Cerebrospinal fluid-derived Semaphorin3B orients neuroepithelial cell divisions in the apicobasal axis

Plexina4 and cell survival in the developing zebrafish hindbrain

BACKGROUND

Growth factors are important in the developing and mature nervous system to support the survival of neurons. Developmental signaling molecules are known for their roles in controlling neurogenesis and neural circuit formation. Whether or not these molecules also have roles in cell survival in the developing nervous system is poorly understood. Plexins are a family of transmembrane receptors that bind Semaphorin ligands and are known to function in the guidance of developing axons and blood vessels.

RESULTS

In embryonic zebrafish, plexina4 is expressed widely in the brain, becoming largely restricted to the hindbrain as neurogenesis and differentiation proceed. Apoptosis is increased in the embryonic hindbrain of a plexina4ca307/ca307 CRISPR mutant. Based on the literature, we tested the secreted heat shock protein, Clusterin, as a candidate ligand to mediate cell survival through Plexina4. clusterin is expressed by the floor plate of the embryonic zebrafish hindbrain, in proximity to plexina4-expressing hindbrain cells. Morpholino-mediated knockdown of Clusterin increases cell apoptosis in the hindbrain, with additional cell death observed in epistasis experiments where Clusterin is knocked down in a plexina4 mutant background.

CONCLUSIONS

Our data suggest that Plexina4 promotes cell survival in the developing zebrafish hindbrain, likely through a pathway independent of Clusterin.

Identification of Novel Biomarkers of Spinal Muscular Atrophy and Therapeutic Response by Proteomic and Metabolomic Profiling of Human Biological Fluid Samples

Spinal muscular atrophy (SMA) is a neuromuscular disease resulting from mutations or deletions in SMN1 that lead to progressive death of alpha motor neurons, ultimately leading to severe muscle weakness and atrophy, as well as premature death in the absence of treatment. Recent approval of SMN-increasing medications as SMA therapy has altered the natural course of the disease. Thus, accurate biomarkers are needed to predict SMA severity, prognosis, drug response, and overall treatment efficacy. This article reviews novel non-targeted omics strategies that could become useful clinical tools for patients with SMA. Proteomics and metabolomics can provide insights into molecular events underlying disease progression and treatment response. High-throughput omics data have shown that untreated SMA patients have different profiles than controls. In addition, patients who clinically improved after treatment have a different profile than those who did not. These results provide a glimpse on potential markers that could assist in identifying therapy responders, in tracing the course of the disease, and in predicting its outcome. These studies have been restricted by the limited number of patients, but the approaches are feasible and can unravel severity-specific neuro-proteomic and metabolic SMA signatures.

A balance of noncanonical Semaphorin signaling from the cerebrospinal fluid regulates apical cell dynamics during corticogenesis

During corticogenesis, dynamic regulation of apical adhesion is fundamental to generate correct numbers and cell identities. While radial glial cells (RGCs) maintain basal and apical anchors, basal progenitors and neurons detach and settle at distal positions from the apical border. Whether diffusible signals delivered from the cerebrospinal fluid (CSF) contribute to the regulation of apical adhesion dynamics remains fully unknown. Secreted class 3 Semaphorins (Semas) trigger cell responses via Plexin-Neuropilin (Nrp) membrane receptor complexes. Here, we report that unconventional Sema3-Nrp preformed complexes are delivered by the CSF from sources including the choroid plexus to Plexin-expressing RGCs via their apical endfeet. Through analysis of mutant mouse models and various ex vivo assays mimicking ventricular delivery to RGCs, we found that two different complexes, Sema3B/Nrp2 and Sema3F/Nrp1, exert dual effects on apical endfeet dynamics, nuclei positioning, and RGC progeny. This reveals unexpected balance of CSF-delivered guidance molecules during cortical development.

Spatial regulation of amacrine cell genesis by Semaphorin 3f

PURPOSE

The axonal projections of retinal ganglion cells (RGCs) of the eye are topographically organized so that spatial information from visual images is preserved. This retinotopic organization is established during development by secreted morphogens that pattern domains of transcription factor expression within naso-temporal and dorso-ventral quadrants of the embryonic eye. Poorly understood are the downstream signaling molecules that generate the topographically organized retinal cells and circuits. The secreted signaling molecule Semaphorin 3fa (Sema3fa) belongs to the Sema family of molecules that provide positional information to developing cells. Here, we test a role for Sema3fa in cell genesis of the temporal zebrafish retina.

METHODS

We compare retinal cell genesis in wild type and sema3fa CRISPR zebrafish mutants by in situ hybridization and immunohistochemistry.

RESULTS

We find that mRNAs for sema3fa and known receptors, neuropilin2b (nrp2b) and plexina1a (plxna1a), are expressed by progenitors of the temporal, but not nasal zebrafish embryonic retina. In the sema3fa^ca304/ca304 embryo, initially the domains of expression for atoh7 and neurod4, transcription factors necessary for the specification of RGCs and amacrine cells, respectively, are disrupted. Yet, post-embryonically only amacrine cells of the temporal retina are reduced in numbers, with both GABAergic and glycinergic subtypes affected.

CONCLUSIONS

These data suggest that Sema3fa acts early on embryonic temporal progenitors to control in a spatially-dependent manner the production of amacrine cells, possibly to allow the establishment of neural circuits with domain-specific functions. We propose that spatially restricted extrinsic signals in the neural retina control cell genesis in a domain-dependent manner.

CRMP4-mediated fornix development involves Semaphorin-3E signaling pathway

Neurodevelopmental axonal pathfinding plays a central role in correct brain wiring and subsequent cognitive abilities. Within the growth cone, various intracellular effectors transduce axonal guidance signals by remodeling the cytoskeleton. Semaphorin-3E (Sema3E) is a guidance cue implicated in development of the fornix, a neuronal tract connecting the hippocampus to the hypothalamus. Microtubule-associated protein 6 (MAP6) has been shown to be involved in the Sema3E growth-promoting signaling pathway. In this study, we identified the collapsin response mediator protein 4 (CRMP4) as a MAP6 partner and a crucial effector in Sema3E growth-promoting activity. CRMP4-KO mice displayed abnormal fornix development reminiscent of that observed in Sema3E-KO mice. CRMP4 was shown to interact with the Sema3E tripartite receptor complex within detergent-resistant membrane (DRM) domains, and DRM domain integrity was required to transduce Sema3E signaling through the Akt/GSK3 pathway. Finally, we showed that the cytoskeleton-binding domain of CRMP4 is required for Sema3E's growth-promoting activity, suggesting that CRMP4 plays a role at the interface between Sema3E receptors, located in DRM domains, and the cytoskeleton network. As the fornix is affected in many psychiatric diseases, such as schizophrenia, our results provide new insights to better understand the neurodevelopmental components of these diseases.

The emerging roles of semaphorin4D/CD100 in immunological diseases

In vertebrates, the semaphorin family of proteins is composed of 21 members that are divided into five subfamilies, i.e. classes 3 to 7. Semaphorins play crucial roles in regulating multiple biological processes, such as neural remodeling, tissue regeneration, cancer progression, and, especially, in immunological regulation. Semaphorin 4D (SEMA4D), also known as CD100, is an important member of the semaphorin family and was first characterized as a lymphocyte-specific marker. SEMA4D has diverse effects on immunologic processes, including immune cell proliferation, differentiation, activation, and migration, through binding to its specific membrane receptors CD72, PLXNB1, and PLXNB2. Furthermore, SEMA4D and its underlying signaling have been increasingly linked with several immunological diseases. This review focuses on the significant immunoregulatory role of SEMA4D and the associated underlying mechanisms, as well as the potential application of SEMA4D as a diagnostic marker and therapeutic target for the treatment of immunological diseases.

Mechanochemical control of epidermal stem cell divisions by B-plexins

The precise spatiotemporal control of cell proliferation is key to the morphogenesis of epithelial tissues. Epithelial cell divisions lead to tissue crowding and local changes in force distribution, which in turn suppress the rate of cell divisions. However, the molecular mechanisms underlying this mechanical feedback are largely unclear. Here, we identify a critical requirement of B-plexin transmembrane receptors in the response to crowding-induced mechanical forces during embryonic skin development. Epidermal stem cells lacking B-plexins fail to sense mechanical compression, resulting in disinhibition of the transcriptional coactivator YAP, hyperproliferation, and tissue overgrowth. Mechanistically, we show that B-plexins mediate mechanoresponses to crowding through stabilization of adhesive cell junctions and lowering of cortical stiffness. Finally, we provide evidence that the B-plexin-dependent mechanochemical feedback is also pathophysiologically relevant to limit tumor growth in basal cell carcinoma, the most common type of skin cancer. Our data define a central role of B-plexins in mechanosensation to couple cell density and cell division in development and disease.

Mapping the molecular and cellular complexity of cortical malformations

The cerebral cortex is an intricate structure that controls human features such as language and cognition. Cortical functions rely on specialized neurons that emerge during development from complex molecular and cellular interactions. Neurodevelopmental disorders occur when one or several of these steps is incorrectly executed. Although a number of causal genes and disease phenotypes have been identified, the sequence of events linking molecular disruption to clinical expression mostly remains obscure. Here, focusing on human malformations of cortical development, we illustrate how complex interactions at the genetic, cellular, and circuit levels together contribute to diversity and variability in disease phenotypes. Using specific examples and an online resource, we propose that a multilevel assessment of disease processes is key to identifying points of vulnerability and developing new therapeutic strategies.

Semaphorins and Plexins in central nervous system patterning: the key to it all?

Semaphorins and Plexins constitute one of the largest family of guidance molecules and receptors involved in setting critical biological steps for central nervous system development. The role of these molecules in axonal development has been extensively characterized but Semaphorins and Plexins are also involved in a variety of other developmental processes, spanning from cell polarization to migration, laminar segregation and neuronal maturation. In this review, we aim to gather discoveries carried in the field of neurodevelopment over the last decade, during which Semaphorin/Plexin complexes have emerged as key regulators of neurogenesis, neural cell migration and adult gliogenesis. As well, we report mechanisms that brought a better understanding of axonal midline crossing.

SlitC-PlexinA1 mediates iterative inhibition for orderly passage of spinal commissural axons through the floor plate

Spinal commissural axon navigation across the midline in the floor plate requires repulsive forces from local Slit repellents. The long-held view is that Slits push growth cones forward and prevent them from turning back once they became sensitized to these cues after midline crossing. We analyzed with fluorescent reporters Slits distribution and FP glia morphology. We observed clusters of Slit-N and Slit-C fragments decorating a complex architecture of glial basal process ramifications. We found that PC2 proprotein convertase activity contributes to this pattern of ligands. Next, we studied Slit-C acting via PlexinA1 receptor shared with another FP repellent, the Semaphorin3B, through generation of a mouse model baring PlexinA1Y1815F mutation abrogating SlitC but not Sema3B responsiveness, manipulations in the chicken embryo, and ex vivo live imaging. This revealed a guidance mechanism by which SlitC constantly limits growth cone exploration, imposing ordered and forward-directed progression through aligned corridors formed by FP basal ramifications.

Choroid Plexus: The Orchestrator of Long-Range Signalling Within the CNS

Cerebrospinal fluid (CSF) is the liquid that fills the brain ventricles. CSF represents not only a mechanical brain protection but also a rich source of signalling factors modulating diverse processes during brain development and adulthood. The choroid plexus (CP) is a major source of CSF and as such it has recently emerged as an important mediator of extracellular signalling within the brain. Growing interest in the CP revealed its capacity to release a broad variety of bioactive molecules that, via CSF, regulate processes across the whole central nervous system (CNS). Moreover, CP has been also recognized as a sensor, responding to altered composition of CSF associated with changes in the patterns of CNS activity. In this review, we summarize the recent advances in our understanding of the CP as a signalling centre that mediates long-range communication in the CNS. By providing a detailed account of the CP secretory repertoire, we describe how the CP contributes to the regulation of the extracellular environment-in the context of both the embryonal as well as the adult CNS. We highlight the role of the CP as an important regulator of CNS function that acts via CSF-mediated signalling. Further studies of CP-CSF signalling hold the potential to provide key insights into the biology of the CNS, with implications for better understanding and treatment of neuropathological conditions.

Roles of axon guidance molecules in neuronal wiring in the developing spinal cord

The spinal cord receives, relays and processes sensory information from the periphery and integrates this information with descending inputs from supraspinal centres to elicit precise and appropriate behavioural responses and orchestrate body movements. Understanding how the spinal cord circuits that achieve this integration are wired during development is the focus of much research interest. Several families of proteins have well-established roles in guiding developing spinal cord axons, and recent findings have identified new axon guidance molecules. Nevertheless, an integrated view of spinal cord network development is lacking, and many current models have neglected the cellular and functional diversity of spinal cord circuits. Recent advances challenge the existing spinal cord axon guidance dogmas and have provided a more complex, but more faithful, picture of the ontogenesis of vertebrate spinal cord circuits.

Brain Ventricular System and Cerebrospinal Fluid Development and Function: Light at the End of the Tube: A Primer with Latest Insights

The brain ventricular system is a series of connected cavities, filled with cerebrospinal fluid (CSF), that forms within the vertebrate central nervous system (CNS). The hollow neural tube is a hallmark of the chordate CNS, and a closed neural tube is essential for normal development. Development and function of the ventricular system is examined, emphasizing three interdigitating components that form a functional system: ventricle walls, CSF fluid properties, and activity of CSF constituent factors. The cellular lining of the ventricle both can produce and is responsive to CSF. Fluid properties and conserved CSF components contribute to normal CNS development. Anomalies of the CSF/ventricular system serve as diagnostics and may cause CNS disorders, further highlighting their importance. This review focuses on the evolution and development of the brain ventricular system, associated function, and connected pathologies. It is geared as an introduction for scholars with little background in the field.

Dysregulation of CRMP2 Post-Translational Modifications Drive Its Pathological Functions

Collapsin response mediator proteins (CRMPs) are a family of ubiquitously expressed, homologous phosphoproteins best known for coordinating cytoskeletal formation and regulating cellular division, migration, polarity, and synaptic connection. CRMP2, the most studied of the five family members, is best known for its affinity for tubulin heterodimers and function in regulating the microtubule network. These functions are tightly regulated by post-translational modifications including phosphorylation, SUMOylation, oxidation, and O-GlcNAcylation. While CRMP2's physiological functions rely mostly on its non-phosphorylated state, dysregulation of CRMP2 phosphorylation and SUMOylation has been reported to be involved in the pathophysiology of multiple diseases including cancer, chronic pain, spinal cord injury, neurofibromatosis type 1, and others. Here, we provide a consolidated update on what is known about CRMP2 signaling and function, first focusing on axonal growth and neuronal polarity, then illustrating the link between dysregulated CRMP2 post-translational modifications and diseases. We additionally discuss the roles of CRMP2 in non-neuronal cells, both in the CNS and regions of the periphery. Finally, we offer thoughts on the therapeutic implications of modulating CRMP2 function in a variety of diseases.

Eph signaling in mitotic spindle orientation: what´s your angle here?

The orientation of the mitotic spindle is a crucial process during development and adult tissue homeostasis and multiple mechanisms have been shown to intrinsically regulate this process. However, much less is known about the extrinsic cues involved in modulating spindle orientation. We have recently uncovered a novel function of Eph intercellular signaling in regulating spindle alignment by ultimately ensuring the correct cortical distribution of central components within the intrinsic spindle orientation machinery. Here, we comment on these results, novel questions that they open and potential additional research to address in the future.

Evolution of vertebrate spinal cord patterning

The vertebrate spinal cord is organized across three developmental axes, anterior-posterior (AP), dorsal-ventral (DV), and medial-lateral (ML). Patterning of these axes is regulated by canonical intercellular signaling pathways: the AP axis by Wnt, fibroblast growth factor, and retinoic acid (RA), the DV axis by Hedgehog, Tgfβ, and Wnt, and the ML axis where proliferation is controlled by Notch. Developmental time plays an important role in which signal does what and when. Patterning across the three axes is not independent, but linked by interactions between signaling pathway components and their transcriptional targets. Combined this builds a sophisticated organ with many different types of cell in specific AP, DV, and ML positions. Two living lineages share phylum Chordata with vertebrates, amphioxus, and tunicates, while the jawless fish such as lampreys, survive as the most basally divergent vertebrate lineage. Genes and mechanisms shared between lampreys and other vertebrates tell us what predated vertebrates, while those also shared with other chordates tell us what evolved early in chordate evolution. Between these lie vertebrate innovations: genetic and developmental changes linked to evolution of new morphology. These include gene duplications, differences in how signals are received, and new regulatory connections between signaling pathways and their target genes.

Expression of Semaphorin 3B (SEMA3B) in Various Grades of Endometrial Cancer

BACKGROUND SEMA3B is known as an inhibitor of angiogenesis and cell proliferation. During carcinogenesis, the loss of SEMA3B function is observed, which results in the progression of neoplastic changes. The aim of this study was to evaluate the expression profile of SEMA3B in endometrial cancer (G1-G3) in comparison to the control group and to assess whether the observed changes in expression could become a molecular marker in endometrial cancer. MATERIAL AND METHODS The study group consisted of 45 patients diagnosed with endometrial cancer (G1, 17; G2, 15; G3, 13). The control group included 15 patients. SEMA3B expression was assessed using the immunohistochemical method. Statistical analysis was carried out using the Statistica 12 PL program (StatSoft, USA). It included the Kruskal-Wallis test and post hoc Dunn's test (p<0.05). RESULTS Statistically significant differences in the level of SEMA3B expression were observed between all analyzed groups. The expression pattern of SEMA3B was as follows: cancer cells G1>G2>G3; endothelial cells: G3>G1>G2; stromal cells: G2>G1>G3. CONCLUSIONS Analysis of the SEMA3B expression profile shows the complexity of neoplastic transformation, which confirms the different expression of SEMA3B in endometrial cancer cells and endothelial cells. The present results and data in the literature data suggest that SEMA3B expression indicates the progression of carcinogenesis in the context of endometrial cancer.

Eph signaling controls mitotic spindle orientation and cell proliferation in neuroepithelial cells

In this study, Franco and Carmena uncover a function for Eph signaling as a novel extrinsic mechanism controlling mitotic spindle alignment in Drosophila neuroepithelial cells through aPKC activity–dependent myosin II regulation. Additionally, Eph loss leads to a Rho signaling–dependent activation of the PI3K–Akt1 pathway, enhancing cell proliferation within this neuroepithelium.

Mitotic spindle orientation must be tightly regulated during development and adult tissue homeostasis. It determines cell-fate specification and tissue architecture during asymmetric and symmetric cell division, respectively. Here, we uncover a novel role for Ephrin–Eph intercellular signaling in controlling mitotic spindle alignment in Drosophila optic lobe neuroepithelial cells through aPKC activity–dependent myosin II regulation. We show that conserved core components of the mitotic spindle orientation machinery, including Discs Large1, Mud/NuMA, and Canoe/Afadin, mislocalize in dividing Eph mutant neuroepithelial cells and produce spindle alignment defects in these cells when they are down-regulated. In addition, the loss of Eph leads to a Rho signaling–dependent activation of the PI3K–Akt1 pathway, enhancing cell proliferation within this neuroepithelium. Hence, Eph signaling is a novel extrinsic mechanism that regulates both spindle orientation and cell proliferation in the Drosophila optic lobe neuroepithelium. Similar mechanisms could operate in other Drosophila and vertebrate epithelia.

Downregulation of ribosome biogenesis during early forebrain development

Forebrain precursor cells are dynamic during early brain development, yet the underlying molecular changes remain elusive. We observed major differences in transcriptional signatures of precursor cells from mouse forebrain at embryonic days E8.5 vs. E10.5 (before vs. after neural tube closure). Genes encoding protein biosynthetic machinery were strongly downregulated at E10.5. This was matched by decreases in ribosome biogenesis and protein synthesis, together with age-related changes in proteomic content of the adjacent fluids. Notably, c-MYC expression and mTOR pathway signaling were also decreased at E10.5, providing potential drivers for the effects on ribosome biogenesis and protein synthesis. Interference with c-MYC at E8.5 prematurely decreased ribosome biogenesis, while persistent c-MYC expression in cortical progenitors increased transcription of protein biosynthetic machinery and enhanced ribosome biogenesis, as well as enhanced progenitor proliferation leading to subsequent macrocephaly. These findings indicate large, coordinated changes in molecular machinery of forebrain precursors during early brain development.

UTX Affects Neural Stem Cell Proliferation and Differentiation through PTEN Signaling

Neural stem cell (NSC) proliferation and differentiation in the developing brain is a complex process precisely regulated by intrinsic and extrinsic signals. Although epigenetic modification has been reportedly involved in the regulation of the cerebral cortex, whether UTX, an H3K27me3 demethylase, regulates the development of cerebral cortex during the embryonic period is unclear. In this study, we demonstrate that Utx deficiency by knockdown and conditional knockout increases NSC proliferation and decreases terminal mitosis and neuronal differentiation. Furthermore, we find that impairment of cortical development caused by lack of Utx is less significant in males than in females. In addition, UTX demethylates H3K27me3 at the Pten promoter and promotes Pten expression. P-AKT and P-mTOR levels are increased in the Utx conditional knockout cortices. Finally, Utx or Pten overexpression can rescue the impairment of brain development caused by Utx loss. These findings may provide important clues toward deciphering brain diseases.

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UTX is essential for cortical development and neuron production

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Utx suppression impairs normal neocortical neurogenesis in a sex-specific manner

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UTX affects the levels of H3K27 trimethylation at Pten promoters

Playing with the cell cycle to build the spinal cord

A fundamental issue in nervous system development and homeostasis is to understand the mechanisms governing the balance between the maintenance of proliferating progenitors versus their differentiation into post-mitotic neurons. Accumulating data suggest that the cell cycle and core regulators of the cell cycle machinery play a major role in regulating this fine balance. Here, we focus on the interplay between the cell cycle and cellular and molecular events governing spinal cord development. We describe the existing links between the cell cycle and interkinetic nuclear migration (INM). We show how the different morphogens patterning the neural tube also regulate the cell cycle machinery to coordinate proliferation and patterning. We give examples of how cell cycle core regulators regulate transcriptionally, or post-transcriptionally, genes involved in controlling the maintenance versus the differentiation of neural progenitors. Finally, we describe the changes in cell cycle kinetics occurring during neural tube patterning and at the time of neuronal differentiation, and we discuss future research directions to better understand the role of the cell cycle in cell fate decisions.

Semaphorin3A-neuropilin1 signalling is involved in the generation of cortical interneurons

Cortical interneurons are generated predominantly in the medial ganglionic eminence of the ventral telencephalon and migrate to the cortex during embryonic development. These cells express neuropilin (Nrp1 and Nrp2) receptors which mediate their response to the chemorepulsive class 3 semaphorin (Sema) ligands. We show here that semaphorins Sema3A and Sema3F are expressed in layers adjacent to cortical interneuron migratory streams as well as in the striatum, suggesting they may have a role in guiding these cells throughout their journey. Analysis of Sema3A ^-/- and Sema3F ^-/- mice during corticogenesis showed that absence of Sema3A, but not Sema3F, leads to aberrant migration of cortical interneurons through the striatum. Reduced number of cortical interneurons was found in the cortex of Sema3A ^-/-, Nrp1 ^-/- and Nrp2 ^-/- mice, as well as altered distribution in Sema3F ^-/-, Nrp1 ^-/-, Nrp2 ^-/- animals and especially in neuropilin double mutants. The observed decrease in interneurons in Sema3A ^-/- and Nrp1 ^-/- mice was due to altered proliferative activity of their progenitors highlighted by changes in their mitotic spindle positioning and angle of cleavage plane during cell division. These findings point to a novel role for Sema3A-Nrp1 signalling in progenitor cell dynamics and in the generation of interneurons in the ventral telencephalon.

Regulation of embryonic neurogenesis by germinal zone vasculature

In the adult rodent brain, new neurons are born in two germinal regions that are associated with blood vessels, and blood vessels and vessel-derived factors are thought to regulate the activity of adult neural stem cells. Recently, it has been proposed that a vascular niche also regulates prenatal neurogenesis. Here we identify the mouse embryo hindbrain as a powerful model to study embryonic neurogenesis and define the relationship between neural progenitor cell (NPC) behavior and vessel growth. Using this model, we show that a subventricular vascular plexus (SVP) extends through a hindbrain germinal zone populated by NPCs whose peak mitotic activity follows a surge in SVP growth. Hindbrains genetically defective in SVP formation owing to constitutive NRP1 loss showed a premature decline in both NPC activity and hindbrain growth downstream of precocious cell cycle exit, premature neuronal differentiation, and abnormal mitosis patterns. Defective regulation of NPC activity was not observed in mice lacking NRP1 expression by NPCs, but instead in mice lacking NRP1 selectively in endothelial cells, yet was independent of vascular roles in hindbrain oxygenation. Therefore, germinal zone vascularization sustains NPC proliferation in the prenatal brain.

Commissural axon navigation: Control of midline crossing in the vertebrate spinal cord by the semaphorin 3B signaling

The mechanisms governing the navigation of commissural axons during embryonic development have been extensively investigated in the past years, often using the drosophila ventral nerve cord and the spinal cord as model systems. Similarities but also specificities in the general strategies, the molecular signals as well as in the regulatory pathways controlling the response of commissural axons to the guidance cues have been found between species. Whether the semaphorin signaling contributes to midline crossing in the fly nervous system remains unknown, while in contrast, it does play a prominent contribution in vertebrates. In this review we discuss the functions of the semaphorins during commissural axon guidance in the developing spinal cord, focusing on the family member semaphorin 3B (Sema3B) in the context of midline crossing in the spinal cord.

Regulation of mitotic spindle orientation: an integrated view

Mitotic spindle orientation is essential for cell fate decisions, epithelial maintenance, and tissue morphogenesis. In most animal cell types, the dynein motor complex is anchored at the cell cortex and exerts pulling forces on astral microtubules to position the spindle. Early studies identified the evolutionarily conserved Gαi/LGN/NuMA complex as a key regulator that polarizes cortical force generators. In recent years, a combination of genetics, biochemistry, modeling, and live imaging has contributed to decipher the mechanisms of spindle orientation. Here, we highlight the dynamic nature of the assembly of this complex and discuss the molecular regulation of its localization. Remarkably, a number of LGN-independent mechanisms were described recently, whereas NuMA remains central in most pathways involved in recruiting force generators at the cell cortex. We also describe the emerging role of the actin cortex in spindle orientation and discuss how dynamic astral microtubule formation is involved. We further give an overview on instructive external signals that control spindle orientation in tissues. Finally, we discuss the influence of cell geometry and mechanical forces on spindle orientation.

Progressive Differentiation and Instructive Capacities of Amniotic Fluid and Cerebrospinal Fluid Proteomes following Neural Tube Closure

After neural tube closure, amniotic fluid (AF) captured inside the neural tube forms the nascent cerebrospinal fluid (CSF). Neuroepithelial stem cells contact CSF-filled ventricles, proliferate, and differentiate to form the mammalian brain, while neurogenic placodes, which generate cranial sensory neurons, remain in contact with the AF. Using in vivo ultrasound imaging, we quantified the expansion of the embryonic ventricular-CSF space from its inception. We developed tools to obtain pure AF and nascent CSF, before and after neural tube closure, and to define how the AF and CSF proteomes diverge during mouse development. Using embryonic neural explants, we demonstrate that age-matched fluids promote Sox2-positive neurogenic identity in developing forebrain and olfactory epithelia. Nascent CSF also stimulates SOX2-positive self-renewal of forebrain progenitor cells, some of which is attributable to LIFR signaling. Our Resource should facilitate the investigation of fluid-tissue interactions during this highly vulnerable stage of early brain development.

Mitotic spindle orientation: semaphorin-plexin signaling flags the way

The extracellular signals and corresponding receptors that align the mitotic spindle of symmetrically dividing cells within an epithelial sheet are largely unknown. In this issue of Developmental Cell, Xia et al. (2015) identify semaphorin-plexin signaling as a regulator of spindle orientation critical for kidney development and repair.

Development and functions of the choroid plexus-cerebrospinal fluid system

The choroid plexus (ChP) is the principal source of cerebrospinal fluid (CSF), which has accepted roles as a fluid cushion and a sink for nervous system waste in vertebrates. Various animal models have provided insights into how the ChP-CSF system develops and matures. In addition, recent studies have uncovered new, active roles for this dynamic system in the regulation of neural stem cells, critical periods and the overall health of the nervous system. Together, these findings have brought about a paradigm shift in our understanding of brain development and health, and have stimulated new initiatives for the treatment of neurological disease.