Host cell mobilization for in situ tissue regeneration

Enhancing Craniofacial Bone Reconstruction with Clinically Applicable 3D Bioprinted Constructs

Medical imaging and 3D bioprinting can be used to create patient-specific bone scaffolds with complex shapes and controlled inner architectures. This study investigated the effectiveness of a biomimetic approach to scaffold design by employing geometric control. The biomimetic scaffold with a dense external layer showed improved bone regeneration compared to the control scaffold. New bone filled the defected region in the biomimetic scaffolds, while the control scaffolds only presented new bone at the boundary. Histological examination also shows effective bone regeneration in the biomimetic scaffolds, while fibrotic tissue ingrowth is observed in the control scaffolds. These findings suggest that the biomimetic bone scaffold, designed to minimize competition for fibrotic tissue formation in the bony defect, can enhance bone regeneration. This study underscores the notion that patient-specific anatomy can be accurately translated into a 3D bioprinting strategy through medical imaging, leading to the fabrication of constructs with significant clinical relevance.

New dual-function in situ bone repair scaffolds promote osteogenesis and reduce infection

Background

The treatment of infectious bone defects is a difficult problem to be solved in the clinic. In situ bone defect repair scaffolds with anti-infection and osteogenic abilities can effectively deal with infectious bone defects. In this study, an in situ polycaprolactone (PCL) scaffold containing ampicillin (Amp) and Mg microspheres was prepared by 3D printing technology.

Results

Mg and Amp were evenly distributed in PCL scaffolds and could be released slowly to the surrounding defect sites with the degradation of scaffolds. In vitro experiments demonstrated that the PCL scaffold containing Mg and Amp (PCL@Mg/Amp) demonstrated good cell adhesion and proliferation. The osteogenic genes collagen I (COL-I) and Runx2 were upregulated in cells grown on the PCL@Mg/Amp scaffold. The PCL@Mg/Amp scaffold also demonstrated excellent antibacterial ability against E. coli and S. aureus. In vivo experiments showed that the PCL@Mg/Amp scaffold had the strongest ability to promote tibial defect repair in rats compared with the other groups of scaffolds.

Conclusions

This kind of dual-function in situ bone repair scaffold with anti-infection and osteogenic abilities has good application prospects in the field of treating infectious bone defects.

In situ Adipogenesis in Biomaterials Without Cell Seeds: Current Status and Perspectives

For cosmetic and reconstructive purposes in the setting of small-volume adipose tissue damage due to aging, traumatic defects, oncological resections, and degenerative diseases, the current strategies for soft tissue replacement involve autologous fat grafts and tissue fillers with synthetic, bioactive, or tissue-engineered materials. However, they all have drawbacks such as volume shrinkage and foreign-body responses. Aiming to regenerate bioactive vascularized adipose tissue on biomaterial scaffolds, adipose tissue engineering (ATE) has emerged as a suitable substitute for soft tissue repair. The essential components of ATE include scaffolds as support, cells as raw materials for fat formation, and a tolerant local environment to allow regeneration to occur. The commonly loaded seeding cells are adipose-derived stem cells (ASCs), which are expected to induce stable and predictable adipose tissue formation. However, defects in stem cell enrichment, such as donor-site sacrifice, limit their wide application. As a promising alternative approach, cell-free bioactive scaffolds recruit endogenous cells for adipogenesis. In biomaterials without cell seeds, the key to sufficient adipogenesis relies on the recruitment of endogenous host cells and continuous induction of cell homing to scaffolds. Regeneration, rather than repair, is the fundamental dominance of an optimal mature product. To induce in situ adipogenesis, many researchers have focused on the mechanical and biochemical properties of scaffolds. In addition, efforts to regulate an angiogenic and adipogenic microenvironment in cell-free settings involve integrating growth factors or extracellular matrix (ECM) proteins onto bioactive scaffolds. Despite the theoretical feasibility and encouraging results in animal models, few of the reported cell-free biomaterials have been tested in humans, and failures of decellularized adipose tissues in adipogenesis have also been reported. In these cases, the most likely reason was the lack of supporting vasculature. This review summarizes the current status of biomaterials without cell seeds. Related mechanisms and influencing factors of in situ adipogenesis in cell-free biomaterials, dilemma in the development of biomaterials, and future perspectives are also addressed.

Cell-Free Scaffolds as a Monotherapy for Focal Chondral Knee Defects

Chondral knee defects have a limited ability to be repaired. Current surgical interventions have been unable to regenerate articular cartilage with the mechanical properties of native hyaline cartilage. The use of a scaffold-based approach is a potential solution. Scaffolds are often implanted with cells to stimulate cartilage regeneration, but cell-based therapies are associated with additional regulatory restrictions, an additional surgical procedure for cell harvest, time for cell expansion, and the associated costs. To overcome these disadvantages, cell-free scaffolds can be used in isolation allowing native cells to attach over time. This review discusses the optimal properties of scaffolds used for chondral defects, and the evidence for the use of hydrogel scaffolds and hydrogel-synthetic polymer hybrid scaffolds. Preclinical and clinical studies have shown that cell-free scaffolds can support articular cartilage regeneration and have the potential to treat chondral defects. However, there are very few studies in this area and, despite the many biomaterials tested in cell-based scaffolds, most cell-free studies focused on a specific type I collagen scaffold. Future studies on cell-free scaffolds should adopt the modifications made to cell-based scaffolds and replicate them in the clinical setting. More studies are also needed to understand the underlying mechanism of cell-free scaffolds.

Leptin-Induced Angiogenesis of EA.Hy926 Endothelial Cells via the Akt and Wnt Signaling Pathways In Vitro and In Vivo

Angiogenesis involves the activation of endothelial cells followed by capillary formation. Leptin, the protein product of the ob gene, can induce the angiogenic potential of endothelial cells. However, the underlying cellular mechanism still remains to be elicited. We firstly evaluated the in vitro effects of leptin on proliferation and angiogenic differentiation of endothelial cell line EA.hy926. Leptin was found to potently induced cell proliferation, expression of angiogenic gene, migration and tube formation. Then we investigated the roles of the Akt and Wnt signaling pathways in the aforementioned processes. It showed that Akt and Wnt signaling pathways could be activated by leptin, while inhibition of the Akt and Wnt signaling pathways by siRNAs effectively blocked the leptin-induced angiogenesis. Finally, we used electrospinning to fabricated leptin-immobilized linear poly(L-lactide-co-caprolactone) (PLCL)-leptin. The in vivo vessel formation of PLCL-leptin was evaluated using subcutaneous implants in Sprague-Dawley rats. The histological and immunofluorescence revealed that cell infiltration with PLCL-leptin was much more significant than that with the control PLCL group. More importantly, the number of laminin⁺ vessels and CD31⁺ cells in PLCL-leptin grafts was significantly higher than in control grafts. The study demonstrated that it is via Akt and Wnt signaling pathways that leptin promotes the proliferation and angiogenic differentiation of endothelial cells and the capacity of endogenous tissue regeneration makes the novel leptin-conjugated PLCL promising materials for grafts.

Targeted Near-Infrared Fluorescence Imaging for Regenerative Medicine

Background

Advances in tissue engineering and regenerative medicine over the last three decades have made great progress in the development of diagnostic and therapeutic methodologies for damaged tissues. However, regenerative medicine is still not the first line of treatment for patients due to limited understanding of the tissue regeneration process. Therefore, it is prerequisite to develop molecular imaging strategies combined with appropriate contrast agents to validate the therapeutic progress of damaged tissues.

Methods

The goal of this review is to discuss the progress in the development of near-infrared (NIR) contrast agents and their biomedical applications for labeling cells and scaffolds, as well as monitoring the treatment progress of native tissue in living organisms. We also discuss the design consideration of NIR contrast agents for tissue engineering and regenerative medicine in terms of their physicochemical and optical properties.

Results

The use of NIR imaging system and targeted contrast agents can provide high-resolution and high sensitivity imaging to track/monitor the in vivo fate of administered cells, the degradation rate of implanted scaffolds, and the tissue growth and integration of surrounding cells during the therapeutic period.

Conclusion

NIR fluorescence imaging techniques combined with targeted contrast agents can play a significant role in regenerative medicine by monitoring the therapeutic efficacy of implanted cells and scaffolds which would enhance the development of cell therapies and promote their successful clinical translations.

In Situ Tissue Regeneration of Renal Tissue Induced by Collagen Hydrogel Injection

Host stem/progenitor cells can be mobilized and recruited to a target location using biomaterials, and these cells may be used for in situ tissue regeneration. The objective of this study was to investigate whether host biologic resources could be used to regenerate renal tissue in situ. Collagen hydrogel was injected into the kidneys of normal mice, and rat kidneys that had sustained ischemia/reperfusion injury. After injection, the kidneys of both animal models were examined up to 4 weeks for host tissue response. The infiltrating host cells present within the injection regions expressed renal stem/progenitor cell markers, PAX‐2, CD24, and CD133, as well as mesenchymal stem cell marker, CD44. The regenerated renal structures were identified by immunohistochemistry for renal cell specific markers, including synaptopodin and CD31 for glomeruli and cytokeratin and neprilysin for tubules. Quantitatively, the number of glomeruli found in the injected regions was significantly higher when compared to normal regions of renal cortex. This phenomenon occurred in normal and ischemic injured kidneys. Furthermore, the renal function after ischemia/reperfusion injury was recovered after collagen hydrogel injection. These results demonstrate that introduction of biomaterials into the kidney is able to facilitate the regeneration of glomerular and tubular structures in normal and injured kidneys. Such an approach has the potential to become a simple and effective treatment for patients with renal failure. Stem Cells Translational Medicine2018;7:241–250

Kidney regeneration with biomimetic vascular scaffolds based on vascular corrosion casts

We have developed a biomimetic renal vascular scaffold based on a vascular corrosion casting technique. This study evaluated the feasibility of using this novel biomimetic scaffold for kidney regeneration in a rat kidney cortical defect model. Vascular corrosion casts were prepared from normal rat kidneys by perfusion with 10% polycaprolactone (PCL) solution, followed by tissue digestion. The corrosion PCL cast was coated with collagen, and PCL was removed from within the collagen coating, leaving only a hollow collagen-based biomimetic vascular scaffold. The fabricated scaffolds were pre-vascularized with MS1 endothelial cell coating, incorporated into 3D renal constructs, and subsequently implanted either with or without human renal cells in the renal cortex of nude rats. The implanted collagen-based vascular scaffold was easily identified and integrated into native kidney tissue. The biomimetic vascular scaffold coated with endothelial cells (MS1) showed significantly enhanced vascularization, as compared to the uncoated scaffold and hydrogel only groups (P < 0.001). Along with the improved vascularization effects, the MS1-coated scaffolds showed a significant renal cell infiltration from the neighboring host tissue, as compared to the other groups (P < 0.05). Moreover, addition of human renal cells to the MS1-coated scaffold resulted in further enhancement of vascularization and tubular structure regeneration within the implanted constructs. The biomimetic collagen vascular scaffolds coated with endothelial cells are able to enhance vascularization and facilitate the formation of renal tubules after 14 days when combined with human renal cells. This study shows the feasibility of bioengineering vascularized functional renal tissues for kidney regeneration. STATEMENT OF SIGNIFICANCE: Vascularization is one of the major hurdles affecting the survival and integration of implanted three-dimensional tissue constructs in vivo. A novel, biomimetic, collagen-based vascular scaffold that is structurally identical to native kidney tissue was developed and tested. This biomimetic vascularized scaffold system facilitates the development of new vessels and renal cell viability in vivo when implanted in a partial renal defect. The use of this scaffold system could address the challenges associated with vascularization, and may be an ideal treatment strategy for partial augmentation of renal function in patients with chronic kidney disease.

Effect of systemic delivery of Substance P on experimental tooth movement in rats

INTRODUCTION

The purpose of this study was to investigate the effect of systemic delivery of Substance P (SP) on experimental tooth movement.

METHODS

Forty-eight adult Sprague-Dawley rats were randomly divided into 2 groups and their maxillary first molars were mesially moved with the use of closed-coil springs. The experiment group received systemic injection of SP and the control group received phosphate-buffered saline solution. Transportation distances of first molars were measured. Hematoxylin and eosin staining, tartrate-resistant acid phosphatase staining, and immunohistochemistry staining were performed to evaluate alveolar bone remodeling. Then the interferon (IFN) γ and tumor necrosis factor (TNF) α concentrations in peripheral blood and local periodontal tissue were measured. Finally, the effects of SP on bone marrow-derived stem cell (BMSC) proliferation and migration were tested in vitro.

RESULTS

Systemic delivery of SP significantly increased the distance of tooth movement and stimulated both osteoclast and osteoblast activities. The concentrations of IFN-γ and TNF-α increased in peripheral blood at early phases of the experiment and decreased in periodontal tissue at late phases. In vitro, the proliferation and migration of BMSCs were promoted by SP.

CONCLUSIONS

Systemic delivery of SP can accelerate orthodontic tooth movement and promote alveolar bone remodeling potentially through immunomodulation and mobilizing endogenous mesenchymal stem cells.

Synthetic Extracellular Microenvironment for Modulating Stem Cell Behaviors

The innate ability of stem cells to self-renew and differentiate into multiple cell types makes them a promising source for tissue engineering and regenerative medicine applications. Their capacity for self-renewal and differentiation is largely influenced by the combination of physical, chemical, and biological signals found in the stem cell niche, both temporally and spatially. Embryonic and adult stem cells are potentially useful for cell-based approaches; however, regulating stem cell behavior remains a major challenge in their clinical use. Most of the current approaches for controlling stem cell fate do not fully address all of the complex signaling pathways that drive stem cell behaviors in their natural microenvironments. To overcome this limitation, a new generation of biomaterials is being developed for use as three-dimensional synthetic microenvironments that can mimic the regulatory characteristics of natural extracellular matrix (ECM) proteins and ECM-bound growth factors. These synthetic microenvironments are currently being investigated as a substrate with surface immobilization and controlled release of bioactive molecules to direct the stem cell fate in vitro, as a tissue template to guide and improve the neo-tissue formation both in vitro and in vivo, and as a delivery vehicle for cell therapy in vivo. The continued advancement of such an intelligent biomaterial system as the synthetic extracellular microenvironment holds the promise of improved therapies for numerous debilitating medical conditions for which no satisfactory cure exists today.

In situ regeneration of skeletal muscle tissue through host cell recruitment

Standard reconstructive procedures for restoring normal function after skeletal muscle defects involve the use of existing host tissues such as muscular flaps. In many instances, this approach is not feasible and delays the rehabilitation process and restoration of tissue function. Currently, cell-based tissue engineering strategies have been used for reconstruction; however, donor tissue biopsy and ex vivo cell manipulation are required prior to implantation. The present study aimed to overcome these limitations by demonstrating mobilization of muscle cells into a target-specific site for in situ muscle regeneration. First, we investigated whether host muscle cells could be mobilized into an implanted scaffold. Poly(l-lactic acid) (PLLA) scaffolds were implanted in the tibialis anterior (TA) muscle of rats, and the retrieved scaffolds were characterized by examining host cell infiltration in the scaffolds. The host cell infiltrates, including Pax7+ cells, gradually increased with time. Second, we demonstrated that host muscle cells could be enriched by a myogenic factor released from the scaffolds. Gelatin-based scaffolds containing a myogenic factor were implanted in the TA muscle of rats, and the Pax7+ cell infiltration and newly formed muscle fibers were examined. By the second week after implantation, the Pax7+ cell infiltrates and muscle formation were significantly accelerated within the scaffolds containing insulin-like growth factor 1 (IGF-1). Our data suggest an ability of host stem cells to be recruited into the scaffolds with the capability of differentiating to muscle cells. In addition, the myogenic factor effectively promoted host cell recruitment, which resulted in accelerating muscle regeneration in situ.

In situ tissue regeneration through host stem cell recruitment

The field of tissue engineering has made steady progress in translating various tissue applications. Although the classical tissue engineering strategy, which involves the use of culture-expanded cells and scaffolds to produce a tissue construct for implantation, has been validated, this approach involves extensive cell expansion steps, requiring a lot of time and laborious effort before implantation. To bypass this ex vivo process, a new approach has been introduced. In situ tissue regeneration utilizes the body's own regenerating capacity by mobilizing host endogenous stem cells or tissue-specific progenitor cells to the site of injury. This approach relies on development of a target-specific biomaterial scaffolding system that can effectively control the host microenvironment and mobilize host stem/progenitor cells to target tissues. An appropriate microenvironment provided by implanted scaffolds would facilitate recruitment of host cells that can be guided to regenerating structural and functional tissues.

Near-infrared fluorescence imaging for noninvasive trafficking of scaffold degradation

Biodegradable scaffolds could revolutionize tissue engineering and regenerative medicine; however, in vivo matrix degradation and tissue ingrowth processes are not fully understood. Currently a large number of samples and animals are required to track biodegradation of implanted scaffolds, and such nonconsecutive single-time-point information from various batches result in inaccurate conclusions. To overcome this limitation, we developed functional biodegradable scaffolds by employing invisible near-infrared fluorescence and followed their degradation behaviors in vitro and in vivo. Using optical fluorescence imaging, the degradation could be quantified in real-time, while tissue ingrowth was tracked by measuring vascularization using magnetic resonance imaging in the same animal over a month. Moreover, we optimized the in vitro process of enzyme-based biodegradation to predict implanted scaffold behaviors in vivo, which was closely related to the site of inoculation. This combined multimodal imaging will benefit tissue engineers by saving time, reducing animal numbers, and offering more accurate conclusions.

Stem cell recruitment and angiogenesis of neuropeptide substance P coupled with self-assembling peptide nanofiber in a mouse hind limb ischemia model

For the successful treatment of ischemia, it is important to resupply sufficient blood into ischemic regions by inducing angiogenesis. Many stem cell transplantation studies have been reported to enhance angiogenesis, especially those relating to mesenchymal stem cells (MSCs); however cell transplantation has a number of limitations, such as the low rate of cell survival and donor cell shortage. In this study, we developed bioactive peptides by immobilizing substance P into self-assembling peptides, and their MSCs recruiting ability and therapeutic effects were evaluated by using ischemic hind limb models. Limb ischemia was produced in athymic mice, and 1% (wt/vol) peptides were injected into ischemic sites (n = 6 in each group: ischemia, substance P, RADA16-II, RADA16-II + substance P, and RADA16-II + RADA-SP (bioactive peptides)). The tissues were harvested for histological analysis and tissue perfusion measurement at 1, 3, 7, and 28 days after injection. We observed that bioactive peptides assembled themselves (<10 nm nanofibers) and formed 3-dimensional (3D) microenvironments within ischemic regions. In the animal study, it was observed that by applying bioactive peptides, substance P continued to be released at 28 days, and consequently, MSCs were successfully recruited into ischemic regions. Bioactive peptides could prevent fibrosis, promote neovascularization, enhance tissue perfusion, and prevent limb salvages. Our results demonstrated that bioactive peptides are one of the most powerful tools for the treatment of ischemia, through their recruitment of autologous MSCs and promotion of angiogenesis without cells transplantation.

Biomaterial implants mediate autologous stem cell recruitment in mice

Autologous stem cells, recognized as the best cells for stem cell therapy, are associated with difficult extraction procedures which often lead to more traumas for the patients and time-consuming laboratory work, which delays their subsequent application. To combat such challenges, it was recently uncovered that, shortly after biomaterial implantation, following the recruitment of inflammatory cells, substantial numbers of mesenchymal stem cells (MSC) and hematopoietic stem cells (HSC) were recruited to the implantation sites. These multipotent MSC could be differentiated into various lineages in vitro. Inflammatory signals may be responsible for the gathering of stem cells, since there is a good relationship between biomaterial-mediated inflammatory responses and stem cell accumulation in vivo. In addition, the treatment with the anti-inflammatory drug dexamethasone substantially reduced the recruitment of both MSC and HSC. The results from this work support that such strategies could be further developed towards localized recruitment and differentiation of progenitor cells. This may permit the future development of autologous stem cell therapies without the need for tedious cell isolation, culture and transplantation.

Combined systemic and local delivery of stem cell inducing/recruiting factors for in situ tissue regeneration

Whereas the conventional tissue engineering strategy involves the use of scaffolds combined with appropriate cell types to restore normal functions, the concept of in situ tissue regeneration uses host responses to a target-specific scaffold to mobilize host cells to a site of injury without the need for cell seeding. For this purpose, local delivery of bioactive molecules from scaffolds has been generally used. However, this approach has limited stem cell recruitment into the implants. Thus, we developed a combination of systemic delivery of substance P (SP) and local release of stromal-derived factor-1α (SDF-1α) from an implant. In this study, we examined whether this combined system would significantly enhance recruitment of host stem cells into the implants. Flow cytometry and immunohistochemistry for CD29/CD45, CD146/α-smooth muscle actin, and c-kit demonstrated that this system significantly increased the number of stem cell-like cells within the implants when compared with other systems. In vitro culture of the cells that had infiltrated into the scaffolds from the combined system confirmed that host stem cells were recruited into these implants and indicated that they were capable of differentiation into multiple lineages. These results indicate that this combined system may lead to more efficient tissue regeneration.

Tissue engineering of human bladder

There are a number of conditions of the bladder that can lead to loss of function. Many of these require reconstructive procedures. However, current techniques may lead to a number of complications. Replacement of bladder tissues with functionally equivalent ones created in the laboratory could improve the outcome of reconstructive surgery. A review of the literature was conducted using PubMed to identify studies that provide evidence that tissue engineering techniques may be useful in the development of alternatives to current methods of bladder reconstruction. A number of animal studies and several clinical experiences show that it is possible to reconstruct the bladder using tissues and neo-organs produced in the laboratory. Materials that could be used to create functionally equivalent urologic tissues in the laboratory, especially non-autologous cells that have the potential to reject have many technical limitations. Current research suggests that the use of biomaterial-based, bladder-shaped scaffolds seeded with autologous urothelial and smooth muscle cells is currently the best option for bladder tissue engineering. Further research to develop novel biomaterials and cell sources, as well as information gained from developmental biology, signal transduction studies and studies of the wound healing response would be beneficial.

Application of tissue-engineered cartilage with BMP-7 gene to repair knee joint cartilage injury in rabbits

Injured articular cartilage has a poor capacity for spontaneous healing. So far, satisfactory solution to this subsistent problem has not been found, but transgenic therapy may be a promising way. This study aims to evaluate the effectiveness of a tissue-engineered cartilage that was transfected with morphogenetic protein 7 (BMP 7) in repairing the cartilaginous defects of rabbit knee joints. Chondrocytes were transfected with BMP-7 gene (5 x 10(6) cells/ml), inoculated into the collagen-fibrin gel scaffolds, and cultured for 14 days. Then, the scaffolds were implanted onto the created defects (5.0 mm in diameter) in rabbits' knee joints. After 12 weeks, the rabbits were sacrificed and histological sections were evaluated using modified O'Driscoll cartilage scores; In situ hybridization and immunohistochemistry were performed to detect the expression of BMP-7 mRNA and BMP-7 at the implanted site while the content of DNA and GAG was determined as well. A better quality of repairs was observed at the 12th week after implantation when compared to the control group using histological analyses. The content of DNA and specific secretion of GAG in the treatment group is statistically significant different compared with the control group. Gene therapy may be a promising treatment method, but the novel therapy approach needs further studies with respect to a longer follow-up period.