Lhx2/9 and Etv1 Transcription Factors have Complementary roles in Regulating the Expression of Guidance Genes slit1 and sema3a

A neuronal Slit1-dependent program rescues oligodendrocyte differentiation and myelination under chronic hypoxic conditions

Oligodendrocyte maturation arrest in hypoxia-induced white matter injury (WMI) results in long-term neurofunctional disabilities of preterm infants. Although neurons are closely linked to myelination regulation, how neurons respond to the above process remains elusive. Here, we identify a compensatory role of neuronal Slit1-dependent signaling in protecting against hypoxia-induced hypomyelination and ameliorating motor and cognitive disabilities. Conditional ablation of Slit1 in neurons exacerbates hypoxia-induced hypomyelination but is negligible for developmental myelination. Secreted Slit1 from hypoxic neurons directly targets oligodendrocyte, acting through Robo2-srGAP1-RhoA signaling. Pharmacological inhibition of RhoA restores myelination and promotes neurofunctional recovery in adolescent mice. Notably, natural selection analysis and functional validation indicate an adaptive variant with higher Slit1 gene expression in the Tibetan population, which has low oxygen availability. Collectively, these findings show a neuronal Slit1-dependent program of OL differentiation and suggest that targeting the Slit1-Robo2 signaling axis may have therapeutic potential for treatment of preterm infants with hypoxic WMI.

The Role of Pea3 Transcription Factor Subfamily in the Nervous System

ETS domain transcription factor superfamily is highly conserved throughout metazoa and is involved in many aspects of development and tissue morphogenesis, and as such, the deregulation of ETS proteins is quite common in many diseases, including cancer. The PEA3 subfamily in particular has been extensively studied with respect to tumorigenesis and metastasis; however, they are also involved in the development of many tissues with branching morphogenesis, such as lung or kidney development. In this review, we aim to summarize findings from various studies on the role of Pea3 subfamily members in nervous system development in the embryo, as well as their functions in the adult neurons. We further discuss the different signals that were shown to regulate the function of the Pea3 family and indicate how this signal-dependent regulation of Pea3 proteins can generate neuronal circuit specificity through unique gene regulation. Finally, we discuss how these developmental roles of Pea3 proteins relate to their role in tumorigenesis.

Multiomic Analysis of Neurons with Divergent Projection Patterns Identifies Novel Regulators of Axon Pathfinding

Axon pathfinding is a key step in neural circuits formation. However, the transcriptional mechanisms regulating its progression remain poorly understood. The binary decision of crossing or avoiding the midline taken by some neuronal axons during development represents a robust model to investigate the mechanisms that control the selection of axonal trajectories. Here, to identify novel regulators of axon guidance, this work compares the transcriptome and chromatin occupancy profiles of two neuronal subpopulations, ipsilateral (iRGC) and contralateral retinal ganglion cells (cRGC), with similar functions but divergent axon trajectories. These analyses retrieved a number of genes encoding for proteins not previously implicated in axon pathfinding. In vivo functional experiments confirm the implication of some of these candidates in axonal navigation. Among the candidate genes, γ-synuclein is identified as essential for inducing midline crossing. Footprint and luciferase assays demonstrate that this small-sized protein is regulated by the transcription factor (TF) Pou4f1 in cRGCs. It is also shown that Lhx2/9 are specifically expressed in iRGCs and control a program that partially overlaps with that regulated by Zic2, previously described as essential for iRGC specification. Overall, the analyses identify dozens of new molecules potentially involved in axon guidance and reveal the regulatory logic behind the selection of axonal trajectories.

Axonal Projection Patterns of the Dorsal Interneuron Populations in the Embryonic Hindbrain

Unraveling the inner workings of neural circuits entails understanding the cellular origin and axonal pathfinding of various neuronal groups during development. In the embryonic hindbrain, different subtypes of dorsal interneurons (dINs) evolve along the dorsal-ventral (DV) axis of rhombomeres and are imperative for the assembly of central brainstem circuits. dINs are divided into two classes, class A and class B, each containing four neuronal subgroups (dA1-4 and dB1-4) that are born in well-defined DV positions. While all interneurons belonging to class A express the transcription factor Olig3 and become excitatory, all class B interneurons express the transcription factor Lbx1 but are diverse in their excitatory or inhibitory fate. Moreover, within every class, each interneuron subtype displays its own specification genes and axonal projection patterns which are required to govern the stage-by-stage assembly of their connectivity toward their target sites. Remarkably, despite the similar genetic landmark of each dINs subgroup along the anterior-posterior (AP) axis of the hindbrain, genetic fate maps of some dA/dB neuronal subtypes uncovered their contribution to different nuclei centers in relation to their rhombomeric origin. Thus, DV and AP positional information has to be orchestrated in each dA/dB subpopulation to form distinct neuronal circuits in the hindbrain. Over the span of several decades, different axonal routes have been well-documented to dynamically emerge and grow throughout the hindbrain DV and AP positions. Yet, the genetic link between these distinct axonal bundles and their neuronal origin is not fully clear. In this study, we reviewed the available data regarding the association between the specification of early-born dorsal interneuron subpopulations in the hindbrain and their axonal circuitry development and fate, as well as the present existing knowledge on molecular effectors underlying the process of axonal growth.

Xenopus leads the way: Frogs as a pioneering model to understand the human brain

From its long history in the field of embryology to its recent advances in genetics, Xenopus has been an indispensable model for understanding the human brain. Foundational studies that gave us our first insights into major embryonic patterning events serve as a crucial backdrop for newer avenues of investigation into organogenesis and organ function. The vast array of tools available in Xenopus laevis and Xenopus tropicalis allows interrogation of developmental phenomena at all levels, from the molecular to the behavioral, and the application of CRISPR technology has enabled the investigation of human disorder risk genes in a higher-throughput manner. As the only major tetrapod model in which all developmental stages are easily manipulated and observed, frogs provide the unique opportunity to study organ development from the earliest stages. All of these features make Xenopus a premier model for studying the development of the brain, a notoriously complex process that demands an understanding of all stages from fertilization to organogenesis and beyond. Importantly, core processes of brain development are conserved between Xenopus and human, underlining the advantages of this model. This review begins by summarizing discoveries made in amphibians that form the cornerstones of vertebrate neurodevelopmental biology and goes on to discuss recent advances that have catapulted our understanding of brain development in Xenopus and in relation to human development and disease. As we engage in a new era of patient-driven gene discovery, Xenopus offers exceptional potential to uncover conserved biology underlying human brain disorders and move towards rational drug design.