High-efficiency matrix modulus-induced cardiac differentiation of human mesenchymal stem cells inside a thermosensitive hydrogel

Mesenchymal stem cells (MSCs) experience an extremely low rate of cardiac differentiation after transplantation into infarcted hearts, in part due to the inability of stiff scar tissue to support differentiation. We hypothesized that delivering MSCs in a hydrogel with a modulus matched to that of native heart tissue should stimulate MSC differentiation into cardiac cells. We have developed a thermosensitive and injectable hydrogel suitable for the delivery of cells into the heart, and found that the appropriate gel modulus can differentiate MSCs into cardiac cells with high efficiency. The hydrogel was based on N-isopropylacrylamide, N-acryloxysuccinimide, acrylic acid and poly(trimethylene carbonate)-hydroxyethyl methacrylate. The hydrogel solution can be readily injected through needles commonly used for heart injection, and is capable of gelling within 7s at 37°C. The formed gels were highly flexible, with breaking strains (>300%) higher than that of native heart tissue and moduli within the range of native heart tissue (1-140kPa). Controlling the concentration of the hydrogel solution resulted in hydrogels with three different moduli: 16, 45 and 65kPa. The moduli were decoupled from the gel water content and oxygen diffusion, parameters that can also influence cell differentiation. MSCs survived in the hydrogels throughout the entire culture period, and it was observed that gel stiffness did not affect cell survival. After 14days of culture, more than 76% of MSCs had differentiated into cardiac cells in the 45 and 65kPa gels, as confirmed by the expression of cardiac markers at both the gene and protein levels. MSCs in the hydrogel with the 65kPa modulus had the highest differentiation efficiency. The differentiated cells also developed calcium channels that imparted an electrophysiological property, and gap junctions for cell-cell communication. The efficiency of differentiation reported in this study was much higher than for the differentiation approaches described in the literature, such as chemical induction and co-culture of MSCs and cardiomyocytes. These results indicate that the novel hydrogel holds great promise for delivering MSCs into an infarcted heart for the generation of new heart tissue.     

Tissue transglutaminase is essential for integrin-mediated survival of bone marrow-derived mesenchymal stem cells

Autologous mesenchymal stem cell (MSC) transplantation therapy for repair of myocardial injury has inherent limitations due to the poor viability of the stem cells after cell transplantation. Adhesion is a prerequisite for cell survival and also a key factor for the differentiation of MSCs. As a novel prosurvival modification strategy, we genetically engineered MSCs to overexpress tissue transglutaminase (tTG), with intention to enhance adhesion and ultimately cell survival after implantation. tTG-transfected MSCs (tTG-MSCs) showed a 2.7-fold and greater than a twofold increase of tTG expression and surface tTG activity, respectively, leading to a 20% increased adhesion of MSCs on fibronectin (Fn). Spreading and migration of tTG-MSCs were increased 4.75% and 2.52%, respectively. Adhesion of tTG-MSCs on cardiogel, a cardiac fibroblast-derived three-dimensional matrix, showed a 33.1% increase. Downregulation of tTG by transfection of small interfering RNA specific to the tTG resulted in markedly decreased adhesion and spread of MSCs on Fn or cardiogel. tTG-MSCs on Fn significantly increased phosphorylation of focal adhesion related kinases FAK, Src, and PI3K. tTG-MSCs showed significant retention in infarcted myocardium by forming a focal adhesion complex and developed into cardiac myocyte-like cells by the expression of cardiac-specific proteins. Transplantation of 1 x 10(6) MSCs transduced with tTG into the ischemic rat myocardium restored normalized systolic and diastolic cardiac function. tTG-MSCs further restored cardiac function of infarcted myocardium as compared with MSC transplantation alone. These findings suggested that tTG may play an important role in integrin-mediated adhesion of MSCs in implanted tissues. Disclosure of potential conflicts of interest is found at the end of this article.     

Cardiac differentiation of cardiosphere-derived cells in scaffolds mimicking morphology of the cardiac extracellular matrix

Stem cell therapy has the potential to regenerate heart tissue after myocardial infarction (MI). The regeneration is dependent upon cardiac differentiation of the delivered stem cells. We hypothesized that timing of the stem cell delivery determines the extent of cardiac differentiation as cell differentiation is dependent on matrix properties such as biomechanics, structure and morphology, and these properties in cardiac extracellular matrix (ECM) continuously vary with time after MI. In order to elucidate the relationship between ECM properties and cardiac differentiation, we created an in vitro model based on ECM-mimicking fibers and a type of cardiac progenitor cell, cardiosphere-derived cells (CDCs). A simultaneous fiber electrospinning and cell electrospraying technique was utilized to fabricate constructs. By blending a highly soft hydrogel with a relatively stiff polyurethane and modulating fabrication parameters, tissue constructs with similar cell adhesion property but different global modulus, single fiber modulus, fiber density and fiber alignment were achieved. The CDCs remained alive within the constructs during a 1 week culture period. CDC cardiac differentiation was dependent on the scaffold modulus, fiber volume fraction and fiber alignment. Two constructs with relatively low scaffold modulus, ∼50–60 kPa, most significantly directed the CDC differentiation into mature cardiomyocytes as evidenced by gene expressions of cardiac troponin T (cTnT), calcium channel (CACNA1c) and cardiac myosin heavy chain (MYH6), and protein expressions of cardiac troponin I (cTnI) and connexin 43 (CX43). Of these two low-modulus constructs, the extent of differentiation was greater for lower fiber alignment and higher fiber volume fraction. These results suggest that cardiac ECM properties may have an effect on cardiac differentiation of delivered stem cells.     

The effect of cyclic stretch on maturation and 3D tissue formation of human embryonic stem cell-derived cardiomyocytes

The goal of cardiac tissue engineering is to restore function to the damaged myocardium with regenerative constructs. Human embryonic stem cell–derived cardiomyocytes (hESC-CMs) can produce viable, contractile, three-dimensional grafts that function in vivo. We sought to enhance the viability and functional maturation of cardiac tissue constructs by cyclical stretch. hESC-CMs seeded onto gelatin-based scaffolds underwent cyclical stretching. Histological analysis demonstrated a greater proportion of cardiac troponin T–expressing cells in stretched than non-stretched constructs, and flow sorting demonstrated a higher proportion of cardiomyocytes. Ultrastructural assessment showed that cells in stretched constructs had a more mature phenotype, characterized by greater cell elongation, increased gap junction expression, and better contractile elements. Real-time PCR revealed enhanced mRNA expression of genes associated with cardiac maturation as well as genes encoding cardiac ion channels. Calcium imaging confirmed that stretched constructs contracted more frequently, with shorter calcium cycle duration. Epicardial implantation of constructs onto ischemic rat hearts demonstrated the feasibility of this platform, with enhanced survival and engraftment of transplanted cells in the stretched constructs. This uniaxial stretching system may serve as a platform for the production of cardiac tissue-engineered constructs for translational applications.     

In vitro cartilage tissue engineering with 3D porous aqueous-derived silk scaffolds and mesenchymal stem cells

Adult cartilage tissue has limited self-repair capacity, especially in the case of severe damages caused by developmental abnormalities, trauma, or aging-related degeneration like osteoarthritis. Adult mesenchymal stem cells (MSCs) have the potential to differentiate into cells of different lineages including bone, cartilage, and fat. In vitro cartilage tissue engineering using autologous MSCs and three-dimensional (3-D) porous scaffolds has the potential for the successful repair of severe cartilage damage. Ideally, scaffolds designed for cartilage tissue engineering should have optimal structural and mechanical properties, excellent biocompatibility, controlled degradation rate, and good handling characteristics. In the present work, a novel, highly porous silk scaffold was developed by an aqueous process according to these criteria and subsequently combined with MSCs for in vitro cartilage tissue engineering. Chondrogenesis of MSCs in the silk scaffold was evident by real-time RT-PCR analysis for cartilage-specific ECM gene markers, histological and immunohistochemical evaluations of cartilage-specific ECM components. Dexamethasone and TGF-beta3 were essential for the survival, proliferation and chondrogenesis of MSCs in the silk scaffolds. The attachment, proliferation, and differentiation of MSCs in the silk scaffold showed unique characteristics. After 3 weeks of cultivation, the spatial cell arrangement and the collagen type-II distribution in the MSCs-silk scaffold constructs resembles those in native articular cartilage tissue, suggesting promise for these novel 3-D degradable silk-based scaffolds in MSC-based cartilage repair. Further in vivo evaluation is necessary to fully recognize the clinical relevance of these observations.     

人脐带间充质干细胞分化为心肌细胞的实验研究

目的 探讨人脐带间充质干细胞(MSCs)在体外向心肌细胞分化的能力及移植后对急性心肌梗死大鼠心功能恢复的影响.方法 胶原酶胰酶消化法分离脐带MSCs.取第4-6代脐带干细胞,采用5-氮胞苷诱导,免疫组织化学和免疫荧光法对诱导后细胞进行鉴定.建立大鼠心肌梗死模型,并按完全随机法将其分为2组(n=10):细胞移植组和空白对照组.将培养脐带MSCs移植到大鼠梗死心肌周围,4周后,免疫荧光法鉴定移植细胞,并超声检测心功能改变.结果 体外诱导后,细胞的形态不断发生变化,诱导后的细胞表达心肌特异性α-肌动蛋白、肌球蛋白和肌钙蛋白T,阳性率在50%以上.细胞移植4周后,脐带MSCs在缺血心肌内存活并分化为心肌样细胞,心功能检测显示脐带MSCs移植组大鼠在移植后4周的左心室射血分数[(68.4±15.2)%]比对照组大鼠明显增加[(53.2±13.4)%,P＜0.05].结论 人脐带MSCs能够在体内外分化为心肌样细胞,并能促进心脏功能的恢复.     

The effect of autologous mesenchymal stem cells on the biomechanics and histology of gel-collagen sponge constructs used for rabbit patellar tendon repair

The objective of this study was to introduce mesenchymal stem cells (MSCs) into a gel-sponge composite and examine the effect the cells have on repair biomechanics and histology 12 weeks postsurgery. We tested two related hypotheses-adding MSCs would significantly improve repair biomechanics and cellular organization, and would result in higher failure forces than peak in vivo patellar tendon (PT) forces recorded for an inclined hopping activity. Autogenous tissue-engineered constructs were created by seeding MSCs from 15 adult rabbits at 0.1 x 10(6) cells/mL in 2.6 mg/mL of collagen gel in collagen sponges. Acellular constructs were created using the same concentration of collagen gel in matching collagen sponges. These cellular and acellular constructs were implanted in bilateral full-thickness, full-length defects in the central third of patellar tendons. At 12 weeks after surgery, repair tissues were assigned for biomechanical (n = 12 pairs) and histological (n = 3 pairs) analyses. Maximum force and maximum stress for the cellular repairs were about 60 and 50% of corresponding values for the normal central third of the PT, respectively. Likewise, linear stiffness and linear modulus for these cellular repairs averaged 75 and 30% of normal PT values, respectively. By contrast, the acellular repairs exhibited lower percentages of normal PT values for maximum force (40%), maximum stress (25%), linear stiffness (30%), and linear modulus (20%). Histologically, both repairs showed strong staining for collagen types III and V, fibronectin, and decorin. The cellular repairs also showed cellular alignment comparable to that of normal tendon. This study shows that introducing autogenous mesenchymal stem cells into a gel-collagen sponge composite significantly improves tendon repair compared to the use of a gel-sponge composite alone in the range of in vivo loading.     

Pluripotent stem cell-derived cardiac tissue patch with advanced structure and function

Recent advances in pluripotent stem cell research have provided investigators with potent sources of cardiogenic cells. However, tissue engineering methodologies to assemble cardiac progenitors into aligned, 3-dimensional (3D) myocardial tissues capable of physiologically relevant electrical conduction and force generation are lacking. In this study, we introduced 3D cell alignment cues in a fibrin-based hydrogel matrix to engineer highly functional cardiac tissues from genetically purified mouse embryonic stem cell-derived cardiomyocytes (CMs) and cardiovascular progenitors (CVPs). Procedures for CM and CVP derivation, purification, and functional differentiation in monolayer cultures were first optimized to yield robust intercellular coupling and maximize velocity of action potential propagation. A versatile soft-lithography technique was then applied to reproducibly fabricate engineered cardiac tissues with controllable size and 3D architecture. While purified CMs assembled into a functional 3D syncytium only when supplemented with supporting non-myocytes, purified CVPs differentiated into cardiomyocytes, smooth muscle, and endothelial cells, and autonomously supported the formation of functional cardiac tissues. After a total culture time similar to period of mouse embryonic development (21 days), the engineered cardiac tissues exhibited unprecedented levels of 3D organization and functional differentiation characteristic of native neonatal myocardium, including: 1) dense, uniformly aligned, highly differentiated and electromechanically coupled cardiomyocytes, 2) rapid action potential conduction with velocities between 22 and 25 cm/s, and 3) significant contractile forces of up to 2 mN. These results represent an important advancement in stem cell-based cardiac tissue engineering and provide the foundation for exploiting the exciting progress in pluripotent stem cell research in the future tissue engineering therapies for heart disease.     

Enhanced differentiation of mesenchymal stem cells co-cultured with ligament fibroblasts on gelatin/silk fibroin hybrid scaffold

The differentiation of mesenchymal stem cells (MSCs) towards fibroblasts is a crucial issue in ligament tissue engineering. This study aims to investigate the feasibility of using co-culture system to induce the differentiation of MSCs for constructing the tissue-engineered ligament in vitro. A kind of silk cable-reinforced gelatin/silk fibroin hybrid scaffold was used to provide three-dimensional (3-D) culture environments for MSCs. The 3-D co-culture system was set up by culturing MSCs/scaffold and ligament fibroblasts in the transwell insert and lower chamber, respectively. The regulatory effects of fibroblasts on MSCs were determined. After 2 weeks of co-culture the MSCs showed faster proliferation and higher DNA content compared with MSCs non-co-cultured. The MSCs were distributed uniformly throughout the scaffold and showed good viability. The collagen production also increased significantly with culture time. The MSCs in co-culture system were proved to differentiate into ligament fibroblasts by expressing ligament extra-cellular matrix (ECM)-specific genes including collagen I, collagen III, and tenascin-C in mRNA and protein level. The immunohistochemistry staining also confirmed the synthesis of key ligament ECM components. This study reveals that specific regulatory signals released from fibroblasts in 3-D co-culture system can enhance the differentiation of MSCs for ligament tissue engineering.     

A 3D aligned microfibrous myocardial tissue construct cultured under transient perfusion

The goal of this study was to design and develop a myocardial patch to use in the repair of myocardial infarctions or to slow down tissue damage and improve long-term heart function. The basic 3D construct design involved two biodegradable macroporous tubes, to allow transport of growth media to the cells within the construct, and cell seeded, aligned fiber mats wrapped around them. The microfibrous mat housed mesenchymal stem cells (MSCs) from human umbilical cord matrix (Wharton's Jelly) aligned in parallel to each other in a similar way to cell organization in native myocardium. Aligned micron-sized fiber mats were obtained by electrospinning a polyester blend (PHBV (5% HV), P(L-D,L)LA (70:30) and poly(glycerol sebacate) (PGS)). The micron-sized electrospun parallel fibers were effective in Wharton's Jelly (WJ) MSCs alignment and the cells were able to retract the mat. The 3D construct was cultured in a microbioreactor by perfusing the growth media transiently through the macroporous tubing for two weeks and examined by fluorescence microscopy for cell distribution and preservation of alignment. The fluorescence images of thin sections of 3D constructs from static and perfused cultures confirmed enhanced cell viability, uniform cell distribution and alignment due to nutrient provision from inside the 3D structure.     

低氧对MSC成心肌细胞中干细胞存活率的影响及对抗策略与原理

间充质干细胞（mesenchymal stem cells,MSCs）是一种具多向分化潜能的成体干细胞,具备修复心肌组织的能力。国内外学者做出了大量的努力,希望能够将干细胞移植治疗心脏疾病用于临床。但干细胞在移植后可发生大量的死亡,对治疗效果产生了重大影响。低氧是影响干细胞存活力较为重要的因素。低氧能够通过线粒体途径诱导MSC凋亡,使MSC的长期存活力下降。而抗凋亡途径（如磷脂酰肌醇3 激酶/AKT信号通路、核因子 κB途径)和抗凋亡蛋白（如Bcl 2相关蛋白、热休克蛋白、低氧诱导蛋白 1等）能够对抗低氧引起的细胞凋亡。利用这一原理,国内外学者采用了基因工程、预适应、药物联合治疗等策略以提高MSCs的长期存活率。     

心肌条件培养液与5-氮胞苷诱导骨髓基质干细胞向心肌样细胞的分化

背景：骨髓基质干细胞可以明显改善心肌缺血时的心脏功能,但其直接分化为心肌细胞并参与心功能恢复的机制尚不明确。 目的：探讨心肌条件培养液与5-氮胞苷诱导骨髓基质干细胞分化为心肌样细胞的作用。 方法：全骨髓贴壁法分离培养骨髓基质干细胞,将第6代骨髓基质干细胞随机分为4组：5-氮胞苷+心肌条件培养液联合诱导组;5-氮胞苷组;心肌条件培养液组;基础培养基组(空白组)。用心肌条件培养液和5-氮胞苷诱导3周后用免疫组化法测定各组心肌肌钙蛋白表达,并采用单因素方差分析检验进行统计学分析。 结果与结论：在体外心肌条件培养液可以诱导骨髓基质干细胞向心肌细胞分化,提示其中细胞因子可能起关键作用,但心肌条件培养液的这种作用要弱于化学诱导剂5-氮胞苷的诱导作用,且联合诱导作用更强。     

Tenogenesis of bone marrow-, adipose-, and tendon-derived stem cells in a dynamic bioreactor

ABSTRACT

Tendons are frequently damaged and fail to regenerate, leading to pain, loss of function, and reduced quality of life. Mesenchymal stem cells (MSCs) possess clinically useful tissue-regenerative properties and have been exploited for use in tendon tissue engineering and cell therapy. However, MSCs exhibit phenotypic heterogeneity based on the donor tissue used, and the efficacy of cell-based treatment modalities may be improved by optimizing cell source based on relative differentiation capacity. Equine MSCs were isolated from bone marrow (BM), adipose (AD), and tendon (TN), expanded in monolayer prior to seeding on decellularized tendon scaffolds (DTS), and cell-laden constructs were placed in a bioreactor designed to mimic the biophysical environment of the tendon. It was hypothesized that TN MSCs would differentiate toward a tendon cell phenotype better than BM and AD MSCs in response to a conditioning period involving cyclic mechanical stimulation for 1 hour per day at 3% strain and 0.33 Hz. All cell types integrated into DTS adopted an elongated morphology similar to tenocytes, expressed tendon marker genes, and improved tissue mechanical properties after 11 days. TN MSCs expressed the greatest levels of scleraxis, collagen type-I, and cartilage oligomeric matrix protein. Major histocompatibility class-II protein mRNA expression was not detected in any of the MSC types, suggesting low immunogenicity for allogeneic transplantation. The results suggest that TN MSCs are the ideal cell type for regenerative medicine therapies for tendinopathies, exhibiting the most mature tendon-like phenotype in vitro. When TN MSCs are unavailable, BM or AD MSCs may serve as robust alternatives.     

Monocyte chemotactic protein-3 is a myocardial mesenchymal stem cell homing factor

MSCs have received attention for their therapeutic potential in a number of disease states, including bone formation, diabetes, stem cell engraftment after marrow transplantation, graft-versus-host disease, and heart failure. Despite this diverse interest, the molecular signals regulating MSC trafficking to sites of injury are unclear. MSCs are known to transiently home to the freshly infarcted myocardium. To identify MSC homing factors, we determined chemokine expression pattern as a function of time after myocardial infarction (MI). We merged these profiles with chemokine receptors expressed on MSCs but not cardiac fibroblasts, which do not home after MI. This analysis identified monocyte chemotactic protein-3 (MCP-3) as a potential MSC homing factor. Overexpression of MCP-3 1 month after MI restored MSC homing to the heart. After serial infusions of MSCs, cardiac function improved in MCP-3-expressing hearts (88.7%, p < .001) but not in control hearts (8.6%, p = .47). MSC engraftment was not associated with differentiation into cardiac myocytes. Rather, MSC engraftment appeared to result in recruitment of myofibroblasts and remodeling of the collagen matrix. These data indicate that MCP-3 is an MSC homing factor; local overexpression of MCP-3 recruits MSCs to sites of injured tissue and improves cardiac remodeling independent of cardiac myocyte regeneration.     

Regulatory Effects of Mechanical Strain on the Chondrogenic Differentiation of MSCs in a Collagen-GAG Scaffold: Experimental and Computational Analysis

The effective treatment of cartilage defects by tissue engineering requires an improved understanding of the effect of mechanical forces on cell differentiation within three-dimensional (3D) matrices. The objective of this study was to investigate the effects of mechanical constraint and cyclic tensile strain on the chondrogenic differentiation of mesenchymal stem cells (MSCs) in a 3D collagen type I-glycosaminoglycan (GAG) scaffold. A multi-station uniaxial stretching bioreactor was fabricated to facilitate application of cyclic strain to the constructs cultured in a chondrogenic medium. Mechanical constraint, created by uniaxial clamping, prevented the cell-mediated contraction of the scaffolds and resulted in a reduction in the rate of GAG synthesis as measured by [³⁵S] sulfate incorporation relative to unconstrained controls. However, the rate of GAG synthesis was increased following application of continuous 10% cyclic tensile loading at 1 Hz for 7 days. A poroelastic finite element analysis of the 3D scaffold computed a maximum fluid flow of 19 μm/s and maximum principal strains of 8% under 10% stretch suggesting these magnitudes were sufficient to mechano-regulate the chondrogenic differentiation process. Presented, in part, at the American Society of Mechanical Engineers Summer Bioengineering Conference, Florida, June 2006, and the Orthopaedic Research Society, San Diego, February 2007.