Translating biomaterial properties to intracellular signaling

In-situ forming injectable GFOGER-conjugated BMSCs-laden hydrogels for osteochondral regeneration

The collagen-mimetic peptide GFOGER possesses the chondrogenic potential and has been used as a cell adhesion peptide or chondrogenic inducer. Here, we prepared an injectable in situ forming composite hydrogel system comprising methoxy polyethylene glycol-b-polycaprolactone (MPEG-PCL) and GFOGER-conjugated PEG-PCL (GFOGER-PEG-PCL) with various GFOGER concentrations based on our recently patented technology. The conjugation of GFOGER to PEG-PCL was confirmed by ¹H NMR, and the particle size distribution and rheological properties for the sol-gel transition behavior of the samples with respect to the GFOGER content were evaluated systemically. In vitro experiments using rat bone marrow-derived mesenchymal stem cells (BMSCs) revealed that the GFOGER-PEG-PCL hydrogel significantly enhanced expression of integrins (β1, α2, and α11), increased expression of FAK, and induced downstream signaling of ERK and p38. Overexpression of chondrogenic markers suggested that BMSCs have the potential to differentiate into chondrogenic lineages within GFOGER-PEG-PCL samples. In vivo studies using a rat osteochondral defect model revealed that transplanted BMSCs with GFOGER_0.8-PEG-PCL survived at the defect with strong chondrogenic expression after 4 weeks. The stem cell-laden GFOGER_0.8-PEG-PCL hydrogel produced remarkable osteochondral regeneration at 8 weeks of transplantation, as determined by histological findings and micro-CT analysis. The histomorphological score in the GFOGER_0.8-PEG-PCL + BMSCs group was ~1.7-, 2.6-, and 5.3-fold higher than that in the GFOGER_0.8-PEG-PCL, MPEG-PCL, and defect groups, respectively. Taken together, these results provide an important platform for further advanced GFOGER-based stem cell research for osteochondral repair.

A Review of Integrin-Mediated Endothelial Cell Phenotype in the Design of Cardiovascular Devices

Sustained biomaterial thromboresistance has long been a goal and challenge in blood-contacting device design. Endothelialization is one of the most successful strategies to achieve long-term thromboresistance of blood-contacting devices, with the endothelial cell layer providing dynamic hemostatic regulation. It is well established that endothelial cell behavior is influenced by interactions with the underlying extracellular matrix (ECM). Numerous researchers have sought to exploit these interactions to generate improved blood-contacting devices by investigating the expression of hemostatic regulators in endothelial cells on various ECM coatings. The ability to select substrates that promote endothelial cell-mediated thromboresistance is crucial to advancing material design strategies to improve cardiovascular device outcomes. This review provides an overview of endothelial cell regulation of hemostasis, the major components found within the cardiovascular basal lamina, and the interactions of endothelial cells with prominent ECM components of the basement membrane. A summary of ECM-mimetic strategies used in cardiovascular devices is provided with a focus on the effects of key adhesion modalities on endothelial cell regulators of hemostasis.

In vitro response of dental pulp stem cells in 3D scaffolds: A regenerative bone material

Three-dimensional-porous scaffolds of bone graft substitutes play a critical role in both cell targeting and transplantation strategies. These scaffolds provide surfaces that facilitate the response of stem cells related to attachment, survival, migration, proliferation, and differentiation.

Objective

The aim of this study was to evaluate the in vitro behavior of human dental pulp mesenchymal stem cells cultured on scaffolds of polylactic/polyglycolic acid with and without hydroxyapatite.

Method

We performed an in vitro experimental study using dental pulp stem cells obtained from samples of premolars, molars. The cells were cultured on scaffolds with osteogenic differentiation medium. Cell proliferation, adhesion and cell differentiation to an osteoblastic linage in the biomaterial were evaluated at three different time points: 7, 15 and 30 days. Each experiment was performed in triplicate. Analysis of the data was performed with the Split Plot block and MANOVA model.

Results

The differentiation capability of hDPSCs towards the osteoblast lineage was better in the scaffold of PLGA/HA at 7, 15 and 30 days, as indicated by the high expression of osteogenic markers RUNX2, ALP, OPN and COL-I, compared with differentiation in the PLGA scaffold. No statistically significant differences were found in cell adhesion between the two types of scaffolds.

Conclusion

The PLGA/HA scaffold provided better physical and chemical signals, as judged by the ability of dental pulp stem cells to adhere, proliferate and differentiate toward the osteogenic lineage.

Viscoelastic, physical, and bio-degradable properties of dermal scaffolds and related cell behaviour

Dermal scaffolds promote healing of debilitating skin injuries caused by burns and chronic skin conditions. Currently available products present disadvantages and therefore, there is still a clinical need for developing new dermal substitutes. This study aimed at comparing the viscoelastic, physical and bio-degradable properties of two dermal scaffolds, the collagen-based and clinically well established Integra(®) and a novel fibrin-based dermal scaffold developed at our laboratory called Smart Matrix(®), to further evaluate our previous published findings that suggested a higher influx of cells, reduced wound contraction and less scarring for Smart Matrix(®) when used in vivo. Rheological results showed that Integra(®) (G'  =  313.74 kPa) is mechanically stronger than Smart Matrix(®) (G'  =  8.26 kPa), due to the presence of the silicone backing layer in Integra(®). Micro-pores were observed on both dermal scaffolds, although nano-pores as well as densely packed nano-fibres were only observed for Smart Matrix(®). Average surface roughness was higher for Smart Matrix(®) (Sa  =  114.776 nm) than for Integra(®) (Sa  =  75.565 nm). Both scaffolds possess a highly porous structure (80-90%) and display a range of pore micro-sizes that represent the actual in vivo scenario. In vitro proteolytic bio-degradation suggested that Smart Matrix(®) would degrade faster upon implantation in vivo than Integra(®). For both scaffolds, the enzymatic digestion occurs via bulk degradation. These observed differences could affect cell behaviour on both scaffolds. Our results suggest that fine-tuning of scaffolds' viscoelastic, physical and bio-degradable properties can maximise cell behaviour in terms of attachment, proliferation and infiltration, which are essential for tissue repair.

Strategies to engineer tendon/ligament-to-bone interface: Biomaterials, cells and growth factors

Integration between tendon/ligament and bone occurs through a specialized tissue interface called enthesis. The complex and heterogeneous structure of the enthesis is essential to ensure smooth mechanical stress transfer between bone and soft tissues. Following injury, the interface is not regenerated, resulting in high rupture recurrence rates. Tissue engineering is a promising strategy for the regeneration of a functional enthesis. However, the complex structural and cellular composition of the native interface makes enthesis tissue engineering particularly challenging. Thus, it is likely that a combination of biomaterials and cells stimulated with appropriate biochemical and mechanical cues will be needed. The objective of this review is to describe the current state-of-the-art, challenges and future directions in the field of enthesis tissue engineering focusing on four key parameters: (1) scaffold and biomaterials, (2) cells, (3) growth factors and (4) mechanical stimuli.

Cellular behavior as a dynamic field for exploring bone bioengineering: a closer look at cell-biomaterial interface

Bone is a highly dynamic and specialized tissue, capable of regenerating itself spontaneously when afflicted by minor injuries. Nevertheless, when major lesions occur, it becomes necessary to use biomaterials, which are not only able to endure the cellular proliferation and migration, but also to substitute the original tissue or integrate itself to it. With the life expectancy growth, regenerative medicine has been gaining constant attention in the reconstructive field of dentistry and orthopedy. Focusing on broadening the therapeutic possibilities for the regeneration of injured organs, the development of biomaterials allied with the applicability of gene therapy and bone bioengineering has been receiving vast attention over the recent years. The progress of cellular and molecular biology techniques gave way to new-guided therapy possibilities. Supported by multidisciplinary activities, tissue engineering combines the interaction of physicists, chemists, biologists, engineers, biotechnologist, dentists and physicians with common goals: the search for materials that could promote and lead cell activity. A well-oriented combining of scaffolds, promoting factors, cells, together with gene therapy advances may open new avenues to bone healing in the near future. In this review, our target was to write a report bringing overall concepts on tissue bioengineering, with a special attention to decisive biological parameters for the development of biomaterials, as well as to discuss known intracellular signal transduction as a new manner to be explored within this field, aiming to predict in vitro the quality of the host cell/material and thus contributing with the development of regenerative medicine.

Kinome profiling of osteoblasts on hydroxyapatite opens new avenues on biomaterial cell signaling

In degenerative diseases or lesions, bone tissue replacement and regeneration are important clinical goals. The most used bone substitutes today are hydroxyapatite (HA) scaffolds. These scaffolds, developed over the last few decades, present high porosity and good osteointegration, but haven't completely solved issues related to bone defects. Moreover, the exact intracellular mechanisms involved in the response to HA have yet to be addressed. This prompted us to investigate the protein networks responsible for signal transduction during early osteoblast adhesion on synthetic HA scaffolds. By performing a global kinase activity assay, we showed that there is a specific molecular machinery responding to HA contact, immediately triggering pathways leading to cytoskeleton rearrangement due to activation of Adducin 1 (ADD1), protein kinase A (PKA), protein kinase C (PKC), and vascular endothelial growth factor (VEGF). Moreover, we found a significantly increased phosphorylation of the activating site Ser-421 in histone deacetylase 1 (HDAC1), a substrate of Cyclin-Dependent Kinase 5 (CDK5). These phosphorylation events are hallmarks of osteoblast differentiation, pointing to HA surfaces ability to promote differentiation. We also found that AKT was kept active, suggesting the maintenance of survival pathways. Interestingly, though, the substrate sequence of CDK5 also presented higher phosphorylation levels when compared to control conditions. To our knowledge, this kinase has never before been related to osteoblast biology, opening a new avenue of investigation for novel pathways involved in this matter. These results suggest that HA triggers a specific intracellular signal transduction cascade during early osteoblast adhesion, activating proteins involved with cytoskeleton rearrangement, and induction of osteoblast differentiation.

Microdialysis sampling techniques applied to studies of the foreign body reaction

Implanted materials including drug delivery devices and chemical sensors undergo what is termed the foreign body reaction (FBR). Depending on the device and its intended application, the FBR can have differing consequences. An extensive scientific research effort has been devoted to elucidating the cellular and molecular mechanisms that drive the FBR. Important, yet relatively unexplored, research includes the localized tissue biochemistry and the chemical signaling events that occur throughout the FBR. This review provides an overview of the mechanisms of the FBR, describes how the FBR affects different implanted devices, and illustrates the role that microdialysis sampling can play in further elucidating the chemical communication processes that drive FBR outcomes.

Activation of the ERK1/2 signaling pathway during the osteogenic differentiation of mesenchymal stem cells cultured on substrates modified with various chemical groups

The current study examined the influence of culture substrates modified with the functional groups –OH, –COOH, –NH₂, and –CH₃ using SAMs technology, in conjunction with TAAB control, on the osteogenic differentiation of rabbit BMSCs. The CCK-8 assay revealed that BMSCs exhibited substrate-dependent cell viability. The cells plated on –NH₂- and –OH-modified substrates were well spread and homogeneous, but those on the –COOH- and –CH₃-modified substrates showed more rounded phenotype. The mRNA expression of BMSCs revealed that –NH₂-modified substrate promoted the mRNA expression and osteogenic differentiation of the BMSCs. The contribution of ERK1/2 signaling pathway to the osteogenic differentiation of BMSCs cultured on the –NH₂-modified substrate was investigated in vitro. The –NH₂-modified substrate promoted the expression of integrins; the activation of FAK and ERK1/2. Inhibition of ERK1/2 activation by PD98059, a specific inhibitor of the ERK signaling pathway, blocked ERK1/2 activation in a dose-dependent manner, as revealed for expression of Cbfα-1 and ALP. Blockade of ERK1/2 phosphorylation in BMSCs by PD98059 suppressed osteogenic differentiation on chemical surfaces. These findings indicate a potential role for ERK in the osteogenic differentiation of BMSCs on surfaces modified by specific chemical functional groups, indicating that the microenvironment affects the differentiation of BMSCs. This observation has important implications for bone tissue engineering.

Temporal differences in Erk1/2 activity distinguish among combinations of extracellular matrix components

Rational design of biomaterials requires understanding how cells interrogate their microenvironment. In this study, human umbilical vein endothelial cells are cultured on combinations of extracellular matrix (ECM) components (collagen I, collagen IV, vitronectin, fibronectin, laminin, heparan sulfate proteoglycan, chondroitin sulfate proteoglycan), and the phosphorylation of four intracellular signaling kinases (Erk1/2, JNK, Akt1, and NFκB) is quantified. These combinations of ECM components elicit different temporal patterns of Erk1/2 phosphorylation. Collagen I-containing substrates cause Erk1/2 phosphorylation to reach maximal levels at 30 min and remain near maximal levels until 90 min. Collagen IV/laminin substrates elicit maximal phosphorylation at 30-45 min, and then phosphorylation decreases substantially at 60-90 min. All other combinations studied (collagen IV and vitronectin-based combinations) cause an increase in phosphorylation at 30-45 min, but not to maximal levels; maximal phosphorylation is reached by 60-90 min. These temporal patterns of phosphorylation may explain how a limited number of intracellular signaling pathways can distinguish among thousands of possible combinations of microenvironmental cues by adding to the information contained in each cell signaling pathway.

Controlling multipotent stromal cell migration by integrating "course-graining" materials and "fine-tuning" small molecules via decision tree signal-response modeling

Biomimetic scaffolds have been proposed as a means to facilitate tissue regeneration by multi-potent stromal cells (MSCs). Effective scaffold colonization requires a control of multiple MSC responses including survival, proliferation, differentiation, and migration. As MSC migration is relatively unstudied in this context, we present here a multi-level approach to its understanding and control, integratively tuning cell speed and directional persistence to achieve maximal mean free path (MFP) of migration. This approach employs data-driven computational modeling to ascertain small molecule drug treatments that can enhance MFP on a given materials substratum. Using poly(methyl methacrylate)-graft-poly(ethylene oxide) polymer surfaces tethered with epidermal growth factor (tEGF) and systematically adsorbed with fibronectin, vitronectin, or collagen-I to present hTERT-immortalized human MSCs with growth factor and extracellular matrix cues, we measured cell motility properties along with signaling activities of EGFR, ERK, Akt, and FAK on 19 different substrate conditions. Speed was consistent on collagen/tEGF substrates, but low associated directional persistence limited MFP. Decision tree modeling successfully predicted that ERK inhibition should enhance MFP on collagen/tEGF substrates by increasing persistence. Thus, we demonstrated a two-tiered approach to control MSC migration: materials-based "coarse-graining" complemented by small molecule "fine-tuning".

Intracellular signal transduction as a factor in the development of "smart" biomaterials for bone tissue engineering

Signal transduction involves studying the intracellular mechanisms that govern cellular responses to external stimuli such as hormones, cytokines, and also cell adhesion to biomaterials surfaces. Several events have been shown to be responsible for cellular adhesion and adaptation onto different surfaces. For instance, cytoskeletal rearrangements during cell adhesion require the recruitment of specific protein tyrosine kinases into focal adhesion structures that promote transient focal adhesion kinase and Src phosphorylations, initially modulating cell behavior. In addition, the phosphorylation of tyrosine (Y) residues have been generally accepted as a critical regulator of a wide range of cell-related processes, including cell proliferation, migration, differentiation, survival signalling, and energy metabolism. The understanding of the signaling involved on the mechanisms of osteoblast adhesion, proliferation, and differentiation on implant surfaces is fundamental for the successful design of novel "smart" materials, potentially decreasing the repair time, thereby allowing for faster patient rehabilitation.

Multipathway kinase signatures of multipotent stromal cells are predictive for osteogenic differentiation: tissue-specific stem cells

Bone marrow-derived multipotent stromal cells (MSCs) offer great promise for regenerating tissue. Although certain transcription factors have been identified in association with tendency toward particular MSC differentiation phenotypes, the regulatory network of key receptor-mediated signaling pathways activated by extracellular ligands that induce various differentiation responses remains poorly understood. Attempts to predict differentiation fate tendencies from individual pathways in isolation are problematic due to the complex pathway interactions inherent in signaling networks. Accordingly, we have undertaken a multivariate systems approach integrating experimental measurement of multiple kinase pathway activities and osteogenic differentiation in MSCs, together with computational analysis to elucidate quantitative combinations of kinase signals predictive of cell behavior across diverse contexts. In particular, for culture on polymeric biomaterial surfaces presenting tethered epidermal growth factor, type I collagen, neither, or both, we have found that a partial least-squares regression model yields successful prediction of phenotypic behavior on the basis of two principal components comprising the weighted sums of eight intracellular phosphoproteins: phospho-epidermal growth factor receptor, phospho-Akt, phospho-extracellular signal-related kinase 1/2, phospho-heat shock protein 27, phospho-c-Jun, phospho-glycogen synthase kinase 3alpha/beta, phospho-p38, and phospho-signal transducer and activator of transcription 3. This combination provides the strongest predictive capability for 21-day differentiated phenotype status when calculated from day-7 signal measurements; day-4 and day-14 signal measurements are also significantly predictive, indicating a broad time frame during MSC osteogenesis wherein multiple pathways and states of the kinase signaling network are quantitatively integrated to regulate gene expression, cell processes, and ultimately, cell fate.