Biologicals and Biomaterials for Corneal Regeneration and Vision Restoration in Limbal Stem Cell Deficiency

The mammalian cornea is decorated with stem cells bestowed with the life-long task of renewing the epithelium, provided they remain healthy, functional, and in sufficient numbers. If not, a debilitating disease known as limbal stem cell deficiency (LSCD) can develop causing blindness. Decades after the first stem cell (SC) therapy is devised to treat this condition, patients continue to suffer unacceptable failures. During this time, improvements to therapeutics have included identifying better markers to isolate robust SC populations and nurturing them on crudely modified biological or biomaterial scaffolds including human amniotic membrane, fibrin, and contact lenses, prior to their delivery. Researchers are now gathering information about the biomolecular and biomechanical properties of the corneal SC niche to decipher what biological and/or synthetic materials can be incorporated into these carriers. Advances in biomedical engineering including electrospinning and 3D bioprinting with surface functionalization and micropatterning, and self-assembly models, have generated a wealth of biocompatible, biodegradable, integrating scaffolds to choose from, some of which are being tested for their SC delivery capacity in the hope of improving clinical outcomes for patients with LSCD.

Contrast-enhanced Micro-CT 3D visualization of cell distribution in hydrated human cornea

Background

The cornea, a vital component of the human eye, plays a crucial role in maintaining visual clarity. Understanding its ultrastructural organization and cell distribution is fundamental for elucidating corneal physiology and pathology. This study comprehensively examines the microarchitecture of the hydrated human cornea using contrast-enhanced micro-computed tomography (micro-CT).

Method

Fresh human corneal specimens were carefully prepared and hydrated to mimic their in vivo state. Contrast enhancement with Lugol's iodine-enabled high-resolution Micro-CT imaging. The cells' three-dimensional (3D) distribution within the cornea was reconstructed and analyzed.

Results

The micro-CT imaging revealed exquisite details of the corneal ultrastructure, including the spatial arrangement of cells throughout its depth. This novel approach allowed for the visualization of cells' density and distribution in different corneal layers. Notably, our findings highlighted variations in cell distribution between non-hydrated and hydrated corneas.

Conclusions

This study demonstrates the potential of contrast-enhanced micro-CT as a valuable tool for non-destructive, 3D visualization and quantitative analysis of cell distribution in hydrated human corneas. These insights contribute to a better understanding of corneal physiology and may have implications for research in corneal diseases and tissue engineering.

Electro-compacted collagen for corneal epithelial tissue engineering

Bioengineered corneal substitutes offer a solution to the shortage of donor corneal tissue worldwide. As one of the major structural components of the cornea, collagen has shown great potential for tissue-engineered cornea substitutes. Herein, free-standing collagen membranes fabricated using electro-compaction were assessed in corneal bioengineering application by comparing them with nonelectro-compacted collagen (NECC). The well-organized and biomimetic fibril structure resulted in a significant improvement in mechanical properties. A 10-fold increase in tensile and compressive modulus was recorded when compared with NECC membranes. In addition to comparable transparency in the visible light range, the glucose permeability of the electro-compacted collagen (ECC) membrane is higher than that of the native human cornea. Human corneal epithelial cells adhere and proliferate well on the ECC membrane, with a large cell contact area observed. The as-described ECC has appropriate structural, topographic, mechanical, optical, glucose permeable, and cell support properties to provide a platform for a bioengineered cornea; including the outer corneal epithelium and potentially deeper corneal tissues.

The progress in techniques for culturing human limbal epithelial stem cells

In vitro culture of human limbal epithelial stem cells (hLESCs) is crucial to cell therapy in the treatment of limbal stem cell deficiency, a potentially vision-threatening disease that is characterized by persistent corneal epithelial defects and corneal epithelium conjunctivalization. Traditionally, hLESCs are cultivated based on either limbal tissue explants or single-cell suspensions in culture media containing xenogenous components, such as fetal bovine serum and murine 3T3 feeder cells. Plastic culture dishes and human amniotic membranes are classical growth substrates used in conventional hLESC culture systems. The past few decades have witnessed considerable progress and innovations in hLESC culture techniques to ensure a higher level of biosafety and lower immunogenicity for further cell treatment, including complete removal of xenogenous components from culture media, the application of human-derived feeder cells, and the development of novel scaffolds. Three-dimensional artificial niches and three-dimensional culture techniques have also been established to simulate the real microenvironment of limbal crypts for better cell outgrowth and proliferation. All these progresses ensure that in vitro cultured hLESCs are more adaptable to translational stem cell therapy for limbal stem cell deficiency.

Bio-fabrication of stem-cell-incorporated corneal epithelial and stromal equivalents from silk fibroin and gelatin-based biomaterial for canine corneal regeneration

Corneal grafts are the imperative clinical treatment for canine corneal blindness. To serve the growing demand, this study aimed to generate tissue-engineered canine cornea in part of the corneal epithelium and underlying stroma based on canine limbal epithelial stem cells (cLESCs) seeded silk fibroin/gelatin (SF/G) film and canine corneal stromal stem cells (cCSSCs) seeded SF/G scaffold, respectively. Both cell types were successfully isolated by collagenase I. SF/G corneal films and stromal scaffolds served as the prospective substrates for cLESCs and cCSSCs by promoting cell adhesion, cell viability, and cell proliferation. The results revealed the upregulation of tumor protein P63 (P63) and ATP-binding cassette super-family G member 2 (Abcg2) of cLESCs as well as Keratocan (Kera), Lumican (Lum), aldehyde dehydrogenase 3 family member A1 (Aldh3a1) and Aquaporin 1 (Aqp1) of differentiated keratocytes. Moreover, immunohistochemistry illustrated the positive staining of tumor protein P63 (P63), aldehyde dehydrogenase 3 family member A1 (Aldh3a1), lumican (Lum) and collagen I (Col-I), which are considerable for native cornea. This study manifested a feasible platform to construct tissue-engineered canine cornea for functional grafts and positively contributed to the body of knowledge related to canine corneal stem cells.

Core-Shell Pure Collagen Threads Extruded from Highly Concentrated Solutions Promote Colonization and Differentiation of C3H10T1/2 Cells

The elaboration of scaffolds able to efficiently promote cell differentiation toward a given cell type remains challenging. Here, we engineered dense type I collagen threads with the aim of providing scaffolds with specific morphological and mechanical properties for C3H10T1/2 mesenchymal stem cells. Extrusion of pure collagen solutions at different concentrations (15, 30, and 60 mg/mL) in a PBS 5× buffer generated dense fibrillated collagen threads. For the two highest concentrations, threads displayed a core-shell structure with a marked fibril orientation of the outer layer along the longitudinal axis of the threads. Young's modulus and ultimate tensile stress as high as 1 and 0.3 MPa, respectively, were obtained for the most concentrated collagen threads without addition of any cross-linkers. C3H10T1/2 cells oriented themselves with a mean angle of 15-24° with respect to the longitudinal axis of the threads. Cells penetrated the 30 mg/mL scaffolds but remained on the surface of the 60 mg/mL ones. After three weeks of culture, cells displayed strong expression of the tendon differentiation marker Tnmd, especially for the 30 mg/mL threads. These results suggest that both the morphological and mechanical characteristics of collagen threads are key factors in promoting C3H10T1/2 differentiation into tenocytes, offering promising levers to optimize tissue engineering scaffolds for tendon regeneration.

Development, structure, and bioengineering of the human corneal stroma: A review of collagen-based implants

Bio-engineering technologies are currently used to produce biomimetic artificial corneas that should present structural, chemical, optical, and biomechanical properties close to the native tissue. These properties are mainly supported by the corneal stroma which accounts for 90% of corneal thickness and is mainly made of collagen type I. The stromal collagen fibrils are arranged in lamellae that have a plywood-like organization. The fibril diameter is between 25 and 35 nm and the interfibrillar space about 57 nm. The number of lamellae in the central stroma is estimated to be 300. In the anterior part, their size is 10-40 μm. They appear to be larger in the posterior part of the stroma with a size of 60-120 μm. Their thicknesses also vary from 0.2 to 2.5 μm. During development, the acellular corneal stroma, which features a complex pattern of organization, serves as a scaffold for mesenchymal cells that invade and further produce the cellular stroma. Several pathways including Bmp4, Wnt/β-catenin, Notch, retinoic acid, and TGF-β, in addition to EFTFs including the mastering gene Pax-6, are involved in corneal development. Besides, retinoic acid and TGF- β seem to have a crucial role in the neural crest cell migration in the stroma. Several technologies can be used to produce artificial stroma. Taking advantage of the liquid-crystal properties of acid-soluble collagen, it is possible to produce transparent stroma-like matrices with native-like collagen I fibrils and plywood-like organization, where epithelial cells can adhere and proliferate. Other approaches include the use of recombinant collagen, cross-linkers, vitrification, plastically compressed collagen or magnetically aligned collagen, providing interesting optical and mechanical properties. These technologies can be classified according to collagen type and origin, presence of telopeptides and native-like fibrils, structure, and transparency. Collagen matrices feature transparency >80% for the appropriate 500-μm thickness. Non-collagenous matrices made of biopolymers including gelatin, silk, or fish scale have been developed which feature interesting properties but are less biomimetic. These bioengineered matrices still need to be colonized by stromal cells to fully reproduce the native stroma.

Remodeling of aligned fibrous extracellular matrix by encapsulated cells under mechanical stretching

Extracellular matrix (ECM) remodeling is essential for the development and functions of connective tissues (e.g., heart, muscle and the periodontal ligament), and entails the highly anisotropic response of cells and their organized ECM molecules to mechanical stimulation. However, the nature of how cells remodel their surrounding ECM under mechanical stimulation remains elusive. Here, we encapsulated human periodontal ligament stem cells (hPDLSCs) within an aligned rat collagen scaffold labeled with fluorescein isothiocyanate (FITC) and applied mechanical stimulation on the scaffold using magnetic stretching. Through tracking the FITC-labeled rat collagen scaffold and the newly secreted human type I collagen, we studied the effect of magnetic stretching on the mechanism of aligned ECM remodeling by the encapsulated cells. We found that the aligned topography combined with magnetic stretching could significantly promote initial ECM degradation and new ECM secretion: expression of matrix metalloproteinases 1 and 9 is increased markedly, and the elastic modulus of the stretched scaffold (75 kPa) is significantly higher than that of the random scaffold (50 kPa). The data support a model whereby the cells remodel their surrounding ECM under continuous stretching through degradation and then secretion of new ECM to integrate with the aligned ECM and maintain tissue function. Our study offers a valuable basis for future optimized design of biomaterial scaffolds for clinical translation. STATEMENT OF SIGNIFICANCE: Extracellular matrix (ECM) remodeling is essential for the development and functions of connective tissues. However, the nature of how cells remodel their surrounding aligned ECM under mechanical stimulation remains elusive. Herein, we developed a method to reveal the remodeling of aligned rat collagen scaffold by the encapsulated human periodontal ligament stem cells (hPDLSCs) using fluorescence imaging. We found that the aligned topography combined with magnetic stretching could significantly promote initial ECM degradation and new ECM secretion: the expression of matrix metalloproteinase 1 and 9 are significantly higher, and the elastic modulus increases from 50 kPa to 75 kPa as compared to the random collagen scaffold encapsulating hPDLSCs. Our study holds great potential in optimization of bio-scaffold design for clinical translation.

Origin of transparency in scattering biomimetic collagen materials

Living tissues, heterogeneous at the microscale, usually scatter light. Strong scattering is responsible for the whiteness of bones, teeth, and brain and is known to limit severely the performances of biomedical optical imaging. Transparency is also found within collagen-based extracellular tissues such as decalcified ivory, fish scales, or cornea. However, its physical origin is still poorly understood. Here, we unveil the presence of a gap of transparency in scattering fibrillar collagen matrices within a narrow range of concentration in the phase diagram. This precholesteric phase presents a three-dimensional (3D) orientational order biomimetic of that in natural tissues. By quantitatively studying the relation between the 3D fibrillar network and the optical and mechanical properties of the macroscopic matrices, we show that transparency results from structural partial order inhibiting light scattering, while preserving mechanical stability, stiffness, and nonlinearity. The striking similarities between synthetic and natural materials provide insights for better understanding the occurring transparency.

Augmented peripheral nerve regeneration through elastic nerve guidance conduits prepared using a porous PLCL membrane with a 3D printed collagen hydrogel

Longitudinally oriented, 3D printed collagen hydrogel-grafted elastic nerve guidance conduits to promote nerve regeneration in peripheral nerve defects.

Spatiotemporal variations of contact stress between liquid-crystal films and fibroblasts Guide cell fate and skin regeneration

The inductions of both the mechanical microenvironment on cell behaviour and the polymeric scaffold on tissue regeneration have been well-proved. This study is aimed to investigate the possibility of guiding cell fate and tissue regeneration by the spatiotemporal controlling of contact stress between matrix materials and cells and to elucidate the mechanisms underlying. A series liquid crystal polymers of cholesteryl-oligo(lactic acid) (CLA) and an amorphous polymer of poly(lactic acid) were used as the growth substrates for fibroblast and skin tissue regeneration. The cellular and animal experiments show that, in the initial stage of wound healing, the liquid crystal texture of CLA films can provide an induced stress for the formation of focal adhesions and the activation of integrin β1/AKT signal pathway, resulting in advanced phenotypic transformation of fibroblasts to myofibroblasts, promoted collagen secretion and fast wound filling. But the gradually weakening cellular contact stress, induced by the decreasing of liquid crystal domains of matrix polymer during degradation, triggers the apoptosis of fibroblasts and myofibroblasts, resulting in non-excessive collagen accumulation. Finally, the CLA groups exhibit no obvious scar formation, more regular cell arrangement and significantly lower type I collagen proportion in regenerated tissue than other groups. This study may inspire a new, effective and safe strategy for tissue regeneration.

A ZnO-curcumin nanocomposite embedded hybrid collagen scaffold for effective scarless skin regeneration in acute burn injury

Scar formation in severe burn injury is a major health concern. Herein, we developed a hybrid collagen scaffold with an incorporated ZnO-curcumin nanocomposite, which facilitates scarless wound healing. Biocompatibility and hemocompatibility studies unveiled that the hybrid scaffold is apt for in vivo wound healing studies. Histological and immunohistochemical analyses demonstrate that the hybrid scaffold accelerated scarless burn wound healing in albino rats owing to the ZnO-curcumin nanocomposite induced up-regulation of angiogenesis and TGF-β3 expression. The semi-quantitatively measured scar elevation index of the hybrid scaffold-treated animals is on a par with that of the unwounded or normal skin. The studies suggest that the prepared hybrid biomaterial could be a potential candidate for scarless healing in severe burn injuries.

Topotactic Fibrillogenesis of Freeze-Cast Microridged Collagen Scaffolds for 3D Cell Culture

Type I collagen is the main component of the extracellular matrix (ECM). In vitro, under a narrow window of physicochemical conditions, type I collagen self-assembles to form complex supramolecular architectures reminiscent of those found in native ECM. Presently, a major challenge in collagen-based biomaterials is to couple the delicate collagen fibrillogenesis events with a controlled shaping process in non-denaturating conditions. In this work, an ice-templating approach promoting the structuration of collagen into macroporous monoliths is used. Instead of common solvent removal procedures, a new topotactic conversion approach yielding self-assembled ordered fibrous materials is implemented. These collagen-only, non-cross-linked scaffolds exhibit uncommon mechanical properties in the wet state, with a Young's modulus of 33 ± 12 kPa, an ultimate tensile stress of 33 ± 6 kPa, and a strain at failure of 105 ± 28%. With the help of the ice-patterned microridge features, normal human dermal fibroblasts and C2C12 murine myoblasts successfully migrate and form highly aligned populations within the resulting three-dimensional (3D) collagen scaffolds. These results open a new pathway to the development of new tissue engineering scaffolds ordered across various organization levels from the molecule to the macropore and are of particular interest for biomedical applications where large-scale 3D cell alignment is needed such as for muscular or nerve reconstruction.

Interaction of gold nanosurfaces/nanoparticles with collagen-like peptides

Nanotechnology has quickly emerged as a promising research field with potential effects in disease treatments. For example, gold nanoparticles (AuNPs) have been extensively used in diagnostics and therapeutics. When administrated into human tissues, AuNPs first encounter extracellular matrix (ECM) molecules. Amongst all the ECM components, collagen is the main tension-resisting constituent, whose biofunctional and mechanical properties are strongly dependent on its hierarchical structure. Therefore, an in-depth understanding of the structural response of collagen to the presence of gold nanosurfaces (AuNS) and AuNPs is crucial in terms of clinical applications of AuNPs. However, detailed understanding of the molecular-level and atomic-level interaction between AuNS/AuNPs and collagen in the ECM is elusive. In this study, comprehensive molecular dynamics (MD) simulations have been performed to investigate the molecular behaviour of a collagen molecule segment (CMS) in the presence of AuNS/AuNPs in explicit water, aiming to explore the interaction of AuNS/AuNPs with collagen triple helices at the molecular and atomic levels. The results show that the CMS forms a rapid association with AuNS/AuNPs and undergoes a severe unfolding upon adsorption on AuNS/AuNPs, indicating an unfolding propensity of gold surfaces. We conclude that collagen triple helices unfold readily on AuNS and bare AuNPs, due to the interaction of gold surfaces with the protein backbone. The revealed clear unfolding nature and the unravelled atomic-level unfolding mechanism of collagen triple helices onto AuNPs contribute to the development of AuNPs for biomedical and therapeutic applications, and the design of gold-binding proteins.

Poly-l/dl-lactic acid films functionalized with collagen IV as carrier substrata for corneal epithelial stem cells

Limbal epithelial stem cells (LESCs) are responsible for the renewal of corneal epithelium. Cultivated limbal epithelial transplantation is the current treatment of choice for restoring the loss or dysfunction of LESCs. To perform this procedure, a substratum is necessary for in vitro culturing of limbal epithelial cells and their subsequent transplantation onto the ocular surface. In this work, we evaluated poly-L/DL-lactic acid 70:30 (PLA) films functionalized with type IV collagen (col IV) as potential in vitro carrier substrata for LESCs. We first demonstrated that PLA-col IV films were biocompatible and suitable for the proliferation of human corneal epithelial cells. Subsequently, limbal epithelial cell suspensions, isolated from human limbal rings, were cultivated using culture medium that did not contain animal components. The cells adhered significantly faster to PLA-col IV films than to tissue culture plastic (TCP). The mRNA expression levels for the LESC specific markers, K15, P63α and ABCG2 were similar or greater (significantly in the case of K15) in limbal epithelial cells cultured on PLA-col IV films than limbal epithelial cells cultured on TCP. The percentage of cells expressing the corneal (K3, K12) and the LESC (P63α, ABCG2) specific markers was similar for both substrata. These results suggest that the PLA-col IV films promoted LESC attachment and helped to maintain their undifferentiated stem cell phenotype. Consequently, these substrata offer an alternative for the transplantation of limbal cells onto the ocular surface.

Collagen-based materials combined with microRNA for repairing cornea wounds and inhibiting scar formation

AuNP/miR-133b can be released from cornea regeneration materials and entered into stromal cells to inhibit cornea scar formation.

Hierarchical Characterization and Nanomechanical Assessment of Biomimetic Scaffolds Mimicking Lamellar Bone via Atomic Force Microscopy Cantilever-Based Nanoindentation

The hierarchical structure of bone and intrinsic material properties of its two primary constituents, carbonated apatite and fibrillar collagen, when being synergistically organized into an interpenetrating hard-soft composite, contribute to its excellent mechanical properties. Lamellar bone is the predominant structural motif in mammalian hard tissues; therefore, we believe the fabrication of a collagen/apatite composite with a hierarchical structure that emulates bone, consisting of a dense lamellar microstructure and a mineralized collagen fibril nanostructure, is an important first step toward the goal of regenerative bone tissue engineering. In this work, we exploit the liquid crystalline properties of collagen to fabricate dense matrices that assemble with cholesteric organization. The matrices were crosslinked via carbodiimide chemistry to improve mechanical properties, and are subsequently mineralized via the polymer-induced liquid-precursor (PILP) process to promote intrafibrillar mineralization. Neither the crosslinking procedure nor the mineralization affected the cholesteric collagen microstructures; notably, there was a positive trend toward higher stiffness with increasing crosslink density when measured by cantilever-based atomic force microscopy (AFM) nanoindentation. In the dry state, the average moduli of moderately (X51; 4.8 ± 4.3 GPa) and highly (X76; 7.8 ± 6.7 GPa) crosslinked PILP-mineralized liquid crystalline collagen (LCC) scaffolds were higher than the average modulus of bovine bone (5.5 ± 5.6 GPa).