) is a premature aging disorder caused by mutations in LMNA, which encodes the nuclear scaffold proteins lamin A and C. In HGPS and related progerias, processing of prelamin A is blocked at a critical step mediated by the zinc metalloprotease ZMPSTE24. LMNA-linked progerias can be grouped into two classes: (1) the processing-deficient, early onset "typical" progerias (e.g., HGPS), and (2) the processing-proficient "atypical" progeria syndromes (APS) that are later in onset. Here we describe a previously unrecognized progeria syndrome with prominent cutaneous and cardiovascular manifestations belonging to the second class. We suggest the name LMNA-associated cardiocutaneous progeria syndrome (LCPS) for this disorder. Affected patients are normal at birth but undergo progressive cutaneous changes in childhood and die in middle age of cardiovascular complications, including accelerated atherosclerosis, calcific valve disease, and cardiomyopathy. In addition, the proband demonstrated cancer susceptibility, a phenotype rarely described for LMNA-based progeria disorders. The LMNA mutation that caused LCPS in this family is a heterozygous c.899A>G (p.D300G) mutation predicted to alter the coiled-coil domain of lamin A/C. In skin fibroblasts isolated from the proband, the processing and levels of lamin A and C are normal. However, nuclear morphology is aberrant and rescued by treatment with farnesyltransferase inhibitors, as is also the case for HGPS and other laminopathies. Our findings advance knowledge of human LMNA progeria syndromes, and raise the possibility that typical and atypical progerias may converge upon a common mechanism to cause premature aging disease.

Force-induced changes in subnuclear movement and rheology

Extracellular mechanical forces result in changes in gene expression, but it is unclear how cells are able to permanently adapt to new mechanical environments because chemical signaling pathways are short-lived. We visualize force-induced changes in nuclear rheology to examine short- and long-time genome organization and movements. Punctate labels in the nuclear interior of HeLa, human umbilical vein endothelial, and osteosarcoma (Saos-2) cells allow tracking of nuclear movements in cells under varying levels of shear and compressive force. Under adequate shear stress two distinct regimes develop in cells under mechanical stimulation: an initial event of increased intranuclear movement followed by a regime of intranuclear movements that reflect the dose of applied force. At early times there is a nondirectionally oriented response with a small increase in nuclear translocations. After 30 min, there is a significant increase in nuclear movements, which scales with the amount of shear or compressive stress. The similarities in the nuclear response to shear and compressive stress suggest that the nucleus is a mechanosensitive element within the cell. Thus, applied extracellular forces stimulate intranuclear movements, resulting in repositioning of nuclear bodies and the associated chromatin within the nucleus.

Mechanotransduction in embryonic vascular development

A plethora of biochemical signals provides spatial and temporal cues that carefully orchestrate the complex process of vertebrate embryonic development. The embryonic vasculature develops not only in the context of these biochemical cues, but also in the context of the biomechanical forces imparted by blood flow. In the mature vasculature, different blood flow regimes induce distinct genetic programs, and significant progress has been made toward understanding how these forces are perceived by endothelial cells and transduced into biochemical signals. However, it cannot be assumed that paradigms that govern the mature vasculature are pertinent to the developing embryonic vasculature. The embryonic vasculature can respond to the mechanical forces of blood flow, and these responses are critical in vascular remodeling, certain aspects of sprouting angiogenesis, and maintenance of arterial-venous identity. Here, we review data regarding mechanistic aspects of endothelial cell mechanotransduction, with a focus on the response to shear stress, and elaborate upon the multifarious effects of shear stress on the embryonic vasculature. In addition, we discuss emerging predictive vascular growth models and highlight the prospect of combining signaling pathway information with computational modeling. We assert that correlation of precise measurements of hemodynamic parameters with effects on endothelial cell gene expression and cell behavior is required for fully understanding how blood flow-induced loading governs normal vascular development and shapes congenital cardiovascular abnormalities.

Mechanical regulation of nuclear structure and function

Mechanical loading induces both nuclear distortion and alterations in gene expression in a variety of cell types. Mechanotransduction is the process by which extracellular mechanical forces can activate a number of well-studied cytoplasmic signaling cascades. Inevitably, such signals are transduced to the nucleus and induce transcription factor-mediated changes in gene expression. However, gene expression also can be regulated through alterations in nuclear architecture, providing direct control of genome function. One putative transduction mechanism for this phenomenon involves alterations in nuclear architecture that result from the mechanical perturbation of the cell. This perturbation is associated with direct mechanical strain or osmotic stress, which is transferred to the nucleus. This review describes the current state of knowledge relating the nuclear architecture and the transfer of mechanical forces to the nucleus mediated by the cytoskeleton, the nucleoskeleton, and the LINC (linker of the nucleoskeleton and cytoskeleton) complex. Moreover, remodeling of the nucleus induces alterations in nuclear stiffness, which may be associated with cell differentiation. These phenomena are discussed in relation to the potential influence of nuclear architecture-mediated mechanoregulation of transcription and cell fate.

Computational image analysis of nuclear morphology associated with various nuclear-specific aging disorders

Computational image analysis is used in many areas of biological and medical research, but advanced techniques including machine learning remain underutilized. Here, we used automated segmentation and shape analyses, with pre-defined features and with computer generated components, to compare nuclei from various premature aging disorders caused by alterations in nuclear proteins. We considered cells from patients with Hutchinson-Gilford progeria syndrome (HGPS) with an altered nucleoskeletal protein; a mouse model of XFE progeroid syndrome caused by a deficiency of ERCC1-XPF DNA repair nuclease; and patients with Werner syndrome (WS) lacking a functional WRN exonuclease and helicase protein. Using feature space analysis, including circularity, eccentricity, and solidity, we found that XFE nuclei were larger and significantly more elongated than control nuclei. HGPS nuclei were smaller and rounder than the control nuclei with features suggesting small bumps. WS nuclei did not show any significant shape changes from control. We also performed principle component analysis (PCA) and a geometric, contour based metric. PCA allowed direct visualization of morphological changes in diseased nuclei, whereas standard, feature-based approaches required pre-defined parameters and indirect interpretation of multiple parameters. Both methods yielded similar results, but PCA proves to be a powerful pre-analysis methodology for unknown systems.

The interaction between nanoscale surface features and mechanical loading and its effect on osteoblast-like cells behavior

Osteoblasts respond to mechanical stimulation by changing morphology, gene expression and matrix mineralization. Introducing surface topography on biomaterials, independently of mechanical loading, has been reported to give similar effects. In the current study, using a nanotextured surface, and mechanical loading, we aimed to develop a multi-factorial model in which both parameters interact. Mechanical stimulation to osteoblast-like cells was applied by longitudinal stretch in parallel direction to the nanotexture (300 nm wide and 60 nm deep grooves), with frequency of 1 Hz and stretch magnitude varying from 1% to 8%. Scanning electron microscopy showed that osteoblast-like cells subjected to mechanical loading oriented perpendicularly to the stretch direction. When cultured on nanotextured surfaces, cells aligned parallel to the texture. However, the parallel cell direction to the nanotextured surface was lost and turned to perpendicular when parallel stretch to the nanotexture, greater than 3% was applied to the cells. This phenomenon could not be achieved when a texture with micro-sized dimensions was used. Moreover, a significant synergistic effect on upregulation of fibronectin and Cfba was observed when dual stimulation was used. These findings can lead to a development of new biomimetic materials that can guide morphogenesis in tissue repair and bone remodeling.

Mechanobiology and the microcirculation: cellular, nuclear and fluid mechanics

Endothelial cells are stimulated by shear stress throughout the vasculature and respond with changes in gene expression and by morphological reorganization. Mechanical sensors of the cell are varied and include cell surface sensors that activate intracellular chemical signaling pathways. Here, possible mechanical sensors of the cell including reorganization of the cytoskeleton and the nucleus are discussed in relation to shear flow. A mutation in the nuclear structural protein lamin A, related to Hutchinson-Gilford progeria syndrome, is reviewed specifically as the mutation results in altered nuclear structure and stiffer nuclei; animal models also suggest significantly altered vascular structure. Nuclear and cellular deformation of endothelial cells in response to shear stress provides partial understanding of possible mechanical regulation in the microcirculation. Increasing sophistication of fluid flow simulations inside the vessel is also an emerging area relevant to the microcirculation as visualization in situ is difficult. This integrated approach to study--including medicine, molecular and cell biology, biophysics and engineering--provides a unique understanding of multi-scale interactions in the microcirculation.

Tissue stretch induces nuclear remodeling in connective tissue fibroblasts

Studies in cultured cells have shown that nuclear shape is an important factor influencing nuclear function, and that mechanical forces applied to the cell can directly affect nuclear shape. In a previous study, we demonstrated that stretching of whole mouse subcutaneous tissue causes dynamic cytoskeletal remodeling with perinuclear redistribution of alpha-actin in fibroblasts within the tissue. We have further shown that the nuclei of these fibroblasts have deep invaginations containing alpha-actin. In the current study, we hypothesized that tissue stretch would cause nuclear remodeling with a reduced amount of nuclear invagination, measurable as a change in nuclear concavity. Subcutaneous areolar connective tissue samples were excised from 28 mice and randomized to either tissue stretch or no stretch for 30 min, then examined with histochemistry and confocal microscopy. In stretched tissue (vs. non-stretched), fibroblast nuclei had a larger cross-sectional area (P < 0.001), smaller thickness (P < 0.03) in the plane of the tissue, and smaller relative concavity (P < 0.005) indicating an increase in nuclear convexity. The stretch-induced loss of invaginations may have important influences on gene expression, RNA trafficking and/or cell differentiation.

Intermediate filaments take the heat as stress proteins

Intermediate filament (IF) proteins and heat shock proteins (HSPs) are large multimember families that share several features, including protein abundance, significant upregulation in response to a variety of stresses, cytoprotective functions, and the phenocopying of several human diseases after IF protein or HSP mutation. We are now coming to understand that these common elements point to IFs as important cellular stress proteins with some roles akin to those already well-characterized for HSPs. Unique functional roles for IFs include protection from mechanical stress, whereas HSPs are characteristically involved in protein folding and as chaperones. Shared IF and HSP cytoprotective roles include inhibition of apoptosis, organelle homeostasis, and scaffolding. In this report, we review data that corroborate the view that IFs function as highly specialized cytoskeletal stress proteins that promote cellular organization and homeostasis.