A novel apparatus applying long term intermittent cyclic hydrostatic pressure to in vitro cell cultures

Bioreactors with hydrostatic pressures imitating physiological environments in intervertebral discs

Intervertebral discs are normally exposed to a variety of loads and stresses but hydrostatic pressure (HP) could be the main biosignal for chondrogenic cell differentiation and maintenance of this tissue. Although there are simple approaches to intermittently expose cell cultures to HP in separate material testing devices, utilization of biomimetic bioreactors aiming to provide in vitro conditions mimicking those found in vivo, attracts special attention. However, design of such bioreactors is complex due to the requirement of high HP magnitudes (up to 3 MPa) applied in different regimes mimicking pressures arising in intervertebral disc during normal daily activities. Furthermore, efficient mass transfer has to be facilitated to cells within 3D scaffolds, and the engineering challenges include avoidance or removal of gas bubbles in the culture medium before pressurization as well as selection of appropriate, biocompatible construction materials and maintenance of sterility during cultivation. Here, we review approaches to induce HP in 2D and 3D cell cultures categorized into 5 groups: (I) discontinuous systems with direct pressurization of the cultivation medium by a piston, (II) discontinuous systems with indirect pressurization by a compression fluid, (III) continuous systems with direct pressurization of the cultivation medium, static culture, (IV) continuous systems with culture perfusion, and (V) systems applying HP in conjunction with other physical signals. Although the complexity is increasing as additional features are added to the systems, the need to understand HP effects on cells and tissues in a physiologically relevant, yet precisely controlled, environment together with current technological advancements are leading towards innovative bioreactor solutions.

Embryonic stem cell-derived osteocytes are capable of responding to mechanical oscillatory hydrostatic pressure

Osteoblasts can be derived from embryonic stem cells (ESCs) by a 30 day differentiation process, whereupon cells spontaneously differentiate upon removal of LIF and respond to exogenously added 1,25α(OH)2 vitamin D3 with enhanced matrix mineralization. However, bone is a load-bearing tissue that has to perform under dynamic pressure changes during daily movement, a capacity that is executed by osteocytes. At present, it is unclear whether ESC-derived osteogenic cultures contain osteocytes and whether these are capable of responding to a relevant cyclic hydrostatic compression stimulus. Here, we show that ESC-osteoblastogenesis is followed by the generation of osteocytes and then mechanically load ESC-derived osteogenic cultures in a compression chamber using a cyclic loading protocol. Following mechanical loading of the cells, iNOS mRNA was upregulated 31-fold, which was consistent with a role for iNOS as an immediate early mechanoresponsive gene. Further analysis of matrix and bone-specific genes suggested a cellular response in favor of matrix remodeling. Immediate iNOS upregulation also correlated with a concomitant increase in Ctnnb1 and Tcf7l2 mRNAs along with increased nuclear TCF transcriptional activity, while the mRNA for the repressive Tcf7l1 was downregulated, providing a possible mechanistic explanation for the noted matrix remodeling. We conclude that ESC-derived osteocytes are capable of responding to relevant mechanical cues, at least such that mimic oscillatory compression stress, which not only provides new basic understanding, but also information that likely will be important for their use in cell-based regenerative therapies.

Estrogen receptor beta and truncated variants enhance the expression of transfected MMP-1 promoter constructs in response to specific mechanical loading

Background

Joint diseases such as osteoarthritis (OA) predominantly afflict post-menopausal women, suggesting a pertinent role for female hormones. Estrogen receptor beta (ER-β) has been detected in connective tissues of the knee joint suggesting that these tissues are responsive to the hormone estrogen. Matrix metalloproteinase-1 (MMP-1) activity contributes to cartilage degradation, a key factor leading to OA development in synovial joints. Two polymorphic forms of MMP-1 exist due to a deletion/insertion of the guanine residue in the promoter, and the 2G allelic variant of MMP-1 exhibits more activity than the 1G allele. Previous studies have demonstrated that the polymorphic forms of the human MMP-1 are influenced by the modulating effects of estrogen receptor isoforms. In addition to hormonal influences, physiological factors such as altered mechanical loading are also contributory features of OA. In the present study, the combined influence of biomechanical and hormonal variables on the activity of MMP-1 isoforms was evaluated. We hypothesized that the combined effects of ER-β and sheer stress will differentially activate the two allelic forms of MMP-1 in a hormone-independent manner.

Methods

HIG-82 synoviocytes were transiently transfected with 1G or 2G alleles (±) ER-β and subjected to either shear or equibiaxial stress. Next, 1G/2G promoter activity was measured to determine the combined influence of physiological stimuli. Truncated ER-β constructs were used to determine the importance of different domains of ER-β on 1G/2G activation.

Results

The 2G allele exhibited a constitutively higher activity than the 1G allele, which was further increased when the transfected cells were subject to shear stress, but not equibiaxial stress. Moreover, the combination of ER-β and shear stress further increased the activity levels of the 1G/2G allelic variants. Additionally, select AF-2 truncated ER-β variants led to increased activity levels for the 2G allele, indicating the AF-1 domain was likely involved in the response to mechanical stimulation.

Conclusions

These results suggest that the 1G/2G alleles of MMP-1 are influenced by specific mechanical stimuli like shear stress, as well as the ER-β receptor. These findings contribute to the potential allelic involvement in connective tissue diseases such as OA in females compared to males.

Hydrostatic pressure stimulation of human mesenchymal stem cells seeded on collagen-based artificial extracellular matrices

Human mesenchymal stem cells (hMSCs) from bone marrow are considered a promising cell source for bone tissue engineering applications because of their ability to differentiate into cells of the osteoblastic lineage. Mechanical stimulation is able to promote osteogenic differentiation of hMSC; however, the use of hydrostatic pressure (HP) has not been well studied. Artificial extracellular matrices containing collagen and chondroitin sulfate (CS) have promoted the expression of an osteoblastic phenotype by hMSCs. However, there has been little research into the combined effects of biochemical stimulation by matrices and simultaneous mechanical stimulation. In this study, artificial extracellular matrices generated from collagen and/or CS were coated onto polycaprolactone-co-lactide substrates, seeded with hMSCs and subjected to cyclic HP at various time points during 21 days after cell seeding to investigate the effects of biochemical, mechanical, and combined biochemical and mechanical stimulations. Cell differentiation was assessed by analyzing the expression of alkaline phosphatase (ALP) at the protein- and mRNA levels, as well as for calcium accumulation. The timing of HP stimulation affected hMSC proliferation and expression of ALP activity. HP stimulation after 6 days was most effective at promoting ALP activity. CS-containing matrices promoted the osteogenic differentiation of hMSCs. A combination of both CS-containing matrices and cyclic HP yields optimal effects on osteogenic differentiation of hMSCs on scaffolds compared with individual responses.

An in-vitro traumatic model to evaluate the response of myelinated cultures to sustained hydrostatic compression injury

While a variety of in-vitro models have been employed to investigate the response of load-bearing tissues to hydrostatic pressure, long-term studies are limited by the need to provide for adequate gas exchange during pressurization. Applying compression in vitro may alter the equilibrium of the system and thereby disrupt the gas exchange kinetics. To address this, several sophisticated compression chamber designs have been developed. However, these systems are limited in the magnitude of pressure that can be applied and may require frequent media changes, thereby eliminating critical autocrine and paracrine signaling factors. To better isolate the cellular response to long-term compression, we created a model that features continuous gas flow through the chamber during pressurization, and a negative feedback control system to rigorously control dissolved oxygen levels. Monitoring dissolved oxygen continuously during pressurization, we find that the ensuing response exhibits characteristics of a second- or higher-order system which can be mathematically modeled using a second-order differential equation. Finally, we use the system to model chronic nerve compression injuries, such as carpal tunnel syndrome and spinal nerve root stenosis, with myelinated neuron-Schwann cell co-cultures. Cell membrane integrity assay results show that co-cultures respond differently to hydrostatic pressure, depending on the magnitude and duration of stimulation. In addition, we find that myelinated Schwann cells proliferate in response to applied hydrostatic compression.

Development and Validation of a System for the Growth of Cells and Tissues Under Intermittent Hydrostatic Pressure

Various cell populations have been shown to respond to hydrostatic pressure; however, many of the culture systems suffer from shortcomings in design or methodology. Of particular interest to us is the potential role of pressure and other environmental factors in modulating stem cell behavior in intervertebral disk repair. A system was developed for the growth of cells and tissues under intermittent hydrostatic pressure. The system was validated with NIH 3T3 fibroblasts for sterilizability and cytotoxicity. Further experiments were conducted with canine mesenchymal stem cells under various levels of pressure, oxygen, glucose, and conditioned medium. The culture system showed no cytotoxicity and was able to demonstrate that the proliferation and metabolism of mesenchymal stem cells are sensitive to medium glucose and oxygen concentration and hydrostatic pressure. The cells exposed to hydrostatic pressure differed in their morphology from nonexposed cells. The system is capable of supporting long-term cell culture and examining the role of mechanical and environmental stimulation in vivo.