Bioengineered niches that recreate physiological extracellular matrix organisation to support long-term haematopoietic stem cells

Long-term reconstituting haematopoietic stem cells (LT-HSCs) are used to treat blood disorders via stem cell transplantation. The very low abundance of LT-HSCs and their rapid differentiation during in vitro culture hinders their clinical utility. Previous developments using stromal feeder layers, defined media cocktails, and bioengineering have enabled HSC expansion in culture, but of mostly short-term HSCs and progenitor populations at the expense of naive LT-HSCs. Here, we report the creation of a bioengineered LT-HSC maintenance niche that recreates physiological extracellular matrix organisation, using soft collagen type-I hydrogels to drive nestin expression in perivascular stromal cells (PerSCs). We demonstrate that nestin, which is expressed by HSC-supportive bone marrow stromal cells, is cytoprotective and, via regulation of metabolism, is important for HIF-1α expression in PerSCs. When CD34+ve HSCs were added to the bioengineered niches comprising nestin/HIF-1α expressing PerSCs, LT-HSC numbers were maintained with normal clonal and in vivo reconstitution potential, without media supplementation. We provide proof-of-concept that our bioengineered niches can support the survival of CRISPR edited HSCs. Successful editing of LT-HSCs ex vivo can have potential impact on the treatment of blood disorders.

Complexities of modeling the bone marrow microenvironment to facilitate hematopoietic research

Hematopoiesis occurs in the bone marrow (BM), within a specialized microenvironment referred to as the stem cell niche, where the hematopoietic stem cells (HSCs) reside and are regulated for quiescence, self-renewal and differentiation through intrinsic and extrinsic mechanisms. The BM contains at least two distinctive HSC-supportive niches: an endosteal osteoblastic niche that supports quiescence and self-renewal and a more vascular/perisinusoidal niche that promotes proliferation and differentiation. Both associate with supporting mesenchymal stromal cells. Within the more hypoxic osteoblastic niche, HSCs specifically interact with the osteoblasts that line the endosteal surface, which secrete several important HSC quiescence and maintenance regulatory factors. In vivo imaging indicates that the HSCs and progenitors located further away, in the vicinity of sinusoidal endothelial cells, are more proliferative. Here, HSCs interact with endothelial cells via specific cell adhesion molecules. Endothelial cells also secrete several factors important for HSC homeostasis and proliferation. In addition, HSCs and mesenchymal stromal cells are embedded within the extracellular matrix (ECM), an important network of proteins such as collagen, elastin, laminin, proteoglycans, vitronectin, and fibronectin. The ECM provides mechanical characteristics such as stiffness and elasticity important for cell behavior regulation. ECM proteins are also able to bind, sequester, display, and distribute growth factors across the BM, thus directly affecting stem cell fate and regulation of hematopoiesis. These important physical and chemical features of the BM require careful consideration when creating three-dimensional models of the BM.

Building bones for blood and beyond: the growing field of bone marrow niche model development

The bone marrow (BM) niche is a complex microenvironment that provides the signals required for regulation of hematopoietic stem cells (HSCs) and the process of hematopoiesis they are responsible for. Bioengineered models of the BM niche incorporate various elements of the in vivo BM microenvironment, including cellular components, soluble factors, a three-dimensional environment, mechanical stimulation of included cells, and perfusion. Recent advances in the bioengineering field have resulted in a spate of new models that shed light on BM function and are approaching precise imitation of the BM niche. These models promise to improve our understanding of the in vivo microenvironment in health and disease. They also aim to serve as platforms for HSC manipulation or as preclinical models for screening novel therapies for BM-associated disorders and diseases.

Synthetic extracellular matrices with function-encoding peptides

The communication of cells with their surroundings is mostly encoded in the epitopes of structural and signalling proteins present in the extracellular matrix (ECM). These peptide epitopes can be incorporated in biomaterials to serve as function-encoding molecules to modulate cell-cell and cell-ECM interactions. In this Review, we discuss natural and synthetic peptide epitopes as molecular tools to bioengineer bioactive hydrogel materials. We present a library of functional peptide sequences that selectively communicate with cells and the ECM to coordinate biological processes, including epitopes that directly signal to cells, that bind ECM components that subsequently signal to cells, and that regulate ECM turnover. We highlight how these epitopes can be incorporated in different biomaterials as individual or multiple signals, working synergistically or additively. This molecular toolbox can be applied in the design of biomaterials aimed at regulating or controlling cellular and tissue function, repair and regeneration.

Fibronectin matrix assembly and TGFβ1 presentation for chondrogenesis of patient derived pericytes for microtia repair

Tissue engineered cartilage for external ear reconstruction of congenital deformities, such as microtia or resulting from trauma, remains a significant challenge for plastic and reconstructive surgeons. Current strategies involve harvesting autologous costal cartilage or expanding autologous chondrocytes ex vivo. However, these procedures often lead to donor site morbidity and a cell source with limited expansion capacity. Stromal stem cells such as perivascular stem cells (pericytes) offer an attractive alternative cell source, as they can be isolated from many human tissues, readily expanded in vitro and possess chondrogenic differentiation potential. Here, we successfully isolate CD146+ pericytes from the microtia remnant from patients undergoing reconstructive surgery (Microtia pericytes; MPs). Then we investigate their chondrogenic potential using the polymer poly(ethyl acrylate) (PEA) to unfold the extracellular matrix protein fibronectin (FN). FN unfolding exposes key growth factor (GF) and integrin binding sites on the molecule, allowing tethering of the chondrogenic GF transforming growth factor beta 1 (TGFβ1). This system leads to solid-phase, matrix-bound, GF presentation in a more physiological-like manner than that of typical chondrogenic induction media (CM) formulations that tend to lead to off-target effects. This simple and controlled material-based approach demonstrates similar chondrogenic potential to CM, while minimising proclivity toward hypertrophy, without the need for complex induction media formulations.

Design of Recombinant Spider Silk Proteins for Cell Type Specific Binding

Cytophilic (cell-adhesive) materials are very important for tissue engineering and regenerative medicine. However, for engineering hierarchically organized tissue structures comprising different cell types, cell-specific attachment and guidance are decisive. In this context, materials made of recombinant spider silk proteins are promising scaffolds, since they exhibit high biocompatibility, biodegradability, and the underlying proteins can be genetically functionalized. Here, previously established spider silk variants based on the engineered Araneus diadematus fibroin 4 (eADF4(C16)) are genetically modified with cell adhesive peptide sequences from extracellular matrix proteins, including IKVAV, YIGSR, QHREDGS, and KGD. Interestingly, eADF4(C16)-KGD as one of 18 tested variants is cell-selective for C2C12 mouse myoblasts, one out of 11 tested cell lines. Co-culturing with B50 rat neuronal cells confirms the cell-specificity of eADF4(C16)-KGD material surfaces for C2C12 mouse myoblast adhesion.

Mimicking Bone Extracellular Matrix: From BMP-2-Derived Sequences to Osteogenic-Multifunctional Coatings

Cell-material interactions are regulated by mimicking bone extracellular matrix on the surface of biomaterials. In this regard, reproducing the extracellular conditions that promote integrin and growth factor (GF) signaling is a major goal to trigger bone regeneration. Thus, the use of synthetic osteogenic domains derived from bone morphogenetic protein 2 (BMP-2) is gaining increasing attention, as this strategy is devoid of the clinical risks associated with this molecule. In this work, the wrist and knuckle epitopes of BMP-2 are screened to identify peptides with potential osteogenic properties. The most active sequences (the DWIVA motif and its cyclic version) are combined with the cell adhesive RGD peptide (linear and cyclic variants), to produce tailor-made biomimetic peptides presenting the bioactive cues in a chemically and geometrically defined manner. Such multifunctional peptides are next used to functionalize titanium surfaces. Biological characterization with mesenchymal stem cells demonstrates the ability of the biointerfaces to synergistically enhance cell adhesion and osteogenic differentiation. Furthermore, in vivo studies in rat calvarial defects prove the capacity of the biomimetic coatings to improve new bone formation and reduce fibrous tissue thickness. These results highlight the potential of mimicking integrin-GF signaling with synthetic peptides, without the need for exogenous GFs.

Interleukin 1β and lipopolysaccharides induction dictate chondrocyte morphological properties and reduce cellular roughness and adhesion energy comparatively

Osteoarthritis (OA) is a whole joint disease marked by the degradation of the articular cartilage (AC) tissue, chronic inflammation, and bone remodeling. Upon AC's injury, proinflammatory mediators including interleukin 1β (IL1β) and lipopolysaccharides (LPS) play major roles in the onset and progression of OA. The objective of this study was to mechanistically detect and compare the effects of IL1β and LPS, separately, on the morphological and nanomechanical properties of bovine chondrocytes. Cells were seeded overnight in a full serum medium and the next day divided into three main groups: A negative control (NC) of a reduced serum medium and 10 ng/ml IL1ß or 10 ng/ml LPS-modified media. Cells were induced for 24 h. Nanomechanical properties (elastic modulus and adhesion energy) and roughness were quantified using atomic force microscopy. Nitric oxide, prostaglandin 2 (PGE2), and matrix metalloproteinases 3 (MMP3) contents; viability of cells; and extracellular matrix components were quantified. Our data revealed that viability of the cells was not affected by inflammatory induction and IL1ß induction increased PGE2. Elastic moduli of cells were similar among IL1β and NC while LPS significantly decreased the elasticity compared to NC. IL1ß induction resulted in least cellular roughness while LPS induction resulted in least adhesion energy compared to NC. Our images suggest that IL1ß and LPS inflammation affect cellular morphology with cytoskeleton rearrangements and the presence of stress fibers. Finally, our results suggest that the two investigated inflammatory mediators modulated chondrocytes' immediate responses to inflammation in variable ways.

Tissue Engineered Neurovascularization Strategies for Craniofacial Tissue Regeneration

Craniofacial tissue injuries, diseases, and defects, including those within bone, dental, and periodontal tissues and salivary glands, impact an estimated 1 billion patients globally. Craniofacial tissue dysfunction significantly reduces quality of life, and successful repair of damaged tissues remains a significant challenge. Blood vessels and nerves are colocalized within craniofacial tissues and act synergistically during tissue regeneration. Therefore, the success of craniofacial regenerative approaches is predicated on successful recruitment, regeneration, or integration of both vascularization and innervation. Tissue engineering strategies have been widely used to encourage vascularization and, more recently, to improve innervation through host tissue recruitment or prevascularization/innervation of engineered tissues. However, current scaffold designs and cell or growth factor delivery approaches often fail to synergistically coordinate both vascularization and innervation to orchestrate successful tissue regeneration. Additionally, tissue engineering approaches are typically investigated separately for vascularization and innervation. Since both tissues act in concert to improve craniofacial tissue regeneration outcomes, a revised approach for development of engineered materials is required. This review aims to provide an overview of neurovascularization in craniofacial tissues and strategies to target either process thus far. Finally, key design principles are described for engineering approaches that will support both vascularization and innervation for successful craniofacial tissue regeneration.

New trends in the development of multifunctional peptides to functionalize biomaterials

Improving cell-material interactions is a major goal in tissue engineering. In this regard, functionalization of biomaterials with cell instructive molecules from the extracellular matrix stands out as a powerful strategy to enhance their bioactivity and achieve optimal tissue integration. However, current functionalization strategies, like the use of native full-length proteins, are associated with drawbacks, thus urging the need of developing new methodologies. In this regard, the use of synthetic peptides encompassing specific bioactive regions of proteins represents a promising alternative. In particular, the combination of peptide sequences with complementary or synergistic effects makes it possible to address more than one biological target at the biomaterial surface. In this review, an overview of the main strategies using peptides to install multifunctionality on biomaterials is presented, mostly focusing on the combination of the RGD motif with other peptides sequences. The evolution of these approaches, starting from simple methods, like using peptide mixtures, to more advanced systems of peptide presentation, with very well defined chemical properties, are explained. For each system of peptide's presentation, three main aspects of multifunctionality-improving receptor selectivity, mimicking the extracellular matrix and preventing bacterial colonization while improving cell adhesion-are highlighted.

An Engineered Biomimetic Peptide Regulates Cell Behavior by Synergistic Integrin and Growth Factor Signaling

Recreating the healing microenvironment is essential to regulate cell-material interactions and ensure the integration of biomaterials. To repair bone, such bioactivity can be achieved by mimicking its extracellular matrix (ECM) and by stimulating integrin and growth factor (GF) signaling. However, current approaches relying on the use of GFs, such as bone morphogenetic protein 2 (BMP-2), entail clinical risks. Here, a biomimetic peptide integrating the RGD cell adhesive sequence and the osteogenic DWIVA motif derived from the wrist epitope of BMP-2 is presented. The approach offers the advantage of having a spatial control over the single binding of integrins and BMP receptors. Such multifunctional platform is designed to incorporate 3,4-dihydroxyphenylalanine to bind metallic oxides with high affinity in a one step process. Functionalization of glass substrates with the engineered peptide is characterized by physicochemical methods, proving a successful surface modification. The biomimetic interfaces significantly improve the adhesion of C2C12 cells, inhibit myotube formation, and activate the BMP-dependent signaling via p38. These effects are not observed on surfaces displaying only one bioactive motif, a mixture of both motifs or soluble DWIVA. These data prove the biological potential of recreating the ECM and engaging in integrin and GF crosstalk via molecular-based mimics.

A versatile click chemistry-based approach for functionalizing biomaterials of diverse nature with bioactive peptides

A novel and versatile toolkit approach for the functionalization of biomaterials of different nature is described. This methodology is based on the solid-phase conjugation of specific anchoring units onto a resin-bound azido-functionalized peptide by using click chemistry. A synergistic multifunctional peptidic scaffold with cell adhesive properties was used as a model compound to showcase the versatility of this new approach. Titanium, gold and polylactic acid surfaces were biofunctionalized by this method, as validated by physicochemical surface characterization with XPS. In vitro assays using mesenchymal stem cells showed enhanced cell adhesion on the functionalized samples, proving the capacity of this strategy to efficiently bioactivate different types of biomaterials.

Challenges and effective routes for formulating and delivery of epidermal growth factors in skin care

Epidermal growth factors are important morphogenetic proteins that instruct cell behaviour. They have many inferred applications within regenerative medicine and have shown promising results in skincare treatments. Many growth factors are known to have skin anti-ageing benefits along with further potential in resolving scarring, acne and other skin disorders. Incorporation of these biologics into skincare formulations has been greatly hindered by low transdermal delivery efficacy, intricate material interactions and protein instability - especially within common cosmetic emulsions. This review explores the cosmeceutical capability of growth factors in skin care, current understandings of constituent interactions and advantageous delivery approaches for more effective topical delivery. The first section highlights the influences growth factor product formulation has on stability and synergy. Current understandings of growth factor formulating techniques in cosmetic products is limited, and the performance of other protein structures is an adequate point of reference. The second section examines emerging drug delivery systems to overcome the challenges of topical growth factor treatment. It is important to consider the coaction and durability of all components in a formulation simultaneously: active ingredients, product format and delivery vehicle, in order to engineer an optimal cosmeceutical product.

Learning from BMPs and their biophysical extracellular matrix microenvironment for biomaterial design

It is nowadays well-accepted that the extracellular matrix (ECM) is not a simple reservoir for growth factors but is an organization center of their biological activity. In this review, we focus on the ability of the ECM to regulate the biological activity of BMPs. In particular, we survey the role of the ECM components, notably the glycosaminoglycans and fibrillary ECM proteins, which can be promoters or repressors of the biological activities mediated by the BMPs. We examine how a process called mechano-transduction induced by the ECM can affect BMP signaling, including BMP internalization by the cells. We also focus on the spatio-temporal regulation of the BMPs, including their release from the ECM, which enables to modulate their spatial localization as well as their local concentration. We highlight how biomaterials can recapitulate some aspects of the BMPs/ECM interactions and help to answer fundamental questions to reveal previously unknown molecular mechanisms. Finally, the design of new biomaterials inspired by the ECM to better present BMPs is discussed, and their use for a more efficient bone regeneration in vivo is also highlighted.

Biocompatible PCL-nanofibers scaffold with immobilized fibronectin and laminin for neuronal tissue regeneration

Recent advances in regenerative medicine have given hope in overcoming and rehabilitating complex medical conditions. In this regard, the biopolymer poly-ε-caprolactone (PCL) may be a promising candidate for tissue regeneration, despite lacking the essential bioactivity. The present study used PCL nanofibers (NFs) scaffold decorated with the extracellular matrix proteins fibronectin and laminin combined for neuronal regeneration. The potential for the dual proteins to support neuronal cells and promote axonal growth was investigated. Two NFs scaffolds were produced with PLC concentrations of 12% or 15%. Under scanning electron microscopy, both scaffolds evidenced uniform diameter distribution in the range of 358 nm and 887 nm, respectively, with >80% porosity. The Brunauer-Emmett-Teller (BET) test confirmed that the fabricated NFs mats had a high surface area, especially for the 12% NFs with 652 m²/g compared to 254 m²/g for the 15% NFs. The proteins of interest were successfully conjugated to the 12% PCL scaffold through chemical carbodiimide reaction as confirmed by Fourier-transform infrared spectroscopy. The addition of fibronectin and laminin together was shown to be the most favorable for cellular attachment and elongation of neuroblastoma SH-SY5Y cells compared to other formulations. Light microscopy revealed longer neurite outgrowth, higher cellular projected area, and lower shape index for the cells cultured on the combined proteins conjugated fibers, indicating enhanced cellular spread on the scaffold. This preliminary study suggests that PCL nanoscaffolding conjugated with matrix proteins can support neuronal cell viability and neurite growth.

What Caging Force Cells Feel in 3D Hydrogels: A Rheological Perspective

It has been established that the mechanical properties of hydrogels control the fate of (stem) cells. However, despite its importance, a one-to-one correspondence between gels' stiffness and cell behavior is still missing from literature. In this work, the viscoelastic properties of poly(ethylene-glycol) (PEG)-based hydrogels are investigated by means of rheological measurements performed at different length scales. The outcomes of this work reveal that PEG-based hydrogels show significant stiffening when subjected to a compressional deformation, implying that conventional bulk rheology measurements may overestimate the stiffness of hydrogels by up to an order of magnitude. It is hypothesized that this apparent stiffening is caused by an induced "tensional state" of the gel network, due to the application of a compressional normal force during sample loading. Moreover, it is shown that the actual stiffness of the hydrogels is instead accurately determined by means of both passive-video-particle-tracking (PVPT) microrheology and nanoindentation measurements, which are inherently performed at the cell's length scale and in absence of any externally applied force in the case of PVPT. These results underpin a methodology for measuring hydrogels' linear viscoelastic properties that are representative of the mechanical constraints perceived by cells in 3D hydrogel cultures.

Multifunctional Biomaterials: Combining Material Modification Strategies for Engineering of Cell-Contacting Surfaces

In the human body, cells in a tissue are exposed to signals derived from their specific extracellular matrix (ECM), such as architectural structure, mechanical properties, and chemical composition (proteins, growth factors). Research on biomaterials in tissue engineering and regenerative medicine aims to recreate such stimuli using engineered materials to induce a specific response of cells at the interface. Although traditional biomaterials design has been mostly limited to varying individual signals, increasing interest has arisen on combining several features in recent years to improve the mimicry of extracellular matrix properties. Tremendous progress in combinatorial surface modification exploiting, for example, topographical features or variations in mechanics combined with biochemical cues has enabled the identification of their key regulatory characteristics on various cell fate decisions. Gradients especially facilitated such research by enabling the investigation of combined continuous changes of different signals. Despite unravelling important synergies for cellular responses, challenges arise in terms of fabrication and characterization of multifunctional engineered materials. This review summarizes recent work on combinatorial surface modifications that aim to control biological responses. Modification and characterization methods for enhanced control over multifunctional material properties are highlighted and discussed. Thereby, this review deepens the understanding and knowledge of biomimetic combinatorial material modification, their challenges but especially their potential.

Functional nanoarrays for investigating stem cell fate and function

Stem cells show excellent potential in the field of tissue engineering and regenerative medicine based on their excellent capability to not only self-renew but also differentiate into a specialized cell type of interest. However, the lack of a non-destructive monitoring system makes it challenging to identify and characterize differentiated cells before their transplantation without compromising cell viability. Thus, the development of a non-destructive monitoring method for analyzing cell function is highly desired and can significantly benefit stem cell-based therapies. Recently, nanomaterial-based scaffolds (e.g., nanoarrays) have made possible considerable advances in controlling the differentiation of stem cells and characterization of the differentiation status sensitively in real time. This review provides a selective overview of the recent progress in the synthesis methods of nanoarrays and their applications in controlling stem cell fate and monitoring live cell functions electrochemically. We believe that the topics discussed in this review can provide brief and concise guidelines for the development of novel nanoarrays and promote the interest in live cell study applications. A method which can not only control but also monitor stem cell fate and function will be a promising technology that can accelerate stem cell therapies.

Beyond temperature: controlling collagen fibrillogenesis under physiological conditions via interaction with cucurbit[7]uril

Under physiological ionic strength and pH, temperature has long been appreciated as the only stimulus that can be applied to induce in vitro self-assembly of tropocollagen. Here, we report a second, mechanistically new control strategy that uses non-covalent and selective binding of cucurbit[7]uril, a macrocyclic cavitand, with midchain aromatic residues on the tropocollagen surface. This strategy directly demonstrates the decisive role hydrophobic interactions play in collagen fibrillogenesis. It also points the way to the temporally-controllable formation of collagen fibrils in vivo that is highly desirable, yet challenging, in some biomedical scenarios.

Tripeptide-induced modulation of mesenchymal stem cell biomechanics stimulates proliferation and wound healing

We demonstrate the ability of two tripeptides to promote proliferation and modulate the mechanical properties of human mesenchymal stem cells (hMSCs). Notably, Young's modulus of peptide-treated hMSCs was found to be ∼2 fold higher compared to the control group. These peptides promoted wound healing in hMSCs, without stimulating osteogenic and adipogenic differentiation, thus showing high potential in vascular tissue engineering applications.

Copresentation of BMP-6 and RGD Ligands Enhances Cell Adhesion and BMP-Mediated Signaling

We report on the covalent immobilization of bone morphogenetic protein 6 (BMP-6) and its co-presentation with integrin ligands on a nanopatterned platform to study cell adhesion and signaling responses which regulate the transdifferentiation of myoblasts into osteogenic cells. To immobilize BMP-6, the heterobifunctional linker MU-NHS is coupled to amine residues of the growth factor; this prevents its internalization while ensuring that its biological activity is maintained. Additionally, to allow cells to adhere to such platform and study signaling events arising from the contact to the surface, we used click-chemistry to immobilize cyclic-RGD carrying an azido group reacting with PEG-alkyne spacers via copper-catalyzed 1,3-dipolar cycloaddition. We show that the copresentation of BMP-6 and RGD favors focal adhesion formation and promotes Smad 1/5/8 phosphorylation. When presented in low amounts, BMP-6 added to culture media of cells adhering to the RGD ligands is less effective than BMP-6 immobilized on the surfaces in inducing Smad complex activation and in inhibiting myotube formation. Our results suggest that a local control of ligand density and cell signaling is crucial for modulating cell response.

Designing stem cell niches for differentiation and self-renewal

Mesenchymal stem cells, characterized by their ability to differentiate into skeletal tissues and self-renew, hold great promise for both regenerative medicine and novel therapeutic discovery. However, their regenerative capacity is retained only when in contact with their specialized microenvironment, termed the stem cell niche Niches provide structural and functional cues that are both biochemical and biophysical, stem cells integrate this complex array of signals with intrinsic regulatory networks to meet physiological demands. Although, some of these regulatory mechanisms remain poorly understood or difficult to harness with traditional culture systems. Biomaterial strategies are being developed that aim to recapitulate stem cell niches, by engineering microenvironments with physiological-like niche properties that aim to elucidate stem cell-regulatory mechanisms, and to harness their regenerative capacity in vitro In the future, engineered niches will prove important tools for both regenerative medicine and therapeutic discoveries.

Coupling Microfluidic Platforms, Microfabrication, and Tissue Engineered Scaffolds to Investigate Tumor Cells Mechanobiology

The tumor microenvironment (TME) is composed of dynamic and complex networks composed of matrix substrates, extracellular matrix (ECM), non-malignant cells, and tumor cells. The TME is in constant evolution during the disease progression, most notably through gradual stiffening of the stroma. Within the tumor, increased ECM stiffness drives tumor growth and metastatic events. However, classic in vitro strategies to study the TME in cancer lack the complexity to fully replicate the TME. The quest to understand how the mechanical, geometrical, and biochemical environment of cells impacts their behavior and fate has been a major force driving the recent development of new technologies in cell biology research. Despite rapid advances in this field, many challenges remain in order to bridge the gap between the classical culture dish and the biological reality of actual tissue. Microfabrication coupled with microfluidic approaches aim to engineer the actual complexity of the TME. Moreover, TME bioengineering allows artificial modulations with single or multiple cues to study different phenomena occurring in vivo. Some innovative cutting-edge tools and new microfluidic approaches could have an important impact on the fields of biology and medicine by bringing deeper understanding of the TME, cell behavior, and drug effects.

Heparin-Conjugated Decellularized Bone Particles Promote Enhanced Osteogenic Signaling of PDGF-BB to Adipose-Derived Stem Cells in Tissue Engineered Bone Grafts

Adipose-derived stem cells (ASCs) are a promising cell source for regenerating critical-sized craniofacial bone defects, but their clinical use is limited due to the supraphysiological levels of bone morphogenetic protein-2 required to induce bone formation in large grafts. It has been recently reported that platelet-derived growth factor-BB (PDGF) directly enhances the osteogenesis of ASCs when applied at physiological concentrations. In this study, a biomimetic delivery system that tethers PDGF to decellularized bone matrix (DCB) is developed to enhance osteogenic signaling in bone grafts by colocalizing PDGF-extracellular matrix cues. Heparin is conjugated to DCB particles (HC-DCB) to promote sustained binding of PDGF via electrostatic interactions. HC-DCB particles bind to PDGF with >99% efficiency and release significantly less PDGF over 21 days compared to nonconjugated DCB particles (1.1% vs 22.8%). HC-DCB-PDGF signaling in polycaprolactone (PCL)-fibrin grafts promotes >40 µg Ca²⁺ µg^-1 DNA deposition by ASCs during in vitro osteogenic culture compared to grafts without HC-DCB or PDGF. Furthermore, more bone formation is observed in grafts with HC-DCB-PDGF at 12 weeks following implantation of grafts into murine critical-sized calvarial defects. Collectively, these results demonstrate that HC-DCB enhances the osteogenic signaling of PDGF to ASCs and may be applied to promote ASC-mediated bone regeneration in critical-sized defects.

RGD Mutation of the Heparin Binding II Fragment of Fibronectin for Guiding Mesenchymal Stem Cell Behavior on Titanium Surfaces

Installing bioactivity on metallic biomaterials by mimicking the extracellular matrix (ECM) is crucial for stimulating specific cellular responses to ultimately promote tissue regeneration. Fibronectin is an ECM protein commonly used for biomaterial functionalization. The use of fibronectin recombinant fragments is an attractive alternate to the use of full-length fibronectin because of the relatively low cost and facility of purification. However, it is necessary to combine more than one fragment, for example, the cell attachment site and the heparin binding II (HBII), either mixed or in one molecule, to obtain complete activity. In the present study, we proposed to install adhesion capacity to the HBII fragment by an RGD gain-of-function DNA mutation, retaining its cell differentiation capacity and thereby producing a small and very active protein fragment. The novel molecule, covalently immobilized onto titanium surfaces, maintained the growth factor-binding capacity and stimulated cell spreading, osteoblastic cell differentiation, and mineralization of human mesenchymal stem cells compared to the HBII native protein. These results highlight the potential capacity of gain-of-function DNA mutations in the design of novel molecules for the improvement of osseointegration properties of metallic implant surfaces.

Nanoscale Coatings for Ultralow Dose BMP-2-Driven Regeneration of Critical-Sized Bone Defects

While new biomaterials for regenerative therapies are being reported in the literature, clinical translation is slow. Some existing regenerative approaches rely on high doses of growth factors, such as bone morphogenetic protein-2 (BMP-2) in bone regeneration, which can cause serious side effects. An ultralow-dose growth factor technology is described yielding high bioactivity based on a simple polymer, poly(ethyl acrylate) (PEA), and mechanisms to drive stem cell differentiation and bone regeneration in a critical-sized murine defect model with translation to a clinical veterinary setting are reported. This material-based technology triggers spontaneous fibronectin organization and stimulates growth factor signalling, enabling synergistic integrin and BMP-2 receptor activation in mesenchymal stem cells. To translate this technology, plasma-polymerized PEA is used on 2D and 3D substrates to enhance cell signalling in vitro, showing the complete healing of a critical-sized bone injury in mice in vivo. Efficacy is demonstrated in a Münsterländer dog with a nonhealing humerus fracture, establishing the clinical translation of advanced ultralow-dose growth factor treatment.

Materials Science and Design Principles of Growth Factor Delivery Systems in Tissue Engineering and Regenerative Medicine

Growth factors (GFs) are signaling molecules that direct cell development by providing biochemical cues for stem cell proliferation, migration, and differentiation. GFs play a key role in tissue regeneration, but one major limitation of GF-based therapies is dosage-related adverse effects. Additionally, the clinical applications and efficacy of GFs are significantly affected by the efficiency of delivery systems and other pharmacokinetic factors. Hence, it is crucial to design delivery systems that provide optimal activity, stability, and tunable delivery for GFs. Understanding the physicochemical properties of the GFs and the biomaterials utilized for the development of biomimetic GF delivery systems is critical for GF-based regeneration. Many different delivery systems have been developed to achieve tunable delivery kinetics for single or multiple GFs. The identification of ideal biomaterials with tunable properties for spatiotemporal delivery of GFs is still challenging. This review characterizes the types, properties, and functions of GFs, the materials science of widely used biomaterials, and various GF loading strategies to comprehensively summarize the current delivery systems for tunable spatiotemporal delivery of GFs aimed for tissue regeneration applications. This review concludes by discussing fundamental design principles for GF delivery vehicles based on the interactive physicochemical properties of the proteins and biomaterials.

Multifunctional Coatings and Nanotopographies: Toward Cell Instructive and Antibacterial Implants

In biomaterials science, it is nowadays well accepted that improving the biointegration of dental and orthopedic implants with surrounding tissues is a major goal. However, implant surfaces that support osteointegration may also favor colonization of bacterial cells. Infection of biomaterials and subsequent biofilm formation can have devastating effects and reduce patient quality of life, representing an emerging concern in healthcare. Conversely, efforts toward inhibiting bacterial colonization may impair biomaterial-tissue integration. Therefore, to improve the long-term success of medical implants, biomaterial surfaces should ideally discourage the attachment of bacteria without affecting eukaryotic cell functions. However, most current strategies seldom investigate a combined goal. This work reviews recent strategies of surface modification to simultaneously address implant biointegration while mitigating bacterial infections. To this end, two emerging solutions are considered, multifunctional chemical coatings and nanotopographical features.

RhBMP-2 and concomitant rapid material degradation synergistically promote bone repair and regeneration with collagen-hydroxyapatite nanocomposites

The effective treatment of bone defects is still a great challenge in clinical practice. Synthetic bone-grafting substitutes of composition and structure analogous to bone as well as incorporated with growth factors are considered to be a promising solution. In this study, a collagen-hydroxyapatite (CHA) nanocomposite scaffold was developed by collagen self-assembly with simultaneous HA synthesis. The physicochemical properties such as morphology, inorganic phase, thermal decomposition, specific surface area and pore size distribution were characterized. The osteogenicity of CHA in the absence or presence of recombinant human bone morphogenetic protein-2 (rhBMP-2) was assessed both by cell culturing and animal implantation experiments. The gene expression results showed that the osteogenic differentiation capacity of rat bone mesenchymal stem cells (rBMSCs) has been enhanced both by CHA and rhBMP-2. The efficient bone regeneration of femoral defects in rabbits was achieved with CHA and CHA pre-absorbed rhBMP-2 (CHA/B), confirmed by micro-computed tomography measurements, histological observation and immunohistochemical analyses. The CHA nanocomposite was completely degraded within 8 weeks and replaced by new bone. It was found that rhBMP-2 not only accelerated and enhanced bone formation, but also expedited the degradation of CHA. It is believed that the rhBMP-2 and concomitant rapid material degradation synergistically promote bone repair and regeneration with CHA. The biodegradation behavior of CHA in the presence of rhBMP-2 can be further investigated to gain an in-depth understanding of the complex interplays among biomaterials, growth factors and their target cells. The relevant knowledge will facilitate the search for a reasonable, safe and efficient methodology for the introduction of growth factors to biomaterials so as to achieve satisfactory tissue regeneration.

Design of protein delivery systems by mimicking extracellular mechanisms for protection of growth factors

Heparin sulfate proteoglycans (HSPGs) are responsible for the storage and stabilization of numerous growth factors in the extracellular matrix. In this complex native environment, the efficient binding of the growth factors is determined by multivalent, specific and reversible electrostatic interactions between the sulfate groups of HSPGs and the positively charged amino acids of the growth factor. Inspired by this naturally occurring stabilization process, we propose the use of diblock copolymers of heparin and polyethylene glycol (Hep-b-PEG) for protection and delivery of FGF-2. We describe the encapsulation of FGF-2 into spontaneously assembling polyelectrolyte complexes (PECs) with Hep-b-PEG in which the Hep block ensures the formation of the PECs, while the PEG moiety confers stability of the generated complex by a stealth corona. Our results demonstrate that by this method we can generate homogeneous complexes (ca. 400nm diameter, PDI 0.29±0.07) with a very high encapsulation efficiency (about 99% encapsulated FGF-2). The release of the growth factor in response to different stimuli such as pH, ionic strength or presence of heparinase was also studied. We report a sustained release of up to 80% during 28days which is not influenced by the presence of heparinase - a result that clearly demonstrates the protective effect of the stealth corona. We also show that FGF-2 remains bioactive as it influences the morphology of bone marrow mesenchymal stem cells.

STATEMENT OF SIGNIFICANCE

We describe a biopolymer that uses the way the cells shield a type of proteins (growth factors) to simultaneously assemble, slowly deliver and shield the protein in a "nanocarrier". Growth factors are essential for the regeneration of cartilage, bones by stem cell therapies but have a short life time as when added directly to tissues. Our design makes use of the heparin bioactivity towards such proteins in combination with a polyethylene glycol moiety (PEG) that makes a protecting shell. PEG, is biocompatible and used in approved medicines and countless cosmetic products. The highest novelty is the reaction (oxime click) used to bound these molecules that does not require modification of heparin and allows preservation of its bioactivity.

Exploring the Role of RGD-Recognizing Integrins in Cancer

Integrins are key regulators of communication between cells and with their microenvironment. Eight members of the integrin superfamily recognize the tripeptide motif Arg-Gly-Asp (RGD) within extracelluar matrix (ECM) proteins. These integrins constitute an important subfamily and play a major role in cancer progression and metastasis via their tumor biological functions. Such transmembrane adhesion and signaling receptors are thus recognized as promising and well accessible targets for novel diagnostic and therapeutic applications for directly attacking cancer cells and their fatal microenvironment. Recently, specific small peptidic and peptidomimetic ligands as well as antibodies binding to distinct integrin subtypes have been developed and synthesized as new drug candidates for cancer treatment. Understanding the distinct functions and interplay of integrin subtypes is a prerequisite for selective intervention in integrin-mediated diseases. Integrin subtype-specific ligands labelled with radioisotopes or fluorescent molecules allows the characterization of the integrin patterns in vivo and later the medical intervention via subtype specific drugs. The coating of nanoparticles, larger proteins, or encapsulating agents by integrin ligands are being explored to guide cytotoxic reagents directly to the cancer cell surface. These ligands are currently under investigation in clinical studies for their efficacy in interference with tumor cell adhesion, migration/invasion, proliferation, signaling, and survival, opening new treatment approaches in personalized medicine.

Mechanotransduction and Growth Factor Signalling to Engineer Cellular Microenvironments

Engineering cellular microenvironments involves biochemical factors, the extracellular matrix (ECM) and the interaction with neighbouring cells. This progress report provides a critical overview of key studies that incorporate growth factor (GF) signalling and mechanotransduction into the design of advanced microenvironments. Materials systems have been developed for surface-bound presentation of GFs, either covalently tethered or sequestered through physico-chemical affinity to the matrix, as an alternative to soluble GFs. Furthermore, some materials contain both GF and integrin binding regions and thereby enable synergistic signalling between the two. Mechanotransduction refers to the ability of the cells to sense physical properties of the ECM and to transduce them into biochemical signals. Various aspects of the physics of the ECM, i.e. stiffness, geometry and ligand spacing, as well as time-dependent properties, such as matrix stiffening, degradability, viscoelasticity, surface mobility as well as spatial patterns and gradients of physical cues are discussed. To conclude, various examples illustrate the potential for cooperative signalling of growth factors and the physical properties of the microenvironment for potential applications in regenerative medicine, cancer research and drug testing.

Current approaches for modulation of the nanoscale interface in the regulation of cell behavior

Regulation of cell behavior in response to nanoscale features has been the focus of much research in recent years and the successful generation of nanoscale features capable of mimicking the natural nanoscale interface has been of great interest in the field of biomaterials research. In this review, we discuss relevant nanofabrication techniques and how they are combined with bioengineering applications to mimic the natural extracellular matrix (ECM) and create valuable nanoscale interfaces.

Engineered microenvironments for synergistic VEGF - Integrin signalling during vascularization

We have engineered polymer-based microenvironments that promote vasculogenesis both in vitro and in vivo through synergistic integrin-growth factor receptor signalling. Poly(ethyl acrylate) (PEA) triggers spontaneous organization of fibronectin (FN) into nanonetworks which provide availability of critical binding domains. Importantly, the growth factor binding (FNIII_12-14) and integrin binding (FNIII_9-10) regions are simultaneously available on FN fibrils assembled on PEA. This material platform promotes synergistic integrin/VEGF signalling which is highly effective for vascularization events in vitro with low concentrations of VEGF. VEGF specifically binds to FN fibrils on PEA compared to control polymers (poly(methyl acrylate), PMA) where FN remains in a globular conformation and integrin/GF binding domains are not simultaneously available. The vasculogenic response of human endothelial cells seeded on these synergistic interfaces (VEGF bound to FN assembled on PEA) was significantly improved compared to soluble administration of VEGF at higher doses. Early onset of VEGF signalling (PLCγ1 phosphorylation) and both integrin and VEGF signalling (ERK1/2 phosphorylation) were increased only when VEGF was bound to FN nanonetworks on PEA, while soluble VEGF did not influence early signalling. Experiments with mutant FN molecules with impaired integrin binding site (FN-RGE) confirmed the role of the integrin binding site of FN on the vasculogenic response via combined integrin/VEGF signalling. In vivo experiments using 3D scaffolds coated with FN and VEGF implanted in the murine fat pad demonstrated pro-vascularization signalling by enhanced formation of new tissue inside scaffold pores. PEA-driven organization of FN promotes efficient presentation of VEGF to promote vascularization in regenerative medicine applications.