Morphological and Functional Changes of Roof Plate Cells in Spinal Cord Development

From neural tube to spinal cord: The dynamic journey of the dorsal neuroepithelium

In a developing embryo, formation of tissues and organs is remarkably precise in both time and space. Through cell-cell interactions, neighboring progenitors coordinate their activities, sequentially generating distinct types of cells. At present, we only have limited knowledge, rather than a systematic understanding, of the underlying logic and mechanisms responsible for cell fate transitions. The formation of the dorsal aspect of the spinal cord is an outstanding model to tackle these dynamics, as it first generates the peripheral nervous system and is later responsible for transmitting sensory information from the periphery to the brain and for coordinating local reflexes. This is reflected first by the ontogeny of neural crest cells, progenitors of the peripheral nervous system, followed by formation of the definitive roof plate of the central nervous system and specification of adjacent interneurons, then a transformation of roof plate into dorsal radial glia and ependyma lining the forming central canal. How do these peripheral and central neural branches segregate from common progenitors? How are dorsal radial glia established concomitant with transformation of the neural tube lumen into a central canal? How do the dorsal radial glia influence neighboring cells? This is only a partial list of questions whose clarification requires the implementation of experimental paradigms in which precise control of timing is crucial. Here, we outline some available answers and still open issues, while highlighting the contributions of avian models and their potential to address mechanisms of neural patterning and function.

Rspo1 and Rspo3 are required for sensory lineage neural crest formation in mouse embryos

BACKGROUND

R-spondins (Rspos) are secreted proteins that modulate Wnt/β-catenin signaling. At the early stages of spinal cord development, Wnts (Wnt1, Wnt3a) and Rspos (Rspo1, Rspo3) are co-expressed in the roof plate, suggesting that Rspos are involved in development of dorsal spinal cord and neural crest cells in cooperation with Wnt ligands.

RESULTS

Here, we found that Rspo1 and Rspo3, as well as Wnt1 and Wnt3a, maintained roof-plate-specific expression until late embryonic stages. Rspo1- and Rspo3-double-knock-out (dKO) embryos partially exhibited the phenotype of Wnt1 and Wnt3a dKO embryos. While the number of Ngn2-positive sensory lineage neural crest cells is reduced in Rspo-dKO embryos, development of dorsal spinal cord, including its size and dorso-ventral patterning in early development, elongation of the roof plate, and proliferation of ependymal cells, proceeded normally. Consistent with these slight defects, Wnt/β-catenin signaling was not obviously changed in developing spinal cord of dKO embryos.

CONCLUSIONS

Our results show that Rspo1 and Rspo3 are dispensable for most developmental processes involving roof plate-derived Wnt ligands, except for specification of a subtype of neural crest cells. Thus, Rspos may modulate Wnt/β-catenin signaling in a context-dependent manner.

Cell Transplantation for Repair of the Spinal Cord and Prospects for Generating Region-Specific Exogenic Neuronal Cells

Spinal cord injury (SCI) is associated with currently irreversible consequences in several functional components of the central nervous system. Despite the severity of injury, there remains no approved treatment to restore function. However, with a growing number of preclinical studies and clinical trials, cell transplantation has gained significant potential as a treatment for SCI. Researchers have identified several cell types as potential candidates for transplantation. To optimize successful functional outcomes after transplantation, one key factor concerns generating neuronal cells with regional and subtype specificity, thus calling on the developmental transcriptome patterning of spinal cord cells. A potential source of spinal cord cells for transplantation is the generation of exogenic neuronal progenitor cells via the emerging technologies of gene editing and blastocyst complementation. This review highlights the use of cell transplantation to treat SCI in the context of relevant developmental gene expression patterns useful for producing regionally specific exogenic spinal cells via in vitro differentiation and blastocyst complementation.

Regulation of axon pathfinding by astroglia across genetic model organisms

Glia and neurons are intimately associated throughout bilaterian nervous systems, and were early proposed to interact for patterning circuit assembly. The investigations of circuit formation progressed from early hypotheses of intermediate guideposts and a "glia blueprint", to recent genetic and cell manipulations, and visualizations in vivo. An array of molecular factors are implicated in axon pathfinding but their number appears small relatively to circuit complexity. Comprehending this circuit complexity requires to identify unknown factors and dissect molecular topographies. Glia contribute to both aspects and certain studies provide molecular and functional insights into these contributions. Here, I survey glial roles in guiding axon navigation in vivo, emphasizing analogies, differences and open questions across major genetic models. I highlight studies pioneering the topic, and dissect recent findings that further advance our current molecular understanding. Circuits of the vertebrate forebrain, visual system and neural tube in zebrafish, mouse and chick, the Drosophila ventral cord and the C. elegans brain-like neuropil emerge as major contexts to study glial cell functions in axon navigation. I present astroglial cell types in these models, and their molecular and cellular interactions that drive axon guidance. I underline shared principles across models, conceptual or technical complications, and open questions that await investigation. Glia of the radial-astrocyte lineage, emerge as regulators of axon pathfinding, often employing common molecular factors across models. Yet this survey also highlights different involvements of glia in embryonic navigation or pioneer axon pathfinding, and unknowns in the molecular underpinnings of glial cell functions. Future cellular and molecular investigations should complete the comprehensive view of glial roles in circuit assembly.

Single-cell transcriptomic landscape of the developing human spinal cord

Understanding spinal cord assembly is essential to elucidate how motor behavior is controlled and how disorders arise. The human spinal cord is exquisitely organized, and this complex organization contributes to the diversity and intricacy of motor behavior and sensory processing. But how this complexity arises at the cellular level in the human spinal cord remains unknown. Here we transcriptomically profiled the midgestation human spinal cord with single-cell resolution and discovered remarkable heterogeneity across and within cell types. Glia displayed diversity related to positional identity along the dorso-ventral and rostro-caudal axes, while astrocytes with specialized transcriptional programs mapped into white and gray matter subtypes. Motor neurons clustered at this stage into groups suggestive of alpha and gamma neurons. We also integrated our data with multiple existing datasets of the developing human spinal cord spanning 22 weeks of gestation to investigate the cell diversity over time. Together with mapping of disease-related genes, this transcriptomic mapping of the developing human spinal cord opens new avenues for interrogating the cellular basis of motor control in humans and guides human stem cell-based models of disease.

Scientific Papers by Developmental Biologists in Japan

We have assembled ten interesting manuscripts submitted by developmental biologists in Japan [...]

Stability of Pattern Formation in Systems with Dynamic Source Regions

We explain the principles of gene expression pattern stabilization in systems of interacting, diffusible morphogens, with dynamically established source regions. Using a reaction-diffusion model with a step-function production term, we identify the phase transition between low-precision indeterminate patterning and the phase in which a traveling, well-defined contact zone between two domains is formed. Our model analytically explains single- and two-gene domain dynamics and provides pattern stability conditions for all possible two-gene regulatory network motifs.

An ependymal cell census identifies heterogeneous and ongoing cell maturation in the adult mouse spinal cord that changes dynamically on injury

The adult spinal cord stem cell potential resides within the ependymal cell population and declines with age. Ependymal cells are, however, heterogeneous, and the biological diversity this represents and how it changes with age remain unknown. Here, we present a single-cell transcriptomic census of spinal cord ependymal cells from adult and aged mice, identifying not only all known ependymal cell subtypes but also immature as well as mature cell states. By comparing transcriptomes of spinal cord and brain ependymal cells, which lack stem cell abilities, we identify immature cells as potential spinal cord stem cells. Following spinal cord injury, these cells re-enter the cell cycle, which is accompanied by a short-lived reversal of ependymal cell maturation. We further analyze ependymal cells in the human spinal cord and identify widespread cell maturation and altered cell identities. This in-depth characterization of spinal cord ependymal cells provides insight into their biology and informs strategies for spinal cord repair.