pH-Sensitive nanocarrier assisted delivery of adenosine to treat osteoporotic bone loss

Bone tissue undergoes continuous remodeling via osteoclast-mediated bone resorption and osteoblast-mediated bone formation. An imbalance in this process with enhanced osteoclastic activity can lead to excessive bone resorption, resulting in bone thinning. Once activated, osteoclasts bind to the bone surface and acidify the local niche. This acidic environment could serve as a potential trigger for the delivery of therapeutic agents into the osteoporotic bone tissue. To this end, we developed a pH-responsive nanocarrier-based drug delivery system that binds to the bone tissue and delivers an osteoanabolic molecule, adenosine. Adenosine is incorporated into a hyaluronic acid (HA)-based nanocarrier through a pH-sensitive ketal group. The HA-nanocarrier is further functionalized with alendronate moieties to improve binding to the bone tissues. Systemic administration of the nanocarrier containing adenosine attenuated bone loss in ovariectomized mice and showed comparable bone qualities to that of healthy mice. Delivery of osteoanabolic small molecules that can contribute to bone formation and inhibit excessive osteoclast activity by leveraging the tissue-specific milieu could serve as viable therapeutics for osteoporosis.

In Vivo Sequestration of Innate Small Molecules to Promote Bone Healing

Approaches that enable innate repair mechanisms hold great potential for tissue repair. Herein, biomaterial-assisted sequestration of small molecules is described to localize pro-regenerative signaling at the injury site. Specifically, a synthetic biomaterial containing boronate molecules is designed to sequester adenosine, a small molecule ubiquitously present in the human body. The biomaterial-assisted sequestration of adenosine leverages the transient surge of extracellular adenosine following injury to prolong local adenosine signaling. It is demonstrated that implantation of the biomaterial patch following injury establishes an in situ stockpile of adenosine, resulting in accelerated healing by promoting both osteoblastogenesis and angiogenesis. The adenosine content within the patch recedes to the physiological level as the tissue regenerates. In addition to sequestering endogenous adenosine, the biomaterial is also able to deliver exogenous adenosine to the site of injury, offering a versatile solution to utilizing adenosine as a potential therapeutic for tissue repair.

Phenylboronic Acid-polymers for Biomedical Applications

Background:

Phenylboronic acid-polymers (PBA-polymers) have attracted tremendous attention as potential stimuli-responsive materials with applications in drug-delivery depots, scaffolds for tissue engineering, HIV barriers, and biomolecule-detecting/sensing platforms. The unique aspect of PBA-polymers is their interactions with diols, which result in reversible, covalent bond formation. This very nature of reversible bonding between boronic acids and diols has been fundamental to their applications in the biomedical area.

Methods:

We have searched peer-reviewed articles including reviews from Scopus, PubMed, and Google Scholar with a focus on the 1) chemistry of PBA, 2) synthesis of PBA-polymers, and 3) their biomedical applications.

Results:

We have summarized approximately 179 papers in this review. Most of the applications described in this review are focused on the unique ability of PBA molecules to interact with diol molecules and the dynamic nature of the resulting boronate esters. The strong sensitivity of boronate ester groups towards the surrounding pH also makes these molecules stimuli-responsive. In addition, we also discuss how the re-arrangement of the dynamic boronate ester bonds renders PBA-based materials with other unique features such as self-healing and shear thinning.

Conclusion:

The presence of PBA in the polymer chain can render it with diverse functions/ relativities without changing their intrinsic properties. In this review, we discuss the development of PBA polymers with diverse functions and their biomedical applications with a specific focus on the dynamic nature of boronate ester groups.

Dysregulation of ectonucleotidase-mediated extracellular adenosine during postmenopausal bone loss

Adenosine and its receptors play a key role in bone homeostasis and regeneration. Extracellular adenosine is generated from CD39 and CD73 activity in the cell membrane, through conversion of adenosine triphosphate to adenosine monophosphate (AMP) and AMP to adenosine, respectively. Despite the relevance of CD39/CD73 to bone health, the roles of these enzymes in bona fide skeletal disorders remain unknown. We demonstrate that CD39/CD73 expression and extracellular adenosine levels in the bone marrow are substantially decreased in animals with osteoporotic bone loss. Knockdown of estrogen receptors ESR1 and ESR2 in primary osteoprogenitors and osteoclasts undergoing differentiation showed decreased coexpression of membrane-bound CD39 and CD73 and lower extracellular adenosine. Targeting the adenosine A2B receptor using an agonist attenuated bone loss in ovariectomized mice. Together, these findings suggest a pathological association of purine metabolism with estrogen deficiency and highlight the potential of A2B receptor as a target to treat osteoporosis.

In Situ Gene Therapy via AAV-CRISPR-Cas9-Mediated Targeted Gene Regulation

Development of efficacious in vivo delivery platforms for CRISPR-Cas9-based epigenome engineering will be critical to enable the ability to target human diseases without permanent modification of the genome. Toward this, we utilized split-Cas9 systems to develop a modular adeno-associated viral (AAV) vector platform for CRISPR-Cas9 delivery to enable the full spectrum of targeted in situ gene regulation functionalities, demonstrating robust transcriptional repression (up to 80%) and activation (up to 6-fold) of target genes in cell culture and mice. We also applied our platform for targeted in vivo gene-repression-mediated gene therapy for retinitis pigmentosa. Specifically, we engineered targeted repression of Nrl, a master regulator of rod photoreceptor determination, and demonstrated Nrl knockdown mediates in situ reprogramming of rod cells into cone-like cells that are resistant to retinitis pigmentosa-specific mutations, with concomitant prevention of secondary cone loss. Furthermore, we benchmarked our results from Nrl knockdown with those from in vivo Nrl knockout via gene editing. Taken together, our AAV-CRISPR-Cas9 platform for in vivo epigenome engineering enables a robust approach to target disease in a genomically scarless and potentially reversible manner.

Mineralized Biomaterials Mediated Repair of Bone Defects Through Endogenous Cells

Synthetic biomaterials that create a dynamic calcium (Ca²⁺)-, phosphate (PO₄^3-) ion-, and calcium phosphate (CaP)-rich microenvironment, similar to that found in native bone tissue, have been shown to promote osteogenic commitment of stem cells in vitro and in vivo. The intrinsic osteoconductivity and osteoinductivity of such biomaterials make them promising bone grafts for the treatment of bone defects. We thus aimed to evaluate the potential of mineralized biomaterials to induce bone repair of a critical-sized cranial defect in the absence of exogenous cells and growth factors. Our results demonstrate that the mineralized biomaterial alone can support complete bone formation within critical-sized bone defects through recruitment of endogenous cells and neo-bone tissue formation in mice. The newly formed bone tissue recapitulated many key characteristics of native bone such as formation of bone minerals reaching similar bone mineral density, presence of bone-forming osteoblasts and tartrate-resistant acid phosphatase-expressing osteoclasts, as well as vascular networks. Biomaterials that recruit endogenous cells and provide a tissue-specific microenvironment to modulate cellular behavior and support generation of functional tissues are a key step forward in moving bench-side tissue engineering approaches to the bedside. Such tissue engineering strategies could eventually pave the path toward readily available therapies that significantly reduce patient cost of care and improve overall clinical outcomes.

Direct Conversion of Human Pluripotent Stem Cells to Osteoblasts With a Small Molecule

Human pluripotent stem cells (hPSCs), which exhibit unlimited self-renewal ability and can differentiate into all cell types in the human body, are a promising cell source for cell-based therapies and regenerative medicine. Small molecules hold great potential in the derivation of tissue-specific cells from hPSCs owing to their cost-effectiveness and scalability. Here, we describe a protocol for deriving osteoblasts from hPSCs by using a single, natural small molecule: adenosine. This simple and effective experimental protocol allows one to obtain large numbers of osteoblasts or osteoprogenitor cells, with the ability to form functional bone tissues, from hPSCs, including human embryonic stem cells and induced pluripotent stem cells. This protocol could potentially enable studies of tissue regeneration and skeletal diseases. © 2018 by John Wiley & Sons, Inc.

In vivo comparison of biomineralized scaffold-directed osteogenic differentiation of human embryonic and mesenchymal stem cells

Human pluripotent stem cells such as embryonic stem cells (hESCs) and multipotent stem cells like mesenchymal stem cells (hMSCs) hold great promise as potential cell sources for bone tissue regeneration. Comparing the in vivo osteogenesis of hESCs and hMSCs by biomaterial-based cues provides insight into the differentiation kinetics of these cells as well as their potential to contribute to bone tissue repair in vivo. Here, we compared in vivo osteogenic differentiation of hESCs and hMSCs within osteoinductive calcium phosphate (CaP)-bearing biomineralized scaffolds that recapitulate a bone-specific mineral microenvironment. Both hESCs and hMSCs underwent osteogenic differentiation responding to the biomaterial-based instructive cues. Furthermore, hMSCs underwent earlier in vivo osteogenesis compared to hESCs, but both stem cell types acquired a similar osteogenic maturation by 8 weeks of implantation.

Small molecule-driven direct conversion of human pluripotent stem cells into functional osteoblasts

The abilities of human pluripotent stem cells (hPSCs) to proliferate without phenotypic alteration and to differentiate into tissue-specific progeny make them a promising cell source for regenerative medicine and development of physiologically relevant in vitro platforms. Despite this potential, efficient conversion of hPSCs into tissue-specific cells still remains a challenge. Herein, we report direct conversion of hPSCs into functional osteoblasts through the use of adenosine, a naturally occurring nucleoside in the human body. The hPSCs treated with adenosine not only expressed the molecular signatures of osteoblasts but also produced calcified bone matrix. Our findings show that the adenosine-mediated osteogenesis of hPSCs involved the adenosine A2bR. When implanted in vivo, using macroporous synthetic matrices, the human induced pluripotent stem cell (hiPSC)-derived donor cells participated in the repair of critical-sized bone defects through the formation of neobone tissue without teratoma formation. The newly formed bone tissues exhibited various attributes of the native tissue, including vascularization and bone resorption. To our knowledge, this is the first demonstration of adenosine-induced differentiation of hPSCs into functional osteoblasts and their subsequent use to regenerate bone tissues in vivo. This approach that uses a physiologically relevant single small molecule to generate hPSC-derived progenitor cells is highly appealing because of its simplicity, cost-effectiveness, scalability, and impact in cell manufacturing, all of which are decisive factors for successful translational applications of hPSCs.

A Single-Cell Assay for Time Lapse Studies of Exosome Secretion and Cell Behaviors

To understand the inhomogeneity of cells in biological systems, there is a growing demand on the capability of characterizing the properties of individual single cells. Since single-cell studies require continuous monitoring of the cell behaviors, an effective single-cell assay that can support time lapsed studies in a high throughput manner is desired. Most currently available single-cell technologies cannot provide proper environments to sustain cell growth and, proliferation of single cells and convenient, noninvasive tests of single-cell behaviors from molecular markers. Here, a highly versatile single-cell assay is presented that can accommodate different cellular types, enable easy and efficient single-cell loading and culturing, and be suitable for the study of effects of in vitro environmental factors in combination with drug screening. One salient feature of the assay is the noninvasive collection and surveying of single-cell secretions at different time points, producing unprecedented insight of single-cell behaviors based on the biomarker signals from individual cells under given perturbations. Above all, the acquired information is quantitative, for example, measured by the number of exosomes each single-cell secretes for a given time period. Therefore, our single-cell assay provides a convenient, low-cost, and enabling tool for quantitative, time lapsed studies of single-cell properties.

Adenosine Signaling Mediates Osteogenic Differentiation of Human Embryonic Stem Cells on Mineralized Matrices

Human embryonic stem cells (hESCs) are attractive cell sources for tissue engineering and regenerative medicine due to their self-renewal and differentiation ability. Design of biomaterials with an intrinsic ability that promotes hESC differentiation to the targeted cell type boasts significant advantages for tissue regeneration. We have previously developed biomineralized calcium phosphate (CaP) matrices that inherently direct osteogenic differentiation of hESCs without the need of osteogenic-inducing chemicals or growth factors. Here, we show that CaP matrix-driven osteogenic differentiation of hESCs occurs through A2b adenosine receptor (A2bR). The inhibition of the receptor with an A2bR-specific antagonist attenuated mineralized matrix-mediated osteogenic differentiation of hESCs. In addition, when cultured on matrices in an environment deficient of CaP minerals, exogenous adenosine promoted osteogenic differentiation of hESCs, but was attenuated by the inhibition of A2bR. Such synthetic matrices that intrinsically support osteogenic commitment of hESCs are not only beneficial for bone tissue engineering but can also be used as a platform to study the effect of the physical and chemical cues to the extracellular milieu on stem cell commitment. Insights into the cell signaling during matrix-induced differentiation of stem cells will also help define the key processes and enable discovery of new targets that promote differentiation of pluripotent stem cells for bone tissue engineering.

Biomineralized matrices dominate soluble cues to direct osteogenic differentiation of human mesenchymal stem cells through adenosine signaling

Stem cell differentiation is determined by a repertoire of signals from its microenvironment, which includes the extracellular matrix (ECM) and soluble cues. The ability of mesenchymal stem cells (MSCs), a common precursor for the skeletal system, to differentiate into osteoblasts and adipocytes in response to their local cues plays an important role in skeletal tissue regeneration and homeostasis. In this study, we investigated whether a bone-specific calcium phosphate (CaP) mineral environment could induce osteogenic differentiation of human MSCs, while inhibiting their adipogenic differentiation, in the presence of adipogenic-inducing medium. We also examined the mechanism through which the mineralized matrix suppresses adipogenesis of hMSCs to promote their osteogenic differentiation. Our results show that hMSCs cultured on mineralized matrices underwent osteogenic differentiation despite being cultured in the presence of adipogenic medium, which indicates the dominance of matrix-based cues of the mineralized matrix in directing osteogenic commitment of stem cells. Furthermore, the mineralized matrix-driven attenuation of adipogenesis was reversed with the inhibition of A2b adenosine receptor (A2bR), implicating a role of adenosine signaling in mineralized environment-mediated inhibition of adipogenesis. Such synthetic matrices with an intrinsic ability to direct differentiation of multipotent adult stem cells toward a targeted phenotype while inhibiting their differentiation into other lineages not only will be a powerful tool in delineating the role of complex microenvironmental cues on stem cell commitment but also will contribute to functional tissue engineering and their translational applications.

Mineralized gelatin methacrylate-based matrices induce osteogenic differentiation of human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSC) are a promising cell source with pluripotency and self-renewal properties. Design of simple and robust biomaterials with an innate ability to induce lineage-specificity of hiPSC is desirable to realize their application in regenerative medicine. In this study, the potential of biomaterials containing calcium phosphate minerals to induce osteogenic differentiation of hiPSC was investigated. hiPSC cultured using mineralized gelatin methacrylate-based matrices underwent osteogenic differentiation ex vivo, in both two-dimensional and three-dimensional cultures, in growth medium devoid of any osteogenic-inducing chemical components or growth factors. The findings that osteogenic differentiation of hiPSC can be achieved through biomaterial-based cues alone present new avenues for personalized regenerative medicine. Such biomaterials that could not only act as structural scaffolds, but could also provide tissue-specific functions such as directing stem cell differentiation commitment, have great potential in bone tissue engineering.

Biomineralized matrix-assisted osteogenic differentiation of human embryonic stem cells

The physical and chemical properties of a matrix play an important role in determining various cellular behaviors, including lineage specificity. We demonstrate that the differentiation commitment of human embryonic stem cells (hESCs), both in vitro and in vivo, can be solely achieved through synthetic biomaterials. hESCs were cultured using mineralized synthetic matrices mimicking a calcium phosphate (CaP)-rich bone environment differentiated into osteoblasts in the absence of any osteogenic inducing supplements. When implanted in vivo, these hESC-laden mineralized matrices contributed to ectopic bone tissue formation. In contrast, cells within the corresponding non-mineralized matrices underwent either osteogenic or adipogenic fate depending upon the local cues present in the microenvironment. To our knowledge, this is the first demonstration where synthetic matrices are shown to induce terminal cell fate specification of hESCs exclusively by biomaterial-based cues both in vitro and in vivo. Technologies that utilize tissue specific cell-matrix interactions to control stem cell fate could be a powerful tool in regenerative medicine. Such approaches can be used as a tool to advance our basic understanding and assess the translational potential of stem cells.

Regulation of the fate of human mesenchymal stem cells by mechanical and stereo-topographical cues provided by silicon nanowires

Extracellular stimuli imposed on stem cells enable efficient initiation of mechanotransductive signaling to regulate stem cell fates; however, how such physical cues conferred by the stereo-topographical matrix govern the fate of stem cells still remains unknown. The purpose of this study is to delineate the effects of stereotopography and its various relevant physical properties on the fate regulation of human mesenchymal stem cells (hMSCs). Stereo-topographical silicon nanowires (SiNWs) that were precisely controlled with respect to their various dimensions and their growth orientation were used in this study. hMSCs cultured on stereo SiNWs of different lengths in the absence of biochemical osteogenic induction cues displayed a spherical and less-elongated morphology and showed an approximately 10% loss of cell viability compared to those grown on two-dimensional (2-D) flat Si. Moreover, osteogenic gene expression of COL1A1 and Runx2 in hMSCs cultured on the shortest SiNWs was significantly higher than those grown on the longer SiNWs and 2-D flat Si. hMSCs grown on shorter SiNWs also demonstrated higher expression levels for F-actin, phosphorylated focal adhesion kinase (pFAK), vinculin and alpha 2 integrin. Stereo-topographical cues provided by SiNWs are able to regulate osteogenic differentiation of hMSCs via cytoskeleton remodeling and this is correlated with the differential expression of alpha 2/beta 1 integrin heterodimers and the focal adhesion molecules pFAK and vinculin. The findings in this study provide insights in terms of the design of stereo-topographical structures for use in tissue engineering, bone regeneration and relevant medical applications.

Hypoxia promotes proliferation and osteogenic differentiation potentials of human mesenchymal stem cells

Mesenchymal stem cells (MSCs), which can be isolated from bone marrow and other somatic tissues, are residing in an environment with relative low oxygen tension. The purpose of this study is to investigate the effects of hypoxia on MSCs, and we hypothesize that oxygen concentration regulates the intricate balance between cellular proliferation and commitment towards differentiation. In this study, human bone marrow-derived MSCs were cultured under hypoxia with 1% O(2). The proliferation ability of MSCs was increased after a 7-day hypoxic culture period. Migration assay showed that hypoxia enhanced the migration capabilities of MSCs. Moreover, expression of stemness genes Oct4, Nanog, Sall4 and Klf4 was increased under hypoxia. Furthermore, the differentiation ability of MSCs under hypoxia favored osteogenesis while adipogenesis was inhibited during a 4-week induction period. Cytokine antibody array analysis showed that a number of growth factors were up-regulated after a 7-day hypoxic incubation and the differential expression of growth factors may account for the increased proliferation and osteogenic potentials of MSCs under hypoxic condition. Taken together, hypoxia provides a favorable culture condition to promote proliferation as well as osteogenesis of MSCs through differential growth factor production.

Matrix stiffness regulation of integrin-mediated mechanotransduction during osteogenic differentiation of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) cultured on extracellular matrices with different stiffness have been shown to possess diverse lineage commitment owing to the extracellular mechanical stimuli sensed by the cells. The aim of this study was to further delineate how matrix stiffness affects intracellular signaling through the mechanotransducers Rho kinase (ROCK) and focal adhesion kinase (FAK) and subsequently regulates the osteogenic phenotype of MSCs. MSCs were cultured in osteogenic medium on tunable polyacrylamide hydrogels coated with type I collagen with elasticities corresponding to Young's modulus of 7.0 ± 1.2 and 42.1 ± 3.2 kPa. Osteogenic differentiation was increased on stiffer matrices, as evident by type I collagen, osteocalcin, and Runx2 gene expressions and alizarin red S staining for mineralization. Western blot analysis demonstrated an increase in kinase activities of ROCK, FAK, and ERK1/2 on stiffer matrices. Inhibition of FAK, an important mediator of osteogenic differentiation, and inhibition of ROCK, a known mechanotransducer of matrix stiffness during osteogenesis, resulted in decreased expression of osteogenic markers during osteogenic induction. In addition, FAK affects osteogenic differentiation through ERK1/2, whereas ROCK regulates both FAK and ERK1/2. Furthermore, α(2)-integrin was upregulated on stiffer matrices during osteogenic induction, and its knockdown by siRNA downregulated the osteogenic phenotype through ROCK, FAK, and ERK1/2. Taken together, our results provide evidence that the matrix rigidity affects the osteogenic outcome of MSCs through mechanotransduction events that are mediated by α(2)-integrin.

Coordinated changes of mitochondrial biogenesis and antioxidant enzymes during osteogenic differentiation of human mesenchymal stem cells

The multidifferentiation ability of mesenchymal stem cells holds great promise for cell therapy. Numerous studies have focused on the establishment of differentiation protocols, whereas little attention has been paid to the metabolic changes during the differentiation process. Mitochondria, the powerhouse of mammalian cells, vary in their number and function in different cell types with different energy demands, but how these variations are associated with cell differentiation remains elusive. In this study, we investigated the changes of mitochondrial biogenesis and bioenergetic function using human mesenchymal stem cells (hMSCs) because of their well-defined differentiation potentials. Upon osteogenic induction, the copy number of mitochondrial DNA, protein subunits of the respiratory enzymes, oxygen consumption rate, and intracellular ATP content were increased, indicating the upregulation of aerobic mitochondrial metabolism. On the other hand, undifferentiated hMSCs showed higher levels of glycolytic enzymes and lactate production rate, suggesting that hMSCs rely more on glycolysis for energy supply in comparison with hMSC-differentiated osteoblasts. In addition, we observed a dramatic decrease of intracellular reactive oxygen species (ROS) as a consequence of upregulation of two antioxidant enzymes, manganese-dependent superoxide dismutase and catalase. Finally, we found that exogenous H(2)O(2) and mitochondrial inhibitors could retard the osteogenic differentiation. These findings suggested an energy production transition from glycolysis to oxidative phosphorylation in hMSCs upon osteogenic induction. Meanwhile, antioxidant enzymes were concurrently upregulated to prevent the accumulation of intracellular ROS. Together, our findings suggest that coordinated regulation of mitochondrial biogenesis and antioxidant enzymes occurs synergistically during osteogenic differentiation of hMSCs.

Growth of mesenchymal stem cells on electrospun type I collagen nanofibers

We reconstituted type I collagen nanofibers prepared by electrospin technology and examined the morphology, growth, adhesion, cell motility, and osteogenic differentiation of human bone marrow-derived mesenchymal stem cells (MSCs) on three nano-sized diameters (50-200, 200-500, and 500-1,000 nm). Results from scanning electron microscopy showed that cells on the nanofibers had a more polygonal and flattened cell morphology. MTS (3-[4,5-dimethythiazol-2-yl]-5-[3-carboxy-methoxyphenyl]-2-[4-sul-fophenyl]-2H-tetrazolium compound) assay demonstrated that the MSCs grown on 500-1,000-nm nanofibers had significantly higher cell viability than the tissue culture polystyrene control. A decreased amount of focal adhesion formation was apparent in which quantifiable staining area of the cytoplasmic protein vinculin for the 200-500-nm nanofibers was 39% less compared with control, whereas the area of quantifiable vinculin staining was 45% less for both the 200-500-nm and 500-1,000-nm nanofibers. The distances of cell migration were quantified on green fluorescent protein-nucleofected cells and was 56.7%, 37.3%, and 46.3% for 50-200, 200-500, and 500-1,000 nm, respectively, compared with those on the control. Alkaline phosphatase activity demonstrated no differences after 12 days of osteogenic differentiation, and reverse transcription-polymerase chain reaction (RT-PCR) analysis showed comparable osteogenic gene expression of osteocalcin, osteonectin, and ostepontin between cells differentiated on polystyrene and nanofiber surfaces. Moreover, single-cell RT-PCR of type I collagen gene expression demonstrated higher expression on cells seeded on the nanofibers. Therefore, type I collagen nanofibers support the growth of MSCs without compromising their osteogenic differentiation capability and can be used as a scaffold for bone tissue engineering to facilitate intramembranous bone formation. Further efforts are necessary to enhance their biomimetic properties.