Novel methods for the quantification of changes in actin organization in chondrocytes using fluorescent imaging and linear profiling

Super-resolution imaging for monitoring cytoskeleton dynamics

The cytoskeleton is a key cellular structure that is important in the control of cellular movement, structure, and sensing. To successfully image the individual cytoskeleton components, high resolution and super-resolution fluorescence imaging methods are needed. This review covers the three basic cytoskeletal elements and the relative benefits and drawbacks of fixed versus live cell imaging before moving on to recent studies using high resolution and super-resolution techniques. The techniques covered include the near-diffraction limited imaging methods of confocal microscopy and TIRF microscopy and the super-resolution fluorescence imaging methods of STORM, PALM, and STED.

Tough and biocompatible hydrogels based on in situ interpenetrating networks of dithiol-connected graphene oxide and poly(vinyl alcohol)

An interpenetrating network (IPN) strategy has been widely facilitated to construct strong and tough hydrogels, but most of the efforts have been focused on organic/organic networks. Herein, aqueous dispersible 2,2'-(ethylenedioxy)-diethanethiol (EDDET) cross-linked graphene oxide (E-cGO) skeleton was in situ incorporated into a PVA matrix, resulting in novel inorganic/organic IPN hydrogels with super mechanical and chondrocyte cell-adhesion properties. The unique interpenetrating structure and hydrogen bonding were demonstrated to play critical roles in enhancing the compressive property of the IPN hydrogels, in comparison to the GO and thermally reduced graphene oxide (T-rGO) filled hydrogels. It is critical that the E-cGO/PVA hydrogels have been demonstrated as being biocompatible, which make the E-cGO/PVA hydrogels promising candidate biomaterials for load-bearing biotissue substitution.

The rate of hypo-osmotic challenge influences regulatory volume decrease (RVD) and mechanical properties of articular chondrocytes

OBJECTIVES

Osteoarthritis (OA) is associated with a gradual reduction in the interstitial osmotic pressure within articular cartilage. The aim of this study was to compare the effects of sudden and gradual hypo-osmotic challenge on chondrocyte morphology and biomechanics.

METHODS

Bovine articular chondrocytes were exposed to a reduction in extracellular osmolality from 327 to 153 mOsmol/kg applied either suddenly (<5 s) or gradually (over 180 min). Temporal changes in cell diameter and the existence of regulatory volume decrease (RVD) were quantified along with changes in cortical actin and chromatin condensation. The cellular viscoelastic mechanical properties were determined by micropipette aspiration.

RESULTS

In response to a sudden hypo-osmotic stress, 66% of chondrocytes exhibited an increase in diameter followed by RVD, whilst 25% showed no RVD. By contrast, cells exposed to gradual hypo-osmotic stress exhibited reduced cell swelling without subsequent RVD. There was an increase in the equilibrium modulus for cells exposed to sudden hypo-osmotic stress. However, gradual hypo-osmotic challenge had no effect on cell mechanical properties. This cell stiffening response to sudden hypo-osmotic challenge was abolished when actin organization was disrupted with cytochalasin D or RVD inhibited with REV5901. Both sudden and gradual hypo-osmotic challenge reduced cortical F-actin distribution and caused chromatin decondensation.

CONCLUSIONS

Sudden hypo-osmotic challenge increases chondrocyte mechanics by activation of RVD and interaction with the actin cytoskeleton. Moreover, the rate of hypo-osmotic challenge is shown to have a profound effect on chondrocyte morphology and biomechanics. This important phenomenon needs to be considered when studying the response of chondrocytes to pathological hypo-osmotic stress.

Validation of an algorithm to quantify changes in actin cytoskeletal organization

The actin cytoskeleton plays an important role in most, if not all, processes necessary for cell survival. Given the fundamental role that the actin cytoskeleton plays in the progression of cancer, it is an ideal target for chemotherapy. Although it is possible to image the actin cytoskeleton in a high-throughput manner, there is currently no validated method to quantify changes in the cytoskeleton in the same capacity, which makes research into its organization and the development of anticytoskeletal drugs difficult. We have validated the use of a linear feature detection algorithm, allowing us to measure changes in actin filament organization. Its ability to quantify changes associated with cytoskeletal disruption will make it a valuable tool in the development of compounds that target the cytoskeleton in cancer. Our results show that this algorithm can quantify cytoskeletal changes in a cell-based system after addition of both well-established and novel anticytoskeletal agents using either fluorescence microscopy or a high-content imaging approach. This novel method gives us the potential to screen compounds in a high-throughput manner for cancer and other diseases in which the cytoskeleton plays a key role.

Primary human chondrocyte extracellular matrix formation and phenotype maintenance using RGD-derivatized PEGDM hydrogels possessing a continuous Young's modulus gradient

Efficient ex vivo methods for expanding primary human chondrocytes while maintaining the phenotype is critical to advancing the sourcing of autologous cells for tissue engineering applications. While there has been significant research reported in the literature, systematic approaches are necessary to determine and optimize the chemical and mechanical scaffold properties for hyaline cartilage generation using limited cell numbers. Functionalized hydrogels possessing continuous variations in physico-chemical properties are, therefore, an efficient three-dimensional platform for studying several properties simultaneously. Herein we describe a polyethylene glycol dimethacrylate (PEGDM) hydrogel system with a modulus gradient (~27,000-3800 Pa) containing a uniform concentration of arginine-glycine-aspartic acid (RGD) peptide to enhance cell adhesion in order to correlate primary human osteoarthritic chondrocyte proliferation, phenotype maintenance, and extracellular matrix (ECM) production with hydrogel properties. Cell number and chondrogenic phenotype (CD14:CD90 ratios) were found to decline in regions with a higher storage modulus (>13,100 Pa), while regions with a lower storage modulus maintained their cell number and phenotype. Over 3 weeks culture hydrogel regions possessing a lower Young's modulus experienced an increase in ECM content (~200%) compared with regions with a higher storage modulus. Variations in the amount and organization of the cytoskeletal markers actin and vinculin were observed within the modulus gradient, which are indicative of differences in chondrogenic phenotype maintenance and ECM expression. Thus scaffold mechanical properties have a significant impact in modulating human osteoarthritic chondrocyte behavior and tissue formation.