Experimental teratoma: at the crossroad of fetal- and onco-development

Evaluating Strategies to Assess the Differentiation Potential of Human Pluripotent Stem Cells: A Review, Analysis and Call for Innovation

Pluripotent stem cells have the ability to differentiate into all cells and tissues within the human body, and as a result they are attractive resources for use in basic research, drug discovery and regenerative medicine. In order to successfully achieve this application, starting cell sources ideally require in-depth characterisation to confirm their pluripotent status and their ability to differentiate into tissues representative of the three developmental germ layers. Many different methods to assess potency are employed, each having its own distinct advantages and limitations. Some aspects of this characterisation process are not always well standardised, particularly techniques used to assess pluripotency as a function. In this article, we consider the methods used to establish cellular pluripotency and subsequently analyse characterisation data for over 1590 human pluripotent cell lines from publicly available repositories in the UK and USA. In particular, we focus on the teratoma xenograft assay, its use and protocols, demonstrating the level of variation and the frequency with which it is used. Finally, we reflect on the implications of the findings, and suggest in vitro alternatives using modern innovative technology as a way forward.

Recreating Tissue Structures Representative of Teratomas In Vitro Using a Combination of 3D Cell Culture Technology and Human Embryonic Stem Cells

In vitro studies using human embryonic stem cells (hESCs) are a valuable method to study aspects of embryogenesis, avoiding ethical issues when using embryonic materials and species dissimilarities. The xenograft teratoma assay is often traditionally used to establish pluripotency in putative PSC populations, but also has additional applications, including the study of tissue differentiation. The stem cell field has long sought an alternative due to various well-established issues with the in vivo technique, including significant protocol variability and animal usage. We have established a two-step culture method which combines PSC-derived embryoid bodies (EBs) with porous scaffolds to enhance their viability, prolonging the time these structures can be maintained, and therefore, permitting more complex, mature differentiation. Here, we have utilised human embryonic stem cell-derived EBs, demonstrating the formation of tissue rudiments of increasing complexity over time and the ability to manipulate their differentiation through the application of exogenous morphogens to achieve specific lineages. Crucially, these EB-derived tissues are highly reminiscent of xenograft teratoma samples derived from the same cell line. We believe this in vitro approach represents a reproducible, animal-free alternative to the teratoma assay, which can be used to study human tissue development.

To Better Generate Organoids, What Can We Learn From Teratomas?

The genomic profile of animal models is not completely matched with the genomic profile of humans, and 2D cultures do not represent the cellular heterogeneity and tissue architecture found in tissues of their origin. Derived from 3D culture systems, organoids establish a crucial bridge between 2D cell cultures and in vivo animal models. Organoids have wide and promising applications in developmental research, disease modeling, drug screening, precision therapy, and regenerative medicine. However, current organoids represent only single or partial components of a tissue, which lack blood vessels, native microenvironment, communication with near tissues, and a continuous dorsal-ventral axis within 3D culture systems. Although efforts have been made to solve these problems, unfortunately, there is no ideal method. Teratoma, which has been frequently studied in pathological conditions, was recently discovered as a new in vivo model for developmental studies. In contrast to organoids, teratomas have vascularized 3D structures and regions of complex tissue-like organization. Studies have demonstrated that teratomas can be used to mimic multilineage human development, enrich specific somatic progenitor/stem cells, and even generate brain organoids. These results provide unique opportunities to promote our understanding of the vascularization and maturation of organoids. In this review, we first summarize the basic characteristics, applications, and limitations of both organoids and teratomas and further discuss the possibility that in vivo teratoma systems can be used to promote the vascularization and maturation of organoids within an in vitro 3D culture system.

Using Advanced Cell Culture Techniques to Differentiate Pluripotent Stem Cells and Recreate Tissue Structures Representative of Teratoma Xenografts

Various methods are currently used to investigate human tissue differentiation, including human embryo culture and studies utilising pluripotent stem cells (PSCs) such as in vitro embryoid body formation and in vivo teratoma assays. Each method has its own distinct advantages, yet many are limited due to being unable to achieve the complexity and maturity of tissue structures observed in the developed human. The teratoma xenograft assay allows maturation of more complex tissue derivatives, but this method has ethical issues surrounding animal usage and significant protocol variation. In this study, we have combined three-dimensional (3D) in vitro cell technologies including the common technique of embryoid body (EB) formation with a novel porous scaffold membrane, in order to prolong cell viability and extend the differentiation of PSC derived EBs. This approach enables the formation of more complex morphologically identifiable 3D tissue structures representative of all three primary germ layers. Preliminary in vitro work with the human embryonal carcinoma line TERA2.SP12 demonstrated improved EB viability and enhanced tissue structure formation, comparable to teratocarcinoma xenografts derived in vivo from the same cell line. This is thought to be due to reduced diffusion distances as the shape of the spherical EB transforms and flattens, allowing for improved nutritional/oxygen support to the developing structures over extended periods. Further work with EBs derived from murine embryonic stem cells demonstrated that the formation of a wide range of complex, recognisable tissue structures could be achieved within 2–3 weeks of culture. Rudimentary tissue structures from all three germ layers were present, including epidermal, cartilage and epithelial tissues, again, strongly resembling tissue structure of teratoma xenografts of the same cell line. Proof of concept work with EBs derived from the human embryonic stem cell line H9 also showed the ability to form complex tissue structures within this system. This novel yet simple model offers a controllable, reproducible method to achieve complex tissue formation in vitro. It has the potential to be used to study human developmental processes, as well as offering an animal free alternative method to the teratoma assay to assess the developmental potential of novel stem cell lines.

Derivation of neural stem cells from human teratomas

Human teratoma is a germ cell tumor that contains normal tissues (e.g., hair, skin or cartilage) differentiated from embryonal germ layers. Because of the feature of this tumor, we hypothesized that human teratomas contain multipotent stem cells that can develop into various non-cancerous normal tissues. In this study, we cultured neurospheres originally derived from a human infantile teratoma tissue, and the sphere cells were found to possess the characteristics of neural stem cells. Tumor tissues were obtained from an infantile immature teratoma at the time of surgical resection. In the primary cell culture, colonies were formed in two weeks and were individually cultured in serum-free conditioned neural stem cell medium (NSC medium). Colonies changed into spheres and grew in smooth round forms, or attached to the bottom of the dishes and extended processes and filaments around. Sphere cells were dissociated into single cells, and new spheres (secondary spheres) were formed in NSC medium. Cell differentiation was induced by culturing cells in serum-containing medium (differentiation medium), as cells spread and attached to the bottom of dishes and changed form. The expression of Nestin, Sox2, CXCR4, and (stem cell markers), β3-tubulin (a neural marker) GFAP (a glial marker) CNPase, SOX10 (oligodendrocyte markers) and NF-L in cells was analyzed by immunofluorescence and a Q-PCR. Nestin, SOX2, CXCR4 were abundant in both primary and secondary spheres. Neural and glial markers (β3-tubulin and GFAP, respectively) were increased in cells cultured in differentiation medium while stem cell markers were diminished. The oligodendrocyte markers SOX10 and CNPase were also found in both spheres and differentiated cells. In conclusion, spheres with the characteristics of neural stem cells were obtained from the primary culture of a human infantile teratoma. These spheres are considered to have the potential to undergo a natural course of neural development in humans.

Doxorubicin-provoked increase of mitotic activity and concomitant drain of G0-pool in therapy-resistant BE(2)-C neuroblastoma

In this study chemotherapy response in neuroblastoma (NB) was assessed for the first time in a transplantation model comprising non-malignant human embryonic microenvironment of pluripotent stem cell teratoma (PSCT) derived from diploid bona fide hESC. Two NB cell lines with known high-risk phenotypes; the multi-resistant BE(2)-C and the drug sensitive IMR-32, were transplanted to the PSCT model and the tumour growth was exposed to single or repeated treatments with doxorubicin, and thereafter evaluated for cell death, apoptosis, and proliferation. Dose dependent cytotoxic effects were observed, this way corroborating the experimental platform for this type of analysis. Notably, analysis of doxorubicin-resilient BE(2)-C growth in the PSCT model revealed an unexpected 1,5-fold increase in Ki67-index (p<0.05), indicating that non-cycling (G0) cells entered the cell cycle following the doxorubicin exposure. Support for this notion was obtained also in vitro. A pharmacologically relevant dose (1μM) resulted in a marked accumulation of Ki67 positive BE(2)-C cells (p<0.0001), as well as a >3-fold increase in active cell cycle (i.e. cells positive staining for PH3 together with incorporation of EdU) (p<0.01). Considering the clinical challenge for treating high-risk NB, the discovery of a therapy-provoked growth-stimulating effect in the multi-resistant and p53-mutated BE(2)-C cell line, but not in the drug-sensitive p53wt IMR-32 cell line, warrants further studies concerning generality and clinical significance of this new observation.

Management of fetal teratomas

Fetal teratomas are the most common tumors diagnosed prenatally. The majority of these tumors are benign and cured by complete resection of the mass during the neonatal period. Prenatal diagnosis has improved the perinatal management of these lesions and especially for the teratomas that might benefit from fetal intervention. A comprehensive prenatal evaluation including conventional ultrasounds, Doppler, echocardiography and fetal MRI, is essential for an effective counseling and perinatal management. Antenatal counseling helps the parents to better understand the natural history, fetal intervention, and perinatal management of these tumors, which differ dramatically depending on their size and location. Fetal surgical debulking improves survival in cases of sacrococcygeal teratoma with cardiac decompensation. Additionally, the use of an EXIT procedure reduces the morbidity and mortality if a complicated delivery in cases of cervical and mediastinal teratomas. Here, we offer an overview of all fetal teratomas and their recommended management, with emphasis on in utero treatment options.

Ino80 is essential for proximal-distal axis asymmetry in part by regulating Bmp4 expression

Background

Understanding how embryos specify asymmetric axes is a major focus of biology. While much has been done to discover signaling pathways and transcription factors important for axis specification, comparatively little is known about how epigenetic regulators are involved. Epigenetic regulators operate downstream of signaling pathways and transcription factors to promote nuclear processes, most prominently transcription. To discover novel functions for these complexes in axis establishment during early embryonic development, we characterized phenotypes of a mouse knockout (KO) allele of the chromatin remodeling Ino80 ATPase.

Results

Ino80 KO embryos implant, but fail to develop beyond the egg cylinder stage. Ino80 KO embryonic stem cells (ESCs) are viable and maintain alkaline phosphatase activity, which is suggestive of pluripotency, but they fail to fully differentiate as either embryoid bodies or teratomas. Gene expression analysis of Ino80 KO early embryos by in situ hybridization and embryoid bodies by RT-PCR shows elevated Bmp4 expression and reduced expression of distal visceral endoderm (DVE) markers Cer1, Hex, and Lefty1. In culture, Bmp4 maintains stem cell pluripotency and when overexpressed is a known negative regulator of DVE differentiation in the early embryo. Consistent with the early embryo, we observed upregulated Bmp4 expression and down-regulated Cer1, Hex, and Lefty1 expression when Ino80 KO ESCs are differentiated in a monolayer. Molecular studies in these same cells demonstrate that Ino80 bound to the Bmp4 promoter regulates its chromatin structure, which correlates with enhanced SP1 binding. These results in combination suggest that Ino80 directly regulates the chromatin structure of the Bmp4 promoter with consequences to gene expression.

Conclusions

In contrast to Ino80 KO differentiated cells, our experiments show that undifferentiated Ino80 KO ESCs are viable, but fail to differentiate in culture and in the early embryo. Ino80 KO ESCs and the early embryo up-regulate Bmp4 expression and down-regulate the expression of DVE markers Cer1, Hex and Lefty1. Based on this data, we propose a model where the Ino80 chromatin remodeling complex represses Bmp4 expression in the early embryo, thus promoting DVE differentiation and successful proximal-distal axis establishment. These results are significant because they show that epigenetic regulators have specific roles in establishing embryonic axes. By further characterizing these complexes, we will deepen our understanding of how the mammalian embryo is patterned by epigenetic regulators.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0238-5) contains supplementary material, which is available to authorized users.