Deciphering Nuclear Mechanobiology in Laminopathy

Microfluidic device reveals new insights into impairment of neutrophil transmigration in patients with sepsis

Neutrophils need to migrate through tight tissue spaces to eliminate pathogens, but their movement is often hindered by their large and stiff nuclei. Neutrophil migration is impaired in sepsis patients, but it is unclear whether this defect is related to the deformability of their nuclei. Herein, we designed microfluidic devices with micron-scale narrow slits to simulate biological barriers. This setup allowed us to observe and record neutrophil movement and nuclear deformation in real-time. We also developed a method for morphological analysis to quantify nucleus deformation in numerous individual cells. Our studies showed that neutrophils from healthy individuals could adjust their nuclear shape to squeeze through these constrictions, whereas those from sepsis patients demonstrated less flexibility. Neutrophils with rigid nuclei struggled to pass through narrow gaps and were more likely to rupture under pressure. These findings suggest that the migration defects of neutrophils observed in sepsis may be attributed to the inability of neutrophils to deform their nuclei, highlighting the crucial role of microfluidic technologies in offering new insights into migration defects under pathological conditions.

Enhanced cell viscosity: A new phenotype associated with lamin A/C alterations

Lamin A/C is a well-established key contributor to nuclear stiffness and its role in nucleus mechanical properties has been extensively studied. However, its impact on whole-cell mechanics has been poorly addressed, particularly concerning measurable physical parameters. In this study, we combined microfluidic experiments with theoretical analyses to quantitatively estimate the whole-cell mechanical properties. This allowed us to characterize the mechanical changes induced in cells by lamin A/C alterations and prelamin A accumulation resulting from atazanavir treatment or lipodystrophy-associated LMNA R482W pathogenic variant. Our results reveal a distinctive increase in long-time viscosity as a signature of cells affected by lamin A/C alterations. Furthermore, they show that the whole-cell response to mechanical stress is driven not only by the nucleus but also by the nucleo-cytoskeleton links and the microtubule network. The enhanced cell viscosity assessed with our microfluidic assay could serve as a valuable diagnosis marker for lamin-related diseases.

Nuclear pore complex and nucleocytoplasmic transport disruption in neurodegeneration

Nuclear pore complexes (NPCs) play a critical role in maintaining the equilibrium between the nucleus and cytoplasm, enabling bidirectional transport across the nuclear envelope, and are essential for proper nuclear organization and gene regulation. Perturbations in the regulatory mechanisms governing NPCs and nuclear envelope homeostasis have been implicated in the pathogenesis of several neurodegenerative diseases. The ESCRT-III pathway emerges as a critical player in the surveillance and preservation of well-assembled, functional NPCs, as well as nuclear envelope sealing. Recent studies have provided insights into the involvement of nuclear ESCRT-III in the selective reduction of specific nucleoporins associated with neurodegenerative pathologies. Thus, maintaining quality control of the nuclear envelope and NPCs represents a pivotal element in the pathological cascade leading to neurodegenerative diseases. This review describes the constituents of the nuclear-cytoplasmic transport machinery, encompassing the nuclear envelope, NPC, and ESCRT proteins, and how their structural and functional alterations contribute to the development of neurodegenerative diseases.

Centripetal nuclear shape fluctuations associate with chromatin condensation in early prophase

The nucleus plays a central role in several key cellular processes, including chromosome organisation, DNA replication and gene transcription. Recent work suggests an association between nuclear mechanics and cell-cycle progression, but many aspects of this connection remain unexplored. Here, by monitoring nuclear shape fluctuations at different cell cycle stages, we uncover increasing inward fluctuations in late G2 and in early prophase, which are initially transient, but develop into instabilities when approaching the nuclear-envelope breakdown. We demonstrate that such deformations correlate with chromatin condensation by perturbing both the chromatin and the cytoskeletal structures. We propose that the contrasting forces between an extensile stress and centripetal pulling from chromatin condensation could mechanically link chromosome condensation with nuclear-envelope breakdown, two main nuclear processes occurring during mitosis.

Progerinin, an Inhibitor of Progerin, Alleviates Cardiac Abnormalities in a Model Mouse of Hutchinson-Gilford Progeria Syndrome

Hutchinson-Gilford Progeria Syndrome (HGPS) is an ultra-rare human premature aging disorder that precipitates death because of cardiac disease. Almost all cases of HGPS are caused by aberrant splicing of the LMNA gene that results in the production of a mutant Lamin A protein termed progerin. In our previous study, treatment with Progerinin has been shown to reduce progerin expression and improve aging phenotypes in vitro and in vivo HGPS models. In this record, cardiac parameters (stroke volume (SV), ejection fraction (EF), fractional shortening (FS), etc.) were acquired in Lmna^WT/WT and Lmna^G609G/WT mice fed with either a vehicle diet or a Progerinin diet by echocardiography (from 38 weeks to 50 weeks at various ages), and then the cardiac function was analyzed. We also acquired the tissue samples and blood serum of Lmna^WT/WT and Lmna^G609G/WT mice for pathological analysis at the end of echocardiography. From these data, we suggest that the administration of Progerinin in the HGPS model mouse can restore cardiac function and correct arterial abnormalities. These observations provide encouraging evidence for the efficacy of Progerinin for cardiac dysfunction in HGPS.

Allele pairing at Sun1-enriched domains at the nuclear periphery via T1A3 tandem DNA repeats

Spatiotemporal gene regulation is fundamental to the biology of diploid cells. Therefore, effective communication between two alleles and their geometry in the nucleus is important. However, the mechanism that fine-tunes the expression from each of the two alleles of an autosome is enigmatic. Establishing an allele-specific gene expression visualization system in living cells, we show that alleles of biallelically expressed Cth and Ttc4 genes are paired prior to acquiring monoallelic expression. We found that active alleles of monoallelic genes are preferentially localized at Sun1-enriched domains at the nuclear periphery. These peripherally localized active DNA loci are enriched with adenine-thymidine-rich tandem repeats that interact with Hnrnpd and reside in a Hi-C-defined A compartment within the B compartment. Our results demonstrate the biological significance of T ₁ A ₃ tandem repeat sequences in genome organization and how the regulation of gene expression, at the level of individual alleles, relates to their spatial arrangement.

Combined alteration of lamin and nuclear morphology influences the localization of the tumor-associated factor AKTIP

Background

Lamins, key nuclear lamina components, have been proposed as candidate risk biomarkers in different types of cancer but their accuracy is still debated. AKTIP is a telomeric protein with the property of being enriched at the nuclear lamina. AKTIP has similarity with the tumor susceptibility gene TSG101. AKTIP deficiency generates genome instability and, in p53^−/− mice, the reduction of the mouse counterpart of AKTIP induces the exacerbation of lymphomas. Here, we asked whether the distribution of AKTIP is altered in cancer cells and whether this is associated with alterations of lamins.

Methods

We performed super-resolution imaging, quantification of lamin expression and nuclear morphology on HeLa, MCF7, and A549 tumor cells, and on non-transformed fibroblasts from healthy donor and HGPS (LMNA c.1824C > T p.Gly608Gly) and EDMD2 (LMNA c.775 T > G) patients. As proof of principle model combining a defined lamin alteration with a tumor cell setting, we produced HeLa cells exogenously expressing the HGPS lamin mutant progerin that alters nuclear morphology.

Results

In HeLa cells, AKTIP locates at less than 0.5 µm from the nuclear rim and co-localizes with lamin A/C. As compared to HeLa, there is a reduced co-localization of AKTIP with lamin A/C in both MCF7 and A549. Additionally, MCF7 display lower amounts of AKTIP at the rim. The analyses in non-transformed fibroblasts show that AKTIP mislocalizes in HGPS cells but not in EDMD2. The integrated analysis of lamin expression, nuclear morphology, and AKTIP topology shows that positioning of AKTIP is influenced not only by lamin expression, but also by nuclear morphology. This conclusion is validated by progerin-expressing HeLa cells in which nuclei are morphologically altered and AKTIP is mislocalized.

Conclusions

Our data show that the combined alteration of lamin and nuclear morphology influences the localization of the tumor-associated factor AKTIP. The results also point to the fact that lamin alterations per se are not predictive of AKTIP mislocalization, in both non-transformed and tumor cells. In more general terms, this study supports the thesis that a combined analytical approach should be preferred to predict lamin-associated changes in tumor cells. This paves the way of next translational evaluation to validate the use of this combined analytical approach as risk biomarker.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02480-5.

Palmdelphin Regulates Nuclear Resilience to Mechanical Stress in the Endothelium

Supplemental Digital Content is available in the text.

PALMD (palmdelphin) belongs to the family of paralemmin proteins implicated in cytoskeletal regulation. Single nucleotide polymorphisms in the PALMD locus that result in reduced expression are strong risk factors for development of calcific aortic valve stenosis and predict severity of the disease.

Thiolene- and Polycaprolactone Methacrylate-Based Polymerized High Internal Phase Emulsion (PolyHIPE) Scaffolds for Tissue Engineering

Highly porous emulsion templated polymers (PolyHIPEs) provide a number of potential advantages in the fabrication of scaffolds for tissue engineering and regenerative medicine. Porosity enables cell ingrowth and nutrient diffusion within, as well as waste removal from, the scaffold. The properties offered by emulsion templating alone include the provision of high interconnected porosity, and, in combination with additive manufacturing, the opportunity to introduce controlled multiscale porosity to complex or custom structures. However, the majority of monomer systems reported for PolyHIPE preparation are unsuitable for clinical applications as they are nondegradable. Thiol-ene chemistry is a promising route to produce biodegradable photocurable PolyHIPEs for the fabrication of scaffolds using conventional or additive manufacturing methods; however, relatively little research has been reported on this approach. This study reports the groundwork to fabricate thiol- and polycaprolactone (PCL)-based PolyHIPE materials via a photoinitiated thiolene click reaction. Two different formulations, either three-arm PCL methacrylate (3PCLMA) or four-arm PCL methacrylate (4PCLMA) moieties, were used in the PolyHIPE formulation. Biocompatibility of the PolyHIPEs was investigated using human dermal fibroblasts (HDFs) and human osteosarcoma cell line (MG-63) by DNA quantification assay, and developed PolyHIPEs were shown to be capable of supporting cell attachment and viability.

Compromised Biomechanical Properties, Cell-Cell Adhesion and Nanotubes Communication in Cardiac Fibroblasts Carrying the Lamin A/C D192G Mutation

Clinical effects induced by arrhythmogenic cardiomyopathy (ACM) originate from a large spectrum of genetic variations, including the missense mutation of the lamin A/C gene (LMNA), LMNA D192G. The aim of our study was to investigate the biophysical and biomechanical impact of the LMNA D192G mutation on neonatal rat ventricular fibroblasts (NRVF). The main findings in mutated NRVFs were: (i) cytoskeleton disorganization (actin and intermediate filaments); (ii) decreased elasticity of NRVFs; (iii) altered cell-cell adhesion properties, that highlighted a strong effect on cellular communication, in particular on tunneling nanotubes (TNTs). In mutant-expressing fibroblasts, these nanotubes were weakened with altered mechanical properties as shown by atomic force microscopy (AFM) and optical tweezers. These outcomes complement prior investigations on LMNA mutant cardiomyocytes and suggest that the LMNA D192G mutation impacts the biomechanical properties of both cardiomyocytes and cardiac fibroblasts. These observations could explain how this mutation influences cardiac biomechanical pathology and the severity of ACM in LMNA-cardiomyopathy.

Advanced mechanotherapy: Biotensegrity for governing metastatic tumor cell fate via modulating the extracellular matrix

Mechano-transduction is the procedure of mechanical stimulus translation via cells, among substrate shear flow, topography, and stiffness into a biochemical answer. TAZ and YAP are transcriptional coactivators which are recognized as relay proteins that promote mechano-transduction within the Hippo pathway. With regard to healthy cells in homeostasis, mechano-transduction regularly restricts proliferation, and TAZ and YAP are totally inactive. During cancer development a YAP/TAZ - stimulating positive response loop is formed between the growing tumor and the stiffening ECM. As tumor developments, local stromal and cancerous cells take advantage of mechanotransduction to enhance proliferation, induce their migratory into remote tissues, and promote chemotherapeutic resistance. As a newly progresses paradigm, nanoparticle-conjunctions (such as magnetic nanoparticles, and graphene derivatives nanoparticles) hold significant promises for remote regulation of cells and their relevant events at molecular scale. Despite outstanding developments in employing nanoparticles for drug targeting studies, the role of nanoparticles on cellular behaviors (proliferation, migration, and differentiation) has still required more evaluations in the field of mechanotherapy. In this paper, the in-depth contribution of mechano-transduction is discussed during tumor progression, and how these consequences can be evaluated in vitro.

Interphase microtubules in nuclear organization and genome maintenance

Microtubules are major cytoskeletal components mediating fundamental cellular processes, including cell division. Recent evidence suggests that microtubules also regulate the nucleus during the cell cycle's interphase stage. Deciphering such roles of microtubules should uncover direct crosstalk between the nucleus and cytoplasm, impacting genome function and organismal health. Here, we review emerging roles for microtubules in interphase genome regulation. We explore how microtubules exert cytoplasmic forces on the nucleus or transport molecular cargo, including DNA, into or within the nucleus. We also describe how microtubules perform these functions by establishing transient or stable connections with nuclear envelope elements. Lastly, we discuss how the regulation of the nucleus by microtubules impacts genome organization and repair. Together, the literature indicates that interphase microtubules are critical regulators of nuclear structure and genome stability.

Escherichia coli vacuolating factor, involved in avian cellulitis, induces actin contraction and binds to cytoskeleton proteins in fibroblasts

Background

Avian pathogenic Escherichia coli (APEC) isolated from avian cellulitis lesions produces a toxin, named Escherichia coli vacuolating factor (ECVF), that causes cell vacuolization and induces inflammatory response in broiler chicken.

Methods

We investigated the intracellular activities of ECVF in avian fibroblasts using fluorescence staining, electron microscopy, MTT and LDH measurements. As ECVF act specifically in avian cells, we performed blotting assay followed by mass spectrometry to better understand its initial intracellular protein recognition.

Results

ECVF induced actin contraction, mitochondrial damage and membrane permeability alterations. Ultrastructural analysis showed intracellular alterations, as nuclear lobulation and the presence of degraded structures inside the vacuoles. Moreover, ECVF induced cell death in fibroblasts. ECVF-biotin associates to at least two proteins only in avian cell lysates: alpha-actinin 4 and vinculin, both involved in cytoskeleton structure.

Conclusion

These findings demonstrated that ECVF plays an important role in avian cellulitis, markedly in initial steps of infection. Taken together, the results place this toxin as a target for drug and/or vaccine development, instead of the use of large amounts antibiotics.

Estimation of three-dimensional chromatin morphology for nuclear classification and characterisation

Classification and characterisation of cellular morphological states are vital for understanding cell differentiation, development, proliferation and diverse pathological conditions. As the onset of morphological changes transpires following genetic alterations in the chromatin configuration inside the nucleus, the nuclear texture as one of the low-level properties if detected and quantified accurately has the potential to provide insights on nuclear organisation and enable early diagnosis and prognosis. This study presents a three dimensional (3D) nuclear texture description method for cell nucleus classification and variation measurement in chromatin patterns on the transition to another phenotypic state. The proposed approach includes third plane information using hyperplanes into the design of the Sorted Random Projections (SRP) texture feature and is evaluated on publicly available 3D image datasets of human fibroblast and human prostate cancer cell lines obtained from the Statistics Online Computational Resource. Results show that 3D SRP and 3D Local Binary Pattern provide better classification results than other feature descriptors. In addition, the proposed metrics based on 3D SRP validate the change in intensity and aggregation of heterochromatin on transition to another state and characterise the intermediate and ultimate phenotypic states.

Cell-ECM contact-guided intracellular polarization is mediated via lamin A/C dependent nucleus-cytoskeletal connection

Cell polarization plays a crucial role in dynamic cellular events, such as cell proliferation, differentiation, and directional migration in response to diverse extracellular and intracellular signals. Although it is well known that cell polarization entails highly orchestrated intracellular molecular reorganization, the underlying mechanism of repositioning by intracellular organelles in the presence of multiple stimuli is still unclear. Here, we show that front-rear cell polarization based on the relative positions of nucleus and microtubule organizing center is precisely controlled by mechanical interactions including cellular adhesion to extracellular matrix and nucleus-cytoskeletal connections. By modulating the size and distribution of fibronectin-coated adhesive spots located in the polarized cell shape mimicking micropatterns, we monitored the alterations in cell polarity. We found that the localization of individual adhesive spots is more dominant than the cell shape itself to induce intracellular polarization. Further, the degree of cell polarization was diminished significantly by disrupting nuclear lamin A/C. We further confirm that geometrical cue-guided intracellular polarization determines directional cell migration via local activation of Cdc42. These findings provide novel insights into the role of nucleus-cytoskeletal connections in single cell polarization under a combination of physical, molecular, and genetic cues, where lamin A/C acts as a critical molecular mediator in ECM sensing and signal transduction via nucleus-cytoskeletal connection.

De Novo Variants in LMNB1 Cause Pronounced Syndromic Microcephaly and Disruption of Nuclear Envelope Integrity

Lamin B1 plays an important role in the nuclear envelope stability, the regulation of gene expression, and neural development. Duplication of LMNB1, or missense mutations increasing LMNB1 expression, are associated with autosomal-dominant leukodystrophy. On the basis of its role in neurogenesis, it has been postulated that LMNB1 variants could cause microcephaly. Here, we confirm this hypothesis with the identification of de novo mutations in LMNB1 in seven individuals with pronounced primary microcephaly (ranging from -3.6 to -12 SD) associated with relative short stature and variable degree of intellectual disability and neurological features as the core symptoms. Simplified gyral pattern of the cortex and abnormal corpus callosum were noted on MRI of three individuals, and these individuals also presented with a more severe phenotype. Functional analysis of the three missense mutations showed impaired formation of the LMNB1 nuclear lamina. The two variants located within the head group of LMNB1 result in a decrease in the nuclear localization of the protein and an increase in misshapen nuclei. We further demonstrate that another mutation, located in the coil region, leads to increased frequency of condensed nuclei and lower steady-state levels of lamin B1 in proband lymphoblasts. Our findings collectively indicate that de novo mutations in LMNB1 result in a dominant and damaging effect on nuclear envelope formation that correlates with microcephaly in humans. This adds LMNB1 to the growing list of genes implicated in severe autosomal-dominant microcephaly and broadens the phenotypic spectrum of the laminopathies.

Loss of MTX2 causes mandibuloacral dysplasia and links mitochondrial dysfunction to altered nuclear morphology

Mandibuloacral dysplasia syndromes are mainly due to recessive LMNA or ZMPSTE24 mutations, with cardinal nuclear morphological abnormalities and dysfunction. We report five homozygous null mutations in MTX2, encoding Metaxin-2 (MTX2), an outer mitochondrial membrane protein, in patients presenting with a severe laminopathy-like mandibuloacral dysplasia characterized by growth retardation, bone resorption, arterial calcification, renal glomerulosclerosis and severe hypertension. Loss of MTX2 in patients' primary fibroblasts leads to loss of Metaxin-1 (MTX1) and mitochondrial dysfunction, including network fragmentation and oxidative phosphorylation impairment. Furthermore, patients' fibroblasts are resistant to induced apoptosis, leading to increased cell senescence and mitophagy and reduced proliferation. Interestingly, secondary nuclear morphological defects are observed in both MTX2-mutant fibroblasts and mtx-2-depleted C. elegans. We thus report the identification of a severe premature aging syndrome revealing an unsuspected link between mitochondrial composition and function and nuclear morphology, establishing a pathophysiological link with premature aging laminopathies and likely explaining common clinical features.

Biological Aging Modulates Cell Migration via Lamin A/C-Dependent Nuclear Motion

Aging is a progressive functional decline in organs and tissues over time and typically represents the accumulation of psychological and social changes in a human being. Diverse diseases, such as cardiovascular, musculoskeletal, and neurodegenerative disorders, are now understood to be caused by aging. While biological assessment of aging mainly focuses on the gradual changes that occur either on the molecular scale, for example, alteration of gene expression and epigenetic modification, or on larger scales, for example, changes in muscle strength and cardiac function, the mechanics that regulates the behavior of individual cells and interactions between the internal elements of cells, are largely missing. In this study, we show that the dynamic features of migrating cells across different human ages could help to establish the underlying mechanism of biological age-dependent cellular functional decline. To determine the relationship between cellular dynamics and human age, we identify the characteristic relationship between cell migration and nuclear motion which is tightly regulated by nucleus-bound cytoskeletal organization. This analysis demonstrates that actomyosin contractility-dependent nuclear motion plays a key role in cell migration. We anticipate this study to provide noble biophysical insights on biological aging in order to precisely diagnose age-related chronic diseases.

Molecular and Mechanobiological Pathways Related to the Physiopathology of FPLD2

Laminopathies are rare and heterogeneous diseases affecting one to almost all tissues, as in Progeria, and sharing certain features such as metabolic disorders and a predisposition to atherosclerotic cardiovascular diseases. These two features are the main characteristics of the adipose tissue-specific laminopathy called familial partial lipodystrophy type 2 (FPLD2). The only gene that is involved in FPLD2 physiopathology is the LMNA gene, with at least 20 mutations that are considered pathogenic. LMNA encodes the type V intermediate filament lamin A/C, which is incorporated into the lamina meshwork lining the inner membrane of the nuclear envelope. Lamin A/C is involved in the regulation of cellular mechanical properties through the control of nuclear rigidity and deformability, gene modulation and chromatin organization. While recent studies have described new potential signaling pathways dependent on lamin A/C and associated with FPLD2 physiopathology, the whole picture of how the syndrome develops remains unknown. In this review, we summarize the signaling pathways involving lamin A/C that are associated with the progression of FPLD2. We also explore the links between alterations of the cellular mechanical properties and FPLD2 physiopathology. Finally, we introduce potential tools based on the exploration of cellular mechanical properties that could be redirected for FPLD2 diagnosis.

Mutated lamin A modulates stiffness in muscle cells

The cytoskeleton is a complex network interlinking filaments that extend throughout the cytoplasm from the nucleus to the plasma membrane. Three major types of filaments are found in the cytoskeleton: actin filaments, microtubules, and intermediate filaments. They play a key role in the ability of cells to both resist mechanical stress and generate force. However, the precise involvement of intermediate filament proteins in these processes remains unclear. Here, we focused on nuclear A-type lamins, which are connected to the cytoskeleton via the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex. Using micro-constriction rheology, we investigated the impact of A-type lamins (p.H222P) mutation on the mechanical properties of muscle cells. We demonstrate that the expression of point mutation of lamin A in muscle cells increases cellular stiffness compared with cells expressing wild type lamin A and that the chemical agent selumetinib, an inhibitor of the ERK1/2 signaling, reversed the mechanical alterations in mutated cells. These results highlight the interplay between A-type lamins and mechano-signaling, which are supported by cell biology measurements.

Lamin A/C promotes DNA base excision repair

The A-type lamins (lamin A/C), encoded by the LMNA gene, are important structural components of the nuclear lamina. LMNA mutations lead to degenerative disorders known as laminopathies, including the premature aging disease Hutchinson-Gilford progeria syndrome. In addition, altered lamin A/C expression is found in various cancers. Reports indicate that lamin A/C plays a role in DNA double strand break repair, but a role in DNA base excision repair (BER) has not been described. We provide evidence for reduced BER efficiency in lamin A/C-depleted cells (Lmna null MEFs and lamin A/C-knockdown U2OS). The mechanism involves impairment of the APE1 and POLβ BER activities, partly effectuated by associated reduction in poly-ADP-ribose chain formation. Also, Lmna null MEFs displayed reduced expression of several core BER enzymes (PARP1, LIG3 and POLβ). Absence of Lmna led to accumulation of 8-oxoguanine (8-oxoG) lesions, and to an increased frequency of substitution mutations induced by chronic oxidative stress including GC>TA transversions (a fingerprint of 8-oxoG:A mismatches). Collectively, our results provide novel insights into the functional interplay between the nuclear lamina and cellular defenses against oxidative DNA damage, with implications for cancer and aging.

Keratin intermediate filaments: intermediaries of epithelial cell migration

Migration of epithelial cells is fundamental to multiple developmental processes, epithelial tissue morphogenesis and maintenance, wound healing and metastasis. While migrating epithelial cells utilize the basic acto-myosin based machinery as do other non-epithelial cells, they are distinguished by their copious keratin intermediate filament (KF) cytoskeleton, which comprises differentially expressed members of two large multigene families and presents highly complex patterns of post-translational modification. We will discuss how the unique mechanophysical and biochemical properties conferred by the different keratin isotypes and their modifications serve as finely tunable modulators of epithelial cell migration. We will furthermore argue that KFs together with their associated desmosomal cell-cell junctions and hemidesmosomal cell-extracellular matrix (ECM) adhesions serve as important counterbalances to the contractile acto-myosin apparatus either allowing and optimizing directed cell migration or preventing it. The differential keratin expression in leaders and followers of collectively migrating epithelial cell sheets provides a compelling example of isotype-specific keratin functions. Taken together, we conclude that the expression levels and specific combination of keratins impinge on cell migration by conferring biomechanical properties on any given epithelial cell affecting cytoplasmic viscoelasticity and adhesion to neighboring cells and the ECM.

The role of the cell nucleus in mechanotransduction

Mechanical forces are known to influence cellular processes with consequences at the cellular and physiological level. The cell nucleus is the largest and stiffest organelle, and it is connected to the cytoskeleton for proper cellular function. The connection between the nucleus and the cytoskeleton is in most cases mediated by the linker of nucleoskeleton and cytoskeleton (LINC) complex. Not surprisingly, the nucleus and the associated cytoskeleton are implicated in multiple mechanotransduction pathways important for cellular activities. Herein, we review recent advances describing how the LINC complex, the nuclear lamina, and nuclear pore complexes are involved in nuclear mechanotransduction. We will also discuss how the perinuclear actin cytoskeleton is important for the regulation of nuclear mechanotransduction. Additionally, we discuss the relevance of nuclear mechanotransduction for cell migration, development, and how nuclear mechanotransduction impairment leads to multiple disorders.

Regulation of keratin network dynamics by the mechanical properties of the environment in migrating cells

Keratin intermediate filaments provide mechanical resilience for epithelia. They are nevertheless highly dynamic and turn over continuously, even in sessile keratinocytes. The aim of this study was to characterize and understand how the dynamic behavior of the keratin cytoskeleton is integrated in migrating cells. By imaging human primary keratinocytes producing fluorescent reporters and by using standardized image analysis we detect inward-directed keratin flow with highest rates in the cell periphery. The keratin flow correlates with speed and trajectory of migration. Changes in fibronectin-coating density and substrate stiffness induces concordant changes in migration speed and keratin flow. When keratinocytes are pseudo-confined on stripes, migration speed and keratin flow are reduced affecting the latter disproportionately. The regulation of keratin flow is linked to the regulation of actin flow. Local speed and direction of keratin and actin flow are very similar in migrating keratinocytes with keratin flow lagging behind actin flow. Conversely, reduced actin flow in areas of high keratin density indicates an inhibitory function of keratins on actin dynamics. Together, we propose that keratins enhance persistence of migration by directing actin dynamics and that the interplay of keratin and actin dynamics is modulated by matrix adhesions.

Cell engineering: Biophysical regulation of the nucleus

Cells live in a complex and dynamic microenvironment, and a variety of microenvironmental cues can regulate cell behavior. In addition to biochemical signals, biophysical cues can induce not only immediate intracellular responses, but also long-term effects on phenotypic changes such as stem cell differentiation, immune cell activation and somatic cell reprogramming. Cells respond to mechanical stimuli via an outside-in and inside-out feedback loop, and the cell nucleus plays an important role in this process. The mechanical properties of the nucleus can directly or indirectly modulate mechanotransduction, and the physical coupling of the cell nucleus with the cytoskeleton can affect chromatin structure and regulate the epigenetic state, gene expression and cell function. In this review, we will highlight the recent progress in nuclear biomechanics and mechanobiology in the context of cell engineering, tissue remodeling and disease development.