The transcriptional portraits of the neural crest at the individual cell level

Head and neck paraganglioma in Pacak-Zhuang syndrome

BACKGROUND

Head and neck paragangliomas (HNPGLs) are typically slow-growing, hormonally inactive tumors of parasympathetic paraganglia. Inactivation of prolyl-hydroxylase domain-containing 2 protein causing indirect gain-of-function of hypoxia-inducible factor-2α (HIF-2α), encoded by EPAS1, was recently shown to cause carotid body hyperplasia. We previously described a syndrome with multiple sympathetic paragangliomas caused by direct gain-of-function variants in EPAS1 (Pacak-Zhuang syndrome, PZS) and developed a corresponding mouse model.

METHODS

We evaluated a cohort of patients with PZS (n = 9) for HNPGL by positron emission tomography, magnetic resonance imaging, and computed tomography and measured carotid body size compared to literature reference values. Resected tumors were evaluated by histologic sectioning and staining. We evaluated the corresponding mouse model at multiple developmental stages (P8 and adult) for lesions of the head and neck by high resolution ex vivo imaging and performed immunohistochemical staining on histologic sections of the identified lesions.

RESULTS

hree patients had imaging consistent with HNPGL, one of which warranted resection and was confirmed on histology. Three additional patients had carotid body enlargement (Z-score > 2.0), and 3 had carotid artery malformations. We found that 9 of 10 adult variant mice had carotid body tumors and 6 of 8 had a paraganglioma on the cranio-caval vein, the murine homologue of the superior vena cava; these were also found in 4 of 5 variant mice at post-natal day 8. These tumors and the one resected from a patient were positive for tyrosine hydroxylase, synaptophysin, and chromogranin A. Brown fat in a resected patient tumor carried the EPAS1 pathogenic variant.

CONCLUSIONS

These findings (1) suggest HNPGL as a feature of PZS and (2) show that these pathogenic variants are sufficient to cause the development of these tumors, which we believe represents a continuous spectrum of disease starting from hyperplasia.

Applying single-cell and single-nucleus genomics to studies of cellular heterogeneity and cell fate transitions in the nervous system

Single-cell and single-nucleus genomic approaches can provide unbiased and multimodal insights. Here, we discuss what constitutes a molecular cell atlas and how to leverage single-cell omics data to generate hypotheses and gain insights into cell transitions in development and disease of the nervous system. We share points of reflection on what to consider during study design and implementation as well as limitations and pitfalls.

Causes and consequences: development and pathophysiology of Hirschsprung disease

Hirschsprung disease (HSCR) is a congenital enteric neuropathy in which the enteric nervous system (ENS) fails to develop along variable lengths of the distal gastrointestinal (GI) tract. This aganglionosis results in a functional bowel obstruction and requires surgical resection of the aganglionic segment. Despite surgery, however, long-term bowel dysfunction affects many patients. Understanding the embryologic causes and pathophysiologic consequences of HSCR is critical to improving its diagnosis and treatment. During normal gut development, the ENS arises from neural crest cells (NCCs) that delaminate from the neural tube to populate the entire GI tract with enteric neurons and glia. This process requires NCCs to undergo proliferation, migration and differentiation to form the complex neuroglial network that regulates gut motility and other intestinal functions. This review discusses the cellular and molecular processes that control normal ENS formation and what goes awry to give rise to HSCR. The complex pathophysiologic consequences of aganglionosis are discussed, including recent observations that describe novel aspects of HSCR beyond the absence of ganglion cells. This review aims to expand the understanding of HSCR and to stimulate new ideas on how to improve current management of the disease.

Neural crest precursors from the skin are the primary source of directly reprogrammed neurons

Direct reprogramming involves the conversion of differentiated cell types without returning to an earlier developmental state. Here, we explore how heterogeneity in developmental lineage and maturity of the starting cell population contributes to direct reprogramming using the conversion of murine fibroblasts into neurons. Our hypothesis is that a single lineage of cells contributes to most reprogramming and that a rare elite precursor with intrinsic bias is the source of reprogrammed neurons. We find that nearly all reprogrammed neurons are derived from the neural crest (NC) lineage. Moreover, when rare proliferating NC precursors are selectively ablated, there is a large reduction in the number of reprogrammed neurons. Previous interpretations of this paradigm are that it demonstrates a cell fate conversion across embryonic germ layers (mesoderm to ectoderm). Our interpretation is that this is actually directed differentiation of a neural lineage stem cell in the skin that has intrinsic bias to produce neuronal progeny.

Motor innervation directs the correct development of the mouse sympathetic nervous system

The sympathetic nervous system controls bodily functions including vascular tone, cardiac rhythm, and the "fight-or-flight response". Sympathetic chain ganglia develop in parallel with preganglionic motor nerves extending from the neural tube, raising the question of whether axon targeting contributes to sympathetic chain formation. Using nerve-selective genetic ablations and lineage tracing in mouse, we reveal that motor nerve-associated Schwann cell precursors (SCPs) contribute sympathetic neurons and satellite glia after the initial seeding of sympathetic ganglia by neural crest. Motor nerve ablation causes mispositioning of SCP-derived sympathoblasts as well as sympathetic chain hypoplasia and fragmentation. Sympathetic neurons in motor-ablated embryos project precociously and abnormally towards dorsal root ganglia, eventually resulting in fusion of sympathetic and sensory ganglia. Cell interaction analysis identifies semaphorins as potential motor nerve-derived signaling molecules regulating sympathoblast positioning and outgrowth. Overall, central innervation functions both as infrastructure and regulatory niche to ensure the integrity of peripheral ganglia morphogenesis.

Melanocyte lineage dynamics in development, growth and disease

Melanocytes evolved to produce the melanin that gives colour to our hair, eyes and skin. The melanocyte lineage also gives rise to melanoma, the most lethal form of skin cancer. The melanocyte lineage differentiates from neural crest cells during development, and most melanocytes reside in the skin and hair, where they are replenished by melanocyte stem cells. Because the molecular mechanisms necessary for melanocyte specification, migration, proliferation and differentiation are co-opted during melanoma initiation and progression, studying melanocyte development is directly relevant to human disease. Here, through the lens of advances in cellular omic and genomic technologies, we review the latest findings in melanocyte development and differentiation, and how these developmental pathways become dysregulated in disease.

Molecular Characterization of Nodose Ganglia Development Reveals a Novel Population of Phox2b+ Glial Progenitors in Mice

The vagal ganglia, comprised of the superior (jugular) and inferior (nodose) ganglia of the vagus nerve, receive somatosensory information from the head and neck or viscerosensory information from the inner organs, respectively. Developmentally, the cranial neural crest gives rise to all vagal glial cells and to neurons of the jugular ganglia, while the epibranchial placode gives rise to neurons of the nodose ganglia. Crest-derived nodose glial progenitors can additionally generate autonomic neurons in the peripheral nervous system, but how these progenitors generate neurons is unknown. Here, we found that some Sox10+ neural crest-derived cells in, and surrounding, the nodose ganglion transiently expressed Phox2b, a master regulator of autonomic nervous system development, during early embryonic life. Our genetic lineage-tracing analysis in mice of either sex revealed that despite their common developmental origin and extreme spatial proximity, a substantial proportion of glial cells in the nodose, but not in the neighboring jugular ganglia, have a history of Phox2b expression. We used single-cell RNA-sequencing to demonstrate that these progenitors give rise to all major glial subtypes in the nodose ganglia, including Schwann cells, satellite glia, and glial precursors, and mapped their spatial distribution by in situ hybridization. Lastly, integration analysis revealed transcriptomic similarities between nodose and dorsal root ganglia glial subtypes and revealed immature nodose glial subtypes. Our work demonstrates that these crest-derived nodose glial progenitors transiently express Phox2b, give rise to the entire complement of nodose glial cells, and display a transcriptional program that may underlie their bipotent nature.

Piezo1 and Piezo2 collectively regulate jawbone development

Piezo1 and Piezo2 are recently reported mechanosensory ion channels that transduce mechanical stimuli from the environment into intracellular biochemical signals in various tissues and organ systems. Here, we show that Piezo1 and Piezo2 display a robust expression during jawbone development. Deletion of Piezo1 in neural crest cells causes jawbone malformations in a small but significant number of mice. We further demonstrate that disruption of Piezo1 and Piezo2 in neural crest cells causes more striking defects in jawbone development than any single knockout, suggesting essential but partially redundant roles of Piezo1 and Piezo2. In addition, we observe defects in other neural crest derivatives such as malformation of the vascular smooth muscle in double knockout mice. Moreover, TUNEL examinations reveal excessive cell death in osteogenic cells of the maxillary and mandibular arches of the double knockout mice, suggesting that Piezo1 and Piezo2 together regulate cell survival during jawbone development. We further demonstrate that Yoda1, a Piezo1 agonist, promotes mineralization in the mandibular arches. Altogether, these data firmly establish that Piezo channels play important roles in regulating jawbone formation and maintenance.

Differential developmental blueprints of organ-intrinsic nervous systems

The organ-intrinsic nervous system is a major interface between visceral organs and the brain, mediating important sensory and regulatory functions in the body-brain axis and serving as critical local processors for organ homeostasis. Molecularly, anatomically, and functionally, organ-intrinsic neurons are highly specialized for their host organs. However, the underlying mechanism that drives this specialization is largely unknown. Here, we describe the differential strategies utilized to achieve organ-specific organization between the enteric nervous system (ENS) 1 and the intrinsic cardiac nervous system (ICNS) 2 , a neuronal network essential for heart performance but poorly characterized. Integrating high-resolution whole-embryo imaging, single-cell genomics, spatial transcriptomics, proteomics, and bioinformatics, we uncover that unlike the ENS which is highly mobile and colonizes the entire gastrointestinal (GI) tract, the ICNS uses a rich set of extracellular matrix (ECM) genes that match with surrounding heart cells and an intermediate dedicated neuronal progenitor state to stabilize itself for a 'beads-on-the-necklace' organization on heart atria. While ICNS- and ENS-precursors are genetically similar, their differentiation paths are influenced by their host-organs, leading to distinct mature neuron types. Co-culturing ENS-precursors with heart cells shifts their identity towards the ICNS and induces the expression of heart-matching ECM genes. Our cross-organ study thus reveals fundamental principles for the maturation and specialization of organ-intrinsic neurons.

Glucose oxidation drives trunk neural crest cell development and fate

Bioenergetic metabolism is a key regulator of cellular function and signaling, but how it can instruct the behavior of cells and their fate during embryonic development remains largely unknown. Here, we investigated the role of glucose metabolism in the development of avian trunk neural crest cells (NCCs), a migratory stem cell population of the vertebrate embryo. We uncovered that trunk NCCs display glucose oxidation as a prominent metabolic phenotype, in contrast to what is seen for cranial NCCs, which instead rely on aerobic glycolysis. In addition, only one pathway downstream of glucose uptake is not sufficient for trunk NCC development. Indeed, glycolysis, mitochondrial respiration and the pentose phosphate pathway are all mobilized and integrated for the coordinated execution of diverse cellular programs, epithelial-to-mesenchymal transition, adhesion, locomotion, proliferation and differentiation, through regulation of specific gene expression. In the absence of glucose, the OXPHOS pathway fueled by pyruvate failed to promote trunk NCC adaptation to environmental stiffness, stemness maintenance and fate-decision making. These findings highlight the need for trunk NCCs to make the most of the glucose pathway potential to meet the high metabolic demands appropriate for their development.

Mesenchymal properties of iPSC-derived neural progenitors that generate undesired grafts after transplantation

Although neural stem/progenitor cells derived from human induced pluripotent stem cells (hiPSC-NS/PCs) are expected to be a cell source for cell-based therapy, tumorigenesis of hiPSC-NS/PCs is a potential problem for clinical applications. Therefore, to understand the mechanisms of tumorigenicity in NS/PCs, we clarified the cell populations of NS/PCs. We established single cell-derived NS/PC clones (scNS/PCs) from hiPSC-NS/PCs that generated undesired grafts. Additionally, we performed bioassays on scNS/PCs, which classified cell types within parental hiPSC-NS/PCs. Interestingly, we found unique subsets of scNS/PCs, which exhibited the transcriptome signature of mesenchymal lineages. Furthermore, these scNS/PCs expressed both neural (PSA-NCAM) and mesenchymal (CD73 and CD105) markers, and had an osteogenic differentiation capacity. Notably, eliminating CD73⁺ CD105⁺ cells from among parental hiPSC-NS/PCs ensured the quality of hiPSC-NS/PCs. Taken together, the existence of unexpected cell populations among NS/PCs may explain their tumorigenicity leading to potential safety issues of hiPSC-NS/PCs for future regenerative medicine.

Development and In Vitro Differentiation of Schwann Cells

Schwann cells are glial cells of the peripheral nervous system. They exist in several subtypes and perform a variety of functions in nerves. Their derivation and culture in vitro are interesting for applications ranging from disease modeling to tissue engineering. Since primary human Schwann cells are challenging to obtain in large quantities, in vitro differentiation from other cell types presents an alternative. Here, we first review the current knowledge on the developmental signaling mechanisms that determine neural crest and Schwann cell differentiation in vivo. Next, an overview of studies on the in vitro differentiation of Schwann cells from multipotent stem cell sources is provided. The molecules frequently used in those protocols and their involvement in the relevant signaling pathways are put into context and discussed. Focusing on hiPSC- and hESC-based studies, different protocols are described and compared, regarding cell sources, differentiation methods, characterization of cells, and protocol efficiency. A brief insight into developments regarding the culture and differentiation of Schwann cells in 3D is given. In summary, this contribution provides an overview of the current resources and methods for the differentiation of Schwann cells, it supports the comparison and refinement of protocols and aids the choice of suitable methods for specific applications.

Schwann cell precursors: a hub of neural crest development

Schwann cell precursors (SCPs) are transient glial progenitors that are important for the formation of late neural crest derivatives, yet their heterogeneity and developmental potential remain incompletely understood. In this issue, Kastriti, Faure, von Ahsen et al (2022) use comprehensive single-cell RNA sequencing analyses to identify a transient "hub" state common to SCPs and neural crest cells (NCCs), revealing a striking similarity of SCPs to late migrating NCCs. These results raise important questions about the potential role of such a state in adult tissue regeneration and tumourigenesis.