Assessing dystrophies and other muscle diseases at the nanometer scale by atomic force microscopy

The extracellular matrix of dystrophic mouse diaphragm accounts for the majority of its passive stiffness and is resistant to collagenase digestion

The healthy skeletal muscle extracellular matrix (ECM) has several functions including providing structural integrity to myofibers, enabling lateral force transmission, and contributing to overall passive mechanical properties. In diseases such as Duchenne Muscular dystrophy, there is accumulation of ECM materials, primarily collagen, which results in fibrosis. Previous studies have shown that fibrotic muscle is often stiffer than healthy muscle, in part due to the increased number and altered architecture of collagen fibers within the ECM. This would imply that the fibrotic matrix is stiffer than the healthy matrix. However, while previous studies have attempted to quantify the extracellular contribution to passive stiffness in muscle, the outcomes are dependent on the type of method used. Thus, the goals of this study were to compare the stiffness of healthy and fibrotic muscle ECM and to demonstrate the efficacy of two methods for quantifying extracellular-based stiffness in muscle, namely decellularization and collagenase digestion. These methods have been demonstrated to remove the muscle fibers or ablate collagen fiber integrity, respectively, while maintaining the contents of the extracellular matrix. Using these methods in conjunction with mechanical testing on wildtype and D2.mdx mice, we found that a majority of passive stiffness in the diaphragm is dependent on the ECM, and the D2.mdx diaphragm ECM is resistant to digestion by bacterial collagenase. We propose that this resistance is due to the increased collagen cross-links and collagen packing density in the ECM of the D2.mdx diaphragm. Taken altogether, while we did not find increased stiffness of the fibrotic ECM, we did observe that the D2.mdx diaphragm conveyed resistance against collagenase digestion. These findings demonstrate how different methods for measuring ECM-based stiffness each have their own limitations and can produce different results.

Expression of Myomaker and Myomerger in myofibers causes muscle pathology

BACKGROUND

Skeletal muscle development and regeneration depend on cellular fusion of myogenic progenitors to generate multinucleated myofibers. These progenitors utilize two muscle-specific fusogens, Myomaker and Myomerger, which function by remodeling cell membranes to fuse to each other or to existing myofibers. Myomaker and Myomerger expression is restricted to differentiating progenitor cells as they are not detected in adult myofibers. However, Myomaker remains expressed in myofibers from mice with muscular dystrophy. Ablation of Myomaker from dystrophic myofibers results in reduced membrane damage, leading to a model where persistent fusogen expression in myofibers, in contrast to myoblasts, is harmful.

METHODS

Dox-inducible transgenic mice were developed to ectopically express Myomaker or Myomerger in the myofiber compartment of skeletal muscle. We quantified indices of myofiber membrane damage, such as serum creatine kinase and IgM+ myofibers, and assessed general muscle histology, including central nucleation, myofiber size, and fibrosis.

RESULTS

Myomaker or Myomerger expression in myofibers independently caused membrane damage at acute time points. This damage led to muscle pathology, manifesting with centrally nucleated myofibers and muscle atrophy. Dual expression of both Myomaker and Myomerger in myofibers exacerbated several aspects of muscle pathology compared to expression of either fusogen by itself.

CONCLUSIONS

These data reveal that while myofibers can tolerate some level of Myomaker and Myomerger, expression of a single fusogen above a threshold or co-expression of both fusogens is damaging to myofibers. These results explain the paradigm that their expression in myofibers can have deleterious consequences in muscle pathologies and highlight the need for their highly restricted expression during myogenesis and fusion.

Uncovering the cytotoxic effects of air pollution with multi-modal imaging of in vitro respiratory models

Annually, an estimated seven million deaths are linked to exposure to airborne pollutants. Despite extensive epidemiological evidence supporting clear associations between poor air quality and a range of short- and long-term health effects, there are considerable gaps in our understanding of the specific mechanisms by which pollutant exposure induces adverse biological responses at the cellular and tissue levels. The development of more complex, predictive, in vitro respiratory models, including two- and three-dimensional cell cultures, spheroids, organoids and tissue cultures, along with more realistic aerosol exposure systems, offers new opportunities to investigate the cytotoxic effects of airborne particulates under controlled laboratory conditions. Parallel advances in high-resolution microscopy have resulted in a range of in vitro imaging tools capable of visualizing and analysing biological systems across unprecedented scales of length, time and complexity. This article considers state-of-the-art in vitro respiratory models and aerosol exposure systems and how they can be interrogated using high-resolution microscopy techniques to investigate cell-pollutant interactions, from the uptake and trafficking of particles to structural and functional modification of subcellular organelles and cells. These data can provide a mechanistic basis from which to advance our understanding of the health effects of airborne particulate pollution and develop improved mitigation measures.

Myoscaffolds reveal laminin scarring is detrimental for stem cell function while sarcospan induces compensatory fibrosis

We developed an on-slide decellularization approach to generate acellular extracellular matrix (ECM) myoscaffolds that can be repopulated with various cell types to interrogate cell-ECM interactions. Using this platform, we investigated whether fibrotic ECM scarring affected human skeletal muscle progenitor cell (SMPC) functions that are essential for myoregeneration. SMPCs exhibited robust adhesion, motility, and differentiation on healthy muscle-derived myoscaffolds. All SPMC interactions with fibrotic myoscaffolds from dystrophic muscle were severely blunted including reduced motility rate and migration. Furthermore, SMPCs were unable to remodel laminin dense fibrotic scars within diseased myoscaffolds. Proteomics and structural analysis revealed that excessive collagen deposition alone is not pathological, and can be compensatory, as revealed by overexpression of sarcospan and its associated ECM receptors in dystrophic muscle. Our in vivo data also supported that ECM remodeling is important for SMPC engraftment and that fibrotic scars may represent one barrier to efficient cell therapy.

Sensing red blood cell nano-mechanics: Toward a novel blood biomarker for Alzheimer's disease

Red blood cells (RBCs) are characterized by a remarkable elasticity, which allows them to undergo very large deformation when passing through small vessels and capillaries. This extreme deformability is altered in various clinical conditions, suggesting that the analysis of red blood cell (RBC) mechanics has potential applications in the search for non-invasive and cost-effective blood biomarkers. Here, we provide a comparative study of the mechanical response of RBCs in patients with Alzheimer's disease (AD) and healthy subjects. For this purpose, RBC viscoelastic response was investigated using atomic force microscopy (AFM) in the force spectroscopy mode. Two types of analyses were performed: (i) a conventional analysis of AFM force-distance (FD) curves, which allowed us to retrieve the apparent Young's modulus, E; and (ii) a more in-depth analysis of time-dependent relaxation curves in the framework of the standard linear solid (SLS) model, which allowed us to estimate cell viscosity and elasticity, independently. Our data demonstrate that, while conventional analysis of AFM FD curves fails in distinguishing the two groups, the mechanical parameters obtained with the SLS model show a very good classification ability. The diagnostic performance of mechanical parameters was assessed using receiving operator characteristic (ROC) curves, showing very large areas under the curves (AUC) for selected biomarkers (AUC > 0.9). Taken all together, the data presented here demonstrate that RBC mechanics are significantly altered in AD, also highlighting the key role played by viscous forces. These RBC abnormalities in AD, which include both a modified elasticity and viscosity, could be considered a potential source of plasmatic biomarkers in the field of liquid biopsy to be used in combination with more established indicators of the pathology.

Exon skipping induces uniform dystrophin rescue with dose-dependent restoration of serum miRNA biomarkers and muscle biophysical properties

Therapies that restore dystrophin expression are presumed to correct Duchenne muscular dystrophy (DMD), with antisense-mediated exon skipping being the leading approach. Here we aimed to determine whether exon skipping using a peptide-phosphorodiamidate morpholino oligonucleotide (PPMO) conjugate results in dose-dependent restoration of uniform dystrophin localization, together with correction of putative DMD serum and muscle biomarkers. Dystrophin-deficient mdx mice were treated with a PPMO (Pip9b2-PMO) designed to induce Dmd exon 23 skipping at single, ascending intravenous doses (3, 6, or 12 mg/kg) and sacrificed 2 weeks later. Dose-dependent exon skipping and dystrophin protein restoration were observed, with dystrophin uniformly distributed at the sarcolemma of corrected myofibers at all doses. Serum microRNA biomarkers (i.e., miR-1a-3p, miR-133a-3p, miR-206-3p, miR-483-3p) and creatinine kinase levels were restored toward wild-type levels after treatment in a dose-dependent manner. All biomarkers were strongly anti-correlated with both exon skipping level and dystrophin expression. Dystrophin rescue was also strongly positively correlated with muscle stiffness (i.e., Young's modulus) as determined by atomic force microscopy (AFM) nanoindentation assay. These data demonstrate that PPMO-mediated exon skipping generates myofibers with uniform dystrophin expression and that both serum microRNA biomarkers and muscle AFM have potential utility as pharmacodynamic biomarkers of dystrophin restoration therapy in DMD.

Biomechanical changes in the liver tissue induced by a mouse model of multiple sclerosis (EAE) and the effect of anti-VLA-4 mAb treatment

Experimental autoimmune encephalomyelitis (EAE) is a mouse model of demyelinating diseases, such as multiple sclerosis (MS). MS can be accompanied by autoimmune hepatitis. In this study, nanomechanical, biorheological and histological examinations were carried out by atomic force microscopy (AFM), rheology, and immunofluorescence microscopy to investigate changes in the liver tissue of EAE mice and the effect of natalizumab, a monoclonal antibody against α4-integrin (VLA-4) cell adhesion molecule, used in MS therapy. Liver samples collected from EAE mice in three successive phases of the disease showed inflammatory changes manifested by leukocyte infiltrations and elevated levels of proinflammatory cytokine IL-1β. Liver stiffness and viscoelasticity increased in the onset phase of EAE, decreased in the peak phase and increased again in the chronic phase to reach the highest values. These changes were not associated with inflammation parameters which increased in the peak phase and decreased to the lowest values in the chronic phase. Moreover, anti-VLA treatment, which reduced the inflammation parameters, had an ambiguous effect on stiffness and viscoelasticity: it increased them in the peak phase but decreased in the chronic phase. The observed discrepancies can result from a complex network of interactions between inflammation and fibrosis, as well as between liver cells and the extracellular matrix influencing the biomechanical properties of the liver tissue.

In vivo PS-OCT needle probe scan of human skeletal muscle

Polarization-sensitive optical coherence tomography (PS-OCT) derived birefringence values effectively identify skeletal muscle structural disruption due to muscular dystrophy and exercise-related muscle damage in animal models in ex vivo tissue. The purpose of this investigation was to determine if a PS-OCT needle probe inserted into the leg of a human subject could accurately identify various anatomical structures with implications for use as a diagnostic tool for the determination of skeletal muscle pathology. A healthy middle-aged subject participated in this study. A custom-built PS-OCT system was interfaced with a side-viewing fiber-optic needle probe inserted into the subject's vastus lateralis muscle via a motorized stage for 3D data acquisition via rotation and stepwise pullback. The deepest recorded PS-OCT images correspond to a depth of 6 mm beneath the dermis with structural images showing uniform, striated muscle tissue. Multiple highly birefringent band-like structures with definite orientation representing connective tissue of the superficial aponeurosis appeared as the depth of the needle decreased. Superficial to these structures the dominating appearance was that of adipose tissue and low birefringent but homogeneous scattering tissue. The data indicate that a PS-OCT needle probe can be inserted into live human skeletal muscle for the identification of relevant anatomical structures that could be utilized to diagnose significant skeletal muscle pathology.

Viscoelasticity, Like Forces, Plays a Role in Mechanotransduction

Viscoelasticity and its alteration in time and space has turned out to act as a key element in fundamental biological processes in living systems, such as morphogenesis and motility. Based on experimental and theoretical findings it can be proposed that viscoelasticity of cells, spheroids and tissues seems to be a collective characteristic that demands macromolecular, intracellular component and intercellular interactions. A major challenge is to couple the alterations in the macroscopic structural or material characteristics of cells, spheroids and tissues, such as cell and tissue phase transitions, to the microscopic interferences of their elements. Therefore, the biophysical technologies need to be improved, advanced and connected to classical biological assays. In this review, the viscoelastic nature of cytoskeletal, extracellular and cellular networks is presented and discussed. Viscoelasticity is conceptualized as a major contributor to cell migration and invasion and it is discussed whether it can serve as a biomarker for the cells’ migratory capacity in several biological contexts. It can be hypothesized that the statistical mechanics of intra- and extracellular networks may be applied in the future as a powerful tool to explore quantitatively the biomechanical foundation of viscoelasticity over a broad range of time and length scales. Finally, the importance of the cellular viscoelasticity is illustrated in identifying and characterizing multiple disorders, such as cancer, tissue injuries, acute or chronic inflammations or fibrotic diseases.

Rheological properties of skeletal muscles in a Duchenne muscular dystrophy murine model before and after autologous cell therapy

Duchenne muscular dystrophy (DMD) is still an incurable muscle degenerative disease; thus, numerous studies focused on novel therapeutic approaches. However, a simple assay of muscle function restoration remains needed. Herein, we used an oscillatory shear rheometer to evaluate changes in rheological properties of mouse muscles (tibialis anterior, TA) and their restoration upon autologous cell therapy by comparing the viscoelastic properties of normal, diseased and treated muscles. Amplitude sweep tests of muscle samples were performed under 20% compression over a range of shear strain between 0.01 and 2% and frequency of 1 rad/s. The samples were tested in plane-plane geometry and horizontal myofiber alignment. Typical linear viscoelastic region (LVER) patterns were found for each muscle type. For healthy muscles, a broad LVER between shear deformations (γ) of 0.013-0.62% was observed. The LVER of DMD mdx/SCID muscles was found at 0.14% to 0.46% shear deformation, and no shear dependence of storage (G') and loss (G") moduli at γ range changing from 0.034% to 0.26% was found for transplanted tissues. G'_LVER and G"_LVER moduli of healthy muscles were significantly higher than G'_LVER and G"_LVER of dystrophic tissues. Additionally, muscle resistance assessment by rheometer indicated that muscles transplanted with stem cells restored elastic properties to levels close to those of healthy muscles. Interestingly, histological staining and rheological data indicate that the loss factor is strongly related to structural changes of examined muscles.

An insight on Drosophila myogenesis and its assessment techniques

Movement assisted by muscles forms the basis of various behavioural traits seen in Drosophila. Myogenesis involves developmental processes like cellular specification, differentiation, migration, fusion, adherence to tendons and neuronal innervation in a series of coordinated event well defined in body space and time. Gene regulatory networks are switched on-off, fine tuning at the right developmental stage to assist each cellular event. Drosophila is a holometabolous organism that undergoes myogenesis waves at two developmental stages, and is ideal for comparative analysis of the role of genes and genetic pathways conserved across phyla. In this review we have summarized myogenic events from the embryo to adult focussing on the somatic muscle development during the early embryonic stage and then on indirect flight muscles (IFM) formation required for adult life, emphasizing on recent trends of analysing muscle mutants and advances in Drosophila muscle biology.

Spatial mapping of the collagen distribution in human and mouse tissues by force volume atomic force microscopy

Changes in the elastic properties of living tissues during normal development and in pathological processes are often due to modifications of the collagen component of the extracellular matrix at various length scales. Force volume AFM can precisely capture the mechanical properties of biological samples with force sensitivity and spatial resolution. The integration of AFM data with data of the molecular composition contributes to understanding the interplay between tissue biochemistry, organization and function. The detection of micrometer-size, heterogeneous domains at different elastic moduli in tissue sections by AFM has remained elusive so far, due to the lack of correlations with histological, optical and biochemical assessments. In this work, force volume AFM is used to identify collagen-enriched domains, naturally present in human and mouse tissues, by their elastic modulus. Collagen identification is obtained in a robust way and affordable timescales, through an optimal design of the sample preparation method and AFM parameters for faster scan with micrometer resolution. The choice of a separate reference sample stained for collagen allows correlating elastic modulus with collagen amount and position with high statistical significance. The proposed preparation method ensures safe handling of the tissue sections guarantees the preservation of their micromechanical characteristics over time and makes it much easier to perform correlation experiments with different biomarkers independently.

Measuring viscoelasticity of soft biological samples using atomic force microscopy

Mechanical properties play important roles at different scales in biology. At the level of a single cell, the mechanical properties mediate mechanosensing and mechanotransduction, while at the tissue and organ levels, changes in mechanical properties are closely connected to disease and physiological processes. Over the past three decades, atomic force microscopy (AFM) has become one of the most widely used tools in the mechanical characterization of soft samples, ranging from molecules, cell organoids and cells to whole tissue. AFM methods can be used to quantify both elastic and viscoelastic properties, and significant recent developments in the latter have been enabled by the introduction of new techniques and models for data analysis. Here, we review AFM techniques developed in recent years for examining the viscoelastic properties of cells and soft gels, describe the main steps in typical data acquisition and analysis protocols, and discuss relevant viscoelastic models and how these have been used to characterize the specific features of cellular and other biological samples. We also discuss recent trends and potential directions for this field.

Changes in spinal cord stiffness in the course of experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis

Experimental autoimmune encephalomyelitis (EAE) is a commonly used mouse model of multiple sclerosis, a chronic inflammatory disease of the central nervous system (CNS) characterized by demyelination leading to brain and spinal cord malfunctions. We postulate that not only biological but also biomechanical properties play an important role in impairements of CNS function. Atomic force microscopy (AFM) was applied to investigate mechanical properties of spinal cords collected from EAE mice in preonset, onset, peak, and chronic disease phases. Biomechanical changes were compared with histopathological alterations observed in the successive phases. The deformability of gray matter did not change, while rigidity of white matter increased during the onset phase, remained at the same level in the peak phase and decreased in the chronic phase. Inflammatory infiltration and laminin content accompanied the tissue rigidity increase, whereas demyelination and axonal damage showed an opposite effect. The increase in white matter rigidity can be regarded as an early signature of EAE.

The matrix environmental and cell mechanical properties regulate cell migration and contribute to the invasive phenotype of cancer cells

The minimal structural unit of a solid tumor is a single cell or a cellular compartment such as the nucleus. A closer look inside the cells reveals that there are functional compartments or even structural domains determining the overall properties of a cell such as the mechanical phenotype. The mechanical interaction of these living cells leads to the complex organization such as compartments, tissues and organs of organisms including mammals. In contrast to passive non-living materials, living cells actively respond to the mechanical perturbations occurring in their microenvironment during diseases such as fibrosis and cancer. The transformation of single cancer cells in highly aggressive and hence malignant cancer cells during malignant cancer progression encompasses the basement membrane crossing, the invasion of connective tissue, the stroma microenvironments and transbarrier migration, which all require the immediate interaction of the aggressive and invasive cancer cells with the surrounding extracellular matrix environment including normal embedded neighboring cells. All these steps of the metastatic pathway seem to involve mechanical interactions between cancer cells and their microenvironment. The pathology of cancer due to a broad heterogeneity of cancer types is still not fully understood. Hence it is necessary to reveal the signaling pathways such as mechanotransduction pathways that seem to be commonly involved in the development and establishment of the metastatic and mechanical phenotype in several carcinoma cells. We still do not know whether there exist distinct metastatic genes regulating the progression of tumors. These metastatic genes may then be activated either during the progression of cancer by themselves on their migration path or in earlier stages of oncogenesis through activated oncogenes or inactivated tumor suppressor genes, both of which promote the metastatic phenotype. In more detail, the adhesion of cancer cells to their surrounding stroma induces the generation of intracellular contraction forces that deform their microenvironments by alignment of fibers. The amplitude of these forces can adapt to the mechanical properties of the microenvironment. Moreover, the adhesion strength of cancer cells seems to determine whether a cancer cell is able to migrate through connective tissue or across barriers such as the basement membrane or endothelial cell linings of blood or lymph vessels in order to metastasize. In turn, exposure of adherent cancer cells to physical forces, such as shear flow in vessels or compression forces around tumors, reinforces cell adhesion, regulates cell contractility and restructures the ordering of the local stroma matrix that leads subsequently to secretion of crosslinking proteins or matrix degrading enzymes. Hence invasive cancer cells alter the mechanical properties of their microenvironment. From a mechanobiological point-of-view, the recognized physical signals are transduced into biochemical signaling events that guide cellular responses such as cancer progression after the malignant transition of cancer cells from an epithelial and non-motile phenotype to a mesenchymal and motile (invasive) phenotype providing cellular motility. This transition can also be described as the physical attempt to relate this cancer cell transitional behavior to a T1 phase transition such as the jamming to unjamming transition. During the invasion of cancer cells, cell adaptation occurs to mechanical alterations of the local stroma, such as enhanced stroma upon fibrosis, and therefore we need to uncover underlying mechano-coupling and mechano-regulating functional processes that reinforce the invasion of cancer cells. Moreover, these mechanisms may also be responsible for the awakening of dormant residual cancer cells within the microenvironment. Physicists were initially tempted to consider the steps of the cancer metastasis cascade as single events caused by a single mechanical alteration of the overall properties of the cancer cell. However, this general and simple view has been challenged by the finding that several mechanical properties of cancer cells and their microenvironment influence each other and continuously contribute to tumor growth and cancer progression. In addition, basement membrane crossing, cell invasion and transbarrier migration during cancer progression is explained in physical terms by applying physical principles on living cells regardless of their complexity and individual differences of cancer types. As a novel approach, the impact of the individual microenvironment surrounding cancer cells is also included. Moreover, new theories and models are still needed to understand why certain cancers are malignant and aggressive, while others stay still benign. However, due to the broad variety of cancer types, there may be various pathways solely suitable for specific cancer types and distinct steps in the process of cancer progression. In this review, physical concepts and hypotheses of cancer initiation and progression including cancer cell basement membrane crossing, invasion and transbarrier migration are presented and discussed from a biophysical point-of-view. In addition, the crosstalk between cancer cells and a chronically altered microenvironment, such as fibrosis, is discussed including the basic physical concepts of fibrosis and the cellular responses to mechanical stress caused by the mechanically altered microenvironment. Here, is highlighted how biophysical approaches, both experimentally and theoretically, have an impact on classical hallmarks of cancer and fibrosis and how they contribute to the understanding of the regulation of cancer and its progression by sensing and responding to the physical environmental properties through mechanotransduction processes. Finally, this review discusses various physical models of cell migration such as blebbing, nuclear piston, protrusive force and unjamming transition migration modes and how they contribute to cancer progression. Moreover, these cellular migration modes are influenced by microenvironmental perturbances such as fibrosis that can induce mechanical alterations in cancer cells, which in turn may impact the environment. Hence, the classical hallmarks of cancer need to be refined by including biomechanical properties of cells, cell clusters and tissues and their microenvironment to understand mechano-regulatory processes within cancer cells and the entire organism.

A novel method for post-mortem interval estimation based on tissue nano-mechanics

Forensic estimation of post-mortem interval relies on different methods, most of which, however, have practical limitations or provide insufficient results, still lacking a gold standard method. In order to better understand the phenomenon of rigor mortis and its applicability to the post-mortem interval estimation, we decided to use atomic force microscopy, a tool often employed to measure mechanical properties of adherent cells. Thus, we surgically removed skeletal muscle samples of three forensic cases from 0 to 120 h post-mortem and quantitatively evaluate two parameters: the Young's modulus (E), which gives information about the sample stiffness, and the hysteresis (H), which estimates the contribution of viscous forces. Despite being a preliminary study, the obtained results show that the temporal behavior of E well correlates with the expected evolution of rigor mortis between 0 and 48 h post-mortem, and then monotonically decreases over time. Unfortunately, it is strongly affected by inter-individual variability. However, we found that H provides measurable data along a time-dependent curve back to the starting point, and these data measured on different subjects collapse onto a single master curve, getting rid of the inter-individual variability. Although a larger sampling should be performed to improve the result reliability, this finding is strongly suggestive that the evaluation of rigor mortis should involve the measure of the nanoscale dissipative behavior of muscular tissues.

AFM assessing of nanomechanical fingerprints for cancer early diagnosis and classification: from single cell to tissue level

Cancer development and progression are closely associated with changes both in the mechano-cellular phenotype of cancer and stromal cells and in the extracellular matrix (ECM) structure, composition, and mechanics. In this paper, we review the use of atomic force microscopy (AFM) as a tool for assessing the nanomechanical fingerprints of solid tumors, so as to be potentially used as a diagnostic biomarker for more accurate identification and early cancer grading/classification. The development of such a methodology is expected to provide new insights and a novel approach for cancer diagnosis. We propose that AFM measurements could be employed to complement standard biopsy procedures, offering an objective, novel and quantitative diagnostic approach with the properties of a blind assay, allowing unbiased evaluation of the sample.

AFM-based nanomechanical characterization of bronchoscopic samples in asthma patients

Asthma is not a single disease, but recently, it is considered as a syndrome characterized through various clinical presentations and different etiopathologies. Large degree of the disease heterogeneity manifests in distinct characteristics that translate into variability of properties at single cell and molecular levels. Here, we conducted measurements of mechanical properties of bronchial tissue samples collected from patients suffering from asthma. The results obtained from different applied protocols for sample preparation may indicate that deep freezing and storage in liquid nitrogen, followed by consecutive unfreezing of tissue samples, preserve tissue mechanical properties as indicated by a parameter referred here as a tissue relative stiffness index. Tissue relative stiffness index quantifies both the degree of heterogeneity and deformability of tissue samples regarding healthy one. These studies demonstrate that the freezing protocol, optimized towards asthma tissue, can facilitate atomic force microscopy use what, together with recent findings on standardization of elasticity measurements, enables the measurements of large group of samples with minimized influence of errors stemming from the applied methodology of tissue stiffness determination.

Myotube elasticity of an amyotrophic lateral sclerosis mouse model

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that affects the motor system leading to generalized paralysis and death of patients. The understanding of early pathogenic mechanisms will help to define early diagnostics criteria that will eventually provide basis for efficient therapeutics. Early symptoms of ALS usually include muscle weakness or stiffness. Therefore, mechanical response of differentiated myotubes from primary cultures of mice, expressing the ALS-causing SOD1^G93A mutation, was examined by atomic force microscopy. Simultaneous acquisition of topography and cell elasticity of ALS myotubes was performed by force mapping method, compared with healthy myotubes and supplemented with immunofluorescence and qRT-PCR studies. Wild type myotubes reveal a significant difference in elasticity between a narrow and a wide population, consistent with maturation occurring with higher actin expression relative to myosin together with larger myotube width. However, this is not true for SOD1^G93A expressing myotubes, where a significant shift of thin population towards higher elastic modulus values was observed. We provide evidence that SOD1 mutant induces structural changes that occurs very early in muscle development and well before symptomatic stage of the disease. These findings could significantly contribute to the understanding of the role of skeletal muscle in ALS pathogenesis.

Autologous Cell Therapy Approach for Duchenne Muscular Dystrophy using PiggyBac Transposons and Mesoangioblasts

Duchenne muscular dystrophy (DMD) is a lethal muscle-wasting disease currently without cure. We investigated the use of the PiggyBac transposon for full-length dystrophin expression in murine mesoangioblast (MABs) progenitor cells. DMD murine MABs were transfected with transposable expression vectors for full-length dystrophin and transplanted intramuscularly or intra-arterially into mdx/SCID mice. Intra-arterial delivery indicated that the MABs could migrate to regenerating muscles to mediate dystrophin expression. Intramuscular transplantation yielded dystrophin expression in 11%-44% of myofibers in murine muscles, which remained stable for the assessed period of 5 months. The satellite cells isolated from transplanted muscles comprised a fraction of MAB-derived cells, indicating that the transfected MABs may colonize the satellite stem cell niche. Transposon integration site mapping by whole-genome sequencing indicated that 70% of the integrations were intergenic, while none was observed in an exon. Muscle resistance assessment by atomic force microscopy indicated that 80% of fibers showed elasticity properties restored to those of wild-type muscles. As measured in vivo, transplanted muscles became more resistant to fatigue. This study thus provides a proof-of-principle that PiggyBac transposon vectors may mediate full-length dystrophin expression as well as functional amelioration of the dystrophic muscles within a potential autologous cell-based therapeutic approach of DMD.

NADPH oxidase mediates microtubule alterations and diaphragm dysfunction in dystrophic mice

Skeletal muscle from mdx mice is characterized by increased Nox2 ROS, altered microtubule network, increased muscle stiffness, and decreased muscle/respiratory function. While microtubule de-tyrosination has been suggested to increase stiffness and Nox2 ROS production in isolated single myofibers, its role in altering tissue stiffness and muscle function has not been established. Because Nox2 ROS production is upregulated prior to microtubule network alterations and ROS affect microtubule formation, we investigated the role of Nox2 ROS in diaphragm tissue microtubule organization, stiffness and muscle/respiratory function. Eliminating Nox2 ROS prevents microtubule disorganization and reduces fibrosis and muscle stiffness in mdx diaphragm. Fibrosis accounts for the majority of variance in diaphragm stiffness and decreased function, implicating altered extracellular matrix and not microtubule de-tyrosination as a modulator of diaphragm tissue function. Ultimately, inhibiting Nox2 ROS production increased force and respiratory function in dystrophic diaphragm, establishing Nox2 as a potential therapeutic target in Duchenne muscular dystrophy.

Measuring nanoscale viscoelastic parameters of cells directly from AFM force-displacement curves

Force-displacement (F-Z) curves are the most commonly used Atomic Force Microscopy (AFM) mode to measure the local, nanoscale elastic properties of soft materials like living cells. Yet a theoretical framework has been lacking that allows the post-processing of F-Z data to extract their viscoelastic constitutive parameters. Here, we propose a new method to extract nanoscale viscoelastic properties of soft samples like living cells and hydrogels directly from conventional AFM F-Z experiments, thereby creating a common platform for the analysis of cell elastic and viscoelastic properties with arbitrary linear constitutive relations. The method based on the elastic-viscoelastic correspondence principle was validated using finite element (FE) simulations and by comparison with the existed AFM techniques on living cells and hydrogels. The method also allows a discrimination of which viscoelastic relaxation model, for example, standard linear solid (SLS) or power-law rheology (PLR), best suits the experimental data. The method was used to extract the viscoelastic properties of benign and cancerous cell lines (NIH 3T3 fibroblasts, NMuMG epithelial, MDA-MB-231 and MCF-7 breast cancer cells). Finally, we studied the changes in viscoelastic properties related to tumorigenesis including TGF-β induced epithelial-to-mesenchymal transition on NMuMG cells and Syk expression induced phenotype changes in MDA-MB-231 cells.

Discrimination Between Normal and Cancerous Cells Using AFM

Currently, biomechanics of living cells is in the focus of interest due to noticeable capability of such techniques like atomic force microscopy (AFM) to probe cellular properties at the single cell level directly on living cells. The research carried out, so far, delivered data showing, on the one hand, the use of cellular mechanics as a biomarker of various pathological changes, which, on the other hand, reveal relative nature of biomechanics. In the AFM, the elastic properties of living cells are delivered from indentation experiments and described quantitatively by Young's modulus defined here as a measure of cellular deformability. Here, the AFM studies directly comparing the mechanical properties of normal and cancerous cells are summarized and presented together with a few important issues related to the relativeness of Young's modulus.