Dynamic contrast-enhanced MRI to quantify VEGF-enhanced tissue-engineered bladder graft neovascularization: pilot study

A bioactive injectable bulking material; a potential therapeutic approach for stress urinary incontinence

Stress urinary incontinence (SUI) is a life changing condition, affecting 20 million women worldwide. In this study, we developed a bioactive, injectable bulking agent that consists of Permacol™ (Medtronic, Switzerland) and recombinant insulin like growth factor-1 conjugated fibrin micro-beads (fib_rIGF-1) for its bulk stability and capacity to induce muscle regeneration. Therefore, Permacol™ formulations were injected in the submucosal space of rabbit bladders. The ability of a bulking material to form a stable and muscle-inducing bulk represents for us a promising therapeutic approach to achieve a long-lasting treatment for SUI. The fib_rIGF-1 showed no adverse effect on human smooth muscle cell metabolic activity and viability in vitro based on AlamarBlue assays and Live/Dead staining. Three months after injection of fib_rIGF-1 together with Permacol™ into the rabbit bladder wall, we observed a smooth muscle tissue like formation within the injected materials. Positive staining for alpha smooth muscle actin, calponin, and caldesmon demonstrated a contractile phenotype of the newly formed smooth muscle tissue. Moreover, the fib_rIGF-1 treated group also improved the neovascularization at the injection site, confirmed by CD31 positive staining compared to bulks made of Permacol^TM only. The results of this study encourage us to further develop this injectable, bioactive bulking material towards a future therapeutic approach for a minimal invasive and long-lasting treatment of SUI.

3D anatomical and perfusion MRI for longitudinal evaluation of biomaterials for bone regeneration of femoral bone defect in rats

Magnetic Resonance Imaging (MRI) appears as a good surrogate to Computed Tomography (CT) scan as it does not involve radiation. In this context, a 3D anatomical and perfusion MR imaging protocol was developed to follow the evolution of bone regeneration and the neo-vascularization in femoral bone defects in rats. For this, three different biomaterials based on Pullulan-Dextran and containing either Fucoidan or HydroxyApatite or both were implanted. In vivo MRI, ex vivo micro-CT and histology were performed 1, 3 and 5 weeks after implantation. The high spatially resolved (156 × 182 × 195 µm) anatomical images showed a high contrast from the defects filled with biomaterials that decreased over time due to bone formation. The 3D Dynamic Contrast Enhanced (DCE) imaging with high temporal resolution (1 image/19 s) enabled to detect a modification in the Area-Under-The-Gadolinium-Curve over the weeks post implantation. The high sensitivity of MRI enabled to distinguish which biomaterial was the least efficient for bone regeneration, which was confirmed by micro-CT images and by a lower vessel density observed by histology. In conclusion, the methodology developed here highlights the efficiency of longitudinal MRI for tissue engineering as a routine small animal exam.

Co-delivery of VEGF and bFGF via a PLGA nanoparticle-modified BAM for effective contracture inhibition of regenerated bladder tissue in rabbits

Graft contracture is a common problem associated with the regeneration processes of tissue-engineered bladders. Currently, most strategies used for incorporating bioactive molecules into biomaterial designs do not work during all phases of tissue regeneration. In this study, we used a growth factor-PLGA nanoparticle thermo-sensitive gel system (i.e., BAM with incorporated VEGF and bFGF-loaded PLGA nanoparticles and mixed with a hydrophilic gel) to promote bladder tissue regeneration in a rabbit model. At 4 and 12 weeks after surgery, contracture rate assessment and histological examination were conducted to evaluate bladder tissue regeneration. The results indicated that the functional composite scaffold continuously and effectively released VEGF and bFGF and promoted bladder reconstruction with a significant decrease in graft contracture. In addition, the number and arrangement of regenerated urothelial cells and smooth muscle cells as well as microvascular density and maturity were improved in the VEGF/bFGF nanoparticle group compared with the single factor VEGF or bFGF nanoparticle group and BAM alone. The nanoparticle thermo-sensitive gel system, which exhibited favourable performance, may effectively inhibit graft contracture and promote bladder tissue regeneration in rabbits.

Bladder Regeneration Using a Smart Acellular Collagen Scaffold with Growth Factors VEGF, FGF2 and HB-EGF

Tissue engineering may become an alternative to current bladder augmentation techniques. Large scaffolds are needed for clinically significant augmentation, but can result in fibrosis and graft shrinkage. The purpose of this study was to investigate whether smart acellular collagen-heparin scaffolds with growth factors (GFs) VEGF, FGF2, and HB-EGF enhance bladder tissue regeneration and bladder capacity in a large animal model of diseased bladder. Scaffolds of bovine type I collagen with heparin and VEGF, FGF2, and HB-EGF measuring 3.2 cm in diameter were prepared. In 23 fetal sheep, a bladder exstrophy was surgically created at 79 days of gestation. One week after birth (at full term), the bladder was reconstructed by primary closure (PC group) or using a collagen-heparin scaffold with GFs (COLGF group) and compared to a historical group reconstructed with a collagen scaffold without GFs (COL group). Functional (video urodynamics) and histological evaluation was performed 1 and 6 months after bladder repair. The overall survival rate was 57%. Cystograms were normal in all animals, except for low-grade reflux in all groups. Urodynamics showed no statistically significant differences in bladder capacity and compliance between groups. Histological evaluation at 1 month revealed increased urothelium formation, improved angiogenesis, and enhanced ingrowth of smooth muscle cells (SMCs) in the COLGF group compared to the COL group. At 6 months, improved SMC ingrowth was found in the COLGF group compared to the COL group; both scaffold groups showed normal urothelial lining and standard extracellular matrix development. Bladder regeneration using a collagen-heparin scaffold with VEGF, FGF2, and HB-EGF improved bladder tissue regeneration in a large animal model of diseased bladder. Larger GF-loaded constructs need to be tested to reach clinically significant augmentation.

Biomatrices for bladder reconstruction

There is a demand for tissue engineering of the bladder needed by patients who experience a neurogenic bladder or idiopathic detrusor overactivity. To avoid complications from augmentation cystoplasty, the field of tissue engineering seeks optimal scaffolds for bladder reconstruction. Naturally derived biomaterials as well as synthetic and natural polymers have been explored as bladder substitutes. To improve regenerative properties, these biomaterials have been conjugated with functional molecules, combined with nanotechology, or seeded with exogenous cells. Although most studies reported complete and functional bladder regeneration in small-animal models, results from large-animal models and human clinical trials varied. For functional bladder regeneration, procedures for biomaterial fabrication, incorporation of biologically active agents, introduction of nanotechnology, and application of stem-cell technology need to be standardized. Advanced molecular and medical technologies such as next generation sequencing and magnetic resonance imaging can be introduced for mechanistic understanding and non-invasive monitoring of regeneration processes, respectively.

Engineering functional epithelium for regenerative medicine and in vitro organ models: a review

Recent advances in the fields of microfabrication, biomaterials, and tissue engineering have provided new opportunities for developing biomimetic and functional tissues with potential applications in disease modeling, drug discovery, and replacing damaged tissues. An intact epithelium plays an indispensable role in the functionality of several organs such as the trachea, esophagus, and cornea. Furthermore, the integrity of the epithelial barrier and its degree of differentiation would define the level of success in tissue engineering of other organs such as the bladder and the skin. In this review, we focus on the challenges and requirements associated with engineering of epithelial layers in different tissues. Functional epithelial layers can be achieved by methods such as cell sheets, cell homing, and in situ epithelialization. However, for organs composed of several tissues, other important factors such as (1) in vivo epithelial cell migration, (2) multicell-type differentiation within the epithelium, and (3) epithelial cell interactions with the underlying mesenchymal cells should also be considered. Recent successful clinical trials in tissue engineering of the trachea have highlighted the importance of a functional epithelium for long-term success and survival of tissue replacements. Hence, using the trachea as a model tissue in clinical use, we describe the optimal structure of an artificial epithelium as well as challenges of obtaining a fully functional epithelium in macroscale. One of the possible remedies to address such challenges is the use of bottom-up fabrication methods to obtain a functional epithelium. Modular approaches for the generation of functional epithelial layers are reviewed and other emerging applications of microscale epithelial tissue models for studying epithelial/mesenchymal interactions in healthy and diseased (e.g., cancer) tissues are described. These models can elucidate the epithelial/mesenchymal tissue interactions at the microscale and provide the necessary tools for the next generation of multicellular engineered tissues and organ-on-a-chip systems.

Urothelial cell culture: stratified urothelial sheet and three-dimensional growth of urothelial structure

Urothelial cells line the urinary tract, including the renal pelvis, ureters, bladder, superior urethra, and the central ducts of the prostate. They are highly specialized epithelial cell types possessing unique features, imparting important functional roles in the urinary system. They act as a permeability barrier and protect underlying muscle tissues from the caustic effects of urine while also expanding with bladder filling to adjust urine pressures. The multilayered urothelium is typically structured with differentiated, mature surface cells and less mature basal cells. The basal cell layer contains tissue-specific stem cells able to self-renew for the lifetime of the mammal and also produces a pool of maturing cells for tissue homeostasis. Maintaining regenerative basal cells in a culture facilitates urothelial cell growth in vitro. Additionally, epithelial-mesenchymal communication, epithelial-matrix interactions, and cytokines/growth factors are required to maintain the normal structure and function of mature urothelial cells in vitro and to induce stem cell differentiation into urothelial cells. These cultures are useful to study the biology and physiology of the urinary tract, particularly for the development of cell-based tissue engineering strategies in urology. This chapter describes methods for the isolation of urothelial cells and their maintenance in monolayer culture, and methods for the production of multilayer urothelial cell sheets and three-dimensional cocultures of urothelial and mesenchymal cells.

Urothelial cell culture

This chapter reviews the use of urothelial cells as a means to enhance tissue regeneration and wound healing in urinary tract system. It addresses the properties of urothelial cells, including their role as a permeability barrier to protect underlying muscle tissue from the caustic effects of urine and as one of the main cell types, along with smooth muscle cells, that are used in urethral or bladder tissue engineering today. This description includes a general overview of various isolation techniques and culture methods that have been developed to improve urinary tract reconstruction in vivo and aid the characterization of growth factor expression in vitro. The chapter then describes various applications using urothelial cells, including production of multilayer urothelial sheets, tissue engineered bladder mucosa, tissue engineered urethra, and tissue engineered bladder. It also outlines the advantages of sandwich and layered coculture of these cells and the effects of epithelial-stromal cell interactions during tissue regeneration or wound healing processes in the urinary tract.

Coadministration of platelet-derived growth factor-BB and vascular endothelial growth factor with bladder acellular matrix enhances smooth muscle regeneration and vascularization for bladder augmentation in a rabbit model

Tissue-engineering techniques have brought a great hope for bladder repair and reconstruction. The crucial requirements of a tissue-engineered bladder are bladder smooth muscle regeneration and vascularization. In this study, partial rabbit bladder (4×5 cm) was removed and replaced with a porcine bladder acellular matrix (BAM) that was equal in size. BAM was incorporated with platelet-derived growth factor-BB (PDGF-BB) and vascular endothelial growth factor (VEGF) in the experimental group while with no bioactive factors in the control group. The bladder tissue strip contractility in the experimental rabbits was better than that in the control ones postoperation. Histological evaluation revealed that smooth muscle regeneration and vascularization in the experimental group were significantly improved compared with those in the control group (p<0.05), while multilayered urothelium was formed in both groups. Muscle strip contractility of neobladder in the experimental group exhibited significantly better than that in the control (p<0.05) assessed with electrical field stimulation and carbachol interference. The activity of matrix metalloproteinase-2 (MMP-2) and MMP-9 in the native bladder tissue around tissue-engineered neobladder in the experimental group was significantly higher than that in the control (p<0.05). This work suggests that smooth muscle regeneration and vascularization in tissue-engineered neobladder and recovery of bladder function could be enhanced by PDGF-BB and VEGF incorporated within BAM, which promoted the upregulation of the activity of MMP-2 and MMP-9 of native bladder tissue around the tissue-engineered neobladder.

Monitoring tissue development in acellular matrix-based regeneration for bladder tissue engineering: multiexponential diffusion and T2* for improved specificity

Cell-seeded acellular matrices (ACMs) are a promising approach for the tissue engineering of soft tissues and organs, such as the urinary bladder. ACM contains site-preferred structural and functional molecules, and degradation products derived from ACM play important roles in tissue remodeling. Regeneration proceeds along concurrent trajectories of cell growth and matrix degradation, characterized by evolving biophysical and biochemical properties. The assessment of tissue development through a noninvasive imaging technique, such as MRI, must therefore be capable of distinguishing these concurrent biophysical and biochemical changes. However, although MRI provides exquisite sensitivity to tissue microstructure, composition and function, specificity remains limited. In this study, multiexponential diffusion and the effective transverse relaxation time T(2)* were investigated for their ability to assess cell growth and tissue composition, respectively. Bladder ACMs prepared with and without hyaluronic acid, and ACMs seeded with smooth muscle cells, were assessed on MRI. The slow diffusion fraction from multiexponential diffusion analysis demonstrated the best correlation with cellularity, with minimal influence from underlying matrix degradation. T(2)* measurements were sensitive to macromolecular content, specifically the presence of hyaluronic acid, without confounding influence from tissue hydration. T(2)* also appeared to be sensitive to cell filling of the matrix pore space. Compared with these metrics, commonly used MRI parameters, such as T(1), T(2) and single diffusion coefficients, were more limited in specificity. The use of T(2) to measure tissue structure and composition is limited by its large dependence on water content, and single diffusion can only reflect the overall characteristics of the extra- and intracellular environment. These findings are important for further development of more specific MRI methods for the monitoring of regeneration in tissue-engineered systems.

Development of a porcine bladder acellular matrix with well-preserved extracellular bioactive factors for tissue engineering

In this study, we compared four decellularization protocols and finally developed an optimized one through which a porcine bladder acellular matrix (BAM) with well-preserved extracellular bioactive factors had been prepared. In this protocol, the intact bladder was treated with trypsin/ethylenediaminetetraacetic acid to remove the urothelium, then with hypotonic buffer and Triton X-100 in hypertonic buffer to remove the membranous and cytoplasmic materials, and finally with nuclease to degrade the cellular nuclear components. Bladder distention and mechanical agitation were simultaneously used to facilitate cell removal. Meanwhile, several preservative techniques, including limitation of wash time, supplement with inhibitors of proteinase, control of the pH value and temperature of the wash buffer, ethylene oxide sterilization, and lyophilization of the scaffold for storage, were used to protect the extracellular bioactive factors. This decellularization protocol had completely removed the cellular materials and well preserved the extracellular collagen, sulfated glycosaminoglycan (GAG), and bioactive factors. The preserved bioactive factors had a great potential of promoting the proliferation and migration of both human bladder smooth muscle cell and human umbilical vein endothelial cell. It was also found that the amount of two representative bioactive factors, platelet-derived growth factor BB and vascular endothelial growth factor, was positively correlated with the sulfated GAG content in the porcine BAM, implying that the amount of sulfated GAG might be a determinant for preservation of bioactive factors in the decellularized tissues. In conclusion, the porcine BAM with well-preserved extracellular bioactive factors might be a favorable scaffold for tissue engineering applications.

Bladder tissue engineering: tissue regeneration and neovascularization of HA-VEGF-incorporated bladder acellular constructs in mouse and porcine animal models

Successful tissue engineering requires appropriate recellularization and vascularization. Herein, we assessed the regenerative and angiogenic effects of porcine bladder acellular matrix (ACM) incorporated with hyaluronic acid (HA) and vascular endothelial growth factor (VEGF) in mouse and porcine models. Prepared HA-ACMs were rehydrated in different concentrations of VEGF (1, 2, 3, 10, and 50 ng/g ACM). Grafts were implanted in mice peritoneum in situ for 1 week. Angiogenesis was quantified with CD31 and Factor VIII immunostaining using Simple PCI. Selected optimal VEGF concentration that induced maximum vascularization was then used in porcine bladder augmentation model. Implants were left in for 4 and 10 weeks. Three groups of six pigs each were implanted with ACM alone, HA-ACM, and HA-VEGF-ACM. Histological, immunohistochemical (Uroplakin III, alpha-SMA, Factor VIII), and immunofluorescence (CD31) analysis were performed to assess graft regenerative capacity and angiogenesis. In mouse model, statistically significant increase in microvascular density was demonstrated in the 2 ng/g ACM group. When this concentration was used in porcine model, recellularization increased significantly from weeks 4 to 10 in HA-VEGF-ACM, with progressive decrease in fibrosis. Significantly increased vascularization, coupled with increased urothelium and smooth muscle cell (SMC) regeneration, was observed in HA-VEGF grafts at week 10 in the center and periphery, compared with week 4. HA-VEGF grafts displayed highest in vivo epithelialization, neovascularization, and SMCs regeneration. A total of 2 ng/g tissue VEGF when incorporated with HA proved effective in stimulating robust graft recellularization and vascularization, coordinated with increased urothelial bladder development and SMC augmentation into bundles by week 10.

Monitoring angiogenesis in soft-tissue engineered constructs for calvarium bone regeneration: an in vivo longitudinal DCE-MRI study

Tissue engineering is a promising technique for bone repair and can overcome the major drawbacks of conventional autogenous bone grafting. In this in vivo longitudinal study, we proposed a new tissue-engineering paradigm: inserting a biological soft-tissue construct within the bone defect to enhance angiogenesis for improved bone regeneration. The construct acts as a resorbable scaffold to support desired angiogenesis and cellular activity and as a vector of vascular endothelial growth factor, known to promote both vessel and bone growth. Dynamic contrast- enhanced magnetic resonance imaging was performed to investigate and characterize angiogenesis necessary for bone formation following the proposed paradigm of inserting a VEGF-impregnated tissue-engineered construct within the critical-sized calvarial defect in the membranous parietal bone of the rabbit. Results show that a model-free quantitative approach, the normalized initial area under the curve metric, provides sensitive and reproducible measures of vascularity that is consistent with known temporal evolution of angiogenesis during bone regeneration.

In vivo imaging demonstrates a time-line for new vessel formation in islet transplantation

Vascularization of transplanted islets must be maintained to provide long-term graft function. In vivo assessment of new vessel formation in islet grafts has been poorly documented. The purpose of this study was to investigate whether neovascularization was detectable in vivo in a Feridex-labeled murine syngeneic subcapsular islet mass using DCE MRI over 180 days. Subcapsular transplants could be visualized at post-transplant days three, seven, 14, and 28 using T2-weighted MRI and at post-transplant day 180 by immunohistochemistry. Injection of the contrast agent gadolinium (Gd)-DTPA for DCE at three, seven, and 14 days showed increased signal in the transplant area consistent with new vessel formation. Areas under contrast enhancement curves suggested peak angiogenesis at 14 days. At 180 days, there was no observable change in signal intensity after contrast injection suggesting established vascularization or islet mass reduction. Immunohistochemistry confirmed MRI and DCE findings. These data suggest that islet angiogenesis occurs early after transplantation and is likely established after one month of transplantation. This study provides an in vivo time-line of neovascularization in subcapsular islet grafts. We anticipate that contrast extravasation captured by MRI may provide useful monitoring of graft angiogenesis if reproduced in a clinically relevant intraportal model.

Intracellular adenosine triphosphate delivery enhanced skin wound healing in rabbits

Small unilamellar lipid vesicles were used to encapsulate adenosine triphosphate (ATP-vesicles) for intracellular energy delivery. This technique was tested in full-thickness skin wounds in 16 adult rabbits. One ear was rendered ischemic by using a minimally invasive surgery. The other ear served as a normal control. Four circular full-thickness wounds were created on the ventral side of each ear. ATP-vesicles or saline was used and the wounds were covered with Tegaderm (3M, St. Paul, MN). Dressing was changed and digital photos were taken daily until all the wounds were healed. The mean healing times of ATP-vesicles-treated wounds were significantly shorter than that of saline-treated wounds on ischemic and nonischemic ears. Histologic study indicated better-developed granular tissue and reepithelialization in the ATP-vesicles-treated wounds. The wounds treated by ATP-vesicles exhibited extremely fast granular tissue growth. More CD31 positive cells were seen in the ATP-vesicles-treated wounds. This preliminary study shows that direct intracellular delivery of ATP can accelerate the healing process of skin wounds on ischemic and nonischemic rabbit ears. The extremely fast granular tissue growth was something never seen or reported in the past.

Feasibility study of a novel urinary bladder bioreactor

We have devised a bioreactor to simulate normal urinary bladder dynamics. The design permits a cell-seeded scaffold made from a modified porcine acellular matrix to be placed between 2 closed chambers filled with culture medium and be mechanically stimulated in a physiologically relevant manner. Specifically designed software increased hydrostatic pressure from 0 to 10 cm of water in a linear fashion in 1 chamber, resulting in mechanical stretch and strain on the scaffold. Pressure was increased over 55 min (filling) and then decreased to 0 over 10 s (voiding). Commercially available small intestinal submucosa scaffolds were used to test the mechanical capabilities of the bioreactor, and pressure waveforms were generated for up to 18 h. Scaffolds were seeded with bladder smooth muscle or urothelial cells and incubated in the bioreactor, which generated pressure waveforms for 6 h. Scaffold integrity was preserved as seen through Masson's trichrome staining. No obvious contamination of the system was noted. Hematoxylin and eosin staining showed presence of cells after incubation in the bioreactor, and immunohistochemistry and real-time reverse transcriptase polymerase chain reaction suggested continued cellular activity. Cellular orientation tended to be perpendicular to the applied pressure. Preliminary results suggest that our bioreactor is a suitable model for simulating normal physiological conditions of bladder cycling in an ex vivo system.

In vivo magnetic resonance imaging of vascularization in islet transplantation

To evaluate changes in neovascularization of transplanted islets in vivo, dynamic contrast (gadolinium) enhanced magnetic resonance imaging (MRI) was used. Both iron (Feridex)-labeled and unlabeled syngeneic murine subcapsular islet grafts were studied. Differences in dynamic contrast enhancement of islet grafts were quantified after gadolinium injection at post-transplant days 3 and 14. Normalized contrast concentrations at day 14 in transplanted islets were increased relative with that on day 3. Time to peak contrast enhancement was faster by 12 min at day 14 compared to day 3 islets (while kidney and muscle peak times remained the same). Areas under the curve for contrast concentration versus time plots were larger in 14-day relative to 3-day islet grafts. In conclusion, noninvasive assessment of neovascularization is achievable. In vivo dynamic contrast-enhanced MRI can be used to detect and quantify changes in vascularization following islet transplantation. This technique may be useful in developing pro-angiogenic strategies to improve the transplantation outcome in experimental and clinical settings.