Early establishment of chloride homeostasis in CRH neurons is altered by prenatal stress leading to fetal HPA axis dysregulation

Corticotropin-releasing hormone (CRH) neurons play an important role in the regulation of neuroendocrine responses to stress. The excitability of CRH neurons is regulated by inhibitory GABAergic inputs. However, it is unclear when GABAergic regulation of CRH neurons is established during fetal brain development. Furthermore, the exact progression of the developmental shift of GABA action from depolarization to hyperpolarization remains unelucidated. Considering the importance of CRH neuron function in subsequent hypothalamic-pituitary-adrenal (HPA) axis regulation during this critical phase of development, we investigated the ontogeny of GABAergic inputs to CRH neurons and consequent development of chloride homeostasis. Both CRH neuron soma in the paraventricular nucleus (PVN) and axons projecting to the median eminence could be identified at embryonic day 15 (E15). Using acute slices containing the PVN of CRF-VenusΔNeo mice, gramicidin perforated-patch clamp-recordings of CRH neurons at E15, postnatal day 0 (P0), and P7 were performed to evaluate the developmental shift of GABA action. The equilibrium potential of GABA (EGABA) was similar between E15 and P0 and showed a further hyperpolarizing shift between P0 and P7 that was comparable to EGABA values in adult CRH neurons. GABA primarily acted as an inhibitory signal at E15 and KCC2 expression was detected in CRH neurons at this age. Activation of the HPA axis has been proposed as the primary mechanism through which prenatal maternal stress shapes fetal development and subsequent long-term disease risk. We therefore examined the impact of maternal food restriction stress on the development of chloride homeostasis in CRH neurons. We observed a depolarization shift of EGABA in CRH neurons of pups exposed to maternal food restriction stress. These results suggest that Cl- homeostasis in early developmental CRH neurons attains mature intracellular Cl- levels, GABA acts primarily as inhibitory, and CRH neurons mature and function early compared with neurons in other brain regions, such as the cortex and hippocampus. Maternal food restriction stress alters chloride homeostasis in CRH neurons of pups, reducing their inhibitory control by GABA. This may contribute to increased CRH neuron activity and cause activation of the HPA axis in pups.

Distinct subdivisions of subcortical U-fiber regions in the gyrencephalic ferret brain

The human cerebrum contains a large amount of cortico-cortical association fibers. Among them, U-fibers are short-range association fibers located in white matter immediately deep to gray matter. Although U-fibers are thought to be crucial for higher cognitive functions, the organization within U-fiber regions are still unclear. Here we investigated the properties of U-fiber regions in the ferret cerebrum using neurochemical, neuronal tracing, immunohistochemical and electron microscopic techniques. We found that U-fiber regions can be subdivided into two regions, which we named outer and inner U-fiber regions. We further uncovered that outer U-fiber regions have smaller-diameter axons with thinner myelin compared with inner U-fiber regions. These findings may indicate functional complexity within U-fiber regions in the cerebrum.

Identification of Tau protein as a novel marker for maturation and pathological changes of oligodendrocytes

Microtubule-associated protein Tau is primarily expressed in axons of neurons, but also in Olig2-positive oligodendrocytes in adult rodent and monkey brains. In this study, we sought to determine at what cell stage Tau becomes expressed in the oligodendrocyte lineage. We performed immunostaining of adult mouse brain sections using well-known markers of oligodendrocyte lineage and found that Tau is expressed in mature oligodendrocytes, but not in oligodendrocyte progenitors and immature pre-oligodendrocytes. We also investigated Tau expression in developing mouse brain. Surprisingly, Tau expression occurred after the peak of myelination and even exceeded GSTπ expression, which has been considered as a marker of myelinating oligodendrocytes. These results suggest Tau as a novel marker of oligodendrocyte maturation. We then investigated whether Tau is important for oligodendrocyte development and/or myelination and how Tau changes in demyelination. First, we found no changes in myelination and oligodendrocyte markers in Tau knockout mice, suggesting that Tau is dispensable. Next, we analyzed the proteolipid protein 1 transgenic model of Pelizaeus-Merzbacher disease, which is a rare leukodystrophy. In hemizygous transgenic mice, the number of Tau-positive cells were significantly increased as compared with wild type mice. These cells were also positive for Olig2, CC1, and GSTπ, but not PDGFRα and GPR17. In stark contrast, the expression level of Tau, as well as GSTπ, was dramatically decreased in the cuprizone-induced model of multiple sclerosis. Taken together, we propose Tau as a new marker of oligodendrocyte lineage and for investigating demyelination lesions.

Shank3a/b isoforms regulate the susceptibility to seizures and thalamocortical development in the early postnatal period of mice

Epileptic seizures are distinct but frequent comorbidities in children with autism spectrum disorder (ASD). The hyperexcitability of cortical and subcortical neurons appears to be involved in both phenotypes. However, little information is available concerning which genes are involved and how they regulate the excitability of the thalamocortical network. In this study, we investigate whether an ASD-associated gene, SH3 and multiple ankyrin repeat domains 3 (Shank3), plays a unique role in the postnatal development of thalamocortical neurons. We herein report that Shank3a/b, the splicing isoforms of mouse Shank3, were uniquely expressed in the thalamic nuclei, peaking from two to four weeks after birth. Shank3a/b-knockout mice showed lower parvalbumin signals in the thalamic nuclei. Consistently, Shank3a/b-knockout mice were more susceptible to generalized seizures than wild-type mice after kainic acid treatments. Together, these data indicate that NT-Ank domain of Shank3a/b regulates molecular pathways that protect thalamocortical neurons from hyperexcitability during the early postnatal period of mice.

A New Technical Approach for Cross-species Examination of Neuronal Wiring and Adult Neuron-glia Functions

Advances in single cell sequencing have enabled the identification of a large number of genes, expressed in many different cell types, and across a variety of model organisms. In particular, the nervous system harbors an immense number of interacting cell types, which are poorly characterized. Future loss- and gain-of-function experiments will be essential in determining how novel genes play critical roles in diverse cellular, as well as evolutionarily adapted, contexts. However, functional analysis across species is often hampered by technical limitations, in non-genetic animal systems. Here, we describe a new single plasmid system, misPiggy. The system is based around the hyperactive piggyBac transposon system, which combines stable genomic integration of transgenes (for long-term expression) with large cargo capacity. Taking full advantage of these characteristics, we engineered novel expression modules into misPiggy that allow for cell-type specific loss- and gain-of-gene function. These modules work widely across species from frog to ferret. As a proof of principle, we present a loss-of-function analysis of the neuronal receptor Deleted in Colorectal Cancer (DCC) in retinal ganglion cells (RGCs) of Xenopus tropicalis tadpoles. Single axon tracings of mosaic knock-out cells reveal a specific cell-intrinsic requirement of DCC, specifically in axonal arborization within the frog tectum, rather than retina-to-brain axon guidance. Furthermore, we report additional technical advances that enable temporal control of knock-down or gain-of-function analysis. We applied this to visualize and manipulate labeled neurons, astrocytes and other glial cells in the central nervous system (CNS) of mouse, rat and ferret. We propose that misPiggy will be a valuable tool for rapid, flexible and cost-effective screening of gene function across a variety of animal models.

Isolation of ferret astrocytes reveals their morphological, transcriptional, and functional differences from mouse astrocytes

Astrocytes play key roles in supporting the central nervous system structure, regulating synaptic functions, and maintaining brain homeostasis. The number of astrocytes in the cerebrum has markedly increased through evolution. However, the manner by which astrocytes change their features during evolution remains unknown. Compared with the rodent brain, the brain of the ferret, a carnivorous animal, has a folded cerebral cortex and higher white to gray matter ratio, which are common features of the human brain. To further clarify the features of ferret astrocytes, we isolated astrocytes from ferret neonatal brains, cultured these cells, and compared their morphology, gene expression, calcium response, and proliferating ability with those of mouse astrocytes. The morphology of cultured ferret astrocytes differed from that of mouse astrocytes. Ferret astrocytes had longer and more branched processes, smaller cell bodies, and different calcium responses to glutamate, as well as had a greater ability to proliferate, compared to mouse astrocytes. RNA sequencing analysis revealed novel ferret astrocyte-specific genes, including several genes that were the same as those in humans. Astrocytes in the ferret brains had larger cell size, longer primary processes in larger numbers, and a higher proliferation rate compared to mouse astrocytes. Our study shows that cultured ferret astrocytes have different features from rodent astrocytes and similar features to human astrocytes, suggesting that they are useful in studying the roles of astrocytes in brain evolution and cognitive functions in higher animals.

Investigation of the Mechanisms Underlying the Development and Evolution of the Cerebral Cortex Using Gyrencephalic Ferrets

The mammalian cerebral cortex has changed significantly during evolution. As a result of the increase in the number of neurons and glial cells in the cerebral cortex, its size has markedly expanded. Moreover, folds, called gyri and sulci, appeared on its surface, and its neuronal circuits have become much more complicated. Although these changes during evolution are considered to have been crucial for the acquisition of higher brain functions, the mechanisms underlying the development and evolution of the cerebral cortex of mammals are still unclear. This is, at least partially, because it is difficult to investigate these mechanisms using mice only. Therefore, genetic manipulation techniques for the cerebral cortex of gyrencephalic carnivore ferrets were developed recently. Furthermore, gene knockout was achieved in the ferret cerebral cortex using the CRISPR/Cas9 system. These techniques enabled molecular investigations using the ferret cerebral cortex. In this review, we will summarize recent findings regarding the mechanisms underlying the development and evolution of the mammalian cerebral cortex, mainly focusing on research using ferrets.

Localized astrogenesis regulates gyrification of the cerebral cortex

The development and evolution of mammalian higher cognition are represented by gyrification of the laminar cerebral cortex and astrocyte development, but their mechanisms and interrelationships remain unknown. Here, we show that localized astrogenesis plays an important role in gyri formation in the gyrencephalic cerebral cortex. In functional genetic experiments, we show that reducing astrocyte number prevents gyri formation in the ferret cortex, while increasing astrocyte number in mice, which do not have cortical folds, can induce gyrus-like protrusions. Morphometric analyses demonstrate that the vertical expansion of deep pallial regions achieved by localized astrogenesis is crucial for gyri formation. Furthermore, our findings suggest that localized astrogenesis by a positive feedback loop of FGF signaling is an important mechanism underlying cortical folding in gyrencephalic mammalian brains. Our findings reveal both the cellular mechanisms and the mechanical principle of gyrification in the mammalian brain.

Multiple Functions of Draxin/Netrin-1 Signaling in the Development of Neural Circuits in the Spinal Cord and the Brain

Axon guidance proteins play key roles in the formation of neural circuits during development. We previously identified an axon guidance cue, named draxin, that has no homology with other axon guidance proteins. Draxin is essential for the development of various neural circuits including the spinal cord commissure, corpus callosum, and thalamocortical projections. Draxin has been shown to not only control axon guidance through netrin-1 receptors, deleted in colorectal cancer (Dcc), and neogenin (Neo1) but also modulate netrin-1-mediated axon guidance and fasciculation. In this review, we summarize the multifaceted functions of draxin and netrin-1 signaling in neural circuit formation in the central nervous system. Furthermore, because recent studies suggest that the distributions and functions of axon guidance cues are highly regulated by glycoproteins such as Dystroglycan and Heparan sulfate proteoglycans, we discuss a possible function of glycoproteins in draxin/netrin-1-mediated axon guidance.

Dysfunction of the proteoglycan Tsukushi causes hydrocephalus through altered neurogenesis in the subventricular zone in mice

The lateral ventricle (LV) is flanked by the subventricular zone (SVZ), a neural stem cell (NSC) niche rich in extrinsic growth factors regulating NSC maintenance, proliferation, and neuronal differentiation. Dysregulation of the SVZ niche causes LV expansion, a condition known as hydrocephalus; however, the underlying pathological mechanisms are unclear. We show that deficiency of the proteoglycan Tsukushi (TSK) in ependymal cells at the LV surface and in the cerebrospinal fluid results in hydrocephalus with neurodevelopmental disorder-like symptoms in mice. These symptoms are accompanied by altered differentiation and survival of the NSC lineage, disrupted ependymal structure, and dysregulated Wnt signaling. Multiple TSK variants found in patients with hydrocephalus exhibit reduced physiological activity in mice in vivo and in vitro. Administration of wild-type TSK protein or Wnt antagonists, but not of hydrocephalus-related TSK variants, in the LV of TSK knockout mice prevented hydrocephalus and preserved SVZ neurogenesis. These observations suggest that TSK plays a crucial role as a niche molecule modulating the fate of SVZ NSCs and point to TSK as a candidate for the diagnosis and therapy of hydrocephalus.

ERAD components Derlin-1 and Derlin-2 are essential for postnatal brain development and motor function

Derlin family members (Derlins) are primarily known as components of the endoplasmic reticulum-associated degradation pathway that eliminates misfolded proteins. Here we report a function of Derlins in the brain development. Deletion of Derlin-1 or Derlin-2 in the central nervous system of mice impaired postnatal brain development, particularly of the cerebellum and striatum, and induced motor control deficits. Derlin-1 or Derlin-2 deficiency reduced neurite outgrowth in vitro and in vivo and surprisingly also inhibited sterol regulatory element binding protein 2 (SREBP-2)-mediated brain cholesterol biosynthesis. In addition, reduced neurite outgrowth due to Derlin-1 deficiency was rescued by SREBP-2 pathway activation. Overall, our findings demonstrate that Derlins sustain brain cholesterol biosynthesis, which is essential for appropriate postnatal brain development and function.

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Derlin-1 and Derlin-2 are essential for postnatal brain development and function

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Chemical chaperon does not ameliorate the phenotype of Derlin-deficient neuron

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Derlin regulates SREBP-2 activation and promotes brain cholesterol biosynthesis

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Derlin-mediated cholesterol biosynthesis is essential for neurite outgrowth

Phosphorylation of GAP-43 T172 is a molecular marker of growing axons in a wide range of mammals including primates

GAP-43 is a vertebrate neuron-specific protein and that is strongly related to axon growth and regeneration; thus, this protein has been utilized as a classical molecular marker of these events and growth cones. Although GAP-43 was biochemically characterized more than a quarter century ago, how this protein is related to these events is still not clear. Recently, we identified many phosphorylation sites in the growth cone membrane proteins of rodent brains. Two phosphorylation sites of GAP-43, S96 and T172, were found within the top 10 hit sites among all proteins. S96 has already been characterized (Kawasaki et al., 2018), and here, phosphorylation of T172 was characterized. In vitro (cultured neurons) and in vivo, an antibody specific to phosphorylated T172 (pT172 antibody) specifically recognized cultured growth cones and growing axons in developing mouse neurons, respectively. Immunoblotting showed that pT172 antigens were more rapidly downregulated throughout development than those of pS96 antibody. From the primary structure, this phosphorylation site was predicted to be conserved in a wide range of animals including primates. In the developing marmoset brainstem and in differentiated neurons derived from human induced pluripotent stem cells, immunoreactivity with pT172 antibody revealed patterns similar to those in mice. pT172 antibody also labeled regenerating axons following sciatic nerve injury. Taken together, the T172 residue is widely conserved in a wide range of mammals including primates, and pT172 is a new candidate molecular marker for growing axons.

The expression of aristaless-related homeobox in neural progenitors of gyrencephalic carnivore ferrets

Aristaless-related homeobox (ARX) has important functions in the development of various organs including the brain. Mutations of the human ARX gene have been associated with malformations of the cerebral cortex such as microcephaly and lissencephaly. Although the expression patterns of ARX in the lissencephalic cerebral cortex of mice have been intensively investigated, those in expanded gyrencephalic brains remained unclear. Here, we show the expression patterns of ARX in the developing cerebral cortex of gyrencephalic carnivore ferrets. We found that ARX was expressed not only in intermediate progenitor (IP) cells but also in outer radial glial (oRG) cells, which are neural progenitors preferentially observed in the gyrencephalic cerebral cortex. We found that the majority of ARX-positive oRG cells expressed the proliferating cell marker Ki-67. These results may indicate that ARX in oRG cells mediates the expansion of the gyrencephalic cerebral cortex during development and evolution.

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We investigated the distribution of ARX in the germinal zone of the ferret cerebrum.

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ARX was abundantly expressed in outer radial glial (oRG) cells.

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Most of the ARX-positive oRG cells were positive for the proliferation marker Ki-67.

A novel third mesh-like myometrial layer connects the longitudinal and circular muscle fibers -A potential stratum to coordinate uterine contractions

Periodic myometrial contraction is one of the important uterine functions to achieve embryo implantation and parturition. Although it is well-known that the mammalian myometrium is composed of longitudinal (outer) and circular (inner) layers, the precise mechanisms that coordinate both muscular contractions to produce peristaltic movements remain unclear. Recently, by treatment with our modified Clear Unobstructed Brain Imaging Cocktails and Computational analysis (CUBIC) tissue-clearing method, we obtained well-contrasted three-dimensional images of the transparent murine ovary using enhanced green fluorescent protein (EGFP) transgenic mice and light-sheet microscopy. Consequently, to investigate accurate anatomical connections between outer and inner myometrial fibers, we observed whole structures of the myometrium using a transparent murine uterus. By this method, we identified a novel muscle layer, a middle layer of the myometrium, which anatomically connects the conventional outer longitudinal and inner circular muscles. This new layer was visualized as a mesh-like structure and this structure was observed throughout the whole uterus from proximal to distal sites. In this area, CD31-positive vessels were abundantly localized around the mesh-like muscle fibers. In addition, CD34-positive uterine telocytes and tubulin β-3-positive nerve fibers were closely located in this middle layer. These findings indicate the presence of a novel mesh-like stratum that connects longitudinal and circular muscle layers, and suggest its coordinating role in myometrial contractions.

A discrete subtype of neural progenitor crucial for cortical folding in the gyrencephalic mammalian brain

An increase in the diversity of neural progenitor subtypes and folding of the cerebral cortex are characteristic features which appeared during the evolution of the mammalian brain. Here, we show that the expansion of a specific subtype of neural progenitor is crucial for cortical folding. We found that outer radial glial (oRG) cells can be subdivided by HOPX expression in the gyrencephalic cerebral cortex of ferrets. Compared with HOPX-negative oRG cells, HOPX-positive oRG cells had high self-renewal activity and were accumulated in prospective gyral regions. Using our in vivo genetic manipulation technique for ferrets, we found that the number of HOPX-positive oRG cells and their self-renewal activity were regulated by sonic hedgehog (Shh) signaling. Importantly, suppressing Shh signaling reduced HOPX-positive oRG cells and cortical folding, while enhancing it had opposing effects. Our results reveal a novel subtype of neural progenitor important for cortical folding in gyrencephalic mammalian cerebral cortex.

Visualization of the Retina in Intact Eyes of Mice and Ferrets Using a Tissue Clearing Method

Purpose

Visualization of specific cells and structures in intact organs would greatly facilitate our knowledge about pathological changes; therefore, a tissue clearing method applicable to the intact eye may be valuable. Here we report a novel imaging method for the retina using the hyperhydration-based tissue clearing technique CUBIC (Clear, Unobstructed Brain/Body Imaging Cocktails and Computational Analysis).

Methods

Eyes of Institute of Cancer Research (ICR) mice, C57BL/6 mice, and normally pigmented sable ferrets (Mustela putorius furo) were used. Intact eyes were subjected to CUBIC, melanin bleaching with H₂O₂, and immunostaining. Images of the retina in intact eyes were taken using epifluorescence microscopes and confocal microscopes.

Results

The combination of melanin bleaching and CUBIC efficiently made the eyes of C57BL/6 mice transparent. By combining melanin bleaching, CUBIC, and immunostaining, we succeeded in visualization of retinal structures from the outside of the intact eyes of mice. Furthermore, we found that our methods were applicable not only to mouse eyes but also to ferret eyes, which are much larger than those of mice.

Conclusions

Our method was useful for visualizing specific cells and structures in the retina of intact eyes with single-cell resolution without making tissue sections.

Translational Relevance

This simple and efficient method can be applicable to various rodent models, including those associated with glaucoma or myopia, and will facilitate evaluating the effects of novel therapy for relevant eye diseases by visualizing changes from the retina to the sclera at both molecular and macroscopic levels simultaneously in a whole-eye preparation.

Characterization of the Inner and Outer Fiber Layers in the Developing Cerebral Cortex of Gyrencephalic Ferrets

Changes in the cerebral cortex of mammals during evolution have been of great interest. Ferrets, monkeys, and humans have more developed cerebral cortices compared with mice. Although the features of progenitors in the developing cortices of these animals have been intensively investigated, those of the fiber layers are still largely elusive. By taking the advantage of our in utero electroporation technique for ferrets, here we systematically investigated the cellular origins and projection patterns of axonal fibers in the developing ferret cortex. We found that ferrets have 2 fiber layers in the developing cerebral cortex, as is the case in monkeys and humans. Axonal fibers in the inner fiber layer projected contralaterally and subcortically, whereas those in the outer fiber layer sent axons to neighboring cortical areas. Furthermore, we performed similar experiments using mice and found unexpected similarities between ferrets and mice. Our results shed light on the cellular origins, the projection patterns, the developmental processes, and the evolution of fiber layers in mammalian brains.

Three-dimensional evaluation of murine ovarian follicles using a modified CUBIC tissue clearing method

BACKGROUND

Recently, we demonstrated the three-dimensional (3D) localization of murine trophoblast giant cells in the pregnant uterus using a modified Clear Unobstructed Brain Imaging Cocktails and Computational analysis (CUBIC) tissue-clearing method and hybrid construct consisting of the cytomegalovirus enhancer fused to the chicken beta-actin promoter (CAG) conjugated enhanced green fluorescent protein (EGFP) transgenic mice. In this study, we applied this method to obtain a transparent whole-image of the ovary and observed the 3D localization of individual oocytes in the developing follicles.

METHODS

Ovarian samples were obtained from EGFP transgenic mice and subjected to nuclear staining with propidium iodide (PI) and CUBIC treatment. The detection of double fluorescence signals (green and red) and subsequent reconstruction of 3D images of the whole ovary were performed by light-sheet microscopy and computer programs, respectively.

RESULTS

The ovary became transparent using the CUBIC method and each nucleus of the follicle component cells was uniformly fluoro-stained by PI perfusion. In contrast, EGFP signals were strong in oocytes, whereas those of surrounding granulosa cells were faint. These signal differences in EGFP expression among oocytes, granulosa cells, and theca-interstitial cells produce well-contrasted images of the growing follicles, providing clear information of the 3D localization of individual oocytes.

CONCLUSION

These results indicate that this procedure is one of the effective approaches to analyze the 3D structure of follicles in the whole ovary.

The chemorepellent draxin is involved in hippocampal mossy fiber projection

Lamina-specific afferent innervation of the mammalian hippocampus is critical for its function. We investigated the relevance of the chemorepellent draxin to the laminar projections of three principal hippocampal afferents: mossy fibers, entorhinal, and associational/commissural fibers. We observed that draxin deficiency led to abnormal projection of mossy fibers but not other afferents. Immunohistochemical analysis indicated that draxin is expressed in the dentate gyrus and cornu ammonis (CA) 3 at postnatal day 0, when dentate granule cells begin to extend mossy fibers towards CA3. Furthermore, a neurite growth assay using dissociated cells of the neonatal dentate gyrus revealed that draxin inhibited the growth of calbindin-D28k-expressing mossy fibers in vitro. Taken together, we conclude that draxin is a key molecule in the regulation of mossy fiber projections.

Expression of Draxin in Lung Carcinomas

Guidance molecules, such as Netrin-1, and their receptors have important roles in controlling axon pathfinding, modulate biological activities of various cancer cells, and may be a useful target for cancer therapy. Dorsal repulsive axon guidance protein (Draxin) is a novel guidance molecule that binds not only common guidance molecule receptors with Netrin-1, but also directly binds the EGF domain of Netrin-1 through a 22-amino-acid peptide (22aa). By immunostaining, Draxin was positively expressed in small cell carcinoma, adenocarcinoma (ADC), and squamous cell carcinoma of the lung. In addition, western blot analysis revealed that Draxin was expressed in all histological types of lung cancer cell lines examined. Knockdown of Draxin in an ADC cell line H358 resulted in altered expression of molecules associated with proliferation and apoptosis. The Ki-67 labeling index of Draxin-knockdown ADC cells was increased compared to that of control ADC cells. In H358 cells, treatment of 22aa induced phosphorylation of histone H3, but did not change apoptosis-associated enzymes. These data suggest that Draxin might be involved in cell proliferation and apoptosis in lung adenocarcinoma cells.

Gyrification of the cerebral cortex requires FGF signaling in the mammalian brain

Although it has been believed that the evolution of cortical folds was a milestone, allowing for an increase in the number of neurons in the cerebral cortex, the mechanisms underlying the formation of cortical folds are largely unknown. Here we show regional differences in the expression of fibroblast growth factor receptors (FGFRs) in the developing cerebral cortex of ferrets even before cortical folds are formed. By taking the advantage of our in utero electroporation technique for ferrets, we found that cortical folding was impaired in the ferret cerebral cortex when FGF signaling was inhibited. We also found that FGF signaling was crucial for producing Pax6-positive neural progenitors in the outer subventricular zone (OSVZ) of the developing cerebral cortex. Furthermore, we found that upper layers of the cerebral cortex were preferentially reduced by inhibiting FGF signaling. Our results shed light on the mechanisms of cortical folding in gyrencephalic mammalian brains.

Pathophysiological analyses of periventricular nodular heterotopia using gyrencephalic mammals

Although periventricular nodular heterotopia (PNH) is often found in the cerebral cortex of people with thanatophoric dysplasia (TD), the pathophysiology of PNH in TD is largely unknown. This is mainly because of difficulties in obtaining brain samples of TD patients and a lack of appropriate animal models for analyzing the pathophysiology of PNH in TD. Here we investigate the pathophysiological mechanisms of PNH in the cerebral cortex of TD by utilizing a ferret TD model which we recently developed. To make TD ferrets, we electroporated fibroblast growth factor 8 (FGF8) into the cerebral cortex of ferrets. Our immunohistochemical analyses showed that PNH nodules in the cerebral cortex of TD ferrets were mostly composed of cortical neurons, including upper layer neurons and GABAergic neurons. We also found disorganizations of radial glial fibers and of the ventricular lining in the TD ferret cortex, indicating that PNH may result from defects in radial migration of cortical neurons along radial glial fibers during development. Our findings provide novel mechanistic insights into the pathogenesis of PNH in TD.

CRISPR/Cas9-Mediated Gene Knockout in the Mouse Brain Using In Utero Electroporation

This unit describes a highly efficient and rapid procedure for brain-specific disruption of genes in the developing mouse brain using pX330 plasmids expressing humanized Cas9 and single-guide RNAs (sgRNAs) against target genes. The pX330 plasmids are delivered into the rodent brain using in utero electroporation. Focusing on the Satb2 gene, which encodes an AT-rich DNA-binding transcription factor, we found that the introduction of pX330-Satb2 induced insertion/deletion (indel) mutations near the predicted cleavage site in the Satb2 gene, resulting in a dramatic reduction of Satb2 expression in post-mitotic neurons. Moreover, introduction of pX330-Satb2 induced abnormalities in axonal projection patterns, which was consistent with the phenotypes observed in Satb2 mutant mice. Thus, the procedure described here, combining the CRISPR/Cas9 system and in utero electroporation, is useful for knocking out genes of interest in the living rodent brain. © 2017 by John Wiley & Sons, Inc.

Inhibitory effects of draxin on axonal outgrowth and migration of precerebellar neurons

The rhombic lip, a dorsal stripe of the neuroepithelium lining the edge of the fourth ventricle, is the site of origin of precerebellar neurons (PCN), which migrate tangentially towards the floor plate. After reaching the floor plate, they project their axons to the cerebellum. Although previous studies have shown that the guidance molecules Netrin/DCC and Slit/Robo have critical roles in PCN migration, the molecular mechanisms underlying this process remain poorly understood. Here, we report that draxin, a repulsive axon guidance protein, is involved in PCN development. We found that draxin is expressed in the rhombic lip and migratory stream of some PCN in the developing hindbrain of mice. In addition, draxin inhibited neurite outgrowth and nuclei migration from rhombic lip explants. These results suggest that draxin functions as a repulsive guidance cue for PCN migration. However, we observed no significant differences in PCN distribution between draxin(-/-) and wild type embryos. Thus, draxin and other axon guidance cues may have redundant roles in PCN migration.

Ancestral functions of Delta/Notch signaling in the formation of body and leg segments in the cricket Gryllus bimaculatus

Delta/Notch signaling controls a wide spectrum of developmental processes, including body and leg segmentation in arthropods. The various functions of Delta/Notch signaling vary among species. For instance, in Cupiennius spiders, Delta/Notch signaling is essential for body and leg segmentation, whereas in Drosophila fruit flies it is involved in leg segmentation but not body segmentation. Therefore, to gain further insight into the functional evolution of Delta/Notch signaling in arthropod body and leg segmentation, we analyzed the function of the Delta (Gb′Delta) and Notch (Gb′Notch) genes in the hemimetabolous, intermediate-germ cricket Gryllus bimaculatus. We found that Gb′Delta and Gb′Notch were expressed in developing legs, and that RNAi silencing of Gb′Notch resulted in a marked reduction in leg length with a loss of joints. Our results suggest that the role of Notch signaling in leg segmentation is conserved in hemimetabolous insects. Furthermore, we found that Gb′Delta was expressed transiently in the posterior growth zone of the germband and in segmental stripes earlier than the appearance of wingless segmental stripes, whereas Gb′Notch was uniformly expressed in early germbands. RNAi knockdown of Gb′Delta or Gb′Notch expression resulted in malformation in body segments and a loss of posterior segments, the latter probably due to a defect in posterior growth. Therefore, in the cricket, Delta/Notch signaling might be required for proper morphogenesis of body segments and posterior elongation, but not for specification of segment boundaries.

Draxin is involved in the proper development of the dI3 interneuron in chick spinal cord

Generation of the appropriate types, numbers and distribution of neurons during the development of the nervous system requires the careful coordination of proliferation, differentiation, and patterning. In this work, we analyzed the roles of a repulsive axon guidance protein, draxin, on the development of chick spinal cord dI3 interneuron. draxin mRNA and/or protein were detected in the roof plate at first and then the boundary region between the ventricular and the mantle zones in chick spinal cord and dorsal basement membrane of the chick spinal cord. Overexpression of draxin caused the decreased and delayed migration of the dI3 interneuron, the reduction of progenitor cell proliferation, and abnormal localization of some ectopic progenitor-like cells in the mantle zone of the spinal cord. Our data reveal that draxin may be involved in the proper development of the dI3 interneuron in chick spinal cord.

Imaging of transgenic cricket embryos reveals cell movements consistent with a syncytial patterning mechanism

The mode of insect embryogenesis varies among species, reflecting adaptations to different life history strategies [1, 2]. In holometabolous insects, which include the model systems, such as the fruit fly and the red flour beetle, a large proportion of the blastoderm produces an embryo, whereas hemimetabolous embryos generally arise from a small region of the blastoderm [3]. Despite their importance in evolutionary studies, information of early developmental dynamics of hemimetabolous insects remains limited. Here, to clarify how maternal and gap gene products act in patterning the embryo of basal hemimetabolous insects, we analyzed the dynamic segmentation process in transgenic embryos of an intermediate-germ insect species, the cricket, Gryllus bimaculatus. Our data based on live imaging of fluorescently labeled embryonic cells and nuclei suggest that the positional specification of the cellular blastoderm may be established in the syncytium, where maternally derived gradients could act fundamentally in a way that is similar to that of Drosophila, namely throughout the egg. Then, the blastoderm cells move dynamically, retaining their positional information to form the posteriorly localized germ anlage. Furthermore, we find that the anterior head region of the cricket embryo is specified by orthodenticle in a cellular environment earlier than the gnathal and thoracic regions. Our findings imply that the syncytial mode of the early segmentation in long-germ insects evolved from a dynamic syncytial-to-cellular mode found in the present study, accompanied by a heterochronic shift of gap gene action.

Olfactory bulb axonal outgrowth is inhibited by draxin

Olfactory bulb (OB) projection neurons receive sensory input from olfactory receptor neurons and precisely relay it through their axons to the olfactory cortex. Thus, olfactory bulb axonal tracts play an important role in relaying information to the higher order of olfactory structures in the brain. Several classes of axon guidance molecules influence the pathfinding of the olfactory bulb axons. Draxin, a recently identified novel class of repulsive axon guidance protein, is essential for the formation of forebrain commissures and can mediate repulsion of diverse classes of neurons from chickens and mice. In this study, we have investigated the draxin expression pattern in the mouse telencephalon and its guidance functions for OB axonal projection to the telencephalon. We have found that draxin is expressed in the neocortex and septum at E13 and E17.5 when OB projection neurons form the lateral olfactory tract (LOT) rostrocaudally along the ventrolateral side of the telencephalon. Draxin inhibits axonal outgrowth from olfactory bulb explants in vitro and draxin-binding activity in the LOT axons in vivo is detected. The LOT develops normally in draxin-/- mice despite subtle defasciculation in the tract of these mutants. These results suggest that draxin functions as an inhibitory guidance cue for OB axons and indicate its contribution to the formation of the LOT.

Draxin, an axon guidance protein, affects chick trunk neural crest migration

The neural crest is a multipotent population of migratory cells that arises in the central nervous system and subsequently migrates along defined stereotypic pathways. In the present work, we analyzed the role of a repulsive axon guidance protein, draxin, in the migration of neural crest cells. Draxin is expressed in the roof plate of the chick trunk spinal cord and around the early migration pathway of neural crest cells. Draxin modulates chick neural crest cell migration in vitro by reducing the polarization of these cells. When exposed to draxin, the velocity of migrating neural crest cells was reduced, and the cells changed direction so frequently that the net migration distance was also reduced. Overexpression of draxin also caused some early migrating neural crest cells to change direction to the dorsolateral pathway in the chick trunk region, presumably due to draxin's inhibitory activity. These results demonstrate that draxin, an axon guidance protein, can also affect trunk neural crest migration in the chick embryo.

Draxin, a repulsive axon guidance protein, is involved in hippocampal development

The hippocampus plays an essential role in learning and memory and is one of the major sites implicated in neural diseases. The proper organization of the hippocampus during development is important for its function. We found that draxin, a repulsive axon guidance cue, was widely expressed in the developing hippocampus and draxin deficient mice possessed a smaller hippocampus, particularly in the anterior part of the structure. Quantification of this reduction revealed that the volume of the dentate gyrus of the mutant was significantly smaller compared to the normal counterpart. This size reduction seemed to be dependent on apoptosis rather than due to a decrease in the rate of cell division.

Draxin, a repulsive guidance protein for spinal cord and forebrain commissures

Axon guidance proteins are critical for the correct wiring of the nervous system during development. Several axon guidance cues and their family members have been well characterized. More unidentified axon guidance cues are assumed to participate in the formation of the extremely complex nervous system. We identified a secreted protein, draxin, that shares no homology with known guidance cues. Draxin inhibited or repelled neurite outgrowth from dorsal spinal cord and cortical explants in vitro. Ectopically expressed draxin inhibited growth or caused misrouting of chick spinal cord commissural axons in vivo. draxin knockout mice showed defasciculation of spinal cord commissural axons and absence of all forebrain commissures. Thus, draxin is a previously unknown chemorepulsive axon guidance molecule required for the development of spinal cord and forebrain commissures.

even-skipped has gap-like, pair-rule-like, and segmental functions in the cricket Gryllus bimaculatus, a basal, intermediate germ insect (Orthoptera)

Developmental mechanisms of segmentation appear to be varied among insects in spite of their conserved body plan. Although the expression patterns of the segment polarity genes in all insects examined imply well conserved function of this class of genes, expression patterns and function of the pair-rule genes tend to exhibit diversity. To gain further insights into the evolution of the segmentation process and the role of pair-rule genes, we have examined expression and function of an ortholog of the Drosophila pair-rule gene even-skipped (eve) in a phylogenetically basal insect, Gryllus bimaculatus (Orthoptera, intermediate germ cricket). We find that Gryllus eve (Gb'eve) is expressed as stripes in each of the prospective gnathal, thoracic, and abdominal segments and as a broad domain in the posterior growth zone. Dynamics of stripe formation vary among Gb'eve stripes, representing one of the three modes, the segmental, incomplete pair-rule, and complete pair-rule mode. Furthermore, we find that RNAi suppression of Gb'eve results in segmentation defects in both anterior and posterior regions of the embryo. Mild depletion of Gb'eve shows a pair-rule-like defect in anterior segments, while stronger depletion causes a gap-like defect showing deletion of anterior and posterior segments. These results suggest that Gb'eve acts as a pair-rule gene at least during anterior segmentation and also has segmental and gap-like functions. Additionally, Gb'eve may be involved in the regulation of hunchback and Krüppel expression. Comparisons with eve functions in other species suggest that the Gb'eve function may represent an intermediate state of the evolution of pair-rule patterning by eve in insects.

brachyenteron is necessary for morphogenesis of the posterior gut but not for anteroposterior axial elongation from the posterior growth zone in the intermediate-germband cricket Gryllus bimaculatus

In the long-germband insect Drosophila, all body segments and posterior terminal structures, including the posterior gut and anal pads, are specified at the blastoderm stage. In short- and intermediate-germband insects, however, posterior segments are sequentially produced from the posterior growth zone, a process resembling somitogenesis in vertebrates, and invagination of the posterior gut starts after anteroposterior (AP) axial elongation from the growth zone. The mechanisms underlying posterior segmentation and terminal patterning in these insects are poorly understood. In order to elucidate these mechanisms, we have investigated the roles of the Brachyury/brachyenteron (Bra/byn) homolog in the intermediate-germband cricket Gryllus bimaculatus. Loss-of-function analysis by RNA interference (RNAi) revealed that Gryllus byn (Gb'byn) is not required for AP axial elongation or normal segment formation, but is required for specification of the posterior gut. We also analyzed Gryllus caudal (Gb'cad) RNAi embryos using in situ hybridization with a Gb'byn probe, and found that Gb'cad is required for internalization of the posterior gut primordium, in addition to AP axial elongation. These results suggest that the functions of byn and cad in posterior terminal patterning are highly conserved in Gryllus and Drosophila despite their divergent posterior patterning. Moreover, because it is thought that the progressive growth of the AP axis from the growth zone, controlled by a genetic program involving Cdx/cad and Bra/byn, might be ancestral to bilaterians, our data suggest that the function of Bra/byn in this process might have been lost in insects.

Krüppel acts as a gap gene regulating expression of hunchback and even-skipped in the intermediate germ cricket Gryllus bimaculatus

In Drosophila, a long germ insect, segmentation occurs simultaneously across the entire body. In contrast, in short and intermediate germ insects, the anterior segments are specified during the blastoderm stage, while the remaining posterior segments are specified during later stages. In Drosophila embryos, the transcriptional factors coded by gap genes, such as Krüppel, diffuse in the syncytial environment and regulate the expression of other gap, pair-rule, and Hox genes. To understand the segmentation mechanisms in short and intermediate germ insects, we investigated the role of Kr ortholog (Gb'Kr) in the development of the intermediate germ insect Gryllus bimaculatus. We found that Gb'Kr is expressed in a gap pattern in the prospective thoracic region after cellularization of the embryo. To determine the function of Gb'Kr in segmentation, we analyzed knockdown phenotypes using RNA interference (RNAi). Gb'Kr RNAi depletion resulted in a gap phenotype in which the posterior of the first thoracic through seventh abdominal segments were deleted. Analysis of the expression patterns of Hox genes in Gb'Kr RNAi embryos indicated that regulatory relationships between Hox genes and Kr in Gryllus differ from those in Oncopeltus, another intermediate germ insect. Furthermore, we found that Gb'Kr regulates expression minimally of hunchback and even-skipped, directly or indirectly, in the prospective thoracic region. Our findings suggest that Gb'Kr is a gap gene that acts in the cellular environment and is required for segmentation in the thoracic and abdominal regions through the regulation of gap and pair-rule gene expression.

Expression patterns of the homeotic genes Scr, Antp, Ubx, and abd-A during embryogenesis of the cricket Gryllus bimaculatus

We have studied embryogenesis of the two-spotted cricket Gryllus bimaculatus as an example of a hemimetabolous, intermediate germ insect, which is a phylogenetically basal insect and may retain primitive features. We observed expression patterns of the orthologs of the Drosophila homeotic genes, Sex combs reduced (Scr), Antennapedia (Antp), Ultrabithorax (Ubx) and abdominal-A (abd-A) during embryogenesis and compared the expression patterns of these genes with the more basal thysanuran insect, Thermobia domestica (the firebrat), and the derived higher dipteran insect, Drosophila melanogaster. Although Scr is expressed commonly in the presumptive posterior maxillary and labial segment in all three insects, the thoracic expression domains vary. Antp is expressed similarly in the three thoracic segments, the limbs, and the anterior abdominal region among these three insects. The early Antp expression in the firebrat and cricket obeys a segmental register in all three thoracic segments, while in Drosophila its initial expression appears in parasegments 4 and 6. Ubx is expressed in the metathoracic (T3) and abdominal segments similarly in the three insects, whereas the expression pattern in the T3 leg differs among them. abd-A is expressed in the posterior compartment of the first abdominal segment and the remaining abdominal segments in all three insects, although its posterior border varies among them.

caudal is required for gnathal and thoracic patterning and for posterior elongation in the intermediate-germband cricket Gryllus bimaculatus

Although the molecular mechanisms directing anteroposterior patterning of the Drosophila embryo (long-germband mode) are well understood, how these mechanisms were evolved from an ancestral mode of insect embryogenesis remains largely unknown. In order to gain insight into mechanisms of evolution in insect embryogenesis, we have examined the expression and function of the orthologue of Drosophila caudal (cad) in the intermediate-germband cricket Gryllus bimaculatus. We observed that a posterior (high) to anterior (low) gradient in the levels of Gryllus bimaculatus cad (Gb' cad) transcript was formed in the early-stage embryo, and then Gb' cad was expressed in the posterior growth zone until the posterior segmentation was completed. Reduction of Gb' cad expression level by RNA interference resulted in deletion of the gnathum, thorax, and abdomen in embryos, remaining only anterior head. We found that the gnathal and thoracic segments are formed by Gb' cad probably through the transcriptional regulation of gap genes including Gb' hunchback and Gb' Kruppel. Furthermore, Gb' cad was found to be involved in the posterior elongation, acting as a downstream gene in the Wingless/Armadillo signalling pathways. These findings indicate that Gb' cad does not function as it does in Drosophila, suggesting that regulatory and functional changes of cad occurred during insect evolution. Since Wnt/Cdx pathways are involved in the posterior patterning of vertebrates, such mechanisms may be conserved in animals that undergo sequential segmentation from the posterior growth zone.

Non-canonical functions of hunchback in segment patterning of the intermediate germ cricket Gryllus bimaculatus

In short and intermediate germ insects, only the anterior segments are specified during the blastoderm stage, leaving the posterior segments to be specified later, during embryogenesis, which differs from the segmentation process in Drosophila, a long germ insect. To elucidate the segmentation mechanisms of short and intermediate germ insects, we have investigated the orthologs of the Drosophila segmentation genes in a phylogenetically basal, intermediate germ insect, Gryllus bimaculatus(Gb). Here, we have focused on its hunchback ortholog(Gb'hb), because Drosophila hb functions as a gap gene during anterior segmentation, referred as a canonical function. Gb'hb is expressed in a gap pattern during the early stages of embryogenesis, and later in the posterior growth zone. By means of embryonic and parental RNA interference for Gb'hb, we found the following: (1) Gb'hb regulates Hox gene expression to specify regional identity in the anterior region, as observed in Drosophila and Oncopeltus; (2) Gb'hb controls germband morphogenesis and segmentation of the anterior region, probably through the pair-rule gene, even-skipped at least; (3) Gb'hb may act as a gap gene in a limited region between the posterior of the prothoracic segment and the anterior of the mesothoracic segment; and (4) Gb'hb is involved in the formation of at least seven abdominal segments, probably through its expression in the posterior growth zone, which is not conserved in Drosophila. These findings suggest that Gb'hb functions in a non-canonical manner in segment patterning. A comparison of our results with the results for other derived species revealed that the canonical hbfunction may have evolved from the non-canonical hb functions during evolution.

Expression patterns of dachshund during head development of Gryllus bimaculatus (cricket)

We report that Gryllus bimaculatus dachshund (Gbdac), a cricket homologue of Drosophila dachshund (Dmdac), is expressed in the developing eye and brain. During brain development, Gbdac was first expressed in the medial head region, corresponding to a part of developing protocephalic region, and expressed in the primordial and adult Kenyon cells. During eye development, Gbdac was first expressed in the lateral head region, becoming to the eye primordium and a part of the deutocerebrum. Then, Gbdac was expressed in the posterior region of the eye primordium, prior to the formation of compound eyes. The expression domain shifted to the anterior domain concomitantly with the movement of morphogenetic furrows. Gbdac was also expressed in the developing optic lobes during differentiation of the retina. These expression patterns were compared with those of Dmdac. We found that although developmental processes of the Gryllus eye and brain differ from those of the Drosophila ones, the expression patterns of Gbdac are essentially similar to those of the Dmdac.

Location of micropyles and early embryonic development of the two-spotted cricket Gryllus bimaculatus (Insecta, Orthoptera)

Early embryogenesis of the two-spotted cricket Gryllus bimaculatus was examined by scanning electron microscopy and several fluorescence staining methods, with special reference to these four issues: (i) the location of micropyles; (ii) the transfer of the female pronucleus following meiosis; (iii) the timing of cellularization; and (iv) the process of the germ primordium formation. Between two and four micropyles lie in the mid-ventral region of the egg. The egg nucleus is at the mid-dorsal periphery of the new laid egg, and meiosis resumes and is completed there. The female pronucleus moves to the mid-ventral side, and fertilization occurs there. Energid starts to proliferate and migrates to the periphery of the egg, initiating blastoderm formation. Actin caps surround each superficial nucleus. Cellularization occurs during the blastoderm stage. At a late blastoderm stage, nuclei aggregate in both the posterolateral patch-like regions of the egg to form a germ primordium. The germ primordium looks like a pair of dumbbells. Both the patches shift towards the ventral side and fuse into a germ primordium. The germ primordium contracts to produce a clearly delineated germ band. Observations on distribution patterns of F-actin indicate that, all through the process, the germ primordium retains that unity, and is not separated into two parts.

piggyBac-mediated somatic transformation of the two-spotted cricket, Gryllus bimaculatus

Transgenic insects have been artificially produced to study functions of interesting developmental genes, using insect transposons such as piggyBac. In the case of the cricket, however, transgenic animals have not yet been successfully artificially produced. In the present study, we examined whether the piggyBac transposon functions as a tool for gene delivery in embryos of Gryllus bimaculatus. We used either a piggyBac helper plasmid or a helper RNA synthesized in vitro as a transposase source. An excision assay revealed that the helper RNA was more effective in early Gryllus eggs to transpose a marker gene of eGFP than the helper plasmid containing the piggyBac transposase gene driven by the G. bimaculatus actin3/4 promoter. Further, only when the helper RNA was used, somatic transformation of the embryo with the eGFP gene was observed. These results suggest that the piggyBac system with the helper RNA may be effective for making transgenic crickets.