Ex vivo organ culture of adipose tissue for in situ mobilization of adipose-derived stem cells and defining the stem cell niche

Generation of functional fat organoid from rat superficial fascia

The organoid is a 3D cell architecture formed by self-organized tissues or cells in vitro with similar cell types, histological structures, and biological functions of the native organ. Depending on the unique organ structures and cell types, producing organoids requires individualized design and is still challenging. Organoids of some tissues, including adipose tissue, remain to generate to be more faithful to their original organ in structure and function. We previously established a new model of the origin of adipose cells originating from non-adipose fascia tissue. Here, we investigated superficial fascia fragments in 3D hydrogel and found they were able to transform into relatively large adipocyte aggregates containing mature unilocular adipocytes, which were virtually "fat organoids". Such fascia-originated fat organoids had a typical structure of adipose tissues and possessed the principal function of adipose cells in the synthesis, storage, hydrolysis of triglycerides and adipokines secretion. Producing fat organoids from superficial fascia can provide a new approach for adipocyte research and strongly evidences that both adipose tissues and cells originate from fascia. Our findings give insights into metabolic regulation by the crosstalk between different organs and tissues and provide new knowledge for investigating novel treatments for obesity, diabetes and other metabolic diseases.Abbreviations: 3D: three dimensional; ASC: adipose-derived stromal cells; C/EBP: CCAAT-enhancer-binding protein; EdU: 5-ethynyl-2-deoxyuridine; FABP4: fatty acid-binding protein 4; FAS: fatty acid synthase; FSCs: fascia-derived stromal cells; Plin1: perilipin-1; Plin2: perilipin-2; PPARγ: peroxisome proliferator-activated receptor γ; WAT: white adipose tissue.

Probing Insulin Sensitivity with Metabolically Competent Human Stem Cell-Derived White Adipose Tissue Microphysiological Systems

Impaired white adipose tissue (WAT) function has been recognized as a critical early event in obesity-driven disorders, but high buoyancy, fragility, and heterogeneity of primary adipocytes have largely prevented their use in drug discovery efforts highlighting the need for human stem cell-based approaches. Here, human stem cells are utilized to derive metabolically functional 3D adipose tissue (iADIPO) in a microphysiological system (MPS). Surprisingly, previously reported WAT differentiation approaches create insulin resistant WAT ill-suited for type-2 diabetes mellitus drug discovery. Using three independent insulin sensitivity assays, i.e., glucose and fatty acid uptake and suppression of lipolysis, as the functional readouts new differentiation conditions yielding hormonally responsive iADIPO are derived. Through concomitant optimization of an iADIPO-MPS, it is abled to obtain WAT with more unilocular and significantly larger (≈40%) lipid droplets compared to iADIPO in 2D culture, increased insulin responsiveness of glucose uptake (≈2-3 fold), fatty acid uptake (≈3-6 fold), and ≈40% suppressing of stimulated lipolysis giving a dynamic range that is competent to current in vivo and ex vivo models, allowing to identify both insulin sensitizers and desensitizers.

A 3D human adipose tissue model within a microfluidic device

An accurate in vitro model of human adipose tissue could assist in the study of adipocyte function and allow for better tools for screening new therapeutic compounds. Cell culture models on two-dimensional surfaces fall short of mimicking the three-dimensional in vivo adipose environment, while three-dimensional culture models are often unable to support long-term cell culture due, in part, to insufficient mass transport. Microfluidic systems have been explored for adipose tissue models. However, current systems have primarily focused on 2D cultured adipocytes. In this work, a 3D human adipose microtissue was engineered within a microfluidic system. Human adipose-derived stem cells (ADSCs) were used as the cell source for generating differentiated adipocytes. The ADSCs differentiated within the microfluidic system formed a dense lipid-loaded mass with the expression of adipose tissue genetic markers. Engineered adipose tissue showed a decreased adiponectin secretion and increased free fatty acid secretion with increasing shear stress. Adipogenesis markers were downregulated with increasing shear stress. Overall, this microfluidic system enables the on-chip differentiation and development of a functional 3D human adipose microtissue supported by the interstitial flow. This system could potentially serve as a platform for in vitro drug testing for adipose tissue-related diseases.

Controlling Differentiation of Stem Cells for Developing Personalized Organ-on-Chip Platforms

Organ-on-chip (OOC) platforms have attracted attentions of pharmaceutical companies as powerful tools for screening of existing drugs and development of new drug candidates. OOCs have primarily used human cell lines or primary cells to develop biomimetic tissue models. However, the ability of human stem cells in unlimited self-renewal and differentiation into multiple lineages has made them attractive for OOCs. The microfluidic technology has enabled precise control of stem cell differentiation using soluble factors, biophysical cues, and electromagnetic signals. This study discusses different tissue- and organ-on-chip platforms (i.e., skin, brain, blood-brain barrier, bone marrow, heart, liver, lung, tumor, and vascular), with an emphasis on the critical role of stem cells in the synthesis of complex tissues. This study further recaps the design, fabrication, high-throughput performance, and improved functionality of stem-cell-based OOCs, technical challenges, obstacles against implementing their potential applications, and future perspectives related to different experimental platforms.

Update on the mechanisms of homing of adipose tissue-derived stem cells

Adipose tissue-derived stem cells (ADSCs), which resemble bone marrow mesenchymal stromal cells (BMSCs), have shown great advantages and promise in the field of regenerative medicine. They can be readily harvested in large numbers with low donor-site morbidity. To date, a great number of preclinical and clinical studies have shown ADSCs' safety and efficacy in regenerative medicine. However, a better understanding of the mechanisms of homing of ADSCs is needed to advance the clinical utility of this therapy. In this review, the reports of the homing of ADSCs were searched using Pubmed and Google Scholar to update our knowledge. ADSCs were proved to interact with endothelial cells by expressing the similar integrins with BMSCs. In addition, ADSCs do not possess the dominant ligand for P-selectin, just like BMSCs. Stromal derived factor-1 (SDF-1)/CXCR4 and CXC ligand-5 (CXCL5)/CXCR2 interactions are the two main axes governing ADSCs extravasation from bone marrow vessels. Some more signaling pathways involved in migration of ADSCs have been investigated, including LPA/LPA1 signaling pathway, MAPK/Erk1/2 signaling pathway, RhoA/Rock signaling pathway and PDGF-BB/PDGFR-β signaling pathway. Status quo of a lack of intensive studies on the details of homing of ADSCs should be improved in the near future before clinical application.

Engineered extracellular microenvironment with a tunable mechanical property for controlling cell behavior and cardiomyogenic fate of cardiac stem cells

Endogenous cardiac stem cells (CSCs) are known to play a certain role in the myocardial homeostasis of the adult heart. The extracellular matrix (ECM) surrounding CSCs provides mechanical signals to regulate a variety of cell behaviors, yet the impact in the adult heart of these mechanical properties of ECM on CSC renewal and fate decisions is mostly unknown. To elucidate CSC mechanoresponses at the individual cell and myocardial level, we used the sol-to-gel transitional gelatin-poly(ethylene glycol)-tyramine (GPT) hydrogel with a tunable mechanical property to construct a three-dimensional (3D) matrix for culturing native myocardium and CSCs. The elastic modulus of the GPT hydrogel was controlled by adjusting cross-linking density using hydrogen peroxide. The GPT hydrogel showed an ability to transduce integrin-mediated signals into the myocardium and to permit myocardial homeostatic processes in vitro, including CSC migration and proliferation into the hydrogel from the myocardium. Decreasing the elastic modulus of the hydrogel resulted in upregulation of phosphorylated integrin-mediated signaling molecules in CSCs, which were associated with significant increases in cell spreading, migration, and proliferation of CSCs in a modulus-dependent manner. However, increasing the elastic modulus of hydrogel induced the arrest of cell growth but led to upregulation of cardiomyocyte-associated mRNAs in CSCs. This work demonstrates that tunable 3D-engineered microenvironments created by GPT hydrogel are able to control CSC behavior and to direct cardiomyogenic fate. Our system may also be appropriate for studying the mechanoresponse of CSCs in a 3D context as well as for developing therapeutic strategies for in situ myocardial regeneration.

STATEMENT OF SIGNIFICANCE

The extracellular matrix (ECM) provides a physical framework of myocardial niches in which endogenous cardiac stem cells (CSCs) reside, renew, differentiate, and replace cardiac cells. Interactions between ECM and CSCs might be critical for the maintenance of myocardial homeostasis in the adult heart. Yet most studies done so far have used irrelevant cell types and have been performed at the individual cell level, none able to reflect the in vivo situation. By the use of a chemically defined hydrogel to create a tunable 3D microenvironment, we succeeded in controlling CSC behavior at the myocardial and individual cell level and directing the cardiomyogenic fate. Our work may provide insight into the design of biomaterials for in situ myocardial regeneration as well as for tissue engineering.

Opportunities and challenges in three-dimensional brown adipogenesis of stem cells

The formation of brown adipose tissue (BAT) via brown adipogenesis has become a notable process due to its ability to expend energy as heat with implications in the treatment of metabolic disorders and obesity. With the advent of complexity within white adipose tissue (WAT) along with inducible brown adipocytes (also known as brite and beige), there has been a surge in deciphering adipocyte biology as well as in vivo adipogenic microenvironments. A therapeutic outcome would benefit from understanding early events in brown adipogenesis, which can be accomplished by studying cellular differentiation. Pluripotent stem cells are an efficient model for differentiation and have been directed towards both white adipogenic and brown adipogenic lineages. The stem cell microenvironment greatly contributes to terminal cell fate and as such, has been mimicked extensively by various polymers including those that can form 3D hydrogel constructs capable of biochemical and/or mechanical modifications and modulations. Using bioengineering approaches towards the creation of 3D cell culture arrangements is more beneficial than traditional 2D culture in that it better recapitulates the native tissue biochemically and biomechanically. In addition, such an approach could potentially protect the tissue formed from necrosis and allow for more efficient implantation. In this review, we highlight the promise of brown adipocytes with a focus on brown adipogenic differentiation of stem cells using bioengineering approaches, along with potential challenges and opportunities that arise when considering the energy expenditure of BAT for prospective therapeutics.

Perivascular cells for regenerative medicine

Mesenchymal stem/stromal cells (MSC) are currently the best candidate therapeutic cells for regenerative medicine related to osteoarticular, muscular, vascular and inflammatory diseases, although these cells remain heterogeneous and necessitate a better biological characterization. We and others recently described that MSC originate from two types of perivascular cells, namely pericytes and adventitial cells and contain the in situ counterpart of MSC in developing and adult human organs, which can be prospectively purified using well defined cell surface markers. Pericytes encircle endothelial cells of capillaries and microvessels and express the adhesion molecule CD146 and the PDGFRβ, but lack endothelial and haematopoietic markers such as CD34, CD31, vWF (von Willebrand factor), the ligand for Ulex europaeus 1 (UEA1) and CD45 respectively. The proteoglycan NG2 is a pericyte marker exclusively associated with the arterial system. Besides its expression in smooth muscle cells, smooth muscle actin (αSMA) is also detected in subsets of pericytes. Adventitial cells surround the largest vessels and, opposite to pericytes, are not closely associated to endothelial cells. Adventitial cells express CD34 and lack αSMA and all endothelial and haematopoietic cell markers, as for pericytes. Altogether, pericytes and adventitial perivascular cells express in situ and in culture markers of MSC and display capacities to differentiate towards osteogenic, adipogenic and chondrogenic cell lineages. Importantly, adventitial cells can differentiate into pericyte-like cells under inductive conditions in vitro. Altogether, using purified perivascular cells instead of MSC may bring higher benefits to regenerative medicine, including the possibility, for the first time, to use these cells uncultured.

Regenerative therapeutic potential of adipose stromal cells in early stage diabetic retinopathy

Diabetic retinopathy (DR) is the leading cause of blindness in working-age adults. Early stage DR involves inflammation, vascular leakage, apoptosis of vascular cells and neurodegeneration. In this study, we hypothesized that cells derived from the stromal fraction of adipose tissue (ASC) could therapeutically rescue early stage DR features. Streptozotocin (STZ) induced diabetic athymic nude rats received single intravitreal injection of human ASC into one eye and saline into the other eye. Two months post onset of diabetes, administration of ASC significantly improved “b” wave amplitude (as measured by electroretinogram) within 1–3 weeks of injection compared to saline treated diabetic eyes. Subsequently, retinal histopathological evaluation revealed a significant decrease in vascular leakage and apoptotic cells around the retinal vessels in the diabetic eyes that received ASC compared to the eyes that received saline injection. In addition, molecular analyses have shown down-regulation in inflammatory gene expression in diabetic retina that received ASC compared to eyes that received saline. Interestingly, ASC were found to be localized near retinal vessels at higher densities than seen in age matched non-diabetic retina that received ASC. In vitro, ASC displayed sustained proliferation and decreased apoptosis under hyperglycemic stress. In addition, ASC in co-culture with retinal endothelial cells enhance endothelial survival and collaborate to form vascular networks. Taken together, our findings suggest that ASC are able to rescue the neural retina from hyperglycemia-induced degeneration, resulting in importantly improved visual function. Our pre-clinical studies support the translational development of adipose stem cell-based therapy for DR to address both retinal capillary and neurodegeneration.

Comparison of Effects of Mechanical Stretching on Osteogenic Potential of ASCs and BMSCs

Mechanical forces play critical roles in the development and remodeling processes of bone. As an alternative cell source for bone engineering, adipose-derived stem cells (ASCs) should be fully investigated for their responses to mechanical stress. Similarly, the osteogenic potential, stimulated by mechanical stress, should be compared with bone marrow stromal cells (BMSCs), which have been clinically used for bone tissue engineering. In this study, ASCs and BMSCs were osteogenic-induced for 48 hours, and then subjected to uniaxial mechanical stretching for 2 or 6 hours. Cell orientation, osteogenic regulatory genes, osteogenic genes and ALP activities were measured and compared between ASCs and BMSCs. ASCs could align in a perpendicular way to the direction of stretching stress, while BMSCs did not present a specific alignment. Both 2 and 6 hours mechanical stretching could enhance the mRNA expression of Osx and Runx2 in BMSCs and ASCs, while OCN mRNA only increased in ASCs after 6 hours mechanical loading. Mechanical stretching enhanced the BMP-2 mRNA expression in ASCs, while only after 6 hours of mechanical loading significantly increased the BMP-2 gene expression in BMSCs. Significant differences only exist between ASCs and BMSCs loaded at 2 hours of mechanical stretching. It is concluded that ASCs are more rapid responders to mechanical stress, and have greater potential than BMSCs in osteogenesis when stimulated by mechanical stretching, indicating their usefulness for bone study in a rat model.

Stem cell niches for skin regeneration

Stem cell-based therapies offer tremendous potential for skin regeneration following injury and disease. Functional stem cell units have been described throughout all layers of human skin and the collective physical and chemical microenvironmental cues that enable this regenerative potential are known as the stem cell niche. Stem cells in the hair follicle bulge, interfollicular epidermis, dermal papillae, and perivascular space have been closely investigated as model systems for niche-driven regeneration. These studies suggest that stem cell strategies for skin engineering must consider the intricate molecular and biologic features of these niches. Innovative biomaterial systems that successfully recapitulate these microenvironments will facilitate progenitor cell-mediated skin repair and regeneration.

Fibrin matrix-supported three-dimensional organ culture of adipose tissue for selective outgrowth, expansion, and isolation of adipose-derived stem cells

Conventional systems for isolating adipose-derived stem cells (ASC) require enzymatic digestion of adipose tissue (AT), followed by monolayer culture to the enrich the stem cell population. However, these systems are hindered by low cell yields and a lack of reproducibility. The present study was aimed at developing a unique strategy for isolating ASC based on fibrin matrix-supported three-dimensional (3-D) organ culture of native AT. Furthermore, we tried to optimize the fibrin composition by adjusting the fibrinogen and thrombin concentrations to allow rapid outgrowth and proliferation of ASC in the 3-D fibrin matrix. Human cutaneous AT fragments were encapsulated within the fibrin matrix to construct a 3-D environment and cultured under dynamic conditions. During in vitro culture the fibrin matrix provided physical support for the AT and also allowed selective outgrowth of ASC from embedded AT fragments. In situ expanded outgrown cells were recovered from the fibrin matrix by selective fibrinolysis and propagated under monolayer culture conditions. The cultured cells fulfilled the following criteria for ASC: adhesion to culture plastic, multipotent differentiation, correct immunophenotypic profile. Fibrin matrix-supported 3-D organ culture produced ASC that with high competency in terms of growth and differentiation capabilities, and resulted in a larger and more consistent cell yield than obtained with conventional culture systems. The fibrinogen and thrombin concentrations inversely affected spreading, migration, and ASC outgrowth from native AT. Our results indicate that this 3-D organ culture system for AT can be used as an efficient and reproducible method for ASC isolation.

Adipose tissue-derived stem cells display a proangiogenic phenotype on 3D scaffolds

Ischemic heart disease is the leading cause of death worldwide. Recent studies suggest that adipose tissue-derived stem cells (ASCs) can be used as a potential source for cardiovascular tissue engineering due to their ability to differentiate along the cardiovascular lineage and to adopt a proangiogenic phenotype. To understand better ASCs' biology, we used a novel 3D culture device. ASCs' and b.END-3 endothelial cell proliferation, migration, and vessel morphogenesis were significantly enhanced compared to 2D culturing techniques. ASCs were isolated from inguinal fat pads of 6-week-old GFP+/BLI+ mice. Early passage ASCs cells (P3-P4), PKH26-labeled murine b.END-3 cells or a co-culture of ASCs and b.END-3 cells were seeded at a density of 1 × 10(5) on three different surface configurations: (a) a 2D surface of tissue culture plastic, (b) Matrigel, and (c) a highly porous 3D scaffold fabricated from inert polystyrene. VEGF expression, cell proliferation, and tubulization, were assessed using optical microscopy, fluorescence microscopy, 3D confocal microscopy, and SEM imaging (n = 6). Increased VEGF levels were seen in conditioned media harvested from co-cultures of ASCs and b.END-3 on either Matrigel or a 3D matrix. Fluorescence, confocal, SEM, bioluminescence revealed improved cell, proliferation, and tubule formation for cells seeded on the 3D polystyrene matrix. Collectively, these data demonstrate that co-culturing ASCs with endothelial cells in a 3D matrix environment enable us to generate prevascularized tissue-engineered constructs. This can potentially help us to surpass the tissue thickness limitations faced by the tissue engineering community today.

Hypoxia and adipose-derived stem cell-based tissue regeneration and engineering

INTRODUCTION

Realization that oxygen is one of the key regulators of development and differentiation has a profound significance on how current cell-based and tissue engineering applications using adipose-derived stem cells (ASCs) can be further improved.

AREAS COVERED

The article provides an overview of mechanisms of hypoxic responses during physiological adaptations and development. Furthermore, a synopsis of the hypoxic responses of ASCs is provided, and this information is presented in context of their utility as a major source of stem cells across the regenerative applications explored to date.

EXPERT OPINION

The reader will obtain insight into a highly specific area of stem cell research focusing on ASCs and hypoxia. In order to enhance the level of comprehension, a broader context with other stem cell and experimental systems is provided. It is emphasized that the pericellular oxygen tension is a critical regulatory factor that should be taken into account when devising novel stem cell-based therapeutic applications along with other parameters, such as biochemical soluble factors and the growth substrates.

Vascularization of prevascularized and non-prevascularized fibrin-based human adipose tissue constructs after implantation in nude mice

Adipose regeneration strategies have been hampered by the inability to supply an adequate vascular supply following implantation. Vascularization in vitro, also called prevascularization, is a promising method that could promote the vascularization of engineered adipose tissue constructs upon implantation. In this study we compared the ability of prevascularized-to-non-prevascularized fibrin-based human adipose tissue to promote vascularization. Human adipose tissue-derived stromal cells (ASCs) and different mixtures (1:1, 1:2 and 1:5) of ASCs with human umbilical vein endothelial cells (HUVECs) were cultured in fibrin at two different densities (1.0 × 10(6) and 10 × 10(6) cells/ml) for 7 days. Histological analysis revealed that prevascular structures formed in 1:5 ASC/HUVEC fibrin-based constructs seeded with a total of 10 × 10(6) cells/ml. These constructs and ASC-only constructs were implanted subcutaneously in athymic mice for 7 days and generated lipid-containing grafts. The numbers and densities of blood vessels within the ASC/HUVEC constructs were similar to those of ASC-only constructs. Furthermore, immunostaining studies demonstrated human-derived vasculature within a few of the ASC/HUVEC and ASC-only constructs. A subset of this human-derived vasculature contained erythrocytes, indicating integration with the host vasculature. In conclusion, our study indicated no difference in the rate of vascularization of prevascularized ASC/HUVEC and non-prevascularized ASC-only fibrin-based constructs, suggesting that prevascularization of these fibrin-based constructs does not promote vascularization. Our results further indicated that not only endothelial cells, but also ASCs may contribute to the formation of vascular lumina upon implantation. This finding is interesting, since it demonstrates the possibility of vascularized adipose tissue engineering from a single cell source.

Placental perivascular cells for human muscle regeneration

Perivascular multipotent mesenchymal progenitors exist in a variety of tissues, including the placenta. Here, we suggest that the abundant vasculature present in the human placenta can serve as a source of myogenic cells to regenerate skeletal muscle. Chorionic villi dissected from the mid-gestation human placenta were first transplanted intact into the gastrocnemius muscles of SCID/mdx mice, where they participated in muscle regeneration by producing myofibers expressing human dystrophin and spectrin. In vitro-cultured placental villi released rapidly adhering and migratory CD146+CD34⁻CD45⁻CD56⁻ cells of putative perivascular origin that expressed mesenchymal stem cell markers. CD146+CD34⁻CD45⁻CD56⁻ perivascular cells isolated and purified from the placental villi by flow cytometry were indeed highly myogenic in culture, and generated dystrophin-positive myofibers, and they promoted angiogenesis after transplantation into SCID/mdx mouse muscles. These observations confirm the existence of mesenchymal progenitor cells within the walls of human blood vessels, and suggest that the richly vascularized human placenta is an abundant source of perivascular myogenic cells able to migrate within dystrophic muscle and regenerate myofibers.

The pivotal role of VEGF in adipose-derived-stem-cell-mediated regeneration

IMPORTANCE OF THE FIELD

Several lines of evidence suggest that VEGF is a key regulator of the paracrine effects of adipose-derived stem cells (ASCs), but the mechanism of action remains to be identified.

AREAS COVERED IN THIS REVIEW

This brief review discusses the following research questions: i) Does VEGF increase the proliferation/migration and differentiation of ASCs?; ii) Does VEGF mediate the paracrine effects of ASCs?; and iii) How is VEGF synthesized, and which factors regulate VEGF secretion?

WHAT THE READER WILL GAIN

External stimuli such as hypoxia may activate receptor tyrosine kinases in the membrane of ASCs, which, in turn, phosphorylate extracellular signal regulated kinase (ERK) and members of the Akt signaling pathway, stabilizing hypoxia inducible factor 1α (HIF-1α) that are primary regulators of VEGF expression. Secreted VEGF directly stimulates ASCs via VEGF receptors in an autocrine manner and regenerates damaged neighboring cells in a paracrine manner.

TAKE HOME MESSAGE

Most studies of stem cell regeneration have focused on differentiation of ASCs and their building block function; however, the paracrine effects of ASCs should also be the focus of attention.