Applying macromolecular crowding to enhance extracellular matrix deposition and its remodeling in vitro for tissue engineering and cell-based therapies

Multiphoton Microscopy for Noninvasive and Label-Free Imaging of Human Skin and Oral Mucosa Equivalents

Multiphoton microscopy has emerged as a powerful modality for noninvasive, spatial, and temporal imaging of biological tissues without the use of labels and/or dyes. It provides complimentary imaging modalities, which include two-photon excited fluorescence (2PEF) and second harmonic generation (SHG). 2PEF from endogenous chromophores such as nicotinamide adenine dinucleotides (NADH), flavins and keratin enable visualization of cellular and subcellular structures. SHG provides visualization of asymmetric macromolecular structures such as collagen. These modalities enable the visualization of biochemical and biological alterations within live tissues in their native state.Organotypic cultures of the skin and oral mucosa equivalents have been increasingly used across basic and translational research. However, assessment of the skin and oral mucosa equivalents is predominantly based on histological techniques which are not suited for real-time imaging and longitudinal studies of the tissues in their native state. 2PEF from endogenous chromophores and SHG from collagen can be effectively used as an imaging tool for noninvasive and label-free acquisition of cellular and matrix structures of live skin and oral mucosa cultures.In this chapter, the methods for noninvasive and label-free imaging of monolayer and organotypic cultures of the skin and oral mucosa using multiphoton microscopy are described.

Predicting Drug Release From Degradable Hydrogels Using Fluorescence Correlation Spectroscopy and Mathematical Modeling

Predicting release from degradable hydrogels is challenging but highly valuable in a multitude of applications such as drug delivery and tissue engineering. In this study, we developed a simple mathematical and computational model that accounts for time-varying diffusivity and geometry to predict solute release profiles from degradable hydrogels. Our approach was to use time snapshots of diffusivity and hydrogel geometry data measured experimentally as inputs to a computational model which predicts release profile. We used two model proteins of varying molecular weights: bovine serum albumin (BSA; 66 kDa) and immunoglobulin G (IgG; 150 kDa). We used fluorescence correlation spectroscopy (FCS) to determine protein diffusivity as a function of hydrogel degradation. We tracked changes in gel geometry over the same time period. Curve fits to the diffusivity and geometry data were used as inputs to the computational model to predict the protein release profiles from the degradable hydrogels. We validated the model using conventional bulk release experiments. Because we approached the hydrogel as a black box, the model is particularly valuable for hydrogel systems whose degradation mechanisms are not known or cannot be accurately modeled.

In Vitro Expansion of Keratinocytes on Human Dermal Fibroblast-Derived Matrix Retains Their Stem-Like Characteristics

The long-term expansion of keratinocytes under conditions that avoid xenogeneic components (i.e. animal serum- and feeder cell-free) generally causes diminished proliferation and increased terminal differentiation. Here we present a culture system free of xenogeneic components that retains the self-renewal capacity of primary human keratinocytes. In vivo the extracellular matrix (ECM) of the tissue microenvironment has a major influence on a cell's fate. We used ECM from human dermal fibroblasts, cultured under macromolecular crowding conditions to facilitate matrix deposition and organisation, in a xenogeneic-free keratinocyte expansion protocol. Phospholipase A2 decellularisation produced ECM whose components resembled the core matrix composition of natural dermis by proteome analyses. Keratinocytes proliferated rapidly on these matrices, retained their small size, expressed p63, lacked keratin 10 and rarely expressed keratin 16. The colony forming efficiency of these keratinocytes was enhanced over that of keratinocytes grown on collagen I, indicating that dermal fibroblast-derived matrices maintain the in vitro expansion of keratinocytes in a stem-like state. Keratinocyte sheets formed on such matrices were multi-layered with superior strength and stability compared to the single-layered sheets formed on collagen I. Thus, keratinocytes expanded using our xenogeneic-free protocol retained a stem-like state, but when triggered by confluence and calcium concentration, they stratified to produce epidermal sheets with a potential clinical use.

Fibrillar fibronectin plays a key role as nucleator of collagen I polymerization during macromolecular crowding-enhanced matrix assembly

Macromolecular crowding is used by tissue engineers to accelerate extracellular matrix assembly in vitro, however, most mechanistic studies focus on the impact of crowding on collagen fiber assembly and largely ignore the highly abundant provisional matrix protein fibronectin. We show that the accelerated collagen I assembly as induced by the neutral crowding molecule Ficoll is regulated by cell access to fibronectin. Ficoll treatment leads to significant increases in the amount of surface adherent fibronectin, which can readily be harvested by cells to speed up fibrillogenesis. FRET studies reveal that Ficoll crowding also upregulates the total amount of fibronectin fibers in a low-tension state through upregulating fibronectin assembly. Since un-stretched fibronectin fibers have more collagen binding sites to nucleate the onset of collagen fibrillogenesis, our data suggest that the Ficoll-induced upregulation of low-tension fibronectin fibers contributes to enhanced collagen assembly in crowded conditions. In contrast, chemical cross-linking of fibronectin to the glass substrate prior to cell seeding prevents early force mediated fibronectin harvesting from the substrate and suppresses upregulation of collagen I assembly in the presence of Ficoll, even though the crowded environment is known to drive enzymatic cleavage of procollagen and collagen fiber formation. To show that our findings can be exploited for tissue engineering applications, we demonstrate that the addition of supplemental fibronectin in the form of an adsorbed coating markedly improves the speed of tissue formation under crowding conditions.

Emerging themes and unifying concepts underlying cell behavior regulation by the pericellular space

Cells reside in a complex three-dimensional (3D) microenvironment where physical, chemical, and architectural features of the pericellular space regulate important cellular functions like migration, differentiation, and morphogenesis. A major goal of tissue engineering is to identify which properties of the pericellular space orchestrate these emergent cell behaviors and how. In this review, we highlight recent studies at the interface of biomaterials and single cell biophysics that are lending deeper insight towards this goal. Advanced methods have enabled the decoupling of architectural and mechanical features of the microenvironment, revealing multiple mechanisms of adhesion and mechanosensing modulation by biomaterials. Such studies are revealing important roles for pericellular space degradability, hydration, and adhesion competition in cell shape, volume, and differentiation regulation. STATEMENT OF SIGNIFICANCE: Cell fate and function are closely regulated by the local extracellular microenvironment. Advanced methods at the interface of single cell biophysics and biomaterials have shed new light on regulators of cell-pericellular space interactions by decoupling more features of the complex pericellular milieu than ever before. These findings lend deeper mechanistic insight into how biomaterials can be designed to fine-tune outcomes like differentiation, migration, and collective morphogenesis.

3D Culture Histology Cryosectioned Well Insert Technology Preserves the Structural Relationship between Cells and Biomaterials for Time-Lapse Analysis of 3D Cultures

When performing histology of softer biomaterials, aspiration disrupts the cellular and molecular location information. This study aims to develop a cryosectionable well insert able to preserve the biomaterial and cell's original 3D conformation from the well to histology analysis. The well insert is composed of a paraffin-coated gelatine pill. Within the coated capsule, the human epithelial cell line (NS-SV-AC) is cultured in Matrigel, GrowDex, Myogel, Myogel + GrowDex, or cell culture media for 14 days. At 0 and 14 days, the samples are frozen in liquid nitrogen and cryotome is used to create sections. The slides are stained by Sirius Red and immunohistochemistry using antibodies human collagens I-V and human Ki-67. Sirius Red shows pink shades of biomaterials and the best cellular vertical distribution throughout the sagittal section of the well is achieved with Matrigel, GrowDex, and Myogel + GrowDex; in Myogel and media, the cells sink. For collagen protein expression, only Matrigel induces a notable difference while in the other materials, collagen staining is weak or difficult to distinguish from endogenous collagens. Ki-67 expression is maintained over time. The 3D-cryo well insert provides a new time-lapse histology perspective of analysis for liquid or gel cultures that maintains cells and macromolecules in their unaltered in-well configuration.

Human Bone Marrow-Derived Mesenchymal Stem Cells Home via the PI3K-Akt, MAPK, and Jak/Stat Signaling Pathways in Response to Platelet-Derived Growth Factor

Mesenchymal stem cells (MSCs) have great potential to improve clinical outcomes for many inflammatory and degenerative diseases either through intravenously delivered MSCs or through mobilization and migration of endogenous MSCs to injury sites, termed "stem cell homing." Stem cell homing involves the processes of attachment to and transmigration through endothelial cells lining the vasculature and migration through the tissue stroma to a site of injury or inflammation. Although the process of leukocyte transendothelial migration (TEM) is well understood, far less is known about stem cell homing. In this study, a transwell-based model was developed to monitor adherence and TEM of human MSCs in response to chemokine exposure. Specifically, transwell membranes lined with human synovial microvascular endothelial cells were partitioned from the tissue injury-mimetic site containing chemokine stromal cell-derived factor-1 (SDF-1). Two population subsets of MSCs were studied: migratory cells that initiated transmigration on the endothelial lining and nonmigratory cells. We hypothesized that cells would adhere to and migrate through the endothelial lining in response to SDF-1 exposure and that gene and protein expression changes would be observed between migratory and nonmigratory cells. We validated a vasculature model for MSC transmigration that showed increased expression of several genes and activation of proteins of the PI3K-Akt, MAPK, and Jak/Stat signaling pathways. These findings showed that MSC homing may be driven by activation of PDGFRA/PI3K/Akt, PDGFRA/MAPK/Grb2, and PDGFRA/Jak2/Stat signaling, as a result of SDF-1-stimulated endothelial cell production of platelet-derived growth factor. This model can be used to further investigate these key regulatory molecules toward the development of targeted therapies.

A high content, phenotypic 'scar-in-a-jar' assay for rapid quantification of collagen fibrillogenesis using disease-derived pulmonary fibroblasts

BACKGROUND

Excessive extracellular matrix (ECM) deposition is a hallmark feature in fibrosis and tissue remodelling diseases. Typically, mesenchymal cells will produce collagens under standard 2D cell culture conditions, however these do not assemble into fibrils. Existing assays for measuring ECM production are often low throughput and not disease relevant. Here we describe a robust, high content, pseudo-3D phenotypic assay to quantify mature fibrillar collagen deposition which is both physiologically relevant and amenable to high throughput compound screening. Using pulmonary fibroblasts derived from patients with idiopathic pulmonary fibrosis (IPF), we developed the 'scar-in-a-jar' assay into a medium-throughput phenotypic assay to robustly quantify collagen type I deposition and other extracellular matrix (ECM) proteins over 72 h.

RESULTS

This assay utilises macromolecular crowding to induce an excluded volume effect and enhance enzyme activity, which in combination with TGF-β1 stimulation significantly accelerates ECM production. Collagen type I is upregulated approximately 5-fold with a negligible effect on cell number. We demonstrate the robustness of the assay achieving a Z prime of approximately 0.5, and % coefficient of variance (CV) of < 5 for the assay controls SB-525334 (ALK5 inhibitor) and CZ415 (mTOR inhibitor). This assay has been used to confirm the potency of a number of potential anti-fibrotic agents. Active compounds from the 'scar-in-a-jar' assay can be further validated for other markers of ECM deposition and fibroblast activation such as collagen type IV and α-smooth muscle actin exhibiting a 4-fold and 3-fold assay window respectively.

CONCLUSION

In conclusion, we have developed 'scar -in-a-jar is' into a robust disease-relevant medium-throughput in vitro assay to accurately quantify ECM deposition. This assay may enable iterative compound profiling for IPF and other fibroproliferative and tissue remodelling diseases.

Effect of Macromolecular Crowding on the FMN-Heme Intraprotein Electron Transfer in Inducible NO Synthase

Previous biochemical studies of nitric oxide synthase enzymes (NOSs) were conducted in diluted solutions. However, the intracellular milieu where the proteins perform their biological functions is crowded with macromolecules. The effect of crowding on the electron transfer kinetics of multidomain proteins is much less understood. Herein, we investigated the effect of macromolecular crowding on the FMN-heme intraprotein interdomain electron transfer (IET), an obligatory step in NOS catalysis. A noticeable increase in the IET rate in the bidomain oxygenase/FMN (oxyFMN) and the holoprotein of human inducible NOS (iNOS) was observed upon addition of Ficoll 70 in a nonsaturable manner. Additionally, the magnitude of IET enhancement for the holoenzyme is much higher than that that of the oxyFMN construct. The crowding effect is also evident at different ionic strengths. Importantly, the enhancing extent is similar for the iNOS oxyFMN protein with added Ficoll 70 and Dextran 70 that give the same solution viscosity, showing that specific interactions do not exist between the NOS protein and the crowder. Moreover, the population of the docked FMN-heme state is significantly increased upon addition of Ficoll 70 and the fluorescence lifetime values do not correspond to those in the absence of Ficoll 70. The steady-state cytochrome c reduction by the holoenzyme is noticeably enhanced by the crowder, while the ferricyanide reduction is unchanged. The NO production activity of the iNOS holoenzyme is stimulated by Ficoll 70. The effect of macromolecular crowding on the kinetics can be rationalized on the basis of the excluded volume effect, with an entropic origin. The intraprotein electron transfer kinetics, fluorescence lifetime, and steady-state enzymatic activity results indicate that macromolecular crowding modulates the NOS electron transfer through multiple pathways. Such a mechanism should be applicable to electron transfer in other multidomain redox proteins.

The extracellular fluid macromolecular composition differentially affects cell-substrate adhesion and cell morphology

Soluble macromolecules present in the tumour microenvironment (TME) alter the physical characteristics of the extracellular fluid and can affect cancer cell behaviour. A fundamental step in cancer progression is the formation of a new vascular network which may originate from both pre-existing normal endothelium and cancer-derived cells. To study the role of extracellular macromolecules in the TME affecting endothelial cells we exposed normal and cancer-derived endothelial cells to inert polymer solutions with different physicochemical characteristics. The cancer cell line SK-HEP-1, but not normal human umbilical vein endothelial cells, responded to high-macromolecular-content solutions by elongating and aligning with other cells, an effect that was molecular weight-dependent. Moreover, we found that neither bulk viscosity, osmotic pressure, nor the fractional volume occupancy of polymers alone account for the induction of these effects. Furthermore, these morphological changes were accompanied by an increased extracellular matrix deposition. Conversely, cell-substrate adhesion was enhanced by polymers increasing the bulk viscosity of the culture medium independently of polymer molecular weight. These results show that the complex macromolecular composition of the extracellular fluid strongly influences cancer-derived endothelial cell behaviour, which may be crucial to understanding the role of the TME in cancer progression.

Polydispersity and negative charge are key modulators of extracellular matrix deposition under macromolecular crowding conditions

Macromolecular crowding is a biophysical phenomenon that stems from the volume excluded by macromolecules, as they undergo steric repulsion and electrostatic interactions. The excluded volume depends on the shape, size, charge and polydispersity of the molecules. Although theoretical/computational models have been used to assess the influence of macromolecular crowding in biological media, real-time experiments are scarce. Herein, we evaluated the influence of hydrodynamic radius, charge and polydispersity of (a) various concentrations of different crowders (carrageenan, Ficoll™ and dextran sulphate); (b) various molecular weights of different crowders (70, 400 and 100 kDa of Ficoll™ and 10, 100 and 500 kDa of dextran sulphate) and (c) various cocktails of the same crowders (cocktails of various concentrations of different molecular weights Ficoll™ and dextran sulphate) on extracellular matrix deposition in human dermal fibroblast culture. The use of crowding cocktails with different molecular weight/concentrations of Ficoll™ or dextran sulphate molecules led to increased polydispersity and enhanced collagen type I deposition in comparison to their mono-domain counterparts. Carrageenan, however, induced the highest deposition of collagen type I due to its negative charge and inherent polydispersity. Our data contribute to a better understanding of the influence of the biophysical properties of the crowders on extracellular matrix deposition in vitro. STATEMENT OF SIGNIFICANCE: Macromolecular crowding is a biophysical phenomenon that accelerates and enhances extracellular matrix deposition in cell culture systems. Herein, we demonstrate that negatively charged and polydispersed macromolecules or cocktails of macromolecules, as opposed to neutral and monodomain macromolecules, induce highest extracellular matrix deposition in human dermal fibroblast cultures.

Macromolecular crowding tunes 3D collagen architecture and cell morphogenesis

Collagen I is the primary extracellular matrix component of most solid tumors and influences metastatic progression. Collagen matrix engineering techniques are useful for understanding how this complex biomaterial regulates cancer cell behavior and for improving in vitro cancer models. Here, we establish an approach to tune collagen fibril architecture using PEG as an inert molecular crowding agent during gelation and cell embedding. We find that crowding produces matrices with tighter fibril networks that are less susceptible to proteinase mediated degradation, but does not significantly alter matrix stiffness. The resulting matrices have the effect of preventing cell spreading, confining cells, and reducing cell contractility. Matrix degradability and fibril length are identified as strong predictors of cell confinement. Further, the degree of confinement predicts whether breast cancer cells will ultimately undergo individual or collective behaviors. Highly confined breast cancer cells undergo morphogenesis to form either invasive networks reminiscent of aggressive tumors or gland and lobule structures reminiscent of normal breast epithelia. This morphological transition is accompanied by expression of cell-cell adhesion genes, including PECAM1 and ICAM1. Our study suggests that cell confinement, mediated by matrix architecture, is a design feature that tunes the transcriptional and morphogenic state of breast cancer cells.

Multifactorial bottom-up bioengineering approaches for the development of living tissue substitutes

Bottom-up bioengineering utilizes the inherent capacity of cells to build highly sophisticated structures with high levels of biomimicry. Despite the significant advancements in the field, monodomain approaches require prolonged culture time to develop an implantable device, usually associated with cell phenotypic drift in culture. Herein, we assessed the simultaneous effect of macromolecular crowding (MMC) and mechanical loading in enhancing extracellular matrix (ECM) deposition while maintaining tenocyte (TC) phenotype and differentiating bone marrow stem cells (BMSCs) or transdifferentiating neonatal and adult dermal fibroblasts toward tenogenic lineage. At d 7, all cell types presented cytoskeleton alignment perpendicular to the applied load independently of the use of MMC. MMC enhanced ECM deposition in all cell types. Gene expression analysis indicated that MMC and mechanical loading maintained TC phenotype, whereas tenogenic differentiation of BMSCs or transdifferentiation of dermal fibroblasts was not achieved. Our data suggest that multifactorial bottom-up bioengineering approaches significantly accelerate the development of biomimetic tissue equivalents.-Gaspar, D., Ryan, C. N. M., Zeugolis, D. I. Multifactorial bottom-up bioengineering approaches for the development of living tissue substitutes.

Preparation and Characterization of Tissue Surrogates Rich in Extracellular Matrix Using the Principles of Macromolecular Crowding

Tissue engineering by self-assembly allows for the fabrication of living tissue surrogates by taking advantage of the cell's inherent ability to produce and deposit tissue-specific extracellular matrix. However, the long culture periods required to build a tissue substitute in conducive to phenotypic drift in vitro microenvironments result in phenotype and function losses. Although several biophysical microenvironmental modulators (e.g., surface topography, substrate stiffness, mechanical stimulation) have been used to address these issues, slow extracellular matrix deposition remains a limiting factor in clinical translation and commercialization of such therapies. Macromolecular crowding is an alternative in vitro microenvironment modulator that has been shown to accelerate extracellular matrix deposition by several orders of magnitude, thereby decreasing culture periods required for the development of an implantable device, while maintaining cell phenotype and function. Herein, we provide protocols for the production of tissue surrogates rich in extracellular matrix from human dermal fibroblasts, equine tenocytes, and equine adipose-derived stem cells using the principles of macromolecular crowding and the subsequent characterization thereof by means of immunofluorescent staining and complementary fluorescence intensity analysis.

Lineage-specific exosomes could override extracellular matrix mediated human mesenchymal stem cell differentiation

Lineage specification is an essential process in stem cell fate, tissue homeostasis and development. Microenvironmental cues provide direct and selective extrinsic signals to regulate lineage specification of stem cells. Microenvironmental milieu consists of two essential components, one being extracellular matrix (ECM) as the substratum, while the other being cell secreted exosomes and growth factors. ECM of differentiated cells modulates phenotypic expression of stem cells, while their exosomes contain phenotype specific instructive factors (miRNA, RNA and proteins) that control stem cell differentiation. This study demonstrates that osteoblasts-derived (Os-Exo) and adipocytes-derived (Ad-Exo) exosomes contain instructive factors that regulate the lineage specification of human mesenchymal stem cells (hMSCs). Analyses of exosomes revealed the presence of transcription factors in the form of RNA and protein for osteoblasts (RUNX2 and OSX) and adipocytes (C/EBPα and PPARγ). In addition, several miRNAs reported to have osteogenic and adipogenic differentiation potentials are also identified in these exosomes. Kinetic and differentiation analyses indicate that both osteoblast and adipocyte exosomes augment ECM-mediated differentiation of hMSCs into the respective lineage. The combination of osteoblast/adipocyte ECM and exosomes turned-on the lineage specific gene expressions at earlier time points of differentiation compared to the respective ECM or exosomes administered individually. Interestingly, the hMSCs differentiated on osteoblast ECM with adipogenic exosomes showed expression of adipogenic lineage genes, while hMSCs differentiated on adipocyte ECM with osteoblast exosomes showed osteogenic lineage genes. Based on these observations, we conclude that exosomes might override the ECM mediated instructive signals during lineage specification of hMSC.

An explanation for the mysterious distribution of melanin in human skin: a rare example of asymmetric (melanin) organelle distribution during mitosis of basal layer progenitor keratinocytes

BACKGROUND

Melanin is synthesized by melanocytes in the basal layer of the epidermis. When transferred to surrounding keratinocytes melanin is the key ultraviolet radiation-protective biopolymer responsible for skin pigmentation. Most melanin is observable in the proliferative basal layer of the epidermis and only sparsely distributed in the stratifying/differentiating epidermis. The latter has been explained as 'melanin degradation' in suprabasal layers.

OBJECTIVES

To re-evaluate the currently accepted basis for melanin distribution in human epidermis and to discover whether this pattern is altered after a regenerative stimulus.

METHODS

Normal epidermis of adult human skin, at rest and after tape-stripping, was analysed by a range of (immuno)histochemical and high-resolution microscopy techniques. In vitro models of melanin granule uptake by human keratinocytes were attempted.

RESULTS

We propose a different fate for melanin in the human epidermis. Our evidence indicates that the bulk of melanin is inherited only by the nondifferentiating daughter cell postmitosis in progenitor keratinocytes via asymmetric organelle inheritance. Moreover, this preferred pattern of melanin distribution can switch to a symmetric or equal daughter cell inheritance mode under conditions of stress, including regeneration.

CONCLUSIONS

In this preliminary report, we provide a plausible and histologically supported explanation for how human skin pigmentation is efficiently organized in the epidermis. Steady-state epidermis pigmentation may involve much less redox-sensitive melanogenesis than previously thought, and at least some premade melanin may be available for reuse. The epidermal melanin unit may be an excellent example with which to study organelle distribution via asymmetric or symmetric inheritance in response to microenvironment and tissue demands.

Soft interaction and excluded volume effect compete as polyethylene glycols modulate enzyme activity

Polyethylene glycols (PEGs) can either preferentially bind to biomolecules or exert excluded volume effect depending upon their chain length and concentration. We have studied the effect of ethylene glycol (EG) and PEGs of different chain lengths (M_n 400 and 4000) on the enzyme efficiency of hen-egg-white lysozyme (HEWL) on Micrococcus lysodeikticus (M. Lys.) cell. The activity shows a bell-like profile as the turnover number increases from ~1.3 × 10⁵ s^-1 M^-1 in water to ~1.7 × 10⁵ s^-1 M^-1 in presence of 2% PEG-400 beyond which it decreases to ~0.7 × 10⁵ s^-1 M^-1 at 20% PEG-400. Solvent polarity, excluded volume effect, soft nonspecific interactions and structural flexibility are found to be the competing factors which govern the overall enzyme activity as evidenced from circular dichroism (CD) and fluorescence measurements. Thermal unfolding temperature (T_m) of HEWL also shows a bell-shaped profile with PEG concentration which establishes possible correlation with its activity. We also observe a minimum in the activation energy barrier for the catalysis at low osmolyte concentrations. The maximum in the enzyme efficiency has been explained on the basis of an optimization between excluded volume effect and soft interaction among the protein and the cosolutes.

The controversial origin of pericytes during angiogenesis - Implications for cell-based therapeutic angiogenesis and cell-based therapies

Pericytes reside within the basement membrane of small vessels and are often in direct cellular contact with endothelial cells, fulfilling important functions during blood vessel formation and homeostasis. Recently, these pericytes have been also identified as mesenchymal stem cells. Mesenchymal stem cells, and especially their specialized subpopulation of pericytes, represent promising candidates for therapeutic angiogenesis applications, and have already been widely applied in pre-clinical and clinical trials. However, cell-based therapies of ischemic diseases (especially of myocardial infarction) have not resulted in significant long-term improvement. Interestingly, pericytes from a hematopoietic origin were observed in embryonic skin and a pericyte sub-population expressing leukocyte and monocyte markers was described during adult angiogenesis in vivo. Since mesenchymal stem cells do not express hematopoietic markers, the latter cell type might represent an alternative pericyte population relevant to angiogenesis. Therefore, we sourced blood-derived angiogenic cells (BDACs) from monocytes that closely resembled hematopoietic pericytes, which had only been observed in vivo thus far. BDACs displayed many pericytic features and exhibited enhanced revascularization and functional tissue regeneration in a pre-clinical model of critical limb ischemia. Comparison between BDACs and mesenchymal pericytes indicated that BDACs (while resembling hematopoietic pericytes) enhanced early stages of angiogenesis, such as endothelial cell sprouting. In contrast, mesenchymal pericytes were responsible for blood vessel maturation and homeostasis, while reducing endothelial sprouting.Since the formation of new blood vessels is crucial during therapeutic angiogenesis or during integration of implants into the host tissue, hematopoietic pericytes (and therefore BDACs) might offer an advantageous addition or even an alternative for cell-based therapies.

Delivery of cellular factors to regulate bone healing

Bone tissue has a strong intrinsic regenerative capacity, thanks to a delicate and complex interplay of cellular and molecular processes, which tightly involve the immune system. Pathological settings of anatomical, biomechanical or inflammatory nature may lead to impaired bone healing. Innovative strategies to enhance bone repair, including the delivery of osteoprogenitor cells or of potent cytokines/morphogens, indicate the potential of 'orthobiologics', but are not fully satisfactory. Here, we review different approaches based on the delivery of regenerative cues produced by cells but in cell-free, possibly off-the-shelf configurations. Such strategies exploit the paracrine effect of the secretome of mesenchymal stem/stromal cells, presented in soluble form, shuttled through extracellular vesicles, or embedded within the network of extracellular matrix molecules. In addition to osteoinductive molecules, attention is given to factors targeting the resident immune cells, to reshape inflammatory and immunity processes from scarring to regenerative patterns.

Towards an in vitro fibrogenesis model of human vocal fold scarring

Background

Vocal fold (VF) scarring remains a therapeutic dilemma and challenge in modern laryngology. To facilitate corresponding research, we aimed to establish an in vitro fibrogenesis model employing human VF fibroblasts (hVFF) and the principles of macromolecular crowding (MMC).

Methods

Fibrogenesis was promoted by addition of transforming growth factor-β1 to standard medium and medium containing inert macromolecules (MMC). Hepatocyte growth factor (HGF) and Botox type A were tested for their antifibrotic properties in various doses. Experiments were analyzed with respect to the biosynthesis of collagen, fibronectin, and α-smooth muscle actin using immunofluorescence, silver stain and western blot.

Results

MMC led to favourable enhanced deposition of collagen and other extracellular matrix components, reflecting fibrotic conditions. Low doses of HGF were able to dampen profibrotic effects. This could not be observed for higher HGF concentrations. Botox type A did not show any effects.

Conclusion

Based on the principles of MMC we could successfully establish a laryngeal fibrogenesis model employing hVFF. Our finding of dose-dependent HGF effects is important before going into clinical trials in humans and has never been shown before. Our model provides a novel option to screen various potential antifibrotic compounds under standardized conditions in a short time.

Electronic supplementary material

The online version of this article (10.1007/s00405-018-4922-7) contains supplementary material, which is available to authorized users.

Self-assembly of collagen-based biomaterials: preparation, characterizations and biomedical applications

By combining regulatory parameters with characterization methods, researchers can selectively fabricate collagenous biomaterials with various functional responses for biomedical applications.

Applying macromolecular crowding to 3D bioprinting: fabrication of 3D hierarchical porous collagen-based hydrogel constructs

3D bioprinting of hierarchical porous structures for tissue engineering.

Cell derived extracellular matrix fibers synthesized using sacrificial hollow fiber membranes

The therapeutic potential of biological scaffolds as adjuncts to synthetic polymers motivates the engineering of fibers formed using the extracellular matrix (ECM) secreted by cells. To capture the ECM secreted by cells during in vitro culture, a solvent degradable hollow fiber membrane (HFM) was created and utilized as a cell culture platform. NIH/3T3 fibroblasts were injected into the narrow (0.986 ± 0.042 mm) lumina of mesoporous polysulfone HFMs and maintained in culture for up to 3 weeks. Following cell culture, HFMs were dissolved using N-methyl-2-pyrrolidone and the accumulated ECM was collected. The ECM retained the filamentous dimensions of the HFM lumen. The process yielded up to 0.89 ± 0.20 mg of ECM for every mm of HFM dissolved. Immunofluorescence, second-harmonic generation microscopy, and tandem mass spectrometry indicated the presence of an array of ECM constituents, including collagen, fibronectin, and proteoglycans, while FTIR spectra suggested thorough HFM material dissolution. Isolated ECM fibers, although fragile, were amenable to handling and exhibited an average elastic modulus of 34.6 ± 15.3 kPa, ultimate tensile strength of 5.2 ± 2.2 kPa, and elongation-at-break of 29% ± 18%. ECM fibers consisted of an interconnected yet porous (32.7% ± 5.8% open space) network which supported the attachment and in vitro proliferation of mammalian cells. ECM fibers were similarly synthesized using muscle and astrocyte cells, suggesting process robustness across different cell types. Ultimately, these ECM fibers could be utilized as an alternative to synthetics for the manufacture of woven meshes targeting wound healing or regenerative medicine applications.

Engineering in vitro models of hepatofibrogenesis

Chronic liver disease is a major cause of morbidity and mortality worldwide marked by chronic inflammation and fibrosis/scarring, resulting in end-stage liver disease and its complications. Hepatic stellate cells (HSCs) are a dominant contributor to liver fibrosis by producing excessive extracellular matrix (ECM), irrespective of the underlying disease aetiologies, and for many decades research has focused on the development of a number of anti-fibrotic strategies targeting this cell. Despite major improvements in two-dimensional systems (2D) by using a variety of cell culture models of different complexity, an efficient anti-fibrogenic therapy has yet to be developed. The development of well-defined three-dimensional (3D) in vitro models, which mimic ECM structures as found in vivo, have demonstrated the importance of cell-matrix bio-mechanics, the complex interactions between HSCs and hepatocytes and other non-parenchymal cells, and this to improve and promote liver cell-specific functions. Henceforth, refinement of these 3D in vitro models, which reproduce the liver microenvironment, will lead to new objectives and to a possible new era in the search for antifibrogenic compounds.

Making microenvironments: A look into incorporating macromolecular crowding into in vitro experiments, to generate biomimetic microenvironments which are capable of directing cell function for tissue engineering applications

Biomimetic microenvironments are key components to successful cell culture and tissue engineering in vitro. One of the most accurate biomimetic microenvironments is that made by the cells themselves. Cell-made microenvironments are most similar to the in vivo state as they are cell-specific and produced by the actual cells which reside in that specific microenvironment. However, cell-made microenvironments have been challenging to re-create in vitro due to the lack of extracellular matrix composition, volume and complexity which are required. By applying macromolecular crowding to current cell culture protocols, cell-made microenvironments, or cell-derived matrices, can be generated at significant rates in vitro. In this review, we will examine the causes and effects of macromolecular crowding and how it has been applied in several in vitro systems including tissue engineering.

Macromolecular crowding for tailoring tissue-derived fibrillated matrices

Tissue-derived fibrillated matrices can be instrumental for the in vitro reconstitution of multiphasic extracellular microenvironments. However, despite of several advantages, the obtained scaffolds so far offer a rather narrow range of materials characteristics only. In this work, we demonstrate how macromolecular crowding (MMC) - the supplementation of matrix reconstitution media with synthetic or natural macromolecules in ways to create excluded volume effects (EVE) - can be employed for tailoring important structural and biophysical characteristics of kidney-derived fibrillated matrices. Porcine kidneys were decellularized, ground and the obtained extracellular matrix (ECM) preparations were reconstituted under varied MMC conditions. We show that MMC strongly influences the fibrillogenesis kinetics and impacts the architecture and the elastic modulus of the reconstituted matrices, with diameters and relative alignment of fibrils increasing at elevated concentrations of the crowding agent Ficoll400, a nonionic synthetic polymer of sucrose. Furthermore, we demonstrate how MMC modulates the distribution of key ECM molecules within the reconstituted matrix scaffolds. As a proof of concept, we compared different variants of kidney-derived fibrillated matrices in cell culture experiments referring to specific requirements of kidney tissue engineering approaches. The results revealed that MMC-tailored matrices support the morphogenesis of human umbilical vein endothelial cells (HUVECs) into capillary networks and of murine kidney stem cells (KSCs) into highly branched aggregates. The established methodology is concluded to provide generally applicable new options for tailoring tissue-specific multiphasic matrices in vitro.

STATEMENT OF SIGNIFICANCE

Tissue-derived fibrillated matrices can be instrumental for the in vitro reconstitution of multiphasic extracellular microenvironments. However, despite of several advantages, the obtained scaffolds so far offer a rather narrow range of materials characteristics only. Using the kidney matrix as a model, we herein report a new approach for tailoring tissue-derived fibrillated matrices by means of macromolecular crowding (MMC), the supplementation of reconstitution media with synthetic or natural macromolecules. MMC-modulation of matrix reconstitution is demonstrated to allow for the adjustment of fibrillation kinetics and nano-architecture, fiber diameter, alignment, and matrix elasticity. Primary human umbilical vein endothelial cells (HUVEC) and murine kidney stem cells (KSC) were cultured within different variants of fibrillated kidney matrix scaffolds. The results showed that MMC-tailored matrices were superior in supporting desired morphogenesis phenomena of both cell types.

Hydrodynamic mobility of a solid particle near a spherical elastic membrane. II. Asymmetric motion

In this paper, we derive analytical expressions for the leading-order hydrodynamic mobility of a small solid particle undergoing motion tangential to a nearby large spherical capsule whose membrane possesses resistance toward shearing and bending. Together with the results obtained in the first part [Daddi-Moussa-Ider and Gekle, Phys. Rev. E 95, 013108 (2017)2470-004510.1103/PhysRevE.95.013108], where the axisymmetric motion perpendicular to the capsule membrane is considered, the solution of the general mobility problem is thus determined. We find that shearing resistance induces a low-frequency peak in the particle self-mobility, resulting from the membrane normal displacement in the same way, although less pronounced, to what has been observed for the axisymmetric motion. In the zero-frequency limit, the self-mobility correction near a hard sphere is recovered only if the membrane has a nonvanishing resistance toward shearing. We further compute the in-plane mean-square displacement of a nearby diffusing particle, finding that the membrane induces a long-lasting subdiffusive regime. Considering capsule motion, we find that the correction to the pair-mobility function is solely determined by membrane shearing properties. Our analytical calculations are compared and validated with fully resolved boundary integral simulations where a very good agreement is obtained.

Effects of Macromolecular Crowding on Human Adipose Stem Cell Culture in Fetal Bovine Serum, Human Serum, and Defined Xeno-Free/Serum-Free Conditions

Microenvironment plays an important role for stem cell proliferation and differentiation. Macromolecular crowding (MMC) was recently shown to assist stem cells in forming their own matrix microenvironment in vitro. The ability of MMC to support adipose stem cell (ASC) proliferation, metabolism, and multilineage differentiation was studied under different conditions: fetal bovine serum- (FBS-) and human serum- (HS-) based media and xeno- and serum-free (XF/SF) media. Furthermore, the immunophenotype of ASCs under MMC was evaluated. The proliferative capacity of ASCs under MMC was attenuated in each condition. However, osteogenic differentiation was enhanced under MMC, shown by increased deposition of mineralized matrix in FBS and HS cultures. Likewise, significantly greater lipid droplet accumulation and increased collagen IV deposition indicated enhanced adipogenesis under MMC in FBS and HS cultures. In contrast, chondrogenic differentiation was attenuated in ASCs expanded under MMC. The ASC immunophenotype was maintained under MMC with significantly higher expression of CD54. However, MMC impaired metabolic activity and differentiation capacity of ASCs in XF/SF conditions. Both the supportive and inhibitory effects of MMC on ASC are culture condition dependent. In the presence of serum, MMC maintains ASC immunophenotype and enhances adipogenic and osteogenic differentiation at the cost of reduced proliferation.

Bioengineered Bruch's-like extracellular matrix promotes retinal pigment epithelial differentiation

In the eye, the retinal pigment epithelium (RPE) adheres to a complex protein matrix known as Bruch's membrane (BrM). The aim of this study was to provide enriched conditions for RPE cell culture through the production of a BrM-like matrix. Our hypothesis was that a human RPE cell line would deposit an extracellular matrix (ECM) resembling BrM. The composition and structure of ECM deposited by ARPE19 cells (ARPE19-ECM) was characterized. To produce ARPE19-ECM, ARPE19 cells were cultured in the presence dextran sulphate. ARPE19-ECM was decellularized using deoxycholate and characterized by immunostaining and western blot analysis. Primary human RPE and induced pluripotent stem cells were seeded onto ARPE19-ECM or geltrex coated surfaces and examined by microscopy or RT-PCR. Culture of ARPE19 cells with dextran sulphate promoted nuclear localization of SOX2, formation of tight junctions and deposition of ECM. ARPE19 cells deposited ECM proteins found in the inner layers of BrM, including fibronectin, vitronectin, collagens IV and V as well as laminin-alpha-5, but not those found in the middle elastic layer (elastin) or the outer layers (collagen VI). ARPE19-ECM promoted pigmentation in human RPE and pluripotent stem cell cultures. Expression of RPE65 was significantly increased on ARPE19-ECM compared with geltrex in differentiating pluripotent stem cell cultures. ARPE19 cells deposit ECM with a composition and structure similar to BrM in the retina. Molecular cues present in ARPE19-ECM promote the acquisition and maintenance of the RPE phenotype. Together, these results demonstrate a simple method for generating a BrM-like surface for enriched RPE cell cultures.

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Macromolecular crowding promoted deposition of extracellular matrix by ARPE19 cells.

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ARPE19 cells deposited matrix proteins found in the inner layers of Bruch's membrane.

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ARPE19-ECM displayed similar microstructure to Bruch's membrane.

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ARPE19-ECM promoted pigmentation in human retinal pigment epithelial cell cultures.

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ARPE19-ECM promoted RPE differentiation from pluripotent stem cells.

The Interplay between Surface Nanotopography and Chemistry Modulates Collagen I and III Deposition by Human Dermal Fibroblasts

The events within the foreign body response are similar to, but ultimately different than, the wound healing cascade. Collagen production by fibroblasts is known to play a vital role in wound healing and device fibrous encapsulation. However, the influence of surface nanotopography on collagen deposition by these cells has not been reported so far. To address this gap, we have developed model substrata having surface nanotopography of controlled height of 16, 38, and 68 nm and tailored outermost surface chemistry of amines, carboxyl acid, and pure hydrocarbon. Fibroblast adhesion was reduced on nanotopographically modified surfaces compared to the smooth control. Furthermore, amine and acid functionalized surfaces showed increased cell proliferation over hydrophobic hydrocarbon surfaces. Collagen III production increased from day 3 to day 8 and then decreased from day 8 to day 16 on all surfaces, while collagen I deposition increased throughout the duration of 16 days. Our data show that the initial collagen I and III deposition can be modulated by selecting desired combinations of surface nanotopography and chemistry. This study provides useful knowledge that could help in tuning fibrous capsule formation and in turn govern the fate of implantable biomaterial devices.

Improving 2D and 3D Skin In Vitro Models Using Macromolecular Crowding

The glycoprotein family of collagens represents the main structural proteins in the human body, and are key components of biomaterials used in modern tissue engineering. A technical bottleneck is the deposition of collagen in vitro, as it is notoriously slow, resulting in sub-optimal formation of connective tissue and subsequent tissue cohesion, particularly in skin models. Here, we describe a method which involves the addition of differentially-sized sucrose co-polymers to skin cultures to generate macromolecular crowding (MMC), which results in a dramatic enhancement of collagen deposition. Particularly, dermal fibroblasts deposited a significant amount of collagen I/IV/VII and fibronectin under MMC in comparison to controls. The protocol also describes a method to decellularize crowded cell layers, exposing significant amounts of extracellular matrix (ECM) which were retained on the culture surface as evidenced by immunocytochemistry. Total matrix mass and distribution pattern was studied using interference reflection microscopy. Interestingly, fibroblasts, keratinocytes and co-cultures produced cell-derived matrices (CDM) of varying composition and morphology. CDM could be used as "bio-scaffolds" for secondary cell seeding, where the current use of coatings or scaffolds, typically from xenogenic animal sources, can be avoided, thus moving towards more clinically relevant applications. In addition, this protocol describes the application of MMC during the submerged phase of a 3D-organotypic skin co-culture model which was sufficient to enhance ECM deposition in the dermo-epidermal junction (DEJ), in particular, collagen VII, the major component of anchoring fibrils. Electron microscopy confirmed the presence of anchoring fibrils in cultures developed with MMC, as compared to controls. This is significant as anchoring fibrils tether the dermis to the epidermis, hence, having a pre-formed mature DEJ may benefit skin graft recipients in terms of graft stability and overall wound healing. Furthermore, culture time was condensed from 5 weeks to 3 weeks to obtain a mature construct, when using MMC, reducing costs.

Molecular crowding impacts the structure of apolipoprotein A-I with potential implications on in vivo metabolism and function

The effect molecular crowding, defined as the volume exclusion exerted by one soluble inert molecule upon another soluble molecule, has on the structure and self-interaction of lipid-free apoA-I were explored. The influence of molecular crowding on lipid-free apoA-I oligomerization and internal dynamics has been analyzed using electron paramagnetic resonance (EPR) spectroscopy measurements of nitroxide spin label at selected positions throughout the protein sequence and at varying concentrations of the crowding agent Ficoll-70. The targeted positions include sites previously shown to be sensitive for detecting intermolecular interaction via spin-spin coupling. Circular dichroism was used to study secondary structural changes in lipid-free apoA-I imposed by increasing concentrations of the crowding agent. Crosslinking and SDS-PAGE gel analysis was employed to further characterize the role molecular crowding plays in inducing apoA-I oligomerization. It was concluded that the dynamic apoA-I structure and oligomeric state was altered in the presence of the crowding agent. It was also found that the C-terminal was slightly more sensitive to molecular crowding. Finally, the data described the region around residue 217 in the C-terminal domain of apoA-I as the most sensitive reporter of the crowding-induced self-association of apoA-I. The implications of this behavior to in vivo functionality are discussed. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 683-692, 2016.

Structure and stability of recombinant bovine odorant-binding protein: III. Peculiarities of the wild type bOBP unfolding in crowded milieu

Contrary to the majority of the members of the lipocalin family, which are stable monomers with the specific OBP fold (a β-barrel consisting of a 8-stranded anti-parallel β-sheet followed by a short α-helical segment, a ninth β-strand, and a disordered C-terminal tail) and a conserved disulfide bond, bovine odorant-binding protein (bOBP) does not have such a disulfide bond and forms a domain-swapped dimer that involves crossing the α-helical region from each monomer over the β-barrel of the other monomer. Furthermore, although natural bOBP isolated from bovine tissues exists as a stable domain-swapped dimer, recombinant bOBP has decreased dimerization potential and therefore exists as a mixture of monomeric and dimeric variants. In this article, we investigated the effect model crowding agents of similar chemical nature but different molecular mass on conformational stability of the recombinant bOBP. These experiments were conducted in order to shed light on the potential influence of model crowded environment on the unfolding-refolding equilibrium. To this end, we looked at the influence of PEG-600, PEG-4000, and PEG-12000 in concentrations of 80, 150, and 300 mg/mL on the equilibrium unfolding and refolding transitions induced in the recombinant bOBP by guanidine hydrochloride. We are showing here that the effect of crowding agents on the structure and conformational stability of the recombinant bOBP depends on the size of the crowder, with the smaller crowding agents being more effective in the stabilization of the bOBP native dimeric state against the guanidine hydrochloride denaturing action. This effect of the crowding agents is concentration dependent, with the high concentrations of the agents being more effective.

ECM microenvironment unlocks brown adipogenic potential of adult human bone marrow-derived MSCs

Key to realizing the diagnostic and therapeutic potential of human brown/brite adipocytes is the identification of a renewable, easily accessible and safe tissue source of progenitor cells, and an efficacious in vitro differentiation protocol. We show that macromolecular crowding (MMC) facilitates brown adipocyte differentiation in adult human bone marrow mesenchymal stem cells (bmMSCs), as evidenced by substantially upregulating uncoupling protein 1 (UCP1) and uncoupled respiration. Moreover, MMC also induced ‘browning’ in bmMSC-derived white adipocytes. Mechanistically, MMC creates a 3D extracellular matrix architecture enshrouding maturing adipocytes in a collagen IV cocoon that is engaged by paxillin-positive focal adhesions also at the apical side of cells, without contact to the stiff support structure. This leads to an enhanced matrix-cell signaling, reflected by increased phosphorylation of ATF2, a key transcription factor in UCP1 regulation. Thus, tuning the dimensionality of the microenvironment in vitro can unlock a strong brown potential dormant in bone marrow.

Protein unfolding in crowded milieu: what crowding can do to a protein undergoing unfolding?

The natural environment of a protein inside a cell is characterized by the almost complete lack of unoccupied space, limited amount of free water, and the tightly packed crowd of various biological macromolecules, such as proteins, nucleic acids, polysaccharides, and complexes thereof. This extremely crowded natural milieu is poorly mimicked by slightly salted aqueous solutions containing low concentrations of a protein of interest. The accepted practice is to model crowded environments by adding high concentrations of various polymers that serve as model "crowding agents" to the solution of a protein of interest. Although studies performed under these model conditions revealed that macromolecular crowding might have noticeable influence on various aspects related to the protein structure, function, folding, conformational stability, and aggregation propensity, the complete picture describing conformational behavior of a protein under these conditions is missing as of yet. Furthermore, there is an accepted belief that the conformational stability of globular proteins increases in the presence crowding agents due to the excluded volume effects. The goal of this study was to conduct a systematic analysis of the effect of high concentrations of PEG-8000 and Dextran-70 on the unfolding behavior of eleven globular proteins belonging to different structural classes.

Effect of Surface Chemical Functionalities on Collagen Deposition by Primary Human Dermal Fibroblasts

Surface modification has been identified as an important technique that could improve the response of the body to implanted medical devices. Collagen production by fibroblasts is known to play a vital role in wound healing and device fibrous encapsulation. However, how surface chemistry affects collagen I and III deposition by these cells has not been systematically studied. Here, we report how surface chemistry influences the deposition of collagen I and III by primary human dermal fibroblasts. Amine (NH3), carboxyl acid (COOH), and hydrocarbon (CH3) surfaces were generated by plasma deposition. This is a practically relevant tool to deposit a functional coating on any type of substrate material. We show that fibroblasts adhere better and proliferate faster on amine-rich surfaces. In addition, the initial collagen I and III production is greater on this type of coating. These data indicates that surface modification can be a promising route for modulating the rate and level of fibrous encapsulation and may be useful in informing the design of implantable biomedical devices to produce more predictable clinical outcomes.

Progress in cell-based therapies for tendon repair

The last decade has seen significant developments in cell therapies, based on permanently differentiated, reprogrammed or engineered stem cells, for tendon injuries and degenerative conditions. In vitro studies assess the influence of biophysical, biochemical and biological signals on tenogenic phenotype maintenance and/or differentiation towards tenogenic lineage. However, the ideal culture environment has yet to be identified due to the lack of standardised experimental setup and readout system. Bone marrow mesenchymal stem cells and tenocytes/dermal fibroblasts appear to be the cell populations of choice for clinical translation in equine and human patients respectively based on circumstantial, rather than on hard evidence. Collaborative, inter- and multi-disciplinary efforts are expected to provide clinically relevant and commercially viable cell-based therapies for tendon repair and regeneration in the years to come.

The past, present and future in scaffold-based tendon treatments

Tendon injuries represent a significant clinical burden on healthcare systems worldwide. As the human population ages and the life expectancy increases, tendon injuries will become more prevalent, especially among young individuals with long life ahead of them. Advancements in engineering, chemistry and biology have made available an array of three-dimensional scaffold-based intervention strategies, natural or synthetic in origin. Further, functionalisation strategies, based on biophysical, biochemical and biological cues, offer control over cellular functions; localisation and sustained release of therapeutics/biologics; and the ability to positively interact with the host to promote repair and regeneration. Herein, we critically discuss current therapies and emerging technologies that aim to transform tendon treatments in the years to come.

Application of AMOR in craniofacial rabbit bone bioengineering

Endogenous molecular and cellular mediators modulate tissue repair and regeneration. We have recently described antibody mediated osseous regeneration (AMOR) as a novel strategy for bioengineering bone in rat calvarial defect. This entails application of anti-BMP-2 antibodies capable of in vivo capturing of endogenous osteogenic BMPs (BMP-2, BMP-4, and BMP-7). The present study sought to investigate the feasibility of AMOR in other animal models. To that end, we examined the efficacy of a panel of anti-BMP-2 monoclonal antibodies (mAbs) and a polyclonal Ab immobilized on absorbable collagen sponge (ACS) to mediate bone regeneration within rabbit calvarial critical size defects. After 6 weeks, de novo bone formation was demonstrated by micro-CT imaging, histology, and histomorphometric analysis. Only certain anti-BMP-2 mAb clones mediated significant in vivo bone regeneration, suggesting that the epitopes with which anti-BMP-2 mAbs react are critical to AMOR. Increased localization of BMP-2 protein and expression of osteocalcin were observed within defects, suggesting accumulation of endogenous BMP-2 and/or increased de novo expression of BMP-2 protein within sites undergoing bone repair by AMOR. Considering the ultimate objective of translation of this therapeutic strategy in humans, preclinical studies will be necessary to demonstrate the feasibility of AMOR in progressively larger animal models.