“Culture shock” from the bone cell's perspective: emulating physiological conditions for mechanobiological investigations

Microfluidic Systems for Neural Cell Studies

Whereas the axons of the peripheral nervous system (PNS) spontaneously regenerate after an injury, the occurring regeneration is rarely successful because axons are usually directed by inappropriate cues. Therefore, finding successful ways to guide neurite outgrowth, in vitro, is essential for neurogenesis. Microfluidic systems reflect more appropriately the in vivo environment of cells in tissues such as the normal fluid flow within the body, consistent nutrient delivery, effective waste removal, and mechanical stimulation due to fluid shear forces. At the same time, it has been well reported that topography affects neuronal outgrowth, orientation, and differentiation. In this review, we demonstrate how topography and microfluidic flow affect neuronal behavior, either separately or in synergy, and highlight the efficacy of microfluidic systems in promoting neuronal outgrowth.

Designing and validation of an automated ex-vivo bioreactor system for long term culture of bone

Several different bioreactors have been developed to study bone biology. Keeping a bone viable for long-term studies is still a challenge. We have developed an ex-vivo bone bioreactor that can keep the ex-vivo live bone viable for more than 4 weeks. Keeping a bone viable for over a month can be used as an alternative model for in-vivo experiments in animals. We hypothesize that the perfusion flow and mechanical load on the bone provide a real-time environment for the bone to survive. Cancellous bones were harvested from the bovine metatarsals and were placed in the dynamic culture with cyclic loading at regular intervals. After a period of week 4, the bone cores were retrieved from the bioreactor and tested for viability using calcein-AM and ethidium homodimer -1 fluorescent dyes and were compared with the cores that were placed in static culture with and without any loads on them and Day 0 bone core that acted as a positive control. The bone blocks were then fixed in 10% formalin, and bone mineral density was evaluated using a DXA scanner before staining them for H&E to study the morphological changes. Results revealed that the bone cultured in the bioreactor was viable as compared to the one in the static culture with and without constant load. Bone cores cultured in our ex-vivo bioreactor system also maintained their morphology and no statistical difference was found in the bone mineral density compared to positive controls and the statistical difference was found when compared with the cores cultured in static culture. This tool can be used to study bone biology for various applications such as bone ingrowth studies, to study the effect of drugs, hormones, or any growth factors, and much more.

Mechanomics Approaches to Understand Cell Behavior in Context of Tissue Neogenesis, During Prenatal Development and Postnatal Healing

Mechanomics represents the natural progression of knowledge at the intersection of mechanics and biology with the aim to codify the role of mechanical environment on biological adaptation. Compared to the mapping of the human genome, the challenge of mapping the mechanome remains unsolved. Solving this grand challenge will require both top down and bottom up R&D approaches using experimental and computational tools to visualize and measure adaptation as it occurs. Akin to a mechanical test of a smart material that changes its mechanical properties and local environment under load, stem cells adapt their shape, cytoskeletal architecture, intrinsic mechanical properties, as well as their own niche, through cytoskeletal adaptation as well as up- and down-regulation of structural proteins that modulate their mechanical milieux. Recent advances in live cell imaging allow for unprecedented study and measurements of displacements, shape and volume changes in stem cells, reconfiguring of cytoskeletal machinery (nucleus, cytoskeleton), in response to controlled mechanical forces and stresses applied at cellular boundaries. Coupled with multiphysics computational and virtual power theoretical approaches, these novel experimental approaches enable mechanical testing of stem cells, multicellular templates, and tissues inhabited by stem cells, while the stem cells themselves evolve over time. The novel approach is paving the way to decipher mechanisms of structural and functional adaptation of stem cells in response to controlled mechanical cues. This mini-review outlines integrated approaches and methodologies implemented to date in a series of studies carried out by our consortium. The consortium's body of work is described in context of current roadblocks in the field and innovative, breakthrough solutions and is designed to encourage discourse and cross disciplinary collaboration in the scientific community.

Mapping the Mechanome-A Protocol for Simultaneous Live Imaging and Quantitative Analysis of Cell Mechanoadaptation and Ingression

Mechanomics, the mechanics equivalent of genomics, is a burgeoning field studying mechanical modulation of stem cell behavior and lineage commitment. Analogous to mechanical testing of a living material as it adapts and evolves, mapping of the mechanome necessitates the development of new protocols to assess changes in structure and function in live stem cells as they adapt and differentiate. Previous techniques have relied on imaging of cellular structures in fixed cells and/or live cell imaging of single cells with separate studies of changes in mechanical and biological properties. Here we present two complementary protocols to study mechanobiology and mechanoadaptation of live stem cells in adherent and motile contexts. First, we developed and tested live imaging protocols for simultaneous visualization and tracking of actin and tubulin mechanoadaptation as well as shape and volume of cells and their nuclei in adherent model embryonic murine mesenchymal stem cells (C3H/10T1/2) and in a neuroblastoma cell line. Then we applied the protocol to enable quantitative study of primary human mesenchymal stem cells in a motile state, e.g., ingression in a three-dimensional, in vitro cell culture model. Together, these protocols enable study of emergent structural mechanoadaptation of the cell's own cytoskeletal machinery while tracking lineage commitment using phenotypic (quantitative morphology measures) and genotypic (e.g., reverse transcription Polymerase Chain Reaction, rtPCR) methods. These tools are expected to facilitate the mapping of the mechanome and incipient mechanistic understanding of stem cell mechanobiology, from the cellular to the tissue and organ length scales.

High glucose alters the secretome of mechanically stimulated osteocyte-like cells affecting osteoclast precursor recruitment and differentiation

Diabetes mellitus (DM) induces bone deterioration, while mechanical stimulation promotes osteocyte-driven bone formation. We aimed to evaluate the interaction of acute exposure (24 h) to high glucose (HG) with both the pro-survival effect conferred to osteocytic MLO-Y4 cells and osteoblastic MC3T3-E1 cells by mechanical stimulation and the interaction of these cells with osteoclast precursor RAW264.7 cells. We found that 24 h of HG (25 mM) pre-exposure prevented both cell survival and ERK and β-catenin nuclear translocation upon mechanical stimulation by fluid flow (FF) (10 min) in both MLO-Y4 and MC3T3-E1 cells. However, migration of RAW 264.7 cells was inhibited by MLO-Y4 cell-conditioned medium (CM), but not by MC3T3-E1 cell-CM, with HG or FF. This inhibitory effect was associated with consistent changes in VEGF, RANTES, MIP-1α, MIP-1β MCP-1, and GM-CSF in MLO-Y4 cell-CM. RAW264.7 proliferation was inhibited by MLO-Y4 CM under static or HG conditions, but it increased by FF-CM with or without HG. In addition, both FF and HG abrogated the capacity of RAW 264.7 cells to differentiate into osteoclasts, but in a different manner. Thus, HG-CM in static condition allowed formation of osteoclast-like cells, which were unable to resorb hydroxyapatite. In contrast, FF-CM prevented osteoclastogenesis even in HG condition. Moreover, HG did not affect basal RANKL or IL-6 secretion or their inhibition induced by FF in MLO-Y4 cells. In conclusion, this in vitro study demonstrates that HG exerts disparate effects on osteocyte mechanotransduction, and provides a novel mechanism by which DM disturbs skeletal metabolism through altered osteocyte-osteoclast communication.

Novel multi-functional fluid flow device for studying cellular mechanotransduction

Cells respond to their mechanical environment by initiating multiple mechanotransduction signaling pathways. Defects in mechanotransduction have been implicated in a number of pathologies; thus, there is need for convenient and efficient methods for studying the mechanisms underlying these processes. A widely used and accepted technique for mechanically stimulating cells in culture is the introduction of fluid flow on cell monolayers. Here, we describe a novel, multifunctional fluid flow device for exposing cells to fluid flow in culture. This device integrates with common lab equipment including routine cell culture plates and peristaltic pumps. Further, it allows the fluid flow treated cells to be examined with outcomes at the cell and molecular level. We validated the device using the biologic response of cultured UMR-106 osteoblast-like cells in comparison to a commercially available system of laminar sheer stress to track live cell calcium influx in response to fluid flow. In addition, we demonstrate the fluid flow-dependent activation of phospho-ERK in these cells, consistent with the findings in other fluid flow devices. This device provides a low cost, multi-functional alternative to currently available systems, while still providing the ability to generate physiologically relevant conditions for studying processes involved in mechanotransduction in vitro.

Flow-induced stress on adherent cells in microfluidic devices

Transduction of mechanical forces and chemical signals affect every cell in the human body. Fluid flow in systems such as the lymphatic or circulatory systems modulates not only cell morphology, but also gene expression patterns, extracellular matrix protein secretion and cell-cell and cell-matrix adhesions. Similar to the role of mechanical forces in adaptation of tissues, shear fluid flow orchestrates collective behaviours of adherent cells found at the interface between tissues and their fluidic environments. These behaviours range from alignment of endothelial cells in the direction of flow to stem cell lineage commitment. Therefore, it is important to characterize quantitatively fluid interface-dependent cell activity. Common macro-scale techniques, such as the parallel plate flow chamber and vertical-step flow methods that apply fluid-induced stress on adherent cells, offer standardization, repeatability and ease of operation. However, in order to achieve improved control over a cell's microenvironment, additional microscale-based techniques are needed. The use of microfluidics for this has been recognized, but its true potential has emerged only recently with the advent of hybrid systems, offering increased throughput, multicellular interactions, substrate functionalization on 3D geometries, and simultaneous control over chemical and mechanical stimulation. In this review, we discuss recent advances in microfluidic flow systems for adherent cells and elaborate on their suitability to mimic physiologic micromechanical environments subjected to fluid flow. We describe device design considerations in light of ongoing discoveries in mechanobiology and point to future trends of this promising technology.

Role of the Parathyroid Hormone Type 1 Receptor (PTH1R) as a Mechanosensor in Osteocyte Survival

Osteocytes have a major role in the control of bone remodeling. Mechanical stimulation decreases osteocyte apoptosis and promotes bone accrual, whereas skeletal unloading is deleterious in both respects. PTH1R ablation or overexpression in osteocytes in mice produces trabecular bone loss or increases bone mass, respectively. The latter effect was related to a decreased osteocyte apoptosis. Here, the putative role of PTH1R activation in osteocyte protection conferred by mechanical stimulation was assessed. Osteocytic MLO-Y4 cells were subjected to mechanical stimuli represented by hypotonic shock (216 mOsm/kg) or pulsatile fluid flow (8 Hz, 10 dynes/cm(2)) for a short pulse (10 min), with or without PTH1R antagonists or after transfection with specific PTHrP or PTH1R siRNA. These mechanical stimuli prevented cell death induced within 6 hours by etoposide (50 μM), related to PTHrP overexpression; and this effect was abolished by the calcium antagonist verapamil (1 μM), a phospholipase C (PLC) inhibitor (U73122; 10 μM), and a PKA activation inhibitor, Rp-cAMPS (25 μM), in these cells. Each mechanical stimulus also rapidly induced β-catenin stabilization and nuclear ERK translocation, which were inhibited by the PTH1R antagonist PTHrP(7-34) (1 μM), or PTH1R siRNA, and mimicked by PTHrP(1-36) (100 nM). Mechanical stretching by hypotonic shock did not affect cAMP production but rapidly (<1 min) stimulated Ca(i)(2+) transients in PTH1R-overexpressing HEK-293 cells and in MLO-Y4 cells, in which calcium signaling was unaffected by the presence of a PTHrP antiserum or PTHrP siRNA but inhibited by knocking down PTH1R. These novel findings indicate that PTH1R is an important component of mechanical signal transduction in osteocytic MLO-Y4 cells, and that PTH1R activation by PTHrP-independent and dependent mechanisms has a relevant role in the prosurvival action of mechanical stimulus in these cells.

Concise review: the periosteum: tapping into a reservoir of clinically useful progenitor cells

Elucidation of the periosteum and its regenerative potential has become a hot topic in orthopedics. Yet few review articles address the unique features of periosteum-derived cells, particularly in light of translational therapies and engineering solutions inspired by the periosteum's remarkable regenerative capacity. This review strives to define periosteum-derived cells in light of cumulative research in the field; in addition, it addresses clinical translation of current insights, hurdles to advancement, and open questions in the field. First, we examine the periosteal niche and its inhabitant cells and the key characteristics of these cells in the context of mesenchymal stem cells and their relevance for clinical translation. We compare periosteum-derived cells with those derived from the marrow niche in in vivo studies, addressing commonalities as well as features unique to periosteum cells that make them potentially ideal candidates for clinical application. Thereafter, we review the differentiation and tissue-building properties of periosteum cells in vitro, evaluating their efficacy in comparison with marrow-derived cells. Finally, we address a new concept of banking periosteum and periosteum-derived cells as a novel alternative to currently available autogenic umbilical blood and perinatal tissue sources of stem cells for today's population of aging adults who were "born too early" to bank their own perinatal tissues. Elucidating similarities and differences inherent to multipotent cells from distinct tissue niches and their differentiation and tissue regeneration capacities will facilitate the use of such cells and their translation to regenerative medicine.

Flow perfusion maintains ex vivo bone viability: a novel model for bone biology research

Encased in lacunae, osteocytes receive nutrition and biomechanical signals through the lacunocanalicular system. We have developed a novel flow-perfusion bioreactor designed to support lacunocanalicular fluid flow. We hypothesize that ex vivo fluid flow can maintain endochondral bone viability and, ultimately, serve as a novel model to study bone biology in vitro. Sprague-Dawley rat femurs were harvested, stripped of soft tissue, loaded into a custom-designed bioreactor and perfused with osteogenic culture medium. After 14 days of flow-perfusion or static culture, the bones were harvested, fixed, decalcified, embedded, sectioned and stained with haematoxylin and eosin. Fresh long bone samples were similarly processed for comparison. Osteocyte viability and function were also evaluated, using thiazolyl blue tetrazolium bromide (MTT), fluorospectrophotometric DNA quantification, alkaline phosphatase (ALP) colorimetric assay and fluorochrome labelling of mineralizing surfaces. All samples remained free of infection throughout the study period. After 14 days of flow perfusion, histological analysis showed normal-appearing bony architecture, with 72% of lacunae being osteocyte-filled compared with 93% in freshly harvested samples and only 36% in static samples. MTT staining and assay confirmed osteocyte viability in the flow-perfusion samples as well as in fresh samples. DNA quantification demonstrated DNA to be preserved in flow-perfused samples when compared with freshly harvested samples. ALP activity in flow-perfusion explants was upregulated compared with fresh and static samples. Fluorochrome-labelled mineralizing surfaces were seen throughout the explanted flow-perfused samples. This is the first demonstration that flow perfusion provides adequate chemotransportation to explanted murine endochondal bones.

Top down and bottom up engineering of bone

The goal of this retrospective article is to place the body of my lab's multiscale mechanobiology work in context of top-down and bottom-up engineering of bone. We have used biosystems engineering, computational modeling and novel experimental approaches to understand bone physiology, in health and disease, and across time (in utero, postnatal growth, maturity, aging and death, as well as evolution) and length scales (a single bone like a femur, m; a sample of bone tissue, mm-cm; a cell and its local environment, μm; down to the length scale of the cell's own skeleton, the cytoskeleton, nm). First we introduce the concept of flow in bone and the three calibers of porosity through which fluid flows. Then we describe, in the context of organ-tissue, tissue-cell and cell-molecule length scales, both multiscale computational models and experimental methods to predict flow in bone and to understand the flow of fluid as a means to deliver chemical and mechanical cues in bone. Addressing a number of studies in the context of multiple length and time scales, the importance of appropriate boundary conditions, site specific material parameters, permeability measures and even micro-nanoanatomically correct geometries are discussed in context of model predictions and their value for understanding multiscale mechanobiology of bone. Insights from these multiscale computational modeling and experimental methods are providing us with a means to predict, engineer and manufacture bone tissue in the laboratory and in the human body.

In situ spatiotemporal mapping of flow fields around seeded stem cells at the subcellular length scale

A major hurdle to understanding and exploiting interactions between the stem cell and its environment is the lack of a tool for precise delivery of mechanical cues concomitant to observing sub-cellular adaptation of structure. These studies demonstrate the use of microscale particle image velocimetry (μ-PIV) for in situ spatiotemporal mapping of flow fields around mesenchymal stem cells, i.e. murine embryonic multipotent cell line C3H10T1/2, at the subcellular length scale, providing a tool for real time observation and analysis of stem cell adaptation to the prevailing mechanical milieu. In the absence of cells, computational fluid dynamics (CFD) predicts flow regimes within 12% of μ-PIV measures, achieving the technical specifications of the chamber and the flow rates necessary to deliver target shear stresses at a particular height from the base of the flow chamber. However, our μ-PIV studies show that the presence of cells per se as well as the density at which cells are seeded significantly influences local flow fields. Furthermore, for any given cell or cell seeding density, flow regimes vary significantly along the vertical profile of the cell. Hence, the mechanical milieu of the stem cell exposed to shape changing shear stresses, induced by fluid drag, varies with respect to proximity of surrounding cells as well as with respect to apical height. The current study addresses a previously unmet need to predict and observe both flow regimes as well as mechanoadaptation of cells in flow chambers designed to deliver precisely controlled mechanical signals to live cells. An understanding of interactions and adaptation in response to forces at the interface between the surface of the cell and its immediate local environment may be key for de novo engineering of functional tissues from stem cell templates as well as for unraveling the mechanisms underlying multiscale development, growth and adaptation of organisms.

Biological basis of bone formation, remodeling, and repair-part I: biochemical signaling molecules

The bony biochemical environment is an active and dynamic system that permits and promotes cellular functions that lead to matrix production and ossification. Each component is capable of conveying important regulatory cues to nearby cells, thus effecting gene expression and changes at the cytostructural level. Here, we review the various signaling molecules that contribute to the active and dynamic nature of the biochemical system. These components include hormones, cytokines, and growth factors. We describe their role in regulating bone metabolism. Certain growth factors (i.e., TGF-beta, IGF-1, and VEGF) are described in greater detail because of their potential importance in developing successful tissue-engineering strategies.

3D collagen cultures under well-defined dynamic strain: a novel strain device with a porous elastomeric support

The field of mechanobiology has grown tremendously in the past few decades, and it is now well accepted that dynamic stresses and strains can impact cell and tissue organization, cell-cell and cell-matrix communication, matrix remodeling, cell proliferation and apoptosis, cell migration, and many other cell behaviors in both physiological and pathophysiological situations. Natural reconstituted matrices like collagen and fibrin are often used for three-dimensional (3D) mechanobiology studies because they naturally form fibrous architectures and are rich in cell adhesion sites; however, they are physically weak and typically contain >99% water, making it difficult to apply dynamic stresses to them in a truly 3D context. Here we present a composite matrix and strain device that can support natural matrices within a macroporous elastic structure of polyurethane. We characterize this system both in terms of its mechanical behavior and its ability to support the growth and in vivo-like behaviors of primary human lung fibroblasts cultured in collagen. The porous polyurethane was created with highly interconnected pores in the hundreds of microm size scale, so that while it did not affect cell behavior in the collagen gel within the pores, it could control the overall elastic behavior of the entire tissue culture system. In this way, a well-defined dynamic strain could be imposed on the 3D collagen and cells within the collagen for several days (with elastic recoil driven by the polyurethane) without the typical matrix contraction by fibroblasts when cultured in 3D collagen gels. We show lung fibroblast-to-myofibroblast differentiation under 30%, 0.1 Hz dynamic strain to validate the model and demonstrate its usefulness for a wide range of tissue engineering applications.

Metastatic breast cancer cells colonize and degrade three-dimensional osteoblastic tissue in vitro

Metastatic breast cancer cells (BCs) colonize a mineralized three-dimensional (3D) osteoblastic tissue (OT) grown from isolated pre-osteoblasts for up to 5 months in a specialized bioreactor. Sequential stages of BC interaction with OT include BC adhesion, penetration, colony formation, and OT reorganization into "Indian files" paralleling BC colonies, heretofore observed only in authentic pathological cancer tissue. BCs permeabilize OT by degrading the extra-cellular collagenous matrix (ECM) in which the osteoblasts are embedded. OT maturity (characterized by culture age and cell phenotype) profoundly affects the patterns of BC colonization. BCs rapidly form colonies on immature OT (higher cell/ECM ratio, osteoblastic phenotype) but fail to completely penetrate OT. By contrast, BCs efficiently penetrate mature OT (lower cell/ECM ratio, osteocytic phenotype) and reorganize OT. BC colonization provokes a strong osteoblast inflammatory response marked by increased expression of the pro-inflammatory cytokine IL-6. Furthermore, BCs inhibit osteoblastic bone formation by down-regulating synthesis of collagen and osteocalcin. Results strongly suggest that breast cancer disrupts the process of osteoblastic bone formation, in addition to upregulating osteoclastic bone resorption as widely reported. These observations may help explain why administration of bisphosphonates to humans with osteolytic metastases slows lesion progression by inhibiting osteoclasts but does not bring about osteoblast-mediated healing.

Mechanotransduction in human bone: in vitro cellular physiology that underpins bone changes with exercise

Bone has a remarkable ability to adjust its mass and architecture in response to a wide range of loads, from low-level gravitational forces to high-level impacts. A variety of types and magnitudes of mechanical stimuli have been shown to influence human bone cell metabolism in vitro, including fluid shear, tensile and compressive strain, altered gravity and vibration. Therefore, the current article aims to synthesize in vitro data regarding the cellular mechanisms underlying the response of human bone cells to mechanical loading. Current data demonstrate commonalities in response to different types of mechanical stimuli on the one hand, along with differential activation of intracellular signalling on the other. A major unanswered question is, how do bone cells sense and distinguish between different types of load? The studies included in the present article suggest that the type and magnitude of loading may be discriminated by overlapping mechanosensory mechanisms including (i) ion channels; (ii) integrins; (iii) G-proteins; and (iv) the cytoskeleton. The downstream signalling pathways identified to date appear to overlap with known growth factor and hormone signals, providing a mechanism of interaction between systemic influences and the local mechanical environment. Finally, the data suggest that exercise should emphasize the amount of load rather than the number of repetitions.

A PLA/calcium phosphate degradable composite material for bone tissue engineering: an in vitro study

Biodegradable polymers reinforced with an inorganic phase such as calcium phosphate glasses may be a promising approach to fulfil the challenging requirements presented by 3D porous scaffolds for tissue engineering. Scaffolds' success depends mainly on their biological behaviour. This work is aimed to the in vitro study of polylactic acid (PLA)/CaP glass 3D porous constructs for bone regeneration. The scaffolds were elaborated using two different techniques, namely solvent-casting and phase-separation. The effect of scaffolds' micro and macrostructure on the biological response of these scaffolds was assayed. Cell proliferation, differentiation and morphology within the scaffolds were studied. Furthermore, polymer/glass scaffolds were seeded under dynamic conditions in a custom-made perfusion bioreactor. Results indicate that the final architecture of the solvent-cast or phase separated scaffolds have a significant effect on cells' behaviour. Solvent-cast scaffolds seem to be the best candidates for bone tissue engineering. Besides, dynamic seeding yielded a higher seeding efficiency in comparison with the static method.

Open access to novel dual flow chamber technology for in vitro cell mechanotransduction, toxicity and pharamacokinetic studies

Background

A major stumbling block for researchers developing experimental models of mechanotransduction is the control of experimental variables, in particular the transmission of the mechanical forces at the cellular level. A previous evaluation of state of the art commercial perfusion chambers showed that flow regimes, applied to impart a defined mechanical stimulus to cells, are poorly controlled and that data from studies in which different chambers are utilized can not be compared, even if the target stress regimes are comparable.

Methods

This study provides a novel chamber design to provide both physiologically-based flow regimes, improvements in control of experimental variables, as well as ease of use compared to commercial chambers. This novel design achieves controlled stresses through five gasket designs and both single- and dual-flow regimes.

Results

The imparted shear stress within the gasket geometry is well controlled. Fifty percent of the entire area of the 10 × 21 mm universal gasket (Gasket I, designed to impart constant magnitude shear stresses in the center of the chamber where outcome measures are taken), is exposed to target stresses. In the 8 mm diameter circular area at the center of the chamber (where outcome measures are made), over 92% of the area is exposed to the target stress (± 2.5%). In addition, other gasket geometries provide specific gradients of stress that vary with distance from the chamber inlet. Bench-top testing of the novel chamber prototype shows improvements, in the ease of use as well as in performance, compared to the other commercial chambers. The design of the chamber eliminates flow deviations due to leakage and bubbles and allows actual flow profiles to better conform with those predicted in computational models.

Conclusion

The novel flow chamber design provides predictable and well defined mechanical forces at the surface of a cell monolayer, showing improvement over previously tested commercial chambers. The predictability of the imparted stress improves both experiment repeatability as well as the accuracy of inter-study comparisons. Carefully controlling the stresses on cells is critical in effectively mimicking in vivo situations. Overall, the improved perfusion flow chamber provides the needed resolution, standardization and in vitro model analogous to in vivo conditions to make the step towards greater use in research and the opportunity to enter the diagnostic and therapeutic market.

Design of Tissue Engineering Scaffolds as Delivery Devices for Mechanical and Mechanically Modulated Signals

New approaches to tissue engineering aim to exploit endogenous strategies such as those occurring in prenatal development and recapitulated during postnatal healing. Defining tissue template specifications to mimic the environment of the condensed mesenchyme during development allows for exploitation of tissue scaffolds as delivery devices for extrinsic cues, including biochemical and mechanical signals, to drive the fate of mesenchymal stem cells seeded within. Although a variety of biochemical signals that modulate stem cell fate have been identified, the mechanical signals conducive to guiding pluripotent cells toward specific lineages are less well characterized. Furthermore, not only is spatial and temporal control of mechanical stimuli to cells challenging, but also tissue template geometries vary with time due to tissue ingrowth and/or scaffold degradation. Hence, a case study was carried out to analyze flow regimes in a testbed scaffold as a first step toward optimizing scaffold architecture. A pressure gradient was applied to produce local (nm-micron) flow fields conducive to migration, adhesion, proliferation, and differentiation of cells seeded within, as well as global flow parameters (micron-mm), including flow velocity and permeability, to enhance directed cell infiltration and augment mass transport. Iterative occlusion of flow channel dimensions was carried out to predict virtually the effect of temporal geometric variation (e.g., due to tissue development and growth) on delivery of local and global mechanical signals. Thereafter, insights from the case study were generalized to present an optimization scheme for future development of scaffolds to be implemented in vitro or in vivo. Although it is likely that manufacture and testing will be required to finalize design specifications, it is expected that the use of the rational design optimization will reduce the number of iterations required to determine final prototype geometries and flow conditions. As the range of mechanical signals conducive to guiding cell fate in situ is further elucidated, these refined design criteria can be integrated into the general optimization rubric, providing a technological platform to exploit nature's endogenous tissue engineering strategies for targeted tissue generation in the lab or the clinic.

Visions for Regenerative Medicine: Interface Between Scientific Fact and Science Fiction

This article gives a brief overview of the authors' views on the future development of tissue engineering with respect to the challenges both to the materials and life sciences. Emphasis will be placed on the advantages of three-dimensional bioresorbable polymers in combination with relevant molecular cues and the application of autologous stem or progenitor cells. There is a requirement for much more diversity in the synthesis of so-called "intelligent" materials, which respond to external stimuli, as well as the development of novel drug and gene delivery systems. In addition, much more basic research is necessary in developmental biology and the application of modern cell and molecular biology to biomaterial questions.

Osteogenic Potentiation of Human Adipose???Derived Stem Cells in a 3-Dimensional Matrix

Adipose-derived stem cells (ADSCs) hold promise for use in tissue engineering. Despite growing enthusiasm for use of ADSCs, there is limited research that has examined their behavior in different in vitro and in vivo systems. The purpose of our study was to evaluate the effect of the extracellular matrix structure and composition on osteogenic differentiation by comparing the osteogenic marker expression of ADSCs grown under 2-dimensional or 3-dimensional cell culture conditions. Group 1 (2-D) included ADSCs raised under conventional cell culture conditions (cells in a 2-D monolayer configuration) (n = 24), and group 2 (3-dimensional) included ADSCs seeded in a collagen gel (cells within a 3-dimensional, biologically active environment) (n = 24). Comparison of ADSC behavior between the 2 groups was analyzed during a 14-day time frame. Osteogenic marker expression (CBFA-1, alkaline phosphatase, osteonectin, osteopontin, Collagen I, and JNK2) was quantified by real-time PCR, and histologic analysis was performed. Histologically, group 1 (2-D) showed cell spreading and deposition of a calcified extracellular matrix. Group 2 (3-dimensional) assumed a disorganized state in the collagen gel, with extension of pseudopodia throughout the matrix. Expression of CBFA-1 was up-regulated immediately in both groups. However, cells in group 2 (3-dimensional) had a more rapid and greater overall expression compared with cells in group 1 (2-D) (250-fold greater at 4 days). At day 14, cells in group 2 (3-dimensional) showed greater expression of all other osteogenic markers than cells in group 1 (2-D) (2.3-fold greater expression of alkaline phosphatase [P < 0.05], 8.4-fold greater expression of osteonectin [P < 0.05], 6.4-fold greater expression of osteopontin [P < 0.05], 2.9-fold greater expression of collagen I [P < 0.05], and 2.5-fold greater expression of JNK2 [P < 0.05]). Our data showed there was a progressive stimulatory effect on ADSCs with regard to osteogenesis when cultured in a 3-dimensional gel compared with a 2-D monolayer.

The imperative for controlled mechanical stresses in unraveling cellular mechanisms of mechanotransduction

Background

In vitro mechanotransduction studies are designed to elucidate cell behavior in response to a well-defined mechanical signal that is imparted to cultured cells, e.g. through fluid flow. Typically, flow rates are calculated based on a parallel plate flow assumption, to achieve a targeted cellular shear stress. This study evaluates the performance of specific flow/perfusion chambers in imparting the targeted stress at the cellular level.

Methods

To evaluate how well actual flow chambers meet their target stresses (set for 1 and 10 dyn/cm² for this study) at a cellular level, computational models were developed to calculate flow velocity components and imparted shear stresses for a given pressure gradient. Computational predictions were validated with micro-particle image velocimetry (μPIV) experiments.

Results

Based on these computational and experimental studies, as few as 66% of cells seeded along the midplane of commonly implemented flow/perfusion chambers are subjected to stresses within ±10% of the target stress. In addition, flow velocities and shear stresses imparted through fluid drag vary as a function of location within each chamber. Hence, not only a limited number of cells are exposed to target stress levels within each chamber, but also neighboring cells may experience different flow regimes. Finally, flow regimes are highly dependent on flow chamber geometry, resulting in significant variation in magnitudes and spatial distributions of stress between chambers.

Conclusion

The results of this study challenge the basic premise of in vitro mechanotransduction studies, i.e. that a controlled flow regime is applied to impart a defined mechanical stimulus to cells. These results also underscore the fact that data from studies in which different chambers are utilized can not be compared, even if the target stress regimes are comparable.

Stretch-induced PTH-related protein gene expression in osteoblasts

Mechanical forces play a critical role in regulating skeletal mass and structure. We report that mechanical loading induces PTHrP in osteoblast-like cells and that TREK-2 stretch-activated potassium channels seem to be involved in this induction. Our data suggest PTHrP as a candidate endogenous mediator of the anabolic effects of mechanical force on bone.

INTRODUCTION

Mechanical force has anabolic effects on bone. The PTH-related protein (PTHrP) gene is known to be mechanically inducible in smooth muscle cells throughout the organism, and N-terminal PTH and PTHrP products have been reported to have anabolic effects in bone. We explored the idea that PTHrP might be a candidate mediator of the effects of mechanical force on bone.

MATERIALS AND METHODS

Mechanical loading was applied by swelling osteoblast-like cells in hypotonic solution and/or by application of cyclical stretch through a FlexerCell apparatus. RNase protection assay and real-time quantitative PCR analysis were used to assay PTHrP gene expression.

RESULTS AND CONCLUSION

Stretching UMR201-10B osteoblast-like cells by swelling in hypotonic solutions rapidly increased PTHrP mRNA. This induction was insensitive to gadolinium and nifedipine, to the removal of extracellular calcium, and to depletion of endoplasmic reticulum calcium, indicating that neither stretch-activated cation channels, L-type calcium channels, nor ER calcium is involved in the induction of PTHrP. The TREK family potassium channels are activated by both stretch and intracellular acidosis, and we identified these channels in osteoblast-like cells by PCR. Intracellular acidification increased PTHrP mRNA expression in UMR-201-10B cells, and siRNA targeted against the TREK-2 gene reduced endogenous TREK-2 expression and dampened PTHrP mRNA induction. Cyclical stretch also induced PTHrP in UMR-201-10B osteoblast-like cells and in MLO-A5 post-osteoblast-pre-osteocyte cells, the latter a stage in the osteoblastic differentiation program that is likely to be a key target of force in vivo. Our evidence suggests PTHrP as a candidate mediator of the anabolic effects of mechanical force on bone.