Viscoelasticity and Volume of Cortical Neurons under Glutamate Excitotoxicity and Osmotic Challenges.

Neural activity depends on the maintenance of ionic and osmotic homeostasis. Under these conditions, the cell volume must be regulated to maintain optimal neural function. A disturbance in the neuronal volume regulation often occurs in pathological conditions such as glutamate excitotoxicity. The cell volume, mechanical properties, and actin cytoskeleton structure are tightly connected to achieve the cell homeostasis. Here, we studied the effects of glutamate-induced excitotoxicity, external osmotic pressure, and inhibition of actin polymerization on the viscoelastic properties and volume of neurons. Atomic force microscopy was used to map the viscoelastic properties of neurons in time-series experiments to observe the dynamical changes and a possible recovery. The data obtained on cultured rat cortical neurons were compared with the data obtained on rat fibroblasts. The neurons were found to be more responsive to the osmotic challenges but less sensitive to the inhibition of actin polymerization than fibroblasts. The alterations of the viscoelastic properties caused by glutamate excitotoxicity were similar to those induced by the hypoosmotic stress, but, in contrast to the latter, they did not recover after the glutamate removal. These data were consistent with the dynamic volume changes estimated using ratiometric fluorescent dyes. The recovery after the glutamate-induced excitotoxicity was slow or absent because of a steady increase in intracellular calcium and sodium concentrations. The viscoelastic parameters and their changes were related to such parameters as the actin cortex stiffness, tension, and cytoplasmic viscosity.

Secondary ossification center induces and protects growth plate structure.

Growth plate and articular cartilage constitute a single anatomical entity early in development but later separate into two distinct structures by the secondary ossification center (SOC). The reason for such separation remains unknown. We found that evolutionarily SOC appears in animals conquering the land - amniotes. Analysis of the ossification pattern in mammals with specialized extremities (whales, bats, jerboa) revealed that SOC development correlates with the extent of mechanical loads. Mathematical modeling revealed that SOC reduces mechanical stress within the growth plate. Functional experiments revealed the high vulnerability of hypertrophic chondrocytes to mechanical stress and showed that SOC protects these cells from apoptosis caused by extensive loading. Atomic force microscopy showed that hypertrophic chondrocytes are the least mechanically stiff cells within the growth plate. Altogether, these findings suggest that SOC has evolved to protect the hypertrophic chondrocytes from the high mechanical stress encountered in the terrestrial environment.

Nanomechanical properties of enucleated cells: contribution of the nucleus to the passive cell mechanics.

BACKGROUND: The nucleus, besides its functions in the gene maintenance and regulation, plays a significant role in the cell mechanosensitivity and mechanotransduction. It is the largest cellular organelle that is often considered as the stiffest cell part as well. Interestingly, the previous studies have revealed that the nucleus might be dispensable for some of the cell properties, like polarization and 1D and 2D migration. Here, we studied how the nanomechanical properties of cells, as measured using nanomechanical mapping by atomic force microscopy (AFM), were affected by the removal of the nucleus. METHODS: The mass enucleation procedure was employed to obtain cytoplasts (enucleated cells) and nucleoplasts (nuclei surrounded by plasma membrane) of two cell lines, REF52 fibroblasts and HT1080 fibrosarcoma cells. High-resolution viscoelastic mapping by AFM was performed to compare the mechanical properties of normal cells, cytoplasts, and nucleoplast. The absence or presence of the nucleus was confirmed with fluorescence microscopy, and the actin cytoskeleton structure was assessed with confocal microscopy. RESULTS: Surprisingly, we did not find the softening of cytoplasts relative to normal cells, and even some degree of stiffening was discovered. Nucleoplasts, as well as the nuclei isolated from cells using a detergent, were substantially softer than both the cytoplasts and normal cells. CONCLUSIONS: The cell can maintain its mechanical properties without the nucleus. Together, the obtained data indicate the dominating role of the actomyosin cytoskeleton over the nucleus in the cell mechanics at small deformations inflicted by AFM.

Mechanical properties of anterior lens capsule assessed with AFM and nanoindenter in relation to human aging, pseudoexfoliation syndrome, and trypan blue staining.

The purpose of this study is the mechanical characterization of the mid-to- old-age human anterior lens capsules (ALCs) obtained by capsulorhexis using Atomic Force Microscopy (AFM) and a nanoindenter at different spatial scales. The dependencies on the human age, presence or absence of pseudoexfoliation syndrome (PEX), and application of trypan blue staining during the surgery were analyzed. The measurements on both the anterior (AS) and epithelial (ES) sides of the ALC were conducted and the effect of cells present on the epithelial side was carefully accounted for. The ES of the ALC had a homogenous distribution of the Young's modulus over the surface as shown by the macroscale mapping with the nanoindenter and local AFM indentations, while the AS was more heterogeneous. Age-related changes were assessed in groups ranging from the mid-age (from 48 years) to old-age (up to 93 years). We found that the ES was always stiffer than the AS, and this difference decreased with age due to a gradual decrease in the Young's modulus of the ES and an increase in the modulus of the AS. No significant changes were found in the mechanical properties of ALCs of PEX patients versus the PEX-free group, as well as in the properties of the ALC with and without trypan blue staining.

Collagen fibrillar structures in vocal fold scarring and repair using stem cell therapy: a detailed histological, immunohistochemical and atomic force microscopy study.

Regenerative medicine opens new opportunities in the repair of cicatricial lesions of the vocal folds. Here, we present a thorough morphological study, with the focus on the collagen structures in the mucosa of the vocal folds, dedicated to the effects of stem cells on the vocal folds repair after cicatricial lesions. We used a conventional experimental model of a mature scar of the rabbit vocal folds, which was surgically excised with a simultaneous implantation of autologous bone marrow-derived mesenchymal stem cells (MSC) into the defect. The restoration of the vocal folds was studied 3 months postimplantation of stem cells and 6 months after the first surgery. The collagen structure assessment included histology, immunohistochemistry and atomic force microscopy (AFM) studies. According to the data of optical microscopy and AFM, as well as to immunohistochemical analysis, MSC implantation into the vocal fold defect leads not only to the general reduction of scarring, normal ratio of collagens type I and type III, but also to a more complete restoration of architecture and ultrastructure of collagen fibres in the mucosa, as compared to the control. The collagen structures in the scar tissue in the vocal folds with implanted MSC are more similar to those in the normal mucosa of the vocal folds than to those of the untreated scars. AFM has proven to be an instrumental technique in the assessment of the ultrastructure restoration in such studies. LAY DESCRIPTION: Regenerative medicine opens new opportunities in the repair of the vocal fold scars. Because collagen is a main component in the vocal fold mucosa responsible for the scar formation and repair, we focus on the collagen structures in the mucosa of the vocal folds, using a thorough morphological study based on histology and atomic force microscopy (AFM). Atomic force microscopy is a scanning microscopic technique which allows revealing the internal structure of a tissue with a resolution up to nanometres. We used a conventional experimental model of a mature scar of the rabbit vocal folds, surgically excised and treated with a mesenchymal stem cells transplant. Our morphological study, primarily AFM, explicitly shows that the collagen structures in the scarred vocal folds almost completely restore after the stem cell treatment. Thus, the modern microscopic methods, and especially AFM are instrumental tools for monitoring the repair of the vocal folds scars.