Using Dihydropyridine-Release Strategies to Enhance Load Effects in Engineered Human Bone Constructs

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

The mechanosensory and mechanotransductive processes mediated by ion channels and the impact on bone metabolism: A systematic review

Mechanical environments were associated with alterations in bone metabolism. Ion channels present on bone cells are indispensable for bone metabolism and can be directly or indirectly activated by mechanical stimulation. This review aimed to discuss the literature reporting the mechanical regulatory effects of ion channels on bone cells and bone tissue. An electronic search was conducted in PubMed, Embase and Web of Science. Studies about mechanically induced alteration of bone cells and bone tissue by ion channels were included. Ion channels including TRP family channels, Ca²⁺ release-activated Ca²⁺ channels (CRACs), Piezo1/2 channels, purinergic receptors, NMDA receptors, voltage-sensitive calcium channels (VSCCs), TREK2 potassium channels, calcium- and voltage-dependent big conductance potassium (BK_Ca) channels, small conductance, calcium-activated potassium (SK_Ca) channels and epithelial sodium channels (ENaCs) present on bone cells and bone tissue participate in the mechanical regulation of bone development in addition to contributing to direct or indirect mechanotransduction such as altered membrane potential and ionic flux. Physiological (beneficial) mechanical stimulation could induce the anabolism of bone cells and bone tissue through ion channels, but abnormal (harmful) mechanical stimulation could also induce the catabolism of bone cells and bone tissue through ion channels. Functional expression of ion channels is vital for the mechanotransduction of bone cells. Mechanical activation (opening) of ion channels triggers ion influx and induces the activation of intracellular modulators that can influence bone metabolism. Therefore, mechanosensitive ion channels provide new insights into therapeutic targets for the treatment of bone-related diseases such as osteopenia and aseptic implant loosening.

Investigating conversion of endplate chondrocytes induced by intermittent cyclic mechanical unconfined compression in three-dimensional cultures

Mechanical stimulation is known to regulate the calcification of endplate chondrocytes. The Ank protein has a strong influence on anti-calcification by transports intracellular inorganic pyrophosphate (PPi) to the extracellular matrix. It is known that TGF-β1 is able to induce Ank gene expression and protect chondrocyte calcification. Intermittent cyclic mechanical tension (ICMT) could induce calcification of endplate chondrocytes by decrease the expression of Ank gene. In this study, we investigated the relation of intermittent cyclic mechanical unconfined compression (ICMC) and Ank gene expression. We found that ICMC decreased the Ank gene expression in the endplate chondrocytes, and there was an decreased in the TGF-β1 expression after ICMC stimulation. The Ank gene expression significantly increased when treated by transforming growth factor alpha 1 (TGF-β1) in a dose-dependent manner and decreased when treated by SB431542 (ALK inhibitor) in a dose-dependent manner. Our results implicate that ICMC-induced downregulation of Ank gene expression may be regulated by TGF-β1 in end-plate chondrocytes.

Biomechanics in bone tissue engineering

Biomechanics may be considered as central in the development of bone tissue engineering. The initial mechanical aspects are essential to the outcome of a functional tissue engineering approach; so are aspects of interface micromotion, bone ingrowths inside the scaffold and finally, the mechanical integrity of the scaffold during its degradation. A proposed view is presented herein on how biomechanical aspects can be synthesised and where future developments are needed. In particular, a distinction is made between the mechanical and the mechanotransductional aspects of bone tissue engineering: the former could be related to osteoconduction, while the latter may be correlated to the osteoinductive properties of the scaffold. This distinction allows biomechanicians to follow a strategy in the development of a scaffold having not only mechanical targets but also incorporating some mechanotransduction principles.

In vivo acceleration of skin growth using a servo-controlled stretching device

Tension is a principal force experienced by skin and serves a critical role in growth and development. Optimal tension application regimens may be an important component for skin tissue engineering and dermatogenesis. In this study, we designed and tested a novel servo-controlled skin-stretching device to apply predetermined tension and waveforms in mice. The effects of static and cyclical stretching forces were compared in 48 mice by measuring epidermal proliferation, angiogenesis, cutaneous perfusion, and principal growth factors using immunohistochemistry, real-time reverse transcriptase-polymerase chain reaction, and hyperspectral imaging. All stretched samples had upregulated epidermal proliferation and angiogenesis. Real-time reverse transcriptase-polymerase chain reaction of epidermal growth factor, transforming growth factor beta1, and nerve growth factor demonstrated greater expression in cyclically stretched skin when compared to static stretch. Hypoxia-induced factor 1alpha was significantly upregulated in cyclically stretched skin, but poststretch analysis demonstrated well-oxygenated tissue, collectively suggesting the presence of transient hypoxia. Waveform-specific mechanical loads may accelerate tissue growth by mechanotransduction and as a result of repeated cycles of temporary hypoxia. Further analysis of mechanotransduction signaling pathways may provide additional insight to improve skin tissue engineering methods and optimize our device.

In vitro bone growth responds to local mechanical strain in three-dimensional polymer scaffolds

Mechanical stimulation plays a key role in healing and remodelling of bone tissue in vivo, and is used in bone tissue regeneration strategies in vitro. Although macroscopic compression of three-dimensional (3-D) seeded constructs can increase bone formation, it is not yet reported how this response is related to differences in local mechanical strains inside the scaffolds. In this study, we experimentally test the hypothesis that differences in local average of heterogeneous strains in a polymer scaffold will correlate with induced differences in the local biological response. Twenty-four poly(L-lactic acid) porous scaffolds seeded with rat bone cells were cultured first for 2 and 3 weeks under static conditions, respectively. Then for 1 week, half of the scaffolds were cyclically compressed (1.5%, 1 Hz), 1 h daily, with continuous perfusion (0.1 ml/min). The remaining half was kept under static conditions. The pore-surface strains in the scaffolds at the start of culture were calculated with micro-finite element modelling based on micro-Computed Tomography (microCT) images. The locations of mineralized nodules were determined from microCT images and coupled to the calculated strains. Detectable mineralized nodules (>10(3) microm3) were only present in the loaded samples. Averages of absolute principal strains at the start of culture were significantly higher at nodule sites than at sites without a nodule. The results support the hypothesis that regenerating bone tissue in a 3-D porous scaffold responds to local mechanical strain. The methodology presented in this study can contribute design optimisation of tissue regeneration strategies relying on mechanical stimulation.

Image-guided tissue engineering

Replication of anatomic shape is a significant challenge in developing implants for regenerative medicine. This has lead to significant interest in using medical imaging techniques such as magnetic resonance imaging and computed tomography to design tissue engineered constructs. Implementation of medical imaging and computer aided design in combination with technologies for rapid prototyping of living implants enables the generation of highly reproducible constructs with spatial resolution up to 25 microm. In this paper, we review the medical imaging modalities available and a paradigm for choosing a particular imaging technique. We also present fabrication techniques and methodologies for producing cellular engineered constructs. Finally, we comment on future challenges involved with image guided tissue engineering and efforts to generate engineered constructs ready for implantation.

Three-dimensional ingrowth of bone cells within biodegradable cryogel scaffolds in bioreactors at different regimes

Three-dimensional cell ingrowth within biodegradable cryogel scaffolds made of cross-linked 2-hydroxyethyl methacrylate (HEMA)-lactate-dextran with interconnected macropores was studied in bioreactors at different regimes (static, perfusion, and compression-perfusion). An osteoblast-like cell line (MG63) was used in these studies. The samples taken after selected times from the bioreactors were examined by microscopy techniques (light, SEM, TEM, and laser scanning confocal). The cell culture conditions were found to have a significant impact not only on the cell morphology, such as the extent of cell attachment and ingrowth, but also on cellular activities. Dynamic conditions (perfusion and/or compression) greatly improved cell ingrowth and extracellular matrix (ECM) synthesis. Alkaline phosphatase activity results confirmed the positive effect of dynamic conditions on bone cells.

Correlating cell morphology and osteoid mineralization relative to strain profile for bone tissue engineering applications

A number of bone tissue engineering strategies use porous three-dimensional scaffolds in combination with bioreactor regimes. The ability to understand cell behaviour relative to strain profile will allow for the effects of mechanical conditioning in bone tissue engineering to be realized and optimized. We have designed a model system to investigate the effects of strain profile on bone cell behaviour. This simplified model has been designed with a view to providing insight into the types of strain distribution occurring across a single pore of a scaffold subjected to perfusion-compression conditioning. Local strains were calculated at the surface of the pore model using finite-element analysis. Scanning electron microscopy was used in secondary electron mode to identify cell morphology within the pore relative to local strains, while backscattered electron detection in combination with X-ray microanalysis was used to identify calcium deposition. Morphology was altered according to the level of strain experienced by bone cells, where cells subjected to compressive strains (up to 0.61%) appeared extremely rounded while those experiencing zero and tensile strain (up to 0.81%) were well spread. Osteoid mineralization was similarly shown to be dose dependent with respect to substrate strain within the pore model, with the highest level of calcium deposition identified in the intermediate zones of tension/compression.

A numerical model of heterogeneous surface strains in polymer scaffolds

In vitro bone tissue growth inside porous scaffolds can be enhanced by macroscopic cyclic compression of the construct, but the heterogeneous strain generated inside the construct must be investigated to determine appropriate levels of compression. For this purpose a linear micro-finite element (muFE) technique based on micro-computed tomography (muCT) was verified for the calculation of local displacements inside polymer scaffolds, from which local strains may be estimated. Local displacements in the axial direction at the surface of microstructures inside the scaffold in 60 locations were calculated with the muFE model, based on compression simulation of a muCT reconstruction of the scaffold. These displacements were compared with accurately measured displacements in the axial direction in the same polymer scaffold at the same 60 locations, using a micro-compression chamber and muCT reconstructions of the scaffold under two fixed levels of compression (5% and 0%). The correlation between the calculated and the measured displacements, after correction for the dependence of the axial displacement on the axial position, was r=0.786 (r2=0.617). From this we conclude that the linear muFE model is suitable to estimate local surface strains inside polymer scaffolds for tissue engineering applications. This technique can not only be used to determine appropriate parameters such as the level of macroscopic compression in experimental design, but also to investigate the cellular response to local surface strains generated inside three-dimensional scaffolds.

Calcium channel antagonists: clinical uses--past, present and future

The calcium channel antagonists are a mature group of drugs directed at cardiovascular diseases including hypertension, angina, peripheral vascular disorders and some arrhythmic conditions. Their sites and mechanisms of actions have been well explored over the past two decades and their interactions at the alpha(1) subunit of L-type channels (Ca(V)1.1-1.4) have made them valuable molecular tools for channel classification and localization. With the realization that other members of the voltage-gated calcium channel family exist--Ca(V)2.1-2.3 and Ca(V)3.1-3.3--considerable effort has been directed to drug discovery at these channel types where therapeutic prospects exist for a variety of disorders including pain, epilepsy, affective disorders, neurodegenerative disorders, etc. In contrast to the situation with the L-type channel antagonists success in developing small molecule antagonists of therapeutic utility for these other channel types has thus far been lacking. The reasons for this are explored and potential new directions are indicated including male fertility, bone growth, immune disorders, cancer and schistosomiasis.

Mechanical strains induced in osteoblasts by use of point femtosecond laser targeting

A study demonstrating how ultrafast laser radiation stimulates osteoblasts is presented. The study employed a custom made optical system that allowed for simultaneous confocal cell imaging and targeted femtosecond pulse laser irradiation. When femtosecond laser light was focused onto a single cell, a rise in intracellular Ca²⁺ levels was observed followed by contraction of the targeted cell. This contraction caused deformation of neighbouring cells leading to a heterogeneous strain field throughout the monolayer. Quantification of the strain fields in the monolayer using digital image correlation revealed local strains much higher than threshold values typically reported to stimulate extracellular bone matrix production in vitro. This use of point targeting with femtosecond pulse lasers could provide a new method for stimulating cell activity in orthopaedic tissue engineering.