In vivo regeneration of renal vessels post whole decellularized kidneys transplantation

Endothelial progenitor cells for fabrication of engineered vascular units and angiogenesis induction

The promotion of vascularization and angiogenesis in the grafts is a crucial phenomenon in the healing process and tissue engineering. It has been shown that stem cells, especially endothelial progenitor cells (EPCs), can stimulate blood vessel formation inside the engineered hydrogels after being transplanted into the target sites. The incorporation of EPCs into the hydrogel can last the retention time, long-term survival, on-target delivery effects, migration and differentiation into mature endothelial cells. Despite these advantages, further modifications are mandatory to increase the dynamic growth and angiogenesis potential of EPCs in in vitro and in vivo conditions. Chemical modifications of distinct composites with distinct physical properties can yield better regenerative potential and angiogenesis during several pathologies. Here, we aimed to collect recent findings related to the application of EPCs in engineered vascular grafts and/or hydrogels for improving vascularization in the grafts. Data from the present article can help us in the application of EPCs as valid cell sources in the tissue engineering of several ischemic tissues.

Polyethylene glycol crosslinked decellularized single liver lobe scaffolds with vascular endothelial growth factor promotes angiogenesis in vivo

BACKGROUND

Improving the mechanical properties and angiogenesis of acellular scaffolds before transplantation is an important challenge facing the development of acellular liver grafts. The present study aimed to evaluate the cytotoxicity and angiogenesis of polyethylene glycol (PEG) crosslinked decellularized single liver lobe scaffolds (DLSs), and establish its suitability as a graft for long-term liver tissue engineering.

METHODS

Using mercaptoacrylate produced by the Michael addition reaction, DLSs were first modified using N-succinimidyl S-acetylthioacetate (SATA), followed by cross-linking with PEG as well as vascular endothelial growth factor (VEGF). The optimal concentration of agents and time of the individual steps were identified in this procedure through biomechanical testing and morphological analysis. Subsequently, human umbilical vein endothelial cells (HUVECs) were seeded on the PEG crosslinked scaffolds to detect the proliferation and viability of cells. The scaffolds were then transplanted into the subcutaneous tissue of Sprague-Dawley rats to evaluate angiogenesis. In addition, the average number of blood vessels was evaluated in the grafts with or without PEG at days 7, 14, and 21 after implantation.

RESULTS

The PEG crosslinked DLS maintained their three-dimensional structure and were more translucent after decellularization than native DLS, which presented a denser and more porous network structure. The results for Young's modulus proved that the mechanical properties of 0.5 PEG crosslinked DLS were the best and close to that of native livers. The PEG-VEGF-DLS could better promote cell proliferation and differentiation of HUVECs compared with the groups without PEG cross-linking. Importantly, the average density of blood vessels was higher in the PEG-VEGF-DLS than that in other groups at days 7, 14, and 21 after implantation in vivo.

CONCLUSIONS

The PEG crosslinked DLS with VEGF could improve the biomechanical properties of native DLS, and most importantly, their lack of cytotoxicity provides a new route to promote the proliferation of cells in vitro and angiogenesis in vivo in liver tissue engineering.

Angiogenesis and Re-endothelialization in decellularized scaffolds: Recent advances and current challenges in tissue engineering

Decellularization of tissues and organs has recently become a promising approach in tissue engineering and regenerative medicine to circumvent the challenges of organ donation and complications of transplantations. However, one main obstacle to reaching this goal is acellular vasculature angiogenesis and endothelialization. Achieving an intact and functional vascular structure as a vital pathway for supplying oxygen and nutrients remains the decisive challenge in the decellularization/re-endothelialization procedure. In order to better understand and overcome this issue, complete and appropriate knowledge of endothelialization and its determining variables is required. Decellularization methods and their effectiveness, biological and mechanical characteristics of acellular scaffolds, artificial and biological bioreactors, and their possible applications, extracellular matrix surface modification, and different types of utilized cells are factors affecting endothelialization consequences. This review focuses on the characteristics of endothelialization and how to optimize them, as well as discussing recent developments in the process of re-endothelialization.

Kidney tissue engineering in preclinical models of renal failure: a systematic review and meta-analysis

Aim: This study aims to compare the efficacy of tissue engineering for kidney reconstruction. Materials & methods: We searched MEDLINE, EMBASE (May 2021), and reference lists of review articles. Results: 19 articles matched our inclusion criteria. A range of natural, synthetic and hybrid scaffolds with or without incorporating cells/growth factors was investigated in 937 animals. More favorable results were observed with a combination of two or more biomaterials, addition of bioactive moieties, and cell seeding. Creatinine concentration, PAX2, collagen type-1, α-SMA, vimentin, IL-1, IL-6 and TNF-α gene expressions were significantly increased compared with native control. Conclusion: Tissue engineering can improve renal function and regeneration; however, further research could benefit from using hybrid scaffolds, stem cells and large animal models.

Engineering cryoelectrospun elastin-alginate scaffolds to serve as stromal extracellular matrices

Scaffold-based regenerative strategies that emulate physical, biochemical, and mechanical properties of the native extracellular matrix (ECM) of the region of interest can influence cell growth and function. Existing ECM-mimicking scaffolds, including nanofiber (NF) mats, sponges, hydrogels, and NF-hydrogel composites are unable to simultaneously mimic typical composition, topography, pore size, porosity, and viscoelastic properties of healthy soft-tissue ECM. In this work, we used cryoelectrospinning to fabricate 3D porous scaffolds with minimal fibrous backbone, pore size and mechanical properties similar to soft-tissue connective tissue ECM. We used salivary glands as our soft tissue model and found the decellularized adult salivary gland (DSG) matrix to have a fibrous backbone, 10-30μm pores, 120 Pa indentation modulus, and ∼200 s relaxation half time. We used elastin and alginate as natural, compliant biomaterials and water as the solvent for cryoelectrospinning scaffolds to mimic the structure and viscoelasticity of the connective tissue ECM of the DSG. Process parameters were optimized to produce scaffolds with desirable topography and compliance similar to DSG, with a high yield of >100 scaffolds/run. Using water as solvent, rather than organic solvents, was critical to generate biocompatible scaffolds with desirable topography; further, it permitted a green chemistry fabrication process. Here, we demonstrate that cryoelectrospun scaffolds (CESs) support penetration of NIH 3T3 fibroblasts 250-450µm into the scaffold, cell survival, and maintenance of a stromal cell phenotype. Thus, we demonstrate that elastin-alginate CESs mimic many structural and functional properties of ECM and have potential for future use in regenerative medicine applications.

Effect of heparinization on promoting angiogenesis of decellularized kidney scaffolds

Native decellularized extracellular matrix provides an adequate platform for tissues and organs and promotes the development of organogenesis and tissue remodeling. However, thrombosis poses a great challenge that hinders the transplantation for a substantial organ in vivo. Therefore, anticoagulation and re-reendothelialization of organ biological scaffolds are the primary concerns to be addressed before orthotopic transplantation. Herein, a heparinized decellularized kidney scaffold (HEP-DKSs) was prepared using end-point attachment technology, followed by binding the vascular endothelial growth factor (VEGF) to greatly improve the hemocompatibility and angiogenesis of DKSs. Based on the anticoagulant, co-culture of human umbilical vein endothelial cells, and subcapsular transplantation of kidney experiments, HEP-VEGF-DKSs are shown to reduce platelet adhesion, which is crucial for subsequent vascularization and slow release of heparin and VEGF, suggesting its ability of improve neovascularization. Taken together, these data indicated an optimal anticoagulation function of HEP-VEGF-DKSs and the potential of vascularization for regeneration of whole decellularized kidney.

EPCs enhance angiogenesis in renal regeneration

Decellularized renal scaffolds have previously been used for renal regeneration following partial nephrectomy, in which angiogenesis played a key role. In this study, rats underwent partial nephrectomy and repaired with decellularized renal scaffolds. Subsequently, the labeled EPCs were intravenously injected into rats in EPCs group, and the control group received an equal amount of phosphate-buffer saline (PBS). We chose 1, 2 and 4 weeks post operation as time point. Average microvascular density (aMVD) analyses revealed higher angiogenesis in EPCs group compared with the control group. The expression of angiogenic growth factors including vascular endothelial growth factor (VEGF), platelet derived growth factor (PDGF) and hypoxia-inducible factors 1-alpha (HIF-1α), was generally higher in the EPCs group in all weeks (1, 2 and 4), and peaked in week 2. EPCs were observed to home into renal injury site, promoting angiogenesis across the renal parenchyma-scaffold interface to be potentially used as bridges for EPCs to migrate into the implanted scaffolds. Administration of exogenous EPCs promotes angiogenesis and vasculogenesis in decellularized renal scaffolds-mediated renal regeneration, providing adequate microenvironment for kidney recovery post renal injury.

The angiogenesis in decellularized scaffold-mediated the renal regeneration

There are increasing numbers of patients underwent partial nephrectomy, and recovery of disturbed renal function is imperative post partial nephrectomy. We previously have demonstrated the decellularized (DC) scaffolds could mediate the residual kidney regeneration and thus improve disturbed renal function after partial nephrectomy. However, the cellular changes including the angiogenesis in the implanted DC scaffold has not yet been elaborated. In this study, we observed that the scaffold promoted the proliferation of human umbilical vein endothelial cells (HUVEC) that adhered to the DC scaffold in vitro. We next examined the pathological changes of the implanted DC graft in vivo, and found a decreased volume of the scaffold and a dramatic angiogenesis within the scaffold. The average microvessel density (aMVD) increased at the early stage, while decreased at the later stage post transplantation. Expression level of vascular endothelial growth factor (VEGF) showed similar dynamic changes. In addition, many endothelial cells (ECs) and endothelial progenitor cells (EPCs) were distributed in the region which contained active angiogenesis in the scaffold. However, the implanted graft became fibrosis and the angiogenesis degraded at final stage roughly 8 weeks post transplantation. Our data indicate that DC scaffold can be vascularized in vivo and possible mechanisms are discussed.

Decellularized scaffolds in regenerative medicine

Allogeneic organ transplantation remains the ultimate solution for end-stage organ failure. Yet, the clinical application is limited by the shortage of donor organs and the need for lifelong immunosuppression, highlighting the importance of developing effective therapeutic strategies. In the field of regenerative medicine, various regenerative technologies have lately been developed using various biomaterials to address these limitations. Decellularized scaffolds, derived mainly from various non-autologous organs, have been proved a regenerative capability in vivo and in vitro and become an emerging treatment approach. However, this regenerative capability varies between scaffolds as a result of the diversity of anatomical structure and cellular composition of organs used for decellularization. Herein, recent advances in scaffolds based on organ regeneration in vivo and in vitro are highlighted along with aspects where further investigations and analyses are needed.