Differentiation of mesenchymal stem cells towards a nucleus pulposus-like phenotype utilizing simulated microgravity In vitro

The effects of microgravity on stem cells and the new insights it brings to tissue engineering and regenerative medicine

Conventional two-dimensional (2D) cell culture techniques may undergo modifications in the future, as life scientists have widely acknowledged the ability of three-dimensional (3D) in vitro culture systems to accurately simulate in vivo biology. In recent years, researchers have discovered that microgravity devices can address many challenges associated with 3D cell culture. Stem cells, being pluripotent cells, are regarded as a promising resource for regenerative medicine. Recent studies have demonstrated that 3D culture in microgravity devices can effectively guide stem cells towards differentiation and facilitate the formation of functional tissue, thereby exhibiting advantages within the field of tissue engineering and regenerative medicine. Furthermore, We delineate the impact of microgravity on the biological behavior of various types of stem cells, while elucidating the underlying mechanisms governing these alterations. These findings offer exciting prospects for diverse applications.

Biomanufacturing of 3D Tissue Constructs in Microgravity and their Applications in Human Pathophysiological Studies

The growing interest in bioengineering in-vivo-like 3D functional tissues has led to novel approaches to the biomanufacturing process as well as expanded applications for these unique tissue constructs. Microgravity, as seen in spaceflight, is a unique environment that may be beneficial to the tissue-engineering process but cannot be completely replicated on Earth. Additionally, the expense and practical challenges of conducting human and animal research in space make bioengineered microphysiological systems an attractive research model. In this review, published research that exploits real and simulated microgravity to improve the biomanufacturing of a wide range of tissue types as well as those studies that use microphysiological systems, such as organ/tissue chips and multicellular organoids, for modeling human diseases in space are summarized. This review discusses real and simulated microgravity platforms and applications in tissue-engineered microphysiological systems across three topics: 1) application of microgravity to improve the biomanufacturing of tissue constructs, 2) use of tissue constructs fabricated in microgravity as models for human diseases on Earth, and 3) investigating the effects of microgravity on human tissues using biofabricated in vitro models. These current achievements represent important progress in understanding the physiological effects of microgravity and exploiting their advantages for tissue biomanufacturing.

MECHANOBIOLOGICAL APPROACHES FOR STIMULATING CHONDROGENESIS OF STEM CELLS

Chondrogenesis is the process of differentiation of stem cells into mature chondrocytes. Such a process consists of chemical, functional, and structural changes which are initiated and mediated by the host environment of the cells. To date, the mechanobiology of chondrogenesis has not been fully elucidated. Hence, experimental activity is focused on recreating specific environmental conditions for stimulating chondrogenesis, and to look for a mechanistic interpretation of the mechanobiological response of cells in the cartilaginous tissues. There are a large number of studies on the topic that vary considerably in their experimental protocols used for providing environmental cues to cells for differentiation, making generalizable conclusions difficult to ascertain. The main objective of this contribution is to review the mechanobiological stimulation of stem cell chondrogenesis and methodological approaches utilized to date to promote chondrogenesis of stem cells in-vitro. In-vivo models will also be explored, but this area is currently limited. An overview of the experimental approaches used by different research groups may help the development of unified testing methods that could be used to overcome existing knowledge gaps, leading to an accelerated translation of experimental findings to clinical practice.

Tissue Engineering of Cartilage Using a Random Positioning Machine

Articular cartilage is a skeletal tissue of avascular nature and limited self-repair capacity. Cartilage-degenerative diseases, such as osteoarthritis (OA), are difficult to treat and often necessitate joint replacement surgery. Cartilage is a tough but flexible material and relatively easy to damage. It is, therefore, of high interest to develop methods allowing chondrocytes to recolonize, to rebuild the cartilage and to restore joint functionality. Here we studied the in vitro production of cartilage-like tissue using human articular chondrocytes exposed to the Random Positioning Machine (RPM), a device to simulate certain aspects of microgravity on Earth. To screen early adoption reactions of chondrocytes exposed to the RPM, we performed quantitative real-time PCR analyses after 24 h on chondrocytes cultured in DMEM/F-12. A significant up-regulation in the gene expression of IL6, RUNX2, RUNX3, SPP1, SOX6, SOX9, and MMP13 was detected, while the levels of IL8, ACAN, PRG4, ITGB1, TGFB1, COL1A1, COL2A1, COL10A1, SOD3, SOX5, MMP1, and MMP2 mRNAs remained unchanged. The STRING (Search Tool for the Retrieval of Interacting Genes/Proteins) analysis demonstrated among others the importance of these differentially regulated genes for cartilage formation. Chondrocytes grown in DMEM/F-12 medium produced three-dimensional (3D) spheroids after five days without the addition of scaffolds. On day 28, the produced tissue constructs reached up to 2 mm in diameter. Using specific chondrocyte growth medium, similar results were achieved within 14 days. Spheroids from both types of culture media showed the typical cartilage morphology with aggrecan positivity. Intermediate filaments form clusters under RPM conditions as detected by vimentin staining after 7 d and 14 d. Larger meshes appear in the network in 28-day samples. Furthermore, they were able to form a confluent chondrocyte monolayer after being transferred back into cell culture flasks in 1 g conditions showing their suitability for transplantation into joints. Our results demonstrate that the cultivation medium has a direct influence on the velocity of tissue formation and tissue composition. The spheroids show properties that make them interesting candidates for cellular cartilage regeneration approaches in trauma and OA therapy.

The effects of microgravity on differentiation and cell growth in stem cells and cancer stem cells

A spaceflight has enormous influence on the health of space voyagers due to the combined effects of microgravity and cosmic radiation. Known effects of microgravity (μg) on cells are changes in differentiation and growth. Considering the commercialization of spaceflight, future space exploration, and long-term manned flights, research focusing on differentiation and growth of stem cells and cancer cells exposed to real (r-) and simulated (s-) μg is of high interest for regenerative medicine and cancer research. In this review, we focus on platforms to study r- and s-μg as well as the impact of μg on cancer stem cells in the field of gastrointestinal cancer, lung cancer, and osteosarcoma. Moreover, we review the current knowledge of different types of stem cells exposed to μg conditions with regard to differentiation and engineering of cartilage, bone, vasculature, heart, skin, and liver constructs.

Stem Cell Culture Under Simulated Microgravity

Challenging environment of space causes several pivotal alterations in living systems, especially due to microgravity. The possibility of simulating microgravity by ground-based systems provides research opportunities that may lead to the understanding of in vitro biological effects of microgravity by eliminating the challenges inherent to spaceflight experiments. Stem cells are one of the most prominent cell types, due to their self-renewal and differentiation capabilities. Research on stem cells under simulated microgravity has generated many important findings, enlightening the impact of microgravity on molecular and cellular processes of stem cells with varying potencies. Simulation techniques including clinostat, random positioning machine, rotating wall vessel and magnetic levitation-based systems have improved our knowledge on the effects of microgravity on morphology, migration, proliferation and differentiation of stem cells. Clarification of the mechanisms underlying such changes offers exciting potential for various applications such as identification of putative therapeutic targets to modulate stem cell function and stem cell based regenerative medicine.

Influence of microgravity-induced intervertebral disc degeneration of rats on expression levels of p53/p16 and proinflammatory factors

Association of expression levels of tumor suppressor proteins p53 and p16 and inflammatory factors in simulated weightlessness with the degree of lumbar disc degeneration of rats was investigated. Magnetic resonance imaging (MRI) examination was performed for rats in control group and experimental group, and the intervertebral disc of rats in both groups was detected and analyzed using hematoxylin and eosin (H&E) histopathological staining. The mRNA expression levels of proinflammatory factors, interleukin-1β (IL-1β), IL-6 and tumor necrosis factor-α (TNF-α) were detected, and p53 and p16 mRNA and protein expression levels were detected. MRI results showed significant intervertebral disc injury in experimental group. Results of H&E staining revealed that the intervertebral disc injury in experimental group was more serious with obvious signs of degeneration than that in control group. The mRNA expression levels of inflammatory factors (IL-1β, IL-6 and TNF-α) in rats in experimental group were significantly increased compared with those in control group, indicating that the degree of lumbar disc degeneration of rats in simulated weightlessness is closely related to the inflammatory factors. RT-PCR and western blotting proved that both p53 and p16 mRNA and protein expression levels in experimental group were obviously increased. Results of t-test manifested that there were statistically significant differences in p53 and p16 expression levels between control group and experimental group (P<0.01). The abnormal expression levels of p53 and p16 genes have close association with the degree of lumbar disc degeneration of rats in simulated weightlessness, and the lumbar disc degeneration is also closely related to the increased expression levels of inflammatory factors.

Tissue Engineering Under Microgravity Conditions-Use of Stem Cells and Specialized Cells

Experimental cell research studying three-dimensional (3D) tissues in space and on Earth using new techniques to simulate microgravity is currently a hot topic in Gravitational Biology and Biomedicine. This review will focus on the current knowledge of the use of stem cells and specialized cells for tissue engineering under simulated microgravity conditions. We will report on recent advancements in the ability to construct 3D aggregates from various cell types using devices originally created to prepare for spaceflights such as the random positioning machine (RPM), the clinostat, or the NASA-developed rotating wall vessel (RWV) bioreactor, to engineer various tissues such as preliminary vessels, eye tissue, bone, cartilage, multicellular cancer spheroids, and others from different cells. In addition, stem cells had been investigated under microgravity for the purpose to engineer adipose tissue, cartilage, or bone. Recent publications have discussed different changes of stem cells when exposed to microgravity and the relevant pathways involved in these biological processes. Tissue engineering in microgravity is a new technique to produce organoids, spheroids, or tissues with and without scaffolds. These 3D aggregates can be used for drug testing studies or for coculture models. Multicellular tumor spheroids may be interesting for radiation experiments in the future and to reduce the need for in vivo experiments. Current achievements using cells from patients engineered on the RWV or on the RPM represent an important step in the advancement of techniques that may be applied in translational Regenerative Medicine.

Bone Marrow Stem Cells in Response to Intervertebral Disc-Like Matrix Acidity and Oxygen Concentration: Implications for Cell-based Regenerative Therapy

STUDY DESIGN

In vitro culture of porcine bone marrow stem cells (BMSCs) in varying pH microenvironments in a three-dimensional hydrogel system.

OBJECTIVE

To characterize the response of BMSCs to varying pH environments (blood [pH 7.4], healthy intervertebral disc (IVD) (pH 7.1), mildly degenerated IVD (pH 6.8), and severely degenerated IVD (pH 6.5) in three-dimensional culture under normoxic (20%) and hypoxic (5%) conditions.

SUMMARY OF BACKGROUND DATA

The IVD is an avascular organ relying on diffusion of essential nutrients through the cartilaginous endplates (CEPs) thereby creating a challenging microenvironment. Within a degenerated IVD, oxygen and glucose concentrations decrease further (<5% oxygen, <5 mmol/L glucose) and matrix acidity (<pH 6.8) increases resulting in especially adverse conditions. This has major implications for injectable cell-based strategies as these adverse microenvironmental conditions might severely affect the survival and regenerative potential of transplanted cells.

METHODS

BMSCs were encapsulated in 1.5% alginate and ionically cross-linked in 102 mmol/L CaCl2 solution to form beads (diameter = 5 mm), which were cultured in different microenvironmental conditions (pH 6.5, 6.8, 7.1, and 7.4; oxygen: 5% and 20%).

RESULTS

This study demonstrated decreased DNA content, increased cell death and minimal sulphated-glycosaminoglycans (sGAG) and collagen accumulation at pH 6.5 with increased proliferation, sustained cell viability and increased sGAG and collagen accumulation in pH 6.8 or higher. These findings suggest that there is a threshold at pH 6.8, below which cells cannot survive and accumulate nucleus pulposus-like matrix components (sGAG and collagen).

CONCLUSION

Translation into a multimodal protocol requires the survival of stem cells and their ability to function normally amidst the harsh microenvironment. This study demonstrates the critical implication of degeneration stage and suggests stratified targeting to identify suitable candidates through measurement of the local pH thereby maximizing the efficacy for IVD cellular regenerative interventions.

LEVEL OF EVIDENCE

N/A.

BMP3 Alone and Together with TGF-β Promote the Differentiation of Human Mesenchymal Stem Cells into a Nucleus Pulposus-Like Phenotype

Human mesenchymal stem cells (MSCs) have the potential to differentiate into nucleus pulposus (NP)-like cells under specific stimulatory conditions. Thus far, the effects of bone morphogenetic protein 3 (BMP3) and the cocktail effects of BMP3 and transforming growth factor (TGF)-β on MSC proliferation and differentiation remain obscure. Therefore, this study was designed to clarify these unknowns. MSCs were cultured with various gradients of BMP3 and BMP3/TGF-β, and compared with cultures in basal and TGF-β media. Cell proliferation, glycosaminoglycan (GAG) content, gene expression, and signaling proteins were measured to assess the effects of BMP3 and BMP3/TGF-β on MSCs. Cell number and GAG content increased upon the addition of BMP3 in a dose-dependent manner. The expression of COL2A1, ACAN, SOX9, and KRT19 increased following induction with BMP3 and TGF-β, in contrast to that of COL1A1, ALP, OPN, and COMP. Smad3 phosphorylation was upregulated by BMP3 and TGF-β, but BMP3 did not affect the phosphorylation of extracellular-signal regulated kinase (ERK) 1/2 or c-Jun N-terminal kinase (JNK). Our results reveal that BMP3 enhances MSC proliferation and differentiation into NP-like cells, as indicated by increased cell numbers and specific gene expressions, and may also cooperate with TGF-β induced positive effects. These actions are likely related to the activation of TGF-β signaling pathway.

Behavior of stem cells under outer-space microgravity and ground-based microgravity simulation

With rapid development of space engineering, research on life sciences in space is being conducted extensively, especially cellular and molecular studies on space medicine. Stem cells, undifferentiated cells that can differentiate into specialized cells, are considered a key resource for regenerative medicine. Research on stem cells under conditions of microgravity during a space flight or a ground-based simulation has generated several excellent findings. To help readers understand the effects of outer space and ground-based simulation conditions on stem cells, we reviewed recent studies on the effects of microgravity (as an obvious environmental factor in space) on morphology, proliferation, migration, and differentiation of stem cells.

Roles of FGF-2 and TGF-beta/FGF-2 on differentiation of human mesenchymal stem cells towards nucleus pulposus-like phenotype

Human mesenchymal stem cells (MSCs) are reported to have the capability of differentiating towards nucleus pulposus (NP)-like phenotype under specific culture conditions. So far, the effects of fibroblast growth factor (FGF)-2 and the cocktail effects of transforming growth factor (TGF)-beta and FGF-2 on MSCs remain unclear. Therefore, we designed this study to clarify these effects. MSCs were cultured in conditioned medium containing FGF-2 or TGF-beta/FGF-2, and compared with basal or TGF-beta medium. The groups with FGF-2 showed the increase of cell proliferation. Functional gene markers and novel NP markers decreased in FGF-2 group, together with functional protein expression. Pho-ERK1/2 and pho-Smad3 differed significantly in the two conditioned groups. All these results suggest FGF-2 promotes MSCs' proliferation, synergistically with TGF-beta. However, FGF-2 plays a negative role in cartilage homeostasis. We also demonstrate that FGF-2 has no positive effect in differentiating MSCs into NP-like cells, but hinders the acceleration effect of TGF-beta.

Differentiation of GFP-Bcl-2-engineered mesenchymal stem cells towards a nucleus pulposus-like phenotype under hypoxia in vitro

Differentiation of bone marrow-derived mesenchymal stem cells (MSCs) into a nucleus pulposus-like phenotype under hypoxia has been proposed as a potential therapeutic approach for intervertebral disc degeneration. However, limited cell viability under hypoxic conditions has restricted MSC differentiation capacity and thus restricted its clinical application. In this study, we genetically modified MSCs with an anti-apoptotic GFP-Bcl-2 gene and evaluated cell survival and functional improvement under hypoxia in vitro. Rat bone marrow MSCs were transfected by lentiviral vectors with the GFP-Bcl-2 gene (GFP-Bcl-2-MSCs). Cell proliferation and apoptosis were assessed, and semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) was carried out to evaluate phenotypic and biosynthetic activities. In addition, Alcian blue staining was used to detect the formation of sulfated glycosaminoglycans (GAGs) in the differentiated cells. We found that the Bcl-2 gene protected MSCs against apoptosis. We also observed that Bcl-2 over-expression reduced apoptosis by 40.61% in non-transfected MSCs and 38.43% in vector-MSCs to 18.33% in Bcl-2-MSCs. At 3days, the number of viable Bcl-2-MSCs was approximately two times higher than the number of MSCs or vector-MSCs under hypoxic conditions. RT-PCR showed higher expression of chondrocyte-related genes (Sox-9, aggrecan and type II collagen) in GFP-Bcl-2-MSCs cultured under hypoxia. The accumulation of proteoglycans in the pellet was 86% higher in GFP-Bcl-2-MSCs than in the control groups. Furthermore, the ratio of proteoglycans/collagen II in GFP-Bcl-2-MSCs was 6.2-fold higher compared to the MSC and vector-MSC groups, which denoted a nucleus pulposus-like differentiation phenotype. Our findings support the hypothesis that anti-apoptotic gene-modified MSCs can differentiate into cells with a nucleus pulposus-like phenotype in vitro, which may have value for the regeneration of intervertebral discs using cell transplantation therapy.