Physicochemical control of adult stem cell differentiation: shedding light on potential molecular mechanisms

Advances in electroactive biomaterials: Through the lens of electrical stimulation promoting bone regeneration strategy

The regenerative capacity of bone is indispensable for growth, given that accidental injury is almost inevitable. Bone regenerative capacity is relevant for the aging population globally and for the repair of large bone defects after osteotomy (e.g., following removal of malignant bone tumours). Among the many therapeutic modalities proposed to bone regeneration, electrical stimulation has attracted significant attention owing to its economic convenience and exceptional curative effects, and various electroactive biomaterials have emerged. This review summarizes the current knowledge and progress regarding electrical stimulation strategies for improving bone repair. Such strategies range from traditional methods of delivering electrical stimulation via electroconductive materials using external power sources to self-powered biomaterials, such as piezoelectric materials and nanogenerators. Electrical stimulation and osteogenesis are related via bone piezoelectricity. This review examines cell behaviour and the potential mechanisms of electrostimulation via electroactive biomaterials in bone healing, aiming to provide new insights regarding the mechanisms of bone regeneration using electroactive biomaterials.

The translational potential of this article

This review examines the roles of electroactive biomaterials in rehabilitating the electrical microenvironment to facilitate bone regeneration, addressing current progress in electrical biomaterials and the mechanisms whereby electrical cues mediate bone regeneration. Interactions between osteogenesis-related cells and electroactive biomaterials are summarized, leading to proposals regarding the use of electrical stimulation-based therapies to accelerate bone healing.

Potential molecular mechanism underlying the harmed haemopoiesis upon Benzo[a]pyrene exposure in Chlamys farreri

Benzo[a]pyrene (B[a]P), a ubiquitous contamination in the marine environments, has the potential to impact the immune response of bivalves by affecting the hemocyte parameters, especially total hemocyte count (THC). THC is mainly determined by haematopoietic mechanisms and apoptosis of hemocytes. Many studies have found that B[a]P can influence the proliferation and differentiation of hemocytes. However, the link between the toxic mechanisms of haematopoietic and environmental pollutants is not explicitly stated. This study is to investigate the toxic effects of B[a]P on haematopoietic mechanisms in C. farreri. Through the tissue expression distribution experiment and EDU assay, gill is identified as a potential haematopoietic tissue in C. farreri. Subsequently, the scallops were exposed to B[a]P (0.05, 0.5, 5 μg/L) for 1d, 3d, 6d, 10d and 15d. Then BPDE content, DNA damage, gene expression of haematopoietic factors and haematopoietic related pathways were determined in gill and hemocytes. The results showed that the expression of CDK2 was significantly decreased under B[a]P exposure through three pathways: RYR/IP3-calcium, BPDE-CHK1 and Notch pathway, resulting in cell cycle arrest. In addition, B[a]P also significantly reduced the number of proliferating hemocytes by affecting the Wnt pathway. Meanwhile, B[a]P can significantly increase the content of ROS, causing a downregulation of FOXO gene expression. The gene expression of Notch pathway and ERK pathway was also detected. The present study suggested that B[a]P disturbed differentiation by multiple pathways. Furthermore, the expression of SOX11 and CD9 were significantly decreased, which directly indicated that differentiation of hemocytes was disturbed. In addition, phagocytosis, phenoloxidase activity and THC were also significant decreased. In summary, the impairment of haematopoietic activity in C. farreri further causes immunotoxicity under B[a]P exposure. This study will improve our understanding of the immunotoxicity mechanism of bivalve under B[a]P exposure.

Store-Operated Ca2+ Entry as a Putative Target of Flecainide for the Treatment of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is a genetic disorder that may lead patients to sudden cell death through the occurrence of ventricular arrhythmias. ACM is characterised by the progressive substitution of cardiomyocytes with fibrofatty scar tissue that predisposes the heart to life-threatening arrhythmic events. Cardiac mesenchymal stromal cells (C-MSCs) contribute to the ACM by differentiating into fibroblasts and adipocytes, thereby supporting aberrant remodelling of the cardiac structure. Flecainide is an Ic antiarrhythmic drug that can be administered in combination with β-adrenergic blockers to treat ACM due to its ability to target both Nav1.5 and type 2 ryanodine receptors (RyR2). However, a recent study showed that flecainide may also prevent fibro-adipogenic differentiation by inhibiting store-operated Ca2+ entry (SOCE) and thereby suppressing spontaneous Ca2+ oscillations in C-MSCs isolated from human ACM patients (ACM C-hMSCs). Herein, we briefly survey ACM pathogenesis and therapies and then recapitulate the main molecular mechanisms targeted by flecainide to mitigate arrhythmic events, including Nav1.5 and RyR2. Subsequently, we describe the role of spontaneous Ca2+ oscillations in determining MSC fate. Next, we discuss recent work showing that spontaneous Ca2+ oscillations in ACM C-hMSCs are accelerated to stimulate their fibro-adipogenic differentiation. Finally, we describe the evidence that flecainide suppresses spontaneous Ca2+ oscillations and fibro-adipogenic differentiation in ACM C-hMSCs by inhibiting constitutive SOCE.

Low-intensity pulsed ultrasound/nanomechanical force generators enhance osteogenesis of BMSCs through microfilaments and TRPM7

BACKGROUND

Low-intensity pulsed ultrasound (LIPUS) has been reported to accelerate fracture healing, but the mechanism is unclear and its efficacy needs to be further optimized. Ultrasound in combination with functionalized microbubbles has been shown to induce local shear forces and controllable mechanical stress in cells, amplifying the mechanical effects of LIPUS. Nanoscale lipid bubbles (nanobubbles) have high stability and good biosafety. However, the effect of LIPUS combined with functionalized nanobubbles on osteogenesis has rarely been studied.

RESULTS

In this study, we report cyclic arginine-glycine-aspartic acid-modified nanobubbles (cRGD-NBs), with a particle size of ~ 500 nm, able to actively target bone marrow mesenchymal stem cells (BMSCs) via integrin receptors. cRGD-NBs can act as nanomechanical force generators on the cell membrane, and further enhance the BMSCs osteogenesis and bone formation promoted by LIPUS. The polymerization of actin microfilaments and the mechanosensitive transient receptor potential melastatin 7 (TRPM7) ion channel play important roles in BMSCs osteogenesis promoted by LIPUS/cRGD-NBs. Moreover, the mutual regulation of TRPM7 and actin microfilaments promote the effect of LIPUS/cRGD-NBs. The extracellular Ca^2 + influx, controlled partly by TRPM7, could participated in the effect of LIPUS/cRGD-NBs on BMSCs.

CONCLUSIONS

The nanomechanical force generators cRGD-NBs could promote osteogenesis of BMSCs and bone formation induced by LIPUS, through regulation TRPM7, actin cytoskeleton, and intracellular calcium oscillations. This study provides new directions for optimizing the efficacy of LIPUS for fracture healing, and a theoretical basis for the further application and development of LIPUS in clinical practice.

The Sodium-Calcium Exchanger Controls the Membrane Potential of AFT024: A Mesenchymal Stem Cell Hematopoietic Niche Forming Line

The aim of this study was to characterize the functional expression of sodium-calcium exchangers on AFT024 cell line, a murine model of mesenchymal stem/stromal cells (MSCs) supporting human primitive hematopoiesis. All current-clamp and voltage-clamp experiments were performed using the perforated patch whole-cell recording technique with amphotericin B. The membrane potential of -14 mV shifted to -35 mV when lowering the external sodium concentration to 0.33 mM and an increase of cytosolic calcium concentration was observed. KB-R7943, a selective blocker of cardiac sodium-calcium exchangers, also named NCX1, induced a hyperpolarization at physiological sodium concentration while it blocked the hyperpolarization observed at low sodium concentration. This demonstrates for the first time the presence of the sodium-calcium exchangers in AFT024 cells and provides initial evidence that the membrane potential of these stromal cells is maintained depolarized by this exchanger. Lowering external sodium concentration and KB-R7943 had no effect on the membrane potential of 2018 cells, a nonhematopoietic-supportive cell line. Since NCX1 is differentially expressed in AFT024 cells as compared with nonhematopoietic supportive cells with more restricted differentiation potential, this study suggests a potential role of this sodium-calcium exchanger, in the differentiation process or hematopoietic support of MSCs.

Electrical waveform dependent osteogenesis on PVDF/BaTiO3 composite using a customized and programmable cell stimulator

Directing cellular functionalities using biomaterial-based bioelectronic stimulation remains a significant constraint in translating research outcomes to address specific clinical challenges. Electrical stimulation is now being clinically used as a therapeutic treatment option to promote bone tissue regeneration and to improve neuromuscular functionalities. However, the nature of the electrical waveforms during the stimulation and underlying biophysical rationale are still not scientifically well explored. Furthermore, bone-mimicking implant-based bioelectrical regulation of osteoinductivity has not been translated to clinics. The present study demonstrates the role of the waveform in electrical signal to direct differentiation of stem cells on an electroactive polymeric substrate, using monophasic DC, square wave, and biphasic wave. In this regard, an in-house electrical stimulation device has been fabricated for the uninterrupted delivery of programmed electrical signals to stem cells in culture. To provide a functional platform for stem cells to differentiate, barium titanate (BaTiO₃ , BT) reinforced PVDF has been developed with mechanical properties similar to bone. The electrical stimulation of human mesenchymal stem cells (hMSCs) on PVDF/BT composite inhibited proliferation rate at day 7, indicating early commitment for differentiation. The phenotypical characteristics of DC stimulated hMSCs provided signatures of differentiation towards osteogenic lineage, which was subsequently confirmed using ALP assay, collagen deposition, matrix mineralization, and genetic expression. Our findings suggest that DC stimulation induced early osteogenesis in hMSCs with a higher level of intracellular reactive oxygen species (ROS), whereas the stimulation with square wave directed late osteogenesis with a lower ROS regeneration. In summary, the present study critically analyzes the role of electrical stimulation and its waveforms in regulating osteogenesis, without external biochemical differentiation inducers, on a bone-mimicking functional substrate. Such a strategy can potentially be adopted to develop orthopedic implant-based bioelectronic medicine for bone regeneration. This article is protected by copyright. All rights reserved.

Control of stem cell differentiation by using extrinsic photobiomodulation in conjunction with cell adhesion pattern

The induction and direction of stem cell differentiation into needed cell phenotypes is the central pillar of tissue engineering for repairing damaged tissues or organs. Conventionally, a special recipe of chemical factors is formulated to achieve this purpose for each specific target cell type. In this work, it is demonstrated that the combination of extrinsic photobiomodulation and collagen-covered microislands could be used to induce differentiation of Wharton’s jelly mesenchymal stem cells (WJ-MSCs) with the differentiation direction dictated by the specific island topography without use of chemical factors. Both neurogenic differentiation and adipogenic differentiation could be attained with a rate surpassing that using chemical factors. Application of this method to other cell types is possible by utilizing microislands with a pattern tailored particularly for each specific cell type, rendering it a versatile modality for initiating and guiding stem cell differentiation.

Time-Dependent Reduction of Calcium Oscillations in Adipose-Derived Stem Cells Differentiating towards Adipogenic and Osteogenic Lineage

Adipose-derived mesenchymal stromal cells (ASCs) are multipotent stem cells which can differentiate into various cell types, including osteocytes and adipocytes. Due to their ease of harvesting, multipotency, and low tumorigenicity, they are a prime candidate for the development of novel interventional approaches in regenerative medicine. ASCs exhibit slow, spontaneous Ca²⁺ oscillations and the manipulation of Ca²⁺ signalling via electrical stimulation was proposed as a potential route for promoting their differentiation in vivo. However, the effects of differentiation-inducing treatments on spontaneous Ca²⁺ oscillations in ASCs are not yet fully characterised. In this study, we used 2-photon live Ca²⁺ imaging to assess the fraction of cells showing spontaneous oscillations and the frequency of the oscillation (measured as interpeak interval—IPI) in ASCs undergoing osteogenic or adipogenic differentiation, using undifferentiated ASCs as controls. The measurements were carried out at 7, 14, and 21 days in vitro (DIV) to assess the effect of time in culture on Ca²⁺ dynamics. We observed that both time and differentiation treatment are important factors associated with a reduced fraction of cells showing Ca²⁺ oscillations, paralleled by increased IPI times, in comparison with untreated ASCs. Both adipogenic and osteogenic differentiation resulted in a reduction in Ca²⁺ dynamics, such as the fraction of cells showing intracellular Ca²⁺ oscillations and their frequency. Adipogenic differentiation was associated with a more pronounced reduction of Ca²⁺ dynamics compared to cells differentiating towards the osteogenic fate. Changes in Ca²⁺ associated oscillations with a specific treatment had already occurred at 7 DIV. Finally, we observed a reduction in Ca²⁺ dynamics over time in untreated ASCs. These data suggest that adipogenic and osteogenic differentiation cell fates are associated with specific changes in spontaneous Ca²⁺ dynamics over time. While this observation is interesting and provides useful information to understand the functional correlates of stem cell differentiation, further studies are required to clarify the molecular and mechanistic correlates of these changes. This will allow us to better understand the causal relationship between Ca²⁺ dynamics and differentiation, potentially leading to the development of novel, more effective interventions for both bone regeneration and control of adipose growth.

Theoretical analysis of the electrochemical systems used for the application of direct current/voltage stimuli on cell cultures

Endogenous electric fields drive many essential functions relating to cell proliferation, motion, differentiation and tissue development. They are usually mimicked in vitro by using electrochemical systems to apply direct current or voltage stimuli to cell cultures. The many studies devoted to this topic have given rise to a wide variety of experimental systems, whose results are often difficult to compare. Here, these systems are analysed from an electrochemical standpoint to help harmonize protocols and facilitate optimal understanding of the data produced. The theoretical analysis of single-electrode systems shows the necessity of measuring the Nernst potential of the electrode and of discussing the results on this basis rather than using the value of the potential gradient. The paper then emphasizes the great complexity that can arise when high cell voltage is applied to a single electrode, because of the possible occurrence of anode and cathode sites. An analysis of two-electrode systems leads to the advice to change experimental practices by applying current instead of voltage. It also suggests that the values of electric fields reported so far may have been considerably overestimated in macro-sized devices. It would consequently be wise to revisit this area by testing considerably lower electric field values.

Insulin-Producing Cell Transplantation Platform for Veterinary Practice

Diabetes mellitus (DM) remains a global concern in both human and veterinary medicine. Type I DM requires prolonged and consistent exogenous insulin administration to address hyperglycemia, which can increase the risk of diabetes complications such as retinopathy, nephropathy, neuropathy, and heart disorders. Cell-based therapies have been successful in human medicine using the Edmonton protocol. These therapies help maintain the production of endogenous insulin and stabilize blood glucose levels and may possibly be adapted to veterinary clinical practice. The limited number of cadaveric pancreas donors and the long-term use of immunosuppressive agents are the main obstacles for this protocol. Over the past decade, the development of potential therapies for DM has mainly focused on the generation of effective insulin-producing cells (IPCs) from various sources of stem cells that can be transplanted into the body. Another successful application of stem cells in type I DM therapies is transplanting generated IPCs. Encapsulation can be an alternative strategy to protect IPCs from rejection by the body due to their immunoisolation properties. This review summarizes current concepts of IPCs and encapsulation technology for veterinary clinical application and proposes a potential stem-cell-based platform for veterinary diabetic regenerative therapy.

Polymeric sheet actuators with programmable bioinstructivity

Stem cells can be conceptualized as computational processors capable of sensing, processing, and converting environmental information (input) to yield a specific differentiation pathway (output). In this study, we employ a temperature-controlled polymer sheet actuator to interpret and transfer information, controlled by the material’s programming, to mesenchymal stem cells. The cell’s interpretation of mechanical, thermal, and biochemical signaling is shown to be dependent on the actuator’s activity, utilized to accelerate differentiation toward bone cells, further elucidating the role of microenvironmental parameters on mammalian cells. Our method provides a unique approach to processing two discrete stimuli into one biochemical signal, calcium ions, providing a basis for the logical control of the flow of biological signals and the design of cellular functions.

Stem cells are capable of sensing and processing environmental inputs, converting this information to output a specific cell lineage through signaling cascades. Despite the combinatorial nature of mechanical, thermal, and biochemical signals, these stimuli have typically been decoupled and applied independently, requiring continuous regulation by controlling units. We employ a programmable polymer actuator sheet to autonomously synchronize thermal and mechanical signals applied to mesenchymal stem cells (MSCs). Using a grid on its underside, the shape change of polymer sheet, as well as cell morphology, calcium (Ca²⁺) influx, and focal adhesion assembly, could be visualized and quantified. This paper gives compelling evidence that the temperature sensing and mechanosensing of MSCs are interconnected via intracellular Ca²⁺. Up-regulated Ca²⁺ levels lead to a remarkable alteration of histone H3K9 acetylation and activation of osteogenic related genes. The interplay of physical, thermal, and biochemical signaling was utilized to accelerate the cell differentiation toward osteogenic lineage. The approach of programmable bioinstructivity provides a fundamental principle for functional biomaterials exhibiting multifaceted stimuli on differentiation programs. Technological impact is expected in the tissue engineering of periosteum for treating bone defects.

Biological response of chemically treated surface of the ultrafine-grained Ti-6Al-7Nb alloy for biomedical applications

Background

Nanophase surface properties of titanium alloys must be obtained for a suitable biological performance, particularly to facilitate cell adhesion and bone tissue formation. Obtaining a bulk nanostructured material using severe plastic deformation is an ideal processing route to improve the mechanical performance of titanium alloys. By decreasing the grain size of a metallic material, a superior strength improvement can be obtained, while surface modification of a nanostructured surface can produce an attractive topography able to induce biological responses in osteoblastic cells.

Methods

Aiming to achieve such an excellent synergetic performance, a processing route, which included equal channel angular pressing (ECAP), hot and cold extrusion, and heat treatments, was used to produce a nanometric and ultrafine-grained (UFG) microstructure in the Ti-6Al-7Nb alloy (around of 200 nm). Additionally, UFG samples were surface-modified with acid etching (UFG-A) to produce a uniform micron and submicron porosity on the surface. Subsequently, alkaline treatment (UFG-AA) produced a sponge-like nanotopographic substrate able to modulate cellular interactions.

Results

After several kinds of biological tests for both treatment conditions (UFG-A and UFG-AA), the main results have shown that there was no cytotoxicity, expressed alkaline phosphatase activity and total protein amounts without statistical differences compared to control. However, the UFG-AA samples presented an attractive effect on the cell membranes, and cell adhesions were preferentially induced as compared with UFG-A. Both conditions demonstrated cell projections, but for UFG-AA, cells were more widely dispersed, and more quantities of filopodia formation could be observed.

Conclusion

Herein, the reasons for such behaviors are discussed, and further results are presented in addition to those mentioned above.

Calcium Spike Patterns Reveal Linkage of Electrical Stimulus and MSC Osteogenic Differentiation

Mesenchymal stromal/stem cells (MSCs) are easily obtained multipotent cells that are widely applied in regenerative medicine. Electrical stimulation (ES) has a promoting effect on bone healing and osteogenic differentiation of MSCs. Direct and alternating currents (AC) are extensively used to promote the osteogenic differentiation of MSCs in vivo and in vitro. However, information on conducting effective differentiation remains scarce. In this paper, we propose a method to optimize ES parameters based on calcium spike patterns of MSCs. Calcium spike frequency decreases as the osteogenic differentiation of MSC progresses. Furthermore, we tested various ES parameters through the real-time monitoring of calcium spike patterns. We efficiently initiated the process of osteogenic differentiation in MSCs by using the optimal parameters of AC, including voltage, signal shapes, frequency, and duty time. This method provides a new approach to optimize osteogenic differentiation and is potentially useful in clinical treatment such as of bone fractures.

In vitro osteoblastic differentiation of mesenchymal stem cells generates cell layers with distinct properties

Background

Differentiation of mesenchymal stem cells to osteoblasts is widely performed in research laboratories. Classical tests to prove this differentiation employ procedures such as cell fixation, cell lysis or cell scraping. Very few studies report gentle dissociation of mesenchymal stem cells undergoing an osteodifferentiation process. Here we used this technique to reveal the presence of several cell layers during osteogenesis and to study their different properties.

Methods

Through the sequential enzymatic detachment of the cells, we confirm the presence of several layers of differentiated cells and we compare them in terms of enzymatic sensitivity for dissociation, expression of cluster of differentiation, cytosolic calcium oscillations and osteogenic potential. Adipogenic and neurogenic differentiations were also performed in order to compare the cell layers.

Results

The cells undergoing differentiation formed one layer in the neurogenic differentiation, two layers in the adipogenic differentiation and at least four layers in the osteogenic differentiation. In the latter, the upper layers, maintained by a collagen I extracellular matrix, can be dissociated using collagenase I, while the remaining lowest layer, attached to the bottom of the dish, is sensitive only to trypsin-versene. The action of collagenase I is more efficient before the mineralization of the extracellular matrix. The collagenase-sensitive and trypsin-sensitive layers differ in their cluster of differentiation expression. The dissociation of the cells on day 15 reveals that cells could resume their growth (increase in cell number) and rapidly differentiate again in osteoblasts, in 2 weeks (instead of 4 weeks). Cells from the upper layers displayed a higher mineralization.

Conclusions

MSCs undergoing osteogenic differentiation form several layers with distinct osteogenic properties. This could allow the investigators to use upper layers to rapidly produce differentiated osteoblasts and the lowest layer to continue growth and differentiation until an ulterior dissociation.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0942-x) contains supplementary material, which is available to authorized users.

Elimination of cancer stem cells and reactivation of latent HIV-1 via AMPK activation: Common mechanism of action linking inhibition of tumorigenesis and the potential eradication of HIV-1

Although promising treatments are currently in development to slow disease progression and increase patient survival, cancer remains the second leading cause of death in the United States. Cancer treatment modalities commonly include chemoradiation and therapies that target components of aberrantly activated signaling pathways. However, treatment resistance is a common occurrence and recent evidence indicates that the existence of cancer stem cells (CSCs) may underlie the limited efficacy and inability of current treatments to effectuate a cure. CSCs, which are largely resistant to chemoradiation therapy, are a subpopulation of cancer cells that exhibit characteristics similar to embryonic stem cells (ESCs), including self-renewal, multi-lineage differentiation, and the ability to initiate tumorigenesis. Interestingly, intracellular mechanisms that sustain quiescence and promote self-renewal in adult stem cells (ASCs) and CSCs likely also function to maintain latency of HIV-1 in CD4⁺ memory T cells. Although antiretroviral therapy is highly effective in controlling HIV-1 replication, the persistence of latent but replication-competent proviruses necessitates the development of compounds that are capable of selectively reactivating the latent virus, a method known as the "shock and kill" approach. Homeostatic proliferation in central CD4⁺ memory T (T_CM) cells, a memory T cell subset that exhibits limited self-renewal and differentiation and is a primary reservoir for latent HIV-1, has been shown to reinforce and stabilize the latent reservoir in the absence of T cell activation and differentiation. HIV-1 has also been found to establish durable and long-lasting latency in a recently discovered subset of CD4⁺ T cells known as T memory stem (T_SCM) cells. T_SCM cells, compared to T_CM cells, exhibit stem cell properties that more closely match those of ESCs and ASCs, including self-renewal and differentiation into all memory T cell subsets. It is our hypothesis that activation of AMPK, a master regulator of cellular metabolism that plays a critical role in T cell activation and differentiation of ESCs and ASCs, will lead to both T cell activation-induced latent HIV-1 reactivation, facilitating virus destruction, as well as "activation", differentiation, and/or apoptosis of CSCs, thus inhibiting tumorigenesis. We also propose the novel observation that compounds that have been shown to both facilitate latent HIV-1 reactivation and promote CSC differentiation/apoptosis (e.g. bryostatin-1, JQ1, metformin, butyrate, etc.) likely do so through a common mechanism of AMPK activation.

Electrical control of calcium oscillations in mesenchymal stem cells using microsecond pulsed electric fields

BACKGROUND

Human mesenchymal stem cells are promising tools for regenerative medicine due to their ability to differentiate into many cellular types such as osteocytes, chondrocytes and adipocytes amongst many other cell types. These cells present spontaneous calcium oscillations implicating calcium channels and pumps of the plasma membrane and the endoplasmic reticulum. These oscillations regulate many basic functions in the cell such as proliferation and differentiation. Therefore, the possibility to mimic or regulate these oscillations might be useful to regulate mesenchymal stem cells biological functions.

METHODS

One or several electric pulses of 100 μs were used to induce Ca2+ spikes caused by the penetration of Ca2+ from the extracellular medium, through the transiently electropermeabilized plasma membrane, in human adipose mesenchymal stem cells from several donors. Attached cells were preloaded with Fluo-4 AM and exposed to the electric pulse(s) under the fluorescence microscope. Viability was also checked.

RESULTS

According to the pulse(s) electric field amplitude, it is possible to generate a supplementary calcium spike with properties close to those of calcium spontaneous oscillations, or, on the contrary, to inhibit the spontaneous calcium oscillations for a very long time compared to the pulse duration. Through that inhibition of the oscillations, Ca2+ oscillations of desired amplitude and frequency could then be imposed on the cells using subsequent electric pulses. None of the pulses used here, even those with the highest amplitude, caused a loss of cell viability.

CONCLUSIONS

An easy way to control Ca2+ oscillations in mesenchymal stem cells, through their cancellation or the addition of supplementary Ca2+ spikes, is reported here. Indeed, the direct link between the microsecond electric pulse(s) delivery and the occurrence/cancellation of cytosolic Ca2+ spikes allowed us to mimic and regulate the Ca2+ oscillations in these cells. Since microsecond electric pulse delivery constitutes a simple technology available in many laboratories, this new tool might be useful to further investigate the role of Ca2+ in human mesenchymal stem cells biological processes such as proliferation and differentiation.

Development of hydroxyapatite nanoparticles loaded with folic acid to induce osteoblastic differentiation

Recently it has been shown that folic acid can have an important role in bone regeneration. For this reason, combining a classic bone regeneration system as, hydroxyapatite, loaded with folic acid, may be an important issue to be developed. To address this issue, hydroxyapatite nanoparticles loaded with folic acid were designed as an effective bone regenerative system, to induce osteoblast differentiation and improve the bone regeneration. HapNP were prepared by a hydrothermal method that used citric acid as a tailoring agent of particles morphology and, simultaneously, had the particularly to let carboxylic pendant groups in the particle surface, which provided a platform for the immobilization of folic acid (FA), producing HapNP-FA. A comparative study among hydroxyapatite nanoparticles loaded and unloaded with folic acid in presence of human mesenchymal stem cells was performed. The results demonstrate, that nanoparticles were able to be internalized by human mesenchymal stem cells. In addition, cell proliferation and viability were not affected in a wide concentration range. Both particles induced the expression of Runx2 and the expression and activity of alkaline phosphatase. However, HapNP-FA caused a significantly higher overexpression of Runx2. The osteoblastic differentiation confirms the potential applicability of HapNP-FA in the local bone regeneration.

Signaling of extracellular matrices for tissue regeneration and therapeutics

Cells receive important regulatory signals from their extracellular matrix (ECM) and the physical property of the ECM regulates important cellular behaviors like cell proliferation, migration and differentiation. A large part of tissue formation and regeneration depends on cellular interaction with its ECM. A comprehensive understanding of the mechanistic biochemical pathway of the ECM components is necessary for the design of a biomaterial scaffold for tissue engineering. Depending on the type of tissue, the ECM requirement might be different and this would influence its downstream intracellular cell signaling. Here, we reviewed the ECM and its signaling pathway by discussing: 1) classification of the ECM into hard, elastic and soft tissue based on its physical properties, 2) proliferation and differentiation control of the ECM, 3) roles of membrane receptor and its intracellular regulation factor, 4) ECM remodeling via inside-out signaling. By providing a comprehensive overview of the ECM's role in mechanotransduction and the self-regulatory effect of cells back on the ECM, we hope to provide a better insight of the physical and biochemical cues from the ECM. A sound understanding on the in vivo ECM has implication on the choice of materials and surface coating of biomimetic scaffolds used for tissue regeneration and therapeutics in a cell-free scaffold.

Optimisation of conductive polymer biomaterials for cardiac progenitor cells

The characterisation of biomaterials for cardiac tissue engineering applications is vital for the development of effective treatments for the repair of cardiac function.

Smart electroconductive bioactive ceramics to promote in situ electrostimulation of bone

Biomaterials can still be reinvented to become simple and universal bone regeneration solutions. Following this roadmap, conductive CNT-based "smart" materials accumulate exciting grafting qualities for tuning the in vitro cellular phenotype. Biphasic electrical stimulation of human osteoblastic cells was performed in vitro on either dielectric bioactive bone grafts or conductive CNT-reinforced composites. The efficiency of the electrical stimuli delivery, as well as the effect of stimulation on cellular functions were investigated. Conductive substrates boosted the local culture medium conductivity and the confinement of the exogenous electrical fields. Hence, bone cell proliferation, DNA content and mRNA expression were maximized on the conductive substrates yielding superior stimuli delivering efficiency over dielectric ones. These findings are suggestive that bioactive bone grafts with electrical conductivity are capable of high spatial and temporal control of bone cell stimulation.

Gold-dotted hydroxyapatite nanoparticles as multifunctional platforms for medical applications

Hydroxyapatite nanoparticles decorated with gold dots, synthesized by a citrate mediated chemical method, enhance the osteogenic differentiation of HMSC.

Improvement of bone regeneration capability of ceramic scaffolds by accelerated release of their calcium ions

To regenerate the bone tissue, the fabrication of scaffolds for better tissue regeneration has attracted a great deal of attention. In fact, growth factors are already used in clinical practice and are being investigated for enhancing the capacity for bone tissue regeneration. However, despite their strong osteoinductive activity, these growth factors have several limitations: safety issues, high treatment costs, and the potential for ectopic bone formation. The aim of this study was therefore to develop ceramic scaffolds that could promote the capacity for bone regeneration without growth factors. Three-dimensional ceramic scaffolds were successfully fabricated from hydroxyapatite (HA) and tricalcium phosphate (TCP) using projection-based microstereolithography, which is an additive manufacturing technology. The effects of calcium ions released from ceramic scaffolds on osteogenic differentiation and bone regeneration were evaluated in vitro and in vivo. The osteogenesis-related gene expression and area of new bone formation in the HA/TCP scaffolds was higher than those in the HA scaffolds. Moreover, regenerated bone tissue in HA/TCP scaffolds were more matured than that in HA scaffolds. Through this study, we were able to enhance the bone regeneration capacity of scaffolds not by growth factors but by calcium ions released from the scaffolds. Ceramic scaffolds developed in this study might be useful for enhancing the capacity for regeneration in complex bone defects.

Influence of conductive polymer doping on the viability of cardiac progenitor cells

Investigating the influence of conductive polymer dopants on surface properties and chemistry, and how they may modify cardiac progenitor cell interactions.

Control of three-dimensional substrate stiffness to manipulate mesenchymal stem cell fate toward neuronal or glial lineages

The unlimited self-renewal and multipotency of stem cells provide great potential for applications in tissue engineering and regenerative medicine. The differentiation of stem cells can be induced by multiple factors including physical, chemical and biological cues. The fate of stem cells can be manipulated by deliberately controlling the interaction between stem cells and their microenvironment. The purpose of this study is to investigate the change in matrix stiffness under the influence of neurogenic differentiation of human mesenchymal stem cells (hMSCs). In this study, three-dimensional (3-D) porous scaffolds were synthesized by type I collagen (Col) and hyaluronic acid (HA). The elastic modulus of the 3-D substrates was modified by adjusting the concentration of 1-ethyl-3(3-dimethylaminopropyl) carbodiimide (EDC) as a crosslinking agent. The mechanical properties of Col-HA scaffolds were evaluated and the induction and characterization of hMSC differentiation toward neural lineages on substrates with different stiffnesses were studied. Using EDC of different concentrations for crosslinking, the stiffness of the matrices can be controlled in the range of 1-10 kPa for soft to stiff substrates, respectively. The results showed that MSCs were likely to differentiate into neuronal lineage in substrate at 1 kPa, while they transformed into glial cells in matrix at 10 kPa. The morphology and proliferation behavior of hMSCs responded to the different stiffnesses of substrates. Using this modifiable matrix, we can investigate the relationship between stem cell behavior and substrate mechanical properties in extracellular matrix-based biomimetic 3-D scaffolds. A substrate with controllable stiffness capable of inducing hMSCs specifically toward neuronal differentiation may be very useful as a tissue-engineered construct or substitute for delivering hMSCs into the brain and spinal cord.

A new method of fabricating robust freeform 3D ceramic scaffolds for bone tissue regeneration

Fabrication of three-dimensional (3D) scaffolds with appropriate mechanical properties and desired architecture for promoting cell growth and new tissue formation is one of the most important efforts in tissue engineering field. Scaffolds fabricated from bioactive ceramic materials such as hydroxyapatite and tricalcium phosphate show promise because of their biological ability to support bone tissue regeneration. However, the use of ceramics as scaffold materials is limited because of their inherent brittleness and difficult processability. The aim of this study was to create robust ceramic scaffolds, which have a desired architecture. Such scaffolds were successfully fabricated by projection-based microstereolithography, and dilatometric analysis was conducted to study the sintering behavior of the ceramic materials. The mechanical properties of the scaffolds were improved by infiltrating them with a polycaprolactone solution. The toughness and compressive strength of these ceramic/polymer scaffolds were about twice those of ceramic scaffolds. Furthermore, the osteogenic gene expression on ceramic/polymer scaffolds was better than that on ceramic scaffolds. Through this study, we overcame the limitations of previous research on fabricating ceramic scaffolds and these new robust ceramic scaffolds may provide a much improved 3D substrate for bone tissue regeneration.

Control of adipogenesis by ezrin, radixin and moesin-dependent biomechanics remodeling

We have recently shown that altered stem cell biomechanics can regulate the lineage commitment through a family of the membrane-cytoskeleton linker proteins (ERM; ezrin, radixin, moesin). The ERM proteins not only modulate the cell stiffness and actin cytoskeleton organization, but also rearrange focal adhesions and therefore influence the biochemically-directed stem cell differentiation. Combining silencing RNA, atomic force microscopy, and fluorescence microscopy, the role of the ERM proteins involved in the regulation of stem cell biomechanics and adipogenic differentiation was quantitatively determined. Transient ERM knockdown by RNAi caused disassembly of actin stress fibers and focal adhesions and a decrease in the cell stiffness. The silencing RNA treatment not only induced mechanical changes in stem cells but impaired adipogenesis in a time-dependent manner. While siRNA ERM treatment at day 0 substantially interfered with adipogenesis, the same treatment at day 3 of adipogenic differentiation significantly facilitated adipogenesis, as assessed by the expression of adipocyte-specific markers. The intact biomechanics homeostasis appears to be critical for the adipogenic induction. These findings may lead to potential biomechanical intervention techniques and methodologies to control the fate and extent of adipogenesis that would likely be involved in stem cell-based therapeutics for soft tissue repair and regeneration.

Electrical stimulation: a novel tool for tissue engineering

New advances in tissue engineering are being made through the application of different types of electrical stimuli to influence cell proliferation and differentiation. Developments made in the last decade have allowed us to improve the structure and functionality of tissue-engineered products through the use of growth factors, hormones, drugs, physical stimuli, bioreactor use, and two-dimensional (2-D) and three-dimensional (3-D) artificial extracellular matrices (with various material properties and topography). Another potential type of stimulus is electricity, which is important in the physiology and development of the majority of all human tissues. Despite its great potential, its role in tissue regeneration and its ability to influence cell migration, orientation, proliferation, and differentiation has rarely been considered in tissue engineering. This review highlights the importance of endogenous electrical stimulation, gathering the current knowledge on its natural occurrence and role in vivo, discussing the novel methods of delivering this stimulus and examining its cellular and tissue level effects, while evaluating how the technique could benefit the tissue engineering discipline in the future.

Adhesive proteins linked with focal adhesion kinase regulate neurite outgrowth of PC12 cells

Adhesive proteins existing in the extracellular matrix (ECM) play important roles in the regulation of neuronal cell behavior, including cell adhesion, motility and neurite outgrowth. Herein we show the effects of a series of adhesive proteins on the neurite outgrowth of PC12 cells and elucidate that this is closely related to the activation of focal adhesion kinase (FAK). For this we prepared culture substrates by coating tissue culture plastic with either collagen (Col), fibronectin (FN) or laminin (LN) and investigated the neurite outgrowth behavior. The results demonstrated that neurite outgrowth was highly dependent on the particular type of adhesive protein. While neurite number was comparable on all the coated surfaces, the length of neurites was greater on the FN- and LN-coated ones (greatest on the LN-coated one). In particular, FAK expression was highly up-regulated in the FN- and LN-coated surfaces, as revealed by Western blot analysis. A knock-down experiment further supported the idea that neurite outgrowth was largely suppressed in cells transfected with a FAK knock-down gene. Taken together, the neurite outgrowth of PC12 cells was greatly affected by adhesive proteins of the ECM, particularly FN and LN, and this is considered to be closely related to FAK intracellular signaling. This study may be useful in the consideration and design of nerve guidance and three-dimensional scaffolds which are appropriate to promote neuronal growth and nerve tissue regeneration.

Altered osteogenic commitment of human mesenchymal stem cells by ERM protein-dependent modulation of cellular biomechanics

Cellular mechanics is known to play an important role in many cellular functions including adhesion, migration, proliferation, and differentiation. Human mesenchymal stem cells (hMSCs) demonstrate unique mechanical properties distinct from fully differentiated cells. This observation suggests that the stem cell mechanics may be modulated to regulate the hMSCs' lineage commitment. Specifically, ERM (ezrin, radixin, moesin) proteins are known to mediate the membrane-cytoskeleton adhesion, cell elasticity, actin cytoskeleton organization, and therefore could serve as potential targets for modulation of the cellular mechanics. Combining silencing RNA, atomic force microscopy, and laser optical tweezers, the role of the ERM proteins involved in the regulation of stem cell biomechanics and osteogenic differentiation was quantitatively determined. Transient ERM knockdown by RNAi causes disassembly of actin stress fibers and focal adhesions, a decrease in the cell stiffness, and membrane separation from the cytoskeleton. The silencing RNA treatment not only induced mechanical changes in stem cells but impaired biochemically-directed osteogenic differentiation. The intact actin cytoskeleton and focal adhesions of hMSCs appear critical for the osteogenic induction. Thus, ERM knockdown modulates the dynamics of cell mechanical changes during hMSC differentiation and regulates the expression of tissue specific molecular markers. These findings are of particular interest for modulation of the cellular biomechanics to control hMSCs' activities and fate in tissue engineering, regenerative medicine, and other stem cell-based therapeutic applications.

Collagen gel three-dimensional matrices combined with adhesive proteins stimulate neuronal differentiation of mesenchymal stem cells

Three-dimensional gel matrices provide specialized microenvironments that mimic native tissues and enable stem cells to grow and differentiate into specific cell types. Here, we show that collagen three-dimensional gel matrices prepared in combination with adhesive proteins, such as fibronectin (FN) and laminin (LN), provide significant cues to the differentiation into neuronal lineage of mesenchymal stem cells (MSCs) derived from rat bone marrow. When cultured within either a three-dimensional collagen gel alone or one containing either FN or LN, and free of nerve growth factor (NGF), the MSCs showed the development of numerous neurite outgrowths. These were, however, not readily observed in two-dimensional culture without the use of NGF. Immunofluorescence staining, western blot and fluorescence-activated cell sorting analyses demonstrated that a large population of cells was positive for NeuN and glial fibrillary acidic protein, which are specific to neuronal cells, when cultured in the three-dimensional collagen gel. The dependence of the neuronal differentiation of MSCs on the adhesive proteins containing three-dimensional gel matrices is considered to be closely related to focal adhesion kinase (FAK) activation through integrin receptor binding, as revealed by an experiment showing no neuronal outgrowth in the FAK-knockdown cells and stimulation of integrin β1 gene. The results provided herein suggest the potential role of three-dimensional collagen-based gel matrices combined with adhesive proteins in the neuronal differentiation of MSCs, even without the use of chemical differentiation factors. Furthermore, these findings suggest that three-dimensional gel matrices might be useful as nerve-regenerative scaffolds.

The effects of mechanical loading on mesenchymal stem cell differentiation and matrix production

Mesenchymal stem cells or stromal cells (MSCs) have the potential to be used therapeutically in tissue engineering and regenerative medicine to replace or restore the function of damaged tissues. Therefore, considerable effort has been ongoing in the research community to optimize culture conditions for predifferentiation of MSCs. All mesenchymal tissues are subjected to mechanical forces in vivo and all fully differentiated mesenchymal lineage cells respond to mechanical stimulation in vivo and in vitro. Therefore, it is not surprising that MSCs are highly mechanosensitive. We present a summary of current methods of mechanical stimulation of MSCs and an overview of the outcomes of the different mechanical culture techniques tested. Tissue engineers and stem cell researchers should be able to harness this mechanosensitivity to modulate MSC differentiation and matrix production; however, more research needs to be undertaken to understand the complex interactions between the mechanosensitive and biochemically stimulated differentiation pathways.