Cell stretchers and the LINC complex in mechanotransduction

LINC complex alterations are a key feature of sporadic and familial ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder that primarily affects motor neurons, leading to progressive muscle weakness and loss of voluntary muscle control. While the exact cause of ALS is not fully understood, emerging research suggests that dysfunction of the nuclear envelope (NE) may contribute to disease pathogenesis and progression. The NE plays a role in ALS through several mechanisms, including nuclear pore defects, nucleocytoplasmic transport impairment, accumulation of mislocalized proteins, and nuclear morphology abnormalities. The LINC complex is the second biggest multi-protein complex in the NE and consists of the SUN1/2 proteins spanning the inner nuclear membrane and Nesprin proteins embedded in the outer membrane. The LINC complex, by interacting with both the nuclear lamina and the cytoskeleton, transmits mechanical forces to the nucleus regulating its morphology and functional homeostasis. In this study we show extensive alterations to the LINC complex in motor and cortical iPSC-derived neurons and spinal cord organoids carrying the ALS causative mutation in the C9ORF72 gene (C9). Importantly, we show that such alterations are present in vivo in a cohort of sporadic ALS and C9-ALS postmortem spinal cord and motor cortex specimens. We also found that LINC complex disruption strongly correlated with nuclear morphological alterations occurring in ALS neurons, independently of TDP43 mislocalization. Altogether, our data establish morphological and functional alterations to the LINC complex as important events in ALS pathogenic cascade, making this pathway a possible target for both biomarker and therapy development.

Engines of change: Nonmuscle myosin II in mechanobiology

The emergence of mechanobiology has unveiled complex mechanisms by which cells adjust intracellular force production to their needs. Most communicable intracellular forces are generated by myosin II, an actin-associated molecular motor that transforms adenosine triphosphate (ATP) hydrolysis into contraction in nonmuscle and muscle cells. Myosin II-dependent force generation is tightly regulated, and deregulation is associated with specific pathologies. Here, we focus on the role of myosin II (nonmuscle myosin II, NMII) in force generation and mechanobiology. We outline the regulation and molecular mechanism of force generation by NMII, focusing on the actual outcome of contraction, that is, force application to trigger mechanosensitive events or the building of dissipative structures. We describe how myosin II-generated forces drive two major types of events: modification of the cellular morphology and/or triggering of genetic programs, which enhance the ability of cells to adapt to, or modify, their microenvironment. Finally, we address whether targeting myosin II to impair or potentiate its activity at the motor level is a viable therapeutic strategy, as illustrated by recent examples aimed at modulating cardiac myosin II function in heart disease.

LINC complex alterations are a hallmark of sporadic and familial ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder that primarily affects motor neurons, leading to progressive muscle weakness and loss of voluntary muscle control. While the exact cause of ALS is not fully understood, emerging research suggests that dysfunction of the nuclear envelope (NE) may contribute to disease pathogenesis and progression. The NE plays a role in ALS through several mechanisms, including nuclear pore defects, nucleocytoplasmic transport impairment, accumulation of mislocalized proteins, and nuclear morphology abnormalities. The LINC complex is the second biggest multi-protein complex in the NE and consists of the SUN1/2 proteins spanning the inner nuclear membrane and Nesprin proteins embedded in the outer membrane. The LINC complex, by interacting with both the nuclear lamina and the cytoskeleton, transmits mechanical forces to the nucleus regulating its morphology and functional homeostasis. In this study we show extensive alterations to the LINC complex in motor and cortical iPSC-derived neurons and spinal cord organoids carrying the ALS causative mutation in the C9ORF72 gene (C9). Importantly, we show that such alterations are present in vivo in a cohort of sporadic ALS and C9-ALS postmortem spinal cord and motor cortex biopsies. We also found that LINC complex disruption strongly correlated with nuclear morphological alterations occurring in ALS neurons, independently of TDP43 mislocalization. Altogether, our data establish morphological and functional alterations to the LINC complex as important events in ALS pathogenic cascade, making this pathway a possible target for both biomarker and therapy development.

Mechanotransduction and epigenetic modulations of chromatin: Role of mechanical signals in gene regulation

Mechanical forces may be generated within a cell due to tissue stiffness, cytoskeletal reorganization, and the changes (even subtle) in the cell's physical surroundings. These changes of forces impose a mechanical tension within the intracellular protein network (both cytosolic and nuclear). Mechanical tension could be released by a series of protein-protein interactions often facilitated by membrane lipids, lectins and sugar molecules and thus generate a type of signal to drive cellular processes, including cell differentiation, polarity, growth, adhesion, movement, and survival. Recent experimental data have accentuated the molecular mechanism of this mechanical signal transduction pathway, dubbed mechanotransduction. Mechanosensitive proteins in the cell's plasma membrane discern the physical forces and channel the information to the cell interior. Cells respond to the message by altering their cytoskeletal arrangement and directly transmitting the signal to the nucleus through the connection of the cytoskeleton and nucleoskeleton before the information despatched to the nucleus by biochemical signaling pathways. Nuclear transmission of the force leads to the activation of chromatin modifiers and modulation of the epigenetic landscape, inducing chromatin reorganization and gene expression regulation; by the time chemical messengers (transcription factors) arrive into the nucleus. While significant research has been done on the role of mechanotransduction in tumor development and cancer progression/metastasis, the mechanistic basis of force-activated carcinogenesis is still enigmatic. Here, in this review, we have discussed the various cues and molecular connections to better comprehend the cellular mechanotransduction pathway, and we also explored the detailed role of some of the multiple players (proteins and macromolecular complexes) involved in mechanotransduction. Thus, we have described an avenue: how mechanical stress directs the epigenetic modifiers to modulate the epigenome of the cells and how aberrant stress leads to the cancer phenotype.

Cell stretching activates an ATM mechano-transduction pathway that remodels cytoskeleton and chromatin

Ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) DNA damage response (DDR) kinases contain elastic domains. ATM also responds to reactive oxygen species (ROS) and ATR to nuclear mechanical stress. Mre11 mediates ATM activation following DNA damage; ATM mutations cause ataxia telangiectasia (A-T). Here, using in vivo imaging, electron microscopy, proteomic, and mechano-biology approaches, we study how ATM responds to mechanical stress. We report that cytoskeleton and ROS, but not Mre11, mediate ATM activation following cell deformation. ATM deficiency causes hyper-stiffness, stress fiber accumulation, Yes-associated protein (YAP) nuclear enrichment, plasma and nuclear membrane alterations during interstitial migration, and H3 hyper-methylation. ATM locates to the actin cytoskeleton and, following cytoskeleton stress, promotes phosphorylation of key cytoskeleton and chromatin regulators. Our data contribute to explain some clinical features of patients with A-T and pinpoint the existence of an integrated mechano-response in which ATM and ATR have distinct roles unrelated to their canonical DDR functions.

PAK1-dependent mechanotransduction enables myofibroblast nuclear adaptation and chromatin organization during fibrosis

Myofibroblasts are responsible for scarring during fibrosis. The scar propagates mechanical signals inducing a radical transformation in myofibroblast cell state and increasing profibrotic phenotype. Here, we show mechanical stress from progressive scarring induces nuclear softening and de-repression of heterochromatin. The parallel loss of H3K9Me3 enables a permissive state for distinct chromatin accessibility and profibrotic gene regulation. Integrating chromatin accessibility profiles with RNA expression provides insight into the transcription network underlying the switch in profibrotic myofibroblast states, emphasizing mechanoadaptive regulation of PAK1 as key drivers. Through genetic manipulation in liver and lung fibrosis, loss of PAK1-dependent signaling impairs the mechanoadaptive response in vitro and dramatically improves fibrosis in vivo. Moreover, we provide human validation for mechanisms underpinning PAK1-mediated mechanotransduction in liver and lung fibrosis. Collectively, these observations provide insight into the nuclear mechanics driving the profibrotic chromatin landscape in fibrosis, highlighting actomyosin-dependent mechanisms as potential therapeutic targets in fibrosis.

Engineering tools for quantifying and manipulating forces in epithelia

The integrity of epithelia is maintained within dynamic mechanical environments during tissue development and homeostasis. Understanding how epithelial cells mechanosignal and respond collectively or individually is critical to providing insight into developmental and (patho)physiological processes. Yet, inferring or mimicking mechanical forces and downstream mechanical signaling as they occur in epithelia presents unique challenges. A variety of in vitro approaches have been used to dissect the role of mechanics in regulating epithelia organization. Here, we review approaches and results from research into how epithelial cells communicate through mechanical cues to maintain tissue organization and integrity. We summarize the unique advantages and disadvantages of various reduced-order model systems to guide researchers in choosing appropriate experimental systems. These model systems include 3D, 2D, and 1D micromanipulation methods, single cell studies, and noninvasive force inference and measurement techniques. We also highlight a number of in silico biophysical models that are informed by in vitro and in vivo observations. Together, a combination of theoretical and experimental models will aid future experiment designs and provide predictive insight into mechanically driven behaviors of epithelial dynamics.

Unravelling the mechanotransduction pathways in Alzheimer's disease

Alzheimer's disease (AD) represents one of the most common and debilitating neurodegenerative disorders. By the end of 2040, AD patients might reach 11.2 million in the USA, around 70% higher than 2022, with severe consequences on the society. As now, we still need research to find effective methods to treat AD. Most studies focused on the tau and amyloid hypothesis, but many other factors are likely involved in the pathophysiology of AD. In this review, we summarize scientific evidence dealing with the mechanotransduction players in AD to highlight the most relevant mechano-responsive elements that play a role in AD pathophysiology. We focused on the AD-related role of extracellular matrix (ECM), nuclear lamina, nuclear transport and synaptic activity. The literature supports that ECM alteration causes the lamin A increment in the AD patients, leading to the formation of nuclear blebs and invaginations. Nuclear blebs have consequences on the nuclear pore complexes, impairing nucleo-cytoplasmic transport. This may result in tau hyperphosphorylation and its consequent self-aggregation in tangles, which impairs the neurotransmitters transport. It all exacerbates in synaptic transmission impairment, leading to the characteristic AD patient's memory loss. Here we related for the first time all the evidence associating the mechanotransduction pathway with neurons. In addition, we highlighted the entire pathway influencing neurodegenerative diseases, paving the way for new research perspectives in the context of AD and related pathologies.

A critical period of prehearing spontaneous Ca2+ spiking is required for hair-bundle maintenance in inner hair cells

Sensory-independent Ca2+ spiking regulates the development of mammalian sensory systems. In the immature cochlea, inner hair cells (IHCs) fire spontaneous Ca2+ action potentials (APs) that are generated either intrinsically or by intercellular Ca2+ waves in the nonsensory cells. The extent to which either or both of these Ca2+ signalling mechansims are required for IHC maturation is unknown. We find that intrinsic Ca2+ APs in IHCs, but not those elicited by Ca2+ waves, regulate the maturation and maintenance of the stereociliary hair bundles. Using a mouse model in which the potassium channel Kir2.1 is reversibly overexpressed in IHCs (Kir2.1-OE), we find that IHC membrane hyperpolarization prevents IHCs from generating intrinsic Ca2+ APs but not APs induced by Ca2+ waves. Absence of intrinsic Ca2+ APs leads to the loss of mechanoelectrical transduction in IHCs prior to hearing onset due to progressive loss or fusion of stereocilia. RNA-sequencing data show that pathways involved in morphogenesis, actin filament-based processes, and Rho-GTPase signaling are upregulated in Kir2.1-OE mice. By manipulating in vivo expression of Kir2.1 channels, we identify a "critical time period" during which intrinsic Ca2+ APs in IHCs regulate hair-bundle function.

The Nuclear Envelope in Ageing and Progeria

Development from embryo to adult, organismal homeostasis and ageing are consecutive processes that rely on several functions of the nuclear envelope (NE). The NE compartmentalises the eukaryotic cells and provides physical stability to the genetic material in the nucleus. It provides spatiotemporal regulation of gene expression by controlling nuclear import and hence access of transcription factors to target genes as well as organisation of the genome into open and closed compartments. In addition, positioning of chromatin relative to the NE is important for DNA replication and repair and thereby also for genome stability. We discuss here the relevance of the NE in two classes of age-related human diseases. Firstly, we focus on the progeria syndromes Hutchinson-Gilford (HGPS) and Nestor-Guillermo (NGPS), which are caused by mutations in the LMNA and BANF1 genes, respectively. Both genes encode ubiquitously expressed components of the nuclear lamina that underlines the nuclear membranes. HGPS and NGPS patients manifest symptoms of accelerated ageing and cells from affected individuals show similar defects as cells from healthy old donors, including signs of increased DNA damage and epigenetic alternations. Secondly, we describe how several age-related neurodegenerative diseases, such as amyotrophic lateral sclerosis and Huntington's disease, are related with defects in nucleocytoplasmic transport. A common feature of this class of diseases is the accumulation of nuclear pore proteins and other transport factors in inclusions. Importantly, genetic manipulations of the nucleocytoplasmic transport machinery can alleviate disease-related phenotypes in cell and animal models, paving the way for potential therapeutic interventions.

Chondrocyte De-Differentiation: Biophysical Cues to Nuclear Alterations

Autologous chondrocyte implantation (ACI) is a cell therapy to repair cartilage defects. In ACI a biopsy is taken from a non-load bearing area of the knee and expanded in-vitro. The expansion process provides the benefit of generating a large number of cells required for implantation; however, during the expansion these cells de-differentiate and lose their chondrocyte phenotype. In this review we focus on examining the de-differentiation phenotype from a mechanobiology and biophysical perspective, highlighting some of the nuclear mechanics and chromatin changes in chondrocytes seen during the expansion process and how this relates to the gene expression profile. We propose that manipulating chondrocyte nuclear architecture and chromatin organization will highlight mechanisms that will help to preserve the chondrocyte phenotype.

Cyclic Stretching Induces Maturation of Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes through Nuclear-Mechanotransduction

BACKGROUND

During cardiogenesis, cardiac cells receive various stimuli, such as biomechanical and chemical cues, from the surrounding microenvironment, and these signals induce the maturation of heart cells. Mechanical force, especially tensile force in the heart, is one of the key stimuli that induce cardiomyocyte (CM) maturation through mechanotransduction, a process through which physical cues are transformed into biological responses. However, the effects and mechanisms of tensile force on cell maturation are poorly studied.

METHODS

In this study, we developed a cyclic stretch system that mimics the mechanical environment of the heart by loading tensile force to human-induced pluripotent stem cell (hiPSC)-derived CMs. hiPSC-CMs cultured with the cyclic stretch system analyzed morphological change, immunofluorescent staining, expression of maturation markers in mRNA, and beating properties compared to static cultures.

RESULTS

hiPSC-CMs cultured with the cyclic stretch system showed increased cell alignment, sarcomere length and expression of maturation markers in mRNA, such as TNNI3, MYL2 and TTN, compared to static cultures. Especially, the expression of genes related to nuclear mechanotransduction, such as Yap1, Lamin A/C, plectin, and desmin, was increased in the cyclically stretched hiPSC-CMs. Furthermore, the volume of the nucleus was increased by as much as 120% in the cyclic stretch group.

CONCLUSION

These results revealed that nuclear mechanotransduction induced by tensile force is involved in CM maturation. Together, these findings provide novel evidence suggesting that nuclear mechanotransduction induced by tensile force is involved in the regulation of cardiac maturation.

The Paradox of Nuclear Lamins in Pathologies: Apparently Controversial Roles Explained by Tissue-Specific Mechanobiology

The nuclear lamina is a complex meshwork of intermediate filaments (lamins) that is located beneath the inner nuclear membrane and the surrounding nucleoplasm. The lamins exert both structural and functional roles in the nucleus and, by interacting with several nuclear proteins, are involved in a wide range of nuclear and cellular activities. Due their pivotal roles in basic cellular processes, lamin gene mutations, or modulations in lamin expression, are often associated with pathological conditions, ranging from rare genetic diseases, such as laminopathies, to cancer. Although a substantial amount of literature describes the effects that are mediated by the deregulation of nuclear lamins, some apparently controversial results have been reported, which may appear to conflict with each other. In this context, we herein provide our explanation of such “controversy”, which, in our opinion, derives from the tissue-specific expression of nuclear lamins and their close correlation with mechanotransduction processes, which could be very different, or even opposite, depending on the specific mechanical conditions that should not be compared (a tissue vs. another tissue, in vivo studies vs. cell cultures on glass/plastic supports, etc.). Moreover, we have stressed the relevance of considering and reproducing the “mechano-environment” in in vitro experimentation. Indeed, when primary cells that are collected from patients or donors are maintained in a culture, the mechanical signals deriving from canonical experimental procedures of cell culturing could alter the lamin expression, thereby profoundly modifying the assessed cell type, in some cases even too much, compared to the cell of origin.

Functions and clinical significance of mechanical tumor microenvironment: cancer cell sensing, mechanobiology and metastasis

Dynamic and heterogeneous interaction between tumor cells and the surrounding microenvironment fuels the occurrence, progression, invasion, and metastasis of solid tumors. In this process, the tumor microenvironment (TME) fractures cellular and matrix architecture normality through biochemical and mechanical means, abetting tumorigenesis and treatment resistance. Tumor cells sense and respond to the strength, direction, and duration of mechanical cues in the TME by various mechanotransduction pathways. However, far less understood is the comprehensive perspective of the functions and mechanisms of mechanotransduction. Due to the great therapeutic difficulties brought by the mechanical changes in the TME, emerging studies have focused on targeting the adverse mechanical factors in the TME to attenuate disease rather than conventionally targeting tumor cells themselves, which has been proven to be a potential therapeutic approach. In this review, we discussed the origins and roles of mechanical factors in the TME, cell sensing, mechano‐biological coupling and signal transduction, in vitro construction of the tumor mechanical microenvironment, applications and clinical significance in the TME.

A Transient Mystery: Nucleolar Channel Systems

The nucleus is a complex organelle with functions beyond being a simple repository for genomic material. For example, its actions in biomechanical sensing, protein synthesis, and epigenomic regulation showcase how the nucleus integrates multiple signaling modalities to intricately regulate gene expression. This innate dynamism is underscored by subnuclear components that facilitate these roles, with elements of the nucleoskeleton, phase-separated nuclear bodies, and chromatin safeguarding by nuclear envelope proteins providing examples of this functional diversity. Among these, one of the lesser characterized nuclear organelles is the nucleolar channel system (NCS), first reported several decades ago in human endometrial biopsies. This tubular structure, believed to be derived from the inner nuclear membrane of the nuclear envelope, was first observed in secretory endometrial cells during a specific phase of the menstrual cycle. Reported as a consistent, yet transient, nuclear organelle, current interpretations of existing data suggest that it serves as a marker of a window for optimal implantation. In spite of this available data, the NCS remains incompletely characterized structurally and functionally, due in part to its transient spatial and temporal expression. As a further complication, evidence exists showing NCS expression in fetal tissue, suggesting that it may not act exclusively as a marker of uterine receptivity, but rather as a hormone sensor sensitive to estrogen and progesterone ratios. To gain a better understanding of the NCS, current technologies can be applied to profile rare cell populations or capture transient structural dynamics, for example, at a level of sensitivity and resolution not previously possible. Moving forward, advanced characterization of the NCS will shed light on an uncharacterized aspect of reproductive physiology, with the potential to refine assisted reproductive techniques.

Mechanosignalling in cartilage: an emerging target for the treatment of osteoarthritis

Mechanical stimuli have fundamental roles in articular cartilage during health and disease. Chondrocytes respond to the physical properties of the cartilage extracellular matrix (ECM) and the mechanical forces exerted on them during joint loading. In osteoarthritis (OA), catabolic processes degrade the functional ECM and the composition and viscoelastic properties of the ECM produced by chondrocytes are altered. The abnormal loading environment created by these alterations propagates cell dysfunction and inflammation. Chondrocytes sense their physical environment via an array of mechanosensitive receptors and channels that activate a complex network of downstream signalling pathways to regulate several cell processes central to OA pathology. Advances in understanding the complex roles of specific mechanosignalling mechanisms in healthy and OA cartilage have highlighted molecular processes that can be therapeutically targeted to interrupt pathological feedback loops. The potential for combining these mechanosignalling targets with the rapidly expanding field of smart mechanoresponsive biomaterials and delivery systems is an emerging paradigm in OA treatment. The continued advances in this field have the potential to enable restoration of healthy mechanical microenvironments and signalling through the development of precision therapeutics, mechanoregulated biomaterials and drug systems in the near future.

Natural Membrane Differentiates Human Adipose-Derived Mesenchymal Stem Cells to Neurospheres by Mechanotransduction Related to YAP and AMOT Proteins

Adipose tissue-derived mesenchymal stem cells (ADMSCs) are promising candidates for regenerative medicine, as they have good cell yield and can differentiate into several cell lines. When induced to the neuronal differentiation, they form neurospheres composed of neural precursors (NPs) that can be an alternative in treating neurodegenerative diseases. This study aimed to characterize NPs from neurospheres obtained after seeding ADMSCs on a natural polyisoprene-based membrane. The ADMSCs were isolated from adipose tissue by enzymatic dissociation, were subjected to trilineage differentiation, and were characterized by flow cytometry for specific ADMSC surface markers. For neuronal differentiation, the cells were seeded on polystyrene flasks coated with the membrane and were characterized by immunocytochemistry and RT-PCR. The results demonstrated that the isolated cells showed characteristics of ADMSCs. At 15 to 25 days, ADMSCs seeded on the natural membrane developed neurospheres. Then, after dissociation, the cells demonstrated characteristic neuronal markers expressed on NPs: nestin, ß-III tubulin, GFAP, NeuN, and the YAP1/AMOT in the cytoplasm. In conclusion, it was demonstrated that this membrane differentiates the ADMSCs to NPs without any induction factors, and suggests that their differentiation mechanisms are related to mechanotransduction regulated by the YAP and AMOT proteins.