Hyperosmotically induced volume change and calcium signaling in intervertebral disk cells: the role of the actin cytoskeleton

Osmolarity regulates gene expression in intervertebral disc cells determined by gene array and real-time quantitative RT-PCR

Intervertebral disc (IVD) cells experience a broad range of physicochemical stimuli under physiologic conditions, including alterations in their osmotic environment. Cellular responses to altered osmolarity have been documented at the transcriptional and post-translational level, but mainly for extracellular matrix proteins. In this study, the gene expression profile of human IVD cells was quantified with gene array technology following exposure to increased osmolarity in order to capture the biological responses for a broad set of targets. A total of 42 genes were identified in IVD cells as significantly changed following culture under hyper-osmotic conditions. Gene expression patterns were verified using RT-PCR. Genes identified in this study include those related to cytoskeleton remodeling and stabilization (ephrin-B2, muskelin), as well as membrane transport (ion transporter SLC21A12, osmolyte transporter SLC5A3, monocarboxylic acid SLC16A6). An unexpected finding was the differential regulation of the gene for the neurotrophin, brain-derived neurotrophic factor, by hyper-osmotic stimuli that suggests a capability of IVD cells to respond to physicochemical stimuli with factors that may regulate discogenic pain.

On the mechanism of cell lysis by deformation

In this study, we identify the extent of deformation that causes cell lysis using a simple technique where a drop of cell suspension is compressed by two flat plates. The viability of human prostatic adenocarcinoma PC-3 cells in solutions of various concentrations of NaCl is determined as a function of the gap size between the plates. The viability declines with decreasing gap size in the following order: 700 mM >150 mM >75 mM NaCl. This is considered to be due to the difference in cell size, which is caused by the osmotic volume change before deformation; cell diameter becomes smaller in a solution of higher NaCl concentration, which appears to increase the survival ratio in a given gap size. The deformation-induced decrease in cell viability is correlated with the cell surface strain, which is dependent on the increase in surface area, irrespective of NaCl concentration. In addition, the treatment of cells with cytochalasin D results in the disappearance of cortical actin filaments and a marked drop in the viability, indicating that cell lysis is closely related to the deformation of the cytoskeleton.

Cell mechanics and mechanobiology in the intervertebral disc

STUDY DESIGN

A review is presented on current knowledge of the micromechanical factors in the intervertebral disc, their role in modifying cell biology, and changes with degeneration.

OBJECTIVES

To identify current knowledge, knowledge gaps, and areas for future research in micromechanics of the intervertebral disc.

SUMMARY OF BACKGROUND DATA

Mechanical factors play important roles in the initiation and progression of intervertebral disc degeneration. Evidence suggests that substantial biologic remodeling occurs in the intervertebral disc in response to mechanical stimuli that may play a critical role in determining the fate of a degenerating intervertebral disc. Information is needed on the precise mechanical stimuli that these cells experience and the mechanisms that govern their responses.

METHODS

A review is presented of cell morphology, cell mechanics, and the internal strains and other mechanical factors predicted to occur at the cell level. A review of intervertebral disc cell responses to well-controlled physical stimuli is also presented with a focus on in vitro studies of explants and isolated cells.

RESULTS

Important differences in cell morphology, mechanics, micromechanical factors, and mechanobiology are noted to occur between cells of the nucleus pulposus and anulus fibrosus. Changes in these features with degeneration are critically understudied, particularly degeneration-associated changes in cell morphology, cell mechanics, and altered physiology with mechanical loading.

CONCLUSIONS

Information on the mechanisms that govern cell responses to mechanical stimuli in the intervertebral disc are just emerging. Studies must address determination of the factors that control micromechanical stimuli, but also mechanisms by which mechanics may interact with genetic factors to regulate expression and remodeling of extracellular matrix molecules, cytokines and mediators of pain and inflammation in degenerating tissue.

Dynamic osmotic loading of chondrocytes using a novel microfluidic device

Many cells exhibit disparate responses to a mechanical stimulus depending on whether it is applied dynamically or statically. In this context, few studies have examined how cells respond to dynamic changes of the extracellular osmolality. In this study, we hypothesized that the cell size change response of cultured articular chondrocytes would be dependent on the frequency of applied osmotic loading. To test this hypothesis, we developed a novel microfluidic device, to apply hydrostatic pressure-driven dynamic osmotic loading by applying composition modulated flow, adapted from Tang and co-workers. This microfluidic device was used to study osmotic loads of +/-180 mOsm at a frequency up to 0.1 Hz with a constant minimal fluid-shear stress, and permit real-time monitoring of cell responses. Bovine articular chondrocytes were observed to exhibit increasing changes in cell volume with decreasing osmotic loading frequency. When the cell volume response was modeled by an exponential function, chondrocytes exhibited significantly different volume change responses to dynamic osmotic loading at 0.0125 Hz and static osmotic loading applied for a period of four minutes (Delta = +/-180 mOsm relative to the isotonic 360 mOsm). The intracellular calcium response at 0.0125 Hz was also monitored and compared with the response to static loading. Coupled with phenomenological or constitutive models, this novel approach could yield new information regarding cell material properties in response to dynamic loading that may contribute new insights into mechanisms of cellular homeostasis and mechanotransduction.