Biomaterials for pluripotent stem cell engineering: From fate determination to vascularization

Effect of magnesium ascorbyl phosphate on collagen stabilization for wound healing application

Wound healing is a complex process which requires appropriate structural support for restoration of tissue continuity and function. Collagen can act as a template for cellular activities but poor physico-chemical properties necessitates the stabilization of collagen without impairing its structure and function. This study investigates the effect of magnesium ascorbyl phosphate (MAP) on collagen with reference to physico-chemical properties. Incorporation of MAP enhanced the rate of collagen fibrillation signifying increased interaction at reduced time interval. MAP did not induce any changes in the secondary structure of collagen while there was an increase in shear viscosity with increase in shear stress at different shear rate. MAP stabilized collagen film exhibited higher denaturation temperature and showed an increase in Young's Modulus when compared with that of collagen film. In vivo studies showed complete wound closure on day 16 in case of stabilized collagen film. Mechanical properties of healed skin revealed that MAP collagen film treated rat skin completely regained its properties similar to that of normal skin thereby making them a potential candidate for wound healing application.

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.

Macroporous Dual-compartment Hydrogels for Minimally Invasive Transplantation of Primary Human Hepatocytes

BACKGROUND

Given the shortage of available organs for whole or partial liver transplantation, hepatocyte cell transplantation has long been considered a potential strategy to treat patients suffering from various liver diseases. Some of the earliest approaches that attempted to deliver hepatocytes via portal vein or spleen achieved little success due to poor engraftment. More recent efforts include transplantation of cell sheets or thin hepatocyte-laden synthetic hydrogels. However, these implants must remain sufficiently thin to ensure that nutrients can diffuse into the implant.

METHODS

To circumvent these limitations, we investigated the use of a vascularizable dual-compartment hydrogel system for minimally invasive transplantation of primary hepatocytes. The dual-compartment system features a macroporous outer polyethylene glycol diacrylate/hyaluronic acid methacrylate hydrogel compartment for seeding supportive cells and facilitating host cell infiltration and vascularization and a hollow inner core to house the primary human hepatocytes.

RESULTS

We show that the subcutaneous implantation of these cell-loaded devices in NOD/SCID mice facilitated vascular formation while supporting viability of the transplanted cells. Furthermore, the presence of human serum albumin in peripheral blood and the immunostaining of excised implants indicated that the hepatocytes maintained function in vivo for at least 1 month, the longest assayed time point.

CONCLUSIONS

Cell transplantation devices that assist the anastomosis of grafts with the host can be potentially used as a minimally invasive ectopic liver accessory to augment liver-specific functions as well as potentially treat various pathologies associated with compromised functions of liver, such as hemophilia B or alpha-1 antitrypsin deficiency.

Biomaterial aided differentiation and maturation of induced pluripotent stem cells

Engineering/reprogramming differentiated adult somatic cells to gain the ability to differentiate into any type of cell lineage are called as induced pluripotent stem cells (iPSCs). Offering unlimited self-renewal and differentiation potential, these iPSC are aspired to meet the growing demands in the field of regenerative medicine, tissue engineering, disease modeling, nanotechnology, and drug discovery. Biomaterial fabrication with the rapid evolution of technology increased their versatility and utility in regenerative medicine and tissue engineering, revolutionizing the stem cell biology research with the property to guide the process of proliferation, differentiation, and morphogenesis. Combining traditional culture platforms of iPSC with biomaterials aids to overcome the limitations associated with derivation, proliferation, and maturation, thereby could improve the clinical translation of iPSC. The present review discusses in brief about the reprogramming techniques for the derivation iPSC and details on several biomaterial guided differentiation of iPSC to different cell types with specific relevance to tissue engineering/regenerative medicine.

[Local injection of angiopoietin 2 promotes angiogenesis in tissue engineered bone and repair of bone defect with autophagy induction in vivo]

Objective

To investigate the mechanism of early vascularization of the tissue engineered bone in the treatment of rabbit radial bone defect by local injection of angiopoietin 2 (Ang-2).

Methods

A single 1.5 cm long radius defect model (left and right sides randomised) was constructed from 48 New Zealand white rabbits. After implantation of hydroxyapatite/collagen scaffolds in bone defects, the rabbits were randomly divided into 2 groups: control group (group A) and Ang-2 group (group B) were injected with 1 mL normal saline and 1 mL saline-soluble 400 ng/mL Ang-2 daily at the bone defect within 2 weeks after operation, respectively. Western blot was used to detect the expressions of autophagy related protein [microtubule associated protein 1 light chain 3 (LC3), Beclin-1], angiogenesis related protein [vascular endothelial growth factor (VEGF)], and autophagy degradable substrate protein (SQSTMl/p62) in callus. X-ray films examination and Lane-Sandhu X-ray scoring were performed to evaluate the bone defect repair at 4, 8, and 12 weeks after operation. The rabbits were sacrificed at 12 weeks after operation for gross observation, and the angiogenesis of bone defect was observed by HE staining.

Results

Western blot assay showed that the relative expression of LC3-II/LC3-I, Beclin-1, and VEGF in group B was significantly higher than that in group A, and the relative expression of SQSTMl/p62 was significantly lower than that in group A ( P<0.05). Radiographic and gross observation of specimens showed that only a small number of callus were formed in group A, the bone defect was not repaired; more callus were formed and complete repair of bone defect was observed in group B. The Lane-Sandhu scores in group B were significantly higher than those in group A at 4, 8, and 12 weeks after operation ( P<0.05). HE staining showed that the Harvard tubes in group B were well arranged and the number of new vessels was significantly higher than that in group A ( t=-11.879, P=0.000).

Conclusion

Local injection of appropriate concentration of Ang-2 may promote early vascularization and bone defect repair of rabbit tissue engineered bone by enhancing autophagy.

Engineering Hydrogel Microenvironments to Recapitulate the Stem Cell Niche

Stem cells are a powerful resource for many applications including regenerative medicine, patient-specific disease modeling, and toxicology screening. However, eliciting the desired behavior from stem cells, such as expansion in a naïve state or differentiation into a particular mature lineage, remains challenging. Drawing inspiration from the native stem cell niche, hydrogel platforms have been developed to regulate stem cell fate by controlling microenvironmental parameters including matrix mechanics, degradability, cell-adhesive ligand presentation, local microstructure, and cell-cell interactions. We survey techniques for modulating hydrogel properties and review the effects of microenvironmental parameters on maintaining stemness and controlling differentiation for a variety of stem cell types. Looking forward, we envision future hydrogel designs spanning a spectrum of complexity, ranging from simple, fully defined materials for industrial expansion of stem cells to complex, biomimetic systems for organotypic cell culture models.

Accelerated host angiogenesis and immune responses by ion release from mesoporous bioactive glass

Angiogenesis represents a major focus for novel therapeutic approaches to the treatment and management of multiple pathological conditions, such as ischemic heart disease and critical-sized bone defect. The complex process of angiogenesis begins when cells within a tissue respond to hypoxia by increasing their production of vascular endothelial growth factor (VEGF). Loading biomaterials with angiogenic therapeutics have emerged as a promising approach for developing superior biomaterials for tissue repair and regeneration due to the possibility of reducing treatment costs and side effects when compared to the use of growth factors or genetic engineering approaches. Trace elements, such as copper (Cu), have been reported to be capable of inhibiting prolyl hydroxylases leading to the accumulation and activation of hypoxia-inducible factor-1α (HIF-1α), a major transcription factor regulating the expression of VEGF. It has also recently been speculated that the artifically induced hypoxic microenvironment may regulate the local immune response, which in turn, further facilitates the tissue repair process. The present study has incorporated ionic Cu²⁺ into mesoporous bioactive glass (MBG), a promising bioactive material system for regenerative medicine, and investigated its effect on angiogenesis and immune responses both in vitro and in vivo. Our results demonstrated that hypoxia-mimicking materials could induce VEGF secretion of bone marrow-derived mesenchymal stromal cells (BMSCs), which provided a positive feedback loop for early blood vessel formation by stimulating migration and tube formation of human umbilical vein endothelial cells (HUVECs). Furthermore, a tissue-regenerative macrophage subtype was triggered by Cu-MBG, leading to superior angiogenic responses (tube formation and angiogenic gene expression) compared to the traditional MBG material. It is concluded that the addition of inorganic ions leads to enhanced angiogenesis and immune responses, which holds promise for the development of functional tissue-engineered constructs to repair and regenerate damaged tissues and organs.

Manufacturing Cell Therapies Using Engineered Biomaterials

Emerging manufacturing processes to generate regenerative advanced therapies can involve extensive genomic and/or epigenomic manipulation of autologous or allogeneic cells. These cell engineering processes need to be carefully controlled and standardized to maximize safety and efficacy in clinical trials. Engineered biomaterials with smart and tunable properties offer an intriguing tool to provide or deliver cues to retain stemness, direct differentiation, promote reprogramming, manipulate the genome, or select functional phenotypes. This review discusses the use of engineered biomaterials to control human cell manufacturing. Future work exploiting engineered biomaterials has the potential to generate manufacturing processes that produce standardized cells with well-defined critical quality attributes appropriate for clinical testing.

Essential design considerations for the resazurin reduction assay to noninvasively quantify cell expansion within perfused extracellular matrix scaffolds

Precise measurement of cellularity within bioartificial tissues and extracellular matrix (ECM) scaffolds is necessary to augment rigorous characterization of cellular behavior, as accurate benchmarking of tissue function to cell number allows for comparison of data across experiments and between laboratories. Resazurin, a soluble dye that is reduced to highly fluorescent resorufin in proportion to the metabolic activity of a cell population, is a valuable, noninvasive tool to measure cell number. We investigated experimental conditions in which resazurin reduction is a reliable indicator of cellularity within three-dimensional (3D) ECM scaffolds. Using three renal cell populations, we demonstrate that correlation of viable cell numbers with the rate of resorufin generation may deviate from linearity at higher cell densities, lower resazurin working volumes, or longer incubation times that all contribute to depleting the pool of resazurin. In conclusion, while the resazurin reduction assay provides a powerful, noninvasive readout of metrics enumerating cellularity and growth within ECM scaffolds, assay conditions may strongly influence its applicability for accurate quantification of cell number. The approach and methodological recommendations presented herein may be used as a guide for application-specific optimization of this assay to obtain rigorous and accurate measurement of cellular content in bioengineered tissues.