Evolution of new cell types at the lateral neural border

Canonical and Non-Canonical Localization of Tight Junction Proteins during Early Murine Cranial Development

This study investigates the intricate composition and spatial distribution of tight junction complex proteins during early mouse neurulation. The analyses focused on the cranial neural tube, which gives rise to all head structures. Neurulation brings about significant changes in the neuronal and non-neuronal ectoderm at a cellular and tissue level. During this process, precise coordination of both epithelial integrity and epithelial dynamics is essential for accurate tissue morphogenesis. Tight junctions are pivotal for epithelial integrity, yet their complex composition in this context remains poorly understood. Our examination of various tight junction proteins in the forebrain region of mouse embryos revealed distinct patterns in the neuronal and non-neuronal ectoderm, as well as mesoderm-derived mesenchymal cells. While claudin-4 exhibited exclusive expression in the non-neuronal ectoderm, we demonstrated a neuronal ectoderm specific localization for claudin-12 in the developing cranial neural tube. Claudin-5 was uniquely present in mesenchymal cells. Regarding the subcellular localization, canonical tight junction localization in the apical junctions was predominant for most tight junction complex proteins. ZO-1 (zona occludens protein-1), claudin-1, claudin-4, claudin-12, and occludin were detected at the apical junction. However, claudin-1 and occludin also appeared in basolateral domains. Intriguingly, claudin-3 displayed a non-canonical localization, overlapping with a nuclear lamina marker. These findings highlight the diverse tissue and subcellular distribution of tight junction proteins and emphasize the need for their precise regulation during the dynamic processes of forebrain development. The study can thereby contribute to a better understanding of the role of tight junction complex proteins in forebrain development.

scRNA-sequencing in chick suggests a probabilistic model for cell fate allocation at the neural plate border

The vertebrate 'neural plate border' is a transient territory located at the edge of the neural plate containing precursors for all ectodermal derivatives: the neural plate, neural crest, placodes and epidermis. Elegant functional experiments in a range of vertebrate models have provided an in-depth understanding of gene regulatory interactions within the ectoderm. However, these experiments conducted at tissue level raise seemingly contradictory models for fate allocation of individual cells. Here, we carry out single cell RNA sequencing of chick ectoderm from primitive streak to neurulation stage, to explore cell state diversity and heterogeneity. We characterise the dynamics of gene modules, allowing us to model the order of molecular events which take place as ectodermal fates segregate. Furthermore, we find that genes previously classified as neural plate border 'specifiers' typically exhibit dynamic expression patterns and are enriched in either neural, neural crest or placodal fates, revealing that the neural plate border should be seen as a heterogeneous ectodermal territory and not a discrete transitional transcriptional state. Analysis of neural, neural crest and placodal markers reveals that individual NPB cells co-express competing transcriptional programmes suggesting that their ultimate identify is not yet fixed. This population of 'border located undecided progenitors' (BLUPs) gradually diminishes as cell fate decisions take place. Considering our findings, we propose a probabilistic model for cell fate choice at the neural plate border. Our data suggest that the probability of a progenitor's daughters to contribute to a given ectodermal derivative is related to the balance of competing transcriptional programmes, which in turn are regulated by the spatiotemporal position of a progenitor.

Time to go: neural crest cell epithelial-to-mesenchymal transition

Neural crest cells (NCCs) are a dynamic, multipotent, vertebrate-specific population of embryonic stem cells. These ectodermally-derived cells contribute to diverse tissue types in developing embryos including craniofacial bone and cartilage, the peripheral and enteric nervous systems and pigment cells, among a host of other cell types. Due to their contribution to a significant number of adult tissue types, the mechanisms that drive their formation, migration and differentiation are highly studied. NCCs have a unique ability to transition from tightly adherent epithelial cells to mesenchymal and migratory cells by altering their polarity, expression of cell-cell adhesion molecules and gaining invasive abilities. In this Review, we discuss classical and emerging factors driving NCC epithelial-to-mesenchymal transition and migration, highlighting the role of signaling and transcription factors, as well as novel modifying factors including chromatin remodelers, small RNAs and post-translational regulators, which control the availability and longevity of major NCC players.

Vertebrate Cell Differentiation, Evolution, and Diseases: The Vertebrate-Specific Developmental Potential Guardians VENTX/NANOG and POU5/OCT4 Enter the Stage

During vertebrate development, embryonic cells pass through a continuum of transitory pluripotent states that precede multi-lineage commitment and morphogenesis. Such states are referred to as “refractory/naïve” and “competent/formative” pluripotency. The molecular mechanisms maintaining refractory pluripotency or driving the transition to competent pluripotency, as well as the cues regulating multi-lineage commitment, are evolutionarily conserved. Vertebrate-specific “Developmental Potential Guardians” (vsDPGs; i.e., VENTX/NANOG, POU5/OCT4), together with MEK1 (MAP2K1), coordinate the pluripotency continuum, competence for multi-lineage commitment and morphogenesis in vivo. During neurulation, vsDPGs empower ectodermal cells of the neuro-epithelial border (NEB) with multipotency and ectomesenchyme potential through an “endogenous reprogramming” process, giving rise to the neural crest cells (NCCs). Furthermore, vsDPGs are expressed in undifferentiated-bipotent neuro-mesodermal progenitor cells (NMPs), which participate in posterior axis elongation and growth. Finally, vsDPGs are involved in carcinogenesis, whereby they confer selective advantage to cancer stem cells (CSCs) and therapeutic resistance. Intriguingly, the heterogenous distribution of vsDPGs in these cell types impact on cellular potential and features. Here, we summarize the findings about the role of vsDPGs during vertebrate development and their selective advantage in evolution. Our aim to present a holistic view regarding vsDPGs as facilitators of both cell plasticity/adaptability and morphological innovation/variation. Moreover, vsDPGs may also be at the heart of carcinogenesis by allowing malignant cells to escape from physiological constraints and surveillance mechanisms.

The conserved core of the nereid brain: Circular CNS, apical nervous system and lhx6-arx-dlx neurons

When bilaterian animals first emerged, an enhanced perception of the Precambrian environment was key to their stunning success. This occurred through the acquisition of an anterior brain, as found in most extant bilaterians. What were the core circuits of the first brain, and how do they relate to today's diversity? With two landmark resources - the full connectome and a multimodal cellular atlas combining gene expression and ultrastructure - the young worm of the marine annelid Platynereis dumerilii takes center stage in comparative bilaterian neuroanatomy. The new data suggest a composite structure of the ancestral bilaterian brain, with the anterior end of a circular CNS fused to a sensory-neurosecretory apical system, and with lhx6-arx-dlx chemosensory circuits giving rise to associative centers in the descending bilaterian lineages.

Riding the crest to get a head: neural crest evolution in vertebrates

In their seminal 1983 paper, Gans and Northcutt proposed that evolution of the vertebrate 'new head' was made possible by the advent of the neural crest and cranial placodes. The neural crest is a stem cell population that arises adjacent to the forming CNS and contributes to important cell types, including components of the peripheral nervous system and craniofacial skeleton and elements of the cardiovascular system. In the past few years, the new head hypothesis has been challenged by the discovery in invertebrate chordates of cells with some, but not all, characteristics of vertebrate neural crest cells. Here, we discuss recent findings regarding how neural crest cells may have evolved during the course of deuterostome evolution. The results suggest that there was progressive addition of cell types to the repertoire of neural crest derivatives throughout vertebrate evolution. Novel genomic tools have enabled higher resolution insight into neural crest evolution, from both a cellular and a gene regulatory perspective. Together, these data provide clues regarding the ancestral neural crest state and how the neural crest continues to evolve to contribute to the success of vertebrates as efficient predators.

A developmental perspective of homology and evolutionary novelty

The development and evolution of multicellular body plans is complex. Many distinct organs and body parts must be reproduced at each generation, and those that are traceable over long time scales are considered homologous. Among the most pressing and least understood phenomena in evolutionary biology is the mode by which new homologs, or "novelties" are introduced to the body plan and whether the developmental changes associated with such evolution deserve special treatment. In this chapter, we address the concepts of homology and evolutionary novelty through the lens of development. We present a series of case studies, within insects and vertebrates, from which we propose a developmental model of multicellular organ identity. With this model in hand, we make predictions regarding the developmental evolution of body plans and highlight the need for more integrative analysis of developing systems.