Derivation of Diverse Hormone-Releasing Pituitary Cells from Human Pluripotent Stem Cells

Benchmark for Setting ACTH Cell Dosage in Clinical Regenerative Medicine for Post-Operative Hypopituitarism

BACKGROUND/OBJECTIVES

Our objective is to develop hormone-producing pituitary cells that can function in the same manner as the human body and provide more effective treatments than current hormone replacement therapy. We have already established a technique for generating hypothalamic-pituitary organoids using feeder-free human pluripotent stem cells (hPSCs) and demonstrated their effectiveness in vivo through transplantation into hypopituitary mouse models. To prospectively determine the upper limit of transplanting adenohypophyseal cells into humans, we investigated the human maximum secretion capacity of adrenocorticotropic hormone (ACTH) and growth hormone (GH).

METHODS

We analyzed data from 28 patients with pituitary adenomas, among whom 16 evinced no abnormality of ACTH secretion and 12 showed no GH secretion on corticotropin-releasing hormone (CRH) and growth hormone-releasing hormone-2 (GHRP-2) stimulation testing.

RESULTS

The average ACTH peak value after CRH stimulation tests was 97.2 pg/mL, and the average GH peak value after GHRP-2 stimulation tests was 25.1 ng/mL.

CONCLUSIONS

These data will likely serve as benchmarks of ACTH and GH secretion when transplanting cultured cells into humans.

Genetic models of Cushing's disease : From cells, in vivo transgenic models to human pituitary organoids

Cushing's disease (CD) is caused by pituitary tumors that overproduce adrenocorticotropic hormone (ACTH); however, effective medical treatments remain limited, significantly impairing patients' quality of life and prognosis. Despite extensive molecular analyses, the pathogenesis of CD remains unclear. Although previous molecular studies have relied heavily on rodent-derived cells and rodent transgenic models, significant species differences exist in the tumorigenesis of CD between humans and rodents. To date, an established human CD cell model is lacking, as human CD cells are limited in availability and sustainability over time. Additionally, the gene modifications used in transgenic models do not necessarily reflect the causative genes in CD. CD tumors exhibit wide phenotypic heterogeneity, which further complicates the development of an ideal genetic model. In this review, we provide an analysis of 11 genetic models used to study CD, outlining their historical development, strengths, and limitations. Additionally, we discuss the ongoing development of human induced pluripotent stem cell (iPSC)-derived pituitary organoids and further describe various models of pituitary organoids as an emerging novel approach to studying CD. By comparing all these models, we highlight the necessity of advancing genetic models to improve our understanding and treatment of CD.

Human Pituitary Organoids: Transcriptional Landscape Deciphered by scRNA-Seq and Stereo-Seq, with Insights into SOX3's Role in Pituitary Development

The 3D human pituitary organoid represents a promising laboratory model for investigating human pituitary diseases. Nonetheless, this technology is still in its nascent stage, with uncertainties regarding the cellular composition, intercellular interactions, and spatial distribution of the human pituitary organoids. To address these gaps, the culture conditions are systematically adjusted and the efficiency of induced pluripotent stem cells' (iPSCs') differentiation into pituitary organoids is successfully improved, achieving results comparable to or exceeding those of previous studies. Additionally, single-cell RNA-sequencing (scRNA-seq) and stereomics sequencing (Stereo-seq) are performed on the pituitary organoids for the first time, and unveil the diverse cell clusters, intricate intercellular interactions, and spatial information within the organoids. Furthermore, the SOX3 gene interference impedes the iPSCs' differentiation into pituitary organoids, thereby highlighting the potential of pituitary organoids as an ideal experimental model. Altogether, the research provides an optimized protocol for the human pituitary organoid culture and a valuable transcriptomic dataset for future explorations, laying the foundation for subsequent research in the field of pituitary organoids or pituitary diseases.

Three-dimensional cell culture using CD9-positive cells isolated from marginal cell layer of intermediate lobe of rats sustains in vivo-like primary niche environment

The adenohypophysis is composed of the anterior and intermediate lobes (AL and IL, respectively), and secretes hormones that play an important role in reproduction. CD9- and SOX2-double (CD9/SOX2) positive cells located in the marginal cell layer (MCL) facing the Rathke's cleft in the AL and IL form the primary stem cell niche in the adult adenohypophysis of rats. In this study, we successfully obtained 3-dimensional (3D) cell aggregates that closely resembled the primary niche of MCL in vivo. After incubation in a Matrigel containing several growth factors, approximately 20% of the cells in the CD9/SOX2-positive cell aggregates were differentiated into hormone-producing cells. The cell aggregates generated in this study may provide insight into the regulation of the pituitary stem/progenitor cell niche and the turnover of hormone-producing cells.

Recent advances and applications of human brain models

Recent advances in human pluripotent stem cell (hPSC) technologies have prompted the emergence of new research fields and applications for human neurons and brain organoids. Brain organoids have gained attention as an in vitro model system that recapitulates the higher structure, cellular diversity and function of the brain to explore brain development, disease modeling, drug screening, and regenerative medicine. This progress has been accelerated by abundant interactions of brain organoid technology with various research fields. A cross-disciplinary approach with human brain organoid technology offers a higher-ordered advance for more accurately understanding the human brain. In this review, we summarize the status of neural induction in two- and three-dimensional culture systems from hPSCs and the modeling of neurodegenerative diseases using brain organoids. We also highlight the latest bioengineered technologies for the assembly of spatially higher-ordered neural tissues and prospects of brain organoid technology toward the understanding of the potential and abilities of the human brain.

Advancing Organoid Engineering for Tissue Regeneration and Biofunctional Reconstruction

Tissue damage and functional abnormalities in organs have become a considerable clinical challenge. Organoids are often applied as disease models and in drug discovery and screening. Indeed, several studies have shown that organoids are an important strategy for achieving tissue repair and biofunction reconstruction. In contrast to established stem cell therapies, organoids have high clinical relevance. However, conventional approaches have limited the application of organoids in clinical regenerative medicine. Engineered organoids might have the capacity to overcome these challenges. Bioengineering-a multidisciplinary field that applies engineering principles to biomedicine-has bridged the gap between engineering and medicine to promote human health. More specifically, bioengineering principles have been applied to organoids to accelerate their clinical translation. In this review, beginning with the basic concepts of organoids, we describe strategies for cultivating engineered organoids and discuss the multiple engineering modes to create conditions for breakthroughs in organoid research. Subsequently, studies on the application of engineered organoids in biofunction reconstruction and tissue repair are presented. Finally, we highlight the limitations and challenges hindering the utilization of engineered organoids in clinical applications. Future research will focus on cultivating engineered organoids using advanced bioengineering tools for personalized tissue repair and biofunction reconstruction.

Pituitary stem cells: past, present and future perspectives

Pituitary cells that express the transcription factor SOX2 are stem cells because they can self-renew and differentiate into multiple pituitary hormone-producing cell types as organoids. Wounding and physiological challenges can activate pituitary stem cells, but cell numbers are not fully restored, and the ability to mobilize stem cells decreases with increasing age. The basis of these limitations is still unknown. The regulation of stem cell quiescence and activation involves many different signalling pathways, including those mediated by WNT, Hippo and several cytokines; more research is needed to understand the interactions between these pathways. Pituitary organoids can be formed from human or mouse embryonic stem cells, or from human induced pluripotent stem cells. Human pituitary organoid transplantation is sufficient to induce corticosterone release in hypophysectomized mice, raising the possibility of therapeutic applications. Today, pituitary organoids have the potential to assess the role of individual genes and genetic variants on hormone production ex vivo, providing an important tool for the advancement of exciting frontiers in pituitary stem cell biology and pituitary organogenesis. In this article, we provide an overview of notable discoveries in pituitary stem cell function and highlight important areas for future research.

In vitro modeling of cranial placode differentiation: Recent advances, challenges, and perspectives

Cranial placodes are transient ectodermal thickenings that contribute to a diverse array of organs in the vertebrate head. They develop from a common territory, the pre-placodal region that over time segregates along the antero-posterior axis into individual placodal domains: the adenohypophyseal, olfactory, lens, trigeminal, otic, and epibranchial placodes. These placodes terminally differentiate into the anterior pituitary, the lens, and contribute to sensory organs including the olfactory epithelium, and inner ear, as well as several cranial ganglia. To study cranial placodes and their derivatives and generate cells for therapeutic purposes, several groups have turned to in vitro derivation of placodal cells from human embryonic stem cells (hESCs) or induced pluripotent stem cells (hiPSCs). In this review, we summarize the signaling cues and mechanisms involved in cranial placode induction, specification, and differentiation in vivo, and discuss how this knowledge has informed protocols to derive cranial placodes in vitro. We also discuss the benefits and limitations of these protocols, and the potential of in vitro cranial placode modeling in regenerative medicine to treat cranial placode-related pathologies.

Organoid models of the pituitary gland in health and disease

The pituitary gland represents the hub of our endocrine system. Its cells produce specific hormones that direct multiple vital physiological processes such as body growth, fertility, and stress. The gland also contains a population of stem cells which are still enigmatic in phenotype and function. Appropriate research models are needed to advance our knowledge on pituitary (stem cell) biology. Over the last decade, 3D organoid models have been established, either derived from the pituitary stem cells or from pluripotent stem cells, covering both healthy and diseased conditions. Here, we summarize the state-of-the-art of pituitary-allied organoid models and discuss applications of these powerful in vitro research and translational tools to study pituitary development, biology, and disease.

Generation and purification of ACTH-secreting hPSC-derived pituitary cells for effective transplantation

Pituitary organoids are promising graft sources for transplantation in treatment of hypopituitarism. Building on development of self-organizing culture to generate pituitary-hypothalamic organoids (PHOs) using human pluripotent stem cells (hPSCs), we established techniques to generate PHOs using feeder-free hPSCs and to purify pituitary cells. The PHOs were uniformly and reliably generated through preconditioning of undifferentiated hPSCs and modulation of Wnt and TGF-β signaling after differentiation. Cell sorting using EpCAM, a pituitary cell-surface marker, successfully purified pituitary cells, reducing off-target cell numbers. EpCAM-expressing purified pituitary cells reaggregated to form three-dimensional pituitary spheres (3D-pituitaries). These exhibited high adrenocorticotropic hormone (ACTH) secretory capacity and responded to both positive and negative regulators. When transplanted into hypopituitary mice, the 3D-pituitaries engrafted, improved ACTH levels, and responded to in vivo stimuli. This method of generating purified pituitary tissue opens new avenues of research for pituitary regenerative medicine.

Recent advances in endocrine organoids for therapeutic application

The endocrine system, consisting of the hypothalamus, pituitary, endocrine glands, and hormones, plays a critical role in hormone metabolic interactions. The complexity of the endocrine system is a significant obstacle to understanding and treating endocrine disorders. Notably, advances in endocrine organoid generation allow a deeper understanding of the endocrine system by providing better comprehension of molecular mechanisms of pathogenesis. Here, we highlight recent advances in endocrine organoids for a wide range of therapeutic applications, from cell transplantation therapy to drug toxicity screening, combined with development in stem cell differentiation and gene editing technologies. In particular, we provide insights into the transplantation of endocrine organoids to reverse endocrine dysfunctions and progress in developing strategies for better engraftments. We also discuss the gap between preclinical and clinical research. Finally, we provide future perspectives for research on endocrine organoids for the development of more effective treatments for endocrine disorders.

Genetically engineered human pituitary corticotroph tumor organoids exhibit divergent responses to glucocorticoid receptor modulators

Cushing's disease (CD) is a serious endocrine disorder attributed to an adrenocorticotropic hormone (ACTH)-secreting pituitary neuroendocrine tumor (PitNET) that that subsequently leads to chronic hypercortisolemia. PitNET regression has been reported following treatment with the investigational selective glucocorticoid receptor (GR) modulator relacorilant, but the mechanisms behind that effect remain unknown. Human PitNET organoid models were generated from induced human pluripotent stem cells (iPSCs) or fresh tissue obtained from CD patient PitNETs (hPITOs). Genetically engineered iPSC derived organoids were used to model the development of corticotroph PitNETs expressing USP48 (iPSCUSP48) or USP8 (iPSCUSP8) somatic mutations. Organoids were treated with the GR antagonist mifepristone or the GR modulator relacorilant with or without somatostatin receptor (SSTR) agonists pasireotide or octreotide. In iPSCUSP48 and iPSCUSP8 cultures, mifepristone induced a predominant expression of SSTR2 with a concomitant increase in ACTH secretion and tumor cell proliferation. Relacorilant predominantly induced SSTR5 expression and tumor cell apoptosis with minimal ACTH induction. Hedgehog signaling mediated the induction of SSTR2 and SSTR5 in response to mifepristone and relacorilant. Relacorilant sensitized PitNET organoid responsiveness to pasireotide. Therefore, our study identified the potential therapeutic use of relacorilant in combination with somatostatin analogs and demonstrated the advantages of relacorilant over mifepristone, supporting its further development for use in the treatment of Cushing's disease patients.

Long-range axonal projections of transplanted mouse embryonic stem cell-derived hypothalamic neurons into adult mouse brain

The hypothalamus is comprised of heterogenous cell populations and includes highly complex neural circuits that regulate the autonomic nerve system. Its dysfunction therefore results in severe endocrine disorders. Although recent experiments have been conducted for in vitro organogenesis of hypothalamic neurons from embryonic stem (ES) or induced pluripotent stem (iPS) cells, whether these stem cell-derived hypothalamic neurons can be useful for regenerative medicine remains unclear. We therefore performed orthotopic transplantation of mouse ES cell (mESC)-derived hypothalamic neurons into adult mouse brains. We generated electrophysiologically functional hypothalamic neurons from mESCs and transplanted them into the supraoptic nucleus of mice. Grafts extended their axons along hypothalamic nerve bundles in host brain, and some of them even projected into the posterior pituitary (PPit), which consists of distal axons of the magnocellular neurons located in hypothalamic supraoptic and paraventricular nuclei. The axonal projections to the PPit were not observed when the mESC-derived hypothalamic neurons were ectopically transplanted into the substantia nigra reticular part. These findings suggest that our stem cell-based orthotopic transplantation approach might contribute to the establishment of regenerative medicine for hypothalamic and pituitary disorders.

Generation of Lens Progenitor Cells and Lentoid Bodies from Pluripotent Stem Cells: Novel Tools for Human Lens Development and Ocular Disease Etiology

In vitro differentiation of human pluripotent stem cells (hPSCs) into specialized tissues and organs represents a powerful approach to gain insight into those cellular and molecular mechanisms regulating human development. Although normal embryonic eye development is a complex process, generation of ocular organoids and specific ocular tissues from pluripotent stem cells has provided invaluable insights into the formation of lineage-committed progenitor cell populations, signal transduction pathways, and self-organization principles. This review provides a comprehensive summary of recent advances in generation of adenohypophyseal, olfactory, and lens placodes, lens progenitor cells and three-dimensional (3D) primitive lenses, "lentoid bodies", and "micro-lenses". These cells are produced alone or "community-grown" with other ocular tissues. Lentoid bodies/micro-lenses generated from human patients carrying mutations in crystallin genes demonstrate proof-of-principle that these cells are suitable for mechanistic studies of cataractogenesis. Taken together, current and emerging advanced in vitro differentiation methods pave the road to understand molecular mechanisms of cataract formation caused by the entire spectrum of mutations in DNA-binding regulatory genes, such as PAX6, SOX2, FOXE3, MAF, PITX3, and HSF4, individual crystallins, and other genes such as BFSP1, BFSP2, EPHA2, GJA3, GJA8, LIM2, MIP, and TDRD7 represented in human cataract patients.

Human stem cell models to study placode development, function and pathology

Placodes are embryonic structures originating from the rostral ectoderm that give rise to highly diverse organs and tissues, comprising the anterior pituitary gland, paired sense organs and cranial sensory ganglia. Their development, including the underlying gene regulatory networks and signalling pathways, have been for the most part characterised in animal models. In this Review, we describe how placode development can be recapitulated by the differentiation of human pluripotent stem cells towards placode progenitors and their derivatives, highlighting the value of this highly scalable platform as an optimal in vitro tool to study the development of human placodes, and identify human-specific mechanisms in their development, function and pathology.

A Molecular Analysis of Neural Olfactory Placode Differentiation in Human Pluripotent Stem Cells

During embryonic development, the olfactory sensory neurons (OSNs) and the gonadotropic-releasing hormone neurons (GNRHNs) migrate from the early nasal cavity, known as the olfactory placode, to the brain. Defects in the development of OSNs and GNRHNs result in neurodevelopmental disorders such as anosmia and congenital hypogonadotropic hypogonadism, respectively. Treatments do not restore the defective neurons in these disorders, and as a result, patients have a diminished sense of smell or a gonadotropin hormone deficiency. Human pluripotent stem cells (hPSCs) can produce any cell type in the body; therefore, they are an invaluable tool for cell replacement therapies. Transplantation of olfactory placode progenitors, derived from hPSCs, is a promising therapeutic to replace OSNs and GNRHNs and restore tissue function. Protocols to generate olfactory placode progenitors are limited, and thus, we describe, in this study, a novel in vitro model for olfactory placode differentiation in hPSCs, which is capable of producing both OSNs and GNRHNs. Our study investigates the major developmental signaling factors that recapitulate the embryonic development of the olfactory tissue. We demonstrate that induction of olfactory placode in hPSCs requires bone morphogenetic protein inhibition, wingless/integrated protein inhibition, retinoic acid inhibition, transforming growth factor alpha activation, and fibroblast growth factor 8 activation. We further show that the protocol transitions hPSCs through the anterior pan-placode ectoderm and neural ectoderm regions in early development while preventing neural crest and non-neural ectoderm regions. Finally, we demonstrate production of OSNs and GNRHNs by day 30 of differentiation. Our study is the first to report on OSN differentiation in hPSCs.

Disease Modeling of Pituitary Adenoma Using Human Pluripotent Stem Cells

Simple Summary

Pituitary adenoma pathophysiology has been studied mainly using murine cell lines, animal models, and pituitary tumor samples. However, the lack of human pituitary cell line is a significant limiting factor in studying the molecular mechanisms of human pituitary tumors. Recently, pituitary induction methods from human-induced pluripotent stem cells (hiPSCs) have been established. These methods can induce human pituitary hormone-producing cells that retain physiological properties. hiPSCs in which tumor-causing gene mutations are introduced using genome-editing techniques, such as CRISPR/Cas9 systems, provide great opportunities to establish in vitro human pituitary adenoma disease models. The models will be a novel platform to discover novel drugs and investigate tumorigenesis and pathophysiology. The purpose of this review is to provide an overview of the applications of iPSCs for pituitary and neoplastic disorder research and genome-editing technologies to create strategies for developing pituitary adenoma models using iPSCs.

Abstract

Pituitary adenomas are characterized by abnormal growth in the pituitary gland. Surgical excision is the first-line treatment for functional (hormone-producing) pituitary adenomas, except for prolactin-producing adenomas; however, complete excision is technically challenging, and many patients require long-term medication after the treatment. In addition, the pathophysiology of pituitary adenomas, such as tumorigenesis, has not been fully understood. Pituitary adenoma pathophysiology has mainly been studied using animal models and animal tumor-derived cell lines. Nevertheless, experimental studies on human pituitary adenomas are difficult because of the significant differences among species and the lack of reliable cell lines. Recently, several methods have been established to differentiate pituitary cells from human pluripotent stem cells (hPSCs). The induced pituitary hormone-producing cells retain the physiological properties already lost in tumor-derived cell lines. Moreover, CRISPR/Cas9 systems have expedited the introduction of causative gene mutations in various malignant tumors into hPSCs. Therefore, hPSC-derived pituitary cells have great potential as a novel platform for studying the pathophysiology of human-specific pituitary adenomas and developing novel drugs. This review presents an overview of the recent progresses in hPSC applications for pituitary research, functional pituitary adenoma pathogenesis, and genome-editing techniques for introducing causative mutations. We also discuss future applications of hPSCs for studying pituitary adenomas.

EpCAM Is a Surface Marker for Enriching Anterior Pituitary Cells From Human Hypothalamic-Pituitary Organoids

Human stem cell-derived organoid culture enables the in vitro analysis of the cellular function in three-dimensional aggregates mimicking native organs, and also provides a valuable source of specific cell types in the human body. We previously established organoid models of the hypothalamic-pituitary (HP) complex using human pluripotent stem cells. Although the models are suitable for investigating developmental and functional HP interactions, we consider that isolated pituitary cells are also useful for basic and translational research on the pituitary gland, such as stem cell biology and regenerative medicine. To develop a method for the purification of pituitary cells in HP organoids, we performed surface marker profiling of organoid cells derived from human induced pluripotent stem cells (iPSCs). Screening of 332 human cell surface markers and a subsequent immunohistochemical analysis identified epithelial cell adhesion molecule (EpCAM) as a surface marker of anterior pituitary cells, as well as their ectodermal precursors. EpCAM was not expressed on hypothalamic lineages; thus, anterior pituitary cells were successfully enriched by magnetic separation of EpCAM+ cells from iPSC-derived HP organoids. The enriched pituitary population contained functional corticotrophs and their progenitors; the former responded normally to a corticotropin-releasing hormone stimulus. Our findings would extend the applicability of organoid culture as a novel source of human anterior pituitary cells, including stem/progenitor cells and their endocrine descendants.

Pituitary lineage differentiation from human-induced pluripotent stem cells in 2D and 3D cultures

Despite its small size, the pituitary gland plays a central role in the maintenance of normal homeostasis of most physiological systems through its regulation of the function of other endocrine glands. The complexity of the anterior pituitary gland, due to its composition of several different hormone-secreting cell types, begets a plethora of disorders and pathologies due primarily to hypo- or hyper-secretion of hormones. The gonadotrophs, which make up less than 5% of the total number of cells in the anterior pituitary, serve to regulate gonad development and sexual reproduction in males and females. Despite the increased research on the development of models to study pituitary function within the last decade, a model specifically designed to study the gonadotrophs is still lacking. The development of organoid technology has facilitated research in the field of personalized medicine and physiological testing using patient-derived cells. The ability to produce pituitary organoids would allow researchers to construct an in vitro model of the human hypothalamic-pituitary-gonadal (HPG) or -adrenal (HPA) axis to use in further fertility or endocrine research. The application of this technology in patients could revolutionize the treatment of infertility and a variety of neuroendocrine disorders. The impetus behind this stud was to develop a pituitary-like organoid consisting only of gonadotrophs. Despite the lack of success in differentiating gonadotrophs, pituitary-like organoids were differentiated from human-induced pluripotent stem cells. In addition, 2D and 3D differentiated cultures were characterized and compared to human adult cadaveric pituitary tissue.

Functional Lactotrophs in Induced Adenohypophysis Differentiated From Human iPS Cells

Prolactin (PRL), a hormone involved in lactation, is mainly produced and secreted by the lactotrophs of the anterior pituitary (AP) gland. We previously reported a method to generate functional adrenocorticotropic hormone-producing cells by differentiating the AP and hypothalamus simultaneously from human induced pluripotent stem cells (iPSCs). However, PRL-producing cells in the induced AP have not been investigated. Here, we confirmed the presence of PRL-producing cells and evaluated their endocrine functions. We differentiated pituitary cells from human iPSCs using serum-free floating culture of embryoid-like aggregates with quick reaggregation (SFEB-q) method and evaluated the appearance and function of PRL-producing cells. Secretion of PRL from the differentiated aggregates was confirmed, which increased with further culture. Fluorescence immunostaining and immunoelectron microscopy revealed PRL-producing cells and PRL-positive secretory granules, respectively. PRL secretion was promoted by various prolactin secretagogues such as thyrotropin-releasing hormone, vasoactive intestinal peptide, and prolactin-releasing peptide, and inhibited by bromocriptine. Moreover, the presence of tyrosine hydroxylase-positive dopaminergic nerves in the hypothalamic tissue area around the center of the aggregates connecting to PRL-producing cells indicated the possibility of recapitulating PRL regulatory mechanisms through the hypothalamus. In conclusion, we generated pituitary lactotrophs from human iPSCs; these displayed similar secretory responsiveness as human pituitary cells in vivo. In the future, this is expected to be used as a model of human PRL-producing cells for various studies, such as drug discovery, prediction of side effects, and elucidation of tumorigenic mechanisms using disease-specific iPSCs. Furthermore, it may help to develop regenerative medicine for the pituitary gland.

Recipe for pituitary organoids

Generation of a variety of organs and tissues from human pluripotent stem cells (hPSCs) has been attempted in vitro. We here present a simple and efficient method for induction of hypothalamic and pituitary tissues from hPSCs. On provision of exogenous agents important for early hypothalamus-pituitary organogenesis, including bone morphogenetic protein 4 and activators of sonic hedgehog, in three-dimensional culture, hPSCs spontaneously form spherical organoids with two distinct tissues, hypothalamus and adenohypophysis. The pituitary tissues derived from hPSCs not only secrete adenocorticotropic hormone, but also retain both positive and negative feedback mechanisms, recapitulating mature endocrine organs in vivo. Furthermore, the results of ectopic transplantation with mouse models of hypopituitarism suggest that these hypothalamus-pituitary organoids have potential as engraftment organs. In addition to their use in transplantation for patients with hypopituitarism they will allow establishment of disease models in vitro and enable research impossible in humans. Hypothalamus-pituitary organoids promise to be a powerful tool in regenerative medicine, drug discovery, and basic research into pituitary development.

Human neural organoids: Models for developmental neurobiology and disease

Human organoids stand at the forefront of basic and translational research, providing experimentally tractable systems to study human development and disease. These stem cell-derived, in vitro cultures can generate a multitude of tissue and organ types, including distinct brain regions and sensory systems. Neural organoid systems have provided fundamental insights into molecular mechanisms governing cell fate specification and neural circuit assembly and serve as promising tools for drug discovery and understanding disease pathogenesis. In this review, we discuss several human neural organoid systems, how they are generated, advances in 3D imaging and bioengineering, and the impact of organoid studies on our understanding of the human nervous system.

Evidence That the Etiology of Congenital Hypopituitarism Has a Major Genetic Component but Is Infrequently Monogenic

Purpose

Congenital hypopituitarism usually occurs sporadically. In most patients, the etiology remains unknown.

Methods

We studied 13 children with sporadic congenital hypopituitarism. Children with non-endocrine, non-familial idiopathic short stature (NFSS) (n = 19) served as a control group. Exome sequencing was performed in probands and both unaffected parents. A burden testing approach was used to compare the number of candidate variants in the two groups.

Results

First, we assessed the frequency of rare, predicted-pathogenic variants in 42 genes previously reported to be associated with pituitary gland development. The average number of variants per individual was greater in probands with congenital hypopituitarism than those with NFSS (1.1 vs. 0.21, mean variants/proband, P = 0.03). The number of probands with at least 1 variant in a pituitary-associated gene was greater in congenital hypopituitarism than in NFSS (62% vs. 21%, P = 0.03). Second, we assessed the frequency of rare, predicted-pathogenic variants in the exome (to capture undiscovered causes) that were inherited in a fashion that could explain the sporadic occurrence of the proband's condition with a monogenic etiology (de novo mutation, autosomal recessive, or X-linked recessive) with complete penetrance. There were fewer monogenic candidates in the probands with congenital hypopituitarism than those with NFSS (1.3 vs. 2.5 candidate variants/proband, P = 0.024). We did not find any candidate variants (0 of 13 probands) in genes previously reported to explain the phenotype in congenital hypopituitarism, unlike NFSS (8 of 19 probands, P = 0.01).

Conclusion

Our findings provide evidence that the etiology of sporadic congenital hypopituitarism has a major genetic component but may be infrequently monogenic with full penetrance, suggesting a more complex etiology.

Organoid technology for tissue engineering

For centuries, attempts have been continuously made to artificially reconstitute counterparts of in vivo organs from their tissues or cells. Only in the recent decade has organoid technology as a whole technological field systematically emerged and been shown to play important roles in tissue engineering. Based on their self-organizing capacities, stem cells of versatile organs, both harvested and induced, can form 3D structures that are structurally and functionally similar to their in vivo counterparts. These organoid models provide a powerful platform for elucidating the development mechanisms, modeling diseases, and screening drug candidates. In this review, we will summarize the advances of this technology for generating various organoids of tissues from the three germ layers and discuss their drawbacks and prospects for tissue engineering.

Pituitary Tumors and Immortalized Cell Lines Generated by Cre-Inducible Expression of SV40 T Antigen

Targeted oncogenesis is the process of driving tumor formation by engineering transgenic mice that express an oncogene under the control of a cell-type specific promoter. Such tumors can be adapted to cell culture, providing immortalized cell lines. To make it feasible to follow the process of tumorigenesis and increase the opportunity for generating cell lines, we developed a mouse strain that expresses SV40 T antigens in response to Cre-recombinase. Using CRISPR/Cas9 we inserted a cassette with coding sequences for SV40 T antigens and an internal ribosome entry site with green fluorescent protein cassette (IRES-GFP) into the Rosa26 locus, downstream from a stop sequence flanked by loxP sites: Rosa26LSL-SV40-GFP. These mice were mated with previously established Prop1-cre and Tshb-cre transgenic lines. Both the Rosa26LSL-SV40-GFP/+; Prop1-cre and Rosa26LSL-SV40-GFP/+; Tshb-cre mice developed fully penetrant dwarfism and large tumors by 4 weeks. Tumors from both of these mouse lines were adapted to growth in cell culture. We have established a progenitor-like cell line (PIT-P1) that expresses Sox2 and Pitx1, and a thyrotrope-like cell line (PIT-T1) that expresses Pou1f1 and Cga. These studies demonstrate the utility of the novel, Rosa26LSL-SV40-GFP mouse line for reliable targeted oncogenesis and development of unique cell lines.

Pituitary stem cells

The anterior pituitary is derived from Rathke's pouch precursors, which differentiate into specific hormone-secreting cell lineages. Sustained low postnatal and adult pituitary cell turnover is governed by stem/progenitor cells that undergo slow mitotic activity and give rise to hormone-secreting cells in response to physiological demands and feedback loops. Pituitary cell populations exhibit stem cell properties, which include stem cell marker expression, non-hormone expression, and the ability to self-renew and to potentially differentiate into any of five hormone-secreting cell lineages. Specific signaling pathways underlie differentiated pituitary cell development and regulation. Several validated pituitary stem cell models have been reported and have the potential for functional regeneration of pituitary hormone-secreting cell functions.

Human pituitary development and application of iPSCs for pituitary disease

The pituitary plays a pivotal role in maintaining systemic homeostasis by secreting several hormones. During fetal development, the pituitary develops from the oral ectoderm in contact with the adjacent hypothalamus. This process is regulated by the fine-tuned expression of transcription and growth factors. Impairments of this process result in congenital pituitary hypoplasia leading to dysfunction of the pituitary. Although animal models such as knockout mice have helped to clarify these underlying mechanisms, the developmental processes of the human pituitary gland and the mechanisms of human pituitary disorders have not been fully understood. This is because, at least in part, of the lack of a human pituitary developmental model. Recently, methods for in vitro induction of the pituitary gland from human pluripotent stem cells were developed. These models can be utilized not only for regenerative medicine but also for human pituitary studies on developmental biology and for modeling of pituitary disorders, such as hypopituitarism and pituitary tumors. In this review, we provide an overview of recent progress in the applications of pluripotent stem cells for pituitary research and discuss further perspectives for pituitary studies.

Pituitary Remodeling Throughout Life: Are Resident Stem Cells Involved?

The pituitary gland has the primordial ability to dynamically adapt its cell composition to changing hormonal needs of the organism throughout life. During the first weeks after birth, an impressive growth and maturation phase is occurring in the gland during which the distinct hormonal cell populations expand. During pubertal growth and development, growth hormone (GH) levels need to peak which requires an adaptive enterprise in the GH-producing somatotrope population. At aging, pituitary function wanes which is associated with organismal decay including the somatopause in which GH levels drop. In addition to these key time points of life, the pituitary's endocrine cell landscape plastically adapts during specific (patho-)physiological conditions such as lactation (need for PRL) and stress (engagement of ACTH). Particular resilience is witnessed after physical injury in the (murine) gland, culminating in regeneration of destroyed cell populations. In many other tissues, adaptive and regenerative processes involve the local stem cells. Over the last 15 years, evidence has accumulated that the pituitary gland houses a resident stem cell compartment. Recent studies propose their involvement in at least some of the cell remodeling processes that occur in the postnatal pituitary but support is still fragmentary and not unequivocal. Many questions remain unsolved such as whether the stem cells are key players in the vivid neonatal growth phase and whether the decline in pituitary function at old age is associated with decreased stem cell fitness. Furthermore, the underlying molecular mechanisms of pituitary plasticity, in particular the stem cell-linked ones, are still largely unknown. Pituitary research heavily relies on transgenic in vivo mouse models. While having proven their value, answers to pituitary stem cell-focused questions may more diligently come from a novel powerful in vitro research model, termed organoids, which grow from pituitary stem cells and recapitulate stem cell phenotype and activation status. In this review, we describe pituitary plasticity conditions and summarize what is known on the involvement and phenotype of pituitary stem cells during these pituitary remodeling events.

Induction of Functional Hypothalamus and Pituitary Tissues From Pluripotent Stem Cells for Regenerative Medicine

The hypothalamus and pituitary have been identified to play essential roles in maintaining homeostasis. Various diseases can disrupt the functions of these systems, which can often result in serious lifelong symptoms. The current treatment for hypopituitarism involves hormone replacement therapy. However, exogenous drug administration cannot mimic the physiological changes that are a result of hormone requirements. Therefore, patients are at a high risk of severe hormone deficiency, including adrenal crisis.

Pluripotent stem cells (PSCs) self-proliferate and differentiate into all types of cells. The generation of endocrine tissues from PSCs has been considered as another new treatment for hypopituitarism. Our colleagues established a 3-dimensional (3D) culture method for embryonic stem cells (ESCs). In this culture, the ESC-derived aggregates exhibit self-organization and spontaneous formation of highly ordered patterning. Recent results have shown that strict removal of exogenous patterning factors during early differentiation efficiently induces rostral hypothalamic progenitors from mouse ESCs. These hypothalamic progenitors generate vasopressinergic neurons, which release neuropeptides upon exogenous stimulation.

Subsequently, we reported adenohypophysis tissue self-formation in 3D cultures of mouse ESCs. The ESCs were found to differentiate into both nonneural oral ectoderm and hypothalamic neuroectoderm in adjacent layers. Interactions between the 2 tissues appear to be critically important for in vitro induction of a Rathke’s pouch-like developing embryo. Various endocrine cells were differentiated from nonneural ectoderm. The induced corticotrophs efficiently secreted adrenocorticotropic hormone when engrafted in vivo, which rescued hypopituitary hosts.

For future regenerative medicine, generation of hypothalamic and pituitary tissues from human PSCs is necessary. We and other groups succeeded in establishing a differentiation method with the use of human PSCs. Researchers could use these methods for models of human diseases to elucidate disease pathology or screen potential therapeutics.

Congenital pituitary hypoplasia model demonstrates hypothalamic OTX2 regulation of pituitary progenitor cells

Pituitary develops from oral ectoderm in contact with adjacent ventral hypothalamus. Impairment in this process results in congenital pituitary hypoplasia (CPH); however, there have been no human disease models for CPH thus far, prohibiting the elucidation of the underlying mechanisms. In this study, we established a disease model of CPH using patient-derived induced pluripotent stem cells (iPSCs) and 3D organoid technique, in which oral ectoderm and hypothalamus develop simultaneously. Interestingly, patient iPSCs with a heterozygous mutation in the orthodenticle homeobox 2 (OTX2) gene showed increased apoptosis in the pituitary progenitor cells, and the differentiation into pituitary hormone-producing cells was severely impaired. As an underlying mechanism, OTX2 in hypothalamus, not in oral ectoderm, was essential for progenitor cell maintenance by regulating LHX3 expression in oral ectoderm via FGF10 expression in the hypothalamus. Convincingly, the phenotype was reversed by the correction of the mutation, and the haploinsufficiency of OTX2 in control iPSCs revealed a similar phenotype, demonstrating that this mutation was responsible. Thus, we established an iPSC-based congenital pituitary disease model, which recapitulated interaction between hypothalamus and oral ectoderm and demonstrated the essential role of hypothalamic OTX2.

Brain Organoids: Tiny Mirrors of Human Neurodevelopment and Neurological Disorders

Unravelling the complexity of the human brain is a challenging task. Nowadays, modern neurobiologists have developed 3D model systems called "brain organoids" to overcome the technical challenges in understanding human brain development and the limitations of animal models to study neurological diseases. Certainly like most model systems in neuroscience, brain organoids too have limitations, as these minuscule brains lack the complex neuronal circuitry required to begin the operational tasks of human brain. However, researchers are hopeful that future endeavors with these 3D brain tissues could provide mechanistic insights into the generation of circuit complexity as well as reproducible creation of different regions of the human brain. Herein, we have presented the contemporary state of brain organoids with special emphasis on their mode of generation and their utility in modelling neurological disorders, drug discovery, and clinical trials.

Recent Progress in Stem Cell Research of the Pituitary Gland and Pituitary Adenoma

Regenerative medicine and anti-tumoral therapy have been developed through understanding tissue stem cells and cancer stem cells (CSCs). The concept of tissue stem cells has been applied to the pituitary gland (PG). Recently, PG stem cells (PGSCs) were successfully differentiated from human embryonic stem cells and induced pluripotent stem cells, showing an in vivo therapeutic effect in a hypopituitary model. Pituitary adenomas (PAs) are common intracranial neoplasms that are generally benign, but treatment resistance remains a major concern. The concept of CSCs applies to PA stem cells (PASCs). Genetic alterations in human PGSCs result in PASC development, leading to treatment-resistant PAs. To determine an efficient treatment against refractory PAs, it is of paramount importance to understand the relationship between PGSCs, PASCs and PAs. The goal of this review is to discuss several new findings about PGSCs and the roles of PASCs in PA tumorigenesis.

Hypothalamic Contribution to Pituitary Functions Is Recapitulated In Vitro Using 3D-Cultured Human iPS Cells

The pituitary is a major hormone center that secretes systemic hormones responding to hypothalamus-derived-releasing hormones. Previously, we reported the independent pituitary induction and hypothalamic differentiation of human embryonic stem cells (ESCs). Here, a functional hypothalamic-pituitary unit is generated using human induced pluripotent stem (iPS) cells in vitro. The adrenocorticotropic hormone (ACTH) secretion capacity of the induced pituitary reached a comparable level to that of adult mouse pituitary because of the simultaneous maturation with hypothalamic neurons within the same aggregates. Corticotropin-releasing hormone (CRH) from the hypothalamic area regulates ACTH cells similarly to our hypothalamic-pituitary axis. Our induced hypothalamic-pituitary units respond to environmental hypoglycemic condition in vitro, which mimics a life-threatening situation in vivo, through the CRH-ACTH pathway, and succeed in increasing ACTH secretion. Thus, we generated powerful hybrid organoids by recapitulating hypothalamic-pituitary development, showing autonomous maturation on the basis of interactions between developing tissues.

Molecular Mechanisms of Pituitary Cell Plasticity

The mechanisms that mediate plasticity in pituitary function have long been a subject of vigorous investigation. Early studies overcame technical barriers and challenged conceptual barriers to identify multipotential and multihormonal cell populations that contribute to diverse pituitary stress responses. Decades of intensive study have challenged the standard model of dedicated, cell type-specific hormone production and have revealed the malleable cellular fates that mediate pituitary responses. Ongoing studies at all levels, from animal physiology to molecular analyses, are identifying the mechanisms underlying this cellular plasticity. This review describes the findings from these studies that utilized state-of-the-art tools and techniques to identify mechanisms of plasticity throughout the pituitary and focuses on the insights brought to our understanding of pituitary function.

From Pituitary Stem Cell Differentiation to Regenerative Medicine

The anterior pituitary gland is comprised of specialized cell-types that produce and secrete polypeptide hormones in response to hypothalamic input and feedback from target organs. These specialized cells arise during embryonic development, from stem cells that express SOX2 and the pituitary transcription factor PROP1, which is necessary to establish the stem cell pool and promote an epithelial to mesenchymal-like transition, releasing progenitors from the niche. Human and mouse embryonic stem cells can differentiate into all major hormone-producing cell types of the anterior lobe in a highly plastic and dynamic manner. More recently human induced pluripotent stem cells (iPSCs) emerged as a viable alternative due to their plasticity and high proliferative capacity. This mini-review gives an overview of the major advances that have been achieved to develop protocols to generate pituitary hormone-producing cell types from stem cells and how these mechanisms are regulated. We also discuss their application in pituitary diseases, such as pituitary hormone deficiencies.

All Together Now: Modeling the Interaction of Neural With Non-neural Systems Using Organoid Models

The complex development of the human nervous system has been traditionally studied using a combination of animal models, human post-mortem brain tissue, and human genetics studies. However, there has been a lack of experimental human cellular models that would allow for a more precise elucidation of the intricate dynamics of early human brain development. The development of stem cell technologies, both embryonic and induced pluripotent stem cells (iPSCs), has given neuroscientists access to the previously inaccessible early stages of human brain development. In particular, the recent development of three-dimensional culturing methodologies provides a platform to study the differentiation of stem cells in both normal development and disease states in a more in vivo like context. Three-dimensional neural models or cerebral organoids possess an innate advantage over two-dimensional neural cultures as they can recapitulate tissue organization and cell type diversity that resemble the developing brain. Brain organoids also provide the exciting opportunity to model the integration of different brain regions in vitro. Furthermore, recent advances in the differentiation of non-neuronal tissue from stem cells provides the opportunity to study the interaction between the developing nervous system and other non-neuronal systems that impact neuronal function. In this review, we discuss the potential and limitations of the organoid system to study in vitro neurological diseases that arise in the neuroendocrine and the enteric nervous system or from interactions with the immune system.

Lipid Deprivation Induces a Stable, Naive-to-Primed Intermediate State of Pluripotency in Human PSCs

Current challenges in capturing naive human pluripotent stem cells (hPSCs) suggest that the factors regulating human naive versus primed pluripotency remain incompletely defined. Here we demonstrate that the widely used Essential 8 minimal medium (E8) captures hPSCs at a naive-to-primed intermediate state of pluripotency expressing several naive-like developmental, bioenergetic, and epigenomic features despite providing primed-state-sustaining growth factor conditions. Transcriptionally, E8 hPSCs are marked by activated lipid biosynthesis and suppressed MAPK/TGF-β gene expression, resulting in endogenous ERK inhibition. These features are dependent on lipid-free culture conditions and are lost upon lipid exposure, whereas short-term pharmacological ERK inhibition restores naive-to-primed intermediate traits even in the presence of lipids. Finally, we identify de novo lipogenesis as a common transcriptional signature of E8 hPSCs and the pre-implantation human epiblast in vivo. These findings implicate exogenous lipid availability in regulating human pluripotency and define E8 hPSCs as a stable, naive-to-primed intermediate (NPI) pluripotent state.

Organoids from pituitary as a novel research model toward pituitary stem cell exploration

The pituitary is the master endocrine gland, harboring stem cells of which the phenotype and role remain poorly characterized. Here, we established organoids from mouse pituitary with the aim to generate a novel research model to study pituitary stem cell biology. The organoids originated from the pituitary cells expressing the stem cell marker SOX2 were long-term expandable, displayed a stemness phenotype during expansive culture and showed specific hormonal differentiation ability, although limited, after subrenal transplantation. Application of the protocol to transgenically injured pituitary harboring an activated stem cell population, resulted in more numerous organoids. Intriguingly, these organoids presented with a cystic morphology, whereas the organoids from undamaged gland were predominantly dense and appeared more limited in expandability. Transcriptomic analysis revealed distinct epithelial phenotypes and showed that cystic organoids more resembled the pituitary phenotype, at least to an immature state, and displayed in vitro differentiation, although yet moderate. Organoid characterization further exposed facets of regulatory pathways of the putative stem cells of the pituitary and advanced new injury-activated markers. Taken together, we established a novel organoid research model revealing new insights into the identity and regulation of the putative pituitary stem cells. This organoid model may eventually lead to an interesting tool to decipher pituitary stem cell biology in both healthy and diseased gland.

The Processes of Anterior Pituitary Hormone Pulse Generation

More than 60 years ago, Geoffrey Harris described his "neurohumoral theory," in which the regulation of pituitary hormone secretion was a "simple" hierarchal relationship, with the hypothalamus as the controller. In models based on this theory, the electrical activity of hypothalamic neurons determines the release of hypophysiotropic hormones into the portal circulation, and the pituitary simply responds with secretion of a pulse of hormone into the bloodstream. The development of methodologies allowing the monitoring of the activities of members of the hypothalamic-vascular-pituitary unit is increasingly allowing dissection of the mechanisms generating hypothalamic and pituitary pulses. These have revealed that whereas hypothalamic input is required, its role as a driver of pulsatile pituitary hormone secretion varies between pituitary axes. The organization of pituitary cells has a key role in the modification of their response to hypophysiotropic factors that can lead to a memory of previous demand and enhanced function. Feedback can lead to oscillatory hormone output that is independent of pulses of hypophysiotropic factors and instead, results from the temporal relationship between pituitary output and target organ response. Thus, the mechanisms underlying the generation of pulses cannot be generalized, and the circularity of feedforward and feedback interactions must be considered to understand both normal physiological function and pathology. We describe some examples of the clinical implications of recognizing the importance of the pituitary and target organs in pulse generation and suggest avenues for future research in both the short and long term.

Human iPSC-derived trigeminal neurons lack constitutive TLR3-dependent immunity that protects cortical neurons from HSV-1 infection

Herpes simplex virus type 1 (HSV-1) encephalitis (HSE) is the most common sporadic viral encephalitis in Western countries. Some HSE children carry inborn errors of the Toll-like receptor 3 (TLR3)-dependent IFN-α/β- and -λ-inducing pathway. Induced pluripotent stem cell (iPSC)-derived cortical neurons with TLR3 pathway mutations are highly susceptible to HSV-1, due to impairment of cell-intrinsic TLR3-IFN immunity. In contrast, the contribution of cell-intrinsic immunity of human trigeminal ganglion (TG) neurons remains unclear. Here, we describe efficient in vitro derivation and purification of TG neurons from human iPSCs via a cranial placode intermediate. The resulting TG neurons are of sensory identity and exhibit robust responses to heat (capsaicin), cold (icilin), and inflammatory pain (ATP). Unlike control cortical neurons, both control and TLR3-deficient TG neurons were highly susceptible to HSV-1. However, pretreatment of control TG neurons with poly(I:C) induced the cells into an anti-HSV-1 state. Moreover, both control and TLR3-deficient TG neurons developed resistance to HSV-1 following pretreatment with IFN-β but not IFN-λ. These data indicate that TG neurons are vulnerable to HSV-1 because they require preemptive stimulation of the TLR3 or IFN-α/β receptors to induce antiviral immunity, whereas cortical neurons possess a TLR3-dependent constitutive resistance that is sufficient to block incoming HSV-1 in the absence of prior antiviral signals. The lack of constitutive resistance in TG neurons in vitro is consistent with their exploitation as a latent virus reservoir in vivo. Our results incriminate deficiencies in the constitutive TLR3-dependent response of cortical neurons in the pathogenesis of HSE.

Regulation of Pituitary Progenitor Differentiation by β-Catenin

The pituitary gland is a critical organ that is necessary for many physiological processes, including growth, reproduction, and stress response. The secretion of pituitary hormones from specific cell types regulates these essential processes. Pituitary hormone cell types arise from a common pool of pituitary progenitors, and mutations that disrupt the formation and differentiation of pituitary progenitors result in hypopituitarism. Canonical WNT signaling through CTNNB1 (β-catenin) is known to regulate the formation of the POU1F1 lineage of pituitary cell types. When β-catenin is deleted during the initial formation of the pituitary progenitors, Pou1f1 is not transcribed, which leads to the loss of the POU1F1 lineage. However, when β-catenin is deleted after lineage specification, there is no observable effect. Similarly, the generation of a β-catenin gain-of-function allele in early pituitary progenitors or stem cells results in the formation of craniopharyngiomas, whereas stimulating β-catenin in differentiated cell types has no effect. PROP1 is a pituitary-specific transcription factor, and the peak of PROP1 expression coincides with a critical time point in pituitary organogenesis-that is, after pituitary progenitor formation but before lineage specification. We used a Prop1-cre to conduct both loss- and gain-of-function studies on β-catenin during this critical time point. Our results demonstrate that pituitary progenitors remain sensitive to both loss and gain of β-catenin at this time point, and that either manipulation results in hypopituitarism.

Molecular Mechanisms Governing Embryonic Differentiation of Pituitary Somatotropes

Pituitary somatotropes secrete growth hormone (GH), which is essential for normal growth and metabolism. Somatotrope defects result in GH deficiency (GHD), leading to short stature in childhood and increased cardiovascular morbidity and mortality in adulthood. Current hormone replacement therapies fail to recapitulate normal pulsatile GH secretion. Stem cell therapies could overcome this problem but are dependent on a thorough understanding of somatotrope differentiation. Although several transcription factors, signaling pathways, and hormones that regulate this process have been identified, the mechanisms of action are not well understood. The purpose of this review is to highlight the known players in somatotrope differentiation while emphasizing the need to better understand these pathways to serve patients with GHD.

A Modular Platform for Differentiation of Human PSCs into All Major Ectodermal Lineages

Directing the fate of human pluripotent stem cells (hPSCs) into different lineages requires variable starting conditions and components with undefined activities, introducing inconsistencies that confound reproducibility and assessment of specific perturbations. Here we introduce a simple, modular protocol for deriving the four main ectodermal lineages from hPSCs. By precisely varying FGF, BMP, WNT, and TGFβ pathway activity in a minimal, chemically defined medium, we show parallel, robust, and reproducible derivation of neuroectoderm, neural crest (NC), cranial placode (CP), and non-neural ectoderm in multiple hPSC lines, on different substrates independently of cell density. We highlight the utility of this system by interrogating the role of TFAP2 transcription factors in ectodermal differentiation, revealing the importance of TFAP2A in NC and CP specification, and performing a small-molecule screen that identified compounds that further enhance CP differentiation. This platform provides a simple stage for systematic derivation of the entire range of ectodermal cell types.

Basic Research Advances on Pituitary Stem Cell Function and Regulation

As a central regulator of major physiological processes, the pituitary gland is a highly dynamic organ, capable of responding to hormonal demand and hypothalamic influence, through adapting secretion as well as remodelling cell numbers among its seven populations of differentiated cells. Stem cells of the pituitary have been shown to actively generate new cells during postnatal development but remain mostly quiescent during adulthood, where they persist as a long-lived population. Despite a significant body of research characterising attributes of anterior pituitary stem cells, the regulation of this population is poorly understood. A better grasp on the signalling mechanisms influencing stem proliferation and cell fate decisions can impact on our future treatments of pituitary gland disorders such as organ failure and pituitary tumours, which can disrupt endocrine homeostasis with life-long consequences. This minireview addresses the current methodologies aiming to understand better the attributes of pituitary stem cells and the normal regulation of this population in the organ, and discusses putative future avenues to manipulate pituitary stem cells during disease states or regenerative medicine approaches.

Pituitary stem cell regulation: who is pulling the strings?

The pituitary gland plays a pivotal role in the endocrine system, steering fundamental processes of growth, metabolism, reproduction and coping with stress. The adult pituitary contains resident stem cells, which are highly quiescent in homeostatic conditions. However, the cells show marked signs of activation during processes of increased cell remodeling in the gland, including maturation at neonatal age, adaptation to physiological demands, regeneration upon injury and growth of local tumors. Although functions of pituitary stem cells are slowly but gradually uncovered, their regulation largely remains virgin territory. Since postnatal stem cells in general reiterate embryonic developmental pathways, attention is first being given to regulatory networks involved in pituitary embryogenesis. Here, we give an overview of the current knowledge on the NOTCH, WNT, epithelial-mesenchymal transition, SHH and Hippo pathways in the pituitary stem/progenitor cell compartment during various (activation) conditions from embryonic over neonatal to adult age. Most information comes from expression analyses of molecular components belonging to these networks, whereas functional extrapolation is still very limited. From this overview, it emerges that the 'big five' embryonic pathways are indeed reiterated in the stem cells of the 'lazy' homeostatic postnatal pituitary, further magnified en route to activation in more energetic, physiological and pathological remodeling conditions. Increasing the knowledge on the molecular players that pull the regulatory strings of the pituitary stem cells will not only provide further fundamental insight in postnatal pituitary homeostasis and activation, but also clues toward the development of regenerative ideas for improving treatment of pituitary deficiency and tumors.

Regulation of pituitary stem cells by epithelial to mesenchymal transition events and signaling pathways

The anterior pituitary gland is comprised of specialized cell-types that produce and secrete polypeptide hormones in response to hypothalamic input and feedback from target organs. These specialized cells arise from stem cells that express SOX2 and the pituitary transcription factor PROP1, which is necessary to establish the stem cell pool and promote an epithelial to mesenchymal-like transition, releasing progenitors from the niche. The adult anterior pituitary responds to physiological challenge by mobilizing the SOX2-expressing progenitor pool and producing additional hormone-producing cells. Knowledge of the role of signaling pathways and extracellular matrix components in these processes may lead to improvements in the efficiency of differentiation of embryonic stem cells or induced pluripotent stem cells into hormone producing cells in vitro. Advances in our basic understanding of pituitary stem cell regulation and differentiation may lead to improved diagnosis and treatment for patients with hypopituitarism.

Bringing Neural Cell Therapies to the Clinic: Past and Future Strategies

Cell replacement therapy in the nervous system has a rich history, with ∼40 years of research and ∼30 years of clinical experience. There is compelling evidence that appropriate cells can integrate and function in the dysfunctioning human nervous system, but the clinical results are mixed in practice. A number of factors conspire to vary patient outcome: the indication, cell source, patient selection, and team performing transplantation are all variables that can affect efficacy. Most early clinical trials have used fetal cells, a limited cell source that resists scale and standardization. Direct fetal cell transplantation creates significant challenges to commercialization that is the ultimate goal of an effective cell therapy. One approach to help scale and standardize fetal cell preparations is the expansion of neural cells in vitro. Expansion is achieved by transformation or through the application of mitogens before cryopreservation. Recently, neural cells derived from pluripotent stem cells have provided a scalable alternative. Pluripotent stem cells are desirable for manufacturing but present alternative concerns and manufacturing obstacles. All cell sources require robust and reproducible manufacturing to make nervous system cell replacement therapy an option for patients. Here, we discuss the challenges and opportunities for cell replacement in the nervous system. In this review, we give an overview of completed and ongoing neural cell transplantation clinical trials, and we discuss the challenges and opportunities for future cell replacement trials with a particular focus on pluripotent stem cell-derived therapies.

Combined small-molecule inhibition accelerates the derivation of functional cortical neurons from human pluripotent stem cells

Considerable progress has been made in converting human pluripotent stem cells (hPSCs) into functional neurons. However, the protracted timing of human neuron specification and functional maturation remains a key challenge that hampers the routine application of hPSC-derived lineages in disease modeling and regenerative medicine. Using a combinatorial small-molecule screen, we previously identified conditions to rapidly differentiate hPSCs into peripheral sensory neurons. Here we generalize the approach to central nervous system (CNS) fates by developing a small-molecule approach for accelerated induction of early-born cortical neurons. Combinatorial application of six pathway inhibitors induces post-mitotic cortical neurons with functional electrophysiological properties by day 16 of differentiation, in the absence of glial cell co-culture. The resulting neurons, transplanted at 8 d of differentiation into the postnatal mouse cortex, are functional and establish long-distance projections, as shown using iDISCO whole-brain imaging. Accelerated differentiation into cortical neuron fates should facilitate hPSC-based strategies for disease modeling and cell therapy in CNS disorders.

CNS disease models with human pluripotent stem cells in the CRISPR age

In vitro differentiation of human pluripotent stem cells provides a systematic platform to investigate the physiological development and function of the human nervous system, as well as the etiology and consequence when these processes go awry. Recent development in three-dimensional (3D) organotypic culture systems allows modeling of the complex structure formation of the human CNS, and the intricate interactions between various resident neuronal and glial cell types. Combined with an ever-expanding genome editing and regulation toolkit such as CRISPR/Cas9, it is now a possibility to study human neurological disease in the relevant molecular, cellular and anatomical context. In this article, we review recent progress in 3D neural culture and the implications for disease modeling.