In Vivo Generation of Neural Stem Cells Through Teratoma Formation

PRRC2B modulates oligodendrocyte progenitor cell development and myelination by stabilizing Sox2 mRNA

Oligodendrocyte progenitor cells (OPCs) differentiate into myelin-producing cells and modulate neuronal activity. Defects in OPC development are associated with neurological diseases. N6-methyladenosine (m6A) contributes to neural development; however, the mechanism by which m6A regulates OPC development remains unclear. Here, we demonstrate that PRRC2B is an m6A reader that regulates OPC development and myelination. Nestin-Cre-mediated Prrc2b deletion affects neural stem cell self-renewal and glial differentiation. Moreover, the oligodendroglia lineage-specific deletion of Prrc2b reduces the numbers of OPCs and oligodendrocytes, causing hypomyelination and impaired motor coordination. Integrative methylated RNA immunoprecipitation sequencing, RNA sequencing, and RNA immunoprecipitation sequencing analyses identify Sox2 as the target of PRRC2B. Notably, PRRC2B, displaying separate and cooperative functions with PRRC2A, stabilizes mRNA by binding to m6A motifs in the coding sequence and 3' UTR of Sox2. In summary, we identify the posttranscriptional regulation of PRRC2B in OPC development, extending the understanding of PRRC2 family proteins and providing a therapeutic target for myelin-related disorders.

Combined 3D bioprinting and tissue-specific ECM system reveals the influence of brain matrix on stem cell differentiation

We have previously shown that human and murine breast extracellular matrix (ECM) can significantly impact cellular behavior, including stem cell fate determination. It has been established that tissue-specific extracellular matrix from the central nervous system has the capacity to support neuronal survival. However, the characterization of its influence on stem cell differentiation and its adaptation to robust 3D culture models is underdeveloped. To address these issues, we combined our 3D bioprinter with hydrogels containing porcine brain extracellular matrix (BMX) to test the influence of the extracellular matrix on stem cell differentiation. Our 3D bioprinting system generated reproducible 3D neural structures derived from mouse embryonic stem cells (mESCs). We demonstrate that the addition of BMX preferentially influences 3D bioprinted mESCs towards neural lineages compared to standard basement membrane (Geltrex/Matrigel) hydrogels alone. Furthermore, we demonstrate that we can transplant these 3D bioprinted neural cellular structures into a mouse's cleared mammary fat pad, where they continue to grow into larger neural outgrowths. Finally, we demonstrate that direct injection of human induced pluripotent stem cells (hiPSCS) and neural stem cells (NSCs) suspended in pure BMX formed neural structures in vivo. Combined, these findings describe a unique system for studying brain ECM/stem cell interactions and demonstrate that BMX can direct pluripotent stem cells to differentiate down a neural cellular lineage without any additional specific differentiation stimuli.

Efficient Generation of Neural Stem Cells from Embryonic Stem Cells Using a Three-Dimensional Differentiation System

Mouse embryonic stem cells (ESCs) are useful tools for studying early embryonic development and tissue formation in mammals. Since neural lineage differentiation is a major subject of organogenesis, the development of efficient techniques to induce neural stem cells (NSCs) from pluripotent stem cells must be preceded. However, the currently available NSC differentiation methods are complicated and time consuming. This study aimed to propose an efficient method for the derivation of NSCs from mouse ESCs; early neural lineage commitment was achieved using a three-dimensional (3D) culture system, followed by a two-dimensional (2D) NSC derivation. To select early neural lineage cell types during differentiation, Sox1-GFP transgenic ESCs were used. They were differentiated into early neural lineage, forming spherical aggregates, which were subsequently picked for the establishment of 2D NSCs. The latter showed a morphology similar to that of brain-derived NSCs and expressed NSC markers, Musashi, Nestin, N-cadherin, and Sox2. Moreover, the NSCs could differentiate into three subtypes of neural lineages, neurons, astrocytes, and oligodendrocytes. The results together suggested that ESCs could efficiently differentiate into tripotent NSCs through specification in 3D culture (for approximately 10 days) followed by 2D culture (for seven days).

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.

Generation of brain organoids from mouse ESCs via teratoma formation

Pluripotent stem cells (PSCs), including embryonic stem cells (ESCs), can differentiate into all cell types in the body; therefore, they are used in the study of development and regenerative medicine. Neural lineage differentiation from PSCs is the initial step to study neurodevelopment and in vitro disease modeling. Brain organoids, which are composed of neural stem cells (NSCs) and differentiated neural lineage cell population, are a powerful in vitro system to mimic the brain tissue. Here, we aimed to establish a new method to generate brain organoids efficiently in a mouse model. We applied the in vivo teratoma formation method as a new approach to generate brain organoids. We induced teratoma formation using Sox1-GFP transgenic ESCs, in which green fluorescence protein (GFP) is expressed under the control of the early NSC marker Sox1. Sox1-GFP-expressing early NSCs were isolated as clumps and further cultured to generate brain organoids. Sox1-GFP ESC-derived brain organoids, composed of multiple layers of distinct cellular components (ventricle, ventricular zone, and cortical layer), were formed within 3 weeks of in vitro culture. We also found that neighboring cells (Sox1-GFP^-) surrounding the Sox1-GFP⁺ clumps are essential for the formation of brain organoids. Thus, in vivo and in vitro conjugated systems-initial commitment in vivo and further specialization in vitro-could be one of the promising platforms for organoid formation that are universally applicable.

Genetic tool for fate mapping of Oct4 (Pou5f1)-expressing cells and their progeny past the pluripotency stage

Background

Methods based on site-specific recombinases are widely used in studying gene activities in vivo and in vitro. In these studies, constitutively active or inducible variants of these recombinases are expressed under the control of either lineage-specific or ubiquitous promoters. However, there is a need for more advanced schemes that combine these features with possibilities to choose a time point from which lineage tracing starts in an autonomous fashion. For example, the key mammalian germline gatekeeper gene Oct4 (Pou5f1) is expressed in the peri-implantation epiblast which gives rise to all cells within embryos. Thus the above techniques are hardly applicable to Oct4 tracing past the epiblast stage, and the establishment of genetic tools addressing such a limitation is a highly relevant pursuit.

Methods

The CRISPR/Cas9 tool was used to manipulate the genome of mouse embryonic stem cells (ESCs), and various cell culture technics—to maintain and differentiate ESCs to neural cell, lentivirus-based reprogramming technique—to generate induced pluripotent stem cells (iPSCs).

Results

In this paper, we have developed a two-component genetic system (referred to as O4S) that allows tracing Oct4 gene activity past the epiblast stage of development. The first component represents a knock-in of an ubiquitous promoter-driven inducible Cre, serving as a stop signal for downstream tdTomato. Upon activation of Cre activity with 4-hydroxytamoxifen (4-OHT) at any given time point, the recombinase excises a stop signal and poses the second component of the system—the FlpO recombinase, knocked into 3’UTR of Oct4, to be expressed upon activation of the latter gene. Oct4-driven expression of FlpO, in turn, triggers the tdTomato expression and thus, permanently marks Oct4⁺ cells and their progeny. We have validated the O4S system in cultured ESCs and shown that it is capable, for example, to timely capture an activation of Oct4 gene during the reprogramming of somatic cells into iPSCs.

Conclusions

The developed O4S system can be used to detect Oct4 activation event, both permanent and transient, in somatic cell types outside the germline. The approach can be equally adjusted to other genes, provided the first component of the system is placed under transcriptional control of these genes, thus, making it a valuable tool for cell fate mapping in mice.

Neural Lineage Differentiation From Pluripotent Stem Cells to Mimic Human Brain Tissues

Recent advances in induced pluripotent stem cell (iPSC) research have turned limitations of prior and current research into possibilities. iPSCs can differentiate into the desired cell types, are easier to obtain than embryonic stem cells (ESCs), and more importantly, in case they are to be used in research on diseases, they can be obtained directly from the patient. With these advantages, differentiation of iPSCs into various cell types has been conducted in the fields of basic development, cell physiology, and cell therapy research. Differentiation of stem cells into nervous cells has been prevalent among all cell types studied. Starting with the monolayer 2D differentiation method where cells were attached to a dish, substantial efforts have been made to better mimic the in vivo environment and produce cells grown in vitro that closely resemble in vivo state cells. Having surpassed the stage of 3D differentiation, we have now reached the stage of creating tissues called organoids that resemble organs, rather than growing simple cells. In this review, we focus on the central nervous system (CNS) and describe the challenges faced in 2D and 3D differentiation research studies and the processes of overcoming them. We also discuss current studies and future perspectives on brain organoid researches.

Murine pluripotent stem cells that escape differentiation inside teratomas maintain pluripotency

Background

Pluripotent stem cells (PSCs) offer immense potential as a source for regenerative therapies. The teratoma assay is widely used in the field of stem cells and regenerative medicine, but the cell composition of teratoma is still elusive.

Methods

We utilized PSCs expressing enhanced green fluorescent protein (EGFP) under the control of the Pou5f1 promoter to study the persistence of potential pluripotent cells during teratoma formation in vivo. OCT4-MES (mouse embryonic stem cells) were isolated from the blastocysts of 3.5-day OCT4-EGFP mice (transgenic mice express EGFP cDNA under the control of the Pou5f1 promoter) embryos, and TG iPS 1-7 (induced pluripotent stem cells) were generated from mouse embryonic fibroblasts (MEFs) from 13.5-day OCT4-EGFP mice embryos by infecting them with a virus carrying OCT4, SOX2, KLF4 and c-MYC. These pluripotent cells were characterized according to their morphology and expression of pluripotency markers. Their differentiation ability was studied with in vivo teratoma formation assays. Further differences between pluripotent cells were examined by real-time quantitative PCR (qPCR).

Results

The results showed that several OCT4-expressing PSCs escaped differentiation inside of teratomas, and these escaped cells (MES-FT, GFP-positive cells separated from OCT4-MES-derived teratomas; and iPS-FT, GFP-positive cells obtained from teratomas formed by TG iPS 1-7) retained their pluripotency. Interestingly, a small number of GFP-positive cells in teratomas formed by MES-FT and iPS-FT (MES-ST, GFP-positive cells isolated from MES-FT-derived teratomas; iPS-ST, GFP-positive cells obtained from teratomas formed by iPS-FT) were still pluripotent, as shown by alkaline phosphatase (AP) staining, immunofluorescent staining and PCR. MES-FT, iPS-FT, MES-ST and iPS-ST cells also expressed several markers associated with germ cell formation, such as Dazl, Stella and Stra8.

Conclusions

In summary, a small number of PSCs escaped differentiation inside of teratomas, and these cells maintained pluripotency and partially developed towards germ cells. Both escaped PSCs and germ cells present a risk of tumor formation. Therefore, medical workers must be careful in preventing tumor formation when stem cells are used to treat specific diseases.

Generation of in vivo neural stem cells using partially reprogrammed cells defective in in vitro differentiation potential

Pluripotent stem cells can be easily differentiated in vitro into a certain lineage through embryoid body formation. Recently, however, we reported partially reprogrammed cells showing some pluripotent characteristics, which failed to differentiate in vitro. Here, we attempted to generate neural stem cells (NSCs) from partially reprogrammed cells using an in vivo differentiation system involving teratoma formation. Partially reprogrammed cells formed teratomas after injection into immunocompromised mice, and NSCs could be isolated from these teratomas. These in vivo NSCs expressed NSC markers and terminally differentiated into neurons and glial cells. Moreover, these NSCs exhibited molecular profiles very similar to those of brain-derived NSCs. These results suggest that partially reprogrammed cells defective in in vitro differentiation ability can differentiate into pure populations of NSCs through an in vivo system.

In vivo differentiation of induced pluripotent stem cells into neural stem cells by chimera formation

Like embryonic stem cells, induced pluripotent stem cells (iPSCs) can differentiate into all three germ layers in an in vitro system. Here, we developed a new technology for obtaining neural stem cells (NSCs) from iPSCs through chimera formation, in an in vivo environment. iPSCs contributed to the neural lineage in the chimera, which could be efficiently purified and directly cultured as NSCs in vitro. The iPSC-derived, in vivo-differentiated NSCs expressed NSC markers, and their gene-expression pattern more closely resembled that of fetal brain-derived NSCs than in vitro-differentiated NSCs. This system could be applied for differentiating pluripotent stem cells into specialized cell types whose differentiation protocols are not well established.