Minwook Kim

High Mesenchymal Stem Cell Seeding Densities in Hyaluronic Acid Hydrogels Produce Engineered Cartilage with Native Tissue Properties

Engineered cartilage based on adult mesenchymal stem cells (MSCs) is an alluring goal for the repair of articular defects. However, efforts to date have failed to generate constructs with sufficient mechanical properties to function in the demanding environment of the joint. Our findings with a novel photocrosslinked hyaluronic acid (HA) hydrogel suggest that stiff gels (high HA concentration, 5% w/v) foster chondrogenic differentiation and matrix production, but limit overall functional maturation due to the inability of the formed matrix to diffuse away from the point of production and form a contiguous network. In the current study, we hypothesized that increasing the MSC seeding density would decrease the required diffusional distance, and so expedite the development of functional properties. To test this hypothesis bovine MSCs were encapsulated at seeding densities of either 20,000,000 or 60,000,000 cells ml(-1) in 1%, 3%, and 5% (w/v) HA hydrogels. Counter to our hypothesis the higher concentration HA gels (3% and 5%) did not develop more rapidly with increased MSC seeding density. However, the biomechanical properties of the low concentration (1%) HA constructs increased markedly (nearly 3-fold with a 3-fold increase in seeding density). To ensure that optimal nutrient access was delivered, we next cultured these constructs under dynamic culture conditions (with orbital shaking) for 9 weeks. Under these conditions 1% HA seeded at 60,000,000 MSCs ml(-1) reached a compressive modulus in excess of 1 MPa (compared with 0.3-0.4 MPa for free swelling constructs). This is the highest level we have reported to date in this HA hydrogel system, and represents a significant advance towards functional stem cell-based tissue engineered cartilage.

Transient exposure to TGF-β3 improves the functional chondrogenesis of MSC-laden hyaluronic acid hydrogels

Tissue engineering with adult stem cells is a promising approach for the restoration of focal defects in articular cartilage. For this, progenitor cells would ideally be delivered to (and maintained within) the defect site via a biocompatible material and in combination with soluble factors to promote initial cell differentiation and subsequent tissue maturation in vivo. While growth factor delivery methods are continually being optimized, most offer only a short (days to weeks) delivery profile at high doses. To address this issue, we investigated mesenchymal stem cell (MSC) differentiation and maturation in photocrosslinkable hyaluronic acid (HA) hydrogels with transient exposure to the pro-chondrogenic molecule transforming growth factor-beta3 (TGF-β3), at varying doses (10, 50 and 100 ng/mL) and durations (3, 7, 21 and 63 days). Mechanical, biochemical, and histological outcomes were evaluated through 9 weeks of culture. Results showed that a brief exposure (7 days) to a very high level (100 ng/mL) of TGF-β3 was sufficient to both induce and maintain cartilage formation in these 3D constructs. Indeed, this short delivery resulted in constructs with mechanical and biochemical properties that exceeded that of continuous exposure to a lower level (10 ng/mL) of TGF-β3 over the entire 9-week time course. Of important note, the total TGF delivery in these two scenarios was roughly equivalent (200 vs. 180 ng), but the timing of delivery differed markedly. These data support the idea that acute exposure to a high dose of TGF will induce functional and long-term differentiation of stem cell populations, and further our efforts to improve cartilage repair in vivo.

Material and mechanical properties of bones deficient for fibrillin-1 or fibrillin-2 microfibrils.

The contribution of non-collagenous components of the extracellular matrix to bone strength is largely undefined. Here we report that deficiency of fibrillin-1 or fibrillin-2 microfibrils causes distinct changes in bone material and mechanical properties. Morphometric examination of mice with hypomorphic or null mutations in fibrillin-1 or fibrillin-2, respectively, revealed appreciable differences in the postnatal shaping and growth of long bones. Fourier transform infrared imaging spectroscopy indicated that fibrillin-1 plays a predominantly greater role than fibrillin-2 in determining the material properties of bones. Biomechanical tests demonstrated that fibrillin-2 exerts a greater positive influence on the mechanical properties of bone than fibrillin-1 assemblies. Published evidence indirectly supports the notion that the above findings are mostly, if not exclusively, related to the differential control of TGFβ family signaling by fibrillin proteins. Our study therefore advances our understanding of the role that extracellular microfibrils play in bone physiology and implicitly, in the pathogenesis of bone loss in human diseases caused by mutations in fibrillin-1 or -2.

Effects of Mesenchymal Stem Cell and Growth Factor Delivery on Cartilage Repair in a Mini-Pig Model

Objective We have recently shown that mesenchymal stem cells (MSCs) embedded in a hyaluronic acid (HA) hydrogel and exposed to chondrogenic factors (transforming growth factor–β3 [TGF-β3]) produce a cartilage-like tissue in vitro. The current objective was to determine if these same factors could be combined immediately prior to implantation to induce a superior healing response in vivo relative to the hydrogel alone. Design Trochlear chondral defects were created in Yucatan mini-pigs (6 months old). Treatment groups included an HA hydrogel alone and hydrogels containing allogeneic MSCs, TGF-β3, or both. Six weeks after surgery, micro-computed tomography was used to quantitatively assess defect fill and subchondral bone remodeling. The quality of cartilage repair was assessed using the ICRS-II histological scoring system and immunohistochemistry for type II collagen. Results Treatment with TGF-β3 led to a marked increase in positive staining for collagen type II within defects (P < 0.05), while delivery of MSCs did not (P > 0.05). Neither condition had an impact on other histological semiquantitative scores (P > 0.05), and inclusion of MSCs led to significantly less defect fill (P < 0.05). For all measurements, no synergistic interaction was found between TGF-β3 and MSC treatment when they were delivered together (P > 0.05). Conclusions At this early healing time point, treatment with TGF-β3 promoted the formation of collagen type II within the defect, while allogeneic MSCs had little benefit. Combination of TGF-β3 and MSCs at the time of surgery did not produce a synergistic effect. An in vitro precultured construct made of these components may be required to enhance in vivo repair in this model system.

Characterization of a Cartilage-Like Engineered Biomass Using a Self-Aggregating Suspension Culture Model: Molecular Composition Using FT-IRIS

Maintenance of chondrocyte phenotype and robust expression and organization of macromolecular components with suitable cartilaginous properties is an ultimate goal in cartilage tissue engineering. We used a self‐aggregating suspension culture (SASC) method to produce an engineered cartilage, “cartilage tissue analog” (CTA). With an objective of understanding the stability of phenotype of the CTA over long periods, we cultured chondrocytes up to 4 years and analyzed the matrix. Both early (eCTAs) (6 months) and aged (aCTAs) (4 years) showed type II collagen throughout with higher concentrations near the edge. Using Fourier transform‐infrared imaging spectroscopy (FT‐IRIS), proteoglycan/collagen ratio of eCTA was 2.8 times greater than native cartilage at 1 week, but the ratio was balanced to native level (p = 0.017) by 36 weeks. Surprisingly, aCTAs maintained the hyaline characteristics, but there was evidence of calcification within the tissue with a distinct range of intensities. Mineral/matrix ratio of those aCTA with “intensive” calcification was significantly higher (p = 0.017) than the “partial,” but when compared to native bone the ratio of “intensive” aCTAs was 2.4 times lower. In this study we utilized the imaging approach of FT‐IRIS and have shown that a biomaterial formed is compositionally closely related to natural cartilage for long periods in culture. We show that this culture platform can maintain a CTA for extended periods of time (4 years) and under those conditions signs of mineralization can be found. This method of cartilage tissue engineering is a promising method to generate cartilaginous biomaterial and may have potential to be utilized in both cartilage and boney repairs.

Brief mechanical ventilation impacts airway cartilage properties in neonatal lambs

Ultrasound imaging allows in vivo assessment of tracheal kinetics and cartilage structure. To date, the impact of mechanical ventilation (MV) on extracellular matrix (ECM) in airway cartilage is unclear, but an indication of its functional and structural change may support the development of protective therapies. The objective of this study was to characterize changes in mechanical properties of the neonatal airway during MV with alterations in cartilage ECM. Trachea segments were isolated in a neonatal lamb model; ultrasound dimensions and pressure–volume relationships were measured on sham (no MV; n = 6) and MV (n = 7) airways for 4 hr. Tracheal cross‐sections were harvested at 4 hr, tissues were fixed and stained, and Fourier transform infrared imaging spectroscopy (FT‐IRIS) was performed. Over 4 hr of MV, bulk modulus (28%) and elastic modulus (282%) increased. The MV tracheae showed higher collagen, proteoglycan content, and collagen integrity (new tissue formation); whereas no changes were seen in the controls. These data are clinically relevant in that airway properties can be correlated with MV and changes in cartilage ECM. MV increases the in vivo dimensions of the trachea and is associated with evidence of airway tissue remodeling. Injury to the neonatal airway from MV may have relevance for the development of tracheomalacia. We demonstrated active airway tissue remodeling during MV using an FT‐IRIS technique which identifies changes in ECM. Pediatr Pulmonol. 2012. 47:763–770.

Enhanced nutrient transport improves the depth-dependent properties of tri-layered engineered cartilage constructs with zonal co-culture of chondrocytes and MSCs.

Biomimetic design in cartilage tissue engineering is a challenge given the complexity of the native tissue. While numerous studies have generated constructs with near-native bulk properties, recapitulating the depth-dependent features of native tissue remains a challenge. Furthermore, limitations in nutrient transport and matrix accumulation in engineered constructs hinders maturation within the central core of large constructs. To overcome these limitations, we fabricated tri-layered constructs that recapitulate the depth-dependent cellular organization and functional properties of native tissue using zonally derived chondrocytes co-cultured with MSCs. We also introduced porous hollow fibers (HFs) and HFs/cotton threads to enhance nutrient transport. Our results showed that tri-layered constructs with depth-dependent organization and properties could be fabricated. The addition of HFs or HFs/threads improved matrix accumulation in the central core region. With HF/threads, the local modulus in the deep region of tri-layered constructs nearly matched that of native tissue, though the properties in the central regions remained lower. These constructs reproduced the zonal organization and depth-dependent properties of native tissue, and demonstrate that a layer-by-layer fabrication scheme holds promise for the biomimetic repair of focal cartilage defects. STATEMENT OF SIGNIFICANCE Articular cartilage is a highly organized tissue driven by zonal heterogeneity of cells, extracellular matrix proteins and fibril orientations, resulting in depth-dependent mechanical properties. Therefore, the recapitulation of the functional properties of native cartilage in a tissue engineered construct requires such a biomimetic design of the morphological organization, and this has remained a challenge in cartilage tissue engineering. This study demonstrates that a layer-by-layer fabrication scheme, including co-cultures of zone-specific articular CHs and MSCs, can reproduce the depth-dependent characteristics and mechanical properties of native cartilage while minimizing the need for large numbers of chondrocytes. In addition, introduction of a porous hollow fiber (combined with a cotton thread) enhanced nutrient transport and depth-dependent properties of the tri-layered construct. Such a tri-layered construct may provide critical advantages for focal cartilage repair. These constructs hold promise for restoring native tissue structure and function, and may be beneficial in terms of zone-to-zone integration with adjacent host tissue and providing more appropriate strain transfer after implantation.

Optimization of macromer density in human MSC-laden hyaluronic acid (HA) hydrogels

Mesenchymal stem cells are attractive cell type and can undergo chondrogenesis in various 3D platforms. Hyaluronic acid (HA) hydrogel, a natural constituent of the cartilage extracellular matrix, provides a biologically relevant interface for encapsulated cells. While MSC-laden HA constructs can produce native mechanical properties using cells from animal sources, clinical repair will depend on successful translation of these findings to human MSCs (hMSCs). To optimize chondrogenesis, we assessed the ability of hMSCs to undergo chondrogenesis in varying macromer concentration HA gels. Variation in this parameter influenced construct mechanical and biochemical properties. In 1% methacrylated HA (MeHA), equilibrium modulus and GAG content were higher (86kPa (EY) and 2.16%) than in 2% MeHA constructs (50 kPa, 1.62% GAG). However, greater contractility occurred in 1% MeHA (-36.25%/-24.25%; Thickness/diameter) compared to 2% MeHA (-20.57%/1.02%) constructs. This study provides new insight into optimized macromer densities for hMSC-based cartilage tissue engineering using HA hydrogels.

Influence of Chondrocyte Zone on Co-Cultures With Mesenchymal Stem Cells in HA Hydrogels for Cartilage Tissue Engineering

Mesenchymal stem cells (MSCs) are an attractive cell type for cartilage tissue engineering in that they can undergo chondrogenesis in a variety of 3D contexts [1]. Focused efforts in MSC-based cartilage tissue engineering have recently culminated in the formation of biologic materials possessing biochemical and functional mechanical properties that match that of the native tissue [2]. These approaches generally involve the continuous or intermittent application of pro-chondrogenic growth factors during in vitro culture. For example, in one recent study, we showed robust construct maturation in MSC-seeded hyaluronic acid (HA) hydrogels transiently exposed to high levels of TGF-β3 [3]. Despite the promise of this approach, MSCs are a multipotent cell type and retain a predilection towards hypertrophic phenotypic conversion (i.e., bone formation) when removed from a pro-chondrogenic environment (e.g., in vivo implantation). Indeed, even in a chondrogenic environment, many MSC-based cultures express pre-hypertrophic markers, including type X collagen, MMP13, and alkaline phosphatase [4]. To address this issue, recent studies have investigated co-culture of human articular chondrocytes and MSCs in both pellet and hydrogel environments. Chondrocytes appear to enhance the initial efficiency of MSC chondrogenic conversion, as well as limit hypertrophic changes in some instances (potentially via secretion of PTHrP and/or other factors) [5–7]. While these findings are intriguing, articular cartilage has a unique depth-dependent morphology including zonal differences in chondrocyte identity. Ng et al. showed that zonal chondrocytes seeded in a bi-layered agarose hydrogel construct can recreate depth-dependent cellular and mechanical heterogeneity, suggesting that these identities are retained with transfer to 3D culture systems [8]. Further, Cheng et al. showed that differences in matrix accumulation and hypertrophy in zonal chondrocytes was controlled by bone morphogenic protein [9]. To determine whether differences in zonal chondrocyte identity influences MSC fate decisions, we evaluated functional properties and phenotypic stability in photocrosslinked hyaluronic acid (HA) hydrogels using distinct, zonal chondrocyte cell fractions co-cultured with bone marrow derived MSCs.Copyright

Role of dexamethasone in the long-term functional maturation of MSC-laden hyaluronic acid hydrogels for cartilage tissue engineering: ROLE OF DEXAMETHASONE IN MSCs

The purpose of study was to investigate the maturation of mesenchymal stem cells (MSC) laden in HA constructs with various combinations of chemically defined medium (CM) components and determine the impact of dexamethasone and serum on construct properties. Constructs were cultured in CM with the addition or withdrawal of media components or were transferred to serum containing media that partially represents an in vivo‐like condition where pro‐inflammatory signals are present. Constructs cultured in CM+ (CM with TGF‐β3) and DEX− (CM+ without dexamethasone) conditions produced robust matrix, while those in ITS/BSA/LA− (CM+ without ITS/BSA/LA) and Serum+ (10% FBS with TGF‐β3) produced little matrix. While construct properties in DEX− were greater than those in CM+ at 4 weeks, properties in CM+ and DEX− reversed by 8 weeks. While construct properties in DEX− were greater than those in CM+ at 4 weeks, the continued absence or removal of dexamethasone resulted in marked GAG loss by 8 weeks. Conversely, the continued presence or new addition of dexamethasone at 4 weeks further improved or maintained construct properties through 8 weeks. Finally, when constructs were converted to Serum (in the continued presence of TGF‐β3 with or without dexamethasone) after pre‐culture in CM+ for 4 weeks, GAG loss was attenuated with addition of dexamethasone. Interestingly, however, collagen content and type was not impacted. In conclusion, dexamethasone influences the functional maturation of MSC‐laden HA constructs, and may help to maintain properties during long‐term culture or with in vivo translation by repressing pro‐inflammatory signals.